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Role of the third- and fourth-harmonic radiation in the formation of a χ(2) grating in Ge-Er fibres made of fused silica

A study was made of the characteristics of the formation, in the absence of seed radiation, of a χ(2) grating with an efficiency of 4% in Ge—Er optical fibres. In a wide dynamic range of the χ(2)-grating 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 χ(3) nonlinearity inside a fibre. The experimentally determined characteristics of the growth and saturation stages of the formation of a grating were in agreement with the proposed physical interpretation based on a model of a coherent photocurrent and on the assumption that the UV harmonic radiation participates in the photoionisation of donor and acceptor centres.