Rate equations studies on coherently combined fiber lasers connected by an optical coupler

Attempt has been made to apply the steady-state rate equations to study two coherently combined Er³⁺-doped fiber lasers joined by an optical fiber coupler. In order to establish the boundary conditions, which are necessary for solving the equations, we have taken into consideration the fact that a constructive interference at the coupler assures that the laser radiations from both constituent lasers are directed to the coupler output port to which a reflection mirror defining both lasers is attached. Consequently, effective reflectivity, provided by the coupler-mirror combination, can be specified in terms of the laser powers of both laser signals, and the rate equations describing the compound system can be solved. Using the derived solutions, quantitative investigations can be made on the influences of different laser parameters. For example, the influence of the length difference between doped fibers, the splitting ratio of the coupler and power difference between the two pump radiations have been given in this report.     

Mutual injection locking and coherent combining of three individual fiber lasers

Mutual injection locking and coherent combining of three individual erbium-doped fiber lasers are reported. Theoretical model of the three fiber lasers under mutual injection is also analyzed. Under free running state, the far field beam profile is a simple intensity superposition as three incoherent beams. Under mutual injection locking, interference lobes with high contrast ratio are obtained in the far field of three fiber lasers, which shows that the three fiber lasers have good interference characteristic under mutual injection. The experimental results are in good agreement with the theoretical results. According to this method, we can also realize coherent beam combining in more fiber lasers.     

Beam quality factor for coherently combined fiber laser beams

The beam quality factor for a coherently combined beam is studied in detail. A theoretical analysis and a numerical calculation are given to illustrate that the M² factor is not a proper means for evaluating the beam quality of a coherently combined beam. The beam propagation factor is introduced to evaluate the beam quality. The effect of phase error on the beam quality is also studied. Results indicate that the beam quality of a coherently combined beam depends mainly on the fill factor of the laser array and not on the number of lasers. It is also revealed that the allowed phase error does not change with the number of lasers in order to ensure the energy encircled in a certain bucket.     

两个半导体激光器交叉注入的理论分析

提出了一个复杂非线性动力学系统－交叉注入锁定半导体激光系统。针对其中最简单的两激光系统,导出了描述它的方程组,解析求解了它们具有相同自由振荡频率时的定态解。结果表明,在相同耦合条件下,锁定的频率范围比主-从方式扩大了大约一倍。从结果可以看出,即使是这种特殊系统,在锁定后频率也可能会改变。     

Passive harmonic mode locking of soliton bunches in a fiber ring laser

We report on the experimental observation of passive harmonic mode locking of bunches of single-pulse solitons or twin-pulse solitons in an Erbium-doped fiber ring laser. Experimental investigations on the phenomenon revealed that, although the soliton interaction between the adjacent single-/twin-pulse solitons in a bunch is weaker than that of the pulse interaction in the twin-pulse solitons, a soliton bunch could also function as a unit and form the state of passively harmonic mode-locking. Harmonic mode-locking is one of the intrinsic characteristics of soliton emission in passively mode-locked fiber ring lasers. It can be formed based on the single-pulse soliton, twin-pulse soliton, or bunch of solitons.     

利用全光纤耦合环实现三路光纤激光器的相位锁定

利用三个2×2的光纤耦合器按一定规则联接成一个全光纤耦合环,该耦合环将三个独立的掺铒光纤激光器连成一个锁相阵列. 由于耦合环的引入,锁相阵列的整体损耗减小,且三个单元激光器之间可以进行有效的能量相互注入耦合,进而实现阵列的锁相输出. 实验中观察到的远场干涉图样和锁相前后的输出光谱均表明三路光纤激光器实现了相位锁定,当三路抽运源的功率均为100 mW时,获得了92 mW的稳定相干输出. 关键词： 光纤激光器 相干合成 光纤耦合器 相位锁定     

32路光纤激光相干阵列的相位锁定

报道了32路光纤激光相干阵列的相位锁定实验研究。搭建了32路光纤激光相干阵列实验系统,基于现场可编程逻辑阵列（FPGA）设计制作了高速高精度相位控制器。当相位控制器执行随机并行梯度下降（SPGD）算法对各路激光的相位进行锁定时,相干阵列输出的激光功率与不进行相位锁定时相比提高了约26倍。     

Dynamic strain monitoring by wavelength locking between two fiber Bragg gratings fiber sensing system

We demonstrated a fiber sensing system using the method of wavelength locking of two identical fibers Bragg gratings (FBG) to interrogate the wavelength shift by directly measuring the intensity of the reflection from the sensing Bragg grating. The light source of the fiber sensing system is an EDFA fiber ring laser pumped by a 980 nm laser diode and a narrow bandwidth fiber Bragg grating for the filter of the ring laser resonator. The wavelength shift is converted to the intensity deviation of the reflection from the sensing FBG under strain variation, and is able to achieve real-time sensing of the dynamic strain sensing in civil engineering. The characteristics and key factors to maintain stability of the dynamic strain sensing system are discussed.     

Influence of the UV-induced fiber loss on the distributed feedback fiber lasers

It was found that the output power of the distributed feedback fiber lasers would be improved after annealing or left unused for several days after the laser had been fabricated, and the output of the fundamental mode would not increase but be clamped while the ±1 order modes would be predominant with the enhancement of the coupling coefficient during the fabrication. The paper discussed the influence of UV-induced fiber loss on the fiber phase-shifted DFB lasers. Due to the gain saturation and fiber internal loss, which included the temperament loss and permanent loss, there was an optimum coupling coefficient for the DFB fiber lasers that the higher internal fiber loss corresponded to the lower optimum values of coupling coefficient.     

注入噪声对光纤激光器锁模启动的影响

 从掺镱光纤激光器腔内的激光场出发,讨论了掺镱光纤激光器腔内的噪声信号对锁模启动的影响,并针对我们设计的8字腔超短脉冲光纤激光器进行了研究。通过对激光器腔内锁模过程的分析发现：在泵浦功率为214.75 mW时,自行研制的8字形腔掺Yb³⁺光纤激光器在未注入噪声信号时,输出端在30～50 s内获得稳定的锁模脉冲激光输出;在腔内注入脉宽30 ns,频率10 kHz,平均功率0.2 mW的脉冲噪声后,输出端在1~2 s内获得同样稳定的锁模脉冲激光输出。实验结果表明,注入激光器谐振腔内一定的噪声信号,可加速锁模的启动。     

两个半导体激光器交叉注入的理论分析

 提出了一个复杂非线性动力学系统－交叉注入锁定半导体激光系统。针对其中最简单的两激光系统,导出了描述它的方程组,解析求解了它们具有相同自由振荡频率时的定态解。结果表明,在相同耦合条件下,锁定的频率范围比主-从方式扩大了大约一倍。从结果可以看出,即使是这种特殊系统,在锁定后频率也可能会改变。     

两个光纤激光器的互相注入锁定

光纤激光器的相干合成技术是实现高功率、高亮度光纤激光系统的重要技术途径.利用分束器分光使两个光纤激光器的能量互相注入，从而使两个光纤激光器达到锁相锁模实现相干输出. 实验研究表明：在独立运行的情况下，两个光纤激光器的远场光斑只是两个激光器光强的简单叠加；在互相注入锁定的情况下，两个光纤激光器的远场光斑是具有高对比度的干涉条纹，并得到两个激光器的输出模式总为异相模.这与理论分析的结果相一致. 关键词： 光纤激光器 相干合成 锁相     

Long-exposure far-field properties of the array of mutually-injected fiber lasers

In this paper, the long-exposure far-field properties of the array of mutually-injected fiber lasers are studied to understand the effect of the multiple phase-locked states on coherent beam combining of the array. It is found that the long-exposure far-field will degenerate into the superposition of the intensities of the elementary lasers when all the phase-locked states of the array are exported with the same probability. It means that the interference pattern will be greatly weakened in the long-exposure far-field because of the export of multiple phase-locked states of the array. This result implies that the multiple phase-locked states of the array go against coherent beam combining of the array, and only some of these phase-locked states should be exported. Then, the effects of the high-order phase-locked states on the far-field are discussed. The result suggests that only the in-phase state should be exported (i.e., all the other phase-locked states should be suppressed) in order to realize high-efficiency (> 80%) coherent beam combining when the number of the elementary lasers of the array is smaller than 19.     

掺Yb3+双包层光纤激光器的数值分析

 对掺Yb³⁺双包层光纤激光器不同参数情况下的输出功率和增益分布进行了数值模拟，分析了一端泵浦和双端泵浦方式下输出特性的差异，激光沿光纤长度方向的分布，输出功率与光纤长度、腔镜反射率及泵浦功率的关系。结果显示：两端泵浦较一端泵浦增益更加平坦，输出功率也稍高；当泵浦光波长为975nm时，输出激光功率对光纤长度更为敏感，最佳光纤长度相对于泵浦光波长为915nm时短且转化效率高；在大功率长光纤的情况下，光纤有损耗时输出功率随输出腔镜反射率的增加单调地减小，无损耗时输出功率不随输出腔镜反射率变化。     

固体激光器与光纤激光器对光子晶体光纤棒耦合的分析与对比

耦合效率的高低与耦合后光斑的好坏直接影响着光子晶体光纤棒的放大效果,因此需要对种子光的耦合效果进行研究,选择合适的激光器作为种子源。本文对光子晶体光纤棒在固体激光器与光纤激光器两种情况下的耦合效率进行了理论分析;模拟计算了两种激光器情况下耦合效率的变化规律以及对准误差对耦合效率的影响;选择合适的透镜或透镜组,使用两种激光器对光子晶体光纤棒进行了耦合实验;对比两种激光器的耦合效果可知:固体激光器的耦合效率最高只能达到62.4%,而光纤激光器的耦合效率可以达到80%以上;在光纤激光器耦合情况下,对不同功率注入时耦合效率,以及耦合后光斑进行了实验分析。得到的实验结果对后续光纤棒的放大实验具有指导作用。     

Stability performance of short cavity Er-doped fiber lasers

An experimental study of stability between two types of short-cavity laser configurations is carried out. The short-cavity lasers are comprised of one or two pairs of wavelength-matched FBGs, encompassing a short length of highly-doped Er-fiber, to create effectively single- or dual-wavelength fiber lasers. The laser performance in terms of the laser output power and laser stability in relation to the gain medium length was investigated experimentally, analyzed and reported in this letter.     

Photonic generation of microwave signals using dual-wavelength single-longitudinal-mode fiber lasers

Photonic generation of microwave signals is demonstrated with a dual-wavelength single-longitudinal-mode erbium-doped fiber ring laser. Microwave signal with a frequency of 21.07 GHz is obtained by aligning the reflection band of a uniform fiber Bragg grating to the two transmission peaks within a fiber Bragg grating-based Fabry-Perot filter. Stable dual-wavelength single-longitudinal-mode operation is guaranteed by means of the combined filtering effect of a saturable absorber and the Fabry-Perot filter. This approach provides an effective solution to the photonic generation of high-frequency microwave signals.     

Coherent beam combination of 137 W fiber amplifier array using single frequency dithering technique

We demonstrate coherent beam combination of 137 W fiber amplifier array using single frequency dithering technique. Four polarization-maintained fiber amplifiers are tiled into 2×2 array and the output power of each amplifier is about 35 W. Single frequency dithering algorithm is run on a signal processor based on field programmable gate array (FPGA) for phase control on the fiber amplifiers. When the phase control system goes into closed-loop, the fringe contrast of far-field intensity pattern is improved by more than 87% from 0 in open-loop, and the residual phase error is less than λ/20.     

Mode locking of an erbium-doped fiber laser with intra-cavity polarization modulation

Erbium-doped fiber lasers are normally actively mode-locked through amplitude modulation or phase modulation. In this paper, we demonstrate that the laser can also be mode-locked by employing polarization modulation with a polarization-dependent cavity loss. We obtain a nearly transform-limited mode-locked pulse train at 10 GHz repetition rate with timing jitter as low as 164 fs. The timing jitter is only limited by the timing jitter of the driving signal.     

Switchable multi-wavelength erbium-doped fiber lasers by using cascaded fiber Bragg gratings written in high birefringence fiber

A novel and simple Erbium-doped fiber laser by using cascaded fiber Bragg gratings (FBGs) written in high-birefringence fibers for switchable multi-wavelength operation is proposed. Due to two-peak reflection with orthogonal polarizations of the FBG, the polarization hole-burning effect in the cavity is greatly enhanced. Experimental results show that the stable dual- and three-wavelength lasing operation with very narrow wavelength separation (0.39 nm) can be generated at room temperature. The configuration is simple and flexible. Only by adjusting polarization controllers (PCs), the laser can be switched among the stable single-, dual- and three-wavelength lasing operations.