Abstract

In this paper we propose a dynamic mechanism of efficient second-harmonic amplification in optical fibers and explain the existence of phenomenon thresholds. We start with the assumption that the second-order susceptibility grating appears in fibers because of the strong electrostatic field caused by coherent photocurrent.¹ We have obtained equations for the amplitudes and phases of this grating and second harmonic that are similar to equations describing dynamic self-diffraction of light beams. We have shown that the phase shift of the grating relative to the "interference" pattern (the pattern of coherent photocurrent oscillations) leads to an amplification of weak second-harmonic seeding. Strong exponential growth takes place when this amplification is greater than the losses. We have obtained a threshold condition connecting pump power, distance from the fiber end, and time from the beginning of fiber preparation. We have shown that the value of the length threshold and the length dependence of second-harmonic intensity are different for various seed intensities because of the dependence of photoconductivity on second-harmonic intensity. The results of numerical simulation of the phenomenon are in close agreement with known experimental results.²

© 1990 Optical Society of America