Abstract:
An optical demultiplexer system for separating adjacent wavelength bands without losing inter-band channels and comprising a first filter stage ( 210 ) of relatively narrow passband and relatively high selectivity in order to extract all of the inter-band channels. The first stage is followed by a second filter stage ( 240 ) of relatively broad passband and relatively low selectivity to extract the wavelength bands delivered by the first filter stage. Such a system makes it possible to use the inter-band channels, in particular for managing the network and the wavelength bands.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to optical fiber transmission technology by spectrum division multiplexing, and more particularly it relates to a system enabling wavelength band separation to be improved in a transmission mode where a large number of channels having the same destination are transported by a respective sequence of different wavelengths that are regularly distributed in the spectrum, all of these wavelengths being transported by the same fiber and constituting a so-called “band”. In other words, the spectrum of a band is in the form of a comb of finite width, and with teeth that are regularly spaced apart. A plurality of bands may be interlaced if the spectrum space between two successive channels in a band is at least equal to the space needed for a channel. 
   In recent years, enormous demand for bandwidth has been initiated by the deployment of modern forms of telecommunications, in particular the Internet and its main application the world wide web, but also all of the private networks of businesses and various other organizations, not forgetting wireless communications, in particular mobile telephony which, in practice, requires high performance terrestrial infrastructure to meet customer expectations. To satisfy this demand, those in charge of implementing the networks needed for deploying such novel forms of communication have rapidly needed to have recourse to transporting signals carrying information in optical form in order to benefit firstly from the low cost of fibers themselves and secondly from the very high data rates that can be achieved in spite of transmission distances that can be measured in kilometers or even tens or hundreds of kilometers, without any need for signal regeneration. Attenuation is very low, in particular in so-called “single mode”, fibers, when compared with the level of attenuation that can be obtained for electrical transmission using copper, for example. Furthermore, optical transmission avoids all problems associated with electromagnetic disturbances which require expensive protection circuits and can lead to frequent errors in transmission. 
   A first step in the use of optical fibers consisted essentially in point-to-point links between two nodes of a network. During that first step, transport took place over a single wavelength, usually around 1550 nanometers (nm) since that is the wavelength that is most suitable for long distance transport. Although optical signals can be modulated up to very high frequencies, which are expressed in gigabits per second (Gb/s, i.e. 10 9  bits per second), the quantity of information that needs to be transported rapidly outgrew the available capacity. Although the fiber itself is of low cost, deploying it can require a large amount of labor and can be extremely expensive. Instead of deploying larger numbers of optical fibers when the capacity of an installed network becomes insufficient, a solution has been found in making better use of those fibers that are already in place. The technique of wavelength division multiplexing (WDM) makes it possible to increase the number of completely independent transmission channels within a single physical fiber by transmitting different wavelengths along the same fiber. In other words, by transmitting light rays of different “colors”, the bandwidth of a single fiber is increased correspondingly. The technique of dense WDM (DWDM) quickly supplanted WDM and now makes it possible to multiplex hundreds of channels, or even more. 
   It has been shown that, under most circumstances, routing wavelengths in groups, i.e. bands (as contrasted with conventional routing by individual wavelengths) leads to a reduction in the overall cost of an optical network insofar as it enables the cost of the filter and switch devices present in nodes to be shared between the various different wavelengths making up a band. Furthermore, amongst the band configurations that are suitable for use for routing purposes, contiguous or adjacent wavelength bands present a major advantage over interlaced wavelength bands in that they use simpler filter devices that are less expensive and also more tolerant from the point of view of the physical constraints they impose (in contrast to interlaced bands which require filter devices that filter each wavelength individually). 
   Such fibers are used for interconnecting optical switching equipment situated at the various nodes of a network. An essential device at a network node is then an optical add/drop multiplexer (OADM). As its name suggest, it serves to remove or demultiplex and to insert or multiplex local traffic optically (e.g. traffic that is local to an inlet/outlet point for a secondary network), while allowing the remainder of the traffic to continue its journey towards other nodes of the network. This assumes that at least one wavelength carrying local information (one channel) can be dropped and added. In practice, for the reasons mentioned above, it is usually wavelength bands (sequences of channels) that need to be dropped and added in this way in order to be able to exchange a sufficient quantity of information with local applications. 
   This is shown in  FIG. 1 , which shows an ordinary optical demultiplexer  100  separately extracting three wavelength bands of a multiplex constituted by a sequence of channels distributed at regular intervals within the spectrum:
         a band  110  constituted by four channels carried respectively by a first group of four wavelengths;   a band  120  constituted by four channels carried respectively by a second group of four wavelengths; and   a band  130  constituted by four channels carried respectively by a third group of four wavelengths.       

   The bands  110  and  120  are separated by a band  139  corresponding to two channels, but which is not used for transmitting signals. The bands  120  and  130  are separated by a band  140  corresponding to two channels, but which is not used for transmitting signals. If ordinary band demultiplexing equipment is used, the bands  110 ,  120 ,  130  cannot be extracted without it being necessary to sacrifice so-called “inter-band” channels between the bands  110  and  120  and between the bands  120  and  130 . This is due to the great proximity of contiguous wavelengths that can be transported in so-called DWDM mode (involving spectrum separations between channels of 100 GHz, 50 GHz, or even 25 GHz), and to the limitations inherent to the spectral discrimination ability (also known as selectivity) of ordinary band demultiplexing equipment, such as the demultiplexer  100 . 
   In  FIG. 1 , the passband of the ordinary demultiplexer  100  is represented by a trapezoid  150  having sides of slope that is not sufficiently steep. This passband is broad enough to cover the channels of a wavelength band (four channels in this example) but is of selectivity that is insufficient for effectively eliminating the inter-band channels constituting the bands  139  and  140 . These inter-band channels therefore cannot be used, thereby significantly reducing the transport capacity on the optical fiber. 
   SUMMARY OF THE INVENTION 
   That is why the object of the invention is to provide an optical demultiplexing system enabling effective use to be made of these inter-band wavelengths that would otherwise be lost. 
   The present invention thus provides an optical demultiplexer system for separating out a single band forming part of a first comb of bands which is interlaced with at least one second comb of bands, each band being constituted by a plurality of channels having adjacent wavelengths, the system being characterized in that it comprises:
         a first spectral demultiplexer stage for separately extracting each of the combs; and   a second spectral demultiplexer stage for separately extracting each of the bands constituting a comb extracted by said first stage.       

   The system of the invention makes it possible to use all of the channels, and in particular the channels conventionally referred to as inter-band channels that were previously not used for transmitting signals. These channels now become usable, in particular for network management and for wavelength band management, since the system of the invention makes it possible to obtain sufficient selectivity, while requiring only means that are simple and inexpensive in each of the two stages. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects, subject matter, characteristics, and advantages of the invention can be seen more clearly from the following detailed description of a preferred embodiment thereof illustrated by the accompanying drawings, in which: 
       FIG. 1 , as described above, shows a prior art mode of transmission in which a large number of wavelengths are transported over the same optical fiber, but in which the inter-band channels are unusable; 
       FIG. 2  shows a first embodiment of the optical demultiplexer system of the invention; and 
       FIG. 3  shows a second embodiment of the optical demultiplexer system of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the embodiment shown in  FIG. 2 , a multiplex  200  arrives on a fiber  205 . It is made up of channels that are distributed at regular intervals. It is constituted by two interlaced combs:
         a first band comb  321  constituted by three bands  321   a ,  321   b ,  321   c  each constituted by four channels used for transmitting signals; and   a second band comb  320  constituted by two bands  320   a  and  320   b  each constituted by two “inter-band” channels, which are also used for transmitting signals.       

   The system comprises:
         a first spectral demultiplexing stage of relatively narrow passband and relatively high selectivity, having as its main function extracting all of the inter-band channels; and   a second spectral demultiplexing stage of relatively broad passband and relatively low selectivity for separating the wavelength bands that remain after all of the inter-band channels have been extracted by the first stage.       

   It is easy to make a first stage having high selectivity because the inter-band channel bands are of narrow width. The second stage can be an ordinary demultiplexer of low selectivity because of the initial highly effective elimination of the inter-band channels that is performed by the first stage. 
   The first stage is constituted by a periodic band deinterlacer  210  for separating the two combs  320  and  321 . A first output from the demultiplexer  210  supplies the comb  320  (bands  320   a  and  320   b ) comprising all of the inter-band channels. These channels are extracted but remain multiplexed in a common multiplex in this first example. The two bands  320   a  and  320   b  can be used for transporting signals, but not separately. A second output from the deinterlacer  210  provides the comb  321  which comprises all of the channels that are used for transporting information. The band deinterlacer  210  is preferably constituted by a periodic filter (or a cascade of periodic filters) of sufficient selectivity to attenuate effectively the bands  320   a  and  320   b  of the inter-band channels from the multiplex (comprising the bands  321   a ,  321   b ,  321   c ) that is applied to the second output. 
   The second stage is constituted by a periodic band demultiplexer  240 . It has three outputs separately delivering the bands  321   a ,  321   b , and  321   c . There is no harm in the demultiplexer constituting the second stage  240  being an ordinary demultiplexer for contiguous wavelength bands, having low spectral selectivity since the inter-band channels (bands  320   a  and  320   b ) have previously been extracted with high selectivity by the first stage  210 . 
   Implementation of the deinterlacer of the stage  210  makes use of techniques that are known to the person skilled in the art. Nevertheless, it is shown in greater detail in an enlarged portion of  FIG. 2 . It comprises a circulator  252  having an input connected to the fiber  205 , and a first output connected to a periodic optical filter  254 , e.g. a Fabry-Perot filter. The spectral separation power of the filter  254  must be sufficient to enable the bands  320   a  and  320   b  of inter-band channels to be extracted. These bands appear on the output  256  of the periodic filter  254 . A second output  258  of the circulator  252  constitutes the second output of the deinterlacer  210  which delivers the bands  321   a ,  321   b , and  321   c . It should be observed that since a circulator  252  is an expensive component, it can advantageously be replaced by an isolator followed by a simple coupler. 
     FIG. 3  reproduces the content of  FIG. 2  for the most part. It shows a demultiplexer system for the situation in which the two inter-band channels  320   a  and  320   b  are used separately. In addition to the means of the first embodiment shown in  FIG. 2 , this second embodiment includes a periodic band demultiplexer  360  having one input connected to the first output of the deinterlacer  210  and having two outputs for separately extracting the bands  320   a  and  320   b  which are also used for transporting signals. 
   Each of the periodic band demultiplexers  240  and  360  preferably comprises a periodic filter made up of deposited thin layers. 
   Thus, the invention enables the inter-band channels to be used, whereas in the past they were normally unusable. In a preferred embodiment of the invention, these channels are used for controlling the network itself, carrying all of the information needed for proper operation of the network and for keeping it at an optimum level of performance. In a DWDM mode of transport based on techniques of mixing numerous wavelengths on optical fibers interconnecting OADM stations where wavelength bands are to be dropped locally while others are added, careful management is essential for them to be used effectively. This management requires one or more control channels which advantageously make use of the inter-band wavelengths that are made usable again by implementing the invention. 
   Finally, it should be observed that the number of inter-band channels is shown in  FIGS. 1 to 3  as being two, both when describing the prior art and when describing the invention, but that it could naturally be different from the number two which is merely a particular example. Specifically, in a preferred embodiment of the invention, it is possible to use only one single inter-band channel by selecting a band demultiplexer  240  which is sufficiently selective for this to be possible or because the wavelengths selected make this possible by being sufficiently spaced apart spectrally. It is then possible to obtain one-to-one correspondence between a wavelength band and one or other of the contiguous inter-band channels, which channels can be used for controlling the wavelength band.