Optical technology is now dominant in long haul communications networks and globally transports >50% of all traffic. The new optical devices and transmission systems now emerging offer revolutionary network options that could eclipse all previous operational gains. Specifically, coherent optics will allow very high traffic densities per fibre and optical amplifiers offer the possibility of full network transparency. Future long haul networks are thus likely to require far less hardware and control, but will offer a far greater utility and reliability than at present.

The purpose of this paper is to review the current status of coherent systems for point-to-point and network applications. The review covers components, subsystems and systems and concludes with an outline of the future prospects.

Laser phase noise manifests itself as a degradation in SNR or BER in coherent optical communication systems. Real systems, particularly those employing phase modulation, often require sub-megahertz linewidth sources to perform satisfactorily. Experimental laboratory systems offer greater flexibility in their realisation and in this paper a novel technique is described to reduce the effective linewidth of semiconductor lasers, which is appropriate to such laboratory experiments.

This paper describes the development of a single mode fibre optic connector with high return loss performance without the use of index matching. Partial reflection of incident light at a fibre optic connector interface is a recognised problem where the result can be increased noise and waveform distortion. This is particularly important for video transmission in subscriber networks which requires a high signal to noise ratio. A number of methods can be used to improve the return loss. The method described here uses a process which angles the connector endfaces. Measurements show typical return losses of -55dB can be achieved for an end angle of 6 degrees. Insertion loss results are also presented.

Coherent optical detection offers the potential for accessing an increased information capacity when -combined with multichannel transmitters. Such coherent multichannel systems are now being investigated with regard to applications in future local broadband networks.

In this work we present a simple self-homodyne technique for the characterisation of wideband photodetectors. This technique transforms the frequency modulation of the laser diode into intensity modulation using a 2-beam interferometer. When the laser diode is modulated by a triangular signal, the power is concentrated in one beating frequency component that can be swept by changing the amplitude of the modulating signal. The experimental and theoretical results are in good agreement. As an example, with the frequency of the modulating signal equals only 15 kHz we have measured a detector response of up to 1.5MHz.

The behaviour of solitons in optical fibres has been an area of study for some time. The interest in this rather fascinating field of nonlinear phenomena has varied from initially modest to the current interest in the possibility of a new type of data communication system . This paper will set out, in an non-mathematical way , the properties of these optical solitons . The type of soliton that will be considered is the envelope soliton, which consists of a carrier wave with a (complex) envelope that is modulated.

Over the last few years the erbium-doped fibre amplifier has emerged as the preferred amplifier for use in transmission systems operating around 1.55 μm. This is due to its high fibre-to-fibre gain, polarisation-insensitivity, quantum-limited 3dB noise figure, low crosstalk and high operating bandwidth. This paper will discuss the characteristics of erbium-doped fibre amplifiers, highlight their advantages and review current applications.

Fast-acting telecommunication systems based on optical solitons will require, for information encoding, the high repetition rate sequential pulses, which can be produced with the aid of induced modulational instability. The disadvantage of the method consists in the accompanying generation of wide pedestal, resulting in the nonlinear interaction between pulses and information losses. The results of our numerical investigations have shown that an employment of the induced Raman effect can suppress it. A sharp increase in contrast (by an order of magnitude) and irregular character of residual background lead to significant depression of sequential pulses interaction. Stability of such pulses generation as well as optimum conditions are discussed in the present work.

Current trends in both long haul transmission and optical networks indicate no slowing down in the move towards increased capacity and network complexity. These requirements have highlighted the limitations of the digital techniques that are the backbone of the current technology. Optical amplifiers offer potential solutions to many of these. This paper reviews the operation of these amplifiers, the current state of their development, and the future prospects for deployment.

The effective exploitation of fibre optics throughout the telecommunications network will demand components which can perform signal routing and control functions in the optical domain under electronic control. In many applications the optical switching elements themselves must be accompanied by high speed electronics and by monitoring photoreceivers to provide for feedback control and self-routing of signals. Cost and reliability considerations point strongly towards the monolithic integration of these functions in an appropriate optoelectronic integrated circuit technology. In this paper we discuss recent progress in our laboratory towards the integration of the three functions discussed above. Experimental data are presented on the performance of optical switching devices, detectors and heterojunction bipolar transistors (HBT's) designed specifically for monolithic integration in the InP/InGaAsP material system.

This paper builds upon earlier work concerned with high definition television (HDTV) transmission for application in the future optical fibre local telephone network. The HDTV signal is introduced by reference to the NHK system originating in Japan and the HD-MAC system supported in Europe. Both digital and analogue intensity modulation schemes for HDTV transmission on single mode fibre are then assessed in relation to transmission bandwidth, receiver signal to noise ratio, multiplexing capabilities, terminal complexity and cost. The practical implementation of a high speed pulse code modulated (P04) HDTV single mode fibre transmission system is then discussed. Details of the implementation of a sub-carrier frequency modulation (SCFM) system are then followed by a description of the practical realisation of a novel wideband pulse frequency modulation (PFM) system again for HDTV transmission on single mode fibre. Modification of this latter system for square wave frequency modulation (SWFM) is then discussed. Finally performance data on the aforementioned systems are presented and compared in order to determine specific trade-offs associated with these modulation techniques for optical fibre HDTV transmission.

Transmission of high definition TV signals over optical fibres will require a careful choice of modulation technique in order to avoid unnecessary bandwidth overhead penalties . This paper reports preliminary results from the use of squarewave frequency modulation in two different optical fibre systems intended for operation with such signals.

On the NASA Space Station, the requirement for high speed data transfer between the exterior experimental bays and the interior research facilities has generated the need for fiberoptics. The adverse vacuum effects in space, temperature extremes, and natural space radiation place extreme conditions on optical fiber interconnects. This report addresses the adverse space environmental effects of temperature and radiation on optical fibers.

Photobleaching at 1.30 by 1.30-micron transmitted light during and following exposure to Co-60 irradiation is evaluated at -55 C in undoped and doped silica fibers. It is found that photobleaching can reduce radiation-induced attenuation by a factor of ten with an irradiance as low as approximately 0.1 W/sq cm in fibers where the 630-nm drawing-induced band is absent. This low-temperature photobleaching is characterized by high-order recovery processes. Little photobleaching was observed in germanium doped fibers until irradiances were above 100 W/sq cm.

The rapid developments in submarine system technology are identified from early trials in 1980 to present plans. System reliability is a key requirement and the methods by which this is achieved are outlined.

An optical fibre cable system is subject to a number of environmental factors which must be considered in order to guarantee survivability over a planned lifetime of typically 25 years. This is particularly true for submarine cables, where the drive to maximise the separation between repeaters is squeezing the operating margin and where the cost of shipboard repair is very high. This paper considers the factors which may limit the life of the optical fibre. These include mechanisms which cause the fibre attenuation to change and failure due to static fatigue.

The temperature-related processes that affect the performance of loose-tube and tight-buffered optical fiber cables are considered with particular attention given to the limits on optical performance. The equations for maximum cable strain are given for loose-tube cables, aerial cables, military tactical cables, and avionic cables, and specific reference is given to the typical range of operating conditions for each. Small perturbations of the fiber axis - brought about by changes in cable length and/or polymer modulus - can cause large radiative losses, and changes are proposed for the fibers that mitigate these effects.

This paper discusses the strength of optical fibre which has been subject to a proof-test. The implications of a strength distribution described by Weibull statistics and modified by flaw growth obeying fracture mechanics are described with the help of Weibull plots.

The loss induced in PM fibers by low dose rate (less than 1 rad/h) irradiations is evaluated, and the radiation response varies from less than 0.4 to about 6 Db/km-krad, depending on the wavelength of measurement and the fiber. The 'permanent' induced loss determined by fitting the recovery of the induced loss following high-dose-rate exposure to nth-order kinetics is shown to be a valid predictor of the low-dose-rate response; both 0.85- and 1.3-micron PM fibers have been found which show virtually no permanent incremental loss and would therefore appear to be resistant to low-dose-rate radiation environments. In addition, explicit measurements are given for the differential incremental attenuations of light traveling in the two orthogonal polarization axes induced by both steady state Co-60 and pulsed electron irradiations. Significant differential attenuation is only evident at high doses and short times.

The ability of coupled waveguide devices to perform in adverse environments is of increasing concern for application in photonic systems. The intent of this paper is to examine the seemingly anomalous or nonlinear responses of two guided wave devices when exposed to the particular adverse condition of ionizing radiation consisting of protons and electrons. Physical models are presented explaining the different responses, and a possible method for partially controlling or mitigating these responses is discussed. The guided wave devices examined include: a polarization maintaining silica fiber and a titanium in-diffused lithium niobate channel waveguide directional coupler.

The effects of neutron irradiation on several properties of both single and multiple stripe laser diodes have been examined. Prior to fast neutron irradiation, total light output as a function of laser current, threshold current, near-field pattern, far-field pattern, and laser output wavelength spectra were measured at room temperature. These measurements were then repeated at intermittent neutron fluence levels. It was observed that the threshold current increased with neutron fluence for all devices examined. In contrast, neutron irradiation had only an indirect effect on the remainder of the laser diode properties in that the higher currents required for operation after irradiation caused variations in these properties.

Exposure of optical fibers to high radiation dose rates can cause a transient increase in the optical attenuation of the fiber as well as induce significant luminescence. High levels of transient attenuation can interrupt the operation of optical data links. Transient radiation induced upset of the transmitter and/or receiver can interrupt the operation of an optical fiber data link by combining the assessments of the three separate parts. The recovery characteristics of the optical fiber, transmitter, and receiver can be used to predict the recovery of a data link.

The successful operation of any system containing an optical fiber depends both on the optical and the mechanical properties of the fiber. When used in applications where ionizing radiations are present, the detrimental effect of the radiation must be evaluated. Although radiation-induced optical attenuation has been studied by many groups, there is negligible quantitative data available on how the mechanical strength of glass optical fibers is affected by gamma irradiation. This paper reports results from an experimental investigation into the tensile strength of standard, polymer coated, glass optical fibers when exposed to gamma-rays from a Co-60 source. The changes in the mean stress at failure, measured from the Weibull plots, showed that fibers protected with acrylate, silicone + acrylate, or polyimide coatings showed little degradation after receiving a total dose of 1 MGy (they retained more than 95 percent of their preirradiated strength). Two other types of fiber, both protected with fluorinated polymers, were severely weakened after 0.1 MGy (their tensile strength being reduced to less than 40 percent of their preirradiated strength). Support is given for the hypothesis that this degradation results from gaseous fluorine containing species chemically attacking the surface of the glass fiber.

Three SM-preforms with the same undoped OH-rich core material [1] and different fluorine content in the cladding are produced. From these preforms SM-fibers with different diameter are drawn and characterized by fiber diameter, cut-off wavelength, and effective V-parameter at 850 nm. The fibers are tested under continuous ⁶⁰Co-irradiation and pulsed irradiation. All measurements are performed at about 850 nm wavelength and room temperature. Due to different power distributions in SM-fibers with different V-parameters, the influence of radiation induced attenuation in the fluorine doped silica cladding is examined and the dependence of this attenuation on the fluorine concentration is shown. In addition MM-fibers with same core material are irradiated as a reference for the core material.

Directives and policies dictate the requirements for nuclear survivable military systems. This report addresses the evolution of component test procedures required to document potential radiation effects on modem fiber optic systems.

Raman spectroscopy has been used to study photosensitive effects in Ge-doped optical fibres. The spectrum obtained for a sample of the original fibre used by Hill et al is shown to have a large broadband fluorescence. A rapid photobleaching of a fluorescence band at 570nm has been observed in the photosensitive fibre. It is shown that the Ge(1) and Ge(2) defects have appropriate properties to produce a refractive index grating via a photobleaching process. A weak photosensitive production of SiH occurs in Ge-doped fibres subjected to 488nm, which have previously been subjected to hydrogen diffusion, the rate of production is found to be linear with the power squared.

Lucas Automotive Advanced Engineering Centre is undertaking research to develop sensors which will provide information about the friction coefficient by measuring the texture and covering of the road surface. This information is required in order to improve braking performance. The optical sensors currently under development include a laser displacement sensor for measuring macrotexture, a surface wetness sensor based on the Brewster angle, a speed-over-ground sensor, and infra-red sensors for measuring road temperature. The information from these sensors is combined with data from vehicle dynamics and environmental sensors in a data fusion approach to solving the problem of friction prediction.

The instrumentation of meteorological measuring stations represents a very significant field of application for fiberoptic sensors. Meteorological measurements specifically call for sensor properties such as freedom from cross sensitivities, long-term stability, freedom from maintenance, and interference immunity, in particular against electromagnetic pulse effects caused for instance by lightning. A fiberoptic measuring system for meteorological stations has advantages over conventional measuring systems especially due to the inherent immunity against lightning interference.

Advances in structurally integrated fiberoptic sensor technology both for damage assessment within composite materials and for strain mapping are reported. This includes the results of work on a fiberoptic test system imbedded within a full-scale aircraft-wing leading edge and the development of compact all-fiber optical strain rosettes for 2D strain mapping within composite structures.

The use of optical waveguides as chemical sensors requires a referencing system to compensate for the effect of contamination on the surface of the sensor. In this paper, we propose the use of two waves with different penetration depths for their evanescent fields in order to correct measurements. Several schemes are considered, including two-wavelength, two-mode and polarisation discrimination techniques. It is shown that good results can be obtained using TE₀ and TM₀ modes in a high index film.

An interferometric technique is demonstrated which allows simultaneous measurement of strain and temperature applied to the same piece of highly birefringent fibre. Second order effects are shown to be important for long sensing lengths or in the presence of high strains or temperature changes. We also describe the results of experiments carried out to verify our theoretical predictions.

The incorporation of laser diodes and monomode fibre optics into an Electronic Speckle Pattern Interferometer (ESPI) has led to the development of a novel holographic surface contouring system. Height contours are obtained by switching between two laser wavelengths. Contour intervals from 0.5-5 mm have been generated by modulating the injection current of a single laser diode source and are demonstrated on an automotive disc brake hub.

The technology of surface acoustic wave optical frequency shifters is discussed. The construction and operation of three types of device each using interdigital transducers on a PZT4 ceramic substrate is described.

Earlier work on the ignition of flammable atmospheres due to loose agglomerates of fine fibres being irradiated by a CW carbon dioxide laser beam is reviewed. An assessment of which fuels are likely to present the greatest hazard as regards the use of optical sensing in flammable atmospheres leads to the development of new apparatus. Results for diethyl ether, carbon disulphide and hydrogen in stoichiometric mixtures with air are presented in the form of minimum igniting radiation flux as a function of the time taken to ignite. Carbon disulphide proves particularly hazardous, leading to ignition at radiation fluxes as low as 22 kW m^-2. It is shown that the hazard cannot be correlated simply with any of the obvious combustion parameters, though the correlation with autoignition temperature is much poorer than that with the critical minimum ignition energy of the mixtures. Lean hydrogen mixtures manifest incandescence due to local reaction under conditions when the flame does not spread to the rest of the volume.

Spectral analysis of broad-band light reflected from a remote object can be used to determine the object's characteristics. The object may be a passive test-sample or a cooperative optical sensor of physical measurands. Here are discussed several approaches to broadband sensing, and in particular to broadband, or "white-light" interferometry.

New sensing techniques based on short coherence interferometry are demonstrated to form the generic basis for a range of absolute, passive sensors.

Limitations on the performance and capabilities of conventional fibre optic Doppler Anemometers (FODAs) are highlighted in the context of fluid flow measurement. In order to overcome these difficulties, a novel optical fibre signal processing scheme is outlined, based on a low coherence interferometric approach using an optical delay cavity matched for the position of the required measurement volume. Both a homodyne and a heterodyne system are discussed and experimental results are presented to verify the feasibility of the proposed method.

The basic electro-optic and magneto-optic effects in monomode fibres are well known. The geometrical flexibility of the optical fibre allows these effects to be used in some novel configurations, for special practical applications.

A novel technique for multiplexing interferometric optical fibre sensors has been developed . The technique uses spread spectrum modulation of a semiconductor laser and electronic correlation detection of the returned signals from a network of interferometric sensors . A network consisting of two interferometric sensors in parallel has been assembled to investigate performance parameters such as cross-talk , and initial results from our experiments are reported.

The Evanescent Mode Coupling phenomenon between two identical fibres has been studied in devices made with three different techniques. All show the coupling constant K to be linearly related to wavelength λ. This is explained in a simple model. In one of them, the BiCore Fibre (BCF), the effects of loading, temperature, bending and twisting are discussed. The BCF is intrinsically insensitive to temperature, bending is a most useful tuning mechanism and loading leads to polarization-dependent sensor effects. Through the wavelength dependence the BCE is shown to act as a distributed sensor. The BCF can be a polarization splitter for a particular length, but twisting allows any length to be used to that effect. In combination with other fibre-optic components the BCF can be used to advantage in all-fibre optical instruments. A reflection polarimeter is shown as an example.