High-brightness diode-pumped waveguide lasers
University of Southampton, Department of Electronics and Computer Science,
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Reported in this thesis are advances toward high-brightness diode-pumped planar waveguide (PW) lasers. Efficient and compact planar waveguide lasers are made possible by their geometry, which is compatible with that of high-power diode lasers, has very good thermal management characteristics, and delivers high optical gains per unit pump power. Thus using the waveguide structure in conjunction with trivalent rare-earth ions, multi-Watt diffraction-limited operation of weak and quasi-three-level laser transitions can be obtained.
Large mode area (LMA) double-clad planar waveguides, fabricated via direct bonding sapphire and YAG, are the primary structures investigated herein. These high numerical aperture waveguides are ideally suited to high-power diode-pumping due to a combination of features related to their slab-like configuration. Furthermore the LMA double-clad planar waveguide is shown to robustly select the fundamental waveguide mode. This general result leads to guided-diffraction-limited output, applicable to a range of oscillating wavelengths.
Two in-plane pumping geometries are detailed; longitudinal or end-pumping and transverse or side-pumping. For end-pumping, the pump is conditioned in one axis to enable launching into the waveguide, and at the same time the free space axis is matched to the fundamental mode of a simple monolithic laser cavity. As such, efficient end-pumped operation of Nd:YAG, Yb:YAG, and Er:YAG LMA-PW lasers were realised. With Nd:YAG for 7.5W of absorbed pump power, 4.3W of CW 1.064µm output was observed. Changing the resonator mirrors to optimise the quasi-three level transition an output of 3.5W at 946nm was measured, corresponding to 6.5W of absorbed pump. In addition the much weaker and rarely studied 1.8µm transition, operated with as much as 0.4W. In a similar configuration, 0.45W of 2.7µm CW output was obtained from a highly doped Er:YAG LMA-PW with 4.6W of absorbed pump power. As the first Er:YAG waveguide laser to be reported, it illustrates the possibility to obtain high output power and good beam quality from weak laser transitions with these structures.
Furthermore by integrating a Cr4+ saturable absorber into a Yb:YAG LMA-PW, an end-pumped passively Q-switched laser was demonstrated with 2.3W average power at 1.03µm in a near-diffraction-limited beam. Pulses of ~2ns temporal width at repetition rates approaching 80kHz produced peak powers of ~20kW. Operation of a similar Nd:YAG LMA PW laser, on both the 1.064µm and 946nm transitions, was also demonstrated and its performance shown to be inferior to that of the Yb3+doped structure in this pumping configuration.
Using the side-pumping scheme higher-powers can be obtained, typically however, at the expense of the resonator complexity when trying to obtain high brightness performance. As such a monolithic Tm:YAG LMA-PW laser, pumped by two proximity-coupled diode bars, produced 15W at 2µm with an asymmetric low brightness beam, despite being diffraction-limited in the guided axis. In addition two separate Nd:YAG LMA-PW lasers were trialled with unstable and external resonator geometries. Watt level output powers with significantly enhanced brightness were observed. Further designs for power scaling these devices are discussed.
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