L.C. Blank’s research while affiliated with University of East London and other places

The paper presents optical duobinary signalling as an alternative to the binary format for improved transmission performance in dispersion-limited systems and summarises results achieved from both experimental and theoretical investigations. The potential advantages of duobinary transmission for dispersion accommodation in high capacity systems are demonstrated in both laboratory experiments and installed optical fibres. Unrepeatered transmission at 10 Gbit/s without dispersion compensation was achieved with penalties of only 1.5 dB after 120 km and 2.5 dB after 138 km. Initial indications of the influence of self-phase modulation (SPM) and stimulated Brillouin scattering (SBS) on duobinary transmission are also presented. Two channel, 2×10 Gbit/s wavelength division multiplex (WDM) operation with mixed binary and duobinary transmission was demonstrated in a field trial. This is the world's first demonstration of using both duobinary and binary signallings at a total capacity of 20 Gbit/s over installed fibre. The investigations show that duobinary signalling should become an important transmission format for future wideband networks where system performance is primarily dispersion dominated

The authors present duobinary optical transmission, as an alternative to binary, for very high speed systems. There are three important advantages of this signalling format. The first is that the dispersion limited transmission distance over nondispersion shifted fibre (NDSF) is substantially increased due to the halving of the transmitter bandwidth. Secondly the constraints of high speed operation for the terminal equipment is significantly reduced due to its lower bandwidth characteristic. Finally the amount of signal processing required for duobinary operation is low, being readily achievable for engineering applications. They detail methods required to realise practical duobinary optical transmission and present results at 10 Gbit/s over NDSF. Implementation issues of this technique in high speed transmission are also addressed

The use of duobinary optic signalling is proposed for extending the dispersion-limited propagation distances at 10 Gbit/s without the use of finely tuned dispersion compensation techniques. Experimental transmission system performance evaluation over 100 km of standard step-index fibre at 1.55 mu m has been achieved with a penalty of only 0.5 dB.

Future networks operation in excess of 10 Gbit/s are likely to require optical multiplexing to obtain such high capacities, linking all major nodes in the inland network. The implementation of optical fibre amplifiers instead of optoelectronic regenerators would future-proof the transmission fabric, permitting capacity upgrades through modifications to terminal equipment. Optical nonlinearity will form an ultimate limit to the capacity of simple wavelength division multiplexed systems. In the longer term, upgrading to an optical time division multiplexed (OTDM) super-trunk network would provide a core transport layer of very high capacity so that services requiring large bandwidth on demand could be enabled. The current status of OTDM systems is surveyed and a scenario is developed for the implementation of OTDM networks within the framework of the synchronous digital hierarchy (SDH).

A 35 km long distributed erbium fibre preamplifier has been used to provide dispersion equalisation and to maintain the signal above the receiver sensitivity level. Remote supply of pump power to the input of the lightly-doped fibre provides significant improvement in amplifier performance.

Nonlinear pulse compression has been used to achieve transmission beyond the linear dispersion limit for 20 Gbit/s optical time-division-multiplexed data. Error free system operation has been achieved over an operating wavelength range of 10 nm above the wavelength of zero dispersion, in the anomalous dispersion regime.

A 20 Gbit/s optical time division multiplexed transmission system based on components and subsystems operating at not more than 5 GHz repetition rate is reported. Full BER measurements indicate a penalty of only 0.8 dB after transmission through 205 km of dispersion-shifted fibre. The penalty was mostly attributed to the accumulation of noise from erbium-doped fibre repeater amplifiers in the system.

In order to fulfil the demand for increasingly sophisticated and bandwidth intensive services, telecommunications operators will require flexible optical transmission networks which will operate at higher data rates than those installed to date. At present, the maximum data capacity of installed optical systems is limited by the bandwidth of the electronics in the terminal and repeater equipment (currently around 2.5Gbit/s). Optical wavelength [1] or time [2] division multiplexing techniques may be used to access data rates substantially over 10Gbit/s, offering a substantial increase in capacity. Both multiplexing techniques could be used in conjunction with switching in either the wavelength or time domains to allow increased network flexibility through, for example, the drop-and-insert function. Discrete erbium-doped fibre amplification is an excellent method of compensating for the loss of the transmission link, providing bit-rate independent amplification. However, the regenerative function of conventional opto-electronic repeaters is no longer available in fibre amplifiers: linear dispersion of the optical pulses now becomes a major system constraint. The availability of fibre amplifiers with excellent performance in the 1550nm window restricts the signal wavelength of optically multiplexed systems to this wavelength range, implying the use of dispersion-shifted transmission fibre. The operation of wavelength division multiplexed transmission with more than a few channels over fibre with low dispersion can lead to significant system penalties due to four wave mixing, even over fibre spans of less than 100km [3]. An alternative approach to ultra-high speed transmission is to use time division multiplexing, requiring a transmitter configuration based on short (≈ps) optical pulses. In this instance, nonlinear optical pulse compression in the transmission fibre may be used to advantage to significantly reduce (or even balance completely [4]) the linear dispersion of the optical pulses, permitting transmission well beyond the usual dispersion limit.