Ultrafast Multi-GHz Waveguide Lasers

In this work we will develop very compact and efficient laser sources with a footprint of just a few square centimetres that are capable of producing more than a billion pulses of light per second, each having a duration of less than one tenth of one trillionth of a second. Such sources have a wide range of applications, including microscopy of biological cells, precise measurement of optical frequencies, spectroscopy and telecommunications, which can all take advantage of both the very short duration of the pulse and its high repetition rate. Many of these applications currently rely on large and relatively inefficient lasers which necessarily limits application design. The miniaturised sources that we propose are based on a waveguide geometry that confines the laser light to very small dimensions, in a similar fashion to that used in glass optical fibres. However, these waveguides are based in crystals such as titanium-doped sapphire and ytterbium-doped tungstate, which have proven themselves capable of providing the very short pulses of light that we are interested in. The waveguide geometry is also capable of supporting components that are integrated into one monolithic device that can both act as the laser gain material necessary to generate the light and provide the ultrafast switching that is required to give short pulses. The waveguides will be fabricated by growing thin layers of doped crystal on undoped substrates, with the dopant providing both the laser gain and the refractive index increase necessary to confine the light to the thin layer. Advanced waveguide structures, based on etching of these layers and re-growth, will be fabricated to give optimum laser performance and allow pumping by high-power diode lasers. The integrated switching components will be based on saturable absorbers that give low loss for high-intensity short optical pulses and high loss for low-intensity continuous wave light. Optimisation of the switching properties of these absorbers and their integration with the waveguide laser will form a major part of this work. We will also investigate the use of the Kerr effect in simple thin-film waveguides to achieve short optical pulse production by using laser resonator designs that take advantage of the fact that the high intensity of the short optical pulses will modify the refractive index such that a focussing effect is achieved. Finally, having developed a number of devices, we will be in a good position to apply them to nonlinear microscopy of biological cells and demonstrate that the high repetition rate of the pulses provides advantages in terms of producing high optical signals without causing damage to the specimens under study.

UK and worldwide photonics industry will benefit from the research outputs of this proposal as they will learn about our advances in multi-GHz ultrafast lasers from our publications, conference papers, web sites, and articles in a 6-monthly newsletter (the Light Times) circulated to a broad range of academics and industrialists. We will specifically target conferences that have both high academic standing and a large industrial attendance, such as CLEO Europe Munich and CLEO USA. UK industry in particular will have the opportunity to commercialise the laser devices as we plan to set-up a UK-based impact panel that will inform us of end-user requirements and aid knowledge transfer. In the first instance, we will look to our project partners, Elforlight Ltd (a commercial sector SME), as a route for transferring technology to UK industry, giving them the opportunity to license the generated intellectual property in the in accordance with our collaboration agreement. We believe the timescale of our project (3-years) is realistic in order for us to realise this immediate benefit through technology transfer. The impact panel will also have members who are more concerned with the application of the proposed devices such as Dr. Helen Margolis from the National Physical Laboratory (as potential public sector users of the technology in the area of optical metrology) and Robert Forster from Nikon UK Instruments (as a potential expoiter of the technology in the area of biomicroscopy). Through our regular meetings with the panel we will be informed of their end-user requirements, which will guide our research so that it is of the most benefit to them and other researchers in their fields. The timescale for these users to benefit will be longer (~5 years), as further work would be required to optimise the devices (such as stabilisation requirements for optical metrology). However, these users could take our work as a starting point for further investigation or indeed further collaborative funding could be applied for to realise these applications. In the longer term (10 years and beyond) the general public will also be beneficiaries as the application areas for our ultrafast devices are of relevance to both healthcare and telecommunications, thereby enhancing quality of life. The staff working on the project will have a full range of professional development courses made available to them by both institutions. In terms of research skills the postdoctoral researchers and student will benefit from gaining experience in ultrafast lasers and technology, waveguide devices, and in photonics cleanroom fabrication skills. They will also benefit from involvement in a collaborative project with strong potential for knowledge transfer. These research and professional skills of of high relevance to potential future employmers in all sectors.