ROLE OF THE 3RD-HARMONIC AND 4TH-HARMONIC RADIATION IN THE FORMATION OF A CHI((2)) GRATING IN GE-ER FIBERS MADE OF FUSED SILICON

A study das made of the characteristics of the formation, in the absen ce of seed radiation, of a chi((2)) grating with an efficiency of 4 % in Ge-Er optical fibres. In a wide dynamic range of the chi((2))-grati ng amplitude, there was a direct relationship between the growth rate of the grating and the intensity of the UV third and fourth harmonics of a neodymium laser generated by the cubic chi((3)) nonlinearity insi de a fibre. The experimentally determined characteristics of the growt h and saturation stages of the formation of a grating were in agreemen t with the proposed physical interpretation based on a model of a cohe rent photocurrent and on the assumption that the UV harmonic radiation participates in the photoionisation of donor and acceptor centres.