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

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


Author: Chung, Weng-hong
Title: Development of fibre grating lasers with tunable wavelength and single polarization mode output for dense wavelength division multiplexing fibre-optic systems
Degree: Ph.D.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
fiber optics
Diffraction gratings
Wavelength division multiplexing
Optical communications
Department: Dept. of Electrical Engineering
Pages: ix, 224 p. : ill. ; 30 cm
Language: English
InnoPac Record:
Abstract: 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. Particular attention is paid on wavelength tunability, tuning accuracy improvement, wavelength stabilization, and output polarization control of FGLs. A multi-wavelength monitoring system capable of measuring both wavelength and power of DWDM channels was developed. The prime advantage of FGL is that the laser wavelength is defined by the Bragg wavelength of the fibre Bragg grating (FBG). Therefore, continuous and mode-hop free laser wavelength tuning can be achieved by altering the FBG's Bragg wavelength either mechanically or thermally. Two tuning techniques were investigated with a view to extend the tuning range and improve the tuning accuracy of FGLs. 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 was demonstrated with a compression length of 350 um. To overcome the hysteresis and backlash of the motorized actuator used in the prototype, a linear variable differential transformer was implemented that measured the compression experienced by the fibre Bragg grating, and a wavelength tuning accuracy of +-5 pm was experimentally demonstrated. 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 and developed to monitor the laser wavelength of a distributed Bragg reflector (DBR) FGL. Long-term frequency fluctuation was reduced from +-480 MHz to less than +-30 MHz by using an appropriate electronic feedback circuit. 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, the laser wavelength can be evaluated and stabilized. 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 single polarization mode operation was 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-Perot laser diode (FP-LD). Since the optical gain of the FP-LD was polarization dependent, injection locking only took place for one of the polarization modes, that is, the output of the injection locked laser was operating in single polarization mode. 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 due to the current fluctuation of the drive circuit, was experimentally investigated. By alternatively switching a scanning Fabry-Perot 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.

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