1.School of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, China 2.Key Laboratory of Optoelectronic Information Science and Technology (Ministry of Education), Tianjin University, Tianjin 300072, China 3.Tianjin Jinhang Institute of Technical Physics, Tianjin 300308, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2017YFF0104603), the National Natural Science Foundation of China (Grant Nos. 61975146, 62075159), and the Key Research and Development Program of Shandong Province, China (Grant No. 2019JZZY020206)
Received Date:14 March 2021
Accepted Date:16 May 2021
Available Online:15 August 2021
Published Online:05 November 2021
Abstract:Fiber laser system in master oscillator power amplifier (MOPA) scheme is a promising technique for high-power narrow-linewidth laser output. With modulation-generated pulsed seed laser, the fiber MOPA benefits the flexible temporal behavior. However, the spectral linewidth broadening induced by self-phase modulation (SPM) is the main obstacle to achieving high-power single-frequency laser output with narrow spectral linewidth, especially for pulsed fiber MOPA in which the kilowatts level peak power results in strong nonlinearity. The SPM induced linewidth broadening is related to the derivative of light intensity with respect to time (dI/dt). Theoretically, if the dI/dt of the laser pulse is a constant, the SPM process will not generate any new frequency components. Hence, the linewidth broadening can be suppressed. In this work, we demonstrate a high-power single-frequency Yb fiber amplifier at 1064 nm, in which a sawtooth laser pulse is employed to suppress the SPM induced linewidth broadening, for obtaining the output with near-transform-limited narrow linewidth. The sawtooth-shaped seed pulse train is generated through using an electro-optic intensity modulator to modulate the continuous-wave (CW) output of a single-frequency fiber laser. After being pre-amplified, the seed laser with a pulse repetition rate of 20 kHz is coupled into the main amplifier, in which a piece of 0.9-m-long Yb-doped silica fiber with core and clad diameters of 35 μm and 250 μm, respectively, is used as a gain medium. The seed laser is enhanced to an average power value of 3.13 W under a launched 976-nm pump power value of 11.3 W before the onset of stimulate Brillouin scattering. The pulse energy 157 μJ and the pulse width 6.5 ns give a peak power of 24 kW. The spectral linewidth measured using a scanning Fabry-Perot interferometer at the maximum power is only 83 MHz, which is quite close to the 76-MHz transform-limited linewidth of the 6.5-ns sawtooth-shaped pulse. For comparison, we also conduct an experiment with a common Gaussian-shaped seed laser, in which the spectral linewidth is broadened significantly with a peak power value of only 1.5 kW. The results here reveal that the using of the sawtooth-shaped pulse is a promising technique to suppress the SPM induced spectral linewidth broadening in high-peak-power fiber amplifiers and acquire near-transform-limited narrow-linewidth laser output. Keywords:single-frequency fiber laser/ pulsed fiber laser/ self-phase modulation/ narrow-linewidth laser
式中, I(t)为随时间变化的激光光强; P和Leff分别为激光脉冲峰值功率和有效光纤长度; γ为非线性参量, γ = n2(ω0)ω0/(cAeff), 其中, n2(ω0)为非线性折射率系数, ω0为激光角频率, c和Aeff分别为真空中的光速和光纤有效模场面积. 对于高斯型等常规脉冲波形, 其光强随时间的变化率$ {{\partial I(t)} / {\partial t}} $是时刻变化的, 这就导致时域上脉冲内不同时刻的激光频率相对其原有中心频率的偏移量不同, 即引起光谱展宽. 对于本实验中所用的锯齿波来说, 由于其下降沿光强随时间线性变化, 也即(1)式中$ {{\partial I(t)} / {\partial t}} $为常数, 因此忽略其很短的上升沿的作用后, 整个脉冲内所产生的非线性频移是一致的, 也就不会产生额外的频率分量, 能够抑制SPM引起的光谱展宽. 用法珀扫描干涉仪(FPI, Thorlabs SA200-8B, 分辨率7.5 MHz)测量激光峰值功率为24 kW时的激光光谱线宽, 结果如图5(a)所示, 此时光谱的半高全宽为83 MHz. 图中示波器记录的FPI波形是由诸多尖峰组成的包络, 这是由于几个ns的激光脉宽远小于ms级的FPI扫描周期, 每个激光脉冲引起的响应在波形上体现为一个尖峰, 而包络体现了激光的光谱线宽. 激光脉宽与光谱的时间带宽积极限和激光脉冲的时域波形有关, 对于常见的高斯、方波和锯齿波等时域波形来说, 其数值各不相同. 对时域锯齿波信号进行傅里叶变换, 可知其时间带宽积极限为0.491, 脉宽为6.5 ns的锯齿波对应变换极限光谱线宽为76 MHz, 24 kW峰值功率下测得的83 MHz的光谱线宽仅比其理论极限宽10%左右, 得益于锯齿波信号光强对时间的变化率为常数的性质, 实验中SPM效应导致的光谱展宽得到了显著的抑制. 而通过信号发生器控制EOIM使种子光脉冲为脉宽7.5 ns的高斯型脉冲时, 种子光经过第2级包层泵浦预放大级、峰值功率为1.5 kW时, 其光谱即发生了非常明显的展宽, 如图5(b). 进一步改变锯齿波信号的脉冲宽度, 使放大器输出脉冲宽度为5.9 ns和7.5 ns, 分别在峰值功率为24 kW和20 kW时测得了88 MHz和70 MHz的光谱线宽. 图6给出上述锯齿波形高功率光纤MOPA输出光谱线宽与其理论变换极限的对比, 可以看到其光谱线宽都接近理论变换极限, 充分验证了采用锯齿波形脉冲是抑制高功率光纤激光MOPA中SPM导致的光谱展宽、获得窄线宽单频激光输出的有效技术途径. 图 5 (a)使用锯齿波形时主放大器最高输出功率为24 kW时的激光线宽; (b)使用高斯波形时预放大级输出的激光线宽(脉宽7.5 ns, 峰值功率1.5 kW) Figure5. Measured spectral linewidths of (a) the sawtooth pulse at the maximum peak power of 24 kW and (b) the Gaussian-shaped pulse after being pre-amplified to a peak power of 1.5 kW with a pulse width of 7.5 ns.
图 6 锯齿波脉冲理论时间带宽积极限(实线)和实验中测得的不同脉宽锯齿波形光谱线宽(圆点) Figure6. Theoretical transform-limited spectral linewidth of the sawtooth pulses (line) and the measured spectral linewidths at different pulse widths (solid circle).
图 1 脉冲单频光纤激光MOPA光路示意图
图 2 不同有源光纤长度时光纤激光MOPA的平均输出功率曲线, 进入放大器种子光功率为220 mW, 重复频率为20 kHz
图 3 平均输出功率为3.13 W、脉冲重复频率为20 kHz时的激光光谱
图 4 (a)调制产生的脉冲种子源的波形; (b)经过预放大进入主放大级之前的波形; (c)主放大器最高输出功率时的波形
图 5 (a)使用锯齿波形时主放大器最高输出功率为24 kW时的激光线宽; (b)使用高斯波形时预放大级输出的激光线宽(脉宽7.5 ns, 峰值功率1.5 kW)
图 6 锯齿波脉冲理论时间带宽积极限(实线)和实验中测得的不同脉宽锯齿波形光谱线宽(圆点)