The linewidth and wavelength tuning characteristics of single wavelength semiconductor laser diodes are described. Continuously tunable sources of this type are useful for multichannel coherent transmission systems.

The static tuning and the small-signal modulation characteristics of multielectrode DBR lasers are analysed. A theoretical model is presented to describe in particular the performance of a three electrode DBR laser with two electrodes in the active region and another electrode in the passive DBR region. Several direct modulation schemes are considered and nonlinear gain is taken into account. A nearly flat FM response over several GHz is observed by a nonuniform current injection and modulation of the lower carrier density section in the active region. A perfect cancellation of spurious intensity modulation together with the removal of the FM resonance peak seems to be possible by push-pull modulation of the active section with an appropriate current splitting ratio. However this situation does not provide generally ideal FM characteristics because of the thermal-effects. Tuning without distortion of the FM characteristics is an attractive feature of this device.

A narrow spectrum transmitter based on a directly modulated DFB laser and FM to AM conversion in a common path interferometer has been investigated theoretically and experimentally. The DFB-LD is locked to the interferometer with a new pattern independent locking scheme in order to give a high extinction ratio of the ASK signal. Experimentally a -20 dB spectral width of 7. 5 GHz 0. 06 nm for a 2. 5 Gbitls ASK signal was obtained using an interferometer with an extinction ratio in excess of 25 dB. The transmitter performance was tested in a direct detection system giving a receiver sensitivity of -28 dBm. I.

A general computer program that a11ow to design and to analyze Ti:LiNho9traveling wave pha5e and amplitude (Mach Zehnder) modu1ator ha been implemented and extensively tested with theoretical and experimental recaltc.

A theoretical study of a novel optical fiber filter comprising a doublecoupler fiber loop mirror and a singlecoupler fiber ring (or loop) resonator with and without degenerate twowave mixing is presented. The circulating and output intensities are computed. The characteristics and potential applications of this filter are discussed. The use of the degenerate twowave mixing improves and changes the output characteristics of the filter significantly.

In this paper we present computer results at 1. 5zm comparing coherent n-ary PPM with coherent PCM. We show that for a given fibre bandwidth there is an optimum word size and this occurs when the equivocations attributed to the error sources in n-ary PPM are equal. We conclude that for moderate to high fibre bandwidths homodyne n-ary PPM should achieve an improvement in sensitivity of typically 5 dB over homodyne PSK PCM.

A new TB/TM integrated polarization splitter made on LiNbO3 for an isotropic cut (X cut with z propagating light) has been theoretically studied and experimentally tested. The anisotropic behaviour is introduced via the electro-optic effect which acts differently on each polarization. At 1. 3 m wavelength a polarization extinction ratio of 25dB has been obtained.

3-dB couplers integrated on InGaAlAs/InP grown by Molecular Beam Epitaxy (MBE) have been studied designed and fabricated. The device structure is based on the modal interference principle modelling has been performed by beam propagation and effective index methods and on this basis devices were designed covering a range of geometrical parameters. Devices have been fabricated on single and double heterostructures by wet chemical etching and characterized over the 1. 47-1. 53 im range by a tunable color center laser. Balanced coupling has been observed in several devices with a best one of output balanced within 0. 1 dB over 15 nm.

A novel 4 level FSK technique for halving the number of transmitter lasers in a multichannel FSK system is described. This is accomplished by appropriately coding two independent binary signals into a 4 level FSK signal pnor to transmission. At the receiver the local oscillator optically decodes the signal prior to envelope detection. This scheme maintains the simplicity of a binary envelope detector and results in no system performance degradation over binary FSK.

The Bit Error Rate (BER) performance of optical phase diversity FSK receivers of both single filtered (LPF1 in Fig. 1) and dualfiltered version (LPF1 and LPF2 in Fig. 1) is analyzed. The effects of several key system parameters (e. g. the bandwidth of LPF1 the amount of laser phase noise the frequency deviation employed the delay time r in the frequency discriminator etc. ) on receiver performance are investigated in detail. Clear system design guide rules have also been obtained. The analysis has been focused on obtaining expressions for the probability distribution functions (pdf''s) of the signal v at the sampler output. The derivation does not use Gaussian approximations and is therefore more accurate. The results are valid for any value of delay time r.

An optical binary phase-shift keying (BPSK) heterodyne receiver using synchronous demodulation by means of a squaring loop carrier synchronizer is analysed. A detailed modelling of the phase error is performed taking into account the influence of additive noise and non-Lorentzian laser phase noise from external cavity semiconductor lasers. The effect of filtering on the baseband signal is also examined. It is found that the filter decreases the impact of laser phase noise namely by reducing the degradation due to relaxation oscillations.

A theoretical model has been applied to predict the performance of lightwave communication systems which incorporate cascaded erbium-doped fiber amplifiers (EDFAs). The model has been applied to direct and coherent detection systems. We have used the model to predict the performance of new direct-detection system configurations which demonstrate the limits of bandwidth x length products which can be obtained in future experimental systems. We have carried out a sensitivity analysis of direct and coherent lightwave communication systems incorporating EDFAs to variations in critical system parameters. These include: optical bandwidth electrical bandwidth bit-rate spontaneous emission factor fiber chromatic dispersion number of cascacTed EDFAS splice efficiency and source laser linewidth. We have also developed a model for a direct-detection system which uses distributed erbium-doped fiber amplifiers (DEDFAS) and used the model to predict the performance of such a system. Results indicate that a high Signal-to-Noise Ratio (SNR) ratio can be maintained throughout the system if the gain per unit length is held at a fairly low level. Compared with a " lumped" amplifier approach we find that the distributed fiber amplifier approach yields a 14 dB improvement in SNR.

This paper proposes and analyses a novel technique to generate a set of optical carriers from an optical reference comb of frequencies. This is achieved by using an optical phase or frequency locked loop as an optical frequency synthesiser. Theoretical models and practical results are presented. The implementation of such a technique is described in the context of an VFDM photonic MAN currently under development. The paper concludes with the merits of both loop designs being considered and contrasted.

Polarisation diversity is an important technique in coherent optical communications. The assembly of cost effective optical networks for realisation of this technique requires low-loss stable and practical polarisation maintaining coupler components. Fused fibre couplers produced from birefringent fibre in which the stressinducing regions have been index matched to the surrounding cladding offers a practical solution. Using this method low-loss polarisation maintaining 3 dB couplers and polarisation beam splitting couplers have been produced and assemblies formed to perform polarisation diversity.

The performance of coherent transmission systems with optical amplifiers is limited by factors such as gain saturation and the accumulation of spontaneous emission. The spontaneous noise is conveniently modelled by the amplifier noise figure which can be used to predict the performance of amplifier cascades. The ultimate transmission loss limit of an optical amplifier cascade is discussed and the requirements of amplifiers to achieve a trans-ocean communication link are described. Other limitations to amplified systems such as dispersion echo signals from reflections and interchannel interference are also discussed.

This paper demonstrates channel selection under embedded microprocessor control within an optical heterodyne receiver. The receiver is for use in a coherent multi-channel system demonstrator having ten channels occupying a total wavelength bandwidth of 0. 8 nm at 1 560nm. . The local oscillator laser in the receiver has to have a sufficiently large continuous wavelength tuning range achieved by electronic methods to ensure that all channels can be received and tuned rapidly. It is demonstrated that a twosegment DFB laser can used to achieve this. Use of computer control of the bias currents and temperature of the laser combined with the flexibility of the tuning method adopted has allowed an adaptive method of channel acquisition and lock to be built. Computer control of other features of the receiver simplifies design and allows relatively straightforward incorporation of infra-red remote control LCD display of date/time/channel information and allows laser ageing effects i. e. long-term drift of wavelength with constant operating conditions to be compensated for.

We propose an optical PSK modulation scheme achieved by direct modulation of a laser diode with a wide flat FM response. We call it Direct-Modulation PSK (DM-PSK). We performed 4-Gb/s DM-PSK and CP-FSK transmission experiments at 135 un over 233 km of conventional 13-. un zero-dispersion fiber. The receiver sensitivity degradation due to chromatic dispersion was 0. 7 dB for DM-PSK smaller than that of 2. 5 dB for CP-FSK.

The progress in selected areas of coherent lightwave systems technology at Bellcore over the past year is reviewed. Studies of the demodulator portion of a polarization diversity receiver show that a polarization sensitivity of less than 0. 5 dB can be achieved and that squaring circuits with wide dynamic range and very high sensitivity can be realized. We also report the results of frequency stability measurements made on packaged DFB laser transmitters over an extended time period. Four-wave-mixing (FWM) experiments show that high power in the range of 5 to 10 mW launched into dispersion shifted fiber can cause FWM crosstalk. Reports of FWM in a semiconductor optical amplifier are also reported. 1. )

Research and development effort on a field deployable coherent lightwave system at Bell Laboratories is reviewed. Detailed features and design of a 1 . 7 Gb/s FSK single filter heterodyne detection system and its performance in a one-month transmission field trial are described. Long distance transmission experiments of such a system with Er-doped fiber amplifiers at 1 . 7 Gb/s and 2. 5 Gb/s are presented. I.