Diode pumped Er:YAG single crystal fiber laser passively Q-switched with Cr:ZnSe saturable absorber emitting at 1645 nm or 1617 nm

Summary form only given. Direct resonant pumping of Er:YAG is a laser configuration studied for compact eye-safe emitters which applications require kilometer range propagation in the atmosphere, like active imaging, Lidar and wind mapping. The laser emission of these cavities naturally occurs at 1645 nm. But a methane absorption line exists at this wavelength. One way to increase the range of the emitter is to use the 1617 nm emission line which is free of absorption. Recent researches gave nice results in active Q-switched regime at 1645 nm [1] and at 1617 nm [2]. But some applications require a compact, efficient and simple setup with mJ level pulse energy. Our recent work focused on passively Q-switching Er:YAG cavity with direct resonant fiber-coupled diode pumping.A 750 μm diameter 30 mm long Er:YAG single crystal fiber with a doping concentration of 0.5 % is inserted in a Taranis module from Fibercryst for efficient cooling and used as gain medium (Fig. 1 left). It was anti-reflection coated on both ends and actively cooled at 12°C. The pump light is provided by a fiber-coupled laser diode with a 400 μm core diameter and a numerical aperture of 0.22, delivering up to 40 W at 1532 nm. Its spectrum is narrowed by an internal grating, down to 1 nm approximately. The beam is collimated by a 40 mm focal length doublet and then focused a few millimeter inside the Er:YAG crystal thanks to another 40 mm focal length doublet. With this setup, we estimate that the pump beam undergoes between 2 and 3 reflections in the single crystal fiber, allowing a higher population inversion and a better spatial overlap between the pump and the laser signal than in standard rods. This higher population inversion along the gain medium favors the emission of the 1617 nm line. In passively Q-switched mode, a Cr:ZnSe saturable absorber with initial transmission of 85 % led to 330 μJ pulse energy, 61 ns pulse duration at a repetition rate of 1460 Hz. A 80 % initial transmission Cr:ZnSe sample led to 510 μJ energy pulses, 41 ns pulse duration at a repetition rate of 820 Hz. From 34 W to 40 W of pump power, no variation of pulse durations has been observed. The sudden wavelength shift from 1645 nm to 1617 nm has already been observed in [4], and comes from the high losses introduced inside the cavity. In addition, the slightly higher Cr:ZnSe absorption at 1645 nm compared to 1617 nm contributes to the wavelength selection. This wavelength shift happens whatever the incident pump power. A numerical model of this cavity has been developped, including pump propagation inside the crystal with ray tracing, temporal evolution of the population inversion, active and passive Q-switch regime, and thermal effects, based on previous work for actively Q-switched four-levels lasers [4]. Comparison of experimental results with numerical simulations will be presented. This is the first reported passively Q-switched diode-pumped Er:YAG laser operating on the 4I13/2 → 4I15/2 transition.