David J. Geisler

Demonstration of 2.1 photon-per-bit sensitivity for BPSK at 9.94-Gb/s with rate-½ FEC

Combining optical-phase-locked loop based coherent detection, interleaving, and powerful rate-½ FEC enabled the error-free transmission of BPSK waveforms at information rates of 9.94-Gb/s and 19.88-Gb/s with sensitivities of 2.1 photons-per-bit and 3.9 photons-per-bit, respectively.

Multi-aperture digital coherent combining for free-space optical communication receivers

Space-to-ground optical communication systems can benefit from reducing the size, weight, and power profiles of space terminals. One way of reducing the required power-aperture product on a space platform is to implement effective, but costly, single-aperture ground terminals with large collection areas. In contrast, we present a ground terminal receiver architecture in which many small less-expensive apertures are efficiently combined to create a large effective aperture while maintaining excellent receiver sensitivity. This is accomplished via coherent detection behind each aperture followed by digitization. The digitized signals are then combined in a digital signal processing chain. Experimental results demonstrate lossless coherent combining of four lasercom signals, at power levels below 0.1 photons/bit/aperture.

Multi-gigabit coherent communications using low-rate FEC to approach the Shannon capacity limit

Combining a rate-¼ forward error-correcting code, a coherent receiver, and an optical phase-locked loop yields near error-free performance with 2-dB photon-per-bit sensitivity, which is <;3-dB from the Shannon limit for a rate-¼, pre-amplified, coherent receiver.

Performance and qualification of a multi-rate DPSK modem

Recently, we demonstrated a multi-rate DPSK modem with high-sensitivity over a wide dynamic range, which can significantly benefit performance and cost of NASA’s Laser Communication Relay Demonstration. This increased flexibility, combined with the need to verify robust operation under challenging free-space environmental conditions, results in a large number of operational states which must be accurately and thoroughly tested. To support this, we developed test and diagnostic capabilities that can be easily reconfigured to assess modem performance across a wide range of data rates and operational modes. These capabilities include internal self-test modes in which test waveforms can be directed from the transmitter into the receiver to determine modem communications performance. We used these self-test capabilities to demonstrate robust performance in realistic environments during thermal-vacuum, shock/vibration, and EMI/EMC testing.

Experimental demonstration of multi-aperture digital coherent combining over a 3.2-km free-space link

The next generation free-space optical communications infrastructure will need to support a wide variety of space-to-ground links. As a result of the limited size, weight, and power on space-borne assets, the ground terminals need to scale efficiently to large collection areas to support extremely long link distances or high data rates. Recent advances in integrated digital coherent receivers enable the coherent combining (i.e., full-field addition) of signals from several small apertures to synthesize an effective single large aperture. In this work, we experimentally demonstrate the coherent combining of signals received by four independent receive chains after propagation through a 3:2-km atmospheric channel. Measured results show the practicality of coherently combining the four received signals via digital signal processing after transmission through a turbulent atmosphere. In particular, near-lossless combining is demonstrated using the technique of maximal ratio combining.

Experimental demonstration of photon efficient coherent temporal combining for data rate scaling

The next generation free-space optical (FSO) communications infrastructure will need to support a wide range of links from space-based terminals at LEO, GEO, and deep space to the ground. Efficiently enabling such a diverse mission set requires a common ground station architecture capable of providing excellent sensitivity (i.e., few photons-per-bit) while supporting a wide range of data rates. One method for achieving excellent sensitivity performance is to use integrated digital coherent receivers. Additionally, coherent receivers provide full-field information, which enables efficient temporal coherent combining of block repeated signals. This method allows system designers to trade excess link margin for increased data rate without requiring hardware modifications. We present experimental results that show a 45-dB scaling in data rate over a 41-dB range of input powers by block-repeating and combining a PRBS sequence up to 36,017 times.

Multi-aperture digital coherent combining for next-generation optical communication receivers

Space terminals for free-space optical communication systems are under constant pressure to reduce their size, weight, and power profiles. Ground terminals with large collection areas are costly, but provide a means to reduce the aperture-power product on a space platform required to close a given link. We present a ground terminal receiver architecture in which many small apertures are coherently combined while maintaining excellent receiver sensitivity. This is accomplished via coherent detection behind each aperture followed by digitization. The digitized signals are then combined in a digital signal processing chain. Experimental results demonstrate lossless coherent combining of low-flux lasercom signals.