4-PORT FIBER FREQUENCY SHIFTER WITH A NULL TAPER COUPLER

A prototype of a new design of fiber frequency shifter, based on a spe cial four-port fused taper coupler with a null maximum coupling ratio, is reported. In this coupler the constituent fibers are so dissimilar that the maximum coupling is (ideally) zero. In the presence of a fle xural acoustic wave, light is coupled between the fibers, acquiring a frequency shift in the process. Near 100% conversion with 30-dB sideba nd and carrier suppression is achieved, in an unoptimized device, for just 1 mW of electrical drive power.