FR2839160A1 - OPTICAL FILTERING DEVICE PROVIDING A PROGRAMMABLE DIFFRACTIVE ELEMENT, SPATIAL ROUTER OF SPECTRAL BANDS, AND CORRESPONDING CHROMATIC DISPERSION COMPENSATION DEVICE - Google Patents

Abstract

The invention relates to an optical filtering device comprising at least one input optical fiber and at least one output optical fiber. According to the invention, such a device comprises transfer means, to at least one of said output optical fibers , at least one spectral band of at least one signal with multiple wavelengths incident by at least one of said input optical fibers, said transfer means implementing at least one programmable diffractive element located on an optical path between said input optical fiber (s) and said output optical fiber (s). The invention also relates to a spectral band router and a chromatic dispersion compensation device using such a filtering device.

Description

less than 500 mm / minute.

1 2839160

  Optical filtering device implementing a programmable diffractive element, spectral band spatial router and device

  corresponding chromatic dispersion compensation.

  The field of the invention is that of optical fiber telecommunications. More specifically, the invention relates to a technique for producing tunable optical filters, in particular used in the design of devices.

  spectral band routing and chromatic dispersion compensation.

  Spectral band routing and chromatic dispersion compensation functions are of particular importance in the implementation of

  the work of new generation optical communication networks.

  Thus, for example, the routing of spectral bands is essential to share a source at multiple wavelengths between the "hub office" (in French, "concentrator" or "multi-repeater") of a service provider and a service provider. content provider, as described in the article by CF Lam et al. entitled "Programmable optical multicasting in a region: metro area network using a wavelength selective optical cross-connect" (in French) "Programmable optical selective scattering in a region: metropolitan network implementing a wavelength selective optical connection device" ), Proc. ECOC 01

  Amsterdam, October 2001, pages 614-615.

  To date, several techniques for routing spectral bands, and in particular that proposed in the article by J. K. Rhee et al. entitled "Variable Pass-band Optical Add-drop Multiplexer Using Wavelength Selective Switch" (in English "Variable Bandwidth Optical Insertion-Optical Multiplexer, Implementing a Selective Wavelength Switch"), Proc. ECOC 01 Amsterdam, October 2001, pages 550-551. This solution, proposed for implementing wavelength selective switches or variable spectral band insertion-extraction multiplexcurs, relies on a free-space optical configuration, implementing liquid crystal spatial modulators.

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  This solution allows the realization of flat profile filters over a wide spectral band and continuous filtering between adjacent channels. The channels are first demultiplexed using fixed diffractive optics and then imaged on a spatial light modulator (MSL), which acts as a spatial filter of wavelengths. The width and selectivity of the channel are determined by the number of pixels or groups of pixels that are activated. The different channels are then recombined

by reverse process.

  This solution has the disadvantage of requiring the implementation of two distinct optical elements, namely first a fixed diffractive optics performing a demultiplexing operation, then a spatial light modulator,

  which performs a spatial filtering operation.

  The routing device designed according to such a technique is therefore little compact. In addition, the use of several distinct optical elements tends to increase the losses affecting the light signal, and thus to reduce the efficiency

  global routing device thus achieved.

  Another disadvantage of this device is that it does not solve

  accurately the width and spectral selectivity of the different channels.

  Like the routing function, chromatic dispersion compensation is a very important feature of next-generation optical communication networks, especially when the transmission rates envisaged are higher than 10Gbits / s, as explained by V. Srikant in "Broadband dispersion and dispersion slope compensation in high bit rate and ultra haul system "(in French) Broadband dispersion, and compensation of the dispersion slope in

  a very long distance broadband system "), OFC 2001, TuH1-1.

  It is recalled that the problem of chromatic dispersion results from the fact that each light pulse comprises multiple wavelengths, each of these wavelengths having propagation characteristics.

  different in the environment.

  Several chromatic dispersion compensation techniques are already known, some of which rely on the use of

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  negative compensation, for others, on techniques based on the excitation of higher order modes of propagation (in English "High order modes"). There are also several known techniques using fibers based on modulated Bragg gratings (in English "chirped Bragg gratings"), as well as, more recently, solutions using a free space configuration of the VIPA type.

(trademark deposit).

  A disadvantage of these different chromatic dispersion compensation techniques, which mainly apply to transport networks, is that they are not adaptive: in other words, they can not be applied to spectral bands having dispersions. chromatic variables. It therefore appears necessary, both in the domain of spectral band routing and in that of chromatic dispersion compensation,

  to design a technique for adaptive selection of variable spectral bands.

  The object of the invention is in particular to satisfy this need and to overcome

  the various disadvantages of the techniques of the prior art.

  More specifically, an object of the invention is to provide a technique for selecting variable spectral bands and spectral band filtering.

tunable wavelength.

  It is another object of the invention to provide such a technique which can be advantageously used for the design of routing devices.

  spectral bands and / or chromatic dispersion compensation.

  The invention also aims to propose such a routing technique implementing a reduced number of optical elements compared to

prior art techniques.

  Yet another object of the invention is to provide an adaptive chromatic dispersion compensation technique which can be applied to

  spectral bands of variable chromatic dispersions.

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  These objectives, as well as others that will appear thereafter, are achieved with the aid of an optical filtering device comprising at least one optical fiber

  input and at least one output optical fiber.

  According to the invention, such a device comprises means for transferring, to at least one of said optical output fibers, at least one spectral band of at least one signal with multiple incident wavelengths by at least one of said fibers. input optical means, said transfer means implementing at least one programmable diffractive element located in an intermediate plane between

  the at least one input optical fiber and the at least one optical output fiber.

  Thus, the invention provides a completely new and inventive approach to selection and filtering of spectral bands. Indeed, the invention is based in particular on the exploitation of the chromatic dependence characteristics of a programmable diffractive element. It therefore advantageously makes it possible to select any spectral band, centered on a wavelength λ; any incident signal at multiple wavelengths, and transfer it to any output optical fiber of the filter device as well as

  constituted by adequate programming of the programmable diffractive element.

  The invention thus makes it possible, unlike the techniques of the prior art, to perform an adaptive selection of spectral bands that vary in width and wavelength, enabling the design of a wavelength-tunable spectral band filter. The filtering device of the invention also has the advantage, compared to the techniques known from the prior art, of being compact and simple, since it only requires the use of an optical element, know a programmable diffractive element. The use of such a device is particularly flexible and adaptable depending on the characteristics

  the incident signal and the filtering that is desired.

  Advantageously, such a device comprises programming means for modifying, in at least one direction, the period

  space of a pattern of said programmable diffractive element.

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  Preferably, said programming means make it possible to configure said programmable diffractive element so that it has a spatial period P in said at least one direction, so that a spectral band centered on a given wavelength Ni is diffracted in said at least one direction by said programmable diffractive element at an angle θ;

  predetermined such that sini = k, where k is an integer.

  According to an advantageous characteristic of the invention, said programming means provide said spatial period P with a perturbation equivalent to a variation less than the size of a pixel of said programmable diffractive element.

  It is thus possible to obtain an infinitesimal variation of the period of the network.

  According to an advantageous embodiment of the invention, said programming means make it possible to configure said programmable diffractive element so that it has a spatial period P comprising: at least one sub-period comprising N pixcls; at least one subperiod including N2 pixels s,

  o N and N2 are two distinct integers.

  According to a first advantageous variant embodiment of the invention, said

  diffractive element is a programmable digital hologram.

  Advantageously, said programmable digital hologram is displayed on a spatial light modulator with amplitude modulation levels or

  phase, said levels being continuous or quantized.

  Preferably, said spatial light modulator can be

  associated with at least one fixed diffractive element.

  According to a first advantageous embodiment of the invention, such a device comprises a collimating lens, said diffractive element acts in reflection and is located in the image focal plane of said collimating lens, and said at least one optical fiber input and are located in the focal plane obj and the collimating lens, so as to form an optical assembly

  in free space type 4-f folded.

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  According to a second advantageous embodiment of the invention, such a device comprises two collimating lenses, called respectively first and second lenses, said diffractive element is located in the image focal plane of said first lens and in the object plane of said second lens. lens, said at least one optical fiber input is located in the object focal plane of said first lens, and said at least one optical output fiber is located in the image focal plane of said second lens, so as to form a mounting

  4-f free-space optics.

  Advantageously, such a device comprises a matrix of at least two optical output fibers, each of said fibers being characterized by its position with respect to the optical axis of said device, so that said device constitutes a battery of at least two filters. tunable, and includes means for holographic adjustment of the spectral selectivity of each of said filters, as a function of said position with respect to the optical axis of said optical fiber of

corresponding output.

  Advantageously, said output optical fibers are monomode fibers. According to an advantageous characteristic of the invention, at least one of said monomode fibers has at least one lens at its end,

  to form a lenticular monomode fiber.

  Preferably, said lens comprises at least one fiber section to

  index gradient reported by assembly and fracture.

  Preferably, said lens further comprises a section of silica fiber between said monomode fiber and said section of gradient fiber

  of index reported by assembly and fracture.

  Advantageously, such a device comprises means for adjusting a

  filtering mask applied to at least one of said wavelengths.

  Advantageously, said filtering template is superimposed on said element

programmable diffractive.

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  Thus, a windowing function is superimposed on the diffractive element.

  The action of such a windowing function is a spatial disagreement of the incident mode on the fiber, having repercussions on the wavelength selectivity of the latter. According to an advantageous variant, said filtering template is included in

  said programmable diffractive element.

  The corrective window is then part of the digital hologram.

  The invention also relates to a spectral band router comprising at least one optical device as described above, the or

  said devices comprising at least two optical output fibers.

  Preferably, in such a spectral band router according to the invention, said diffractive element is dynamically configurable so as to route at least two distinct spectral bands of at least one incident signal,

  respectively to separate output optical fibers Fj.

  Advantageously, said programming means make it possible to configure said programmable diffractive element, so that said programmable diffractive element has, in said at least one direction, a spatial period P corresponding to the combination of a plurality of spatial periods Pi, o each of said spatial periods Pi is such that, when said programmable diffractive element has said spatial period Pi, a spectral band

  centered on j is transferred to said output optical fiber Fj.

  Preferably, said output optical fibers are located on an isochromatic circle, so that the routing of said spectral bands

  is made with constant bandwidth.

  The invention also relates to a dispersion compensation device

  chromatic, comprising an optical device as described above.

  Preferably, at least one of said output optical fibers is

  connected to at least one fiber section with negative chromatic compensation.

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  In a first advantageous embodiment of the invention, said sections of fiber with negative chromatic compensation have an end

reflec hi ss ante.

  According to a first embodiment of the invention, said optical output fibers are located on an isochromatic circle. According to a second variant embodiment of the invention, said optical output fibers are located on at least two distinct isochromatic circles. In a second advantageous embodiment of the invention, said optical fiber section with negative chromatic compensation is connected, at a first end, to a first output optical fiber, and, by a

  second end, to a second output optical fiber.

  Preferably, said first and second output optical fibers

  are two diametrically opposed fibers of a circle of isochromatism.

  Other features and advantages of the invention will appear more

  clearly on reading the following description of an embodiment

  preferred, given by way of a simple illustrative and nonlimiting example, and the appended drawings, among which: FIG. 1 shows a syroptic of an optical filtering device according to the invention, which can be applied, for example, to the routing of bands; spectral or chromatic dispersion compensation; FIG. 2 illustrates measurement results of the bandwidth of the various filters of the device of FIG. 1; FIG. 3 shows two exemplary patterns of the programmable diffractive element of the device of FIG. 1; FIG. 4 illustrates the coupling intensity in an optical fiber output of the device of the invention, as a function of the wavelength, with the two exemplary patterns of FIG. 3; FIG. 5 illustrates the bandwidth of a filter of the device of FIG. 1, as a function of the number of pixels per period of the programmable diffractive element; FIG. 6 shows an example of positioning of the output optical fibers with respect to the optical axis of the device of the invention; FIG. 7 shows an exemplary routing of spectral bands from the optical filtering device of the invention; FIGS. 8a and 8b illustrate, in the form of result curves, the routing operation of FIG. 7; FIGS. 9 and 10 illustrate a first variant embodiment of a chromatic compensation device of the invention, comprising an optical filtering device of FIG. 1, and implementing sections of fibers with negative compensation having a reflecting end. ; FIG. 11 shows a second alternative embodiment of a chromatic compensation device according to the invention, comprising an optical filtering device of FIG. 1, and implementing negative compensation fiber sections connected to two output fibers. order

symmetrical diffraction.

  The general principle of the invention is based on the implementation of a periodic diffractive element (of the phase grating type) or of a thin digital hologram used for the deflection of an incident light beam, which is advantageously exploited. chromatic dependence, in order to achieve a

tunable filtering device.

  With reference to FIG. 1, an embodiment of an optical filtering device of the invention, allowing an adaptive selection of

spectral bands.

  In a preferred embodiment of the invention, use is made of a dual diffraction imaging assembly 1 (also called a 4-f assembly) comprising: a programmable digital hologram 2; an input optical fiber 3;

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  a matrix 4 of optical fibers at the output;

- a collimating lens 5.

  The programmable digital hologram 2 is located in the Fourier plane (2f), that is to say in the focal plane of the collimating lens 5. The matrix 4 of the output optical fibers is arranged in the focal plane of the collimator lens, symmetrically with the digital hologram 2, as shown in Figure 1. The operating principle of the assembly of Figure 1 is to exploit the chromatic dependence of the periodic diffractive element 2 (which can be for example a network phase or a thin digital hologram), when it is used for the deflection of an incident beam by the fiber

optical input 3.

  In other words, we consider the signal at multiple wavelengths 6, incident by the input optical fiber 3. This signal 6 comprises a plurality of distinct components of respective wavelengths j, where i varies from 1 to N. The device of FIG. 1 makes it possible to select any spectral band of this signal 6, centered for example on a wavelength λ, and to transfer it to any one of the optical fibers of the output matrix 4. In the particular example of FIG. 1, it is sought to transfer this spectral band to the fiber

output optical referenced 7.

  In a preferred embodiment of the invention, it is limited to the study of the diffractive properties of the hologram 2 to the first order. However, higher orders (although less energetic than the first order) can also be used because they have the advantage of offering greater angular dispersion. The addressing of an output fiber of the matrix 4 is done by imposing the diffraction angle of the spectral band considered by

the hologram 2.

  The value of this diffraction angle is given, as a function of the central wavelength of the spectral band considered and the spatial period of

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  hologram 2, by the following equation of rescals, valid for a signal at normal incidence on the diffractive element 2: PsinO = k; (1) \, P. 0, and k are respectively the central wavelength of the selected spectral band, the spatial period of grating 2, the angle and the diffraction order. By derivation, we deduce the chromatic dependence of the diffractive network 2 considered:

= = (2)

  And X and H are respectively the chromatic and angular dispersions of the diffractive element 2. It will be noted that the angular dispersion 60 is greater for the small spatial periods d of the grating 2 (that is to say for the large frequencies

  of the diffractive element 2).

  The output fibers of the matrix 4, depending on their opening

  digital, constitute a plurality of spectral band filters.

  These fibers 4 may be conventional mono-mode fibers and / or fibers with extended heart. These aspects will be presented in more detail later in this document. The output fibers of the matrix 4 are characterized by their position relative to the optical axis 8 of the device of FIG. 1. The injection rate of a signal in an optical fiber depends on the following relation: (2w, exp-25 with 5r = w (3) where wr represents the width of the incident beam (or "waist", in French "col"), w0 represents the width of the mode of the fiber, and represents the distance between the center of the incident optical beam and the

  center of the heart of the fiber, still called misalignment.

  The width of the bandwidth of the different filters of the filtering device of FIG. 1 will thus depend on the position of the corresponding output optical fiber of the matrix 4, located more or less far from the optical axis 8 of the system, and the value of for a given wavelength. This dependency

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  is illustrated by the experimental results of FIG. 2, which show that the more the output optical fiber considered is remote from the optical axis of the system ("Out

  # 14 "), and the bandwidth of the corresponding filter is narrow.

  The principle of the device of FIG. 1 consists, in order to produce wavelength-tunable filters from the output optical fibers of the matrix 4, to scroll the wavelength spectrum of the incident signal 6 in front of the

  selected optical output fiber 7.

  In view of the small size of the core of the output fiber 7 (typically of the order of 10 μm), the device 1 must allow sub-micron displacements of the beam in front of the optical fiber 7, so as to allow tuning.

  precisely the bandwidth of the corresponding filter.

  These sub-micron displacements are advantageously made possible, in the device 1, by the use of a digital hologram or a thin diffractive grating 2. Under these conditions, the deflection angle of the

  the incident is determined by the resolution of the diffractive element 2, i.e.

  say by the size of the smallest element of the hologram or echo 2 independently adjustable. More precisely, if p is the size of a pixel of the diffractive element 2, and assuming a plane incident wave, the diffraction angles θ; (i = x, y in the direction of the abscissae and ordinates) of this wave on the diffractive element 2 are given by equation (1) above which is expressed as: sine = k (4)

  o Nj is the number of pixels used per period of the network in dimension i.

  In a preferred embodiment of the invention, it is limited to the study of the first diffraction order in paraxial regime (that is to say in the case ok = 1 and o the diffraction angles Oi are of low value ). For Oi close to zero, the previous expression (4) is simplified to: 0i = (5)

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  The angle of deflection D; can be a priori modified by a variation of the two adjustment parameters that are the wavelength and the average number of pixels per space period Ni: 1 oi éN (6) A constant wavelength (X = 0), a "digital" correction is made: 6ei = Ni éNi (5) The smallest physical variation between two different networks 2 corresponds to one additional pixel per spatial period, ie to éNj = 1 in

equation (5) above.

  However, to allow a finer tuning of the diffraction angle associated with a given wavelength, the inventors of the present application have contemplated using disturbances of the complete pattern of the pattern of the grating, in order to

  to obtain variations ÀNj smaller than the pixel.

  An exemplary embodiment of such a disturbance is illustrated in FIG. 3, which presents two almost identical patterns Gl and G2 of the network 2, in a

direction given.

  The pattern G 1 of the diffractive element 2 is characterized by a number of pixels Ni per period in the direction i, which produces a diffraction angle Bi =, for a given wavelength. (Remember that p is the size of a pixel

of the diffractive element 2).

  The pattern G2 of the diffractive element 2 is produced by the periodic sequence of sNi + 1 pixels in the direction i, comprising s-1 subperiods containing N pixcls followed by a period containing Ni + 1 pixels. The average number of pixels per period for G2 is therefore equal to NiX (S-1) + (Ni + 1) N 1

  diffraction angle =, for a given wavelength.

  The corresponding angular difference between the two patterns G1 and G2 of the diffractive grating 2 is therefore equal to: GI FG2) = SNi + 1 (6)

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  The increase in the angular resolution of the device of FIG. 1, induced by the passage of the pattern G to the pattern G2, compared with the value given by the relation (5) with Ni = 1, can be measured by the following ratio: (G,} (G2) + Ni s (7) The smallest value of the oversampling factor s associated with a disturbance of the complete form of the pattern G of the network 2 is sn, n = 2. The value of s is in in addition to the NpiX number of pixels of the programmable thin diffractive element 2 per dimension at SmaX = NpiX / 2Ni For characteristic values of NpiX = 1000 and Ni = 10, the gain in

  angular resolution, induced by the passage of the pattern G. to the motif G2 is R = 50.

  According to the method described above, it is therefore possible to move, in all directions, the light beam infinitesimally in front of the fiber 7, by simple configuration of the pattern of the programmable diffractive element 2, for example by loading the pseudo-periods. corresponding to the pattern G2 in the programmable digital hologram 2. The output optical fiber 7 then becomes a band filter, centered on a wavelength λ 2. This wavelength / 2 is shifted with respect to the corresponding central / wavelength obtained previously with the pattern G to evaluate the wavelength shift of the passband of the filter 7, induced by the passage of the pattern G on the pattern G2, suffice to note that the diffraction angle Oi must be preserved during the transition from G to G2 (the addressed optical fiber remains the output fiber referenced 7 during the configuration of the hologram 2), which leads to: / 2- = sN (8) The results of FIG. 4 illustrate this wavelength shift induced by the

  passing a pattern G. characterized by Ni = 14 to a pattern G2 characterized by s = 10.

  For a spectral band filter 7 centered, before reconfiguration of the hologram, on a wavelength λ = 1548 nm, a wavelength shift equal to X2 - \, | = 1548 nm / 140 = 11 nm. After reconfiguring the pattern

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  of the hologram, the output optical fiber 7 therefore constitutes a band filter

  centered on a wavelength i2 = 1537 nm.

  If a single-mode fiber array is used to design the matrix

  4 of output optical fibers, a battery of tunable filters is thus produced.

  The spectral selectivity of each of these tunable filters is a function of the offsetting of the corresponding output optical fiber with respect to the axis

optical system.

  Indeed, by exploiting the chromatic dependence of the diffractive element 2 with respect to the positioning of the output optical fibers of the matrix 4 in the imaging plane, it is possible to express the spectral selectivity of each of the tunable filters in accordance with FIG. one of the number of pixels per spatial period of the diffractive element 2. The mode imaged by the diffractive element 2 is centered in the imaging plane at a distance Xj from the optical axis, corresponding to the distance from the center the optical fiber output 7 to the optical axis of the device 1, we have: tan (Oi) = X '/ f (9) In paraxial regime tan (Oj) = ;, and for a given configuration of the diffractive element 2, 0N, = 0. By derivation of equation (9), we obtain: oXi = kf / (Np) o (10)

  o p is the size of a pixcl of the hologram 2.

  If we fix the losses, due to the wavelength shift, to a value, u and for a given coupling value in dB, we have: dil = N (11) ow denotes the "waist" (or in French collar) of the optical fiber mode of

output 7 considered.

  The bandwidth of an output fiber is therefore given by o (, u = -3dB) = 1.66 Nw. We therefore note that the smaller the pitch of the network 2, the more the network is dispersive and the bandwidth of the tunable filter , scaled down. For values of p = 10-s m, w = 5.10-6, f = 40.5.10-3 m and Nj = 2, we obtain: o = 4.1 nm. This value can easily be reduced by taking the

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  higher orders (that is for k> 1) but in return, the penalty in

energy will be important.

  The curve of FIG. 5 illustrates the evolution of the passband ante of a tunable filter of the invention, as a function of the number of pixels per spatial period of the programmable diffractive element 2. The template of the filter can be adjusted (in terms of uniformity and slope), by superimposing a windowing function on the network displayed on the programmable diffractive element. Such a method has notably been proposed, in the case of fixed elements, by JP Laude and S. Louis in "A new Method for Broadening and Flattening the Spectral Shape of Transmission Channels of WDM Multiplexers and Routers", OECC'98, Techn. . Digest, pp. 522-532, Chiba, Japan, July 1998 (in French "a new method to broaden and flatten the answer

  spectral transmission channels of multiplexers and routers WDM ").

  The choice of the output optical fibers of the matrix 4 is an important parameter of the design of the device 1 of the invention. Indeed, as explained above, these optical fibers act as spectral filters, the width of which

  the bandwidth is in particular a function of the size of the core of the fiber.

  The use, in the matrix 4, of monomode lentillated fibers of the type of those described in patent document FR 2,752,623, entitled "Method of manufacturing a collective optical coupling device and device obtained by such a method ", further allows to achieve, according to the technique of the invention, very varicose filter banks. Indeed, it is possible, for example, to construct spectral filters having bandwidths of between 100 nm and 2 nm, while maintaining deflection angles of the beam on the diffractive element 2 which are small (typically less than 3), the value of which is mainly limited by the

  resolution of the programmable digital hologram 2 used.

  It will be recalled that these optical fibers consist of conventional monomode fibers, at the end of which, by assembly and fracture, a segment of graded-index fiber is reported, and possibly a fiber section in which

  silica, in order to achieve a single-mode optical fiber with extended core.

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  These optical fibers have the advantage of allowing diameters of modes ranging from less than 5 μm to several tens of microns, while

  retaining the properties of a single mode fiber.

  These output optical fibers may be arranged, within the matrix 4, according to the diagram of FIG. 6. In this diagram, the matrix 4 comprises 7 optical output fibers 12, which are distributed over two circles of isochromatism respectively referenced 10 and 11. All the optical fibers 12 located on the same circle of isochromatism are located at the same distance from the optical axis 8 of the system: they constitute

  therefore tunable filters of the same spectral band.

  FIG. 7 is an example of an application of the filtering technique described above in connection with FIGS.

  Figures 1 to 6, to the realization of a routing device.

  The device 1 can be completed, in a variant of the invention, to achieve, in addition to the filtering function described above, a routing function for the realization of selectors-routers spectral bands. This embodiment advantageously exploits the property of the

  Digital holograms can address multiple fibers simultaneously.

  For the sake of simplification, a description will now be given of a simple embodiment, in which it is sought to perform a spectral band switching between two output fibers of the device 1. Thus, in a first given configuration of the diffractive element 2, a fiber 71 constitutes a spectral band filter centered on a wavelength λ, and a second fiber 72 constitutes a spectral band filter centered on a wavelength λ 2. It is sought to configure the device 1, so that it routes the spectral band of wavelength X to the second output optical fiber referenced 72, and the

  spectral band centered on \ 2 towards the first fiber referenced 71.

  The functionality of the routing device is of course not limited to the simple permutation of two spectral bands, but is extended to all

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  routing operations that can be performed given a signal incident to

  multiple wavelengths vi, and a plurality of output optical fibers.

  The principle described below in relation to FIG. 7 can therefore be generalized to several fibers, subject to additional power losses induced by the holographic coding. Considering the pattern R1 of the diffractive grating 2, which makes it possible, according to the technique described above in connection with FIG. 3, to switch to the

  fiber referenced 71 a spectral band centered on the wavelength \.

  The pattern R2 of the diffractive grating 2 is also considered, which makes it possible to switch to the fiber referenced 72 a spectral band centered on the length

of wave \ 2 OR \ 1 <72.

  The programmable digital hologram 2 can then be configured so that it corresponds to the combination H of these two network patterns R1 and R2, and thus realizes a routing of the band X1 towards the first reference fiber. 71 and the band X2 to the second fiber referenced 72. Such a combination corresponds to the addition of the two reflection coefficients AAA networks R1 and R2, R = R1 + 2, accompanied by a suitable holographic coding C, operation symbolized by H = CR. This operation is equivalent to a "bridging" operation except that the spectral bands transmitted on

each fiber are different.

  When the fibers referenced 71 and 72 are located on a circle of isochromatism (for example the circle referenced 10 of FIG. 6), the operation of

  routing is then done at constant bandwidth.

  We now try to swap these two spectral bands \ and \ 2. To do this, the pattern disturbance method of the network 2 described is used.

previously with Figure 3.

  In other words, the pattern R1 is slightly modified (using a

  associated pseudo-period R1 ') so as to shift the spectrum to the left (ie

  that is to say in the direction of decreasing wavelengths), according to the arrow referenced 73 of FIG. 7. Thus, the incident spectral band is shifted in wavelength

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  on the fiber referenced 71, so that it is now centered on 72. Similarly, the pattern R2 is modified slightly (using an associated pseudoperiod R2 '), so as to shift the spectrum to the right (ie that is to say in the direction of increasing wavelengths), according to the reference arrow 74 of FIG. 7. It is then possible to form the hologram H ', resulting from the composition of these two pseudo-periods R1' and R2 ', which gives the expected result, as illustrated by FIG. 7: the spectral band centered on X2 is now transmitted to the optical fiber referenced 71, and the spectral band centered on X1 is transmitted to the optical fiber referenced 72. This is done a wavelength routing operation by simply changing the configuration of

  the programmable hologram 2 from H to H '.

  As mentioned before, this operation can of course be extended

  in the case of a plurality of output optical fibers.

  FIGS. 8a and 8b show the curves representative of the losses in the output optical fibers 71 and 72, as a function of the wavelength, respectively for the H and H 'hologram configurations. In FIG. 8a, the bandwidth of referenced fiber 71 is centered at 7 = 1530 nm, and the bandwidth of referenced fiber 72 is centered at approximately λ = 1s5O nm. After reconfiguration of the hologram 2 according to the pattern H ', the fiber referenced 72 has a spectral band centered on X = 1530 nm, and the referenced fiber 71,

  a spectral band centered on 2 = 1550 nm, as illustrated by FIG. 8b.

  FIGS. 9 to 11 show, in connection with FIGS. 9 to 11, examples of application of the tunable spectral band filtering technique of the invention to the production of a chromatic dispersion compensation device. In such a chromatic dispersion compensation device of the invention, the output optical fibers of the matrix 4 of the filtering device 1, operating as spectral band filters, are connected to sections of special fibers with negative chromatic compensation. These fiber sections can

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  have different characteristics in terms of slopes or spectral bands. The value of the chromatic compensation induced by such fiber sections is determined by their length, as explained in the article by M. Hirano et al. titled "Dispersion Compensating Fiber Over 140 nm Bandwidth" (in English "Compensation Fiber with Bandwidth Dispersion greater than

nm "), ECOC October 2001.

  The choice of the compensating optical fiber and its length are conditioned by the quality factor Q that is sought to obtain, defined by the following equation: Q = D / oc (11) where D is the dispersion (in ps / nm / km) and ot attenuation (in dB / km and function of

  of the signal conveyed by the compensation fiber.

  FIGS. 9 and 10 illustrate a first variant embodiment of a chromatic dispersion compensation device, in which the sections of FIG.

  compensating fibers 20, 21 and 22 are used as mirrors.

  Indeed, the filtering device 1 of the invention being designed according to a type 4-f mounting, we obtain a perfect imaging in the plane of the matrix 4 of output fibers (which is consistent with the plane of the "waist", or collar, w0), and fibers

  can then be used as a mirror without loss of information.

  After a run in the dispersion compensating fiber 20 for example, and reflection at the end 23 thereof, the signal is backpropagated and follows the reverse optical path to recombine in the output fiber 24 with the signals that have followed. other routes, ie taken from other sections 21

  or 22 of dispersion compensating fiber.

  Such an output fiber 24 corresponds to the input fiber referenced 3 of the

device of Figure 1.

  The principle described above allows the realization of an adaptive variable compensation. The device of FIG. 9 has the advantage of automatically recombining the signals after compensation in the fiber sections 20 to 22, without it being necessary to implement an additional device of FIG.

recombination.

21 2839160

  In the variant embodiment of FIG. 9, filters of the same spectral width are used, by selecting output optical fibers 30, 31 and 32 on the same isochromatic circle 11. In this device, the adjustable parameters correspond to the choice of the spectral band, which is determined by the configuration of the hologram 2 used, and the value of the compensation,

  determined by the choice of an optical fiber 30, 31 or 32 in the output plane.

  In the variant embodiment of FIG. 10, on the other hand, two different spectral bands are used, selecting two isochromes that are referenced and 11 distinct. Thus, the incident signal is transferred to two output optical fibers referenced 33 and 34 respectively situces on isoschromatism circles 10 and 11. Each of these fibers 33, 34 is connected to a dispersion-compensated fiber section 37, 36, whose end is covered with a reflective treatment 23, so that these fiber sections 36, 37 act as a mirror. As for the assembly of FIG. 9, the signals are backpropagated in the fibers 33 and 34, and then recombined.

in the fiber referenced 24.

FIG. 11 shows a third variant embodiment of the chromatic dispersion compensation device of the invention, which makes it possible to prevent signal back-propagation in the optical fiber output of the matrix 4, and therefore the processing of the sections dispersion-compensated fibers at their ends. According to this variant, a dispersion-compensated fiber section 42 is connected, at each of its ends, to two output optical fibers referenced 40 and 41 on the same isochromatic circle 11. These two optical fibers 40 and 41, diametrically opposite on the circle of isochromatism 11, correspond to two symmetrical orders of diffraction of the hologram

  2 programmable digital device 1 of the invention.

  Such a configuration corresponds for example to the case of a holographic system with a binary phase level. The assembly of FIG. 11 requires a good balancing of the two optical paths, from the input fiber 24 to the optical fiber

22 2839160

  output 41 on the one hand, and the output optical fiber 40 to the input fiber 24 on the other hand, in opposite sounds. However, the error induced by an imbalance of these two optical paths remains low compared to the dispersion compensation

  chromatic provided by the referenced fiber 42.

-

23 2839160

Claims (23)

  An optical wavelength filtering device comprising at least one input optical fiber and at least one output optical fiber, characterized in that it comprises transfer means, to at least one of said output optical fibers. of at least one spectral band of at least one incident multi-wavelength signal by at least one of said input optical fibers, said transfer means implementing at least one programmable diffractive element located in an intermediate plane between the one or more optical fibers   input and the at least one optical output fiber.   2. An optical device according to claim 1, characterized in that it comprises programming means for modifying, in at least one direction, the spatial period of a pattern of said programmable diffractive element. 3. An optical device according to claim 2, characterized in that said programming means make it possible to configure said programmable diffractive element so that it has a spatial period P in said at least one direction, so that a spectral band centered on a given wavelength j is diffracted in said at least one direction by said programmable diffractive element at a predetermined angle such that sin0, -4, ok is an integer.   4. Optical device according to claim 3, characterized in that said programming means provide said spatial period P a disturbance equivalent to a variation less than the size of a pixel of said diffractive element programmable.   Optical device according to one of Claims 3 and 4,   characterized in that said programming means makes it possible to configure said programmable diffractive element so that it has a spatial period P comprising: at least one sub-period comprising N pixcls; 24 2839160   at least one subperiod comprising N2 pixels,   o N. and N2 are two distinct integers.   Optical device according to one of Claims 1 to 5,   characterized in that said diffractive element is a programmable digital hologram. An optical device according to claim 6, characterized in that said programmable digital hologram is displayed on a spatial light modulator with amplitude or phase modulation levels, said levels being continuous or quantified.   Optical device according to claim 7, characterized in that said   spatial light modulator is associated with at least one fixed diffractive element.   Optical device according to one of Claims 1 to 8,   characterized in that it comprises a collimating lens, in that said diffractive element acts in reflection and is located in the image focal plane of said collimating lens, and in that said at least one input and output optical fiber are located in the object focal plane of said collimating lens,   so as to form a folded 4-f free-space optical assembly.   Optical device according to one of Claims 1 to 6 and 8,   characterized in that it comprises two collimating lenses, respectively called first and second lenses, in that said diffractive element is located in the image focal plane of said first lens and in the object focal plane of said second lens, in that said at least one input optical fiber is located in the object focal plane of said first lens, and in that said at least one output optical fiber is located in the image focal plane of said second lens,   to form a free-space optical arrangement of type 4-f.   Optical device according to one of Claims 1 to 10,   characterized in that it comprises a matrix of at least two optical fibers of 28391 60   output, each of said fibers being characterized by its position with respect to the optical axis of said device, so that said device constitutes a battery of at least two tunable filters, and in that it comprises holographic adjustment means of the spectral selectivity of each of said filters, as a function of said position with respect to the axis   optical fiber of said corresponding output optical fiber.   Optical device according to one of Claims 1 to 11,   characterized in that said output optical fibers are monomode fibers.   13. Optical device according to claim 12, characterized in that at least one of said monomode fibers has at least one lens in its   end, so as to form a lenticular monomode fiber.   Optical device according to claim 13, characterized in that said lens comprises at least one segment of graded index fiber reported by assembly and fracture.   15. The optical device as claimed in claim 14, characterized in that said lens further comprises a section of silica fiber between said single-mode fiber and said segment of graded index fiber by assembly and fracture.   Optical device according to one of Claims 1 to 15,   characterized in that it includes means for regulating a filtration system.   applied to at least one of said wavelengths.   Optical device according to claim 16, characterized in that said   Filtering template is superimposed on said programmable diffractive element.   Optical device according to claim 16, characterized in that said   Filtering template is included in said programmable diffractive element.   19. Router spectral bands characterized in that it comprises at least one   An optical device according to any of claims 1 to 18, said one or more   devices comprising at least two output optical fibers. 26 28391 60   A spectral band router according to claim 19, characterized in that said diffractive element is dynamically configurable so as to route at least two distinct spectral bands of at least one incident signal,   respectively to separate output optical fibers Fj.   Spectral band router according to claim 19, characterized in that said programming means make it possible to configure said programmable diffractive element, so that said programmable diffractive element has, in said at least one direction, a spatial period P corresponding to the combination of a plurality of spatial periods Pi, where each of said spatial periods Pj is such that, when said programmable diffractive element has said spatial period Pb a spectral band   Ni-centered is transferred to said output optical fiber Fj.   22. Spectral band router according to any one of claims 19   at 21, characterized in that said output optical fibers are located on an isochromatic circle, so that the routing of said spectral bands   is made with constant bandwidth.   23. A chromatic dispersion compensation device, characterized in that   it comprises an optical device according to any one of claims 1 to   The chromatic dispersion compensating device according to claim 23, characterized in that at least one of said optical output fibers is connected to at least one fiber section with negative chromatic compensation. 25. Chromatic dispersion compensation device according to claim 24, characterized in that said fiber sections with compensation   chromatic negative have a reflective end.   26. Chromatic dispersion compensation device according to one of   any of claims 23 to 25, characterized in that said fibers   output optics are located on a circle of isochromatism.   w 27 2839160 27. Chromatic dispersion compensation device according to one of   any of claims 23 to 25, characterized in that said fibers   Output optics are located on at least two distinct isochromatic circles. 28. Chromatic dispersion compensation device according to one of   some of claims 23 and 24, characterized in that said section of   negative-chromatic-compensated optical fiber is connected at a first end to a first optical output fiber, and at a second end to   a second output optical fiber.   29. Chromatic dispersion compensation device according to claim 28, characterized in that said first and second output optical fibers are two diametrically opposed fibers of a circle.