Development of fibre grating lasers with tunable wavelength and single polarization mode output for dense wavelength division multiplexing fibre-optic systems.

摘要

This thesis describes the investigation and development of fibre grating based fibre laser (FGL) source as well as channel monitoring system for dense wavelength division multiplexing (DWDM) fibre-optic systems.; The prime advantage of FGL is that the laser wavelength can be tuned continuously by altering the FBG's Bragg wavelength. Two tuning techniques were investigated. A novel FGL incorporating a platinum coated fibre Bragg grating was fabricated, by controlling the electric current passing through the platinum coating, repeatable tuning range of 12.3 nm with 1.61 W of electrical input power was achieved. A compression-tuned FGL prototype was also developed, wavelength tuning of more than 20 nm and accuracy of ±5 pm was demonstrated with a compression length of 350 μm.; Two types of wavelength stabilization experiments were carried out. The first one employed a fibre-based scanning Michelson interferometer as a wavelength discriminator, whereby the pseudoheterodyne demodulation technique was investigated to stabilize the laser wavelength of a distributed Bragg reflector (DBR) FGL. Long-term frequency fluctuation was reduced from ±480 MHz to less than ±30 MHz. In the second experiment, hydrogen cyanide gas cell was employed as an absolute wavelength reference. A distributed feedback (DFB) FGL was wavelength modulated around the center wavelength of one of the absorption lines. By measuring the first derivative signal using a lock-in amplifier, it was found that the laser frequency can be stabilized to within 10 MHz.; FGLs fabricated in ordinary circular fibre often consist of two modes with orthogonal linear polarization. This limitation was successfully overcome and demonstrated in two different experiments. In the first experiment, lateral compressive force was applied to one of the gratings of a DBR FGL that introduced additional birefringence to the grating. It was found that one of the polarization lasing modes was suppressed when a force of 0.6 N/mm was applied. In the second experiment, a dual polarization mode DFB FGL was used to injection lock a Faber-Pérot laser diode (FP-LD). By adjusting the injection power of the FGL to the FP-LD, high quality laser output with a degree of polarization larger than 95% and side mode suppression ratio higher than 45 dB was achieved. A 3.8 dB power penalty improvement was also demonstrated in a 41 km, 10 Gbps transmission experiment.; A novel wavelength monitoring system was developed, where a near threshold biased FP-LD was employed as wavelength reference for the system. The optimum bias current of the FP-LD, which minimizes the wavelength drift was experimentally investigated. By alternatively switching a scanning Fabry-Pérot filter between the reference FP-LD and eight DFB laser diode spaced 0.8 nm apart, wavelength and power measurement accuracy of better than ±10 pm and 0.2 dB, respectively, were achieved.