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Details of Grant 

EPSRC Reference: GR/H82907/01
Principal Investigator: O'Mahony, Professor M
Other Investigators:
Siddiqui, Dr S
Researcher Co-Investigators:
Project Partners:
Department: Computing and Electronic Systems1
Organisation: University of Essex
Scheme: Standard Research (Pre-FEC)
Starts: 01 January 1993 Ends: 31 December 1995 Value (£): 175,048
EPSRC Research Topic Classifications:
Networks & Distributed Systems Optical Communications
EPSRC Industrial Sector Classifications:
Related Grants:
Panel History:  
Summary on Grant Application Form
1. To study the non-Linear mechanisms associated with the use of erbium fibre amplifiers in optical transmission systems. 2. To study the limitations on network performance associated with these non-linearities.3. To investigate most appropriate transmission techniques to minimise these effects.Progress:The work commenced in January 1993 on three fronts as outlined in the initial proposal. These were(a) a survey of the latest published work in the field(b) experimental work, and(c) modelling.The practical work started by setting up an experiment to observe and identify the non-linear effects in a system comprising several lasers, an Er-doped fibre amplifier (EDFA) and transmission fibre. The major non-linear effect imposing limitations in such systems was found to be four wave mixing (FWM), caused by the high power associated with the amplifier and the fibre non-linearities. Various parameters involved in FWM such as wavelength separation, polarisation of each optical channel, fibre input power and fibre dispersion were identified and quantified (objectives 1,2). The effect on signal to noise ratio of crosstalk generated via FWM were also measured. The second phase of the experimental work was to construct a subcarrier multiplex comprising two video channels directly modulating a DFB laser whose output was amplified by an EDFA connected to dispersion shifted fibre. The experimental work showed that FWM is not a problem in single optical channel systems regardless of the number of video channels present in the SCM. This directed us to study the effect in multiwavelength systems where both digital and analogue modulated optical sources are amplified by the same EDFA and transmitted over a common fibre. Current results show that the analogue channel with carrier frequencies ranging from-50 MHz to -500 MHz can tolerate crosstalk levels (which could be from non-linearities) as high as -14dBm before video signal impairments become perceptible; however, the digital channel was found to be more sensitive to FWM crosstalk. These measurements are still in hand. These results mean that we can recommend suitable design criteria (objective 3) to minimise effects. The experimental and modelling work undertaken on non-linearities and in particular four wave mixing has had a significant spin off to other projects and publications. The work has highlighted the serious effect of non-linearities in systems using erbium amplifiers with low dispersion and enabled us to quantify the number of wavelengths that can be supported in a system as a function of distance. This turns out to be particularly important for future access networks which may employ large numbers of wavelengths. At the moment of writing six publications are associated with the work carried out in this project, but space prevents listing. As an example the most recent accepted paper is:S. Hamidi and M.J. O'Mahony, Sensitivity of Wavelength Division Multiplex Distribution Systems Incorporating Analogue and Digital Lightwave Channels to Four Wave Mixing Crosstalk . Accepted Cleo '95, 22-26 May 1995, Baltimore, USA.
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Organisation Website: http://www.sx.ac.uk