KEYWORDS: Semiconductor lasers, Fiber lasers, Digital signal processing, Laser stabilization, Signal processing, Interferometry, Signal detection, Laser development, Laser applications, Sensors
Operation and performance are described for tunable 50mW distributed feedback semiconductor laser modules with
feedback frequency stabilization. Frequency stability resulting in average phase noise reduction of over 35 dB is
achieved, comparable to the performance of single-frequency fiber lasers, but with tunability and greatly reduced
sensitivity to microphonic disturbance. The noise performance holds up well in and following harsh environments. These
advantages make this laser module a good low-cost alternative to fiber lasers for applications involving interferometric
sensors and interferometric measurement techniques, such as seismic signal detection for hydrocarbon exploration and
earth movement analysis, underwater acoustic signal sensing, perimeter security, intrusion detection, and others
currently being developed.
In a Raman amplifier having a broadband pump beam which is not exactly collinear with the
Stokes seed beam, the Stokes output spectrum will be "fanned", i.e., angularly dispersed, because the
direction of maximum gain differs for growth of each component of the Stokes spectrum. The
spectral fanning produces transverse phase and intensity variations which degrade the Stokes output.
Approximate analytic expressions are given for beam quality degradation in cases where the Stokes
seed is narrowband, broadband and temporally correlated to the pump beam, -or broadband and
uncorrelated. The effect is predicted to be usually negligible, but can be significant for cases of
very large beam aperture sizes, bandwidths and/or angles. Use of a diffraction grating in the output
Stokes beam or a stepped mirror in the input pump beam are proposed for reducing the effect.
Collinear microwave phase conjugation was observed in an artificial Kerr medium consisting
of short graphite fibers suspended in a binary liquid mixture. Using an 18 GHz pump beam
with up to 20 W continuous power, characterization of the changes in the 94 GHz refractive
index were made by interferometry. A nonperturbative method for describing the response of
the medium was used to analyze the phase-shift measurements for the static berefringence and
the time response as functions of microwave intensity.
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