Large area transmissive modulator for a remotely-interrogated MEMS-based optical tag

The development of a micro-opto-electro-mechanical system (MOEMS) technology employing interference effects to modulate incident light in the near-IR band (1550nm) over a wide angular range (120 degrees) is reported. Modulation is achieved by tuning a large array of Fabry-Perot cavities via the application of an electrostatic force to adjust the gap between a moveable mirror and the underlying silicon substrate. The optical design determines the layer thicknesses; however, the speed and power are determined by the geometry of the individual moveable elements. Electro-mechanical trade-offs will be presented as well as a key innovation of utilising overshoot in the device response in reduced pressure environment to reduce the drive voltage. Devices have been manufactured in a modified polysilicon surface micromachining process with anti-reflection coatings on the back of the silicon substrate. Measurements of individual mirror elements and arrays of mirrors at 1550nm show excellent uniformity across the array. This enables good response to an incident signal over a wide field of view when integrated with a silicon retroreflector in a passive optical tag. In conjunction with appropriate anti-stiction coatings, lifetimes of over 100 million cycles have been demonstrated. Key advantages of the modulator are that it is low cost being based on standard polysilicon micromachining; high speed (>100kHz) and robust due to utilising a massively parallel array of identical compact devices; low power for portable applications; and operates in transmission - allowing simple integration with a retroreflector in a passive tag for halfduplex free-space optical communications to a remote interrogator.