Search Menu
Photonics Media Photonics Buyers' Guide Photonics Spectra BioPhotonics EuroPhotonics Vision Spectra Photonics Showcase Photonics ProdSpec Photonics Handbook
More News

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

Facebook Twitter LinkedIn Email Comments
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.

Nd:GdVO<SUB>4</SUB> Raman Laser Generates More than 1 W in Eye-Safe Region
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:GdVO4 laser.

Photonics Spectra
Jan 2005
The transparent front layer of the eye. Light entering the eye is refracted (converged) by the outer surface of the cornea.
remote sensing
Technique that utilizes electromagnetic energy to detect and quantify information about an object that is not in contact with the sensing apparatus.
1. The photosensitive membrane on the inside of the human eye. 2. A scanning mechanism in optical character generation.
The science of sensing and measuring information at some remote location and transmitting the data to a convenient location for reading or recording.
Basic SciencecorneadefenseDPSS lasersEye-safe lasersremote sensingResearch & Technologyretinatelemetrylasers

back to top
Facebook Twitter Instagram LinkedIn YouTube RSS
©2019 Photonics Media, 100 West St., Pittsfield, MA, 01201 USA,

Photonics Media, Laurin Publishing
x Subscribe to Photonics Spectra magazine - FREE!
We use cookies to improve user experience and analyze our website traffic as stated in our Privacy Policy. By using this website, you agree to the use of cookies unless you have disabled them.