Compact Two-color Ultrafast Yb:fiber Chirped Pulse Amplifiers and Mid-infrared Generation Based on Difference Frequency Generation

Abstract

A powerful mid-infrared frequency comb in the 2-20 μm range plays a very important role in the development of optical frequency metrology, spectroscopy and other fields of technology. However, there is a big gap in the long wavelength range from 10 to 40 μm mainly because of technical challenges. Difference frequency generation has been shown to be able to generate a frequency comb due to its ability to cancel the carrier-envelope phase shift from pulse to pulse. In this thesis, the first task is to develop a compact and powerful two-color chirped pulse amplification system consisting of two Yb:fiber amplification stages. Such system is compact, stable and highly efficient by utilizing a chirped fiber Bragg grating, a all-normal dispersion photonic crystal fiber and fiber splicing. The chirped fiber Bragg grating generates a two-color signal from the supercontinuum produced in the photonic crystal fiber. An average power of 3.5 W of the two-color signal composed of two peaks at 1025 and 1085 nm is achieved after the main Yb:fiber amplifier stage. Moreover, the power ratio between the 1025 and 1085 nm wavelengths can be tunable. Particularly, the best short:long power ratio obtained is 2:1. The second task is to generate a mid-infrared wavelength of around 18 μm in a 1 mm GaSe crystal by employing the difference frequency generation technique. The generated mid-infrared power is in the range of 150-180 μW. Finally, the optical cross correlation technique to measure the timing jitter of the femtosecond Yb:fiber laser at a 65MHz repetition rate is applied for the purpose of synthesizing a frequency comb at the wavelength of 18 μm. We conclude that this Yb:fiber laser could not make a frequency comb due to its high timing jitter at around 450 fs.