Two-Photon Absorption Based Optical Sampling for High Capacity OTDM Communication Systems

Abstract

High capacity optical communication systems based on optical time division multiplexing have the potential to meet the need for increased network capacity and flexibility. In order to implement such a system all-optical signal processing technologies, which are not limited by electrical bandwidths, are required. This thesis presents the development of an ultrashort pulse source and an optical sampler suitable for use in OTDM systems. The optical pulse source is based on a gain-switched laser diode. The timing jitter and spectral quality of this source has been improved with the use of CW injection from an external DFB laser diode source. The optical pulses of ≈15ps produced directly from the gain-switched laser are compressed with an external fibre based compressor. This produces optical pulses of 0.5-3 ps duration that are suitable for OTDM communication systems. The design of this fibre compressor has been greatly simplified and enhanced with the use of the FROG technique. The FROG technique completely characterises the magnitude and phase of the optical pulses. This information is then used to numerically evaluate, via the nonlinear Schrödinger equation, the pulse shaping effects of propagation through various optical fibres. Thus the optimal pulse compressor can be numerically designed, before the physical implementation. The optical sampler is based on the optical nonlinearity provided by two-photon absorption (TPA) in semiconductors. Specifically this sampler utilises two-photon absorption at incident wavelengths of 1.5 µm in a multiple quantum well Fabry-Perot laser diode whose bandgap energy corresponds to a wavelength of 1.3 µm. This device is shown to be a highly sensitive nonlinear detector whose output photocurrent depends quadratically on the input optical intensity. The use of this TPA sampler to carry out a number of applications relevant to OTDM systems is investigated. A highly sensitive TPA based real-time optical sampling system for characterising the temporal intensity profile of short optical pulses at 1.5 µm, such as those used to transmit data in OTDM communication systems, has been developed. This system is able to characterise the temporal profile of optical pulses with energies as low as 400 fJ, with a temporal resolution of 2 ps. The sampling technique is extended to allow for the measurement of a sonogram trace. The sonogram trace allows for the retrieval of the complete temporal intensity and phase of a short optical pulse The TPA. based sonogram technique is used to completely characterise 1.5 µm pulses with energies as low as 12.5pJ. the use of the TPA based optical sampler as a demultiplexer in an OTDM system with an aggregate bit-rate of 80 Gbit/s is demonstrated. The TPA demultiplexer is shown to select out a single 500 Mbit/s channel in an essentially error free manner.