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    • 专利摘要:
    According to one aspect, the present disclosure relates to an optical imaging device (20) of an object (OBJ) by off-axis holography comprising a light source (21) adapted for the emission of an illumination wave (EI). of the object, in transmission or reflection, and a set of one or more thick Bragg gratings (22) for receiving a wave (Eo) from the illuminated object and deflecting a first component (ER) of the wave coming from the object, said reference wave, and to pass without passing a second component (Es) of the wave coming from the object, called the signal wave, so that the deflected reference wave has relative to the non-deflected signal wave predetermined deflection angles defined in two perpendicular planes. The imaging device according to the first aspect further comprises a two-dimensional detection device (23) for acquiring an interferogram resulting from the interference between said deflected reference wave and said signal wave and a calculation unit for determining from said interferogram an amplitude and phase distribution of the signal wave in the plane of the object (hologram).
    公开号:FR3064759A1
    申请号:FR1752775
    申请日:2017-03-31
    公开日:2018-10-05
    发明作者:Michael Atlan；Jean-Pierre Huignard
    申请人:Centre National de la Recherche Scientifique CNRS；Ecole Superieure de Physique et Chimie Industrielles de Ville Paris ；
    IPC主号:
    专利说明:

    Holder (s): NATIONAL CENTER FOR SCIENTIFIC RESEARCH - CNRS Public establishment, HIGHER SCHOOL OF INDUSTRIAL PHYSICS AND CHEMISTRY OF THE CITY OF PARIS.
    Extension request (s)
    Agent (s): CABINET OSHA ET ASSOCIES.
    (041 DEVICES AND METHODS FOR OPTICAL IMAGING BY OFF-AXIS DIGITAL HOLOGRAPHY.
    FR 3,064,759 - A1 (57) According to one aspect, the present description relates to an optical imaging device (20) of an object (OBJ) by off-axis holography comprising a light source (21) adapted for the emission of 'an illumination wave (El) of the object, in transmission or in reflection and a set of one or more thick Bragg grating (s) (22) intended to receive a wave (Eo) coming from the object thus illuminated and to deflect a first component (ER) of the wave coming from the object, called reference wave, and to let pass without deflection a second component (Es) of the wave coming from the object , called signal wave, so that the deflected reference wave has relative to the non-deflected signal wave predetermined deflection angles defined in two perpendicular planes. The imaging device according to the first aspect further comprises a two-dimensional detection device (23) for the acquisition of an interferogram resulting from the interference between said deflected reference wave and said signal wave and a calculation unit for determining , from said interferogram, a distribution of amplitude and phase of the signal wave in the plane of the object (hologram).

    E 0 x / Es L

    i
    Off-axis digital holography optical imaging devices and methods
    STATE OF THE ART
    Technical field of the invention
    The present invention relates to optical imaging devices and methods by off-axis digital holography and applies in particular to microscopic imaging, in particular transparent or reflective objects.
    State of the art
    Phase contrast microscopy was developed in the 1940s for the observation of transparent microscopic objects. Indeed, in the fields of biology for example, most microscopic objects, like living cells, are transparent and differ only very slightly from their surroundings in terms of absorption or color, which results in very small variations in the amplitude of a light wave used in conventional microscopy. Phase contrast microscopy detects variations in the refractive index within the object to form an image. In particular, interferometric techniques based on interference between a light beam transmitted or reflected by the object and a reference light beam, have made it possible to access quantitative measurements of the distribution of the phase within the objects of study. .
    Specifically, digital holography phase microscopy has proven to be a very powerful technique for the quantitative analysis of local variations in the refractive index of a transparent object. Indeed, digital holography which includes the digital recording of an interferogram by a digital camera and the digital reconstruction of holograms, gives simultaneous access to the intensity and the phase of the propagating wave.
    The digital holography on the axis, developed on the basis of the work of Gabor, and described for example in the article by J. Garcia-Sucerquia et al. (“Digital in-line holographie microscopy”, Applied Optics, Vol. 45, N ° 5 (2006)), a diagram of which is reproduced in FIG. 1 A, gave rise to imaging devices which are particularly simple to implement. As illustrated in FIG. IA, a light beam emitted by a laser source L is sent to a hole P forming a source point. A first spherical wave emerges from the source point P and illuminates a study object O; this results in a wave coming from the object, called signal wave and whose electromagnetic field is noted Es. The signal wave interferes with a spherical wave coming from the point P, called reference wave, whose electromagnetic field is noted Er. The acquisition of the interference signal by a two-dimensional detector (detection plane C) results in an interference figure or interferogram I which can be expressed by equation (1) below:
    I = | E S + E r | a 2 = | ES | A 2 + | ER | A 2 + EsEr * + E S * E R (1)
    These systems, allowing those skilled in the art to create Gabor holograms, are however limited in sensitivity due to the spatial superposition of the useful signal EsEr * (where * represents complex conjugation) with the other interference terms. in the interferogram.
    Off-axis digital holography, an example of implementation of which is described for example in the article by P. Marquet et al. {"Digital holography microsocopy: a noninvasive contrast imaging technique allowing quantitative visualization of living cells with subwavelength axial accuracy", Optics letters, vol. 30, N ° 5 (2005)) compared to digital holography on the axis, to gain considerably in sensitivity due to the introduction of a spatial frequency shift on the cross interferometric term of the signal wave and the reference wave (term EsEr * in equation (1)).
    FIG. IB reproduces a diagram of an off-axis digital holography microscopy device as described in the above-mentioned article. The device comprises a Mach-Zehnder interferometer in which separating blades BS and mirrors M are arranged to form a reference arm and an object arm on which the study object O is positioned. A laser source L illuminates the Mach-Zehnder interferometer at a given wavelength λ; a MO microscope objective, arranged on the object arm, makes it possible to select a field of the study object O. The signal and reference waves, whose electromagnetic fields are denoted respectively Es and Er, interfere in a detection plane of a two-dimensional detector C. In this arrangement, the signal and reference waves are angularly separated and their directions of propagation make angles Θ ~ k x / k and h "ky / k, where k x and k y are respectively projections of the wave vector of the reference wave on each of the x and y axes of an orthogonal coordinate system defined in a plane perpendicular to the optical axis, and k is the norm of said wave vector, given by k = 2π / λ where λ denotes the optical wavelength.
    In this case, the interferogram I measured in the detection plan is written:
    I = | Es + Er exp (ik x x + ik y y) | A 2
    Is:
    I = | Es | A 2 + | Er | A 2 + EsEr * exp (-i kx x - ik y y) + Es * E R exp (ik x x + ik y y) (2)
    As it appears in equation (2), the cross interference term EsEr * which represents the useful signal is now spatially separated from the self modulation terms | Es | A 2 and | E R | At 2, which makes it possible to detect it with much better sensitivity.
    The device illustrated in FIG. IB has the disadvantage of being sensitive to the measurement environment; indeed, simple disturbances of the ambient air or vibrations can disturb the reference beam and therefore the measurement.
    The present description proposes a device and a method of optical imaging by off-axis digital holography allowing, compared to known technologies, both a very great simplicity of implementation and an excellent robustness.
    ABSTRACT
    According to a first aspect, the present description relates to a device for optical imaging of an object by off-axis holography comprising:
    - a light source suitable for the emission of an object illumination wave, in transmission or in reflection;
    - a set of one or more thick Bragg grids intended to receive a wave coming from the object thus illuminated and to deflect a first component of the wave coming from the object, called reference wave, and allowing a second component of the wave coming from the object, called the signal wave, to pass without deflection so that the deflected reference wave has defined predetermined deflection angles relative to the non-deflected signal wave in two perpendicular planes,
    - a two-dimensional detection device for the acquisition of an interferogram resulting from the interference between said deflected reference wave and said signal wave; and
    - a calculation unit for determining, from said interferogram, a distribution of amplitude and phase of the signal wave in the plane of the object.
    The applicants have demonstrated that the use of one or more Bragg gratings for optical imaging using off-axis holography makes it possible to guarantee both robustness and quality of the imaging. Indeed, the reference wave is formed directly from the wave coming from the object, thus preventing the device from being subjected to external disturbances. In addition, thick Bragg gratings intrinsically generate low-pass spatial filtering of the deflected wave (reference wave), which contributes to the very good quality of the reconstructed hologram.
    According to one or more exemplary embodiments, said set of one or more thick Bragg grids comprising at least a first thick Bragg grating with index layers arranged in a first direction, with a given first step and at least a second thick Bragg grating with index strata arranged in a second direction, perpendicular to the first direction, with a second given step. This arrangement makes it possible to obtain a deflection of the reference wave relative to the non-deflected signal wave with deflection angles defined in two perpendicular planes.
    According to one or more exemplary embodiments, said set of one or more thick Bragg grids comprising at least a first thick Bragg grating with multiplexed index strata, having index strata arranged in two perpendicular directions, with a step given in each direction. This arrangement also makes it possible to obtain a deflection of the reference wave relative to the non-deflected signal wave with deflection angles defined in two perpendicular planes.
    According to one or more exemplary embodiments, said set of one or more thick Bragg grids further comprises a second thick Bragg grating with multiplexed index strata, having index strata arranged in two perpendicular directions, with a pitch given in each of the directions, so that the total deflection angle of the deflected reference wave, measured in each of said perpendicular planes, is equal to the difference of the deflection angles introduced by each of said Bragg gratings thick multiplexed.
    The association of two thick Bragg gratings with multiplexed index strata allows, for thick Bragg gratings of given thickness, to ensure greater angular selectivity of the deflected reference wave.
    According to one or more exemplary embodiments, the device for optical imaging of an object by off-axis holography further comprises a microscope objective to form an image of a restricted field of the object OBJ with a given magnification. This arrangement allows application of the device in off-axis holography microscopy.
    According to a second aspect, the present description relates to a method of optical imaging of an object by off-axis holography comprising:
    - the illumination of the object in transmission or in reflection by means of an illumination wave coming from a light source;
    - sending on a set of one or more thick Bragg grids (s) of a wave coming from the illuminated object;
    the deflection of a first component of the wave coming from the object, called the reference wave, while a second component of the wave coming from the object, called the signal wave, crosses said set of one or more Bragg grating (s) without being deflected, the deflected reference wave having relative to the non-deflected signal wave predetermined deflection angles defined in two perpendicular planes,
    the acquisition by means of a two-dimensional detection device of an interferogram resulting from the interference between said deflected reference wave and said signal wave; and
    - the calculation, from said interferogram, of a distribution of amplitude and phase of the signal wave in the plane of the object.
    BRIEF DESCRIPTION OF THE FIGURES
    Other advantages and characteristics of the invention will appear on reading the description, illustrated by the following figures which represent:
    - FIGS. 1A and IB (already described), diagrams of microscopy devices respectively on holography on the axis and off axis, according to the prior art;
    - FIG. 2, a diagram of an example of an optical imaging device using off-axis holography according to the present description.
    - FIGS. 3A to 3C, diagrams illustrating the propagation of the beams transmitted and deflected by thick Bragg gratings, respectively with so-called "horizontal" index strata (FIG. 3A), with so-called "vertical" index strata (FIG. 3B ), and with so-called “multiplexed” index strata (FIG. 3C).
    - FIGS. 4A and 4B, series of images obtained experimentally and showing respectively (FIG. 4A) the intensity of the electromagnetic field of the waves transmitted and deflected by the Bragg grating, for different Bragg tuning conditions and (FIG. 4B) interferograms acquired by the camera and holograms reconstructed digitally;
    - FIGS. 5A and 5B, diagrams showing examples of optical imaging device by off-axis holography according to the present description respectively with a microscope objective and with a field pupil;
    - FIG. 6, diagrams illustrating different sets of one or more thick Bragg gratings, for the implementation of the off-axis holography method according to this description;
    FIG. 7, a diagram illustrating an arrangement with two thick Bragg gratings such that the deflection angle (s) resulting from the successive deflection (s) are of the order of the deflection angle (s) sought for the implementation of off-axis holographic imaging according to this description.
    DETAILED DESCRIPTION
    FIG. 2 shows a diagram of an example of an imaging device 20 of an OBJ object according to this description. The object is for example a biological sample, for example a culture of living cells, or a material intended for microelectronics (semiconductor, glass, mirror).
    The imaging device 20 comprises a light source 21 adapted for the emission of an illumination wave of the object OBJ whose electromagnetic field is denoted Ei. Although shown in transmission in FIG. 2, the object can be illuminated in reflection, for example when the object is a semiconductor, glass, or a mirror. The light source is for example a laser, a super luminescent diode (or SLD according to the abbreviation of the English expression “Super-Luminescent Diode”), a light emitting diode (or “LED” according to the abbreviation of the Anglo-Saxon expression "Light-Emitting Diode").
    The imaging device 20 also comprises an assembly 22 of one or more thick Bragg grids (x) intended to receive a wave coming from the object, of electromagnetic field Eo. The wave coming from the object comprises a first component formed by a wave coming from the object, or signal wave, of electromagnetic field Es and a second component formed by a wave coming from the source and not disturbed by l object, called reference wave, of electromagnetic field Er. The wave coming from the object is in practice a wave resulting from the diffraction of the illumination wave by all of the microstructures of the object, the microstructures being able to result for example from inhomogeneities of refractive index in a object observed in transmission. Under certain conditions of incidence of the reference wave, say conditions of agreement of
    Bragg, the reference wave is deflected by the thick Bragg grating 22 in a given direction and spatially filtered due to the intrinsic properties of the thick Bragg grids. The signal wave, emitted by the object in an angular cone defined by the opto-geometrical characteristics of the imaging device, is always transmitted without being deviated due to non-compliance with the Bragg agreement. The reference wave Er deflected and filtered by the thick Bragg grating 22 therefore interferes with the signal wave Es transmitted by the Bragg grating without being deflected. This results in an interference signal formed in a detection plane of a two-dimensional detection device 23; the two-dimensional detection device thus allows the acquisition of an interferogram resulting from said interference signal. A calculation unit 24 then makes it possible to determine, from said interferogram, a distribution of amplitude and phase of the signal wave in the plane of the object.
    The imaging device is therefore indeed an off-axis digital holography device whose characteristics, in particular in terms of pitch of the fringes of the interference signal, orientation of the fringes in the plane of the detector, width of the angular spectrum of the reference wave, are given by the characteristics of the thick Bragg grating (s), as explained below. The “reconstruction” of the signal wave from the interferogram includes for example and in a known manner a Fresnel transformation, and can be done in an identical manner to that carried out in the known techniques of off-axis digital holography.
    It is of course possible to proceed with the acquisition of a series of interferograms in the case for example where one wishes to make a dynamic imagery of the object (video). In this case, a reconstruction of the signal wave is performed for each interferogram.
    For the acquisition of the interferogram (s), the two-dimensional detection device is for example a matrix detector of a CCD (Charge-coupled device) or CMOS (Complementary metal-oxide-semiconductor) camera.
    FIGS. 3A to 3C illustrate the propagation of the reference wave and the signal wave in three thick Bragg gratings, a thick Bragg grating 221 with so-called "horizontal" index strata of step (or period) A x (FIG . 3A), a thick Bragg grating 222 with so-called “vertical” index strata of step A y (FIG. 3B) and a thick Bragg grating 223 with so-called “multiplexed” index strata with steps A x and A y , respectively (FIG. 3C).
    In the present description, a thick Bragg grating with index strata called "horizontal" or "vertical" is used to refer to a thick Bragg grating with index modulation in a single preferred direction, making it possible to define a set of index strata. parallel to each other. The terms "horizontal" and "vertical" are used arbitrarily and simply mean in the present description that the two sets of strata of the two respective Bragg gratings are perpendicular. A thick Bragg grating with so-called "multiplexed" index strata is called a thick Bragg grating with index modulation extending in two preferred directions, making it possible to form two sets of multiplexed strata extending in two perpendicular directions.
    According to an example, the refractive index n (x, y) in a Bragg grating can be written, according to each of the directions x and y perpendicular to each other:
    n (x, y) = n 0 + n x sin (2πχ / Λ χ ) + n y sin (2% y / A y ) (3)
    Where n 0 is the average index, Λ χ and A y the steps (or periods) of the refractive index modulations in the two perpendicular directions x and y respectively and n x and n y the amplitudes of the index modulations of refraction in the two perpendicular directions x and y respectively.
    In practice, to obtain a two-dimensional image of the object, one can use, according to a first example, a thick Bragg grating with multiplexed index strata or one can use, according to a second example , two thick Bragg gratings arranged one behind the other, having index strata perpendicular to each other. This allows in both examples to define a direction of the deflected wave relative to the signal wave by two angles defined in planes perpendicular to each other.
    Thus, in an example of two successive networks with “horizontal” and “vertical” index strata, it will be possible to define, for the first Bragg network with “horizontal” index strata (FIG. 3 A) an angle Θβ of the direction of an incident wave Eo with respect to the plane of the strata of the network in a plane perpendicular to the plane of the strata and containing the direction of the wave vector of the incident wave. We can define, for the second Bragg grating with “vertical” index strata (FIG. 3B) an angle 3b of the direction of an incident wave Eo with respect to the plane of the grating strata in a plane perpendicular to the plane of the strata and containing the direction of the incident wave vector.
    Furthermore, in an example of a thick Bragg grating with multiplexed index strata (FIG. 3C), it is possible to define the angles (Θβ, 3b) of the direction of an incident wave with respect to the planes of the index strata. of a thick Bragg grating with multiplexed index strata, in two planes perpendicular to each other and containing respectively the direction of the incident wave wave vector and the normal to the index strata.
    In practice, it will be possible to work with thick Bragg gratings having index strata characterized by an angle of inclination Φ relative to a direction normal to the entry face of the network. This facilitates the mounting of the Bragg network (s) in the device by working with an incident wave always normal to the input face of the network while respecting the Bragg agreement (s).
    In the first as in the second example, a deflection of the reference wave relative to the direction of propagation of the signal wave may occur provided that the angles (Θβ, 3b) satisfy the Bragg tuning conditions network (s), as described for example in Kogelnik et al. (Coupled wave theory for thick hologram gratings. Bell Labs Technical Journal 48, no. 9 (1969): 2909-2947) and recalled below with reference to FIGS. 3 A - 3C.
    In FIGS. 3A to 3C, we denote by Eo the electromagnetic field of the wave coming from the object, composed of the reference wave and the signal wave. The electromagnetic field of the signal wave, transmitted by the network without deviation, is noted E O o (or Es). As illustrated in FIG. 3A, in the Bragg's agreement conditions, the incident reference wave on the “horizontal” stratum network 221 with an angle Θβ is deflected in a “vertical” plane of 2Θ Β , the “vertical” plane being defined as the plane perpendicular to the plane of the strata of the network with "horizontal" strata, and containing the direction of the wave vector of the incident wave; we note Swelling electromagnetic field of the deflected wave. Furthermore, as illustrated in FIG. 3B, still under Bragg's agreement conditions, the incident reference wave on the "vertical" stratum network 222 with an angle 3b is deflected in a "horizontal" plane of 23b, the "horizontal" plane being defined as the plane perpendicular to the plane of the strata of the “vertical” strata network and containing the direction of the wave vector of the incident wave; we denote E O i the electromagnetic field of the deflected wave. In the case of a thick Bragg grating 223 with “multiplexed” index strata (FIG. 3C), the incident reference wave with the angles (Θβ, 3b) is deflected by 2Θ Β and 23b respectively; we note In the electromagnetic field of the deflected wave.
    The Bragg angles Θβ and 3b satisfy in a known way the relations:
    2sin (0B) = λ / Λ χ (4)
    2sin (3B) = λ / Ay (5)
    Or, in the approximation of small angles:
    2Θβ ~ λ / Λ χ (4 ') ίο
    2θβ ^ λ / Λ ν (5 ')
    It is thus possible, for a given wavelength λ of the object's illumination wave, to choose the steps (or periods) Λ χ and A y of modulations of refractive index in two perpendicular directions between them (“horizontal” and “vertical” strata) respectively, in order to determine the angle (2Θ Β , 2Ùb) formed between the reference wave and the signal wave.
    The four electromagnetic fields E O o, Eio, E O i, and En correspond to light beams detectable by a matrix detector arranged downstream of the Bragg grating (s), as illustrated in FIG. 4A.
    More specifically, FIG. 4A represents experimental results of deflection of an incident wave on an assembly formed by two thick Bragg gratings arranged one behind the other; a first thick Bragg grating with "horizontal" index strata (as illustrated in FIG. 3A) and a second thick Bragg grating with "vertical" index strata (as illustrated in FIG. 3B ). Each of the gratings is characterized by an average refractive index (n 0 = 1.5), a step of the modulation of the refractive index (respectively A x = 18.9 pm, A y = 18.3 pm), a thickness (respectively d x , d y = 8.9 mm) of the grating, a depth of modulation of the refractive index (respectively n x , n y = 1.4 10 ' 5 ), an angle of inclination (respectively Φ χ , Φ ν = 90 °) index strata relative to the input face of the network.
    The incident wave is a wave coming from an object (a test pattern) lit in transmission by means of a laser source emitting at λ = 660 nm. A camera comprising 2048 x 2048 elementary detectors (or pixels) allows the acquisition of images. Images 41 - 44 shown in FIG. 4A are obtained with a collimated incident beam of approximately 1 mm in diameter, by modifying the inclination of the thick Bragg gratings. Image 41 of FIG. 4A corresponds to a case where the Bragg condition is not satisfied for either of the two networks. Only the non-deflected beam E O o (signal wave from the object) is transmitted. Image 42 of FIG. 4A corresponds to a case where the Bragg condition is satisfied only for the network with "horizontal" strata. The non-deflected beam E O o (signal wave from the object) and the beam E i0 deflected by 2Θ Β in a "vertical" plane are transmitted by the network. Image 43 of FIG. 4A corresponds to a case where the Bragg condition is satisfied only for the network with vertical strata. The non-deflected beam E O o (signal wave from the object) and the beam E O i deflected by 2Ùb in a "horizontal" plane are transmitted by the network. Image 44 of FIG. 4A corresponds to a case where the Bragg condition is satisfied for the network with horizontal strata and for the network with vertical strata. The non-deflected beam E O o (signal wave from the object), the deflected Eio beam by 20 B in a "vertical" plane, the deflected E O i beam by 20b in a "horizontal" plane, are transmitted by the network. A fourth beam En, deflected by 20 B in a "vertical" plane and by 23s in a "horizontal" plane is transmitted.
    FIG. 4B represents an interferogram obtained experimentally (images 45, 46), a Fourier transform (image 47) calculated from said interferogram and a hologram (image 48) reconstructed from the calculated Fourier transform. The interferogram shown in image 45 results from the interference between the beams transmitted by the Bragg gratings respectively with strata of index "horizontal" and "vertical" (experimental conditions identical to those described for FIG. 4A), when the Bragg conditions are satisfied for the two gratings (conditions corresponding to image 44 of FIG. 4A), but for an incident beam of larger diameter, approximately 11 mm, illuminating the whole object. Fes four waves of respective electromagnetic fields E O o, Eio, Eoi, and En interfere in the overlap area of the respective beams to form the interferogram illustrated in image 45. E'image 46 is an enlargement of the area delimited by a white square on image 45.
    Ee total field E t is thus the sum of the four transmitted fields:
    E t = Eoo + Eio e A {i kx x} + EOi e A {i ky y} + En e A {ik x x + ik y y} (6)
    Or :
    k x = 4 π Θ Β / λ (7) k y = 4 π / λ (8)
    The interferogram is given by the intensity of the total field:
    I = | E t | A 2 (9)
    The cross interference term E O o En * of the interferogram thus calculated represents the signal useful for the reconstruction of the hologram.
    Image 47 corresponds to the amplitude of the spatial Fourier transform of the interferogram shown in image 45. The useful component of this signal, corresponding cross interference term E O o En *, appears in the corner in top right on the calculated amplitude of the spatial Fourier transform of the interferogram, shown in image 47 of FIG.4B.
    Image 48 is the hologram calculated in the plane of the object by a discrete Fresnel transform of the interferogram; a calculation method is described for example in the article by N. Verrier et al. (Off-axis digital hologram reconstruction: some practical considerations. Applied optics 50, no. 34 (2011): H136-H146). The region of the hologram in the upper right corner of image 48 corresponds to the amplitude of the discrete Fresnel transform of the cross interference term E O o En * calculated in the plane of the object. The phase of this hologram corresponds to the phase difference between the field E O o and the field En calculated in the plane of the object.
    In practice, for the implementation of the digital off-axis holography method according to this description, the choice of the steps Λ χ and A y of the horizontal and vertical strata can be made by taking into account the sampling rules dictated by the Nyquist-Shannon theorem. The deflection angles 2Θ Β and 23 B between the electromagnetic field waves En (deflected reference wave) and E O o (transmitted signal wave) advantageously satisfy the conditions 2Θ Β e [- Os / 2, Os / 2] and 23 B e [-Os / 2, Os / 2], with:
    20 s ~ L / d x (10)
    23 s ~ L / d y (11)
    Where 0s and 3s are called acceptance angles of the coherent detection, and d x and d y respectively the steps between the elementary detectors (or "pixels") of the two-dimensional detector, in the directions x and y, respectively.
    The applicants have shown that the use of thick Bragg gratings for the formation of signal and reference waves in an off-axis digital holography assembly, has a very high robustness, in particular because the reference wave is formed from Found object. Thus, the phase of the hologram strictly corresponds to the phase difference between the field E O o and the field En calculated in the plane of the object. In other words, the phase fluctuations which may be present in the interferometry arrangements with separate arms are here avoided.
    Furthermore, it is remarkable to note that a thick Bragg grating generates spatial deflected wave filtering, which contributes to forming a reference wave filtered angularly low pass and consequently, a hologram of very good quality.
    More precisely, the angular width of the beams deflected by thick Bragg gratings, respectively with strata of index "horizontal" (FIG. 3A) and with strata of index "vertical" (FIG. 3B), of average index n 0 and of thickness d, are respectively Δθι and A3i, with (see Kogelnik et al. Previously cited, as well as Ciapurin et al. Modeling of phase volume diffractive gratings, part 1: sinusoidal uniform gratings transmitting. Optical Engineering 45, no. 1 (2006): 015802-015802.):
    Δθι * η 0 A x / d (12)
    Δθι ~ η 0 Λ γ / ά (13)
    The thickness of a Bragg grating is defined by the thickness intended to be crossed by the incident wave. In a thick Bragg grating intended to work at normal incidence with respect to the input face of the grating (index strata inclined at an angle F with respect to a direction normal to the input face of the grating), l The thickness of the thick Bragg grating could simply be the distance between the entry and exit faces.
    So for example, for a thickness of each Bragg grating d ~ 9 mm, an average index n 0 ~ 1.5 (average index of the glass) and pitch values A x = 18.9 pm and A y = 18 , 3 pm, Δθι = Δθι ~ 3.3 mrad is obtained, which is equivalent, in an interferometer of the “point diffraction interferometer” type as described in the reference Smartt, R. N et al. (Point-Diffraction Interferometer. Journal of the Optical Society of America. 62: 737 (1972)) to filtering by a pupil with a diameter of 330 pm, placed at the focal point of a 10 cm focal lens.
    It is therefore possible to adapt the steps of the strata of index A x and A y and the network thickness d as a function of the angular width (Δθι, Δθι) sought for the deflected reference wave.
    FIGS. 5A and 5B illustrate two exemplary embodiments, with and without a microscope objective, of an off-axis digital holography device according to the present description.
    In each of these examples, the off-axis digital holography device comprises as in the example of FIG. 2, a light source 21 adapted for the emission of an illumination wave from an OBJ object, a set 22 of one or more thick Bragg grids, a two-dimensional detector 23. Although shown in transmission in FIG. 2, the illumination of the object can be done in reflection.
    In the example of FIG. 5A, a microscope objective 51 makes it possible to form an image of a restricted field of the object OBJ with a given magnification. The method implemented by means of the device then becomes a method of optical microscopy by off-axis digital holography. In practice, the microscope objective is arranged so as to return the image of the object to infinity, that is to say that the object is placed at the working distance from a so-called objective. infinite home ”. The microscope objective 51 is defined by a digital aperture NA limiting in practice the angular bandwidths Δθ ο and Δθο of the wave coming from the object. The microscope objective forms a field diaphragm 52 which extends and limits the angular bandwidth of the Eo field wave coming from the object with respect to a detection without objective. More precisely, the angular bandwidth (Δθο, ΔΘΟ) of the wave coming from the object is defined by:
    Δθο = Δθο = 2 arcsin (NA) (14)
    FIG. 5B illustrates another example in which there is no microscope objective. The angular bandwidth of the field wave Eo coming from the object is limited by the dimensions D x , D y in the absence of a field diaphragm, or by the dimensions a x , a y a field diaphragm arranged within the device, as is the case in FIG. 5B.
    In the first case, the angular bandwidth (Δθο, Δθο) of the wave coming from the object is defined by
    Δθο -D x / L (15)
    Δθο-Dy / L (16)
    Where D x and D y are the dimensions of the image field defined by the detector, respectively along the x, y and L axes is the distance between the plane of the object and the detector.
    In the second case, the angular bandwidth (Δθο, Δθο) of the wave coming from the object is defined by
    A0 o ^ a x / t (17)
    A3o ~ a y / f (18)
    Where a x and a y are the dimensions of the field diaphragm 52, respectively along the axes x, y and f is the distance between the plane of the object and the field diaphragm.
    In practice, as described above, it will be possible to use for the implementation of the off-axis holography method according to this description and illustrated in FIG. 6, a thick Bragg grating with multiplexed index strata 61, or an arrangement of two Bragg grids 62, 63, respectively with horizontal index strata and with vertical index strata.
    As will be described in connection with FIG. 7, it may also be advantageous to split each of the networks to obtain a set of two thick Bragg gratings with strata of multiplexed index 64, 65 or a set of four Bragg gratings 66, 67, 68, 69 respectively with strata d 'horizontal indices and vertical index strata. The split network configurations have the advantage of making it possible to increase the spatial filtering power for a fixed total thickness of Bragg grating, compared to a configuration with non-split network (s).
    Such thick Bragg gratings can be manufactured in a known manner, for example by exposure under ultraviolet light to a photo-thermorefractive doped glass material, as described in the following article by Efimov et al. (High-efficiency Bragg gratings in photothermorefractive glass. Applied Optics 38, no. 4 (1999): 619-627).
    FIG. 7 presents in particular a possible configuration according to which two thick Bragg gratings with multiplexed index strata 64, 65 arranged one behind the other are used. Their periods Ai and Λ 2 , and orientations of the phase strata Φι and Φ 2 , are such that the total deflection angle Θ2 - Θ1 is the desired reference wave angle. In other words, in the split network configuration, the deflection angles of the reference wave with respect to the object wave are the algebraic sum of the deflection angles of each network.
    This configuration has the advantage of ensuring greater angular selectivity (Δθι, Δθι) of Fonde En, for given network thicknesses. In fact, in a single thick Bragg grating, the search for small angles of deflection between the reference wave and the object wave results in steps of the strata of significant index (see equations (3) and (4). '' good reference wave filtering power (equations (12), (13)), it is then necessary to increase the thickness d of the network, which is not desirable beyond a certain limit The configuration with split Bragg gratings makes it possible to work with gratings which remain thin.
    For example, two successive networks of periods Ai = 1.5 microns and Λ 2 = 1.9 microns, each 2.25 mm thick, with angles of orientation of the strata Φ | * Φ 2 * 80 degrees, allow an angular selectivity of the order of Δθ ο ~ 1 mrad, and a deflection angle between the reference wave and the object wave of about 1.5 degrees. To obtain an angular selectivity of the order of Δθ ο ~ 1 mrad and a deflection angle between the reference wave and the object wave of approximately 1.5 degrees with a single network, its period must be approximately 20 microns and its thickness must be d '' about 30 mm.
    Although FIG. 7 presents a configuration with two thick Bragg gratings multiplexed, it was quite possible to apply it to thick Bragg gratings with horizontal or vertical strata, in which case we could have a set of four thick Bragg gratings (networks 66 - 69, FIG. 6).
    Although described through a certain number of detailed exemplary embodiments, the method and the device for optical imaging by off-axis digital holography according to the present description include various variants, modifications and improvements which will become obvious to those skilled in the art. the art, it being understood that these different variants, modifications and improvements form part of the scope of the invention, as defined by the claims which follow.
    权利要求:
    Claims (6)
    [1" id="c-fr-0001]
    1. Optical imaging device (20) of an object (OBJ) by off-axis holography comprising:
    a light source (21) adapted for the emission of an illumination wave (Ei) of the object, in transmission or in reflection;
    - a set of one or more thick Bragg grids (x) (22) intended to receive a wave (Eo) coming from the object thus illuminated and to deflect a first component (Er) of the wave coming from the object, called the reference wave, and allowing a second component (Es) of the wave coming from the object, called the signal wave, to pass without deflection, so that the deflected reference wave presents with respect to the undeflected signal wave of the predetermined deflection angles (2Θ Β , 2fl B ) defined in two perpendicular planes,
    a two-dimensional detection device (23) for the acquisition of an interferogram resulting from the interference between said deflected reference wave and said signal wave; and
    - a calculation unit (24) for determining, from said interferogram, a distribution of amplitude and phase of the signal wave in the plane of the object.
    [2" id="c-fr-0002]
    2. An optical imaging device according to claim 1, in which said set of one or more thick Bragg grids comprises at least a first thick Bragg grating with multiplexed index strata, having index strata. arranged in two perpendicular directions, with a given pitch (A ix , A iy ) in each of the directions.
    [3" id="c-fr-0003]
    The optical imaging device according to claim 2, wherein said set of one or more thick Bragg grids further comprises a second thick Bragg grating with multiplexed index strata, having index strata. arranged in two perpendicular directions, with a given pitch (A 2x , A 2y ) in each of the directions, so that the total deflection angle of the deflected reference wave, measured in each of said perpendicular planes, is equal to the difference in the deflection angles introduced by each of said multiplexed thick Bragg gratings.
    [4" id="c-fr-0004]
    4. An optical imaging device according to claim 1, in which said set of one or more thick Bragg grids comprises at least a first thick Bragg grating with index strata arranged according to a first direction, with a first given step (Λ χ ) and at least one second thick Bragg grating with strata of index arranged in a second direction, perpendicular to the first direction, with a second given step (A y ).
    [5" id="c-fr-0005]
    5. An optical imaging device according to any one of the preceding claims, further comprising a microscope objective for forming an image of a restricted field of the object OBJ with a given magnification.
    [6" id="c-fr-0006]
    6. Method of optical imaging of an object by off-axis holography comprising:
    - the illumination of the object in transmission or in reflection by means of an illumination wave (Ei) coming from a light source (21);
    - sending on a set of one or more thick Bragg grids (22) of a wave (Eo) coming from the illuminated object;
    - the deflection of a first component of the wave coming from the object, called the reference wave (E R ), while a second component of the wave coming from the object, called the signal wave (Es ), crosses said set of one or more Bragg grids without being deflected, the deflected reference wave having predetermined deflection angles (2Θ Β , 2θβ) defined in two relative to the undeflected signal wave perpendicular planes,
    the acquisition by means of a two-dimensional detection device (23) of an interferogram resulting from the interference between said deflected reference wave and said signal wave; and
    - the calculation, from said interferogram, of a distribution of amplitude and phase of the signal wave in the plane of the object.
    1/7
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    同族专利:
    公开号 | 公开日
    WO2018178366A1|2018-10-04|
    US11099522B2|2021-08-24|
    IL269742D0|2019-11-28|
    FR3064759B1|2021-02-19|
    US20200233378A1|2020-07-23|
    EP3602201A1|2020-02-05|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    FR2997518A1|2012-10-30|2014-05-02|Centre Nat Rech Scient|SELF-REFERENCE HOLOGRAPHIC IMAGING SYSTEM|
    US20170003650A1|2014-02-06|2017-01-05|Lyncee Tec S.A.|Digital Holographic Device|
    KR101817110B1|2009-10-08|2018-02-21|유니베르시테 리브레 드 브룩크젤즈|Off-axis interferometer|
    FR2998064B1|2012-11-15|2016-11-25|Centre Nat De La Rech Scient - Cnrs|OFF-AXIS DIGITAL HETERODYNE HOLOGRAPHY|US10852472B1|2019-06-18|2020-12-01|Cisco Technology, Inc.|Multiple stage Bragg gratings in multiplexing applications|
    US11002980B1|2020-03-10|2021-05-11|Cisco Technology, Inc.|Cascaded arrangement of two-mode Bragg gratings in multiplexing applications|
    CN112327398B|2020-11-20|2022-03-08|中国科学院上海光学精密机械研究所|Preparation method of vector compensation volume Bragg grating angle deflector|
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    优先权:
    申请号 | 申请日 | 专利标题
    FR1752775|2017-03-31|
    FR1752775A|FR3064759B1|2017-03-31|2017-03-31|OPTICAL IMAGING DEVICES AND METHODS BY OFF-AXIS DIGITAL HOLOGRAPHY|FR1752775A| FR3064759B1|2017-03-31|2017-03-31|OPTICAL IMAGING DEVICES AND METHODS BY OFF-AXIS DIGITAL HOLOGRAPHY|
    EP18719064.0A| EP3602201A1|2017-03-31|2018-03-30|Devices and methods for optical imaging by means of off-axis digital holography|
    PCT/EP2018/058349| WO2018178366A1|2017-03-31|2018-03-30|Devices and methods for optical imaging by means of off-axis digital holography|
    US16/499,770| US11099522B2|2017-03-31|2018-03-30|Devices and methods for optical imaging by means of off-axis digital holography|
    IL26974219A| IL269742D0|2017-03-31|2019-10-02|Device and method for optical imaging by means of off-axis digital holography|
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