A prototype of a new design of fiber frequency shifter, based on a special 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 flexural acoustic wave, light is coupled between the fibers, acquiring a frequency shift in the process. Near 100% conversion with 30-dB sideband and carrier suppression is achieved, in an unoptimized device, for just 1 mW of electrical drive power.

Abstract-We have investigated the suitability of InP-based push-pull modulators to generate flat optical comb spectra. The residual amplitude modulation that was a consequence of driving the modulator well above its design voltage (up to 4V π ) is demonstrated to have no adverse effect for the comb generation. We also demonstrate generation of a comb with spacing equal to twice the repetition rate via control of the relative phase of the RF signals.

We report significant advances in the development of simple all-fiber, tunable acousto-optic spectral filters based on null couplers. The performance agrees well with theoretical predictions and is attributed to improved control of the coupler uniformity. Furthermore, using a double-pass arrangement, we demonstrate filter bandwidth reduction and improved sidelobe suppression of -20 dB . The double-pass configuration is also shown to double the frequency shifts that are obtainable from such devices.

We demonstrate a comb-based transmitter with a potential to be integrated on a single InP photonic chip. Nyquist-shaped polarization-multiplexed 16QAM/64QAM signals are generated and transmitted over 300-km of SMF-28.

An entirely new type of single-mode all-fiber polarizer is reported. The polarizer is based on a narrow circularly fused null taper coupler, which was twisted through 45 degrees after being made. The polarizing effect is due to the excitation of hybrid second modes in the coupler waist. An extinction ratio of 15 dB and an insertion loss of 0.2 dB have been obtained, with scope for further improvement.