Trevor Whitbread

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.

A distributed-feedback fiber-laser-based optical fiber hydrophone system with very high sensitivity

This paper reports the development of a very compact and very-high sensitivity optical fibre hydrophone system using a distributed-feedback fibre laser with a cavity length of 10cm. A theoretical system design making use of a Mach-Zehnder interferometer, homodyne demodulation scheme and digital signal processing is described. At the time of writing, the system is only partially completed; therefore the content of this paper will focus on the distributed-feedback fibre laser sensor head. Results for noise spectrum below 100kHz are presented, as well as discussions on some key issues with designing such hydrophone systems. Although not the intention for the system, initial results also indicate the suitability of the DFBFL for intensity modulated sensing.

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.

Tunable dispersion using linearly chirped polymer optical fiber Bragg gratings with fixed center wavelength

We propose a new scheme for tunable dispersion using linearly chirped polymer optical fiber Bragg gratings made in fiber tapers. The simple tension and uniform heating to the fiber gratings act as the adjustment and control process. Dispersion is tuned by the applied tensile strain. Owing to the unusually large and negative thermooptic coefficient of polymer fiber, the center wavelength shift induced by the applied strain will be offset by the effect of the uniform heating, leading to the fixed center wavelength. Our simulation demonstrates a very large dispersion tuning range, say from 2400 to 110 ps/nm.

Optical properties of a nonlinear p-phenylenevinylene oligomer side chain polymer in films and fiber preforms

Linear and nonlinear optical properties of a new soluble polymer bearing distyrylbenzene chromophore, the alkoxy-sulphono-substituted p-phenylenevinylene oligomer (MTPV-ORSO) attached as a side-chain to the methyl methacrylate backbone are reported. This chromophore was developed for introduction into single-mode nonlinear optical polymer fibres. The molecular second-order and third-order nonlinearities were estimated with quantum chemical calculations (MOPAC). The side-chain polymer shows an optical absorption maximum at 326 nm. The polymer was incorporated into the core of the fibre preform with a procedure which led to a step index profile, as measured with a preform profiler, suitable for a single-mode optical fibre. Second harmonic of the fundamental 1200 nm wave was measured in the corona poled polymer films giving the second-order susceptibility d33 = 0.8 pm/V for the polymer containing about 1.3 wt% of the side-chain chromophore and 0.1 pm/V in a guest-host system containing 0.24 wt% of the chromophore in the fibre preform. The coherence length of the side-chain polymer, equal to 28 μm at 1200 nm, was evaluated from the dispersion of the linear refractive indices. Degenerate four-wave mixing (DFWM) with amplified femtosecond pulses was also applied to measure the nonlinear refractive index of the co-polymer at 800 nm.

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.

A simple strain sensor using polymer fiber Bragg grating and long-period fiber grating

We develop a simple strain sensor using polymer optical fibre Bragg grating and long period fibre grating. The sensor head is formed by a polymer optical fiber Bragg grating. A long period fibre grating is used for strain related wavelength shift demodulation. This particular combination of two quite different gratings could offer very large dynamic, up to tens of thousands micro-strains, for strain sensing. The preliminary experimental results have demonstrated that this sensing scheme provides good linearity, high resolution and large dynamic range.

Spectrally-coded multiplexing in a strain sensor system based on carrier-modulated fiber Bragg gratings

In order to increase the number of channels available to a grating based strain sensor system specialised gratings were designed that would allow the sensor system to be able to distinguish between a number of gratings located in the one WDM channel independently of the amount by which they overlap each other in the wavelength domain. Distinguishing between gratings is achieved by inscribing a carrier frequency in the grating spectrum, so that each grating can be addressed in the Fourier domain via the spectral information centred around the inscribed carrier frequency. Tests performed on the gratings successfully show the ability to distinguish between three spectrally overlapping gratings. The calculated value for Youngs modulus, 72+/-3GPa, was found to be in keeping with the standard value of 70.3GPa for fused silica.

Polymer fiber Bragg gratings tunable dispersion compensation

We propose a new scheme for tunable dispersion with a large tuning range and a fixed center wavelength using linearly chirped polymer fiber Bragg gratings. Simple tension and uniform heating are employed as the control process.