IMAGING THE NONLINEAR GRATING IN FREQUENCY-DOUBLING FIBERS

THE second-order response of a transparent material to intense light c reates an oscillatory electromagnetic field at twice the driving frequ ency. Materials with a strong second-order response can therefore be u sed for frequency-doubling, for example to convert infrared laser ligh t to visible light(1). Although amorphous materials have no significan t intrinsic second-order response, glass fibres can nevertheless exhib it second-harmonic generation after exposure to intense laser irradiat ion(2). Beating between the electromagnetic fields of the laser light at the fundamental frequency and a weak second-harmonic signal (extern ally applied or intrinsic to the fibre) permanently modifies the glass and enhances the second-order response; the high efficiency of the re sponse points to the formation of a periodic electric-field grating wi thin the fibre(3-7). High electric fields have been detected in fibres (8) and the existence of a grating has been confirmed indirectly(9). H ere we present direct images of this grating in germanosilicate optica l fibres, obtained by exposing the fibres to chemical attack by hydrof luoric acid while the grating is in place. The rate of etching is sens itive to the intensity of the internal electric field in the fibres. O ur results are consistent with the idea that the grating results from macroscopic separation of charge at the boundary between the fibre cor e and cladding, rather than from a microscopic reorientation of dipole s throughout the material.