Silva, Claudio Fernandes Castanheira; (2002) Dense WDM channel synthesis using optical frequency comb generation and locked laser filtering techniques. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is concerned with the synthesis of a large number of optical carriers with high frequency stability for wavelength division multiplex (WDM) optical communication systems. Current WDM transmission systems include guard bands between optical carriers to allow for laser frequency drift. To minimise the laser drift and, consequently, reduce the necessary guard band, locking of the laser emission frequency to some reference is commonly used. Close to absolute frequency referencing is possible by the use of absorption peaks of atomic or molecular resonances. However, from considerations of physical dimensions, price and flexibility, application of these techniques for a large number of WDM channel lasers is not viable. Use of simpler frequency references allows guard bands to be reduced to 3 plus minus GHz, enabling commercial systems with 10 Gb/s, 25 GHz channel spacing, corresponding to 0.4 bit/s/Hz spectral efficiency. Polarisation interleaving and special modulation schemes can increase the spectral efficiency, although guard bands are still required to ensure long term system reliability. This work investigates a technique where channel lasers are phase locked to atomic transition referenced optical frequency comb generator (OFCG) output lines. The OFCG generates multiple absolute frequency references from a single atomic absorption stabilised laser source. The fibre based optical injection phase lock loop (OIPLL) technique ensures that channel lasers stay locked for a range of environmental and other parameter variations during the lifetime of the lasers. Experimentally, an improved 0.56 bit/s/Hz spectral efficiency without the use of polarisation interleaving or other special techniques is demonstrated, implying ~ 50% better usage of the available spectrum. Because widely tuneable lasers are used, the technique represents an enabling technology for wavelength-routeable and packet-switching optical networks. The application of the technique for synthesis of low phase noise, high power millimetre wave signals is also demonstrated.

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