Read the thesis in its original form here.

• Written by Marcel Köberlin in the Ultrafast Laser Physics Group ETH Zürich.
• Supervision: Moritz Seidel, Dr. Christopher Phillips
• Supervising Professor: Prof. Dr. Ursula Keller
• Date: October 4, 2024

Table of Contents

This thesis presents the design, construction, and characterisation of a modelocked thin disk laser (TDL) with an average output power of $60$ W , a pulse duration of $1030$ fs , and a repetition rate of $20$ MHz . Mode-locking is achieved using a sapphire-bonded SESAM, and beam stabilisation is implemented with a two-actuator, two-detector system, reducing beam position fluctuations significantly. The stabilised laser is input to a hollow-core photonic crystal fibre (HCPCF). This results in a fibre coupling efficiency of $82\mathrm{\%}$ at $36$ W input power.

Simulations of spectral broadening in the fibre are conducted, incorporating fluctuations in average output power and pressure-dependent nonlinear refractive indices, predicting a noise gain peak of $35$ dB and a spectral broadening of $25$ nm FWHM. A grating spectrometer is also designed and built to achieve a wavelength resolution of up to half a nanometre. Two silicon photodiodes are characterised, and a comprehensive noise characterisation is performed, measuring shot noise at $-162\mathrm{dBc}/\mathrm{Hz}$, limited by the photodiode.

Due to a gas leak in the pressure system, final noise measurements could not be completed, but steps for future work are clearly outlined. After repairs and final measurements are conducted, the project sets the stage for further exploration, potentially achieving a record-low shot noise level of lower than $-190$ dBc/Hz, corresponding to measuring $4$ W of optical power.

Table of contents

1 Introduction
2 Theory
3 Setup, Characterisation and Results
4 Conclusion and Outlook
5 Appendix