J.W. Arkwright

Experimental and theoretical analysis of the resonant nonlinearity in ytterbium-doped fiber

Experimental measurements are described characterizing the nonlinear index change over the range from 500 to 1550 nm induced in an ytterbium (Yb/sup 3+/)-doped twin-core fiber by a 980 nm pump. At 1550 nm, a phase change of /spl pi/ is induced with as little as 14 mW of pump power for a signal loss of only 0.2 dB, By allowing the doped fiber to lase and observing the associated clamping of the induced phase change, we show that a digital nonlinear response can be achieved in which a constant, pump-power-insensitive, phase change is induced for all pump powers above a certain threshold. This lasing induced clamping of the phase change also demonstrates that the nonlinear effect is population dependent as opposed to thermal. The pump-induced phase change is observed to increase for shorter signal wavelengths, which suggests that the effect is due principally to pump-induced changes in the strong ultraviolet (UV) absorptions of Yb/sup 3+/. This observation is accurately predicted by a theoretical analysis that takes into account absorptions in both infrared and ultraviolet regions. This analysis shows that Yb/sup 3+/ may be suitable for low-power all-optical switching applications in both 1300 and 1550 nm telecommunications windows when the speed of response is not a critical parameter.

Nonlinear phase changes at 1310 nm and 1545 nm observed far from resonance in diode pumped ytterbium doped fiber

Resonant nonlinearities at 1310 mm and 1545 nm have been observed in diode pumped ytterbium doped fiber, with greater than /spl pi/ phase changes observed with 18 mW launched pump power. These signal wavelengths lie far from the ytterbium absorption peak centred at 980 nn, and can be explained by considering the effect of strong absorptions in the UV. It has been confirmed that these phase changes are not due to thermal effects in the optically pumped fiber.

High-isolation demultiplexing in bend-tuned twin-core fiber

By applying a cantilever bend to a twin-core optical fiber wavelength division demultiplexer, isolations of >30 dB have been achieved between channels at 1.33-1.54 /spl mu/m. These bend-tuned TCFs exhibit /spl ges/20 dB isolation over a bandwidth of 32 and 21 nm in the two wavelength windows, respectively. This is significantly better than that typically offered by commercially available, single-stage, tapered coupler demultiplexers. The bend-tuning mechanism is demonstrated experimentally and described theoretically using a coupled local-mode analysis.

Optical-to-electrical wavelength demultiplexing detector: design, fabrication, and analysis

An optical-to-electrical wavelength demultiplexing detector has been fabricated using a short length of twin-core optical fiber and an integrated bi-cell detector. The twin-core fiber splits 1325 and 1535 nm input signals onto different output cores, thus directing each demultiplexed channel onto the spatially separated active areas of the bi-cell. We discuss the design, fabrication, and post-tuning techniques used to successfully demonstrate the wavelength demultiplexing functionality of the device and present some preliminary results from an assembled laboratory prototype.

Low-power all-optical broad-band switching device using ytterbium-doped fiber

All-optical photonic switching has been demonstrated at 1310 nm and 1545 nm using an all-fiber Mach-Zehnder interferometer incorporating ytterbium-doped silica fiber. The switching was achieved with 980 nm excitation from a laser diode, with a power-length product of 3.7 mW/spl middot/m and 5.8 mW/spl middot/m at 1310 nm and 1545 mm respectively, thus giving low power all-optical switching at the preferred telecommunications wavelengths.

Resonantly enhanced nonlinearaties in rare-earth-doped fibers and waveguides

The use of rare-earth dopants for all-optical switching is of interest because of the very low pump powers required to achieve full switching. In addition, the ease with which rare-earth ions can be incorporated into silica based fibers and waveguides makes them ideally suited for the fabrication of fiber compatible components. The principal disadvantages of this type of nonlinearity are that the relaxation times are characteristically slow, and relatively long interaction lengths are required to achieve phase changes of the order of (pi) . In this presentation, an overview of the work being carried out in Australia will be given, concentrating on the techniques developed to minimize the relaxation times and power length products of these nonlinearities.

Large off-resonance nonlinear index changes at 1310 nm and 1545 nm observed in ytterbium-doped fiber

The resonant nonlinearity observed in rare-earth doped fiber is of interest for all-optical switching due to the very low pump powers needed to achieve complete switching. Large nonlinear effects have been observed previously in both erbium and neodymium doped fibers, however, these large effects cannot be explained by taking into account the change in absorption of local transitions alone. Two mechanisms have been suggested to account for these anomalously large index changes, namely thermally induced effects and far from resonance population dependent effects. In this work, an ytterbium doped twin-core fiber is used to investigate the tow possible mechanisms, and by allowing the doped fiber to lase and observing the clamping of the induced phase change, it is demonstrated that the effect is population dependent in nature. The observed wavelength response of the effect shows an increase in induced phase towards shorter wavelengths, which suggests that the effect is due principally to changes in strong absorptions in the UV. This far-from-resonance effect has been used to demonstrate large phase changes at the preferred telecommunications windows of 1300nm and 1550nm.

Digital all-optical switching in a ytterbium-doped two-mode fiber interferometer

A resonance-enhanced nonlinearity is used to demonstrate a digital switching response in a two-mode fiber interferometer constructed from ytterbium(III)-doped optical fiber. The fiber used was single-mode at the pump wavelength (980 nm) and two-moded at the signal wavelength (514 nm). Interferometer-based optical switches are generally sensitive to fluctuations in pump power, because of the sinusoidal dependence of the output state on pump power. However by allowing the doped fiber to lase, thus clamping the phase shift, this sensitivity can be eliminated to yield a bi-stable, digital switching response. In our interferometer configuration, a laser cavity was defined in the ytterbium-doped fiber by a matched pair of Bragg gratings, and the magnitude of the clamped phase shift was controlled by stretching one of the gratings to adjust the laser threshold. Digital switching of a 514 nm signal with 11 dB isolation and a power-length product of 1.3 mW.m was demonstrated. The relaxation time of the switch was determined by the ytterbium(III) excited state lifetime, while the rise time could be reduced by increasing the pump power without compromising the isolation. The clamping technique demonstrated in this paper should be applicable to any optical switch incorporating a gain medium.