Title: "Modal instabilities in large mode area fiber amplifiers and lasers"
Principal supervisor, Associate Professor Jesper Lægsgaard, DTU Fotonik
Co-supervisor, PhD Thomas Tanggaard Alkeskjold, NKT Photonics
Professor, Ole Bang, DTU Fotonik (Chairman)
Dr. César Jáuregui Misas, Abbe Center of Photonics, Spain
Associate Professor, Liang Dong, Clemson University, USA
Master of the Ceremony:
Professor, Karsten Rottwitt, DTU Fotonik
Ultra-fast high power large mode area (LMA) rod fiber amplifiers may suffer from transverse modal instabilities (TMIs), that show up as rapid beam fluctuations above an average power threshold. They severely degrade the beam quality, and currently set the upper limit for power scaling. This thesis concerns both numerical and experimental investigations of TMIs. A semi-analytic model estimating the TMI threshold from a set of coupled mode equations is expanded to include the actual mode distributions of a specific fiber design. The thermal effects alter the waveguiding properties and the modal distributions with increasing average output power, and are included in the numerical model. The onset of TMI is estimated as a function of signal power for different fiber amplifier designs, showing that suppression of higher order modes increases the TMI threshold. Especially aeroGAIN-ROD-PM85 (DMF85) fiber is considered in detail. TMIs are estimated to set in at signal output power levels of 300 W - 345 W, corresponding to a heat load at the fiber output end of 58 W/m – 68 W/m. Two LMA rod fiber amplifiers are tested experimentally in a counter pumped high power amplifier setup. The TMI threshold is recorded by considering the pinholed signal with a photodetector. The TMI threshold is recorded and repeating the measurement decreases the threshold until a saturation level is reached. The initial TMI threshold is recovered by thermal annealing, indicating long term processes related to photodarkening. The TMI threshold is measured for the DMF85 rod fiber to be higher than 360 W of signal power limited by available pump power. Repeating the measurements decrease the TMI threshold, that saturates at roughly 240 W of signal power.