Optical communications – Duplex
Reexamination Certificate
1998-07-07
2004-06-15
Pascal, Leslie (Department: 2633)
Optical communications
Duplex
C398S042000
Reexamination Certificate
active
06751414
ABSTRACT:
BACKGROUND TO THE INVENTION
This invention relates, in general, to an optical transmission plan and optical amplifier circuit therefor, and is particularly, but not exclusively, applicable to a bi-directional wave division multiplexed (WDM) amplifier circuit and a channel assignment plan for optical transmissions therein.
SUMMARY OF THE PRIOR ART
method of increasing the transmission capacity of an optical fibre is to use wavelength division multiplexed (WDM) channels. Indeed, in this respect, optical amplifier designs have been optimised for WDM operation. For example, in uni-directional systems, all the WDM channels will be transmitted through the transmission fibre in the same direction, while an alternative, bidirectional transmission configuration has some channels within the fibre travelling in one direction and other channels travelling in an opposite direction. This latter system configuration offers a number of advantages, especially in relation to the number of redundant transmission terminals required for reliability. As such, optical amplified designs are currently being optimised for bidirectional transmission.
One particular mechanism by which bidirectional transmission are optimised is provided by splitting the available channels into distinct operating bands, one for each direction. These bands are sometimes referred to as the ‘blue’ and ‘red’ bands, and can contain varying numbers of channels. For example, the blue band may have channels based within a wavelength range of 1527 to 1540 nanometres (nm), while the red band may have channels based within a wavelength range of 1545nm to 1560nm. Each channel supports information transfer through the modulation of data onto a carrier frequency (generated by a laser). In an optical system, typical modulation techniques include amplitude and phase modulation, and frequency shift keying (FSK).
As will be understood, WDM channel allocation is typically based on the International Telecommunications Union (ITU) standardised wavelength grid, and is therefore subject to specified minimum channel spacing. Specifically, the present ITU standard requires a channel spacing of 100 GigaHertz (GHz), with possible channel combinations using multiples of this 100 GHz channel spacing. In practice (and bearing in mind the constraints imposed by laser light resolution), each band can support sixteen channels having a 100 GHz spacing regime, i.e. thirty-two channels in total.
Unfortunately, as channel spacing decreases, each channel suffers from higher levels of performance degradation due to multi-channel operation. Specifically, the superposition of harmonics results in significant detrimental effects on data integrity within a specific channel. More particularly, in an optical transmission system, densely packed channels exhibit an intermodulation product, known as “four-wave mixing”. Basically, the degradation from the intermodulation product arises from sidebands being superpositioned on or about adjacent optical channels, with each sideband generated as a consequence of the spacing between adjacent channels. Increasing the channel separation therefore improves transmitted signal integrity, but this improved performance only results from sacrificing overall transmission capacity.
One way to maintain the same total number of channels in a bidirectional amplifier, whilst reducing the channel degradation arising from an intermodulation product is to adopt an interleaved channel plan in which adjacent channels propagate in opposite directions. In practice, this means that the channel spacing between adjacent channels in one direction has been doubled, with the effects on adjacent co-propagating channels accordingly reduced. An additional advantage of this scheme arises from the distribution of the “dead band” in which channels cannot be used. Specifically, in relation to a dual band system in which distinct bands are allocated for up-link and down-link transmissions, it is imperative that the bands be separated, (isolated) to prevent corruption of data. Unfortunately, in any attenuated system (as implemented within a filter, for example) isolation is dependent on operating parameters of the actual attenuation devices. In the specific case of filters in an optical system, the filters have a response curve that gradually rises and gradually tails off. Consequently, a minimum extinction zone must exist between the separate channel bands, with the width of the extinction zone necessarily excluding any overlap potentially arising from the lead-in or tail-off profiles of the filter response characteristics. In other words, the rate of increase of attenuation against wavelength (termed “roll-off”) that is achievable between the channel bands through filtering produces a portion of the bandwidth that cannot be used for communication information; this is the overlapping region of the “dead band”. In an interleaved case, the channel spacing in one direction is doubled and so the filter edges are steep enough not to lose a channel slot due to dead bands. Indeed, in practice, the dead band is distributed across the whole of the operating bandwidth, and so the optical system is able to increase the total number of available channels.
Although it will now be understood that interleaving in an optical transmission system is desirable, the problems associated with the provision of a suitable optical amplifier discourage the implementation of such interleaved schemes. Specifically, the design of a suitable optical amplifier is presently both complex and expensive. Consequently, with the development of a new and improved architecture for an optical amplifier, interleaved optical transmission systems could and would be implemented more frequently, which systems would therefore provide an improvement in the standard, quality and rate of information transfer, as will be appreciated.
EP-A-0 680 168 describes a system and method for optimising spectral efficiency using time-frequency-code slicing. More particularly, through the scheduling of the various speed users within the frequency and time domains, the system and method allocates and makes use of the available spectrum.
EP-A-0 668 675 describes a multi-channel optical fibre communication system in which wavelength division multiplexed channels have simple channel-to-channel spacing which prevent substantial coincidence of 4-wave mixing products with channel carriers.
U.S. Pat. No. 5,390,043 describes an optical heterodyne communication system for transmission of multiple optical channels at different wavelengths, with the optical channels grouped into blocks. Spacing between optical channels with a block is equal to or somewhat greater than the channel bandwidth, while the spacing between blocks is such that when the optical signal is heterodyned with a local oscillator lightwave the interference with a selected optical channel from the adjacent block does not exceed a predetermined level.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a method of assigning a band of optical channels to an optical fibre arranged to support bidirectional communication in an up-link and a down-link, the band of optical channels having adjacent wavelengths, the method arranged to reduce intermodulation in each of the up-link and down-link by assigning to at least one of the up-link and the down-link at least one pair of channels having adjacent wavelengths in the band of optical channels, wherein assignment of channels to the up-link and down-link are mutually exclusive and each of the up-link and the down-link comprises at least one other optical channel having a wavelength not adjacent to said at least one pair of channels.
In particular embodiments, the optical channels in the up-link and down-link are separated by no more that two contiguous optical channels. Alternatively or additionally, at least one of the up-link and the down-link comprises at least two pairs of adjacent optical channels, the at least two pairs being separated by no more than two adjacent channels.
I
Davies Fiona
Jolley Nigel Edward
Jones Kevan
Keys Robert
O'Sullivan Maurice
Barnes & Thornburg
Nortel Networks Limited
Pascal Leslie
Singh Dalzid
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