Optical communications for micromachines

A key challenge for sensor networks is to create sensor ‘motes’ that have low power consumption, and hence a long lifetime. Sensors that ‘scavenge’ energy from their environment are attractive for this reason, but these must operate at very low power levels to be sustainable. Most sensor networks use RF communications to transmit information, and this can require substantial amounts of energy for each bit transmitted compared with processing information. This is both due to the complex circuitry required to create the signals, and the low directivity of the RF antennas typically used in such applications, leading to most of the transmitted power being wasted.Optical channels, however, can be very highly directive, so that path loss can be minimized if transmitter and receiver ends of the link can be aligned. If the sensor uses a retro-reflector rather than an optical source, then the only energy required for communications from the sensor is to modulate the retro-reflector. The modulated beam is then returned to the source creating a sensor-source link. The link from source to sensor can be made by modulating the source, and this radiation (along with any ambient illumination there might be) can be used to power the sensor.

We have developed an optical base station that communicates with small sensor motes that contain retro-reflecting transceivers, based on CMOS ICs. Key achievements of the work include

Holographic beamsteering basestation architecture that locates and communicates with sensor 'motes'

Transmission of data from a retro-reflecting  modulator based on a low-voltage liquid crystal shutter to the base station over a distance of 30m

Demonstration of a CMOS IC based mote that is (i) powered by the beam from the base station, (ii) can receive and recover data from the beam and (iii) can control and switch a liquid crystal shutter to modulate the retro-reflected beam.