For the first time a link is shown between the time to failure, the initial equilibrium temperature and high-power damage coating degradation at failure. A fibre that is sensitive to high-power damage has been observed to have a low equilibrium temperature and a coating that is sensitive to thermal ageing.

The recently-installed UK—Channel Islands No 7 cable system is a 132 km unrepeatered, twelve optical fibre, 140 Mbit/s system. In February the cable was used for a series of tests to demonstrate new transmission and receiver equipment developed at British Telecom Research Laboratories (BTRL). The paper describes first the use of erbium fibre power amplifiers to achieve two-way transmission over the system at 2.488 Gbit/s, and then the testing of a novel semi-ruggedised laser PIN receiver.

The advent of erbium doped fibre amplifiers (EDFA) has revolutionised the options available to submarine cable system planners. It has become possible to extend the range of unrepeatered systems operating at 1.55 μm to about 300-400 km. The next generation of optical systems, TAT12/13 and TPC5, which are planned for the Atlantic and Pacific Oceans in 1995/6 will use optical amplifiers and operate at 5 Gbit/s per fibre pair. The authors look at constraints on the operation of long haul optically amplified systems and suggest areas where progress needs to be made in order to allow operation at rates beyond 5 Gbit/s. Gordon-Haus jitter and provision of an adequate SNR have been recognised as the limiting factors for soliton systems. However, guiding filters may allow these constraints to be relaxed and higher speed operation achieved. Soliton systems may also offer the promise of multi-channel operation using either two polarisation states or a few WDM channels. The latter requires that the gain of each wavelength channel can be adequately controlled

The unsaturated (vadose) zone in arid and semiarid regions can be >100 m thick and may receive little or no moisture recharge from the surface. The microbiological properties of the vadose zone are of interest because of the potential for microorganisms to impact the fate and transport of contaminants in these environments. At numerous sites in the western United States, large volumes of wastewater or process water have been disposed of directly to the surface or shallow subsurface and have subsequently migrated through the vadose zone to the groundwater. The purpose of this study was to determine and compare the microbial properties of pristine and impacted vadose zone sediments. Vadose zone sediments from depths ranging from 24 to 90 m were obtained from 3 bore-holes drilled on the Hanford site in south-central Washington State. One bore-hole was located in a pristine area and the other two were located in areas where wastewater had been disposed of directly to the surface and near subsurface. An open-flow centrifugation method was used to determine unsaturated hydraulic conductivity for the sediments, allowing construction of characteristic curves relating the hydraulic conductivity (K) to the volumetric water content (theta). By comparing the volumetric water content of sediments obtained prior to centrifugation to their water content over a range of K(theta), it could be determined which vadose zone samples had been subjected to artificial recharge. Elevated concentrations of nitrate and the presence of carbon tetrachloride in vadose sediments were also used as indicators that sediments had been impacted by past waste disposal practices. Those vadose zone paleosol samples receiving artificial recharge had higher populations of culturable bacteria (log 1.0-6.6 CFU g-1) and were able to mineralize organic substrates more rapidly than a pristine paleosol. Thus, the in situ microbial population was stimulated by increased moisture from artificial recharge and/or from contaminants. In contrast, culturable bacteria were near or below detection in unimpacted fluvial and flood sands. The few culturable microorganisms that were present in unimpacted vadose sediments were either associated with the sediments at the time of deposition or transported from the surface during the last major proglacial flood, approximately 13,000 years ago.

Experiments to determine the effects of exposure to ionizing radiation and hydrogen on erbium fibers and erbium fiber amplifiers are described. A preliminary assignment of the radiation induced losses to components of the fiber composition is made. Both reversible and long term losses resulting from hydrogen permeation have been observed. As in other types of fibers, the presence of hydrogen modifies the response to radiation.

A series of irradiation tests on erbium fibre amplifiers using fibres with different erbium and codopant levels show the range of gain losses induced and permit some conclusions about the relative roles of the fibre constituents.<>

Methods of predicting the performance of ultra-long haul optically amplified nonreturn-to-zero (NRZ) systems are considered, with a brief discussion of the relative strengths of experimental and numerical modeling approaches. Results of numerical simulations are used to highlight some important issues, including the effect of chromatic dispersion and polarization-mode dispersion

Speed limitations of conventional transmission and future research directions are discussed. There has been steady progress for both optical devices and transmission techniques toward transmission above 2.5 Gb/s. Erbium fiber amplifiers, with their extremely wide bandwidth, have provided increased performance for both optical transmitters and receivers as well as offering efficient line amplifiers. Such devices and techniques now make it possible to demonstrate the feasibility of practical systems up to 10 Gb/s, and there are interesting possibilities for doubling and even tripling this rate. At such rates, proper consideration must be given to source spectral characteristics, even for short systems. On very long repeatered systems, such as submarine cables, nonlinear effects also become significant and could severely limit performance. However, by careful design these can be made to counter chromatic dispersion to provide a soliton system

Propagation of 20 ps pulses over 357 km of fibre using erbium doped fibre amplifiers is reported. The pulses are shown to exhibit soliton like propagation, by virtue of their unmodified temporal profile. This is achieved despite the fact that the pulses were propagated through fibre where the dispersion coefficient alternated between -0.2 ps/nm/km and + 14 ps/nm/km, and a large amplifier spacing of 71 km was employed.

Erbium doped fibre amplifiers now allow the possibility of deploying optically transparent systems over trans-oceanic and trans-continental distances. However it is now well established that fibre non-linearities cannot be ignored in such systems. For example, when a single channel system is operated in the anomalous dispersion régime, ie. with λ > λ0 where λ0 is the wavelength of zero chromatic dispersion, self phase modulation (SPM) may lead to pulse compression or pulse break up. When operated with λ < λ0, SPM leads to increased dispersion and pulse broadening. A choice may be made to avoid the non-linearities as far as possible; for example by minimising the amplifier spacing, by using the minimum optical power consistent with the required electrical signal-to-noise ratio (SNR) and by careful choice of the operating wavelength with respect to λ0. Alternatively, one may choose to use actively the fibre non-linearity to design a soliton based system operating in the anomalous dispersion régime. This paper attempts to consider some of the design constraints for such a system. In particular it considers a system operating at 5Gbit/s over a total distance L sys of 6000km.

An optical fibre loop is used to simulate experimentally a transoceanic system employing optical amplifier repeaters. Propagation of a data pattern at 2.5 Gbit/s is demonstrated to 10000 km. Pulse distortion due to the accumulation of nonlinear (Kerr) and dispersive effects was observed.

There appear to be two distinct approaches to the design of future ultra long-span optically-amplified transmission systems. In the first, the signal wavelength is chosen to give operation in the region of anomalous dispersion. This is consistent with soliton operation, although other pulse shapes may propagate with little degradation. This paper is concerned with the second approach, where the operating wavelength is chosen to lie very close to the dispersion minimum. Self-phase modulation is now a source of potential impairment, and must be controlled.

An optical fibre loop is used to simulate experimentally a transoceanic system employing optical amplifier repeaters. Propagation of a data pattern at 2.5 Gbit/s is demonstrated to 10000 km. Pulse distortion due to the accumulation of nonlinear (Kerr) and dispersive effects was observed.<>

The results of a field trial on the UK-Channel Islands No. 7 cable, in which ruggedised, broadband erbium power amplifiers were used to achieve two-way transmission at 2.488 Gbit/s over a 132 km submerged cable is reported.

In optically-amplified systems the propagated signal will accumulate dispersive and non-linear effects along the full length of the fibre. The optical Kerr effect is of particular importance and can lead to significant levels of self-phase and cross-phase modulation which interact with the chromatic dispersion. This paper describes a computer model which simulates the propagation of data signals along such a system and predicts the received signal shape, together with a corresponding penalty figure. The model is capable of simulating a variety of signal formats, and results are presented for some of these

An erbium power amplifier is used to give a system launch power in excess of +20 dBm. In conjunction with an FSK modulation technique this allows unrepeatered transmission over 250 km of step-index fibre with a loss of 50 dB and a total dispersion of 3930 ps nm/sup -1/.<>

A new integrated structure is proposed for generalised connection networks, which requires fewer optical crosspoints for 4*4 and 8*8 broadcast switching than previously known structures.<>

We report the first use of erbium power amplifiers and low chirp transmission techniques on an operational submarine cable. Two way transmission at 2.488Gbit/s was achieved over an unrepeatered distance of 132km on the UK-Channel Islands No. 7 submarine cable.

An optical fibre cable system is subject to a number of environmental factors which must be considered in order to guarantee survivability over a planned lifetime of typically 25 years. This is particularly true for submarine cables, where the drive to maximise the separation between repeaters is squeezing the operating margin and where the cost of shipboard repair is very high. This paper considers the factors which may limit the life of the optical fibre. These include mechanisms which cause the fibre attenuation to change and failure due to static fatigue.