Reflective variable optical attenuators and fibre ring lasers for wavelength-division multiplexing systems

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Reflective variable optical attenuators and fibre ring lasers for wavelength-division multiplexing systems


Author: Liu, Heliang
Title: Reflective variable optical attenuators and fibre ring lasers for wavelength-division multiplexing systems
Degree: Ph.D.
Year: 2005
Subject: Hong Kong Polytechnic University -- Dissertations.
Optical fibers.
Optoelectronic devices.
Wavelength division multiplexing.
Department: Dept. of Electrical Engineering
Pages: iv, 159 p. : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: Wavelength division multiplexing (WDM) optical fibre system is an important enabling technology to fulfill the demands for bandwidth in the modern information age. The main objective of this project is to study novel devices with the potential to enhance the performance of WDM systems. In particular, a novel reflective variable optical attenuator (RVOA) used for dynamic gain equalization (DGE) and fibre lasers based on an entirely new type of erbium-doped fibres with ultrawide tuning range were investigated theoretically and experimentally. We proposed a new type of RVOA device which could be potentially integrated with arrayed waveguide grating (AWG) to reduce the cost of DGE substantially. Initially, fibre-based RVOAs, fabricated with optical fibre components such as fibre coupler and Faraday rotator mirror, were investigated theoretically and experimentally. Larger attenuation range up to 22 dB was realized for fibre coupler-based ROVA with a Faraday rotator mirror and its polarization-dependent loss is about 0.5 dB. Then polymeric waveguide-based RVOAs were investigated theoretically and experimentally. Using an epoxy Novolak resin as core material and an UV-cured resin (Norland's NOA61) as cladding material, a polymeric waveguide RVOA was successfully fabricated. The dynamic 15 dB attenuation range was achieved and the PDL was less than 0.2 dB. The measured insertion loss of the polymeric waveguide RVOA was too large (about 18 dB) and was mainly induced by coupling loss, material loss and poor alignment. In addition, a novel low-loss waveguide crossing design technique based on MMI structure was proposed and investigated by simulations using a two-dimensional beam propagation method. The low-loss waveguide crossing propagation can be achieved when a slab is introduced to the image reformed region of MMI structure, where any perturbations immediately outside the waveguide would have little effect to the propagation of the optical field. The advantage of the new low-loss MMI waveguide crossing technique is shown by comparing with the common taper width waveguide crossing technique. The application of MMI structure in small angle intersection was also demonstrated and angle intersection as small as 20o with low insertion loss (<0.15 dB) can be realized with MMI structure technology. This technique is very useful in the design of compact RVOA array with high channel-count. In the second part of the study, fibre ring lasers with continuous wavelength tuning over wide wavelength range and fibre ring lasers with discrete wavelength tuning were investigated. Tunable lasers are important devices in WDM systems because they could be employed as reserved sources and therefore avoiding the need to stock large inventory of lasers to cover the ITU-wavelength grid. In this project, erbium ions doped bismuth oxide glass fibres instead of the common silicate fibres were used. Due to the high solubility of erbium ions in bismuth oxide based erbium-doped fibre (Bi-EDF), a stable and short-cavity fibre ring laser was realized by employing only 0.85 m of Bi-EDF. Ultrawide continuous wavelength tuning range over 100 nm was obtained by using a tuning MEMS filter or scanning fibre FP filter. A very narrow linewidth, single-longitudinal mode fibre ring laser, using an unpumped EDF acting as saturable absorber in the cavity, was realized. The linewidth of the fibre ring laser was measured by two optical mixing methods and about 1.17 kHz linewidth was measuring by the delayed self-heterodyne technique. In addition, two types of wavelength-switchable fibre ring laser based on polarization wavelength selection were investigated. The principle of polarization wavelength selection to attain the switchable function was explained. The first type employs an unbiased FP-LD to act as the polarization and etalon filter. Eight lasing wavelengths switchable from 1552.98 nm to 1560.80 nm with a separation of 1.1 nm, which is defined by the FP mode of the FP-LD, were achieved. The second design used only 0.85 m of Bi-EDF as gain medium. Thirty-five switching wavelengths can be achieved in the range of 1545 nm to 1573 nm with a wavelength separation of 0.8 nm, which is defined by the free-spectral range of the F-P etalon filter. The laser exhibits high output power of about 0 dBm, high SMSR better than 45 dB and output power flatness of 3 dB. Excellent wavelength and power stability of 1 pm and less than 0.15 dB, respectively, were obtained during a 2-hour stability test.

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