Publications

We have developed and demonstrated a highduty- cycle asynchronous InGaAsP-based photon counting detector system with near-ideal Poisson response, roomtemperature operation, and nanosecond timing resolution for near-infrared applications. The detector is based on an array of Geiger-mode avalanche photodiodes coupled to a custom integrated circuit that provides for lossless readout via an asynchronous, nongated architecture. We present results showing Poisson response for incident photon flux rates up to 10 million photons per second and multiple photons per 3-ns timing bin.

Demand for increased capacity in deep-space to Earth communications systems continues to rise as sensor data rates climb and mission requirements expand. Optical freespace laser communications systems offer the potential for operating at data rates 10 to 1000 times that of current radiofrequency systems. A key element in an optical communications system is the Earth receiver. This paper reviews the design of a distributed photon-counting receiver array composed of four meter-class telescopes, developed as a part of the Mars Laser Communications Demonstration (MLCD) project. This design offers a cost-effective and adaptable alternative approach to traditional large, single-aperture receive elements while preserving the expected improvement in data rates enabled by free-space laser communications systems. Key challenges in developing distributed receivers and details of the MLCD design are discussed.

NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near future. To address this need, a substantial effort has been invested in developing a free-space laser communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We have built an endto- end free-space photon counting testbed to demonstrate many of the key technologies required for a deep space optical receiver. The testbed consists of two independent receivers, each using a Geiger-mode avalanche photodiode detector array. A hardware aggregator combines the photon arrivals from the two receivers and the aggregated photon stream is decoded in real time with a hardware turbo decoder. We have demonstrated signal acquisition, clock synchronization, and error free communications at data rates up to 14 million bits per second while operating within 1 dB of the channel capacity with an efficiency of greater than 1 bit per incident photon.

The process of formulating a remote sensing instrument design from a set of observational requirements involves a series of trade studies during which judgments are made between available design options. The outcome of this process is a system architecture which drives the size, weight, power consumption, cost, and technological risk of the instrument. In this paper, a set of trade studies are described which guided the development of a baseline sensor design to provide vertical profiles (soundings) of atmospheric temperature and humidity from future Geostationary Operational Environmental Satellite (GOES) platforms. Detailed trade studies presented include the choice between an interferometric versus a dispersive spectrometer, the optical design of the IR interferometer and visible imaging channel, the optimization of the instrument spatial response, the selection of detector array materials, operating temperatures, and array size, the thermal design for detector and optics cooling, and the electronics required to process detected interferograms into spectral radiance. The trade study process was validated through simulations of the radiometric performance of the instrument, and through simulated retrievals of vertical profiles of atmospheric temperature and humidity. The flexibility of these system trades is emphasized, highlighting the differing outcomes that occur from this process as system requirements evolve. Observations are made with respect to the reliability and readiness of key technologies. The results of this study were disseminated to industry to assist their interpretation of, and responses to, system requirements provided by the U.S. Government.