Industry standards are being developed for fibre optic LANs for application within buildings and campus environments. An overview of key standards projects was presented, covering FDDI and fibre optic versions of Ethernet and Token-passing networks. The paper also included a brief report on the standardisation of individual components such as optical fibre, connectors and transmission devices.

There are two main structures emerging as candidates for a future fibre-based local network. The star topology with its dedicated feed to each customer has been the mainstay of telecomms networks to the present day. It offers simplicity, flexibility for upgrade, and security (i.e. customer independence), and so remains a major contender as fibre is introduced. However it is the distributed-star network (DSN) structure of Figure 1 that is the most viable form of this topology for the foreseeable future. Here the star connection is from a remotely-sited equipment node near to the customer rather than all the way from the exchange. This reduces the fibre content of the network, and more importantly makes fibre handling more manageable.There are already fibre fed remote telecomms multiplexers and switched-star cable TV schemes being installed. They both match the topology and can come together to make a multi-service distributed-star network.

In this paper it is shown that the multiple-access requirement of optical fibre LANs may be met efficiently by a hybrid protocol scheme combining Wavelength Division Multiplexing (WDM) with CDMA as an inner protocol. The COMA inner protocol enables several users to access the channel simultaneously before any transmission delay is incurred while the WDM protocol allows for the duplication of each COMA channel capacity in the overall LAN. If the WDM protocol can support M simultaneous channels and the CDMA protocol can support K simultaneous users then the total number of users which can be supported concurrently equals MK. This represents MK users more than other multiple-access schemes, for example ALOHA or CSMA/CD, which allow only one user to access the channel at any one time.

The technique of Pulse-Position-Modulation offers greater receiver sensitivity than PCM, and is ideally suited to optical sources. A broad analysis shows that more taps could be made available on an existing multi-point fibre Local-Area-Network at a data rate of 10 Mb/s, by using PPM instead of PCM as a method of signalling.

Data buses are being employed in avionic applications to simplify the task of systems integration. MIL-STD-1553B is a universally accepted standard developed for military systems and it is this standard that has been identified as a suitable candidate for implementation using fibre optic components. A number of candidate topologies have been evaluated which have led to the design of specific components, eg fibre optic star couplers and the fibre optic transceiver chip set. The other components that are required are fibre optic cables and connectors designed for the avionic environment. Together, these components and the chosen topology provide an effective means to integrate avionic systems and provide for future increases in data rate and wavelength division multiplexing of different types of data. WHL has recently been involved in the design and installation of a MIL-STD-1553B based fibre optic transmission system for a research helicopter programme. This paper describes the techniques and practical problems of using fibre optic technology in aircraft applications.

The focal point of the RACE Programme is the Integrated Broadband Communications (IBC), where "Integration" implies the merging of telephony/videotelephony, data, etc together with TV distribution. The main solution to the large bandwidth requirements can be expected through the application of Optical Communications.

This year, 1989, marks the beginning of a new era in international communications since the World's major economies -Japan, North America and Europe - will now be interconnected by digital links of very low cost, very high performance, and very large capacity.

Research into direct optical amplifiers has accelerated in recent years as the potential advantages and applications of such devices have become apparent. Currently semiconductor amplifiers are the most developed, but research into fibre amplifiers is also making rapid progress. This paper considers the progress in amplifier development and discusses device performance expectations and limitations.

It is widely accepted that single-mode (SM) fibre will be used in telecommunication networks and, in particular, in subscriber loops. In such mass applications, the cost and reliability of optical sources are important considerations which, together with recent developments in 1.3 μm edge-emitting LEDs, make the latter increasingly attractive compared with the lasers that traditionally been used in single-mode systems. Wavelength-division multiplexing (WDM) offers many advantages for such applications. Optical WDM devices for single-mode systems are currently available based on a variety of principles - e.g. interference filters, gratings, fused couplers, or a combination of these. The main purpose of this paper is to provide a quantitative comparison of the performance of these various types of WDM devices, when used with single-mode fibre and LED sources in applications such as the subscriber loop. System performance is measured in terms of insertion loss and received optical power levels, and crosstalk between WDM channels, as functions of temperature (-40 to +80°C); an important criterion for such applications. These measures are evaluated numerically, based on a mathematical analysis with parameters taken from published data to characterize the LEDs and WDM devices, and using measured values of the single-mode fibre characteristics. The results demonstrate the superiority of WDM devices using dielectric-thin-film, multilayer interference filters. Using these devices, the LED-based, single-mode-WDM system is shown to perform satisfactorily over the entire temperature range considered.

This paper describes the application of avalanche photodiodes (APD's) as frequency downconverters and high-isolation sampling gates. Frequency downconversion or mixing is necessary in subcarrier multiplexed (SCM) optical networks whilst sampling techniques are required in time-division-multiple-access (TDMA) systems. Two novel subscriber access node receiver designs are considered. The first features a Germanium APD in an optoelectronic phase-locked-loop (OPLL) circuit configuration for SCM networks. This design minimises the high-speed component count by performing the phase comparison (mixing) function within the APD. A second circuit technique, described for the first time, illustrates the use of a III-V APD as a high isolation sampling gate. Complex (and expensive) high-speed logic functions are therefore not required for TDMA broadcast optical networks. A prototype OPLL receiver has shown a sensitivity of -35 dBm at 1.3 μm wavelength (100 % optical modulation depth) for 16.5 dB signal-to-noise ratio in 36 MHz bandwidth.

Over the past decade simulation has almost become a standard tool for the analysis and the design of communication systems. Many simulation packages have been developed for the conventional radio and the satellite communication systems,1 and various improved simulation techniques, more recently, have been considered in order to reduce the long error rate estimation time for systems operating at low error rates (i.e. BER <10^-7).

A hardware concept demonstrator has been built of a fast combined frequency and time hopping RF synthesiser for use in high security radio communications. The design involves the use of a recirculating switch breakable loop incorporating a fibre optic delay line and hop rates of greater than 33,000 Hops/sec have been demonstrated.

In the last 10 years, the trunk telecommunications networks in the USA and Europe have been restructured using fibre optic technology. Fibre optics offer the advantages of almost infinite bandwidth, low loss and interference free transmission. The trunk network operators have used these capabilities to increase their system capacity, with 565 Mbps now a standard transmission rate. The reduction or elimination of repeaters and the increased capacity has allowed the systems to be installed cost effectively. With the fibre optic trunk networks largely complete, the benefits of applying this technology to the customer access connection are being considered.

A self-aligning guided wave breadboard system has been developed, based on preferentially etched silicon, which can splice single mode optical fibre ribbon cable, or connect ribbon fibre pigtails to integrated optic chips containing arrays of channel waveguide components. The system can be used to construct optical delay-line signal processing networks with an extremely simple architecture, involving a single I0 chip and multiple fibre links. The processors may be used to encode a low-bandwidth signal before transmission in an optical LAN. The signal may then be decoded using a similar processor acting as a matched filter, allowing instantaneous access to a bus for a number of users through CDMA , or self-routing. Experimental results are presented for typical interconnection schemes, and a prototype signal encoding/decoding network is demonstrated.

The functionality, operation and performance of an Optical Switching Matrix for Local Area Network applications is described. Through the use of bulk optics and precision beam deflection assemblies, the switching matrix allows the connectivity of between up to ten input ports and ten output ports to be varied. The through loss of the matrix, when used with 50/125 multimode systems, is less than 3dB with crosstalk between adjacent ports constrained to below -40dB.

A bipolar imaging device consisting of a capacitor loaded emitter follower circuit for a photo-transistor has been implemented into linear image sensors, which has capabilities of charge amplification and self-noise-reduction. The linear sensors are demonstrated experimentally to exhibit excellent performance such as a linearity in a wide dynamic range and a high sensitivity.

In contrast to gas lasers, semi-conductor lasers are small, robust and relatively cheap. The wavelength of the semi-conductor laser is also easily tuned by varying the injection current, allowing a number of electronic processing schemes to be used that are not normally possible with a gas laser. As a consequence of these advantages, the semi-conductor laser is widely used for implementing optical sensors, in particular fibre optic interferometric sensors. However, one major limitation of semi-conductor laser diodes is that they exhibit both amplitude and frequency jitter noise that exceed the noise levels present in for example the HeNe laser.

Single-frequency laser sources are needed for optical communication applications. For error-free operation at high bit rates the laser must emit in a single frequency mode and never hop to another frequency throughout its operational life.

The process of measuring power in an optical fibre has to date required access to the end of the fibre. On an installed cable this could entail a lengthy procedure to cut, measure and resplice, assuming the interruption in service could be tolerated. We describe a 'Clip-on' optical power meter which does not require access to the fibre end but instead measures power through the side of the fibre. This testing method has obvious operational advantages over the former procedure.

Television pioneer John Logie Baird, a Scot from Helensburgh near Glasgow, described in his 1928 British patent No. 285,738 a honeycomb bundle of hollow metal tubes or thin solid rods of various types of glass. The tubes or rods were to be used for dissecting optical images in order to allow an ordered scanning of the dissected image pieces in a T.V. transmitter. Each tube or rod carried one small piece of the total image. The dissected image light passed along the tube or rod length, remaining inside the solid rods by internal reflection. By using hollow tubes Baird could also transmit all wavelengths of infra-red as well as visible light. By using hollow metal tubes Baird had ready made waveguides for the control of centimetric or millimetric radio waves. Baird's tube bundles would appear in publications of 1926, as part of the world's first military infra-red night vision units. The 1926 apparatus used on infra-red searchlight to illuminate the target, the reflected rays were dissected, scanned in a desired pattern and then converted into visible images by Baird's T.V. unit which was called a NOCTOVISOR (night vision). The range of the unit was stated as being 25 miles.

There is currently a great deal of interest in the development of new organic and polymeric materials for use in the field of nonlinear optics. This paper will review the advantages of such materials over their inorganic counterparts, and the specific attractions of using polymer-based systems. The status of materials under development for use in electro-optic waveguide switches and couplers will be outlined, and their prospects for commercial exploitation discussed.

The recent advent of materials showing superconducting properties at temperatures above the boiling point of liquid nitrogen (77 K ), and the prospect of room temperature superconductivity, have resulted in a number of suggestions as to how superconductors will penetrate the electronics, telecommunications and sensor marketplaces. Fibre optics and optoelectronics now occupy a significant sector of these markets and are seeking to expand into others such as optical interconnect and sensors. Will super-conductors compete with fibres, or even replace them for low-loss telecommunication cables, ultra-high speed interconnect and high precision sensors? This paper seeks to address these considerations, and to provide some design pointers as to where these different technologies may best be employed.

Because of the unique features of structural composites they are ideal materials to act as the basis of a new family of intelligent or smart materials, which can both sense and respond to changes in external parameters or internal integrity. This paper reviews some of the important materials issues and optical sensing problems to be solved in order to establish a practical nervous system based on fibre optic sensors embedded in composite materials.

Smart Structure is a term that is growing in stature as developments in technology makes it possible, and catastrophic failures provide the incentive. Most failures are invariably due to human error. Even when a structure (for brevity I shall use the term structure to cover everything from a minor structural component to a complex vehicle like an aircraft) fails, it is still human error for not correctly allowing for this possibility and either building in enough redundancy or providing a sensor system that could forewarn of impending failure. The revolution in material and structural engineering termed, Smart Structure, directly addresses the latter modality. The following definitions are proposed to clarify this new technology: Type-1 or "Passive Smart Structure" Possesses a structurally integrated optical microsensor system for determining the state of the structure and in certain instances the environment in which it is operating.

Pure diamond has excellent mechanical and infra red properties and would make an ideal window material were it readily available. Synthetic diamond films can be grown with good infra red transmission and used as protective coatings. Many coating requirements can already be met using diamond-like carbon films.

Some initial results on the implantation of single- and multi-mode optical fibres are described. Fibres are wound onto a bobin system for insertion into the implantation chamber. Long and short lengths of fibres as well as discrete elements along a fibre have been implanted. Refractive index changes of 5 x 10^-3 have been produced in the fibres, leading to new guiding properties. A large dose dependent fluorescence and attenuation increase is also produced in the fibre.

Recent measurements of the effect of gamma radiation on the transmission at 1300 nm of some relatively insensitive optical fibres are presented. The significance and interpretation of these results from the viewpoint of a potential user are briefly discussed.

We report the photosensitive transient increase in transmission of a probe HeNe beam in a HiBi fibre when a counter propagating beam from a frequency doubled Nd. YAG of peak power 150W and 8ns pulse length is propagating in the fibre. Increases in transmission of upto 50%, in 100m of fibre, have been observed. The increase in transmission is observed to decrease when the YAG laser is switched off, but with a long decay time. This behaviour is counter to previously reported effects were attenuation is decreased and remains so by photobleaching, or attenuation is increased by Blue/green light. We propose a tentative model for this effect in terms of the photobleaching and production of Ge defect centres.

Experience with the first optical fibre video-communications networks has highlighted the significance of optical fibre jointing techniques which require:-a) Reliable joints b) Easily field-fitted terminations. Bearing in mind the environmental conditions they are used in together with the awkward situations where they may be installed, e.g. telegraph pole behind facades and in equipment racks. c) Low cost components. Due to the high number of connections per fibre kilometre used in these networks.

This paper will review automotive applications of optical sensors at the present time and in the forseeable future. Applications described will include engine and power train management as well as more advanced concepts such as navigation and collision avoidance systems for road vehicles. Current development work at Lucas on sensors to be applied in some of these areas will be described. There will also be a discussion of the environmental problems to be overcome in the design of automotive systems and components, and the effect of these constraints on the selection of optical hardware for use in sensors.

This paper deals with the concept, design, build and eventual flight trials of an optically signaled flight control system for an Airship Industries Skyship 600. This project has taken place over a number of years, culminating in the first flight of an optically signaled system which has no reversionary technology.

The potential of optical fibre based sensing systems for monitoring physical and chemical conditions in difficult environments such as those with explosion hazards or those susceptible to electromagnetic interference is now well established (e.g. Parr et al (1987)). One such environment which is difficult to monitor fully with conventional electronic instrumentation is that of high voltage power transmission systems. Perhaps the component of such systems which imposes the most severe demands upon monitoring (and therefore is arguably the least well monitored in service) is the extra high voltage circuit breaker. The difficulties in fully monitoring these devices include the need to preserve high levels of electrical insulation (over 525 kV in the UK and higher in other parts of the world) and the need for immunity from strong magnetic fields and from severe electromagnetic interference during circuit breaker operation. A range of parameters need to be addressed which are not only electrical in nature but also thermal, mechanical, aerodynamic and chemical.

A number of techniques for the multiplexing of interferometric fiber sensors have been reported in recent years, and systems with up to eight sensor elements have been experimentally demonstrated. This paper presents a comparison of these various multiplexing approaches in terms of power budget, noise, bandwidth/sampling-rate requirements and crosstalk.

The advantages offered by optical-fibre sensors are well known. Distributed optical-fibre sensors utilise the one-dimensional nature of an optical fibre to offer the additional advantage of the determination of the distribution of the measurand along the path of the fibre, with a spatial resolution 0.1-10m, depending on the system and the application. Most distributed optical-fibre measurement systems which have been studied hitherto have relied on time-domain backscatter (OTDR) techniques. In these, the spatial information is obtained via time resolution of the light backscattered from a pulse propagating in the fibre. These techniques suffer from the fact that backscattered light levels are low; thus signal detection and signal processing become problematic. An alternative approach is to extract the information from forward-scattered light by allowing counter-propagating light waves to interact non-linearly in the fibre. Detected signal levels then become some three orders of magnitude higher. The paper investigates this alternative approach.

Integrated optical instruments and their components are described by comparison with existing technology and the potential advantages of the new systems are outlined. Fundamental problems of material fabrication need to be overcome before a truly integrated circuit becomes feasible but useful advances are being made by combining discrete with integrated components in hybrid systems. A very large investment programme in R & D is underway in Europe, the U.S.A. and Japan. The first products to emerge in instrumental form have not yet had a significant influence on industrial and commercial markets. Future prospects will depend upon improvements in capability, reliability and on cost reductions.

We describe a technique for uniquely recovering the optical path imbalance within an interferometer interferometrically by means of a sinusoidal modulation of the source frequency. The technique is demonstrated in systems configured to measure displacement, intermodal dispersion in a highly birefringent fibre and the refractive index of a number of liquids.

The paper discusses a fiber optic multisensor methane detection system matched to topology and environment of the underground mine. The system involves time domain multiplexed (TDM) methane sensors based on selective absorption of source radiation by atomic/molecular species in the gas sensing heads. A two-wavelength ratiometric approach allows simple self-referencing, cancels out errors arising from other contaminants, and improves the measurement contrast. The laboratory system consists of a high radiance LED source, multimode fiber, optical sensing head, optical bandpass filters, and involves synchronous detection with low noise photodiodes and a lock-in amplifier. Detection sensitivity versus spectral resolution of the optical filters has also been investigated and described. The system performance was evaluated and the results are presented.

A technique is described whereby multipoint temperature measurements may simultaneously be made over long distances (up to 4 km) using a single optical fibre as the sensing element. The technique, which is based on the detection of Raman backscatter, utilizes a state-of-the-art photon-counting receiver plus a novel method for eliminating the effects of wavelength dependency in the fibre attenuation. A practical system is now in the final stages of development, based upon the use of 50/125 0.2NA multimode fibre, and a distance resolution of 1 m.

A novel type of optical hydrophone has been developed which offers a compact and reproducible extrinsic sensor with good noise performance. A conventional piezoelectric transducer is interfaced to an integrated optical modulator having very low drive-voltage. This is an electro-optic device on lithium niobate and thus has potential for reliable and cheap manufacture. Several hydrophones have been constructed and fully packaged. They are approximately 55mm x 13 mm x lOmm in size and are accessed via single-mode fibres. To achieve good noise performance a very low noise laser source has been developed to drive the hydrophone and achieves noise levels close to theoretical limits. Although the work has centred on hydrophones, it is also applicable to other sensing applications requiring a robust sensor with high sensitivity, large dynamic range and low noise.

A non-contacting optical vibration sensor has been developed using a fibre-optic system that is suitable for use as a reference grade device. It is based around a fibre Michelson interferometer configuration in which an optical carrier is produced by imposing a sinusoidal phase modulation of specific amplitude in the interferometer and electronically gating and bandpass filtering the resulting output signal. In order to achieve a primary reference grade sensor, the carrier modulation induced by the vibrating surface is demodulated in the phase domain using a digital phase-tracker, and the time dependence of the surface displacement is recovered relative only to the wavelength of radiation used.

A stable microwave power measurement in the x-band frequency region has been demonstrated with a polarimetric single-fibre probe. The sensor was constructed using high birefringence optical fibres which incorporated thermal compensation technique to enhance the detection sensitivity down to sub-milliwatt microwave power levels.

An optical current sensor has been designed and tested. This device, which employs the Faraday effect in optical fibre to measure electrical current, is intended to be applied in power system protection equipment; it is regarded as a potential replacement for the iron cored current transformer in certain applications. A novel polarization analyser arrangement is included in the optical current sensor. The theory of operation of this component is presented and it is shown that a simple graphical representation of the state of polarization may be produced in real time. The operation of the current sensor is described and pre-installation test results are presented.

A novel dual wavelength intensity modulated optical fibre displacement sensor system is described which employs a single Graded Index (GRIN) rod lensed fibre together with an interference filter and moving mirror element in the sensing head. The dual wavelength approach essentially utilises two spectral slices for a single LED transmission spectrum to provide the measurand and reference signals. In this way the sensor system is fully referenced for all major common-mode variations that may be present within the system components. The terminal transceiver unit is also discussed together with a specially designed LED coupler and demultiplexer device which enables the system to operate with a single fibre connection. The performance characteristics of the prototype dual wavelength sensor system are reported showing a linear displacement range over some 20 mm. In addition the coupling of the sensing head to a Bourdon tube is described which provides accurate pressure measurement over a range 3-15 psi. Finally an initial investigation of the immunity of the system to common-mode variations is presented using the mechanism of fibre bend loss.

An optical fibre with a core which photoluminesces is assessed as a means of detecting light emitted from a spark discharge in air. The spectral properties of both the fibre and the spark show significant overlap and the detection of sparks is demonstrated. At the peak excitation wavelength of the fibre, i.e. 400 nm, the photoluminescent response of the fibre is 7x10^-7 and the output from the fibre increases linearly with incident intensity. The response time of the fibre, when excited by a spark, is nominally 3 ms which is limited by the lifetime of the excited state producing the luminescence. The application of the fibre to detect breakdown in air filled microwave waveguides carrying high powers is discussed.

The performance of optical-fibre intensity sensors, particularly those utilising highly coherent diode laser sources and multi-mode fibres, is susceptible to modal-noise in the fibre. This can become a limiting factor in the sensitivity and stability of fibre-optic gas sensors based on absorption measurements where intensity variations of the order of 10^-4 need to be measured. For long lengths of fibre (several km), where natural mode scrambling in the fibre has occurred, the modal noise has been shown generally not to be a limiting factor. A signal processing scheme is proposed which is shown to reduce the performance dependence of the sensor on modal noise where short lengths of fibres are used. Data from a fibre-optic methanometer utilising this technique is presented demonstrating the improvement in the sensitivity, reproducibility and long term stability of the sensor.

The various techniques for accurate distance measurement using optical means may be subdivided as a) Time of flight measurement b) Intensity modulation c) Interferometric ranging including multi-source heterodyne interferometry and d) heterodyne frequency modulated continuous wave interferometry.

This paper describes a simple and economical sensor which employs fiber optics to detect the presence of hazardous liquid chemicals, particularly undiluted hydrocarbons. The device is best suited to monitor the interstitial space of double walled underground storage tanks. Because the sensor is plastic and is situated at the end of a passive and insulating optical fiber the sensor can be considered inherently safe. The optical fiber used for this device can be up to several hundred meters long.