Monolithic Silicon Photonics Interferometer for Ultra-sensitive MEMS Sensors

Interferometers are devices which can provide accurate position measurements. They work by comparing the distance travelled by light in two (typically) perpendicular arms. When the light from the two beams is combined and interfered, the change in intensity can be compared to a fraction of the wavelength of light. If both light waves are in-phase the light output is a maximum, while if the light waves are out of phase the intensity will be zero.
This project will develop a miniaturised on-chip interferometer suitable for use in Micro-Electro-Mechanical System (MEMS) sensors. The entire interferometer will occupy a size below 10 mm x 10 mm. The light source will be an off-chip 1550 nm laser which is coupled to on-chip silicon waveguides, polarising beamsplitters and Ge photodetectors which are fabricated by reactive ion etching of commercial silicon-on-insulator (SOI) wafers. The interferometer utilises the polarisation of the incoming laser beam to operate two separate interferometers within the same Si components. This is achieved by shifting one of the beams by 90 degrees with respect to the other one with a phaseplate. By plotting the two outputs from the interferometer we achieve a circular pattern whereby the circle is traversed every time the mirrors are moved a relative separation equal to one half the wavelength of light (or lambda/2). If the circle can be subdivided into 1000 sections, the sensitivity is of the order 700 picometres for 1550 nm laser light. In principle it is possible to do even better by accurate modelling of the shape of the resulting circular pattern and we propose to demonstrate a sensitivity of 100 picometres.
Current MEMS sensors are typically monitored with simple electrostatic sensors which measure the capacitance between micro-structured electrodes. Whilst being a relatively simple component, this method has limitations in the achievable sensitivity, and also challenges as electrostatic voltage causes spurious forces which can destabilises the delicate MEMS device and cause it to lock. The on-chip interferometer which is described in this proposal will be combined with a MEMS accelerometer in order to provide an ultra-sensitive device able to sense tiny changes in gravity. To give quantitative measure of the sensitivity, MEMS accelerometers found in iPhones typically have an acceleration sensitivity of 0.015 ms-2 and are able to sense the direction of the screen. The accelerometer we propose here has sensitivity approximately 1 million times better than an iPhone accelerometer. This will allow the opportunity to utilise the optical readout MEMS to monitor the gravitational effects of a variety of bodies. This could include dense objects being smuggled through ports of entry, hidden subterranean tunnels, the location of carbon sequestered under the ground or the location of buried nuclear waste. The device thus has a wide range of applications in the areas of precision sensing and monitoring, detecting hidden dense masses and buried objects/tunnels, and the monitoring of carbon/nuclear waste in geological repositories.

Academic impacts: We expect a broad range of academic impacts to be generated from this project. Once all IP has been protected we expect that a significant output of high quality academic peer reviewed papers suitable for inclusion in future REF exercises would be generated. Furthermore, the investigators will ensure that a high profile is established through visible presence at conferences and workshops which are aligned to the subject area (e.g. photonics, gravity and geophysics, defence). Talks and posters will be utilised to identify the impact of the work within the global challenge areas. This will directly enhance the health of physics and engineering as academic disciplines. We further aim to train highly skilled researchers (1 PhD student and undergraduate project students) in topics which include MEMS fabrication, mechanical design, optical and electronic design and fabrication. The skills learnt during this project will be of the highest quality and significant breadth and will allow the students to excel in their future career path. The project will provide the successful applicant with a high level of individual ownership and offer the opportunity to perform cutting edge research & development. Moreover, the studentship incorporates knowledge transfer aspects and industrial collaboration together with high quality core research training. There will be opportunity for the students to present their work at national/international conferences, publish in high quality journals, and interact with industrial end users, ultimately enhancing the student teaching and learning exerience.

Economic and Societal Impacts: The project will enhance the commercialisation and exploitation opportunities for this technology. Glasgow University spin out company Kelvin Nanotechnology, who provide commercial access to the James Watt Nanofabrication Centre (JWNC), will contribute to the project exploitation with initial low volume manufacture and by providing advice and support for the transfer of the technology to larger MEMS foundries if successful. The technology will be made available to STFC personnel and academics through the Kelvin-Rutherford partnership. In addition we will engage with industry and government bodies to understand their precise needs and enhance knowledge of their applications. This will be achieved through engagement with partners including MoD, SELEX ES, Thales, BP Alternative Energy, Micro-g Lacoste, Bridgeporth, and DEFRA. Prof Paul sits on the Cabinet Office High Impact Threats and Home Office CBRN scientific advisory panels and has sat on MOD DSAC until the end of 2013. He therefore has the required DV (Developed Vetting) clearance to know the operational requirements, and knows the appropriate people related to the UK national security who would be interested in using this technology. The on-chip amplifier and MEMS device will further enhance the ability of the businesses to enhance their economic sustainability via the potential of bringing to market a novel and disruptive technology, with wide ranging applications on the global challenge areas.