利用啁啾脉冲光谱滤波和非线性偏振旋转技术实现高稳定性和开机自启动的全光纤掺Yb^3＋光纤锁模激光器

Highly stable and self-started all-fiber Yb^(3+) doped fiber laser mode-locked by chirped pulse spectral filtering and nonlinear polarization evolution

建立理论模型,讨论了非线性偏振旋转全光纤锁模激光器的锁模过程、谐波过程以及导致激光器锁模运行难以稳定的影响因素.讨论了采用啁啾脉冲光谱滤波产生脉冲自振幅调制、增加激光器锁模稳定性和自启动能力的机理以及非线性偏振旋转与啁啾脉冲光谱滤波相结合实现锁模的物理过程和脉冲演化过程.研制出全光纤结构的超短脉冲掺Yb^(3+)光纤环形激光器,采用非线性偏振旋转和啁啾脉冲光谱滤波相结合的锁模技术,实现了激光器锁模的开机自启动和高稳定运行.对激光器进行了长期运行稳定性、锁模开机自启动能力、锁模输出参数可重复性监测.锁模脉冲中心波长1052.9 nm,谱宽9.1 nm,脉冲能量4.25 nJ,脉冲宽度17.8 ps.运行期间,各参数波动均小于0.3%.开机自启动能力和可重复性测试显示,激光器可实现一键自启动,启动后各参数可重复精度在0.55%以内.

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 π/38 can break the mode-locking completely. Experimentally, after carefully adjusting, singlepulse 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%.