Proc. SPIE, "Atomic and Molecular Pulsed Lasers III", 2000, v. 4071, pp. 128-132. Ti:Sapphire laser pumped with a Cu-vapour laser S.M. Kobtsev, S.V. Kukarin, A.A. Pustovskikh, V.B. Sorokin Novosibirsk State University Pirogova 2, Novosibirsk, 630090, Russia ABSTRACT Developed and studied is a pulsed Ti:Sapphire laser with new active medium parameters. For the first time, short (3–5 mm) Ti 3+ :Al 2 O 3 crystals with relatively high Ti 3+ ion concentrations (0.18–0.22%) have been employed. The developed laser has comparatively high generation power and very simple reliable construction design. The best-achieved differential efficiency of the Ti:Sapphire laser amounted to 30.4%. Considered are the advantages of the proposed laser pumped with a Cu-vapour laser as applied to two-photon photodynamic therapy (PDT). Keywords: Ti:Sapphire laser, Ti 3+ :Al 2 O 3 , Cu-vapour laser. 1. INTRODUCTION Ti:Sapphire lasers are of significant interest to a large area of applications, such as scientific, technological, medical, and other fields. The active medium of the Ti:Sapphire laser (Ti 3+ :Al 2 O 3 crystal) has a broad absorption band, which makes it possible to pump this laser with many types of light sources in the blue-green spectrum region. Cu-vapour laser is a particular example of such sources. It features plug efficiency by an order of magnitude higher than that of the argon laser, average output power reaching 100 W for commercial models. Both of the Cu-laser generation lines (511 and 578 nm) fall into the absorption band of the active crystal (Ti 3+ :Al 2 O 3 ). The output pulse duration of Cu-vapour lasers is typically 10–30 ns at repetition rate in the range of 1–30 kHz (some lasers can generate at up to 200 kHz). Unusually small number of papers are devoted to studies of Cu-laser pumped Ti:Sapphire lasers [1-4]. Probably, this happens because the amount of marketed Cu-vapour lasers is substantially smaller than that of other laser sources (argon and DPSS lasers) suitable for pumping the Ti:Sapphire lasers, and also because the beam quality (transverse intensity distribution, divergence and stability) of Cu-vapour laser is inferior to that of argon and DPSS systems. Another problem to solve when pumping a Ti:Sapphire laser with a Cu-vapour source is coping with a large beam diameter (15–30 mm). To avoid aberrations, such beams must be focused with special optical elements. However, a combination of unstable resonator with a small-aperture discharge channel makes the output of Cu-vapour laser much more suitable source and allows one to use it as an efficient pump for a tunable laser. The first report of pumping a Ti:Sapphire laser with a Cu-vapour source was published in 1 . It featured 1.5 W output power at 15 W pumping. The total efficiency of the pump conversion was 10%. In 2 , different methods of pumping a Ti:Sapphire laser with a Cu-vapour laser were tested, including bi-directional pumping of the Ti 3+ :Al 2 O 3 crystal. Bi-directional pumping is used in cases when pumping through a single surface may lead to its damage due to high power of focused beam. With different experimental configurations of the 4-mirror Ti:Sapphire laser cavity was obtained output powers in the range of 0.76 to 5.5 W, differential efficiency of the pump radiation conversion being 20 to 38.5%. The best result, 38.5% was obtained at unidirectional pumping of the crystal with an 8.45 W beam, the output power of the Ti:Sapphire laser being 2.4 W, and overall efficiency 28.4%. Previous investigations of Ti:Sapphire lasers pumped with Cu-vapour systems were conducted using relatively long (10 mm and more) Ti 3+ :Al 2 O 3 crystals that had a comparatively low concentration of Ti 3+ ions (0.03–0.15%) 1-4 . Progress in Ti 3+ :Al 2 O 3 crystal growth made over last few years allowed one to employ samples with a higher ion concentration (0.20–0.41%) 5 . Application of such crystals as active medium of Ti:Sapphire lasers pumped with Cu-vapour lasers up to now have not yet been explored. This paper was aimed at these applications. 2. EXPERIMENT A schematic diagram of the laser system is shown in Fig. 1. The cavity of the Cu-vapour laser had a telescopic configuration. Its magnification (ratio of the curvature radii of the spherical mirrors M 1 and M 2 ) varied over the 20–60