Invited paper

Silicon based hot electron short wave infrared detection technology (cover paper·invited)
Wen Xinhao, Jia Yu, Yu Leyong, Shao Li, Chen Hui, Xia Chaojie, Tang Linlong, Shi Haofei
2024, 53(4): 20240116. doi: 10.3788/IRLA20240116
[Abstract](42) [FullText HTML] (4) [PDF 6513KB](20)
  Significance  Short wave infrared detectors, as a very important type of detector, play a crucial role in sensing and obtaining target image information. Their notable features include the ability to penetrate smoke, high spatial recognition, all-weather working ability, and applicability in harsh weather conditions, making it widely applicable in multiple fields of national major needs and national economic development. In the military field, shortwave infrared detectors, with their unique night vision and covert reconnaissance functions, have become a key tool for enhancing combat capabilities at night and in adverse weather conditions. In the field of security monitoring, it provides strong technical support for video monitoring under low or no light conditions, significantly enhancing security capabilities. In terms of environmental monitoring, these detectors provide valuable data support for environmental protection and climate research by accurately measuring specific components in the atmosphere. In addition, in the medical field, the application of shortwave infrared detectors in disease diagnosis has opened up new paths for medical technology innovation. Therefore, in-depth research on shortwave infrared detectors has important practical significance.  Progress  This article systematically reviews the photoelectric conversion mechanism of Schottky photodetector, and summarizes and analyzes recent research results at home and abroad around the basic physical processes of hot electrons. This article first introduces the formation and basic characteristics of metal silicon Schottky junctions, and explores the three core processes of hot electron generation, transmission, and injection. Next, in terms of the generation of hot electrons, a review is conducted on the relevant work of researchers to improve the efficiency of hot electron generation through methods such as light absorption enhancement and thermal loss suppression. In terms of the transfer of hot electrons, the current proposed methods to control the initial position, initial energy and momentum, and mean-free path of hot electrons have been summarized to improve the transfer efficiency of hot electrons. In the injection method of hot electrons, strategies to improve injection efficiency such as multiple Schottky junctions and interface engineering were introduced. In addition, considering the crucial impact of dark current on detector performance, this article also explores current methods for suppressing dark current. Finally, this article provides an outlook on the future development direction of this field.  Conclusions and Prospects  Silicon-based hot electron detection technology holds the potential to broaden the response band of silicon to include the short-wave infrared band, while maintaining compatibility with silicon-based semiconductor processes. Its advantages, including low cost and high uniformity, bode well for its significant role in diverse fields such as military applications, security, and environmental monitoring. Looking ahead, it is imperative to delve deeper into the research of novel materials, structures, and mechanisms to further enhance the detector's performance. By focusing on developing new materials that can enhance the mean-free path of electrons and optimize the density of states, the transport efficiency of hot electrons can be boosted. Concurrently, the pursuit of innovative structures that efficiently absorb wide-spectrum infrared light, coupled with the optimization of the Schottky interface to increase hot electron injection efficiency and minimize dark current, is paramount. Moreover, exploring novel photoelectric conversion mechanisms that transcend the constraints of classical frameworks offers a promising avenue for pioneering advancements in infrared detection technology.
Advances of laser range-gated three-dimensional imaging (invited)
Wang Xinwei, Sun Liang, Zhang Yue, Song Bo, Xia Chenhao, Zhou Yan
2024, 53(4): 20240122. doi: 10.3788/IRLA20240122
[Abstract](17) [FullText HTML] (2) [PDF 8522KB](12)
  Significance   Traditional light detection and ranging (LiDAR) can obtain point cloud data of three-dimensional (3D) scenes, but it is often difficult to obtain high-quality intensity images. Therefore, a technical solution that combines LiDAR and cameras is usually used, where LiDAR senses 3D spatial information and cameras obtain high-definition texture images of the scene. However, this composite technical solution faces the problem of heterogeneous data fusion. For example, in self-driving and driver assistance systems there are different working distances of the two sensors under severe weather or low light level conditions, and it is hard to achieve effective data fusion, which leads to performance degradation or failure. With the advent of the artificial intelligence era, light ranging and imaging (LiRAI) that simultaneously obtains high-resolution intensity images and dense 3D images of targets and scenes, has become a development trend of LiDAR. That means a single sensor can realize light ranging and imaging instead of light detection and ranging, and thus the heterogeneous data mismatch problem of LiDAR and camera composite technology can be solved. In essence, laser range-gated 3D imaging (Gated3D) technology is a kind of gated LiRAI, since it can utilize a single gated camera to simultaneously obtain high-quality 2D intensity images and high-resolution 3D images. Gated3D has gained much attention in the applications of long-range surveillance, advanced driving assistance system and underwater imaging, owing to its long working distance, fast imaging speed, high resolution and the ability to suppress medium backscattering noise. Unlike traditional imaging methods that indiscriminately capture targets and backgrounds within the field-of-view, laser range-gated imaging selectively captures targets within a specific distance range-of-interest (ROI), which filters out medium backscattering noise in the imaging chain, as well as background noise outside the ROI, thereby increasing the imaging distance and enhancing the image quality. Moreover, different from traditional scanning LiDAR, the Gated3D technology employs gated cameras beyond megapixels, and thus offers spatial resolutions surpassing mechanical scanning LiDAR and outperforming flash LiDAR based on avalanche photodiode (APD) arrays. Over the past decade, there has been significant progress domestically and internationally in the development of Gated3D technologies. These advancements have led to the achievement of super range resolution 3D imaging, and promoted their applications.  Progress   This paper systematically reviews the advances of Gated3D technologies in conjunction with its applications across various fields. It introduces the working principles of different technologies such as time slicing, gain modulation and range-intensity correlation methods. Their imaging characteristics of working distance, range resolution, imaging speed and depth of field are discussed. In recent years, the applications of Gated3D technologies have been explored in remote surveillance, automatic driving, vegetation measurement, marine life observation, underwater obstacle avoidance and so on. The results indicate that the technology readiness level (TRL) of range-intensity correlation 3D imaging technology is relatively high, generally reaching TRL5-7. It can fully utilize the correlated information between target distance and image intensity in gated images, enabling real-time super-resolution 3D imaging with fewer gated images. The application of deep learning techniques has further improved the performance of range-intensity correlation method. Finally, the paper analyzes the challenges and further development directions and application prospects faced in laser range-gated 3D imaging technology.  Conclusions and Prospects   We believe that LiRAI will be the trend of LiDAR. LiRAI refers that with the help of active illumination, it does not rely on ambient light level, and uses a single sensor to simultaneously obtain high-resolution intensity images that reflect the radiation characteristics and texture characteristics of targets, as well as dense point cloud data/3D images that reflect the 3D spatial information of targets and their scene, and has long working distance with a certain ability of imaging through scattering medium. The Gated3D technology utilizes a single gated camera to simultaneously obtain high-quality 2D intensity images and high-resolution 3D images. The pixels in 2D images correspond one-to-one with the voxels in 3D images, inheriting the technical advantages of laser range-gated imaging through scattering medium. It has great potential to achieve high-performance LiRAI. The development trends of Gated3D are expected to focus on long-distance imaging in fog, rain, snow, smoke, dust, and underwater conditions, high-resolution fast 3D imaging in large depth of view, and high-performance color LiRAI. In the future, with the support of computational imaging and artificial intelligence, Gated3D will achieve faster, higher precision, longer working distance, more imaging functions, higher sensing dimensions, stronger adaptability to complex environments, and thus meet diverse scenario task requirements.
Research progress on polarimetric imaging technology in complex environments based on deep learning (invited)
Hu Haofeng, Huang Yizhao, Zhu Zhen, Ma Qianwen, Zhai Jingsheng, Li Xiaobo
2024, 53(3): 20240057. doi: 10.3788/IRLA20240057
[Abstract](331) [FullText HTML] (29) [PDF 3058KB](166)
  Significance  Polarization information, as one of the fundamental physical characteristics of light waves, can provide information about the intrinsic properties of the target. Polarimetric imaging technology digitizes the polarization information of the measured target field through digital processing. This approach effectively reduces the interference from the light propagation environment, thereby improving the imaging quality of the target and enhancing perception of its characteristics. In complex environments, polarimetric imaging has significant advantages. However, in complex environments such as scattering and low illumination, the degradation mechanism of polarized images exhibits nonlinear characteristics, leading to high complexity in polarized information interpretation methods. Deep learning methods possess powerful feature extraction and learning capabilities, enabling the recovery of polarized information by learning the mapping rules hidden in the large-scale collected data. This approach is particularly suitable for complex signal processing problems like polarimetric imaging, which involves multiple dimensions and interrelated signals.  Progress   First, the basic theory of the polarimetric imaging is introduced, including the principles of polarimetric imaging and a macroscopic description of polarimetric imaging issues in complex environments. Next, the general workflow of deep learning polarization imaging technology in complex environments is introduced. Based on deep learning, polarimetric imaging technology in complex environments uses the multi-dimensional polarimetric parameters collected by the polarimetric imaging system as input data. It leverages the nonlinear feature-fitting capabilities of neural networks to obtain image restoration results. Essentially, this approach transforms the nonlinear inverse problem of polarimetric imaging restoration in complex environments into a pseudo-forward problem, avoiding the challenges associated with solving nonlinear inverse problem algorithms. The representative developments of research in deep learning polarimetric imaging technology in response to scattering and noise, two of the most representative complex imaging environments, have been elaborated. From the inception of research in this field, the developmental trajectory of the field has been systematically outlined. In the early stages, polarimetric imaging technology in complex environments based on deep learning primarily relied on supervised training. Due to the challenges in collecting real-world data, researchers explored solutions using unsupervised, self-supervised, transfer learning, and simulation algorithms. Researchers also delved into the incorporation of prior knowledge and physical models into networks, leading to training approaches embedded with physical models or guided by prior knowledge. Overall, these representative works have made significant contributions to addressing the difficulties in constructing large-scale datasets, enhancing the generalization performance of networks, and exploring the interpretability of the networks. To better illustrate the connections and distinctions among research works, and to streamline the developmental process in this field for reader convenience, a summary has been compiled in the form of a table. The table provides task types, training methods, and characteristics of representative works for easy reference.  Conclusions and Prospects   With the rapid development of deep learning, polarimetric imaging technology in complex environments has achieved remarkable research progress. Existing studies indicate that, due to the multiple parameters and inherent correlations in polarized information, this multi-dimensional and interrelated signal processing problem is well-suited for the application of deep learning. The combination of deep learning and polarimetric imaging technology enables further improvement in optical imaging quality, meeting the imaging demands of complex environments and demonstrating more prominent advantages. The generalization ability, interpretability, and parameter lightweighting of deep learning technology remain areas that require further in-depth research. There is a continued need for refinement in multimodal fusion strategies, exploration of the underlying principles of network polarimetric parameter image restoration, and the design of network structures tailored for polarized multidimensional data to enhance real-time performance. Further efforts are essential to consolidate the feasibility of deep learning models in polarimetric imaging within complex environments, to enhance the adaptability of models to changes in complex environmental conditions, and to make them more universally applicable across different scenarios.
Atmospheric optical turbulence prediction method for satellite-ground laser communication (invited)
Guo Yingchi, Li Lang, Li Chen, Gao Chunqing, Fu Shiyao
2024, 53(3): 20230729. doi: 10.3788/IRLA20230729
[Abstract](94) [FullText HTML] (9) [PDF 3377KB](47)
  Significance  The prediction of atmospheric turbulence has great significance both in science and engineering, which provides key parameters and references for domains like astronomical observation, site selection, satellite-ground laser communication, and remote sensing. Especially in satellite-ground laser communication, predicting key parameters of atmospheric turbulence can schedule satellite-ground data transmission links in advance, and pre-deploy adaptive optical schemes to compensate turbulence effects, so as to establish effective communication links and suppress the performance degradation of data transmission. Therefore, atmospheric turbulence prediction is crucial and become an important issue, which needs to be addressed for most of laser scenarios in atmosphere.  Progress   This review consists of three sections. In the first section, firstly, the widely used meso-scale numerical prediction scheme to forecast atmospheric turbulence is introduced in detail. This scheme is accomplished by turbulence parameterization schemes, which establishes the relationship between the turbulence characteristics and the conventional meteorological parameters output from mesoscale meteorological model. Mesoscale meteorological model has been well developed, the most representative models include Meso-Nh(Non-hydrostatic mesoscale atmospheric model), MM5(Mesoscale Model 5), WRF(Weather Research & Forecasting Model) and Polar WRF. Many achievements have been made in turbulence parameterization schemes, including Hufmagel model, Tatarski model. Then, the relevant work of using mesoscale numerical prediction method to forecast atmospheric turbulence in typical regions is reviewed.　　The second section presents recent advances regarding deep learning in atmospheric turbulence prediction, and discusses its advantages and limitations. This section first introduces the research achievements of deep learning in meteorological forecasting, and then introduces the research advances of deep learning in atmospheric turbulence forecasting. Based on a large amount of data, deep learning scheme can establish a relationship between the input data and the target label without any prior formula. In atmospheric turbulence prediction, deep learning is used to establish the relationship between meteorological parameters and atmospheric turbulence parameters, but the prediction accuracy is also limited by the accuracy of meteorological parameters.　　In the third section, a short-time atmospheric coherence length prediction method called TsVMD-AR is introduced. TsVMD-AR model uses VMD (variational mode decomposition) algorithm and AR (autoregression) algorithm to forecast the short-term atmospheric coherence length. This scheme reduces the interference and coupling between the multi-scale feature information in the dataset, makes the complex internal features of the dataset easier to obtain. The results show that the established TsVMD-AR model is obviously superior to other models and is suitable for daily atmospheric turbulence prediction.  Prospects   We hope this review will provide more valuable information for people who is working in scenarios of laser applications in atmosphere turbulence, and inspire more wonderful ideas towards abilities of more accurate and faster turbulence grasp.
Athermalization design for annular aperture folding imaging system based on light-digital combination (cover paper·invited)
Ma Dechao, Piao Mingxu, Xie Yafeng, Zhao Yuanming, Niu Qun, Zhang Chengran, Wang Zhe, Zhang Bo
2024, 53(3): 20240013. doi: 10.3788/IRLA20240013
[Abstract](146) [FullText HTML] (14) [PDF 4660KB](33)
  Objective  With the advancement and development of science and technology, the demand for miniaturized optical systems is becoming increasingly significant. The total length of the optical system can be reduced by folding the optical path in the annular aperture folding imaging system. However, the annular aperture folding imaging system only uses a piece of base material, so the annular aperture folding imaging system cannot achieve high-quality imaging in a wide temperature range. In order to reduce the impact of temperature on the imaging quality and simplify the optical system structure, the wavefront encoding method is introduced to design a light-digital combined annular aperture folding imaging system.  Methods  The design principle of the annular aperture folding imaging system based on light-digital is studied. The relationship between the obscuration ratio and the phase mask parameters is studied to achieve defocus consistency (Fig.5). In the image decoding part, the image restoration effect is analyzed. The synthetic PSF model is studied through simulated annealing algorithm (Fig.2). High-quality imaging in a wide temperature range is achieved through a light-digital combination method.  Results and Discussions  A long-wave infrared annular aperture folding imaging system is designed (Fig.3). The focal length is 70 mm, the system aperture is 98 mm, the full field of view is 8°, and the total length is 25 mm. The synthesized PSF is constructed by simulated annealing algorithm. When over a wide temperature range, the high-quality image restoration is achieved through a single filter. Although the PSNR of the restored image dropped by 3.572 3 dB at the design temperature, the PSNR of the restored image at −40 ℃ also increased from 19.417 3 dB to 24.461 5 dB, which increased by 5.044 2 dB. The PSNR of the restored image at 60 ℃ also increased from 19.751 9 dB to 24.460 9 dB, which increased by 4.709 0 dB. This method outperforms traditional PSF image restoration at the design temperature. Image artifacts and blur are significantly reduced by this method.  Conclusions  Athermalization of infrared annular aperture folding imaging system is achieved. The light-digital combination method is introduced into the annular aperture folding imaging system. The annular cubic phase mask is introduced into the annular aperture folding imaging system, and the restored image is achieved through image restoration. The relationship between central obscuration and phase mask parameters is studied. The conclusion that increasing central obscuration will reduce PSF consistency is studied. The synthesized PSF is constructed by simulated annealing algorithm. When over a wide temperature range, high-quality image restoration is achieved through a single filter. In order to verify the effectiveness of this theoretical model, an annular aperture folding imaging system based on light-digital combination is designed. The total length is 25 mm, the focal length is 70 mm, the system aperture is 98 mm, and the full field of view is 8°. High-quality image restoration is achieved by synthesizing PSF when the temperature is between −40 ℃ and 60 ℃. Although the PSNR of the restored image dropped by 3.572 3 dB at the design temperature, the PSNR of the restored image at −40 ℃ also increased from 19.417 3 dB to 24.461 5 dB, which increased by 5.044 2 dB. The PSNR of the restored image at 60 ℃ also increased from 19.751 9 dB to 24.460 9 dB, which increased by 4.709 0 dB. Compared with PSF image restoration at design temperature, this method significantly reduces image artifacts. The study not only simplifies the infrared imaging optical system, but also uses a light-digital combination method to overcome the temperature limitations of the annular aperture folding imaging system. A new idea is provided for the miniaturization of infrared systems across a wide temperature range.
Research progress of optoelectronic quantum devices (cover paper·invited)
Song Haizhi, Zhang Zichang, Zhou Qiang, Deng Guangwei, Dai Qian, Wang You
2024, 53(1): 20230560. doi: 10.3788/IRLA20230560
[Abstract](230) [FullText HTML] (48) [PDF 3897KB](88)
  Significance   Quantum information science has now attracted significant attention, since it has been well proved and is believed to support quantum computation, quantum communication and quantum metrology in the near future. Characteristics of quantum states have opened the opportunities to accomplish tasks beyond classical limits, resulting in a frontier field of quantum technologies. Among them, quantum computation technology can accelerate the speed of computers exponentially with respect to the classical machine. Quantum communication technology guarantees completely secure communication, and quantum measurement technology can greatly optimize the sensitivity and/or resolution of many instruments. These potential accomplishments have led to the development of innovative and advanced applications in various fields, and therefore people are presently struggling to construct efficient quantum information systems and quantum networks. To realize practical quantum information systems and quantum networks, fundamental devices must be firstly well developed. The successful fabrication of superconductor quantum circuit chips led to an achievement of constructing quantum computer consists of 127 qubits. Realization of more general quantum computers needs much larger scaled, more robust, more quantum logic circuit chip consisting of probably superconductors, cold atoms, semiconductors, photonic crystals etc. The primary obstacles in establishing a quantum network involve the distribution of entangled qubits among nodes that are physically distant from each other, which needs high-performance entangled photon source and quantum memory. Among various types of quantum devices, optoelectronic devices play a key and central role, since the advanced microelectronic, optical and optoelectronic platforms enable fabricating the building blocks for most of the quantum information processing systems. Technologies based on optoelectronics have the potential to realize a complete product chain in the field of quantum information. This work shows the study or fabrication of optoelectronic quantum devices including single photon sources, photon entanglers, single photon detectors, quantum memories and opto-electro-mechanical sensors.   Progress   Single photon emitters refer to the light sources that release light in the form of individual particles or photons. Single photon emitters are the fundamental devices for quantum communication. They are also well used in quantum detection and photonic quantum computation. In this direction, we have studied single photon emitters based on quantum dot (Fig.1), heralded single-photon sources (Fig.2-3), and a quantum random number generator (Fig.4). Quantum entanglement is a phenomenon that arises when a collection of particles is created, interact, or exist in close proximity to each other in such a manner that the individual quantum states of each particle cannot be figured out independently from the states of the others, even if these particles are widely separated. As a fundamental resource, quantum entangled light sources are widely used in quantum information processing. We have made a comprehensive study on the performance improvement (Fig.5-7), chip integration (Fig.8) and application (Fig.9-10) of entangled photon sources. A single photon detector is a photodetector which can respond to incident light signal as weak as one single photon. Single photon detectors play a widespread role in the field of quantum information processing since they serve as key devices for, e.g., readout in quantum computing, receiving in quantum communication and photon measurement in quantum metrology. This research is focused on specially designed single photon avalanche detectors (Fig.11), focal-plane single photon avalanche detectors (Fig.12), and negative feedback avalanche diodes (Fig.12). Moreover, we have proposed fiber Bragg grating sensing system by utilizing single photon detectors (Fig.13). In addition to the optoelectronic devices described above, we have also conducted abundant research on fiber-based quantum memory (Fig.14), optomechanical quantum device (Fig.15) and nano-opto-electro-mechanical system (Fig.16). All our studies will impact on the application of quantum technologies.   Conclusions and Prospects   In order to realize practical quantum systems in the future, our group have made efforts to create and investigate quantum devices by using optoelectronic techniques. QD-embedded nanocavities were designed to improve the efficiency of and to realize on-demand single photon emitters. Spectral multiplexing technique enabled the fabrication of a heralded single photon source with high purity and speed, approaching on-demand single photon emitting. A quantum random number generator working at room temperature was constructed based on single photon emitting from defects in commercial GaN material. Applying cascaded second-order nonlinear optical process in PPLN waveguides, we developed an entangled photon emitter with fidelity of 97% and noise level nearly 10 times better. Chip-integrated photon entangler with visibility of over 99% was established by fabricating Si3N4 micro-rings via micro/nano-processing. Readout circuits were optimized to help fabricating high quality SPAD devices, and SPAD focal plane devices were improved to 128×32 array for single photon and quantum imaging. A quantum memory was achieved to simultaneously store 1 650 single photons at low temperatures, and a few opto-electro-mechanical devices were experimentally tried to obtain quantum-level measurement ability for minor quantities. Our studies might be a step forward to the realization of practical quantum information networks.
Research on key issues of laser splitting of transparent hard and brittle materials (invited)
Zhao Shusen, He Hongzhi, Han Shifei, Jiang Lu, Du Jiabao, Yu Haijuan, Lin Xuechun, Zhang Guling
2024, 53(1): 20230487. doi: 10.3788/IRLA20230487
[Abstract](181) [FullText HTML] (63) [PDF 3412KB](62)
  Significance   Transparent hard brittle materials have been widely used in the fields of semiconductors and electronics due to their excellent mechanical properties, thermal stability, corrosion resistance, and optoelectronic properties. The traditional slicing method for transparent hard brittle materials has low efficiency and high material loss, which restricts the promotion and application of hard brittle materials. Diamond wire cutting is commonly used in the cutting of high-hardness and brittle materials. The existing substrate processing technology has slow cut speed, and there is a large loss of transparent and brittle materials and cutting lines. Every time a piece of transparent and brittle material is processed, a large amount of wire cutting loss will be caused by wire saw cutting, greatly increasing the cost of splitting transparent and brittle materials. The laser assisted separation technology, which leads to expensive separation processes, is a new method for slicing transparent hard brittle materials in recent years. It revolutionarily utilizes nonlinear optical effects to make laser pass through transparent hard brittle materials, causing a series of physical and chemical processes such as thermal damage and laser induced ionization inside the transparent hard brittle materials, forming a thin modified layer, and ultimately achieving the splitting of transparent hard brittle materials. Compared with traditional diamond wire cutting methods, it greatly improves the slicing efficiency and material utilization of hard and brittle materials. In the field of laser processing of hard and brittle materials, it has developed into a common focus of academic research and industrial applications.   Progress  This article provides an in-depth analysis of the physical process of laser separation of transparent hard brittle materials and summarizes the key scientific issues in the process of laser separation, which are the nonlinear absorption of laser by transparent hard brittle materials, the evolution of the internal microstructure of transparent hard brittle materials under laser action, and the mechanism of the influence of laser field regulation on material modification. Combining special optical design, beam shaping, multi-factor coupling and stripping techniques and based on these scientific issues, this article reviews the research progress of laser separation of different types of transparent hard brittle materials in recent years. At present, materials used for laser separation include semiconductor materials such as SiC (Fig.8), Si, GaN (Fig.12), diamond (Fig.13), and ceramic materials such as sapphire, polycrystalline Al2O3, and zirconia. Laser separation technology has developed multiple splitting methods. For example, ultrafast laser dual pulse induced separate, ultrafast laser chemically assisted splitting, multiple laser composite splitting, etc. Multiple companies and research institutes at home and abroad are actively promoting the research and development of fully automated laser stripping equipment, with laser technology as the core for industrial and specialized manufacturing machines.   Conclusions and Prospects  The physical process of vertical laser detachment is a typical interdisciplinary problem in the thermodynamics of laser materials. Laser splitting can almost completely avoid the cutting loss caused by conventional multi-wire cutting technology. Only the peeled lenses need to be ground and polished, so the loss of each transparent hard brittle material can be significantly reduced to below 100 microns, thereby increasing the production of transparent hard brittle materials. Despite significant breakthroughs and rapid development in experimental results, there is still a lack of in-depth theoretical and numerical simulation research on the process mechanism of laser separate technology. In the future, the vertical laser splitting technology for hard and brittle materials will develop towards ultra-thin material splitting with smaller material losses below 100 microns, low damage of modified layers, and process adaptability. In addition, laser splitting technology can also be applied to the development of transparent hard and brittle materials in areas such as thinning, polishing, and surface modification. This paper provides greater technical support for the rapid development of semiconductors and electronics.
Research progress on enhancing the output power of all-solid-state single-frequency continuous-wave lasers by using intracavity nonlinear loss mode-selecting technology (invited)
Lu Huadong, Li Jiawei, Jin Pixian, Su Jing, Peng Kunchi
2024, 53(1): 20230592. doi: 10.3788/IRLA20230592
[Abstract](125) [FullText HTML] (27) [PDF 2733KB](37)
  Significance:  All-solid-state single-frequency continuous-wave (CW) laser have found extensive applications in diverse domains such as the generation of non-classical light fields, cold atom physics, detection of gravitational waves, and so on, which primarily attributed to their merits of low noise, narrow bandwidth, excellent beam quality, and high power stability. In line with the advancement in science and technology, the output power of the traditional all-solid-state laser (ASSL) cannot satisfy the application requirements of many frontier research fields, so it is necessary to further scale the ASSL power and simultaneously maintain other excellent performance. For the purpose of improving the output power of the ASSL, its pump power has to be primarily elevated. However, with the increasement of the pump power, the laser gain is enhanced, and the non-oscillating laser modes of the ASSL start to oscillate, which results in the mode-hopping or the multi-mode oscillating operation of the ASSL. Moreover, the severe thermal effect of the laser gain medium and its relatively lower damage threshold also further restrict the improvement of the ASSL power. In this paper, an effective method of improving the all-solid-state single-frequency CW laser power via deliberately introducing a nonlinear loss into the resonator was presented. When the nonlinear loss was introduced, the nonlinear loss of the lasing mode was half of that of non-lasing mode, and the non-lasing mode was effectively inhibited, under the mode competition of the laser. As a consequence, the stable single-longitudinal mode operation of the laser can be guaranteed at higher laser gain. In addition, the design of multi-laser-crystal resonator can be adapted to efficiently mitigate the negative impact of the thermal effects of the laser crystal. By combining the nonlinear loss technique and the multi-laser-crystal resonator scheme, the output power of the all-solid-state single-frequency CW laser had been scaled up to 100-watt level and continuously increased.  Progress:   First, the fundamental principle of mode selection implemented by intra-cavity nonlinear loss is presented. When the nonlinear loss is introduced into the resonator, the nonlinear loss of the lasing mode is half of that of the non-lasing mode, and the non-lasing modes are suppressed effectively under the mechanics of mode competition. Based on the principle above, the physical condition of stable SLM operation for ASSL is proposed. The condition depends on the intra-cavity linear and nonlinear losses, which is experimentally validated by changing the transmission of the output coupler. In the experiment, when the output coupler transmission is 19%, and the temperature of the type-I phase matched nonlinear crystal LBO is 149 ℃, the maximal output power of 33.7 W for the stable single-frequency 1064 nm laser is realized. On this basis, the intra-cavity round-trip loss of an ASSL is measured precisely by simply changing the temperature of the nonlinear LBO crystal to manipulate the nonlinear loss within the SLM region of the laser. According to the measured results and the oscillating condition of the ASSL, the output coupler transmission of the designed laser as well as its pump power is further optimized and the maximal output power of 50.3 W for the single-frequency 1064 nm laser is obtained.　　To further increase the output power of the single-frequency laser, the pump power of the laser has to be raised. However, the sever thermal effect of the laser gain medium and its lower damage threshold restrict the continuously increasing of the single-frequency laser power. For the purpose of breaking aforementioned restriction and attaining higher power single-frequency laser, a laser resonator with two identical laser crystals was designed, where the precise mode-reproduction of the two crystals was implemented by a pair of lenses with identical focal length of 100 mm. When the total pump power was 240 W, a single frequency 1064 nm laser with maximal output power of 101 W was realized. In this laser, the focal lengths of the two lenses were fixed, so the laser only would be operated at a given incident pump power, and simultaneously the optical length between the imaging lenses had to be precisely adjusted. To this end, a self-mode-matching laser with four laser crystals in a single resonator was further designed. The total four laser crystals were used for both laser gain media and mode-matching elements. Under an appropriate combination of pump powers on four crystals, a stable CW single-frequency 1064 nm laser with 140 W power was obtained.  Conclusions and Prospects:   Introducing nonlinear losses within the resonator is a robust way to realize SLM laser output, which has been experimentally proved. With multiple gaining crystals inserted in one cavity, the heat load on each crystal is effectively shared, so more total power is tolerable. With suitable mode matching and mode reproducing in the resonant, a high-power single-frequency CW ASSL has been designed and built which can deliver 140 W single-frequency CW laser, this is to our knowledge the highest SLM ASSL power. The progress in high-power single-frequency ASSL has significantly broadened its potential applications and made substantial contributions to the advancement of related disciplines.
Effects of ultrasonic vibration on wear and corrosion resistance of WC particles reinforced coating produced by laser cladding (invited)
Yao Zhehe, Dai Wenke, Zou Pengjin, Yu Peijiong, Wang Fabo, Chi Yiming, Sun Zhenqiang, Zhang Qunli, Yao Jianhua
2024, 53(1): 20230542. doi: 10.3788/IRLA20230542
[Abstract](58) [FullText HTML] (18) [PDF 7605KB](10)
  Objective  Mechanical components in marine and mining fields have long served in harsh environments of mechanical wear and electrochemical corrosion. The interaction of friction and corrosion will accelerate the damage of the component surface and reduce its service life. At present, in order to improve the wear resistance of components, the method of preparing ceramic particle reinforced metal-based composite coating on the surface of the substrate is widely used. Due to the excellent chemical stability, wettability and adhesion, WC particles have become one of the most commonly used ceramic particles in reinforced coating produced by laser cladding. However, under the action of high-energy laser beam, the dissolution of WC particles will change the phase composition and microstructure of the reinforced coating, thus affecting its corrosion resistance. In order to solve the problem that the WC particles reinforced coating is difficult to have both high wear and high corrosion resistance produced by laser cladding, ultrasound is introduced into laser melt injection process. The effects of ultrasonic vibration on the microstructure, microhardness, wear and corrosion resistance of the coating were analyzed. The study provides reference for the preparation of WC particles reinforced coating with high wear and high corrosion resistance.   Methods  The experimental setup for ultrasonic-assisted laser cladding (Fig.1) is mainly composed of fiber-coupled semiconductor laser, cooling system, motion control system, powder feeder and ultrasonic vibration device. The substrate used in the experiments is 316L stainless steel plate. The powder used in the experiments is a mixed powder of 316 powder and WC particles with the mass ratio of 1 : 4, while the particles size are 70-100 μm and 50-100 μm (Fig.1). Based on the developed experimental setup, the laser cladding experiments with and without ultrasound are carried out. After the experiments, the cross section (perpendicular to the laser scanning direction) and the longitudinal section (parallel to the laser scanning direction) of the laser cladding layer are sampled, polished and etched. The microstructure of the sample was characterized by optical microscope, scanning electron microscope and the chemical composition was determined by EDS analysis. Meanwhile, the hardness, wear and corrosion resistance of the cladding layer were tested.   Results and Discussions   After ultrasonic assisted laser cladding, the average grain size around WC decreased from 101.0 μm to 59.6 μm, and the surrounding structure and elements are more uniform (Fig.2-4). Due to the effect of ultrasound, the precipitation of fishbone carbide around WC is inhibited. At the same time, the alloy reaction layer on the surface of the WC is dissolved, resulting in the average microhardness of the sample increasing from 310 HV0.1 to 425 HV0.1, and the hardness distribution around the tungsten carbide particles is more uniform (Fig.6). The wear resistance of the composite coating was further improved by increasing the hardness (Fig.7-8). The mass loss and wear rate of the sample without ultrasonic assisted laser cladding were 8.8 mg and 0.043 8 mg/m, respectively, and the maximum depth of the wear mark was about 53 μm. The mass loss and wear rate of the sample with ultrasonic are 6.5 mg and 0.032 3 mg/m, respectively, and the maximum depth of the wear marks is about 26 μm. The addition of ultrasound reduced the wear rate by 26.2%. In addition, the introduction of ultrasound did not change the overall corrosion open circuit potential and pitting potential of the cladding layer, but it reduced the corrosion current density (Fig.9), improved the penetration resistance of the corrosive medium on the surface of the coating (Fig.10), and improved the corrosion resistance. Ultrasonic vibration assisted laser cladding can dissolve the alloy reaction layer on the surface of tungsten carbide and increase the hardness of the coating through the uniform distribution of acoustic flow, thus improving the wear resistance of the cladding layer. At the same time, due to the cavitation of ultrasound, the epitaxial growth of columnar dendrites is broken, the grains are refined, and a denser grain boundary is formed. In a corrosive environment, a stable and continuous passivation film can be formed faster because of the increase of the grain boundary, thus improving the corrosion resistance of the WC particles reinforced coating.   Conclusions  In this paper, ultrasonic assisted laser cladding technology was used to prepare WC particles reinforced coating. The microstructure, hardness, wear and corrosion resistance of the coating under the influence of ultrasound were compared and analyzed. In the non-ultrasonic cladding layer, a large number of columnar crystals existed around WC, accompanied by some element segregation bands, due to the acoustic cavitation effect of ultrasound. The average grain size around WC in the ultrasonic cladding layer is refined from 101.0 μm to 59.6 μm, and there is no obvious segregation phenomenon; The average microhardness of WC particles strengthened coating without ultrasonic is 310 HV0.1, and the hardness around WC decreases from 480 HV0.1 to 320 HV0.1. The average microhardness of WC particles strengthened coating with ultrasonic is 425 HV0.1, and the hardness around WC decreases from 426 HV0.1 to 413 HV0.1. The weight loss and wear rate of samples without ultrasound were 8.8 mg and 0.0438 mg/m, respectively. The mass loss and wear rate of samples with ultrasound were 6.5 mg and 0.0323 mg/m, respectively. The maximum depth of samples without ultrasonic scratches was about 53 μm, and the maximum depth of samples with ultrasonic was only about 26 μm. The introduction of ultrasound reduced the wear rate by 26.2%. The corrosion current densities of the electrochemical samples with and without ultrasonic are 2.13 μA /cm2 and 5.20 μA /cm2, respectively. Ultrasonic assisted laser cladding of WC particles reinforced coating has better wear and corrosion resistance.
640×512 HgTe colloidal quantum-dot mid-wave infrared focal plane array (invited)
Tan Yimei, Zhang Shuo, Luo Yuning, Hao Qun, Chen Menglu, Liu Yanfei, Tang Xin
2023, 52(7): 20230377. doi: 10.3788/IRLA20230377
[Abstract](668) [FullText HTML] (64) [PDF 2549KB](168)
  Objective  Mid-wave infrared imaging plays an important role in various fields including military reconnaissance, remote sensing, and aerospace. The existing mid-wave infrared focal planes mainly use bulk semiconductor materials such as mercury cadmium telluride, type-II superlattices, and indium antimonide, which have excellent performance and high stability. However, the complex material preparation and flip-chip bonding processes limit the production volume and their usage in cost-sensitive application. As an emerging infrared semiconductor material, colloidal quantum dots (CQDs) have the advantages of wide spectral tunability, large-scale synthesis, and low-cost preparation, providing a new route towards high-performance and low-cost infrared focal plane arrays. For this purpose, HgTe CQDs have been investigated and a mid-wave infrared focal plane array imager has been proposed in this paper.   Methods  Oleylamine was used as the reaction solvent for the synthesis of HgTe CQDs. Inorganic mercury salts and tellurium were dissolved in oleylamine and trioctylphosphine, respectively, at 100 ℃. After mixing them in an anhydrous and oxygen-free environment, the size of the HgTe CQDs can be precisely controlled by the reaction time, thus the response wavelength can be accurately adjusted. The transmission electron microscopy (TEM) image of the HgTe quantum dots used in this experiment is shown (Fig.1), with a diameter of about 8 nm. The response spectra of quantum dots at room temperature and 80 K are shown (Fig.2). The response cut-off wavelength of the quantum dot detector reaches 4.6 μm at 80 K. The HgTe CQDs mid-wave infrared detector uses a trapping-mode photodetector configuration. The device structure and energy band diagram are shown (Fig.3).   Results and Discussions   The diagram of signal extraction and dewar test package is shown (Fig.4). The performance of the trapping-mode infrared focal plane detector is quantitatively analyzed by testing parameters including photoresponse non-uniformity, noise voltage, specific detectivity, and operable pixel rate. A calibrated blackbody is used as the excitation light source, and the temperature of the blackbody is stabilized with a feedback control circuit. The blackbody emitting cavity is about 4 cm in diameter and the distance between the imager and the emitting cavity is about 25 cm. The experimental results show that the non-uniformity of the photoresponse of the focal plane array device is as low as 3.42% (Fig.5(a)). The noise of the detector is an important indicator of performance, which is determined by the noise of the readout circuit itself and the uniformity of the film thickness of the detector pixel points. The overall noise of the detector is low, and the average noise voltage is as low as 0.66 mV at an integration time of 2 ms and a device bias of 2.3 V (Fig.5(b)). The distribution of the specific detectivity, and the average peak specific detectivity is about 2 × 1010 Jones (Fig.5(c)). The operable pixel rate can reach 99.99% (Fig.6).   Conclusions  In this paper, we report a CMOS-compatible trapping-mode HgTe CQDs mid-wave infrared focal plane and demonstrate the infrared thermal imaging capability. With a noise equivalent temperature difference of 51.26 mK (F#=2), a low photoresponse nonuniformity of 3.42%, an operable pixel rate of 99.99%, a response cutoff wavelength of 4.6 μm, and a peak specific detectivity of 2×1010 Jones at 80 K, the HgTe CQDs-based focal plane array is expected to potentially solve the bottlenecks faced by traditional bulk semiconductors. In the future, HgTe CQDs will be combined with 3D nanostructure embossing and other processing technologies to develop multi-functional and multi-mode infrared detectors.
Recent progress and prospect of laser imaging processing technology （invited）
Hu Yihua, Zhao Luda
2023, 52(6): 20230169. doi: 10.3788/IRLA20230169
[Abstract](641) [FullText HTML] (202) [PDF 3162KB](203)
  Significance   Laser imaging refers to an imaging method that emits a specially designed laser signal, receives the laser echo, and processes it to obtain attribute information such as an image of the target. Laser imaging has wide applications in target detection, satellite surveying, smart agriculture, national defense and aerospace, and other fields. It contains a series of signals and information processing processes, including denoising, radiation, geometric correction, point cloud processing of laser echo signals, and subsequent data processing of various imaging tasks (such as laser ranging, laser image reconstruction, target detection, etc.), and have a critical impact on imaging quality and play a crucial role in the application of imaging information. Currently, with the continuous development of imaging systems and imaging hardware, laser imaging processing technology has increasingly high requirements for processing accuracy and speed, and involves a wider range of technical fields. Especially with the rapid development of machine learning technology represented by deep learning, it has achieved better results than traditional technologies in many classic problems, and has also been successfully applied in laser imaging processing technology, providing a new development direction for laser imaging processing.   Progress  This paper first introduces the characteristics of laser imaging processing technology of typical imaging system (Fig.1). We explained the characteristics of imaging processing technologies under various laser imaging systems, identified the similarities and differences between laser imaging processing technologies under different systems, and conducted a comparative analysis of laser imaging processing technologies under typical imaging systems (Tab.1). In summary, it can be found that although there are differences in the names of signal and information processing contents corresponding to different systems, the common contents of laser imaging signal processing can be summarized into four aspects of signal denoising, radiation correction, geometric correction, and point cloud processing. The common contents of imaging information processing can be summarized into three common processing contents of laser ranging, image reconstruction, and object detection. 　　Based on the summarized common methods of laser imaging signals and information processing technology, we conducted separate studies. In the current research status of laser imaging signal processing technology, we focus on the laser signal denoising, correction and laser point cloud processing technology. In the research of signal denoising, we have conducted research based on wavelet transform, empirical mode decomposition, variational mode decomposition, and hybrid methods. We have also conducted specialized research on the application of deep learning algorithms in laser signal denoising. Representative algorithms are shown (Fig.5). The laser signal correction focuses on two aspects of laser signal radiation and geometric correction. And in point cloud signal processing, we mainly summarized the work on denoising and background removal, and focused on the work based on deep learning. Besides, we have organized and summarized the research on laser information processing for laser ranging, image reconstruction and target detection information processing technology. In the section of laser image reconstruction, we conducted research on three aspects of stereo matching, point cloud data stitching, and laser reflection tomography reconstruction. In object detection, the traditional method and deep-learning based method were elaborated, and classic point cloud object detection algorithms based on deep learning algorithms were studied (Fig.9-10). 　　Based on the classification of laser imaging processing technology in this paper, we finally analyzed the current challenges and future development directions of laser imaging processing technologies, and summarized the current development of laser imaging technology and future laser imaging processing technology examples. It is hoped that it can provide some reference for the research related to laser imaging.   Conclusions and Prospects  Laser imaging has always been a hot topic in the field of optical imaging and signal processing. In the past 20 years, laser imaging signal and information processing technology has made great progress. In the previous studies, deep learning has been deeply applied to laser imaging processing. Through the powerful representation learning ability of deep learning, great improvements have been made in laser imaging processing quality, precision, robustness and other aspects. In the future research on different signal and information processing tasks, the standardization of large-scale data sets for imaging tasks and more robust deep neural network processing paradigm will be the further development direction of the research. It should be noted that laser imaging processing technology is not limited to the contents in this paper. There are many other signal and information processing technologies not involved in this paper, which worth further study and exploration by researchers.
Fiber laser from interdisciplinary perspective: review and prospect (invited)
Zhou Pu, Jiang Min, Wu Hanshuo, Deng Yu, Chang Hongxiang, Huang Liangjin, Wu Jian, Xu Jiangming, Wang Xiaolin, Leng Jinyong
2023, 52(6): 20230334. doi: 10.3788/IRLA20230334
[Abstract](422) [FullText HTML] (107) [PDF 2742KB](144)
　　Since the turn of the century, China has been a research hub for fiber lasers. The National University of Defense Technology's research into fiber lasers began during the "11th Five-Year Plan" period and has lasted approximately 15 years, yielding a number of peer-reviewed research outputs. The optical engineering field underpins the majority of fiber laser research at the institution. Optical engineering is one of the university's dominating fields, with good results in recent discipline review, providing a high-level scientific research platform and talent team for fiber laser research. On the other hand, fiber laser development benefits from the advantages of reasonably complete subject categories as well as helpful exploration and practice in interdisciplinary aspects. From an interdisciplinary standpoint, this paper sorts out several important breakthroughs in the interdiscipline of fiber laser and electronics, materials, control, intelligence, nano, and other disciplines in the university, and analyzes the opportunities faced by interdisciplinary scientific research and interdisciplinary construction from four perspectives: the evolution of scientific research paradigm, subject driving, application demand traction, and the inception of interdisciplinary scientific research and interdisciplinary construction.  Significance & Progress  The National University of Defense Technology's main fiber laser research is based on the discipline of optical engineering; Research in the fiber laser began during the "11th Five-Year Plan" period, has been about 15 years, and has achieved a series of peer-recognized research results. During the "11th Five-Year Plan" period, the university concentrated on scientific research in the fields of fiber laser coherent synthesis and supercontinuum fiber light source, and officially began related work in the fiber laser discipline, achieving research achievements represented by kilowatt fiber laser coherent synthesis system and high-power near-infrared supercontinuum light source. During the "12th Five-Year Plan" period, the university focused on high-power fiber lasers, gradually expanding its research into high-power fiber lasers, fiber passive devices, and so on, and achieved innovative results in cascade pumping high-power fiber lasers, special wavelength fiber lasers, high-brightness laser bunders, and high-power ultrafine lasers. Since the "13th Five-Year Plan," the research focus has shifted to the development of laser fiber materials and software, as well as the development of laser full machines to form a complete chain. As a representative of the corporation, we have obtained independent intellectual property software, virtual simulation courses, various types of laser fiber, high power and high beam quality single frequency/narrow linewidth/broadband fiber laser, cascade pump/semiconductor direct pump high power and high beam quality fiber laser, high power visible light/near infrared/mid-infrared supercontinuum light source, thousand-beam laser phase control/high power fiber coherent synthesis system. 　　This study examines the prospects for cross-disciplinary research and construction from four perspectives: the growth of scientific research paradigms, subject driving, application demand pulling, and science-education integration. The university's fiber laser approach has produced a number of notable research results, with input from other disciplines playing an essential role. The in-depth analysis, however, reveals that relevant research is primarily "reference" and "inspiration" between disciplines, such as the development of fiber simulation software mentioned in the introduction and ultra-long-term stable high-performance pulsed fiber laser, etc., which is more the result of personnel familiar with "optical engineering," "software engineering," and "nanoscience" working together to promote. In reality, there aren't many cross-disciplinary construction and cross-scientific research items. There are basically no examples like Logan Wright who have made significant contributions to various disciplines such as nonlinear fiber optics and artificial intelligence at the moment. The scientific foundation of relevant researchers must be strengthened further. However, with the rapid advancement of a new round of scientific and technological revolution, industrial change, and continuous innovation of education and teaching methods, it provides new opportunities for cross-scientific research and cross-disciplinary construction, as well as a broad space for the development of the fiber laser direction, and the fiber laser direction will continue to produce more innovative results.  Conclusions and Prospects   After approximately 15 years of development, the university's research of fiber laser has accomplished a number of significant outcomes that are strongly tied to the support of the optical engineering discipline and the deep cross-integration of other associated disciplines. The continuous human science and technology development in this century is continuous comprehensive development, from comprehensive to more comprehensive, and this integration tendency is reflected in scientific research, discipline construction, personnel training, and other aspects. Although the development of fiber laser has experienced many problems and challenges, the research of fiber laser will continue to create more novel outcomes with the continued advancement of interdisciplinary construction and cross-scientific research.
Research progress in levitated optomechanical sensing technology (invited)
Zhang Haoming, Xiong Wei, Han Xiang, Chen Xinlin, Kuang Tengfang, Peng Miao, Yuan Jie, Tan Zhongqi, Xiao Guangzong, Luo Hui
2023, 52(6): 20230193. doi: 10.3788/IRLA20230193
[Abstract](339) [FullText HTML] (91) [PDF 5072KB](77)
  Significance   With the rapid development of laser technology in the last century, microscopic optomechanical effects have gradually been discovered by researchers. In 1971, Arthur Ashkin in Bell Laboratory discovered the acceleration and trapping of particles by radiation pressure, and first proposed the concept of "optical potential wells", also known as "optical trap". In 1976, Ashkin achieved optical levitation of a fused quartz sphere in ultrahigh vacuum and pointed out its feasibility of high-precision sensing in low-damping environments. In 1986, Ashkin constructed an optical gradient potential trap using tightly focused beams to capture particles, which announced the birth of optical tweezers and raised a new era of levitated optomechanical sensing technology. Thanks to the pioneering work of Ashkin, and with the development of vacuum technology, levitated optomechanical sensing technology emerged. The technology has great characteristics of non-contact, high sensitivity, and feasible integration. Compared to previous quantum sensing based on the cold atom interference or nuclear magnetic resonance, this new technology involves larger particles with much more uniform atoms, which allows intuitive observation of particle morphology. Meanwhile, levitated optomechanical sensing technology enables ultra-high sensitive detection at room temperature without the need of the complex cryogenic environment. Therefore, the levitated optomechanical system can be considered as an "ideal platform" for precise measurements, where its accuracy is gradually approaching the standard quantum limits. The technology has also played significant roles in many cutting-edge fields including microscopic thermodynamics, dark-matter explorations, and macroscopic quantum state preparations.   Progress   Firstly, we describe the basic theory of the levitated optomechanical sensing. Tested physical quantity can be measured by sensing the motion parameters of the optical-trapped particles. Relevant key sensing technologies contain the loading of the particles, the enhancement of the optical forces, the displacement detections, the calibration of the voltage coefficient and the feedback cooling. These specific technologies are remarkably developed in recent years. For instance, feedback cooling has achieved occupation numbers below 1, which opens the door to quantum ground-state at room temperature. During the last decades, levitated optomechanical sensing is widely used in the measurements of the basic physical quantities, such as the extremely weak forces, the accelerations, the microscopic mass, the residual electrical quantities, and ultra-small torques. We have listed the typical applications of levitated optomechanical sensing. It can realize a force sensitivity of ~10−21 N\begin{document}$ /\sqrt{\mathrm{H}\mathrm{z}} $\end{document} and acceleration sensitivity of ~100 ng\begin{document}$ /\sqrt{\mathrm{H}\mathrm{z}} $\end{document}. It also has achieved microscopic mass resolutions of 10−12 gram and an electric intensity sensitivity of 1 μV/(cm·\begin{document}$\sqrt{\mathrm{H}\mathrm{z}} $\end{document}). When the particles are optically driven to high-speed rotation, accurate torque measurements can be achieved with a sensitivity of ~10−29 N·m\begin{document}$ /\sqrt{\mathrm{H}\mathrm{z}} $\end{document}.   Conclusions and Prospects   The trends of the technology are summarized and relevant suggestions are given. With the progress of its engineering, levitated optomechanical sensing is moving towards practical applications. The current levitated optomechanical sensing is developed in two routes of high-precision and integration. The former orients towards the demand for basic research, mainly using spatial optical components and pursuing lower noise floors. The latter orients towards practical applications using integrated optics and micro-nano processing. In the next step, we need to pay more attention to effective combination of levitated optomechanical sensing technology and other disciplines, and continue to strengthen the engineering practical research. We hope to achieve technical breakthroughs and practical applications of relevant sensors such as the light force accelerometers and the optomechanical gyroscopes.
Research progress of ultra-narrow-linewidth Brillouin fiber laser (invited)
Chen Mo, Wang Jianfei, Lu Yang, Hu Xiaoyang, Chen Wei, Meng Zhou
2023, 52(6): 20230131. doi: 10.3788/IRLA20230131
[Abstract](242) [FullText HTML] (58) [PDF 4452KB](89)
  Significance   Ultra-narrow-linewidth (~ kHz) lasers have attracted much research interest because of their wide applications in optical communications, fiber sensors, and so on. The linewidth of lasers affects the performance of the systems, such as the communication length, the minimum detection signal, and the measurement accuracy. Brillouin fiber lasers (BFLs), based on stimulated Brillouin scattering (SBS) in fibers, present Hz-scale ultra-narrow linewidth due to their intrinsic linewidth-narrowing effect. With its development in the past several decades, the compact Brillouin/erbium fiber laser (BEFL) becomes the frontier of the research on BFLs. Unlike the erbium-doped fiber lasers or laser diodes, the compact BEFL presents Hz-scale linewidth without complicated feedback loop or extremely precise isolation from the temperature and vibration variations. Besides the advantage of ultra-narrow linewidth, the BEFL simultaneously presents very stable central frequency and stable fast tuning. These outstanding performances make the compact BEFL a very ideal laser source for many applications, especially for phase-generation-carrier (PGC) interferometric fiber sensors.   Progress  The development of narrow-linewidth BFLs went through three stages, i.e. the early-days BFLs, the traditional BEFLs, and the compact BEFLs. The BFLs were introduced according to the three development stages. In the early days, the BFL was based on SBS in a single-mode fiber resonator (Fig.1-4). The Brillouin pump (BP) was injected in the cavity and generates SBS in the single-mode fibers. The BFL needed high pump threshold or critical pump coupled resonator due to the small Brillouin coefficient. The Brillouin/erbium fiber laser (BEFL) was then proposed to overcome the need of a pump couple resonator by introducing an erbium-doped fiber amplifier in the resonator (Fig.5). The BEFL presents low threshold and high output power (Fig.6(b)), but it needed over 100-m single-mode fibers as the Brillouin gain medium. Long cavity causes mode hopping easily. Many studies were carried out to establish single-longitudinal BEFL, such as multi-resonance-cavity BEFL (Fig.7), short-cavity BEFL based on high-nonlinearity special fibers (Fig.8), single-mode BEFL based on Brillouin pump preamplification (Fig.9). Short-cavity, low-threshold BEFLs were desirable. Until 2012, the compact BEFL was proposed based on a length of erbium-doped fiber (EDF) providing both the Brillouin gain and linear gain (Fig.10). It presented short cavity and low threshold. In 2013, an all-polarization-maintained ultra-short-ring-cavity compact BEFL was reported (Fig.11). The mechanism, characteristics, and applications of the compact BEFL were studied in the following 10 years. A series of progress has been achieved on the studies of compact BEFL. This kind of fiber laser showed 3-Hz ultra-narrow linewdith (Fig.16), stable central frequency, and stable fast tuning (Fig.26). The phase noise of the BEFL is lower than the state-of-the-art commercial laser diodes (Fig.18). The outstanding performance of the compact BEFL leads to many important potential applications, such as in high-accuracy interferometric fiber sensors (Fig.27-28) and Brillouin distributed fiber sensors (Fig.29-30).   Conclusions and Prospects  The optical communication and sensing systems are in great need of high-performance ultra-narrow-linewidth lasers. The BFLs, based on SBS in fibers, present Hz-scale ultra-narrow linewidth. The BFLs have already been developed to the stage of the compact BEFLs, which present ultra-narrow linewidth, stable central frequency, and stable fast tuning simultaneously. Compared with the state-of-the-art narrow-linewidth external-cavity laser diodes, the compact BEFL presents even lower phase noise. The applications in interferometric fiber sensors and distributed fiber sensors are validated for the compact BEFL. The advantages of the applications of the compact BEFLs are verified. The compact BEFL has a fully independent intellectual property, it has great significance for the localization of producing many important optoelectric information systems. The research aims to provide some reference for the study and applications of narrow-linewidth lasers in the future. It is expected that the compact BEFL will be modularized so that it could be widely used in more applications.
Review of backscattering problems in optical gyros (invited)
Tan Zhongqi, Ji Hongteng, Mao Yuanhao, Wu Geng, Jiang Xiaowei, Guan Shiyu, Chen Dingbo, Quan Yuchuan
2023, 52(6): 20230181. doi: 10.3788/IRLA20230181
[Abstract](342) [FullText HTML] (109) [PDF 4507KB](102)
  Significance   Optical gyro, as an indispensable part of the modern inertial navigation technology, has been widely used in aerospace, military equipment and even civil field. It is based on Sagnac effect, which induces the optical difference between two signals propagating in opposite directions within the optical path rotation. So far, optical gyro has developed from traditional laser gyro to fiber optic gyro and integrated optical gyro, which are smaller and consumes less power. In terms of working principle, optical gyro could be divided into two categories, active optical gyro and passive optical gyro. The former is essentially a laser, using the frequency beat of the opposite propagating light to derive the external rotation signal, while the latter is based on the phase or frequency difference generated by the outside laser to measure the rotation. However, both of them are necessary when considering the backscattering problems in gyro during the process of improving the working accuracy.   Progress  In order to better understand the backscattering mechanism in optical gyro, it could start with self-consistent equation under semi-classical theory, followed by analyzing the coupling mode of opposite propagating light wave. And the practical research of backscattering mechanism in optical gyro mainly focuses on lock-in effect and noise characteristics under macro conditions, taking corresponding methods to suppress them. For laser gyros, the lock-in effect caused by backscattering mechanism is usually solved by frequency bias. With the development of optical fiber, fiber optic gyroscope is invented. According to different working principles, it can be divided into interference fiber optic gyroscope, resonant fiber optic gyroscope and Brillouin fiber optic gyroscope. Their manifestation of backscattering is also slightly different. For the fiber optic gyroscope, it is based on the phase difference generated by the interference of the opposite propagating light wave. Therefore, the noise of backscattering disturbs the actual phase measurement. The main solution is to use the wide spectrum light source or phase modulation. For resonator fiber optic gyroscope, the noise caused by backscattering can be divided into two aspects. One is the light intensity of the scattered light wave itself, and the other is the mutual interference between the scattered light and the signal. The former will lead to the nonlinearity of the gyroscope output, while the latter results in the bias noise of the frequency measurement. The common solution is to carry out frequency modulation or phase modulation on the system, which suppresses the effect of backscattering noise. For Brillouin fiber optic gyro, it is an active resonant gyro in essence, like laser gyro. Therefore, frequency bias and phase modulation are often used to overcome the influence of lock-in effect. At present, integrated optical gyro with more fine structure and smaller volume is also a valuable research direction. However, it is identical to fiber optic gyros except for the waveguide resonator or micro resonator. Therefore, there is no great difference with fiber optic gyros in analysis methods for backscattering problems. It is worth noting that the Brillouin gyros on chip, which benefit from the reduction of stimulated Brillouin scattering threshold, can spontaneously generate Stokes light with a large frequency difference, so as to fundamentally solve the problem caused by the lock-in effect.   Conclusions and Prospects  Backscattering in optical gyro is a common phenomenon, which will bring in the inevitable noise or lock-in effect leading to the deterioration of the gyro performance. At present, the common research focuses on using different modulations to suppress the adverse effects of backscattering, with the purpose to improve the performance of gyro. However, there are still some unsolved problems. One is the precision measurement of backscattering in the optical gyro, and the other is to minimize the magnitude of backscattering in the optical gyro through reasonable design and processing. Both of them are beneficial to better understand the backscattering mechanism of optical gyro at the micro level, reduce the adverse effects of backscattering fundamentally, and improve the actual working efficiency of optical gyro. In addition, it is also worth thinking about how to cleverly use the characteristics of backscattering, remove its adverse aspects, and promote its application in precision measurement.
Interferometric test of coaxial folded mirrors for visible/near-infrared imaging systems (invited)
Xiong Yupeng, Lu Wenwen, Huang Cheng, Chen Fulei, Chen Shanyong
2023, 52(6): 20230175. doi: 10.3788/IRLA20230175
[Abstract](272) [FullText HTML] (71) [PDF 5602KB](49)
  Objective  Photoelectric imaging system serves as the “eye” of all kinds of equipment, which plays an indispensable role in scene detection and target recognition. To acquire more abundant target information, one of the development directions is multi-band fusion detection. However, the existing multi-band imaging system mostly adopts the discrete structure, with large system volume architecture, high manufacturing cost, and lack of spatial consistency due to parallax between the discrete systems. The challenges pose difficulties in image fusion and other back-end processing. Multi-band common aperture, also a common configuration, is generally used to split the front optical path with optical components, and subsequently respond to the detection requirements of different bands through the discrete rear optical path. To address these issues, coaxial folded mirrors for visible/near-infrared imaging systems are designed in this paper.   Methods  To guarantee the surface accuracy and relative orientation accuracy for multiple mirrors, an interferometric null test with a computer-generated hologram (CGH) is proposed (Fig.5). Diamond turning technology is applied to machining the mirrors. In this approach, two CGHs are designed for the null test of the monolithic primary/tertiary mirrors and the monolithic secondary/fourth mirrors (Fig.6, Fig.9). Ghost image of disturbance orders of diffraction is effectively separated by properly choosing the power carrier and the axial position of the CGH. A single CGH is capable of simultaneously measuring both the surface error and the relative orientation error of multiple mirrors (Fig.8). The result of the interferometric null test shows multiple mirrors are measured with nearly null fringes, indicating high accuracy in terms of surface form and orientation. Moreover, no ghost disturbance is observed.   Results and Discussions   The optical components undergo the diamond turning process, and the mirror blank is shared among the primary mirror and the three additional mirrors, allowing for simultaneous processing (Fig.12). After processing, a CGH is used to conduct zero compensation measurements on both mirrors (Fig.13). The measured surface shape error is shown (Fig.14), and the primary mirror and the three mirrors demonstrate a combined surface shaper error of PV 0.87λ, RMS 0.12λ; Interference diagram reveals that the ghost image stripes only exist outside the main mirror and the three mirror stripes, and they do not form interference. The primary mirror and the three mirrors reach a near-zero fringe state at the same time, indicating a high level of surface shape accuracy and mutual pose accuracy (reaching the sub-wavelength level), which meets the imaging requirements of the system.   Conclusions  The study proposes an interferometric null test with a CGH for the coaxial folded mirrors in visible/near-infrared imaging systems. The method involves the creation of multiple holographic regions with different functions on the same CGH substrate, which allows for the generation of the aspheric wavefronts of different shapes after the diffraction of the incident test wavefront. Consequently, the zero position of different mirror shapes can be tested at the same time. Following ultra-precision machining based on CGH compensation measurement, the mirror shape accuracy and pose accuracy attain a sub-wavelength level, which realizes direct assembly without additional assembly and adjustment for optimal imaging performance. Similarly, by positioning reference processing, multiple similar systems are nested coaxially, which enables multi-band coaxial imaging from visible light to near-infrared. Such capability holds obvious advantages for unmanned platform target detection and fast image fusion processing.
Design, simulation and implementation of direct LD pumped high-brightness fiber laser (invited)
Wang Xiaolin, Wang Peng, Wu Hanshuo, Ye Yun, Zeng Lingfa, Yang Baolai, Xi Xiaoming, Zhang Hanwei, Shi Chen, Xi Fengjie, Wang Zefeng, Han Kai, Zhou Pu, Xu Xiaojun, Chen Jinbao
2023, 52(6): 20230242. doi: 10.3788/IRLA20230242
[Abstract](1076) [FullText HTML] (90) [PDF 16977KB](136)
  Significance  Direct LD pumped high power fiber lasers have the advantages of low cost, high conversion efficiency, and good beam quality, finding applications as diverse as industrial processing, medical treatment and fundamental research, etc. The brightness, which is determined by the output power and beam quality, is a crucial parameter of fiber lasers that affects the application effectiveness. However, the power scaling of high-brightness fiber lasers is mainly constrained by the nonlinear effects and the transverse mode instability (TMI). It is noteworthy that IPG Photonics announced the 10 kW and 20 kW single-mode fiber laser in 2009 and 2013, respectively, but no domestic fiber laser products with > 6 kW output power and beam quality factor M2<2.0 are commercially available until now (Mar.2023). There are multiple reasons for the huge gap between the domestic high-brightness fiber laser industry and its foreign counterparts. In addition to the late start time and the less advanced material processing technology, the lack of theoretical guidance and original theoretical-based solutions make it difficult to overcome the technical bottleneck for power scaling of high-brightness fiber lasers, and the lack of fiber laser simulation software also slowed down the process from fundamental research and laboratory demonstration to industrial products. Therefore, it is of significant necessity to develop fiber laser software to aid the fiber laser design and accelerate the process from simulation results to industrial products.   Progress   To overcome the aforementioned difficulties, researchers from National University of Defense Technology have conducted fundamental theoretical research and developed fiber laser simulation software SeeFiberLaser with independent intellectual property rights. The software can simulate the generation, amplification and transmission of fiber lasers with different time domain characteristics and effects such as amplified spontaneous radiation, stimulated Raman scattering (SRS), stimulated Brillouin scattering and transverse mode competition can be considered in the simulation. The simulation results can output data of the power, spectrum, spot pattern, and time domain as required, which greatly helps the study of fiber laser theory, engineering design and scientific research. With the aim of suppressing SRS and TMI, systematic solutions are proposed, including exploiting the backward pump scheme, optimizing the pump wavelength, employing spindle-shaped fiber design, etc., which are proven effective to improve the performance of fiber lasers through theoretical simulation. Then, industrial fiber oscillators are simulated and optimized based on the SeeFiberLaser software by studying the effects of the active fiber length, operating wavelength, the reflectivity of the output coupling fiber Bragg grating, and the pump wavelength. Furthermore, high-brightness fiber amplifiers that are capable of delivering 8-10 kW output power are simulated and optimized based on the SeeFiberLaser software by considering the active fiber's length and pump absorption coefficient, the backward pump power as well as the core diameter and length of the quartz block head. Furthermore, experimental studies are carried out to verify the effectiveness of the above-mentioned theoretical solutions, including optimizing the pump wavelength as well as employing backward pumping for improved TMI threshold, and exploiting spindle-shaped fiber for SRS and TMI mitigation. Moreover, a 6 kW oscillating-amplifying high-brightness fiber laser based on pump wavelength optimization, 7 kW backward pumped high-brightness fiber laser, and 10 kW fiber laser based on fiber with a small core-to-cladding ratio are experimentally demonstrated, fully proving the functionality and great potential of the proposed solutions. Last but not least, the technical schemes of higher brightness fiber laser are prospected, which include employing integrated multifunctional passive devices on a single piece of passive fiber, adopting ytterbium-doped and energy transfer integrated fiber, and exploiting gain-resonator integrated design scheme, facilitating better good beam quality and improved stability.   Conclusions and Prospects  Power scaling of high-brightness fiber laser is a complex yet challenging work, which requires comprehensive investigation and optimization. This paper analyzes the impact of various factors on the laser in the fiber laser design process and proposes methods, such as variable core diameter fiber, optimized pump wavelength, etc., to improve the performance of fiber lasers. Noteworthily, the SeeFiberLaser software is developed to bridge the fundamental research outcomes of fiber laser technology and industrial products, which were proven quite effective in high-power fiber laser optimization for both industrial and research use. In addition, 6-10 kW high-brightness fiber lasers have been experimentally demonstrated based on the proposed optimization solutions. Looking forward, higher-brightness robust fiber laser could be expected by developing integrated multifunctional passive devices, ytterbium-doped and energy transfer integrated fiber design, and gain-resonator integrated design scheme.
Optimization of quantum-enhanced receiving method for weak optical signal (invited)
Dong Chen, Guo Chang, Wu Tianyi, Ran Yang, Dang Kezheng, Li Fuquan, Zhou Zichao
2023, 52(6): 20230189. doi: 10.3788/IRLA20230189
[Abstract](170) [FullText HTML] (47) [PDF 2068KB](44)
　　Under the framework of classical theory, the performance of coherence detection is limited by the standard quantum limit (SQL) corresponding to the shot noise. However, the quantum-enhanced receiving technology can break the SQL and approach the Helstrom limit by introducing the displacement operation and converting the classical measurement into the measurement of photon number states. Unambiguous state discrimination (USD) is one of the commonly used discrimination strategies for quantum-enhanced reception. However, due to the limited energy of weak signals, the traditional USD quantum-enhanced receiving method has a high error rate of weak signal recognition. A hybrid measurement scheme for quadrature phase-shift-keying (QPSK) coherent states is developed. The scheme firstly converts the discrimination of QPSK coherent states into the distinction of BPSK coherent states by homodyne detector (HD), and then realizes the unambiguous discrimination of coherent states by BPSK quantum-enhanced receiving measurement. The simulation results show that the hybrid scheme is superior to the classical measurement scheme in the average photon number between 3.2 and 11.3, and has a larger signal range than the traditional QPSK quantum-enhanced receiving scheme.   Objective  Coherent states are a critical carrier in optic communication and quantum information processing due to their intrinsic resilience to the loss of coherence. Unambiguous state discrimination (USD), which aims to realize the error-free discrimination of coherent states by outputting "no result" for the finite ambiguous results, is especially essential in quantum key distribution and quantum digital signatures. Recently, Becerra first experimentally demonstrated a generalized quantum measurement for USD of four non-orthogonal coherent states with a displacement operator and single-photon detector (SPD). As the unambiguously correct probability is still significantly low, Ref. introduces an adaptive feedback strategy and presents an adaptive generalized measurement scheme for USD of QPSK coherent states. However, the adaptive measurement scheme still needs to be improved to realize better performance. Thus, it is crucial to develop new schemes that can unambiguously discriminate coherent states with performance surpassing the ideal heterodyne strategy. This paper presents a new hybrid scheme that unites a homodyne detector (HD) and a quantum measurement scheme for USD of QPSK coherent states.   Methods  To realize the unambiguous state discrimination, this paper presents a new hybrid measurement scheme based on these (Fig.1). The scheme consists of two successive measurements toward the coherent states. The first measurement is conducted by a homodyne detector, which can exclude half of the four possible states of the QPSK coherent states. The result of the first measurement gives a feed-forward to the second measurement. And the second measurement is conducted by a quantum measurement scheme and finally discriminates the signal states. The received QPSK coherent state \begin{document}$\left|{\alpha }_{m}\right\rangle=\left|\alpha \right|{{\rm{e}}}^{i\left(m-1/2\right)\pi /2}, {m}=\mathrm{1,2},\mathrm{3,4}$\end{document} is first divided by a beam splitter (BS) with transmittance \begin{document}$ T $\end{document} and reflectivity \begin{document}$ R $\end{document}. To simplify the calculation, we use \begin{document}$ {t}^{2}=T $\end{document} and \begin{document}$ {r}^{2}=R $\end{document} to denote the transmittance and reflectivity, respectively. The transmitted part \begin{document}$\left|{\alpha }_{Tm}\right\rangle=\left|t\alpha \right|{{\rm{e}}}^{i\left(m-1/2\right)\pi /2}$\end{document} and reflected part \begin{document}$\left|{\alpha }_{Rm}\right\rangle=\left|r\alpha \right|{{\rm{e}}}^{i\left(m-1/2\right)\pi /2}$\end{document} of the received signal state are respectively output to the quantum measurement stage and HD stage. We can change the partitional ratio \begin{document}$ {R}_{Q,H}={t}^{2}/\left({r}^{2}+{t}^{2}\right)\approx {t}^{2} $\end{document} by selecting the appropriate beam splitter.   Results and Discussions  The \begin{document}$ M=1+2 $\end{document} hybrid scheme can realize a lower error ratio than the heterodyne strategy when achieving the same correct unambiguous results probability for coherent states with mean photon number \begin{document}$ 4.2\leqslant {\left|\alpha \right|}^{2}\leqslant 12.6 $\end{document}(Fig.8). And \begin{document}$ M=4 $\end{document} quantum measurement scheme can only beat the heterodyne strategy for coherent state with mean photon number \begin{document}$ {\left|\alpha \right|}^{2}\leqslant7.41 $\end{document} while the \begin{document}$ M=1+3 $\end{document} hybrid scheme can realize it with mean photon number \begin{document}$ 3.2\leqslant {\left|\alpha \right|}^{2}\leqslant 11.3 $\end{document}(Fig.10). Furthermore, the hybrid scheme has a lower error ratio for coherent state with a mean photon number more than 6.7 compared with the quantum measurement scheme. This phenomenon notes that the hybrid scheme has a more excellent application range than the quantum scheme.   Conclusions  A hybrid measurement scheme with an adaptive feedback strategy is proposed to unambiguously discriminate the QPSK coherent states, which converts the discrimination of four coherent states to two coherent states by HD and conducts the final discrimination by quantum measurement scheme. Here, we fully consider the non-ideal factors in the practical implementations, such as detection efficiency and dark count rate of detectors, visibility and transmittance of displacements, and develop the model of the hybrid measurement scheme. The simulation results clearly show that the hybrid scheme with \begin{document}$ M=1+2 $\end{document} can beat the heterodyne strategy for coherent states with \begin{document}$ 4.2 \leqslant {\left|\alpha \right|}^{2} \leqslant 12.6 $\end{document}. Furthermore, compared with the quantum measurement scheme, the hybrid scheme with \begin{document}$ M=1+3 $\end{document} has a higher probability of correct unambiguous results and a lower error ratio for coherent states with \begin{document}$ 6.7\leqslant {\left|\alpha \right|}^{2} $\end{document}. The hybrid scheme uses HD with less energy to convert a complex four-state discrimination problem to a simple two-state discrimination problem, which improves the probability of obtaining an unambiguous conclusion. However, this scheme is also limited by the HD and quantum measurements and can only achieve better performance within a specific range.
Mid-infrared BaGa4Se7 optical parametric oscillator with high conversion efficiency (invited)
Bian Jintian, Kong Hui, Ye Qing, Yao Jiyong, Lv Guorui, Xu Haiping, Zhou Quan, Wen Kaihua
2023, 52(6): 20230178. doi: 10.3788/IRLA20230178
[Abstract](200) [FullText HTML] (67) [PDF 2085KB](35)
　　The LiB3O5(LBO) was inserted into the branch of the L-shaped BaGa4Se7(BGSe) optical parametric oscillator (OPO) to improve the conversion efficiency for the first time. When the pump laser energy is 115 mJ (1.06 μm), the idler light (3.5 μm) energy was 16.18 mJ, corresponding to the conversion efficiency of 14.06%, and the slope efficiency was 18.4%, which was the highest conversion efficiency of BGSe OPO pumped by 1 μm laser. The signal, idler, and pump wave waveform in BGSe L OPO cavity with and without LBO crystals was simulated, and the output waveform of idler light was given. Compared with traditional OPO cavities, L-type OPO cavities (with frequency doubling crystals) suppress the inverse conversion under high-energy pumping conditions, achieving higher idle frequency light conversion efficiency.   Objective  The mid-infrared (IR) coherent sources in the 3-5 μm have always been intensively demanded for a wide range of scientific and technological applications in remote sensing, spectrum analysis, materials diagnostics, aerospace fields, etc. Optical parametric oscillation is an attractive approach, especially when high energy and average power are demanded simultaneously. However, there is reverse conversion in the OPO cavity.When the pump energy is high, the signal light and idle frequency light generated are also strong. At this time, the signal light and idle frequency light will be converted to the pump light, which seriously affects the conversion efficiency of OPO. In addition, due to the high intensity of signal light in the cavity during the reverse conversion, it is easy to damage the nonlinear crystal or its coating.Therefore, how to suppress reverse conversion in the OPO cavity under high-energy pumping conditions and improve the conversion efficiency of OPO has always been the focus of research.   Methods  To suppress the inverse conversion in the OPO cavity, we proposed a method of inserting a frequency doubling crystal into the L-type OPO cavity to suppress the signal light intensity (Fig.1). All three mirrors of the L-shaped cavity are coated with a high-reflection coating for the signal laser, and crystals are inserted in the L-branch to achieve intracavity frequency doubling of the signal laser. When the energy density of the signal laser in the OPO cavity is high, the signal laser is converted into red light by the frequency doubling crystal and output from the L branch. At the same time, the signal laser is attenuated, reverse conversion is suppressed, and the efficiency of idle laser conversion is improved.   Results and Discussions  The idler laser energy was 16.18 mJ at a pump energy of 115 mJ, corresponding to an optical-to-optical conversion efficiency of 14.06% and a slope efficiency of 18.4% (Fig.2). It is the highest conversion efficiency for BaGa4Se7 (BGSe) OPO pumped by a 1.06 μm laser, to the best of our knowledge. The energy density of the three waves at the output of the OPO cavity is simulated. The simulation results show that the optical-to-optical conversion efficiency of the idler laser with the LiB3O5 (LBO) inserted in the cavity is 1.20 times higher than that without LBO in the cavity at a pump energy of 80 mJ (Fig.3). The OPO output wavelength could be tuned by adjusting the angle of the BGSe crystal (Fig.5). When the θ angle of the crystal is changed, the experimental peak wavelength agrees well with the theoretical simulation curve, and the measured \begin{document}$ \Delta {{\lambda }}_{2}/\Delta \mathrm{\theta } $\end{document} is −231.81 nm/(°) . When changing the φ angle of the crystal, the measured \begin{document}$ \Delta {{\lambda }}_{2}/\Delta {\varphi} $\end{document} of −6.25 nm/(°) deviates from the theoretical value of −1.25 nm/(°) because the incident direction of the pump laser is difficult to exactly coincide with the θ=56.3° line of BGSe.   Conclusions  The conversion efficiency of idler light in OPO cavity was improved by inserting a signal laser frequency doubling crystal into the L-shaped OPO cavity for the first time. When the pump energy is 115 mJ, the 16.18 mJ of the idler laser energy was obtained in BGSe OPO. The optical-to-optical conversion efficiency was 14.06%, and the slope efficiency was 18.4%, which is the highest conversion efficiency of BGSe OPO pumped by a 1.06 μm laser. The output wavelength of BGSe OPO with high conversion efficiency can also be tuned.
Progresses in infrared detection based on spectrum transducing (invited)
Zhou Zhiyuan, Shi Baosen
2023, 52(5): 20230165. doi: 10.3788/IRLA20230165
[Abstract](312) [FullText HTML] (82) [PDF 2623KB](112)
  Significance   In this paper, spectrum transducing detection of infrared light with silicon detectors is systematic reviewed. Traditional infrared detection is based on semiconductor photonic detectors, such as AsGaIn and HgCdTe. These detectors have low detection sensitivities and relatively high noise at room temperature, and deep cooling is required to get better sensitivity. While the detection performances of silicon detectors are much better than those of the infrared detectors. Therefore, an effective method to detect infrared light is to transfer the wavelength of the infrared light to the detection window of silicon detector. Based on this principle, spectrum transducing detection of infrared light with silicon detectors is developed by using frequency up-conversion via sum frequency generation. This new detection scheme has the potential to offer single photon detection sensitivity at room temperature, which is very promising to be used in remote sensing at infrared regime.  Progress  The main progress for spectrum transducing detection of infrared light can be divided into two groups. The first group is aimed at improving the key parameters in frequency conversion, which are quantum efficiency, noise, frequency bandwidth and spatial bandwidth. The conversion efficiency in frequency transducing can be enhanced by using cavity and waveguide (Fig.4), both configurations are demonstrated to achieve near unity internal conversion efficiencies; Noise in frequency conversion is mainly caused by spontaneous Raman scattering and parametric down conversion of strong pump beam, which can be measured at different pump configuration, and some effective methods can be used to sufficiently reduce the noise. These methods include: long wavelength pump laser, narrow band filters and reduction of the operation temperature of the nonlinear crystals. The frequency bandwidth is strongly dependent on the phase matching conditions, therefore effective methods such as chirped poling and multi-angle cut crystal can be used to enhance the frequency bandwidth in frequency conversion (Fig.5). The spatial bandwidth is dependent on crystal dimensions and phase matching, crystals with large optical aperture and large phase matching angles are preferred for spectrum transducing detection of image with large field of view, about 30 degree field of view is realized in mid-infrared up-conversion based on chirped PPLN crystal (Fig.6). The second groups of progresses aimed at applications of the spectrum transducing detection in different fields, these fields are: single photon detection at mid-infrared regime and quantum frequency interface (Fig.7) for applications in quantum information processing; classical optical imaging such as large field of view and high frame rate imaging in the mid-infrared regime, phase contrast imaging (Fig.8) and spectrum analysis for material sciences.  Conclusions and Prospects   For the mutual restrictions between different key parameters in spectrum transducing detection, one need to balance between different parameters for specific applications. Though the performance of spectrum transducing detection at the near infrared regime is high enough for some mentioned applications, the performances at mid-infrared is still not satisfied for typical applications, great efforts should be taken to improve the performance at this wavelength regime. For imaging detection based on spectrum transducing plane detectors, most studies are focused on coherent illumination, many key problems for illuminating with large bandwidth incoherent blackbody radiations are still not solved yet. In summaries, there are still opportunities for researches inspectrum transducing detection, these opportunities are: (1) to extending quantum optics and quantum spectroscopy to mid-infrared regime; (2) by combining spectrum transducing in interferometers to realize detection of infrared signal with undetected photons and optical phase amplification; (3) to transduce all other spectrums to the detection windows of silicon detectors and greatly reducing the detection complexity in large optical systems.
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