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.     

Determine locking range of coherently combined fiber lasers joined by fiber couplers

When the wavelength difference of two fiber lasers, sharing a common reflection mirror with the aid of an optical coupler, is tuned to fall into a certain range, the array lasers can be locked to generate coherent radiation with a wavelength somewhere between the two constituent field wavelengths. Based on the fact that the laser should oscillate at the wavelength with the lowest losses and the band widths of the reflection curves of the laser tuning elements are finite, the locking range of the laser array has been quantified. For the tuning element (i.e., fiber gratings) selected in this work, the calculations show that the locking range is mainly dependent on the coupling coefficients of the gratings which ever is smaller.     

Analytical solutions to rate equations including losses describing threshold pumped fiber lasers

An exact analytical expression to the threshold pump power has been deduced for rate equations including loss coefficients applicable to fiber lasers doped with quasi two-energy-level ions. Following the derivation of a closed-form expression for the variation of the pump power along the fiber, together with analytical solutions to the position dependent atom population density and laser power in terms of the pump power, we have analytically and fully solved rate equations describing threshold pumped fiber lasers. A comparison has also been made between the exact analytical result with the previous reported results, which indicates that various approximation have underestimated the threshold pump powers.     

Threshold analysis of fiber-coupler-combined fiber lasers

Threshold conditions of coherently combined fiber lasers, joint by fiber couplers (FCs), have been studied. After considering the phase change of a light wave crossing the fiber inside the FC, it has been shown that, at the two input ports of the coupler connecting with the amplifying fibers, the phase difference between the two incoming fields should be equal to −π/2 if the threshold gain of the compound system is to be minimized. Starting from these input ports, following the circulations of waves, threshold conditions can be established. Expressions for the output and leakage powers, and optimum coupler splitting ratio nullifying the leakage when constituent fiber lasers giving different free-running powers have been derived. And, the threshold gain reductions of individual lasers after being coherently combined have been predicted.     

Coherent beam combination of an optical array using adaptive fiber optics collimators

A novel adaptive-fiber-optics-collimator (AFOC) compensating both piston-type and tip/tilt-type phase errors of output beam is introduced, and has been employed in experiments of coherent beam combination (CBC) of a delta distributed fiber array. Feedback control is realized using stochastic parallel gradient descent (SPGD) algorithm. Excellent CBC effect has been achieved when piston and tip/tilt errors among beamlets corrected. The necessity of wavefront tip/tilt control in CBC is verified. Experimental results exhibit great potential applications of this kind of AFOC in fiber amplifier arrays.     

准二能级阈值泵浦光纤激光器的理论分析

 根据含损耗系数的稳态速率方程组,对掺准二能级稀土粒子光纤激光器的泵浦阈值进行了分析和完全解析求解。求得了泵浦阈值功率的显函数解析式,得到了泵浦光阈值功率随光纤位置变化的隐函数表达式,以及信号光功率和上能级粒子数随光纤位置变化的解析表达式。对所得的解进行了数值分析,并与已有的理论和实验结果进行了比较分析。结果表明：由于忽略了信号光和泵浦光损耗系数,不同近似条件下给出的泵浦阈值功率均比实验值低,但所导出的理论研究结果与已有实验结果的差异最小。     

一种实现高功率输出的新型光纤激光器

介绍了一种实现光纤激光器高功率输出的新方法--采用螺旋芯光纤,这种光纤是在光纤制作过程中按照一定的弯曲半径使纤芯呈螺旋状排布在光纤包层中.介绍了这种螺旋芯光纤的原理和其提高输出功率的能力,以及目前的理论与实验研究现状.基于该光纤,提出了一种光纤激光器相干合成新思想.     

一种实现光纤激光器高功率输出的方法

 介绍了一种实现光纤激光器高功率输出的新方法——采用螺旋芯光纤，这种光纤是在光纤制作过程中按照一定的弯曲半径使纤芯呈螺旋状排布在光纤包层中。介绍了这种螺旋芯光纤的原理和其提高输出功率的能力，以及目前的理论与实验研究现状。基于该光纤，提出了一种光纤激光器相干合成新思想。     

高功率光纤激光的时间部分相干性对相干合成的影响

 将多个光纤激光器的输出光束进行相干合成是获得高功率、高光束质量激光的有效途径。利用准部分相干光(PPCB)模型,计算了高功率光纤激光器阵列发出的部分相干光在远场的能量分布,分析了激光的时间部分相干性对相干合成的影响。计算结果表明,随着单根激光器输出光束线宽的增大,远场光斑的图样基本保持不变,但峰值功率和Strehl比随之减小。对于采用的算例,要保证远场光斑的Strehl比大于0.8,单根光纤激光器的线宽不能超过5 nm。     

基于功率合束器的多路光纤激光自组织相干合成

提出了基于功率合束器的多路光纤激光自组织被动相干合成结构,研究了多模状态下泵浦功率、合成路数对激光合成效率和光束质量的影响,并对2,3,4路激光合成进行了实验研究,合成效率大于90%。实验结果表明：激光中心主瓣所占能量比随着泵浦功率的增加而增大,随激光合成路数增多而增大;在相同的输出功率下,单路激光的主瓣所占能量比比合成激光主瓣所占能量比要高;降低输出光栅反射率,激光输出能量增强,但合成效率有所降低;增加功率和增加路数可以增加中心亮斑的能量比,但增加幅度变低。     

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.     

Coherent combining of two fiber lasers in a Michelson-type coupled cavity

Coherent combining of two fiber lasers using a Michelson-type coupled cavity has been demonstrated experimentally. To make the fiber lasers combined efficiently, two convergent lenses are introduced into the two arms of the coupled cavity, and an f-f arrangement is specially configured for the cavity to improve internal feedback. When the lenses are taken away, the output power decreases to nearly 78.5% of its original value, and the coherent state become less stable than before. Through tuning the FBG (fiber Bragg grating) of one fiber laser, the converting process from incoherent state to coherent state is investigated, and the spectrums, output power and the beam patterns all indicate that the brightness of coherent state increases to nearly twice of incoherent state’s, i.e. four times of component laser’s.     

三能级光纤激光器速率方程组的阈值解析解

 在考虑了泵浦损耗及激光损耗的情况下,解析求解了阈值泵浦下的三能级光纤激光器速率方程组,获得了泵浦功率阈值的显函数解析表达式,泵浦功率沿光纤变化的隐函数表达式,以及用功率变化表示的各个能级的粒子数密度随位置变化的显函数表达式。并利用这些表达式对光纤激光器的阈值特性进行了研究,研究表明：泵浦功率阈值随泵浦损耗系数增大而增大;激光上能级粒子数密度随归一化位置呈下降趋势,而下能级粒子数密度的变化与此相反;泵浦功率阈值越大(光纤长度越长),净增益系数随归一位置下降就越快。所获得的结果适用于单包层和双包层光纤激光器。     

两路光纤激光器自组织相干合成实验研究

 建立了两路光纤激光器自组织相干合成的理论模型,并进行实验验证。结果表明,通过能量的相互注入,可以实现光纤激光器的自组织相干合成,合成效率达到92%。当外界基本没有干扰时,远场光斑为稳定、高对比度的干涉条纹,激光器阵列输出模式为异相模,与理论分析的结果一致;当外界环境存在轻微干扰时,实验出现条纹移动现象,输出模式也不再是恒定的异相模。对外界干扰对自组织相干合成的影响进行了数值分析,结果表明：外界干扰的幅度与频率越大,两束激光的相位差的变化幅度与频率也就越大;当外界干扰超过一定程度,两束激光之间的相位差将不能被锁定,始终处于变化状态。     

LaB6在低压强氮气和氦气中的放电特性

研究了LaB6在1~10 Pa氮气和氦气中的直流和脉冲放电特性以及放电过程对电极的影响。结果表明，电极直径为5 mm的LaB6氦气放电管在脉冲工作状态下可以长期稳定放电。在脉冲电压为2.2 kV、脉冲宽度10 ms、频率13.3 Hz下，脉冲峰值放电电流超过120 A。氦气放电管在放电过程中，阴极表面有离子的清洗和活化作用，可以使电极的表面逸出功降低，提高放电管的发射能力和稳定性。LaB6作为气体放电电极具有寿命长、延迟时间短、放电电流大等优点，可用于重复强流脉冲气体放电的高压高速开关器件。     

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.     

High sensitivity conductivity-temperature-depth sensing based on an optical microfiber coupler combined fiber loop

In order to meet the practical needs of all-fiber conductivity-temperature-depth sensors with high sensitivity,compact structure,and easy packaging,this Letter uses a microfiber coupler combined with fiber loop(MCFL)reflective photonic device to conduct salinity,temperature,and deep sensing experiments.These MCFLs’dynamic range and resolution of salinity,temperature,and depth can meet the requirements of actual marine environment monitoring.This structure opens up a new design idea for the practical research of microfiber coupler-based marine environmental parameter sensors.     

Ytterbium-doped Y2O3 nanoparticle silica optical fibers for high power fiber lasers with suppressed photodarkening

We report efficient laser demonstration and spectroscopic characteristics of a Yb-doped Y₂O₃ (or Y₃Al₅O₁₂) nanoparticle silica fiber developed by conventional fiber fabrication technique. The spectroscopy study evidences modification in the environment of Yb ions by the Y₂O₃ nanoparticles. As a result, photodarkening induced loss is reduced by 20 times relative to Yb-doped aluminosilicate fibers. The fiber is suitable for power scaling with good laser slope efficiency of 79%.     

Switchable dual-wavelength polarization-maintaining erbium-doped fiber laser using an optical circulator as the all-reflecting mirror

A simple and effective switchable dual-wavelength polarization-maintaining erbium-doped fiber laser is proposed and demonstrated. Only using a uniform fiber Bragg grating written directly in a segment of the photosensitive and polarization-maintaining erbium-doped fiber as the wavelength selector and an optical circulator as the all-reflecting mirror, a stable simultaneous dual-wavelength oscillation is achieved with a wavelength spacing of 0.364 nm experimentally by exploiting the polarization hole burning effect at room temperature. The output can be switched between single- and dual-wavelength by adjusting the polarization controller. The 3-dB bandwidth and the side mode suppression ratio of the laser’s outputs are measured to be less than 0.01 nm and more than 50 dB. The power fluctuation and wavelength drift are measured to be less than 0.50 dB and 0.020 nm over an hour.     

Numerical analysis of Ytterbium-doped double-clad fiber lasers based on the temperature-dependent rate equation

The effects of temperature on the output power and optimal conditions of Ytterbium-doped double-cladding fiber lasers (YDDFLs) are discussed. Temperature-dependent rate equations for YDDFLs based on Boltzmann distribution are built in two-end pump model. The results show that the output power and the slope efficiency decrease with the increase of temperature. The increase of pump power results in that the effect of the temperature on the output performance increases, and the fiber length is a factor that the effect of the temperature depends on really. Moreover we combine the heat distribution formula with the rate equations and compare the numerical results considering with that not considering the temperature-dependence inside laser cavity, the effect of temperature can be ignored only when the pump power is lower, and the optimal fiber length becomes longer when considering the temperature-dependence.