WO2004038362A2

WO2004038362A2 - Method and device for the polarimetric analysis of the three-dimensional infrared scattering from an object using a parabolic mirror assembly

Info

Publication number: WO2004038362A2
Application number: PCT/FR2003/050100
Authority: WO
Inventors: Stéphane MAINGUY; Thomas Lanternier
Original assignee: Commissariat A L'energie Atomique
Priority date: 2002-10-23
Filing date: 2003-10-21
Publication date: 2004-05-06
Also published as: FR2846417B1; FR2846417A1; WO2004038362A3

Abstract

The invention relates to a method and a device for the polarimetric analysis of the three-dimensional infrared scattering from an object using a parabolic mirror assembly. The object (2) is illuminated by incident infrared radiation (32) by means of the mirror (30) using a determined angle of incidence. The mirror reflects the radiation towards the object as well as the radiation (36) scattered by said object. The inventive method consists in measuring the radiation scattered along a determined scattering angle and varying the scattering angle and, optionally, the angle of incidence. According to the invention, the incident radiation and scattered radiation are polarised and, in order to analyse the three-dimensional infrared scattering from the object, the polarisations are also varied. The scattered radiation is measured according to the values of the incident radiation polarisation, the scattered radiation polarisation, the scattering angle and, optionally, the angle of incidence, and the measurements are processed in order to obtain the desired properties of the object. The invention is particularly suitable for characterising materials.

Description

METHOD AND DEVICE FOR POLARIMETRIC ANALYSIS OF THE

THREE-DIMENSIONAL INFRARED BROADCAST OF AN OBJECT IN

MEANS OF A PARABOLIC MIRROR MOUNTING

DESCRIPTION

TECHNICAL AREA

The present invention relates to a method and a device for optical analysis of the three-dimensional infrared scattering of an object by means of an assembly comprising a parabolic mirror.

It applies in particular to the equipment of infrared spectrometers for the infrared characterization of materials. It is also possible to manufacture a device in accordance with the invention, of small size, with a view to studying coatings of objects, in particular large parts, located in a given operating environment. The invention also finds applications in the aeronautical and space fields where thermo-optical data are needed to solve, in particular, remote sensing problems. The invention also applies to all industrial sectors where non-destructive testing of materials is carried out by infrared means, for example with regard to the geometry, the defects and the more or less reflective nature of these materials. STATE OF THE PRIOR ART

It is known to analyze the infrared diffusion of objects using assemblies allowing only measurements in an incidence plane where the zenith angle of an incident radiation is simply varied. These arrangements only allow two-dimensional measurements at constant azimuth angle.

There is also known a device for optical analysis of the infrared scattering of an object by means of a parabolic mirror assembly, by the following documents to which we will refer:

[1] S. Mainguy et al., “Description and calibration of a fully automated infrared scatterometer”, SPIE Proceedings, vol. 1530, Ed. J.C. Stover, 1991, pages 269 to 282.

[2] S. Mainguy, Doctoral thesis, "Measurement and modeling of the scattering of optical radiation by rough surfaces, application to the infrared properties of boron ceramic", 1994.

In these documents, we examine the problems of parallelism of an infrared beam incident on the parabolic mirror and the problems of collection of the reflected beam. It follows from these documents that the parabolic mirror makes it possible to explore the reflection half-space very widely, the exploration being carried out continuously according to the zenith angles and the azimuthal angles, and to reduce the three-dimensional angular scans to scans in a plane, that is to say to two-dimensional scans, which limits the movements of the collection-detection arm that the device described in this document comprises.

It should however be noted that the technique disclosed by these documents uses, for the study of the object, a radiation whose polarization is fixed. It will also be recalled that different techniques have been developed involving a simple polarimetric analysis, which only concerns the rectilinear polarization states p and s.

Indeed, the components of the electric field are written:

E _x = Re [ae ^{i (ωt} - ^kz -4 ^>) ] (la)

E _y = Re [ae ^{i (m} - ^ - ^φ - ^φ) ] (ib)

By placing themselves in any plane z, these components can be put in the following form:

E _x = Re [ae ^{i (ωt +} ^] (2a)

E = Re [ae ^{i (ωt + φy}} '] (2b) or again:

The Jones J vector thus defined contains all the information necessary (phase and amplitude) to perfectly define the field vector electric. It can therefore represent all possible polarization states (rectilinear, circular and elliptical).

For the problem considered in the present invention, the relevant information is in fact in the transition from an incident Jones vector to a scattered Jones vector J '. This passage can be represented by the following matrix M, called Jones matrix or diffusion matrix:

I ≈ M J (4)

Which also says:

The diffusion matrix is a 2x2 matrix with four components. The components E _pp and E _ββ are the co-polarization components of the diffusion matrix. E _pΞ and E _sp are the cross-polarization components of the diffusion matrix. All the necessary information is contained in the Jones matrix. However, the components of this matrix are fields and not intensities. However, in optics, we do not know how to measure fields but intensities.

Most polarimetric techniques exploit homogeneous quantities squared in the modulus of the components of the Jones matrix, such as for example the specular reflection factors R _g and R _p . As an example of an application, mention may be made of ellipsometric techniques for characterizing the optical index of materials. This level of analysis polarimetric is reduced and therefore the applications which result from it can only be limited.

PRESENTATION OF THE INVENTION The aim of the present invention is to improve the technique disclosed by the documents mentioned above by proposing a method and a device allowing a much more complete analysis than that which is permitted by this known technique. To do this, the invention combines the good geometrical properties of the parabolic mirror with a polarized measurement technique: in the invention, the polarizations of the light beam used for the study of the object (preferably a laser beam) are controlled , respectively upstream and downstream of the parabolic mirror.

In the present invention, use is also made of the fact that a parabolic mirror little polarizes a light which it reflects and therefore has, in polarized light, optical properties which are well suited to the invention, that is to say to polarized three-dimensional infrared scattering measurements.

Specifically, the present invention relates to a method for analyzing the three-dimensional infrared scattering of an object, method in which

- this object is illuminated by incident infrared radiation, by means of a parabolic mirror, at a determined angle of incidence, this parabolic mirror being provided for reflecting the incident infrared radiation towards the object, this object then providing scattered radiation, the parabolic mirror being further provided for reflecting this scattered radiation,

- the scattered radiation is measured according to a determined scattering angle and

- the angle of scattering and possibly the angle of incidence are varied, this method being characterized in that the incident radiation and the scattered radiation are further polarized and, to analyze the three-dimensional infrared scattering of the object , the polarization of this incident radiation and the polarization of this scattered radiation are also varied, the scattered radiation is measured, as a function of the values of the polarization of the incident radiation, of the polarization of the scattered radiation, of the scattering angle and possibly the angle of incidence and the measurements are processed to obtain the desired properties of the object. The present invention also relates to a device for analyzing the three-dimensional infrared diffusion of an object, this device comprising

- a source of infrared radiation,

a parabolic mirror, the source being provided to illuminate the object by incident infrared radiation, by means of the parabolic mirror, at a determined angle of incidence, this parabolic mirror being provided for reflecting the incident infrared radiation towards the object, this object then providing scattered radiation, the mirror being further provided to reflect this scattered radiation,

- means for detecting the scattered radiation at a determined scattering angle; and - means for varying the scattering angle and possibly the angle of incidence, this device being characterized in that it further comprises first polarization means, provided for polarizing the incident radiation, second polarization means, provided for polarizing the scattered radiation,

means for controlling the first and second polarization means, provided for varying the polarization of this incident radiation and the polarization of this scattered radiation, for analyzing the three-dimensional infrared diffusion of the object, and means for processing the signals supplied by the detection means, as a function of the values of the polarization of the incident radiation, of the polarization of the scattered radiation, of the scattering angle and possibly of the angle of incidence, in order to obtain desired properties of the object. According to a first particular embodiment of the device which is the subject of the invention, the first polarization means comprise a first polarizer and a first quarter-wave plate, the second polarization means comprise a second polarizer and a second quarter-wave plate and the control means include means for rotation of these first and second quarter wave blades.

In this case, the first and second polarizers can be chosen from rectilinear polarizers, circular polarizers and elliptical polarizers.

According to a second particular embodiment of the device which is the subject of the invention, the first polarization means comprise at least one Pockels cell, the second polarization means also comprise at least one Pockels cell and the control means are provided for controlling each Pockels cell.

According to a preferred embodiment of the device which is the subject of the invention, the first polarization means comprise at least two Pockels cells, the second polarization means also comprise at least two Pockels cells and the control means are provided for controlling each Pockels cell.

Preferably, the infrared radiation source comprises at least one infrared emission laser.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limiting, with reference to the appended drawings, in which: FIG. 1 is a schematic perspective view of a particular embodiment of the device which is the subject of the invention,

FIG. 2 is a schematic view of an example of polarization means which can be used in the device of FIG. 1,

- Figure 3 is a schematic view of another example of polarization means that can also be used in the device of Figure 1, and - Figure 4 is an evaluation of the Mueller matrix of the parabolic mirror of a device according to the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The example of the device which is the subject of the invention, which is schematically represented in FIG. 1, is a polarized three-dimensional infrared diffusion measurement installation. This installation makes it possible to study samples of various materials, such as sample 2.

This sample 2 is placed on a support 4. The latter is provided with rotation means which are symbolized by the arrow 6 and make it possible to rotate the support 4, and therefore the sample 2, around an axis Z.

The support 4 is further provided with translation means which are symbolized by the arrow 7 and make it possible to move the support 4, and therefore the sample 2, along the axis 2. The device of FIG. 1 also includes a source 8 infrared radiation. This source includes a C0 ₂ 10 laser which allows measurements to be made at a wavelength of 10.6 μm.

In the example shown, the source also includes a CO 12 laser which makes it possible to make measurements at a wavelength of 5.4 μm. A semi-transparent plate 14 makes it possible to obtain an infrared beam 16 containing the radiation of that of the two lasers which is in operation.

The lasers 10, 12 and the semi-transparent plate 14 are arranged so that the beam 16 propagates in a plane P which is orthogonal to the axis Z.

Part of this beam 16 is transmitted by another semi-transparent plate 18 and picked up by a detector 20, such as a pyrometric detector, intended to measure the power of the infrared beam 16.

The other part of this beam 16 is reflected by the semi-transparent plate 18. It is then reflected by a mirror 22, along an axis X of the plane P, then successively passes through an adjustable diaphragm 24 and a focusing lens 26 before d be reflected parallel to the Z axis by another mirror 28. The device of FIG. 1 also includes a parabolic mirror 30. The axis of this parabolic mirror is the Z axis and meets the X axis at a point O This parabolic mirror is placed above the sample 2 and the surface of this sample is placed in its focus (using the translation means 7). This mirror 30 is provided for reflecting the light coming from the mirror towards the sample 2. This sample 2 thus receives, from the mirror 30, an infrared beam 32 whose angle of incidence, counted with respect to the axis Z, is noted θi.

Under the effect of its illumination, sample 2 diffuses infrared light. The latter is reflected by the mirror 30 parallel to the axis Z. Another mirror 34 is provided for picking up an infrared beam 36, constituting part of the scattered light, after reflection of this beam by the mirror 30, and for reflecting the beam it receives along an axis Y of the plane P. The axis Y meets the axis X at the point O and makes an angle φ with this axis X.

The beam 36, which is scattered by the sample 2, makes a scattering angle θr with the axis Z. In addition, the plane PI, containing the axis Z and the axis of the beam 32, makes the angle φ with the plane P2, containing the axis Z and the axis of the beam 36.

The infrared beam 38, reflected by the mirror 34 along the Y axis, is focused by a lens 40 on an infrared radiation detector 42, for example a detector based on HgCdTe. The lens 26 and the mirror 28 are fixed to a frame 44. A stepping motor, symbolized by the arrow 46, allows the displacement of this frame in translation along the axis X to vary the angle θi. It is specified that the lens 26 is a pre-focusing lens, allowing the arrival, on sample 2, of a beam with parallel rays instead of a focused beam.

The mirror 34, the lens 40, and the detector 42 are fixed to a frame 48. A stepping motor, symbolized by the arrow 50, allows the displacement of this frame in translation along the Y axis to select a another beam scattered by the sample.

Another stepping motor, symbolized by the arrow 52, makes it possible to rotate the frame 48, and therefore the Y axis, around the Z axis, and consequently to vary the angle φ.

We can therefore vary the angle θr by translating the frame 48 along the Y axis and / or rotating this frame around the Z axis.

The device of FIG. 1 also comprises means 54 for controlling the stepping motors, means 56 for amplifying the signal supplied by the detector 42, a chopper 58 which is placed on the trajectory of the beam reflected by the semi-transparent plate 18, and

- a modulator 60, for example in the frequency range [10 Hz; 1000 Hz], which controls this chopper.

FIG. 1 also shows electronic and computer means 62 provided for

- controlling the device (in particular the means 54 for controlling the stepping motors), - acquiring the measurements made with this device, - store the results of these measurements and

- exploit these results.

These means 62 receive in particular the signals supplied by the detector 20 and by the amplifier 56 and cooperate with the modulator and the chopper to allow synchronous detection of the signals supplied by the detector 42, in order to eliminate parasitic noises, due in particular to the infrared emission of the sample 2. In accordance with the present invention, the device further comprises means 64 for controlling the polarization of the incident beam 32 which is sent to the sample 2 and - means 66 for controlling the polarization of the scattered beam 36 that the detector 42 receives.

The means 64 are for example placed on the path of the beam 16, before the semi-transparent plate 18, while the means 66 are placed on the path of the beam 38, on the frame 48, between the lens 40 and the detector 42.

It is specified that the adjustments of these means

64 and 66 are not necessarily identical: the polarization of the incident beam 32 and the polarization of the scattered beam 36 (after passing through the means

66) may be different from each other.

The device of FIG. 1 makes it possible to envisage a large number of polarization states of the incident beam and, independently of these, a large number of polarization states of the scattered beam.

FIG. 2 shows a first example of the polarization control means 64 and 66.

In this example, the means 64 or 66 comprise a linear polarizer 68 or 70 and a quarter-wave plate 72 or 74 which is movable in rotation around the axis of the beam that the polarizer polarizes. This polarizer 68 or 70, which is associated with the quarter-wave plate 72 or 74, can precede the latter.

FIG. 2 also shows means 76 or 78 for controlling the rotation of the quarter-wave plate 72 or 74, these means 76 and 78 being themselves controlled by the electronic means 62.

In FIG. 3, a second example of the polarization control means 64 and 66 has been shown. In this second example, the means 64 and the means 66 comprise at least one Pockels cell controlled by means making it possible to vary them. frequency .

Preferably, as can be seen in FIG. 3, these means comprise several Pockels cells. For example, the means 64 comprise two Pockels cells 80,82 and the means 66 comprise two other Pockels cells 84,86. Means 88 for controlling all these Pockels cells, intended to vary the time frequency of each Pockels cell, are also provided. Indeed, the processing of the signals supplied by the amplifier 56 generally requires more than one frequency to be satisfactory, in particular in the case where it is a processing by inverse Fourier transform.

The latter requires at least four frequencies to be satisfactory and there are indeed four frequencies if four Pockels cells are used. To analyze sample 2 with the device in Figure 1, we therefore have the following parameters:

Xl = pl: polarization of the incident beam X2 = p2: polarization of the scattered beam X3 = θi: angle of incidence of the incident beam

X4 = φ: angle between the X and Y axes X5 = θr: scattering angle of the scattered beam. We wish to vary these parameters respectively in predefined intervals, each parameter taking Ni values in the interval which corresponds to it, for i ranging from 1 to 5.

The signal S supplied by the detector 42 is then measured for each of the N = NlxN2x 3xN4xN5 possible combinations of the values of the parameters and thus N values of S are obtained which are stored in the electronic means 62 and which are processed then to determine the desired properties of sample 2. However, it should be noted that in practice we prefer to work at X3 = θi given.

The device according to the invention, which has just been described, makes it possible to carry out a very thorough optical analysis of the diffusion properties of a sample of given material. This polarimetric analysis makes it possible to characterize exhaustively the optical response capacities of a given coating which is illuminated by infrared light.

This optical response is essential data for evaluating the infrared signature of an object having such a coating.

It also makes it possible to access information on the composition of the surface of this coating, in particular as regards the dielectric properties of the material, at the wavelength of the infrared illumination source, and microgeometric data such as parameters representative of material defects.

This device according to the invention makes it possible to acquire polarimetric optical properties thanks to

1) a "movable" detection in the half-space which overhangs the surface of the studied sample, so as to carry out measurements in three dimensions according to directions coming from the sample, each direction being defined by a couple (azimuthal angle ; zenith angle) and 2) control of the polarization properties of the incident and scattered beams by the sample.

The use of a metallic parabolic mirror, weakly polarizing in the infrared, makes it possible to combine these two points.

Details of the present invention are given below.

A complete description of the diffusion properties of a structure, for any incident polarization state, is interesting for several reasons. Typically, in the case of diffractive optics (plates whose thickness is a fraction of the wavelength, multilayers, gratings, new periodic nanometric structures, ...), the incident and reflected polarization states can be very different . In addition, for several years, the polarimetric analysis of a structure in the field of optics has been the subject of in-depth studies. Applications range from military or civilian remote sensing to the detection and control of contamination in microelectronics or laser optics.

The complete description of the properties of diffusion of a structure, which one wants to study using the device object of the invention, requires the exploitation of the formalism of the vectors of Stokes and the matrixes of Mueller. These quantities are expressed from the intensities, which are measurable. The Stokes vector have four components and not two, namely:

These four components I, Q, U and V are expressed as a function of the amplitudes (a, b) of the field as well as of the phase shift ^~ y - φ &, according to the following formulas:

I = a ² + b ² = E _p E _p ^* + E _s E _s ^* (7a)

*

Q = a ² -b ² = E p _D E ^{^} p E _S E _S (7b)

U = 2abcos (φ) = E _p E _s + E _s E _p (7c)

V = 2ab sin (φ) = i (E _p E _s ^* - E _s E _ρ ) <7d)

This vector can therefore completely represent the electric field vector. The four components I, Q, U, N can also be expressed as a function of the intensities, according to the following formulas:

Q = ι _// -ι X (8b)

^{r == I} _cd ~ E _cg (8d)

I represents the total intensity, Q the intensity difference according to the two main rectilinear polarizations (TE and TM), U the intensity difference according to two rectilinear polarizations at 45 ° and V the difference between the two components of right circular polarization and left circular. The passage of an incident Stokes vector

S to a diffused Stokes vector S 'is made thanks to a matrix M called Mueller matrix of the studied sample

S '= MS (S) or also:

This matrix is a 4x4 matrix which therefore has sixteen elements. These sixteen elements are intensities and not fields. Each of these components represents the intensity diffused in a polarization for an incident polarization (straight or circular). All the information necessary for a polarimetric study is contained in the Mueller matrix. It is therefore this quantity that we measure.

Because any polarization state can be broken down into two rectilinear polarization states (p and s), we show that the coefficients of the Mueller matrix are expressed as a function of the four coefficients of the Jones matrix E _pp , E _sa , E _pa and E s, p -

One can in particular evaluate the Mueller matrix of a plane mirror of aluminum in the visible and the infrared, this plane mirror corresponding to the portions of the parabolic mirror used in a device according to the invention, for example the device which is shown in Figure 1. Depending on the angular configurations of incidence and reflection on the sample, the angle of incidence on the parabolic mirror portion varies from 5 ° to 45 °. This Mueller M _MP matrix has the following form:

The Mueller matrix Mg _C h of the sample is linked to the Mueller matrix of the parabolic mirror

M _MP and to incident Stokes vectors S and scattered * S "by:

S '= MMPM ^ MMP S (12) hence sH

M ≈ _exp iM pY'siM _M _M p S) (13)

We deduce that the use of a parabolic mirror is perfectly compatible with the polarimetric analysis method proposed in the present invention. The interest of the parabolic mirror is to carry out this polarimetric study in three dimensions.

FIG. 4 is an evaluation of the Mueller M _MP matrix of the parabolic mirror of the device according to the invention. The coefficients trii _j are represented in matrix form (see formula (11)). On the abscissa, we have the local angle of incidence on the parabolic mirror. The results are given for the wavelengths: 10.6 μm (curve R), 5.4 μm (curve V, almost identical to the curve R) and 0.633 μm (curve B).

Claims

1. Method for analyzing the three-dimensional infrared diffusion of an object (2), method in which - this object is illuminated by incident infrared radiation (32), by means of a parabolic mirror (30), according to a determined angle of incidence (θi), this parabolic mirror being provided for reflecting the incident infrared radiation towards the object, this object then providing a scattered radiation, the parabolic mirror being also provided for reflecting this scattered radiation, the scattered radiation (36) according to a determined scattering angle (θr) and - the scattering angle and possibly the angle of incidence are varied, this process being characterized in that the incident radiation is further polarized and the scattered radiation and, to analyze the three-dimensional infrared scattering of the object, the polarization of this incident radiation and the polarization of this scattered radiation are also varied, we measure the scattered radiation, as a function of the values of the polarization of the incident radiation, of the polarization of the scattered radiation, of the scattering angle and possibly of the angle of incidence and the measurements are processed to obtain desired properties of the object.

2. Device for analyzing the three-dimensional infrared diffusion of an object (2), this device comprising

- a source (8) of infrared radiation, - a parabolic mirror (30), the source being provided to illuminate the object by incident infrared radiation (32), via the parabolic mirror, at an angle of incidence determined

(θi), this parabolic mirror being provided for reflecting the incident infrared radiation towards the object, this object then providing scattered radiation, the parabolic mirror being further provided for reflecting this scattered radiation, means (42) for detecting the radiation scattered (36) at a determined scattering angle (θr) and

- Means (46, 50, 52) for varying the scattering angle and possibly the angle of incidence, this device being characterized in that it further comprises

- first polarization means (68-72, 80-82), provided for polarizing the incident radiation, - second polarization means (70-

74, 84-86), intended to polarize the scattered radiation,

means (76-78, 88) for controlling the first and second polarization means, provided for varying the polarization of this incident radiation and the polarization of this scattered radiation, to analyze the three-dimensional infrared scattering of the object, and

means (62) for processing the signals supplied by the detection means as a function of the values of the polarization of the incident radiation, the polarization of the scattered radiation, the scattering angle and possibly the angle of incidence, to obtain desired properties of the object.

3. Device according to claim 2, in which the first polarization means comprise a first polarizer (68) and a first quarter wave plate (72), the second polarization means comprise a second polarizer (70) and a second blade quarter wave (74) and the control means (76, 78) comprise means for rotating these first and second quarter wave blades.

4. Device according to claim 3, in which the first and second polarizers (68,

70) are chosen from rectilinear polarizers, circular polarizers and elliptical polarizers.

5. Device according to claim 2, in which the first polarization means comprise at least one Pockels cell (80-82), the second polarization means also comprise at least one Pockels cell (84-86) and the means for control (88) are provided to control each Pockels cell.

6. Device according to claim 5, in which the first polarization means comprise at least two Pockels cells (80-82), the second polarization means also comprise at least two Pockels cells (84-86) and the means for control (88) are provided to control each Pockels cell.

7. Device according to any one of claims 2 to 6, wherein the infrared radiation source (8) comprises at least one infrared emission laser (10, 12).