Free space optical communication has emerged in recent years as an attractive alternative to the conventional Radio Frequency (RF) approach. This emergence is due to the increasing maturity of lasers and compact optical systems that enable exploitation of the inherent advantages of the much shorter wavelengths characteristic of optical and near-infrared carriers.
Inherent advantages of optical communications:
- Large bandwidth
- Low probability of intercept
- Immunity from interference or jamming
- Frequency allocation relief
- Smaller, lighter payloads
For a conventional optical link, a good to high quality telescope that provides relatively accurate pointing and tracking capability and a robust laser with sufficient power, temperature stabilization, and requisite electronics are needed in addition to the usual modulation/demodulation and control and acquisition instrumentation and software.
Modulating retro-reflector systems couple an optical retro-reflector and an electro-optic shutter to allow two-way optical communications.
By coupling an optical retro-reflector with a Multiple Quantum Well (MQW) modulator and providing the data-gathering instrumentation signals to the driver, an elegantly simple, low power, and compact communications transceiver can be configured.
The retro-reflector with its related electronics are mounted on the data-gathering platform. A remotely located transmit/receive system consisting of a laser, telescope, and detector of sufficient power, gain, and sensitivity provides the means to interrogate the platform-mounted modulating retro-reflector. By virtue of the inherent nature of the retro-reflector, the incident light from the transmit system, carrying the modulated signal imposed by the shutter, is reflected to the co-located receiver.
The Field-of-View (FOV) of a typical mounted device is of the order of 25 degrees. The FOV can be expanded arbitrarily using an array of devices to accommodate pointing and acquisition requirements of the interrogator. The mass increases to some extent, but a five-element device with a FOV of 60 degrees can be configured at less than 100 grams.
The NRL modulating retro-reflector uses a semiconductor based MQW shutter capable of modulation rates up to 10 Mbps, depending on link characteristics. See "Modulating Retro-reflector Using Multiple Quantum Well Technology", U.S. Patent No. 6,154,299, awarded November, 2000. The technology enables the use of near-infrared frequencies that provide communications immune to frequency allocation problems. The Multiple Quantum Well modulating retro-reflector has the added advantages of being compact, lightweight, and requires very little power. Up to an order of magnitude in onboard power can be saved using a small array of these devices instead of the RF equivalent.