Special issue-Laser technology and its application

Development and prospect of fiber grating in high-power continuous fiber laser
Shen Hua, Zhu Rihong, Bian Yinxu
2022, 51(2): 20210908. doi: 10.3788/IRLA20210908
[Abstract](516) [FullText HTML] (166) [PDF 2285KB](138)
At present, the applications of fiber gratings in high-power fiber lasers mainly include two aspects, be used as cavity mirrors and to suppress nonlinear effects. The development of fiber gratings as cavity mirror was discussed firstly, and then the applications of special structure fiber gratings with nonlinear effects suppression function was focused on. The stimulated Raman scattering and stimulated Brillouin scattering suppression method were introduced based on tilted fiber Bragg grating in detail. And the feasibility of suppressing stimulated Raman scattering by long-period fiber grating was also discussed. Further, a novel method of suppressing self-phase modulation or four-wave mixing by utilizing phase-shifted long-period fiber grating was presented. Finally, a prospect of the applications of fiber gratings in high-power fiber laser was provided. Higher power durability and longer wavelength were the definite development directions. And also considered that the femtosecond laser lithography technology, cascaded fiber grating that can suppress multiple nonlinear effects, and polarization control technology based on special structure fiber gratings will become a new research hotspot.
Home-made 6.7 kW narrow linewidth triple-cladding fiber laser
Han Zhigang, Zheng Yunhan, Wang Haoye, Li Fangxin, Chen Jiale, Zhu Rihong
2022, 51(2): 20210849. doi: 10.3788/IRLA20210849
[Abstract](419) [FullText HTML] (244) [PDF 1662KB](108)
In order to achieve high-power output of fiber lasers with narrow linewidth, the thermal effect and the four-wave mixing (FWM) effect of the Yb3+ doped large mode area triple-cladding fiber (LMA-YTF) based fiber laser was investigated. Based on the FWM effect model, the influences on the spectral broadening of the large mode area triple-cladding fiber (LMA-TCF) amplifiers were simulated and analyzed. Thermal distribution model of the LMA-YTF was modeled. The effects of the power ratio in the second cladding on the fiber temperature characteristics and the upper limit of the pump power were analyzed. The influence of the thermal conductivity of polymer coatings and external temperature on fiber temperature was discussed. The upper limits of the pump power of different backward combiners were compared experimentally. The experimental results indicate that the (6+1)×1 triple-cladding backward combiner with a lower power ratio in the second cladding has a higher pump power upper limit than the (9+1)×1 combiner, which agree with the simulation results. A triple-cladding fiber laser is constructed with fully home-made devices, achieving an output power of 6.7 kW and a 3 dB linewidth of 0.32 nm.
Light beam induced current mapping to characterize damage characteristics of silicon solar cell irradiated by continuous-wave laser
Lu Jian, Xie Zhijian, Zhang Hongchao
2022, 51(2): 20220022. doi: 10.3788/IRLA20220022
[Abstract](326) [FullText HTML] (122) [PDF 1363KB](54)
Aiming at the damage characteristics of silicon solar cells irradiated by continuous-wave (CW) laser, a light beam induced current (LBIC) mapping system was applied to characterize the damage characteristics of solar cells, and the damage characteristics were analyzed. A 1 070 nm CW laser was used to focus on the surface of silicon solar cell to induce damage. A LBIC system was use to obtain the photocurrent spatial distribution of the laser irradiation area of solar cell to analyze the damage. In order to characterize the damage of solar cell at different depths, the LBIC system used 650 nm and 980 nm lasers as probe light sources respectively. The results show that when 1 070 nm CW laser irradiates the non-finger part of a silicon solar cell, the solar cell damage first occurs inside; with the increase of power density, there is an invalid region inside the solar cell before its surface melts. When the laser irradiates fingers, the finger will melting. It will result in a decrease in the photocurrent at the side of the irradiated position which away from the electrode lead. In severe cases, the solar cell will have cracks which perpendicular to fingers, and the cracks will invalidate the cell on the side away from the electrode lead. The results can provide a reference for the research on the damage mechanism of CW laser irradiated solar cells.
Nanosecond laser-induced periodic structures on polyimide film
Lin Suying, Liao Xiaojie, Han Bing
2022, 51(2): 20210911. doi: 10.3788/IRLA20210911
[Abstract](214) [FullText HTML] (165) [PDF 1604KB](51)
Laser-induced periodic surface structures (LIPSS) were created by a 355 nm linearly polarized laser with the pulse duration of 7 ns and the pulse repetition rate of 1 Hz on polyimide films. The influences of different laser parameters on the morphology of the formed LIPSS were investigated. It is found that there is a certain fluence threshold and pulse number threshold for the generation of periodic structure. When the laser fluence is in the range of 54-586 mJ/cm², and the pulse number is between 1-50, the well-ordered LIPSS are formed with the periods between 4-6.65 μm. The pulse number and the laser fluence are changed in the experiment. With the same laser fluence, increasing the pulse number, or with the same pulse number, increasing the laser fluence, the period of the ripples can be increased. It is also observed that the formation of LIPSS is accompanied by the removal of materials, with the increase of the pulse number, the depth of the crater increases, and LIPSS can continue to appear at the bottom of the crater. In addition, in order to analyze the possible causes of the formation of periodic structure, the physical state of the material is discussed by establishing the heat conduction model when the periodic structure is formed. These works provide potential support for studying the improvement of material surface wettability, friction mechanics, and optical properties.
Analysis and treatment of thermal effects in 10 kW-level optic switch
Li Zhijie, Kong Qingqing, Zhang Mingdong, Jing Ziheng, Bian Yinxu, Shen Hua, Zhu Rihong
2022, 51(2): 20210909. doi: 10.3788/IRLA20210909
[Abstract](226) [FullText HTML] (154) [PDF 1554KB](57)
The optic switch can output the single laser beam output by the fiber laser through multiple channels to realize the "one device and multiple applications" of the laser. It is the key device of modern laser intelligent manufacturing. As the power of the optic switch is up to 10 kW level, thermal effects happen easily in the coupling system, which affects the performance of the optic switch. In order to solve the problem of effective control of thermal effects, ensure the efficient coupling and high-quality output of the 10 kW-level optic switch, the physical mechanism of thermal effects in the coupling system was studied by finite element analysis. And a novel method based on water circulation and flow around cooling was proposed to control the thermal effects of the system. Through the verification of serial 10 kW-level experiments, it indicated that under the guarantee of the method proposed in this paper, the high-power optic switch was able to carry 10 kW-level power for a long time. The coupling efficiency was maintained above 98%. The thermal aberration of the system was suppressed, and the stability of the beam quality of the laser output by the optic switch can be guaranteed. This study provides an effective means for the analysis and treatment of thermal effects in the high-power laser system.
Evolution mechanism of transient optical properties of ultrafast laser-induced monocrystalline silicon
Liao Xiaojie, Lin Suying, Han Bing
2022, 51(2): 20210907. doi: 10.3788/IRLA20210907
[Abstract](264) [FullText HTML] (139) [PDF 1598KB](47)
The evolution pattern of the transient optical properties on the surface of monocrystalline silicon materials under the action of lasers with different pulse widths and different energy densities in the sub-picosecond to picosecond range was studied. This research was based on a dual temperature equation, carrier number density model, that considered the latent heat of phase transition. The carrier temperature, lattice temperature, permittivity, and the number density of excited carriers during laser irradiation were calculated, energy transfers processes from photons to electrons and electrons to phonons was simulated. In the end, the variation results of refractive index and extinction coefficient of the monocrystalline silicon surface were obtained. This result helps to reveal the evolution mechanism of the transient optical properties of monocrystalline silicon materials under the irradiation of ultrashort pulse lasers in the sub-picosecond to picosecond pulse width range. Theoretical calculations show that if a single laser pulse cannot melt monocrystalline silicon, the effects of different laser energy densities and laser pulse widths on the minimum refractive index and extinction coefficient are minimal. In the laser energy density range from 0.3 J/cm2 to 0.4 J/cm2, the minimum refractive index change is less than 0.5% per 0.01 J/cm2 change in energy density. Suppose a single laser pulse can melt monocrystalline silicon. In that case, different laser energy densities and pulse widths have different degrees of influence on the silicon surface's refractive index and extinction coefficient. This research results can provide some theoretical guidance for the processing and surface modification of monocrystalline silicon materials based on ultrashort pulse laser.
Performance test of solar cell under laser energy transmission and signal transmission
Sun Zhiyu, Lu Jian, Zhang Hongchao, Li Guangji, Xie Zhijian
2022, 51(2): 20210888. doi: 10.3788/IRLA20210888
[Abstract](350) [FullText HTML] (115) [PDF 1810KB](61)
Laser wireless energy transmission has potential applications prospects in supplying energy for long-distance equipment. And laser wireless communication with energy transmission has important application value. For GaAs solar cell, the laser wireless communication performance of the laser energy transmission system was tested during wireless energy transmission. A wavelength of 808 nm laser to achieve the energy transmission of the GaAs solar cell was used in the experiment, and a wavelength of 650 nm laser was used as the signal transmission. The output characteristics of GaAs solar cell under three conditions of single energy transmission, single signal transmission and energy and signal simultaneous transmission were tested respectively. The results show that when the single energy is transmitted, the performance of the solar cell is closely related to the laser power density. In the range of 54.9-90 mW/cm2 of the laser power density, the maximum energy conversion efficiency is 46.6%; when the single signal is transmitted, by measuring the frequency response of the system, the 3 dB bandwidth of the GaAs solar cell is about 3.7 kHz. And by designing the amplifier circuit, the communication performance of the system is improved and the output waveform is optimized, so that the transmission rate of the system is increased from 10 kbps to 240 kbps, and the output voltage peak-to-peak reaches 7.2 V. Finally, the achievable signal transmission rates under different laser intensities were measured experimentally. When the laser power density is 59.5 mW/cm2, the signal transmission rate of 140 kbps is achieved, so that the laser charging system can perform signal transmission under wireless energy transmission.
Perforation effect of CW laser irradiation on aluminum alloy under subsonic flow
Zhang Xiaoteng, Li Zewen, Zhou Yiqing, Shen Zhonghua
2022, 51(2): 20210883. doi: 10.3788/IRLA20210883
[Abstract](256) [FullText HTML] (117) [PDF 2736KB](38)
The perforation effect of laser irradiated target is different under different airflow velocity. The perforation effect of 7075 aluminum alloy irradiated by 1070 nm CW laser under subsonic airflow(0-0.7 Ma) was experimentally studied. The temperature history, perforation time, perforation aperture and surface morphology of the center point of the aluminum alloy were analyzed. The results show that under the same airflow velocity, with the increase of the incident laser power density, the temperature rise rate of the aluminum alloy surface increases and the equilibrium temperature of the final melting layer increases. The perforation time of aluminum alloy decreases exponentially; the increase rate of pore size decreases exponentially. At the same laser power density, with the increase of airflow velocity, the perforation time of aluminum alloy increases first and then decreases to a stable and then increases. Both the removal rate of melt and the cooling effect of airflow lead to the longest perforation time near 0.1 Ma and the shortest perforation time near 0.3 Ma. The perforation time of 0.6 Ma is roughly equal to that of 0 Ma about 5.5 s. With the increase of airflow velocity, the cooling effect increases, and there is no perforation in the aluminum alloy after 0.7 Ma. Convection cooling leads to rapid condensation of the melt, and the removed melt concentrates in the downstream area of the airflow.
Design of new-style driving and temperature control circuit for low power diode laser
Yang Tao, Li Wusen, Chen Wenjian
2022, 51(2): 20210764. doi: 10.3788/IRLA20210764
[Abstract](538) [FullText HTML] (251) [PDF 2312KB](113)
In order to deal with the influence of the stability of the output optical power of diode laser (LD) on the flatness measurement accuracy of the strip laser flatness measuring instrument, a high stability diode laser constant current driving circuit, temperature control circuit and protective circuit were designed. FPGA was used as the system control core. Based on the principle of deep negative feedback, the accurate current control was achieved by the constant current driving circuit. The effective control of LD operating temperature was realized by temperature control circuit based on ADN8830. Driving current was slowly and linearly increased to preset value by improved slow-start circuit, and the slow-start time can be controlled precisely. High reliability and high security were realized by current limiting and electrostatic protection circuit. The results show that the driving current can be adjusted continuously from 0 to 75 mA, the adjustment accuracy can reach 0.025 mA, the short-term stability of current can reach 0.014% and the long-term stability can reach 0.016%. When the operating temperature is controlled at 25 ℃, the stability of the output optical power is 0.205%.
Application of electrostatic jet-print technology in terahertz metasurface devices fabrication
Feng Chunjie, Zhu Xiaobo, Wu Yanghui, Fu Chen, Chang Huiyu, Yue Yutao, Gu Wenhua
2022, 51(2): 20210878. doi: 10.3788/IRLA20210878
[Abstract](264) [FullText HTML] (163) [PDF 1425KB](58)
In recent years, terahertz technology has been developed rapidly, and terahertz devices based on metasurfaces have received widespread attention and have been applicated in many areas, including terahertz imaging, spectroscopy, biosensing, and so on. However, the fabrication of the terahertz metasurface devices is complex and costly, while electrostatic jet-print technology has the advantages of mask-free, low cost, high precision, and special-shaped curved surface conformability. In this work, a terahertz absorber based on the electrostatic jet-print technology was designed and fabricated, and characterized by using a reflective terahertz time-domain spectrometer (THz-TDS). The result shows an absorption rate greater than 90% in the range of 0.098-0.353 THz, which is basically consistent with the simulation result. In addition, a terahertz polarization converter was also designed. The conversion efficiency in the range of 0.167-0.355 THz is greater than 95%, while the relative bandwidth is about 72%. The fabrication process conditions of the designed terahertz polarization converter was then analyzed, and the electrostatic jet-print technology was verified to be a promising fabrication method. The research results show that the electrostatic jet-print technology has broad application prospects in the fabrication of terahertz metasurface devices.