红外与激光工程

Verification of demodulation method for differential optical Doppler velocimetry data

Zhang Zhijun, Song Ran, Jiang Lili, Zhang Xinyu, Li Bingbing, Chen Shenggong, Su Juan, Wu Chi

2024, 53(4): 20240094. doi: 10.3788/IRLA20240094

[Abstract](28) [FullText HTML] (8) [PDF 1007KB](7)

Objective In the field of physical oceanographic research, seawater flow velocity is one of the key parameters, primarily measured using acoustic Doppler velocimeters. In recent years, laser Doppler technology has made significant advancement in seawater flow velocity measurement. Laser Doppler velocimetry, with its simple and integrable structure, is expected to be a complementary technique with acoustic Doppler velocimeters in marine applications. Compared to acoustic velocity measurement techniques, laser Doppler velocimeters offer several advantages: their shorter wavelength (in the micron range) allows for the study of smaller-scale water features, and they can resist noise interference generated by underwater vehicles when used with unmanned underwater vehicles. However, due to seawater absorption and scattering, the detected signal is extremely weak and buried in strong noise, posing challenges for Doppler signal demodulation. Moreover, limited by the sampling frequency, there exists an error between the peak position of the obtained data spectrum and the true frequency. Therefore, effectively removing noise interference and improving measurement accuracy are crucial for laser Doppler velocimeters. In this paper, an adaptive filtering algorithm is employed to denoise the collected signal, followed by fast Fourier transform to enhance the signal-to-noise ratio. Three peak-finding algorithms are compared, and the Gaussian-LM algorithm is selected to process the power spectrum of the signal, bringing the peak position closer to the real peak value and thereby improving the demodulation accuracy of the Doppler signal and significantly reducing the error caused by noise. Methods The principle of laser Doppler velocimetry is illustrated in Fig.1(a). A laser beam is split into two equal beams by an optical fiber splitter after passing through a single-mode optical fiber. These two beams are then collimated into parallel beams by a collimator and directed onto a plano-convex lens at the end, which focuses the parallel beams onto a specific point outside the instrument, generating interference fringes at this focal point. When particles in the water pass through these interference fringes, they scatter light, which is collected by the plano-convex lens and converted into parallel light. This scattered light is then collected by an avalanche photodetector and converted into an electrical signal, which is acquired by an oscilloscope. The acquired signal undergoes algorithm processing to demodulate the flow velocity. Fig. 1(b) is a field photo of the optical system prototype being tested in the Marine environment off Qingdao. The key to signal processing is accurately extracting the Doppler frequency shift from a large amount of noise, and the noise in the Doppler signal is non-stationary. Therefore, the least mean square error algorithm can be utilized to effectively denoise the Doppler signal. Fast Fourier transform shifts the focus of the research from the time domain to the frequency domain, where it is easier to analyze the regularity of the Doppler frequency. Further, the Gaussian-LM algorithm is employed to perform peak finding on the Doppler signal, obtaining accurate frequency information. Results and Discussions Through simulation, the optimal peak finding algorithm was selected. The Monte Carlo algorithm, Gaussian fitting algorithm, and Gaussian-LM algorithm were employed to perform peak finding on Gaussian signals with added noise, and their measurement accuracies were compared, as shown in Fig.2(a). Peak finding calculations were conducted on multiple datasets, and their standard deviations are illustrated in Fig.2(b). The results indicate that the Monte Carlo algorithm exhibited the lowest peak finding accuracy, while the Gaussian-LM algorithm demonstrated the highest accuracy. Moreover, the Gaussian-LM algorithm exhibited smaller standard deviation compared to other algorithms, with a lower fluctuation range, indicating greater stability. Therefore, the Gaussian-LM algorithm was chosen for peak finding in the Doppler signal. A comparative experiment on seawater velocity was conducted at the Zhongyuan Tourist Dock in Qingdao, China, using a home-made optical Doppler velocimetry (LDV) and an acoustic Doppler velocimeter (ADV model: SonTek Argonaut-ADV). Algorithmic research was carried out on the obtained seawater velocity measurement data. Considering the different sampling rates of the two instruments, the data were first averaged over 30 minutes. From Fig.3(a), it can be observed that the data before algorithm processing roughly align with the trend of velocity values measured by ADV, but there are still discrepancies. However, the data after algorithm processing shows a higher degree of fitting with the data measured by ADV. Fig.3(b) illustrates the errors obtained by ADV for the data before and after processing, and presents the calculation of the average error. Through error analysis, it shows that the average error between the pre-processed LDV and ADV velocity measurements was 0.2905 cm/s, while the average error between the post-processed LDV and ADV velocity measurements was 0.2163 cm/s, indicating a reduction in error of 25.5%. Conclusions The signal of light scattering from suspended particles in seawater is extremely weak. Extracting signals submerged in noise and demodulating them to obtain velocity information poses a challenge for accurate measurements with laser Doppler velocimeters. In this paper, demodulation algorithms based on velocity data obtained from experiments in the near-shore of Qingdao were studied. Initially, through simulation and optimization, the Gaussian-LM algorithm was selected as the peak finding algorithm. Subsequently, signal denoising was performed based on the Least Mean Square (LMS) algorithm on the actual velocity data obtained during sea trials, combined with the Gaussian-LM algorithm for peak finding, achieving high-precision demodulation. Comparative experiments between home-made laser Doppler velocimeter and a well-known commercial acoustic Doppler velocimeter indicate that the post-processed velocity measurement error based on this algorithm is 0.21 cm/s, representing a 25.5% error reduction compared to the pre-processing velocity measurement error.

Metal water-triple-point automatic reproduction control system for in-situ online calibration of temperature sensors

Qiao Zhigang, Gao Dexin, Zhang Muzi, Zhao Shanshan, Wu Jiali, Su Juan, Chen Shenggong, Jing Chao, Liu Hailing, Yang Bo, Wu Chi

2024, 53(4): 20240096. doi: 10.3788/IRLA20240096

[Abstract](45) [FullText HTML] (14) [PDF 960KB](11)

Objective The triple point of water refers to the state where water, ice, and vapor coexist simultaneously, with an equilibrium temperature of 273.16 K (0.01 ℃). In the International Temperature Scale, the triple point of water serves as the sole reference point for defining the thermodynamic temperature unit Kelvin, and it is one of the most important fixed points in ITS-90 ^[1-2]. The thermodynamic temperature reproduction of water's triple point is crucial for practical temperature measurements ^[3].The reproduction of water's triple point is achieved by freezing an ice mantle inside a triple point of water cell. Widely used in the ITS-90 guidelines are triple point of water cells with borosilicate glass or fused silica shells. Traditional reproduction methods include the ice-salt mixture cooling method, dry ice cooling method, and liquid nitrogen cooling method. These methods all require the cooling of the triple point of water cell using dry ice, liquid nitrogen, or other cryogenic media, followed by freezing the high-purity water inside the cell and then storing it in an ice bath. While these traditional methods offer high reproduction accuracy and good results, they are complex, operationally difficult, and demand high standards for operators and the environment, making them inconvenient for on-site calibration and integrated applications ^[2-3]. Addressing the limitations of traditional triple point of water cells and reproduction methods for in-situ applications, such as the on-site calibration of temperature sensors in the deep sea, this paper investigates a miniaturized triple point reproduction control system suitable for the automatic calibration of temperature sensors, based on a self-developed miniature metal water triple point cell. Methods This control system utilizes the principle of spontaneous phase transition of high-purity water in a metal water triple point container, combined with a thermoelectric cooler (TEC) based on the semiconductor Peltier effect and a temperature control circuit, to achieve the automatic reproduction and maintenance of the water triple point. Temperature phase transition monitoring is achieved through the use of thermistors and temperature detection circuits. By employing a dual thermistor setup and TEC in a closed-loop control, the system adjusts the driving power of the TEC based on the temperature difference detected by the feedback resistors, thereby realizing the automatic reproduction and maintenance of the water triple point. Results and Discussions Figures 1(a) and (b) respectively illustrate the control schematic of the automatic reproduction system for the metal water triple point bottle and a photograph of the actual metal water triple point bottle. The research employed a miniaturized metal water triple point bottle, utilizing the principle of spontaneous phase transition of high-purity water, along with a thermoelectric cooler (TEC) based on the semiconductor Peltier effect and a temperature control circuit, to achieve the reproduction and maintenance of the water triple point. High sensitivity thermistors combined with a temperature detection circuit were used for monitoring the phase transition of high-purity water. A closed-loop control consisting of dual thermistors and the TEC was utilized. Based on the temperature difference detected by the feedback resistors, the study investigated the cooling demand of the high-purity water phase transition and established a thermodynamic model for the triple point bottle cooling system. By appropriately adjusting the TEC's driving power, the state of the water triple point was reproduced and maintained for an extended period. The measurement results in Figure 2 indicate that, significant supercooling of the high-purity water inside the metal water triple point bottle was observed. It remained unfrozen at the liquid-solid phase equilibrium temperature (0 ℃) and suddenly underwent a phase transition when the temperature reached the transition temperature (approximately −7.3 ℃), causing a rapid increase in the internal trap temperature, which then stabilized, with a stability duration of 20 minutes and a temperature fluctuation of ±1mK. The analysis of the experiment demonstrates that the miniaturized triple point temperature automatic reproduction control system based on the metal water triple point bottle can achieve spontaneous phase transition of high-purity water and maintain a stable temperature plateau for a certain period, facilitating high-precision in-situ temperature calibration of temperature sensors. Conclusions This study indicates that combining the metal water triple point bottle with properly arranged temperature monitoring sensors, a TEC cooling system, and a refrigeration control circuit and algorithm can automatically reproduce and maintain the high-purity water triple point state for 20 minutes, with a temperature fluctuation of ±1 mK. This provides an accurate, stable, and sustainable environment for in-situ calibration of temperature sensors, serving high-precision in-situ temperature calibration in deep-sea and deep-space environments.

Application of 632 nm FMCW lidar for simultaneous velocity and distance measurement in humid environment

Zhang Xinyu, Jiang Lili, Song Ran, Zhang Zhijun, Li Bingbing, Su Juan, Wu Qi

2024, 53(3): 20240093. doi: 10.3788/IRLA20240093

[Abstract](41) [FullText HTML] (10) [PDF 1090KB](20)

Objective In high sea conditions and complex environments, for achieving safe landing of unmanned aerial vehicles (UAV) on unmanned surface vehicles (USV), it is necessary to accurately measure the distance and speed simultaneously between them and provide real-time feedback to the control system. However, commonly used GPS navigation and vision-based navigation approaches often suffer from insufficient dynamic positioning. These technologies could only measure distance but not the relative speed between UAV and USV. Frequency modulated continuous wave (FMCW) lidar, which could simultaneously measure both velocity and distance, has great potential for application in autonomous landing between UAV and USV during high sea conditions.For different applications, FMCW lidar could utilize different frequency modulation schemes on the optical carrier. Commonly used frequency modulation scheme includes the triangular, the sawtooth, and the sinusoidal waveforms. It is an economical and convenient approach to measure velocity and distance simultaneously using a triangular waveform modulated FMCW lidar. Considering eye safety, FMCW lidar normally deploys lasers with wavelengths longer than 1550 nm. However, the light absorption in the moisture above air/sea surface is too large for the infrared wavelength range. Methods In this paper, the lidar system employing a 632 nm distributed Bragg reflector (DBR) semiconductor laser which operating the water vapor transmission wavelength band and an FMCW technology was proposed and experimentally demonstrated. The symmetric triangular waveform modulation was achieved by directly modulating the laser injection current. The schematic diagram of the system is shown in Fig.1(a), and the experimental setup of the system is shown in Fig.1(b). The modulated laser output, with continuous frequency tuning, was further divided into two beams by a beam splitter (BS). Of which, one beam is used as the reference light, and another beam is used as the detection beam and incident on the measurement target. The detection light, which is reflected (or scattered) by the target, is collected and mixed with the reference light by a photo-detector (PD) for coherent heterodyne detection. The beating signal that carries information about the distance and velocity of the target is recorded by a digital storage oscilloscope (DSO). By performing operations such as fast Fourier transform (FFT), the beat frequencies of the up-sweep and down-sweep bands could be obtained, and then the distance and velocity information of the target could be calculated. The experimental results show that the modulation bandwidth of the system is 12.5 GHz without mode hopping using the internal modulation scheme of direct current injection, and the modulation period is 5 kHz（0.2 ms）. Results and Discussions The measurement accuracy of distance of this FMCW system was tested by moving the target with 5 cm a step, ranging from 10 cm to 130 cm. The measured distances of the target were compared to the reference distances, as shown in Fig.2(a). The results demonstrate a strong correlation between the measured distance and the reference distances, with a linear fitting curve slope of 1.00121, R-squared value of 1, and a maximum relative standard deviation (RSD) of 0.3. The RSD is defined by the following formula, where S is the standard deviation (also denoted as SD) and

\begin{document}$ \bar{x} $\end{document}

is the mean value.　　　　　　　　　　　　　　　　

\begin{document}$RSD=\dfrac{S}{\bar{x}}\times 100\mathrm{\%}=\dfrac{\sqrt{{\displaystyle\sum_{i=1}^{n}{\left({x}_{i}-\bar{x}\right)}^{2}}/({n-1})}}{\bar{x}}\times 100\mathrm{\%} $\end{document}

　　Further reducing the step size for movement to test distance resolution, experimental verification showed that the system's distance resolution is 1.5 cm. The accuracy of velocity measurement of the FMCW system was verified by measuring the linear speed of a scattering point on the standard rotating disc with high-precision control of the rotational frequency. A comparative experiment was conducted with a continuous wave (CW) system using the same laser operated under continuous wave. The results of the speed measurement are shown in Fig. 2(b). The linear fitting results show that within the velocity range of 10 cm/s to 125 cm/s, the FMCW system has a linear fitting curve slop of 0.99991 and an R-squared value of 0.99999 when compared to the reference velocity. The measurement resolution is 0.5 cm/s with RSD of 0.6%. On the other hand, The CW system has a fitting curve slope of 1.00214, an R-squared value of 0.99999, and a RSD of 1.2%, which is higher than that of the FMCW system. The experimental verification demonstrates that the FMCW system not only achieves synchronous measurement of target velocity and distance, but also provides a higher speed measurement accuracy than the continuous wave (CW) laser speed measurement system. Conclusions When UAV is performing precise landing on USV in high sea conditions, it is important to measure both the velocity and distance between them simultaneously. In response to this requirement, this paper proposed and experimentally demonstrated a lidar system based on the 632 nm laser which was frequency-modulated by continuous wave (FMCW) for simultaneous measurement of both velocity and distance. The 632 nm semiconductor laser was modulated by a directly injected triangular-wave current. The modulated light was incident on the moving target. The beat frequency signal generated by the interference of the scattered light from the target and the reference light was demodulated to extract information about the velocity and distance. The experimental results show that the FMCW lidar system has a measured distance range of 10 cm to 130 cm, with a resolution of 1.5 cm and a relative standard deviation (RSD) of 1.5%. The measured speed range was from 10 cm/s to 125 cm/s, with a resolution of 0.5 cm/s and a relative standard deviation (RSD) of 0.6%.

Correction of pressure effect in calibrating nitrate concentration of seawater

Zhang Naixin, Zhu Xingyue, Shan Baoyi, Xu Jian, Wu Qi

2024, 53(3): 20240095. doi: 10.3788/IRLA20240095

[Abstract](34) [FullText HTML] (9) [PDF 1147KB](9)

Objective Optical nitrate sensors have advantages for in-situ exploration and the potential for long-term observation in deep-sea environment. However, measurement of optical nitrate is influenced by substrates in seawater, especially bromide ions (Br^-), within the relevant spectral range, causing a spectral shift in the ultraviolet (UV) intensity spectrum. In recent years, the pressure coefficient of the UV absorption spectrum of bromine under seawater pressure of 2 000 meters has been studied and experimentally confirmed, that the bromide in seawater affects the UV absorption spectrum. Considering that the seabed minerals are primarily distributed undersea at depths ranging from 1000 to 6000 meters, achieving accurate in-situ detection of nitrate concentration becomes crucial for the assessment of impact of seabed mining on the marine eco-system, establishment of an early warning system for the marine mining environment. Currently, products of nitrate sensors are limited to the submerged depths of approximately 2000 meters. No nitrate sensor product is available beyond this depth. This paper reports a calibration method for nitrate measurements within the pressure range of 0-50 MPa (0-5000 meters), aiming to improve the accuracy of nitrate measurements in deep-sea environments. Methods A system capable of measuring the UV spectrum of seawater under deep-sea pressure is constructed in this work. The light emitted from a deuterium lamp is transmitted through a fiber-optic beam splitter, dividing it into two paths. One path passes through a fiber-optic attenuator, while the other path goes through a pressure vessel. The two signals are then combined at an optical switch and selectively transmitted through it. Finally, the data processing module performs data acquisition and calculation. To simulate the deep-sea environment, the pressure vessel is connected to a weight manometer. The UV absorption spectra at different pressure are measured by controlling the external pressure. A continuous flow analyser was used to calibrate the nitrate concentration in the seawater samples collected from Aoshan Bay. Different levels of nitrate (0-50 μmol/L) were added to the Aoshan Bay seawater, and these seawater samples with different nitrate concentrations were measured by the measurement system. Results and Discussions The measurement results revealed a decrease in the absorbance of seawater samples with an increase in pressure. To investigate the pressure-induced changes in different substrates in seawater, identical pressure tests were conducted for nitrate solution (50 μmol/L), Aoshan Bay seawater, and sodium bromide solution (840 μmol/L). The absorbance results obtained are depicted in Fig.2(a). Notably, the absorbance of seawater and bromide under pressure exhibited a similar trend, whereas the absorbance of nitrate remained largely unaffected by pressure. Subsequently, pressure correction of seawater UV absorption spectra was conducted at pressures ranging from 0 to 50 MPa using two algorithms for spectral pre-processing, including standard normal variate transform (SNV) and multiplicative scatter correction (MSC), and regression prediction with the partial least squares regression (PLS) algorithm. The results are presented in Fig. 2(b). It is evident that the R² is 0.991, MAE is 1.980 μmol/L, MBE is −0.042 μmol/L, and root mean square error (RMSE) is 2.505 without using any pressure correction. The R² is 0.997, MAE is 1.294 μmol/L, MBE is 0.037 μmol/L, and RMSE is 1.620 using the MSC-PLS algorithm. The R² is 0.989, MAE is 2.308 μmol/L, MBE is 0.098 μmol/L, and RMSE is 3.085 μmol/L using the SNV-PLS algorithm. Therefore, with the utilization of the MSC-PLS pressure correction algorithm, the prediction results are superior to those without using any pressure correction. This suggests that the pressure correction algorithm improves measurement accuracy. The MSC-PLS algorithm has the highest R² and the smallest error range, indicating its superior pressure correction and data prediction capabilities. Conclusions The primary objective of this study is to enhance the accuracy of optical nitrate measurements in the deep-sea environment by addressing the influence of substrates such as bromide on UV absorption spectra. A system capable of measuring the UV spectrum of seawater under deep-sea pressure is constructed, utilizing a deuterium lamp, fiber-optic components, and a pressure vessel. The experimental results demonstrate variations in UV absorption spectra between 200-240 nm under different pressure conditions at the same nitrate concentration. The SNV and MSC algorithms are employed for pressure correction, and MSC-PLS algorithm exhibits superiority in predicting nitrate concentrations under the pressure range of 0-100 MPa (R² of 0.997). Therefore, the proposed method offers potential applications in mining exploration and environmental monitoring.

500 Hz Joule-level output by sub-nanosecond Zig-Zag slab laser

Li Kai, Song Changyu, Yue Jianfeng, Jia Mengyu, Xu Zhipeng, Wu Di, Cao Chen, Bai Zhenxu, Yu Yu, Wang Yulei, Lv Zhiwei

2023, 52(8): 20230423. doi: 10.3788/IRLA20230423

[Abstract](110) [FullText HTML] (30) [PDF 1342KB](43)

Objective The high-energy, high-repetition-rate sub-nanosecond lasers have been widely applied in various fields such as industry, military, and scientific research due to their superior peak power compared to nanosecond lasers and enhanced stability compared to femtosecond lasers. Researchers have discovered that sub-nanosecond lasers have a lower threshold for causing complete damage to optoelectronic devices compared to nanosecond and femtosecond lasers. Therefore, high-repetition-rate, high-energy sub-nanosecond solid-state lasers offer significant advantages in the field of optoelectronic countermeasures. Currently, under conventional water cooling conditions, the single-pulse energy of high-repetition-rate lasers has surpassed the hundred-millijoule level. However, for optoelectronic countermeasure applications, higher repetition rates and higher single-pulse energies are required to improve the hit rate on rapidly moving targets. Thus, the breakthrough of higher repetition rates and high-energy sub-nanosecond lasers is urgently needed. Methods This paper presents the realization of Joule-level sub-nanosecond laser output by combining end-pumped microchip crystal picosecond laser generation technology and multi-pass multi-stage slab laser amplification techniques. Initially, the microchip laser is pre-amplified through a three-stage end-pumping process, scaling the microjoule-level energy to millijoule-level. Subsequently, the shaped laser beam with a size of 2×18 mm² is injected into a first-stage single-end-pumped slab amplifier system. The amplified laser is then transmitted through a first-stage imaging and beam expanding system before being injected into a second-stage dual-end-pumped double-pass amplifier system. Finally, the laser is further amplified through a third-stage single-pass booster amplifier in the slab configuration. This study presents the design of a dual-end pumping structure (Fig.1), with the omission of the isolation system. Results and Discussions In the dual-end-pumping structure, the energy of the leaked pump light can directly cause damage to the pumping module. In this study, experimental results revealed significant fluctuations in the energy of the leaked pump light with variations in the pump current and pump module cooling temperature (Fig.2(a)). Therefore, by controlling the cooling temperature of the pumping module, it is possible to regulate the energy of the leaked pump light at different pump currents, thereby avoiding damage to the pumping module and eliminating the need for complex isolation devices. Using the temperature-controlled dual-end pumping technique, the research team amplified the seed light from 3.12 mJ at 500 Hz to 952 mJ (Fig.2(b)) with pulse width of 680 ps, under the conditions of a first-stage and second-stage pump cooling temperature of 25 ℃, and a second-stage pump cooling temperature of 23 ℃. The amplification energy levels in Fig.2(b) were measured after the output of the third-stage slab amplifier. The sub-nanosecond laser output with Joule-level energy and a repetition rate in the hundreds of hertz achieved by this system represents the highest parameters currently achieved in the field of slab lasers. Conclusions This paper reports a high-repetition-rate, high-energy Nd:YAG low-doped slab laser. The laser utilizes a high-power master oscillator power amplifier (MOPA) structure, with a single longitudinal mode microchip laser as the seed source, and achieves amplification through a three-stage slab system with beam shaping. The study demonstrates that the leaked pump light in the dual-end-pumped slab amplifier can damage the pumping module. However, precise control over the energy of the leaked pump light can be achieved by controlling the cooling temperature of the pumping module, effectively avoiding damage to the pumping module. In this work, using temperature-controlled dual-end pumping technique, we amplify the seed light from 3.12 mJ at 500 Hz to 952 mJ with a pulse width of 680 ps. This work provides an effective pump cooling solution for high-energy, short-pulse lasers, ensuring their stable operation, thereby paving the way for the application of high-repetition-rate, high-energy sub-nanosecond slab lasers in the field of optoelectronic countermeasures.

Generation of high-efficiency hundred-millijoule stimulated Brillouin scattering in fused silica

Chen Bin, Bai Zhenxu, Zhao Guijuan, Wang Yulei, Lu Zhiwei

2023, 52(8): 20230421. doi: 10.3788/IRLA20230421

[Abstract](71) [FullText HTML] (22) [PDF 1230KB](30)

Objective Stimulated Brillouin scattering (SBS) is a powerful tool for serving as a phase conjugation mirror (PCM) due to its inherent properties of high gain, small frequency shift, and phase conjugation. Solid-state gain media offer the advantages of high stability and high repetition rate SBS, compared to liquid and gas gain media. However, solid gain media face the challenge of recovery once breakdown occurs. Currently, there is limited research on achieving high-efficiency and high-energy SBS generation in solid media, which restricts the application of solid gain media in high-energy SBS. In this study, we experimentally investigate an SBS generator based on bulk fused silica to provide guidance for the development and application of all solid-state SBS systems with high efficiency. Methods The experimental setup is illustrated (Fig.1). A passively Q-switched nanosecond laser, based on a ring cavity, is used as the pump source, delivering a pulse width of 10 ns. A Fabry-Perot etalon is inserted into the cavity to control the number of longitudinal modes. Lenses L₁ and L₂ are utilized to adjust the beam diameter from 3.2 mm to 5.6 mm, while the focal length of L₃ is 250 mm. Fused silica, with a length of 200 mm, serves as the Brillouin gain medium. The output characteristics of SBS generation, including threshold, slope efficiency, damage threshold, and beam profile, are studied by varying the pump mode and pump intensity. Results and Discussions Compared to a single longitudinal-mode (SLM) pump, the SBS threshold for a multi-longitudinal-mode (MLM) pump is 14% higher, and the damage threshold is only 34 mJ (Fig.2(a)). A phase-conjugate reflectivity of up to 81.0%, with a slope efficiency of 85.8%, is achieved when the pump single pulse energy is 183.1 mJ. The results indicate that MLM pulse spikes are the key factor causing optical breakdown in the SBS process, while SLM pumping can effectively prevent the optical breakdown in solid media. The narrowest Stokes pulse width of 5.5 ns is obtained at an energy reflectivity of 15%; While the waveform maintains good fidelity at the highest input pump energy (Fig.2(b)). The Stokes beam profile exhibits a good cleanup effect under low-energy pumping conditions (Fig.2(c)). However, it gradually evolves towards the pumping profile as the energy increases. This suggests that high reflectivity also leads to high beam quality fidelity of Stokes. Conclusions In this study, we have demonstrated the feasibility of achieving high-efficiency and high-energy SBS output in fused silica. A Stokes energy of 183.1 mJ with a slope efficiency of 85.8% was obtained when the pump energy was 226 mJ. This research lays the foundation for optimizing the characteristics of SBS-PCM based on solid Brillouin gain media, as well as its expansion in pulse compression, Brillouin amplification, and beam combination. It has important implications for achieving high-power all-solid-state SBS lasers.

Full control of structured light with liquid-crystal geometric phase

Li Chunyu, Yu Bingshi, Zhao Bo, Carmelo Rosales-Guzmán, Bai Zhenxu, Zhu Zhihan

2023, 52(8): 20230396. doi: 10.3788/IRLA20230396

[Abstract](189) [FullText HTML] (49) [PDF 1440KB](38)

Significance Flat optics elements based on geometric phase, owing to their low cost, integrability, and versatility, have been widely used in shaping of light's spatial structure. Notably, current SOC (spin-orbit coupling) devices, such as the best-known q-plates, provide only spatial phase modulation with SoP (state of polarization)-switchable behavior. The absence of amplitude control prevents research scholars from accessing light's full spatial degrees of freedom, thus limiting their application in corresponding studies. This team demonstrates a series of novel flat optics elements with liquid-crystal geometric phase, which unlocks the full-ﬁeld control of paraxial structured light, providing a powerful toolbox for relevant experimental studies and especially for high-dimensional classical/quantum information. Progress To control a paraxial SOC state in all its spatial degrees of freedom, spin-dependent complex amplitude modulation provides an essential alternative. But up to now, it has remained elusive with flat optics. This paper fills this gap by putting forward a new type of geometric phase element termed structured geometric-phase grating (SGPG), featuring a spatially-varying grating cycle, depth and orientation (Fig.1). In addition, the joint team also demonstrated the vector wavefront control technology based on the geometric phase of liquid crystals, and developed a series of liquid crystal geometric phase elements (e.g., mode convertor (Fig.2(a)) and high-order spatial mode generator (Fig.2(b)) with the full dimensional control ability. Conclusions and Prospects Such a crucial advance, compared with the present geometric phase elements, unlocks the control of paraxial structured light in all spatial dimensions, and paves the way for arbitrary SOC conversion via flat optics. This capability makes it a key extra-/intracavity component to build a structured laser that has greater tunability in beam structure, compared with reported systems based on q-plate and metasurface. For quantum optics, the proposed reciprocal SOC interface allows to implement a Bell measurement for arbitrary SOC states, which is the basis for the teleportation scheme for SOC photon pairs. Moreover, owing to the capability of full-field spatial mode control, the device also paves the way for quantum control of high-dimension photonic skyrmions. Beyond single-beam vector mode control, this principle can further realize multiple vector mode control through the addition of a Dammann grating structure. This represents a promising way to develop information exchange and processing units working for photonic SOC states, that is, vector-mode multiplexers and demultiplexers.

Four times linewidth narrowing has been achieved in diamond Brillouin laser

Jin Duo, Bai Zhenxu, Fan Wenqiang, Qi Yaoyao, Ding Jie, Yan Bingzheng, Wang Yulei, Lv Zhiwei

2023, 52(8): 20230295. doi: 10.3788/IRLA20230295

[Abstract](149) [FullText HTML] (45) [PDF 1054KB](36)

Objective Brillouin laser is an important technological approach for achieving high coherence and low noise lasing, among which Brillouin lasers in free space have been proven to generate high-power single-frequency laser radiation. However, unlike the widely studied guided-wave-based Brillouin lasers, no studies on the linewidth properties have been reported for Brillouin lasers in free space. In this paper, a series of research works have been conducted on the generation, parameter regulation, and performance optimization of the Brillouin lasers in free space using diamond as gain media. We experimentally studied the feasibility of realizing linewidth narrowing of the Brillouin laser in free space. Methods The structure of the spatial Brillouin laser and the corresponding linewidth measurement device is shown respectively (Fig.1(a), (b)). The Brillouin laser uses a diamond crystal as the Brillouin gain medium, which has the highest known thermal conductivity and transmission range. A directly pumped ring cavity structure is used for the experiments, where the linewidth of the pumped light is 7.36 kHz. The linewidth behavior of the Stokes light is comparatively investigated by choosing three different sets of coupled mirror reflectivity: R₁ = 96%, R₁ = 97% and R₁ = 98.5% for the experiments. Results and Discussions The measurement results (Fig.2) show that the Stokes linewidth becomes narrower as the coupler reflectivity increases. The Stokes linewidths corresponding to three sets of coupler reflectivity are 3.2 kHz, 2.43 kHz and 1.77 kHz, respectively, and all of them realize linewidth compression compared with the pump, with the highest compression ratio of 4.1. Theoretically, the output efficiency and linewidth compression can be improved at the same time by decreasing the insertion loss of the intracavity element, and the analysis shows that, at the pump power of 60 W and coupled-mirror reflectivity of 96%, the linewidth of 1.6 kHz and up to 80% can be achieved by decreasing the insertion loss of the intracavity element. The analysis shows that at a pump power of 60 W and a coupling mirror reflectivity of 96%, a linewidth of 1.6 kHz and a Stokes output with an optical conversion efficiency of up to 80% can be realized by reducing the insertion loss of the intracavity components. In the future, when realizing ultra-narrow linewidth laser radiation, the technical noise introduced in the system will be the main obstacle limiting the further reduction of the fundamental linewidth. Conclusions For the first time, we have verified the feasibility of realizing linewidth-narrowed Brillouin laser output in a free-space optical transport structure. The study provides a feasible technical solution for obtaining high-power, narrow-linewidth lasing with a wide wavelength range. The result is of great significance for promoting the development of diamond laser technology and advancing the application of highly coherent light sources.

Nonlinear control of structured light in all spatial degrees of freedom

Wu Haijun, Yu Bingshi, Jiang Jiaqi, Zhao Bo, Carmelo Rosales-Guzmán, Bai Zhenxu, Zhu Zhihan, Shi Baosen

2023, 52(8): 20230397. doi: 10.3788/IRLA20230397

[Abstract](174) [FullText HTML] (42) [PDF 1227KB](42)

Significance Driven by the recent gradual maturity of digital holography and flat optics with geometric phase, great advances have been made in shaping and application of structured light in the linear optics. In comparison, relevant study based on nonlinear optics, although enabling many crucial functions, such as information exchange between light fields or photons, is still in its infancy. Focusing on this frontier topic of structured nonlinear optics-i.e., nonlinear generation, transformation and interface of classical/quantum states encoded by complex spatial modes-some theoretical and technical bottlenecks to parametrically control all the spatial dimensions of light have been broken. These results lay a solid foundation for future relevant studies on high-dimensional quantum optics experiments. Progress Advances in parametrically controlling all the spatial dimensions of light can be divided into theoretical and applied aspects. On the theoretical side, a parameter transformation theory for the full-field selection rule of spatial modes in cylindrical coordinates during small signal three-wave mixing (Fig.1(a)) and a theoretical model of spin-orbit-coupling-mediated nonlinear polarizations (Fig.1(b)) were first proposed. These two theoretical tools can be used together to describe and predict the nonlinear propagation of the vector spatial structure of light fields (amplitude, phase, and polarization) in any paraxial second-order parameter process. Guided by theoretical tools, a nonlinear astigmatism frequency interface has been proposed (Fig.1(c)). In this parametric astigmatism system, an unexpected new physical effect called anomalous orbital angular momentum conservation has been uncovered. This discovery renewed the perception of the nonlinear orbital angular momentum conservation. On the applied side, a frequency conversion technique based on a Sagnac nonlinear interferometer pumped by a super-Gauss mode was first proposed to achieve a spatial polarization independent conformal frequency interface (Fig.2(a)). On this basis, the apparatus can also act as a spatial-amplitude independent frequency interface for orbital angular momentum, which enables a simultaneous conversion of frequency and orbital angular momentum without impacting on the radial mode of signals, with the introduction of vortex super-Gaussian modes (Fig.2(b)). What's more, the new theory has also inspired a new application called spatially-resolved autocorrelation technique, which is based on spatially multimodal nonlinear optical effects. This technique allows for the characterization of the temporal envelope and spatial modes of ultrafast light simultaneously (Fig.2(c)). Conclusions and Prospects These systematic research results fill the key theoretical gaps in the field of nonlinear control of structured light in all spatial degrees of freedom. They also provide inspiration for new ideas in the field of light field shaping and have significant implications for related studies, such as structured laser technology, modulation of high-dimensional quantum states and polarization-resolved up-conversion imaging.

Twelvefold phase superresolution interferometric measurement in real time via nonlinear light field control

Zhang Xinyu, Wu Haijun, Carmelo Rosales-Guzmán, Bai Zhenxu, Zhu Zhihan, Hu Xiaopeng, Zhu Shining

2023, 52(8): 20230398. doi: 10.3788/IRLA20230398

[Abstract](135) [FullText HTML] (40) [PDF 1404KB](30)

Objective Optical interferometry metrology techniques and devices are pillars of modern precision metrology. With the development of laser and light field shaping technologies, their performance has achieved significant improvements across multiple orders of magnitude. However, they are still limited by the wavelength of the light source. Due to the easy absorption and difficult manipulation of extremely short-wavelength optical fields, the resolution of interferometers cannot be infinitely improved by simply reducing the wavelength. "Phase superresolution" refers to the technological means to overcome the limitation imposed by the light source wavelength. Currently, research on phase superresolution mainly focuses on manipulating N-photon entangled states to achieve this goal. However, the extremely high difficulty in preparing and controlling N-photon entangled states, as well as the low efficiency of coincidence counting, renders this approach impractical for actual measurements. Therefore, it is necessary to realize real-time phase superresolution measurements to meet practical application requirements. Methods To overcome these aforementioned cutting-edge challenges, the collaborative team has taken a novel approach of utilizing the modal structure evolution of orbital angular momentum (OAM) coherent states during parametric conversion processes to simulate the behavior of N00N states in SU(2). Consequently, they have achieved a more efficient means of actively preparing multi-photon amplitude signals carrying interferometer arm phase information. Results and Discussions Phase superresolution signal carried by coherent states with an N-fold enhancement (N=4) has been achieved in a single artificial metamaterial crystal using multiple quasi-phase matching in quasi-periodic optical superlattice (Fig.1(a)). By cascading parametric conversions of the superresolution signal, a phase superresolution interference signal with enhanced resolution of up to N=12 has been realized. Remarkably, the signal intensity remains visible to the naked eye, and real-time recording can be achieved with low-cost photodetectors. In the near future, using this scheme with appropriate technical improvement (Geometric phase elements) with N>100, corresponding to an extreme-ultrviolet de Broglie wavelength, is expected to be an attainable goal. Conclusions The preparation of N-photon entangled states typically involves the use of spontaneous parametric down-conversion (SPDC) process to convert the short-wavelength pump light into a photon stream with extremely low efficiency. The target signal is then selected using inefficient photon coincidence counting systems. As a result, the performance of such systems is significantly lower compared to interferometers that directly utilize the pump light source for sensing. In the approach proposed by the collaborative team, spatial modes are employed to encode phase information into the pump light field. By actively constructing multiphoton amplitudes through a strong stimulated parametric process, the signal power loss incurred in cascaded nonlinearities can be regained through phase-sensitive amplification, leading to a significant improvement in system performance. Therefore, their result paves a promising way for the development of practical phase superresolution interferometry techniques and instruments for metrology.

5 kW-level narrow linewidth fiber laser output realized by homemade polarization-maintained fiber

Ren Shuai, Ma Pengfei, Chen Yisha, Li Wei, Wang guangjian, Liu wei, Huang Liangjin, Pan Zhiyong, Yao Tianfu, Zhou Pu

2023, 52(2): 20220900. doi: 10.3788/IRLA20220900

[Abstract](310) [FullText HTML] (80) [PDF 828KB](115)

2022, 51(6): 20220032. doi: 10.3788/IRLA20220032

[Abstract](192) [FullText HTML] (45) [PDF 742KB](58)

2022, 51(6): 20220293. doi: 10.3788/IRLA20220293

[Abstract](208) [FullText HTML] (35) [PDF 3367KB](48)

2022, 51(4): 20211103. doi: 10.3788/IRLA20211103

[Abstract](277) [FullText HTML] (33) [PDF 880KB](50)

Wu Hanshuo, Li Ruixian, Xiao Hu, Huang Liangjin, Yan Huan, Pan Zhiyong, Leng Jinyong, Zhou Pu

2021, 50(9): 20210353. doi: 10.3788/IRLA20210353

[Abstract](228) [FullText HTML] (103) [PDF 794KB](61)

2021, 50(9): 20210621. doi: 10.3788/IRLA20210621

[Abstract](334) [FullText HTML] (118) [PDF 649KB](95)

Global topology optimized metagrating beam splitter based on deep learning

Deng Renjun, Shi Tan, Li Xiangping, Deng Zilan

2021, 50(5): 20211028. doi: 10.3788/IRLA20211028

[Abstract](728) [FullText HTML] (243) [PDF 1061KB](90)

With the help of the deep learning model applied in the inverse design of the metagrating beam splitter, good uniformity and high diffraction efficiency can be obtained. The structure design, diffraction efficiency and uniformity of the metagrating beam splitter was studied by using the global topology optimization neural networks. Under the working wavelength of 900 nm, the beam splitter with splitting angle of 120° and 150° designed based on the global topology optimization networks had high diffraction efficiencies of 95% for 120° and 85% for 150°.

2021, 50(11): 20210668. doi: 10.3788/IRLA20210668

[Abstract](235) [FullText HTML] (91) [PDF 859KB](40)

2021, 50(11): 20210822. doi: 10.3788/IRLA20210822

[Abstract](184) [FullText HTML] (54) [PDF 4127KB](56)

Beyond 4 kW narrow-linewidth and near single-mode fiber laser

Ma Pengfei, Xiao hu, Leng jinyong, Li san, Chen zilun, Wang xiaolin, Wang zefeng, Zhou pu, Chen jinbao

2021, 50(1): 20200421. doi: 10.3788/IRLA20200421

[Abstract](457) [FullText HTML] (134) [PDF 714KB](538)

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