FR2838187A1 - OPTICAL SPECTRUM ANALYSIS DEVICE - Google Patents

Abstract

This device applies in particular to optical telecommunications and comprises at least one assembly (38) comprising at least one means (40) for separating an input signal, at least one Bragg grating (44) used in reflection, at least. least one phase-jump Bragg grating (46) used in transmission, the separation means being connected to the gratings, and at least one actuator (52, 54) for wavelength tuning of the gratings, and at least one means (50) for detecting the signal (s) supplied by this assembly.

Description

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OPTICAL SPECTRUM ANALYSIS DEVICE
DESCRIPTION TECHNICAL FIELD
The present invention relates to an optical spectrum analysis device which is also called an optical spectrum analyzer.

 As will be seen later, the device of the invention uses at least one device called "spectral extractor with high finesse", allowing the extraction of a thin spectral band of an input optical signal.

 The device of the invention allows the analysis of this signal with a good resolution (of the order of a few picometers) and a limited size. The complexity of this device depends on the spectral range desired for the analysis.

 The invention applies in particular to the field of optical telecommunications and, more particularly, to those which implement DWDM or dense multiplexing wavelength ("Dense Wavelength Division Multiplexing").

STATE OF THE PRIOR ART
Apparatuses for analyzing optical spectra are already known from the following documents: [1] FR 2 794858A, "Optical spectrum analyzer device with integrated optical diffraction grating", invention of D. Trouchet.

 [2] "Tunable Analyzer Monitors Multiwavelength Systems", Fiber Systems, February 1999

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 [3] WO 00/49376A, "Optical spectrum analyzer".

 The device disclosed by document [1] uses an integrated optical component, spectrally dispersive, so as to distribute, by means of an appropriate optical assembly, the different spectral components of the signal analyzed on a set of photodetectors of CCD array type.

 The integrated optical component, which is used in this device, is a "phasar" or optical phase array.

 This known device has a disadvantage: it does not allow to obtain a high resolution.

 The device disclosed by document [2] uses a Fabry-Perot tunable filter with optical fiber, without lens ("lensless optical fiber FabryPerot tunable filter").

 This known device has a disadvantage: the Fabry-Perot filter used in this device can not be integrated into the optical fiber so that the manufacture of the device is relatively complex and expensive.

 The device disclosed by document [3] uses at least one Bragg grating ("grating grating") operating by reflection. This network is arranged at the output of an optical coupler, the two input ports of which are respectively connected to the signal to be analyzed and to a photodetector which picks up the light reflected by the network.

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 This known device has a disadvantage. because of its structure, the resolution it achieves is not very large.

STATEMENT OF THE INVENTION
The present invention aims to overcome the above disadvantages by providing an optical spectrum analysis device that uses at least one spectral extractor.

 As a result, the device that is the subject of the invention makes it possible to obtain a better resolution than the device known from document [1].

 The device which is the subject of the invention is therefore particularly suitable in the case where a high precision is required on the measurement of the spectral components of an optical signal.

 In addition, the spectral extractor is writable in the optical fibers so that the device of the invention is less complex and less expensive to manufacture than the device known from document [2].

 In addition, the device according to the invention, which uses a spectral extractor, has a better resolution than the device known from document [3].

 In the preferred embodiment of the invention in which the spectral extractor comprises an apodized Bragg grating and a phase-jump Bragg grating, the resolution on the measurement of an optical spectrum is even better and the device which is the subject of the invention. 'invention

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 is then significantly more efficient than the device known from document [3].

 Specifically, the subject of the present invention is an optical fiber optical spectrum analysis device, this device being characterized in that it comprises: at least one assembly forming a spectral extractor with a high fineness which is tunable in length this set being intended to receive an optical input signal, guided by an optical fiber, and to supply an optical output signal, the input optical signal having a spectral band which it is desired to analyze, this set comprising: at least one means for separating the input optical signal, at least one first Bragg grating used in reflection, at least one second Bragg grating, forming a phase jump Bragg grating and used in transmission, the separating means an input optical signal being connected to the first and second Bragg gratings, and at least one actuator for wavelength tuning the first and second Bragg gratings, and at least one light detecting means provided for detect the optical output signal that is provided by this set.

 Preferably, the means for separating the input optical signal comprises a circulator.

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 According to a preferred embodiment of the device according to the invention, the first Bragg grating, which is used in reflection, is an apodized Bragg grating.

 According to a first particular embodiment of the device which is the subject of the invention, this device comprises a plurality of sets each forming a spectral extractor with a high fineness, tunable in wavelength, these sets being respectively mounted on parallel tracks, the device further comprising an optical switch adapted to select each of these channels.

 According to a second particular embodiment, the device comprises a plurality of sets each forming a wavelength-tunable, high-finesse spectral extractor, these sets being used in parallel with: a plurality of pairs of Bragg gratings, used in transmission and reflection, these Bragg gratings having different Bragg wavelengths so as to cover the entire spectral band of the input optical signal, a circulator adapted to receive the optical input signal and for to separate it, this circulator having two output channels which are respectively connected to two optical switches, each of these two optical switches being able to establish a connection with any one of the spectral extractors with a high fineness, and - a plurality of light detection means which are respectively connected to the outputs of the

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 extractors and provided to detect the output signals of these extractors.

 According to a third particular embodiment, this device comprises a plurality of sets each forming a wavelength-tunable, high-finesse spectral extractor, these sets being used in parallel with: a plurality of pairs of Bragg gratings, used in transmission and reflection, these Bragg gratings having different Bragg wavelengths so as to cover the entire spectral band of the input optical signal, a circulator adapted to receive the optical input signal and for to separate it, this circulator having two output channels which are respectively connected to two optical switches, each of these two optical switches being able to establish a connection with any one of the spectral extractors, - a single light detection means , and another optical switch provided to establish a connection between the output of each extractor and this single detecting means. n light.

 According to a fourth particular embodiment, the device comprises a plurality of sets each forming a spectral extractor with a high fineness, wavelength tunable, these sets being used in series with: a plurality of pairs of Bragg gratings, used in transmission and reflection, these

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 Bragg gratings having different Bragg wavelengths so as to cover the entire spectral band of the input optical signal, a single circulator associated with the set of extractors, this single circulator having two output channels, the extractors being distributed on these two output channels, - a single light detection means connected to the outputs of all the extractors, - an actuator which is common to all the Bragg gratings used in reflection, or in transmission, this common actuator being capable of wavelength tuning each Bragg grating on a spectral analysis range that is specific to this Bragg grating and on the other hand the whole of the Bragg gratings over the entire band spectrum that we want to analyze, a separate actuator for all Bragg gratings used in transmission, or in reflection, this distinct actuator being simultaneously provided for the wavelength tuning of each a Bragg grating, on a spectral range of analysis which is specific to this Bragg grating, and of the set of Bragg gratings over the whole of the spectral band which one wants to analyze, and a spectral overmodulation on a small spectral width and a modulation frequency specific to each of the networks, and - a selective detection means, provided for the selective detection between the optical signals

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 from the various extractors, this selective detection being obtained by processing the output signal using synchronous detection on the different modulation frequencies of the extractors.

 Preferably, each actuator comprises a piezoelectric element.

BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood on reading the description of embodiments given below, for purely indicative and in no way limiting, with reference to the accompanying drawings in which: - Figure 1 is a schematic view of a three-way circulator, Figure 2 schematically illustrates an actuator for varying the stress and / or temperature applied to a Bragg grating, Figures 3 and 4 are schematic views of extractors, - Figure 5 shows the spectrum. output of an extractor, in dB, as a function of the difference at the wavelength, - FIG. 6 illustrates the modification of the shape of the spectral response of the extractor as a function of different phase jump networks. 7 schematically illustrates an assembly of an optical coupler and an optical isolator, suitable for replacing a circulator; FIG. 8 is a diagrammatic view of an extractor which can be modulated in power;

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 FIG. 9 illustrates the principle of power modulation on the spectral response of an extractor; FIG. 10 is a schematic view of an example of the invention that uses a very fine wavelength extractor. and in which the Bragg gratings are attached to actuators subjected to one or more voltage checks, - Figure 11 schematically illustrates an example of the invention which uses a combination of four extractors in a single configuration, FIG. schematic and partial an example of the invention that uses a combination of three couplers to replace the switch 1x4 of Figure 11, - Figure 13 schematically illustrates an example of the invention that uses a combination of four extractors in parallel and three switches and a photodetector, Figure 14 schematically illustrates an example of the invention that uses a combination of four extractors para Thus, there are two switches and four photodetectors, FIG. 15 schematically illustrates an example of the invention using a combination of four extractors in a paired multifrequency configuration, FIG. 16 schematically illustrates an example of the invention that uses a combination of four extractors in unpaired multifrequency configuration,

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 FIG. 17 schematically illustrates an example of the invention that uses a complex architecture for the spectral analysis of four optical channels, and FIGS. 18 and 19 schematically illustrate the operation of a spectrum analyzer according to the invention, having a multi-frequency configuration.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS
The invention uses at least one spectral extractor with high fineness. This device, also called wavelength extractor of great fineness, is capable of extracting, from a guided optical input signal, a band of low spectral width ##, centered around a so-called wavelength. extraction wavelength and noted #e.

 Such an extraction device is connected, on one side, to an input optical guide and, on the other, to a photodetector.

 The optical signal, having a given spectrum, is sent to the device via the input guide and this device provides an output optical signal, limited to the low spectral width band ## around #e. This output signal is transmitted by the output guide.

 Preferably, the input and output guides are optical fibers.

 Let S (,) (respectively S '(#)) be the power spectral density of the input optical signal

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 (respectively output) as a function of wavelength #.

S' (À) = S (À) XET ( À) ""S ( À e ) (1)
Un extracteur accordable en longueur d'onde repose sur un principe identique au précédent mis à part que la longueur d'onde d'extraction peut être ajustée par l'utilisateur. After passing through the extractor, a simple line is observed at the output whose amplitude is defined by the amplitude of the incident signal at the extraction wavelength. If we write #T (#) the spectral response of the extractor in transmission, we have:

S '(A) = S (TO) XET (A) "" S (To e) (1)
A wavelength tunable extractor is based on a principle identical to the previous except that the extraction wavelength can be adjusted by the user.

 In the spectral band of the input signal, which can be noted [# 1; #f], we can vary, as a function of time t, this extraction wavelength, for example from a value #e (t) to a value #e (t + # t).

S' (k, t) = S (1) XF'!'(;, t) S ( À e ( t) ) (2)
Une autre fonctionnalité peut être ajoutée au dispositif d'extraction simple. Il s'agit de la possibilité de moduler la puissance de la réponse donnée par le dispositif. If the control of the tunability of the extractor is done according to the function #e (t), equation (1) becomes:

S '(k, t) = S (1) XF'! '(;, T) S (To e (t)) (2)
Another feature can be added to the simple extraction device. This is the ability to modulate the power of the response given by the device.

 A power adjustable extractor is thus obtained: it is possible to vary, as a function of time, the height of the peak of width at half-height ##, centered on #e.

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S' À, t ) = S (À) x.r ( À, t) zS (Àe) x1 ( t ) (3)
Un extracteur complet est un dispositif qui intègre les différentes fonctionnalités présentées. If the control of the level of the output signal is done according to the function T (t), the equation (1) becomes:

S ', t) = S (λ) xr (λ, t) zS (λe) x1 (t) (3)
A complete extractor is a device that integrates the different features presented.

The user can thus extract from any guided optical signal "any wavelength" over the tunability range and modulate it according to his needs.

 An extractor uses a circulator, a Bragg grating inscribed in an optical fiber (preferably an apodized grating) and a phase jump Bragg grating.

 With regard to such networks, reference will be made, for example, to the following document: [4] WO 99/59009, "Bragg grating registration method, device for implementing this method and Bragg grating devices obtained by this process ", invention of Pierre Ferdinand, Sylvain Magne and Christophe Martinez.

 We return below to the component called circulator.

 Consider the example of a fiber optic circulator. It is a fiber component that allows to direct a guided signal in three ways, following specific paths. Figure 1 shows a diagram of this circulator. The signal which enters the circulator 2 by the channel ("port") v1 thereof exits via the channel v2. The signal which enters via the channel v2 exits via the channel v3.

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 The other passages from one channel to another are isolated: there is no signal from channel v2 to channel v1 nor from channel v1 to channel v3 nor from channel v3 to channel v1, this channel insulation is generally obtained at 20dB or 30dB.

 With an extractor, one can use at least one actuator, component on which we return in the following.

 The tunability of a Bragg grating can be obtained in different ways. The most common ones take advantage of the variation of the Bragg #B wavelength according to the application of a constraint (elongation or compression) or a change of temperature.

To characterize the effect of the constraint, we introduce the notion of relative elongation expressed in micro-deformations (#)

Under stress, a fiber of 1m in length that elongates by 1..un undergoes a relative elongation of 1 #.

 The variation of the Bragg wavelength #B (in pm) as a function of the relative elongation # (in #) and of a variation of temperature AT (in K) is given by the following relation, calculated for # B neighbor of 1550nm: ## B = 1,2 # + 12AT

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An actuator is therefore a device capable of varying the stresses # and / or the temperature T applied to a Bragg grating.

 FIG. 2 is a schematic view of an exemplary actuator 8. The Bragg grating 10, formed in an optical fiber 12, is represented by an alternating zone of dark and light fringes.

The actuator 8 is provided with control means 14 and comprises a material 16 allowing the transduction of stresses and / or temperature changes AT, from this actuator 8 to the Bragg grating 10.

 Various examples of extractors are now given.

un coefficient Trsp (iv,) ) . La réponse spectrale ET (À) du dispositif, donnée en transmission, est ainsi : ET (À) =Trsp ( /l. ) xRr a ( / )
L'association entre ces deux réseaux de Bragg se fait de préférence par l'intermédiaire d'un circulateur. An extractor comprises a Bragg grating (preferably an apodized grating) used in reflection (characterized by a coefficient Rra (#)) and a phase jump grating used in transmission (characterized by

a coefficient Trsp (iv,)). The spectral response ET (A) of the device, given in transmission, is thus: AND (A) = Trsp (/ l.) XRr a (/)
The association between these two Bragg gratings is preferably via a circulator.

 Two configurations, RT and TR, can then be considered depending on whether the guided wave is first reflected or, conversely, transmitted by the first network it encounters.

 An extractor, in a configuration RT, is schematically represented in FIG. 3 and comprises an apodized network 18 and a phase-jump network 20, which are mounted on a support 22, as well as

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 a three-way circulator 24 ("ports") v1, v2 and v3.

 The input of the signal S to be spectrally filtered is via an optical fiber 26 on the channel v1 of the circulator. The guided wave is thus directed to the v2 path. There she meets the apodized network.

 Part of the signal corresponding to the filter band of this network is reflected towards the channel v2 and then directed to the channel v3.

 The remaining signal is dispersed in a medium 28 consisting of an index matching liquid without which the Fresnel reflections at the end of the fiber may redirect a significant part of the input signal in the device.

 In a variant, the end of the fiber on which the network 18 is formed can be cleaved at an angle. The index matching liquid and such cleavage can also be used.

modulation d'indice 6.nmod' avec L=10mm et Ànn,od=4xlCT4 dans l'exemple, et rencontre le réseau à saut de phase caractérisé, dans l'exemple, par un saut de phase ## égal à #, une longueur L égale à 3mm et une amplitude de modulation #nmod égale à 15x10-4. The spectral band reflected in path v3 is given by the spectrum of the apodized network which is characterized by a length L and an amplitude of

modulation of index 6.nmod 'with L = 10mm and Ànn, od = 4xlCT4 in the example, and encounters the phase-jump network characterized, in the example, by a phase jump ## equal to #, a length L equal to 3mm and a modulation amplitude #nmod equal to 15x10-4.

 If the spectra of these networks are well tuned, only the central peak of the phase-jump network is extracted from the signal.

 The signal reflected by the latter is lost in the insulation of the circulator. No effect of

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 resonance (of the Fabry-Perot type) is therefore observed between the two networks.

 The signal thus extracted from S is provided by the optical fiber on which the phase-jump network is formed.

 The TR configuration is similar to the previous one. An extractor, using this configuration TR, is diagrammatically represented in FIG. 4 and uses the same components as the extractor of FIG. 3, arranged as can be seen in FIG.

 The input signal S is guided in a fiber 30 to the phase-jump network 20. The transmitted signal enters via the channel v1 into the circulator which directs it to the channel v2 where it meets the apodized network 18. The signal reflected spring by the v3 path of the circulator.

 The output of the end peak of transmission of the phase jump network is found at the output.

 An index adapter fluid and / or an angled cleavage of the optical fiber carrying the output signal s prevents the signal transmitted by the two networks from being reflected at the output of the device.

 The circulator also blocks the resonance between the two networks in the extractor of FIG.

 The RT configuration is characterized by the isolation of its input. No signal entering it can be seen through the entrance. The TR configuration is itself isolated at the output: a signal that

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 enters the device through the output has no effect.

 The extractors of Figures 3 and 4 actually work as explained above. This requires that the filter band of the apodized network is included in the filter band of the phase-jump network.

phase : déphasage de n, longueur 3mm et Anmod=15xlO~4 ; réseau apodisé : apodisation sinus, longueur 10mm, #nmod=4x10-4), on a une largeur de bande de filtrage ## peu différente de lOpm (à -3dB), pas de perte d'insertion et un taux de réjection important puisque l'atténuation est de 35dB sur une bande de 0,4nm. In the example under consideration (jump network

phase: phase shift of n, length 3 mm and Anmod = 15 × 10 -4; apodized network: sinus apodization, length 10mm, # nmod = 4x10-4), we have a filter bandwidth ## little different from lOpm (at -3dB), no insertion loss and a high rejection rate since the attenuation is 35 dB on a 0.4 nm band.

 This is shown in FIG. 5 which shows the output spectrum Tt of an extractor, in RT or TR configuration, expressed in decibels, as a function of the wavelength difference, denoted ## (in nm).

 The choice of the characteristic spectra of the apodized network and the phase jump network makes it possible to modify the shape of the filter of the extractor.

 FIG. 6 shows examples of different extractor responses as a function of different phase jump networks all having a length of 2mm but modulation amplitudes #nmod of 2x10-4 (response I), 4x10-4 (response II) and 8x10-4 (answer III). We notice that the width ## can be granted. It specifies that the apodized network associated with a length of 4mm and a parameter #nmod equal to 4x10-4.

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 In the extractors of Figures 3 and 4, the circulator 24 may be replaced by an optical coupler provided with an insulator. The coupler allows the separation of the signal from one channel to the other and the isolator avoids the creation of cavity effects between the two networks.

 This is schematically illustrated in Figure 7 where we see the optical coupler 32 having four input-output channels. One of these channels is connected to one end of the optical isolator 34.

The other end of this insulator forms a channel v1 of the coupler-insulator combination.

 Another coupler path is not used and placed in an index adapter fluid 36 and / or cleaved at an angle.

 The other two channels of the coupler have references v2 and v3.

 The channels v1, v2 and v3 of FIG. 7 correspond respectively to the channels v2, v1 and v3 of FIG.

 From the point of view of insertion loss, the use of the circulator is advantageous compared to that of the coupler-isolator combination because one of the coupler channels is not used and part of the signal is lost. because of this way.

 The principle of a tunable extractor is very simple, in view of the above: if the two networks of FIG. 3 or of FIG. 4 are fixed on the same actuator 8a, which is itself fixed on the support 22 and of the kind of the actuator 8 of FIG. 2 and which is provided with control means 14a, it

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 It is possible to modify the Bragg wavelength simultaneously on both networks.

 The spectral response of the extractor is therefore translated spectrally over the tunability range.

 We thus find the principle of the tunable extractor as defined above. The sensitivity and scan interval depend on both the type of actuator and the selected Bragg gratings.

 A power adjustable extractor requires a configuration different from the previous one.

An example of this extractor is schematically shown in FIG. 8.

 This extractor of FIG. 8 is deduced from the extractor of FIG. 3 as follows: the apodized network 18 is replaced by an apodized network 18a whose spectral response is adapted to the power modulation in a manner explained further .

 In addition, the network 18a is attached to an actuator 8b, the type of the actuator 8 of Figure 2 and controlled by means 14b.

 In the example of FIG. 8, the network 18a is thus fixed on the actuator 8b, itself fixed on the support 22.

 The differential tunability of the two networks 18a and 20 causes the thin transmission band of the phase-jump network to be modulated by the response of the apodized network.

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 This is schematically illustrated in FIG. 9. The transmission of the response of the phase-jump grating 20 is shown in transmission (T) and in the reflection (R) the response of the apodized grating 18a (L = 10mm and # nmod = 2x10-4). for different spectral offsets, namely Onm (I), 0.07nm (II) and 0.14nm (III).

 The response of the spectral extractor is the product of T by R.

 The total spectral response of the device decreases of course from case 1 to case III.

 Note therefore that a modulation is actually obtained without changing the position of the extraction peak. The apodized network 18a must have a lower spectral width than the network 18 since it must move in the transmission band of the phase-jump network. As a result, an apodized network of modulation amplitude lower than the network 18, namely 2x10-4 instead of 4x10-4, is used.

 It is possible to produce an extractor which is both wavelength-tunable and power-adjustable. To do this, two actuators are used so as to allow both the Bragg wavelength tunability for the two networks and the low modulation of the Bragg wavelength of the apodized network, allowing the modulation of the output power.

 An example of such an extractor can be obtained from that of FIG. 8, by fixing the grating 20 to an actuator 8c of the actuator type

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 8 of FIG. 2 and controlled by means 14c, this actuator 8c being itself fixed on the support 22.

 The optical spectrum analyzer according to the invention, which is schematically represented in FIG. 10, comprises a wavelength extractor of very high fineness 38.

 In the latter, thanks to a circulator 40, an optical signal S guided in an input optical fiber 42 first undergoes reflection by an apodized Bragg grating 44 and then a transmission by a Bragg grating with phase jump. The spectral responses of these two networks are such that the output signal corresponds to a thin spectral band of a few picometers in width at mid-height.

 It should be noted that, in the example of Figure 10 as in the examples of the following figures, an extractor in RT (Reflection and Transmission) configuration is used, but in all cases a TR configuration can also be used.

 The extractor 38 of FIG. 10 also comprises an index adapter liquid 48 and / or an oblique cleavage at the end of the optical fiber in which the apodized grating 44 is formed.

 In addition, the spectrum analyzer of FIG. 1 further comprises a photodetector 50 for detecting the optical signal provided by the extractor 38.

 The Bragg gratings 44 and 46 are respectively fixed to actuators 52 and 54 making it possible to vary the wavelengths of

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 Corresponding bragg.

 The analyzer also comprises means 56 for controlling these actuators 52 and 54, these means being in particular able to provide a voltage ramp.

 The analyzer of FIG. 10 further comprises electronic means 57 for processing the signal supplied by the photodetector 50.

 This treatment consists of a synchronized time detection on the control means 56.

 Figure 5 gives a theoretical representation of the transmission response (in dB) of an extractor. For this answer, we notice a half-height width of less than 10 μm and an extinction rate of -35 dB at 0.2 nm of the central wavelength.

 The extraction wavelength can be tuned to a range ## around a central wavelength #c, following a function #c + ## (t). The measurement of the output signal of the extractor during a scanning of the extraction wavelength thus makes it possible to measure the spectrum of the input optical signal over the tunability range ##.

 This is the principle of the spectrum analyzer object of the invention.

 If the total measurement range ## t is greater than ##, we use a series of extractors whose scanning ranges are superimposed to ensure the continuity of the measurement over the entire spectrum to be acquired.

d'onde [À1; ÀfJ dont la largeur vaut 07T=.f-71, des extracteurs successifs #k sont choisis avec des plages For scanning a range of lengths

wave [A1; Ff whose width is 07T = .f-71, successive extractors #k are chosen with ranges

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d' accordabilité Ikk,k-1; .x, x+1] . Préférentiellement, un léger recouvrement est prévu entre les plages de deux extracteurs successifs, ce recouvrement s'exprimant par la condition supplémentaire #k+1,k<#k,k+1.

tunability Ikk, k - 1; .x, x + 1]. Preferably, a slight overlap is provided between the ranges of two successive extractors, this recovery being expressed by the additional condition # k + 1, k <# k, k + 1.

d' accordabilité [?1, o; 1/2] avec 1/0=A,i; le second extracteur E2 aura une plage d'accordabilité [#2,1; 2,3]; le troisième extracteur E3 aura une plage d' accordabilité [#3,2; 3,4]; enfin le quatrième extracteur E4 aura une plage d'accordabilité {#4,3; #4,5] avec #4,5=#f. For example, if to cover a range of wavelengths [# 1; #f] we use four extractors E1, E2, E3 and E4, the first extractor El will have a range

of tunability [? 1, o; 1/2] with 1/0 = A, i; the second extractor E2 will have a tuning range [# 2.1; 2,3]; the third extractor E3 will have a tunability range [# 3.2; 3,4]; finally the fourth extractor E4 will have a tuning range {# 4.3; # 4,5] with # 4,5 = # f.

 In the following examples are given of the invention to achieve the combination of different extractors and, in particular, two architectures for this association.

 The first is a simple configuration in which the extractors are paralleled. The second requires a signal processing more elaborate than the first because it is based on a modulation of the extractors on several frequencies; why it is called multifrequency configuration.

 The example of Figure 11 is a simple configuration that includes a combination of four extractors connected in parallel. In this case, the input signal S is distributed over the channels of the different extractors E1, E2, E3 and E4.

 We also see a piezoelectric actuator 58, on which are fixed the networks of

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 Bragg extractors, and means 60 for controlling this actuator, these means being able to provide a voltage ramp.

 In addition, photodetectors P1, P2, P3 and P4 are provided which are respectively provided for detecting the output signals of the extractors E1, E2, E3 and E4. Each photodetector Pi, 1 <i <4, makes it possible to study a part of the spectrum to be analyzed, around a wavelength # 1.

 The analyzer of FIG. 11 further comprises electronic means 63 for processing the signals supplied by the photodetectors p1, p2, p3 and p4.

 This processing consists of a synchronized time detection on the control means 60.

 In the example of FIG. 11, it has been chosen to place an optical switch 62 of the sort 1x4 which switches the input signal S on each of the analysis channels comprising an extractor. The advantage of such a switch is that it makes it possible to analyze the totality of the signal on each of the channels. There is no loss of power.

 It should be noted, however, that the measurement is not done instantly: the scanning ranges of the extractors are interrogated successively.

 Instead of the switch can be used a sequence of couplers or any other system of division of a guided optical wave.

 In the case of couplers, for a combination of 2n extractors, use 2n - 1

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 couplers.

 In the example of Figure 12, n is 2.

Three couplers C1, C2 and C3 are therefore used. These are type 2 to 2 couplers. These couplers are cascaded and only three channels of each coupler are used.

 In FIG. 12 (as in FIG. 11), the references e1, e2, e3, and e4 denote the input channels of the extractors E1, E2, E3 and E4.

 The latter simultaneously receive a fraction of the input signal S.

 If 50/50 couplers are used, this association has the advantage of dividing the signal into equal parts on each of the output channels. Contrary to the case of the switch, each channel then receives (0, 5) p times the initial power of the signal (where p is the number of couplers that the signal must cross before reaching the extractor), hence a constraint more great on the sensitivity of the photodetectors used.

The acquisition of the two spectra is cons cons simultaneously, which is an advantage over the switch.

 With the simple configuration, all the extractors can be tuned by a single actuator or by several actuators controlled by the same control voltage.

 The extractors can also be controlled independently. The interest then lies in the simplicity of the architecture. This, however, is accompanied by the integration of a large number of components.

<Desc / Clms Page number 26>

 In the following, we present other more complex configurations, allowing the use of fewer components.

 It should be noted that the simple configuration may have other forms than that of Figure 11.

 FIG. 13 thus gives the example of a simple configuration using only a single photodetector 64 and a single circulator 66 but requiring the use of three optical switches 68, 70 and 72 of the type 1 x 4 before the photodetector 64. This configuration amounts to associating the extractors in parallel.

 The switches must then be controlled in the same way to always interrogate the right pair of networks.

 In the case of FIG. 13, it is specified that the four apodized networks al, a2, a3 and a4 (respectively the four phase-jump networks si, s2, s3 and s4) respectively belonging to the four extractors are fixed on the same actuator. piezoelectric 74 (respectively 76).

 The actuators 74 and 76 are controlled by the same control means 78 which are able to provide a voltage ramp.

 The apodized networks (respectively the phase jump networks) are connected, on one side, to the outputs of the switch 70 (respectively 68) and, on the other side, to index adapter media (respectively to the inputs of the switch 72 whose output is connected to the photodetector 64.

<Desc / Clms Page number 27>

 In addition, as seen in FIG. 13, the channel v1 of the circulator receives the signal to be analyzed S and its two other channels v2 and v3 respectively connected to the inputs of the switches 70 and 68.

 In order to use the spectrum analyzer of FIG. 13, the switches 68 and 70 are controlled so as to connect the two networks of the same extractor via the circulator and the switch 72 is further controlled so as to recover the wavelength thus filtered.

 The analyzer of FIG. 13 also comprises electronic means 79 for processing the signal supplied by the photodetector 64.

 This processing consists of a synchronized time detection on the control means of the actuators and switches.

 Given the cost of the switches, a slightly different configuration may also be interesting as shown in Figure 14.

 If the cost of a photodetector, multiplied by the number of outputs, remains lower than the cost of a switch, we can remove the output switch and provide a photodetector at the end of each extractor.

 For FIGS. 13 and 14, the same elements have the same references and it can be seen that, in the spectrum analyzer of FIG. 14, the switch assembly 72-photodetector 64 of FIG. 13 has been replaced by four photodetectors. 80, 82, 84 and 86 which are respectively connected to the phase jump networks.

<Desc / Clms Page number 28>

 The signals provided by these photodetectors are processed by electronic processing means 87.

 This processing consists of a synchronized time detection on the control means of the actuators and switches.

 It is specified that the control means of the switches 68, 70 and 72 of FIG. 13 have the reference 78a in this FIG. 13 and that the control means 68 and 70 of FIG. 14 have the reference 78b in this FIG.

 Table I summarizes the various elements that make it possible to make simple configurations of the type of the examples of FIGS. 11, 13 and 14.

TABLE I

<Tb>
<tb> Configuration <SEP> 11 <SEP> 13 <SEP> 14
<tb> corresponding
<tb> to <SEP> the <SEP> figure
<tb> Number <SEP> 2n <SEP> 2n <SEP> 2n
<tb> extractors
<tb> Number <SEP> 1 <SEP> 1 <SEP> 1 <SEP>
<tb> minimum
<tb> of actuators
<tb> Number <SEP> of <SEP> 2n <SEP> 2n
<tb> photodetectors
<tb> Number <SEP> of <SEP> 2n <SEP> 1 <SEP> 1
<tb> circulators
<tb> Number <SEP> of <SEP> 1 <SEP> of <SEP> genus <SEP> 3 <SEP> of <SEP> genus <SEP> 2 <SEP> of <SEP> genus
<tb> switches <SEP> 1x2n <SEP> 1x2n <SEP> 1x2n
<Tb>

<Desc / Clms Page number 29>

The configurations of the type of those of FIGS. 13 and 14 may therefore be particularly advantageous in the case of the use of a number of extractors greater than 4.

 It is possible to modulate the filtered signal by means of an extractor by inducing a differential spectral shift between the two networks of this extractor.

 In the present invention this property can be used to use a serial type configuration for the combination of extractors.

 In this case, at the level of the photodetectors of analysis, the recognition of the signal filtered by an extractor i is not done in the optical spectrum at the wavelength # 1 associated with this extractor: it is done by the recognition of the signal modulation at the frequency f1 imposed on this extractor.

 Synchronous detections or any other type of frequency filtering (by Fourier transform for example) can be used to extract information from each extractor.

 Two configurations can be considered, one paired and the other unmatched.

In the following examples we take the case of a division of the spectral analysis range into four parts.

 The paired configuration is an intermediate version between a serial architecture and a parallel architecture.

 As can be seen in FIG. 15, the signal S that one wants to analyze is firstly divided into

<Desc / Clms Page number 30>

 two parts, through coupler 88 50/50 type in the example of Figure 15. Each part of the signal S then reaches an extractor with two stages.

 More precisely, in the example of FIG. 15, a first (respectively a second) extractor comprises, as can be seen, a circulator 90 (respectively 92) whose first channel receives a portion of the signal S, a second channel is associated to two apodized Bragg gratings 94 and 96 (respectively 98 and 100) in series and - a third channel is associated with two phase jump Bragg gratings 102 and 104 (respectively 106 and 108).

 The end of the optical fiber carrying the network 96 (respectively 100) terminates with an index adapter fluid and / or a skew cleavage and the end of the optical fiber carrying the network 104 (respectively 108) ends with a photodetector 110 (respectively 112).

 In addition, the networks 94 and 100 (respectively 96 and 98) are fixed parallel to each other on an actuator 114 (respectively 116) and the networks 102, 104, 106 and 108 are fixed parallel to each other on an actuator 118.

 The actuators 114 and 116 are identical.

 The spectrum analyzer of FIG. 15 also comprises means 120 for controlling the actuator 118, these means being able to provide a voltage ramp.

<Desc / Clms Page number 31>

 These means 120 also control the actuator 116 (respectively 114) via a signal summator 122 (respectively 124), an input of which therefore receives the output signal of these means 120 and whose other input receives the signal output of a frequency generator 126, at the frequency f1 (respectively 128, at the frequency f2), the frequencies f1 and f2 being different.

 Each extractor thus withdraws from the signal to be analyzed S two thin strips respectively centered on two distinct wavelengths, denoted # 1 and # 3 for one of the extractors and # 2 and # 4 for the other extractor.

 In addition, these extractors are, as we have seen, used in power modulation according to the distinct frequencies f1 and f2, f1 being associated with # 1 and # 2 while f2 is associated with # 3 and # 4.

 The operating principle of the spectrum analyzer of FIG. 15 is based on the idea of frequency separation of each stage of an extractor.

 In the example given, a first two-stage extractor sweeps the spectral bands n 1 and 3 (networks corresponding to # 1 and # 3). Each stage is modulated by the two frequencies f1 and f2. (But note that the floors of this extractor could also scan spectral ranges that overlap, unlike the case of the example).

 The second extractor works on the same principle as the first.

<Desc / Clms Page number 32>

 The use of two identical actuators for the two extractors imposes the modulation on the same frequencies.

 The photodetectors 110 and 112 thus receive the sum of the modulated signals coming from the different stages. Signal processing means 130 are therefore provided to separate the frequency domains according to the frequency, which allows, indirectly, the spectral separation according to the wavelength.

 Table II summarizes the different elements necessary to achieve a paired multifrequency configuration.

TABLE II

<Tb>
<tb> Number <SEP> of extractors <SEP> 2n
<tb> Number <SEP> minimum
<tb> of actuators <SEP> 2n-1 + 1
<tb> Number <SEP> of <SEP> photodetectors <SEP> 2
<tb> Number <SEP> of <SEP> Circulators <SEP> 2
<tb> Number <SEP> of <SEP><SEP> Generators
<tb> frequency <SEP> 2n-1
<tb> Distribution <SEP> of the <SEP> signal
<tb> input <SEP> 1 <SEP> coupler
<Tb>

The unpaired configuration is a completely serial-type architecture where the different extractors are associated so as to form only one, having several stages.

 Each stage i is modulated at a frequency f1 that is specific to it. One photodetector

<Desc / Clms Page number 33>

 is then necessary to receive and process all the signals. At the output of this photodetector a synchronous detection or a Fourier analysis of the signal allows the separation of the different wavelength ranges.

 FIG. 16 gives an example of a spectrum analyzer according to the invention, using such a configuration and based on the association of four extractors.

 More precisely, this spectrum analyzer comprises a circulator 132 of which a first channel receives the signal to be analyzed S, a second channel is associated with a series connection of four apodized Bragg gratings 134, 136, 138 and 140, this assembly being terminated by a index adapter liquid and / or angled cleavage of the optical fiber carrying these four networks, and a third path is associated with a series connection of four phase jump Bragg gratings 142,144, 146 and 148, this arrangement being terminated by a photodetector 150 connected to electronic means 152 for processing the signals supplied by this photodetector.

 The networks 134, 136, 138 and 140 are respectively fixed to four piezoelectric actuators 154, 156, 158 and 160 while the four networks 142, 144, 146 and 148 are fixed parallel to each other on the same piezoelectric actuator 162.

 The spectrum analyzer of Figure 16

<Desc / Clms Page number 34>

 also includes means 164 for controlling the actuator 162, these means being able to provide a voltage ramp.

 These means 164 also control the actuators 154, 156, 158 and 160 respectively via four signal summers 166, 168, 170 and 172 whose input receives the output signal from these means 164 and whose other input receives the output signal. a frequency generator.

 The latter has the reference 173,174, 176 or 178 and operates at a frequency f-, f2, f3 or f4 depending on whether it is associated with the actuator 154, 156, 158 or 160.

 In addition, the frequencies f1, f2, f3 and f4 are different and are respectively associated with four different wavelengths # 1, # 2, # 3 and # 4 which respectively correspond to the pairs of networks 134-148, 136-146, 138 -144 and 140-142.

 The function of the actuator 162 is as follows: allow the tunability of all the networks on their spectral range of variation.

 The function of each of the actuators 154, 156, 158 and 160 is as follows: allow modulation at a frequency f around the tunable wavelength for each apodized network.

 In the means 152, the processing of the signal supplied by the photodetector 150 consists of a time detection, for example a synchronous detection, which makes it possible to separate the signals received at the frequencies f1, f2, f3, f4.

 The spectrum analyzer of FIG. 16 operates in a similar manner to that of FIG.

<Desc / Clms Page number 35>

 this is because all the extractors are assembled in one floor. The photodetector 150 thus receives the sum of the 4 signals corresponding to the frequencies f1, f2, f3 and f4.

 Table III summarizes the various elements necessary for carrying out an unpaired multifrequency configuration.

TABLE III

<Tb>
<tb> Number <SEP> of extractors <SEP> 2n
<tb> Number <SEP> minimum
<tb> of actuators <SEP> 2n + l
<tb> Number <SEP> of <SEP> photodetectors <SEP> 1
<tb> Number <SEP> of <SEP> circulators <SEP> 1
<tb> Number <SEP> of <SEP><SEP> Generators
<tb> frequency <SEP> 2n
<tb> Distribution <SEP> of the <SEP> signal
<tb> input
<Tb>

It should be noted that the multi-frequency configuration requires signal processing means provided by the photodetector. This processing consists of a time detection, for example a synchronous detection.

 However, this multifrequency configuration does not require division of the input signal. We can therefore analyze the entire spectrum simultaneously without significant reduction of the signal to be detected.

 In what follows, we give the characteristics that can have analyzers of

<Desc / Clms Page number 36>

 spectrum according to the present invention.

The spectral range of measurement is fixed by the number N of extractors used. For a tunability of between 10 nm and 20 nm per extractor, the following formula can be used for the definition of the measuring range: lOxN (nm) #### 20xN (nm)
Proper conditioning of the optical fibers used can increase their elongation limit from 1% to 2%, which doubles the range of the spectral range for the same number of extractors.

 The ## resolution is set by the half-width of the spectral peak and depends on the shape of the two Bragg gratings that are paired. It can range from a few picometers to a few tens of picometers.

 To overcome thermal and mechanical drift problems on Bragg gratings and actuators, a calibration is used.

 In a conventional way, to implement this calibration, it is possible to use a light source of fixed wavelength which is regularly connected to the input of a spectrum analyzer according to the invention. The accuracy of the spectrum analyzer then depends on the stability of this source.

 Insertion losses are very dependent on the chosen configuration. In theory, the unpaired multi-frequency configuration is the one with the least losses. If we assume that the networks are almost saturated, only the

<Desc / Clms Page number 37>

 circulator gives losses, of the order 1 to l, 3dB.

 The simple configuration has more losses than the other configurations since the signal must be divided into as many channels as there are extractors.

 The sensitivity of a spectrum analyzer according to the invention, ie the minimum power that a signal must have in order to be analyzed, depends mainly on the type of photodetector chosen, the desired resolution (plus the peak of extraction is wide, the more the signal is detected) and the configuration chosen for the spectrum analyzer (because of insertion losses).

If one takes the example of a wide source emitting 1mW (0 dBm) over a range of 60 nm (typical case of an application of the invention to the sensors), the power coming out of a resolution extractor ## is:
Pext = - 48 (dBm / pm) + 10log (## (pm))
If ## is equal to 2pm, an output signal of -45dBm (33nW) is obtained and if ## is equal to 10pm, the signal goes up to -38dBm (0.16W).

 This sensitivity is also a function of the insertion losses caused by the use of optical couplers and other components of the spectrum analyzer.

 In the case of the simple configuration the losses are of the order of 7dB. The power to detect drops respectively to -52dBm and -45dBm.

 This sensitivity is further considerably reduced by the frequency

<Desc / Clms Page number 38>

 acquisition. The integration time on the band ## is decisive according to the level of the signal to be detected.

In the case of a measurement of a telecommunication signal, which is much more important since the power emitted generally extends over 100 pm, the power coming out of the resolution extractor ## thus becomes:
Pext = - 20 (dBm / pm) + 10log (## (pm))
If ## is 2pm then the output signal is -18dBm (16W).

 Given the fact that the signal is higher, the constraint on the acquisition frequency is less strong.

 The acquisition frequency depends on the level of the signal to be detected but also the modulation frequencies in the case of a multifrequency configuration (to avoid the phenomena of beats).

 Now consider the actuators, preferably the piezoelectric actuators, which are important components of the invention.

 An actuator suitable for pulling an optical fiber is commercially available from the Jena Company under the reference PX 500 or PX 1500.

 The displacement / force curve of this actuator is a line of slope 0.06 N / m. In the case of a fiber stretched between two points 14 mm apart, the slope of this same straight line is 0.061N / m, hence an identical slope.

<Desc / Clms Page number 39>

 In certain configurations of the invention, the same actuator must allow the tunability of several networks. Such actuators are commercially available from the Jena Company.

 By way of example, the piezoelectric actuator whose reference is PX 400HL has a blocking force of 120N and a maximum elongation of 400 m and is suitable for an association comprising eight optical fibers.

 To reduce the number of actuators, which are expensive, the simplest is to use a single actuator capable of cooperating with several optical fibers, for example 8 fibers. In this case, we find a simple configuration.

 To reduce the number of circulators, which are also expensive, it is necessary to consider a multi-frequency spectrum analyzer.

 In this case, the optimization of the use of the actuators leads to a complex analyzer.

 The simple configuration follows the principle of Figure 14. The main characteristics of the resulting spectrum analyzer are then: - an analysis channel - analysis range: 40nm #### 80nm - non-instantaneous measurement.

 A multi-channel configuration (for example four channels) takes advantage of the possibility of controlling several actuators (for example eight actuators) from the same control module.

<Desc / Clms Page number 40>

 So 8 actuators are used to operate 8 optical fibers (corresponding to 64 Bragg gratings in all). The device thus obtained is therefore very complex.

 To limit the number of circulators, one chooses the multi-frequency configuration.

 A simple example of the invention, corresponding to such a configuration, is schematically shown in FIG. 17.

 In this example, one is able to simultaneously analyze four channels on 8 extractors.

 Each of the channels v1, v2, v3 and v4 is divided in two to go to twice four extractors, tuned to the modulation frequencies f1, f2, f3 and f4. Each channel therefore concerns 8 Bragg wavelengths. The filtering by the two photodetectors relating to the same channel makes it possible to analyze the spectrum almost instantaneously in ranges of 10 to 20 nm.

 It is specified that in FIG. 17 - the references A1, A2, A3, A4, A5, A6, A7 and A8 represent the actuators, - the reference CA represents the control means of the actuators A1, A2, A3 and A4, able to to provide these latter with a voltage ramp, the references F1, F2, F3 and F4 represent signal summers and frequency generators which are respectively associated with the frequencies f1, f2, f3 and f4 and respectively connect the actuator Al to the actuator A5, the actuator A2 to the actuator A6, the actuator A3 to

<Desc / Clms Page number 41>

 the actuator A7 and the actuator A4 to the actuator A8, these actuators having the function of performing both the wavelength tunability and the power modulation described above, the references d1, d2, d3, d4, d5, d6, d7 and d8 represent the photodetectors of the analyzer of FIG. 17, the reference MT represents the electronic means for processing the signals supplied by these photodetectors, the processing consisting in temporally detecting the signals at the frequencies f1, f2 , f3 and f4, in particular by synchronous detection.

 It is furthermore specified that in FIG. 17 the references r1, r2, r3, r4, r5, r6, r7 and r8 are intended to identify the apodized networks of the first two stages of extractors.

 The characteristics of the device of FIG. 17 are therefore the following: four analysis channels, 80 nm analysis range, approximately instantaneous measurement.

 The device of FIG. 17 offers a spectral range twice as large and four times as many analysis channels as the previous solution. This device can therefore be considered as the integration of four spectrum analyzers, broadband and high resolution.

 By dividing the spectral analysis range by two, the same device (without 1x2 couplers) would allow eight-channel analysis for equivalent cost.

<Desc / Clms Page number 42>

 In the following, information is given enabling a better understanding of the operation of a spectrum analyzer according to the invention, in a multi-frequency configuration, and the choice of certain parameters is also specified.

 We rely on FIG. 16 and consider the case where the signal to be analyzed is emitted by a monochromatic source of width ##, centered in the scanning band of the first extractor.

 At first, we consider this first extractor. By scanning, it covers a range # 11- # 12 thanks to a voltage ramp (see Figure 18 where the time t is plotted on the abscissa and the wavelengths on the ordinate).

 The peculiarity of the multifrequency configuration means that a frequency modulation f1 is added to the voltage ramp.

 In practice this results in a "closed-off" ("on-off") switching operation of the signal extracted by the first extractor, with a switching frequency f1 or a switching time # 1.

 FIG. 18 is a schematic representation of the wavelength sweep of the signal extracted by the extractor. Due to the switching on-off, the scanning is discontinuous. Each line of the ramp I represents the extractor in position "on" and each white represents the "off" position of this extractor. Two successive lines are separated by the switching time # 1.

 The time T1 represents the duration of the

<Desc / Clms Page number 43>

 scanning.

 It is considered that the signal entering the analyzer has a spectrum centered in #i with a ## width. If the latter is large enough the extractor switches several times during the acquisition of the signal.

 This principle is illustrated in FIG. 19 which shows the variations of the power P supplied by the detector as a function of time.

commutation (T1/11=kx (À, 12-:1) /5A. avec k très supérieur à 1) . Note that the switching effect samples the signal at frequency f1. A detection tuned on this frequency therefore gives a signal at a time Ti provided that: the detection window expressed in frequency covers several switching periods (# T = kx # 1 with k much greater than 1), the width of the spectrum of entry is sufficient to cover several periods of

switching (T1 / 11 = kx (λ, 12-: 1) / 5A with k much greater than 1).

balayage), .z-.lï=l0nm, cS,=5pm (résolution) et k=10 et l'on obtient : #1=1/20000=50 s. For example, we take Tl = ls (duration of

scan), .z-.l = l0nm, cS, = 5pm (resolution) and k = 10 and we obtain: # 1 = 1/20000 = 50 s.

 If we consider the four extractors of Figure 16, we must consider four switching frequencies. All these frequencies must be greater than 20kHz if we take the previous example.

 As far as the detection is concerned, we have a treatment window of width #T equal to 0.5ms. In

<Desc / Clms Page number 44>

 this window must find enough measuring points to ensure the treatment. If 1,000 measurement points are used, the photodetector must be fast at 2MHz. Such photodetectors are commercially available.

Claims (8)

1. A device for optical fiber optical spectrum analysis, this device being characterized in that it comprises: at least one set (38, E1-E2-E3-E4) forming a spectral extractor with a high fineness which is tunable wavelength, this assembly being adapted to receive an optical input signal (S), guided by an optical fiber (42), and to provide an output optical signal (s), the input optical signal having a spectral band that it is desired to analyze, this set comprising: at least one means (40, 66, 90-92) for separating the input optical signal, at least one first Bragg grating (44, al to a4, 94-96-98-100, 134-136-138-140) used in reflection, at least one second Bragg grating, forming a Bragg grating with a jump of phase (46, if at s4,102-104-106-108, 142-144-146-148) and used in transmission, the input optical signal separating means being connected to the first and second Bragg gratings, and . at least one actuator (52-54, 58, 74-76, 114-116-118, 154-156-158-160-162, A1 to A8) for wavelength tuning the first and second Bragg gratings, and at least one light detection means (50, P1-P2-P3-P4, 64, 80-82-84-86, 110-112, 150, d1 to <Desc / Clms Page number 46>  d8) provided for detecting the output optical signal that is provided by this set.  2. Device according to claim 1, wherein the means for separating the input optical signal comprises a circulator (40,66, 90-92).  3. Device according to any one of claims 1 and 2, wherein the first Bragg grating, which is used in reflection, is an apodized Bragg grating (44, 94-96-98-100, 134-136-138). -140).  4. Device according to any one of claims 1 to 3, comprising a plurality of sets (E1, E2, E3, E4) each forming a spectral extractor with high finesse, tunable wavelength, these sets being respectively mounted on parallel channels, the device further comprising an optical switch (62) adapted to select each of these channels.  5. Device according to any one of claims 1 to 3, comprising a plurality of sets each forming a spectral extractor finesse, wavelength tunable, these sets being used in parallel with: - a plurality of pairs of Bragg gratings (al-sl, a2-s2, a3-s3, a4-s4) used in transmission and reflection, these Bragg gratings having different Bragg wavelengths so as to cover the entire band spectral signal of the input optical signal, - a circulator (66) arranged to receive the input optical signal and to separate it, this circulator having two output channels which are <Desc / Clms Page number 47>  respectively connected to two optical switches (68-70), each of these two optical switches being adapted to establish a connection with any one of the high-accuracy spectral extractors, and - a plurality of light detection means (80-82 -84-86) which are respectively connected to the outputs of the extractors and provided for detecting the output signals of these extractors.  6. Device according to any one of claims 1 to 3, comprising a plurality of sets each forming a wavelength tunable spectral extractor very fine, these sets being used in parallel with: - a plurality of pairs of Bragg gratings (al-sl, a2-s2, a3-s3, a4-s4) used in transmission and reflection, these Bragg gratings having different Bragg wavelengths so as to cover the entire band spectral signal of the input optical signal, - a circulator (66) arranged to receive the input optical signal and to separate it, this circulator having two output channels which are respectively connected to two optical switches (68-70). each of these two optical switches being adapted to establish a connection with any one of the spectral extractors, - a single light detection means (64), and - another optical switch (72) arranged to establish a connection between the exit each extractor and this single light detection means. <Desc / Clms Page number 48>  7. Device according to any one of claims 1 to 3, comprising a plurality of sets each forming a wavelength tunable spectral extractor very finely, these sets being used in series with: - a plurality of pairs of Bragg gratings (134-148,136-146, 138-144,140-142) used in transmission and reflection, these Bragg gratings having different Bragg wavelengths so as to cover the entire spectral band of the optical signal input, a single circulator (132) associated with all the extractors, this single circulator having two output channels, the extractors being distributed on these two output channels, - a single light detection means (150) connected at the outputs of all the extractors, - an actuator (162) which is common to all the Bragg gratings used in reflection, or in transmission, this common actuator being able to match in wavelength on the one hand each resistor Bragg water on a spectral range of analysis which is specific to this Bragg grating and on the other hand the set of Bragg gratings over the whole of the spectral band which one wants to analyze, - a separate actuator ( 154, 156, 158, 160) for all Bragg gratings used in transmission, or in reflection, this distinct actuator being simultaneously provided for the wavelength tuning of each Bragg grating, over a spectral range of analysis which <Desc / Clms Page number 49>  is specific to this Bragg grating, and of the set of Bragg gratings over the whole of the spectral band that we want to analyze, and a spectral over-modulation on a small spectral width and on a modulation frequency specific to each networks, and - a means (152) for selective detection, provided for the selective detection between the optical signals from the different extractors, this selective detection being obtained by a processing of the output signal using a synchronous detection on the different modulation frequencies extractors.  Apparatus according to any one of claims 1 to 7, wherein each actuator comprises a piezoelectric element (52-54,58, 74-76, 114-116-118, 154-156-158-160-162, Al to A8).