FR2862452A1 - OPTICAL AMPLIFICATION DEVICE AND ASSEMBLY COMPRISING THE SAME - Google Patents

Abstract

The invention relates to a device for the optical amplification of a working light signal (4), characterized in that it comprises a source (1) of optical pumping illumination (5) and a device (2) forming a cavity. optical pumping, said device being able to be placed upstream of an optical detector (3) of the working light signal. The invention also relates to an assembly comprising such a device.

Description

OPTICAL AMPLIFICATION DEVICE AND TOGETHER

WITH

  GENERAL FIELD OF THE INVENTION The invention relates to the general technical field of optical amplification devices.

  More particularly, it relates to the technical field of optical amplification devices by pumping.

STATE OF THE ART

  In general, optical amplification devices receive a signal from an optical detector in the form of an electrical signal that they amplify.

  Such amplification devices, however, are not entirely satisfactory. Indeed, if they provide correct amplification for optical signals of relatively high intensity, namely signals which generate a significant signal-to-noise ratio at the output of the detector, problems arise for signals of very low optical intensity. . Amplifiers can no longer amplify the signal properly since they also amplify the noise.

  PRESENTATION OF THE INVENTION The purpose of the invention is to overcome the above disadvantages.

  One of the aims of the invention is to propose a device allowing the observation of optical signals of very low intensities but whose wavelength is precisely known.

  Another object of the invention is to provide a device for amplifying optical signals of very low intensities but whose wavelength is precisely known.

  Another object of the invention is to provide a device for filtering a light signal in addition to the amplification.

  For this purpose, the invention proposes a device for optical amplification of a working light signal, characterized in that it comprises an optical pump illumination source and an optical pump cavity device, said device being capable of being placed upstream of an optical detector of the working light signal.

  The invention is advantageously completed by the following characteristics, taken alone or in any technically possible combination: the source of pump illumination is located opposite the pumping cavity device or laterally with respect to the pumping cavity; the optical pump cavity device comprises at least three layers superimposed on each other, the input layer of the working signal comprising a semi-reflecting mirror of the order of magnitude of a fraction of the wavelength of the pump illumination, the output layer comprising a semi-reflective mirror of the order of magnitude of a fraction of the working wavelength, the intermediate layer comprising, for its part, a material suitable for optical pumping; the optical pump cavity device comprises at least three layers superimposed on each other, the input layer of the working signal comprising a semi-reflective mirror of thickness suitable for laser amplification of the working wavelength , the output layer comprising a semi-reflective mirror of the order of magnitude of a fraction of the wavelength of work to be able to laser amplification of the working wavelength while blocking the wavelength of the pump illumination, the intermediate layer having in turn a material suitable for optical pumping; the source of pump illumination is such that the wavelength of the pump illumination has a wavelength much shorter than the working wavelength of the working signal.

  The invention also relates to an assembly comprising such a device.

PRESENTATION OF FIGURES

  Other characteristics, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and nonlimiting, and which should be read with reference to the accompanying drawings, in which: FIG. 1 schematically represents a possible embodiment of the invention. an optical amplification device according to the invention; and Figure 2 shows schematically the energy levels involved in a device according to the invention.

DETAILED DESCRIPTION

  The invention is advantageously applied to optical signals of low intensities but whose wavelength is precisely known, such as for example astronomical optical signals.

  The principle of the invention is to amplify the optical signal before detection by a detector.

  Admittedly, an amplification of the signal upstream of a detector can do nothing against fish noise related to the low number of photons at the arrival on the detector. On the other hand, the amplification can increase the ratio of the signal to the thermal noise and / or allow the use of less efficient detectors.

  Thus, FIG. 1 schematically represents a longitudinal section 20 of a detector 3 of an optical signal 4 of work to be observed and of an amplification device 10 according to the invention.

  Thus, there is an amplifier 10 in front of an optical detector 3. The detector 3 can be of any type. For example, it may be a space telescope objective type device comprising a device with a charge-coupled matrix (CCD) according to the English terminology.

  The amplifier 10 comprises firstly a planar device 2 consisting of several layers 23, 22, 21 superimposed on each other. The plane device 2 extends substantially parallel to the receiving surface of the receiver 3.

  It also comprises a device 1 generating pump illumination 5.

  The illumination device 1 can be located, as described below, firstly opposite the device 2, so as to send the pumping signal 5 parallel to the working signal 4 and thus perform a front illumination of the receiver 3 and of the device 2.

  It may, secondly, be located laterally, in order to effect lateral illumination of the device 2, that is to say illumination by the edge of the plane device 2. The illumination by the wafer then takes place in a direction perpendicular to the longitudinal plane of the detector 3 contained in the plane of FIG.

  The first layer 23 of the planar device 2 is furthest from the detector 3. It comprises a semi-reflective mirror of small thickness.

  The third and last layer 21 is a half-reflective mirror thicker than the first, for reasons which are explained below.

  The second layer 22 located below the layer 23 and above the layer 21 is sandwiched between the layers 21 and 23. It comprises a suitable material for optical pumping known to those skilled in the art.

  The space between the two mirrors 23 and 21, materialized by the layer 22, is a laser amplification cavity between the two mirrors 23 and 21.

  The adjustment of the laser amplification allows the selection of the wavelength chosen to be amplified among the working signal 4. The layer 22 thus exerts the filtering function.

  The optical pumping material of the layer 22 and / or its thickness are chosen according to the pumping wavelength 5, which must be of a much shorter wavelength than the working wavelength of the signal 4.

  The pumping wavelength should be about 10 times less than the working wavelength. Indeed, the layer 21, which has an exponentially attenuating coefficient, must be able to stop the radiation very efficiently. at the pump wavelength 5 while allowing the radiation to pass at the working wavelength 4.

  It will be recalled, as shown in FIG. 2, that the pump A generated by the illumination 5 results in a change in the energy state level of the electrons incident on the device 2. Thus, the pump A has passed the electrons of the pumping for example initially on an energy level EO to a much higher energy level, denoted E2, which is relaxed rapidly to a lower intermediate level El, but relatively close to the fundamental level E0. The transition B between the lower level El and the fundamental level EO constitutes the wavelength 4 of work. The transition between EO and E2 is the pumping wavelength.

  The operation of the device 10 is as follows.

  Pumping by the short pumping wavelength produced by the device 1 can be achieved in two ways.

  The first way uses frontal illumination. In this case, the pump illumination 5 passes the first mirror 23, whose small thickness passes a substantial proportion of the illumination 5 at the pumping wavelength. The thickness of the input mirror 23 is therefore typically a fraction of the pump wavelength 5.

  The second way uses the lateral illumination, that is to say by the edge of the device 2 and in the layer 22. The pumping cavity tuned to the working wavelength, that is to say the wavelength to filter and amplify, then allows a significant multiplicative gain on each photon arriving on the device 2 with the right wavelength.

  The output mirror 21, which is thicker, is such that the working wavelength 4 can pass through it, but the pumping wavelength 5 can not. This prevents the detection of the pumping signal by the detector 3. The thickness of the output mirror 21 is typically a fraction of the working wavelength 4.

  In the case of a frontal illumination, the input layer 23 of the working signal comprising a semi-reflecting mirror of thickness of the order of magnitude of a fraction of the wavelength of the pump illumination 5. The output layer 21 comprises a semi-reflecting mirror of the order of magnitude of a fraction of the working wavelength 4. The intermediate layer 22 comprises a suitable material for pumping. optical.

  In the case of lateral illumination, the input layer 23 of the working signal comprising a semi-reflecting mirror of thickness suitable for the laser amplification of the working wavelength 4. There is no stress on the layer 23 for the pumping wavelength 5. The output layer 21 comprises a semi-reflective mirror of the order of magnitude of a fraction of the working wavelength 4 for be capable of laser amplification of the working wavelength 4. It must be thick enough, however, to block the wavelength of the pump illumination 5. The intermediate layer 22 comprises a material suitable for a optical pumping.

  The detector 3 located behind the device 2 therefore only sees the photons of the wavelength 4 of work.

  In the case where the observation laser radiation 4 is polarized, the discriminant efficiency of the device can be increased by making the polarization-sensitive, and therefore opaque, cross-polarized input and output mirrors 23.

  In addition, the polarization can also serve as an observation of any object, by the observation in the cross polarization or the simultaneous observation in two different polarizations. In this case, several detection / filtering / amplification devices are required. It is also possible to envisage an assembly comprising several detectors amplifying several different wavelengths under different polarizations.

  The main advantage of a filtering and amplifying device 10 is that it is located upstream of the detector. It thus allows an amplification of monochromatic optical signals of very low intensity, but whose wavelength is known precisely, in particular, but not exclusively, in astronomical, earth observation or low-lit or unlit landscape applications.

  Thus, the foregoing developments advantageously apply to assemblies comprising a Fresnel lens in orbit and focusing light radiation in space towards a detector also in orbit.

  The assembly comprises means for transmitting a working light signal 4. Preferably, the transmission means comprise at least one monochromatic laser source.

  One can thus consider emitting the wavelength of work under low intensity towards a landscape and use the device to observe the landscape.

  It is of course understood that such a device can be advantageously applied to any optical detection assembly, and even to detection assemblies that are not in orbit. It is thus possible to have an optical observation assembly comprising a detector and convergence means other than Fresnel-type lenses, or Fresnel lenses not in orbit, and using such a filtering and amplification system.

Claims (10)

CLAIMS.   1 device for optical amplification of a working light signal (4), characterized in that it comprises a source (1) for illumination (5) for optical pumping and a device (2) forming an optical pumping cavity, said device being able to be placed upstream of an optical detector (3) of the working light signal.   2. Device according to claim 1, characterized in that the pump illumination source 10 (5) is located opposite the device (2) forming a pumping cavity or laterally with respect to the pumping cavity.   3. Device according to one of claims 1 or 2, characterized in that the device (2) forming optical pumping cavity comprises at least three layers (23, 22, 21) superimposed on each other, the layer of input (23) of the working signal having a semireflective mirror of the order of magnitude of a fraction of the wavelength of the pump illumination (5), the output layer (21) having a semi-reflecting mirror of the order of magnitude of a fraction of the working wavelength (4), the intermediate layer (22) having in turn a material suitable for optical pumping.   4. Device according to one of claims 1 or 2, characterized in that the device (2) forming optical pumping cavity comprises at least three layers (23, 22, 21) superimposed on each other, the layer of input (23) of the working signal comprising a semi-reflective mirror of thickness suitable for laser amplification of the working wavelength (4), the output layer (21) comprising a semireflective mirror of thickness of the order of magnitude of a fraction of the working wavelength (4) to be able to laser amplification of the working wavelength (4) while blocking the wavelength of the pump illumination, the intermediate layer (22) having in turn a material suitable for optical pumping.   5. Device according to one of claims 1 to 4, characterized in that the source of pump illumination is such that the wavelength of the pump illumination (5) has a much shorter wavelength 10 as the working wavelength of the working signal (4).   6. An assembly comprising a detector (3) of an optical working signal, characterized in that it comprises, upstream of the detector, a device according to one of claims 1 to 5.   7. The assembly of claim 6, characterized in that it is placed in orbit around an object in space.   8. Assembly according to one of claims 6 or 7, characterized in that it comprises a Fresnel lens comprising alternating transparent areas and opaque areas of certain widths to converge the working signal to the receiver (3). ).   9. Assembly according to one of claims 6 to 8, characterized in that it comprises means for transmitting a working light signal (4).   10. The assembly of claim 9, characterized in that the transmitting means comprise at least one monochromatic laser source.