Optronic helical phase deviation measurement device.

Abstract

Optronic deviation measurement device (101) comprising an optical system (102), an optronic sensor (104), and an electronic weighing circuit (105) connected to the optronic sensor. The optronic sensor comprises at least four adjacent photodiodes which provide a signal proportional to an amount of light energy of an incident light beam having at least one wavelength of interest to which the photodiodes are sensitive and which have fields combining to form the field of the optronic sensor. The optical system comprises a helical phase optical element (106). Vehicle and pointing device provided with such a device. FIGURE OF THE ABSTRACT: Fig.2

Description

Title of the invention: Optronic helical phase deviation measurement device.

[0001] The present invention relates to the field of optronics and more particularly to deviation measurement.

[0002] BACKGROUND OF THE INVENTION

[0003] This type of device is notably used to guide a vehicle towards a light spot projected onto an object by a laser.

[0004] Optronic deviation measurement devices are known comprising an optical system having an optical axis on which an optronic sensor is arranged. The optronic sensor comprises four photodiodes which provide a signal proportional to a quantity of light energy received and which have fields combining to form the field of the sensor. A distinction will be made between the total field of the sensor, which is the optical field in which a light spot can be detected by at least one of the photodiodes, and the linear field, which is the part of the total field in which several photodiodes observe the light spot, thus allowing a deviation measurement. To facilitate the detection of the light spot and the deviation measurement, it is known to arrange the optical system to defocus the spot on the sensor.

[0005] The device is associated with a calculation circuit arranged to carry out a deviation measurement, that is to say to determine the position of a light spot located in the linear field of the sensor by calculating a barycenter of the light spot in the linear field of the sensor from the energies detected by the photodiodes. Indeed:

[0006] - when the light spot is in the center of the sensor field, the four pho todiodes will measure the same light energy so that they provide signals of the same value, apart from measurement noise. The calculated barycenter is therefore also at the center of the linear field;

[0007] - if the light spot is shifted towards one of the photodiodes, said photodiode will provide a larger signal than the other photodiodes and the calculated barycenter of the detected spot will be shifted towards said photodiode.

[0008] After defocusing the spot on the sensor, the distribution of the flux in the image spot is representative of the distribution of the incident flux in the entrance pupil of the optical path. However, the incident laser flux in the pupil is very often non-uniform, taking into account the Speckle noise, or speckle, linked to the laser coherence of the source, to the surface state of the illuminated target, to the diameter of the illuminated target and to atmospheric scintillation (variation of index of the atmosphere on the path between the illuminated target by the laser and the optical system). This lack of uniformity degrades the accuracy of the deviation weighing measurement.

[0009] SUBJECT OF THE INVENTION

[0010] An object of the invention is to provide a means for improving the deviation measurement.

[0011] BRIEF DESCRIPTION OF THE INVENTION

[0012] To this end, according to the invention, an optronic deviation measurement device is provided comprising an optical system, an optronic sensor, and an electronic weighing circuit connected to the optronic sensor. The optronic sensor comprises at least four adjacent photodiodes which provide a signal proportional to a quantity of light energy of an incident light beam having at least one wavelength of interest to which the photodiodes are sensitive and which have fields combining to form the field of the optronic sensor. The optical system comprises an optical element arranged to generate at least one helical phase around an optical axis of the optical system.

[0013] The helical phase optical element will confer on the light beam passing through it at least one orbital angular momentum. At the output of the helical phase optical element, the light beam has at least one helical wavefront centered on the optical axis with a pitch equal to the wavelength. The distribution of the intensity of this beam on the sensor has an annular shape, the beam having a zone of zero intensity along the optical axis. The helical phase optical element thus causes a structuring of a hypergeometric Gaussian mode. The helical phase optical element can be arranged in such a way that the beam has at the output of the helical phase optical element several helical phases, or several helical wavefronts: this is referred to as a multi-helical phase.

[0014] We recall that the equation for the propagation of a wave is written:

[0015] E = -))

[0016] In which: - Eo is the amplitude, - i is the imaginary number, - co is the time pulse, - t is time, - k is the spatial pulsation, - z is the position along the propagation direction, - y(x, y, z) is the phase as a function of the position (x, y, z) in space.

[0017] The helical phase optical element will add a phase y»(x, y, z) to the wavefront (we recall that this phase can be positive or negative and is defined modulo 211 i.e. that + (p = + (p + V n GZ). The helical phase optical element-

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[0022]

[0023] The licoidal system thus creates an optical vortex which will form on the sensor an annular image spot (defined by a point spread function) having a uniformity relatively insensitive to the uniformity of illumination of the entrance pupil. The optical system is therefore less sensitive to variations in illumination due for example to atmospheric scintillation and the Speckle effect linked to the coherence of the laser beam returned by the target. The invention also relates to a vehicle, such as an aircraft, provided with such a device. The invention also relates to a pointing device comprising a frame, a steerable support mounted on the frame to be steerable relative to the frame via a motor, and such a device mounted on the steerable support. Such a pointing device is, for example, an optronic turret used for observing the environment of the structure on which it is installed or for orienting low-field communication equipment carried by the turret. Such a device can also be a gyro-stabilized optronic ball. Other characteristics and advantages of the invention will emerge from reading the following description of particular non-limiting embodiments of the invention. BRIEF DESCRIPTION OF THE FIGURES Reference will be made to the attached drawings, including: - [Fig.l] [Fig.l] is a partial schematic view of a vehicle equipped with an optronic deviation measurement device according to the invention; - [Fig.2] [Fig.2] is a schematic view in longitudinal section of this device ; - [Fig.3] [Fig.3] is a schematic front view of an arrangement of refractive zones capable of generating the helical phase; - [Fig.4] [Fig.4] is a partial schematic view, in axial section, of a optical element according to an embodiment variant, which is provided with a surface structuring to generate the helical phase; - [Fig.5] [Fig.5] is a view similar to [Fig.2] and illustrates a first structure of an optical system according to the invention; - [Fig.6] [Fig.6] is a view similar to [Fig.2] and illustrates a second structure of an optical system according to the invention; - [Fig.7] [Fig.7] is a partial schematic view of a pointing device equipped with an optronic deviation measurement device according to the invention. DETAILED DESCRIPTION OF THE INVENTION With reference to [Fig.l], the invention is described here in application to a vehicle, here a missile generally designated M, comprising a fuselage F, an engine E, flight surfaces S steerable via actuators A and an electronic guidance unit generally designated G connected to the actuators A to steer the flight surfaces S and direct the missile M towards a target. The structure and operation of these elements are known in themselves and will not be described further here.

[0024] The target is designated by a laser beam forming a spot on the target and the electronic guidance unit G is connected to an optronic deviation measurement device, generally designated 101, sending to the electronic guidance unit G signals representative of a difference between the direction of the spot and the direction in which the missile M is pointing. The optronic guidance device 101 is mounted in a front part Ff of the fuselage F. The electronic guidance unit G is arranged, in a manner known per se, to orient the flight surfaces S so that the trajectory of the missile M intercepts the target, for example by ensuring that this difference is reduced.

[0025] Also referring to [Fig.2], the optronic deviation measurement device 101 comprises: - an optical system, generally designated 102, having an optical axis 103, - an optronic sensor 104 arranged on the optical axis 103 behind the system optics 102, and - an electronic weighing circuit 105 connected to the optronic sensor 104.

[0026] The optronic sensor 104 comprises a detector comprising, in a manner known per se, four adjacent photodiodes which are arranged in quadrants and which are each associated with an optical group having a field which combines with that of the optical groups of the other photodiodes to define the field of the detector and therefore of the optronic sensor 104. The photodiodes are here monophotodiodes defining together the sensitive surface of the optronic sensor 104.

[0027] The electronic weighing circuit 105 comprises an electronic card comprising, in a manner known per se, a processor and a memory containing an operating program for the optronic device 101. The four photodiodes provide the electronic weighing circuit 105 with a signal proportional to a quantity of light energy of an incident light beam having at least one wavelength of interest (for simplification; this is in fact a range of wavelengths which will preferably be narrow) to which the photodiodes are sensitive. The electronic weighing circuit 105 is arranged to carry out a deviation measurement known per se from the signals coming from the optronic sensor 104.

[0028] The optical system 102 comprises a helical phase optical element 106 which is centered on the optical axis 103 and which has two opposite faces 106.1, 106.2. More precisely, the optical element 106 comprises a plurality of refractive zones 1 arranged (shape and distribution) in such a way that the optical element 106 forms a helical phase plate: the refractive zones 1 are thus distributed to have phases increasing in a direction of rotation around the optical axis. The refractive zones 1 are obtained here by dividing the surface into N1 angular sectors and N2 rings to obtain a number N=N1*N2 of refractive zones 1. In the example shown in [Fig.3], the surface comprises 5 angular sectors and 10 rings, i.e. 50 refractive zones 1 in the form of a crown section, the crown sections of a ring being angularly offset relative to the crown sections of the adjacent rings.

[0029] In each of the sectors, according to the angle d (defined according to the cylindrical coordinate system centered on the optical axis), we define the phase rp(x, y)= rp(r, d)= rp(d) such that the phase rp(d) is for example: - equal to a constant; or - proportional to the angle; or - conforming to another relation to the angle (for example linked to the angle by the sine, the cosine, the tangent, a polynomial relation, etc.).

[0030] In the example of [Fig.3], the phase rp(d) is proportional to the angle. More precisely, the phase is proportional to the angle with a coefficient of proportionality established so that, in each crown section, a difference of 2ir is obtained between the 2 angular ends.

[0031] The optical element 106 thus corresponds to a phase map and here comprises a hologram defining the refractive zones 1. The hologram is here produced in a manner known per se by insolation and etching of an interference field on a photosensitive material covering the optical element 106.

[0032] In the variant of [Fig.4], the optical element 106 is a circular lens centered on the optical axis 103, having two opposite faces 106.1, 106.2 of shapes such that the optical element 106 here forms a power component of the optical system 102. The surface 106.1 is here oriented opposite the optronic sensor 104 and the surface 106.2 is therefore oriented towards the optronic sensor 104.

[0033] The surface 106.1 is provided with a surface structuring, symbolized at 107, comprising a plurality of refractive zones 1 covering the surface 106.1 of the optical element 106. The surface structuring forms a layer having a thickness of the order of magnitude of the wavelengths of the light radiation of interest or even less than these wavelengths (this is then referred to as a sub-wavelength thickness). The layer has a variable refractive index in section through a plane containing the optical axis. This variation is for example the result of the presence of reliefs having sub-wavelength dimensions transversely to the optical axis. These layers are called META-SURFACES. In the present case, the surface structuring 107 forms a layer having a sub-wavelength thickness of interest (this thickness is less than the smallest wavelength at which the photodiodes are sensitive) and comprises a plurality of sub-wavelength reliefs of interest (the size of which is exaggerated in [Fig.4]), that is to say that the reliefs of the surface structuring 107 have a larger dimension less than the smallest wavelength to which the photodiodes are sensitive.

[0034] The reliefs of the surface structuring 107 are arranged (shape and distribution) to form the refractive zones 1 to generate a helical phase. The refractive zones 1 are here distributed to have increasing phases according to a direction of rotation around the optical axis. The refractive zones 1 are obtained here by dividing the surface into N1 angular sectors and into N2 rings to obtain a number N=N1*N2 of refractive zones 1. In the example shown, the surface comprises 5 angular sectors and 10 rings, i.e. 50 refractive zones 1 in the form of a crown section, the crown sections of a ring being angularly offset relative to the crown sections of the adjacent rings. The reliefs have, for example, a stud, pillar, L or other shape. The wavefront to be obtained is defined at least by values ​​of angular orientation, curvature and spherical aberration.The reliefs are, for example, arranged so that the transfer function produces an annular image spot on the sensor, the ideal shape of which is dictated by the desired measurement precision over the linear field and the total field of the sensor.

[0035] It is recalled that the design and manufacture of meta-surfaces for optical applications are known in themselves, for example from the following documents: - Hui-Hsin Hsiao, Cheng Hung Chu, Ding Ping Tsai, “Fundamentals and Applications of Metasurfaces”, Small Methods, WILEY-VCH Verlag GmbH & Co KGaA, Weinheim, 2017; - Mohammadreza Khorasaninejad, Zhujun Shi, Alexander Zhu, Wei Ting Chen, Vyshakh Sanjeev, Federico Capasso, “Achromatic Metalens over 60 nm Bandwidth in the Visible and Metalens with Reverse Chromatic Dispersion”, Nano Lett., DOI 10.1021 / acs.nanolett.6b05137, Jan 26, 2017; - Benedikt Groever, Wei Ting Chen, and Federico Capasso, “Meta-lens doublet in the visible”, Nano Lett., Just Accepted Manuscript • DOI: 10.1021 / acs.nanolett.7b01888 • Publication Date (Web): 29 Jun 2017.

[0036] The surface structuring 107 is here covered with a layer 108 which is made of a material with an index different from the index of the surface structuring 107 and which has a smooth surface opposite the surface structuring 107. The smooth surface is covered with an anti-reflection layer 109. Other layers could be imagined providing other functions. It is noted that one of the advantages of meta-surfaces is that there is no edge effect in a meta-surface as there could be between microlenses, whereas this effect would be very detrimental for a precise deviation measurement.

[0037] Figures 5 and 6 show two possible structures, among others, of the optical system 102 of the device according to the invention.

[0038] In the first structure, the optical system 102 comprises a converging meniscus 121 arranged towards the entrance of the optical system 102 and secured to a plano-convex lens 122 arranged towards the optronic sensor 104. The pupil is represented by the line bearing the reference 120.

[0039] In the second structure, the optical system 102 comprises a blade 131 arranged towards the entrance of the optical system 102, a plano-convex lens 132 and a converging meniscus 133 arranged towards the optronic sensor 104. The pupil is represented by the line bearing the reference 130.

[0040] In these two figures, different possible positions for the optical element 106, which is symbolized by a double dot-dash line, have been indicated.

[0041] Thus, in the first structure, the optical element 106 can be: - positioned on the entrance pupil 120, - arranged in the plano-convex lens 122, - arranged on the flat surface of the plano-convex lens 122.

[0042] In the second structure, the optical element 106 can be: - arranged on the rear flat surface of the blade 131, - positioned on the entrance pupil 130, - positioned between the plano-convex lens 132 and the converging meniscus 133, - positioned between the converging meniscus 133 and the sensor 104.

[0043] Of course, the invention is not limited to the embodiments described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0044] In particular, the device may have a structure different from that described.

[0045] The optical element 106 may be provided with a hologram, a meta-surface or be a traditional optical element obtained by molding and / or machining.

[0046] The refractive zones 1 may have a shape other than that mentioned, for example triangular or annular. The number of zones may be different from that mentioned with in all cases N1>1 and N2>2. For example, we may have the following numbers of zones: - 2 zones (N 1=1 and N2=2); - 4 zones (N 1=1 and N2=4); - 10 zones (Nl=1 and N2=10); - 32 zones (Nl=4 and N2=8); - 80 zones (Nl=10 and N2=8);

[0047] The distribution of the refractive zones can also be different. The crown sections of a ring can, for example, be angularly aligned with respect to the crown sections of adjacent rings.

[0048] The phase of the same zone can be progressive according to a direction of rotation around the optical axis 103.

[0049] In the case of a meta-surface, the optical element may comprise a layer of sub-wavelength thickness whose zones are made of different materials to present different transfer functions. This layer may also comprise reliefs or not.

[0050] The reliefs may have transverse dimensions greater than the smallest length of interest.

[0051] The thickness of the layer can be of the order of magnitude of the wavelengths of interest.

[0052] The number of photodiodes may be different from four and they may be arranged differently. The device may for example comprise nine or sixteen photodiodes arranged in a matrix, or seven photodiodes in a hexagonal structure or a different number of photodiodes in angular sectors...

[0053] The optronic sensor 104 may comprise several detectors having fields which accumulate to define the field of the optronic sensor.

[0054] To simplify the explanation, the optical system 102 has been described and shown with a reduced number of optical elements. It goes without saying that the optical system can comprise any number of optical elements and that the helical phase optical element can be arranged at any location in this sequence of optical elements.

[0055] The optical system 102 preferably comprises at least one diopter respecting the Snell-Descartes relationship to optimize the properties of the image spot as a function of the field.

[0056] The optical system 102 can be dioptric (lens system only) or cata-dioptric (lens and mirror systems).

[0057] The optical system 102 may include one or more power components and the optical element 106 may or may not be a power component.

[0058] The optical element 106 is preferably positioned in or near the entrance pupil, but may nevertheless be positioned elsewhere.

[0059] The electronic weighing circuit 105 can be analog or digital.

[0060] The optronic device can equip any type of vehicle, and for example land, air, naval vehicles or drones, but more generally any type of vector.

[0061] The device can also be integrated into a pointing device, for example for laser designators or "Laser Spot Trackers". It is also possible to integrate it into a pointing device for low-field communication equipment, in particular those using a collaborative laser beacon. in [Fig.7] such a pointing device a frame 200, an orientable support 201 mounted on the frame 200 to be orientable relative to the frame via a motorization 202, and an optronic device 101 such as that described above mounted on the orientable support 201.

Claims

Claims

1. Optronic deviation measurement device (101) comprising an optical system (102), an optronic sensor (104), and an electronic weighing circuit (105) connected to the optronic sensor, the optronic sensor comprising at least four adjacent photodiodes which provide a signal proportional to a quantity of light energy of an incident light beam having at least one wavelength of interest to which the photodiodes are sensitive and which have fields combining to form the field of the optronic sensor, characterized in that the optical system comprises an optical element (106) arranged to generate at least one helical phase around an optical axis of the optical system.

2. Device according to claim 1, wherein the optical element (106) comprises a plurality of refractive zones (1) having increasing phases in a direction of rotation around the optical axis of the optical system (102).

3. A device according to claim 2, wherein each refractive zone (1) has a crown section shape, the refractive zones having an increasing circumferential dimension from a center of the optical element (106) to a periphery of the optical element (106).

4. Device according to claim 1, in which the optical element (106) is arranged to define as a function of an angle (d) in a cylindrical coordinate system centered on the optical axis, the phase such that the phase is: - equal to a constant; or - proportional to the angle; or - linked by a relationship of another type to the angle.

5. The device of claim 1, wherein the optical element (106) is disposed in an entrance pupil (120, 130) of the optical system (102).

6. The device of claim 1, wherein the optical element (106) comprises a hologram.

7. Device according to claim 1, in which the optical element (106) is traditionally produced by molding and / or machining.

8. The device of claim 1, wherein the optical element (106) comprises a first layer (107) which has a thickness of the order of magnitude of the wavelength of interest or less and which comprises a plurality of refractive zones (1) covering the surface of the optical element (106) and arranged to create the helical phase.

9. Vehicle (M) comprising a fuselage (F) and an electronic guidance unit (G) connected to a device (101) according to any one of the preceding claims, the device being positioned in the vicinity of a front part (Ff) of the fuselage.

10. A pointing device comprising a frame (200), a steerable support (201) mounted on the frame to be steerable relative to the frame via a motorization (202), and a device (101) according to any one of claims 1 to 8 mounted on the steerable support.

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