FR2923028A1 - WAVELENGTH SUB-LENGTH COMPUTING IMAGING DEVICE - Google Patents

Abstract

The invention relates to an imaging device comprising at least a first optical subsystem for forming the image of an object, at least one sensitive detector in a range of wavelengths (lambda0-Deltalambda, lambda0 + Deltalambda ) having a so-called reference wavelength (lambda0) for detecting said image and image post-processing means for reproducing a quality image, characterized in that it further comprises a blazed phase mask binary having a phase function at least invariant with respect to a given parameter (wavelength type, small field optics, wide field optics, zoom function, ...), said mask: - having patterns of dimensions smaller than said reference wavelength and; - being combined with the post-processing means, making it possible to correct aberrations introduced by the first optics.

Description

The field of the invention is that of optical imaging devices including imaging in the infrared allowing the development of low cost thermal imaging cameras for applications. for example of type: zone protection, starting fire monitoring, ....

In general, an imaging device comprises at least one lens-type optics making it possible to form the image of an object on a detector. The simplest optics of the lens type introduces a set of important defects that are not generally revolutionary. It is thus necessary to use complementary optical elements generating significant phase differences. The combination with optical elements generating phase differences that are not of revolution make it possible to compensate for the defects introduced by said lens, by minimizing the number of elements. Currently imaging devices are defined from an architecture of optical elements that implements the minimum number of dioptres, said ideal configuration. However, when we associate the problem posed (object and image field aperture, resolution, ...) with the environmental constraints of performance and spectral range of use, we generally end up with the implementation of a combination that puts a number of elements higher than that of the so-called ideal combination. There are coded pupil techniques which make it possible to simplify combinations but which require the realization of complex phase function. This increase in components leads to increase not only the cost but also the weight and bulk of the systems. This constraint may prove to be very problematic in certain cases, in particular for embedded systems, even prohibitive to access to new technologies, for example in the case of worn equipment such as binoculars or helmet visuals. as much as one seeks to operate multi-spectral systems. It should be noted that for the realization of multi-spectral systems, it may be envisaged to use pupillary coding principles, making it possible to reduce the wavelength sensitivity of the optics used.

It has already been proposed, to meet all the aforementioned constraints, to use diffractive optics as illustrated in FIG. 1, in which a diffractive lens LD coupled to a second optic L2 makes it possible to form the image of an object. On a detector C. Such diffractive optics generate by nature a phase function and are generally machined by a diamond tip to achieve all of the patterns within a material chosen for its transparency properties in a range of lengths. given usage wave. Nevertheless, a number of disadvantages remain with this type of component. Diamond machining techniques are only compatible with revolution phase functions. Moreover, there exist multi-level binary techniques that require many masking steps to be effective (typically greater than four to obtain an efficiency of more than 90%), which induces both a cost and rates of deviation to the calculated profile rather important, related to the limitations of both the equipment used and the positioning tolerances of the different masks. A major disadvantage lies in the fact that such structures are by nature chromatic, providing chromatic phase functions, which does not allow them to be used in active or passive imaging applications integrating monochromatic or multispectral channels (operating typically at several wavelengths).

In this context, the present invention proposes a new type of imaging device using on the one hand an optical system capable of providing a phase function rendering the image invariant with respect to a given parameter (of the type for example: length d wave, small-field optics, wide-field optics, zoom function, etc.), thereby enabling part of the optical corrections necessary for the formation of an image and, secondly, image processing means. coupled to the detector, in order to lighten the entire imaging device while ensuring the presentation of a high quality image.

More specifically, the subject of the present invention is an imaging device comprising at least a first optical subsystem making it possible to form the image of an object, and at least one sensitive detector in a wavelength range comprising a length. so-called reference waveform for detecting said image and image post-processing means for reproducing a quality image, characterized in that it further comprises a binary phase blazed mask having a phase function at less invariant with respect to a given parameter (wavelength type, mufti-spectral or multi-field optical, aperture parameters, zoom function, ...), said mask: - having patterns of dimensions smaller than said Reference wavelength and; and being combined with the post-processing means, making it possible to correct aberrations introduced by the first optical subsystem. According to a variant of the invention, the phase mask has an invariant phase function by defocusing. According to one variant of the invention, the function may be of the polynomial type or of the exponential type. According to a variant of the invention, the phase mask has a polynomial function corresponding to the following equation: c (x, y) = ax (x3 + y3) With x and y the coordinates perpendicular to the optical axis Z of the device. According to a variant of the invention, the image post-processing means provide a filter function which inverts the percusional response (PSF) delivered by the binary coding operation introduced by the phase mask. According to a variant of the invention, the phase mask comprises a blazed network. According to a variant of the invention, the blazed network is binary at two levels and comprises ladders, each ladder comprising periodic patterns of variable size and distributed over a given period of time less than the reference wavelength. According to a variant of the invention, the reference wavelength is in the infrared range.

According to a variant of the invention, the reference wavelength is in the X band. According to a variant of the invention, the reference wavelength is in the ultraviolet domain.

According to a variant of the invention, the reference wavelength is of the order of a few hundred microns. According to one variant of the invention, the imaging device comprises: a multi-spectral detector sensitive to at least: in a first wavelength range X1 + AX1) having a first so-called reference wavelength ( II); In a second wavelength range (X2-0X2, X2 + 0X2) having a second so-called reference wavelength λ 1, 2); a phase mask comprising patterns of dimensions smaller than the smallest of the reference wavelengths.

According to a variant of the invention, the first range of wavelengths is defined in the vicinity of 3-5 microns, the second wavelength range is defined in the vicinity of 8-12 microns. According to a variant of the invention, the phase mask is made of a GaAs type material, for example, or Zns, ZnSe or GaSNR® from Umicore. According to a variant of the invention, the first optical subsystem is of the reflective or refractive lens type. According to a variant of the invention, the first optical subsystem is of the Fresnel lens type. According to a variant of the invention, the first optical subsystem 30 and the phase mask are on both sides of the same optical component, one face providing a diffractive lens function, the other side providing the mask function. phase. The invention also relates to an embedded system comprising an imaging device according to the invention. The invention will be better understood and other advantages will become apparent on reading the following description given by way of non-limiting example and with reference to the appended figures in which: FIG. 1 illustrates an imaging device comprising an optical device; diffractive according to the known art; FIG. 2 illustrates a first example of an imaging device according to the invention involving a phase coding; FIG. 3 illustrates an exemplary phase function of an exemplary phase mask used in an imaging device of the invention; FIG. 4 illustrates the diffraction efficiency for a conventional ladder diffractive optical element and of the ladder grating type respectively with a binary synthesis of this element, by means of pillar-type microstructures of a diffractive optical element. conventional, as a function of the ratio of the illumination wavelength to the nominal wavelength; FIGS. 5a and 5b respectively illustrate a conventional diffractive optic element with echelette and of the echelette grating type with a binary synthesis of this element, by means of microstructures of the pillar type.

In general, the present invention provides an imaging device that combines a pupil-coded imaging approach and an approach to use of linear blazed linear sub-wavelength optical surfaces. FIG. 2 illustrates a first example of an imaging device according to the invention. This device comprises a first optical LI, a phase mask Mp, a second optical L2 and a sensitive detector C in a wavelength range (ko-A2., X0 + 4X), said detector being coupled at the output to image processing means Ti making it possible to ensure the quality of the images in support of said phase mask. Indeed, the phase mask provides a phase law to correct some of the optical aberrations, the other part being provided by the digital image processing. More precisely, the phase mask provides an invariant image of an object O, for example by defocusing (the image at the output of the component Mp appears fuzzy) and is coupled to image processing means which make it possible to restore the image by applying for example a filter that reverses the coding (effect induced by the coding on the percusional response). It is thus possible to obtain a clear image for a large focusing interval. An example of a phase law can be: cD (x, y) = ax (x3 + y3) With x and y the coordinates perpendicular to the optical axis Z of the imaging device. FIG. 3 illustrates an example of phase function evolution with a coefficient a = 90.41 which shows the significant amplitude of the phase law.

The advantage of such a solution is to simplify the complexity of the imaging chain leading to a solution closer to that of an imaging system implementing a minimal number of optical elements, relying on the advantages specific to the two following techniques: an optical coding which implements the use of a phase law making it possible to make the optical combination insensitive to certain types of aberration and a frequency filtering in the detection plane to restore the quality of the image formed on the detection plane; performing part of the optical function with sub-length binary blazed type diffractive elements, so as to efficiently obtain a phase function for the pupil coding, or even a part of the optical combination.

Such subwavelength binary blazed type diffractive elements have in particular been described in the publication Extended depth of field wave-front coding: Edward R. Dowski, Jr. and W. Thomas Cathey (April 1995, Vol. 34, No. 11 @Applied Optics) and will be explained in more detail below.35 In general, with conventional diffractive optics such as Fresnel lenses and echelette or multilevel gratings, Blaze effect sought is obtained by a gradual change in the depth of a constant index material. The surface profile of these elements thus consists of continuous reliefs separated by discontinuities. These elements are designed for a certain wavelength, called the design wavelength, noted 1o. These elements have a significant limitation to their use, because they are in practice blazed only at the design wavelength. At the design wavelength, the diffraction efficiency is 100% at Fresnel losses, but as soon as the wavelength of the incident light deviates from this nominal value, the efficiency in the order blaze (order 1) drops considerably (effect magnified by the difficulty of technological achievement).

This is represented on the curves 4a and 4b of FIG. 4, which show the efficiency curve for a conventional diffractive optic element with echelette and of the echelette grating type with a binary synthesis of this element, on means of pillar-type microstructures of a conventional diffractive optical element, as a function of the ratio of the illumination wavelength to the nominal wavelength. In the scalar domain, it can indeed be shown that the diffraction efficiency as a function of the wavelength of any diffractive optical element blazed in the order 1 is given by the following equation: II (2) = sinc2a, r (1- ° 11 Eq (1) where sinc (x) = sin (x), whose curve 1 is the graphical representation x Light lost in the order of blaze (order 1) is transmitted in orders If we take the example of a hybrid lens, this phenomenon results in the transmission of a parasitic light, which is detrimental to the quality of the imagery.This is reflected in a practical way by the appearance of light For example, the output image is not clear or has low contrast because of the noise level provided by the spurious orders, especially if there are hot spots at the level of the image. scene that we are trying to image.

It is shown that this drop in efficiency is due to the low dispersion of the material, which means that for a small difference in wavelength, the phase difference 4 (2) induced in the structure deviates from 27c, whereas it is 27r at the wavelength of design a, o 5 or blaze. For example, if we consider the echelette array RE shown in Figure 5a, Aço (2) represents the phase difference between the low b and the high t of a step e of the network RE. This difference is equal to 27r for an incident light at X. In the scalar approximation, neglecting the Fresnel losses, and placing itself at normal incidence of the incident beam, the phase difference as a function of the wavelength Aço ( 2) and the efficiency as a function of the wavelength 1 (9 ^,) are in fact given by: Op (ll) = 27r (.I.) Eq. (2) (Ai)) q ( 2) = sinc2 [7r (1-Agp (.1) / 27r)], Eq. (3) where An (?) = (N (X) û na; r is (n (X) û 1), for a diffractive optical element etched in a refractive index material n For these diffractive optical elements, it can be considered that An (X) = An (Xo), since the dispersion of the material is negligible: the index The refraction rate varies slightly around 4. Eq (2) therefore becomes: çc ^ (.) = 27r-- ° Eq. (4)

and we obtain the equation Eq (1) seen above and shown in Figure 4, replacing Aço (2) by this expression in Eq (2). Thus, the low dispersion of the optical material in conventional diffractive optical elements results in a drop in diffraction efficiency with the wavelength 2 # 2 expressed by equation Eq (1). These conventional diffractive optical elements are therefore not broadband spectral. They can not be used in optical systems dedicated to broadband spectral applications, such as hybrid optical systems, composed of refractive and diffractive optics. Other elements of diffractive optics known as binary microstructure, also called binary blazed networks, or sub-wavelength diffractive optics (SWDOE) are known. binary synthesis of a conventional diffractive optical element: we start from the classical diffractive optical element that we want to synthesize and we sample this network to obtain points, to which we can associate an index value The sampling must be done at a period less than the design wavelength, to obtain a sub-wavelength grating.The various calculation techniques used are known to the man of the These techniques make it possible, for example, for a blazed network ~ o with echelettes such as the network RE represented in FIG. 5a, to define a blazed network b. As shown in Figure 5b, if we take again Figure 5a, two rungs of a grating RE network of period A (or not of the network) are represented. These steps are etched in an optical material of index n. A blazed binary network corresponding to the RE network of Figure 5a is shown in Figure 5b. The RE network is sampled at the chosen AS period less than the design wavelength X0. A certain number of points are obtained for each period A of the network. At each point a given fill factor is matched for a given type of microstructure (hole, pillar): this fill factor is equal to the d dimension of the microstructure relative to the sampling period of the grating: f = d / AS. The filling factor of each microstructure is defined, by known calculations, to locally give a phase shift value cD (x) similar to that of the echelette grating at the sampled point, and equal in known manner to ~ (x) = 27r ( n1) h (x, where x is the coordinate of the sampled point on the Ox axis of the grating In the example of Figure 5b, the binary microstructures are of the abutment type, resulting in a set of binary microstructures which encode the pattern This set of microstructures is repeated at the period A of the echelette grating of Figure 5a, and the use of this type of structure makes it possible to produce materials with an effective index whose index value allows to have a wider range of materials (in first approximation varying between the index of the substrate and the air, so typically can vary between 3 and less than 1.5 considering the materials used in particular in the infrared Moreover, such materials offer a chromatic dispersion that depends on the geometry of the structures, which has the advantage of producing achromatic phase functions and thus sampling of the echelette type without loss of efficiency as a function of the length of the structure. 'wave. It is furthermore possible in this type of coding of the diffraction function to use only one level of masking during an etching operation, which may be of the RIE etching type, leading to the production of type of phase mask blazed binary. It is also possible to use replication methods for mass production, such as vitreous material molding technology, resin embossing deposited on the optical component or CD-type optical replication which have been proven in terms of resolution. The advantages of this new class of optical components makes it possible to envisage the realization of more complex optical functions than those used in the design of traditional imaging systems, in particular it makes it possible to envisage carrying out the complex aspherical functions and to high amplitude (typically a few tens of wavelengths) necessary for the implementation of pupil-coded imaging principles. In particular, it makes it possible to benefit from the achromatism properties that allow the wavelength scale reduction, the amplitude of the difference in gait difference. The present invention may also be of interest in the context of new technologies, for example in the case of worn equipment such as binoculars or helmet visuals, in which multi-spectral systems are used. Thus and according to the invention, the imaging device comprises for example a multi-spectral detector, that is to say both sensitive in a first range of wavelengths (? I-AX1, 2 ^ .1 + AX1) having a first so-called reference wavelength (X1) and being sensitive in a second wavelength range (X2-AX2,% 2 + AX2) having a second so-called reference wavelength (1,2).

A single phase mask Mp having patterns smaller than the smallest of the reference wavelengths is positioned upstream of the multi-spectral detector.

In conclusion, the ability to implement pupil-coded imaging techniques, where part of the correction is provided by the signal processing, also opens the way for the simplification of optical systems on the basis of a criterion of different optimization, that of a spatial frequency invariance transfer function, which offers additional degrees of freedom ~ o, thus making it possible to dispense with certain optical elements known as athermalisation, or to avoid the switching of an optical block in the case of multi-spectral imaging.

Claims (18)

An imaging device comprising at least a first optical subsystem (LI) for forming the image of an object, at least one sensitive detector in a range of wavelengths (Xo-AX, Xo + AX ) having a so-called reference wavelength (Xo) for detecting said image and image post-processing means making it possible to restore a quality image, characterized in that it also comprises a blazed phase mask binary (Mn) having a phase function at least invariant with respect to a given parameter (wavelength, bi-spectral optical, bi-field optical, wide field optical, zoom function, ...), said mask : having patterns of dimensions smaller than said reference wavelength and; being combined with the post-processing means for correcting aberrations introduced by the first optical subsystem. Imaging device according to claim 1, characterized in that the phase mask has an invariant phase function by defocusing. 3. Imaging device according to claim 2, characterized in that the phase mask has a function of polynomial or exponential type. 4. An imaging device according to claim 3, characterized in that the polynomial function corresponds to the following equation: t (x, y) = ax (x3 + y3) With x and y the coordinates perpendicular to the axis optical Z of said imaging device. 5. Imaging device according to one of claims 1 to 4, characterized in that the image post-processing means provide a filter function which reverses the result from the binary coding operation introduced by the mask. phase. 6. Device according to one of claims 1 to 5, characterized in that 5 the phase mask comprises a blazed network. 7. Device according to claim 6, characterized in that the blazed binary network comprises echelettes, each scale having periodic patterns of period (As) less than the wavelength of 1 o reference. 8. Imaging device according to one of claims 1 to 7, characterized in that the reference wavelength is in the field of infra-red. Imaging device according to one of claims 1 to 7, characterized in that the reference wavelength is in the X band. 10. Imaging device according to one of claims 1 to 7, characterized in that the reference wavelength is in the field of ultraviolet. 11. Imaging device according to one of claims 1 to 7, characterized in that the reference wavelength is of the order of a few hundred microns. 12. Imaging device according to one of claims 1 to 11, characterized in that it comprises: - a sensitive multispectral detector: • in a first wavelength range (X1-AX1, XI + AX1) having a first reference wavelength (XI); In a second wavelength range (X2-AX2, X2 + 0X2) having a second reference wavelength (X2); a phase mask having patterns of dimensions less than the smallest of the reference wavelengths. Imaging device according to claim 12, characterized in that the first wavelength range is defined in the vicinity of 3-5 microns, the second wavelength range is defined in the vicinity of 8-12. microns. 14. Imaging device according to one of the preceding claims, characterized in that the phase mask is made of a GaAs type material, ZnS, ZnSe. 15. Imaging device according to one of claims 1 to 14, characterized in that the first optical subsystem is of reflective or refractive lens type. 20 16. Imaging device according to one of claims 1 to 15 characterized in that the first optical subsystem is of the Fresnel lens type. 17. An imaging device according to one of claims 1 to 16 characterized in that the first optical subsystem and the phase mask are on both sides of the same optical component, a face providing a lens function. diffractive, the other side providing the function of phase mask. 30 18. Embedded system characterized in that it comprises an imaging device according to one of claims 1 to 17. 15