Highly stable and self-started all-fiber Yb3+ doped fiber laser mode-locked by chirped pulse spectral filtering and nonlinear polarization evolution
Authors: Zhang Pan-Zheng  , Wang Xiao-Chao  , Li Jing-Hui  , Feng Tao  , Zhang Zhi-Xiang  , Fan Wei  , Zhou Shen-Lei  , Ma Wei-Xin  , Zhu Jian  , Lin Zun-Qi  ,   Key Laboratory of High Power Laser and Physics, CAS, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China;,   Shanghai Institute of Laser Plasma, China Academy of Engineering Physics, Shanghai 201800, China
Published: December 2020

Without discrete optical components influencing the fiber format, all-fiber mode-locked laser has tremendous potential practical applications due to its advantages of better stability, alignment free, and better compaction. All-fiber laser mode-locked by nonlinear polarization evolution（NPE) can obtain good performances in terms of pulse duration and spectrum. But the effective saturable absorption mirror can be overdriven at high peak power, which leads to multiple pulses, limiting the output pulse energy. And there is a trade-off between avoiding overdriving the NPE and ease of self-starting. In addition, the polarization of the pulse propagating in a long fiber is so sensitive to the environment vibration that it is difficult to implement a stable lone-time operation.All-fiber ring laser mode-locked by NPE alone is analyzed and realized. The simulation results show that even a polarization vibration of up/38 can break the mode-locking completely. Experimentally, after carefully adjusting, single-pulse mode-locking is achieved with the spectrum centered at 1053.4 nm and a maximum pulse energy of 82 pJ. But the output parameters change continually during operating. After 60 min, the mode-locking is broken. The conclusion is obtained that instability and unreliability of self-starting are inevitable for such a laser.Here, we show significant improvements of the pulse energy, operating stability, and self-starting reliability from an all-fiber Yb-doped mode-locked fiber laser. The laser is mode-locked by NPE combined with chirped pulse spectral filtering（CPSF). In order to easily self-start and stabilize mode locking, a spectral filter is employed in the all-normal group velocity dispersion NPE cavity to provide additional amplitude modulation. Combined effects of NPE and CPSF result in desirable pulse output, desirable operating stability, and reliable self-starting simultaneously. Stable mode-locking centered at 1053 nm is achieved with a 3 dB spectral bandwidth of 9.1 nm and pulse duration of 17.8 ps. The average output power is 66.9 mW at a repetition rate of 15.2 MHz, corresponding to a pulse energy of 4.25 nJ. Especially, high operating stability and easily one-button self-starting are achieved simultaneously. The fluctuations of output parameters including pulse energy, pulse duration, and spectrum are within 0.3% during 150-min operation. Self-starting reliability is tested. The testing time lasts two weeks. During the two weeks, the laser is turned off and turned on 48 times by using a power supplying button, without any adjustment. And the re-turned on intervals change randomly. Each time, the mode-locking can start itself. The repeatabilities of output parameters including pulse energy, pulse duration, and spectrum are within 0.55%.