# The effect of cavity length detuning on the output characteristics for the middle infrared FEL oscillator of FELiChEM

## TL;DR

This study numerically investigates how cavity length detuning affects the output pulse duration and spectral bandwidth of the middle-infrared FEL oscillator at FELiChEM, revealing tunable pulse characteristics and conditions for comb structure formation.

## Contribution

It provides detailed analysis of cavity length detuning effects on pulse properties and spectral features, offering guidance for optimizing FEL oscillator performance.

## Key findings

- Pulse duration can be tuned from 1 to 6 ps.
- Spectral bandwidth varies from 13% to 0.2%.
- Comb structure may form under specific conditions.

## Abstract

FELiChEM is an infrared free electron laser (FEL) facility under construction, which consists of two oscillators generating middle-infrared and far-infrared laser covering the spectral range of 2.5-200 ${\rm{\mu m}}$. In this paper, we numerically study the output characteristics of the middle-infrared oscillator with the 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 ${\rm{\mu m}}$ and 5 ${\rm{\mu m}}$ 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.