Hybrid Laser Expands Imaging Range

Laurel M. Sheppard

There is potential for a laser that simultaneously produces output at multiple wavelengths in remote sensing, environmental testing and biomedical imaging. Such a device would be a boon for the military as well; applications such as surveillance and the spectroscopic detection of biological and chemical agents would benefit from multispectral illumination. Of particular interest is its use in lidar systems; the spectral information that a multiwavelength laser imaging system affords would improve the ability to identify targets.

Xybion's hybrid laser prototype could offer multispectral lidar applications a smaller, more efficient solution.

One approach to designing such a laser uses stimulated Raman scattering to shift the wavelength of a frequency-doubled Nd:YAG source and thereby simultaneously produce several laser lines. However, this design has several drawbacks, including bulkiness, a nonuniform output energy and low-pulse repetition rates. Simultaneous multiple-wavelength lasing is also possible in solid-state lasers that have a single laser rod in combination with doped crystals. Though these lasers are compact and can be operated at a high repetition rate, their emission efficiency is lower than those in an undoped configuration.

A better approach, the hybrid laser, incorporates at least two lasers of different absorption bands within the same flashlamp pump cavity. Compared with two individual lasers, a simultaneous multiple-wavelength output of higher efficiency is possible with the hybrid laser. Other advantages include size and weight savings, because many of the components can be shared by the two lasers.

Researchers at Xybion Electronic Systems Corp., with support from the Naval Surface Warfare Center in Panama City, Fla., have used this approach to build a prototype hybrid laser as the active illumination source for long-range imaging lidar applications.

This hybrid laser uses two materials -- Nd:YAG and Cr:LiSAF, with twice the active volume of Nd to Cr -- that are complementary in both high-power output wavelengths and absorption bands, so that the absorption of the flashlamp pump energy is more efficient with the combined materials than it would be with just one of them.

The laser has produced simultaneous outputs that span the spectral range from the UV to near-IR (425, 532, 850 and 1064 nm). For active imaging applications, this spectral range translates into video cameras with enhanced sensitivity. According to tests, the hybrid laser has an overall efficiency of 2.4 percent when it is Q-switched, but the researchers expect higher efficiencies to come with improvements in laser crystal quality and the pump cavity configuration. Xybion is building a self-contained multispectral hybrid laser that incorporates Cr:LiSAF and Nd:YAG rods.

Tests of a range-gated multispectral lidar based on the hybrid laser are under way. "The primary goal for this test is to compare the images taken by multispectral range-gated ladar with images taken by single-wavelength range-gated ladar," explained Manvel A. Andriasyan, senior scientist at Xybion and one of the authors of the results published in the March 20 issue of Applied Optics. "It is hoped that the comparative results will show the benefits of the multispectral system."