基于模式耦合器的锁模掺镱光纤激光器

尧 涵，石 帆，黄译平，王 腾，曾祥龙

基于模式耦合器的锁模掺镱光纤激光器

尧 涵，石 帆，黄译平，王 腾，曾祥龙*

特种光纤与光接入网重点实验室，特种光纤与先进通信国际合作联合实验室，上海先进通信与数据科学研究院，上海大学，上海 200444

本文提出了一种可以实现光纤高阶模式(HOM)在激光腔内振荡的锁模掺镱光纤激光器。通过使用一对级联的模式选择耦合器(MSC)作为有效的模式转换器，获得光纤锁模激光腔内HOM产生。其中，制备的MSC中心波长为1064 nm，可实现80 nm的模式转换带宽和94%的高阶模式纯度。通过搭建掺镱锁模光纤激光器，实验获得了3 dB谱宽7.4 nm、脉冲重复频率10.9 MHz、射频信噪比55 dB的锁模脉冲激光，输出功率的斜率效率为2.3%。实验证明，这种方法可在激光器内部通过模式级联转换，且能参与腔内锁模过程获得脉冲HOM激光。

模式选择耦合器；高阶模式；掺镱光纤；锁模光纤激光器

1 引 言

模分复用技术利用少模光纤(few model fiber，FMF)中传输的不同高阶模式(high-order mode，HOM)，突破了传统单模光纤(signal model fiber，SMF)通信系统的容量极限，使进一步探索光束的幅度、相位、波长和偏振等多维自由度成为可能[1-3]。此外，光纤中传输的HOM比基模具有更大的横向传播面积，为光纤激光器提供更多可用的色散管理和相位匹配过程中的能量。因此，HOM激光的产生为1.0 µm和1.55 µm超快光纤激光器提供了新的研究方向[4-7]。

光纤中传输的HOM包括LP11、LP21、LP02、LP31以及更高阶的模式。其中简并线偏振LP11模最受关注，它由四个传播常数相近的矢量模式组成：径向偏振(TM01)，角向偏振(TE01)和混合态偏振(HE21even和HE21odd)。这四个矢量模式的空间强度和偏振分布呈环形且轴对称，也被称作柱矢量光束(cylindrical vector beams，CVB)。特别地HE21odd和HE21even模式的组合又可形成涡旋光(orbital angular momentum，OAM)，其波前相位呈中心对称螺旋形分布，中心轴线处光场强度为零。CVB和OAM具有独特的空间强度和偏振分布，其在光镊[8]、遥感[9]、光纤通信[10-11]、高分辨率测量[12]、激光加工[13]等领域具有广泛应用前景。

锁模光纤激光器具有峰值功率高、脉冲宽度小、紧凑性高、成本低等特点。当前很多研究将模式转换器件与光纤激光器结合，获取短脉冲型HOM。香港城市大学的Dong等人将SMF和FMF错位以激发HOM，少模布拉格光栅将HOM输出，而将基模反射至激光器内继续传输[4]。然而，错位拼接法在模式转换的过程中易引入大量损耗，使激光器效率大大降低。Wang等人使用长周期光纤光栅(long period fiber grate，LPFG)将基模转换为HOM并从激光器输出[6]。而LPFG的模式转换波段覆盖范围小，且中心波长易受温度和微弯扰动的影响。产生HOM的方法还有自由空间法，即使用空间光调制器、Q板(Q-plate)、涡旋相位板等空间光器件[14-16]产生HOM。这些器件价格比较昂贵，不仅提高了实验成本，还打破了光纤激光器全光纤结构的特性和优势，因此不适用于光纤激光器中。之后超快高阶模光纤激光器相继被报道[17-19]，但都只是在激光腔外输出HOM，激光器内传输的仍然是基模。

本文使用一对全光纤模式选择耦合器(mode selective couplers，MSC)产生HOM，并将MSC接入掺镱光纤激光器(Yb-doped fiber laser，YDFL)，使HOM在激光腔内传输，并输出脉冲型CVB和一阶OAM。MSC为从LP01模到LP11模的模式转换器，有极宽的带宽、高的转换效率和高阶模式纯度。利用MSC的可逆性，可将LP11模转换成LP01模，并从SMF端口输出。该方法实现了1064 nm波段的HOM在激光腔内直接振荡的锁模掺镱光纤激光器。

2 MSC的结构和特性

根据耦合模理论[20-21]，当光纤MSC满足相位匹配条件时，SMF中传输的基模光束转换为特定的HOM并从FMF中输出。MSC由SMF (HI-1060)和FMF(core=5 μm，ring=15 μm，cladding=125 μm)构成，其结构如图1(a)的MSC1所示。采用熔融拉锥法，1400 ℃的氢氧焰将两根光纤熔融在一起，加热的部分形成耦合区域(图1(a))。功率计分别连接两根光纤的尾端，以监测其功率变化。当两根光纤的传输光束的功率比达到拉锥机预先设置的耦合比时，拉锥机停止熔融。使用U型管和热缩管封装，避免外界杂质进入耦合区而影响MSC性能。当基模从MSC1的单模端(Port1)输入，光束经过耦合区后转化为LP11模，并在FMF的输出端(Port3)中输出，未发生模式转换的能量从单模输出端(Port2)输出。根据MSC可逆性，将LP11模通入MSC的少模输出端，在单模输入端(Port4)输出基模，在少模输入端(Port5)获得未发生模式转化的LP11模。因此，连接两个MSC的少模输出端，可在FMF部分传输HOM。即基模从Port1输入，经过两个耦合区域后，在Port4输出LP01模式，Port5输出LP11模式。在FMF部分加一个偏振控制器(polarization controller，PC)，由PC挤压强度改变FMF中传输模式的偏振态[5,22]，用于提高MSC2的模式转换效率和HOM的模式纯度。

将MSC的单模输入端连接宽带光源，SMF和FMF输出端依次接入光谱分析仪，可测量MSC的传输谱(图1(b))，其中心波长在1064 nm左右，LP11模的最大功率强度接近0 dB，表明MSC的插入损耗接近0 dB，LP01模式的最小功率强度被衰减到-27 dB，高的功率消光比表明模式耦合效率达98%，且传输谱工作带宽达80 nm。MSC的宽光谱特性保证了在80 nm的波长范围内都有高的模式转换效率，由于脉冲型光纤激光器具有宽光谱范围和窄脉宽特性，因此MSC既作为脉冲激光器光源，也作为传输脉冲光束器件。一对级联MSC的传输谱如图1(c)，级联后的MSC仍有宽的工作带宽，其插入损耗约为3 dB，光谱条纹有轻微波动是由于LP11模的本征矢量模式之间竞争干扰引起[23]。

图1 (a) 两个MSC级联的结构图；(b) 单个MSC的传输谱；(c) 级联一对MSC的传输谱

3 实验装置

将两个MSC接入YDFL腔中实现模式转换和锁模，实验装置如图2所示。波长为980 nm的泵浦光源产生的980 nm激光通过980/1060 nm波分复用器(wavelength division multiplexer，WDM)进入YDFL腔内，长度为0.25 m的掺镱光纤(YDF：LIEKKI Yb1200-4/125)提供功率增益。10:90的单模光纤耦合器(fiber optic coupler，OC)用来提取10%的腔内能量，并从输出端output1输出。输出激光由功率计、光谱分析仪(OSA, YOKOGAWA AQ6370C)、示波器(OSC，Tektronix MSO4104)和射频仪(Siglent SSA 3032X)接收，以监测激光器的锁模状态。两个PC和一个偏振相关隔离器(PD-ISO)可实现基于非线性偏振旋转(NPR)效应的锁模。MSC用来产生HOM，调节PC3的压力和旋转角度，可改变HOM的偏振状态和纯度。CCD监测输出端output2的模场分布。中心波长为1064 nm的带通滤波器(BPF)提供稳定的锁模状态[24]。除了两个MSC的FMF部分外，其他器件通过单模光纤(HI-1060)连接，激光器总腔长为15.8 m。

图2 基于模式选择耦合器的锁模掺镱光纤激光器示意图。插图：激光器内传输的模式

4 实验结果

当泵浦功率大于250 mW时，激光器开始达到锁模状态。在泵浦功率为320 mW时，我们监测输出端output1以获取锁模激光的输出特性。OSA用于测量YDFL的光谱，如图3(a)所示，其3 dB带宽为7.4 nm，为矩形状光谱结构，该光谱是正色散腔中耗散孤子的典型光谱。带宽为1 GHz的OSC可表征YDFL的时域特性，其测得锁模脉冲序列的重复频率为10.9 MHz，如图3(b)所示。射频仪测量得到以10.9 MHz为中心频率的脉冲序列的信噪比为55 dB。射频谱中有两个旁瓣这是由于该锁模脉冲为类噪声脉冲，它的底座中存在多个随机分布的脉冲串，其对应于射频谱上的两个旁瓣。插图为频率范围在0~300 MHz的频谱图，强度均匀的频谱表明锁模达到稳定状态，如图3(c)所示。功率计测量了泵浦功率从250 mW增加到600 mW时，激光器的输出功率随泵浦功率变化的情况，随着泵浦功率的增大，输出的锁模激光功率也线性增大，斜率效率约为2.3%，如图3(d)所示。值得一提的是，脉冲的重复频率并不随泵浦功率的增大而变化，说明YDFL输出的是锁模脉冲激光。根据射频谱图，可推算出激光器脉宽可能为类噪声脉冲，两边有底座。根据脉宽推算公式[25]：

为了证明YDFL腔内的FMF部分传输的是HOM，我们使用CCD监测output2的模场分布，如图4(a)所示，模场截面为LP11模式，这表明LP11模式在激光器的FMF部分振荡并参与锁模过程。基于数值分析的模式分解和重构方法[26]，检测了该LP11模式的重构光场及其在总输出模式中的占比。计算得到LP11模的纯度大于94%，这表明我们制作的MSC能得到高纯度的LP11模。

在锁模状态下保持PC1和PC2的偏转角度不变，轻微调节PC3可消除线偏LP11模的简并度，在不同偏振态下能激发单个环状矢量光束：TM01，TE01，HE21even和HE21odd模，即CVB。起偏器用于区分这四种矢量光束，只有与起偏器方向一致的光束才能被通过。图4(b)显示的是实验所测的矢量光束，第一列是不加起偏器的环状分布，后四列是在CCD前放置一个起偏器，光束在不同的起偏方向下得到的模场分布。实验表明，TM01和TE01模、HE21even和HE21odd模相互正交。进一步调节PC3，以改变LP11模的相位，该方法可引入π/2相位差[5]，进而获得OAM光束，如图4(c)所示，光束的形状也为环型。用另一束同频的基模作为干涉光对该OAM光束进行矢量叠加，可获得干涉条纹，如图第三列所示。顺时针和逆时针方向的干涉条纹分别表示OAM光束的拓扑荷数为+1和-1。偏振控制器PC3调控光束的光场而获得CVB和一阶OAM，再次证明YDFL中FMF部分振荡的HOM为LP11模。

图4 Output2端口监测模式的模场分布图。(a) 测量和重构光场的线偏振LP11模式的模场分布，以及LP11模式的纯度；(b) 柱矢量光束TM01、TE01、HE21even和HE21odd的模场分布；(c) 线偏振LP11模、环状涡旋光及其干涉条纹的模场分布

5 总 结

本文通过实验提出了一种基于全光纤MSC的锁模掺镱光纤激光器直接振荡高阶模式的方法。光纤激光器内部接入两个MSC，将其少模输出端连接在一起形成一个模式转换器，使激光腔内产生并能稳定传输1064 nm波段的LP11模式，并根据NPR锁模机制输出脉冲型CVB和一阶OAM。结果表明，MSC有着80 nm的带宽，98%的模式转换效率和94%的高阶模式纯度。激光器输出的锁模激光有着7.4 nm的3 dB谱宽，10.9 MHz的重频，55 dB的信噪比和2.3%的斜率效率。该方法使全少模超快掺镱光纤激光器直接产生并全腔振荡高纯度、高模式转换效率的高阶模式成为可能。

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Mode-locked Yb-doped fiber laser based on mode coupler

Yao Han, Shi Fan, Huang Yiping, Wang Teng,Zeng Xianglong*

Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and Advanced Communication, Shanghai Institute for Advanced Communication and Data Science, Shanghai University, Shanghai 200444, China

Experimental setup of the mode-locked Yb-doped fiber laser based on mode coupler

Overview:High-order modes (HOMs), surpassing the capacity barrier in the traditional single-mode fiber (SMF) communication system, have attracted extensive attention and been widely applied in the fiber laser, optical communication, particle trapping, remote sensing technology, and so on. The HOMs include linear polarization LP11, LP21, LP02, LP31, and even higher-order modes, which can be generated by using free-space and fiber-based mode conversion devices. LP11mode is one of the most important HOMs, which has four vector eigenmodes. These eigenmodes are called as cylindrical vector beams (CVBs) with the axially symmetric polarization and circular intensity distribution. A polarization controller (PC) added on the fiber can effectively eliminate the degeneracy of LP11mode to excite individual vector modes in different polarization states. Additionally, the orbital angular momentum (OAM) characterized by helical wavefront can be generated by superimposing two orthogonal vector modes. Recently, ultrafast fiber lasers combined by HOMs have been reported owing to their outstanding characteristics, such as compactness, high peak power, narrow pulse width, and low cost. However, the HOMs were converted outside the laser, and the fundamental mode (LP01) was still transmitted in the laser cavity.

In this paper, a HOM directly oscillating in a mode-locked Yb-doped fiber laser (YDFL) is demonstrated. Two PCs and a polarization-dependent isolator are used to achieve the mode-locked mechanism of nonlinear polarization rotation. A pair of home-made mode selective couplers (MSCs) connecting through their few-mode fiber (FMF) ports, acts as an efficient mode convertor to generate and oscillate HOMs in the FMF section of the YDFL. A MSC is composed of a SMF and a FMF, which are fused by using hydrogen oxygen flame technology to keep two fiber cores close to each other. The claddings of two fibers are partly fused to form a coupling region. If the phase matching condition is satisfied, the LP01mode is transferred to the LP11mode in the coupling region. The MSC has a central wavelength of 1064 nm, a mode conversion bandwidth of 80 nm, and a HOM purity of 94%. Meanwhile, according to the reversibility of MSCs, the LP11mode can be lunched in the FMF port and output the LP01mode in the SMF port. The pulsed laser with a 3 dB spectral width of 7.4 nm, a pulse repetition frequency of 10.9 MHz, and a signal-to-noise ratio of radio frequency of 55 dB is obtained, and the slope efficiency of the pump and output power is 2.3%. The pulse LP11mode, CVB, and first-order OAM are obtained from the YDFL. These results demonstrate that the HOM can be generated by the MSC and be directly oscillated in the YDFL, and this approach is promising for directly generating pure and efficient HOMs in all-FMF ultrafast Yb-doped fiber lasers.

Citation: Yao H, Shi F, Huang Y P,. Mode-locked Yb-doped fiber laser based on mode coupler[J]., 2020,47(11): 200040

Mode-locked Yb-doped fiber laser based on mode coupler

Yao Han, Shi Fan, Huang Yiping, Wang Teng, Zeng Xianglong*

Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and Advanced Communication, Shanghai Institute for Advanced Communication and Data Science, Shanghai University, Shanghai 200444, China

We demonstrate a mode-locked Yb-doped fiber laser (YDFL) that enables fiber high-order mode (HOM) oscillation inside the ring cavity, by using a pair of mode selective couplers (MSCs) as an effective mode converter, the optical fiber HOM is obtained.The central wavelength of MSC is located at 1064 nm, which can achieve 80 nm mode conversion bandwidth and 94% high-order mode purity. A mode-locked pulsed fiber laser with a 3 dB spectral width of 7.4 nm, a pulse repetition frequency of 10.9 MHz, and a radio frequency signal-to-noise ratio of 55 dB is obtained, and the slope efficiency of the output power is 2.3%. These results show that the HOM can be directly oscillated by the cascaded MSCs in the fiber laser and participated in the mode-locking process to obtain a pulsed HOM laser.

mode-selective couplers; high-order mode; Yb-doped fibers; mode-locking fiber lasers

TN248

A

尧涵，曾祥龙，石帆，等. 基于模式耦合器的锁模掺镱光纤激光器[J]. 光电工程，2020，47(11): 200040

10.12086/oee.2020.200040

: Yao H, Shi F, Huang Y P,Mode-locked Yb-doped fiber laser based on mode coupler[J]., 2020, 47(11): 200040

2020-02-06；

2020-03-11

国家自然科学基金资助项目(91750108)；上海市科学技术委员会资助项目(20JC1415700, 16520720900)；上海市高等学校特聘教授(东方学者)项目；高等学校学科创新引智计划(111)资助(D20031)

尧涵(1995-)，女，硕士，主要从事特种光纤器件和激光器的研究。E-mail：yaohan_super@163.com

曾祥龙(1977-)，男，博士，教授，主要从事非线性超快光学、特种光纤及其传感技术的研究。E-mail：zenglong@shu.edu.cn

Supported by National Natural Science Foundation of China (91750108), Science and Technology Commission of Shanghai Municipality (16520720900), and Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning

* E-mail: zenglong@shu.edu.cn