# Generation of optical frequency combs via four-wave mixing processes for low- and medium-resolution astronomy

## TL;DR

This paper presents a method to generate optical frequency combs using four-wave mixing in nonlinear fibres, optimized for astronomical calibration, demonstrating both modeling and experimental results across infrared and visible spectra.

## Contribution

The study introduces a novel fiber-based system for generating stable optical frequency combs suitable for low- and medium-resolution astronomy calibration.

## Key findings

- Successful experimental generation of frequency combs in near-infrared and visible ranges.
- Optimized fiber parameters reduce comb noise and improve line stability.
- Demonstrated applicability for astronomical wavelength calibration.

## Abstract

We investigate the generation of optical frequency combs through a cascade of four-wave mixing processes in nonlinear fibres with optimised parameters. The initial optical field consists of two continuous-wave lasers with frequency separation larger than 40 GHz (312.7 pm at 1531 nm). It propagates through three nonlinear fibres. The first fibre serves to pulse shape the initial sinusoidal-square pulse, while a strong pulse compression down to sub-100 fs takes place in the second fibre which is an amplifying erbium-doped fibre. The last stage is a low-dispersion highly nonlinear fibre where the frequency comb bandwidth is increased and the line intensity is equalised. We model this system using the generalised nonlinear Schr\"odinger equation and investigate it in terms of fibre lengths, fibre dispersion, laser frequency separation and input powers with the aim to minimise the frequency comb noise. With the support of the numerical results, a frequency comb is experimentally generated, first in the near infra-red and then it is frequency-doubled into the visible spectral range. Using a MUSE-type spectrograph, we evaluate the comb performance for astronomical wavelength calibration in terms of equidistancy of the comb lines and their stability.