Q-Switched CO2 Laser Intended for Heterodyne-Detection Lidar

Breck Hitz

Waveguide CO₂ lasers excited by radio-frequency discharges have found broad application as the sources for laser radar systems. In particular, pulsed heterodyne-detection lidar requires a pair of frequency-offset, pulsed lasers. Q-switched, waveguide CO₂ lasers are good candidates for this application, but thermal effects in the Q-switch have been a major hindrance. Now a group of scientists at Sichuan University and at Southwest Institute of Technical Physics, both in Chengdu, China, has shown that careful temperature control of the Q-switch and associated intracavity optics is the key to stable laser operation.

Figure 1. The CdTe Q-switch enabled the laser to operate in a normal Q-switched mode, or in a cavity-dumped mode. In the cavity-dumped mode, the output beam reflected off the Brewster plate.

The scientists studied a grating-narrowed CO₂ laser with an intracavity CdTe, electro-optic Q-switch (see figure). The Q-switch acted as a loss modulator by rotating the polarization of the intracavity circulating power so that it was reflected out of the resonator by the Brewster plate. The λ/4 plate between the Q-switch and the output mirror allowed the Q-switch to operate with its λ/4 voltage (2.65 kV) off most of the time; the high voltage was applied to the crystal only for the several microseconds that its loss was minimized. The lifetime of the CdTe is extended by minimizing the time it is exposed to the high voltage.

The laser could operate either in a cavity-dumped or in a normal Q-switched mode. In the cavity-dumped mode, circulating power built up in the resonator while the Q-switch was in its low-loss state, and was reflected out of the resonator by the Brewster plate when the Q-switch was switched to the high-loss state.

In the laser's normal Q-switched mode, the CdTe crystal functioned like any other Q-switch: It blocked lasing as energy accumulated in the population inversion, then switched to the low-loss state to allow the laser to emit all of the accumulated energy in a single, giant pulse. The group observed that, if the low-loss state lasted less than a microsecond, there was insufficient time for the Q-switched pulse to build up from spontaneous emission, and the laser produced little output. On the other hand, if the low-loss condition lasted more than 5 or 6 µs, the population inversion that had been depleted by the Q-switched pulse reached threshold again, and a second pulse was emitted. Under optimum conditions -- the Q-switch in its low-loss state for 2 or 3 µs -- stable Q-switched pulses of about 150 ns and 360-W peak power were obtained.

However, steady-state operation at 42 kHz could not be obtained without cooling the CdTe crystal. Instead, the scientists saw oscillatory behavior in the laser's output power. As the CdTe crystal absorbed power from the intracavity beam, it introduced thermal lensing that distorted the resonator mode. As the mode distorted, the laser's output power -- and circulating power -- diminished. As the circulating power dropped, the thermal lensing decreased, reducing the distortion and allowing the circulating power to build up again.

This cycle repeated itself within an approximate 12-minute period. It completely disappeared, however, when the scientists provided adequate cooling to the CdTe crystal. When the Q-switch was actively cooled, the laser operated without observable thermal cycling.