Invited paper-Beam combining of fibre laser

Review on the progress in coherent beam combining of ultra-short fiber lasers(Invited)
Su Rongtao, Zhou Pu, Zhang Pengfei, Wang Xiaolin, Ma Yanxing, Ma Pengfei
2018, 47(1): 103001. doi: 10.3788/IRLA201847.0103001
[Abstract](572) [PDF 1946KB](287)
Coherent beam combining (CBC) of ultra-short pulsed lasers is a promising way to overcome the power and pulse width limitations from a single fiber. In this paper, the principles and technologies of coherent combining of ultra-short pulsed lasers in spatial, time and spectral domains were described. The current status of ultra-short laser CBC systems and their key technologies were reviewed, several future perspectives were pointed out. This paper can be a reference for future development of CBC of ultra-short pulsed lasers.
Recent progress of key technologies for spectral beam combining of fiber laser with dual-gratings configuration(Invited)
Ma Yi, Yan Hong, Sun Yinhong, Peng Wanjing, Li Jianmin, Wang Shufeng, Li Tenglong, Wang Yanshan, Tang Chun, Zhang Kai
2018, 47(1): 103002. doi: 10.3788/IRLA201847.0103002
[Abstract](706) [PDF 3018KB](206)
A spectral beam combination(SBC) system with dual Multi-Layer Dielectric(MLD) grating dispersion compensation configuration, which can combine multiple fiber lasers into a common aperture beam with high beam quality and relax the linewidth requirement of the individual fiber laser, has become gradually one of the most important technique route of fiber laser beam combining. The basic principle and the key technologies of the SBC system with dual-MLD-grating configuration were introduced and analyzed briefly. The recent progress of the main key technologies about high power beam combinable narrow linewidth fiber laser, high power high efficiency short wavelength narrow linewidth fiber laser, large dispersion high efficiency MLD grating and high integration dense beam combining were reviewed. The recent progress on the key technologies of SBC with dual-MLD-configuration in Institute of Applied Electronics, China Academy of Engineering Physics, were also presented. The potential of the SBC with dual-MLD-configuration was prospected.
Research progress of laser phased array technique based on fiber adaptive manipulation(Invited)
Geng Chao, Li Feng, Huang Guan, Yang Yan, Liu Jiaying, Li Xinyang
2018, 47(1): 103003. doi: 10.3788/IRLA201847.0103003
[Abstract](465) [PDF 3707KB](157)
Laser phased array technique on coherent combining of multiple beams faces challenges when applied in propagation through the long-range atmosphere. Aberrations in such transmission systems include turbulence-induced dynamic aberrations located at the path from the fiber laser array to the target, besides the inherent phase errors like phase noises and tip/tilt errors. Effective bandwidth for eliminating such aberrations is limited by the optical transmission delay and the increment of the array scale. Existing techniques, e.g., target-in-the-loop and delayed stochastic parallel gradient descent, are difficult to deal with the fast-changing turbulence-induced tip/tilt aberrations. But correcting these aberrations is critical for obtaining combined laser beams on the target with the best beam quality. In this paper, research progress of multi-aperture laser transceiving control for beam combining applications in IOE, CAS was This method provided efficient ways to achieve tip/tilt control for the beam coupling from space to fiber and the outgoing laser beams in the beam combining applications. Correction of the aberrations outside the fiber laser array based on active multi-aperture wavefront sensing of the fiber laser array was proposed. These novel techniques presented here would promote the application of laser phased array technique in atmospheric propagation.
Advances in ultrashort divided-pulse amplification systems(Invited)
Yang Kangwen, Hao Qiang, Zeng Heping
2018, 47(1): 103004. doi: 10.3788/IRLA201847.0103004
[Abstract](493) [PDF 1208KB](127)
The new development of using divided-pulse amplification system in controlling the fiber nonlinearity, enhancing the pulse energy and breaking through the damage threshold of gain material was reviewed, including the basic types of pulse divider and optical arrangement of amplifiers. The latest advances in high repetition rate, high power Er-doped fiber divided-pulse amplifier was introduced. Based on the double-pass configuration and multi-stage pulse divider, nonlinear amplification and pulse compression were achieved simultaneously, resulting in frequency-doubled output pulses with an average power of hundreds of milliwatts and pulse width less than one hundred femtosecond at 780 nm. This divided-pulse Er-doped amplification method has proven a valuable tool to develop compact, user-friendly and environment stable femtosecond source, which provides an alternative to Ti:sapphire laser in applications to the generation of Terahertz radiation, biological imaging and optical nonlinearity.
Review of all-fiber signal combiner for high power fiber lasers(Invited)
Chen Zilun, Zhou Xuanfeng, Wang Zefeng, Xu Xiaojun
2018, 47(1): 103005. doi: 10.3788/IRLA201746.0103005
[Abstract](521) [PDF 1523KB](284)
All-fiber signal combiner is one of the key components to break through the power scaling limitation of a single fiber laser by combing several high power fibers into a single fiber to realize the higher output power. The basic structures and the research progress of the all-fiber combiner at home and abroad were sumarized. The technical program and features were discussed in detail, existing problem and future research direction were put forward.
Research progress of liquid crystal optical phased array in high power laser applications(Invited)
Wang Xiangru, Zhou Zhuangqi
2018, 47(1): 103006. doi: 10.3788/IRLA201847.0103006
[Abstract](409) [PDF 3913KB](135)
The progress of liquid crystal optics phased array devices in high power laser applications was reviewed in this paper. Based on the realization principle of the device, the reason of the device in the high power application was illustrated. Based on the multi-layer structure of the device, the working condition and limitation of the high power laser were introduced, including the transparent conductive layer, alignment layer and liquid crystal. Meanwhile, the latest research progress of these typical functional layers was carried out. At the same time, the relationship between laser threshold and the operating mode or the heat dissipation structure was also analyzed in detail. The related research progress was also reviewed.
Polarization control of combined laser beams based on optical homodyne polarization detection and phase locking
Shen Hui, Quan Zhao, Yang Yifeng, Zhao Xiang, Bai Gang, He Bing, Zhou Jun
2018, 47(1): 103007. doi: 10.3788/IRLA201746.0103007
[Abstract](409) [PDF 975KB](123)
As a critical technique for increasing the brightness of narrow-linewidth fiber lasers, coherent polarization beam combining can achieve common aperture joining of multiple lasers, while maintaining higher beam quality and linear polarization state. A polarization control system of combined lasers was investigated based on the linear phase locking technique. A physical model for optical homodyne polarization detection and a mathematical equation for linear phase-locking loop were established and analyzed in detail. The polarization phase of combined lasers was checked by employing high-precision optical homodyne method and then fed back to phase lock lasers in real time. A linear polarization laser was output with the power of 279 mW. After phase locking, the polarization extinction ratio of combined lasers attained 19.3 dB and the control bandwidth was 39.6 kHz. The residual phase noise is 710-4 rad/Hz(1 Hz) and 310-4 rad/Hz. When the power was increased to 1 W, the polarization extinction ratio was -15 dB, which was restricted by power-induced phase noise and spatial mismatch of beam spots.
Spectral beam combining system of fiber laser by external-cavity fiber oscillator
Zhang Jingpu, Yang Yifeng, Zhao Xiang, Bai Gang, He Bing, Zhou Jun
2018, 47(1): 103008. doi: 10.3788/IRLA201746.0103008
[Abstract](441) [PDF 1091KB](113)
Spectral beam combining is an effective way to break through the power limitation of a monolithic fiber laser and achieve high brightness fiber laser output.A spectral beam combining scheme was presented that the single element was structured by external-cavity fiber oscillator.This scheme had more compact structure and was able to contain more elements,compared with the current MOPA structure element.A theoretical analysis was conducted about the relationship between the wavelength and position of each element,and a numerical simulation was carried out about the influence of aberrations in the transform lens on cavity loss and combining effects.An experimental system was built,and the experimental results agree well with the theoretical analysis.This work has important guiding significance for the building of next generation spectral beam combining system.
Automatic alignment method and realization of space optical coupling
Ke Xizheng, Luo Jing, Lei Sichen
2018, 47(1): 103009. doi: 10.3788/IRLA201847.0103009
[Abstract](375) [PDF 1208KB](99)
The sticking point of spatial light-single-mode fiber is to find the best position in the transverse plane of fiber precisely. Based on optical fiber migration's influence on the spatial light-single-mode fiber coupling efficiency, an auto-coupling system was proposed, which mainly consisted of two-dimensional piezoceramics, driver, controller, photoelectric detector which was a closed loop control system and coupling lens. The automatic alignment coupling of the spatial light-fiber was realized by combining with simulated annealing algorithm. The coupling efficiency for free-space laser coupling into the fibers and simulated annealing algorithm were analyzed theoretically. Analysis results of experiment show that the method can make the system search the best position automatically based on the power of the light which is coupled into the single-mode fiber. The optimal coupling efficiency is about 51.4%, which can be obtained in short times by the system and it is improved by 6.5% compared with no automatic alignment before. The scheme is practical and it is of great significance for the study of the automatic coupling of spatial light-single-mode fiber.
Analysis of beam characteristic in spectral beam combining of laser array with pointing deviation
Bai Gang, Shen Hui, Yang Yifeng, Zhao Xiang, Zhang Jingpu, He Bing, Zhou Jun
2018, 47(1): 103010. doi: 10.3788/IRLA201847.0103010
[Abstract](472) [PDF 1348KB](111)
Maintaining excellent beam quality is very important for the construction of the experimental spectral beam combining(SBC) system. The influence of pointing deviation on the beam characteristic was studied in SBC of fiber laser array theoretically. The incident light field of the laser array with the pointing deviation was corrected, and the variation rule of the combined beam quality with uniformly distributed random perturbations was discussed by the beam propagation model and the statistical analysis. The simulation result shows that the pointing deviation has a significant effect on the output characteristics of the SBC system. When the maximum deflection angle of the laser array is only 0.05, the beam quality of SBC system will increase to(6.491.73). In order to achieve the brightness scaling of combined beam and gradually expand the scale of laser array, the combined beam quality tends to stabilize. As a reference for the array scale(30 fiber lasers) with the changes of relative stability, the variation tendency of the M2 factor and maximum pointing deflection angle is fitted for building the experimental system of SBC.
Beam combining of high energy fibre lasers
Cheng Xue, Wang Jianli, Liu Changhua
2018, 47(1): 103011. doi: 10.3788/IRLA201847.0103011
[Abstract](554) [PDF 2808KB](246)
Beam combining of high energy fibre lasers is the research hotspot especially for the directed energy application in recent years, which can overcome the output power lever limits of one single-mode fibre laser, and establish the theoretical foundation for the application of laser weapons with high power and perfect beam quality. Research status of fibre laser incoherent combining and coherent combining were presented. In the section of incoherent combining, the combining principle and combining level of beam overlap and spectral beam combining were introduced. In the section of coherent combining, the key combining apparatus of transmission-type and reflection-type equivalent large aperture laser array output and optical element of single aperture output were analyzed in detail. The advantages and disadvantages and range of application of high power fibre laser beam coherent combining and incoherent combining were compared briefly.