888 nm pumping of Nd:YVO4 for high-power TEM00 lasers

For the last decade, neodymium-doped orthovanadate has established itself as the activematerial of choice for commercial solid-state lasers emitting in the 1 µm range, with output powersfrom several hundred milliwatts to a few tens of watts, in continuous-wave, short nanosecondQ-switched, or picosecond modelocked pulsed regimes. Its main advantages over other Nd-dopedhosts such as YAG are a large stimulated-emission cross section leading to a high gain, a strongpump absorption allowing the efficient mode-matching of tightly-focused pump light, and a naturalbirefringence resulting in a continuously polarized output. The main drawbacks, however,are rather poor mechanical characteristics and strong thermal lensing, effectively limiting themaximum applicable pump power before excessively strong and aberrated thermal lensing preventsan efficient operation in a diffraction-limited beam, and ultimately the crystal's fracture.Put aside the power limitation, the association of vanadate with diode end pumping allows forthe realization of highly efficient and reliable laser sources based on well-known technologies,which provides an advantage in terms of manufacturability and cost-effectiveness over otherhigh-potential technologies such as disks and fibers.This thesis introduces a novel pumping technique for Nd:YVO4 that allows for the realizationof significantly higher-power laser sources with a high optical-to-optical efficiency anddiffraction-limited beam quality, while keeping the benefits of a well-established technology. Itconsists in pumping at a wavelength of 888 nm instead of the classic 808 nm, providing a lowand isotropic absorption, which results in a smooth distribution of the absorbed pump light inlong crystals, effectively limiting the deleterious effects of high inversion density such as crystalend-facet bulging, high crystal temperature, aberrated thermal lensing, and upconversion. Afterpresenting vanadate's spectroscopic and physical characteristics, a complete analysis of the heatgeneratingeffects is performed, allowing for side-by-side simulations of the thermal effects inpractical 808 nm and 888 nm pumped systems, and for an evaluation of their respective thermallensing behaviors.Continuous-wave operation was thoroughly investigated, first in a multi-transversal modeoscillator to assess the maximum optical efficiency with optimum pump-mode matching and thethermal lensing characteristics. A TEM00 resonator was then developed with a single crystaland one pump diode, providing 60 W of output power with an optical efficiency of 55% anda beam quality of M2 = 1.05. This resonator was symmetrically replicated to form a periodicresonator, providing 120 W of output with the same optical efficiency and beam quality. Thistwo-crystal configuration was then modified to an oscillator-amplifier configuration, providing asingle-pass extraction efficiency of 53% and a total oscillator-amplifier output of 117 W withoutany beam-quality degradation. Intracavity doubling of the one and two-crystal configurationswas achieved by inserting a non-critically phase-matched LiB5O3 (LBO) non-linear crystal inthe resonator, providing up to 62 W of diffraction-limited green light at 532 nm with low-noisecharacteristics thanks to a large number of oscillating modes, thus limiting the effects of the"green problem¿.A strong industrial interest resides in Q-switched lasers emitting nanosecond pulses, particularlywith a high average power, high pulse repetition rate, and pulse durations of a fewto several tens of nanoseconds. Achieving high-frequency and short-pulse operation both requirea high gain, which explains the domination of Nd:YVO4 over lower-gain materials such asNd:YAG or Yb:YAG. Thus, an acousto-optically Q-switched oscillator was demonstrated with50 W output power and 28 ns pulse duration at 50 kHz. Pulse duration, however, is inverselyproportional to the pulse energy, so that an increase in repetition rate inevitably results in analmost linear increase in pulse width. A cavity-dumped Q-switched oscillator was built to circumventthis limitation, the pulse length being defined by the cavity roundtrip time and theelectro-optic cell switching time. It provided a constant pulse duration of 6 ns up to a repetitionrate of 100 kHz and a maximum output power of 47 W. Such short pulse durations are normallyavailable with output powers of a few watts from Q-switched lasers, and conversely Q-switchedlasers of similarly high output power deliver pulses of several tens to over 100 ns in duration.There exists another strong interest in high average power quasi-cw picosecond sources,which allow for the efficient generation of green and UV radiation, or even red-green-blue forlaser video projection. Passive mode locking with a semiconductor saturable absorber mirror(SESAM) is the preferred technique employed for the stable and self-starting generation ofpicosecond pulse trains, yet a high gain is necessary for achieving high repetition rates whileavoiding the Q-switched mode-locking regime. Thus SESAM mode locking was applied to an888 nm pumped oscillator, achieving 57 W of output power at a repetition-rate of 110 MHzand a pulse duration of 33 ps. Its output was efficiently amplified in a single pass up to 111 Wwithout any beam quality, temporal, or spectral degradation. The high peak power of 30 kWallowed for the generation of 87 W of second harmonic at 532 nm with an efficiency of 80%, and35 W of 355 nm third harmonic with a conversion efficiency of 33% in LBO crystals.The wide range of high-power systems demonstrated in this work illustrate the benefits ofthe optimized pumping of Nd:YVO4 at 888 nm, maintaining its highly-desirable characteristicssuch as a high gain and a polarized output while extending its power capabilities far beyondregular 808 nm pumped systems. This improvement should allow Nd:YVO4 systems to competewith high-power technologies such as disks and fibers, which often struggle in the generation ofshort pulses because of their low gain and strong non-linear effects, respectively.

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