Multi-photon imaging is a ubiquitous tool in life sciences research, where pulsed tuneable lasers are required for precision microscopy. The majority of multi-photon imaging research employs two-photon fluorescent techniques, however three-photon fluorescence is emerging as a powerful instrument for deep-tissue (> 1mm) imaging as it offers reduced tissue scattering and enables access to a wide variety of fluorescent dyes and proteins. Non-destructive and non-invasive high-resolution imaging of cells through surrounding tissue and bone would be groundbreaking for research into areas including regenerative medicine and leukemia. The ideal three-photon excitation source is a low repetition frequency, high-energy femtosecond laser tuneable in the near-infrared with low average power to avoid tissue heating. The laser industry is focused on the two-photon imaging market, serviced by well-proven ~100MHz fixed wavelength and tunable sources. Three-photon excitation systems based on optical parametric amplifiers (OPAs) are available from select manufacturers, however these are highly inefficient and are prohibitively expensive for the majority of research facilities. A collaboration between an industrial laser manufacturer (Chromacity, UK) and STFC-funded academic research in photonics (McCracken, Heriot-Watt University), this project will demonstrate prototype cost-efficient lasers for three-photon microscopy, addressing this customer-driven demand by exploring two novel laser architectures to realize few-MHz optical parametric oscillators (OPOs), pumped by Chromacity's robust fiber laser technology. We will combine patented HWU IP in the generation of few-MHz high-energy OPO pulses with know-how in the construction of dispersion- controlled compact cavities to develop a commercial alternative to the dominant market offering. Working directly with early-adopters (Packer, Oxford; Lo Celso, Imperial; Williams, Edinburgh) and industrial beneficiaries (Scientifica), we will evaluate our OPO and develop it to a level where it can be brought to market in a compressed timeframe.

At a glance: Microstructured optical fibres are transforming science and technology in fields spanning telecommunications through to healthcare. Their unique offering of guiding properties continues to push the limits of established photonics and drive novel innovation and scientific discovery. However, a limit to this potential is approaching because many theoretically transformative fibres cannot be realised in practice due to manufacturing challenges. With this fellowship, I aim to unlock this unmet potential by developing a freeform optical fibre manufacturing process, which is unbound from conventional manufacturing constraints. The vast majority of optical fibre is produced for the telecommunications sector to satisfy exponentially rising data capacity needs. The type of fibre used in telecoms is typically conventional step-index fibre, comprised of a silica glass core surrounded by a lower-index doped-silica cladding. Solid fibre is inexpensive and guides with reasonably low-loss, but is fundamentally limited in performance by material absorption, scattering and high-dispersion amongst other factors. Over the past few decades, another type of optical fibre has emerged - microstructured optical fibre (MOF). MOF utilises a structured-material core-cladding in which light is guided through complex waveguiding mechanisms. Depending on the type, MOF can offer several advantages over conventional fibre including broad spectral transmission, low bend-loss, low latency and high-power delivery. Remarkably, certain MOFs guide light within a hollow region of the fibre. These so-called hollow-core fibres overcome problems faced by solid-core fibres such as material absorption, dispersion, optical damage and latency, as well as enabling an innovation-rich field of gas-filled sensors and light sources. MOF is manufactured by an approach known as stack-and-draw. Stack-and-draw is a two-step process: firstly, circular glass capillaries, rods and tubes are stacked laterally, often with added spacers, to form a scaled-up approximation of the fibre known as a preform. Secondly, the preform is drawn to fibre through a high-temperature furnace. The design of MOF developed so far has been heavily steered by the restrictive stacking process, e.g., hexagonally-packed Kagomé fibre and circle-tubular antiresonant fibre. Unfortunately, several types of MOF that have shown huge potential theoretically cannot be reasonably stacked, and so the vast applicability of MOF is beginning to plateau. To unlock this potential, we will develop a new preform manufacturing process capable of producing freeform fibre, i.e., fibre with arbitrarily structured cross-section, without compromising on fibre quality. In the proposed approach, short segments of the preform are precisely and arbitrarily machined using tailored laser-manufacturing methods. These segments are then bonded axially to form the preform which is drawn to fibre using traditional methods. Building upon a recent early feasibility demonstration, the fellowship will facilitate an overhaul of the laser-based approach to fabricating preforms and investigation of optimal glass bonding techniques. Amongst a trove of benefits, freeform fibre will bring drastically lower loss, increased stability, faster data transfer speeds and novel spectral guidance. The later stages of the fellowship will focus on developing fibre with unprecedented guiding performance and exploring applications of fibre with novel geometry. We aim to develop an industry-ready manufacturing method for freeform silica optical fibre, and further improve high-resolution glass macro-fabrication and advanced bonding and assembly capabilities. This work is expected to open up a new field of fibre optics research and nurture a team of dedicated researchers.

Multi-photon imaging is a ubiquitous tool in life sciences research, where pulsed tuneable lasers are required for precision microscopy. The majority of multi-photon imaging research employs two-photon fluorescent techniques, however three-photon fluorescence is emerging as a powerful instrument for deep-tissue (> 1mm) imaging as it offers reduced tissue scattering and enables access to a wide variety of fluorescent dyes and proteins. Non-destructive and non-invasive high-resolution imaging of cells through surrounding tissue and bone would be groundbreaking for research into areas including regenerative medicine and leukemia. The ideal three-photon excitation source is a low repetition frequency, high-energy femtosecond laser tuneable in the near-infrared with low average power to avoid tissue heating. The laser industry is focused on the two-photon imaging market, serviced by well-proven ~100MHz fixed wavelength and tunable sources. Three-photon excitation systems based on optical parametric amplifiers (OPAs) are available from select manufacturers, however these are highly inefficient and are prohibitively expensive for the majority of research facilities. A collaboration between an industrial laser manufacturer (Chromacity, UK) and STFC-funded academic research in photonics (McCracken, Heriot-Watt University), this project will demonstrate prototype cost-efficient lasers for three-photon microscopy, addressing this customer-driven demand by exploring two novel laser architectures to realize few-MHz optical parametric oscillators (OPOs), pumped by Chromacity's robust fiber laser technology. We will combine patented HWU IP in the generation of few-MHz high-energy OPO pulses with know-how in the construction of dispersion- controlled compact cavities to develop a commercial alternative to the dominant market offering. Working directly with early-adopters (Packer, Oxford; Lo Celso, Imperial; Williams, Edinburgh) and industrial beneficiaries (Scientifica), we will evaluate our OPO and develop it to a level where it can be brought to market in a compressed timeframe.

The ability to accurately measure the power and frequency (or wavelength) distribution of an optical signal is crucial to a vast range of applications, for spectroscopy in medicine, ensuring the safety of food or pharmaceuticals to remote sensing of gasses and fundamental science, e.g. characterising short laser pulses or finding the atmospheres of extrasolar planets. Currently, this is achieved using Optical Spectrum analyzers or optical monochromators, which have a key limitation. To achieve high-resolution they need a large optical path length and therefore large footprint (optical path length on the order of 0.5-1 m is common). Thus these devices are bulky and expensive. While not an issue for lab-based low-volume applications, this excludes their use - and thus the use of high-resolution spectroscopy - in large volume, or footprint and weight-sensitive applications, e.g. integration into lab-on-a-chip devices, mobile phones and low mass satellites (e.g. cube-sat). These applications can only be served by integrated on-chip spectrometers. Here the use of speckle spectrometers, using the random scattering of light to achieve a high wavelength resolution in an ultra-small footprint would be highly promising if it were not for the case that typical the multiple scattering needed to create the speckle results in most of the light being scattered out of the device before it can be detected. However, over the last decade, several groups (including myself) have shown that the statistical distribution of scattering sites can be used to control the amount and direction (e.g. within the plane of the device vs out-of-plane) of light scattering. In this project we merge these advances with speckle spectrometers, i.e. using controlled disorder to efficiently generate a speckle pattern, while virtually eliminating out-of-plane scattering and optical losses. Building on this advance we will demonstrate a high resolution, low footprint on-chip spectrometer that outperforms the state of the art by orders of magnitude (in device footprint) without sacrificing the device resolution. We will also demonstrate that these devices are suitable for future large scale manufacturing, using pre-existing CMOS facilities, are suitable for gas spectroscopy and laser pulse spectrum analysis and compatible with future integration with optical detectors for a direct electronic readout. This would present a game-changing advance in the field of integrated spectrometers and lay the foundation for future commercialization of integrated speckle spectrometers.

This project is a joint proposal between researchers in Photonics (Reid, Heriot-Watt University) and Robotics (Ramamoorthy, University of Edinburgh). It is a cross-disciplinary collaboration, which is necessary in order to tackle in a new and exciting way the problem of fugitive emissions of methane and volatile hydrocarbons from installations such as refineries, petrochemical plants, carbon-capture and storage facilities and landfill sites. These emissions cost the energy sector up to $5B per year, account for 12% of greenhouse gas emissions and jeopardise worker safety and public health. Our idea uses mid-infrared laser light to sense the presence of hydrocarbons by looking for characteristic absorptions at wavelengths specific to individual chemical species. Such "gas absorption spectroscopy" is far from new, but we will implement it in a radically different way to conventional approaches. Normally, optical gas detection works by transmitting a single-wavelength through a gas and looking for an intensity change. For fugitive emissions sensing, this is implemented using a technique called DIAL, which shines an intense beam into the air and detects the weak backscattering of this light from particles in the air (Mie scattering). By looking for small differences in the backscattered intensity between two closely-spaced wavelengths, DIAL can sense the presence of one (and only one) chemical species. Its main drawbacks are the weakness of the returned light (after all, air is a very poor reflector!) and its sensitivity only to one chemical species in any given set-up. The gold standard for lab-based chemical identification is Fourier-transform spectroscopy (FTS), which uses a source similar to a filament light-bulb to explore absorptions over a massive wavelength range all at once. Sadly, such thermal light sources have very poor beam quality, so cannot be transmitted over the long distances appropriate to environmental sensing. In 2004, Heriot-Watt demonstrated that broadband laser light could be used for FTS, combining the wavelength coverage of a thermal source with the beam quality of a laser. This is a game changer for implementing FTS over a long path length as required in environmental sensing, but (for reasons of signal-to-noise) is incompatible with a geometry in which the returned light is very weak. Unmanned aerial vehicle (UAV, or drone) technology has now reached a level of maturity that we can conceive of flying a retroreflector on a UAV to provide a highly efficient means of returning the laser light to a ground-based detector. This concept, which we call DRone-Assisted FTS (DRAFTS), immediately offers improved capabilities over the current state-of-the-art including: 1. Acquisition of concentration and flux maps of multiple chemicals, enabled by using broadband mid-infrared light and allowing correlations to be established and causal effects to be inferred. 2. Sensing with greater range and in diverse atmospheric conditions, since the UAV-mounted retroreflector eliminates the reliance on airborne particles and offers 10,000 times greater efficiency. 3. Deployment in a wider range of scenarios, exploiting the compactness of solid-state lasers, such as using a travelling laser source tracked by the UAV to survey emissions along a road or pipeline. Working with two key partners -- NPL (a leader in fugitive emissions sensing) and Chromacity (a femtosecond laser manufacturer) -- we aim to evaluate DRAFTS and develop it to a level where we can prove its utility in a simulated fugitive emissions field trial. Our partners are contributing £85K toward the project, and span the supply chain from manufacturer to end-user, thus providing critical opportunities for early commercialization of the DRAFTS concept.