Effect of cavity length detuning on the output characteristics for the middle infrared FEL oscillator of FELiChEM

FELiChEM is an infrared free electron laser (FEL) facility currently under construction, which consists of two oscillators generating middle-infrared and far-infrared laser covering the spectral range of 2.5-200 μm. In this paper, we numerically study the output characteristics of the middle-infrared oscillator with accurate cavity length detuning. Emphasis is put on the temporal structure of the micropulse and the corresponding spectral bandwidth. Taking the radiation wavelengths of 50 μm and 5 μm as examples, we show that the output pulse duration can be tuned in the range of 1-6 ps with corresponding bandwidth of 13%-0.2% by adjusting the cavity length detuning. In addition, a special discussion on the comb structure is presented, and it is indicated that the comb structure may arise in the output optical pulse when the normalized slippage length is much smaller than unity. This work has reference value for the operation of FELiChEM and other FEL oscillators.