Test of photovoltaic model of photoinduced second-harmonic generation in optical fibers

It is shown that second-harmonic generation efficiency in a photovoltaic model depends on the relationship of illuminated size to grating period. Model test experiments are proposed and realized. To explain the photo-induced second-harmonic generation (SHG) in glass optical fibers, two groups of models have been suggested, some based on separation of charges and appearance of a strong electrostatic field the others based on orientation of defects. However, a strong electrostatic field (approximately equals 104 V/cm) appears also in the last models. That is why neither the experiment on the discovery of such a field nor the experiment on the measurement of the component (chi)(2) tensor ratio gives an answer to the preference of either model. This paper shows that the efficiency of SHG by a (chi)(2) grating, resulting from a coherent photocurrent, depends strongly (approximately equals (r0/(Lambda) )-4) on the ratio of the transverse size r0 of the light beam, used for the grating preparation, and the grating period (Lambda). So we think that an experiment on SHG in a bulk sample to check this point will make it possible to clear up the mechanism of (chi)(2) grating formation. Another possibility is an experiment with the (chi)(2) grating writing by pump and second-harmonic waves, propagating in opposite directions in a fiber. In this case the grating period will decrease to (lambda)p/4 approximately equals 0.25 micrometers.