Progress to Date
The NRL MQW modulating retro-reflector has been shown to support 10 Mbps at a Bit Error Rate (BER) of better than 10-6. The device was also demonstrated to support uncompressed near-realtime black and white video over a 17 meter link in 1998.
After the field tests, the driver and receiver electronics were modified to support JPEG compressed video across a modulating retro-reflector link in the laboratory. This was done to demonstrate that a higher quality video link could be easily supported by a modulating retro-reflector link than was previously shown. In this demonstration, compressed color video was transmitted at 8 frames per second at a rate of 1.2 Mbps across several meters in the laboratory.
The 980 nm transmission modulator with retro-reflector used in the field tests was used in these bench tests as well. The driver electronics were substantially modified, to drive the modulator at the requisite rates. The receive software was modified as well.
The compression and decompression were performed with an off-the-shelf card from Linux Media Labs containing the Zoran 36060 CODEC. Two major modifications were required to the receiver electronics and software. We found it necessary to use "bit stuffing" on the outgoing video stream to further reduce problems with a running stream of 1's or 0's. We also found it necessary to modify the driver for the Linux Media Labs compression card such that it would recover from bit errors. Frame recovery rate was ultimately limited by electronics not by the device, which in previous tests demonstrated performance at 12 Mbps.
Compressed color video was also transmitted over a modulating retro-reflector optical system at Mbps rates. Eight frames per second is certainly adequate to communicate scene intelligence and situational awareness to the interrogator location.
An important question for potential space-borne systems is the radiation tolerance of the MQW modulator, which is the principle active component. To investigate this subject, we irradiated three 0.5 cm diameter InGaAs/AlGaAs modulators using a sequence of bombardments of 1 MeV protons. One of the devices was irradiated while under a normal operating reverse bias voltage of 15 V; the other devices were unbiased. After each exposure the electronic, optical and modulation characteristics of the modulators were evaluated. No degradation was observed until a cumulative fluence of 1 x 1014 protons/cm2, equivalent to an ionizing radiation dose of approximately 200 Mrad(Si).
Three 0.5-cm diameter InGaAs/AlGaAs modulators were used in this series of tests. The modulators were designed for surface normal transmissive operation. All three were fabricated from the same wafer. Two of the modulators were segmented by etching through the top contact and MQW layers. Modulator 1 [PGG1]was unsegmented, modulator 2 [PGG2]was segmented into 4 pixels and modulator 3 [PGG3]was segmented into 9 pixels. Segmentation allows for a reduction of sheet resistance resulting in an increase in speed, and a decrease in power consumption. Segmentation can also result in an increase in device yield by allowing isolation of any electrical defects.
The results are shown above in Table 1. As shown explicitly in the table, the irradiation levels studied here are equivalent to many years in Earth orbit, indicating that the present experiments significantly over-tested the devices. This was purposefully done to ensure that all of the damage modes in the devices were exercised. Thus, the QW modulators will be expected to operate with essentially no degradation for the duration of a standard space mission. Furthermore, these devices can be expected to operate satisfactorily in more harsh radiation environments such as medium Earth Orbits (MEO).