Abstract
A phenomenological theory describing the second harmonic generation (SHG) in optical fibers is presented. The SHG phenomenon is explained as a result of a photoelectric convective instability leading to spontaneous formation of χ (2) grating. It is shown that spontaneous SHG is due to stationary amplification of noise. The theory explains the observed strong fluctuation of the output harmonic signal and the spatial location of the χ (2) grating. It is shown that the region where the χ (2) grating is written is not restricted by the coherence length determined by the walk-off or self-phase modulation effects. Saturation of SHG due to nonlinearity is studied. New effects in SHG dynamics are predicted which could be checked experimentally.
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