GB2281669A

GB2281669A - WDM Optical transmission systems

Info

Publication number: GB2281669A
Application number: GB9318106A
Authority: GB
Inventors: Paul Roos Morkel
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1993-09-01
Filing date: 1993-09-01
Publication date: 1995-03-08
Anticipated expiration: 2013-09-01
Also published as: GB9318106D0; GB2281669B

Abstract

In a wavelength division multiplexed optical transmission system extending from a WDM transmitter (10) to a WDM receiver (12), the transmission path (11) includes a set of optically pumped optically amplifying fibre optical amplifiers (13) arranged optically in series. The amplifying fibres are of the same type, but exhibit a spread of spectral gain characteristics as the result of being operated under different conditions of amplifier length, or pump power and optical input signal, so that the overall variation of gain with wavelength is reduced compared to that which would prevail were all the amplifiers identical. <IMAGE>

Description

WDM Optical Transmission Systems This invention relates to wavelength division multiplexed (WDM) optical transmission systems incorporating optically pumped optical amplifiers. Wavelength division multiplexing- may be used in a transmission system to give increased information carrying capacity without putting up the bit rate. In an optical transmission system incorporating optical amplifiers, a problem with WDM is liable to arise if the gain spectrum of these amplifiers is not sufficiently flat over the full spectral range of the multiplexing. If the transmitted signals have to pass through a chain of amplifiers with substantially identical spectral gain characteristics spaced at intervals along the transmission path, and one channel is amplified slightly less than other at each amplifier, then the amplification deficit is multiplied by the number of amplifiers in the chain.In this way even quite a small deficit can assume significant consequences in a long chain.

The effect of the concatenation of these amplifiers is to provide the system with a gain window that is narrowed in relation to that of a single one of the amplifiers. The present invention is directed to reducing that narrowing effect.

According to the present invention there is provided a wavelength division multiplexed optical transmission system having an optical transmission path with a set of optically pumped amplifying fibre optical amplifiers optically in series therein, wherein substantially the same fibre composition is employed for the amplifying fibre of each member of the set of amplifiers, and wherein the bandwidth narrowing effect that would otherwise result from the use of amplifiers with substantially identical non-flat spectral gain characteristics is substantially reduced by the use of amplifiers adapted by the use of different operating conditions to provide a spread of different spectral gain characteristics, said different operating conditions being selected from the group comprising employing amplifiers with different lengths of amplifying fibre, and employing amplifiers that are pumped with different levels of optical pump power in conjunction with locating the amplifiers at intervals in the transmission path providing those amplifiers with different levels of signal input to their respective amplifying fibres.

The invention is based upon an appreciation that optical fibre amplifiers that are constructed from a single source of optical amplifier fibre, and that do not have flat gain spectra, can be operated under different conditions arranged to promote individual gain spectra that are sufficiently different one from another to promote, in a concatenation of such amplifiers, a significant reduction in the narrowing of the gain window, as compared with the gain narrowing that would have occurred had all the amplifiers had substantially identical gain spectra.

The use of two types of optical amplifier with differing spectral flow characteristics in an optical transmission system is already disclosed in a paper by C R Giles et al entitled, 'Dynamic Gain Equalization in Two-Stage Fiber Amplifiers', IEEE Photonics Technology Letters, Vol. 2 No. 12 December 1990, pp 866-868. It is however to be noted that that disclosure was specifically in the context of the use of only two different types of amplifier with differing spectral gain characteristic; these being used in pairs to secure equal amplification for a WDM system having only two wavelength-separated channels, and moreover that the spectral diversity was achieved by using two different recipes of optical fibre amplifier.This contrasts strongly with the present invention, in which a variety of different spectral gain characteristics is obtained from a single recipe of amplifier fibre, and wherein there is no requirement specifically to balance the amplification at n different wavelengths by the use of n different types of amplifier arranged in groups of n.

There follows a description of a WDM transmission system embodying the invention in a preferred from. The description refers to the accompanying drawing which is a block diagram of the system.

The system comprises a WDM transmitter 10 typically arranged to launch three or more WDM optical signals into one end of an optical fibre transmission path 11. At the far end of this path these signals are detected by a WDM receiver 12. At spaced intervals along the transmission path 11 are inserted a set of optical amplifiers 13. Each optical amplifier 13 is an optically pumped optical fibre amplifier, and the optically amplifying fibre of each amplifier employs substantially the same optical fibre composition (recipe), typically all being drawn from the same optical fibre preform.

Although all the amplifiers of the chain use the same type of amplifying fibre, they are arranged for operation so that they do not all provide substantially identical spectral gain characteristics.

Instead they are deliberately operated to provide a spread of spectral gain characteristics. Such a spread does not necessarily imply that the characteristic of every amplifier must be different from that of every other amplifier. Indeed there can be advantage in arranging for the full spread to be accommodated in a subset of the total number of amplifiers in the chain, these being arranged in a specific sequence which is repeated one or more times in the full set of amplifiers of the chain.

A particularly convenient way of providing the desired spread in amplifiers using germanic or alumina doped silica optical fibres additionally doped with erbium is to employ different lengths of amplifier fibre in different amplifiers. Thus by changing the amplifier length by a factor of about two, while keeping the other relevant parameters constant, the peak in the spectral gain characteristic can be shifted by several namometers. Such changes to any given amplifier can, if desired, be augmented by additionally changing either its pump power level, or its input signal power level, or both of these power levels. The level of pump power at any given amplifier can readily be adjusted by a factor of at least two to give a significant shift in the peak of the spectral gain characteristic.

Similarly an adjustment by a factor of about an order of magnitude in the power level of the input signal applied to the amplifier fibre of any amplifier will give a significant shift in the spectral gain characteristic peak wavelengths if, as is usual, the amplifier is operated with a degree of gain saturation.

Different levels of input signal power to the amplifier fibres of individual amplifiers can conveniently be provided by arranging to have different attenuation values between different consecutive pairs of amplifiers. Typically these would be provided by having different lengths of transmission path between consecutive pairs of amplifiers.

In place of having different lengths of amplifier fibre in different amplifiers a second preferred option for achieving the desired spread in spectral gain characteristics is to employ the same length of amplifier fibre in each amplifier, but to employ different levels of pump power in conjunction with different levels of signal input power.

Claims

CLAIMS;

1. A wavelength division multiplexed optical transmission system having an optical transmission path with a set of optically pumped amplifying fibre optical amplifiers optically in series therein, wherein substantially the same fibre composition is employed for the amplifying fibre of each member of the set of amplifiers, and wherein the bandwidth narrowing effect that would otherwise result from the use of amplifiers with substantially identical non-flat spectral gain characteristics is substantially reduced by the use of amplifiers adapted by the use of different operating conditions to provide a spread of different spectral gain characteristics, said different operating conditions being selected from the group comprising employing amplifiers with different lengths of amplifying fibre, and employing amplifiers that are pumped with different levels of optical pump power in conjunction with locating the amplifiers at intervals in the transmission path providing those amplifiers with different levels of signal input to their respective amplifying fibres.

2. A transmission system as claimed in claim 1, wherein different amplifiers of the set are adapted to operate with both different lengths of amplifying fibre and with different levels of optical pump power.

3. A transmission system as claimed in claim 1 or 2, wherein different amplifiers of the set are adapted to operate with both different lengths of amplifying fibre and with different levels of signal input to their respective amplifying fibres.

4. A transmission system as claimed in claim 1, 2 or 3, wherein the number of wavelength multiplexed channels is at least three.

5. A transmission system substantially as hereinbefore described with reference to the accompanying drawing.