Figure 1: Imaging data obtained over a town in northern California is shown. Transmission through smoke and haze is evident at 1.55 microns but blocked in the visible. Data was obtained using the JPL AVIRIS hyperspectral camera.
Figure 1: Imaging data obtained over a town in northern California is shown. Transmission through smoke and haze is evident at 1.55 microns but blocked in the visible. Data was obtained using the JPL AVIRIS hyperspectral camera.

In practice, except for close ranges, the link rather than the modulator limits the performance of a modulating retro-reflector system. For a conventional corner-cube modulating retro-reflector, MQW technology should allow data rates in the tens of megabits per second, depending on the range and the interrogator system.

For a diffraction-limited system, the optical power retro-reflected from the small platform back to the large platform scales as:

(Plaser•Dretro4•Drec2•Tatm2)/(θdiv2•R4)

where Plaser is the power of the transmitter on the large platform, Dretro is the diameter of the modulating retro-reflector on the small platform, Drec is the diameter of the receiver telescope on the large platform, and Tatm is the loss due to transmission through the atmosphere, θdiv is its divergence of the transmit beam, and R is the range between the two platforms.

The strongest dependencies are on the range and the retro-reflector diameter, both of which scale as fourth powers. Retro-reflector links fall off more strongly with range than conventional links because of their bi-directional nature. The strong dependence on retro-reflector diameter occurs because increasing the size of the retro-reflector both increases the optical power intercepted and decreases the divergence of the returned optical beam. The link is very clearly a compromise between a large retro-reflector aperture to maximize the returned optical power and a small modulator to maintain data rate while keeping the consumed electrical power low. This trade is mitigated to some extent by using segmented devices. When all of the segments are driven in parallel, the power consumption may be comparable to a monolithic device, but the modulation rate of the smaller device will be exploited while enabling the larger aperture.

Figure 2: Transmission curves are shown for a clear day (red) and for a misty day (green). These are two-way transmission data through the entire atmospheric channel and include losses from absorption and from Rayleigh and Mie scattering. It can be seen that at 1.06 microns and at 1.55 microns, there is a transmission "window" for viable data links.
Figure 2: Transmission curves are shown for a clear day (red) and for a misty day (green). These are two-way transmission data through the entire atmospheric channel and include losses from absorption and from Rayleigh and Mie scattering. It can be seen that at 1.06 microns and at 1.55 microns, there is a transmission "window" for viable data links.

Channel losses are an additional consideration, particularly for terrestrial applications. Propagation through the atmosphere can induce losses due to absorption and scattering. In addition, the use of a near-infrared link means that it will not operate under all weather conditions. However, operability in the one micron regime is greater than in the visible. Figure 1 compares visibility through smoke and haze in the visible to visibility at 1.55 microns. These images are of a fire over a town in northern California taken by the JPL Airborne Visual Infrared Imaging Spectrometer (AVIRIS), which records data in 224 wavelength bands with both the resolution and center-to-center band spacing of about 10 nm. It is clear from these photos that operation in the near-infrared extends the range of viable operation for an application such as remotely observed video. Propagation through the atmosphere at different wavelengths is compared in Figure 2. In this figure, propagation on a clear day is compared with propagation on a hazy day. The traces include absorption, Rayleigh scattering, and aerosol scattering losses for a retro-reflected link through the full atmospheric channel using a NRL model.