Fiber optic dosimeter for space environment and dosimetry process

Abstract

The invention relates to a dosimeter (100) comprising a light source (1) capable of generating a light beam (10), an optical coupler-splitter (2), a radiosensitive optical fiber (3), a first photodetector (5) arranged to record a power measurement (21) of the light beam transmitted through the radiosensitive optical fiber (3), a reference optical arm (4), a second photodetector (6) arranged to record a reference power measurement (22) of the light beam transmitted through the reference optical arm (4), and an electronic system (20) configured to extract a differential measurement of radiation-induced attenuation in the radiosensitive optical fiber (3) with respect to the reference optical arm (4). According to the invention, the light beam (10) is unpolarized or depolarized, or, respectively, the light beam (10) is polarized and the radiosensitive optical fiber (3) is a polarization-maintaining fiber.Figure for the abridged version: Fig. 1.

Description

Title of the invention: Fiber optic dosimeter for space environment and dosimetry method Technical field

[0001] The present invention relates to the technical field of dosimetry apparatus, devices and methods for measuring irradiation doses, expressed in Gray (1 Gy = 1 J / kg), in particular in a mixed environment comprising ions, protons, electrons, neutrons and / or photons.

[0002] More specifically, the present invention relates to a point dosimeter for providing a high-precision irradiation dose measurement in real time. The invention also relates to a dosimeter for providing a total ionizing dose (TID) measurement over a wide dynamic range and / or over a long duration.

[0003] In particular, such a dosimetry apparatus, device and method finds applications in the space field, in any type of space radiation environment, or even in the medical field, scientific in high energy physics, as a reference sensor for other dosimeters, or in the nuclear field. Prior art

[0004] It is important to be able to perform dosimetry in any type of space radiation environment, i.e. on any type of platform and in all orbits. Indeed, manned flights, missions to the surface of the moon or planets, space probes, samples taken during space missions or even flights in stratospheric balloons are exposed to different radiation environments, called space radiation environments. For example, it is important to be able to measure the radiation dose received by an astronaut in a space station at each moment and cumulatively over the entire duration of a mission. Time-resolved dosimetry can also allow the detection of solar flares or the monitoring of neutrons received during an extravehicular mission, whether lunar or on a planet.

[0005] There are different types of passive or active point dosimeters. Passive dosimeters based on thermoluminescent materials are known, which can be stimulated optically or by measuring the concentration of defects by electronic paramagnetic resonance. These passive dosimeters are read after irradiation but do not allow precise dose measurement in real time. There are also active dosimeters, for example based on microelectronic components.

[0006] It is also known to use a fiber optic dosimeter based on a measurement of attenuation induced by irradiation (or RIA for "Radiation Induced Attenuation" in English) also called measurement of losses induced by irradiation for different types of radiation: ions, protons, electrons, neutrons and / or photons. In particular, it is known to use a dosimeter comprising a radiosensitive optical fiber, a light source and a photodetector to measure, as a function of time, the optical power losses of the light beam transmitted through the optical fiber to deduce therefrom, thanks to a pre-calibration, the attenuation induced by irradiation in the optical fiber. An optical fiber is made radiosensitive by doping the core and / or the optical cladding, for example with phosphorus, which makes the optical fiber sensitive to ionizing radiation.

[0007] Some dosimeters based on the measurement of RIA in an optical fiber make it possible to achieve high sensitivity at low irradiation doses, for example with a sensitivity of the order of 1 dB.km '.Gy 1 and an accuracy of 20%. However, the RIA generally depends on the temperature and the dose rate and decreases significantly after the end of exposure to irradiation. In addition, the intensity of the light source limits the measurement dynamics.

[0008] In many terrestrial applications, a dosimeter may comprise a part exposed to irradiation and another part protected from irradiation. On the contrary, in a space environment, the entire system is subject to irradiation. The space environment imposes drastic operating conditions: small size, low mass and low power consumption, mechanical resistance to shocks and vibrations and insensitivity to the strong thermal variations of the space environment. These conditions make the use of fiber optic dosimeters in a space environment very difficult for accurate dose measurements.

[0009] One of the aims of the invention is to propose a high-precision active dosimeter over a wide dynamic range, in particular at very low doses and dose rates of radiation. Another aim of the invention is to propose a point dosimeter having a response independent of temperature and dose rate, in particular for applications in a space environment. Yet another aim of the invention is to propose a dosimeter sensitive to the irradiation of electrons, protons, photons, neutrons and / or ions. Statement of the invention

[0010] To this end, the invention relates to a dosimeter comprising a light source capable of generating a light beam, an optical coupler-splitter capable of receiving the light beam emitted by the light source and of separately transmitting a first part of the light beam and a second part of the light beam, a radiosensitive optical fiber, the radiosensitive optical fiber being arranged to receive the first part of the light beam, a first photodetector arranged to record a power measurement of the light beam transmitted through the radiosensitive optical fiber, a reference optical arm of optical length less than that of the radiosensitive optical fiber, the reference optical arm being arranged to receive the second part of the light beam, a second photodetector arranged to record a reference power measurement of the light beam transmitted through the reference optical arm, and an electronic system capable of simultaneously receiving the power measurement of the first photodetector and the reference power measurement of the second photodetector, the electronic system being configured to extract a differential measurement of attenuation induced by irradiation in the radiosensitive optical fiber relative to the reference optical arm.

[0011] According to the invention, the light beam is unpolarized or depolarized, or, respectively, the light beam is polarized and the radiosensitive optical fiber is a polarization-maintaining fiber.

[0012] According to a particular and advantageous aspect, the light source comprises an amplified spontaneous emission source generating the light beam by amplified stimulated emission, the light beam being unpolarized.

[0013] According to another particular and advantageous aspect, the dosimeter comprises an active or passive optical device arranged downstream of the light source and configured so as to depolarize the light beam.

[0014] In an exemplary embodiment, the passive optical device comprises a depolarizer, for example a Lyot depolarizer or a polarization combiner and two polarization-maintaining optical fibers of different lengths.

[0015] In another exemplary embodiment, the active optical device comprises an optical phase modulator, for example an electro-optical modulator.

[0016] Advantageously, the light source comprises a laser diode or a superluminescent diode.

[0017] Advantageously, the radiosensitive optical fiber is a single-mode or multi-mode fiber. In an exemplary embodiment, the reference optical arm comprises a section of optical fiber.

[0018] According to a particularly advantageous aspect, the electronic system comprises a logarithmic amplifier configured to simultaneously receive the power measurement of the first photodetector and the reference power measurement of the second photodetector, the logarithmic amplifier being capable of extracting the differential measurement of attenuation induced by irradiation in the radiosensitive optical fiber.

[0019] In an exemplary embodiment, the light source is capable of emitting the light beam at a first wavelength and at a second wavelength distinct from the first wavelength, the first photodetector being capable of recording a power measurement of the light beam transmitted through the optical fiber. radiosensitive to the first wavelength and the second wavelength, the second photodetector being adapted to record the reference power measurement at the first wavelength and the second wavelength, and the electronic system is adapted to extract the differential measurement of attenuation induced by irradiation at the first wavelength and the second wavelength.

[0020] According to a particular and advantageous aspect, the dosimeter comprises at least one temperature sensor arranged to acquire at least one temperature measurement at at least one point of the dosimeter, and the electronic system is adapted to correct the differential measurement of attenuation induced by irradiation as a function of said at least one temperature measurement.

[0021] Advantageously, the dosimeter comprises a reflective element arranged at a distal end of the radiosensitive optical fiber, the first photodetector being arranged to record the power measurement of the light beam transmitted after a round trip through the radiosensitive optical fiber.

[0022] According to a particular aspect, the first photodetector and the second photodetector each comprise a photodiode.

[0023] According to another particular and advantageous aspect, the dosimeter comprises another radiosensitive optical fiber and a beam splitter arranged between the light source, the radiosensitive optical fiber and the other radiosensitive optical fiber, the beam splitter being able to split the light beam between the radiosensitive optical fiber and the other radiosensitive optical fiber, another photodetector arranged to record another power measurement of the light beam transmitted through the other radiosensitive optical fiber, the electronic system being adapted to receive the other power measurement from the other photodetector, the electronic system being configured to extract another differential measurement of attenuation induced by irradiation in the other radiosensitive optical fiber relative to the reference optical arm.

[0024] The invention also relates to a dosimetry method comprising the following steps: emitting a light beam; separating the light beam into a first part of the light beam and a second part of the light beam; injecting the first part of the light beam into a radiosensitive optical fiber, the light beam being depolarized, or, respectively, the light beam being polarized and the radiosensitive optical fiber being a polarization-maintaining fiber; injecting the second part of the light beam into a reference optical arm of optical length less than that of the radiosensitive optical fiber, recording on a first photodetector a measurement of the power of the light beam transmitted through the radiosensitive optical fiber; recording on a second photodetector a measurement of the reference power of the light beam transmitted through the reference optical arm;and electronic processing; from the power measurement of the first photodetector and the reference power measurement of the second photodetector to extract therefrom a differential measurement of attenuation induced by irradiation in the radiosensitive optical fiber relative to the reference optical arm.

[0025] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations provided that they are not incompatible or mutually exclusive. Brief description of the drawings

[0026] Furthermore, various other characteristics of the invention emerge from the appended description given with reference to the drawings which illustrate non-limiting forms of embodiment of the invention and where:

[0027] [Fig.l] schematically represents a dosimeter according to an exemplary embodiment,

[0028] [Fig.2] schematically represents a dosimeter according to another example of realization,

[0029] [Fig.3] schematically represents an example of a non-polarized source.

[0030] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references. Detailed description

[0031] The present disclosure provides a point dosimeter based on a measurement of attenuation induced by irradiation in a radiosensitive optical fiber. The dosimeter is configured to have high sensitivity to low irradiation doses, high accuracy and high measurement dynamics. Advantageously, the dosimeter is stable with respect to variations in ambient temperature.

[0032] Here, the term "point dosimeter" is understood to mean a dosimeter capable of carrying out induced attenuation measurements which are integrated over the entire length of the optical fiber. Such a point dosimeter is distinguished from a fiber optic reflectometer, which makes it possible to measure attenuation locally in the fiber and to determine the position of this attenuation as a function of the longitudinal direction of the fiber.

[0033] [Fig.l] illustrates the structure of a dosimeter 100. The dosimeter 100 comprises a light source 1, an optical coupler-splitter 2, a radiosensitive optical fiber 3, a reference arm 4, a first photodetector 5, a second photodetector 6 and an electronic system 20. The principle of the measurement is based on a differential measurement of the optical losses by RIA between two measuring arms of different optical lengths comprising on the one hand the radiosensitive optical fiber 3 and on the other hand the reference arm 4.

[0034] The radiosensitive optical fiber 3 is a fiber whose optical transmission properties vary with the dose of irradiation received. For example, the optical fiber radiosensitive optical fiber 3 is an optical fiber with a core doped with phosphorus or aluminum. For example, the radiosensitive optical fiber 3 is a phosphosilicate or aluminosilicate fiber. Such a fiber is sensitive to ionizing radiation in a neutron, X-ray or gamma ray energy range. The radiosensitive optical fiber 3 has a long length, between 50 m and 10 km, for example 100 m, 2 km, 5 km or 7 km. The optical fiber length is optimized by taking into account the sensitivity coefficient of this optical fiber at the wavelength(s) of the source used, expressed for example in dB.km '.Gy '. In the example illustrated in [Fig.l], the radiosensitive optical fiber 3 selected is a phosphosilicate fiber 5 km long. The radiosensitive optical fiber 3 is wound in the form of a coil to reduce its size. The diameter of the radiosensitive optical fiber coil 3 is generally between 20 mm and 200 mm, for example around 40 mm.

[0035] As described in detail later in the present disclosure, the radiosensitive optical fiber may be, depending on the embodiments, a single-mode (SM) fiber, a multi-mode (MM) fiber or a polarization-maintaining (PM) fiber.

[0036] In a variant illustrated in [Fig.2], a reflector 9 is arranged at the distal end of the radiosensitive optical fiber 3. The reflector 9 comprises for example a mirror arranged facing the distal end of the radiosensitive optical fiber 3. The reflector 9 can also be made up of a reflective treatment applied to the distal end of the radiosensitive optical fiber 3. The reflector 9 can also be made up of a fiber Bragg reflector.

[0037] The reference arm 4 is an arm which has a much shorter optical path than the radiosensitive optical fiber 3. The reference arm 4 comprises, for example, a section of optical fiber of short length. In one example, the section of optical fiber of the reference arm 4 is of the same structure and / or composition as the radiosensitive optical fiber 3 and of much shorter length than that of the radiosensitive optical fiber 3. However, the optical fiber in the reference arm is not necessarily a radiosensitive fiber. For example, the reference arm 4 comprises a section of optical fiber having a length of 1 m.

[0038] The first photodetector 5 and the second photodetector 6 comprise, for example, photodiodes.

[0039] The electronic system 20 makes it possible to control and command the active elements, such as the light source 1 and the photodetectors 5, 6. The electronic system 20 also makes it possible to process the measurements of the powers transmitted in the radiosensitive optical fiber and in the reference arm, to calculate the losses induced by irradiation and the corresponding irradiation dose. The electronic system 20 also makes it possible to transfer to the user the data 30 including in particular the dose measurement result D extracted from the differential measurement of RIA 27. The dosimeter performs real-time measurements, for example with acquisition of a measurement point on the two photodetectors 5, 6 every microsecond (in other words at an acquisition rate of MHz), depending on the performance of the detection electronics. This system makes it possible to detect very brief radiative events. This system also makes it possible to carry out and read differential RIA measurements in real time.

[0040] Optionally, the dosimeter 100 comprises one or more temperature sensors. For example, [Fig.l] illustrates a temperature sensor 13 arranged near the light source 1, another temperature sensor 14 arranged near the optical coupler-splitter 2 and / or another temperature sensor 15 arranged near the radiosensitive optical fiber coil 3 and / or another temperature sensor 16 arranged near the first photodetector 5 and / or the second photodetector 6. Particularly advantageously, the photodiodes 5, 6 are placed near each other so that they have the same temperature and their thermal dependence partly compensates for each other. In practice, the most useful sensor is the temperature sensor 16 located near the photodetectors 5, 6. The signals 23, 24, 25, 26 coming respectively from the temperature sensors 13, 14, 15, 16 are transmitted to the electronic system 20.A calculator 8 uses a model for converting optical losses into dose which uses these point temperature measurements to correct certain drifts due to thermal effects on the components of the dosimeter, in particular on the photodiodes 5, 6 and / or respectively on the radiosensitive optical fiber 3.

[0041] We will explain the operation of the dosimeter 100. The light source 1 emits a light beam 10. The light beam 10 is in the visible or infrared range. The optical coupler-splitter 2 receives the light beam 10 and separates it into a first part of the light beam 11 and a second part of the light beam 12. The first part of the light beam 11 is injected at a proximal end of the radiosensitive optical fiber 3. Simultaneously, the second part of the light beam 12 is injected into the reference arm 4. The optical coupler-splitter 2 is for example a 90:10 coupler so as to inject 90% of the optical power into the radiosensitive optical fiber 3 and 10% of the remaining optical power into the reference arm 4. Advantageously, a 50 / 50 coupler is used which has a stable optical coupling ratio by symmetry.Other optical coupling ratios are conceivable without departing from the scope of this disclosure.

[0042] The first photodetector 5 receives the first part of the light beam 11 exiting the radiosensitive optical fiber 3 and the second photodetector 6 receives the second part of the light beam 12 exiting the reference arm 4. The first detector 5 thus measures the power 21 of the first part of the light beam 11 after transmission through the radiosensitive optical fiber 3. The second photodetector 6 measures the power 22 of the second part of the light beam 12 after transmission through the reference arm 4.

[0043] In the variant illustrated in [Fig.2], the reflector 9 reflects the first part of the light beam 11 transmitted a first time through the radiosensitive optical fiber 3 so that it propagates a second time, in the opposite direction, through the radiosensitive optical fiber 3. The first detector 5 then measures the power 21 of the first part of the light beam 11 after a round trip through the radiosensitive optical fiber 3. This variant makes it possible to double the length of the optical path through the radiosensitive optical fiber 3 without increasing its physical length.

[0044] D denotes the dose of radiation received by the radiosensitive optical fiber 3. Generally speaking, for example in the case of a dosimeter illustrated in [Fig.l], the dose measurement is deduced from the absorption A due to the radiation in the optical fiber via a mathematical model, based for example on the following formula: [00451

[0046] Where q is the sensitivity of the fiber to radiation, (for example q=4 dB / km / Gy(SiO2 ) at 1550 nm for a phosphosilicate fiber) and L is the length of the radiosensitive optical fiber 3. This general formula must be adapted for example in the case of a dosimeter using a mirror as illustrated in [Fig.2], where the length of the optical path is equal to twice the length of the optical fiber. A precalibration makes it possible to adjust the model over the entire measurement dynamic range. It is advantageous to use a long fiber length L, for a given accuracy on the absorption reading A. However, the longer the length L of the radiosensitive optical fiber 3, the greater the absorption A due to radiation in the sensitive fiber. However, at the wavelength of 650 nm, with a length L of 2 km, the initial absorption (before radiation) of the fiber is already of the order of 20 dB.In order to obtain an accurate measurement of optical attenuation, it is advantageous to use a single light source 1 followed by the passive coupler-splitter 2 which divides the optical power between the second part of the light beam 12 towards the reference arm and the first part of the light beam 11 on the arm which passes through the radiosensitive optical fiber 3 whose attenuation A is to be measured. This architecture makes it possible to overcome fluctuations in the power of the optical source. However, the difference in optical power measured by the two photodetectors 5 and 6 can be very significant, in particular when the length L of the radiosensitive optical fiber 3 is significant, generally several km long, for example 5 km.

[0047] The electronic system 20 is designed to perform a ratio between the power 21 of the first part of the light beam 11 and the power 22 of the second part of the light beam 12 so as to obtain a differential attenuation measurement. induced by irradiation 27. The computer 8 then uses the differential measurement of RIA 27 to deduce the dose D of irradiation received, on the basis of a model for example as described above. The fact of normalizing at each instant the power 21 transmitted via the radiosensitive optical fiber 3 with respect to the power 22 transmitted via the reference arm 4 makes it possible to overcome the variations in power of the light source 1 as a function of time.

[0048] According to a particular and advantageous aspect, the electronic system 20 comprises a logarithmic amplifier 7. The logarithmic amplifier 7 is for example an integrated analog DC logarithmic amplifier (or “integrated DC logarithmic amplifier” in English). The logarithmic amplifier 7 simultaneously receives on two separate input channels the power 21 of the first part of the light beam 11 and the power 22 of the second part of the light beam 12. For example, with two photodiodes 5, 6, the logarithmic amplifier 7 receives on one input channel the photocurrent emitted by the signal photodiode 5 and on another input channel the photocurrent emitted by the reference photodiode 6. The logarithmic amplifier 7 thus makes it possible to compare these photocurrents.More precisely, the logarithmic amplifier 7 directly performs a logarithmic ratio analog operation that allows the differential measurement of RIA 27 to be directly provided in decibels. Indeed, the logarithmic amplifier 7 uses the exponential relationship between voltage and current in a PN junction to have a relationship between an input current and an output voltage that is intrinsically logarithmic, which allows for an accurate attenuation measurement even when the photocurrent varies by several orders of magnitude. On the contrary, an electronic system based on the use of two independent transimpedance amplifiers generates a large difference between the two photocurrents, and requires the transimpedance amplifiers to be adjusted in a very unbalanced way to compensate for this large difference.

[0049] Unlike electronic systems in which each analog signal is first digitized via an analog-to-digital converter before applying a digital division calculation, the operations are performed directly and mainly on analog signals. As a result, the measurement accuracy is not limited by the resolution or the dynamics of an analog-to-digital converter. In addition, the data flow at the output of the logarithmic amplifier 7 is not limited by the size of the memory blocks. The use of such a logarithmic amplifier 7 makes it possible to significantly increase the measurement accuracy and dynamics of the RIA signal by several orders of magnitude, particularly towards low irradiation doses.

[0050] However, according to one aspect of the present disclosure, power measurement instabilities related to polarization effects limit the detection of low doses of radiation. In general, the polarization of light is not preserved during propagation in a single-mode (SM) fiber that is not polarization-maintaining. The polarization of a light beam changes in particular very strongly depending on the mechanical stress of the fiber or the temperature. If a polarized light beam is injected into a single-mode fiber, the polarization of the light beam at the fiber output is difficult to control and changes rapidly, in particular due to temperature fluctuations. In addition, certain optical components essential for the attenuation measurement, such as the coupler-splitter 2 and the photodiodes 6, 7, have a response that depends slightly on the polarization.The difference in polarization dependent loss (PDL) between the two orthogonal polarizations is estimated to be of the order of at least 0.1 dB. Unexpectedly, and although this difference is apparently small, this phenomenon is likely to limit the accuracy of a radiation-induced attenuation measurement in a single-mode (SM) fiber. Indeed, polarization fluctuations are converted into power fluctuations. It follows from the present disclosure that polarization effects directly and strongly affect the detection limit and accuracy of a fiber optic dosimeter exploiting the radiation-induced attenuation (RIA) phenomenon to track the temporal evolution of the deposited dose.

[0051] Different solutions are proposed here to reduce or eliminate the effects of polarization in a fiber optic dosimeter.

[0052] According to a first embodiment, the light source 1 is a non-polarized optical source. Different non-polarized sources are envisaged. As a non-limiting example, an amplified spontaneous emission (ASE) source is used, illustrated in [Fig. 3]. The source 1 of [Fig. 3] comprises a pump diode 40 connected via an optical fiber 41 to a coupler-multiplexer 43. The pump diode 40 emits a pump signal at a pump wavelength, for example 980 nm. A first Bragg grating 42 is arranged upstream of the coupler-multiplexer 43 and configured to promote the emission of the laser diode at a specific wavelength, here 980 nm. The coupler-multiplexer 43 receives the filtered pump signal at 980 nm on an input channel. The coupler-multiplexer 43 receives on another input channel 44 a signal to be amplified at a wavelength determined as a function of the amplifying medium.In the case of an erbium ASE source, the wavelength of the signal to be amplified is 1550 nm. A second Bragg grating 45 is arranged downstream of the coupler-multiplexer 43 and configured to promote stable emission at 1550 nm by the optical amplifying medium 47, here consisting of an erbium-doped optical fiber. At the output of the coupler- . multiplexer 43, the pump signal at 980 nm and the signal to be amplified at 1550 nm are injected into the erbium-doped amplifying fiber. An optical isolator 48 is arranged at the output of the amplifying fiber to transmit the light beam 10 at 1550 nm emitted by the source 1. The source 1 thus performs the optical conversion of the polarized pump beam emitted by the pump laser diode, for example at 980 nm or at 1480 nm, into a light beam at a wavelength in the telecom band (1532-1550 nm). Below the lasing threshold, the photons emitted at the telecom wavelength are emitted spontaneously so that the light beam emitted in the telecom band is unpolarized. This light source is then unpolarized. In other words, the light beam 10 generated by amplified stimulated emission is unpolarized. In this case, the optical coupler-splitter 2 is neither polarization-maintaining nor polarizing.The first part of the light beam 11 and the second part of the light beam 12 are unpolarized. The dosimeter 100 based on such an unpolarized source can use standard optical components and in particular a radiosensitive optical fiber 3 of the single-mode (SM) or multi-mode (MM) type which is not polarization-maintaining. Similarly, the reference arm 4 is then based on components which are not polarization-maintaining, for example a section of standard optical fiber (SM) or a section of multi-mode optical fiber (MM). There are polarimeters for example based on rotating polarizers which make it possible to measure the polarization state of a light beam. For an unpolarized light beam, for example, an identical power is measured in the two transverse polarization modes and no phase coherence between them. The use of an ASE erbium source makes it possible to eliminate polarization fluctuations on the RIA measurements.

[0053] According to a variant of the first embodiment, a polarized source 1 is used combined with a passive or active optical device to average the polarization of the source light beam 10. By way of non-limiting example, a source comprising a superluminescent diode (SLED) which emits a polarized light beam is used. Downstream of the polarized source, an active or passive optical component is provided to depolarize the light beam emitted by the source.

[0054] In one example, the passive optical device is of the Lyot depolarizer type. The principle of the Lyot depolarizer is to make the two components of the transverse polarization follow different optical paths, the optical path length of which is greater than the coherence length of the source, and then to recombine the two polarizations. The Lyot depolarizer can be made by butt-welding two sections of polarization-maintaining optical fibers with their own axes misaligned by 45 degrees relative to each other. In this case, the two optical paths correspond to the two indices of the PM fiber. Alternatively, if the coherence length of the source is large, a polarization combiner is used to inject the two transverse polarization components in different PM fibers and recombine them at the output. Such a passive depolarizer is particularly well suited to a light source with a relatively short coherence length, such as a superluminescent diode (SLED).

[0055] In another example, the active optical device is of the polarization jammer type. This active optical device is configured to modulate the phase randomly in order to break the coherence of the source and its polarization. The active optical device comprises an optical phase modulator, for example an electro-optical modulator. Alternatively, the active optical device is formed by applying a suitable mechanical stress to the fiber itself to modulate the phase randomly.

[0056] According to a second embodiment, the source 1 is polarized, the radiosensitive optical fiber 3 is a polarization-maintaining (PM) fiber and the entire optical chain between the source 1 and the photodetectors 5, 6 is polarization-maintaining. Advantageously, this radiosensitive optical fiber 3 is manufactured by doping the core or the optical cladding of a conventional PM fiber, for example with phosphorus doping. In this case, the optical coupler-splitter 2 is polarization-maintaining. The first part of the light beam 11 and the second part of the light beam 12 are polarized. The source-side and coil-side welds are made so as to align the natural axes of the polarization-maintaining radiosensitive optical fiber 3 with respect to the natural axes of the polarized source. Similarly, the natural axes of the reference arm 4 are aligned with the natural axes of the polarized source.The first part of the light beam 11 and the second part of the light beam 12 are thus polarized. The polarization of the light beam being then preserved during propagation in the radiosensitive optical fiber 3 and in the reference arm 4, the PDL has no influence on the measurement of attenuation induced by irradiation.

[0057] Alternatively, the dosimeter comprises at least two sources emitting at two distinct wavelengths, each source being connected to a suitable optical fiber. For example, the two sources are polarized and the two radiosensitive optical fiber coils are polarization-maintaining optical fibers. In another example, the two sources are apolarized or depolarized. In yet another example, the dosimeter comprises a first polarized source associated with a polarization-maintaining radiosensitive optical fiber and a second apolarized or depolarized source. The combination of two sources at two different wavelengths allows for measurement redundancy and therefore improved accuracy. Alternatively, a single source is adapted to emit the light beam at several wavelengths, for example at 16 distinct wavelengths.

[0058] According to another particular and advantageous aspect, a single light source 1 is used, coupled simultaneously to several coils of radiosensitive optical fibers, for example to two or three coils of optical fiber of different lengths and / or having different sensitivity to ionizing radiation, in addition to the reference arm 4. For this purpose, for example, another radiosensitive optical fiber and a beam splitter arranged between the light source 1, the radiosensitive optical fiber 3 and the other radiosensitive optical fiber are used. The beam splitter receives the light beam 10 from the light source and divides it between the radiosensitive optical fiber 3 and the other radiosensitive optical fiber. In this case, the splitter is located upstream of the coupler 2. It may be an IxN color type power divider or a multiplexer which makes it possible to address the different coils sequentially. Each coil needs its own reference arm.In a manner similar to the first detector associated with the radiosensitive optical fiber, another photodetector is associated with the other radiosensitive optical fiber and arranged to record another power measurement of the light beam transmitted through the other radiosensitive optical fiber. The electronic system 20 is here adapted to receive the other power measurement from the other photodetector, simultaneously with the measurement from the reference arm. The electronic system 20 is configured to extract another differential measurement of attenuation induced by irradiation in the other radiosensitive optical fiber relative to the reference optical arm. Such a configuration advantageously makes it possible to read several dosimeters with a single optical source so as to reduce costs.

[0059] The drastic reduction of the effects of polarization in the fiber optic dosimeter makes it possible to make the dosimeter less sensitive to ambient thermal variations. The reduction of polarization fluctuations in the fiber optic dosimeter thus makes it possible to lower the detection threshold for low doses of radiation, to improve the accuracy of RIA measurements and also to increase the dose measurement dynamics.

[0060] In total, the combination of an electronic system based on a logarithmic amplifier and the reduction of polarization effects makes it possible to lower the threshold for detecting low doses of radiation down to a few tens of microGy, to improve the accuracy of RIA measurements and also to increase the dose measurement dynamics to extend between 1 Gy and 100 Gy for example. In an exemplary embodiment, the dosimeter of the present disclosure makes it possible to reduce the detection noise by 1 to 2 orders of magnitude. The measured dose rate is in a range between 100 gray / d and 1 Mgray / d. The dosimeter of the present disclosure thus allows measurements of irradiation doses with remarkable accuracy of the order of 10 4 in relative terms over a dynamic range extended over 10 dB.

[0061] This architecture makes it possible to overcome the power fluctuations of the light source 1. The differential measurement of RIA makes it possible to choose a light source 1 whose intensity is not necessarily stabilized as a function of time. For example, we choose an ASE source or a laser diode.

[0062] The dosimeter can be packaged in a very compact case, for example a cylinder about 40 mm in diameter and 20 mm high and of low weight. The dosimeter can thus be carried on board a space station or even in the belt of an astronaut for an extravehicular activity. The dosimeter provides instantaneous measurements. The case can be configured to transmit the dose measurements, for example, via a wireless link to a display. These measurements make it possible, for example, to anticipate a solar flare likely to damage sensitive equipment.

[0063] This point dosimeter can also find applications in the medical field for the point measurement of radiation dose produced by medical equipment, for example in pulsed radiotherapy. This point dosimeter also finds applications in the nuclear field, for the control or dismantling of all nuclear installations of the fusion or fission type.

[0064] Of course, various other modifications may be made to the invention within the scope of the appended claims.

Claims

1. Claims Dosimeter (100) comprising: - a light source (1) capable of generating a light beam (10), - an optical coupler-splitter (2) capable of receiving the light beam (10) emitted by the light source (1) and of separately transmitting a first part of the light beam (11) and a second part of the light beam (12), - a radiosensitive optical fiber (3), the radiosensitive optical fiber (3) being arranged to receive the first part of the light beam (H), - a first photodetector (5) arranged to record a power measurement (21) of the light beam transmitted through the radiosensitive optical fiber (3), - a reference optical arm (4) of optical length less than that of the radiosensitive optical fiber (3), the reference optical arm (4) being arranged to receive the second part of the light beam (12), - a second photodetector (6) arranged to record a reference power measurement (22) of the light beam transmitted through the reference optical arm (4), and - an electronic system (20) capable of simultaneously receiving the power measurement (21) of the first photodetector (5) and the reference power measurement (22) of the second photodetector (6), the electronic system (20) being configured to extract a differential measurement of attenuation induced by irradiation in the radiosensitive optical fiber (3) relative to the reference optical arm (4), characterized in that it comprises another radiosensitive optical fiber and a beam splitter arranged between the light source (1), the radiosensitive optical fiber (3) and the other radiosensitive optical fiber, the beam splitter being capable of dividing the light beam (10) between the radiosensitive optical fiber (3) and the other radiosensitive optical fiber, another photodetector arranged to record another power measurement of the light beam transmitted through the other radiosensitive optical fiber,the electronic system (20) being adapted to receive the other power measurement from the other photodetector, the electronic system (20) being configured to extract another differential measurement of attenuation induced by, irradiation in the other radiosensitive optical fiber relative to the reference optical arm and in that - the light beam (10) is unpolarized or depolarized or - the light beam (10) is polarized and the radiosensitive optical fiber (3) is a polarization-maintaining fiber.

2. A dosimeter (100) according to claim 1 wherein the light source (1) comprises an amplified spontaneous emission source generating the light beam by amplified stimulated emission, the light beam (10) being unpolarized.

3. Dosimeter (100) according to claim 1 comprising an active or passive optical device arranged downstream of the light source (1) and configured so as to depolarize the light beam (10).

4. The dosimeter (100) of claim 3 wherein the passive optical device comprises a Lyot depolarizer or polarization combiner and two polarization-maintaining optical fibers of different lengths.

5. The dosimeter (100) of claim 3 wherein the active optical device comprises an optical phase modulator.

6. Dosimeter (100) according to one of claims 3 to 5 wherein the light source (1) comprises a laser diode or a superluminescent diode.

7. Dosimeter (100) according to any one of claims 1 to 6 wherein the radiosensitive optical fiber (3) is a single-mode or multi-mode fiber.

8. A dosimeter (100) according to any one of claims 1 to 7 wherein the reference optical arm (4) comprises a section of optical fiber.

9. Dosimeter (100) according to any one of claims 1 to 8 wherein the electronic system (20) comprises a logarithmic amplifier (7) configured to simultaneously receive the power measurement (21) of the first photodetector (5) and the reference power measurement (22) of the second photodetector (6), the logarithmic amplifier (7) being capable of extracting the differential measurement of attenuation induced by irradiation (27) in the radiosensitive optical fiber.

10. A dosimeter (100) according to any one of claims 1 to 9 wherein the light source (1) is capable of emitting the light beam (10) at a first wavelength and at a second wavelength distinct from the first wavelength, the first photodetector (5) being adapted to record a power measurement (21) of the light beam transmitted through the radiosensitive optical fiber (3) at the first wavelength and at the second wavelength, the second photodetector (6) being adapted to record the reference power measurement (21) at the first wavelength and at the second wavelength, and in that the electronic system (20) is able to extract the differential measurement of attenuation induced by irradiation at the first wavelength and at the second wavelength.

11. Dosimeter (100) according to any one of claims 1 to 10 comprising at least one temperature sensor (13, 14, 15) arranged to acquire at least one temperature measurement (23, 24, 25) at at least one point of the dosimeter, and in that the electronic system (20) is adapted to correct the differential measurement of attenuation induced by irradiation as a function of said at least one temperature measurement (23, 24, 25).

12. A dosimetry method comprising the following steps: - emitting a light beam (10), - separating the light beam (10) into a first part of the light beam (11) and a second part of the light beam (12), - injecting the first part of the light beam (11) into a radiosensitive optical fiber (3) and into another radiosensitive optical fiber, the light beam (10) being depolarized, or, respectively, the light beam (10) being polarized and the radiosensitive optical fiber (3) being a polarization-maintaining fiber; - injecting the second part of the light beam (12) into a reference optical arm (4) of optical length less than that of the radiosensitive optical fiber (3), - recording on a first photodetector (5) a power measurement (21) of the light beam transmitted through the radiosensitive optical fiber (3);- recording on another photodetector another measurement of the power of the light beam transmitted through the other radiosensitive optical fiber; - recording on a second photodetector (6) a reference power measurement (22) of the light beam transmitted through the reference optical arm (4); - electronic processing of the power measurement (21) of the first photodetector (5) and of the reference power measurement (22) of the second photodetector (6) to extract therefrom a differential measurement of attenuation induced by irradiation in the radiosensitive optical fiber (3) relative to the reference optical arm (4) - electronic processing of the other power measurement of the other photodetector and of the reference power measurement (22) of the second photodetector (6) to extract therefrom another differential measurement of attenuation induced by irradiation in the other radiosensitive optical fiber relative to the reference optical arm (4).

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