John E. Kaufmann

An Opto-Mechanical Subsystem For Space Based Coherent Optical Communication

This paper provides an overview of the opto-mechanical subsystem (OMS) for the Lincoln Laboratory Laser Intersatellite Transmission Experiment. The OMS contains the telescope, relay optics, and beam steering mechanisms. The optical, mechanical and thermal aspects of the OMS design are discussed and the predicted design performance is presented.

Coherent optical intersatellite crosslink systems

Key system and technology issues for heterodyne laser intersatellite communication systems are reviewed, including telescope/aperture, laser transmitter, and heterodyne receiver technology; spatial acquisition and tracking; optical/mechanical/thermal analysis design; and modulation/demodulation coding. It is concluded that heterodyne technology with semiconductor lasers has evolved to the point where it is now feasible to implement heterodyne systems for approximately 100 Mb/s with a single-laser transmitter of modest power (30 mW) and with small aperture (20 cm). It is suggested that technology advances, particularly in the areas of higher power laser sources, will extend system capabilities beyond 1 Gb/s in the future.<<ETX>>

Analog link design

In some applications signals originate in analog form and are to be delivered to a remote destination as analog signals. In such cases, it is sometimes more economical or technically feasible to transmit these signals in their original analog form than to convert to a digital format, particularly when the signal bandwidth and/or dynamic range is large enough to stress the capabilities of current digital-to-analog and analog-to-digital converters. This paper discusses analog signal transmission at optical wavelengths in the context of free space links. The technology of optical modulators, amplifiers, and receivers is discussed and an example of an analog crosslink design is given.

Performance limits of high-rate space-to-ground optical communications through the turbulent atmospheric channel

Atmospheric turbulence corrupts both the amplitude and phase of an optical field propagating from space to an earth-based receiver. While aperture averaging can mitigate amplitude scintillation effects, the performance of single spatial-mode receiver systems such as coherent detection or preamplified direction detection can be significantly degraded by the corrupted phase when the ratio of aperture diameter D to atmospheric coherence length r0 exceeds unity. Although adaptive optics may be employed to correct the wavefront, in practice the correction is imperfect and the residual phase errors induce a communications performance loss. That loss is quantified here by Monte Carlo simulation techniques. Single-mode-receiver fade statistics for imperfect phase correction are calculated in terms of the atmospheric Greenwood frequency fg, the adaptive optic servo loop cutoff frequency fc, and the ratio D/r0. From these statistics, link bit-error rate (BER) performance is calculated. The results reveal that conventional performance measures such as Strehl ratio or mean signal-to- noise ratio loss can significantly underestimate receiver BER losses. Only when the ratio fg/fc is 0.1 or less will communications losses be small (about 0.5 dB) over a wide range of D/r0.

Integrated heterodyne receiver and spatial tracker for binary FSK communication

In order to simplify system architectures and make efficient use of laser power, space lasercom system designers often try to consolidate the receiver subsystems. In this paper, we present a receiver which uses a single subsystem for both spatial tracking error sensing and communication signal reception. It makes use of an electro-optic crystal as a conical scanner for tracking error measurement, couples the scanned light into a single mode fiber, and uses standard fiber-based heterodyne techniques to derive an intermediate frequency signal. This signal is processed to retrieve both the binary FSK signal and the tracking error signal, as well as an estimate of signal power for use in normalizing the tracking error. The fiber-coupled receiver makes possible a modular architecture, whereby the transmitter, receiver, and telescope subsystems can reside in different parts of the spacecraft. Such an architecture is known to have a number of desirable properties. We present a discussion of the frequency plan, data demodulation, frequency tracking, spatial tracking, and gain control subsystems. Design considerations and experimental results are presented.

Coherent Optical Intersatellite Crosslink Systems

High-capacity intersatellite communication crosslinks will allow more efficient and reliable operation of satellite systems. This paper surveys present and future coherent optical communication technology and its application to intersatellite links.

A near-optimum discriminator demodulator for binary FSK with wide tone spacing

Optical FSK communication systems often require large tone spacings to reduce bit-error-rate (BER) degradation from laser-linewidth-induced crosstalk. Until now, discriminator detection of FSK for such wide tone spacings has fallen short of matched filter performance because of the suboptimal choice of a prefilter. A near-optimum demodulator for 240 Mb/s, three-times minimum orthogonal CPFSK (continuous-phase frequency-shift keying) has been constructed, with a measured performance that is 0.5 dB from matched filter theory at 10/sup -9/ BER. The design can be scaled to other data rates and tone spacings. The demodulator incorporates a frequency tracking loop that has good performance at low signal levels and no data-pattern dependence.<<ETX>>

Free-space optical code-division multiple-access system design

This paper describes an optical direct-detection multiple access communications system for free-space satellite networks utilizing code-division multiple-access (CDMA) and forward error correction (FEC) coding. System performance is characterized by how many simultaneous users operating at data rate R can be accommodated in a signaling bandwidth W. The performance of two CDMA schemes, optical orthogonal codes (OOC) with FEC and orthogonal convolutional codes (OCC), is calculated and compared to information-theoretic capacity bounds. The calculations include the effects of background and detector noise as well as nonzero transmitter extinction ratio and power imbalance among users. A system design for 10 kbps multiple-access communications between low-earth orbit satellites is given. With near- term receiver technology and representative system losses, a 15 W peak-power transmitter provides 10-6 BER performance with seven interfering users and full moon background in the receiver FOV. The receiver employs an array of discrete wide-area avalanche photodiodes (APD) for wide field of view coverage. Issues of user acquisition and synchronization, implementation technology, and system scalability are also discussed.

Sensitive clock recovery for multi-rate DPSK optical receivers

Robust clock recovery is essential for optical receivers that can operate over a wide range of data rates. The multi-rate burst-mode DPSK format that is finding applications in NASAs Laser Communications Relay Demonstration (LCRD) leverages a single optically-preamplified receiver that can maintain nearly theoretical performance for data rates spanning more than two orders of magnitude. Clock recovery of this versatile format needs to function both in low signal-to-noise ratio (SNR) and in high-power regimes, and needs to be able to accommodate a wide variation of rate-dependent peak-to-average powers as well as operation over intermittent fading channels. This can be achieved with traditional analog phase-locked-loop techniques as well as gated approaches that offer lower jitter, which is helpful for stressing low-data-rate applications. This paper presents an overview of clock recovery approaches for multi-rate DPSK receivers, an analysis of expected performance, and measured results.

Using a low-noise interferometric fiber optic gyro in a pointing, acquisition, and tracking system

Heritage pointing, acquisition, and tracking (PAT) systems have relied on optical tracking with a cooperative remote terminal to stabilize the line-of-sight of optical communications links. A hybrid approach, using new interferometric fiberoptic gyro (IFOG) technology to sense and correct local angular disturbances, blended with optical tracking, is shown to yield two significant advantages over traditional all-optical tracking: (1) line-of-sight stabilization over a very wide disturbance frequency range, down to extremely low frequencies (<<1 Hz), without the need for any optical signal power or cooperation from the remote terminal, and (2) a significant reduction in signal power required for the optical tracker. This paper will present fundamental performance analyses of a hybrid IFOG/optical tracking system and will derive simple design rules that the system designer can use to architect an optimal hybrid IFOG/optical PAT system. In addition, flow-down benefits that can simplify PAT system hardware will be discussed.