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Fluoro-aluminate amplifier for second telecom window


The majority of terrestrial optical fibre networks utilise the second transmission window of silica which lies at the wavelengths of 1280-1340 nm. World-wide, 55 million kilometres of second window silica fibres have been installed. Currently 80% of UK telephone traffic is being carried by optical fibres operating in the second window. At these wavelengths no efficient and marketable optical amplifier is at present available, necessitating the use of electronic repeaters. Compared with optical amplifiers, electronic repeaters are more expensive, slower and less flexible in operation. Therefore in order to take full advantage of the potential capacities of optical fibre communications, it is necessary to upgrade the networks to all optical systems. The main objective of this project is therefore to realise a Neodymium (Nd)-doped optical fibre amplifier for the second telecommunications window and to demonstrate a fully characterised industrial prototype for future network trials of these components. Specific objectives are:
To optimise the composition of the core glass so as to obtain a significant blue-shifting of the emission and Excited State Absorption (ESA) curves. This will lead directly to the achievement of high gain in the centre of the second window.
To develop purification processes leading to high-volume production of high purity fluoride starting materials. This procedure will allow fabrication of fibres containing low levels of impurities and hence exhibiting low background loss.
To optimise the manufacture of fluoro-aluminate preforms and fibres and to obtain high quality low-loss single mode Nd3+-doped fibres.
To investigate a range of techniques for in-fibre suppression of 1050 nm Amplified Spontaneous Emission (ASE) and to implement the most effective method.
To fabricate a second window fibre amplifier laboratory prototype device which will then be fully tested and characterised.
To carry out a comprehensive assessment of the mechanical strength and environmental durability of the high quality single mode fibres.
To fully engineer and refine a prototype commercial second window amplifier device.
To accomplish full technology transfer from the academic participants to the industrial partners.
To promote a rapid deployment of the amplifier device in the networks.
At the present state of the project, glass compositions have been carefully optimised with significantly reduced ESA compared with other commercially available glasses. Moreover new methods have been developed for host glass design allowing the blue-shifting of the emission spectrum and the achievement of a flat gain curve on the long wavelength side, thus making possible WDM applications.
As an example, the gain curves at 1.3 um of three Nd3+-doped fluoro-aluminate glasses. Glass AlF70 (hollow circles) was developed previously; glasses AlF117 (solid triangles) and AlF123 (solid squares) were recently developed by the FAST project. Two highly important facts are immediately apparent. First, the peak of the gain curve in the new glasses is blue-shifted by ~ 5 nm compared with the old glass. Second, the effect of ESA is greatly reduced in the new glasses, especially so in glass AlF123. This is shown by the gradual decrease of gain on the short-wavelength side, as compared with the abrupt drop in AlF70.

Fibre drawing is a core activity of the FAST project; after having improved the preform fabrication process a fibre with significantly reduced losses has been obtained. Further technology refinement will allow achievement of low loss single mode doped fibre suitable for an amplifier device.
Expected Impact
The results obtained are to be used in developing plans for network systems. The concepts developed will ensure that operators will be in a position to provide materials, know-how and technologies to support a large scale realisation of second window optical amplifiers. Once working and engineered prototypes are made, the second window amplifier will be introduced as a major breakthrough in the market of optical amplifiers, aiming particularly at the CATV and access network business, taking advantage of the low cost of pump laser diodes. Other interesting applications will be: optically amplified multi-channel links with several optical carriers multiplexed in the second window, to be used especially where the lack of fibres requires the maximum exploitation of the fibre bandwidth, and bench-top and module amplifiers for laboratory use and instrumentation.

Main contributions to the programme objectives:
Main deliverables
A prototype Neodymium-doped optical fibre amplifier
Contribution to the programme
Supports the development of cost-effective second window optical fibre amplifiers
Technical Approach
The project is divided into tasks assigned to partners according to their expertise, as described below. Brunel University has carried out compositional modifications aimed at optimising the spectroscopic characteristics of the core glass. In particular Brunel has developed new glasses which demonstrated a significant blue-shifted gain curve thus achieving one of the main objectives towards the realisation of efficient and commercial second window amplifiers. Merck Ltd supplies high purity materials necessary for glass-making, whilst the University of Southampton has assessed the gain parameters of the new compositions. The most successful core compositions has been incorporated into preforms by Leeds University, and fibre drawing has been carried out by Southampton resulting in low loss fibre. The University of Southampton in collaboration with Pirelli Cavi will engage in developing efficient methods of ASE suppression focusing in particular on two promising techniques: namely, absorption by a co-dopant, and filtering by an in-fibre Bragg grating. Finally Pirelli Cavi will realise the 1.3 micron optical amplifier prototype by developing activities on polishing, pigtailing and packaging. Its associated partner, University of Parma, has developed numerical models in order to provide reliable tools to analyse the amplifier behaviour, including gain and noise characterisation.
Summary of Trial
The FAST project intends to identify, purify, dope and characterise materials in order to realise a second window optical fibre amplifier prototype and further to fully engineer the device itself. The FAST consortium believes it can realise the 1300 nm fibre amplifier prototype within the two year project length. Being a component development project, demonstration of the final amplifier will be initially in the laboratories of the partners, but liaison will be maintained with other ACTS projects with a view to demonstrating the amplifier within a system trial should that prove feasible.
Key Issues
Good quality single mode fibre drawing.
Optimisation of techniques for suppression of ASE at 1050 nm.
Numerical modelling of second window amplifier in order to provide useful tools for amplifier design.
Packaging of the active fibre in order to have a reliable and ruggedized device.

Funding Scheme

CSC - Cost-sharing contracts


Pirelli Cavi SpA
Viale Sarca 222
20126 Milano

Participants (5)

United Kingdom
Kingston Lane
Merck Ltd
United Kingdom
United Kingdom
Woodhouse Lane
LS2 9JT Leeds
United Kingdom
Southampton Ocenagraphic Centre, European Way
SO14 3ZH Southampton
Università degli Studi di Parma
Via Cavestro 7
43100 Parma