Light communication is nothing new (see the Fraunhofer Institute’s VLC system), but new advancements are making technologies such as free-space retroreflectors more power, efficient, as well as giving them an over-all boost in speed. One of the major draw-backs of using free-space optical communication systems, on space-craft for example, is that they require a sizable power supply to transmit data over significant distances and space for those is often limited.
This is where CCR’s (Corner Cube Retroreflectors) become handy. Light can be emitted from a base station that has no power constraints, and the CCR can ‘bounce’ light back to the source with little scattering. They function by using two vertical separately angled mirrors and one horizontal that together form a concaved cube which pulses light from the source to the hub resulting in an ‘on’ state and diverts the light using a piezoelectric cantilever to redirect the horizontal mirror which creates an ‘off’ state. However, they too have their draw-backs in terms of the high-voltage needed to redirect the (off) light-source, but a team of engineers from Kwangwoon University in Seoul have tackled the voltage problem with the help of MEMS (Micro Electrical Mechanical Systems) technology.
(Top left) Concept of CCR unit (top right) tunneling microscope image (bottom left) CCR on state (bottom right) CCR off state (via J. Park, et al. © 2012 IOP Publishing Ltd)
The teams CCR design uses perfectly aligned silicon mirrors fabricated using a double SOI (Silicon on Insulation) wafer with an anisotropic KOH (potassium hydroxide) wet etching technique which makes them almost perfectly flat. The team then micro-fabricated a lead zirconate titanate (PZT) piezoelectric cantilever that runs on a voltage of only 5 volts to actuate the horizontal mirror. The reduction in overall voltage is attributed to the almost perfect alignment of the specially designed mirrors. In addition to being more accurate and energy conservative, they can also be outfitted with sensors as well as CMOS circuitry giving them additional micro-optical systems applications such as coded data communications for the military. While the team's new CCR design is extremely efficient over previous iterations, it still has its faults. Its range is limited due to a slight curvature in the horizontal mirror, but the engineers state that the problem will be overcome through ‘optimization of the material selection and the process conditions.'