Frequency stabilization is one of the most important problems in building a practical multichannel coherent system where the transmitter frequency spacing may be as small as a few GHz. In this paper, we review the problem of frequency stability in lasers and describe several stabilization techniques which are used in multichannel systems. Both absolute and relative frequency stabilization techniques are described.

Characteristics and problems of laser diode FM response, strongly affecting coherent optical transmissions with FSK modulation, are briefly reviewed. A measurement set up, that is based on the use of a plane mirror Fabry-Perot resonator as a frequency discriminator, and that allows accurate measurement of laser FM characteristics, is described. A thorough survey has been carried out on a group of fourteen commercial DFB laser diodes, with five different structures, and relevant results are presented here. Significant FM response distortion, both in amplitude and phase, has been found, mainly concentrated in the range 0.1-10 MHz. From a system point of view, this calls for either equalization circuits or transmission codes with small energy content at low frequency, where the distortion is higher. Yet, although common features can be roughly recognized, it is very difficult to identify, a "typical" behaviour, due to the large dispersion of results, even inside the same class of devices. This means that, with present DFB laser diodes, the equalization approach does not seem practical, since individual tailoring of the equalization to each laser would be necessary. As a result of this investigation, the introduction of proper transmission codes (AMI, CMI,...) seems therefore the only realistic solution, at least at present.

A measurement technique has been developed, that allows the accurate characterization of laser diode intensity noise up to microwave frequencies, with high sensitivity. Relative intensity noise levels lower than 10^-15 Hz^-1 can be measured, with good accuracy, up to 10 GHz. Several examples are presented, showing the effects of reflections on laser noise.

Semiconductor lasers with arbitrary external optical feedback are considered. The more relevant aspects to optical communications are particularly emphasized. Topics covered include: intensity and frequency noise, linewidth and stability, small-signal intensity and frequency modulation responses, chirp reduction and influence on the frequency and damping rate of relaxation oscillations. Some novel results are presented, specially as regards Fabry-Perot and DFB lasers with strong optical feedback.

A novel fiber phase-shifter is proposed. It is based on the birefringence effect induced when a polarization-maintaining optical fiber is deformed periodically. The relative phase delay between two fiber modes is altered by fiber axis perturbations to produce a light beam deflection of the far-field pattern by 20 degrees.

In this paper, we review polarization-diversity and polarization scrambling techniques and also include recent progress in these areas. The advantages and disadvantages of these techniques and several experimental and theoretical results reported to date are also discussed.

Recent demonstrations of endless polarisation control are briefly reviewed. Schemes based on three different types of transducer are compared, whose behaviour is exemplified by (i) fibre squeezers, (ii) fractional-wave plates and (iii) an integrated optic lithium niobate device. The algebra of polarisation transforma-tions is developed, and it is shown how control schemes based on the three types of transducer are closely related, sharing many common features.

This paper describes the design and construction of a polarisation control device which uses four liquid crystal cells. This device has a low fibre to fibre insertion loss and a low drive voltage. A novel polarisation control algorithm has been developed which produces the required state of polarisation and simultaneously optimises the voltage applied to each element. The algorithm avoids the need for careful calibration of the elements. Tests have been carried out which demonstrate that the controller and algorithm can successfully track polarisation changes.

An original high speed polarization modulator based on an optical guided wave two-by-two switch is demonstrated. Computer simulations and experimental results, obtained in real operation conditions, confirm the excellent performance of the electro-optic device. Applications to polarization phase shift keying systems and to polarization diversity receivers are suggested.

Computer simulation of coherent optical systems is described. Simulation models of a PSK heterodyne coherent optical receiver including both laser phase noise and shot noise are developed and a semi-analytical approach for the estimation of low Bit Error Rates (BER) (<10-9) is described. The developed models and the semi-analytical BER estimation approach are successfully applied to the examination of IF filtering effects on the BER performance. Linewidth requirements for various IF filters are also estimated and compared with the literature.

The performance degradation of an optical DPSK receiver in the presence of optical channel noise is analyzed. We effectively convert the optical channel noise into an equivalent additive Gaussian noise at the output of the IF filter of the receiver. It is found that the equivalent noise depends on the LO power but is independent of the signal and LO phase noise. The performance degradation is significant if strong channel noise is present.

It is widely known that heterodyne detection provides an improved receiver sensitivity and selectivity compared to direct detection. Therefore heterodyne systems are very interesting for both long haul transmission and local networks. For long haul transmission the up to 15 dB improvement in receiver sensitivity at high data rates will be the most important issue. This means that the receiver spacing will increase by about 75 km. The most sensitive PSK and DPSK modulation formats are of interest for this application. In LANs and distribution networks it will be very advantageous to transmit a great number of densely spaced channels over one fiber. In the subscriber premises, a heterodyne receiver selects one of them without any switching element. Since the FSK modulation format is very tolerant to laser linewidth and since low cost is an important issue for broadcast systems, FSK systems at moderate data rates with tunable DFB lasers will be used for this application. We demonstrate two transmission experiments with 565 Mbit/s DPSK and 140 Mbit/s FSK modulation, respectively, using different laser types.

Two novel concepts for suppression of laser phase noise in coherent optical communication systems are presented, based on compensation after heterodyne detection. As a result, the normally imposed requirements on laser linewidth to bit rate ratio can be diminished considerably. Discussion of both techniques is given, together with computer simulations and experiments demonstrating the principles of operation.

The heterodyne systems with polarization modulation presented so far are reviewed both from the theoreti-cal and experimental viewpoint. The probability distribution for the basic schemes is determined through a rigorous derivation. Consequently the detection sensitivity for these particular cases is shown equal to that of FSK (although with reduced bandwidth occupancy) . This fundamental equivalence is also extended in a qualitative way to all the configurations using orthogonal polarization vectors for modulation. Open problems regarding the analy-sis of advanced versions and the development of numerical simulations are also discussed in detail.

565 Mbit/s DPSK optical transmission experiments with a balanced phase diversity receiver are reported. The receiver was realized employing two different optical 90° hybrids, an integrated optics hybrid on LiNb03 and a compact bulk optical hybrid. A silicon bipolar multiplier IC was used in the demodulator and in the AFC-stage of the receiver. Long term stability at a BER of 10-9 with a receiver sensitivity of -48.5 dBm was obtained.

The results for coherent fiber optic digital subcarrier multiplexed (SCM) system experiments are described. With a frequency shift keying (FSK) format, 20 video channels at 100 Mb/s each were transmitted on one optical carrier using microwave subcarriers in a multi-octave configuration. A receiver sensitivity of -27 dBm was obtained when using narrow linewidth Nd YAG lasers, representing a 14 dB improvement over a 20 channel direct detection SCM system. It is shown that intermodulation distortion, in addition to shot noise and thermal noise, limits the obtainable receiver sensitivity. Theoretical and experimental results are compared and conditions for optimal performance as a function of channel number and phase modulation index are given. Analog system experiments using standard DFB lasers with a combined linewidth of 50 MHz were also performed. By employing a simple phase noise cancellation circuit, 12 FM video channels with a 30 MHz bandwidth and a 40 MHz channel spacing were successfully transmitted.

This paper describes the design, construction and characterisation of an engineered, rack mounted, coherent transmission system. This system demonstrates a 9.6dB link budget advantage over the same transmit laser and front end receiver operated as a direct detection link.

This paper reports on the UCOL concept which is currently under development as a laboratory prototype under the framework of the European programme ESPRIT; the acronym UCOL stands for Ultra-wideband Coherent Optical L.A.N. The first section sets out the basic rationale on which this project was started and reviews the system architecture. The following sections address the individual building blocks of the network and review the constraints set by multichannel capability on network operation and capacity.

A proposed high capacity optical Coherent Multi-Carrier (CMC) switching node for broad-band application is described. The switching of signals is performed in the optical domain by routing the carriers through broadband and selective stars, by frequency conversion and tunable heterodyn-receivers. Experimental results of a switching element are presented and a simple calculation shows, that more than 10,000 broadband carriers could be processed.

The CSI has developed analytical techniques for constructing pseudo orthogonal words suitable for temporal CDMA. The performance of these sets of words have been studied with respect to bit-error rate and with respect to other coding methods. The CSI methodology and algorithms permit generalized solutions for any combination of users (words), weights, and taps. CLS has taken these solutions and has developed design rules for implementing the encoders and decoders and for comparing them with theoretical predictions. The design rules show that, depending on data rate, number of users, and bit error rate, there are regimes where an optical or an electrical implementation of the encoders/decoders is preferred.

This paper reviews recent technological progress on coherent systems. Both devices and system developments are described, though the accent is placed on systems applications. Subjects investigated recently include stabilisation and modulation of LEC lasers, optical amplifiers, FSK based systems and phase modulation in amplifiers.

Research and development within the Plessey Company in the area of coherent optical communications is aimed at both free space and fibre based applications. Fibre based system design and experimentation covers the areas of high sensitivity single channel systems and multichannel systems, the latter being addressed in the main for the subscriber access within future broadband telecommunications networks. In this context a major system demonstration is being constructed in collaboration with partners within Europe as part of a RACE project. Modulation techniques of FSK, CPFSK, and DPSK have been addressed. Supporting R&D in devices and components includes: narrow linewidth and tunable laser sources of the DFB and external cavity types, integrated optic devices for polarisation control, polarisation diversity, and amplitude and phase modulation using lithium niobate, and broadband dual detector receiver front-ends.

We review the recent progress of a 16-channel coherent broadcast network at 155.52 Mb/s, that uses packaged 1.54 μ m DFB lasers stabilized at absolute frequencies and a tunable, polarization-insensitive receiver. System experiments conducted show a 0.3 dB power penalty incurred at a channel spacing of 8.5 Ghz. Results using an erbium-doped fiber amplifier gave a fiber-to-fiber gain of 22 dB and an amplifier dynamic range of 15 dB, enabling the distribution of sixteen 155 Mb/s signals to one of 256 end-users at a distance of 102 km.

Recent advances in the theory and practice of optical heterodyne detection components and systems has lead to demonstration communications systems which have potential throughputs far greater than those of conventional systems. The ultimate goal is the attainment of a practical technology for accessing the more than 20,000 gigahertz of low-loss transmission bandwidth available with present day single-mode optical fiber.

PSK coherent systems are very attractive because of their highly sensitive characteristics and widely applicable features. In this paper, our recent progress on various modulation/demodulation schemes of optical PSK /DPSK systems are described. High receiver sensitivities of -46.9 dBm for 1.2 Gb/s DPSK, -52.5 dBm for 560 Mb/s DPSK and -51.6 dBm for 560 Mb/s carrier recovery PSK have been achieved using external fiber cavity DFB-LD modules and a balanced optical receiver. Transmission experiments over 200 km of single mode fiber have been performed using these systems. To overcome the signal fading caused by polarization fluctuation in the transmitting fiber, a polarization diversity receiver was developed and demonstrated for 1.2 Gb/s DPSK transmission over 100 km of single mode fiber. A phase-noise-canceling demodulator using carrier recovery was also developed and an effective suppression of the laser phase noise by this demodulator was confirmed. This scheme is applicable to a demodulator for multi-value PSK transmission. Also described are a multi-channel transmission of HDTV signals using a DPSK polarization diversity tunable receiver, and an image rejection receiver for optical frequency division multiplexing heterodyne transmission.