Nd:GdVO4 Raman Laser Generates More than 1 W in Eye-Safe Region

Breck Hitz

Eye-safe lasers, whose output is absorbed in the cornea rather than focused onto the retina, are important in remote sensing and telemetry, as well as in military applications such as rangefinding and target designating. A scientist at National Chiao Tung University in Hsinchu, Taiwan, has developed an Nd:GdVO₄ laser that utilizes stimulated Raman scattering in the laser crystal to shift the output from 1.34 µm to the eye-safe wavelength of 1.52 µm. The laser produces more than 1 W and has advantages in terms of efficiency and compactness over other eye-safe lasers.

Figure 1. Stimulated Raman scattering within the laser crystal converts the intracavity 1.34-µm laser radiation to 1.52 µm. (top) Both mirrors are highly reflective at the laser wavelength, but the output mirror is partially transmissive at the Raman output wavelength. (bottom) The laser's output contains frequency components at both the laser and the Raman wavelengths, but the latter is much stronger.

A fiber-coupled 808-nm laser diode provides up to 25 W of pump power to the Nd:GdVO₄ laser (Figure 1a). A 9-mm-long Nd:GdVO₄ crystal with 0.15 percent atomic doping produced the best experimental results. The back mirror is highly transmissive at the 808-nm pump wavelength and highly reflective at both the lasing wavelength (1.34 µm) and the Raman-shifted wavelength (1.52 µm), while the output mirror is highly reflective at the laser wavelength and 67 percent reflective at the Raman wavelength. Both mirrors are transmissive at 1.06 µm to keep the strong Nd line at that wavelength below threshold. An acousto-optic modulator Q-switches the laser and generates pulses in the 5- to 10-ns range.

The output spectrum of the laser shows that the output is Stokes-shifted by 882 cm²¹ from the laser line at 1.34 µm (Figure 1b). The output beam from the laser has an M² value of less than 2.

The efficiency of the eye-safe laser decreases with increasing pump power at low Q-switching frequencies more than it does at higher frequencies (Figure 2). The reason for this falloff is not clear, but the scientist believes it may be due to a thermal lensing effect. He notes that other workers have shown that thermal effects can broaden the linewidth and decrease the gain of stimulated Raman scattering.

Figure 2. The cause of the output-power saturation at lower Q-switching rates is not well-understood, but it may be due to a thermal lensing effect.

The greatest average output power from the Nd:GdVO₄, 1.2 W, was obtained at the highest repetition frequency, 20 kHz, and the highest peak power at that frequency was ~7.5 kW. At 10 kHz, the average power was only 1.0 W, but the peak power was 20 kW.

This compares favorably with the two older approaches to eye-safe lasers. Erbium-doped lasers lase directly in the eye-safe region at approximately 1.5 µm, but their optical efficiency (from the diode pump to the output) of roughly 2.5 percent is inferior to the 8.7 percent of the Nd:GdVO₄ laser. Optical parametric oscillators pumped with the 1-µm output of diode-pumped solid-state lasers are capable of an overall optical efficiency similar to that of the Nd:GdVO₄ Raman laser, but these devices are more bulky and complex than the straightforward Nd:GdVO₄laser.