Monolithic Silicon Photonics Interferometer for Ultra-sensitive MEMS Sensors

干涉仪是一种能够提供精确位置测量的设备。它们的工作原理是比较光在两个(通常)垂直的手臂上行进的距离。当两束光合并和干涉时，强度的变化可以比作光波长的一小部分。如果两个光波是同相的，则光输出是最大值，而如果两个光波是异相的，则强度为零。该项目将开发一种适用于微型机电系统(MEMS)传感器的小型化片上干涉仪。整个干涉仪的尺寸将小于10 mm×10 mm。光源将是一个芯片外1550 nm激光器，它耦合到片上硅波导、偏振分束器和Ge光电探测器，这些都是通过商业绝缘体上硅(SOI)晶片的反应离子刻蚀而制成的。干涉仪利用入射激光的偏振来操作同一硅组件内的两个单独的干涉仪。这是通过利用相位板将其中一个光束相对于另一个光束移动90度来实现的。通过绘制干涉仪的两个输出，我们得到了一个圆形图案，在这个图案中，每当反射镜移动时，圆都会被穿过，相对间隔等于光波长的一半(或波长/2)。如果圆可以细分为1000个部分，则对于1550 nm的激光，灵敏度约为700皮米。原则上，通过精确建模所产生的圆形图案的形状可以做得更好，我们建议展示100皮米的灵敏度。目前的MEMS传感器通常使用简单的静电传感器来监测，该传感器测量微结构电极之间的电容。虽然这是一个相对简单的组件，但这种方法在可实现的灵敏度方面存在局限性，而且还面临挑战，因为静电电压会产生杂散力，从而破坏精密MEMS设备的稳定并导致其锁定。本方案中描述的片上干涉仪将与MEMS加速度计相结合，以提供能够感知重力微小变化的超灵敏设备。为了定量测量灵敏度，iPhone中的微机械加速度计的加速度灵敏度通常为0.015 ms-2，并能够感知屏幕的方向。我们在这里提出的加速度计的灵敏度大约是iPhone加速度计的100万倍。这将使我们有机会利用光学读出MEMS来监测各种物体的引力效应。这可能包括通过入境口岸、隐藏的地下隧道、地下封存碳的位置或掩埋的核废料的位置走私的密集物体。因此，该装置在精密传感和监测、探测隐藏的致密物质和埋藏物/隧道以及监测地质储存库中的碳/核废料等领域有着广泛的应用。

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.