Abstract:
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.