Electric Field Poling Of Quasi-Phase-Matched Optical Fibres

Ten years have passed since the discovery of photoinduced quasi-phase-matched second harmonic generation in optical fibres [1-3]. However, until fairly recently, second harmonic generation in optical fibres has been more of scientific than practical interest, owing to the low levels of induced nonlinearity (~10-3 pm/V, which is four orders of magnitude less than in inorganic crystals, e.g. lithium niobate). The mystery of photoinduced χ(2) gratings was finally solved on the basis of the coherent photogalvanic effect [4]: a high (104-5 V/cm) spatially oscillating electrostatic field appears in glass as a result of charge separation induced by coherent photocurrent, oscillating with a period determined by the coherence length; this electric field produces a quasi-phase-matching χ(2) grating in proportion to χ(3). The conversion efficiency reported in the first experiments on photoinduced SHG-~ 5 % from a peak pump power of ~ 20 kW-is still among the highest conversion efficiencies achieved so far in optical fibres. The reasons for these relatively high conversion efficiencies has to be searched in the long length (tens of centimetres) and good uniformity of the photoinduced gratings.