FR2697352A1 - Electromagnetic energy concentrator with frequency shift - uses dopant that responds to electromagnetic excitation by emitting light at different frequency - Google Patents

Abstract

The concentrator has a transparent matrix (4) with a coating over parts of the matrix. The coating has a high reflectivity at one wavelength and a high transmissivity at another wavelength. The transparent zone has optically active dopants absorbing energy at one wavelength and transmitting it at another wavelength. The excitation wavelength is in the transmission band for the reflective coating, and the emission wavelength is in the reflective band for the coating. The concentrator thus acts as an 'electromagnetic diode', showing different behaviour dependent on the direction of arrival of the electromagnetic signal. ADVANTAGE - Improved detection of illumination by laser to warn military target that it has been scanned, or can be used to improve efficiency of photovoltaic cells.

Description

ELECTROMAGNETIC ENERGY CONCENTRATOR A
FREQUENCY CHANGE CONSTITUTING BETWEEN ANOTHER DIODE
ELECTROMAGNETIC.

 The present invention relates to an optical device having an electromagnetic diode effect, that is to say having a different behavior depending on the direction of incidence of the radiation source. Such a device is used in particular in the military field, for the detection of a laser telemetry beam and the alert of the pilot of a land, naval or air vehicle, or the triggering of countermeasures. In the current state of the art, there is no means to determine with sufficient spectral and spatial discrimination, whether the machine is the subject of an illumination by a rangefinder or illuminator for the acquisition of the target, the parameter of a missile and the guidance of said missile. As a result, military vehicles present an indisputable vulnerability with regard to weapon systems implementing illumination techniques or laser telemetry.

 The device according to the invention also behaves, according to certain embodiments, like an electromagnetic flux concentrator.

 The invention relates more particularly to a device comprising a substantially transparent matrix and, on a part of the external surfaces of said matrix, a coating having a high reflection rate in a first wavelength band and a high transmission rate in a second wavelength band. The doped matrix comprises at least one optically active dopant absorbing electromagnetic energy in one or more wavelength bands, and re-emitting energy in one or more wavelengths, one at less of the remission wavelengths being different from the excitation wavelengths, said matrix further comprising a filtering coating having a maximum transmission in one or more absorption wavelength bands of the doped matrix, and having a maximum reflection in at least one of the re-emission wavelength bands of the doped matrix.

Advantageously, the invention relates to an optical device comprising a substantially transparent matrix and, on a part of the external surfaces of said external matrix, a coating having a high reflection rate in a first wavelength band and a high transmission rate in a second wavelength band, characterized in that the transparent matrix comprises optically active dopants absorbing light energy in a wavelength band ka + AXa and re-emitting in a wavelength band ke + Ake , the harfrilha excitation band being included in the transmission range of the coating and the Xe + AX re-emission band being included in the reflection range of said coating. The optically active dopants are integrated in the matrix or are deposited on a part of the external surfaces opposite to the entrance face of the incident beam of a transparent matrix.

 When the incident beam comprises two lines Xa and Xe, the output beam of the device according to the invention has only one line Xe, when the incident beam meets the face comprising said coating.

 On the other hand, when the incident beam meets the face opposite to the treated beam, the device does not allow any of the two lines to pass.

 According to a first variant, the doped matrix is coated with a bandpass filter whose cut-off wavelength is between w and Xe.

 According to a second variant, the doped matrix is coated with a bandpass filter constituted by a dichroic filter.

 According to another variant, the doped matrix is coated with a bandpass filter constituted by an interference filter.

 According to a particular embodiment, the doped matrix also has a portion of the outer surfaces coated with a reflective material in the re-emission wavelength band and in the excitation wavelength band of the dopants, as well as 'a transparent area.

 In this embodiment, the device behaves like a light flux concentrator, because it allows all the effective energy received by the treated surfaces to be transferred to the transparent area.

 Advantageously, the doped matrix is of rectangular shape, the coating having a high reflection rate in a first wavelength band and a high transmission rate in a second wavelength band being provided on the input face. of said device.

 According to a variant, the doped matrix is of paraboloidal shape, the coating having a high reflection rate in a first wavelength band and a high transmission rate in a second wavelength band being provided on the lateral faces .

 According to a variant, the optically active dopants are deposited on the surface of a transparent matrix.

 The invention also relates to a device consisting of a plurality of cells each formed by a matrix having on its input face a dichroic filter, the cell of index n comprises a matrix doped with dopants absorbing light energy in a band. of wavelength Xan + AXan which corresponds to Xe (n-1) - AX (nl) and re-emitting in a wavelength band henf AXen, the excitation band Xan + AXan being included in the transmission range of the dichroic coating and the Xen + Aken retransmission band being included in the reflection range of said coating. This embodiment makes it possible to discriminate the incident wavelengths, and is particularly advantageous for discriminating different types of illumination beams laser to identify the type of hazard.

The invention will be better understood on reading the description which follows, referring to the drawings in which:
- Figure 1 shows the response curve in transmission and reflection, of the coating
- Figure 2 shows the response curve of the dopant;
- Figure 3 shows a block diagram of the device according to the invention
- Figure 4 shows a sectional view of a device according to the paraboloid embodiment;
- Figures 5 and 6 show two exemplary embodiments of photovoltaic generators;
Figures 7 and 8 show two particular embodiments of the invention;
- Figures 9 and 10 show sectional and front views of an embodiment of a security device;
- Figure 11 shows an example of spectral response of a fiduciary marking.

 FIGS. 1 and 2 represent, on the same graph, the spectral response of the filtering coating covering part of the transparent matrix, and the spectral behavior of the dopant.

 The upper curve (1) represents the transmission and reflection rate of the coating placed on the input face of the device, as a function of the wavelength of the incident beam.

 In the example described, the filter coating has a transmission rate close to 100% for wavelengths less than Xc nanometers. For wavelengths greater than Xc nanometers, the reflection rate is close to 100%, and therefore the transmission rate close to 0. The cutoff slope is more or less steep depending on the quality of the filter.

 Curve (2) represents the spectral behavior of the dopants included in the transparent matrix.

 These dopants have an excitation spectrum in the wavelength band kathXa and a re-emission spectrum in the wavelength band + axis.

 The dopants consist, for example, of photoluminescent products, in particular luminescent crystalline or organic materials.

 FIG. 3 represents the block diagram of a device according to the invention.

 The incident radiation in the example described comprises two lines X1 and k2, X1 being less than the cut-off wavelength Xc of the filtering coating of the device is substantially equal to Xa, wavelength of excitation of the dopants. 2 is greater than this cut-off wavelength Xc, and substantially equal to Xe, the dopant re-emission wavelength.

 Component 2 is fully reflected by the filtering layer (3), and consequently, only component 1 enters the doped matrix (4). This incident beam interacts with the dopants (5) included in the matrix, and is therefore converted into a diffuse beam of wavelength Xe.

 At the output of the device, a diffuse radiation of wavelength 4 will therefore be observed.

 In the event that the incident beam enters the device through a surface not coated with the filtering layer (3), the wavelength line Xe cannot exit from the side of the filtering coating, since it is reflected by said coating (3 ). The wavelength line Xa is absorbed by the dopants and converted into radiation of wavelength Xe, which is therefore also reflected by the filtering layer (3).

 Consequently, the device does not allow the beam entering the device to pass through a surface not coated with the filtering layer.

 FIG. 4 represents a sectional view of a device according to an embodiment of the paraboloid type.

 The tubular part of the envelope of the doped matrix (4) is coated with a selective reflection layer (3). The parabolic dome (8) is covered with a completely reflecting layer (9). The planar end (10) of the doped matrix (4) is also coated with a completely reflecting layer (12), with the exception of a transparent zone (11).

As an example, the doped matrix is made of polymethamethacrilate doped with the following compounds:
PPO 0.5 moles per liter
OB 0.1 moles per liter
GE 0.04 moles per liter
PPO, OB and GE being the usual commercial names of aromatic cyclic molecules.

The cut-off wavelength Xc of the selective reflection layer is 400 nanometers.

FIG. 5 represents an exemplary embodiment in which the matrix (13) is transparent, and in which the dopants are deposited on the external surface of said matrix (13). The dopants form with the reflective layer an envelope (14) surrounding the matrix (13) except for an exit window (16) and the surface covered by the dichroic filter (15).
The device according to this embodiment has a high photonic efficiency and includes a light energy concentrator. The window (16) can be equipped with a shutter or an optical chopper. Wavelength transfer phenomena occur in this embodiment on the surfaces of the matrix.

 According to another embodiment represented in FIG. 6, the device is constituted by a series association of several cells constituted by a doped matrix coated with a dichroic filter. The first cell (17) comprises a matrix doped with dopants absorbing light energy in a band of wavelength kaliAWal and re-emitting in a band of wavelength kel + AXel, the excitation band XaliAXal being included in the transmission range of the dichroic coating and the re-emission band le1 + Dlel being included in the reflection range of said coating. The second cell (18) comprises a matrix doped with dopants absorbing light energy in a band of wavelength Xa2 * AXa2 which corresponds heif AXel and re-emitting in a wavelength band Xe2 + # Xe2, the excitation band # a2 + AXa2 being included in the transmission range of the dichroic coating and the re-emission band Xe2 + AXe2 being included in the reflection range of said coating.

Similarly, the third cell (19) comprises a matrix doped with dopants absorbing light energy in a wavelength band Xa3 + txa3 which corresponds to Xe2I AXe2 and re-emitting in a wavelength band ke3 + AXe3, the excitation band # a3 +
AXa3 being included in the transmission range of the dichroic coating and the Xe3 + AXe3 retransmission band being included in the reflection range of said coating.

 Such a device makes it possible to discriminate different types of incident beams. In particular, for laser detection and alert applications, a device consisting of a plurality of juxtaposed cells makes it possible to deliver a specific light signal for each type of incident laser beam, and to signal the type of acquisition in progress.

 For laser detection and alert applications, several implementation methods can be envisaged.

 The first mode of implementation consists in producing a sensor constituted by a matrix covered by a dichroic filter transparent in the emission wavelengths of the telemetry lasers, ie around 1.06 micrometers, the filter being further reflective of the wavelength bands between 1.2 and 1.65 microns, and being absorbent or reflective, but not transparent, in the wavelength ranges between 0.75 and 0.95 micrometers.

 The incident laser beam is thus trapped in the blade formed by the dichroic filter on the front face, the luminescent optical material and the reflective screen covering the other faces of the matrix, the photosensitive detector allowing multidirectional monitoring of the laser beams.

 According to a second embodiment, the device allows the detection of two laser beams of different wavelengths, for example at 1.06 microns and 1.54 microns.

For this mode of implementation, the dichroic filter has the following characteristics:
- Transparency to laser radiation likely to be detected, for example at 1.06 and 1.54 micrometers,
- Reflection between 1.2 and 1.45 micrometers
- Non-transparency, that is to say absorption or reflection between 0.75 and 0.95 micrometers.

 This embodiment makes it possible to detect two different types of laser.

 According to a third embodiment, the device comprises two consecutive systems, the first being constituted by a reflective filter corresponding to the wavelength of reemission of the dopants of the first matrix whose excitation wavelength corresponds to the wavelength of one of the type of laser to be detected, the second system being placed at the rear of the first system and comprising a dichroic filter reflecting the wavelengths of remission of the dopant from the second matrix, dopant of which the excitation wavelength corresponds to the wavelength of the second type of laser to be detected. Each system includes a photosensitive detector placed for example on the edge of the matrix, and emits a signal making it possible to detect the lasers likely to constitute a danger, and to identify the type of laser employed. Advantageously, the systems are mounted on a support pivoting about an axis perpendicular to the entry face coated by the dichroic filter.

 The number of similar systems, tuned to a specific wavelength band, can be increased, the device constituting in this case a spectral analyzer.

 Another application of the invention relates to the generation of energy by photovoltaic effect, and more precisely the increase in the coefficient of transformation of light energy into electrical energy, without recourse to cells with high efficiency and high cost. For such an application, the solar cells are disposed at the rear of the doped matrix, or are embedded in the mass of the doped matrix. The doped matrix comprises dopants absorbing light energy in the wavelength range between approximately 360 and 485 nanometers, in which the photovoltaic cells have a low efficiency, and re-emit in a wavelength band substantially between 825 and 875 nanometers corresponding to the range of greatest sensitivity of single crystal silicon cells.

 A dichroic filter covers at least part of the surfaces of the matrix. This dichroic filter is constituted by a coating of thin layers having a maximum reflectance in the length band set on 850 nanometers + 10 nanometers. This wavelength band corresponds to a low level of solar energy emission, thus making it possible to reduce the losses by external reflection, but in which the energy re-emitted by the optically active dopants is completely reflected by the dichroic filter. Photon trapping is thus carried out in the wavelength range of higher efficiency of the solar cells.

Thus, the energy efficiency is increased by the wavelength conversion combined with the increase in the optical paths ensuring a maximum rate of photonic interactions.

 FIG. 7 represents a view of an exemplary embodiment of a photovoltaic generator according to the invention. It is constituted by a multilayer structure having a transparent sheet (20) constituted for example by extra-white glass, ensuring the rigidity of the assembly and protecting a layer (21) constituting a dichroic filter. The doped matrix (22) includes dopants and diffusers. The photovoltaic cells (23) are arranged in the doped matrix (22) and form a planar network. A neutral layer (24) allows the passage of the connecting wires. The device also has a reflective layer (25) on its lower surface.

 The doped matrix (22) includes dopants of PPO, OB and 8G type (trade names).

 The grouping of the two OB and 8G dopants has an excitation band of between 360 and 480 nanometers, and a remission band of between 470 and 550 nanometers, with a peak corresponding to VS% of emission between 480 and 520 nanometers.

 Furthermore, the molecule marketed under the name of 8G absorbs wavelengths between 480 and 560 nanometers, to re-emit them between 560 and 670 nanometers (50% of the peak) and mainly between 580 and 650 nanometers (75% of the peak ).

 The dichroic filter (21) upstream of the solar cells (23) lets through the wavelengths between 350 and 480 nanometers, the wavelengths between 520 and 580 nanometers, as well as the wavelengths above 650 nanometers , and reflects the wavelengths between 490 and 520 nanometers, as well as between 580 and 650 nanometers.

 Silicon solar cells have a spectral band whose maximum is around 870 nanometers. With a device according to the invention, energy is transferred from short wavelengths (less than 500 nanometers) to the wavelength bands corresponding to the maximum efficiency of the solar cells.

 Another embodiment represented in FIG. 8 consists in superimposing inside the matrix several strata of photovoltaic cells, arranged on planes (25 to 27) perpendicular to the input face (28) having a dichroic filter (29 ). The lateral faces (30 to 33) of the doped matrix (34) which are not liable to receive incident light are covered by a reflective layer. This "volume" embodiment makes it possible to produce compact photoelectric generators.

 Another application of the invention relates to the production of an active retro-reflector type device, for example for safety signaling. An exemplary embodiment is shown in Figure 9, in sectional view and in Figure 10, in front view.

 The device comprises a doped matrix (40) covered on its front face by a dichroic filter (41) having untreated areas (42), for example totally reflecting areas. The other faces of the matrix (40) are covered by a reflective layer (43).

 The matrix is doped with excitable dopants in the wavelength band between 360 and 450 nanometers, and reemitting light in a wavelength band close to 620 nanometers. Preferably, the matrix comprises long-lasting remanence dopants.

 The untreated areas (42) of the front face are distributed to form a geometric figure, or iconographic or text messages. FIG. 10 represents an exemplary embodiment in which the untreated zones (42) form a planar network.

 When a flash of white light illuminates the device, the light will be reflected in the wavelengths between 420 and 720 nanometers by the dichroic filter (41) and for the other wavelengths absorbed by the doped matrix (41) and re-emitted at a wavelength of 620 nanometers.

 A derived application consists in implementing this embodiment for securing documents, in particular for fiduciary marking, or for the identification of payment cards.

 The principle of marking consists in implementing optical effects detectable by electronic means designed by the transmitter of authentic products, but almost impossible to analyze by laboratory means, and even less with the naked eye.

 The marking uses a dichroic filter associated with several fluorescent and / or remanent dopants and explained by an ultraviolet illuminator, or by illuminator with a narrow spectral domain in the visible or in the infrared. Preferably, the matrix comprises one or more fluorescent dopants and one or more residual dopants, associated with a dichroic filter having a high transmission coefficient in the excitation wavelength bands of the dopants, and a high reflection coefficient in the remission wavelength bands of said dopants.

 FIG. 11 represents an example of spectral response of an object comprising an authentication sign in accordance with the invention.

 The OZ axis corresponds to the temporal variation, the OX axis corresponds to the wavelength of the response of the marked device to an excitation by an ultraviolet flash at a given time TO, and the OY axis corresponds to the intensity measured.

 In this example, which corresponds to an optimal situation, the remission spectrum of the fluorescent dopant completely covers the remission spectrum of the remanent dopant, the top of the remanence peak being nevertheless offset with respect to the fluorescence peak. This situation prevents the analysis of the components used in the doped matrix, but allows satisfactory discrimination by the specific analysis equipment.

The intensity of the fluorescent response, shown in dotted lines in FIG. 11, exhibits an intensity greater than the remanent response, even at To, and thus saturates the visual response due to the retinal persistence. If you do a fluorescent analysis using a spectroadiometer or spectrofluorimeter, it will be difficult to distinguish the afterglow.

Some examples of combinations of dopants are given below (the names of the products correspond to the usual trade name):
In the blue wavelength range
Short-term remanent .............. DMABN
1st blue fluorescent ............... OB
2nd blue fluorescent .............. PPO
3rd blue fluorescent ................... 2205
In the range of green-blue
Short-term remnant .............. DMABN
Fluorescent green 52012
or
Short-lived green reman 2330
Green fluorescent (mineral dopant) 52012
or
Short-lived green remnant 2230
Fluorescent green 52012
In the red wavelength range
Short-term remnant .............. 2305
1st red fluorescent .............. P22
2nd yellow fluorescent ................ 6 G
3rd orange fluorescent ............. 8 G
Another application concerns the improvement of vision in certain wavelength bands, in particular in the band centered on 540 nanometers for which the human eye is most sensitive. This improvement makes it possible to correct certain chromaticity defects, or to improve night vision. This application consists in covering a spectacle lens with a dichroic reflective filter for wavelengths
Xe + AX, and transmitting the different wavelengths of this wavelength window. The optical glass is doped with optically active dopants, excited in the wavelengths Xa ± AXa, for example in the near ultraviolet, and re-emitting in the wavelengths Xe + # AX, for example around 540 nanometers.

 It is understood that the present invention is not limited to the above, but extends to all applications and variants within the competence of the skilled person.

Claims (13)

RESELL ICAT IONS  1 - Device comprising a substantially transparent matrix (4) and, on part of the external surfaces of said matrix (4), a coating having a high reflection rate in a first wavelength band and a high transmission rate in a second wavelength band, characterized in that the doped matrix (4) comprises at least one optically active dopant absorbing electromagnetic energy in one or more wavelength bands, and re-emitting energy in one or more several wavelengths, at least one of the remission wavelengths being different from the excitation wavelengths, said matrix further comprising a filtering coating (3) having maximum transmission in one or more bands of length d absorption wave of the doped matrix, and having a maximum reflection in at least one of the bands of wavelength of remission of the doped matrix.  2 - Optical device comprising a substantially transparent matrix (4) and, on part of the external surfaces of said matrix (4), a coating having a high reflection rate in a first wavelength band and a high transmission rate in a second wavelength band, characterized in that the transparent matrix (4) comprises optically active dopants absorbing light energy in a wavelength band Xa + # AXa and re-emitting in a length band Xe ± dhe wave, the excitation band Xa + MXa being included in the transmission range of the coating and the re-emission band Xe + AX being included in the reflection range of said coating (3).  3 - Optical device according to the main claim characterized in that the doped matrix (4) is coated with a bandpass filter whose cut-off wavelength is between Xa and Xe.  4 - Optical device according to claim 2 characterized in that the doped matrix (4) is coated with a bandpass filter constituted by a dichroic filter.  5 - Optical device according to claim 2 characterized in that the doped matrix (4) is coated with a bandpass filter constituted by an interference filter.  6 - Optical device according to any one of the preceding claims, characterized in that the doped matrix (4) also has part of the external surfaces coated with a reflective material in the band of re-emission wavelength and in the band of dopant excitation wavelength, as well as a transparent zone.  7 - Optical device according to any one of the preceding claims, characterized in that the doped matrix (4) is of rectangular shape, the coating having a high reflection rate in a first wavelength band and a high transmission rate in a second wavelength band being provided on the input face of said device.  8 - Optical device according to any one of claims 1 to 5 characterized in that the doped matrix (4) is of paraboloidal shape, the coating having a high rate of reflection in a first wavelength band and a rate of high transmission in a second wavelength band being provided on the side faces.  9 - Optical device according to any one of the preceding claims, characterized in that the optically active dopants are deposited on the surface of a transparent matrix.  10 - Optical device characterized in that it consists of a plurality of cells each formed by a matrix (14) having on its input face a dichroic filter, the cell of index n comprises a matrix doped with absorbing dopants the light energy in a wavelength band Ban + an is substantially equal to Xe (nl) + e (n-1) and re-emitting in an wavelength band Xen + # Xen, the excitation band Xan + AXan being included in the transmission range of the dichroic coating and the Xen + # Xen retransmission band being included in the reflection range of said coating.  11 - Application of the device according to any one of the preceding claims for the production of photovoltaic energy characterized in that the matrix includes a plurality of photovoltaic cells  12 - Application of the device according to any one of claims 1 to 10 for securing documents characterized in that the front face has a dichroic filter comprising geometrically defined areas not treated.  13 - Marking method for verifying authenticity, characterized in that one of the faces of the product to be authenticated comprises a matrix (4) which is substantially transparent and, on part of the external surfaces of said matrix (4), a dichroic coating having a high reflection rate in at least a first wavelength band and a high transmission rate in at least a second wavelength band, the transparent matrix (4) comprising at least a first optically dopant active with short-term remanence, absorbing light energy in a wavelength band Xa + MXa and re-emitting in a wavelength band Xe + ffXe, the excitation band ka- + AXa being included in the coating transmission range and the re-emission band #e - + ## e being included in the reflection range of said dichroic coating, as well as at least one second optically active dopant with remanence of duration longer than i mportant, absorbing light energy in a band of wavelength # 'a - + ##' a and re-emitting in a band of wavelength 'e- + AQ'e, the excitation band' a- + A'Xa being included in the transmission range of the coating and the re-emission band 'e + A'Xe being included in one of the reflection ranges of said dichroic coating, the length of re-emission of the remanent material hr e being between #e - ## e and Xe + AX.