A fibre optic frequency shifter has been developed which generates aheterodyne frequency that is used to facilitate electronic demodulation ofoptical information. The operation of this device is analogous to an acoustoopticdevice such as a Bragg cell. This frequency shifter works on the principleof mixing two stimulated Brillouin scattering signals (generated in optical fibrering resonators) which have slightly different frequencies. Dual ring resonatorand single ring resonator topologies have been used. For the former systema conversion efficiency of 16% was obtained. The beat frequency was tunablebetween 218.4 MHz and 414.6 MHz for a 40'C change in temperature. Atemperature coefficient of 5+0.2 MHzK-1 was measured. The later configurationprovides a highly stable carrier frequency (11MHz) with a temperaturecoefficient of 6.7+0-5 kHzK-1. A 20% conversion efficiency was obtained. Thisdemonstrates that this technique offers a practical, fibre efficient, low opticalpower requirement method for producing a frequency shifter. One of the mainadvantages of the system is that no electrical power is required to produce thetravelling acoustic wave.A novel technique to characterize the frequency response of optical detectoramplifiercombinations, used in this project to detect these high frequencies,is also demonstrated. The technique is based on the wavelength modulationof a laser diode source in a path length imbalanced two-beam interferometer.A robust configuration using a low finesse Fabry-Perot interferometer madefrom birefringent optical fibre has been implemented. Measurements forseveral detector circuits are presented for the frequency range DC to about 30MHz. Results are compared with direct modulation of the laser intensity andalso with a circuit simulation programme (PSpice) and found to be in closeagreement.
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