超窄线宽布里渊光纤激光器研究进展(特邀)

Research progress of ultra-narrow-linewidth Brillouin fiber laser(invited)

基于光纤受激布里渊散射的布里渊光纤激光器以其Hz量级甚至亚Hz量级的超窄线宽特性,自问世以来便吸引了广泛的研究关注。超窄线宽布里渊光纤激光器主要经历三个发展阶段,从最初的基于单模光纤谐振腔的布里渊光纤激光器,到向腔内引入掺铒光纤放大器的布里渊掺铒光纤激光器,再到利用一段普通掺铒光纤同时提供布里渊增益与线性增益的紧凑型布里渊掺铒光纤激光器,激光器的性能不断得到发展,相关理论研究也不断得到丰富。近10年,紧凑型布里渊掺铒光纤激光器的研究取得了一系列的进展,在高精度光纤传感等诸多领域有着十分重要的应用前景。按照三个发展阶段依次梳理和总结了布里渊光纤激光器的研究进展,重点阐述了紧凑型布里渊掺铒光纤激光器的机理、特性和应用,并对其未来发展方向进行展望。

Significance Ultra-narrow-linewidth(~kHz)lasers have attracted much research interest because of their wide applications in optical communications,fiber sensors,and so on.The linewidth of lasers affects the performance of the systems,such as the communication length,the minimum detection signal,and the measurement accuracy.Brillouin fiber lasers(BFLs),based on stimulated Brillouin scattering(SBS)in fibers,present Hz-scale ultranarrow linewidth due to their intrinsic linewidth-narrowing effect.With its development in the past several decades,the compact Brillouin/erbium fiber laser(BEFL)becomes the frontier of the research on BFLs.Unlike the erbium-doped fiber lasers or laser diodes,the compact BEFL presents Hz-scale linewidth without complicated feedback loop or extremely precise isolation from the temperature and vibration variations.Besides the advantage of ultra-narrow linewidth,the BEFL simultaneously presents very stable central frequency and stable fast tuning.These outstanding performances make the compact BEFL a very ideal laser source for many applications,especially for phase-generation-carrier(PGC)interferometric fiber sensors.Progress The development of narrow-linewidth BFLs went through three stages,i.e.the early-days BFLs,the traditional BEFLs,and the compact BEFLs.The BFLs were introduced according to the three development stages.In the early days,the BFL was based on SBS in a single-mode fiber resonator(Fig.1-4).The Brillouin pump(BP)was injected in the cavity and generates SBS in the single-mode fibers.The BFL needed high pump threshold or critical pump coupled resonator due to the small Brillouin coefficient.The Brillouin/erbium fiber laser(BEFL)was then proposed to overcome the need of a pump couple resonator by introducing an erbiumdoped fiber amplifier in the resonator(Fig.5).The BEFL presents low threshold and high output power(Fig.6(b)),but it needed over 100-m single-mode fibers as the Brillouin gain medium.Long cavity causes mode hopping easily.Many studies were carried out to establish single-longitudinal BEFL,such as multi-resonance-cavity BEFL(Fig.7),short-cavity BEFL based on high-nonlinearity special fibers(Fig.8),single-mode BEFL based on Brillouin pump preamplification(Fig.9).Short-cavity,low-threshold BEFLs were desirable.Until 2012,the compact BEFL was proposed based on a length of erbium-doped fiber(EDF)providing both the Brillouin gain and linear gain(Fig.10).It presented short cavity and low threshold.In 2013,an all-polarization-maintained ultra-short-ring-cavity compact BEFL was reported(Fig.11).The mechanism,characteristics,and applications of the compact BEFL were studied in the following 10 years.A series of progress has been achieved on the studies of compact BEFL.This kind of fiber laser showed 3-Hz ultra-narrow linewdith(Fig.16),stable central frequency,and stable fast tuning(Fig.26).The phase noise of the BEFL is lower than the state-of-the-art commercial laser diodes(Fig.18).The outstanding performance of the compact BEFL leads to many important potential applications,such as in high-accuracy interferometric fiber sensors(Fig.27-28)and Brillouin distributed fiber sensors(Fig.29-30).Conclusions and Prospects The optical communication and sensing systems are in great need of highperformance ultra-narrow-linewidth lasers.The BFLs,based on SBS in fibers,present Hz-scale ultra-narrow linewidth.The BFLs have already been developed to the stage of the compact BEFLs,which present ultra-narrow linewidth,stable central frequency,and stable fast tuning simultaneously.Compared with the state-of-the-art narrow-linewidth external-cavity laser diodes,the compact BEFL presents even lower phase noise.The applications in interferometric fiber sensors and distributed fiber sensors are validated for the compact BEFL.The advantages of the applications of the compact BEFLs are verified.The compact BEFL has a fully independent intellectual property,it has great significance for the localization of producing many important optoelectric information systems.The research aims to provide some reference for the study and applications of narrowlinewidth lasers in the future.It is expected that the compact BEFL will be modularized so that it could be widely used in more applications.