基于非线性环形镜锁模的全保偏光纤光梳系统

All Polarization Maintaining Fiber-Based Optical Comb System with Nonlinear Loop Mirror Mode-Locking

探究了一种基于非线性放大环形镜锁模的全保偏掺铒光纤光梳系统,重复频率(f_(r))为75.27 MHz,f_(r)与载波包络偏移频率(f_(0))的标准偏差为分别为358μHz和248 mHz。该系统通过温度反馈控制锁模振荡器局部环境、电压反馈控制压电陶瓷伸缩量、电流反馈控制泵浦源功率实现了对f_(r)和f_(0)的精确锁定,在10℃温度波动的外部环境中实现约100 h的稳定工作。最后,将该光梳应用于混合气体光谱检测实验,获得的乙炔吸收谱线与HITRAN数据库模拟结果一致,表明该光梳系统可初步用于光谱分析等室外应用。

Objective Compared with the traditional laser,fiber laser has the advantages of simple structure,high stability,and small size,becoming the research focus of practical optical comb.The structure of fiber-based optical comb with nonlinear polarization rotation(NPR)mode-locking is simple,but the refractive index of non polarization-maintaining fiber is easily affected by vibration and temperature,and the long-term stability of mode-locked state and phaselocked state of the oscillator is poor.The optical frequency comb based on semiconductor saturable absorption mirror(SESAM)can improve the influence of environmental noise due to the fully polarization maintaining fiber structure and appropriate damping measures.However,SESAM has the risk of light-induced damage during long-term use.It has a relaxation time of several hundred femtoseconds to picoseconds,bringing additional phase noise and instability to the optical comb.Most of the reports about the long-term stable operation of the comb were obtained in the constant temperature laboratory.In this study,we report a compact optical frequency comb with all polarization maintaining fiber structure.The system had kept stable operation for a long time in the external environment of 10℃ temperature fluctuation,and the mixed gas detection experiment was completed.This paper provides a feasible method for realizing practical optical frequency comb.Methods In the study,a mode-locked fiber laser with nonlinear amplifying loop mirror(NALM)was used as the seed source of the optical comb,which was placed in a sealed cavity to achieve isolation from the external environment.Then,the optical power amplification,pulse width compression,and spectrum broadening were studied by using cascaded erbium-doped fiber amplifier,single-mode polarization-maintaining fiber(PMF),and highly nonlinear fiber(HNLF).The broadened spectrum was injected into the f-2 f self-referenced detector to detect the carrier envelope offset frequency(f_(0))signal.Then,by precision controlling the local temperature of the modelocked fiber oscillator,the length of a piezoelectric transducer,and the current of the pump diode,long-term repetition rate(fr)and f_(0) locking of the optical frequency comb was achieved.Finally,a high non-linear dispersion shifted fiber(HN-DSF)was connected to the comb application end to generate flat spectrum at 1350--1550 nm.Besides,agas sample cell filled with ^(12)CO and ^(12)C_(2)H_(2) was used to exam the mixed gas detection experiment.Results and Discussions The repetition rate of the mode-locked laser was 75.27 MHz,and the average power was4.1 mW.The pulsed light was amplified to 196 mW by a cascaded erbium-doped fiber amplifier,and then the pulse width was compressed to less than 100 fs by a section of PMF fiber.The supercontinuum of the compressed pulse was realized by a segment of HNLF fiber from which the spectrum covered 1000--2200 nm[Fig.2(a)].The f-2 f selfreferenced detector detected f_(0) signal with signal to noise ratio of 40 dB and line width of 5 kHz[Fig.2(b)--2(c)].In long-term operation,the drifts of frand f_(0)(24 h)in the open-loop were reduced from 6.2 kHz and 310 MHz to 0.51 kHz and 26.9 MHz,respectively[Fig.4(a)and(b)].The drifts were further reduced to 10 Hz and 700 kHz in the partially feed-back loop,which satisfied the servo capability of the phase-locked circuit of the system[Fig.5(a)and(b)].Finally,the standard deviations of frand f_(0) after locking were 358μHz and 248 mHz in 100 hours operation,respectively[Fig.6(a)].In the experiment of gas mixture detection based on the optical frequency comb,a flat spectrum from 1250--1650 nm was generated from a segment of HN-DSF fiber,covering the absorption spectra of various gas molecules including ^(12)C_(2)H_(2),^(12)CO,and H_(2)O[Fig.2(d)].The obtained ^(12)C_(2)H_(2)gas detection data was basically consistent with the simulation results of HITRAN database,and the standard deviation of their deviations was only 2.4%[Fig.7(c)].Conclusions An all polarization-maintaining fiber optical frequency comb for outdoor application is proposed.The size of the optical frequency comb is 330 mm×340 mm×65 mm.The frequency stability of the optical frequency comb system is about 4×10^(-12)in 1 s,and the system can keep frand f_(0) stable for a long time under the temperature fluctuation of(22±5)℃.In addition,in the experiment of wide spectrum detection of mixed gas based on the optical comb,the gas detection data obtained is consistent with the simulation results of HITRAN database,indicating that the optical comb system can be rudimentarily used for outdoor applications such as spectral analysis.