基于模式耦合器的鎖模摻鐿光纖激光器

堯 涵，石 帆，黃譯平，王 騰，曾祥龍

基于模式耦合器的鎖模摻鐿光纖激光器

堯 涵，石 帆，黃譯平，王 騰，曾祥龍*

特種光纖與光接入網(wǎng)重點實驗室，特種光纖與先進通信國際合作聯(lián)合實驗室，上海先進通信與數(shù)據(jù)科學研究院，上海大學，上海 200444

本文提出了一種可以實現(xiàn)光纖高階模式(HOM)在激光腔內(nèi)振蕩的鎖模摻鐿光纖激光器。通過使用一對級聯(lián)的模式選擇耦合器(MSC)作為有效的模式轉(zhuǎn)換器，獲得光纖鎖模激光腔內(nèi)HOM產(chǎn)生。其中，制備的MSC中心波長為1064 nm，可實現(xiàn)80 nm的模式轉(zhuǎn)換帶寬和94%的高階模式純度。通過搭建摻鐿鎖模光纖激光器，實驗獲得了3 dB譜寬7.4 nm、脈沖重復頻率10.9 MHz、射頻信噪比55 dB的鎖模脈沖激光，輸出功率的斜率效率為2.3%。實驗證明，這種方法可在激光器內(nèi)部通過模式級聯(lián)轉(zhuǎn)換，且能參與腔內(nèi)鎖模過程獲得脈沖HOM激光。

模式選擇耦合器；高階模式；摻鐿光纖；鎖模光纖激光器

1 引 言

模分復用技術(shù)利用少模光纖(few model fiber，F(xiàn)MF)中傳輸?shù)牟煌唠A模式(high-order mode，HOM)，突破了傳統(tǒng)單模光纖(signal model fiber，SMF)通信系統(tǒng)的容量極限，使進一步探索光束的幅度、相位、波長和偏振等多維自由度成為可能[1-3]。此外，光纖中傳輸?shù)腍OM比基模具有更大的橫向傳播面積，為光纖激光器提供更多可用的色散管理和相位匹配過程中的能量。因此，HOM激光的產(chǎn)生為1.0 μm和1.55 μm超快光纖激光器提供了新的研究方向[4-7]。

光纖中傳輸?shù)腍OM包括LP11、LP21、LP02、LP31以及更高階的模式。其中簡并線偏振LP11模最受關(guān)注，它由四個傳播常數(shù)相近的矢量模式組成：徑向偏振(TM01)，角向偏振(TE01)和混合態(tài)偏振(HE21even和HE21odd)。這四個矢量模式的空間強度和偏振分布呈環(huán)形且軸對稱，也被稱作柱矢量光束(cylindrical vector beams，CVB)。特別地HE21odd和HE21even模式的組合又可形成渦旋光(orbital angular momentum，OAM)，其波前相位呈中心對稱螺旋形分布，中心軸線處光場強度為零。CVB和OAM具有獨特的空間強度和偏振分布，其在光鑷[8]、遙感[9]、光纖通信[10-11]、高分辨率測量[12]、激光加工[13]等領(lǐng)域具有廣泛應用前景。

鎖模光纖激光器具有峰值功率高、脈沖寬度小、緊湊性高、成本低等特點。當前很多研究將模式轉(zhuǎn)換器件與光纖激光器結(jié)合，獲取短脈沖型HOM。香港城市大學的Dong等人將SMF和FMF錯位以激發(fā)HOM，少模布拉格光柵將HOM輸出，而將基模反射至激光器內(nèi)繼續(xù)傳輸[4]。然而，錯位拼接法在模式轉(zhuǎn)換的過程中易引入大量損耗，使激光器效率大大降低。Wang等人使用長周期光纖光柵(long period fiber grate，LPFG)將基模轉(zhuǎn)換為HOM并從激光器輸出[6]。而LPFG的模式轉(zhuǎn)換波段覆蓋范圍小，且中心波長易受溫度和微彎擾動的影響。產(chǎn)生HOM的方法還有自由空間法，即使用空間光調(diào)制器、Q板(Q-plate)、渦旋相位板等空間光器件[14-16]產(chǎn)生HOM。這些器件價格比較昂貴，不僅提高了實驗成本，還打破了光纖激光器全光纖結(jié)構(gòu)的特性和優(yōu)勢，因此不適用于光纖激光器中。之后超快高階模光纖激光器相繼被報道[17-19]，但都只是在激光腔外輸出HOM，激光器內(nèi)傳輸?shù)娜匀皇腔！?/p>

本文使用一對全光纖模式選擇耦合器(mode selective couplers，MSC)產(chǎn)生HOM，并將MSC接入摻鐿光纖激光器(Yb-doped fiber laser，YDFL)，使HOM在激光腔內(nèi)傳輸，并輸出脈沖型CVB和一階OAM。MSC為從LP01模到LP11模的模式轉(zhuǎn)換器，有極寬的帶寬、高的轉(zhuǎn)換效率和高階模式純度。利用MSC的可逆性，可將LP11模轉(zhuǎn)換成LP01模，并從SMF端口輸出。該方法實現(xiàn)了1064 nm波段的HOM在激光腔內(nèi)直接振蕩的鎖模摻鐿光纖激光器。

2 MSC的結(jié)構(gòu)和特性

根據(jù)耦合模理論[20-21]，當光纖MSC滿足相位匹配條件時，SMF中傳輸?shù)幕９馐D(zhuǎn)換為特定的HOM并從FMF中輸出。MSC由SMF (HI-1060)和FMF(core=5 μm，ring=15 μm，cladding=125 μm)構(gòu)成，其結(jié)構(gòu)如圖1(a)的MSC1所示。采用熔融拉錐法，1400 ℃的氫氧焰將兩根光纖熔融在一起，加熱的部分形成耦合區(qū)域(圖1(a))。功率計分別連接兩根光纖的尾端，以監(jiān)測其功率變化。當兩根光纖的傳輸光束的功率比達到拉錐機預先設(shè)置的耦合比時，拉錐機停止熔融。使用U型管和熱縮管封裝，避免外界雜質(zhì)進入耦合區(qū)而影響MSC性能。當基模從MSC1的單模端(Port1)輸入，光束經(jīng)過耦合區(qū)后轉(zhuǎn)化為LP11模，并在FMF的輸出端(Port3)中輸出，未發(fā)生模式轉(zhuǎn)換的能量從單模輸出端(Port2)輸出。根據(jù)MSC可逆性，將LP11模通入MSC的少模輸出端，在單模輸入端(Port4)輸出基模，在少模輸入端(Port5)獲得未發(fā)生模式轉(zhuǎn)化的LP11模。因此，連接兩個MSC的少模輸出端，可在FMF部分傳輸HOM。即基模從Port1輸入，經(jīng)過兩個耦合區(qū)域后，在Port4輸出LP01模式，Port5輸出LP11模式。在FMF部分加一個偏振控制器(polarization controller，PC)，由PC擠壓強度改變FMF中傳輸模式的偏振態(tài)[5,22]，用于提高MSC2的模式轉(zhuǎn)換效率和HOM的模式純度。

將MSC的單模輸入端連接寬帶光源，SMF和FMF輸出端依次接入光譜分析儀，可測量MSC的傳輸譜(圖1(b))，其中心波長在1064 nm左右，LP11模的最大功率強度接近0 dB，表明MSC的插入損耗接近0 dB，LP01模式的最小功率強度被衰減到-27 dB，高的功率消光比表明模式耦合效率達98%，且傳輸譜工作帶寬達80 nm。MSC的寬光譜特性保證了在80 nm的波長范圍內(nèi)都有高的模式轉(zhuǎn)換效率，由于脈沖型光纖激光器具有寬光譜范圍和窄脈寬特性，因此MSC既作為脈沖激光器光源，也作為傳輸脈沖光束器件。一對級聯(lián)MSC的傳輸譜如圖1(c)，級聯(lián)后的MSC仍有寬的工作帶寬，其插入損耗約為3 dB，光譜條紋有輕微波動是由于LP11模的本征矢量模式之間競爭干擾引起[23]。

圖1 (a) 兩個MSC級聯(lián)的結(jié)構(gòu)圖；(b) 單個MSC的傳輸譜；(c) 級聯(lián)一對MSC的傳輸譜

3 實驗裝置

將兩個MSC接入YDFL腔中實現(xiàn)模式轉(zhuǎn)換和鎖模，實驗裝置如圖2所示。波長為980 nm的泵浦光源產(chǎn)生的980 nm激光通過980/1060 nm波分復用器(wavelength division multiplexer，WDM)進入YDFL腔內(nèi)，長度為0.25 m的摻鐿光纖(YDF：LIEKKI Yb1200-4/125)提供功率增益。10:90的單模光纖耦合器(fiber optic coupler，OC)用來提取10%的腔內(nèi)能量，并從輸出端output1輸出。輸出激光由功率計、光譜分析儀(OSA, YOKOGAWA AQ6370C)、示波器(OSC，Tektronix MSO4104)和射頻儀(Siglent SSA 3032X)接收，以監(jiān)測激光器的鎖模狀態(tài)。兩個PC和一個偏振相關(guān)隔離器(PD-ISO)可實現(xiàn)基于非線性偏振旋轉(zhuǎn)(NPR)效應的鎖模。MSC用來產(chǎn)生HOM，調(diào)節(jié)PC3的壓力和旋轉(zhuǎn)角度，可改變HOM的偏振狀態(tài)和純度。CCD監(jiān)測輸出端output2的模場分布。中心波長為1064 nm的帶通濾波器(BPF)提供穩(wěn)定的鎖模狀態(tài)[24]。除了兩個MSC的FMF部分外，其他器件通過單模光纖(HI-1060)連接，激光器總腔長為15.8 m。

圖2 基于模式選擇耦合器的鎖模摻鐿光纖激光器示意圖。插圖：激光器內(nèi)傳輸?shù)哪Ｊ?/p>

4 實驗結(jié)果

當泵浦功率大于250 mW時，激光器開始達到鎖模狀態(tài)。在泵浦功率為320 mW時，我們監(jiān)測輸出端output1以獲取鎖模激光的輸出特性。OSA用于測量YDFL的光譜，如圖3(a)所示，其3 dB帶寬為7.4 nm，為矩形狀光譜結(jié)構(gòu)，該光譜是正色散腔中耗散孤子的典型光譜。帶寬為1 GHz的OSC可表征YDFL的時域特性，其測得鎖模脈沖序列的重復頻率為10.9 MHz，如圖3(b)所示。射頻儀測量得到以10.9 MHz為中心頻率的脈沖序列的信噪比為55 dB。射頻譜中有兩個旁瓣這是由于該鎖模脈沖為類噪聲脈沖，它的底座中存在多個隨機分布的脈沖串，其對應于射頻譜上的兩個旁瓣。插圖為頻率范圍在0~300 MHz的頻譜圖，強度均勻的頻譜表明鎖模達到穩(wěn)定狀態(tài)，如圖3(c)所示。功率計測量了泵浦功率從250 mW增加到600 mW時，激光器的輸出功率隨泵浦功率變化的情況，隨著泵浦功率的增大，輸出的鎖模激光功率也線性增大，斜率效率約為2.3%，如圖3(d)所示。值得一提的是，脈沖的重復頻率并不隨泵浦功率的增大而變化，說明YDFL輸出的是鎖模脈沖激光。根據(jù)射頻譜圖，可推算出激光器脈寬可能為類噪聲脈沖，兩邊有底座。根據(jù)脈寬推算公式[25]：

為了證明YDFL腔內(nèi)的FMF部分傳輸?shù)氖荋OM，我們使用CCD監(jiān)測output2的模場分布，如圖4(a)所示，模場截面為LP11模式，這表明LP11模式在激光器的FMF部分振蕩并參與鎖模過程?；跀?shù)值分析的模式分解和重構(gòu)方法[26]，檢測了該LP11模式的重構(gòu)光場及其在總輸出模式中的占比。計算得到LP11模的純度大于94%，這表明我們制作的MSC能得到高純度的LP11模。

在鎖模狀態(tài)下保持PC1和PC2的偏轉(zhuǎn)角度不變，輕微調(diào)節(jié)PC3可消除線偏LP11模的簡并度，在不同偏振態(tài)下能激發(fā)單個環(huán)狀矢量光束：TM01，TE01，HE21even和HE21odd模，即CVB。起偏器用于區(qū)分這四種矢量光束，只有與起偏器方向一致的光束才能被通過。圖4(b)顯示的是實驗所測的矢量光束，第一列是不加起偏器的環(huán)狀分布，后四列是在CCD前放置一個起偏器，光束在不同的起偏方向下得到的模場分布。實驗表明，TM01和TE01模、HE21even和HE21odd模相互正交。進一步調(diào)節(jié)PC3，以改變LP11模的相位，該方法可引入π/2相位差[5]，進而獲得OAM光束，如圖4(c)所示，光束的形狀也為環(huán)型。用另一束同頻的基模作為干涉光對該OAM光束進行矢量疊加，可獲得干涉條紋，如圖第三列所示。順時針和逆時針方向的干涉條紋分別表示OAM光束的拓撲荷數(shù)為+1和-1。偏振控制器PC3調(diào)控光束的光場而獲得CVB和一階OAM，再次證明YDFL中FMF部分振蕩的HOM為LP11模。

圖4 Output2端口監(jiān)測模式的模場分布圖。(a) 測量和重構(gòu)光場的線偏振LP11模式的模場分布，以及LP11模式的純度；(b) 柱矢量光束TM01、TE01、HE21even和HE21odd的模場分布；(c) 線偏振LP11模、環(huán)狀渦旋光及其干涉條紋的模場分布

5 總 結(jié)

本文通過實驗提出了一種基于全光纖MSC的鎖模摻鐿光纖激光器直接振蕩高階模式的方法。光纖激光器內(nèi)部接入兩個MSC，將其少模輸出端連接在一起形成一個模式轉(zhuǎn)換器，使激光腔內(nèi)產(chǎn)生并能穩(wěn)定傳輸1064 nm波段的LP11模式，并根據(jù)NPR鎖模機制輸出脈沖型CVB和一階OAM。結(jié)果表明，MSC有著80 nm的帶寬，98%的模式轉(zhuǎn)換效率和94%的高階模式純度。激光器輸出的鎖模激光有著7.4 nm的3 dB譜寬，10.9 MHz的重頻，55 dB的信噪比和2.3%的斜率效率。該方法使全少模超快摻鐿光纖激光器直接產(chǎn)生并全腔振蕩高純度、高模式轉(zhuǎn)換效率的高階模式成為可能。

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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

堯涵，曾祥龍，石帆，等. 基于模式耦合器的鎖模摻鐿光纖激光器[J]. 光電工程，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

國家自然科學基金資助項目(91750108)；上海市科學技術(shù)委員會資助項目(20JC1415700, 16520720900)；上海市高等學校特聘教授(東方學者)項目；高等學校學科創(chuàng)新引智計劃(111)資助(D20031)

堯涵(1995-)，女，碩士，主要從事特種光纖器件和激光器的研究。E-mail：yaohan_super@163.com

曾祥龍(1977-)，男，博士，教授，主要從事非線性超快光學、特種光纖及其傳感技術(shù)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