Degenerate Four-Wave Mixing Using Phase Conjugate Pump Waves
The most commonly used techniques for generating phase conjugate (PC) signals are degenerate four-wave mixing (DFWM) and stimulated Brillouin scattering (SBS). However, these two processes have contrasting requirements for producing high quality wavefront correction. Using DFWM, high quality pump beams are required for good reproduction of the probe beam. On the other hand, while SBS readily conjugates aberrated pump beams, unlike DFWM it is a threshold process and usually requires much higher intensities. For the majority of common laboratory laser sources these high intensities can usually only be generated by tight focusing of the laser output. This need for tight focusing and the associated reduction of the image size in the Brillouin medium clearly limits the usefulness of SBS for phase conjugation in applications, such as photolithography, in which the phase aberrations across the beam are on a ‘fine’ scale. To our knowledge, reports of the smallest aberration feature which have been corrected for using an SBS phase conjugate mirror are ~ 20 µm.1 However, features as small as ~ 0.5 µm have been reproduced using a DFWM phase conjugate mirror.2
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