Optimised multiplexer/demultiplexer optical structure

An optical demultiplexing structure and method for demultiplexing channels from a wavelength multiplexed optical signal comprising a first optical filter (115), a second optical filter (117), and at least one third optical filter (120), wherein the first filter has a band-pass characteristic for extracting a first set of channels from the optical signal, and the second filter has a cut-off wavelength corresponding to a wavelength within the pass-band of the first filter, and wherein the first filter is connected to the second filter, the second filter extracting a second set of channels from the channels that remain after extraction of first the set of channels by the first filter, and wherein the at least one third filter has a passband to extract a subset of channels from the second set of channels spaced apart from the first set of channels. The corresponding multiplexing structure and method are also described.

This invention relates to a Multiplexer/Demultiplexer (MUX/DEMUX) passive modular optical structure.

Passive optical systems are used for performing multiplexing and/or demultiplexing of channels separated in frequency (for example Wave Division Multiplexed (WDM) signals). This enables a plurality of channels to be allocated different wavelengths (and hence frequencies) such that they can be transmitted down a single transmission path without interfering with each other. Cascaded interconnected passive optical filters are used to filter individual channels.

In the realization of such cascaded optical filters it is important to limit the deterioration of channels due to passive filtering, that is losses that are experienced by a channel due to a filter as the channel passes through, or is reflected by, the filter. Deterioration can be due to insertion losses, reflection losses and/or chromatic dispersion losses introduced by the filters that each channel traverses.

A first known solution for realizing a passive MUX/DEMUX is to use cascaded passive filters, each with a bandwidth corresponding to one channel wherein each passive filter is centred on a different one of the channels. There are as many filters as there are channels. However, this causes very high deterioration of some channels compared to other channels. Indeed, in cascaded optical filters, all of the remaining wavelengths except the one on which the filter is centred are reflected to the next filter and each channel experiences deterioration as it is either reflected/rejected or passed. The channel that is intended for the last filter of the cascade has therefore been reflected as many times as there are filters preceding it.

For example, in a cascaded filter system for multiplexing and/or demultiplexing a WDM signal with 40 channels, the deterioration of the last channel, due only to the reflection losses (without considering other deterioration losses), would be of the order of forty times the deterioration of the first channel of the chain. In many situations this is unacceptable. A channel with such deterioration may require further amplification such that the quality of the channel is retained.

One prior art system that attempts to reduce the deterioration of channels comprises a MUX/DEMUX passive modular structure (for example, with 40 channels spaced by 100 GHz) based on the use of cascaded type 8skip0 filters. 8skip0 filters extract adjacent bands of 8-channels, and then cascaded channel filters are used to extract individual channels from each 8-channel band. In general, the term ‘Xskip0 filter’ is a filter with a pass-band bandwidth of X channels.

This modular solution reduces the number of filters that each channel traverses when compared to the previous solution. There are standard proposals for use with a WDM signal with 40 channels that use five cascaded 8skip0 filters, wherein each 8skip0 filter has eight cascaded channel filters associated with it to separate the groups of eight channels into individual channels.

One problem with this solution is that the channels at the edges of the 8skip0 filters are subjected to maximum chromatic dispersion from that filter plus the chromatic dispersion of the adjacent 8skip0 filter. In other words, channels at the edges of the band-pass characteristic curve (transmission profile) of each 8skip0 filter experience chromatic dispersion from both the 8skip0 filter that has passed that channel and the neighbouring 8skip0 filter. This means that the channels that are nearest the cut-off wavelengths/frequencies of the filter, and therefore are at the edges of a group/band of channels that have been extracted by the band-pass filter, experience double the chromatic dispersion to that experienced by channels that are not at the edge of a group of eight channels with an adjacent 8skip0 filter.

It will be appreciated that equivalent multiplexers are known.

Another difficulty in realizing passive MUX/DEMUX systems is that in addition to meeting the criteria of low channel deterioration, it is also desired to have modular structures that allow the number of channels handled by the system to be easily increased. For example, to upgrade the systems as the need for channels increases. In addition, it is generally preferable that the additional channels are positioned towards the outside of the channels that are already present in a system, and preferably adjacent to an outer channel that is already present in a system.

It is an aim of embodiments of the present invention to ameliorate the above-mentioned shortcomings by making available a MUX/DEMUX structure with passive filters that limits the deterioration of the channels compared to known solutions. Another aim of embodiments of the invention is to provide a modular structure that enables the number of channels to be increased easily.

According to a first aspect of the invention there is provided an optical demultiplexing structure for demultiplexing channels from a wavelength multiplexed optical signal comprising a first optical filter, a second optical filter, and at least one third optical filter, wherein the first optical filter has a band-pass characteristic for extracting a first set of channels from the optical signal, and the second optical filter has a cut-off wavelength that corresponds to a wavelength that is within the pass-band of the first optical filter, and wherein the first optical filter is connected to the second optical filter such that the channels that remain after extraction of the set of channels by the first optical filter are filtered by the second optical filter such that a second set of channels remains, and wherein the at least one third optical filter has a pass-band arranged to extract a subset of channels from the second set of channels that is spaced apart from the first set of channels.

Preferably, the subset of channels is not adjacent in wavelength to the first set of channels. There may be at least one channel between the subset of channels and the first set of channels. Preferably at least one channel, and more preferably at least two channels, will be located between the pass-bands of any two band-pass filters such that any deterioration that is suffered by channels that are near to a cut-off wavelength of a filter is not added to by any deterioration caused by further filters. For a band-pass filter, the characteristic curve has two cut-off wavelengths at an upper and lower region of the band-pass wavelengths.

Having a structure in which the band-pass filters are arranged so as not to extract adjacent bands/groups of channels enables the effects of chromatic dispersion to be reduced when compare with prior art systems. Also, the total number of filters that a channel must traverse in the structure/system is reduced when compared with the prior art.

Preferably, the channels that are nearest a cut-off wavelength of the third optical filter are spaced apart in wavelength from the channels that are nearest a cut-off wavelength of the first optical filter. Providing the channels that are nearest a cut-off wavelength of the third optical filter such that they are spaced apart from the channels that are nearest a cut-off wavelength of the first optical filter may result in the pass-band of the first optical filter being spaced apart in wavelength from the pass-band of the third optical filter, and preferably the two pass-bands do not overlap.

The second optical filter is arranged such that the cut-off wavelength is located within the pass-band of the first optical filter such that any imperfections in the transmission profile of the second optical filter are also located in the gap that has been created by the extraction of the first optical filter. This means that the imperfections may not negatively affect the remaining channels, or at least may reduce the effects of the imperfections on the remaining channels. In some embodiments this can mean that a less expensive, and more economical, second optical filter with a lower technical specification can be used without detracting substantially from the performance of the demultiplexer. With prior art systems a choice may have to be made between using an expensive optical filter with a good transmission profile and allowing some of the neighbouring channels to be deteriorated. In other prior art systems, one or more channels may simply be not used, and this obviously reduces the overall bandwidth capacity of the system.

Preferably the structure further comprises a first optical channel extraction unit and a second optical channel extraction unit arranged to extract individual channels from the sets and subsets of channels. The optical extraction units may be arranged to extract individual channels from a group/set/band of channels, and preferably isolate the individual channels onto separate transmission paths so that they can be provided on output ports/pins. The first and/or second optical channel extraction units may comprise at least one cascade of channel filters.

The first optical filter may be an Xskip0 type optical filter with X equal to a preset number of channels, and/or the at least one third optical filter may be an Xskip0 type optical filter with X equal to a preset number of channels. Xskip0 filter provide convenient band-pass filters for extracting a set or subset of channels, which may be the same or different to the preset number of channels of the first optical filter.

Preferably, the first set of channels comprises intermediate channels that are intermediate the full set of channels in the WDM signal, and the second optical filter is a high/low wavelength optical splitter having an intersection wavelength that corresponds to a wavelength that is within the pass-band of the first optical filter, and is arranged to divide the channels that remain after the extraction of the first intermediate set of channels such that the second and third sets of channels comprise channels that are above and below the first set of extracted channels respectively.

The intermediate first set of channels are not at an end of the WDM signal, that is, there is at least one channel that is at a higher wavelength than the wavelengths of the first set of channels, and at least one channel that is at a lower wavelength than the wavelengths of the first set of channels.

Using a system where the first set of channels is an intermediate set of channels enables a second and third set of channels to be obtained that are above and below the first set of channels in terms of wavelength/frequency. This provides the advantage that the second and third sets of channels can be demultiplexed in parallel, and further serves to reduce the number of filters that a channel must be traversed before being completely demultiplexed when compared with the prior art.

The structure may further comprise a third optical extraction unit arranged to extract individual channels from the third set of channels.

The structure may further comprise a fourth optical filter associated with the third optical filter, wherein the fourth optical filter has a transmission wavelength that is within the pass-band of the third optical filter, and is arranged such that the channels that remain after extraction of the first subset of channels by the third optical filter are divided into a second subset of channels by the fourth optical filter.

Subsequent filters may be provided that drill down into the sets of channels such that a set, or a subset, may be divided into further subsets any number of times until a certain number of channels remain in the subset/set. The certain number of channels may be a threshold value at which it is more beneficial to extract the individual channels rather than subdivide the set into further subsets. One or more channel extraction units may be used once the number of channels within a set drops below, or equals, the threshold value. The threshold value may be 16, 8, 6, or 4 channels, as an example. In other embodiments, the number of channels extracted by an upstream optical filter with a band-pass characteristic may be selected such that a predefined number, or less than a predefined number, of channels will be left in a downstream subset. Preferably, using an upstream optical filter to extract a predefined number of channels controls the number of channels left in a final subset for individual channel extraction. The predefined number of channels may correspond to a convenient number of channels that a channel extraction unit can extract.

Preferably, the fourth optical filter is a high/low wavelength optical splitter having an intersection wavelength that corresponds to a wavelength that is within the pass-band of the third optical filter, and is arranged to divide the remaining channels into a second and third subset of channels comprising sets of channels that are above and below the first intermediate subset of channels that are within the pass-band of the third optical filter.

The intersection wavelength of the wavelength optical splitter is a cut-off wavelength/frequency whereby any channels having a wavelength that is greater than the cut-off wavelength are grouped into one signal, and any channels having a wavelength that is less than the cut-off wavelength are grouped into another different signal.

Each of the optical channel extraction units may be assembled on a respective extraction module in order to provide a modular composition of the structure. This can allow for the total number of channels that the multiplexer/demultiplexer to be easily increased or decreased by adding or subtracting further modules to the structure.

According to a second aspect of the invention there is provided an optical multiplexing structure for multiplexing channels to obtain a wavelength division multiplexed optical signal comprising a first optical filter, a second optical filter, and at least one third optical filter, wherein the first optical filter has a band-pass characteristic for combining/grouping a first set of channels having wavelengths within the pass-band with a second set of channels having wavelengths that are outside of the pass-band to form a wavelength division multiplexed optical signal, the second optical filter has a cut-off wavelength that is within the pass-band of the first optical filter and is arranged to combine/group the second set of channels, and wherein the at least one third optical filter has a pass-band and is arranged to combine/group a first subset of channels having wavelengths within its pass-band, wherein the first subset of channels forms a subset of the second set of channels, and wherein the first subset of channels is spaced apart from the first set of channels.

Preferably, the channels that are nearest a cut-off wavelength of the third optical filter are spaced apart in wavelength from the channels that are nearest a cut-off wavelength of the first optical filter

The structure may further comprise a first optical grouping/concatenation unit and a second optical grouping/concatenation unit arranged to concatenate/group individual channels into the sets and subsets of channels. The first and/or second optical channel concatenation units may comprise at least one cascade of individual channel filters.

The first optical filter may be an Xskip0 type filter with X equal to a preset number of channels, and/or the at least one third optical filter may be an Xskip0 type filter with X equal to a preset number of channels.

Preferably, the first set of channels is an intermediate set of channels and the second optical filter is an optical splitter used in a reverse configuration with an intersection zone at a wavelength that corresponds to a wavelength that is within the pass-band of the first optical filter. The optical splitter may be arranged to combine the second set of channels with a third set of channels wherein the channels present in the second and third sets of channels correspond to wavelengths that are above and below the first set of channels and wherein the first optical filter is further arranged to combine the first, second and third sets of channels.

The multiplexing structure may further comprise a third optical concatenation unit.

Preferably, the structure further comprises a fourth optical filter associated with the third optical filter, wherein the fourth optical filter has a cut-off wavelength that corresponds to a wavelength that is within the pass-band of the third optical filter, and is arranged to combine a second subset of channels and supply the second subset of channels to the third optical filter where the second subset of channels is combined with the first subset of channels.

The fourth optical filter may be an optical splitter associated with the third optical filter, and may be arranged such that the intersection wavelength of the optical splitter corresponds to a wavelength that is within the pass-band of the associated third optical filter, and is arranged to combine the second subset of channels with a third subset of channels that comprise sets of channels that are above and below the first subset of channels that is within the pass-band of the third optical filter.

The structure may comprise “n” further downstream optical filters, which comprise band-pass filters with associated optical splitters, arranged to extract a band of channels from a subset, and subdivide the remaining channels into further subsets of channels.

The concatenation units may be each mounted on a respective multiplexing module for a modular composition of the structure. The band characteristics of the Xskip0 filter may be selected for combining at least four channels. The number of channels extracted by the Xskip0 filter may be eight.

According to a third aspect of the invention, there is provided a method for optically demultiplexing channels from a wavelength multiplexed optical signal comprising the steps of:

a) extracting a first set of channels from the wavelength multiplexed optical signal,

b) dividing the remaining channels into a second set of channels at a cut-off wavelength that corresponds to a channel within the first set of channels; and

c) extracting a subset of channels from the second set of channels that are spaced apart from the first set of channels. Preferably, a channel at an end of the subset of channels is spaced apart in wavelength from a channel at an end of the first set of channels.

The method may further comprise:

d) separating the sets and subsets of channels into individual channels.

The demultiplexing method may further comprising repeating steps a) to c) with the remaining sets of channels after extracting a first set of channels until the number of channels in the remaining sets is less than or equal to a preset number of channels.

Preferably, the first set of channels is an intermediate set of channels and step b) further comprises dividing the remaining channels into a second and third set of channels that are above and below the first set of channels respectively.

According to a fourth aspect of the invention, there is provided a method of generating a wavelength division multiplexed optical signal comprising the steps of:

a) combining/grouping at least one channel to form a first set of channels;

b) combining/grouping at least one different channel to form a subset of channels, wherein the first subset of channels is spaced apart from the first set of channels;

c) combining/grouping the subset of channels with at least one different channel at a cut-off wavelength that corresponds to a channel within the first set of channels to form a second set of channels; and

d) combining/grouping the first and second sets of channels to generate the wavelength division multiplexed optical signal.

Preferably, a channel at an end of the subset of channels is spaced apart in wavelength from a channel at an end of the first set of channels Any of the at least one channels may comprise a plurality of channels.

Grouping channels and subsets of channels may comprise combining the channels onto a single transmission path and/or allocating as belonging to the same group of channels.

The method may further comprise repeating steps a) to d) by combining a plurality of channels or subsets of channels to further multiplex a number of wavelength division multiplexed optical signals.

Preferably, the first set of channels is an intermediate set of channels and the method further comprises combining a different set of channels to form a third set of channels wherein the second and third sets of channels are above and below the first intermediate set of channels, and wherein step c) further comprises combining the second set of channels with the third set of channels at the cut-off wavelength that corresponds to a channel within the first set of channels, and step d) further comprises combining the first, second and third set of channels to generate the wavelength division multiplexed signal.

Preferably, the method further comprises concatenating individual channels into the sets and subsets of channels. In other embodiments there may be no predefined sequential order in the channels, and the channels may be allocated into sets and subsets with their sequential order being determined as the channels are multiplexed/demultiplexed.

According to a further aspect of the invention there is provided a method of demultiplexing a wavelength division multiplexed signal, comprising:

a) using a first Xskip0 filter to extract a first set of channels having a wavelength that is not adjacent to the wavelength of any previously extracted set of channels;

b) dividing the remaining signals into two further sets of signals;

c) repeating steps a) and b) for each subsequent divided set of signals until the wavelength of each set of signals is smaller than a threshold size; and

d) extracting the individual channels from each of the sets of channels.

According to a further aspect of the invention there is provided a method of multiplexing comprising combining/allocating a plurality of input signals to a corresponding plurality of frequency signals, and producing a multiplexed signal comprised of the frequency signals by:—

transducing a first plurality of input signals to a corresponding first group of a plurality of frequency signals of contiguous adjacent frequencies;

transducing a second plurality of input signals to a corresponding second group of a plurality of frequency signals of contiguous adjacent frequencies;

transducing a third plurality of input signals to a corresponding third group of a plurality of frequency signals of contiguous adjacent frequencies, the first and second groups of contiguous frequency signals being disposed in frequency either side of the range of frequencies of the third group of frequencies; and

adding the first and second groups of signals together to form an intermediate combined signal and then subsequently adding the third group of signals to the intermediate combined signal to form a multiplexed signal.

According to a further aspect of the invention there is provided a demultiplexer arranged to split a signal into two by using an extraction unit to extract a central span of channels and a splitter, splitting at a wavelength in the extracted band, arranged to split signal into a below-central span group and an above-central span group, and use extraction units to extract signals from the above-central span group and from the below-central span group at a wavelength range in each group that is disposed away from the central span so as to leave signals in the above-central span group and below-central span group that are closer to the central span.

The central span may not necessarily be the real centre of a wavelength range of the signal, but in some embodiments it can be.

The above-central and below-central groups may or may not be symmetrically disposed about the gap/central span. That is, the above-central and below-central groups may or may not have the same number of channels in them.

There may be provided an optical demultiplexing structure for demultiplexing channels from a wavelength multiplexed optical signal comprising an optical filter having a band characteristic for extracting a first intermediate set of channels from the optical signal characterized in that it comprises a second optical filter having a transition wavelength within the band of the first filter and connected to receive from the first filter the signal with the channels remaining after extraction of the intermediate set of channels and for dividing said remaining channels into a second and third sets of channels.

There may also be provided a wavelength optical multiplexing structure of channels to obtain a wavelength multiplexed optical signal comprising a first and a second optical unit for multiplexing respectively a first and a second set of channels separated by a band gap suited to receiving exactly a third set of multiplexed channels by means of a third optical unit with a first optical filter having band characteristics for containing said preset number of channels of the third set and to have a band contained in said band gap and with a second optical filter used as an adder having transition wavelength inside the band of the first optical filter and that receives the first and second sets of channels and sends the sum to the first optical filter connected for receiving said sum and insert therein the third set of channels in said band gap and thus obtain an optical signal formed from the multiplexing of the first, second and third set of channels.

Some embodiments of the invention may provide a method of optical demultiplexing of channels from a wavelength multiplexed optical signal in comprising the steps of, a) with an optical filter extract an intermediate set of channels, b) dividing the channels remaining after extraction into two other channel groups by means of an optical filter having transition wavelength within the band of the first filter and for each of the two other channel groups if the number of channels of the other group is greater than a preset number of channels, c) extract from the group with an optical filter another set of channels not adjacent to a set of channels already extracted, d) repeat steps b) to d) with the channels remaining after extraction.

Further embodiments of the invention may provide a method for wavelength optical multiplexing of channels in an optical signal comprising the steps of, a) form a first and second sets of multiplexed channels separated by a band gap suited to receiving exactly a third set of multiplexed channels, b) add the first and second sets of channels using an adding optical filter having transition wavelength falling within said band gap, c) use another optical filter having band characteristics to contain said preset number of channels of the third set and to have a band contained in said band gap to receive the sum outlet from the adding filter and insert therein the third set of channels in said band gap and thus obtain an optical signal formed from the multiplexing of the first, second and third sets.

The second optical filter may comprise a high/low wavelength optical splitter having splitter cut within the band of said intermediate set of channels and connected for receiving from the first optical filter the signal with the remaining channels after extraction of the intermediate set of channels and for dividing said remaining channels in said second and third set of channels formed respectively from channels above and below the first group of extracted channels.

The second and/or third optical units may comprise at least one other first optical filter having band positioned for extracting a different other set of channels with the bands of said other optical filters being selected so that the sets of channels extracted thereby are not adjacent to each other and to said first set of channels.

With at least some of the other first optical filters may be associated another second optical filter having transition wavelength within the band of the associated other first filter and that it is connected to receive therefrom the signal with the remaining channels after extraction of the respective channels and to divide such remaining channels in two other sets of channels.

The other second optical filter (417,517,617) may comprise a high/low wavelength optical splitter having splitter cut inside the band of the corresponding other first filter and connected for receiving therefrom the signal with the remaining channels after extraction of the respective channels and dividing said remaining channels of said two other sets of remaining channels formed respectively from the channels over and under the respective extracted channels.

The first, second and/or third optical extraction units may comprise, individually or in cascade, extraction units selected from among:a cascade of individual channel filters separating the individual channels of the signal sent to it;an Xskip0 type optical filter having band characteristics for extracting another set of channels from the signal sent to it and with an associated second filter having transition wavelength within the filter band and connected to receive from the first filter the signal with the remaining channels after extraction of the intermediate set of channels and to divide said remaining channels into a second and third set of channels, andan Xskip0 optical filter having band characteristics for extracting another set of channels from the signal sent to it and with at least one associated cascade of individual channel filters separating the individual channels of the other set of channels and/or of the set of channels remaining after extraction of the other set of channels.
There may be provided a structure for wavelength optical multiplexing of channels to obtain a wavelength multiplexed optical signal comprising:a first and a second optical unit for multiplexing respectively a first and a second set of channels separated by a band gap suitable to receiving exactly a third set of multiplexed channels by means of a third optical unit;a first optical filter having band characteristics for containing said preset number of channels of the third set and for having band contained in said band gap; anda second optical filter used as adder having transition wavelength within the band of the first optical filter and that receives the first and second channel sets and sends the sum to the first optical filter that is connected to receive said sum and insert therein the third set of channels in said band gap and thus obtain an optical signal formed by multiplexing the channels of the first, second and third sets.

The first optical filter may be an Xskip0 type filter with X equal to the preset number of channels of the third set.

The second optical filter may comprise an optical splitter used as adder with splitter cut inside the band of first optical filter.

The first, second and/or third optical units may comprise at least one cascade of channel optical filters with each filter of the cascade receiving a channel for composing the first, second and/or third set of channels respectively.

The first and/or second optical unit may comprise at least one other optical filter having band positioned for inserting a different other set of channels with the bands of said other filters being selected so that the channel sets inserted thereby are not adjacent to each other and to said third set of channels.

There may be provided one other optical filter having its transition wavelength within the band of the associated other first filter is associated with at least some of the other optical filters and that it is connected to receive two subsets of channels and send their sum as a set of channels to the associated optical filter.

The other second optical filter may comprise an optical splitter used as adder having splitter cut inside the band of the corresponding other filter and that it is connected to receive two subsets of channels and send the sum as set of channels to the associated optical filter.

The first, second and/or third optical groups may comprise individually or in cascade multiplexing groups selected from among:a cascade of individual channel filters which constitute together individual channels sent to it;an Xskip0 optical filter with X equal to a preset number of channels and band characteristics for inserting another set of X channels into the signal sent to it and with an associated optical splitter used as adder and with splitter cut inside the band of the associated Xskip0 filter and that is connected to receive two subsets of channels and send their sum as a set of channel to the associated Xskip0 optical filter;an Xskip0 optical filter with X equal to a preset number of channels and band characteristics for inserting another set of X channels into the signal sent to it and with at least one cascade of associated individual channel filters that constitute together the individual channels sent to it and send their sum as a set of channels to the associated Xskip0 filter.

There may be provided a method for optical demultiplexing of channels from a wavelength multiplexed optical signal with the method comprising the steps of:a) extract with an optical filter an intermediate set of channels,b) after extraction, divide the remaining channels into two other channel groups by means of an optical filter having a transition wavelength within the band of the first filter;
for each of said two other groups of channels, if the number of channels in the other group is higher than a preset number of channels:c) extract from the group by means of a optical filter another set of channels not adjacent to a set of channels already extracted; andd) repeat steps b) to d) with the channels remaining after extraction.

If the number of channels of another group is less than or equal to the preset number of channels, the channels of the group may be directly separated from each other by passage in a cascade of channel filters.

There may be provided a method for wavelength optical multiplexing of channels in an optical signal comprising the steps of:a) form a first and second multiplexed channel optical sets separated by a band gap suitable to receive exactly a third set of multiplexed channels;b) add the first and second channel sets by using an optical filter adder having transition wavelength falling within said band gap; andc) use another optical filter for receiving the sum outlet from the adder filter and insert therein the third set of channels in said band gap and obtain thus an optical signal formed by multiplexing of the first, second and third channel sets with the other optical filter having band characteristics for containing said preset number of channels of the third set and for having a band contained in said band gap.

The optical filter adder may be an optical splitter with splitter cut within said band gap.

The channel sets may be obtained by adding to a set of multiplexed channel at last one subset of channels while using for the sum another Xskip0 optical filter with X equal o the number of channels of the subset and choosing the bands of said other filters so that the subsets of channels inserted by them are not adjacent to each other and to said third set of channels.

At least some of the other Xskip0 optical filters may be associated an optical splitter used as adder with said splitter having its splitter cut in the band of the corresponding other Xskip0 filter and being connected to receive two other subsets of channels that are separated by the band width of the corresponding Xskip0 filter and to send the sum thereof as a set of channels to the associated Xskip0 optical filter.

It will be appreciated that any of the features described above may be equally applicable to any of the other aspects of the invention, and/or any of the embodiments of the invention.

In this application the term ‘Xskip0’ will be used to mean a skip0-type filter with any number of X separate channels that are filtered from the signal as a single group or band. The number to be given to X in a particular filter will be defined when necessary each time. It will be appreciated that in other embodiments of the invention, X can be given different values to those indicated in the following description.

With reference to the figures,FIG. 1shows an example of a prior art MUX/DEMUX designated as a whole by reference number10with 40 optical channels. In this example each channel has a bandwidth of 100 GHz

The multiplexer15and the demultiplexer16each use a cascade of five 8skip0 filters13. This enables five contiguous groups of eight contiguous channels to be extracted from the modulated signal for the demultiplexer16, and five contiguous groups of eight contiguous channels to be combined for the multiplexer15.

Each group of eight channels is sent to, or received from, a cascade of eight channel filters (indicated as a whole by reference number14). The DEMUX circuit16(in the lower part of the drawing and with input11) is symmetrical with the MUX circuit15(in the upper part of the drawing and with output12). The difference between the multiplexer15and the demultiplexer16is the direction of propagation of the signals in the filters13,14.

FIG. 2ashows diagrammatically the operation of a first embodiment of the present invention that comprises an all-optical demultiplexer (DEMUX) example (designated as a whole by reference number110).FIG. 2bshows diagrammatically the operation of a multiplexer (MUX)130that is equivalent to the demultiplexer of the first embodiment of the present invention shown inFIG. 2a. The MUX/DEMUX is for use with signals comprising 40 channels, andFIG. 3shows an embodiment of a block diagram that can perform the operations of multiplexing and demultiplexing forty channels as illustrated inFIGS. 2aand2b.

FIG. 2ashows the operations performed in a demultiplexing operation to extract the forty individual channels from a signal containing forty wavelength division multiplexed channels. A WDM signal supplied at111is applied to a first optical filter, which in this embodiment is an 8skip0 filter115in the MASTER module, which is centred at a frequency such that it can extract from the signal a first set of channels that comprises a band of eight intermediate channels corresponding to central channels17to24.

The group of eight channels extracted by the 8skip0 filter115are sent to a first optical extraction unit, which in this embodiment comprises eight cascaded channel filters116, each with an appropriate band to extract each of the eight channels. A channel filter is a filter with a bandwidth of one channel and a central wavelength centred on the preset channel to be extracted. The channels separated by cascaded filters116are each sent to respective outputs D17-D24. Extracting the eight intermediate channels17to24leaves a gap in the signal between channels16and25.

The signal with the remaining channels (that is channels1to16and25-40) is reflected by the 8skip0 filter115and applied to a second optical filter, which in this embodiment is a wavelength optical splitter117that is also in the MASTER module. The splitting operation divides the signal with the first set of channels extracted into two optical signals, which comprise a second and third set of channels each of which is directed down a different optical path118and119.

The wavelength optical splitter117is arranged such that the wavelength at which the split is made, and the associated intersection zone, is located at a wavelength that is within the gap in the signal that has been left by the extraction of the eight channels by the 8skip0 filter115. Therefore, the wavelength of the intersection zone can be as large as the bandwidth of the channels that have been extracted by the 8skip0 filter115(i.e. corresponding to a maximum of eight channels in this embodiment) without affecting any remaining channels (that is1to16and25to40) and without reducing the performance of the circuit. This enables a relatively inexpensive economical splitter.

The sixteen channels on path118(that is the second set of channels corresponding to channels25to40) are sent to a third optical filter, which in this embodiment is an 8skip0 filter120in module BLUE1. The 8skip0 filter120is arranged such that it's pass-band is positioned to extract a subset of eight channels (channels33-40) that are not adjacent to the channels that have been extracted by the 8skip0 filter115in module MASTER.

The channels (25to32) that remain following the extraction by the 8skip0 filter120are sent to a second optical extraction unit, which in this embodiment comprises eight cascaded channel filters122to divide the remaining signal into individual channels. These individual channels are then sent to the respective outputs D25-D32. The 8skip0 filter120and the eight cascaded channel filters122are assembled in a module named BLUE1.

The channels extracted by the 8skip0 filter120in module BLUE1are individually separated by a further second optical extraction unit, which comprises eight cascaded channel filters121in module BLUE2, and then sent to the respective outputs D33-D40.

The channels lower than the gap created by the first 8skip0 filter115in the MASTER module are sent over the path119to a further third optical filter, which comprises an 8skip0 filter123in module RED1. The 8skip0 filter123in module RED1is arranged such that it's pass-band is positioned to extract the group of eight channels corresponding to channels1to8. Again, channels1to8are not adjacent to the gap, and therefore are not adjacent to the set of channels that had previously been extracted by the 8skip0 filter115in the MASTER module.

The remaining channels (channels9to16) are sent to a third optical extraction unit comprising eight cascaded channel filters124, also in module RED1, to divide the signal into individual channels and send the individual channels to the respective outputs D09-D16.

The signal extracted by the 8skip0 filter123in module RED1is applied to module RED2where it is filtered by a further third optical extraction unit comprising eight cascaded channel filters125to obtain the eight channels1to8, and these channels are then sent to respective outputs D01-D08.

Due to the channel gap in the signal that has been created by the first 8skip0 filter115in the MASTER module, and the subsequent use of the splitter117, it is possible to create a parallel structure operating on the two paths118,119derived from the initial signal. This allows a reduction in the number of filters traversed by the channels before being extracted individually, and also allows easy implementation of a modular structure that easily enables expansion of the number of channels handled. Operating on the signals on the two paths118,119in parallel can also reduce the time taken for the WDM signal to be demulitplexed.

In addition, none of the 8skip0 filters are used on any neighbouring channel groups, and therefore the doubling effect of the chromatic dispersion of the channels at the edges of a filter is avoided.

The modules MASTER, BLUE1, BLUE2, RED1and RED2all comprise a single channel extraction module arranged to extract eight individual channels. The modular composition of the structure can enable further modules to be added to the structure to increase the number of channels that are demultiplexed, if required. The separate modules may be mounted on separate cards, on separate printed circuit boards (pcb's), in single housings, or may constitute separate integrated circuits, for example.

FIG. 2bshows the operations performed in a multiplexing operation to concatenate forty individual wavelength division multiplexed channels into a single signal on a single transmission path. The forty individual signals are supplied as individual channels on forty transmission paths as inputs D01to D40. Eight of the input channels are applied to each of the individual modules, that is: MASTER, BLUE1, BLUE2, RED1and RED2.

The eight individual channels033to040comprise a20subset of channels are supplied as inputs to module BLUE2, which are applied to a second optical grouping/concatenation unit comprising eight cascaded channel optical filters121′ where they are concatenated to form a band of the eight input channels33to40such that channels33to40are combined onto a single transmission path. The group of eight concatenated input signals33to40are then supplied from module BLUE2to module BLUE1.

Channels25to32are supplied directly to inputs D25to D32in module BLUE1, where they are concatenated by a further second optical concatenation unit comprising eight cascaded channel optical filters122′. The eight concatenated channels33to40and25to32are then applied on a single transmission path, as a single signal, to a third optical filter comprising an 8skip0 filter120′ where they are concatenated to form a single signal comprising a second set of channels that comprise a band of sixteen channels,25to40.

Modules RED1and RED2with filters125′,123′ and124′ are similarly used to generate a single signal comprising a band of sixteen channels01to16from the signals applied at inputs D01to D16.

The second set of channels comprising the band of sixteen channels,01to16is referenced119′, and the third set of channels comprising the band of sixteen channels,25to40is referenced as118′. The two bands of sixteen channels118′,119′ are both supplied to a second optical filter that comprises a wavelength optical splitter/combiner117′ in the MASTER module.

For multiplexing operations, the wavelength splitter117′ is used in a reverse configuration such that the two signals that are supplied to the wavelength splitter117′ are combined into a single signal instead of a single signal being split into two signals. Combining the two signals118′ and119′, each comprising sixteen channels, generates a signal containing the second set of channels1to16and the third set of channels25to40. The wavelength splitter117is arranged such that the intersection zone between the combined signals (corresponding to the wavelength at which a signal is split when the wavelength splitter is in a forward configuration when demultiplexing) is located in the gap between the two groups of sixteen channels118′ and119′. That is, the signals118′ and119′ are combined at an intersection wavelength between channels16and25.

Also supplied to the MASTER module at inputs D17to D24are signals representing channels17to24. The signals17to24are supplied to a first optical grouping/concatenation unit comprising eight cascaded channel optical filters116′ in the MASTER module in order to generate a first set of channels that comprise the band of the eight concatenated channels17to24.

The output of the wavelength splitter117′ and the output of the eight cascaded optical filters116′ are fed to a first optical filter, which comprises an 8skip0 filter115′ within the MASTER module. The 8skip0 filter is arranged to combine the first, second and third sets of channels, and to generate a WDM signal that comprises the forty channels,01to40, at adjacent wavelengths. This signal can then be transmitted as a wavelength division multiplexed signal.

It will be appreciated that each of the three 8skip0 filters120′,123′ and115′ are centred on bands of 25 channels that are not adjacent to each other. 8skip0 filter120′ is centred on the band of channels33to40, 8skip0 filter123′ is centred on the band of channels01to08, and 8skip0 filter115′ is centred on the band of channels17to24. By not using 8skip0 filters on adjacent channel bands, the chromatic dispersion of the channels at the edges of a filter is not doubled, as can be the case with the prior art.

It will be appreciated that the operations performed in the multiplexing operation are very similar to those performed in the demultiplexing operation. This is shown inFIG. 3where the block diagram for the multiplexer110is shown above the demultiplexer130, and the similarity of the components is illustrated.

The MUX/DEMUX comprises an input111for the signal containing the channels to be demultiplexed by the DEMUX130part of the circuit, and an output112for the signal containing the channels that have been multiplexed by the MUX110part of the circuit.

With regard to the MUX part110ofFIG. 3, the input channels are grouped in bands of eight channels supplied to respective optical grouping/concatenation units, which in this embodiment comprise cascaded channel filters121′,122′,116′,124′,125′ so as to form five optical signals each containing a groups of eight channels. The end channel groups (channels1-8and33-40) are applied to 8skip0 filters120′,123′ in modules BLUE1and RED1respectively. Two groups of channels25-32and9-16are also applied to the respective 8skip0 filters120′,123′ to make up a signal composed of the respective two channel groups of sixteen channels (channels25to40, and1to16) on each path118′ and119′.

The splitter117′ receives the two signal paths118′ and119′ such that a gap exists in the combined signal that is between the channels that have been supplied on optical paths118′ and119′. The gap comprises the central channels17-24. 8skip0 filter115′ is used to fill the gap with channels17to24that correspond to signals that have been received on inputs D17to D24and subsequently filtered by cascaded channel filters116′. Thus the signal comprising all forty channels is obtained at112.

In accordance with the principles of this invention the total number of channels in the signal can vary, and the number of channels extracted as a group can also vary.

FIGS. 4 and 5show a second embodiment of the invention, in which 32 channels are to be multiplexed/demultiplexed, and a combination of 8skip0 and 6skip0 filters are used. For the sake of simplicityFIG. 4shows only the DEMUX part of the MUX/DEMUX (designated as a whole by210) of the second embodiment.

The overall WDM signal with 32 channels is applied at211and reaches a first 8skip0 optical filter215in the MASTER module that is arranged to extract a group of eight intermediate channels (channels13to20) from the signal. Extracting channels13to20leaves a gap in the remaining signal. The group of eight channels thus extracted is sent to eight cascaded channel optical filters216that are also in the MASTER module and are arranged such that the eight channels can be extracted individually. The individually extracted channels are each sent to respective outputs D13-D20.

The signal with the remaining channels is reflected from the 8skip0 filter215and sent to a wavelength optical splitter217that is also in the MASTER module. The splitter217divides the reflected signal into two sets/groups of channels. One of the groups comprises channels1to12, and the other group comprises channels21to32. One of the groups of channels have wavelengths that are above the gap of channels created by the 8skip0 filter215, and the other group of channels have wavelengths that are below the gap. The two groups of channels are sent down separate optical paths218,219.

Apart from the settings of the filters215,216and the splitter217, the structure of the MASTER module ofFIG. 4is the same as that of the MASTER module of the first embodiment of the invention shown inFIG. 2a.

The twelve channels that are above the gap (channels21to32) are sent to 6skip0 filter220in module BLUE1on path218. The 6skip0 filter220is arranged to extract the band/group of six channels32-27. Channels32-27are not adjacent to the channels that have been extracted by the previous 8skip0 filter215in the MASTER module such that chromatic dispersion caused by both of any two of the Xskip0 filters does not effect a single channel. The remaining six channels, channels21to26, that is the channels that are between the bands that have been extracted by 8skip0 filter215and 6skip0 filter220, are sent to six cascaded channel filters222in module BLUE1that are arranged to divide the signal into the individual channels and send them to respective outputs D21-D26.

The channels extracted by the 6skip0 filter220are sent to module BLUE2where they are separated by six cascaded channel filters221and sent to the respective outputs D27-D32.

In a similar manner, the twelve channels below the gap created by the 8skip0 filter215in the MASTER module are sent on path219to 6skip0 filter223in module RED1. 6skip0 filter223is arranged with a band positioned to extract six channels (channels1-6) that are not adjacent to the channels extracted by the 8skip0 filter215in the MASTER module. The remaining channels (channels7to12) are sent to six cascaded channel filters224in module RED1to divide the band of remaining channels into individual channels, and send them to respective outputs D07-D12.

The channels extracted by the 6skip0 filter223in module RED1are sent to module RED2where they are individually separated by six cascaded channel filters225and sent to respective outputs D01-D06.

It will be appreciated from the block diagram ofFIG. 5, that a multiplexer that corresponds to the demultiplexer ofFIG. 4will comprise the same components as the demultiplexer ofFIG. 4, only with the direction of travel of the signals reversed.

The structures illustrated inFIGS. 2 to 5have an optimised number of filters, whilst providing minimised deterioration of the channels. However, it will be appreciated that other structures can be realized in accordance with this invention depending on the availability of filters and specific modularity requirements.

FIGS. 6,7and8show a third embodiment of a MUX/DEMUX (designated as a whole by reference number310) of the invention. The MUX/DEMUX operates on forty channels in total, but with extraction of groups of four channels at a time. Although it is clear that this structure entails the use of a greater number of filters than the first embodiment of the invention, the number of channel filters that the last channel extracted for each group has to traverse is still reduced when compared with the prior art systems. Doubling of the chromatic dispersion that would by caused by the use of adjacent Xskip0 filters is also avoided.

As shown inFIG. 6, the overall forty-channel WDM signal is applied at311and reaches a first 4skip0 optical filter315in module MASTER A that is centred to extract channels21to24that comprise a group of four intermediate channels from the signal. Extracting the group of four channels leaves a gap in the original signal.

The group of four channels thus extracted are sent to four cascaded channel optical-filters316in module MASTER A so as to separate and send each channel to a respective output D21-D24.

The signal with the remaining channels is reflected from the 4skip0 filter315and is sent to a wavelength optical splitter317to divide the reflected signal onto two optical paths318,319.

The sixteen ‘high’ channels on path318are sent to a 4skip0 filter415in module BLUE1A, arranged to extract a band comprising channels29-32that is not adjacent to the channels extracted by the 4skip0 filter315in the MASTER A module. The extracted band is sent to four cascaded channel filters421in module BLUE1B, to divide the band into individual channels and send them to the respective outputs D29-D32.

The remaining channels are reflected towards a splitter417in module BLUE1A to divide the reflected signal onto two paths418,419, wherein each of the paths418,419comprises channels that are either above or below the gap created by the 4skip0 filter415in module BLUE1A.

The low channels (channels25to28) on the path419, are supplied to four cascaded channel filters416in module BLUE1A that divide the channels individually to respective outputs D25-D28.

The high channels (channels33to40) on path418are sent to a 4skip0 filter520in module BLUE2A to extract the group of channels (channels37-40) that are not adjacent to the group extracted by the 4skip0 filter415in module BLUE1A. The extracted signal is then sent to four cascaded channel filters521in module BLUE2B to divide them individually to respective outputs D37-D40. The remaining channels (channels33to36), are reflected by the 4skip0 filter520in module BLUE2A and are sent to four cascaded channel filters522in module BLUE2A that extract the individual channels and sends the individual channels to the respective outputs D33-D36.

As shown inFIG. 7, the twenty ‘low’ channels initially conveyed on the path319reach a 4skip0 filter515in module MASTER B with a band positioned to extract another group of four intermediate channels (channels13-16) that are not adjacent to the channels extracted by the 4skip0 filter315in module MASTER A. The extracted signal is sent to four cascaded channel filters525in module RED1A to divide the extracted signal into individual channels and then send them to the respective outputs D13-D16.

The remaining channels are reflected by the 4skip0 filter515in module MASTER B to a splitter517in module MASTER B to divide the signal into the channels that are above and below the gap created by the 4skip0 filter515in module MASTER B, and send the groups of channels over two paths518,519.

The high channels (channels17to20) on the path518are applied to four cascaded channel filters516in module MASTER B to divide them individually to respective outputs D17-D20.

The low channels (from 1 to 12) on the path519are sent to 4skip0 filter615in module RED1B to extract the set of channels (channels8-5) that are not adjacent to the group of channels that have been extracted by the 4skip0 filter515in module MASTER B. The extracted channels are sent to four cascaded channel filters625in module RED2A to divide them individually among respective outputs D5-D8.

The remaining channels, that is those reflected by the 4skip0 filter615in module RED1B, are applied to a splitter617in module RED1B to divide the signal into the channels that are above and below the gap produced by the 4skip0 filter615in module RED1B. The divided signals are applied to paths618and619.

The two groups of channels (both of four channels each) thus obtained on the paths618and619are each applied to four cascaded channel filters. The signal on path618is applied to four cascaded channel filters616in module RED1B, and the signal on path619is applied to four cascaded channel filters725in module RED2B. The cascaded channel filters616and725each supply the individual channels to outputs D9to D12and D1to D4respectively. Separation of all the channels starting from the initial WDM signal is thus obtained.

FIG. 8shows the modular demultiplexer circuit752that realizes the demultiplexing operation described inFIGS. 6 and 7.FIG. 8also comprises a multiplexer circuit750that corresponds to the demultiplexer circuit752. The MUX circuit750comprises components that correspond to the DEMUX circuit753and uses the same numbering of components with an apostrophe (').

Use of the structure of embodiments of the invention allows a low cost system, with a lower maximum chromatic dispersion, lower maximum insertion loss, and a better uniformity when compared with the prior art.

As an example of the improved performance,FIGS. 9 and 10show a comparison between the results of chromatic dispersion and loss of insertion of the prior art system ofFIG. 1with the embodiment of the invention ofFIG. 3.

BothFIG. 9andFIG. 10relate to the band L, from 187.0 to 190.9 THz, but it will be appreciated that the same structure with the same advantages applies to other optical bands of interest such as for example band C from 192.1 to 196 THz.

To generate the dispersion graphs ofFIG. 9, it was assumed that the filters add a chromatic dispersion of 15 ps/nm on the channels at the edges of filtered bands (even the reflected channels), 2 ps/nm to the channels that are positioned in a pass-band, and 1 ps/nm to the other channels (and then zero).FIG. 9shows that the chromatic dispersion on the channels at the edges of bands/groups of the embodiment of the invitation is halved compared with that of the prior art system.

To generate the graphs of the insertion losses ofFIG. 10, it is assumed that the loss of transmission of each of the filters is 0.6 dB, and the loss of reflection is 0.3 Db, while loss of connection is 0.2 dB (average). Comparison of the two solutions shows that the losses on the upper channels are considerably reduced by applying the principles of the invention and, especially, are made more uniform with those of the lower channels.

It will be appreciated that the invention is not limited to the above described preferred embodiments.

Although the embodiments ofFIGS. 2 and 3use 8skip0 filters to optimise costs, expandability and performance, it will be appreciated that other band-pass filters, and other Xskip0 filters with a different value for X, could be used. Structures and circuits in accordance with the principles of this invention provide high modularity, such that it is possible to combine modules with different numbers of extracted channels. For example, in the embodiment ofFIG. 6, the group of eight channels on the path418obtained in the BLUE1A module could instead be sent to a module for extraction with eight cascaded channel filters, such as that shown in module BLUE2ofFIG. 2. The same applies for the corresponding MUX.

In some embodiments Xskip0 filters having different values for X, and therefore being arranged to extract different sized wavelength bands of channels, may be used on different sets and subsets of channels within the same multiplexer/demultiplexer. The wavelength gap created by the extraction of channels by a band-pass filter need not necessarily be located at the centre of the remaining channels. The gap could be asymmetrically disposed in the channels in order to provide conveniently sized subsets for further processing. In some embodiments the wavelength gap is symmetrically disposed.

The choice among the various possibilities of the number and types of filters used may depend on the factors of cost, modularity and signal deterioration, and what values are considered acceptable by an administrator/user.

The number of channels (X) extracted by the Xskip0 filter preceding the splitter should be sufficiently high for the characteristic curve/transmission profile of the splitter to fit into the ‘gap’ of channels created by the Xskip0 without attenuating any of the channels to either side of the gap. The quality of the splitter that is to be used may determine how many channels need to be extracted by the Xskip0 filter to create the gap.

In a preferred structure in accordance with this invention it is possible to use a recursive structure; that is:

a) using a first Xskip0 filter to extract a first set of channels that is not adjacent to any previously extracted set of channels;

b) dividing the remaining signals into two further sets of signals;

c) repeating steps a) and b) for each subsequent divided set of signals until each set of signals is smaller than a threshold size; and

d) extracting the individual channels from each of the sets of channels.

It will be appreciated that a first set of channels extracted by a first optical filter need not necessarily be towards the middle of the WDM signal. In some embodiments the first set of channels can be located towards an edge of the WDM signal. What is important is that the next downstream filter has a cut-off/transition wavelength that is located at a wavelength that corresponds to a channel that has been extracted by the first optical filter. In such embodiments a third optical filter (and in some embodiments, further still optical filters) with a band-pass characteristic can still be used to extract a further group/band of channels that is not adjacent to a band/group of channels that has been extracted by the first optical filter, or any other group/band of channels.

It will be appreciated that any of the features/advantages discussed in relation to a multiplexer, or multiplexing operation, are equally applicable to a demultiplexer, or demultiplexing operation, and vice versa.

It will also be appreciated that any feature in any embodiment of the invention described above may be a feature that can be used with any other embodiment of the invention described above.