1040nm泵浦的光子晶体光纤超连续光谱产生性能退化研究

光子晶体光纤已经被广泛应用于由飞秒脉冲激光源产生超连续光谱。当激光源的重复频率较低时，由光子晶体光纤产生的超连续光谱随时间的变化过程较为缓慢，通常不被注意到。而在天文光谱仪定标等应用中，需要使用GHz至几十GHz量级的高重复频率激光源。此时，可观察到光子晶体光纤的超连续光谱产生性能在有限时间内产生显著的退化。在1 040 nm飞秒激光泵浦条件下，通过测试三种不同气孔占空比的光子晶体光纤的超连续光谱产生性能演化，发现超连续光谱的退化进程随光纤气孔占空比的增大而加速。观察发生光谱退化后的光子晶体光纤样品，发现在光纤上超连续光被产生的区段出现多个不同颜色的亮点，呈现有方向性的光泄露现象。针对光泄露现象，通过测量光纤的吸收光谱线，证实了实验中超连续光谱退化的主因并非是光纤熔融石英材料中大量非桥氧色心产生。针对光泄露具有方向性这一特征，提出了经由多光子吸收作用在光纤纤芯中形成长周期光栅的理论。为探究影响光子晶体光纤超连续光谱产生性能的退化的因素，以达到光谱退化抑制的目的，首先通过改变了光纤的拉锥参数，期望增强光纤熔融石英材料的光子耐受性。实验结果证实了该方法的有效性较为有限。其次，从保持激光源的平均功率，降低激光脉冲的峰值功率和保持激光脉冲的峰值功率，降低激光源平均功率两个方面入手，对激光源进行调制。实验结果证明，光纤单位时间内接受的高峰值功率脉冲总量是影响其超连续光谱产生性能的最重要因素。在天文光谱仪定标的应用中，对超连续光谱光功率的需求并不高，使用斩波器降低光子晶体光纤入射光的平均功率是减缓超连续光谱产生性能退化过程的有效且简单可行的方法。

Supercontinuum Generation Degradation of 1 040 nm Laser Pumped Photonic Crystal Fibers

Photonic crystal fibers have been widely used in the supercontinuum generation of femtosecond pulse laser sources. When the repetition rate of a laser source is low, the evolution of supercontinuum over time is slow, which is usually not noticed. In applications such as calibrations of astronomical spectrometers, high repetition rate laser sources of the order of gigahertz to tens of gigahertz are required. In this case, the supercontinuum degradation is significant within a limited time period. Using 1040nm femtosecond laser as the pump source, by testing the evolutions of supercontinua of three photonic crystal fibers with different air-filling fractions, it is found that the degradation process accelerates with the increase of the air-filling fraction. Accompanying the degradation of supercontinuum, multiple bright spots of different colors appear in the section where the supercontinuum is generated on the fiber. It implies a directional light leakage phenomenon. Observing the spectral absorption of the spectrally degraded fiber confirmed that the main reason for the degradation is not the generation of massive non-bridged oxygen color centers in the fused silica material. Based on the directional characteristic of the light leakage, a theory that a long-period grating formation in the fiber core by multiphoton absorption is proposed. In order to search for the factors that affect the supercontinuum generations of the photonic crystal fibers, so that the goal of suppressing the degradation can be achieved, firstly, parameters of the fiber tapering are changed. It is expected that the photon tolerance of the fused silica material of the fibers can be enhanced. The experimental results show that the effectiveness is scant. Then, experiments are carried out with maintaining the average power of the laser source, reducing the peak power of the laser pulse and maintaining the peak power of the laser pulse, reducing the average power of the laser source. It is shown that the total amount of high peak power pulses coupled into the optical fiber in a certain time period is the most important factor affecting the supercontinuum degradation. In the application of astronomical spectrometer calibration, the demand for optical power of supercontinuum is not high. Using a chopper to reduce the average power of the incident light of the photonic crystal fiber is an effective, simple and feasible method to slow down the supercontinuum degradation.