S.B. Alexander

A precompetitive consortium on wide-band all-optical networks

The technical core of a precompetitive consortium formed by AT&T, DEC, and MIT to study the technology, architecture and applications of wideband all-optical networks of local to national (or international) extent is described. A general introduction to all-optical networks is given, and some proposed applications are discussed. The architecture, technology and testbed portions of this effort are described. >

Passive equalization of semiconductor diode laser frequency modulation

Optical heterodyne communication systems using direct-frequency-modulated (FM) semiconductor diode lasers often exhibit system degradation because of the nonuniform injection-current-to-FM transfer function present in most semiconductor diode lasers. In a FSK system, a nonuniform FM transfer function causes tone drift which results in increased crosstalk between time slots and a corresponding degradation in system bit-error rate (BER). Using simple passive electrical networks it is possible to equalize the nonuniform FM response and substantially reduce this transfer-function-induced BER degradation. The theory, computer optimation, construction, and test of various equalization networks and their use in realizing an FM-equalized transmitter for a 100-Mb/s binary FSK communication system is described. >

Design of wide-band optical heterodyne balanced mixer receivers

The optical heterodyne balanced mixer, or dual-detector receiver, offers significant advantages over a single detector receiver. Balanced mixer receivers are particularly attractive for use in optical heterodyne communication systems because they conserve local oscillator power and cancel excess intensity noise present in the local oscillator. Simple circuit models that illustrate the noise performance, small signal gain, and bandwidth of a balanced mixer receiver are developed. A figure-of-merit for receiver noise performance is also derived. An example design of a gigahertz bandwidth optical heterodyne balanced mixer receiver and the techniques used to characterize near-quantum-limited receiver performance are discussed.

Phase-noise-canceled differential phase-shift-keying (PNC-DPSK) for coherent optical communication systems

A modulation scheme using a nonlinear demodulation process, phase-noise-canceled differential phase-shift keying (PNC-DPSK), is used to circumvent the effects of phase noise in a phase-shift keying (PSK) system. A theoretical description of the system is given, and results of an experimental test system are shown to be in good agreement with the predictions. In particular, it is shown that the phase-noise-induced bit error rate (BER) floor can be eliminated despite using lasers with linewidths comparable to the data rate. Furthermore, for an optimized PNC-DPSK system, the degradation at high signal-to-noise compared to heterodyne PSK with ideal sources (i.e. no phase noise) should only be approximately=3 dB. >

Wideband frequency noise reduction and FM equalization in AlGaAs lasers using electrical feedback.

With the use of an optical frequency discriminator and negative electrical feedback, wide-bandwidth frequency noise suppression with simultaneous FM equalization of a semiconductor laser is achieved. With a heterodyne optical phase-lock loop the intermediate-frequency linewidth of two lasers, each using negative electrical feedback, was reduced from more than 25 MHz to less than 0.01 Hz.

Frequency-noise cancellation in semiconductor lasers by nonlinear heterodyne detection

The bit-error-rate (BER) performance of conventional noncoherent, heterodyne frequency-shift-keyed (FSK) optical communications systems can be surpassed by the use of a differential FSK modulation format and nonlinear postdetection processing at the receiver. A BER floor exists for conventional frequency-shift keying because of the frequency noise of the transmitter and local oscillator. The use of differential frequency-shift keying with nonlinear postdetection processing suppresses this BER floor for the semiconductor laser system considered here.

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.

Gbps-class optical communications systems for free-space applications

For very high data rates, optical communications holds a potential performance edge over other technologies, especially for space applications where size, weight, and power are of prime importance. We report demonstrations of several Gigabit-per-second (Gbps) class all- semiconductor optical communications systems which have been developed for free-space satellite crosslink applications. These systems are based on the master-oscillator-power- amplifier (MOPA) transmitter architecture which resolves the conflicting requirements of high speed and high power on a single-laser coherent transmitter. A 1 Gbps, 1 Watt system operating at 973 nm with a frequency-shift-keyed (FSK) modulation format is the highest power coherent optical communications system using all semiconductor lasers reported to date. A 3 Gbps differential-phase-shift-keyed (DPSK) system uses a 2-stage injection-locked diode array as a power amplifier at 830 nm. At a wavelength of 1.5 micrometers , an optically- preamplified direct-detection on-off-keyed (OOK) receiver was demonstrated at both 3 and 10 Gbps. A 3 Gbps optically-preamplified direct-detection DPSK receiver was also demonstrated and represents, to our knowledge, the highest sensitivity DPSK receiver reported to date for data rates above 2 Gbps.

Phase-noise-canceled differential-phase-shift-keying PNC-DPSK modulation for coherent optical communication systems

A major problem encountered in using AlGaAs lasers for optical heterodyne phase-shift-keying communication is that of broad linewidth caused by phase noise in the laser. Typical linewidth/bit rate ratio requirements are ≤0.1 % for PSK and ≤0.3% for DPSK. When these are met, the performance degradation due to the linewidth of the source becomes negligible. For AlGaAs lasers with 10-20-MHz linewidths this means a transmission rate in excess of several gigahertz, which exceeds the needs of many applications and which is difficult to achieve. For more common data rates (50-500 MHz) the linewidth becomes a serious problem.

High power single emitters for coherent optical communication

The application of high-power single-emitter lasers to coherent optical communication in studied. Measurements of laser properties essential to heterodyne detection systems are presented for high-power lasers and compared to standard single-mode 30-mW laser diodes. The advantages of the new high-power laser structures are demonstrated in a heterodyne frequency-shift-keyed communication system.