Coherent Multispectral LIDAR in Silicon Photonics using Optical Phased Arrays

The project will investigate the different aspects of developing a LIDAR sensor on a low-cost, compact silicon chip under the established CMOS fabrication foundry model. The resulting photonic integrated circuit will comprise a large variety of optical components including an on-chip laser, waveguides, metal heaters, amplifiers and photodetectors, but the early focus will be on developing an optical phased array (OPA) to steer and shape the beam in the far-field. In order to have a viable LIDAR sensor with an acceptable system performance using OPAs, certain criteria must be met and satisfied. The potential impact of a fully-integrated LIDAR device on a silicon chip is huge; these devices could replace the bulky LIDAR systems seen on autonomous vehicles and drones up until now at a fraction of the SWaP (Size, weight and power requirements).

An optical phased array requires splitting light from a bus waveguide via a multi-mode interference coupler, for example, into an array of waveguides with optical phase shifters attached to each channel to control the phase of each beam separately, and hence steer the beam in free-space in the desired far-field direction. Several issues have to be addressed in order to produce a high quality, narrow divergence output beam via constructive interference of the different beams from each waveguide channel in the array.

The first problem is that the phase shifters, operating using the thermo-optic effect, will incur thermal crosstalk issues between adjacent waveguides and disrupt the phases of beams in adjacent channels. This problem can be alleviated by increasing the gap between the waveguides at the location of the shifters. However, this leads to a second problem in that the waveguides will be spaced too far apart at the output of the array, leading to a large beam width in the far-field. At small spacings however the modes between adjacent waveguides will interact and transfer power as in directional waveguide couplers. The waveguides will also have different optical path lengths due to the divergent and convergent path sections. Solutions to all of these problems in order to realise a large optical phased array will be investigated, simulated and then fabricated. The factors will be simulated in Lumerical MODE solutions. There will also be phase nonuniformities induced due to fabrication errors in addition to various other factors. These factors will be monitored and tested post-fabrication to find optimum voltage values for the phase shifters to counter phase nonuniformities.
The later parts of the project could involve looking at different wavelengths of lasers for a multispectral analysis of a scene, which would deliver additional target information. Frequency chirping of a continuous wave source will also be studied and its feasibility of on-chip operation compared to using a pulsed source considering factors such as eye-safe operation and efficiency of target detection.

Academic
The development of a cohort of fifty young doctoral researchers who in the programme will acquire a unique set of technical abilities allied with working practice, managerial and enterprise skills.
The research to be undertaken by the cohort incorporates areas such as photonics, meta-materials, functional materials and plasmonics. The research programme is targeted at developing a suite of integrative technologies that address the requirements of heat assisted magnetic recording (HAMR). HAMR requires a highly manufacturable, rugged heterogeneous integration platform, encompassing semiconductor lasers, passive waveguides, rugged plasmonic devices and advanced magnetic materials. The successful development of HAMR will see a paradigm shift in the performance of data storage devices.
Advances in the above areas will see the CDT, supervisory staff and cohort develop a reputation and output profile that will lead to further basic research funding in the Universities and to the launch of academic careers for some of the cohort through attaining post-doctoral positions.

Industrial
The CDT brings together key companies who could form a complete UK manufacturing supply chain for HAMR technology. These companies include as founder partners - IQE as a supplier of custom epitaxy, Oclaro for volume laser production and Seagate for volume manufacturing of magnetic recording heads.
The CDT will result in the development and adoption of this low-cost heterogeneous integration technology, a technology than can also be applied in multiple markets. Although the HAMR environment is particularly harsh, many other consumer and society driven applications (such as widely deployable high speed internet) also require operation in harsh environments. The technology developed here will allow migration away from traditional expensive solutions such as laser packaging in temperature stabilised, gold plated, hermetic boxes.

Societal
The Engagement & Outreach Committee of the CDT along with the leadership, supervisory staff and cohort will proactively engage with the wider society to raise awareness of the underpinning science and engineering. The CDT will demonstrate how it supports a high technology manufacturing supply chain in which UK activity has a global significance and brings benefit to a large part of society. Notwithstanding other commercial applications that our end-users have, we will be able to highlight how the integration of underpinning science and engineering lies at the core of much high technology.

Economic.
Our key partner has a significant presence in the UK through employment of some 1500 people in manufacturing and R&D. The current operation is centred on a capital base of some £1.5B and contributes around £100M GVA p.a. to the UK economy. The societal need for increased data storage places this operation as a nexus of the global economy and consequently offers significant supply chain opportunity for the UK. The need to develop HAMR requires the development of the integration technology that lies at the core of our CDT. The outcomes of the CDT will inform future decisions that will underpin further corporate investment of £10M's to equip the partner and to recruit the necessary staff. We note that the key partner, in their letter of support, could absorb the entire cohort into employment over the next few years. Our other project partners will also benefit beyond HAMR. As examples; CST Global would apply novel lasers and integrated solutions to niche applications, Kelvin Nanotechnology will be able to exploit new integration expertise, OIPT need a pipeline of trained personnel that is currently not available in the UK, JEOL and FEI have interests in new imaging and metrology associated with new material and integration technologies. All the partners would benefit from a flow of PhD graduates trained in advanced material assessment.