IMAGING THE NONLINEAR GRATING IN FREQUENCY-DOUBLING FIBERS

THE second-order response of a transparent material to intense light creates an oscillatory electromagnetic field at twice the driving frequency. Materials with a strong second-order response can therefore be used for frequency-doubling, for example to convert infrared laser light to visible light(1). Although amorphous materials have no significant intrinsic second-order response, glass fibres can nevertheless exhibit second-harmonic generation after exposure to intense laser irradiation(2). Beating between the electromagnetic fields of the laser light at the fundamental frequency and a weak second-harmonic signal (externally applied or intrinsic to the fibre) permanently modifies the glass and enhances the second-order response; the high efficiency of the response points to the formation of a periodic electric-field grating within the fibre(3-7). High electric fields have been detected in fibres(8) and the existence of a grating has been confirmed indirectly(9). Here we present direct images of this grating in germanosilicate optical fibres, obtained by exposing the fibres to chemical attack by hydrofluoric acid while the grating is in place. The rate of etching is sensitive to the intensity of the internal electric field in the fibres. Our results are consistent with the idea that the grating results from macroscopic separation of charge at the boundary between the fibre core and cladding, rather than from a microscopic reorientation of dipoles throughout the material.