(Left) An illustration and (right) an SEM image of
the piezoelectrically actuated CCR. Image credit: J. Park, et al. © 2012
IOP Publishing Ltd
In free-space optical communications (FSO), data is wirelessly transmitted by light propagating through open space. Among their applications, FSO systems are used for communications between spacecraft and have the potential to serve as the “last mile” for fiber optics broadband services. However, one challenge they face is that the light sources used to encode the data require power, and a power supply is often limited. Devices that reflect light, called corner cube retroreflectors (CCRs), can overcome this problem because they can transmit data without their own light source, simply by reflecting incident light from a base station.
Although CCRs were introduced more than 10 years ago as a solution for FSO systems, they face their own challenges, most notably the need for a high voltage. In a new study, a team of researchers from Seoul, South Korea, has improved the CCR design so that it operates with an ultra-low voltage and negligible power consumption. The new design, which is based on MEMS technology, could make the devices more attractive for FSO systems.
As the researchers explain in their study, CCRs work by digitally modulating incident laser light. The device reflects laser light coming from the base station back toward the source in the “on” state, and scatters the light away from the source in the “off” state. By quickly switching between states, the CCR can transmit data at high speeds.
The CCR device itself consists of three mirrors: two vertical mirrors form a cross on top of a horizontal mirror, resulting in four concave corners. The alignment of the mirrors is essential, since perfect alignment constitutes the “on” state to reflect light. Misalignment occurs when a piezoelectric cantilever causes the horizontal mirror to lift a few micrometers above the substrate like a seesaw, which causes incident light to scatter.
The difference between the CCR's “on” and “off” states. Image credit: J. Park, et al. © 2012 IOP Publishing Ltd
In
previous designs, the piezoelectric cantilever required high voltages
to move the mirror, but the new design can work with ultra-low voltages
due to improved mirror alignment. To do this, the researchers added two
supporting cantilevers in addition to the actuating cantilever, which
helps improve the initial alignment of the horizontal mirror. A
simplified fabrication process also improves the alignment and flatness
of the mirrors.
“The developed CCR is the first device fabricated by combining a
cross-shaped vertical silicon mirror and a piezoelectrically actuated
horizontal mirror,” Jae Park, Professor of Electrical Engineering at
Kwangwoon University, told Phys.org. “There are two significant
advantages in its fabrication. Firstly, the cross-shaped vertical mirror
was simply fabricated by using a double-SOI wafer and anisotropic KOH
etching technique. This method provides good surface roughness and
accurate angular alignment of reflective surfaces in the vertical mirror
with mass productivity. Secondly, microfabricated piezoelectric
cantilever actuator was applied to actuate the horizontal mirror. The
piezoelectric cantilever provides larger angular displacement at a lower
induced voltage than a conventional electrostatic actuator without
power consumption.”
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