Theses 2020

Bauer, Carolin

Summary: The main motivation behind the present work roots in bringing into operation an NPS experimental set-up intended to be used to characterize the full electric field of few-cycle laser pulses generated by soliton-e_ect self compression in an ARR HC-PCF. Short pulses in general are among many other possible applications important to the field of ultrafast optical science as their employment as temporal gates in any kind of optical measurements sets the temporal resolution. In our case, the implemented experimental set-up might for instance open the door for studying pulse propagation dynamics in PCFs evolving on a sub-fs timescale. Hereafter, I will quickly summarize the work which has been done, so far.

A Ti:Sapphire laser system served as our laser source emitting 25 fs long pulses at a repetition rate of 1 kHz at a central wavelength of 800 nm. With respect to the generation of the ultrashort pulses, I set up a compression stage with a ARR HC-PCF in a gas filled cell. One of the main tasks towards proper pulse compression was finding a suitable fibre which made us end up with a 12-2 capillary single-ring HC-PCF. It was the most suitable fibre among those in stock for e_cient temporal compression of pulses with about 10 _J at the fibre input.

This brings us to the other main task which was related to finding the pulse compression parameters such that we get short enough pulses to have high enough field strength at the dielectric microjunction without damaging the fibre. The field strength at the dielectric must be high enough to pave the way for carriers to be injected and a current to be measured. In the end, wewere enabled to generate a spectrumbroadband enough to theoretically support pulse durations in the order of a few fs. We justify this claim by the Fourier limited pulse duration we deduced from a spectroscopic measurement as well as a d-scan measurement. As already slight dispersion changes in the order of a few fs2 strongly influenced the spectral power of the generated SH, we assume that the dispersion change equivalently had to have had influenced the pulse duration which can be the case only for very short pulses. Moreover, also simulations ran based on the UPPE confirmed that such short pulses can be generated with the applied compression parameters.

As we learned, few-cycle pulses require the consideration of the CEP. To that end, I have implemented an f-to-2f interferometer in direct proximity to the compression 63 stage and the NPS set-up. The evaluation of the detected spectrogram was accomplished with the help of a self-written algorithm based on the FTSI algorithm. In view of our accomplished CEP stability we suspect it stable enough to not wash out any

subsequent current measurement with the NPS set-up.

In the course of my master project, I have continued to advance our experimental NPS set-up. Thereby, I also tried out di_erent methods to position the dielectric microjunction relative to the focal spot size. To date, the NPS set-up itself is completely implemented and several test measurements have been run indicating that the electric circuit of the sample and all the electronics are well working. Yet, I have to cease the work on our version of the NPS set-up without succeeding in detecting an opticallyinduced current or in characterizing laser pulses. Nevertheless, we have been able to figure out some of its issues including the proposition of potential solutions.

Mardani Samani, Masoud

Abstract: In this thesis a passively modelocked fibre laser oscillator, operating at near zero net dispersion and passive self-similar regimes, is developed and characterized. The oscillator operates at repetition rate of 40 MHz and delivers pulses with 2.5 nJ energy when operating in the self-similar modelocking regime or 0.33 nJ energy when the net group velocity dispersion is set close to zero. Nonlinear Polarization Rotation (NPR) through 4.4 meters of passive and Yb3+doped fibre is used as a fast artificial saturable absorber while dispersion management and polarization manipulation of the pulse takes place in the free space part of the cavity. The Relative Intensity Noise (RIN) and the phase noise of the oscillator in both operating regimes are measured and presented in this thesis. In order to understand pulse shaping mechanisms and visualize the temporal and spectral dynamics of pulse evolution in dispersion-managed NPR based modelocked fibre lasers, scalar and vectorial numerical simulations are performed for the average soliton, zero net dispersion and passive self-similar regimes.