S.G. Farwell

Four-port fiber frequency shifter with a null taper coupler

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.

2 × 2 Single-mode fiber routing switch

An all-fiber 2 x 2 acousto-optic routing switch is described that has an insertion loss of 0.11 dB, a drive power of 1.5 mW, and a switching time of 80 micros. The switch has a very simple construction and so has the potential to be low in cost.

ALL-FIBER POLARIZER BASED ON A NULL TAPER COUPLER

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.

Low-loss all-fiber acousto-optic tunable filter

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.

2/spl times/2 fused fiber null couplers with asymmetric waist cross sections for polarization independent (<0.01 dB) switching

Polarization independent (<0.01 dB) switching is achieved at 1.55 /spl mu/m in a strongly guiding, fused tapered fiber null coupler acoustooptic switch. An intermediate degree of fusion of the dissimilarly sized fibers is shown experimentally to have the form birefringence required to render the mode-splitting coupler polarization insensitive. The effect of the degree of fiber fusion on the acoustic wavelength of flexural waves is studied theoretically and experimentally, and precise control over the degree of fiber fusion is obtained for coupler waist cross-sectional dimensions as small as 6 /spl mu/m.

3 x 3 all-fiber routing switch

An all-fiber acoustooptic switch has been constructed from a 3/spl times/3 null fused coupler. The maximum drive power required is 4 mW, the switching time is 100 /spl mu/s and the insertion loss is less than 0.5 dB and has a cross talk of 20 dB. The switch shares the advantages of an earlier 2/spl times/2 switch but with more fiber ports, and should permit the design of compound routing arrays with fewer switching elements per channel.

All-fiber 2/spl times/2 polarization-insensitive switch

We have demonstrated a broadband, polarization-independent all-fiber switch. It is based on twisting the waist of a null taper coupler. The polarization sensitivity is reduced from 17 dB to 0.2 dB. The device has a low drive power requirement, low insertion loss and a conversion efficiency of /spl sim/100%. The null coupler is fabricated from standard telecommunications fiber and is, therefore, compatible with any existing network.

Fibre acousto-optic frequency shifter based on a null coupler

All-fibre acousto-optic frequency shifters have in the past been based on two-mode fibres1,2,3, the frequency- shifted light appearing in the second of the two modes. Mode convertors and filters are necessary to separate the residual carrier from the shifted signal and give a single-mode output.

Nondestructive measurement of the degree of fiber fusion in fused tapered couplers

Summary form only given. The enormous diversity of optical signal processing schemes available with fused tapered coupler devices, either in the passive or active state, explains why these devices have become one of the essential building blocks of fiberoptic systems. This large diversity of applications also justifies why it is important to be able to fully characterize this range of devices. Depending on the required bandwidth of the coupler, the waist region of the coupler may vary between 40 /spl mu/m diameter for large bandwidth splitters to 2 /spl mu/m diameter for grating-assisted filters, and depending on the polarization insensitivity required of the signal processing, the couplers waist cross section may vary between circularly fused to dumbbell-shaped cross section. The degree of fiber fusion of the waist region of a fused coupler therefore strongly influences the output characteristics of these devices in terms of polarization sensitivity and coupling coefficients, and any characterization method must be able to be function within this wide range of parameters. Using the acousto-optic effect, it is possible to nondestructively determine the degree of fiber fusion at a given position along a fiber coupler.

Polarization-independent (<0.01 dB) switching in a 2 x 2 fused fiber null coupler using asymmetric dumbbell cross section

Polarisation independent (<0.01dB) switching at 1.55µm is achieved in a strongly guiding tapered fibre null coupler acousto-optic switch. A strongly fused asymmetric dumbbell cross-section gives the form birefringence required to render the null coupler polarisation insensitive.