Diffractive-optics-based beam combination of a phase-locked fiber laser array

A diffractive optical element (DOE) is used as a beam combiner for an actively phase-locked array of fiber lasers. Use of a DOE eliminates the far-field sidelobes and the accompanying loss of beam quality typically observed in tiled coherent laser arrays. Using this technique, we demonstrated coherent combination of five fiber lasers with 91% efficiency and M2=1.04. Combination efficiency and phase locking is robust even with large amplitude and phase fluctuations on the input laser array elements. Calculations and power handling measurements suggest that this approach can scale to both high channel counts and high powers.     

Application of fiber interferometer in coherent Doppler lidar

A novel coherent Doppler lidar (CDL) system based on single-mode fiber (SMF) components and instruments is presented to measure the speed of target. A fiber interferometer used in CDL system is reported.This fiber mixer is employed as a coherent receiver to resolve the shifts of signal-to-noise ratio (SNR) and mixing efficiency induced by backscattered field's wavefront error. For a certain wavelength, the maximum coupling efficiency between signal and SMF is determined by the ratio of pupil diameter to focal length of the coupling lens. The legible interference patterns and spectrum signals show that fiber interferometer is suitable to compensate for amplitude and phase vibrations. This robust coherent receiver can achieve improved CDL system performance with less transmitter power.     

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     

基于公共环形腔耦合的光纤激光器相干合成技术

 为提高光纤激光器无源自调整相干合成阵列的效率、稳定性和可扩展性,提出了基于公共光纤环形腔耦合与单模光纤滤波的光纤激光器相干合成方案。将多个2×2的光纤耦合器分别插入各单元激光器的谐振腔,利用耦合器余下的端口,两两相连构成公共环形耦合腔。采用单模光纤滤波技术,提高了各输出激光束之间相位锁定的稳定性。利用该方案在实验上实现了三路光纤激光器的被动锁相输出,实验测得的远场干涉光斑、输出功率及光谱均表明该方案适于构建性能较好的光纤激光器相干合成阵列。     

Experimental study on phase-locking of two high-power all-fiber lasers

By using a novel mutual injection technique, phase-locking and coherent combining of two high-power all-fiber lasers are realized and experimentally demonstrated. Steady interference strips with high visibility of 46% are observed. The coherent combined 407 W CW output power with a power-combining efficiency of up to 98% is obtained. The laser array works well with excellent stability. In the long time of high-power operation no thermal distortions or damages are observed. The proposed technique can be used to further scale up the coherent combined output power of high-power fiber lasers.     

两光子晶体光纤激光器相干锁定的实验研究

利用自成像腔技术进行光子晶体光纤(PCF)激光器光束相干合成的实验研究,实现了两光子晶体光纤激光器的相位锁定.在不使用滤波器的情况下,实验仍能观测到清晰的下涉图样,且在高功率输出状态以及环境噪声情况下干涉图样仍可保持稳定,表明具有单模大模场特性的光子晶体光纤在实现光束相干合成方面比传统的双包层光纤(DCF)有明显的优越性.实验还表明耦合输出镜的反射率对相干输出功率有一定影响,当反射镜的反射率分别为5%,10%和15%时,两台激光器相干输出斜率效率分别为63.8%,61.6%和60.2%.在抽运功率为150 W和耦合输出镜的反射率为5%时,获得95.8 W的最大相干功率输出,相干功率合成效率为90.2%.实验中无任何热光效应产生,有望利用该方法获得更高的相干输出功率.     

基于迈克尔逊型耦合腔光纤激光器相干合成的理论与实验研究

为使光纤激光器在自由空间中进行有效的相干合成,提出了耦合臂上带有会聚透镜的迈克尔逊型耦合腔,并将外腔配置为f－f结构以增强内部反馈.利用高斯光束的ABCD定律分析了外腔的传输特性,详细研究了外腔偏离f－f结构对相互注入耦合效率的影响,指出输出光纤端面与透镜前焦平面的偏差是影响反馈耦合效率的关键因素.基于此特别设计的迈克尔逊型耦合腔,实验上实现了两路光纤激光的有效相干合成.当透镜被移走时,阵列输出功率下降为原始值的78.5％,且相干态变得不稳定.     

利用全光纤结构Michelson腔实现两路高功率双包层光纤激光器相干合成

Michelson腔技术是实现激光相干合成的有效方案. 利用Michelson腔技术, 实现了全光纤结构下的两路光纤激光器的相干合成, 获得了功率为11.75 W, 效率为95.76%的相干输出. 实验研究了相干合成效率与激光腔长差的关系以及抽运对称性对相干合成效率的影响. 实验表明相干合成效率极易受激光腔长差的影响, 不同腔长差下相干合成效率差异可达18%以上, 且存在最佳腔长差; 抽运对称性对相干合成效率的影响在2%以内.     

Experimental investigation of a seven-element hexagonal fiber coherent array

The phase noises of a 10-m polarization-maintaining （PM） fiber and a 10-W PM fiber amplifier are experimentally measured. The results indicate that the 10-m PM fiber with similar phase noise could be used to investigate the architecture of master oscillator power amplifier coherent combination. A seven-element hexagonal fiber coherent array is developed to investigate the far-field distribution and phase controlling technique of a coherently combining fiber laser array. A hexagonal prism is designed as the combining and splitting component to achieve a fill factor of 0.66. The hill climbing method is employed to detect and lock the element phase.     

两根大芯双包层光纤激光器获得60 W相干输出

报道了一个相位锁定的光纤激光器阵列获得60W的输出,同相模式输出时的斜率效率为37%。对于两种不同的纤芯距离都观测到稳定的高对比度干涉条纹。整个系统在高功率操作时,所用的空间滤波器没有观察到明显的热问题,这充分说明了采用这种方法能够进一步提高相干输出功率。     

Simulataneous coherent and spectral addition of fiber lasers

A new approach for the simultaneous coherent and spectral addition of a two-dimensional array of fiber lasers is presented. A combining efficiency of over 80% and a combined beam profile with M(2)=1.15 were experimentally obtained with an array of four fiber lasers. This approach can lead to significant upscaling in fiber laser additions.     

量子点单光子源的光纤耦合

半导体量子点在低温下产生谱线细锐的激子发光可制备单光子源.光纤耦合可避免低温共聚焦装置扫描定位和振动影响,是实现单光子源即插即用和组件化的关键技术.在耦合工艺上,基于微区定位标记发展出拉锥光纤与光子晶体腔或波导侧向耦合、大数值孔径锥形端面光纤与量子点样片垂直耦合等技术;然而,上述工艺需要多维度精密调节以避免柔软光纤的畸形弯曲实现对准和高效耦合.陶瓷插针或石英V槽封装的光纤无弯曲且具有大平滑端面,只要与单量子点样片对准贴合就可保证垂直收光, V槽封装的排式光纤还可通过盲对粘合避免扫描对准,耦合简单.本文在前期排式光纤粘合少对数分布Bragg反射镜(distributed Bragg reflector, DBR)微柱样片实现单光子输出基础上,经理论模拟采用多对数DBR腔提升样片垂直出光和光纤收光效率,使光纤输出单光子计数率大大提升.     

类视网膜阵列结构的光纤相控阵方法研究

出射光束阵列布局对光纤相控阵的高功率激光相干合成性能有着重要影响。针对现有光纤相控方法的阵元间距难以突破半个波长,导致能量合成效率低的问题,提出了类视网膜多环形光纤相控方法,设计了具有圆形对称分布的结构,在保持中心阵元以及最外环阵元位置不变的情况下,通过粒子群算法优化环间其余阵元间距以获得最佳的远场相干合成性能。仿真结果表明,与传统光纤相控方法相比,提出的阵列布局可将能量集中度从0.562提升至0.921,峰值旁瓣水平从0.212压缩至0.043。     

Generation of a high-quality partially coherent dark hollow beam with a multimode fiber

We report the experimental generation of a high-quality partially coherent dark hollow beam (DHB) by coupling a partially coherent beam into a multimode fiber (MMF) with a suitable incidence angle. The interference experiment of the generated partially coherent DHB passing through double slits is demonstrated. It is found that the coupling efficiency of the MMF, the quality, and the coherence of the generated partially coherent DHB are closely controlled by the coherence of the input beam.     

1018 nm短波长掺镱光纤激光高功率自组织相干合成

搭建了两台高功率、低量子损耗的1018 nm短波长掺镱光纤激光器,进行了全光纤结构下两路光纤激光器的相干合成实验。获得了功率为55 W、合成效率为90.2%的相干输出,这是当前严格单模1018 nm光纤激光器的最高功率水平。同时,验证了Michelson腔自组织相干合成技术能够实现光纤激光器的高功率单模输出。     

Optimal Truncation of Element Beam in a Coherent Fiber Laser Array

The beam quality of a coherent fiber laser array often suffers from the low fill factor of the Gaussian laser array. One simple and effective approach to improve the fill factor is to truncate the array element properly. An analytical expression for far-field intensity distribution of a truncated coherent fiber laser array is derived. Optimal truncation of the element beam in different coherent fiber laser arrays is obtained by using energy encircled in the far-field central-lobe as the beam quality criterion. By optimal truncation, energy encircled in the central-lobe can be 97% compared with the ideal case. The shift in optimal truncation parameter in the case of phase noise is also analyzed.     

二维固体激光阵列逆达曼光栅相干合束技术模拟研究

采用逆达曼光栅将二维锁相相干的激光阵列进行相干合束并进行孔径装填是获得远场单一主瓣大功率高光束质量激光输出的一种有效技术方案,将其应用于大尺寸5×5固体激光相干阵列相干合束中,并进行了固体激光阵列合束孔径装填的理论分析和原理性验证实验,测量了系统的实际合束效率,同时进行了后焦面逆达曼光栅的加工和放置误差对合束效率影响的详细分析。实验结果表明,逆达曼光栅用于固体激光阵列相干合束是一种有效的技术方案,且可以通过调节光栅周期和傅里叶透镜焦距来适应系统对激光阵列占空比的要求。这对于开发基于逆达曼光栅相干合束的高功率高光质量的全固态激光系统具有重要的意义。     

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.     

Batch Assembly Micro-Ball Lens Array for Si-Based Optical Coupling Platform in Free Space

The paper presents novel concepts of batch fabrication of micro-ball lens array technology integrated on the silicon-based wafer which can improve the appropriate distance for optical fiber coupling. The silicon-based optical coupling platform consists of a self-parking framework (including flat-topped mesa and intersection of two v-grooves) and micro-ball lens array for optical fiber coupling purposes. The structure of optical coupling platform is able to improve the distance between fiber and micro-ball lens and to increase coupling efficiency. Optical measurement reveals an insertion loss below 2.5 dB with wavelength 635 nm in singlemode fiber. The micro-ball lens array is batch fabricated by polymeric material and thick photoresist, and then bonded onto a flat-topped mesa that adjoins to the v-groove. The corner compensation offers a method to fabricate the flat-topped mesa in the intersection of two v-grooves. This fabrication process not only provides accurate coupling distance between fibers and micro-ball lens, but also reduces micro-assembly cost.This paper was originally presented at the 2001 International Conference (2^nd Joint OSJ-SPIE Conference) on Optical Engineering for Sensing and Nanotechnology, ICOSN 2001 which was held June 6-8, 2001 at the Pacifico-Yokohama Conference Center, Yokohama, Japan.     

Tolerance on tilt error for coherent combining of fiber lasers

Limited by the precision of optical machining and assembling, the optical axes of lasers in an array cannot be strictly parallel to each other, which will result in the beam quality degradation of the combined beam. The tolerance on tilt error for coherent combining of fiber lasers is studied in detail. The complex amplitude distribution in the far field for the Gaussian beam with tilt angle is obtained by a novel coordinate transform method. Effect of tilt error on coherent combining is modelled analytically. Beam propagation factor is used to evaluate the effect of coherent combining. Numerical results show that for ring-distributed fiber laser array with central wavelength A and geometry size D, if the root-mean-square （RMS） value of the tilt error is smaller than 0.72A/D, the energy encircled in the diffraction-limited bucket can be ensured to be more than 50% of the value when there is no tilt error. The results are helpful to the designing and manufacturing of fiber array for coherent combining.