# Passive alignment stability and auto-alignment of multipass amplifiers based on Fourier transforms

## TL;DR

This paper investigates the stability of Fourier-based multipass amplifiers against misalignments, proposes an auto-alignment system to enhance tilt stability, and demonstrates its effectiveness for high-power laser applications.

## Contribution

It introduces a method to quantify tilt sensitivity in Fourier-based amplifiers and presents a simple auto-alignment system that significantly improves stability.

## Key findings

- Tilt stability improved by over an order of magnitude.
- Method for sensitivity quantification based on small signal gain.
- Validated auto-alignment system through experimental testing.

## Abstract

The stability properties of Fourier-based multipass amplifier to misalignments (tilts) of its optical components has been investigated. For this purpose, a method to quantify the sensitivity to tilts based on the amplifier small signal gain has been elaborated and compared with measurements. To improve on the tilt stability by more than an order of magnitude a simple auto-alignment system has been proposed and tested. This study, combined with other investigations devoted to the stability of the output beam to variations of aperture and thermal lens effects of the active medium, qualifies the Fourier-based amplifier for the high-energy and the high-power sector.