Development of a laser transmitter for space applications is a highly challenging task. The laser must be rugged, reliable, lightweight, compact and energy efficient. Most of these features are inherently achieved by diode pumping of solid state lasers. Overall system reliability can further be improved by appropriate optical design of the laser resonator besides selection of suitable electro-optical and opto-mechanical components. This paper presents the design details and the theoretically estimated performance of a crossed-porro prism based, folded Z-shaped laser resonator. A symmetrically pumped Nd: YAG laser rod of 3 mm diameter and 60 mm length is placed in the gain arm with total input peak power of 1800 W from laser diode arrays. Electro-optical Q-switching is achieved through a combination of a polarizer, a fractional waveplate and LiNbO<sub>3</sub> Q-switch crystal (9 x 9 x 25 mm) placed in the feedback arm. Polarization coupled output is obtained by optimizing azimuth angle of quarter wave plate placed in the gain arm. Theoretical estimation of laser output energy and pulse width has been carried out by varying input power levels and resonator length to analyse the performance tolerances. The designed system is capable of meeting the objective of generating laser pulses of 10 ns duration and 30 mJ energy @ 10 Hz.
Laser Science and Technology Center (LASTEC), Delhi, is developing a space qualified diode pumped
Nd: YAG laser transmitter capable of generating 10 ns pulses of 30 mJ energy @ 10 pps. This paper presents the results of experiments for comparative studies between electro-optic and passively Q-switched Nd: YAG laser in a crossed porro prism based laser resonator. Experimental studies have been performed by developing an economical bench model of flash lamp pumped Nd: YAG laser (rod dimension, &nullset; 3 X 50 mm). Electro-optic (EO) and Passive Q-switching were performed employing LiNbO<sub>3</sub> crystal (9 x 9 x 25 mm) and Cr: YAG (7 &nullset; 10 mm) saturable absorber respectively. Laser output of 30 mJ was achieved in EO Q-switching mode by optimizing Pockels cell operation. More than 80 % Q-switching efficiency was achieved. However, at the same input level in passive Q-switching mode at optimized initial transmission of Cr: YAG, only 36% efficiency could be achieved. Comparative studies were made for output pulse energy at different input levels. In passive Q-switched mode, deviation from the optimum flash lamp input either stops the lasing action or leads to multiple pulsing. Thus in view of the very stringent requirements of reliability and efficiency of space-based system, the electro-optical method of Q-switching has been adopted in the design.