Modulating retro-reflector
[2] Free space optical communication technology has emerged in recent years as an attractive alternative to the conventional radio frequency (RF) systems.
These approaches have many advantages and disadvantages relative to one another with respect to such features as power use, speed, modulation range, compactness, retroreflection divergence, cost, and many others.
In a typical optical communications arrangement, the MRR with its related electronics is mounted on a convenient platform and connected to a host computer which has the data that are to be transferred.
However, MRRs are still not widely used, and most research and development in that area is confined to rather exploratory military applications, as free-space optical communications in general tends to be a rather specialized niche technology.
Some modulating retro-reflector systems are desired to operate at data rates of megabits per second (Mbit/s) and higher and over large temperature ranges characteristic of installation out-of-doors and in space.
Semiconductor MQW modulators are one of the few technologies that meet all the requirements need for United States Navy applications, and consequently the Naval Research Laboratory is particularly active in developing and promoting that approach.
[1] When a moderate (~15V) voltage is placed across the shutter in reverse bias, the absorption feature changes, shifting to longer wavelengths and dropping in magnitude.
Thus, the transmission of the device near this absorption feature changes dramatically, allowing a signal can be encoded in an on-off-keying format onto the carrier interrogation beam.
The device is grown on an n-type GaAs wafer and is capped by a p-type contact layer, thus forming a PIN diode.
[1] Once grown, the wafer is fabricated into discrete devices using a multi-step photolithography process consisting of etching and metallization steps.
Its magnitude depends on the drive voltage applied to the device and the wavelength of the interrogating laser relative to the exciton peak.
In perfect quantum well material this is not a problem, but the presence of a defect in the semiconductor crystal can cause the device to break down at voltages below those necessary for operation.
If a single "pixel" is lost due to defects but is one of nine or sixteen, the contrast ratio necessary to provide the requisite signal-to-noise to close a link is still high.
If the modulator causes aberrations in the beam, the returned optical signal will be attenuated and insufficient light may be present to close the link.
