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Customer access photonics: an integrated technology for active, low-cost devices

Objectif

to establish the viability of a low-cost photonic component technology, based on sol-gel glasses on silicon;
to demonstrate the superior functionality of this technology, by fabrication and development of advanced active materials and devices;
to bring successful basic research in this technology forward towards commercial exploitation.
low loss sol-gel waveguides and passive components on silicon;
new glass materials for integrated optical amplifiers;
planar waveguide optical amplifiers;
automated equipment for sol-gel waveguiding film deposition;
demonstration of cost/performance advantages of the technology.
Expected Impact
The direct result of CAPITAL, if it is successful, will be to support the confidence in the technology which is necessary for commercial development and exploitation. The availability of low cost, advanced functionality components would then greatly increase the flexibility and scope for systems designers, particularly to address advanced access network needs.

Main contributions to the programme objectives:
Main deliverables
Establish the industrial viability of sol-gel glasses on silicon as a low cost photonic component technology
Contribution to the programme
Combination of a broad and advanced functionality with low cost, in components for photonic networks
Technical Approach
The basis of silica-on-silicon photonics is to deposit multi-micron silica-based glass layers on silicon substrates, to define single-mode channel waveguides in this material, and to couple these guides to external fibres with minimal loss.
In CAPITAL, the layers are deposited by spin coating using the sol-gel process. This involves the use of liquid metallorganic precursors of the desired glass composition; for example, tetra-ethyl silane for SiO2. Reaction of the precursors in solution forms suspended nano-particles of the glass; this suspension is spun to a thin layer on the substrate, whereupon solvent evaporation causes the film to 'gel'. The gel is heated to remove residual organics and to complete densification; this induces large tensile stresses which must be annealed out to prevent cracking. Iteration of this process is used to produce a multi-micron film.
By varying the precursor ratios in the sol, glasses of widely varying composition, and thus index, can be achieved. By this technique a bilayer is formed consisting of a lower index buffer and a higher index guiding layer (core). This guiding layer may also be doped with a great variety of functional materials and other dopants; for example, rare-earths for optical gain. Waveguides are then defined by photolithography followed by reactive ion etching to produce channels; these can be reflowed to improve shape and surface quality, and then buried under a top cladding glass. Alternatively, light propagating in the core is guided by shallow ridges on the upper cladding layer (strip loading); in this way etching and reflow of the active layer are avoided.
Work in Progress

Fabrication Equipment and Processes
The iterative deposition process, when carried out manually, is excessively labour intensive, and insufficiently repeatable. A key objective is to design, construct and test an automated machine for this purpose, and to develop processes for producing various film compositions with it. Construction of the machine is now complete, and initial tests have been made; further development will allow wafer and device yield issues to be assessed, and thus add evidence and detail to our cost analysis.

New Host Glasses
Silicate glasses are not ideal hosts for integrated erbium doped amplifiers, where a short path length is needed. Other oxides and sulfides, having low phonon energies and thus offering reduced non-radiative de-excitation, may provide enhanced performance. We have been developing non-silicate oxide waveguides, with a particular focus on germanate glasses. Key technical challenges involve avoiding crystallisation during annealing, and in minimising OH contamination. These glasses already show promising characteristics; waveguides have been fabricated with high fluorescence lifetimes.

Dopants in Nanocrystalline Hosts
In previous work by the partners, techniques were developed to dope sol-gel films with semiconductor nanocrystals. We are now investigating doping with rare earth containing nanocrystals, to achieve strong amplification over narrow wavelength bands, for notch amplifiers, lasers and active filters. This can involve the crystallite acting as an enhanced matrix in which the active ion is a dopant, or a self-activated crystallite of which the rare-earth is an intrinsic part. Dopants investigated are praseodymium and erbium; both systems have been fabricated, and in the latter, high fluorescence lifetimes have been achieved at high concentrations

Amplifiers
The central component objective is the fabrication of erbium-doped integrated amplifiers, for application in lossless splitters, and other system requirements. A key goal is obtaining the required net gain, e.g. > 10 dB, within a short path length, to minimise device size. This requires strong confinement through large index difference guides. We have fabricated strip-loaded channel waveguides based on silica-titania guiding layers, and through the use of codopants have achieved high quenching concentration (0.5 at. %), and good spectroscopic performance, indicating the potential for short amplifiers. Gain has been achieved. By further reducing OH impurity, scattering losses and coupling losses, we anticipate achieving the first sol-gel amplifier in the first half of '97. Later we aim to fabricate an amplifier based on new materials developed in the project, to illustrate the wide functionality and flexibility of the sol-gel system.

Passive Devices
Work on passive components has shown that these can be fabricated using the CAPITAL sol-gel technology. Components have been fabricated based on buried channel waveguides in borophosphosilicate glass, giving excellent fibre compatibility and thus low fibre-to-fibre loss (< 2 dB). These include passive splitters as well as thermo-optic switches. Key future goals are to develop constituent functions required for fully integrated amplifiers; in particular, a tapered coupler for efficient power transfer between fibres and strong confinement guides, and a wavelength selective filter for coupling pump and signal wavelengths. A key issue is the need to achieve minimal loss with high confinement waveguides.

Integration and Applications
The long-term exploitation potential of this technology will come from the incorporation of active and passive functions in advanced systems components. CAPITAL is studying the design and specification requirements of such components in specific applications, and feeding this information back into the ongoing planning of material and fabrication workpackages. Links with systems projects through Domain activities plays a key role in this guidance.
Summary of Trial
The project is mainly concerned with developing photonic components and with related materials issues. Trials will involve testing of materials and devices within the consortium, with application targets developed in concertation with other ACTS projects. Where possible and appropriate, components will be evaluated in trials organised with other projects. Such plans are under development.
Key Issues
maximising passive and active optical quality of amplifier materials;
cost/performance values relative to alternative component technologies;
linking of component development to key systems requirements;
integration of functions using low cost processes.

Appel à propositions

Data not available

Régime de financement

CSC - Cost-sharing contracts

Coordinateur

Imperial College
Contribution de l’UE
Aucune donnée
Adresse

SW7 2BT London
Royaume-Uni

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Coût total
Aucune donnée

Participants (7)