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