Cacophonous lasers and their applications

• Main Author: Couch, P. R.
• Published: University of Cambridge 1988
• Subjects: 535; Optical signal processing
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.232984

Chaos, an unstable steady-state phenomenon, arises in apparently random optical sequences from semiconductor lasers subjected to reflection. This condition, referred to as cacophony, might provide a new pseudo-random source for use in coherent fibre optic systems. Coherent optical signal processing is expected to find substantially increased application, especially in local data networks. An optical spread-spectrum source may suit two apparent needs of these networks: 1) a high resolution optical time-domain reflectometer, using correlation of sequences, which can identify the closely spaced features found in these systems; and 2) data security through optical frequency-hopping encryption, especially in broadcast data networks. The link between cacophony and chaotic processes suggests that, although noise like, the spectral evolution of cacophony is deterministic. This implied reproducibility, akin the binary pseudo-random sequences, would be advantageous in spread-spectrum applications. Experimental examination of reflection effects on lasers has explored various lasing and external reflection conditions. Computer simulation of cacophonous generators supplement the experimental work with quick trials of experiments under typical, hypothetical, or even unrealisable conditions. A new in-phase and quadrature equivalent circuit models optical magnitude with phase information, and with modest computing requirements. Cacophony has been generated experimentally and in the computer model, and reproducible sequences up to 10ns long have been demonstrated. Modelling shows that reproducibility may be improved if conditions, especially at the start of lasing, are better controlled. It is concluded that, in order to reach the kind of optical sequence reproducibility that is called for in the applications described above, it is probably necessary to introduce quantisation into the generator. The work has attempted to characterize optical cacophony, and has perhaps added some knowledge to the general problems of coherent optical signal processing.