WO2007072428A2

WO2007072428A2 - Spectrophotometer and spectrophotometric process using fabry-perot resonator

Info

Publication number: WO2007072428A2
Application number: PCT/IB2006/054955
Authority: WO
Inventors: Marco Pisani; Massimo Zucco
Original assignee: I.N.Ri.M. - Istituto Nazionale Di Ricerca Metrologica
Priority date: 2005-12-20
Filing date: 2006-12-19
Publication date: 2007-06-28
Also published as: EP1971836A2; ITTO20050887A1; WO2007072428A3; WO2007072428A9

Abstract

A spectrophotometer (1), in particular for the spectral analysis of a source (5), comprising a Fabry-Perot resonator (2) including a partially transparent entry mirror (7) suitable for being crossed by radiation originating from the source (5), and a partially transparent output mirror (8). The spectrophotometer (1) also comprises a modulator (9) suitable for modulating the optical path of the radiation between mirrors (7, 8) and a photosensor matrix (3) arranged downstream of the exit mirror (8) of the Fabry-Perot resonator (2). The photosensors (3) are suitable to supply electrical signals representative of a function which correlates the intensity (I) of the radiation emerging from the Fabry-Perot resonator (2) with the length of the optical path (d) between the mirrors (7, 8) thereof. The spectrophotometer (1) finally also includes a processing and control unit (4) connected to the photosensor matrix (3) and arranged to generate data representative of the emission spectrum of the source.

Description

Spectrophotometer and spectrophotometry process using a Fabry-Perot resonator

The present invention relates to a spectrophotometer based on a Fabry-Perot resonator integrated into an optoelectronic system, in particular for spectral analysis of every point (pixel) of a given two dimensional image. This image may be, in particular, generated by an optical system from a two dimensional or three dimensional object or scenario.

It is known to determine the emission or absorption spectrum of electromagnetic radiation using the Fourier transform of an interferogram obtained with a Michelson interferometer. This technique consists of dividing into two parts of equal intensity, using a separating plate, a beam of light originating from a source and, after making the two beams follow two optical paths of different length, superimpose them by means of the same plate and observe the interference downstream of the interferometer. When the relative length of the two paths is varied, the different spectral components of the light recombine on the plate giving rise to constructive or destructive interference according to the known laws of wave interference. The result obtained, as the length of the two paths is varied, is a variation in the intensity of the light coming from the plate, which is a function of the spectral composition of the light source. This variation and/or its graphic representation is called interferogram. It is demonstrated that a mathematical procedure comprising the Fourier transform of this interferogram corresponds to the power spectrum of the light source.

The Michelson interferometer, used in instruments which employ this principle, requires a complicated construction process and it is also difficult to integrate with optoelectronic devices .

An object of the present invention is to propose an innovative spectrophotometer which is easy to construct and which can be integrated into an optical-electronic system.

This and other objects are achieved according to the invention with a spectrophotometer whose main features are defined in claim 1.

The invention also relates .to an innovative spectrophotometric method or process whose salient features are defined in claim 25.

Other features and advantages of the invention will become clear from the following detailed description which is provided purely by way of non-limiting example, with reference to the appended drawings, in which:

Fig. 1 is a block diagram of a spectrophotometer according to the invention;

Fig. 2 is a detailed diagram of a spectrophotometer according to the invention;

Fig. 3 is a representation of a variation of the Fabry-Perot cavity according to the invention;

Fig. 4 shows an example of an interferogram; and

Fig. 5 illustrates an initial segment of an interferogram.

In figure 1 is overall shown a diagram of a spectrophotometer 1 in which are present a Fabry-Perot resonator 2, sensors 3 and a processing and control unit 4 connected sequentially to one another . In figure 2 is shown an external light source 5 suitable for emitting light which is focused on the sensors 3 by the objective lens 6, passing through the Fabry-Perot cavity 2.

Alternatively to the diagram given in Fig. 2, the Fabry-Perot cavity 2 may be placed between the external light source 5 and the objective lens 6.

The source 5 may be an object or a three dimensional scenario, appropriately illuminated, of which it is desired to obtain a spectrum analysis.

The light arrives inside the Fabry-Perot cavity 2 by crossing the entry mirror 7 which is partially transparent . When the light has reached the interior of the Fabry-Perot cavity 2 it leaves it passing through the partially transparent output mirror 8. The semi-transparent mirrors 7 and 8 have an internal coating which must have the most uniform possible reflectivity and phase delay for all wavelengths and may be made, for example, of INCONEL (metal alloy) . The mirrors 7 and 8 may have planar rigid surfaces of glass or other transparent material or may be composed of a film of plastics material .

Alternatively, the resonator 2 may comprise a plurality of microcavities in MEMS (Micro Electrical Mechanical Systems) technology. In this case may be produced a matrix of semi- reflecting micromirrors which are electrostatically actuated and which can vary their distance from a common mirror. In each of the preceding configurations, one of the two semi- reflecting mirrors may be the same surface of the sensors 3 on which a semi-reflecting layer 11 is deposited (see Fig. The light transmitted from the Pabry-Perot cavity 2 may be considered as the superimposition of a plurality of components which combine and interfere in output : there is a component 7a which directly crosses the mirrors, a component 7b which is reflected once by both mirrors (travelling an additional path equal to twice the distance between the mirrors) , a component 7c which undergoes a further reflection by both mirrors and so on. In the embodiment illustrated the relative distance between the entry mirror 7 and the output mirror 8 is varied by means of an electrically-controlled actuator 9, controlled by a control unit 10.

The light transmitted from the Fabry-Perot cavity is sensed by a two dimensional sensor matrix 3 of CCD (Charge Coupled Device) or CMOS (Complementary Metal-Oxide Semiconductor) sensors which supply electrical output * signals representative of the intensity of the radiation emerging from the cavity of the resonator 2. Changes in the intensity I of the light beam emerging from the resonator 2 when the distance d between the mirrors 7 and 8 is varied are defined an interferogram 12 (see Fig. 4) and is a function of the spectral composition of the light emitted from the source 5.

The actuator 9 may be of electromechanical, piezoelectric, pneumatic or electrostatic type .

The distance d between the mirrors may be varied from a minimum value equal to zero (contact between the two mirrors) up to a predetermined maximum value on which the resolution of the device will depend.

Alternatively to the embodiment described above and illustrated in the drawings, it is possible to modulate, i.e. to vary, the length of the optical path of the radiation in the resonator 2 by using a material with a refractive index which can be varied in a controlled manner, for example liquid crystal-based, interposed between two stationary mirrors .

In any case, the processing and control unit 4 then processes the signals supplied by the sensor matrix 3, the processing corresponding to a mathematical process comprising a Fourier transformation of the interferogram 12, which, takes into account the dispersion caused by the mirrors, and in this way generates data representative of the ^"spectrum of the light emitted by the image.

In a Fabry-Perot spectrophotometer 1 it may in practice be impossible to determine the value of the intensity I of the radiation for a value d=0 of length of the optical path. Indeed, even if .the surfaces of the mirrors of the resonator are in contact (zero distance between them) , there still remains a difference in path between the various components of the light beam. In the case of "dielectric" mirrors, the reflection is produced by a succession of dielectric layers which force the light to penetrate for a certain distance into the mirror, while in the case of metal mirrors the light undergoes a phase delay comparable to -a physical penetration of the mirror. This in fact produces an incomplete interferogram. The incompleteness is evident in the initial segment 13 (see Fig. 5) of the interferogram 12.

The processing unit 4 however, in order to be able to perform the Fourier transformation, requires a complete interferogram 12. In order to solve this problem a possible process will be illustrated below for the reconstruction of the initial missing part of the interferogram 12. It has been verified experimentally that in order to reconstruct the initial segment of an interferogram from two to four points, according to experimental conditions, are necessary and sufficient. It has been demonstrated experimentally that the set of points subsequently obtained is unique and that the Fourier transform of the reconstructed interferogram 12 corresponds to the power spectrum of the light ^■ analysed. If the reflecting properties of the semi-reflecting mirrors 7 and 8 are known, by measuring the average intensity of the interferogram 12 it is possible to calculate the corresponding luminous intensity at point 13b of zero distance between the mirrors. The maximum intensity (point 13b of the interferogram) may be obtained by measuring the average intensity transmitted from the cavity and multiplying it by the division between the maximum transmissivity and the average transmissivity of the cavity, which are known because of the construction.

In addition, by ensuring, through the use of selective filters, that determined portions of the spectrum are at zero intensity, the missing points 13a of the interferogram are found by minimising the value of the Fourier transform of the portions referred to above .

According to a variation, since the interferogram 12 obtainable with the Fabry-Perot spectrophotometer is equivalent to a complete symmetrical Michelson interferogram to which is applied a window of zero weight in the centre, the two spectra are linked by convolutions of particular functions. If the procedure is applied more generally, the convolution of the spectrum obtained with the Fabry-Perot spectrophotometer may be considered as a means for obtaining spectra obtainable from other types of interferometer.

Moreover, the use of the weighting window ("apodization" or "tapering") applied to the interferogram makes it possible to increase the frequency resolution (windows with narrow central lobe) or the amplitude resolution (windows with low lateral lobes) .

In order to be able to apply the Fourier transformation without errors, it is necessary to know, for each point of the interferogram 12 sampled, the exact distance of the mirrors at the moment of sampling. If the actuator 9 were perfectly linear and repeatable, an initial calibration of the system would be sufficient. Moreover, the mirrors 7 and 8 should be perfectly planar and the movement perfectly parallel so that this distance is equal in every point. In the real case, the actuator 9 is not perfectly linear and repeatable, and the mirrors 7 and 8 are not perfectly planar and it is therefore necessary to arrange a periodical calibration of the device.

The calibration may be carried out by illuminating the objective lens 6 with a monochromatic source of known wavelength and varying the distance between the mirrors 7 and 8 by means of the actuator 9. In this- way for each point of the image an interferogram 12 is obtained which is a function of the control signal supplied by the control unit 10 to the actuator 9. Since the form of the interferogram 12 for a given wavelength is already known, it is possible, by comparing the interferogram 12 obtained and the theoretical interferogram, to obtain the function which determines the relationship between the control signal given to the actuator 9 and the effective distance of the mirrors. If the mirrors 7 and 8 are not planar and the movement is not parallel, this function is unique for each pixel and will be used as the basis for the Fourier transformation of the interferogram corresponding to that pixel . Alternatively, if the mirrors are sufficiently planar, it is possible to take as a basis for the distance between the mirrors a local measurement of that distance. By integrating for example onto the mirrors themselves the electrodes of one or more capacitive sensors, the distance of the reflecting surfaces during the movement may be measured very precisely. This measurement may be used to correct the movement of the actuator 9 to render it linear and repeatable, or to associate the correct distance value with every sampling point of the interferogram 12.

Alternatively to the use of a mathematical procedure based on Fourier transforms, which takes into account the dispersion of the mirrors, it is possible to obtain the emission spectrum of the source with a process which uses a "generalized least squares estimation" method (GLS) . With this procedure the system is calibrated first of all by measuring (or calculating) the interferograms associated with a series of substantially monochromatic reference sources, having respective predetermined equally spaced frequencies in a range of frequencies which cover the entire interval concerned. After the initial calibration phase, the interferogram 12 of the source 5 is measured and is equalized to a weighted linear combination of the interferograms of the reference sources. At this point, the weight coefficients of said linear combination are calculated using the "generalized least squares estimation" method and the emission spectrum of the source 5 is determined as comprising all the frequencies of the reference sources, each weighted with the weight coefficient calculated for the respective interferogram.

The spectrophotometer according to the invention opens up great possibilities in the field of spectroscopy and colourimetry. The Fabry-Perot resonator is in fact extremely compact and can easily be interposed between objective and sensor in an image acquisition system so that, for each point (pixel) of the image, the spectral composition of the light originating from it can be obtained. The instrument may be seen either as the implementation of classic trichromatic photography in terms of information added to the colour, or as the implementation of classic spectrophotometry in terms of the number of spectra acquired at the same time . This spectrophotometer may be used in a variety of applications including, for example, thermography, fluorescent microscopy in the biological field, spectroscopy, measurements in the environmental field, measurement of the spectrum of reflection of pictures or frescoes for conservation and/or restoration of cultural objects in general, photography or for industrial processes .

Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated, which have been given purely by way of example, without thereby departing from the scope of the invention as defined in the appended claims .

Claims

1. A spectrophotometer (1) , in particular for spectral analysis of a source (5) , comprising: a Fabry-Perot resonator (2) including a partially transparent entry mirror (7) suitable for being crossed by the radiation originating from the source (5) , and a partially transparent output mirror (8) ;

- a modulator (9) suitable for modulating the optical path of the radiation between said mirrors (7, 8) ;

- a photosensor matrix (3) arranged downstream of the exit mirror (8) of the Fabry-Perot resonator (2) and suitable for providing electrical signals representative of an interferogram function (12) which correlates the intensity

(1) of the radiation emerging from the Fabry-Perot resonator

(2) with the length of the optical .path (d) between the mirrors (7, 8) thereof; and

- means of processing and control (4) connected to said photosensor matrix (3) and arranged to generate, according to predetermined conditions, signals or data representative of the emission spectrum of the source (5) .

2. A spectrophotometer according to claim 1, wherein said means of processing and control (4) are arranged to generate signals or data representative of the emission spectrum of said source (5) by means of calculation of the Fourier transform of the aforesaid interferogram function (12) .

3. A spectrophotometer according to claim 1, wherein the means of processing and control (4) are arranged to:

- consider the interferogram function (12) of said source (5) as equivalent to a weighted linear combination of the interferogram functions of a plurality of substantially monochromatic reference sources, having respective predetermined frequencies; calculate the weight coefficient of the interferogram functions in said linear combination; and

- determine the emission spectrum of said source (5) as comprising all the frequencies of the reference sources, each weighted with the weight coefficient calculated for the respective interferogram function.

4. A spectrophotometer according to claim 3 , wherein the means of processing and control (4) are arranged to calculate the weight coefficient of the interferogram functions of the reference sources using the "generalized least squares estimation" method.

5. A spectrophotometer according to either claim 3 or 4 , wherein the frequencies of the reference sources are equally spaced in a predetermined range of frequencies .

6. A spectrophotometer according to any one of the preceding claims, wherein between the source (5) and the entry mirror (7) of the Fabry-Perot resonator (2) is interposed an objective lens (6) suitable for focusing the radiation originating from the source (5) on the photosensor matrix

(3) .

7. A spectrophotometer according to any one of claims 1 to 5 , wherein between the output mirror (8) of the Fabry-Perot resonator (2) and the photosensor matrix (3) is interposed an objective lens (6) suitable for focusing the radiation originating from the source (5) on the photosensor matrix (3) .

8. A spectrophotometer according to any one of the preceding claims, wherein the entry mirror (7) and the output mirror (8) are planar rigid mirrors.

9. A spectrophotometer according to any one of the preceding claims, wherein the entry mirror (7) and/or the output mirror (8) are composed of a film of plastics material.

10. A spectrophotometer according to any one of claims 1 to 8, wherein the Fabry-Perot resonator (2) comprises a plurality of microcavities in MEMS technology.

11. A spectrophotometer according to any one of the preceding claims, wherein the output mirror (8) is made with a partially transparent layer (11) deposited on the surface of the photosensors (3) .

12. A spectrophotometer according to any one of the preceding claims, wherein said modulator (9) comprises an actuator suitable for modifying the distance between said mirrors (7, 8) .

13. A spectrophotometer according to claim 12 , wherein the actuator (9) is suitable for being ^• actuated by a signal originating from a control unit (10) and is capable of varying the relative distance between the two mirrors (7, 8) from a minimum value (contact between the mirrors) to a predetermined maximum value .

14. A spectrophotometer according to either claim 12 or 13, wherein the actuator (9) is of electromechanical type.

15. A spectrophotometer according to either claim 12 or 13, wherein the actuator (9) is of piezoelectrical type.

16. A spectrophotometer according to either claim 12 or 13 , wherein the actuator (9) is of pneumatic type.

17. A spectrophotometer according to either claim 12 or 13, wherein the actuator (9) is of electrostatic type.

18. A spectrophotometer according to any one of claims 1^" to 11, wherein between the mirrors (7, 8) of the resonator (2) is interposed a material having a refractive index which may be varied by an electrical control, and said modulator comprises means of control suitable for causing controlled variations in the refractive index of said material .

19. A spectrophotometer according to claim 18, wherein said material is liquid crystal-based.

20. A spectrophotometer according to any one of the preceding claims, wherein the modulator (9) is calibrated by coupling to the spectrophotometer (1) a monochromatic light source of known wavelength, of which the interferogram is known, and comparing the interferogram thus measured (12) with the known interferogram.

21. A spectrophotometer according to any one of claims 12 to 17, comprising capacitive sensor means integrated into the entry and output mirrors (7, 8) and -wherein said means of processing and control (4) are arranged to measure the displacement of said mirrors (7, 8) to correct the movement of the actuator (9) or to associate a correct value for the distance (d) between the mirrors (7, 8) at each point of the interferogram (12) .

22. A spectrophotometer according to either claim 1 or 2, or a claim dependent on 1 or 2, wherein said means of processing and control (4) are arranged to calculate the value of said interferogram function (12) corresponding to the zero value of the length (d) of the optical path according to predetermined conditions, on the basis of the reflecting properties of the mirrors (7, 8) of the resonator (2) and of the asymptotic value of said function (I) .

23. A spectrophotometer according to claim 22, wherein said means of processing and control (4) are arranged to reconstruct values of said function (I) in a range of lengths

(d) of the optical path close to zero making the Fourier transform of said function (I) zero for values of the length

(d) of the optical path for which the transmissivity of the spectrophotometer (1) or the intensity of the radiation entering the resonator (2) is zero.

24. A spectrophotometer according to any one of claims 1 to 21, wherein said means of processing and control (4) are arranged to transform the spectrum obtained with the Fabry- Perot cavity spectrophotometer into a spectrum obtainable with other types of spectrophotometer or to increase the frequency or amplitude resolution.

25. A spectrophotometric process, in particular for spectral analysis of a source (5), comprising the operations of: setting up a Fabry-Perot resonator (2) including a partially transparent entry mirror (7) and a partially transparent output mirror (8) ;

- passing through the entry mirror (7) of said resonator (2) the radiation originating from the source (5) ;

- modulating the optical path of the radiation between said mirrors ( 7 , 8) ;

- sensing the radiation emerging from the output mirror (8) of the resonator and generating electrical signals representative of an interferogram function (12) which defines the intensity (I) of said radiation emerging in relation to the length of the optical path (d) between the mirrors (7, 8) , and

- generating, on the basis of said interferogram function (12) data representative of the emission spectrum of the source (5) .

26. A spectrophotometric process according to claim 25, in which are generated signals or data representative of the emission spectrum of said source (5) by calculation of the Fourier transform of said interferogram function (12) .

27. A procedure according to claim 25, wherein: the interferogram function (12) of said source (5) is considered as equivalent to a weighted linear combination of the interferogram functions of a plurality of substantially monochromatic reference sources, having respective predetermined frequencies;

- weight coefficients of the interferogram function in said linear combination are calculated; and ^•

- the emission spectrum of said source (5) is determined as comprising all the frequencies of the reference sources, each weighted with the weight coefficient calculated for the respective interferogram function.

28. A spectrophotometric process according to claim 27, wherein the weight coefficients of the interferogram function of the reference sources are calculated using the "generalized least squares estimation" method.

29. A spectrophotometric procedure according to either claim 27 or 28, wherein the frequencies of the reference sources are equally spaced in a predetermined range of frequencies .