Modulating retro-reflector

A modulating retro-reflector (MRR) system combines an optical retro-reflector and an optical modulator to allow optical communications and sometimes other functions such as programmable signage. Free space optical communication technology has emerged in recent years as an attractive alternative to the conventional radio frequency (RF) systems. This emergence is due in large part to the increasing maturity of lasers and compact optical systems that enable exploitation of the inherent advantages (over RF) of the much shorter wavelengths characteristic of optical and near-infrared carriers:

Larger bandwidth Low probability of intercept Immunity from interference or jamming Frequency spectrum allocation issue relief Smaller, lighter, lower power

Technology An MRR couples or combines an optical retroreflector with a modulator to reflect modulated optical signals directly back to an optical receiver or transceiver, allowing the MRR to function as an optical communications device without emitting its own optical power. This can allow the MRR to communicate optically over long distances without needing substantial on-board power supplies. The function of the retroreflection component is to direct the reflection back to or near to the source of the light. The modulation component changes the intensity of the reflection. The idea applies to optical communication in a broad sense including not only laser-based data communications but also human observers and road signs. A number of technologies have been proposed, investigated, and developed for the modulation component, including actuated micromirrors, frustrated total internal reflection, electro-optic modulators (EOMs), piezo-actuated deflectors, multiple quantum well (MQW) devices, and liquid crystal modulators, though any one of numerous known optical modulation technologies could be used in theory. 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. A remotely located optical transmitter/receiver system usually consisting of a laser, telescope, and detector provides an optical signal to the modulating retro-reflector. The incident light from the transmitter system is both modulated by the MRR and reflected directly back toward the transmitter (via the retroreflection property). Figure 1 illustrates the concept. One modulating retro-reflector at the Naval Research Laboratory (NRL) in the United States uses a semiconductor based MQW shutter capable of modulation rates up to 10 Mbit/s, depending on link characteristics. (See "Modulating Retro-reflector Using Multiple Quantum Well Technology", U.S. Patent No. 6,154,299, awarded November, 2000.) The optical nature of the technology provides communications that are not susceptible to issues related to electromagnetic frequency allocation. The multiple quantum well modulating retro-reflector has the added advantages of being compact, lightweight, and requires very little power. The small-array MRR provides up to an order of magnitude in consumed power savings over an equivalent RF system. However, MQW modulators also have relatively small modulation ranges compared to other technologies. The concept of a modulating retro-reflector is not new, dating back to the 1940s. Various demonstrations of such devices have been built over the years, though the demonstration of the first MQW MRR in 1993 was notable in achieving significant data rates. 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. Qualities often considered desirable in MRRs (obviously depending on the application) include a high switching speed, low power consumption, large area, wide field-of-view, and high optical quality. It should also function at certain wavelengths where appropriate laser sources are available, be radiation-tolerant (for non-terrestrial applications), and be rugged. Mechanical shutters and ferroelectric liquid crystal (FLC) devices, for example, are too slow, heavy, or are not robust enough for many applications. 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.

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