光子晶体光纤中高效的反斯托克斯信号产生

基于四波混频的反斯托克斯变换, 被广泛应用于短波辐射高分辨率成像以及直接激发分子的电子跃迁等方面. 为了实现更加高效的反斯托克斯变换, 利用中心波长为810 nm脉冲宽度为120 fs的钛蓝宝石(Ti: sapphire)飞秒激光器作为抽运光源, 在长度为0.5 m和3 m的光子晶体光纤中分别实现了高阶模和基膜的简并四波混频. 实验中, 采用的光子晶体光纤的零色散波长在820 nm附近. 在基模相位匹配条件下, 在560 nm附近实现了高效地反斯托克斯信号的产生, 反斯托克斯信号与残余抽运信号的最大功率比为33:1; 反斯托克斯信号和斯托克斯信号的最大功率比25:1; 反斯托克斯信号最大功率转换效率P_a/P_p0为34%. 抽运波长从790 nm逐渐增加到810 nm过程中, 在长为3 m的光子晶体光纤中相位从不匹配状态转化为高阶模匹配状态后, 再转化为基模匹配状态. 通过实验研究得出了相位匹配程度随抽运功率、波长和光纤长度的变化规律, 同时分析了造成理论计算与实验结果存在差异的主要因素. 本文为研究在光子晶体光纤基模中实现相位匹配和产生高效反斯托克斯信号提供了理论和实验依据.

High efficient anti-Stokes signal conversion in photonic crystal fiber

The anti-Stokes frequency conversion based on four-wave mixing (FWM) has been widely used to generate short-wavelength radiation for high resolution imaging, direct excitation of electronic molecular transitions, and so on. For achieving more effective anti-Stokes conversion, we use the Ti: sapphire laser with a central wavelength of 810 nm and a pulse width of 120 fs as a pump source, and the degenerated FWMs of the higher mode and the fundamental mode are achieved respectively in 0.5 m long and 3 m long photonic crystal fibers (PCFs) with a zero dispersion wavelength of fundamental mode around 820nm in our experiment. The anti-Stokes signals around 560nm are generated efficiently at the fundamental phase matching. The maximum power ratios of anti-Stokes signal at 562 nm to the residual pump component and the Stokes signal are above 33:1 and 25:1, respectively. The maximum conversion efficiencies are achieved to be up to 48% and 34% in theory and experiment, respectively. And then the variation laws of the phase matching and the output spectrum with pump power, wavelength and the fiber length are obtained and the discrepancy between theoretical and experimental results is analyzed. Moreover, the effects of more factors on experimental results are discussed.