Covers issues involved in improving the present range of systems and technology of optical fibre based telecommunications services operating with analogue-sourced signals.
Inspec keywords: optoelectronic devices; pulse position modulation; optical receivers; radio-over-fibre; video communication; wavelength division multiplexing; subcarrier multiplexing; solitons; mobile communication; millimetre waves; optical fibre amplifiers; intensity modulation; optical fibre networks
Other keywords: millimetre-wave radio over fibre system; optical fibre video transmission; optical fibre SCM system; optical receiver design; analogue intensity modulation; optical fibre digital pulse position modulation; analogue signal transmission; optoelectronics; performance assessment; analogue optical fibre communication network; subcarrier multiplexed optical system; mobile communication; soliton pulse position modulation; optical fibre amplifier; WDM
Subjects: Optical communication equipment; Optical fibre networks; Modulation and coding methods; Multiplexing and switching in optical communication; Fibre lasers and amplifiers
In this chapter, the principles behind subcarrier multiplexing (SCM), the factors affecting component and system performance and its application potential are discussed. SCM makes efficient use of available bandwidth and, even when techniques to reduce bandwidth in purely digital systems are considered, SCM remains less complex and expensive. The RF/microwave components required are often readily available from other systems (e.g. radio, radar, satellite) again reducing costs. The performance requirements for SCM systems are different to those for typical digital optical communications systems. Generally, these are more stringent regarding noise and nonlinearities, especially for multichannel applications. However, the correct choice of components and system design, perhaps with the use of compensation techniques, does allow these requirements to be met. The compatibility of SCM with many other types of RF/microwave systems and its flexibility makes it useful in a variety of applications, from antenna remoting and CATV to local-area networks. Probably the most significant feature of this flexibility, for the near-term application of SCM techniques, is its ability to allow an evolutionary development of existing networks into broadband-ISDN with low initial installed costs.
Most optical fibre analogue transmission systems use intensity modulation of the optical source with direct detection of the modulated optical signal in a depletion photo-detector. In this chapter, coherent techniques are introduced, and their advantages and disadvantages relative to intensity modulation/direct detection (IMDD) systems are considered. Coherent techniques are defined broadly to include all transmission methods where the precise frequency of the optical signal is important.
The widespread deployment of analogue optical fibre links started in the late 1980s with their incorporation as fibre trunk lines into coaxial cable-based CATV networks, particularly in North America. These coaxial cable television networks provide the multichannel amplitude modulated-vestigial sideband (AM-VSB) modulation over the frequency range from 50 to 88 MHz and from 120 to 550 MHz, whilst the band from 88 to 120 MHz is reserved for frequency modulated (FM) radio broadcast. Vestigial sideband amplitude modulation was employed within the CATV networks as it exhibited the good low-frequency baseband characteristics of double-sideband amplitude modulation whilst conserving bandwidth, and hence it has been widely utilised for the electrical transmission of TV and similar analogue signals.
This chapter has surveyed and reviewed the range of PTM techniques available for fibre transmission and outlined their advantages from the standpoint of bandwidth-efficient modulation of analogue-sourced signals. A general classification method has been described by which different members of the PTM family may be categorised. Equations have been presented to describe the modulation spectrum for each of the different methods. To explore the high frequency potential of PTM the interaction between distortion performance and minimum sampling ratio as a result of sidetone overlap has been examined. Calculations have indicated that, operating above threshold, SWFM could, for example, offer an improvement factor of almost 20 dB for a system restricted to a transmission channel bandwidth of 1 GHz operating with a maximum input signal frequency of 100 MHz.
The phenomenon of a train of optical fibre solitons, modulated in some way by impressed disturbances, is increasingly being identified as a new type of dynamical system, with interesting dynamical behaviour which can be studied both experimentally and mathematically. This perception leads rather naturally to an investigation of the behaviour of this system when subjected to various forms of pulse-analogue modulation, to which this chapter is devoted. The principal motivation for the study of solitons in pulse position modulation (PPM) arises from the fact that PPM with short pulses permits a greater transmission rate than longer pulses, and intense pulses exhibit higher signal-to-noise ratio than weak pulses. Consequently, intense short pulses are desirable for PPM, but fibre dispersion limits the minimum width an individual pulse can have after linear transmission. This consideration leads naturally to speculation on solitons in PPM because the soliton does not suffer dispersion. However, this is only one example of a general interest which is developing in the understanding of dynamical phenomena involving solitons, to which this work is addressed.
A performance analysis has been presented for digital PPM transmitted over an optical fibre channel and detected using both optimum and sub-optimum pre-detection filters. Receiver sensitivity calculations, carried out at a bit-rate of 140Mbit/s and a wavelength of 1.3 μm, show that the optimum digital PPM system considered offers an 8.6 dB improvement over a typical PCM system. The sub-optimum pre-detection filters considered were a matched filter, an optimised 3-pole filter and a third-order Butterworth filter. These led to sensitivity degradations of 0.4 dB, 0.9 dB and 1.1 dB respectively. This clearly illustrates that receiver complexity can be simplified without large reductions in sensitivity. In particular, the well known and simple Butterworth filter can be employed with only 1.1 dB degradation in sensitivity. The timing requirements for digital optical fibre PPM have been analysed. An original spectral characterisation of the PPM format using its cyclostationary properties has been presented. The characterisation was used to evaluate the inherent systematic jitter associated with the extracted slot clock. An optimisation of the extracted slot clock timing variance and system wrong slot errors (due to imperfect slot synchronisation) was shown to be feasible in terms of the PLL bandwidth and the PPM order. Frame synchronisation was analysed for an original class of frame synchronisers that utilises natural sequences. The extracted frame clock timing variance was evaluated and the probability of wrong slot errors due to the non-ideal frame clock was assessed. The frame clock timing variance and wrong slot errors were shown to be minimisable provided that the proper number of natural sequences is tracked and the appropriate PLL bandwidth is utilised. The analysis has provided a performance evaluation of the optical fibre PPM system in the presence of inherent systematic slot and frame jitter.
The work presented here is concerned with an investigation of the impact of source nonlinearity on the performance of multichannel subcarrier multiplexed optical fibre systems. There has been much interest recently in the capabilities of subcarrier multiplexing (SCM) as a means of realising economical multichannel systems which can be deployed in the short to medium term to support a wide range of analogue and digital services. The attractive feature of SCM is that it provides a way of exploiting the multi-gigahertz bandwidth potential of high speed lasers using conventional and established microwave techniques. Also it is very flexible being capable of simultaneously transmitting conventional baseband and microwave signals with the same fibre and detector. Moreover, SCM systems can be combined with coherent techniques and wavelength division multiplexing to utilise fully the tens of terahertz capacity of single-mode fibre.
Radio over fibre (RoF) transmission systems, characterised by having elements of free-space radio and optical fibre, are expected to find an increasing role in telecommunication networks over the next decade, due to their ability to provide operational benefits in a variety of applications. These can range from present day niche applications such as antenna remoting at satellite earth stations, to future mainstream applications such as a cordless or mobile periphery to an optical fibre network infrastructure. Future communication expectations are very likely to revolve around cordlessness or mobility while, in the same timeframe, major network operators are expected to expand their optical fibre infrastructure more and more into the access network. RoF systems provide good synergy between optics and radio, and are ideally placed, therefore, to allow an efficient means for these two seemingly disparate technologies to merge. This chapter sets out to cover some of the recent developments towards realising RoF systems operating at millimetre-wave frequencies, where the spectral resource that these future applications are likely to require can be obtained.
Future mobile communications is expected to make use of mm-wave frequencies, spectral congestion at lower frequencies making these effectively unavailable for broadband applications. The potential of optical fibre techniques to support the generation and remote delivery of mm-wave based mobile broadband services has been outlined, with particular reference to developments effected within the context of the EU RACE programme by project MODAL. It has been shown that such systems can be developed using optical and microwave/mm-wave technology which is readily available today. By employing optical techniques the flexibility for systems deployment is greatly increased, with optical fibre allowing signal transport over tens of kilometers, effecting benefits in base station interconnection and antenna siting and so contributing to low construction, installation and maintenance costs. The remainder of this introductory section will outline possible electrical methods for distributed mm wave generation and briefly outline some of the advantages that could be enjoyed if an optically-based method for the generation and distribution of mm-waves were to be adopted.
In this chapter, the basis for the design and realisation of optical fibre receivers suited to subcarrier multiplexing (SCM) applications is considered, and circuit noise modelling and optimisation are discussed in detail. A relation has been established that allows the optical receiver's equivalent input noise current spectral density to be calculated by using the noise-matching network's small signal parameters and the front-end noise parameters Fmin, Yopt, and Rn. From this, it is possible to derive the optimum noise matching condition for an optical receiver which differs from the optimum matching for the minimum noise figure.
Optical amplifiers have become a major element in the design of advanced optical systems. Their main characteristic is that they provide direct optical gain over a very wide bandwidth. In addition, they can support very high output powers and so are ideal for a number of analogue applications, such as CATV distribution. The wide bandwidth enables the simultaneous amplification of many signals at different wavelengths, making them ideal components to use in wavelength division multiplexed systems