Device for measuring the performance of an optical detector and associated measuring method

Description

DOMAINE DE L'INVENTION

[0001] The present invention relates to a device for measuring the performance of an optical detector and the associated measurement method. More specifically, the invention relates to detectors implementing quantum phenomena operating at low temperature, the detector having to be cooled to very low temperatures, typically between 50 and 200 K.

[0002] The invention is intended to be applied to measure the performance of a detector in the visible, near infrared range, i.e. the wavelength of which is between 1 and 2.5 micrometers (also called SWIR for the Anglo-Saxon expression " Short- Wavelength InfraRed "), mid-infrared, whose wavelength is between 3 and 8 micrometers (also called MWIR for the Anglo-Saxon expression « Mid-Wavelength InfraRed "), and far infrared, with a wavelength between 8 and 14 micrometers (also called LWIR for the Anglo-Saxon expression " Long-Wavelength InfraRed ").

[0003] Thus, the invention can be applied to a large number of detectors and for a large number of distinct domains. More particularly, the invention aims to provide high-precision measurements, for example to characterize the performance of a detector intended for very demanding applications, such as applications in the space domain. ETAT ANTERIEUR DE LA TECHNIQUE

[0004] To characterize the performance of a quantum-type detector, particularly before it is embarked on a satellite, several distinct measurements can be carried out, such as: radiometric measurement; remanence measurement; dark current measurement; and spectral response measurement.

[0005] The aim of radiometric measurement is to measure the response of the detector output level as a function of the flux of photons incident on the detector 20. To do this, as illustrated in the figure 1a , a radiometric measuring bench 100 typically includes a light source 101 allowing to obtain a quasi-black body emission law with an emission peak around 1 micrometer.

[0006] Typically, this light source 101 can have an optical power of a few tens of Watts with a color temperature of approximately 2700 Kelvin. For the purposes of the invention, the "color temperature" characterizes a light source by comparison with an ideal material emitting light solely by the effect of heat. Such a light source 101 classically comprises coils forming a resistance. However, light L11 generated by this type of light source 101is highly inhomogeneous and it is possible to observe, in the far field, variations in light due to the shape of the turns of the resistance of the light source 101.

[0007] To correct this defect, an integrating sphere 102 is used at the output of the light source 101 so as to obtain a luminous flux L12 more homogeneous. More precisely, this integrating sphere 102 is powered by the light source 101 through an adjustable iris 103. Indeed, the adjustable iris 103 allows the quantity of photons to be adjusted while maintaining a constant current for the light source 101 so as to guarantee the stability of said light source 101. In addition, the adjustable iris 103 does not modify the distribution of rays at the exit of the integrating sphere 102.

[0008] To control the amount of photons entering the integrating sphere 102, a reference detector 104 is placed on the integrating sphere 102 so as to measure the quantity of photons present in the integrating sphere 102. A controller 110 controls the power of the light source 101 and the iris opening rate 103 depending on the quantity of photons sought and measured by the reference detector 104.

[0009] The luminous flux L12 at the exit of the integrating sphere 102 enters a thermal enclosure 105 whose internal walls are intended to absorb radiation in the wavelength range of interest. This thermalized enclosure 105 integrates screens 106 to limit the viewing angle of the detector 20. More precisely, stray light fluxes are captured by screens 106and are absorbed on the walls of the thermalized enclosure 105 so that only the luminous flux transmitted directly by the integrating sphere 102 reaches the detector 20.

[0010] Thermalization of the thermalized enclosure 105 allows the emission capacity of the walls to be limited. For example, the enclosure can be maintained at a temperature of 12°C while the ambient temperature is approximately 22°C.

[0011] Thus, the luminous flux L13 at the exit of the thermal enclosure 105 directly reaches the detector 20. The detector 20 is integrated into a cryostat 107 intended to reproduce the operating conditions of the detector 20. For example, for space applications, the detector 20 can operate in vacuum with cryogenic temperatures.

[0012] More specifically, the vacuum level in the cryostat107 can be 10 -5< mbars while the temperature of the cryostat 107 can be close to 180K for a near infrared detector, between 150K and 110K for a mid infrared detector, and between 60K and 100K for a far infrared detector.

[0013] To limit the propagation of the luminous flux L13 inside the cryostat 107, a screen 112 can also be placed around the detector 20. For this radiometric measuring bench 100, the screen 112 is open to receive the luminous flux L13.

[0014] Considering the pressure and temperature differences between the cryostat 107 and the thermal enclosure 105; the cryostat 107 is isolated from the thermal enclosure 105. The luminous flux L13 enters the cryostat 107 through an optical window 111. Between the optical window 111 and the screen112, the cryostat typically includes a filter specifically sized to allow only the range of wavelengths used by the detector to pass through 20.

[0015] Furthermore, a detector 20 typically includes a set of sensitive elements or pixels. It is typically sought to measure the output level of these sensitive elements as a function of the flux of incident photons. To do this, the detector 20 is mounted on a mobile support 108 by means of a moving member 109 so as to adjust the central point P10 of light flux reception L13 so that this central point P10 be placed in the center of the detector 20 before measuring the detector output level 20.

[0016] To perform the radiometric measurement, an operator begins by mounting the detector 20 on mobile support108 of the cryostat 107, in a first step 120 illustrated on the figure 1b . He then orders the cryostat to be placed under vacuum. 107, during a second stage 121.

[0017] Given the high level of vacuum required, this vacuuming of the cryostat 107 is particularly long and typically lasts between 4 and 8 hours, a period during which frequent pressure and temperature checks must be carried out. When the desired vacuum level is reached, the cryostat 107 is cooled to the desired temperature, in a third step 122. This third stage 122 cryostat cooling 107 usually lasts between 1 and 3 hours.

[0018] When these steps of preparing the cryostat 107are completed, the operator proceeds to the various radiometric measurements for the different sensitive elements, in a step 123.

[0019] To do this, it controls the movement of the mobile support 108 so as to adjust the central point P10 of light flux reception L13 so that this central point P10 be placed in the center of the detector 20. Measuring the output level of sensitive elements P10 is then carried out for several quantities of photons, by varying the opening of the iris 103.

[0020] Thus, the measurements are chained together to measure the output level of all the sensitive elements of the detector 20. Typically, all of these measurements take about a day for an operator to complete.

[0021] At the end of the measurements, the temperature should be gradually and slowly increased so that the cryostat107 returns to room temperature, in one step 124. To protect the physical integrity of the detector, this step 124 requires about a day of handling during which frequent pressure and temperature checks must be carried out. The final step 125 consists of raising the vacuum level before being able to remove the cryostat 107 of the radiometric measuring bench 100. In total, all the steps 120 has 125 require approximately one week of handling for an operator.

[0022] Before or after the detector performance 20 are obtained on the radiometric measuring bench 100, this detector 20 can be analyzed on another measuring bench, for example a remanence measuring bench 200, as illustrated on the figure 2a . To do this, the cryostat 107is dismantled from the radiometric measuring bench 100 to be placed in the remanence measurement bench 200.

[0023] Prior to installing the cryostat 107 integrating the detector 20 on the remanence measuring bench 200, the filter mounted on the screen 112 of the cryostat 107 can be modified.

[0024] The purpose of afterglow measurement is to observe the effects of temporal variations of a scene on the images of a detector. To do this, it is sought to see whether an image previously captured by a detector influences the image being read (afterglow effect) and, for detectors performing integration during reading, whether the image being integrated by the detector influences the image being read (pre-afterglow effect).

[0025] To perform this measurement, it is necessary to use a light source 201whose light intensity varies temporally in a controlled manner. Thus, the light source 201 is associated with a mechanical shutter 202 able to close the exit of the light source 201 with a response time of the order of a millisecond.

[0026] The light source 201 may be identical to the light source 101 of the radiometric measuring bench 100.

[0027] To perform the remanence measurement, an operator begins by mounting the cryostat 107 on the remanence measuring bench 200, in a first step 220 illustrated on the figure 2b . He then orders the cryostat to be placed under vacuum. 107, in a second stage 221, and cooling of the cryostat 107, in a third stage 222.

[0028] When these steps of preparing the cryostat 107are completed, the operator proceeds to the various remanence measurements, in a step 223. To do this, it orders the acquisition of several successive images modified during the moments when the shutter is closed. 202, and it compares the acquired images with reference images so as to detect the effects of pre-afterglow or afterglow on the acquired images. The afterglow effect appears when an acquired image presents artifacts linked to the influence of the previous image while the pre-afterglow effect appears, for detectors performing integration during reading, when an image being read presents artifacts linked to the influence of the image being integrated (acquisition in IWR mode for the English acronym « Integrate While Read ").

[0029] In the same way as for the radiometric measuring bench 100,when the remanence measurements are completed, the operator orders the increase in the temperature of the cryostat 107, in one step 224, and the rise in pressure, in a step 225. In total, all the steps 220 has 225 require a few days of handling for an operator.

[0030] Before or after the detector performance 20 are obtained on the remanence measurement bench 200, this detector 20 can be analyzed on a dark current measuring bench 300, as illustrated on the figure 3a . To do this, the cryostat 107 is dismantled from the remanence bench 200 to be placed on the dark current bench 300.

[0031] The purpose of dark measurement or dark current measurement is to measure the dark current of the detector 20,i.e. the leakage current of components integrated in the detector 20 without the presence of flux. The dark current level influences the detector's performance. Indeed, the higher the dark current, the higher the number of defective pixels, and the more the dark current noise impacts the overall noise of the detector. 20.

[0032] Thus, the measurement of the dark current partly fixes the operating temperature of the detector 20. To measure dark current, the current inherent in the flow coming from the test bench must be negligible compared to the dark current to be measured.

[0033] It is therefore also important to vary the temperature during this measurement to ensure that the measured dark current does not consist of a background flux.

[0034] To do this, prior to installing the cryostat 107integrating the detector 20 on the dark current bench 300, the filter is removed from the screen 112 of the cryostat 107 to be replaced by a shutter 313 so as to isolate the detector 20. In addition, the optical window 111 can also be covered with an opaque cap 311 so as to further isolate the detector 20.

[0035] These two levels of light isolation make it possible to measure the leakage current of the sensitive elements of the detector 20 by varying the temperature of the cryostat 107.

[0036] To do this, as illustrated in the figure 3b , an operator still has to mount the detector 20 in the cryostat 107, in a first step 320, then order the cryostat to be evacuated 107, in a second stage 321, and cooling of the cryostat 107,in a third stage 322. After having carried out the measurements of the leakage current of each sensitive element, in a step 323, the operator commands the increase in the temperature of the cryostat 107, in one step 324, and the rise in pressure, in a step 325. In total, all the steps 320 has 325 still require about a week of handling for an operator.

[0037] The last measurement bench aims to obtain the spectral response of the detector 20. The measurement of the spectral response makes it possible to obtain the wavelength response of the detector pixels 20. In fact, whatever the technology used, the detector's response 20 is not homogeneous depending on the wavelength. It can depend on many technological parameters.

[0038] It is often necessary to know the spectral response, especially when it is desired to use optical filters in front of the detector. 20. Thus, it is possible to predict whether certain wavelengths need to be filtered or not depending on the desired application.

[0039] To measure the spectral response, the wavelength of a monochrome luminous flux must be varied L42 and to measure the ratio between the detector response 20 and the response of a reference detector 404 whose spectral response is known.

[0040] To do this, as illustrated in the figure 4a , the spectral response measurement bench 400 typically includes a light source 401 whose luminous flux is modulated in wavelength by a controller 410. The light source 401 may be identical to the light source 101of the radiometric measuring bench 100.

[0041] The luminous flux L41 generated by the light source 401 is transmitted to a diffraction grating 414 allowing to obtain a monochrome luminous flux L42. Monochrome luminous flux L42 is then processed by a filter 415, to remove the influence of higher orders of the diffraction grating 414, before being inserted into an integrating sphere 402. This integrating sphere 402 allows a light flux to be generated for the reference detector 404 as well as a luminous flux L44 to the detector 20. To do this, the cryostat 107 of the detector has a screen 112 which is open and an optical window 111, also open. A filter can optionally be mounted on the screen 112.

[0042] To use the spectral response measurement bench 400, an operator still needs to mount the detector 20 in the cryostat 107, in a first step 420, then order the cryostat to be evacuated 107, in a second stage 421, and cooling of the cryostat 107, in a third stage 422, as illustrated on the figure 4b . After performing the spectral response measurements, in one step 423, the operator commands the increase in the temperature of the cryostat 107, in one step 424, and the rise in pressure, in a step 425. In total, all the steps 420 has 425 still require about a week of handling for an operator.

[0043] Thus, to fully characterize a detector 20, the cryostat 107 is transported between the benches 100, 200, 300 And 400,and one or more operators must perform all the steps 120-125, 220-225, 320-325 And 420-425. It follows that the complete characterization of a detector 20 today requires about a month of work for an operator.

[0044] In addition, these different measuring benches 100, 200, 300 And 400 also have a significant footprint.

[0045] CHORIER PHILIPPE ET AL: "Sofradir latest developments for infrared space detectors", INFRARED TECHNOLOGY AND APPLICATIONS XXXVII, vol. 8012, no. 1, May 13, 2011, pages 1-12 discloses an example of prior art.

[0046] The technical problem of the invention is therefore to characterize a detector with several distinct measurements, such as radiometric measurement, remanence measurement, dark current measurement, and spectral response measurement, in a simpler and faster manner. EXPOSE DE L'INVENTION

[0047] To address this technical problem, the invention proposes a unique measuring bench allowing the various measurements to be carried out without moving the detector. To do this, the detector is placed in a cryostat into which a single-mode optical fiber is inserted, intended to transmit a light beam opposite the detector.

[0048] The single-mode optical fiber has two ends: a first end placed inside the cryostat, and a second end placed outside the cryostat and to which several modules for generating a light flux integrating a fiber source can be connected so as to carry out several distinct measurements, such as radiometric measurement, remanence measurement, dark current measurement, and spectral response measurement.

[0049] To this end, according to a first aspect, the invention relates to a device for measuring the performance of an optical detector, comprising: a cryostat comprising vacuum means and cooling means capable of placing the detector in expected operating conditions; means for measuring the performance of the detector; a support capable of receiving the detector, fixed inside the cryostat; and a screen arranged around the support so as to limit the radiation which can reach the support in a range of wavelengths of the detector.

[0050] The invention is characterized in that the measuring device also comprises: a single-mode optical fiber in the operating wavelength range of the detector, said optical fiber being inserted into an opening of the cryostat, and comprising: a first end fixed at an opening of the screen, so as to project a light flux onto all or part of the detector; and a second end, external to the cryostat, intended to receive a light flux; and at least two modules for generating a light flux integrating a fiber source capable of generating the light flux in the second end of the optical fiber; the means for measuring the performance of the detector being capable of measuring said performance as a function of the light flux received at the second end, several separate measurements being able to be carried out by interchanging the module connected to the second end.

[0051] For the purposes of the invention, a "fiber source" is a source whose generated light beam is coupled with a mode of the optical fiber associated with the source. This fiber source may be a light-emitting diode, a super-continuum source or a laser for example. A super-continuum source is obtained by directing a laser beam onto a material having non-linear optical transmission properties so that the passage of the laser beam through the material modifies the spectrum of the laser beam.

[0052] A fiber source is the coupling of a light source with an optical fiber; the light flux from the primary source is guided inside the optical fiber and to the fiber outlet. The coupling between the source and the fiber can be done by aligning the two components and / or by means of an intermediate component(s) (lens, prism, etc.)

[0053] The invention thus makes it possible to carry out several measurements without moving the detector within the cryostat, and without requiring repeated vacuuming of the latter, by modifying the light flux injected into the optical fiber by means of the module for generating said light flux. It follows that it is no longer necessary to carry out a large number of pressure mounting / reduction and temperature mounting / reduction phases because the cryostat can remain on the same measuring bench and in the same state during changes in light flux.

[0054] The present invention therefore consists of replacing the bulky elements of the radiometric measurement and spectral response measurement benches, i.e. the light sources and the integrating spheres, with a module integrating a fiber source and a single-mode optical fiber.

[0055] Indeed, the luminous flux generated by a fiber source, for example a light-emitting diode, typically has a lower power than state-of-the-art light sources and an inhomogeneous spatial distribution due to its structure. It would be possible to use an integrating sphere to achieve homogenization. However, using an integrating sphere results in a significant loss of power and it would then be difficult to obtain the desired power levels.

[0056] To solve this problem, the invention proposes to use a single-mode optical fiber, which spatially filters the light flux coming from the fiber source and allows a homogeneous divergence at its output, for example in the form of a Gaussian beam, without losing light power.

[0057] Thus, the single-mode nature of the optical fiber makes it possible to correct inhomogeneities, so that at the output of the single-mode optical fiber, the luminous flux transmitted to the detector is equivalent to the luminous fluxes used in the state of the art.

[0058] As is known, the single-mode character of the optical fiber is obtained by selecting the index step (NA) and the diameter (a) of the core of the optical fiber so that the normalized frequency (V) is of the order of 1 to 2.405, according to the following relationship: V = 2 πaNA λ with λ corresponding to the wavelength of interest of the detector.

[0059] For the purposes of the invention, a "single-mode" optical fiber is an optical fiber whose standardized frequency (V) is between 1.2 and 3.8 in the wavelength range of interest. Indeed, in this standardized frequency range, the majority of the wavelength range exhibits a single-mode behavior and the optical fiber is very little subject to coupling of external light fluxes. Thus, the optical fiber exhibits effective isolation from external disturbances.

[0060] There are other advantages to using fiber optics as well.

[0061] Indeed, the optical fiber can also be selected to have an optical cladding capable of very effectively filtering external radiation and promoting the transmission of the light flux in the wavelength range of interest of the detector. Thus, for applications in the mid or far infrared, an optical fiber with a silica core is not optimal due to the absorption of silica.

[0062] Preferably, the optical fiber is made with a core of fluoride glass, for example, Zblan, chalcogenide glass, or a polycrystalline material such as silver chloride or silver bromide. Zblan takes its name from the elements contained in fluoride glass, including zirconium, barium, lanthanum, aluminum, and sodium, and is very effective for applications in the mid-infrared region, the wavelength of which is between 3 and 8 micrometers. Chalcogenide glass optical fibers have a wide transparency range in the infrared and significant non-linear optical properties. Thus, chalcogenide glass optical fibers are suitable for applications in the mid-infrared and far-infrared regions.Optical fibers with a polycrystalline core are widely developed; they are therefore relatively low cost and ensure the homogeneity of the light beam transmitted to the detector in all infrared regions.

[0063] Additionally, the optical fiber can be integrated into the cryostat with a very small aperture so as to limit the risk of temperature or pressure loss.

[0064] Furthermore, the use of optical fiber allows the use of integrated optical functions from telecommunications.

[0065] Thus, to carry out the radiometric measurement, one of the light flux generation modules, intended to be connected to the second end of the optical fiber, includes: a fiber source intended to generate a light flux inside a first feed optical fiber; an optical attenuator, capable of limiting the light flux, connected between the first feed optical fiber and a second feed optical fiber; an optical splitter, connected to the second feed optical fiber, capable of separating the light flux from the second feed optical fiber onto a third and a fourth feed optical fiber; the third feed optical fiber being connected to the second end of the optical fiber inserted into the cryostat; and a reference detector connected to the fourth feed optical fiber, capable of measuring the light flux at the output of the optical splitter so as to control the power of the fiber source and of the optical attenuator to obtain an expected measurement light power.

[0066] It follows that by restrictively implementing small optical elements, it is possible to carry out a radiometric measurement as efficient as the radiometric measurements carried out on state-of-the-art radiometric measuring benches.

[0067] To perform a remanence measurement, one of the light flux generation modules, intended to be connected to the second end of the optical fiber, includes: a fiber source intended to generate a light flux inside a first feed optical fiber; an optical shutter capable of blocking the light flux, said shutter being connected between the first feed optical fiber and a second feed optical fiber; an optical splitter connected to the second feed optical fiber, capable of separating the light flux from the second feed optical fiber onto a third and a fourth feed optical fiber; the third feed optical fiber being connected to the second end of the optical fiber inserted into the cryostat; and a reference detector connected to the fourth feed optical fiber, capable of controlling the light flux at the output of the optical splitter.

[0068] Unlike a conventional remanence measuring bench, this embodiment allows the use of an optical or acousto-optic shutter instead of a mechanical shutter. However, an optical or acousto-optic shutter has a response time of the order of a microsecond, whereas a mechanical shutter of the " shutter » for example, has a response time of the order of milliseconds. It follows that the remanence measurement can be more precise.

[0069] Furthermore, the implemented light flux generation module can be used to perform a dark current measurement by cutting off the power supply to the fiber source. Indeed, by using an optical fiber with a normalized frequency (V) between 1.5 and 3 in the wavelength range of interest, the wavelengths of the luminous fluxes of the ambient lights cannot be coupled with the optical fiber. Thus, it is not necessary to use a cap.

[0070] To perform a measurement of the spectral response, one of the light flux generation modules, intended to be connected to the second end of the optical fiber, includes: a fiber source intended to generate a light flux inside a first supply optical fiber; a diffraction grating connected to the first supply optical fiber; a filter connected to the output of the diffraction grating; and a second supply optical fiber connected to the output of the second filter.

[0071] Thus, radiometric measurement, remanence measurement, dark current measurement, and spectral response measurement can be performed by fiber sources connected to the optical fiber inserted into the cryostat so that it is not necessary to move the detector between two measurements.

[0072] It is therefore sufficient to change the operating mode and / or the light flux generation module to switch from one measurement to another, without needing to modify the vacuum level or the temperature of the cryostat. The device therefore makes it possible to carry out all the required measurements in a simplified manner.

[0073] Thus, according to a second aspect, the invention relates to a method for measuring the performance of an optical detector by means of a measuring device of the type previously described. This method comprises the following steps: mounting the detector whose performance is to be measured on the cryostat support; placing the cryostat under vacuum so as to reach a desired operating vacuum level of the detector; cooling the cryostat so as to reach a desired operating temperature of the detector; connecting a first light flux generation module to the second end of the optical fiber; measuring the performance of the detector; disconnecting the first module from the second end of the optical fiber; connecting a second light flux generation module to the second end of the optical fiber; measuring the performance of the detector; reheating the cryostat so as to reach ambient temperature; and putting the cryostat back under ambient pressure and dismantling the detector.

[0074] Preferably, four separate measurements are made so that the method comprises the following steps: connecting a first light flux generation module to perform a radiometric measurement on the second end of the optical fiber; radiometric measurement of the detector; disconnecting the first light flux generation module from the second end of the optical fiber; connecting a second light flux generation module to perform a remanence measurement on the second end of the optical fiber; measuring the remanence of the detector; measuring the dark current of the detector; disconnecting the second light flux generation module from the second end of the optical fiber; connecting a third light flux generation module to perform a measurement of the spectral response on the second end of the optical fiber; and measuring the spectral response of the detector. BREVE DESCRIPTION DES FIGURES

[0075] The invention will be clearly understood upon reading the following description, the details of which are given solely by way of example, and developed in relation to the appended figures, in which identical references refer to identical elements: There figure 1a is a schematic representation of a state-of-the-art radiometric measuring bench; The figure 1b illustrates the steps involved in carrying out a radiometric measurement from the bench of the figure 1a ; There figure 2a is a schematic representation of a state-of-the-art remanence measuring bench; The figure 2b illustrates the steps for carrying out a remanence measurement from the bench of the figure 2a ; There figure 3a is a schematic representation of a state-of-the-art dark current measuring bench; figure 3b illustrates the steps for performing a dark current measurement from the bench of the figure 3a ; There figure 4a is a schematic representation of a state-of-the-art spectral measurement bench; The figure 4b illustrates the steps involved in carrying out a spectral measurement from the bench of the figure 4a ; There figure 5 is a schematic representation of a device for measuring the performance of an optical detector according to one embodiment of the invention; The figure 6 schematically illustrates a configuration of the measuring device of the figure 5 to obtain a radiometric measurement; The figure 7 illustrates a configuration of the measuring device of the figure 5 to obtain a measurement of remanence; The figure 8 illustrates a configuration of the measuring device of the figure 5 to obtain a spectral measurement; The figure 9 illustrates the evolution of the luminous flux at the output of a fiber source in the configuration of the figure 6 ; There figure 10 illustrates the evolution of the luminous flux at the output of an optical fiber inserted in a cryostat in the configuration of the figure 6 ; There figure 11 illustrates the evolution of the luminous flux on the detector in the configuration of the figure 6 ; There figure 12 illustrates the evolution of the luminous flux at the output of an integrating sphere in the state-of-the-art radiometric measuring bench of the figure 1a ; There figure 13 illustrates the evolution of the luminous flux at the input of an optical window in the state-of-the-art radiometric measuring bench of the figure 1a ; There figure 14 illustrates the evolution of the luminous flux on the detector in the state-of-the-art radiometric measuring bench of the figure 1a ; and The figure 15 illustrates the steps of carrying out a radiometric measurement, remanence measurement, a dark current measurement and a spectral measurement according to an embodiment of the method of the invention. DESCRIPTION DETAILLEE DE L'INVENTION

[0076] As illustrated on the figure 5 , the invention relates to a measuring device 10 allowing the performance of an optical detector to be characterized 20. To do this, the optical detector 20 is mounted in a cryostat 17 by means of a support 19. The support 19 may consist of a plate intended to receive an electronic circuit on which the detector 20 is mounted. Thus, the support 19 can accommodate power supply connectors for the detector mounting electronics 20.

[0077] This support 19 is connected to an internal wall of the cryostat 17 by means of feet 18. Typically, four feet 18 are arranged at the four corners of a support plate 19. To carry the power supply and feedback signals from the detector 20,the cryostat 17 is classically crossed by a set of connectors, not shown on the figure 5 . So when the detector 20 is mounted on the support 19, this can be powered and controlled in a manner analogous to its operation under the conditions expected when operating the detector 20.

[0078] For example, the detector 20 may be intended for space applications in which the detector 20 operates at very low temperatures and very high vacuum levels. Thus, the cryostat includes vacuum means and cooling means, allowing the detector to be placed 20 in its expected operating conditions.

[0079] The power supply and performance measurement devices of the detector are conventionally arranged outside the cryostat 17,as well as the means of cooling and vacuuming the cryostat 17.

[0080] Around the detector 20, the cryostat 17 includes a screen intended to limit the radiation that can reach the support 19 in the wavelength range of the detector 20. In the example of the figure 5 , the screen is made in two parts. A parallelepiped or cylindrical part 12a is fixed on an internal wall of the cryostat 17 or to the support 19. More specifically, the parallelepiped or cylindrical part 12a of the screen extends above the height of the stand 19 relative to the internal wall of the cryostat on which the feet 18 are fixed. Above this first parallelepiped or cylindrical part 12a, a second pyramidal or conical part 12b is reported above the detector 20.At the top of this pyramidal or conical part 12b, the screen has an opening receiving a first end 14 of an optical fiber 16. This optical fiber 16 also passes through an opening 15 of the cryostat, arranged opposite the opening of the pyramidal or conical portion 12b of the screen.

[0081] Of course, the shape of the screen may vary without changing the invention. For example, the screen may comprise a first cylindrical portion surmounted by a truncated cone-shaped portion or any other shape making it possible to provide a small opening opposite the detector. 20. In fact, the invention gives two distinct roles to the cryostat screen. 17 : a primary role in blocking unwanted light fluxes present in the cryostat 17 ; and a second role of positioning the first end 14 optical fiber 16opposite the detector 20.

[0082] Indeed, the first end 14 optical fiber 16 must be arranged so that the luminous flux coming out of this first end 14 mainly covers the focal plane of the detector 20. To do this, the dimensions of the cryostat must be adapted to the guidance needs of this first end 14 optical fiber 16. For example, the volume of the cryostat 17 can be between 250 cm 3 and 1000 cm 3 . It follows that the volume of the cryostat 17 can be three times larger than that of a state-of-the-art cryostat, and this volume depends on the type of optical fiber 16 used and its numerical aperture at the first end 14.The numerical aperture characterizes the acceptance cone at the fiber input and the propagation cone at the fiber output. For example, the numerical aperture of optical fiber 16 can be selected between 0.1 and 0.4 in the monomodal regime.

[0083] Preferably, the materials of the inner wall of the cryostat 17, of the screen 12a And 12b, of the support 19 and feet 18 are selected to limit the desorption of molecules when the detector is placed under vacuum 20. Likewise, the opening 15 of the cryostat 17 is preferentially adapted to the diameter of the optical fiber 16, so as to limit the incorporation of parasitic light fluxes into the cryostat 17.

[0084] More specifically, optical fiber 16 is a predominantly single-mode optical fiber in the detector wavelength range 20.This single-mode character of optical fiber 16 indicates that the optical fiber preferably has a standardized frequency between 1.2 and 3.8 in the wavelength range of interest. For example, the optical fiber can be made with a core made of Zblan, chalcogenide glass, or polycrystalline material. In addition, the optical fiber 16 can be covered with a specially selected sheath to limit the risk of transmission of stray light fluxes to the detector 20, for example an acrylate sheath. The sheath can also be formed from the same material as the optical fiber 16, This material is then associated with dopants making it possible to define the transmission window and therefore the spectral band on which the optical fiber 16 can be used.

[0085] Using an optical fiber with a normalized frequency between 1.2 and 3.8 in the wavelength range of interest of the detector 20, optical fiber 16 is particularly resistant to the integration of external parasitic flows from one of its ends, since these parasitic flows have wavelengths that are very distinct from the wavelengths that can be coupled with the optical fiber 16. In addition, the sheath limits the risk of integration of parasitic flows outside the ends of the optical fiber 16.

[0086] To transmit a light flux inside the optical fiber 16, a second end 22 optical fiber 16 is intended to receive a luminous flux from a fiber source 23, 33, 43.

[0087] This fiber source is preferably integrated into a module for generating a luminous flux used according to the desired measurement. Thus, the detector 20 can be mounted in the cryostat 17 and the second end 22 the optical fiber can receive different light flux generation modules depending on the measurements required.

[0088] This fiber source intended to transmit the light flux to the second end 22 optical fiber 16 is for example made up of a light-emitting diode, a super-continuum source or a laser source.

[0089] When it is desired to perform a radiometric measurement, as illustrated in the figure 6 , the luminous flux generation module 50 includes for example a fiber source 23 connected to a first power optical fiber 28.This feeder optical fiber is connected to an optical attenuator 24 to limit the luminous flux transmitted on the first optical supply fiber 28. At the output of the attenuator 24, a second optical power fiber 29 is implemented to transmit the luminous flux at the output of the optical attenuator 24 to an optical splitter 25. This optical splitter 25 has two outputs: a first output connected to a third optical power supply fiber 30, itself connected on the second end 22 optical fiber 16 ; and a fourth power optical fiber 31 connected to a reference detector 26.

[0090] A controller 11 controls the power of the fiber source 23 and the attenuation rate of the optical attenuator 24depending on the quantity of photons sought and measured by the reference detector 26.

[0091] At the output of the fiber source 23, the spectral distribution of the luminous flux generated in the optical fiber 23 is illustrated on the figure 9 . On the figures 9 à 14 , the spectral distribution of the light flux at the fiber output is represented in W / cm 2< as a function of the wavelength in nanometers. This spectral distribution can be measured by a spectroradiometer type device. In addition, this spectral distribution of the light flux at the fiber output is also known as "spectral density of the energy flux".

[0092] In the example of the figure 9 , the use of the fiber source 23 and optical fiber 28already allows to obtain a luminous flux particularly centered in the wavelength range of interest and presenting a very acceptable luminous power.

[0093] After passing through the optical attenuator, the luminous flux L1 at the output of the optical fiber 16 at the cryostat level 17 is illustrated on the figure 10 . We note that this luminous flux L1 retained its spectral width but decreased in luminous intensity. After propagation of this luminous flux L1 inside the cryostat, the luminous flux measured at the detector 20 is illustrated on the figure 11 . Again, the luminous flux measured at the detector 20 retained a very precise spectral width and simply lost some of its luminous power.

[0094] Thus, the use of a fiber source 23 and optical fibers 28, 29, 30, 16adapted to the transmission mode of the fiber source 23, allows for efficient transmission of light flux with elements that have a very small footprint. Comparatively, the figure 12 illustrates the luminous flux L12 at the exit of the integrating sphere 102 of the figure 1a ; we note that this luminous flux has a spectral width that is much less precise than the luminous flux transmitted by the association of the fiber source 23 and optical fibers 28, 29, 30, 16.

[0095] Similarly, in the example of the figure 1a , the luminous flux L13 at the entrance of the cryostat is illustrated on the figure 13 and still has a significant spectral width. It is only after passing through the screen filter 112, that the spectral width of the luminous flux of the state of the art is reduced. Thus, as illustrated in the figure 14 , the luminous flux transmitted to the detector 20in the state-of-the-art radiometric measuring bench also has a spectral width close to that of the invention but with a lower optical power level.

[0096] It follows that with less expensive, less bulky and less energy-consuming elements, the invention makes it possible to generate a more powerful optical flow and as precise as that transmitted in the radiometric measuring bench of the state of the art, as illustrated in the figure 1a .

[0097] In addition, the possibility of modifying the luminous flux generation module without the need to move the cryostat 17 allows for a significant improvement in measurement speed compared to the use of several successive state-of-the-art benches.

[0098] Besides the module 50 illustrated on the figure 6 to carry out a radiometric measurement, it is also possible to connect a module 51to perform a remanence measurement.

[0099] This module 51 includes a fiber source 33, for example a source identical to the source 23 of the module 50, or a source with distinct properties. This fiber source 33 is connected to a first optical power fiber, itself connected to an optical shutter 35. The optical shutter 35 is connected to a second power optical fiber 29, itself connected to an optical splitter 25. This optical splitter 25 has two outputs: a first output connected to a third power optical fiber 30, itself connected on the second end 22 optical fiber 16 ; and a second output connected to a fourth power optical fiber 31 connected to a reference detector 34.

[0100] The controller 11 allows to control the luminous flux emitted by the fiber source and the optical shutter 35 to measure the afterglow and pre-afterglow effects by comparing the images acquired by the detector 20 and by the reference detector 34.

[0101] To obtain the dark current measurement, it is not necessary to use a dedicated luminous flux generation module, and it is sufficient to turn off the fiber source 23 Or 33 of the light generation module 50 Or 51.

[0102] Indeed, without luminous flux generated by the fiber source 23 Or 33, The wavelengths of ambient light fluxes cannot be coupled into the optical fiber so that it is possible to effectively measure the dark current.

[0103] Regarding the spectral response, a luminous flux generation module 52 can be connected to the second end 22 optical fiber 16, as illustrated on the figure 8 . For example, this module 52 includes a fiber source 43 connected to a first power optical fiber 44. This first optical power supply fiber 44 is also connected to a diffraction grating 46. The output of the diffraction grating 46 is projected onto a filter 47 allowing to limit the diffraction orders of the luminous flux at the output of the diffraction grating 46. This filtered light flux is integrated into a second optical power fiber 30. This second optical power supply fiber 30 is connected on the second end 22 optical fiber 16.

[0104] So, a controller 11can control the diffraction grating parameters 46 to perform a measurement of the spectral response.

[0105] To measure the spectral response, the wavelength of the light flux at the fiber output must be varied. 16 and to measure the ratio between the detector response 20 and the response of a reference detector whose spectral response is known.

[0106] To do this, the reference detector can be positioned in the cryostat next to the detector 20 in order to directly obtain the ratio between the detector response 20 and the response of the reference detector.

[0107] Alternatively, the reference detector may be positioned in a second cryostat having properties analogous to the cryostat 17 of the detector 20, this second cryostat also integrating an optical fiber 16able to receive the luminous flux. After or before carrying out the measurements on the detector 20, optical fiber 30 can thus be connected to the optical fiber of the second cryostat to carry out the same measurements on the reference detector. The ratio between the detector response 20 and the response of the reference detector is thus calculated at the end of two measurement phases, on the detector 20 and on the reference detector.

[0108] As illustrated on the figure 15 , the method of measuring the performance of an optical detector 20 can include a first step 60 detector mounting 20 on the support 19 of the cryostat. A second stage 61 consists of putting the cryostat under vacuum to achieve the desired operating vacuum level of the detector 20. The means of cooling the cryostat 17are then implemented in a step 62 and, at the same time, a first light flux generation module can be connected to the second end 22 optical fiber 16, in one step 63. When a module 50 has 52 luminous flux generation is connected on the second end 22 optical fiber 16, detector performance measurements 20 can be carried out, in one step 64.

[0109] Then, when all the detector performance measures 20 are carried out using the module 50 has 52 connected, the module 50 has 52 can be disconnected, in one step 65. When disconnecting the module 50 has 52, there is no need to change the temperature or vacuum level of the cryostat 17, so that a second module 50 has52 can then be connected to the optical fiber 16, in one step 66. Detector performance measurements 20 can then be carried out, in one step 67, based on the new module 50 has 52 connected to optical fiber 16.

[0110] Several modules 50 has 52 can thus be connected successively to the optical fiber 16 without needing to change the temperature or vacuum level of the cryostat 17.

[0111] For example, we can have the following sequence of steps: stage 63 : connection of the generation module 50 allowing a radiometric measurement to be carried out; step 64 : radiometric measurement of detector performance 20 ; stage 65 : module disconnection 50 ; stage 66 : module connection 51to measure the remanence; step 67 : measurement of the detector's remanence; step 68 : measurement of dark current of the detector by switching off the fiber source 33 of the module 51.

[0112] When the remanence and dark current measurements are performed by the module 51, we then have the following sequence of steps: stage 69 : module disconnection 51 ; stage 70 : module connection 52 allowing the spectral response to be measured; step 71 : measurement of the spectral response of the detector 20.

[0113] Regardless of the number of modules 50 has 52 connected to optical fiber 16 to characterize the detector 20, the process ends with a step 72 cryostat heating 17 and a step 73 of repressurizing the cryostat under ambient pressure 17and disassembly of the detector 20.

[0114] The invention thus makes it possible to obtain a measuring device 10 performance of an optical detector 20 having a smaller footprint and a reduced cost compared to the use of multiple measuring benches. Indeed, the invention allows the light flux generation modules to be interchanged to couple them to an optical fiber partially integrated into the cryostat 17. This possibility of coupling different modules 50 has 52 on optical fiber 16 improves the speed and time required to complete all measurements.

[0115] For example, when it is desired to characterize a detector 20by carrying out a radiometric measurement, a remanence measurement, a dark current measurement and a spectral response measurement, the invention makes it possible to obtain a division of the measurement time by a factor of between 5 and 10.

[0116] Furthermore, it was found that the accuracy of the measurements obtained is also better than that obtained in the state of the art. Indeed, the repeatability rate of the measurements was estimated at 0.1% with the devices of the state of the art and the invention made it possible to obtain a repeatability rate of 0.01%. This repeatability rate is calculated by repeating a similar measurement a large number of times and calculating the divergence between these measurements.

Claims

1. A device (10) for measuring the performance of an optical detector (20) comprising: - a cryostat (17) comprising means for placing it under vacuum and cooling it capable of placing the detector (20) in its anticipated operating conditions; - a holder (19) capable of receiving the detector (20), secured to the inside of the cryostat (17); - means for measuring the performance of the detector (20); and - a screen (12a-12b) arranged around the holder (19) capable of limiting the radiation likely to reach the holder in a wavelength range of the detector (20); characterized in that the measurement device (10) also comprises: - a single-mode optical fiber (16) in the wavelength range of the detector (20); the optical fiber (16) being inserted in an opening (15) of the cryostat (17) and comprising: • a first end (14) secured to an opening of the screen (12a-12b), to project a luminous flux onto all or part of the detector (20); and • a second end (22), external to the cryostat (17), intended to receive a luminous flux; and - at least two modules (50-52) for generating a luminous flux that incorporate a fibered source (23, 33, 43) capable of generating the luminous flux in the second end (22) of the optical fiber (16); the means for measuring the performance of the detector (20) being capable of measuring said performance according to the luminous flux received on the second end (22); where a plurality of distinct measurements may be performed by interchanging the module (50-52) connected on the second end (22).

2. The device for measuring the performance of an optical detector (20) according to claim 1, wherein the fibered source (23, 33, 43) is formed of a light-emitting diode, of a supercontinuum source, or of a laser.

3. The device for measuring the performance of an optical detector (20) according to claim 1 or 2, wherein the optical fiber (16) is formed with a core made of fluorinated glass, of chalcogenide glass, or of polycrystalline material.

4. The device for measuring the performance of an optical detector (20) according to any of claims 1 to 3, wherein one of the luminous flux generation modules (50), intended to be connected on the second end (22) of the optical fiber (16) to perform a radiometric measurement, comprises: - a fibered source (23) intended to generate a luminous flux inside of a first feeder optical fiber (28); - an optical attenuator (24), capable of limiting the luminous flux, said attenuator (24) being connected between the first feeder optical fiber (28) and a second feeder optical fiber (29); - an optical splitter (25), connected to the second feeder optical fiber (29), capable of splitting the luminous flux of the second feeder optical fiber (29) onto a third (30) and a fourth (31) feeder optical fiber; the third feeder optical fiber (30) being connected on the second end (22) of the optical fiber (16) inserted in the cryostat (17); and - a reference detector (26), connected on the fourth feeder optical fiber (31), and capable of measuring the luminous flux at the output of the optical splitter (25) in order to control the power of the fibered source (23) and the optical attenuation (24) to obtain an anticipated measurement luminous power.

5. The device for measuring the performance of an optical detector (20) according to any of claims 1 to 4, wherein one of the luminous flux generation modules (51), intended to be connected on the second end (22) of the optical fiber (16) to perform a remanence measurement, comprises: - a fibered source (33) intended to generate a luminous flux inside of a first feeder optical fiber (28); - an optical shutter (35), capable of blocking the luminous flux, connected between the first feeder optical fiber (28) and a second feeder optical fiber (29); - an optical splitter (25), connected to the second feeder optical fiber (29), capable of splitting the luminous flux of the second feeder optical fiber (29) onto a third (30) and a fourth (31) feeder optical fiber; the third feeder optical fiber (30) being connected on the second end (22) of the optical fiber (16) inserted in the cryostat (17); and - a reference detector (34), connected to the fourth feeder optical fiber (31), capable of controlling the luminous flux at the output of the optical splitter (25).

6. The device for measuring the performance of an optical detector (20) according to claim 5, wherein the luminous flux generation module (51), intended to be connected on the second end (22) of the optical fiber (16) to perform a remanence measurement, is also implemented to perform a dark current measurement by cutting off the electric power supply of the fibered source (33).

7. The device for measuring the performance of an optical detector (20) according to any of claims 1 to 5, wherein the optical fiber (16) has a normalized frequency (V) in the range from 1.2 to 3.8 in the wavelength range of interest.

8. The device for measuring the performance of an optical detector (20) according to any of claims 1 to 7, wherein one of the luminous flux generation modules (52), intended to be connected on the second end (22) of the optical fiber (16) to perform a spectral response measurement, comprises: - a fibered source (43) intended to generate a luminous flux inside of a first feeder optical fiber (44); - a diffraction grating (46) connected to the first feeder optical fiber (44); - a filter (47) connected at the output of the diffraction grating (46); and - a second feeder optical fiber (48) connected at the output of the second filter (47).

9. A method of measuring the performance of an optical detector (20) by means of a measurement device (10) according to any of claims 1 to 8, said method comprising the following steps: - assembly (60) of the detector (20) to the holder (19) of the cryostat (17); - placing under vacuum (61) of the cryostat (17) to reach a desired operating vacuum level of the detector (20); - cooling (62) of the cryostat (17) to reach a desired operating temperature of the detector (20); - connection (63) of a first luminous flux generation module (50-52) on the second end (22) of the optical fiber (16); - measurement (64) of the performance of the detector (20); - disconnection (65) of the first luminous flux generation module (50-52) from the second end (22) of the optical fiber (16); - connection (66) of a second luminous flux generation module (50-52) on the second end (22) of the optical fiber (16); - measurement (67) of the performance of the detector (20); - heating (72) of the cryostat (17) to reach the room temperature; and - placing back under ambient pressure of the cryostat (17) and disassembly (73) of the detector (20).

10. The method of measuring the performance of an optical detector according to claim 9, said method comprising the following steps: - connection (63) of a first luminous flux generation module (50) to perform a radiometric measurement on the second end (22) of the optical fiber (16); - radiometric measurement (64) of the detector (20); - disconnection (65) of the first luminous flux generation module (50) from the second end (22) of the optical fiber (16); - connection (66) of a second luminous flux generation module (51) to perform a remanence measurement on the second end (22) of the optical fiber (16); - measurement of the remanence (67) of the detector (20); - measurement (68) of the dark current of the detector (20); - disconnection (69) of the second luminous flux generation module (51) from the second end (22) of the optical fiber (16); - connection (70) of a third luminous flux generation module (52) to perform a spectral response measurement on the second end (22) of the optical fiber (16); and - measurement (71) of the spectral response of the detector (20).