Investagation of OPO cavities for high average power generation in the mid-infrared ; Etude des cavités OPO pour la génération de hautes puissances dans l'infrarouge moyen

Abstract

High repetition rate, high average power, coherent light sources emitting in the mid infrared (mid-IR) spectral region (3-5 μm) are important for many civiland military applications, such as remote sensing, spectroscopy and defense countermeasures. Currently, the most mature technique to deliver such radiation is through frequency down-conversion in Optical Parametric Oscillators (OPOs). Despite many studies of different research groups involved in this field, developing compact and reliable systems delivering tens of watts with good beam quality is still challenging. The thermal effects in nonlinear crystals and optical damage threshold of components involved in such high power systems are the limiting factors.The aim of this thesis is to demonstrate a compact and robust system delivering mid-IR high average power (in the order of tens of watts) with goodbeam quality (M² ≤ 2). The thesis can be divided in two parts. The first part o this work is dedicated to the investigation of different OPO resonators based on nonlinear crystals of Zinc Germanium Phosphide (ZGP). The OPO pump source is a 10 kHz, Ho3+ :LLF MOPA (Master Oscillator Power Amplifier) system, developed at ISL and delivering 68.7 W of total output power at a wavelength of 2.065 μm with a diffraction limited beam quality. The study includes linear resonators, and two different non-planar ring cavities, the so-called RISTRA and FIRE resonators.We explore the advantages and drawbacks of each design and a comparison between them is reported. A maximum output power of 38 W with an M² = 2.2 is demonstrated using a linear resonator, while the beam quality is improved down to an M² = 1.4, in the FIRE resonator, at an output power of 25 W.In the second part, this thesis shows how the performances of these devices can be further enhanced, both in power and output beam quality. At first, reducing the pump spot size allows us to decrease the oscillation threshold and up-scale the average power, obtaining 30 W in RISTRA and reaching one of the limitation of our ...