高非线性光子晶体光纤中优化产生宽带紫外三次谐波

Optimazation of broadband third-harmonic UV generation in highly nonlinear photonic crystal ?ber

通过非线性光学频率转换技术产生不同频率的脉冲辐射是实现具有短波波长的激光光源的有效手段.近年来,光子晶体光纤技术的发展为解决传统的基于非线性晶体的频率转换系统面临的难以维护、转换效率低、不易推广等问题带来了新的解决思路.在频率转换研究领域中,紫外光波段的脉冲辐射产生一直以来都受到学者的广泛关注.国外已报道过利用超短脉冲抽运光子晶体光纤实现三次谐波产生,从而输出具有高灵敏度和高分辨率的窄带紫外脉冲辐射,但其紫外光转换效率较低,且光谱的可调谐能力有限,而这些缺陷恰恰可以由宽带紫外脉冲辐射的获得来改善.宽带紫外脉冲辐射的有效获得不仅意味着紫外光转换效率可大幅提高,并且若加以合适的滤波手段,还可获得内任意波长下的窄带宽的脉冲辐射,从而增加窄带紫外脉冲辐射的可调谐度,但目前相关报道较为有限.本文将中心波长为1035 nm,脉冲重复频率为50 MHz的飞秒激光耦合至一定长度的高非线性光子晶体光纤中,将其产生的拉曼自频移孤子作为三次谐波产生的抽运源,通过相位匹配作用,在深紫外波段产生高阶模式传导下的三次谐波.随后让超短脉冲以偏离光纤轴心一定角度入射,可进一步激发出具有更短波长的超高阶紫外光模式.通过激发多个邻近的超高阶紫外光模式,在一定连续范围内实现相位匹配,获得超高阶紫外光模式传输下紫外光转换效率为3.6%的宽带(32—360 nm)深紫外脉冲激光.实验结果与理论模拟结果相一致.

The generation of pulse radiation with different frequency based on nonlinear optical frequency conversion technology is an effective method to produce lasers with the wavelength in the visible light or ultraviolet （UV） light range. In recent years, the developments of photonic crystal fiber （PCF） technology and ultra-short pulse technology have brought new solutions to the problems that the system needs great maintenance work, has low frequency conversion rate and much difficulty in popularizing, which the traditional frequency conversion system based on nonlinear crystal is confronting. Research on UV pulse radiation has been consistently attracting much attention of many academics. Particularly, narrowband and broadband UV pulse radiation sources are complementary, each having its own characteristics and scope of applications. The generation of narrowband UV pulse radiation of high sensitivity and high resolution through third harmonic generation （THG） in PCF has already been reported. However, the frequency conversion rate of narrowband UV pulse radiation is relatively low and the tunable ability of the spectrum is limited. These imperfections can be exactly completed by broadband UV pulse radiation. Broadband UV pulse radiation based on THG in PCF can be realized efficiently in PCF. This means that the conversion of UV light increases substantially, and simultaneously, the narrowband UV radiation of any wavelength in a certain range can be acquired more easily and the tunable ability of narrowband UV pulse radiation can be enhanced further. In this paper, the femtosecond pulse with a central wavelength of 1035 nm at a pulse repetition rate of 50 MHz is coupled into a highly nonlinear photonic crystal fiber with an appropriate length. The Raman self-frequency shift soliton produced from the ultra-short input pulse acts as a pump resource of third harmonic, transmitting through fundamental mode in PCF. Phase-matching between the fundamental mode and the high order modes is achieved and the third harmonic transmitted by specific high order modes （such as HE13） at deep UV wavelength is acquired effectively. Besides, the very high order UV mode （HOUVM） transmitting third harmonic with shorter wavelength is stimulated when intentionally inputting the ultra-short pulse into the PCF in the direction of a certain angle deviating from the axis of fiber core. Broadband deep UV （320-360 nm） pulse radiation with a UV light conversion rate of 3.6% can be acquired effectively in nonlinear PCF by stimulating a number of adjacent HOUVMs and achieving phase matching between the modes. Good agreement between theoretical results and experimental results is achieved.