Abstract

In recent years laser diodes have grown in popularity as efficient, compact, and robust sources of coherent radiation. These characteristics make them a natural choice for communications systems, particularly in space-based systems where issues of weight and reliability are paramount. At the same time, photorefractive materials have been suggested for a wide variety of applications in dynamic signal processing. There have been several studies of the use of laser diodes with photorefractives, including two beam coupling,¹ phase conjugation,² beam combination,³ as well as line narrowing.⁴ However, laser diodes have spectral characteristics that can cause problems in the temporal stability of some photorefractive effects. Laser diode arrays typically emit several supermodes with a total bandwidth on the order of several nanometers. Single-mode laser diodes have very narrow instantaneous bandwidths, but are subject to longitudinal mode hops caused by temperature or current fluctuations. The frequency fluctuations of the laser diode can become fluctuations in the photorefractive effect because of two mechanisms: phase changes of the interference pattern and Bragg mismatch. The first effect arises in systems that use a conventional beam splitting interferometer when there is a difference in path length between the two arms. In this case, a change in frequency causes a phase shift of the interference pattern. Careful stabilization^1,5 or control² of the temperature and input current of a single-mode laser diode can reduce mode hops, but such stabilization adds to the complexity and weight of a system and may be undesirable in many applications. In addition, if the diode laser is modulated the mode hops may be unavoidable. As an alternative to active stabilization, we have examined the use of achromatic gratings as a simple passive stabilization technique. An achromatic grating is one in which the phase, direction, and grating spacing of the interference pattern are independent of the wavelength of the interfering beams.⁶ The formation of an achromatic grating can be understood as imaging a portion of a Ronchi ruling illuminated by a laser diode into the crystal.

© 1993 Optical Society of America