In Line Optical Fibre Frequency Shifter Project

Abstract

Abstract : This report describes the results of research directed towards improving the performance of the torsional-birefringent optical fibre frequency shifter. A detailed account is given of the experimental procedures required to construct and test a fibre-optic frequency shifter utilising a travelling torsional acoustic wave to couple the polarisation eigenmodes of a highly linearly birefringent optical fibre. The frequency shifter configurations such that the acoustic wave generator and optical fibre are co-axial. The device is thus more rugged than previous 'free fibre' frequency shifters where the acoustic wave is coupled into the fibre via an acoustic horn bonded to the edge of the optical fibre. Results are presented for the best device constructed thus far. The optical frequency shift was 3.204 MHz and an optical coupling efficiency of 12% was obtained with 4W of electrical power applied to the acoustic transducer. Carrier and sideband suppressions of 25 and 30 dB were attained. The coupling efficiency, carrier and sideband suppressions achieved for this device, while significantly improved on previous similar devices, are significantly lower than theoretical prediction and this is attributed to the difficulties in manufacturing the unit.