基于光栅反馈技术的掺铥光纤随机激光器

Tm^(3+)-doped Fiber Random Laser Based on Fiber Grating Feedback Technology

提出一种基于光栅反馈技术的掺铥光纤随机激光器。激光器采用半开腔设计,封闭端采用中心波长为1 940 nm的高反射率光纤光栅为激光器系统提供强反馈,增益介质采用1.5 m长的掺铥光纤,泵浦源采用793 nm半导体激光器,开放端采用光纤随机光栅提供随机分布反馈。该光纤随机光栅由飞秒激光逐点刻写技术制备,在10 cm单模光纤上刻写超过6 000个间距随机分布的折射率畸变点,以增强光纤的后向瑞利散射效应。实验测得中心波长为1 940 nm的随机激光输出,其泵浦阈值为2.33 W,在3.8 W泵浦功率下的输出功率为57 mW,光信噪比达56 dB。输出激光在1 h内的波长偏移量小于0.1 nm,功率变化约0.26 dB,具有良好的稳定性。

Random Fiber Lasers(RFLs) based on random distributed feedback can operate without a precise resonant cavity,leading to the advantages of simple structure and low production cost.In previous work,random fiber lasers operating in the band of 1.0~1.6 μm have been widely investigated.However,limited by the high transmission loss of ~30 dB/km and the weak Rayleigh scattering efficiency in normal silica fibers,random fiber lasers operating in the band of 2 μm are rarely reported.It’s of great fundamental interest to push the random fiber lasers to 2 μm mid-infrared band for their potential applications in the fields including medical surgery,nonlinear optics,material processing,and remote sensing.In this work,a random fiber laser operating in 2 μm band is developed by using a 1.5 m long thulium-doped fiber as the gain medium and a fiber random grating for random distributed feedback with enhanced Rayleigh scattering efficiency.The proposed random fiber laser adopts the half-open cavity design by using a high reflectivity fiber Bragg grating with a central wavelength of 1 940 nm to provide strong feedback to the laser system.A793 nm semiconductor laser is employed as the pump laser source.The fiber random grating containing over 6 000 refractive index distortion spots was inscribed point by point along with a 10 cm long singlemode fiber by using a Ti:sapphire femtosecond regenerative amplifier with an operation wavelength of800 nm,a repetition rate of 100 Hz and a pulse duration of 80 fs.The neighboring refractive index distortion points were spaced at a random distance between 7.5 and 12.5 μm.Experimental results show that random laser output at the wavelength of 1 940 nm is achieved with a relatively low threshold power of2.33 W.Benefit from the enhanced Rayleigh scattering efficiency of the fiber random grating,the pump threshold of the random fiber laser is much lower than that of the previously reported random fiber laser in2 μm region.With increasing the pump power,an output power of the random fiber laser increases nearly linearly with a slope efficiency of 4%.When the pump power reaches 3.8 W,the output power is 57 mW and the optical signal-to-noise ratio is up to 56 dB.The laser output wavelength remains quite stable during the change of pump power.To further test the stability of the random fiber laser,laser output spectra and powers were measured at an interval of 5 min and one second respectively within 60 min under the fixed pump power of 3.8 W.Good wavelength stability of 0.1 nm and power stability of fluctuation less than0.26 dB are achieved.The good performance in stability should be related to the good wavelength selectivity and stability of the high-reflectivity fiber Bragg grating in both wavelength and reflectivity.It was fabricated on ordinary single-mode fiber,not the thulium-doped fiber,so its reflection wavelength and reflectivity can keep stable even when the pump laser reaches new heights and changes the temperature of the thulium-doped fiber.The slope efficiency is relatively low if compared with that of the common thulium-doped fiber lasers.It should be related to the relatively large insertion losses,7.5 dB in total,of the two fiber fusion splicing points between the pump laser source and the thulium-doped fiber.The fiber parameters of the lead-out fiber of the pump laser and the thulium-doped fiber are much different from those of the single-mode fiber of the ports of the wavelength-division multiplexer.However,it can be improved by customizing a wavelength-division multiplexer with matching fiber parameters.Anyway,the proposed random fiber laser provides an effective technical method to develop random fiber lasers in the2 μm wavelength band with relatively low pump threshold and better performances.