Silicon core fibres: extending the reach of nonlinear fibre systems

Optical fibre systems that can generate, amplify or manipulate light signals across a broad range of wavelengths and powers are highly desired for applications spanning optical communications to quantum processing and sensing. Although conventional silica glass fibres are routinely used in applications to transport signals in the 400-2000 nm spectral region, their high losses in the mid-infrared wavelength region (>2000 nm) excludes their use in emerging areas such as environmental monitoring, weather-resilient free-space communications, absorption spectroscopy, and quantum sensing. Additionally, the capabilities of silica fibres to manipulate the signals (e.g., modulate, convert, switch, regenerate) are limited. This is because optical signal processing relies on the ability to alter the transmission properties of the fibre by the presence of high power (nonlinear) light, and the nonlinear coefficient of silica is low. Although the low nonlinearity of silica can be overcome to some extent by using long fibre lengths (hundreds of metres) and/or high-power control beams (kilowatts), the resulting systems are typically bulky and expensive.

This project aims to address these key limitations to extend the application of nonlinear fibre systems by using a new class of fibre where the silica core has been replaced by a crystalline silicon material. Compared to traditional all-silica glass fibres, the silicon core offers a significantly higher nonlinear coefficient (> 100 times) and an extended transmission window covering much of the near to mid-infrared spectral regions (1200-8000 nm). By developing methods to reduce the losses, optimise the nonlinear conversion efficiency and robustly connect the silicon core fibres to commercially available glass fibre components (conventional silica fibres up to 2000 nm, and hollow core fibres or fluoride fibres for longer wavelengths), nonlinear systems can be constructed that support operation over a range of powers and signal wavelengths, as required by many practical applications.

Within the project, we aim to design and test the all-fibre connected silicon fibre systems for high performance and ease of use across various applications within the areas of optical communications and quantum technologies. For example, we will design devices that can triple the amplification bandwidth of telecom signals compared to existing technologies, thus enabling transmission of three times more data over the same optical fibre. We will use the extended transparency of the core to generate mid-infrared signals that can be used in high performance free-space data transmission, even in adverse weather conditions such as fog or rain. And finally, we will exploit the low losses and extended spectral coverage of the interconnected silicon fibres to produce alignment-free sources of quantum states of light for applications reaching beyond traditional quantum information systems and into exciting areas such as daylight satellite-to-ground secure communication, enhanced sensing through fog or smoke, and squeezed-state metrology in the mid-infrared. As well as opening up new avenues of exploration for nonlinear fibre systems, we expect this work will also help to increase the wide-spread adoption of silicon fibres within diverse research groups and photonic industries seeking robust, compact and flexible systems.