Objective Picosecond laser pulses,characterized by their high peak power and spectral purity,hold significant importance in numerous applications across various fields.Mode-locked fiber lasers,with their compact structure,maintenance-free operation,and superior anti-interference ability,have emerged as one of the most vital sources of picosecond pulse lasers.Among these,SESAM and Figure-9 mode-locked fiber lasers have attracted significant attention owing to their exceptional self-start performance.Furthermore,in precision measurement applications,such as lidar and precision distance measurement,it is imperative to maintain a locked repetition rate of the laser pulse.However,practical engineering applications present novel challenges due to their complex environments,which include temperature changes ranging from-40 ℃ to 50 ℃ across different seasons,violent vibrations during transportation and usage,and stringent requirements concerning volume,weight,and power consumption.These conditions pose challenges to maintaining a locked repetition rate of the picosecond mode-locked fiber laser while ensuring the self-start function.Consequently,the design and development of a picosecond fiber laser with rapid self-start and repetition-rate-locking capabilities becomes a significant issue that warrants further exploration and research.Methods The configuration of the Figure-9 fiber laser was chosen,and the intracavity nonlinearity was optimized to realize the fast self-start mode-locking function for the laser.A"constant temperature"local-environment for the optical module was established by adiabatically packaging with low-thermal-conductivity materials.This approach significantly relaxed the requirement of the tuning range for the piezoelectric transducer(PZT)frequency tuning mechanism to lock the repetition rate of the fiber laser operating in the outdoor environment.Based on these advancements,a prototype mode-locked fiber laser weighing only 3 kg was designed and developed.This prototype showcased a typical repetition rate and pulse width of 10 MHz and 20 ps,respectively.Results At room temperature,the measured pulse train,which displays a repetition rate of 10 MHz,is illustrated in Fig.4(a).The intensity autocorrelation trace indicates the pulse width to be 20 ps[see Fig.4(b)].The pulse's center wavelength is 1064 nm with a 3 dB bandwidth of 0.2 nm(Fig.4c).The repetition rate's fluctuation is less than 7.5 mHz over a 10 h test period[see Fig.5(a)].The corresponding Allan variance of repetition-rate instability corresponds to 2.1×10-11@1 s,8.5×10-12@10 s,and 3.6×10-11@1000 s[see Fig.5(b)].Repetition rates as a function of time are depicted in Fig.6(a)when the repetition rates are locked at-40 ℃,0 ℃,and 50 ℃,respectively.The fluctuations of the repetition rates remain less than 15 mHz over a 30 min test period.Correspondingly,the Allan variance of repetition-rate instability is 4.3× 10-11,5× 10-11,and 2.8× 10-11 for 1 s at-40 ℃,0 ℃,and 50 ℃,respectively[see Fig.6(b)].Fluctuations of repetition rates as functions of time are illustrated in Fig.7(b)when the vibration is superimposed on each of the three coordinate axes,and the Allan variance for 1 s across these axes remains better than 2.5×10-10[see Fig.7(b)].In an outdoor environment,the prototype's repetition rate can be locked for more than 3 h[see Fig.8©and Fig.8(d)],suggesting that the prototype can withstand temperature fluctuations of approximately 10 ℃ in an outdoor setting.Conclusions A repetition-rate-locked picosecond pulsed fiber laser,designed for operation in outdoor environments,has been reported.The configuration of the Figure-9 fiber laser was chosen,and the intracavity nonlinearity was optimized to realize a fast self-start mode-locking function for the laser.A prototype of the mode-locked fiber laser,weighing only 3 kg and typically exhibiting a repetition rate and pulse width of 10 MHz and 20 ps respectively,was developed.Under varying conditions,such as room temperature,extreme ambient temperatures(-40 ℃ or 50 ℃),and environments experiencing vibrations of 1.5g,the prototype still managed to maintain self-start mode-locking and repetition-rate-locking.Furthermore,the prototype's repetition-rate-locking function demonstrated resistance to a 10 ℃ ambient temperature fluctuation when operating in high-temperature outdoor environments during summer.
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