Tech Briefs

Scientists are developing nonlinear frequency conversion devices based upon epitaxially grown gallium arsenide.

The US Air Force has a need for improved tunable laser sources—both in the midinfrared region, for developing infrared countermeasure (IRCM) applications, and in the longinfrared region, for addressing an increasing variety of threat sensors. Since few direct lasers exist in these spectral regions, scientists generally use nonlinear frequency conversion techniques to convert the output of available lasers into the desired longer

wavelengths.

ImageFor many years, AFRL has been instrumental in developing nonlinear frequency conversion devices for IRCM, from the devices of the late 1980s, which employed birefringently phasematched crystals (e.g., silver gallium selenide, zinc germanium diphosphide, and potassium titanyl phosphate), to those emerging during the 1990s, which were based on quasi-phasematched periodically poled lithium niobate (PPLN) and similar poled ferroelectric materials. While all such materials have contributed to advancing the state of the art in nonlinear frequency conversion technology, each possesses inherent limitations. For example, crystals that achieve phase-matching as a result of birefringence often experience beam walk-off, which limits their output and efficiency. They also exhibit other problematic characteristics, including slow and limited-range frequency tuning by means of crystal rotation; inadequate power handling, resulting in thermal lensing and damage; and the presence of impurities, leading to scatter or absorption. Although poled ferroelectric materials use quasi-phasematching to avoid many of these problems, their intrinsic absorption prevents their practical use at wavelengths longer than 4 μm.