Passive infrared imaging system based on parametric upconversion

Abstract

The application provides a passive infrared imaging system based on parametric up-conversion, and relates to the technical field of infrared detection, and comprises: a pump laser for outputting pump light; a thermal radiation modulation assembly for receiving and modulating the radiation light of an object to be imaged and outputting modulated signal light to a back-wall mode microcavity chip; an optical filter; a photoelectric detector for detecting the output signal of the back-wall mode microcavity chip to generate light intensity information; the back-wall mode microcavity chip generates up-conversion light; a control and calculation imaging module for controlling the modulation process of the thermal radiation modulation assembly and receiving corresponding light intensity information, so as to obtain single-pixel up-conversion light intensity information and perform infrared image reconstruction. The application realizes high-sensitivity passive imaging of far-infrared spontaneous radiation light in an object at room temperature, and has the advantages of high integrability and imaging not affected by the size of a detector array.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of infrared detection technology, in particular to a passive infrared imaging system based on parametric up-conversion. BACKGROUND

[0002] All objects with temperature higher than absolute zero can spontaneously radiate electromagnetic waves, and this spontaneous radiation is also called thermal radiation. The thermal radiation energy of a normal temperature object is mainly concentrated in the infrared waveband with a longer wavelength. Passive infrared imaging mainly refers to imaging by measuring the infrared light spontaneously radiated by an object. Compared with visible and near-infrared wavebands, the research and application of mid-infrared waveband emitted by a normal temperature black body are still relatively lagging behind, and there is a lack of high-sensitivity and large-array imaging devices.

[0003] At present, passive infrared imaging is mainly realized by using thermal sensitive devices. They mainly use the thermal effect of infrared radiation to change the electrical characteristics of the devices to perform detection and imaging. However, the sensitivity of thermal sensitive devices is often low, and they are not suitable for environments with high precision requirements. Semiconductor photosensitive devices, such as mercury cadmium telluride detectors, not only need ultralow-temperature refrigeration in actual use, but also are difficult to realize large-scale imaging arrays and are high in cost.

[0004] Parametric up-conversion is to convert low-frequency mid-infrared light to high-frequency near-infrared and visible light wavebands through optical nonlinear effects. The near-infrared and visible light wavebands have low-cost, superior performance, and single-photon-level detection sensitivity. However, existing parametric conversion is mostly realized by using nonlinear crystals. In order to improve the conversion efficiency, pulsed light is often used to irradiate the imaging object to measure the reflected light, which is not suitable for passive infrared imaging. In addition, the field of view size, shape and quality of imaging are limited by the phase matching condition, and it is difficult to achieve efficient and complete infrared imaging. SUMMARY

[0005] The present application aims to at least solve one of the problems in the prior art or related art, and provides a passive infrared imaging system based on parametric up-conversion. The system not only realizes high-sensitivity passive imaging of the spontaneous radiation of mid-infrared light of an object at normal temperature, but also has the advantages of strong integrability and imaging not affected by the size of the detector array.

[0006] The application is implemented through the following technical scheme: a passive infrared imaging system based on parametric up-conversion, comprising: a pump laser connected with an input end of an echo-wall mode microcavity chip through an optical fiber and used for outputting pump light; a thermal radiation modulation assembly connected with the input end of the echo-wall mode microcavity chip through an optical fiber and used for receiving and modulating radiation light of an object to be imaged and outputting modulated signal light to the echo-wall mode microcavity chip; an optical filter connected with an output end of the echo-wall mode microcavity chip through an optical fiber; a photodetector connected with an output end of the optical filter and used for detecting an output signal of the echo-wall mode microcavity chip to generate light intensity information; the echo-wall mode microcavity chip receives the pump light and the modulated signal light, and generates up-conversion light in the cavity by means of second-order sum frequency effect in nonlinear optics; and a control and calculation imaging module connected with the thermal radiation modulation assembly and the output end of the photodetector, used for controlling a modulation process of the thermal radiation modulation assembly and receiving corresponding light intensity information, so as to obtain single-pixel up-conversion light intensity information and perform infrared image reconstruction.

[0007] In the technical scheme, the system comprises two optical paths: the light source of the optical path one is the pump laser, which is connected with the echo-wall mode microcavity chip, the optical filter and the photodetector through the optical fiber in sequence; the optical path two is infrared radiation light emitted by the object to be imaged, which enters the mid-infrared optical fiber after being modulated by the thermal radiation modulation assembly, and finally the infrared radiation light is coupled into the waveguide on the echo-wall mode microcavity chip, and the pump light and the infrared radiation light generate high-frequency up-conversion light in the microcavity through nonlinear frequency conversion. The light field is coupled into and out of the microcavity chip through the lens optical fiber. The control and calculation imaging module controls the modulation process according to the preset program, receives the up-conversion light intensity information measured by the photodetector, and finally obtains the infrared radiation image of the object through calculation. The system takes the echo-wall mode microcavity as a nonlinear optical platform, modulates the mid-infrared light emitted by the object to be measured in space, and injects the modulated mid-infrared light and the pump light in the near-infrared or visible light band into the microcavity. Then, the second-order sum frequency effect in nonlinear optics is used to convert the measured mid-infrared light into up-conversion light in the near-infrared or visible light band in the cavity. Then, the up-conversion light intensity is obtained by filtering and using the photodetector. Each measurement obtains the up-conversion light intensity under a certain spatial modulation state, that is, the intensity information of a single pixel. A plurality of modulation states are programmed according to a specific rule, and a multi-state scan is performed to obtain a plurality of single-pixel intensity information, and finally the infrared image of the object with spatial resolution is reconstructed through calculation.

[0008] According to the passive infrared imaging system based on parametric up-conversion provided by the application, preferably, the thermal radiation modulation assembly specifically comprises: an optical imaging and filtering lens group, which receives the radiation light of the object to be imaged; a spatial light modulator, which modulates the radiation light according to the control signal of the control and calculation imaging module; and a focusing lens, which converges the output light of the spatial light modulator.

[0009] In this technical solution, the pump laser injects pump light into one waveguide on the chip via a lensed fiber, and then enters the ring microcavity via evanescent field coupling. The thermal radiation signal light emitted by the object to be imaged is first filtered by a filter group to remove background noise, and the light field of the measurement spectrum is selected. Then, the two-dimensional signal light field is programmably binary masked and modulated by a spatial light modulator to construct the detected single-pixel image intensity. Next, the spatial light field is focused by a focusing lens into a mid-infrared fiber, and then coupled into another mid-infrared waveguide on the chip via the mid-infrared lensed fiber, and finally enters the microcavity via evanescent field coupling as well.

[0010] According to the passive infrared imaging system based on parametric upconversion provided by the present invention, preferably, the infrared image reconstruction step specifically includes: while keeping the pump light power constant, performing different mask encodings on the radiation light field, and obtaining the single-pixel upconverted light intensity information of the corresponding mask encoding to characterize the single-pixel intensity of the two-dimensional radiation light field under the mask encoding; assuming that the pixelated image I of the object to be imaged has a sparse representation in a certain basis R, image I is represented as:

[0011] I = RT

[0012] Where T is a set of coefficients, and the basis R is a basis that is considered to fit the expected spatial properties of the image, including wavelet basis, Fourier basis or discrete cosine basis.

[0013] The single-pixel intensity matrix S is:

[0014] S = PI

[0015] Where P is the binary base loaded on the spatial light modulator, and PRT = S;

[0016] The reconstructed pixelated image I is calculated * It can be represented as

[0017] I * =RT *

[0018] T in the formula * The following relationship must be satisfied:

[0019] T * =argmin||T||1.

[0020] According to the passive infrared imaging system based on parametric upconversion provided by the present invention, preferably, the step of generating upconversion light specifically includes: determining the geometry of the microcavity and the mode distribution of the optical field based on the wavelength of the modulation signal light and the wavelength of the pump light, such that the frequency of the optical field and the effective refractive index of the mode satisfy the following relationship:

[0021] ω s +ω p = ω up

[0022] n s ω s + n p ω p = n up ω up

[0023] where ω s , ω p , ω up are the frequencies of the modulated signal light, pump light, and up-converted light, and the effective refractive indices of the corresponding optical modes are denoted as n s , n p , n up , respectively.

[0024] In the technical solution, the optical field in the microcavity can efficiently generate the sum-frequency effect in the three-wave mixing as long as it satisfies certain conditions. First, the injected signal light, pump light, and generated up-converted light all need to approximately satisfy the resonance condition of the microcavity, i.e. the integer multiple of the optical wavelength is equal to the optical path length around the cavity at the corresponding wavelength. The single resonance peak of the microcavity has a certain bandwidth, and the size of the bandwidth is inversely proportional to the Q value of the microcavity. The optical field can resonate within a certain bandwidth, and the loss of the optical field is different, so the resonance condition does not need to be satisfied very strictly.

[0025] According to the passive infrared imaging system based on parametric up-conversion provided by the application, preferably, the whispering gallery mode microcavity chip comprises a micro-ring cavity structure, the micro-ring cavity structure is a GaAs waveguide with isosceles trapezoidal shape, the waveguide cross-section size is lower base 10 μm and height 3 μm, the trapezoidal wedge angle is 60°, and the microcavity radius is 50 μm-1500 μm.

[0026] According to the passive infrared imaging system based on parametric up-conversion provided by the application, preferably, the preparation process of the whispering gallery mode microcavity chip comprises: sequentially growing a 10 μm thick Al 0.8 Ga 0.2 As layer and a 3 μm thick GaAs film on a 500 μm thick GaAs substrate; plasma-activating the chip, and then bonding the GaAs film layer on a SiO2 layer on the surface of a 3 μm thick thermal silicon wafer; and removing the GaAs substrate and the Al 0.8 Ga 0.2As layer; depositing SiO2 on the GaAs film layer, and writing the cavity geometry into the deposited SiO2 layer by developing technology; removing the excess SiO2 and GaAs by chemical etching, and controlling the wedge angle size by etching time and dose size; depositing another SiO2 layer on the sample, so that the ring cavity of the GaAs is entirely wrapped by the SiO2.

[0027] The parametric up-conversion based passive infrared imaging system provided by the application has the advantages that the spatial light modulator is preferably a digital micromirror device composed of 1024*768 closely arranged mirror arrays.

[0028] The parametric up-conversion based passive infrared imaging system provided by the application has the advantages that the pump light is preferably single-frequency continuous light, and the wavelength is located in the near-infrared or visible light band.

[0029] The parametric up-conversion based passive infrared imaging system provided by the application has the advantages that the working waveband of the photoelectric detector is preferably the visible light band or the near-infrared light band.

[0030] The parametric up-conversion based passive infrared imaging system provided by the application has the advantages that the optical filter is preferably a fiber Bragg grating.

[0031] The parametric up-conversion based passive infrared imaging system provided by the application has the advantages that the working waveband of the photoelectric detector is preferably the visible light band or the near-infrared light band.

[0032] (1) The process of processing the GaAs film into an echo wall mode microcavity is relatively mature, and the optical platform is easy to prepare by using existing technical means.

[0033] (2) The system has the potential of on-chip integration, and is conducive to realizing a small-sized, portable high-sensitivity infrared passive imaging system.

[0034] (3) The scheme of combining the nonlinear parametric up-conversion with a commercial near-infrared and visible light detector with high sensitivity can realize detection and imaging at room temperature, avoids the problem that the traditional mid-infrared high-sensitivity detector needs super-low temperature refrigeration, and does not need a large-volume temperature control and cooling device, thereby reducing the cost.

[0035] (4) The echo wall mode microcavity is used as a nonlinear platform, the super-high Q value and the great light field energy density of the echo wall mode microcavity make the power threshold required for the parametric up-conversion effect to occur be significantly reduced, and the scheme can convert the continuous wave of the spontaneous radiation of an object, thereby realizing passive imaging, which is different from the use of a nonlinear crystal to perform up-conversion, which often needs to use a pulsed light source to irradiate.

[0036] (5) The single-pixel imaging is realized by using the spatial light modulator and algorithm demodulation, which is compatible with the microcavity parametric up-conversion, does not need a large-scale imaging array, and avoids the problem that there is a lack of a large-scale detector array in the current mid-infrared waveband. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 A structure schematic diagram of a passive infrared imaging system based on parametric up-conversion according to an embodiment of the present application is shown.

[0038] Figure 2 A simulation result diagram according to an embodiment of the present application is shown. DETAILED DESCRIPTION

[0039] In order to enable a more clearly understanding of the above-mentioned purposes, features and advantages of the present application, the present application will be further described in detail below in combination with the drawings and specific embodiments.

[0040] In the description of the present application, it should be understood that the terms “center”, “longitudinal”, “lateral”, “upper”, “lower”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inner”, “outer” and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. For those of ordinary skill in the art, the specific meanings of the above-mentioned terms in the present application can be understood through specific circumstances.

[0041] As shown in Figure 1 The present application provides a passive infrared imaging system based on parametric up-conversion, which comprises a pump laser (1), an object to be imaged (2), an optical imaging and filtering lens group (3), a spatial light modulator (4), a focusing lens (5), an acoustic whispering gallery mode microcavity chip (6), an optical filter (7), a photodetector (8), and a control and calculation imaging module (9).

[0042] In this embodiment, the output laser of the pump laser is single-frequency continuous light, and the wavelength is in the near-infrared and visible light band, and the output wavelength is preferably 1550 nm. The spatial light modulator is preferably a digital micromirror device composed of 1024x768 closely arranged mirror arrays. The acoustic whispering gallery mode microcavity is preferably designed as an isosceles trapezoidal gallium arsenide (GaAs) waveguide, and the waveguide preferably has a cross-sectional size of 10 μm in lower base and 3 μm in height, a trapezoidal wedge angle of 60°, and a microcavity radius of 50 to 1500 μm. The working wavelength band of the photodetector is the visible or near-infrared light band, and the photodetector is preferably an indium gallium arsenide (InGaAs) detector. The control and calculation imaging module can realize related functions by a customized electronic calculation chip, so that the entire system can realize higher integration and portability.

[0043] The pump laser is a continuous-wave laser connected to an optical fiber. During operation, it emits high-power single-frequency near-infrared or visible light. The pump light is injected into a waveguide on the microcavity chip via a lensed fiber, and then enters the ring microcavity via evanescent field coupling. The thermal radiation signal light emitted by the object to be imaged is first filtered to remove background noise, selecting the light field of the measurement spectrum. Then, the two-dimensional signal light field is programmably binary masked and modulated by a spatial light modulator to construct the detected single-pixel image intensity. Next, the spatial light field is focused by a focusing lens into a mid-infrared fiber, coupled through a mid-infrared lensed fiber into another mid-infrared waveguide on the microcavity chip, and finally enters the microcavity via evanescent field coupling. At this point, the light field in the microcavity can efficiently undergo the sum-frequency effect in three-wave mixing if certain conditions are met. First, the injected signal light, pump light, and generated upconversion light all need to approximately satisfy the microcavity's resonance condition, i.e., an integer multiple of the light wavelength equals the optical path length of one revolution within the cavity at the corresponding wavelength. A single resonant peak in a microcavity has a certain bandwidth, the size of which is inversely proportional to the Q value of the microcavity. Within a certain bandwidth, it can resonate with the microcavity, only the optical field loss varies. Therefore, the resonance condition does not need to be perfectly satisfied. Second, parametric conversion requires the energy conservation relationship to be satisfied, and high-efficiency conversion requires phase matching. Let the frequencies of the signal light, pump light, and upconversion light be ω... s ω p ω up The effective refractive indices of the corresponding optical modes are n s n p n up Then the constraint can be expressed as:

[0044] ω s +ω p =ω up

[0045] n s ω s +n p ω p =n up ω up

[0046] Therefore, based on the above two equations, and combining the wavelengths of the signal light and pump light to be converted, we need to determine the geometry of the microcavity and the mode distribution of each optical field through simulation calculations, so that the frequencies of the three optical fields and the effective refractive index of the modes satisfy the above relationship.

[0047] like Figure 2The finite element simulation results of the mode field energy distribution of a GaAs microcavity are shown, which satisfy the energy conservation and almost perfect phase matching relationship. The figure shows a GaAs waveguide microcavity with a radius of 200 μm along the radial cross section, which is a isosceles trapezoid. The trapezoidal geometry size is 10 μm for the lower base, 3 μm for the height, and 60° for the wedge angle. Figure 2 The three parts (a), (b), (c) in the middle are the electric field intensity distributions of the signal light to be measured, the pump light, and the upconversion light mode respectively. The darker the color, the stronger the relative electric field intensity. The wavelengths of the three are 5.1980 μm, 1.5497 μm and 1.1938 μm respectively, and the corresponding equivalent refractive indices calculated by simulation are 3.2595, 3.4825 and 3.4318 respectively.

[0048] The upconversion light formed in the microcavity through the parametric upconversion process is also in the near-infrared or visible light band, which can be coupled out of the microcavity by the waveguide of the pump light, and then collected into the optical fiber through the lens fiber and transmitted to the optical filter. The optical filter can be an optical fiber Bragg grating, which filters out other wavelengths of light other than the upconversion light. The low-noise upconversion light obtained is finally received by the photodetector and the single-pixel light field intensity information is measured.

[0049] The control and calculation imaging module controls the spatial light modulator to perform different mask coding on the signal light field, and receives the single-pixel upconversion light intensity information corresponding to the mask coding obtained by the photodetector. In the case of keeping the pump light power unchanged, the generated upconversion light intensity will be in a certain proportion to the injected signal light intensity, so the measured upconversion light intensity can represent the single-pixel intensity of the two-dimensional signal light field under the mask coding.

[0050] Assuming that the pixelated image I to be measured has a sparse representation in a certain basis R, the image I can be represented as:

[0051] I = RT

[0052] T is a set of coefficients, and the selected basis R is a basis that is considered suitable for the expected spatial properties of the image, such as a wavelet basis, a Fourier basis or a discrete cosine basis. In actual measurement, the single-pixel intensity matrix S obtained can be represented as:

[0053] S = PI

[0054] where P is a binary basis loaded on the spatial light modulator, and PRT = S. Therefore, the reconstructed pixelated image I * can be represented as:

[0055] I * = RT *

[0056] T in the formula * satisfying the following relationship:

[0057] T * = argmin||T||1

[0058] According to the above mathematical relationship, the infrared image of the object can be reconstructed according to the measured single-pixel intensity matrix S, and passive infrared imaging of the object is realized.

[0059] Another embodiment of the present application also discloses specific steps for preparing the echo wall mode microcavity chip: the GaAs microcavity chip is made of a wafer prepared by metal organic chemical vapor deposition technology. The chip is preferably made in the following manner: first, a 10 μm thick Al 0.8 Ga 0.2 As layer and a 3 μm thick GaAs film are grown on a 500 μm thick GaAs substrate in sequence; second, the chip is plasma activated, and then the GaAs film layer is bonded to a SiO2 layer on the surface of a 3 μm thick thermal silicon wafer; third, the GaAs substrate and the Al 0.8 Ga 0.2 As layer are removed by mechanical polishing and chemical etching; fourth, SiO2 is deposited on the GaAs film layer, and the cavity geometry is written on the deposited SiO2 layer by developing technology; fifth, the excess SiO2 and GaAs are removed by chemical etching, and the wedge angle size is controlled by etching time and dose size; sixth, another layer of SiO2 is deposited on the sample, so that the ring-shaped cavity of GaAs is entirely wrapped by SiO2, and has high stability.

[0060] The above only describes preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A passive infrared imaging system based on parametric upconversion, characterized in that, The application relates to a single-pixel infrared imaging system based on a whispering gallery mode microcavity chip. The system comprises the following components: a pump laser connected with an input end of a whispering gallery mode microcavity chip through an optical fiber, used for outputting pump light; a thermal radiation modulation assembly connected with the input end of the whispering gallery mode microcavity chip through an optical fiber, used for receiving and modulating radiation light of an object to be imaged and outputting modulated signal light to the whispering gallery mode microcavity chip; an optical filter connected with an output end of the whispering gallery mode microcavity chip through an optical fiber; a photodetector connected with an output end of the optical filter, used for detecting output signal of the whispering gallery mode microcavity chip to generate light intensity information; the whispering gallery mode microcavity chip receives the pump light and the modulated signal light, and generates upconversion light in the cavity by a second-order sum frequency effect in nonlinear optics; a control and calculation imaging module connected with the thermal radiation modulation assembly and an output end of the photodetector, used for controlling a modulation process of the thermal radiation modulation assembly and receiving corresponding light intensity information, so as to acquire single-pixel upconversion light intensity information and perform infrared image reconstruction, and the infrared image reconstruction specifically comprises the following steps: under the condition that the pump light power is kept unchanged, different mask coding is performed on the radiation light field, and single-pixel upconversion light intensity information corresponding to the mask coding is acquired, so as to represent single-pixel intensity of the two-dimensional radiation light field under the mask coding; assuming that a pixelated image I of the object to be imaged has a sparse representation in a certain base R, the image I is represented as: I=RT; wherein T is a set of coefficients, the base R is a base considered to be suitable for expected spatial properties of the image, including a wavelet base, a Fourier base or a discrete cosine base; a single-pixel intensity matrix S is: S=PI; by calculating the reconstructed pixelated image I * may be expressed as: I * = RT * ; T in the formula * satisfies the following relationship: T * = argmin ||T||1.

2. The passive infrared imaging system based on parametric upconversion according to claim 1, characterized in that, wherein P is a binary base loaded on a spatial light modulator, and PRT=S; the thermal radiation modulation assembly specifically comprises the following components: an optical imaging and filtering lens group, which receives the radiation light of the object to be imaged; a spatial light modulator, which modulates the radiation light according to a control signal of the control and calculation imaging module; 3. The passive infrared imaging system based on parametric upconversion according to claim 1, wherein, a focusing lens, which converges output light of the spatial light modulator. The step of generating upconversion light specifically comprises the following steps: ω s +ω p =ω up n s ω s +n p ω p = n up ω up ; Where, ω s ω p ω up To modulate the frequencies of the signal light, pump light, and upconversion light, the effective refractive index of the corresponding optical modes is denoted as n, respectively. s n p n up .

4. The passive infrared imaging system based on parametric upconversion of claim 1, wherein, the geometric structure of the microcavity and the mode distribution of the light field are determined according to the wavelength of the modulated signal light and the wavelength of the pump light, so that the frequency of the light field and the effective refractive index of the mode satisfy the following relationship:

5. The passive infrared imaging system based on parametric upconversion according to claim 4, characterized in that, the whispering gallery mode microcavity chip comprises a micro-ring cavity structure, the micro-ring cavity structure is a GaAs waveguide with an isosceles trapezoidal shape, the waveguide cross-section size is 10 mu m for the lower base and 3 mu m for the height, the trapezoidal wedge angle is 60 degrees, and the microcavity radius is 50 mu m to 1500 mu m. A 10 μm thick Al 0.8 Ga 0.2 As layer was grown on a 500 μm thick GaAs substrate followed by a 3 μm thick GaAs film. The preparation process of the whispering gallery mode microcavity chip comprises the following steps: GaAs substrate and Al by mechanical polishing and chemical etching 0.8 Ga 0.2 As layer; the chip is plasma-activated, and then a GaAs film layer is bonded on a SiO2 layer on the surface of a 3 mu m thick thermal silicon wafer; SiO2 is deposited on the GaAs film layer, and a cavity geometric shape is written on the deposited SiO2 layer through a developing technology; excessive SiO2 and GaAs are removed through chemical etching, and the wedge angle size is controlled by etching time and dose size; another layer of SiO2 is deposited on the sample, so that the ring cavity of the GaAs is entirely wrapped by the SiO2.

6. The passive infrared imaging system based on parametric upconversion according to claim 2, wherein, The spatial light modulator is a digital micromirror device composed of closely arranged mirror arrays.

7. The passive infrared imaging system based on parametric upconversion according to any of claims 1 to 6, characterized in that, The pump light is single-frequency continuous light, and the wavelength is in the near-infrared or visible light band.

8. The passive infrared imaging system based on parametric upconversion according to any of claims 1 to 6, characterized in that, The working waveband of the photodetector is the visible light band or the near-infrared light band.

9. The passive infrared imaging system based on parametric upconversion according to any of claims 1 to 6, characterized in that, The optical filter is a fiber Bragg grating.

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