John M. Senior

Developments in wavelength division multiple access networking

The last few years have seen a growing interest in WDM for optical networks in order to increase the capacity and overcome the electronic bottleneck. Alongside the improvement in the WDM components there has been continuing development of WDMA networks. A large number of different WDMA strategies have been investigated in terms of both network architecture and the associated protocol requirements. This article identifies major themes and provides examples of experimental and theoretical developments which are anticipated to facilitate WDMA networking.

Performance evaluation of optically pre-amplified PPM systems

An original analysis is presented for an optical fiber n-ary pulse position modulation (PPM) system employing optical preamplification. The theoretical results, from calculations performed at a bit-rate of 622 Mbit/s and a wavelength of 1.53 /spl mu/m, demonstrate that the PPM system offers a 7.5 dB sensitivity benefit in comparison with an equivalent PCM system. Furthermore the results illustrate that optically preamplified PPM offers sensitivities significantly better than the fundamental limit of an optically preamplified PCM system and in fact approaches that of coherent detection.<<ETX>>

Misalignment losses at multimode graded-index fiber splices and GRIN rod lens couplers

A geometric optics model to describe transverse offset splice loss in multimode graded-index fiber, which has been adapted to evaluate misalignment losses at a graded-index rod (GRIN rod) lens-to-lens coupler, is reported. In both cases, the model gives close agreement with experimental measurements. Finally, in contrast with fiber splice losses, invariance of the lens coupler loss with the optical launch and receive conditions is shown.

An optically preamplified intersatellite PPM receiver employing maximum likelihood detection

An original analysis, facilitated by the use of Gaussian approximation (GA) and Chernoff bound (CB) techniques, is presented for an optically preamplified intersatellite pulse position modulation (PPM) receiver employing maximum likelihood detection (MLD). The theoretical results, from calculations performed at a wavelength of 1.54 /spl mu/m and bit rate of 25 Mb/s demonstrate that the receiver is approximately 1.5 dB more sensitive than an earlier optically preamplified PPM receiver and also more sensitive than the fundamental limit of optically preamplified on-off keyed (OOK) NRZ signalling. Thus, it is demonstrated that the proposed receiver configuration is a strong contender for implementation in future laser intersatellite communication systems.

Dual wavelength referencing of optical fibre sensors

Dual wavelength configurations normally employed to eradicate the effects of variable signal losses in intensity-dependent optical sensors are discussed. Particular attention is paid to the referencing capability of each configuration in relation to the major sources of measurement contamination. Illustrative examples of generic type sensor system implementations are provided and assessed to identify areas of any serious shortcomings. It is found that while full referencing can be achieved with both single and dual LED systems the maximum achievable performance will be dependent on the extent of differential spectral effects which can occur because of a finite wavelength separation between the two optical signals.

Angular tilt misalignment loss at a GRIN rod lens coupler.

A geometric optics model for evaluating end separation and transverse offset losses at a graded-index (GRIN) rod lens to lens coupler is extended to the case of angular tilt misalignment loss. Two different fiber–lens combinations are examined, and the model is verified by comparison to experimental measurements, which show close agreement to the theoretical predictions. In addition, invariance of the angular tilt loss to differing optical launch and receive conditions is shown, and a possible explanation for this phenomenon is provided. Finally, the theoretical model is used to predict fiber–lens combinations which minimize possible angular tilt losses.

Methods for providing stable optical signals in dual wavelength referenced LED based sensors

In general LED based dual wavelength referenced intensity-modulated optical fiber sensors do not take account of the thermal drifts of the LED source(s). An optical design concept termed wavelength thermal matching is described that can be applied to extract a pair of stable optical signals from the emission of a single LED. This strategy ensures the selection of two separate wavelength bands within which the optical signal variations due to LED thermal effects will be very similar. Hence the application of wavelength thermal matching principle in the design of a dual wavelength referenced system provides referencing effectiveness against both the optical propagation losses and the optical source fluctuations.<<ETX>>

WDM crosstalk analysis for systems employing spectrally-sliced LED sources

Linear crosstalk due to interchannel spectral overlapping in wavelength division multiplexed (WDM) systems employing spectrally-sliced LED sources is examined with the aid of a new analytical approach. Mathematical expressions are obtained that predict that, in general, the crosstalk in such WDM systems will be approximately six times the square of the channel separation expressed as a fraction of the spectral bandwidth of the channels. Experimental optical crosstalk measurements are also reported for a dual-channel spectrally-sliced LED sourced WDM system that support the theoretical predictions.

Method for extracting thermally stable optical signals from a GaAlAs LED source

A self-compensating scheme is described that eliminates the need for temperature control devices employed in many LED-based optical test and measurement instruments to ensure optical signal stability. Thermal behavior of GaAlAs LED sources is exploited to provide an optical wavelength band signal with 0.1%//spl deg/C power level stability.<<ETX>>

Single LED based dual wavelength referenced optical fibre sensor system using intensity modulation

Abstract An intensity-modulated optical fibre sensor system is described which employs a single LED source to provide the measurand and the reference signals with two separate wavelength bands. The allocated wavelength bands are selected to minimize any differential intensity effects that may arise from LED thermal variations. The sensor system, comprising a transceiver unit connected to an optical displacement sensor using a single optical fibre, provides an output fully referenced for all major common-mode variations that are likely to occur. Performance characteristics for the prototype system are reported which show a linear displacement range of over 20 mm.