In short distance optical fibre communication systems, especially those using LED sources and a large number of connectors, the optical power transmitted is limited by the core size and numerical aperture (NA) of the fibre. For military and some civil applications the cabled fibre must provide stable operation over a wide temperature range and resist degradation in a variety of environments, including exposure to nuclear radiation. Two approaches to satisfy the fibre requirements have been taken. The first is based on the well established radiation resistance of high purity silica core fibres. The second exploits recent improvements in the control of radiation induced attenuation in doped silica materials.
The optical waveguide is becoming increasingly important. Its capability can only be put to widespread practical use, when a simple and reliable method for joining fibers is also available. Siemens has developed a completely new fusion splicer for multimode silica glass optical waveguides in single fiber buffer and minibundle buffer design. (See Figure 8.)
The following objectives were pursued:
. minimum splice loss . suitability for field use
. a self-contained unit in a portable case
. easy maintenance service
. independence from AC mains voltage and instant readiness for operation
The units constructed according to these specifications are mass-produced. Under laboratory conditions and in the field, they have performed reliably. Using fibers of the same type, splice losses averaging 0.1 dB were achieved.
Triangular index fibers have generated considerable interest because of their reported low loss behavior in dispersion shifted (λο = 1.55μm) monomode fibers. Furthermore, the high V-number in such fibers allows core diameters ameters larger than the corresponding step index fibers. However, the sensitivity of several system parameters to the variations in the index structure have not been emphasized. The paper discusses these aspects and outlines a possible mechanism for the observed low loss behavior.
Because of their immunity from electromagnetic interference, optical fibres are being used increasingly in composite cables which perform the dual roles of power transmission and data communication. Applications in railway telecommunications, coal mining, underwater cables and overhead power lines are described, also the design methods utilised to ensure fibre protection and high optical performance within these complex cable structures.
General dimensionless forms of the evolution equation of nonlinear pulses propagating in an optical waveguide are given. The main aim of the paper is to provide a quantitative framework within which various factors that could contribute to the pulse development can be assessed. After showing that both waveguide and material dispersion are important, and are automatically included in the simulation, several specimen examples containing enhanced effects are given. These show the effects of dispersion and self-steepening effects that can produce pulse asymmetry and, ultimately, optical shock formation. In particular the effect of the shock term on the N=1 and N=2 soliton is given that shows the shock term becoming more significant for higher order solitons provided that third order dispersion can be neglected.
The low loss window and zero chromatic dispersion at 1.3 pm wavelength in optical fibres has made the use of light emitting diodes (LEDs) as sources a practical solution in multimode telecommunication systems. The microlensed surface emitting LED (SLED) has become an established device in commercial systems operating at 34 Mb/s over repeater distances of up to 10.5 kml. However, the drive for higher data rates (140 Mb/s) and longer repeater distances requires narrower spectral linewidths and higher coupled powers than can be achieved with SLEDs, thus provid-ing the impetus for the development of the Edge Emitting LED (ELED) as a higher output power device. The stimulated emission in the plane of the junction (that contributes to radiance saturation in SLEDs2) can enhance both coupled power and switching speed in the ELED structure. The linewidth of this device is inherently narrower due to self absorption. We have previously reported3 a superluminescent device utilising a deep diffused structure capable of very high launch powers (340 μW into 0.2 NA 50 p core fibre at 250 mA) and fast switching (2.2-2.6ns TR and TF). However, a penalty for placing too much reliance on stimulated emission can be an unacceptable temperature derating of some device performance parameters.
Optical sources using GaAlAs emitting at 850nm for fibre optic communication have been the subject of intensive research and development. Lasers have proved themselves ideal for long haul high bandwidth applications, where their high power and fast response time are unmatched. However, high radiance LEDs capable of launching several hundred microwatts of power into optical fibres will play an important role in medium and low bandwidth systems over relatively short transmission lengths.
InGaAs photodiodes for detection in the 1.0 to 1.7 µm spectral region are now widely used in fibre optics and other applications. Properties, performance characteristics, and fabrication parameters for diodes grown by both Vapour-phase a1d6 Liquid-Phase-Epitaxial techniques have been described in a number of published reports . It is the purpose of this paper to present some recent results from a number of device types fabricated using Vapour-Phase-Fpitaxial-Growth techniques. The structure is shown in Figure 1. Successive layers of n- In.53Ca.47As (nominally undoped) and p+ InP are grown on a low-dislocation-density n-type InP substrate. Diffusion of zinc from the p+ InP layer results in the formation of a p-n junction in the InGaAs layer, about 1-2μm below the InP layer. The p+-InP serves as a passivation-layer for the InGaAs surface, and is transpa-rent for wavelengths above about 1pm. Standard photolithographic techniques are used to define p-side contacts (evaporated AuZn), and to mask for etching of the mesa. Deposited silicon dioxide has been used as an anti-reflection coating. The use of a Cr-Au metallization on the lower surface of the substrate has made it possible to solder the diode chips onto an appropriate carrier.
The coupling efficiency between semiconductor lasers and single mode fibres can be greatly increased if a microlens of appropriate focal length is fabricated at the end of the fibre, the reason being that the lens can effectively improve the mode matching of the laser and fibre fields. Using simple techniques we have fabricated hemispherical microlenses at the end of fibres. Coupling efficiencies of about 50% have been achieved. The thoery used to predict the lens parameters for maximum coupling regards the lens as a 'thick lens' and shows that spherical aberration contributes to coupling loss.
This paper describes the construction of connectors, both single and multiway, employing the expanded-beam principle. These connectors are robust and environmentally sealed, and have no delicate precision parts in the mating components, and the mating surface is manufactured to reasonable machining tolerances.
Ultra low loss monomode fibre in the 1.3µm to 1.6µm wavelength range offers the possibility of achieving very long repeater section lengths for high speed digital communication systems. Experiments have been performed over 32 and 62km of monomode fibre cable installed in BT duct at 140 and 650Mbit/s using a semiconductor laser at the fibre's minimum dispersion wavelength of 1.3um. Unrepeatered transmission of 140Mbit/s over 102km of monomode fibre and 90.6km of cabled fibre has been achieved at 1.52um using semiconductor lasers; the spectral width being controlled by injection locking and external cavity techniques.
This paper describes a multiplexer system which is compatible with the IBM 3274 controller and enables the 32 long runs of coaxial cable normally used to connect the controller to each peripheral to be replaced by a single core fibreoptic cable. This enables users to obtain drastic reductions in required duct space, gives total immunity to Electromagnetic Interference (EMI) and a significantly enhanced level of security of data transmission.
Early experience has been gained with the switched-star type of network in the Fibrevision cable TV trial at Milton Keynes, and British Telecom are progressing towards a full-scale multi-star wideband local network. This paper discusses both the present and future use of fibre optics in this type of network.
The Plessey BICC 140 Mb/s multimode London-Birmingham long wavelength route is one system meeting BT trunk network requirements. This 205 km route is divided into 25 sections and comprises Terminal and Intermediate Station Equipment in TEP1E equipment practice and buried and in-station regenerators. The system employs LED sources and PINFET receiver modules. A microprocessor based fault locating supervisory system is provided. The cable sub-system uses loosely tubed fibres jointed using a 3-Rod splice. There are also a number of conductors to provide for ancilliary services. The overal cable joint closure uses an injection welding technique. The fibre has a nominal minimum bandwidth specification of 800 MHz-km and a nominal maximum attenuation of 1.6 dB/km.
Since the culmination of early work on 140 Mbit/s optical systems, a number of experimental links have been installed. The first, a nine kilometre link between Stevenage and Hitchin just north of London, in 1977, was chosen to demonstrate the suitability of optical communications for trunk networks. In 1979 British Telecom awarded contracts for the first phase of proprietary optical line systems, (POLS 1) for installation in the UK network between 1980 and 1982. POLS 2 and POLS 3 awards included seventy 140 Mbit/s systems, mainly using multimode optical fibre cables.
Optical fibre as a telecommunication transmission medium has become one of the most significant developments so far of the digital era. A major contributory factor to the availability of optical fibre transmission systems has been the outstanding progress in the technology of manufacturing high quality glass fibre having relatively low attenuation and broad bandwidths. This and other achievements has enabled manufacturers in the United Kingdom to develop a range of optical fibre cables and transmission equipment for operation over short and long distances at the CCITT recommended digital hierarchical rates of 2, 8, 34 and 140Mbit/s which have been adopted by British Telecom (BT) for modernisation to an all digital transmission network. These optical fibre systems are now becoming increasingly price and performance competitive with conventional technology in many parts of telecommunications networks. In the British Telecom (BT) network the 140Mbit/s optical fibre systems are now economically competitive with equivalent systems on coaxial cable and microwave radio. The economics of 8 and 34Mbit/s systems are not yet sufficiently attractive, compared with 2Mbit/s digital line systems on pair type copper cables, for large scale use in rural and local distribution networks and are presently only justified on cost in and around large cities. As system costs decrease large applications are predicted particularly in support of future high speed data, cable television and visual information services.
Laser sources offer important technical advantages in long haul and/or high bit rate optical fibre communication systems because of their narrow spectral line-widths, high speed modulation capability and high power output compared with LEDs.
Methods of determining the LP11 mode cut-off wavelength, λc, have been examined using a range of monomode fibre samples. The results obtained show that bending of the fibre in an uncoated condition can produce large changes in the measured value of λc. In view of the problems associated with the measurement of this parameter doubt is expressed about its use in fibre characterisation.
This paper reviews some of the current Military applications of fibre optics in the UK Armed Forces. The emphasis is on those aspects of fibre optics which have particular significance in a Military environment, for example security and Electro Magnetic Pulse Protection (EMPP) considerations. In addition, those normal attributes of fibre optics, for example low weight, which are of particular relevance, will be highlighted. During the course of the review implementations of fibre optics will be described to illustrate ways in which the particular attributes of fibre optics have been utilised in these Military systems.
A novel holographic system is described, in which the functions of all the conventional optical components commonly used to produce holograms - such as beam splitters, mirrors and beaA expanding spatial filters - are performed with single mode optical fibres. Problems caused by strain induced birefringence and stimulated scattering processes within the fibre are considered. An electronic servo system which provides compensation for thermally induced phase shifts is discussed.
1.0 CURRENT FIBRE OPTIC EQUIPMENT 1.1 Motorway signs - communicating to the motorist Fibre optic technology has been used in motorway changeable message signs since the early 1970s. Although non fibre optic units of the bulb matrix type are also used, fibre optic signs have some considerable advantages, namely:
a) The ability to project colour legends and if required multi-colour per legend
b) Power consumption is only about half of the equivalent bulb matrix types, typically 70 watts
c) Due to the use of tungsten halogen lamps the output is of a much whiter light
d) Maintenance is considerably reduced as only one lamp per legend is used and this can easily be replaced via an access panel at the side of the sign case
e) Lamp failure detection and stand-by legends
f) Legends can be reprogrammed at the factory
Endoscopes are long thin optical instruments used for remotely viewing internal voids through small access holes. They have to be capable of transmitting light to the area being inspected and relaying back to the observer an image of that area in order for the inspection to be completed. Endoscopes are also known as borescopes, industrial telescopes and intrascopes.
The use of optical fibres as light guides in astronomy promises to increase the efficiency of data collection in certain types of observing programmes. Fibres can also be used to simplify the construction of telescopes and associated instrumentation replacing complicated optical systems. At the Anglo-Australian Observatory (AAO) we have developed a number of fibre systems which spatially rearrange images at the 3.9m Anglo-Australian Telescope's (AAT) focal plane so as to optimise the input of light into the spectrograph. One system known as a fibre optically coupled aperture plate (FOCAP) allows the spectra of 50 objects to be gathered simultaneously. This unique application of fibres has revealed a number of problems not normally encountered in fibre data transmission systems. When fibres are inputted with a slow f-ratio beam, transmission along the fibre results in a spreading of the output beam beyond that of the input. This so-called focal ratio degradation is the major source of loss in these fibre systems.
The choice of single mode semiconductor diode lasers for the source in fiber optic interferometers is primarily determined by the size, power requirements and potential cost of these devices. In interferometer systems (both bulk and fiber) powered by GaAlAs diode lasers, the noise properties of the laser primarily govern the noise properties of the interferometer. Since 1980 there has been a continuing interest in the low frequency properties of these devices. In this paper, the noise properties of the radiation emitted by these devices will be reviewed. Methods of improving the noise properties of these devices will be discussed. Properties of broadband sources suitable Sagnac interferometry will also be briefly discussed.
This review paper describes the various means in which optical fibres may be used in transducer systems, either as intrinsic sensors themselves or in their more normal role as a light guiding medium coupled to more conventional optical sensors. As the variety of these transducers increases, it has become necessary when reviewing applications to concentrate on relatively few types of major practical interest.
The fibre optic gyroscope (FOG) offers the potential of a low cost, high performance strap down inertial navigational instrument. In this article, the principles of the FOG are reviewed and the factors which limit the performance of the system are discussed. The basic Sagnac interferometer is shown to be susceptible to polarisation effects, optically induced nonlinearaties and both spatial and temporal coherence within the optical source and within the passive gyroscope optics. However, with due attention to these phenomena, a high performance instrument is feasible. An assessment of the applications role of this instrument is presented together with some speculations on the required technological developments to produce the instrument.
Passive optical devices offer valuable technical and economic advantages for power-system current and voltage measurement. Such devices can also be used to measure a variety of other power-system parameters (e.g. temperature, pressure, strain, etc.). The main advantages of all such devices may be summarized as follows:
(i) No powering is required at high voltage points.
(ii) There is freedom from saturation effects.
(iii) They have large measurement bandwith.
(iv) The measurement sensitivity is high.
(v) The costs are low and are not strongly dependent on system voltages.
(vi) They are readily interfaced with interference-immune communications links.
The purpose of the demodulation scheme in fibre optic interferometers is to transform the optical output of the interferometer into an electrical signal proportional to the amplitude of the relative phase shift. Typical requirements of the detection scheme1,20,4 are: to be able to resolve signals corresponding to 10-6 rad phase shift, to have a linear response and to have a large dynamic range (106 107). Specific requirements such as packaging and low power consumption are also important. In this talk, four types of detection schemes will be discussed, two active homodyne techniques using a) a piezoelectric fibre stretcher) and b) laser tuning2 and two passive homodyne techniques using a) phase modulation3 and b) a (3x3) directional coupler.4 Each of these schemes have achieved prad performance, however, each scheme has specific advantages and disadvantages which will be briefly described below and discussed at the conference.
A simple displacement fibre optic sensor is proposed which is based upon the principle of frustrated total internal reflection. The simple designs considered overcome the problems inherent with the small distances involved with frustrated total internal reflection.