Abstract

One-way real-time distortion correction of a laser beam, which may originate from transmission through atmospheric turbulence, has been studied by optical engineers for many years. This paper presents the concept of real-time passive wave front converters (WFC) by using nonlinear two-wave mixing (2WM) and four-wave mixing (4WM) in photorefractive crystals. The 2WM-WFC is demonstrated by the unidirectional ring oscillator.¹ The input distorted laser beam is used to pump the oscillation inside the ring cavity. By inserting an aperture inside the ring cavity to control the oscillating mode, a clean laser beam is obtained. A 15% conversion efficiency was measured. For the 4WM-WFCs the input distorted laser beam is phase conjugately reflected back by a passive phase conjugate mirror (PPCM).² Then, this pair of phase conjugate beams is used to pump the laser oscillation, by means of 4WM, between a photorefractive crystal and an external mirror. A portion of the oscillating beam is coupled to the output of the device. We also demonstrate that we can use only one crystal to serve both as the PPCM and the crystal in the oscillator. The response time of these WFCs depends on the response time of the photorefractive crystals which ranges from a few seconds to microseconds. A comparison between the 2WM-WFC and the 4WM-WFC is also made.

© 1985 Optical Society of America