Self-adaptive, passively Q-switched, diode-side-pumped Nd:YAG slab laser

Abstract

ABSTRACT Passively Q -switched, closed -loop, self-adaptive resonator with a Nd:YAG as an active medium is presented . For maximal pump energy of 840 mJ Q -switched generation provided 5 pulse series with total energy of 120 mJ. Single pulse width was 24 ns. The beam quality parameter M 2 was 1.6. Four -wave mixing and linear resonators were compared. Keywords: solid-state, neodymium, Q -switched, self-adaptive, slab laser 1. INTRODUCTION Coherent sources operating at ~1 µm wavelength, including bulk [ e.g. 1-4] and fiber-based [e.g. 5-11] laser systems, have gained considerable attention in many industrial and scientific applications such as material processing, remote sensing, range findin g, defense. Furthermore, the past two decades have seen significant increases in the output pulse energy and peak power limited only by the active material damage or available pump power. An important objective has also been the design of optical cavity and pump -lase scheme providing suitable optical parameters of the laser output. Efficient extraction of a near -diffraction-limited beam from a gain medium also seems to be the one of the most important and challenging tasks of laser technology . The phase -conjugate mirror (PCM) [ 12,13] concept seems to be one of the most elegant and efficient approaches to realize such a task. The four -wave mixing process as the physical basis of PCM has been realized in several ways in solid -state lasers for the last 30 yea rs (see e.g., [14Ð24]). Some renaissance of this subject was observed with the advent of efficient high -power laser -diode bars and two -dimensional (2D) stacks in the 1990s. Nowadays, commercially available 2D laser -diode stacks can deliver several kilowatt s of pump radiation that can basically be confined to the area of a few square millimeters. The gain element, even if it is made of a low -quantum -defect medium [e.g., Yb:YAG], suffers from high heat -source density, resulting in a complex, thermal -optical distortion. Thermal -optical effects and strong transverse inhomogeneities of inversion result in severe worsening of beam quality. In case of cw side -pumping of slab -gain media, the effect of severe vertical temperature gradients could be basically compensa ted for by means of additional cylindrical lenses. However, the horizontal asymmetrical profile of inversion, which in a case of quasi -cw pumping has transient behavior, requires a different approach. It can be compensated for by adaptive optics closed -loop systems or exploiting the inherent physical effect of PCM. The aim of this work was to find the most efficient method of extraction of a near -diffraction-limited output beam from a side-pumped Nd:YAG slab laser. Low -duty cycle quasi -cw 2D stacks of lase r diodes were used as a pump source. In this case the phase distortions are dominated by a transient asymmetrical inversion distribution. The stationary temperature gradients limit the available repetition rate. The most promising as well as challenging ap proach to beam cleaning seems to be the use of a Oself -adaptiveO cavity, in which the PCM inside the gain medium is exploited. Two groups of self -adaptive resonators were studied in the past: nonreciprocal ones [ 17Ð21], in which one direction of propagation is enforced due to additional nonreciprocal elements, and reciprocal resonators [ 22,23], in which the two beams inside a cavity travel in counter -propagating directions. The most advanced results of the former group were demonstrated in [21], in which a 100 mJ diffraction -limited, single frequency beam in the self -started, self-Q-switched, diode -side-pumped Nd:YAG laser was demonstrated. In case of a reciprocal cavity, open -loop resonators were used, as a rule. In the open-loop case, the cavity has no closed -loop feedback and the laser beam is generated from the noise, and the thresholds of operation are therefore relatively high. Moreover, the demonstrated efficiencies of open -loop lasers are not