Novel Pulsed Laser Sourses - Dr Mifta Ganija

High-energy pulsed lasers operating at around 2 µm are of interest for a variety of applications: LIDAR, materials processing, medical and for non-linear conversion into the mid-IR using OPOs. The most promising approaches to nonlinear conversion use zinc germanium phosphide (ZGP) and cadmium silicon phosphide (CSP) but, for efficient conversion and low risk of optical damage, the pump source must have a wavelength > 2.05 μm and excellent beam quality.

Holmium-doped gain media are commonly used to achieve lasing in this wavelength regime. However, materials such as Ho:YLF and Ho:YAG are quasi-3-level gain media at room temperature and thus require high pump intensities to reach transparency, resulting in large internal thermal stress and wavefront distortion. Additionally, it prevents power scaling by increasing the mode size.

Cryogenic cooling of these gain media converts them to a 4-level system, dramatically reducing the lasing threshold and the associated parasitic heating, and improves the thermal conductivity, and thermal expansion and thermo-optics coefficients. Large mode area, high power Ho lasers with excellent beam quality should thus be possible.

The benefits of cryogenic cooling were demonstrated in the early 1980s. However, the lack of narrow-band pump sources at 1.9 μm necessitated the use of flash-lamp pumping and co-doped gain media, resulting in poor slope efficiencies and highly multimode laser outputs.

Subsequently, alternative pumping schemes were tried, including laser diodes, Tm:YLF and Tm:fiber lasers. While this has resulted in significant improvements, generally the cryogenic Ho lasers have limited average power and poor beam quality, due primarily to the narrow absorption linewidth of cryogenic Ho gain media and the mechanical mounting of the gain medium.

I shall discuss our high-energy pulsed laser projects and outline our ultra-short/short pulsed laser program, including new mode-locked 2um sources.