Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
  • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
  • G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
  • G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
  • G02B6/29317—Light guides of the optical fibre type
  • G02B6/29319—With a cascade of diffractive elements or of diffraction operations
  • G02B6/2932—With a cascade of diffractive elements or of diffraction operations comprising a directional router, e.g. directional coupler, circulator
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
  • G01J3/0256—Compact construction
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/12—Generating the spectrum; Monochromators
  • G01J3/18—Generating the spectrum; Monochromators using diffraction elements, e.g. grating
  • G01J3/1895—Generating the spectrum; Monochromators using diffraction elements, e.g. grating using fiber Bragg gratings or gratings integrated in a waveguide
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
  • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
  • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
  • G02B6/29358—Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator

Definitions

• the present invention relates to an optical spectrum analysis device which is also called an optical spectrum analyzer.
• the device which is the subject of the invention uses at least one device called a "very fine spectral extractor", allowing the extraction of a fine spectral band from an optical input signal.
• the device which is the subject of the invention allows the analysis of this signal with 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 wavelength division multiplexing ("Dense
• the device disclosed in document [1] uses an integrated, spectrally dispersive optical component, so as to distribute, by means of an appropriate optical arrangement, the different spectral components of the signal analyzed on a set of photodetectors of the CCD strip type.
• the integrated optics component which is used in this device, is a "phasar" or optical phase array.
• the device disclosed in the document [2] uses a tunable Fabry-Perot fiber optic filter, without lens ("lensless optical fiber Fabry-Perot tunable filter").
• This known device has a drawback: the Fabry-Perot filter used in this device cannot be integrated into the optical fiber so that the manufacturing of the device is relatively complex and expensive.
• the device disclosed by document [3] uses at least one Bragg grating ("Bragg grating”) operating by reflection.
• This network is arranged at the output of an optical coupler, the two input channels ("ports") of which are respectively connected to the signal to be analyzed and to a photodetector which collects the light reflected by the network.
• This known device has a drawback: due to its structure, the resolution which it makes it possible to obtain is not very large.
• the present invention aims to remedy the above drawbacks by proposing an optical spectrum analysis device which uses at least one spectral extractor.
• the device which is the subject of the invention makes it possible to obtain better resolution than the device known from document [1].
• the device which is the subject of the invention is therefore particularly indicated in the case where high precision is required on the measurement of the spectral components of an optical signal.
• the spectral extractor is writable in the optical fibers so that the device which is the subject of the invention is less complex and less expensive to manufacture than the device known from document [2].
• the device which is the subject of the invention which uses a spectral extractor, has better resolution than the device known from document [3].
• the spectral extractor comprises an apodized Bragg grating and a phase-shifted Bragg grating
• 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 very fine spectral extractor which is tunable in length d wave, this assembly being intended to receive an optical input signal, guided by an optical fiber, and to provide an optical output signal, the optical input signal having a spectral band which we want to analyze, this assembly including:.
• the means for separating the optical input signal comprises a circulator.
• the first Bragg grating, which is used in reflection is an apodized Bragg grating.
• this device comprises a plurality (“a plurality”) of sets each forming a spectral extractor of great finesse, tunable in wavelength, these sets being respectively mounted on parallel channels, the device further comprising an optical switch able to select each of these channels.
• the device comprises a plurality of assemblies each forming a very fine spectral extractor, tunable in wavelength, these assemblies being used in parallel with:
• this device comprises a plurality of assemblies each forming a spectral extractor with great finesse, tunable in wavelength, these assemblies being used in parallel with: - a plurality of pairs of gratings
• Bragg 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 designed to receive the optical signal from input and 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,
• the device comprises a plurality of assemblies each forming a spectral extractor with great finesse, tunable in wavelength, these assemblies being used in series with: a plurality of pairs of networks of
• these Bragg gratings having different Bragg wavelengths so as to cover the entire spectral band of the input optical signal, a single circulator associated with all of the extractors, this single circulator having two outlet channels, the extractors being distributed over these two outlet 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 able to tune in wavelength on the one hand each Bragg grating over a spectral range of analysis which is proper to this Bragg grating and on the other hand the set of Bragg grids on the whole of the spectral band which we want to analyze, - a separate actuator for all the Bragg grids used in transmission, or in reflection, this separate actuator being simultaneously provided for the wavelength tuning of each Bragg grating, over a spectral analysis range which is specific to this Bragg grating, and of the set of Bragg grids over the whole the spectral band that we want to analyze, and a spectral over-modulation over a narrow spectral width and over a modulation frequency specific to each of the networks, and a selective detection means, provided for the selective detection between the optical signals coming from the different extractors, this selective detection being obtained by processing the output signal using synchronous detection on the different modulation frequencies of the extractors.
• each actuator comprises a piezoelectric element.
• FIG. 1 is a schematic view of a three-way circulator
• FIG. 2 schematically illustrates an actuator making it possible to vary the stress and / or the temperature applied to a Bragg grating
• FIGS. 3 and 4 are schematic views of extractors
• FIG. 5 shows the output spectrum of an extractor, in dB, as a function of the difference in wavelength
• FIG. 6 illustrates the modification of the shape of the spectral response of the extractor as a function of different phase jump networks
• FIG. 7 schematically illustrates an assembly of an optical coupler and an optical isolator, capable of replacing a circulator
• FIG. 8 is a schematic view of an extractor which is adjustable in power
• 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 which uses a very fine wavelength extractor and in which the Bragg gratings are fixed 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 simple configuration
• Figure 12 illustrates schematically and partially an example of the invention which uses a combination of three couplers to replace the switch 1x4 of figure 11
• FIG. 13 schematically illustrates an example of the invention which uses a combination of four extractors in parallel as well as three switches and a photodetector
• FIG. 14 schematically illustrates an example of the invention which uses a combination of four extractors in parallel as well as two switches and four photodetectors,
• FIG. 15 schematically illustrates an example of the invention which uses a combination of four extractors in paired multifrequency configuration
• - Figure 16 schematically illustrates an example of the invention which uses a combination of four extractors in unmatched multi-frequency configuration
• Figure 17 schematically illustrates an example of the invention which uses a complex architecture for one spectral analysis of four optical channels
• Figures 18 and 19 schematically illustrate the operation of a spectrum analyzer according to the invention, having multi-frequency configuration.
• the invention uses at least one very fine spectral extractor.
• This device also called a very fine wavelength extractor, is capable of extracting, from a guided optical input signal, a band of small spectral width ⁇ , centered around a wavelength called length. extraction wave and noted ⁇ e .
• Such an extraction device is connected, on one side, to an optical input 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 optical output signal, limited to the band of small spectral width ⁇ around ⁇ e .
• This output signal is transmitted by the output guide.
• the inlet and outlet guides are optical fibers.
• S ( ⁇ ) (respectively S '( ⁇ )) be the power spectral density of the input optical signal (respectively output) as a function of the wavelength ⁇ .
• a wavelength tunable extractor is based on a principle identical to the previous one except that the extraction wavelength can be adjusted by the user.
• the spectral band of the input signal which can be noted [ ⁇ i; ⁇ f ]
• this extraction wavelength for example from a value ⁇ e (t) to a value ⁇ e (t + ⁇ t). If the extractor is tunable according to the function ⁇ e (t), equation (1) becomes:
• equation (1) If the output signal level is checked according to the function ⁇ (t), equation (1) becomes:
• a complete extractor is a device which integrates the various functionalities presented. The user can thus extract from a guided optical signal "any wavelength" over the tunability range and modulate it in power according to his needs.
• An extractor uses a circulator, a Bragg grating inscribed in an optical fiber (preferably an apodized grating) and a Bragg grating with phase jump.
• FIG. 1 shows a diagram of this circulator.
• At least one actuator can be used, a component to which we return in the following.
• the tunability of a Bragg grating can be obtained in different ways. The most common take advantage of the variation in the wavelength of
• Bragg ⁇ B depending on the application of a stress (elongation or compression) or a change in temperature.
• ⁇ B l, 2 ⁇ + 12 ⁇ T
• 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 example of an actuator 8.
• the Bragg grating 10, formed in an optical fiber 12, is represented by a zone of alternation of dark and light fringes.
• the actuator 8 is provided with control means 14 and comprises a material 16 allowing the transduction of the stresses and / or temperature changes ⁇ T, from this actuator 8 to the Bragg grating 10.
• An extractor comprises a Bragg grating (preferably an apodized grating) used in reflection (characterized by a coefficient R ra ( ⁇ )) and a phase jump grating used in transmission (characterized by a coefficient T rsp ( ⁇ )).
• the association between these two Bragg gratings is preferably made by means of a circulator.
• RT and TR Two configurations, RT and TR, can then be considered depending on whether the guided wave is first reflected or, on the contrary, transmitted by the first network it encounters.
• An extractor in an RT configuration, 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, thus than a three-way circulator 24 ("ports") vl, v2 and v3.
• the input of the signal S to be spectrally filtered is done by an optical fiber 26 on the channel v1 of the circulator.
• the guided wave is therefore directed towards channel v2. There she meets the apodized network.
• Part of the signal corresponding to the filter band of this network is reflected towards channel v2 and then directed towards channel v3.
• the remaining signal disperses in a medium 28 consisting of an index matching liquid without which the Fresnel reflections at the end of the fiber risk redirecting a non-negligible part of the input signal into the device.
• the end of the fiber, on which the network 18 is formed can be cleaved at an angle. It is also possible to use the index adaptation liquid and such a cleavage.
• the signal s thus extracted from S is supplied 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 schematically represented in FIG. 4 and uses the same components as the extractor of FIG. 3, arranged as seen in FIG. 4.
• the input signal S is guided in a fiber 30 to the phase jump network 20.
• the transmitted signal enters • the channel v1 in the circulator which directs it to the channel v2 where it meets the apodized network 18.
• the reflected signal comes out through channel v3 of the circulator.
• the output peak of the phase jump network is found at the output.
• An index adapter liquid and / or a cleavage at an angle to the optical fiber transporting 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. 4.
• the RT configuration is characterized by the isolation of its input. No signal entering it comes out through the input channel.
• the TR configuration is isolated at the output: a signal which enters the device through the outlet has no effect.
• phase jump network phase shift of ⁇
• filter bandwidth ⁇ little different from lOpm (at -3dB)
• no insertion loss and a significant rejection rate since l attenuation is 35dB on a 0.4nm band.
• Figure 5 shows the output spectrum T t of an extractor, in RT or TR configuration, expressed in decibels, depending on the deviation from the wavelength, noted ⁇ (in nm).
• Figure 6 shows examples of different extractor responses as a function of different phase jump networks all having a length of 2mm but modulation amplitudes ⁇ n m ⁇ d of 2xl0 "4 (response I), 4xl0 ⁇ 4 (response II) and 8xl0 "4 (response III). It is thus noted that the width ⁇ can be granted. It is specified that the apodized network associated with a length of 4mm and a parameter ⁇ n m ⁇ d equal to 4xl0 "4 .
• the circulator 24 can be replaced by an optical coupler provided with an isolator. 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.
• FIG. 7 This is schematically illustrated in FIG. 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 isolator forms a channel v1 of the coupler-isolator association.
• Another coupler channel is not used and placed in an adapter liquid of index 36 and / or cleaved at an angle.
• the other two channels of the coupler have the references v2 and v3.
• the channels v1, v2 and v3 in FIG. 7 correspond respectively to the channels v2, vl and v3 in FIG. 3.
• the use of the circulator is advantageous compared to that of the coupler-isolator association because one of the coupler channels is not used and part of the signal is lost. because of this path.
• the principle of a tunable extractor is very simple, taking into account 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 it is possible to modify the Bragg wavelength simultaneously on both networks.
• the spectral response of the extractor is therefore translated spectrally over the range of tunability.
• the sensitivity and the scanning interval depends both on the type of actuator 1 and selected Bragg gratings.
• a power modular extractor requires a configuration different from the previous one.
• An example of this extractor is schematically represented in FIG. 8.
• This extractor of FIG. 8 is deduced from the extractor of FIG. 3 in the following manner: the apodized network 18 is replaced by an apodized network 18a whose spectral response is adapted to power modulation in a manner explained below.
• the network 18a is fixed to an actuator 8b, of the type of the actuator 8 in FIG. 2 and controlled by means 14b.
• 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 means that the fine transmission band of the phase jump network is modulated by the response of the apodized network.
• This is schematically illustrated in FIG. 9.
• the response of the spectral extractor is the product of T by R.
• 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. Therefore, an apodized network with a lower amplitude of modulation is used than network 18, namely 2xl0 ⁇ 4 instead of 4xl0 "4.
• An extractor can be produced which is both tunable in wavelength and scalable in power. To do this, two actuators are used so as to allow both tunability of the Bragg wavelength for the two networks and low modulation of the Bragg wavelength of the apodized network, allowing modulation of the output power.
• An example of such an extractor can be obtained from that of FIG. 8, by fixing the network 20 to an actuator 8c of the kind of the actuator. 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, includes a very fine wavelength extractor 38.
• an optical signal S guided in an input optical fiber 42 is first of all reflected by an apodized Bragg grating 44 and then transmitted by a phase jump Bragg grating 46
• the spectral responses of these two networks are such that the output signal s corresponds to a fine spectral band of a few picometers from width to half height.
• the extractor 38 of FIG. 10 also comprises an adapter liquid of index 48 and / or a cleavage at an angle at the end of the optical fiber in which the apodized network 44 is formed.
• the spectrum analyzer of FIG. 1 further comprises a photodetector 50 intended to detect the optical signal s supplied 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 Corresponding Bragg.
• the analyzer also comprises means 56 for controlling these actuators 52 and 54, these means being in particular capable of providing a voltage ramp.
• the analyzer of FIG. 10 further comprises electronic means 57 for processing the signal supplied by the photodetector 50.
• This processing consists of a synchronized time detection on the control means 56.
• FIG. 5 gives a theoretical representation of the transmission response (in dB) of an extractor. For this response, we notice a width at half height less than 10 pm and an extinction rate of -35 dB at 0.2 nm from the central wavelength.
• the extraction wavelength can be tuned over a range ⁇ around a central wavelength ⁇ c , according to a function ⁇ c + ⁇ (t). The measurement of the extractor output signal during a scanning of the extraction wavelength thus makes it possible to measure the spectrum of the optical input signal over the range of tunability ⁇ .
• the first is a simple configuration in which the extractors are placed in parallel.
• the second requires a more elaborate signal processing than the first because it is based on a modulation of the extractors on several frequencies; this is why it is called multi-frequency configuration.
• the example in FIG. 11 is a simple configuration which includes a combination of four extractors mounted in parallel.
• the input signal S is distributed over the channels of the different extractors Ei, E 2 , E 3 and E 4 .
• piezoelectric actuator 58 on which are fixed the networks of Bragg extractors, and means 60 for controlling this actuator, these means being capable of providing a voltage ramp.
• photodetectors P1, P2, P3 and P4 which are respectively designed to detect the signals of. output of extractors Ei, E 2 .
• Each photodetector Pi, l ⁇ i ⁇ 4, makes it possible to study part of the spectrum to be analyzed, around a wavelength ⁇ j.
• the analyzer of FIG. 11 further comprises electronic means 63 for processing the signals supplied by the photodetectors pi, p2, p3 and p4.
• This processing consists of a synchronized time detection on the control means 60.
• couplers instead of the switch, it is possible to use a series of couplers or any other system for dividing a guided optical wave.
• couplers for a combination of 2 n extractors, use 2 n - 1 couplers.
• n 2
• 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.
• references el, e2, e3, and e4 denote the input channels of the extractors Ei, E 2 , E 3 and E 4 .
• the latter simultaneously receive a fraction of the input signal S.
• the extractors can also be ordered independently.
• the interest then lies in the simplicity of the architecture. This is however accompanied by the integration of a large number of components. In the following, we present other more complex configurations, allowing the use of fewer components.
• FIG. 13 an example is thus given 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 1 ⁇ 4 type 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.
• the actuators 74 and 76 are controlled by the same control means 78 which are capable of providing a voltage ramp.
• Apodized networks are connected, on one side, to the outputs of switch 70 (respectively 68) and, on the other side, to index adapter media (respectively to inputs of the switch 72, the output of which is connected to the photodetector 64.
• 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.
• 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 also controlled so as to recover the wavelength thus filtered.
• the analyzer of FIG. 13 also includes 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.
• This processing consists of a synchronized time detection on the control means of the actuators and switches.
• 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. 14.
• Table I summarizes the various elements which make it possible to carry out simple configurations of the kind of the examples of FIGS. 11, 13 and 14.
• this property can be used to use a configuration of the series type for the association of extractors.
• the recognition of the signal filtered by an extractor i is not done in the optical spectrum at the wavelength ⁇ i associated with this extractor: it is done by the recognition of the signal of modulation at the frequency f ⁇ imposed on this extractor.
• Synchronous detections or any other type of frequency filtering can be used to extract the information from each extractor.
• Two configurations can be considered, one paired and the other unpaired.
• the paired configuration is an intermediate version between a serial architecture and a parallel architecture. As seen in Figure 15 the signal
• a first (respectively a second) extractor comprises, as can be seen, a circulator 90 (respectively 92) of which
• a first channel receives part of the signal S
• a second channel is associated with two apodized Bragg gratings 94 and 96 (respectively 98 and 100) in series and
• phase jump 102 and 104 respectively 106 and 108.
• the end of the optical fiber carrying the network 96 (respectively 100) ends in an index-matching liquid and / or a cleavage at an angle and the end of the optical fiber carrying the network 104 (respectively 108) ends in a photodetector 110 (respectively 112).
• networks 94 and 100 are fixed parallel to each other on an actuator 114 (respectively 116) and 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 includes means 120 for controlling the actuator 118, these means being capable of providing a voltage ramp.
• These means 120 also control the actuator 116 (respectively 114) by means of a signal summator 122 (respectively 124), one input of which therefore receives the output signal from these means 120 and of which the other input receives the signal output of a frequency generator 126, at the frequency f x (respectively 128, at the frequency f 2 ), the frequencies fi and f 2 being different.
• Each extractor therefore removes from the signal to be analyzed S two fine bands respectively centered on two distinct wavelengths, denoted ⁇ ⁇ and ⁇ 3 for one of the extractors and ⁇ 2 and ⁇ 4 for the other extractor.
• these extractors are, as we have seen, used in power modulation according to the distinct frequencies fi and f 2 , fi being associated with ⁇ x and ⁇ 2 while f 2 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.
• a first two-stage extractor scans the spectral bands n ° 1 and 3 (networks corresponding to ⁇ i and ⁇ 3 ). Each stage is modulated by the two frequencies fi and f 2 . (But note that the stages of this extractor could also sweep overlapping spectral ranges, unlike the case of the example).
• the second extractor works on the same principle as the first. The use of two identical actuators for the two extractors requires modulation on the same frequencies.
• the photodetectors 110 and 112 therefore receive the sum of the modulated signals coming from the different stages.
• Signal processing means 130 are therefore provided for separating 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 multifrequential configuration.
• the unpaired configuration is a completely series-like architecture where the different extractors are combined so as to form a single one, having several stages.
• Each stage ⁇ L is modulated ' according to a frequency f ⁇ which is specific to it.
• a single photodetector is then necessary to receive and process all the signals.
• synchronous detection or Fourier analysis of the signal allows the separation of the different wavelength ranges.
• FIG. 16 gives an example of a spectrum analyzer in accordance with the invention, using such a configuration and based on the association of four extractors. More specifically, this spectrum analyzer comprises a circulator 132, a first channel of which receives the signal to be analyzed S, a second channel is associated with a series arrangement of four apodized Bragg gratings 134, 136, 138 and 140, this arrangement being completed by an index adapter liquid and / or a bias cleavage of the optical fiber carrying these four networks, and a third channel is associated with a series connection of four Bragg networks with phase jump 142, 144, 146 and 148 , this assembly being completed by a photodetector 150 connected to electronic means 152 for processing the signals supplied by this photodetector.
• a circulator 132 a first channel of which receives the signal to be analyzed S
• a second channel is associated with a series arrangement of four apodized Bragg gratings 134, 136, 138 and 140
• 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 also comprises means 164 for controlling the actuator 162, these means being capable of providing a voltage ramp.
• These means 164 also control the actuators 154, 156, 158 and 160 respectively by means of four signal summers 166, 168, 170 and 172, one input of which receives the output signal of these means 164 and the other input of which receives the output signal from a frequency generator.
• the latter has the reference 173, 174, 176 or
• 178 and operates at a frequency f x , f 2 , f 3 or f 4 depending on whether it is associated with the actuator 154, 156, 158 or 160.
• the frequencies fi, f 2 , f 3 and f 4 are different and are respectively associated with four different wavelengths ⁇ l, ⁇ 2, ⁇ 3 and ⁇ 4 which correspond respectively to the pairs of networks 134-148, 136-146, 138-144 and 140-142.
• the function of the actuator 162 is as follows: to allow the tunability of all the networks over their spectral range of variation.
• each of the actuators 154, 156, 158 and 160 is as follows: to allow modulation at a frequency f around the tunable wavelength for each apodized network.
• 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 frequencies fl, f2, f3, f4.
• the spectrum analyzer of figure 16 works in a similar way to that of figure 15, except except that all of the extractors are combined in a single stage.
• the photodetector 150 therefore receives the sum of the 4 signals corresponding to the frequencies f1, f2, £ 3 and £ 4.
• Table III summarizes the different elements necessary to achieve an unpaired multifrequential configuration.
• the multi-frequency configuration requires means for processing the signal supplied by the photodetector.
• This processing consists of a temporal detection, for example a synchronous detection.
• this multi-frequency configuration does not require the division of the input signal. It is therefore possible to analyze the entire spectrum simultaneously without significant reduction in the signal to be detected.
• the spectral range of measurement is fixed by the number N of extractors used.
• N the number of extractors used.
• the following formula can be used to define the measurement range:
• Adequate conditioning of the optical fibers used can reduce their elongation limit from 1% to 2%, which makes it possible to double the extent of the spectral range for the same number of extractors.
• the resolution ⁇ is fixed by the width at half-height of the spectral extraction peak and depends on the shape of the two Bragg gratings which are paired. It can range from a few picometers to a few tens of picometers.
• a light source of fixed wavelength can be used 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.
• the insertion losses depend very much on the configuration chosen.
• the unpaired multi-frequency configuration is the one with the least losses. If we assume that the networks are almost saturated, only the circulator gives losses, of the order 1 to l, 3dB.
• the sensitivity of a spectrum analyzer according to the invention depends mainly on the type of photodetector chosen, on the desired resolution (plus the peak of The larger the extraction, the more the signal is detected) and the configuration chosen for the spectrum analyzer (due to insertion losses).
• This sensitivity is also a function of the insertion losses caused by the use of optical couplers and of the other components of the spectrum analyzer.
• the losses are around 7dB.
• the power to be detected therefore drops to -52dBm and -45dBm respectively.
• This sensitivity is also considerably reduced by the frequency acquisition.
• the integration time on the ⁇ band is decisive depending on the level of the signal to be detected.
• the acquisition frequency depends on the level of the signal to be detected but also on the modulation frequencies in the case of an ultra-frequency configuration (to avoid beat phenomena).
• the actuators preferably the piezoelectric actuators, which are important components of the invention.
• An actuator suitable for the traction of 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 straight line with a slope of 0.06 N / ⁇ m.
• the slope of this same straight line is 0.061N / ⁇ m, hence an identical slope.
• the same actuator must allow
• the piezoelectric actuator whose reference is PX 400HL has a blocking force of 12ON and a maximum elongation of
• the simplest consists in using a single actuator capable of cooperating with several optical fibers, for example 8 fibers. In this case, there is a simple configuration.
• a configuration with several channels takes advantage of the possibility of controlling several actuators (for example eight actuators) from the same control module. 8 actuators are therefore used to actuate 8 optical fibers (corresponding to 64 Bragg gratings in all).
• the device thus obtained is therefore very complex.
• FIG. 17 A simple example of the invention, corresponding to such a configuration, is schematically represented 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 f l7 f 2 , f 3 and f 4 .
• Each channel therefore concerns 8 Bragg wavelengths. 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. Note that in Figure 17
• 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, capable of supplying the latter with a voltage ramp
• the references FI, F2, F3 and F4 represent signal summers and frequency generators which are respectively associated with the frequencies f x , f 2 , f 3 and f 4 and respectively connect the actuator Al to the actuator A5, the actuator A2 to the actuator A6, the actuator A3 to 1 actuator A7 and actuator A4 to actuator A8, these actuators having the function of achieving both wavelength tunability and the power modulation described above
• the references dl, 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 frequencies fl, f2 , f3 and f4, in particular by synchronous detection.
• references ri, 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 in FIG. 17 are therefore the following:
• the device of FIG. 17 offers a spectral range twice as large and four times more analysis channels than the previous solution. This device can therefore be considered as the integration of four spectrum analyzers, broadband and high resolution.
• the time Tl represents the duration of the scanning.

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• Spectroscopy & Molecular Physics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Communication System (AREA)
• Spectrometry And Color Measurement (AREA)
• Testing Of Optical Devices Or Fibers (AREA)

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