Free-space laser communication systems have the potential to provide flexible, high-speed connectivity suitable for long-haul intersatellite and deep-space links. For these applications, power-efficient transmitter and receiver designs are essential for cost-effective implementation. State-of-the-art designs can leverage many of the recent advances in optical communication technologies that have led to global wide-band fiber-optic networks with multiple Tbit/s capacities. While spectral efficiency has long been a key design parameter in the telecommunications industry, the many THz of excess channel bandwidth in the optical regime can be used to improve receiver sensitivities where photon efficiency is a design driver. Furthermore, the combination of excess bandwidth and average-power-limited optical transmitters has led to a new paradigm in transmitter and receiver design that can extend optimized performance of a single receiver to accommodate multiple data rates.

Laser communication transmitter and receiver design

The development of a Nd : YAG space laser communication system is discussed with respect to history, potential applications, and present status. The basic design of an Air Force Space Communications Flight Test operable at a data rate of 1000 Mbit/s is described at both the system and component level. An engineering feasibility model of this system has been completed, and the results of tests data are reported. Communications at 1000 Mbit/s with a receiver sensitivity of 20 photoelectrons/bit was demonstrated for a bit error rate of 10-6, interterminal tracking with angle erros less than 1 µrad, and station to station acquisition in less than 6 s. A simulated range of 40 000 km was used.

Space optical communications with the Nd: YAG laser

Photon-efficient optical communications using variable-duty-cycle M-ary pulse-position modulation (M-PPM) with coding is investigated experimentally using a simple, multi-rate nearly quantum-limited receiver with throughputs ranging from 1.25 Gbit/s, in the binary case, to 78 Mbit/s, for M=256.

Demonstration of 2.5-Gslot/s optically-preamplified M-PPM with 4 photons/bit receiver sensitivity

Gain-switched pulses of InGaAs double-quantum-well lasers fabricated from identical epitaxial laser wafers were measured under both current injection and optical pumping conditions. The shortest output pulse widths were nearly identical (about 40 ps) both for current injection and optical pumping; this result attributed the dominant pulse-width limitation factor to the intrinsic gain properties of the lasers. We quantitatively compared the experimental results with theoretical calculations based on rate equations incorporating gain nonlinearities. Close consistency between the experimental data and the calculations was obtained only when we assumed a dynamically suppressed gain value deviated from the steady-state gain value supported by standard microscopic theories.

Gain-switched pulses from InGaAs ridge-quantum-well lasers limited by intrinsic dynamical gain suppression.

There has been a recent surge in interest for optical satellite communication (SatCom) utilizing lasers. It is clear to see why, as optical SatCom is capable of higher speed, lighter weight, higher directionality, and higher efficiency versus their radio-based counterparts. Research into optical SatCom has focused on devices operating in the short-wave infrared (SWIR), which is due to the maturity and commercial availability of such component’s thanks to significant development in terrestrial telecommunications networks. However, SWIR performs poorly in fog and heavy weather, prompting investigations into longer mid-wave and long-wave infrared bands for optical communication instead due to reduced atmospheric losses. This paper provides a comprehensive review of laser transmitters, detectors, and the science behind selecting longer wavelengths for optical SatCom to boost optical SatCom between ground stations and low earth orbit satellite constellations being deployed.

Mid-wave and long-wave infrared transmitters and detectors for optical satellite communications—a review

Experiments in short-pulse optical communications have been performed at high data rates (107bit/s) with M-ary modulation formats and, using a subnanosecond internally gated receiver, at very high data rates (2 × 108bit/s) with binary modulation formats to experimentally verify the potential of short-pulse low-duty-cycle direct-detection formats. High resolution TV pictures and error rate data are given for two M-ary formats, pulse interval modulation and pulse position modulation, and error rate data and gated optical receiver characteristics are given for the 200-Mbit/s pulsed gated binary modulation. Results demonstrate the capability of short-pulse laser-communication systems to discriminate against background light and to efficiently convey information. Subnanosecond gating of the receiver was achieved in the 200-Mbit/s experiments. In the M-ary experiments, as many as 12 bit/pulses were transmitted, using 4095 digitally selected 1 -ns time slots. High-quality TV pictures were transmitted at average detected signal to background ratios of less than 0.01.

S. I. Green

Papers

Short-pulse optical communications experiments

Short-pulse optical communications experiments

Multiplexed TV and high-data-rate pulse-gated binary modulation communications experiments