A n infrared data link between a ground station and a small uninhabited helicopter in flight has been demonstrated in an effort to develop a type of system for free-space optical communication between (1) a larger and relatively stationary platform, and (2) a smaller and relatively mobile platform. In a system of this type, rather than using laser transmitters with their associated gimbaled telescopes and pointing/tracking subsystems on both platforms, one uses only a single such laser transmitter on the larger platform (in this case, the ground station). The single laser transmitter is capable of tracking the smaller platform (in this case, the helicopter) and transmitting data to the smaller platform in the conventional way via modulation of the outgoing laser beam. The field of view of the receiver on the smaller platform is wide enough to capture the laser beam, without need for a large receiving telescope and its aiming subsystem. For transmitting data from the smaller to the larger platform, a large fraction of the laser power incident on the smaller platform is modulated and retroreflected to the larger platform, by means of an InGaAs-based multiple-quantum-well (MQW) light modulator on a cornercube retroreflector (see Figure 1).

Figure 1. The Modulating Retroreflector of the present system consists of an MQW light modulator on a corner-cube reflector, depicted here in simplified schematic form.
The laser transmitter is also designed to have sufficient power that when the laser beam is transmitted unmodulated from the larger platform and is modulated at the smaller platform and retroreflected to the larger platform, the modulated signal returning to the larger platform is strong enough to convey data from the smaller platform to the larger one at an acceptably low error rate. There is no need for a transmitting telescope and its aiming subsystem on the smaller platform because a retroreflector inherently reflects light in the reverse of the direction of incidence. If desired, multiple retroreflectors can be mounted in a hemispherical array to increase the acceptance angle.

The basic idea of a modulating retroreflector on the smaller platform is not new. What is new is the use of an MQW device as the light modulator. An MQW light modulator typically contains several hundred layers (each of the order of 10 nm thick) of semiconductor materials like those of laser diodes. Electrically, an MQW modulator resembles a positive/intrinsic/ negative diode; optically, the MQW modulator exhibits a sharp absorption feature at a wavelength determined by the constituent materials and the numbers and thickness of layers. When a moderate reverse bias potential (≈15 V) is applied, the absorption feature shifts to greater wavelengths and its magnitude decreases. Hence, if the device is designed to position the absorption feature near the laser wavelength, the laser power transmitted through the device can be modulated significantly by varying the reverse bias potential.

Of all semiconductor electro-optical modulators, only MQW modulators offer the combination of high switching speed, low power consumption, low weight, large area, wide field of view, high optical quality, functionality in the desired infrared wavelength range, and ruggedness required for an infrared data link of the type undergoing development. It has been estimated that because of the MQW modulator, the power consumption of the modulatingretroreflector- based data-transmission subsystem aboard the smaller platform can be an order of magnitude smaller than that of an equivalent radio-frequency data-transmission subsystem.

Figure 2. The Uninhabited Helicopter, about 1 m long, carried a data-communication subsystem that included the modulating reflector, which was mounted on the tail facing downward toward a ground station.
In a field test of this system, the modulating retroreflector, which had an aperture diameter of 0.5 cm, was mounted on the tail of the helicopter pointing downward (see Figure 2). The helicopter was flown at an altitude of 50 to 100 ft. (≈15 to ≈30 m) and a range 100 to 200 ft. (≈30 to ≈61 m) from the ground station. A pseudorandom bit stream at a rate of 400 Kb/s was transmitted from the helicopter to the ground station, using a modulator power of only 40 mW. However, it was estimated that the modulator and detector bandwidths and the strength of the return signal were sufficient to sustain a data rate of 3 Mb/s. It is planned to demonstrate even higher rates in future tests.

This work was done by W. S. Rabinovich, G. C. Gilbreath, Chris Bovais, Kerry Cochrell, H. R. Burris, Mena Ferraro, Michael Vilcheck, Rita Mahon, Kim Goins, Ilene Sokolsky, John Vasquez, Timothy Meehan, Robin Barbehenn, D. S. Katzer, and K. Ikossi Ansatasiou of the Naval Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. NRL-0010

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Infrared Data Link Using an MQW Modulator on a Retroreflector

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This article first appeared in the June, 2007 issue of Defense Tech Briefs Magazine.

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