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
  • 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
  • G02B6/29379—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 characterised by the function or use of the complete device
  • G02B6/29389—Bandpass filtering, e.g. 1x1 device rejecting or passing certain wavelengths

Definitions

• the present invention relates to an optical filtering device of the bandpass type, which operates by transmission, and more precisely to a device called a "high finesse spectral extractor", allowing the extraction of a fine spectral band from an optical signal d 'Entrance.
• this device can be easily made tunable in wavelength over a spectral range ⁇ and / or adjustable in power at a maximum frequency Fmax.
• the latter and the spectral range are defined by the users, preferably by means of an actuator.
• the invention has various applications to which we will return later, in particular in the field of optical telecommunications.
• a filter comprising a "phasar" or optical phase array.
• the width of its passband is generally greater than a few tens of picometers and it is difficult to make it tunable in wavelength and / or adjustable in power.
• this component has drawbacks: it provides a signal comprising several orders and does not allow the power of the filtered signal to be modulated in a simple manner.
• the object of the present invention is to remedy the above drawbacks by proposing an optical filtering device which can be integrated into optical waveguides (preferably optical fibers) and which is capable of being easily tuned in wavelength and / or power modulated.
• this device is capable of providing a filtered signal comprising only a single peak of very small spectral width around a Bragg wavelength, without simultaneously providing filtering peaks in the vicinity of the central peak, such as the would make the Fabry-Perot type devices.
• the subject of the present invention is an optical filtering device, characterized in that it comprises: at least one assembly comprising first and second components, the first component being an apodized Bragg grating ("apodized Bragg grating”) ), used in reflection and having a first filter band, the second component being a Bragg grating with at least one phase shift, used in transmission and having at least a second filter band and comprising at least one transmission peak of great fineness, that is to say at least a transmission peak whose width at half height is less than 100 pm but preferably of the order of 20 pm or less (this width being typically between 10 pm and 20 pm) the first filter band being included in the second filter band , and
• the first and second components of the assembly cooperating with each other and with the optical link means to allow an optical signal, which interacts with one of these first and second components , then interact with the other and be filtered by this set which thus extracts only the part corresponding to the transmission peak of the second component.
• the relative arrangement of the optical connection means and the first and second components is provided so that the optical signal interacts first with the first component then with the second component or, conversely, first with the second component then with the first component.
• this device further comprises an actuator, to which the first and second components are fixed and which is designed to simultaneously modify the respective Bragg wavelengths of these first and second components, so that you can tune the device spectrally.
• this device further comprises an actuator, to which the first component is fixed and which is provided for modifying the Bragg wavelength of this first component, so as to be able to modulate the amplitude of the signal filtered, supplied by the device.
• this device further comprises a pair of actuators, to which the first and second components are fixed respectively and which is provided for differentially modifying the respective Bragg wavelengths of the first and second components, so as to be able to both spectrally tune the device and modulate the amplitude of the filtered signal.
• Each actuator may include an element chosen from piezoelectric transducers operating in traction, piezoelectric transducers operating in compression, thermal transducers, mechanical deformation means and elements made of shape memory alloys.
• the optical connection means can comprise an optical coupler, provided with at least one optical isolator, or an optical circulator.
• the device which is the subject of the invention comprises a plurality of sets of first and second components, the first components are mounted in series, the second components are also mounted in series and the second filter bands corresponding respectively to the second components are spaced from one another, to allow the optical signal to be filtered by the assemblies which thus extract therefrom only the parts corresponding respectively to the transmission peaks of the second components.
• the device which is the subject of the invention comprises a plurality of sets of first and second components, the first components are mounted in parallel, the second components are also mounted in parallel and the optical connection means comprise optical switching means provided for successively interacting the optical signal with each of the assemblies so as to extract only the parts of this signal corresponding respectively to the transmission peaks of the second components.
• FIG. 1 shows the impulse response of an extractor , given in transmission
• FIG. 2 shows the shape of the spectral reflection coefficient of a Bragg grating
• FIG. 3 shows the spectral response, given in reflection, of a Bragg grating apodized by a sinusoidal function
• FIG. 5 is a schematic view of a three-way circulator
• FIG. 6 is a schematic view of an optical isolator
• FIG. 7 is a schematic view of an optical fiber coupler
• FIG. 8 schematically illustrates an actuator making it possible to vary the stress and / or the temperature applied to a Bragg grating
• FIGS. 9 and 10 are schematic views of extractors in accordance with the invention.
• FIG. 11 shows the output spectrum of an extractor according to the invention, in dB, as a function of the difference in the wavelength
• FIG. 12 illustrates the modification of the shape of the extractor spectral response as a function of different phase-jumped gratings
• FIG. 13 schematically illustrates an assembly of an optical coupler and an optical isolator, capable of replacing a circulator
• FIG. 14 is a schematic view of an extractor according to the invention, which is adjustable in power
• FIG. 15 shows the spectral response of an extractor according to the invention, which is adjustable in power
• FIG. 16 is a schematic view of a device according to the invention, using extractors mounted in series, and
• FIG. 17 is a schematic view of another device according to the invention, using extractors mounted in parallel.
• the invention relates to a very fine spectral extractor which is a device capable of extracting, from a guided optical input signal, a band of small spectral width ⁇ , centered around a wavelength called length of extraction wave and denoted ⁇ e .
• a very fine spectral extractor which is a device capable of extracting, from a guided optical input signal, a band of small spectral width ⁇ , centered around a wavelength called length of extraction wave and denoted ⁇ e .
• Such an extraction device is connected, on one side, to an optical input guide and, on the other, to an optical output guide.
• 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.
• equation (1) If the extractor is tunable according to the function ⁇ e (t), equation (1) becomes:
• Another functionality can be added to the simple extraction device. This is the possibility of modulating the power of the response given by the device.
• a complete extractor is a device that integrates the different functions 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 optical filtering device, or extractor, according to the invention can be characterized by the following parameters: ⁇ : width of the filtering band pi: loss on insertion ⁇ : rejection rate
• ⁇ minimum increment
• ⁇ scanning interval (spectral range)
• sp frequency of spectral modulation
• / pu power modulation frequency
• the filter band characterizes the finesse with which the extractor filters the input signal.
• the rejection rate ⁇ r (expressed in dB) accounts for the isolation of the peak in the filter band in terms of power. It is defined, for a given distance ⁇ r relative to the extraction wavelength (see Figure 1) by the relation:
• the minimum increment ⁇ is a parameter specific to the tunable extractor. It is the spectral increment separating two successive positions of the wavelength.
• the scanning interval ⁇ relates to the tunable extractor. It characterizes the spectral band separating the smallest wavelength from the longest wavelength that can be extracted over the entire spectrum by the device.
• the spectral modulation frequency / sp is the frequency at which the wavelength can be tuned over the entire spectral range ⁇ .
• the power modulation frequency relates to the modular power extractor in the case where the modulation is done over a time period.
• the frequency / max is the power modulation frequency which can be applied to the extractor.
• the power modulation amplitude is the value of the maximum modulation amplitude ⁇ max which can be applied to the device.
• Bragg gratings is a diffracting structure produced longitudinally in an optical fiber by a periodic modification of the refractive index of the core of the fiber. This index can then be represented by the following relation, which involves the step ⁇ of the resulting modulation:
• ⁇ n avg can depend on z.
• these two parameters are rectangular functions, defined over the length L of the grating.
• Figure 2 gives the typical shape of the spectral reflection coefficient R of a grating of
• Apodized Bragg grating As can be seen on In Figure 2, the spectrum of a uniform network has large side lobes. The creation of an apodized network is based on the principle of the reduction of these lobes.
• ⁇ n mod (z) is no longer a rectangle function but a symmetrical function with respect to the center of the network, gradually decreasing towards the edges of it.
• ⁇ n raod (z) is a Gaussian (formula (8)) or sinusoidal (formula (9)) function:
• Figure 3 shows the typical shape of the spectral reflection coefficient R of a Bragg grating apodized by a sinusoidal function, as a function of the wavelength difference ⁇ (nm) compared to the abscissa of the maximum of R .
• the network parameters are: length
• a phase jump Bragg grating is characterized by the presence of a double resonance.
• the first is the classical resonance of the Bragg grating, which creates the filter band which was previously discussed.
• the second resonance comes from a punctual change in the phase of the guided signal when it crosses the network. This second phenomenon can cause a counter-resonance at the wavelength of
• FIG. 5 shows a diagram of this circulator.
• An optical isolator is one. fiber component which allows the passage of a guided wave in a single direction from a vl channel to a v2 channel (insulation from 35dB to 40dB from v2 to vl).
• the diagram of an example of an optical isolator 4 is shown in FIG. 6.
• FIG. 7 An example of an optical coupler is illustrated in FIG. 7.
• This coupler 6 has four channels v1, v2, v'I and v'2. It distributes an input signal to two output channels symmetrically.
• an incident signal on the channel v1 is distributed in the channels v'I and v'2 with coupling coefficients which are generally equal to 0.5 and 0.5.
• An incident signal on channel v'I is distributed identically in channels vl and v2.
• the tunability of a Bragg grating can be obtained in different ways. The most common take advantage of the variation in the wavelength of
• a fiber of lm in length which lengthens by l ⁇ m undergoes a relative elongation by l ⁇ .
• FIG. 8 is a schematic view of an example of an actuator 8.
• the Bragg grating 10, formed in an optical fiber 12, is represented by an alternation zone 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.
• a device comprises an apodized network used in reflection (characterized by a coefficient R ra ( ⁇ )) and a phase jump network used in transmission (characterized by a coefficient T rsp ( ⁇ )).
• optical component which can be a circulator or a coupler.
• a device according to the invention in an RT configuration, is schematically represented in FIG. 9 and comprises an apodized network 18 and a phase jump network 20, which are mounted on a support 22, as well as a circulator 24 to three way
• 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.
• 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.
• phase jump network is extracted from the signal.
• the signal reflected by the latter is lost in the insulation of the circulator. No resonance effect (of the Fabry-Perot type) is therefore observed between the two networks.
• 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.
• a device according to the invention, using this configuration TR, is schematically represented in FIG. 10 and uses the same components as the device in FIG. 9, arranged as seen in FIG. 10.
• the input signal S is guided in a fiber 30 to the phase jump network 20.
• the transmitted signal enters via the channel v1 in the circulator which directs it to the channel v2 where it meets the apodized network 18.
• the signal reflected spring 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 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 device of FIG. 10.
• 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 that enters the device through the output has no effect.
• FIG. 11 shows the output spectrum T of an extractor according to the invention, in RT or TR configuration, expressed in decibels, as a function of the difference in wavelength, denoted ⁇ ( in nm).
• Figure 12 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 (answer III). It is thus noted that the width ⁇ can be granted. It is specified that the associated apodized network has a length of 4mm and a parameter ⁇ n m0d 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.
• 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 vl, v2 and v3 in Figure 13 correspond respectively to the channels v3, v2 and vl in Figure 9 or the channels vl, v2 and v3 in Figure 10.
• a second isolator can be used at the input or output of the component in order to improve its insulation.
• the use of the circulator is advantageous compared to that of the coupler-isolator association because one of the coupler's channels is not used and part of the signal is lost because of this channel.
• the principle of a tunable extractor according to the invention is very simple, taking into account the above: if the two networks of FIG. 9 or of FIG. 10 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. 8 and which is provided with control means 14a, it is possible to modify the Bragg wavelength simultaneously on the two networks.
• the spectral response of the extractor is therefore translated spectrally over the range of tunability.
• the sensitivity and the scanning interval depend both on the type of actuator and on the Bragg gratings chosen.
• a device in accordance with the invention, forming a power-adjustable extractor, requires a configuration different from the previous one.
• An example of this device is schematically represented in FIG. 14.
• This device of FIG. 14 can be deduced from the device of FIG. 9 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 below.
• the network 18a is fixed to an actuator 8b, of the type of the actuator 8 in FIG. 8 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.
• 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.
• an apodized network with a lower amplitude of modulation than network 18 is used, namely 2xl0 "4 instead of 4xl0 " 4 .
• An extractor can be produced according to
• An example of such an extractor can be obtained from the device of FIG. 14, by fixing the network 20 to an actuator 8c of the type of the actuator 8 of FIG. 8 and controlled by means 14c, this actuator 8c being itself fixed on the support 22.
• FIG. 16 This is technically illustrated in FIG. 16 where a device according to the invention uses three extractors E1, E2, and E3 in series, in the RT configuration.
• Each extractor E1 or E2 or E3 comprises an apodized network 18 ⁇ or 18 2 or 18 3 as well as a phase jump network 20i or 20 2 or 20 3 , these two networks being mounted on a support 22 ⁇ or 22 2 or 22 3 .
• the networks 18 ⁇ , 18 2 and 18 3 (respectively 20 ⁇ , 20 2 and 20 3 ) are formed in series on the same optical fiber 38 (respectively 40).
• the circulator 24 of FIGS. 9 and 10 is also used, the channel v2 (respectively v3) of which is connected to one end of the fiber 38 (respectively 40).
• the other end of the fiber 38 is provided with an adapter liquid of index 42 and / or cleaved at an angle.
• the signal S to be filtered is also sent on the channel v1 of the circulator and the filtered signal s_ is available at the other end of the optical fiber 40.
• the three extractors are respectively tuned to extraction wavelengths ⁇ el, ⁇ e2 and ⁇ e3.
• the isolator can be replaced by an optical coupler provided with an optical isolator.
• a parallel association of extractors is also possible.
• it requires the use of three optical switches of the type 1 to N, where N is the number of extractors placed in parallel.
• N is the number of extractors placed in parallel.
• FIG. 17 where a device according to the invention, of RT configuration, uses: three extractors which are mounted in parallel, are respectively tuned on three different wavelengths ⁇ el, ⁇ e2 and ⁇ e3 and each include a apodized network 44 ⁇ or 44 2 or 44 3 as well as a phase jump network 46 ⁇ or 46 2 or 46 3 ,
• the apodized networks are fixed to a support 54 and the networks are of phase are fixed to a support 56.
• the channel v1 of the circulator receives the signal
• Channel v2 (respectively v3) is connected to the input of the optical switch 48 (respectively 50).
• the three apodized networks are respectively connected, on one side, to the three outputs of the switch 48 and, on the other side, to three adapter media of index 58, 60 and 62 (and / or the ends of the optical fibers on which are formed these networks are cleaved at an angle).
• the three phase jump networks are respectively connected, on one side, to the three outputs of the switch 50 and, on the other side, to the three inputs of the switch 52.
• the output of the latter provides the filtered signal s.
• the switches 48 and 50 of the control are controlled. so as to connect the two networks of the same extractor via the circulator and the switch 52 is also controlled so as to recover the wavelength thus filtered. In the example shown, we recover ⁇ i by connecting the 44 ⁇ and 46 ⁇ networks to each other.
• the isolator can be replaced by an optical coupler provided with an optical isolator.
• the invention has many applications: it can be used not only in the field of optical telecommunications but also in the field of instrumentation, for manufacturing fiber optic sensors.
• the invention also applies to the manufacture of a tunable wavelength modulator for use in the field of telecommunications which implement DWDM or dense wavelength division multiplexing ("Dense Wavelength Division Multiplexing") .
• a tunable wavelength modulator for use in the field of telecommunications which implement DWDM or dense wavelength division multiplexing ("Dense Wavelength Division Multiplexing") .
• the combination of an apodized network and a phase jump network makes it possible to constitute a sensor in the same way as a Bragg network but with great sensitivity due to the very low bandwidth of the spectral response.
• the filter band has a width ⁇ which can extend from a few picometers to a few tens of picometers.
• Losses on insertion depend on the type of extractor chosen and the quality of the circulator used. This circulator introduces typical losses of 0.8dB from one channel to the other. A minimum value of 1.6dB (approximately 31% loss) must therefore be assigned to the pump. In the case of the modular extractor, the association of the two networks can lead to losses of up to 20%, whence a total of 45% of losses.
• the extinction rate depends on the spectral characteristics of the networks used. This rate can change in particular during the scanning of the apodized network with respect to the phase jump. However, a typical theoretical value is ⁇ r ⁇ 30dB at 0.2nm around ⁇ B. 2) Parameters 1 tunability / modulation
• the parameters linked to the tunability and modulation properties of the extractors are linked to the characteristics of the actuator (or actuators) used.
• actuator or actuators
• two cases of agreement are considered in a non-exhaustive manner, namely an agreement obtained by a Peltier element and a tuning obtained by a piezoelectric element.
• Shape memory alloys can also be used, as can any system based on the deformation of an object, for example by curvature or by bending.
• the means generally used for thermal transduction is a Peltier element.
• a Peltier element thermal cooler can be considered a heat pump.
• thermocouples which constitute it When the thermocouples which constitute it are connected to a direct current source, it generates a transfer of heat from one of its sides to the other. One side therefore cools while the other heats up. A temperature variation ⁇ T is thus obtained between one face and the other. By reversing the current, the temperature distribution is also reversed.
• the temperature variation ⁇ T remains limited, however.
• a typical maximum value is equal to 70 ° C, hence a tunability ⁇ equal to 0.8nm.
• Peltier elements are available from the DLK Company.
• the Peltier element is more generally used to stabilize a temperature circuit.
• the tunability of networks thus preferably uses piezoelectric transducers, which are faster than thermal transducers.
• a piezoelectric transducer is formed from ceramic materials which have piezoelectric properties: under the effect of an electric field its ceramic structure is deformed. A component such as an optical fiber, which is fixed to the transducer, therefore undergoes elongation (or compression) accompanied by a certain force.
• the relative elongation and the applied force are linked, for a given tension, by a linear relation which is defined by two characteristic points: the blocking force F b at a given tension, which corresponds to the maximum force developed by the piezo transducer -electric when it is blocked in extension, and
• This optical fiber also admits a curve of evolution of its elongation as a function of the applied force. If these two quantities occur along the same axis (generally the fiber axis) we have the relationship:
• the curves relating to the piezoelectric transducer and to the fiber thus define an operating point making it possible to predict the tunability of a Bragg grating.
• the work developed is maximum when the operating point corresponds to the midpoint of the curve of the piezoelectric transducer.
• Piezoelectric transducers are available from Piezo Systems, Inc.
• the Young's modulus E of silica is 7 ⁇ 10 10 N / m.
• this value can be raised to 2xl0 4 ⁇ , or even more for Bragg gratings of very good quality. It should be noted that, for compression, the limit is much higher and is worth approximately 10 5 ⁇ .
• the midpoint criterion requires a piezoelectric transducer whose blocking force
• F b is approximately 18N and the maximum elongation 200 ⁇ m (such transducers being commercially available). This results in an amplitude ⁇ of between 10 nm and 20 nm for the scanning interval.
• the actuator which verifies the characteristics given above has a sub-nanometric resolution. For a 10mm long network, this gives a sensitivity ⁇ less than 1.2pm over ⁇ B.
• the modulation frequency depends on the other characteristics of the device, especially if tuning is to be considered.
• the power modulation requiring a special differential tuning over a small range (about lOOpm), / pu is much greater than / sp .
• frequencies f pu of the order of a few kilohertz can be proposed.
• a resistive or capacitive gauge In order to control the stresses generated by the actuator, a resistive or capacitive gauge must be added to it. Indeed, the response of piezoelectric ceramic to an input current is not linear: it is affected by a hysteresis phenomenon.
• this device is sensitive to temperature.
• this device for measurements over a long period or in a thermally disturbed environment, it is preferable to place this device in an isothermal enclosure or to provide it with temperature regulation means.
• examples of the invention have been considered in which the Bragg grating with phase jump only comprises one phase jump.
• the invention it is also possible to use a Bragg grating comprising more than one phase jump.

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