Integration scores more GOALS
In October the European Commission initiated a 12-month EUR155,000 project (with EUR100,000 funding) called GOALS (Gallium-arsenide with Orientation-patterning for All-optical non-Linear Signal-processing). The project (coded IST-2001-37127) is part of the Future & Emerging Technologies section of the EC's Fifth Framework Programme.
The tailoring of non-linear optical properties of gallium arsenide (GaAs) wafers, used to manufacture guided-wave photonic components, is a key to competitive high-speed wavelength management in telecoms (box 1). Targeting breakthroughs in conversion efficiency and versatility, the GOALS project will explore a novel processing concept and carry out quantitative assessment of key parameters for a wide range of applications.
The GOALS consortium (box 2) aims to promote the use of the high intrinsic quadratic non-linearity (or χ(2)) of commonly used semiconductor materials to manufacture a wide range of ultra-fast all-optical data processing devices.
The corresponding switching, WDM-oriented frequency conversion and regeneration functions will render practical future high-capacity telecoms networks.
The basic technique involves parametric amplification in waveguides engineered with the appropriate spatial modulation of their non-linear optical (NLO) properties.
This can be achieved through a preliminary patterning of the crystalline orientation of the semiconductor wafers on which the guiding structures are fabricated (fig.1), according to the laws of quasi-phase matching.
The project aims to assess the feasibility of such orientation-patterned (OP) waveguides with standard fabrication techniques, using GaAs as the starting material.
At a time when the role of ultra-fast modules relying on optical functions is increasing — provided that some critical requirements such as low waveguide losses and unaltered NLO coefficients are met — GOALS opens up some very rich research that will take advantage of the large libraries of semiconductor processing parameters that are available. GOALS' aim for the long term is the monolithic integration of active pump diodes with waveguides, turning them into broadband multi-functional all-optical data processing chips.
GOALS partners will focus on studying the main obstacles to applying orientation-patterned GaAs waveguides.
The following risks, starting from most critical issues, have been identified will be used to evaluate the project's output:
- Final scattering and propagation losses should be kept low;
- The effective non-linear coefficient of quasi-phase matched interactions, taking into account actual orientation-patterning and waveguide quality, should reach at least 50% of the theoretical value calculated for a perfect structure;
- The thickness and homogeneity of GaAs from the thick growth step should not impinge on the polishing step and limit the size of the components (diced from 2in wafers).
The chosen method involves component processing, modelling and optical measurements on the guiding structures:
- Fabrication will start with orientation patterning of the initial wafers, followed by re-growth of GaAs, surface preparation and final fabrication of waveguides. Some samples will be characterised during the process, giving feedback for subsequent batches;
- Waveguide design will rely on data from literature and early measurements;
- Optical characterisation, both linear (evaluation of losses, effective index measurements) and non-linear (derivation of effective NLO coefficient from second-harmonic-generation efficiency, difference frequency generation) will be implemented.
Arnaud Grisard of prime contractor Thales says that GOALS should be followed in 2003 by another proposal under the EC's Sixth Framework Programme, possibly based on a larger partnership, to further benefit from the main advantages of the technology (polarisation-independent, room-temperature broadband frequency conversion, possibly with gain).
- Waveband conversion, from one set of WDM channels to another set of adjacent channels, is needed in high-bit-rate networks, but currently poorly handled by SOAs. (The spectral characteristics of this function also enable dispersion compensation.)
- C-to-L band conversion (and vice versa) could benefit, especially with gain at moderate pump power.
- All-optical switching is easily implemented by shifting data from one WDM channel to another.
- Optical parametric signal regeneration, including re-amplification, re-timing and re-shaping is limited by the lack of high c(2) waveguides, which GOALS could provide.
- Generation of correlated photon pairs for quantum information processing should be eased.
- Demonstrations of true all-optical switching (including promising optical transistors) based on cascaded quadratic nonlinear effects also await the same kind of material properties.
- All-optical data processing chips integrating lasers, nonlinear waveguides, AWG filters and tapers for fibre coupling.
The prime contractor of GOALS is Paris-based Thales Research and Technology — France (TRT-FR) (e-mail: firstname.lastname@example.org).
Other contractors include:
- Université Blaise Pascal, via Evelyne Gil-Lafon in the CNRS' Laboratoire des Sciences et Matériaux pour l'Electronique in Clermont-Ferrand (e-mail: email@example.com)
- Universita degli Studi Roma Tre, via Gaetano Assanto of the Laboratory of Optoelectronics in the Department of Electronic Engineering in Rome (e-mail: firstname.lastname@example.org)