G.R. Walker

Polarization control for coherent communications

Two practical implementations of endless polarization control are described. The first approach uses polarization maintaining fiber transducer, while the second uses an integrated optic lithium niobate device. In order to characterize the lithium niobate device in detail, a technique for analyzing the birefringence as a function of the applied voltages was developed, and the measured performance of the devices is presented. Fractional-wave controllers for both static and endless control applications are considered, and design rules for maximum wavelength-window and minimum loss fiber-loop polarization adjusters are given. Endless control using various configurations of the three types of transducers is discussed, and it is shown how apparently different schemes are closely related. >

Sensitivity of optically preamplified receivers with optical filtering

The noise spectral densities resulting from detection of preamplified signals using an optical filter are discussed. The authors also carried out a simplified analysis of preamplified receiver sensitivities with ASK and FSK signals. The analysis shows that total optical bandwidths of less than 50 times the data rate result in (10/sup -9/ BER) sensitivities close to the limit set by the amplifier noise figure. The development of low-noise high-gain amplifiers in conjunction with narrowband tunable optical filters enable high-sensitivity FDM systems to be constructed, without the complication of a local oscillator or polarization control/diversity networks which are required for heterodyne coherent detection systems.<<ETX>>

Erbium-doped fiber amplifier cascade for multichannel coherent optical transmission

The requirements and practicalities of amplifier cascades for multichannel coherent transmission are discussed. A four-amplifier cascade which maintains an overall noise figure of less than 13 dB across a 20-nm optical bandwidth for total input powers up to -10 dBm is described. A potential capacity of 100 wavelength division multiplexing channels is demonstrated in a 300-km 622-Mb/s DPSK (differential phase shift keying) coherent transmission experiment. >

Progress towards the field deployment of coherent optical fiber systems

A description is given of the first demonstration of a coherent optical transmission system deployed in an operational network. The miniaturized external cavity laser and the automated endless polarization control scheme which made it possible to mount this demonstration are described in detail, together with specific system results. A discussion is presented of the possible field deployment of other coherent system options, based on FSK modulation of DFB and DBR lasers, and polarisation diversity reception. The considerable potential afforded by coherent techniques for meeting future network requirements is highlighted. The demonstrated advantages of greater power budget and wavelength selectivity, combined with the use of optical amplifier multiwavelength repeaters, make coherent techniques particularly relevant to the growing demand for greater transmission capacity, transparency, and network flexibility. An increasingly urgent need for robust coherent optical technology can thus be anticipated, for wideband transmission and passive routing, for both telecommunications and computer networks of the near future. >

Optical PSK Transmission Experiment At 565Mbit/S Incorporating An Endless Polarisation Control System

The successful application of an endless polarisation control scheme was demonstrated by its use in a high sensitivity PSK heterodyne optical communication system. The synchronous demodulation of PSK signals was investigated and the practical implementation of a phase locked loop which is able to track the phase noise of optical signals is discussed. A large loop bandwidth is shown to be necessary and a loop filter of modified first order type was found to have advantages over a second order type. A synchronous demodulation system not using a phase locked loop is also proposed.

High-sensitivity FSK signal detection with an erbium-doped fiber preamplifier and Fabry-Perot etalon demodulation

A report is presented on a 622-Mb/s optical transmission system with demodulation of alternate mark invert (AMI) encoded frequency shift keying (FSK) signals by a Fabry-Perot etalon. A 25-dB gain erbium-doped fiber preamplifier gave a receiver sensitivity of -40 dBm at 10/sup -9/ BER, which was improved to -44 dBm with the addition of a narrowband optical filter. The theoretical sensitivity of the preamplified receiver and the effect of the etalon on the amplified spontaneous emission beat noise are discussed and related to experiment.<<ETX>>

Performance limitations of amplified coherent optical transmission systems due to interfering echoes

Interfering echoes produced in semiconductor amplifier cascades are known to cause bit error rate (BER) degradations in optical transmission systems. An experimental investigation of the effect of interfering echoes in a coherent differential phase-shift keying (DPSK) system is described and results in good agreement with theoretical predictions are reported. It is concluded that interfering echoes limit the minimum allowable inter-amplifier attenuation, which in turn severely limits the number of semiconductor laser amplifiers that can be cascaded without optical isolation.<<ETX>>

Transparency in optical amplifier repeaters

Transmission links with optical amplifier repeaters are potentially transparent to system upgrades such as increased bit rate or the addition of wavelength division multiplexed (WDM) channels. The potential ease of upgrade in the capacity of an optically amplified transmission link, from a single channel to one hundred channels, is demonstrated in a 300 km 622 Mb/s coherent transmission experiment. The authors consider the limitations imposed by amplifier noise and discuss the degree of transparency that can be achieved with currently available optical amplifiers.<<ETX>>

Recent Progress In Coherent Systems Research At British Telecom

This paper reviews recent technological progress on coherent systems. Both devices and system developments are described, though the accent is placed on systems applications. Subjects investigated recently include stabilisation and modulation of LEC lasers, optical amplifiers, FSK based systems and phase modulation in amplifiers.

Field demonstration of coherent WDM with a fibre amplifier repeater, for transparent optical network applications

A series of field experiments performed with two-wavelength transmissions at 622 Mb/s over 200-km of installed single-mode fiber using a diode-pumped erbium-doped fiber amplifier repeater are described. Both differential phase-shift keying (DPSK) modulation and frequency-shift keying (FSK) modulation were employed, along with two implementations of automated endless polarization control. In a further field experiment, a power budget of 64 dB was demonstrated in the form of repeaterless DPSK transmission at 622 Mb/s over 264 km of installed fiber. The use of such a large power budget to accommodate the sharing of power between many wavelength channels and the accumulation of spontaneous emission noise through cascaded optical amplifier repeaters is discussed.<<ETX>>