WO2005043785A1

WO2005043785A1 - Optical filter chain

Info

Publication number: WO2005043785A1
Application number: PCT/EP2004/052692
Authority: WO
Inventors: Julien Nicholas Vincent Poirrier
Original assignee: Marconi Communications Gmbh
Priority date: 2003-10-28
Filing date: 2004-10-28
Publication date: 2005-05-12
Also published as: DE10350282A1

Abstract

A filter chain which is selectively transparent for at least one carrier wavelength of a wavelength-division multiplex signal comprises at least one flat-top filter, the transmission characteristic of which comprises at least one payload band, lateral zones surrounding the payload band and having a slightly higher transmission than in the payload band, and blocking regions surrounding the lateral zones, and, in series with the flat-top filter, at least one second filter, the transmission band of which coincides with that of the flat-top filter and which has less transmission in the lateral zones than in the payload band.

Description

Optical Filter Chain

The present invention relates to an optical filter chain in an optical telecommunication system in which telecommunication signals are transmitted as wavelength-division multiplex signals between network nodes.

Each wavelength-division multiplex signal is formed of a plurality of channels at different carrier wavelengths. The channels transmitted on an optical fibre between two nodes of the network may have different start and target nodes. In order to route these channels independently through the network, the nodes are equipped with various filters which enable an incoming multiplex signal to be separated into its individual channels and a plurality of channels to be combined into an outgoing multiplex signal after passing through a switching station of the node.

An ideal filter should have a transmission of 1 in a wavelength range corresponding to a channel of the multiplexer and a transmission of 0 outside of this range. The filters that exist in practice can only approximate such an ideal characteristic more or less exactly. A simple filter type is known as a Gaussian filter. Its transfer function, expressed in decibels as a function of frequency, has essentially the shape of a Gaussian curve in the transmission band and its surroundings. The 3dB-bandwidth of the Gaussian curve, that is, the width of the transmission band, is defined by the need to transmit the desired channel as completely and as free from distortion as possible and to suppress as completely as possible channels at adjacent wavelengths. The transfer function of a series combination of several Gaussian filters corresponds to the sum of the fransfer functions of the individual filters. The greater the number of filters included in the series, the less is the bandwidth of the overall assembly. If a channel passes a plurality of Gaussian filters on its path through the network, the bandwidth of the accumulated transfer functions thereof may be noticeably less than that of the channel, which may cause a substantial distortion of the telecommunication signal and may even make it unusable.

For this reason, it is usually attempted to avoid filters with Gaussian characteristic in an optical telecommunication network and to use so-called flat-top filters instead, the transfer function of which better approximates the ideal rectangular characteristic. The transfer function of such a flat-top filter comprises a payload band centred on the channel to be filtered, lateral zones surrounding the payload band, in which the transmission is even slightly higher than in the payload band, and beyond these lateral zones, blocking regions in which the transmission drops more steeply than with a Gaussian filter. Due to this sharp drop, the decrease of bandwidth in a series of several flat-top filters is noticeably less than with Gaussian filters, which makes them better suited for use in an optical telecommunication network. However, problems are caused by the transmission increase in the lateral zones, which may also cause signal distortions if a larger number of filters is mounted in series. Therefore, the ideal flat-top filter should preferably have a payload band with a width corresponding to the bandwidth of the telecommunication channel, as narrow lateral zones as possible and, in these lateral zones, as little transmission increase as possible. However, the stricter the requirements for these parameters, the more laborious and costly the realization of the filter becomes, and so does that of the network components which use these filters.

The object of the invention is therefore to provide a filter chain for an optical wavelength-division multiplex transmission network, which achieves a good transmission characteristic with simple, economic filters.

The object is achieved by a filter chain having the features of claim 1.

By combining the various filter types, the disturbing transmission increase in the lateral zones of the flat-top filters is avoided. As the second filter, a simple Gaussian filter may be used.

Regardless of the type of the second filter, it is always possible to choose a number of the flat-top filters such that the amount of the difference between the transmission in the payload band and in one of the lateral zones is not higher for the entire filter chain than for one of its flat-top filters. That is, it is always possible to improve the transmission characteristic of the filter chain, if the transmission of the second filter in the lateral zones is less than in the centre of the payload band.

In particular if the second filter has a Gaussian characteristic, the amount of the transmission differences between the centre of the payload band and the lateral zones at equal width of the transmission band in the second filter is approximately three times as large as in a flat-top filter, so that an excellent transmission characteristic may be achieved if the second filter is combined in series with three flat-top filters.

A total number of four filters is also quite advantageous for the entire filter chain since with this number of filters a complete transmission line may be formed in which the four filters are conventionally formed by an interleaver, a de-interleaver, a multiplexer and a demultiplexer.

Such a transmission line may be formed by an optical transmission fibre for a wavelength-division multiplex signal and a multiplexer and an interleaver at its input, and a de-interleaver and a demultiplexer at its output. Also the transmission path from an input of a network node via de-interleaver and demultiplexer to a switching fabric and from there via multiplexer and interleaver to an output of the node may be regarded as a transmission line in the sense of the invention.

Further features and advantages of the invention become apparent from the subsequent description of embodiments referring to the appended Figures.

Figure 1 illustrates an architecture of an optical transmission network with filters and transmission lines according to the invention;

Figure 2 illusfrates a transfer cuive of a flat-top filter;

Figure 3 illustrates a transfer curve of a Gaussian filter having the same bandwidth; and Figure 4 illustrates the transfer characteristic of a series connection of three flat-top filters and a Gaussian filter.

Figure 1 schematically shows an optical telecommunication network in which the present invention is applied. The network comprises a plurality of nodes 1, 2, 3, which are interconnected by optical transmission fibres 4. In the fibres, intermediate amplifiers are inserted at regular intervals according to need, here in the form of units comprising a pre-amplifier 6, a dispersion compensator 7 and post-amplifier 8, which are passed through by the entire wavelength-division multiplex signal propagating on the fibre 4.

A typical node of the network, such as the node 2, has a plurality of inputs 9 which receive a wavelength-division multiplex signal from a fibre 4. At each of these inputs 9 a de-interleaver 10 is connected for demulitplexing the optical wavelength-division multiplex signal. The optical wavelength-division multiplex signal comprises a plurality of uniformly spaced carrier channels numbered according to their frequency. The signal is separated into two partial multiplex signals, one of which comprises the odd- numbered carrier wavelengths and the other the even-numbered carrier wavelengths of the original incoming wavelength-division multiplex signal. The de-interleaver 10 thus has two transmission paths, each of which has an approximately comb-shaped transfer function wherein the transmission bands of the one transfer function coincide with blocking regions of the other. The de-interleaver 10 thus forms a first optical filter, which, according to the exact shape of the transfer function in a transmission band, may have an influence on the shape of the communication signal modulated onto the carrier wave in this transmission band.

The two outputs of each de-interleaver 10 lead to two demultiplexers 11, only one of which is shown in the Figure. The demultiplexer 11 demultiplexes the partial multiplex signal supplied to it into the individual carrier waves from which it is formed, and supplies each of these to an associated input of a switching fabric 12. The demultiplexer 11 thus forms a further filter for each carrier wavelength.

The communication signals that have been switched in the switching fabric 12 and eventually been transferred to other carrier wavelengths are combined by multiplexers 13 connected to the outputs of the switching fabric, so as to form even-numbered or odd-numbered partial multiplex signals. The passage through the multiplexer 13 involves a filtering of the individual carrier waves, just like the passage through the demultiplexer 11 does.

Every two multiplexers 13 (only one of which is shown in the Figure) are connected to a same output 15 by an interleaver 14, whereby the complete wavelength-division multiplex signal formed by the multiplexer 13 from the two partial multiplex signals is output to output fibre 4. Also the interleaver 14, being the symmetric counterpart to interleaver 10, functions as a filter for each single carrier wavelength. The passage through the node 2 may be regarded as a four-fold filtering for each single carrier wavelength. At least part of the nodes of the network, here the nodes 1 and 3, have transponders 16, 17 connected to them which form a source and a sink, respectively, for a communication signal. The communication between the transponders is usually bidirectional, but for the comprehension of the present invention it is sufficient to consider only unidirectional transmission from the transponder 16 of the node 1 to the fransponder 17 of the node 3, so that in the following, the transponders 16, 17 are referred as fransmission and receiver transponders, respectively.

For the sake of simplicity, the transmitter transponder 16 is shown to be directly connected to multiplexer 13 in Figure 1, but obviously, the optical output signal of fransponder 16 might as well be switched in the switching fabric 12 of this node before reaching the multiplexer 13. Similarly, the switching fabric might also be located between the demultiplexer 11 and the receiver transponder 17 in node 3.

The path of a communication signal from fransmitter fransponder 16 to receiver transponder 17 may be divided into a plurality of stages, which extend between switching fabrics 12 of adjacent nodes (or between a transponder 16 or 17 and a switching fabric 12 of an adjacent node). Each of these stages comprises four filters mounted in series, namely multiplexer 13 and interleaver 14 of a first node and de- interleaver 10 and demultiplexer 11 of the same or an adjacent node. Narious types of transfer characteristics of such filters are known.

Figure 2 shows a typical shape of a characteristic of a so-called flat-top filter. The characteristic is represented in the diagram as the transmission T, expressed in decibels, as a function of the frequency f, a value of 0 being set for the transmission maximum. The transmission curve has a central region, referred to as payload band 21, in which the transmission is high and nearly independent of frequency, but not maximum. The maximum value of transmission is reached in lateral regions 22 surrounding the payload band 21. Beyond the transmission maximum, the transmission drops sharply with increasing distance from the payload band 21. The fast drop of the transmission allows a neat separation of the individual carrier wavelength from one another, but the fact that the transmission is higher in the lateral regions 22 than in the payload band 21 proper may cause a distortion of the communication signals modulated onto the carrier, if a plurality of flat-top filters is connected in series.

In a Gaussian-shaped transfer characteristic, as shown in Figure 3, the maximum of transmission is in the centre of the transmission range, and at both sides of this maximum the transmission drops. The transmission drop is less pronounced than in a flat-top filter having the same 3dB bandwidth. If several filters of this type are connected in series, the 3dB bandwidth decreases with increasing number of filters, and if it becomes less than the data rate of the communication signal conveyed on this particular carrier wavelength, the communication signal is partially suppressed by the filtering and becomes unusable.

The ideal transfer function for avoiding a deterioration of the communication signal would be a rectangular function; filters having such a transfer function might be mounted in series in arbitrary numbers without affecting the communication signal. Instead of trying to approximate the transfer function of every single filter as far as possible to this ideal shape, the present invention considers their series connection by serially combining one or more flat-top filters with a Gaussian filter which compensates the excess of transmission of the flat-top filters in the lateral regions 22 by its fransfer function which continuously drops from the centre of the payload band towards its edges. Since in the common flat-top filters, the difference between the fransmission in the payload band 21 and the maximum in the lateral regions 22 is about a third of the difference between the transmission in the centre of the payload band and in the lateral regions in a Gaussian filter having the same 3dB bandwidth, a transfer function which approximates well the ideal rectangular shape may simply be obtained by combining three flat-top filters in series with a Gaussian filter. For example, in a network node such as the node 2, among the four filter-type components de-interleaver 10, demultiplexer 11, multiplexer 13 and interleaver 14, three have a flat-top transfer characteristic, and the fourth has a Gaussian transfer characteristic. It is not important which one of the four filter components has the Gaussian transfer characteristic.

Figure 4 shows as a solid line the fransfer characteristic which results from the series combination of three flat-top filters having the transfer function of Figure 2 and a Gaussian filter having the transfer function of Figure 3. The transfer function of Figure 3 is shown as a dashed curve 24 in Figure 4, and a dash-dot curve 25 indicates the shape of the transfer function for a series connection of three flat-top filters of the type shown in Figure 2. In the resulting transfer characteristic 26 shown in Figure 4, transmission maxima are distinguished in the lateral regions surrounding the payload band, as in case of Figure 2, but the payload band in which the transmission is practically independent of the wavelength is somewhat broader than in the individual filter of Figure 2, and the difference between the transmissions in the centre of the payload band on the one hand and in the lateral regions on the other is decreased with respect to the case of Figure 2. If the transmission increase in the lateral regions was slightly less in the flat-top filter of Figure 2, it might even disappear completely in the transfer characteristic of the series combination in Figure 4.

Obviously, the invention is not limited to the case in which the transmission difference between the centre of the payload band and lateral regions in the flat top filters is only a third of that of the Gaussian filter. If flat-top filters are used in which this difference is smaller, a compensation of this difference might straightforwardly be achieved by combining a larger number of flat-top filters in series with a Gaussian filter. However, this would have the disadvantage that the filters of such a series connection must be distributed to several network nodes, which would make the structure of the network more complicated. The difference considered within the present invention has the advantage that the number of filters that have to be combined is four, so that in each network node 1, 2 or 3 the same component 10, 11, 13 or 14 may be selected as the Gaussian filter, and all network nodes may have a same structure.

Claims

CLAIMS 1. An optical filter chain which is selectively transparent for at least one carrier wavelength of a wavelength-division multiplex signal, comprising at least one flat-top filter, the transmission characteristic of which comprises at least a payload band (21), lateral zones (22) surrounding the payload band (21) having a slightly higher transmission than in the payload band (21), and blocking regions surrounding the lateral zones (22), characterized in that the flat-top filter is combined in series with at least a second filter, the transmission band of which coincides with that of the flat-top filter and which has less transmission in the lateral zones than in the payload band (21).

2. The optical filter chain of claim 1, characterized in that the second filter is a Gaussian filter.

3. The optical filter chain according to one of the preceding claims, characterized in that the amount of the difference between the transmission in the payload band (21) and in one of the lateral zones (22) is not larger for the filter chain than for one of its flat-top filters.

4. The optical filter chain of claim 1 or 2, characterized in that the second filter is combined in series with a plurality of flat-top filters, preferably with three flat-top filters.

5. The optical filter chain according to one of the preceding claims, characterized in that the filters of the filter chain comprise an interleaver (14), a de-interleaver (10), a multiplexer (13) and a demultiplexer (11).

6. The optical filter chain according to one of the preceding claims, characterized in that its filters are distributed to the beginning and the end of a transmission fibre (4) for an optical wavelength-division multiplex signal.

7. The optical filter chain according to one claims 1 to 5, characterized in that its filters (10, 11, 13, 14) are distributed between input (9) and output (15) of an optical network node (2).