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

Abstract

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.