Cryogenically-cooled neodymium-doped solid-state lasers

Abstract

The central idea of this thesis is to study cryogenically-cooled neodymium (Nd³⁺) doped lasers operating on the ⁴F_3/2 → ⁴I_9/2 transition around 0.95 microns, known as a quasi-four-level (QFL) transition. The QFL transition has unavoidable reabsorption loss at room temperature that introduces additional saturable losses into the laser cavity and must be overcome to achieve efficient operation. In general, this particular transition has lower gain than the dominant four-level ⁴F_3/2 → ⁴I_11/2 transition, around 1 micron. While the advantages of cryogenically cooled gain media have been recognised since the birth of the laser, in more recent times there has been a trend in exploiting these advantages for power-scaling QFL lasers, such as Yb-doped gain media.

The first part of the thesis explores the extensive characterization of the spectroscopic properties of Nd³⁺ doped crystals. The present work tracks the spectroscopic changes over the temperature range from 77K to 450K. A number of crystals (YAG, GSAG, YVO4, GdVO4, KGW, YLF) hosting Nd³⁺ ions have been studied. The absorption cross section spectra for 800nm and 870nm bands were determined with 0.1nm resolution. The absorbance was measured exploiting the Beer-Lambert law and a bespoke set-up using two separate broadband light sources. The fluorescence spectrum was collected and characterized for the transitions to ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 energy-levels from the metastable level, ⁴F_3/2, from which we calculated the stimulated emission cross section of the various crystals and over the temperature range by applying the Füchtbauer-Ladenburg equation, with the measured fluorescence lifetime. Furthermore, in this report we determine the energy transfer upconversion parameters, for the same set of crystals, using the Z-scan technique. The technique measures the change in transmittance of a probe beam tuned to an absorption peak of crystal sample. The measured transmitted power changes as the intensity incident on the crystal is varied via scanning the beam size and correlated to the saturation intensity through a spatially-dependent rate equation model, we found excellent fit between experiment and simulation.

The second part of this thesis reports the development of cryogenic lasers. The design strategies are described by end-pumped and side-pumped systems, with two different crystal geometries reported. Conventional radially-cooled rods are first reported with an end-pumping arrangement, then with a slab (Brewster angled, and afterwards AR coated), and finally a side-pumped Zigzag slab configuration. A rod geometry is tested using a Nd:YAG and a Nd:GSAG crystal for generation of QFL laser emission. The first end-pumped Nd:YAG rod have demonstrated 3.8W at 946nm for 12.8W of pump being absorbed and slope efficiency of 47%. Similar experiment was duplicated with Nd:GSAG rod demonstrating 3.5W at 942nm for 10.5W of pump absorbed. Both suffered significant modal instability during laser oscillation, which afterwards for the Nd:GSAG crystal was found to be due to AR-coating damage. A Nd:YAG slab crystal was tested for the both pumping configurations, using a wavelength-locked 869nm diode bar as a pump source. For this in-band pump-source, the quantum defect is only 8%, in the case of the main QFL transition. A top/bottom-face cooled slab presented effective mitigation of the previously observed modal stability, assumed to be associated with reducing birefringence losses. Despite the coating damage/contamination, which was repeatedly encountered, 946nm emission for both configurations was demonstrated. An end-pumping configuration has demonstrated 5.5W for 13.6W of absorbed pump with 47% slope efficiency. While the side-pumping the zigzag slab produced 6.3W for 30W of absorbed pump with 30% slope efficiency. It is expected that with improvements in the cleanliness within the vacuum chamber used for the cryogenic setup, better results in terms of slope efficiency, output power and beam quality will be realised in the near future.

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