FR2743430A1 - Switching matrix router for optical communications networks - Google Patents

Abstract

The router (1) has input signals (11,2,16,17,21,22,6,27) which are fed to optical couplers (10,15,20,25). The optical couplers pass the signals to an optical switch (30, 40). Switching of the incoming signals to an output (35-38,45-48) is performed dependent on the optical wavelength of the incoming signals. An n*n optical matrix router is produced, with each comb of wavelength responses being different to the next. The "teeth" of the combs are included in inserted wavebands.

Description

 The present invention relates to a N x N wavelength router, more particularly intended for a communications network.

 The important developments of optical teletransmissions lead to connecting each user to a transmission or relay center via transmission optical fibers provided, at each of their ends, with an optical component for connecting the transmitter. of light modulated to the fiber to penetrate the light, or at the other end, to collect the light to direct it to a detector that will decode the optical signal to transform it into an electrical or electronic signal usable in a conventional receiver .

 The telecommunications networks thus obtained make it necessary to perform switching in large numbers by means of complex devices of the optical switch type.

 To achieve such an optical switch, specific components have been developed with N inputs and N outputs, able to connect any input to any output by wavelength adjustment. This component, called the Wavelength Division Multiplexer (Wavelength Division Multiplexer) N x N, usually has N input diffracting elements connected respectively to the inputs, and N output diffracting elements arranged with respect to the diffracting elements of the input. input and respectively connected to the outputs. Each of the input diffracting elements is adapted to produce for a given signal transmitted at a wavelength, an image disposed in a focal plane corresponding to any of the output diffracting elements. The selection of the output element is subordinate to the wavelength of the signal.

 Each pair of input fiber-output fiber is therefore associated with a transmission wavelength. The overall transmission spectrum of the entire N x N router is thus in the form of a comb with teeth located at these wavelengths. For identical diffracting elements arranged at equal intervals in an input line and an output line, 2N-1 distinct wavelengths are used in the transmission spectrum.

 A disadvantage of these traditional routers is their size and complexity, 2N diffracting elements being necessary.

 In addition, the wavelength ranges between the teeth are lost areas. Since the usable spectral band is restricted, this results in a limitation of the number of transmission wavelengths. The possible number of users is thus considerably reduced. By way of example, for a total spectral band limited to 30 nm, and the spacing between two transmission wavelengths being 1 nm, the number of transmission wavelengths can not exceed 30, which limits the number of users to fifteen.

 On the other hand, the teeth of the transmission spectrum have a half width or FWHM (Full Width Half-Maximum) usually low compared to the spacing between transmission wavelengths, which makes the selection of an emission wavelength, since the positioning in a tooth of the spectrum requires a high degree of accuracy.

 To reduce the lost areas and increase the useful area in wavelengths, one way is to bring the heart of the fibers. The ratio R of the width at mid-height of the teeth on their spacing increases with the ratio of the diameter of the core of the fibers over their distance. Thus, the ratio R can reach a value of 0.4 when fibers having hearts of diameter 10 μm are spaced successively from 22 clam.

 However, such fiber approximations are technically limited, and moreover cause a phenomenon of crosstalk, light passing directly from a fiber core to another neighbor.

 In order to increase the capacity of the routers, it has been proposed to associate several elementary routers. In existing devices, the routers are input to achromatic couplers via time multiplexing and demultiplexing units.

The inputs of each of the couplers can thus be connected to the inputs of each of the routers by means of an interconnection network.

The capacity of the router thus obtained is substantially increased compared to that of the elementary routers, the total number of users being the sum of the expected users for the different elementary routers.

 However, such a network produces significant losses. In addition, it involves the implementation of complex processes in order to perform time division multiplexing. Moreover, the previously mentioned problem of wavelength selections remains present.

 The present invention aims a N x N wavelength router valid for a large number of users, involving low losses and simple to implement and implement.

 The present invention also aims at such a N x N router avoiding any problem of crosstalk, and having a ratio width to mid-high spacing between wavelengths of high transmission.

 The invention also aims a communications network based on optical means, valid for a large number of users, reliable, involving low energy losses, and simple to achieve and use.

 For this purpose, the invention relates to a N x N wavelength router having N inputs and N outputs, for transmitting optical signals each having a wavelength, inputs to the outputs.

The router N x N also comprises switching means capable of switching each of the optical signals of any one of the N inputs to any of the N outputs as a function of the wavelength of the signal.

The router N x N is characterized in that the switching means comprise:
n mxm couplers each having m inputs and m outputs, N being equal to nxm and the N inputs of the n mxm couplers being the inputs of the router N x N,
nxn routers each having n inputs and n outputs, each of the n inputs being connected to one of the outputs of, respectively, the n mxm couplers and the N outputs of the nxn m routers being the outputs of the router N x N. Each of the routers nxn is able to steer an optical signal having a wavelength, from any of its n inputs to any one of its n outputs, depending on the wavelength of the signal, the router nxn having a transmission spectrum as a function of the wavelength in the form of a comb.

 The combs of the routers n x n are different from each other.

 The transmission spectrum of an optical router is the spectral distribution of the light flux at its outputs, produced by an extended spectrum light flux over the entire operating band of the router applied to all its inputs.

 The router according to the invention implements different spectra according to the different routers nxn making it possible to select as a function of the wavelength, for a given input, of the nxn router concerned and within it, of the channel target output. This device therefore makes no time division multiplexing necessary and makes it possible to use, upstream of the routers n x n, a simple circuitry for routing tracks such as the one claimed.

 In its entirety, the N x N router can admit a transmission spectrum in the form of a comb with very close teeth.

For a given range of usable wavelengths, the number N of inputs and outputs can thus be substantially increased, without the risk of crosstalk and without particular technical problems.

 In addition, the ratio R for the overall transmission spectrum can be considerably increased. The relative widening of the width of the teeth thus allows a selection of emission wavelengths less strict than in the existing devices.

 The larger the number of routers n x n, the closer the teeth of this spectrum can be, but at the cost of an increased complexity of the assembly.

 Preferably, the combs of the routers n x n have teeth included in interleaved strips.

 This configuration makes it possible to globally obtain a transmission spectrum comprising complementary bands coming from the different routers n x n. This can be particularly advantageous when transmission wavelengths are packaged in packets in each of the routers n x n.

 In an advantageous embodiment of this configuration, the combs of the routers n x n have their teeth interposed.

 In this case, the teeth of the overall transmission spectrum originate alternately from a distinct router nxn, which makes it possible to obtain, with respect to each of the routers nxn taken in isolation, a closer approximation of the teeth, the greater the number of routers nxn is big. The ratio R can thus be multiplied by m.

 In an advantageous embodiment, the combs of the n x n m routers have completely dissociated teeth.

 The teeth of the overall transmission spectrum can thus be clearly distinguished from each other, while being very close.

However, tooth overlaps from the different spectra of routers n x n are also possible.

 According to a first embodiment of the N x N router, each of the mxm couplers is an achromatic coupler with a uniform distribution of energy between its outputs.

 In this way, inexpensive and customary elements are used.

 According to a second embodiment, each of the mxm couplers is a wavelength division coupler able to transmit energy of an optical signal having a wavelength, solely by the output of the mxm coupler connected to the router. nxn whose comb includes this wavelength.

 This second embodiment is advantageous in that it avoids any loss of energy due to the couplers m × m, by channeling the signals upstream of the routers n × n as a function of the wavelength of the optical signal.

 In a preferred embodiment of the N x N router, the m x m couplers are composed of 2 x 2 couplers each having two inputs and two outputs.

 Such 2 × 2 couplers, also known as X-couplers, are elements that are currently available and can easily be used in combination.

 In this preferred embodiment, the router N x N advantageously comprises a first level of 2 x 2 couplers whose inputs are the inputs of the router N x N, and a last level of 2 x 2 couplers whose outputs are connected to the inputs routers nx n. The inputs of the 2 x 2 couplers that do not belong to the first level are connected to outputs of 2 x 2 couplers, and the outputs of the 2 x 2 couplers that do not belong to the last level are connected to inputs of 2 x 2 couplers.

 It is interesting that the router N x N has Y couplers each having a main arm and two secondary arms, the inputs and outputs of the N x N router being connected to the main arms of the Y couplers. The N x N router allows to send optical signals simultaneously from its inputs to its outputs and its outputs to its inputs, the optical signals flowing in opposite directions in the two secondary arms of each of the Y couplers.

 This last device makes it possible to gain a factor of 2 in the number of available inputs and outputs, because the same wavelength allows communications in one direction and in the other of the router N x N. The characteristic claimed thus contributes to increase the number of users of a transmission system in a simple way.

Preferably, each of the routers nxn comprises
a transmission medium comprising a first row of n inputs and a second row of n outputs, the first and second rows being arranged in parallel and the transmission medium having a flat transmission surface,
a diffractive element arranged facing the transmission surface,
a focussing optics placed between the transmission medium and the diffracting element
The diffractive element and the focusing optics being able to produce from any of the inputs an image corresponding to any one of the outputs, as a function of the wavelength of the optical signals.

 This configuration of nxn routers is both compact and economical.

 The invention also relates to a communications network comprising at least one N x N router having any of the above characteristics, or several of them in combination.

The invention will be better understood with reference to particular applications given by way of example and represented by the appended drawings:
FIGS. 1A and 1B are diagrammatic representations of a wavelength 4 × 4 router used in an N × N router according to the invention, FIG. 1A showing an overview of the 4 × 4 router and the Figure 1B, a front view of its fiber plane;
2 represents an 8 x 8 wavelength router according to FIG.
The invention, comprising four 2x2 couplers and two 4x4 routers such as that shown schematically in Figures 1A and 1B;
Figure 3 shows the respective transmission spectra of the two 4x4 routers of the 8x8 router of Figure 2.
- Figure 4 is a schematic representation of a 12 x 12 wavelength router according to the invention, comprising three 4 x 4 couplers and four routers 3 X 3, and two directions of transmission;
FIG. 5 is a detailed view of one of the 4 × 4 couplers of the 12 × 12 router of FIG. 4;
Figure 6 shows the respective transmission spectra of the four 3x3 routers of the 12x12 router of Figure 4.

 An elementary 4 x 4 router used in an N x N router according to the invention comprises a diffraction grating 50 arranged with respect to a plane 62 of fibers, as can be seen in FIG. 1A. The 4x4 router also includes focusing optics such as a lens 59.

Input fibers 51, 52, 53, 54 terminate at the plane 62 of fibers being arranged in line, as are exit fibers 55, 56, 57, 58, the plane 62 of fibers being represented frontally on the Figure 1 B. The input fibers 51-54 are arranged parallel to the output fibers 55-58 and facing the plane 62. The network 50 is able to produce any input fibers 51-54 an image disposed in a focal plane which corresponds to any of the output fibers 55-58 depending on the wavelength. An incident ray 60 containing information transmitted by one of the input fibers from a transmitting user is thus transformed into a returned ray 61 by means of the lens 59 and the grating 50 which is directed to a receiving user by one of the output fibers. Depending on the wavelength of the transmitted signal, any one of the four transmitting users may communicate with any one of the four receiving users.

 The wavelength-to-pass relationships of any of the input fibers to any of the output fibers can be schematized in a 4 × 4 matrix of the type shown below. The lines correspond to input fibers 51-54 and columns to output fibers 55-58. All communications across the 4 x 4 router require 7 wavelengths Xi, i ranging from 1 to 7, ranked in ascending order.

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<tb> 2 <SEP> k6 <SEP> B5 <SEP> x4 <SEP> x3 <SEP>
<tb> 3 <SEP> A5 <SEP> A4 <SEP> A3 <SEP> x2 <SEP>
<tb> 4 <SEP> A4 <SEP> x <SEP> A2 <SEP>
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The realization of an elementary nxn router, similar to that described, with any n, can be carried out as for the 4 x 4 router. In this case, if the fibers are identical and periodically spaced, 2n-1 wavelengths of transmission are necessary.

 The diffraction grating 50 may be replaced or supplemented by a set of multi-dielectric filters, or alternatively by a set of phase gratings (phased array grating).

 In addition, instead of fibers, one or more laser arrays can be used as input switching means, and one or more photodetector arrays as output receiving means.

 A single input and output fiber array may also be employed.

In a particular embodiment of an nxn router, the latter comprises a mirror and a diffraction grating, the mirror, the grating and the fibers being fixed in a solid silica block. An array of input fibers and a strip of parallel output fibers are placed in front of a photoengraved slot on a plane reflection grating perpendicular to grooves. A concave mirror is adapted to transform a diverging beam from any of the input fibers into a parallel beam. The network is itself able to transform this beam by dispersing it angularly towards the concave mirror. This beam then has an image formed at one end of one of the output fibers at a position which depends on the wavelength. This configuration is aplanatic, afocal and has an amplification of 1. Thus, all angles to and from the fibers being identical, the best conditions are obtained with a high coupling efficiency,
The achromatism is perfect and the aberrations are practically nil when the mirror is parabolic.

 For the calculation of specifications, the network is advantageously in a Littrow configuration. In addition, an OP (Out of Plane) configuration is preferably used to avoid a direct return to an optical amplifier located at a short distance. In this configuration, the component uses an input fiber array at the focus of the dispersive array device, so that no light can be coupled directly to the output fibers at a wavelength corresponding to a Littrow condition. . The array of output fibers is arranged for this above that of the input fibers.

 An 8 x 8 router referenced 1 according to the invention, shown in Figure 2, comprises eight inputs E1-E8 and eight outputs S1-S8. This 8 x 8 router is intended to transmit optical signals of any input Ei to any output S, in the direction 5, the path of the optical signal being determined by its wavelength. The router 1 comprises two 4 × 4 routers referenced respectively 30 and 40 which are 4 × 4 routers such as the one previously described (FIGS. 1A and 1B).

Optionally, it can also be traditional 4 x 4 routers. The routers 30 and 40 respectively comprise four inputs 3134 and 41-44, and four outputs 35-38 and 45-48. The outputs 35-38, 45-48 of the routers 30 and 40 lead directly to the outputs S1-S8 of the router 1.

 The router 1 also comprises four X achromatic couplers referenced 10, 15, 20, 25 each respectively having two inputs 11, 12; 16.17; 21, 22 and 26, 27, and two outputs 13, 14; 18.19; 23, 24 and 28, 29. The inputs 11,12; 16, 17; 21, 22; 26, 27 of the couplers 10, 15, 20, 25 in X respectively correspond to the inputs E1-E8 of the router 1. Each of the couplers 10, 15, 20, 25 has moreover one of its outputs, respectively 13, 18, 23, 28, connected to one of the inputs, respectively 31, 32, 33, 34, of a first router 30, and its other output, respectively 14, 19, 24, 29, connected to one of the inputs, respectively 41, 42, 43, 44, of the second router 40.

The respective transmission spectra 67 and 68 of the routers 30 and 40, shown in FIG. 3, are in the form of combs with teeth 71-73 and 74-76, respectively. The spectra 67 and 68 being plotted along axes 65 and 66, respectively of wavelengths X and of transmission T, have their teeth 71-76 centered on transmission wavelengths of routers 30 and 40. correspondence matrix previously given, the teeth 71, 72 and 73 of the spectrum 67 are, for example, respectively centered on the wavelengths x 2. A3, wavelengths Xi being ranked in ascending order.Similarly, the teeth 74, 75 and 76 of the spectrum 68 are centered on the wavelengths h'l, X 3
In the exemplary embodiment, the teeth 71-76 have heights approximately equal to and equal to H. Particularities of the spectra 67 and 68 are constituted by the widths L of the teeth 71-76 at mid-height 2 and by the spacings D between the successive transmission wavelengths Xi. As a first approximation, the widths L at mid-height are close for all the teeth1 as well as the spacings D. The ratio R = L is significant of the capacities of the routers 30 and 40. The greater this ratio, the more the capacities of Processing routers 30 and 40 are high.

The routers 30 and 40 are chosen so that their transmission wavelengths Rj and X'i are interspersed. So we have:
x 1 <Xl <Y2 <X2 <Y3 <
In operation, a signal at a given wavelength reaches any one of the inputs Ei of the router 1, then to one of the couplers 10, 15, 20, 25. The signal is then transmitted to the two outputs of this coupler with in each an energy divided by two, the couplers being achromatic. The optical signal thus arrives at an input of each of the routers 30, 40. Since the wavelength of the signal corresponds to a transmission wavelength of only one of the spectra 67, 68 of the routers 30, 40 , the signal is transmitted only in this router, to the appropriate output selected according to the wavelength. The output Sj referred to is thus reached by the signal.

The router 1 thus has a global transmission spectrum which jointly admits the teeth 71-76 of the two spectrums 67 and 68 of the routers 30 and 40. At constant half-height, equal to L,
The spacing D inter-wavelengths of transmission is thus substantially reduced, this reduction being up to a factor of 2.

For a given available spectral band, the number of transmission wavelengths can therefore be multiplied by 2, and consequently also the number of users. The ratio R can itself be multiplied by 2, which makes it easier to select a transmission wavelength.

 A second embodiment of an N x N router according to the invention is provided by a 12 x 12 wavelength router referenced 2, shown in FIG. 4. This router has twelve P1-P12 inputs and twelve P13 outputs. -P24. It also comprises four routers 3 x 3 of wavelength referenced 130, 140, 150, 160, each having three inputs 131-133, 141-143, 151-153, 161-163 and three outputs 134-136, 144 respectively. -146, 154-156 and 164-166. The outputs of the routers 130, 140, 150, 160 respectively correspond to the outputs P13-P24 of the router 2.

 The router 2 also includes three achromatic couplers 4 x 4, referenced 80, 90 and 100. These couplers each have four inputs, respectively 81-84, 91-94 and 101-104 and four outputs, respectively 85-88, 95-98 and 105-108. The inputs of the couplers 80, 95, 100 respectively correspond to the inputs P1-P12 of the router 2, and the four outputs of each of the couplers 80, 90, 100 are each connected to an input of one of the routers 130, 140, 150, 160.

 Each of the couplers 80, 90, 100 is actually made up of four X-couplers. Thus, as can be seen in FIG. 5, the coupler 80 comprises four 2 × 2 couplers 111-114 each comprising two inputs, respectively 81, 82; 83, 84; 125, 126; 127, 128 and two outputs, respectively 121, 122; 123, 124; 85, 86; 87, 88. Two of the X-couplers, 111, 112, are arranged at a first level, their inputs 81-84 being those of the coupler 80. The two other X-couplers, 113 and 114, are arranged at a second level, their outputs 85-88 being those of the coupler 80. In addition, the outputs 121124 of each of the two couplers 111, 112 of the first level are each connected to an input 125-128 of one of the couplers 113, 114 of the second level. The other two 4 x 4, 90 and 100 couplers are made in a similar manner.

Router 2 furthermore comprises 24 Y achromatic couplers referenced 120, each of them comprising a main arm 115 and two secondary arms 116 and 117. Each of the inputs P1-P12 and outputs
P13-P24 of the router 2 is connected to the main arm 115 of one of the Y couplers 120. The couplers 120 are intended to serve as transmission means, in two directions of propagation, the two branches 116, 117 of each of the couplers 120 being provided for opposite directions 118, 119 of signal propagation. The router 2 is thus intended for signal transmissions both in one direction 3 of the inputs P1-P12 to the outputs P13-P24, and in an opposite direction.

Routers 130, 140, 150, 160 have transmission spectra 175, 180, 183, 186 having comb shapes having teeth, respectively 176-178; 181,182; 184.185; 187, 188, centered on wavelengths # 1, X3; 2; X'2;2; X "2; 2 X"'2
The teeth of the four spectra 175, 180, 183, 186 of the routers 130, 140, 150, 160 are alternated, that is to say that we have:
xi <x'1 <Y'1 <Y "1 <Y2 <Y'2 <Y112 <X3
In operation, a signal at a given wavelength reaches one of the inputs of one of the couplers 80, 90 or 100 and is transmitted to the four outputs of this 4x4 coupler with a reduction of energy by a factor of 4. The optical signal is thus transmitted to an input of each of the four routers 130, 140, 150, 160. Since the wavelength of the signal is only a transmission wavelength of only one of the spectra 175, 180, 183, 186 of the 3 x 3 routers, the transmission of the signal takes place only in this router, to the selected output according to the wavelength of the signal.

 The router 2 therefore has a general transmission spectrum having all the teeth 176-178, 181, 182, 184, 185, 187, 188 of the spectra 175, 180, 183, 186 of the four routers 130, 140, 150, 160 The width at mid-height of the teeth is thus preserved, and the spacing of the transmission wave lengths is substantially reduced, this reduction being able to reach a factor of 4. Moreover, signals can reach P13-P24 outputs simultaneously at the same time. inputs P1-P12, according to the same principle. The same wavelength can thus be used in the two directions of communication between two users, which doubles the capacity of the router 2.

 The energy losses of routers 1 and 2 are low, respectively corresponding to twisting factors of 2 and 4. The presence of optical amplifiers in the switching devices compensates for the effects of these losses. It is however possible to make the losses due to the couplers negligible, by associating the couplers with wavelength selection means. These means can be obtained, for example, by a two-wave interferometer, such as a Michelson interferometer which can be arranged in an integrated optical wafer. In this case, the optical signal output from one of the couplers is transmitted selectively only to the router having a transmission wavelength corresponding to the wavelength of the signal.

Although the teeth of the separate nxn router combs have been shown dissociated, they may also overlap,
The important being the centering of each of the teeth on a given transmission wavelength. In the examples shown, spectra were shown whose teeth of the different routers are intercalated. However, any other means of obtaining complementary spectrum coverage may be suitable. In particular, the alternation of spectral components from one router to another may relate to bands and not to teeth, each band comprising several teeth. This last configuration is advantageous especially if the inter-wavelength spacing of transmission is irregular, the teeth being grouped in packets.

 It will be understood that the 4 × 4 and 3 × 3 elementary routers used respectively in routers 1 and 2 may themselves be routers according to the invention.

 This possibility is of particular interest in the presence of a greater number of inputs and outputs, the invention being cascadable on two or more levels.

 The router of the invention can be realized in a technology based on optical fibers or in integrated optics.

Example 1
In this first example of application, the router according to the invention is a 40 x 40 router comprising two routers 20 x 20 and 20 X achromatic couplers. Each of the 20 x 20 routers corresponds to a silica component, with a focal length of 119.512 mm, a network of 600 lines / mm, with a double fiber bar fiber holder spaced 42.54 lit. The first router is self-collimating at the center of the field at 1548.5145 nm, and the second router has a half-period offset spectrum due to autocollimation at 1548.5145 + 0.4 nm.

A double bar of 20 fibers equally spaced is disposed at the focus.

 The transmission wavelengths of the first 20 x 20 router range from 1533.2 nm to 1563.9 nm with a wavelength inter-wavelength spacing of 0.8 nm.

The spectra of the 20x20 routers are such that the half-height width of the teeth, which have approximately a Gaussian shape, is 0.25 nm + 0.02 nm. In addition, the 20x20 routers have the following properties:
- losses of 4 dB + 1 dB;
- intrinsic crosstalk <- 55 dB from one channel to another
-back loss <- 25 dB;
- polarization sensitivity <1 dB;
- network pulse width equal to 38.6 ps; - wavelength channel accuracy + + 0.05 nm.

 The spacing between transmission wavelengths (0.8 nm) is chosen constant, which avoids additional losses of up to 3 dB.

 The 20x20 routers are compatible with 10 Gbit / second rates on each channel.

 The 20x20 wavelength matrix of the first 20x20 router was calculated by taking into account the exact silica index of each wavelength, by iterative adjustment to reference values. The matrix obtained is represented in the appendix, the columns corresponding to the inputs, and the 20 lines to the outputs. The values indicated are expressed in nm. The lack of adjustment remains below 0.008 nm, which is negligible.

Example 2
An 80x80 router includes two 40x40 routers and 40 X achromatic couplers.

The 40 x 40 routers verify the following specifications:
- inter-wavelength spacing of transmission: 0.4 nm
- width at mid-height of teeth: 0.25 nm + 0.03 nm - losses <7 dB 12 dB;
- crosstalk from one channel to another <- 60 dB
These 40 x 40 routers are compatible with 10 Gbit / s on each channel.

Example 3
A 6x6 router has two 3x3 routers made using 6 multiplexers, each made of shared optical function technology with a focal length of 119.512 mm, a fiber space of 32 μm and a network of 300 lines / mm.

 The multiplexers of the first 3 X 3 router are associated with the wavelengths 1552.517 nm, 1550.916 nm and 1549.315 nm. Those of the second 3 x 3 router, at the same wavelengths shifted by + 0.8 nm.

 The 6x6 router also features 3 x fused coupler couplers.

ANNEX:

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Claims (11)

 1. Router (1, 2) N x N wavelength having N inputs (E1-E8, P1-P12) and N outputs (S1-S8, P13-P24), for transmitting optical signals each having a wavelength, inputs to the outputs, the N x N router (1, 2) also comprising switching means capable of switching each of the optical signals of any of the N inputs to any of the N outputs as a function of the wavelength of said signal,  characterized in that the switching means comprise:  n mx m couplers (10, 15, 20, 25, 80, 90, 100) each having m inputs (11, 12, 16, 17, 21, 22, 26, 27, 81-84, 91-94, 101 -104) and m outputs (13, 14, 18, 19, 23, 24, 28, 29, 85-88, 95-98, 105-108), N being equal to nxm and the N inputs of the n couplers mx m (10, 15, 20, 25, 80, 90, 100) being the inputs (E1-E8, P1-P12) of the N x N router (1, 2),  nxn routers (30, 40, 130, 140, 150, 160) each having n inputs (31-34, 41-44, 131-133, 141-143, 151-153, 161-163) and n outputs ( 35-38, 45-48, 134-136, 144-146, 154-156, 164-166), each of said n inputs being connected to one of the outputs of, respectively, the n mxm couplers (10, 15, 20, 25, 80, 90, 100) and the N outputs of the routers nxn (30, 40, 130, 140, 150, 160) being the outputs (S1-S8, P13-P24) of the router N x N, each of the nxn routers being able to steer an optical signal having a wavelength, of any of its n inputs to any of its n outputs, depending on the wavelength of said signal, said router nxn (30, 40, 130, 140, 150, 160) having a transmission spectrum (67, 68, 175, 180, 183, 186) as a function of the wavelength in the form of a comb, the combs m nxn routers being different from each other.  2. Router N x N according to claim 1, characterized in that the combs (67, 68, 175, 180, 183, 186) of the routers nxn (30, 40, 130, 140, 150, 160) have teeth (71-76, 176-178, 181, 182, 184, 185, 187, 188) included in intercalated strips.  3. N x N router according to claim 2, characterized in that the combs (67, 68, 175, 180, 183, 186) of the nxn routers (30, 40, 130, 140, 150, 160) have their teeth. (71-76, 176-178, 181, 182, 184, 185, 187, 188) intercalated.  4. Router N x N according to any one of the preceding claims, characterized in that the combs (67, 68, 175, 180, 183, 186) of the routers nxn (30, 40, 130, 140, 150, 160) have teeth (71 76,176-178,181,182, 184,185,187,188) completely dissociated.  5. N x N router according to any one of the preceding claims, characterized in that each of the mxm couplers (10, 15, 20, 25, 80, 90, 100) is an achromatic coupler with a uniform distribution of energy between its outputs (13, 14, 18, 19, 23, 24, 28, 29, 85-88, 95-98, 105-108).  6. Router N x N according to any one of claims 1 to 4, characterized in that each of the couplers mxm (10,15, 20, 25, 80, 90, 100) is a coupler with wavelength distribution capable of transmitting energy of an optical signal having a wavelength, only by the output of said mxm coupler connected to the router nxn (30, 40, 130, 140, 150, 160) whose comb (67, 68 , 175, 180, 183, 186) comprises said wavelength.  7. N x N router according to any one of the preceding claims, characterized in that the mxm couplers (10, 15, 20, 80, 90, 100) are composed of 2x 2 couplers (10, 15, 20, 111- 114) each having two inputs (11, 12, 16, 17, 21, 22, 26, 27, 81-84, 125-128) and two outputs (13, 14, 18, 19, 23, 24, 28, 29 85-88, 121-124).  8. Router N x N according to claim 7, characterized in that it comprises a first level of 2 x 2 couplers (111, 112) whose inputs (81-84) are the inputs (P1-P4) of the router N x N (2), and a last level of 2 x 2 couplers (113, 114) whose outputs (85-88) are connected to the inputs (131, 141, 151, 161) of the routers nxn (130, 140, 150 160), the inputs (125-128) of the 2x2 couplers (113, 114) which do not belong to said first level being connected to outputs (121124) of 2x2 couplers (111, 112), and the outputs (121-124). 2 x 2 couplers (111, 112) which do not belong to said last level being connected to inputs (125-128) of 2 x 2 couplers (113, 114).  9. Router N x N according to any one of the preceding claims, characterized in that it comprises Y couplers (120) each having a main arm (115) and two secondary arms (116,117), the inputs (P1- P12) and the outputs (P13-P24) of the N x N router (2) being connected to the main arms (115) of said Y couplers (120), said N x N router (2) for sending optical signals simultaneously from its inputs to its outputs and from its outputs to its inputs, said optical signals traveling in opposite directions (3, 4) in the two secondary arms (116, 117) of each of the Y couplers.  10. Router N x N according to any one of the preceding claims, characterized in that each router n x n comprises:  a transmission medium comprising a first row of n inputs and a second row of n outputs, the first and second rows being arranged in parallel and the transmission medium having a flat transmission surface,  a diffractive element disposed facing said transmission surface,  a focusing optics placed between the transmission medium and the diffracting element, The diffractive element and the focusing optics being capable of producing any of the inputs, an image corresponding to any one of the outputs, as a function of the wavelength of said optical signals.  11. Communications network comprising at least one router N x N (1, 2) according to any one of claims 1 to 10.