Compact Nd:YAG Laser Developed for Airborne, Space Applications

Breck Hitz

Compact, robust, efficient nanosecond sources are essential in many emerging airborne and spacecraft applications, including lidar, altimetry and remote spectroscopy, where high voltages and bulky heat exchangers are not practical. Recently, a collaboration of scientists at EADS Deutschland GmbH in Munich and at Dilas Diodenlaser GmbH in Mainz-Hechtsheim, both in Germany, demonstrated an Nd:YAG, fiber-coupled oscillator-amplifier capable of producing stable, ∼5-ns, 68-mJ pulses at a 20-Hz repetition rate.

The system comprised a passively Q-switched oscillator, a quadruple-pass amplifier and a final, double-pass power amplifier (Figure 1). It was pumped by two remote diode laser modules, which were coupled to the laser head with optical fibers. There were two eight-bar stacked arrays of AlGaAs lasers in each module, producing a quasi-continuous output of 1600 W at 807 nm. Thermal considerations limited the quasi-continuous duty cycle to 2 percent. A prism array in each module reshaped the asymmetric beam into a near-circular one that could be coupled into the 0.22-NA fiber. About two-thirds — or ∼1 kW — of the diode power from each module was transmitted through the corresponding fiber to the laser head.

Figure 1. Two diode laser pump modules powered the oscillator-amplifier system. The output of one module was divided between the oscillator and the first amplifier, and the output of the second went to the final power amplifier. Images ©OSA.

Eighty percent of the incoming pump power on one fiber was diverted to the first amplifier, while the remaining ~200 W pumped the oscillator. A diaphragm between the fiber and the oscillator provided a mechanism to independently adjust the pump power to the oscillator, thereby fine-tuning the timing of the passively Q-switched pulse relative to the population inversion in the amplifier stages. A small crystal of Cr⁴⁺:YAG, whose small-signal transmission was 20 percent, provided the saturable absorption necessary to Q-switch the oscillator.

Four carefully aligned mirrors reflected the pulses from the oscillator back and forth for two double passes through the 6-mm-diameter, 20-mm-long Nd:YAG rod in the first amplifier stage. Each double pass followed a V-shaped path through the rod, and the two V’s were in orthogonal planes so that there was minimal spatial overlap between passes. To suppress parasitic lasing, the researchers wedged antireflection-coated end faces of the amplifier rods about 1° from normal.

They initially operated the system with only the first amplifier and obtained as much as 43.5 mJ, with an energy stability of ±3 percent measured over 20,000 pulses. The beam quality from the amplifier matched that of the oscillator, with an M² of approximately 1.5.

Figure 2. The output energy of the final amplifier, at constant input from the first amplifier, was linear with pump energy. The straight line is a theoretical prediction, and the highest three data points were obtained after realigning the system.

A final power amplifier stage, providing a double pass through another 6-mm-diameter, 20-mm-long Nd:YAG rod, boosted the output energy to as much as 68 mJ (Figure 2). The beam quality was not diminished from that of the first amplifier. The scientists calculated the overall electrical efficiency of the passively cooled system at ~6 percent.

Optics Letters, July 1, 2006, pp. 1991-1993.