Tapered Semiconductor Fibres for Nonlinear Photonics Applications

Semiconductor photonics is a field that is currently revolutionizing the future of modern optoelectronic devices. Although semiconductor materials are more commonly associated with electronic functionality (e.g., popular gadgets that use semiconductor microelectronic processors include cell phones, computers, and digital radios), to date a number of important photonic devices have been demonstrated using planar based substrates on a chip including silicon lasers and germanium photodetectors. More recently, however, the incorporation of semiconductor materials into the core of optical fibres has generated much interest as it provides a unique opportunity to completely integrate this technology with existing silica fibre infrastructures used in data transmission networks. Fiberized semiconductor devices offer some notable advantages over those developed on-chip such as simple, low cost fabrication (i.e., no need for multimillion dollar cleanroom based lithography) and robust and versatile waveguide geometries. Furthermore, the wide (visible to far-infrared) optical transparency of the semiconductor materials that can be incorporated into the fibre geometry ensures that their applications will extend far beyond optical communications to disciplines such as medicine, sensing, spectroscopy and security monitoring.

The work described in this proposal seeks to combine this exciting new semiconductor fibre platform with a key waveguide technology: tapered optical fibres. Conventional tapered silica fibres, which have varying waveguide dimensions along the length, have been exploited for a wealth of applications such as, optical signal processing, supercontinuum generation, remote sensing, as well as for optimized mode coupling between devices. The extension of these structures to incorporate semiconductor materials with rich optoelectronic functionality into the tapered cores will present new degrees of design flexibility for the optimization of semiconductor optical waveguides. The primary goal of this work will thus be to develop the procedures for the fabrication of high quality tapered semiconductor fibre structures and to demonstrate their potential for nonlinear photonic applications. This highly innovative project has the potential to lead to the development of a number of technologically disruptive all-fibre optoelectronic devices, for example, ultra-compact broadband mid-infrared laser sources for healthcare, frequency combs for chemical analysis, and highly nonlinear optical couplers and switches for ultrafast telecommunications.

This proposal seeks to develop the procedures for the fabrication of a new class of tapered semiconductor fibre structures and to demonstrate their use for nonlinear photonic applications. The success of this project will thus be measured via the development of technologically disruptive all-fibre optoelectronic devices. We have identified two areas within the EPSRC key research themes, namely information technology and healthcare, which will benefit significantly from these outcomes eventually leading to substantial economic and societal impact.

Within the proposed body of work we have described a number of nonlinear optical processes that could be used to develop devices which have the potential to increase the speed and capacity of data transmission rates, for example, switches, couplers and signal regenerators. These all-fibre integrated devices not only circumvent the complex coupling optics required to link between transport fibres and semiconductor processing chips, but the all-optical signal processing techniques on which they are based will also negate the need to convert between optical and electronic signals which is slow, costly and energy inefficient. Improved energy efficiency in data processing is essential if transfer rates are to continue to increase at the current level to reduce the load on electricity production, and thus the carbon footprint of communication networks. This aspect of "Green Photonics" could also have a significant impact on the environment, since currently ICT is responsible for 2% of global carbon emissions (equivalent to that of the aviation industry) and is expected to grow sharply in the future. Furthermore, we also expect that the tapered waveguide designs will be applicable for future developments in silicon devices on-chip, which are currently the subject of extensive research within some of the world's leading technological companies, including Intel, IBM and Luxtera. Leveraging off the multibillion-dollar silicon based electronics platform, silicon photonics is itself predicted to be a billion-dollar industry by 2020. The beneficiaries of these device developments will largely be information and communications companies and their consumers, who comprise much of the world's population.

The development of compact and inexpensive mid-infrared laser sources will also be of use for the telecommunications industry owing to the need for ultra-broadband systems to mitigate the looming data capacity crunch. In this wavelength regime, the extension to semiconductor fibre core materials is an important advance as for wavelengths >2um standard silica based fibres suffer from high losses. Examples of nonlinear processes that could lead to increased bandwidths include parametric wavelength conversion and mid-infrared Raman amplifiers. In addition, we anticipate that these sources will have a valuable societal impact through their employment in biological and medical applications. Specifically we have identified mid-infrared broadband supercontinuum sources for use in tissue imaging and analysis, and frequency comb generators for spectroscopy (used in disease diagnosis and for drug analysis). In the near term this research is most likely to benefit medical researchers, though we expect that longer term beneficiaries will include healthcare practitioners and patients.

As a research field, semiconductor fibre photonics is still very much in its infancy and, as such, it is difficult to predict all of the potential areas of impact. By training new staff in this area we will ensure that the UK has a solid skill and knowledge base for any future industrial and academic developments. The Optoelectronics Research Centre has a large network of industrial collaborators and so is well placed to ensure that the relevant industrial beneficiaries will hear about our research.