EP1523663A1

EP1523663A1 - Device and method for optical spectroscopy, optical sensor and use of said device

Info

Publication number: EP1523663A1
Application number: EP02807594A
Authority: EP
Inventors: Thilo Weitzel
Original assignee: Campus Technologies AG
Current assignee: Campus Technologies AG
Priority date: 2002-07-15
Filing date: 2002-07-15
Publication date: 2005-04-20
Also published as: JP2005533244A; US20050248769A1; WO2004008092A1; US7466421B2

Abstract

The invention relates to a device and a method for optical spectroscopy, optical sensor and use of said device. According to the invention, a device with high spectral resolution yet with comparatively low quality requirements for the optical components may be achieved, whereby the device for optical spectroscopy comprises means for generation of an interference pattern, means for injection of the light field under investigation such that only one or individual spatial modes of the field are transmitted and a detector which can recorded the intensity of the interference pattern generated in a number of different spatial positions, whereby the wavefronts and/or the dispersion direction of at least one of the light fields in the interference pattern are altered as a function of wavelength due to spectral dispersive or diffractive optical elements. The invention further relates to a method for determining the optical spectrum and/or measured values encoded or transmitted by means of an optical spectrum by analysis of the interference pattern measured with said device, or by using said device.

Description

Device and method for optical spectroscopy and optical sensors and use of the device

The invention relates to devices and methods for optical spectroscopy and optical sensors.

Optical spectrometers can be divided into dispersive or diffractive spectrometers and Fourier transform spectrometers.

Dispersive (from prism) or diffractive (grating) spectrometers break down the incident light beam into its spectral components due to the wavelength dependence of a diffraction or reflection angle. The different spectral components are thereby spatially separated and the spectral component to be determined can be selected (monochromator). A spectrum is then recorded with the help of moving parts, in that the various spectral components are selected and measured in succession.

The most common are monochromators with a Czerny-Turner beam path, ie with a rotatable plane grating (diffraction grating in reflection) between an entrance and an exit slit and mutually independent collimator or collector mirrors. Collimator and collector create an image of the entry gap in the plane of the exit gap. The diffraction grating is located in the Fourier transform plane of this imaging system.

The development of spatially resolving detectors (CCD, diode array) now allows the simultaneous measurement of all spectral components by providing a separate element of the detector for each spectral component. Such an arrangement does not need any moving parts and uses the available incident light much more efficiently.

Fourier transform spectrometers are based on an interferometer, in which the difference in the optical path lengths of the partial beams brought into interference can be set with high precision. The spectrum can be determined from a measurement of the interference signal over a suitable range of path length differences by Fourier transformation.

Devices are usually constructed in the manner of a Michelson or Twyman Green interferometer. Technically, the mechanical components for adjusting the optical path lengths by means of movable mirrors or tiltable mirror pairs as well as the necessary collimator for generating flat wavefronts are particularly demanding.

Another variant of spectrometers uses static interference patterns generated by light beams which are brought to interference at a certain angle, e.g. Fizeau interferometer. The spectrum can be calculated by counting the interference fringes or by determining the spatial frequencies of the interference pattern with the aid of a numerical Fourier transformation.

A disadvantage of these interferometric spectrometers (both for Michelson-Twyman-Green interferometers with variable path lengths and for static interferometers with spatial interference patterns) is the fact that the relative spectral resolution is determined directly by the number of line pairs measured in the interference pattern (Fizeau stripes) is determined. Become N line pairs for counting a certain wavelength λ, the spectral resolution is on the order of λ / N.

A newer variant of Fourier transform spectrometers ("spatial heterodyne spectrometer") uses dispersive or diffractive optical elements (diffraction gratings) to change the angle between two collimated partial beams of a static interferometer depending on the wavelength and thus to increase the spectral resolution ,

The superposition of flat wavefronts is mandatory here in order to obtain Fizeau interferograms (Fizeau strips), which can be broken down into their spectral components after the measurement by means of a numerical Fourier transformation.

Such arrangements are further based on the translational invariance of the optical Fourier transform. The incident light is first collimated by a collimator. The collimated beam (flat wavefronts) is split (amplitude division) and guided over spectrally dispersive or diffractive elements, e.g. a diffraction grating. The spectrally dispersive optical element lies in the Fourier plane of the collimator. The again superimposed partial beams are then imaged by a collector and a further Fourier transform lens in such a way that a spatially resolving detector comes to rest in a Fourier transform plane of the entrance aperture.

Such arrangements, like Fourier transform spectrometers or conventional monochromators, are therefore dependent on high-quality imaging optical systems. In particular, relatively large focal lengths of the optical systems are required.

The possible performance of dispersive or diffractive spectrometers is dependent on certain parameters, in particular the dimensions of inlet or Exit slit, the focal length and aperture of the imaging elements and the Properties of the dispersive or diffractive element itself. Modern devices almost reach these physically set limits.

The possible performance of Fourier transform spectrometers is correspondingly determined by certain parameters and here in particular by the distance and the step size for the variation of the optical path lengths. The performance of Fourier transform spectrometers far exceeds the possibilities of dispersive or diffractive spectrometers.

Fourier transform spectrometers can also almost reach the physical limits of their performance, but the technical effort may be very high. Since Fourier transform spectrometers are based on an interferometer, all optical components and especially the moving parts must be manufactured and positioned with a precision of fractions of the wavelengths to be measured.

Spatially heterodyne spectrometers are technically less complex, but also require high quality imaging as well as dispersive or diffractive optical components.

The spectral resolution dλ at a wavelength λ of all the spectrometers mentioned is directly related to a corresponding coherence length I = λ ² / dλ.

In order to achieve a certain spectral resolution, the spectrometric arrangement must generate defined differences in the optical path lengths of at least length I.

Common to all the spectrometers mentioned is the need for a collimation of the incident light. The collimator is an imaging optical element of a certain focal length f, for example a concave mirror or a lens. The entry opening of the spectrometer is at the focal point of the collimator.

The spectrometers now explicitly use the special properties of the optical Fourier transformation, in particular the translational invariance of the Fourier transformation, i.e. the transformation of a translation in the focal plane to a change in the direction of propagation in the Fourier plane of the collimator.

Monochromators ("4f system": entrance slit - f - collimator - f - diffraction grating - f - collector - f - exit slit) influence the direction of light propagation in the Fourier plane of the imaging system through a diffraction grating and thus generate the desired spectral dispersion without the image to be significantly disturbed from the entrance slit to the exit slit or detector (I is defined by the geometry of the grating in the beam path, f »I). The collimator carries out an optical Fourier transformation, the collector takes over the optical inverse transformation and thus brings about the optical imaging of the entrance slit in the plane of the exit slit or of the detector.

Fourier transform spectrometers (2f system) absolutely require the collimator (usually with f much larger than I) in order to maintain the interference despite optical paths of different lengths, i.e. to match the wave fronts on the detector. In particular, the translational invariance of the Fourier transform is used here.

In a Fourier transform spectrometer, the numerical Fourier transform replaces the optical inverse transform used in the monochromator.

Fourier transform spectrometers with dispersive elements that evaluate a spatial interference pattern (spatial heterodyne spectrometer) require the collimator explicitly in the context of an optical Fourier transformation, on the one hand to avoid smearing the interference pattern despite a finally large entrance opening (translation invariance), on the other hand to the defined and clear attribution to establish the relationship between the optical spectrum and proportions of spatial frequencies in the resulting pattern, which forms the basis of the numerical back transformation.

These spectrometers also require additional imaging optics ("6f system": entrance slit - f - collimator - f - interferometer with diffraction grating - f - collector - f - exit aperture - f - imaging element - f detector level)

Since both interferometric arrangements and high-resolution imaging systems have to be realized by high-quality optics with possibly large focal lengths and a minimum size of the components or path lengths - depending on the respective exact arrangement - by the above. If the value I is fixed, the technical effort increases rapidly with increasing demands on the spectral resolution. A characteristic variable here is the so-called spectral aperture broadening (collimation broadening) that occurs despite collimation.

The object of the present invention is to provide a device and a method for realizing spectrometers with high spectral resolution and at the same time substantially lower demands on the quality of the optical components.

According to the invention the object is achieved by an interferometric device according to claim 1 and by the use and method claims.

The coupling of the light via defined spatial modes or a mono-mode coupling is essential for the implementation of an inexpensive and spectrally high-resolution spectrometer or sensor according to the invention. Under these circumstances, the aperture broadening disappears, in particular the interference pattern remains recognizable by a collimator even without optical Fourier transformation and can be evaluated with the aid of the methods shown. Such an optical spectrometer in combination with dispersive or diffractive optical elements for wavelength-dependent influencing of the wavefronts allows much more compact and flexible designs than previous approaches with imaging optical elements.

A prerequisite for the functioning of such structures is the measurement method shown or the method shown for orthogonalizing the measured interference patterns, since these cannot be evaluated directly using a numerical Fourier transformation.

Preferred embodiments of the invention result from the subclaims 2 to 34 following the main claim. Uses according to the invention result from claims 35 to 38 and a method according to the invention and preferred variants of variants derive from claims 39 to 48.

The invention comprises a device which combines dispersive or diffractive optical elements with an interferometer with the coupling of individual spatial modes and with a detector which can measure the intensity of the resulting interference pattern at a plurality of spatial positions, and a method which it allows the spectrum of the incident light or directly measured values that can be derived from such a spectrum to be reconstructed from an interference pattern measured in this way.

The device according to the invention is designed in such a way that the interference patterns of different spectral components of the spectral range to be examined differ greatly from one another. Such an interference pattern assigned to a specific spectral component is referred to below as the basic pattern. The patterns can be viewed one-dimensionally or two-dimensionally. An interference pattern generated by a device according to the invention is considered to be a superimposition of a number of different basic patterns. The interference pattern is recorded by the detector by measuring the intensities at a large number of discrete spatial positions. An interference pattern is therefore always in the form of a fixed number of (measured) values. Accuracy and representable spatial frequencies follow from the sampling theorem.

In the method according to the invention, an interference pattern is interpreted as a series of (measurement) values and thus in the context of linear algebra as a vector or in particular as an element of a sequence space of the corresponding dimension. In the context of linear algebra, the basic patterns introduced above are initially interpreted as linearly independent basic vectors of this sequence space.

The method according to the invention is based on the possibility of determining the respectively required basic pattern for a device according to the invention either by calculation or by measurement. In the method according to the invention, the spectrum of the incident light can then be obtained by breaking down the interference pattern into these basic patterns.

The particular advantages of the device and method for the realization of high-resolution or very compact optical spectrometers result from the optical mono-mode coupling, which allows the translational invariance of the optical transformation to be dispensed with and thus a collimator to be dispensed with. The device can therefore be implemented entirely without the use of imaging optical elements. This is possible in combination with the described methods, which use the fact that at least approximately a numerical inverse transformation of the interference signal measured at the detector to the sought spectrum can be found for almost any, sufficiently complicated optical transformations.

The process can be implemented in different variants, we introduce the following definitions for discussion: s be a spectrum represented by discrete spectral components of a certain intensity, ie as a vector with the components s _n n: 1..N.

s comprises a certain spectral range of the optical spectrum, the individual components are spectrally close to the spectral resolution under consideration.

i be the interference pattern measured at the detector, i is therefore a vector which, for example, represents the individual elements of an array detector with the components i _m m: 1..M

o be the spectrum reconstructed by the method as a measurement result or a vector that directly represents the measurement values derived from a spectrum, represented according to s by components O k: 1..K. If o represents a spectrum usually with K = N.

The optical transformation T can be represented as a matrix by T s = i. The evaluation is initially shown as a back transformation R by R i = o.

Under very favorable circumstances (good signal / noise ratio, fixed phase position, "spectrally dense" basic pattern) a direct (approximate) calculation of R as an inverse of T could take place, o then (approximately) becomes s.

The components (vectors) of the matrix T can be determined on the basis of the relationship Te _n = t _n , the e _n being the unit vectors of the spectral components. Of particular interest is the possibility to actually generate the spectral components e _n with the help of a monochromatic reference light source and to determine the t _n and thus the matrix T experimentally (reference or calibration measurement).

As a rule, it is not possible to determine R by inversion of the (measured) matrix T, but with known t _n the back transformation can approximate done by correlation. Different correlation methods are possible, a common method is "cross-correlation" based on the scalar product of the discrete Fourier transforms of the sequences or vectors to be compared. With the discrete Fourier transform F, o and thus approximately s can be calculated as o _n = ', F (i) F ^"1 (t _n )].

In the event that the optical transformation is an exact Fourier transformation, only one component of the expression F ^"1 (t _n ) will be non-0, namely that which represents the respective spatial frequency and thus directly a spectral component of the spectrum. Here are the base vectors t _{n are} not only linearly independent but also orthogonal and also form the unit vectors of the spatial frequencies, so for this special case the calculation from o is reduced to the Fourier transform of i.

However, the following two options deserve special interest:

The properties of the optical transformation may be similar to those of a Fourier transform, or the optical transformation may be completely irregular, i.e. e.g. Form so-called "speckle patterns" ("granulation").

The first case can be represented by a grossly faulty optical Fourier transformation, for example generated by an optical arrangement according to the invention without a collimator and with very inexpensive optical elements. Due to the systematic generation, the basic patterns are still linearly independent but only approximately orthogonal.

The second case can be represented by an optical arrangement according to the invention with an interferometer based on a scratched piece of broken glass (extremely inexpensive). The base vectors can be assumed to be statistically distributed here. In the first case, the method represents a correction, ie the poor quality of the optical transformation can be largely compensated for by an adapted reverse transformation.

In the second case, the spectrum is determined by a purely statistical correlation of the measured values with the base vectors. In this case, a high number of elements of the detector should be assumed, in particular it is favorable to choose M very much larger than N, for example by using a two-dimensional detector array. The basic patterns are not linearly independent due to their statistical nature. Nevertheless, the correlation for large N shows good results. Very good results are obtained for very large M, because in this case, i.e. the statistical distribution of N base vectors in an M-dimensional space, the base vectors become at least approximately linearly independent.

In this context, other correlation functions for the method can also be considered, in particular stochastic correlations.

A further calculation or refinement of the results by deconvolution is particularly advantageous if the selected method can be applied to a set of different transfer functions.

If the arrangement according to the invention is used as a sensor, it may be advantageous to aim for the results of the calculations and not the spectrum, but rather the measured values sought.

For a chemical sensor, the base vectors are then determined not by measuring spectral components but by recording spectra of the substances sought. A base vector and thus a component of the result vector thus does not represent a single spectral component but directly the measured value sought, ie, for example, the concentration of a specific substance in accordance with an absorption spectrum. The same applies, for example, to the measurement of layer thicknesses on the basis of the characteristic spectral modulation of light transmitted or reflected by thin layers.

This adaptive approach allows the implementation of optical sensors for a variety of applications. The evaluation of the measurements by correlation with previously recorded basic patterns allows the desired quantities to be determined directly without having to go through an analysis of the optical spectrum.

As far as the interference pattern, i.e. the basic pattern for the spectral components in question are linearly independent within the scope of the resolution and accuracy of the measurement, the respective spectral components of the incident light and thus the spectrum can be determined by correlating the respective basic pattern with the recorded interference pattern.

If the properties of all components of the device are determined with sufficient precision, the required set of basic patterns can be calculated.

Of particular interest is the possibility of using a suitable, adjustable monochromatic reference light source to measure a set of basic patterns for the respective specific structure of the device. Since the basic patterns in this case already contain all types of optical aberrations occurring in the respective device, the demands on the optical quality of the components of the device are relatively low, provided the basic patterns remain approximately linearly independent.

In Fourier transform spectrometers, the "perfect" interference patterns recorded are linearly independent (superposition of sinusoidal components) and the Fourier transformation represents an orthogonalization method. The individual Fourier coefficients represent the spectral components of the measured spectrum. A direct Fourier transformation of the patterns recorded with an arrangement according to the invention is pointless, but orthogonalization with respect to spectral components is possible after a suitable transformation of the recorded interference patterns. For this purpose, the relative path length difference of the partial beams brought to interference must be determined for each measuring point.

According to a preferred embodiment of the invention, the interference pattern can be generated by dividing the amplitude of the incident light field with the aid of a semitransparent mirror or a suitable grating (optionally in more than two partial beams) and then superimposing the partial fields on the location of the detector. All classical interferometers come into question here, which may be supplemented by dispersive or diffractive elements, for example: Michelson, Mach-Zehnder, Sagnac, Fabry-Perot or shear interferometers. Any arrangement that generates interference patterns with spatial periods that the respective detector can resolve is also suitable. By suitably dimensioning the device, the spatial frequencies occurring at the detector can be selected independently of the wavelength range to be examined in each case.

Furthermore, particularly favored by the restriction to individual spatial modes of the light field, the generation of the partial fields by dividing the wavefront is also possible, for example by means of a Fresnell biprism, other combinations of prisms or mirrors, with the help of irregularly shaped surfaces or also with the help of diffractive ones Elements.

The required spectral dispersion can in all cases be introduced by a suitable design of the beam splitter itself or by additional optical elements.

The detector can be moved through the interference pattern with a suitably small aperture (scan). It is also possible by moving other components of the device or with the help of an additional movable one Spiegel to record the different measuring points one after the other. This method is particularly suitable for extremely high-resolution measurements or in wavelength ranges for which no suitable spatially resolving detectors are available.

In the one-dimensional case, a suitable diode array or a CCD line is suitable as a spatially resolving detector.

The use of two-dimensional detectors (CCD or others) is particularly interesting, since in this case, with increasing the number of measured values, there is considerably greater scope for the properties of the basic functions and, in the case of “better” linearly independent functions, the respective correlations can be calculated more sharply.

The figures show preferred embodiments of the invention in different combinations of the different claims.

Figure 1 shows an extremely compact arrangement according to claim 1, wherein the optical components are integrated in a monolithic glass block. The light coupling (M) takes place directly from a monomode glass fiber into the block, so that the field initially develops as a spherical wave. A diffraction structure (G) applied directly to the glass block divides the amplitude of the wave according to claim 2 into a diffracted and a reflected component, which each run to one of the mirrors (S1, S2) applied directly to the glass block. The diffraction structure acts according to claim 27 both as a beam splitter and as a spectrally highly dispersive optical element which changes the wavefront of the diffracted beam in a spectrally dependent manner. In the further course, the subfields are reflected and overlaid again. The arrangement shown here works according to claims 28 to 30. The resulting field leaves the glass block over the free area. A second field consisting of non-used diffracted portions essentially strikes the surface of the glass body over which the spherical shaft is coupled. followed. This proportion should be absorbed by a suitable coating on this surface.

The detector (D) has a small spatial extension or has a suitable aperture and is located on a movable arm, shown with a pivot point (P). The detector is moved by the light field and records its intensity at a number of spatial positions one after the other. In the arrangement shown, the arm is moved by means of an eccentric (X) which is driven by a motor (R).

A set of such measurements, i.e. a set of measured values recorded at defined positions forms a pattern which can be evaluated with the aid of the methods according to claims 39-48.

An arrangement according to Figure 2 using a separate beam splitter (S) for dividing the amplitude of the waves according to claim 2 and two dispersive elements (G1, G2) in the arms of the interferometer is possible by a monomode coupling (M) according to claim 4. An aperture diaphragm (A) as shown is advantageous. Such an arrangement does not require Fourier transform optics or completely without imaging optical elements, since the translational invariance of the Fourier transform can be dispensed with. The evaluation of the interference pattern which such an arrangement produces cannot therefore be carried out directly by means of a numerical Fourier transformation, but rather requires one of the methods presented in claims 39 to 48. The arrangement shown in Figure 2 uses a spatially resolving detector (CCD) according to claim 9. A phase modulator (P) according to claim 14 has a particularly advantageous effect, for example in the form of the piezo actuator symbolized in the figure.

The possibility of recording interference patterns in the case of a large number of different relative phase positions of the fields involved offers the methods presented considerable advantages. In this case, the amount of the intensities recorded in each case by the spatially resolving detector forms a pattern which can be evaluated with the aid of the methods according to claims 39-48.

In addition to the advantages of arrangements according to claim 1, which arise from the possible complete omission of imaging optical elements, a monomode coupling in particular also permits interferometric arrangements based on a division of the wavefront according to claim 3. In addition to dispensing with imaging optical elements, this also allows dispensing with a beam splitter as a discrete optical element.

Figure 3 shows an arrangement according to claims 1 and 3. The prerequisite is a coupling (M), for example according to claim 4. The coupled light field propagates from M as a spherical wave. In the arrangement shown, the mirror (S) has a suitable opening through which the coupled field can pass. Part of the wave hits a diffraction grating (G1), another part hits a diffraction grating (G2), so the wavefront is divided. An aperture diaphragm (A) as shown is advantageous. The gratings deflect the light back onto the movable mirror (S) with the highest possible efficiency, where the wave fields are superimposed.

According to claim 8, the movable mirror reflects the resulting field onto the detector (D) which, depending on the position of the mirror, can record the intensity of the field at a number of different positions.

It is cheap but not absolutely necessary to provide a phase modulator according to claim 14, for example in the form of the piezo actuator (P) shown.

An alternative possibility according to claim 15 for generating different interference patterns, which can be used in the illustrated methods, can be realized in such an arrangement simply by spatially displacing the coupler. In this case, the pattern to be evaluated by a method according to one of Claims 39 to 48 is given by a set of measured values which have been measured for different positions of the mirror S.

The performance of the device and of the method described below can be significantly improved if the relative phase position of the partial beams can be suitably influenced. This can be done, for example, by using a mirror which can be displaced linearly over a distance of the order of the wavelength, by means of which the relative phase position of the reflected light can be changed with great accuracy, or e.g. in the case of a shear interferometer or e.g. in the case of a grating with several spatial frequency components as a beam splitter by means of a suitable "lateral" displacement of the components.

The interferometric devices shown can also be designed or developed in such a way that the differences in the optical path lengths under which the partial beams are brought to interference differ by a degree introduced by the dispersive element or elements. The interference is then limited to components of the incident light with a correspondingly high coherence length or small bandwidth.

An interference signal is only generated when the incident radiation shows coherence properties or autocorrelation properties in the region of the optical path length differences. When used in the field of optical spectroscopy, line spectra can be recorded selectively in this way. In this case, only spectrally narrow-band components of the incident radiation with correspondingly large coherence lengths contribute to the measured signal.

When used in the field of optical data transmission, carriers with defined autocorrelation properties can be selectively recorded or measured. will be. This is particularly interesting for an application in the field of coherence length multiplexing.

For both areas of application, the arrangement has the particular advantage that the spectral resolution (spectroscopy) or bandwidth (data transmission) can be set independently of the line width to be selected (spectroscopy) or autocorrelation length (data transmission).

Figure 4 shows an extremely compact and inexpensive possibility of realizing an arrangement according to the invention. A diffracting optical element (D) according to claim 11 is used in a function according to claim 27, in this case a diffuser with a granularity of a suitable size. A prerequisite for operation is that the light field (M) is coupled in the form of only one or fewer spatial modes according to claims 4 to 6. A suitable aperture diaphragm (A) as shown is advantageous. The variant shown expediently has an imaging detector (CCD) according to claim 10. Depending on the application, the diffuser can be replaced by diffractive elements which can generate a highly structured interference field. In this context, a variant of the Talbot or Lau effect can also be used, in particular the ability of certain structures to map themselves. If necessary, different interference fields can be generated by spatially displacing the coupling or displacing or tilting the diffuser according to claim 15.

This arrangement is advantageously operated with a very large number of measuring points for the interference field in combination with the statistical methods shown.

The selectivity of the arrangements can be improved by parts interacting repeatedly with the light fields, in particular if the arrangement allows multiple reflections or forms a resonator. Figure 5 shows an arrangement according to the invention according to claim 16 with this property. Again, the light field (M) must be coupled in according to one of claims 4 to 6 in order to generate recognizable interference fields. A suitable aperture diaphragm (A) as shown is advantageous. The resonator is formed according to claim 17 by the beam splitter (S) and a diffractive element (G) which simultaneously serves as a beam splitter via different diffraction orders. The field is coupled into the resonator via the beam splitter (S), and the resulting interference field is coupled out towards the detector (CCD) via the diffractive element (G). Further multiple reflected partial beams also contribute to the interference.

As a diffractive element, in addition to simple gratings on the one hand and complex diffraction structures on the other hand, multiplex gratings (superimposition of several spatial frequencies) or multiply divided gratings, for example as shown in Figure 6, are suitable diffractive element (G) in the form shown is realized by stripes lying next to one another with different lattice constants. The part of the field reflected by the respective gratings (0th diffraction order) leaves the resonator, while the part of the light field diffracted by the gratings (provided a suitable wavelength is provided) initially remains in the resonator and partly again the diffractive element via the beam splitter (S) reached.

The technical design of the resonator is of minor importance. In addition to simple resonators with only two components, all types of resonators, in particular ring cavities, are also possible.

The multiple reflections result in very complex patterns, which are preferably treated with the aid of the statistical methods mentioned in the method claims (cross correlation) with a large number of measured values. A further embodiment according to the invention provides that the device has means for rotating the interferometer or means for changing or selecting the angle of incidence, which enable the spatial frequency or the spatial frequencies of the generated interference pattern to be set.

The wavelength range that the arrangement can detect without moving parts is given by the ability of the detector to detect the corresponding spatial frequencies in the interference pattern. It can be particularly advantageous for a technical implementation of the arrangement to select a wavelength range, i.e. in this case, the setting of the interferometer such that the spatial frequencies resulting for this wavelength range can be detected by the detector by rotating the interferometer as a whole or by a suitable change in the angle of incidence. For this type of construction, the interferometer itself does not need any movable elements, apart from the means that may be required for phase modulation, and can nevertheless be used for different wavelength ranges.

In this case, the components of the interferometer can be fixed against each other, which has an advantageous effect on the stability of the adjustment. A prerequisite for the wavelength adjustment via the angle of incidence is that the angle at which the subfields are superimposed in the interferometer shows a suitable dependence on the angle of incidence. This is e.g. the case when the subfields are superimposed in mirror image, i.e. the subfields must be guided in an asymmetrical interferometer in this regard over a number of mirrors that is different by 1 in each case.

According to a further advantageous embodiment of the invention, this situation can be achieved in the case of symmetrical interferometers by using a dihedral or retroreflector. Figure 7 shows a particularly advantageous arrangement according to claim 30. The light field is coupled in according to one of claims 4 to 6 (M). The aperture diaphragm (A) limits the solid angle to avoid stray light.

The light field then strikes a diffractive structure according to claim 27 or 28 (diffraction grating), preferably designed as a grating or multiplex grating. Holographic optical elements can be used very advantageously at this point. The reflected part of the field hits a mirror (S2), the diffracted part of the field hits another mirror (S1). Portions of the respective subfields are reflected back from the mirrors to the diffractive element and are superimposed there by partial reflection and diffraction to form two interference fields. One of these interference fields reaches the detector (CCD) as described in claim 30. The patterns recorded by the detector can then be processed numerically in the manner already shown. Other parts of the fields leave the arrangement unused. The actuator (phase shifter) shown in one of the mirrors (S2) enables interference patterns to be recorded at different relative phase positions of the subfields.

A particularly advantageous combination is the arrangement shown in Figure 8. In addition to the element already shown in Figure 7 for coupling the light field (M), an aperture diaphragm (A), mirror (S1, S2), a diffractive element (diffraction grating) and the detector (CCD), an imaging optical can Element (L) and an exit aperture (A2) can be used. The exit aperture limits the variability of the interference patterns that occur. In the event that the diffractive element is a diffraction grating, the exit aperture can also restrict the wavelength range of the fields that can reach the detector.

The correlation of a measured interference pattern required for a measurement with the interference pattern known for a specific spectral component or a group of spectral components can very advantageously be immediate optically with the aid of a mask and, if appropriate, suitable phase modulation or other detuning of the interferometer.

In particular, the interference pattern of a spectral fingerprint with many spectral components can already be contained in a single mask.

The multiple recording of the interference pattern through the mask upstream of the detector with different relative phase positions of the partial beams shows a strong dependence of the respectively measured integrated total intensity of the signal on the relative phase position only for those spectral components of the incident light with the resulting interference patterns the mask correlates.

A direct optical correlation is far superior to numerical methods under favorable circumstances. This design of the arrangement becomes particularly interesting when using a variable mask, such as an LCD screen (spatial light modulator, SLM). A variable amplitude mask (SLM), which can represent different patterns for optical correlation, is relatively easy to implement, since the mask is no longer part of the actual interferometer.

According to a further advantageous embodiment of the invention, the change in the relative phase position of the interfering subfields and the change in the spatial frequency or the spatial frequencies of the generated interference pattern are carried out jointly by moving at least one component of the device.

It is advantageous to carry out measurements with different relative phase positions of the subfields. If the optical path lengths of the subfields are not equal and / or the tilting of the optical elements leads to a change in the difference in the optical path lengths of the subfields, then the relative phase position of the interference pattern also changes when the wavelength is set. This effect can be used directly for measurements at different phase positions the. This is particularly advantageous for a technical implementation, since a separate mechanism for modulating the phase position can then be omitted.

The rotation of one of the optical elements around a base P outside the beam path causes not only the change in the angle and thus the setting of the selected wavelength, but also a change in the optical path length and thus a modulation of the relative phase position.

According to a further advantageous embodiment of the invention, the spectrally dispersive or diffractive element is a multiplex grating, a multiplex hologram, a holographic-optical element or a computer-generated hologram (CGH).

When using a two-dimensionally resolving detector, it can be particularly advantageous to use spectrally dispersive elements which not only cause a simple deflection of the respective partial beam. The generation of more complicated interference patterns appears to be advantageous, particularly in connection with the correlation methods shown. Such complex patterns may show a sharper correlation signal as a simple stripe pattern.

When using a periodic diffraction grating (in contrast to a normal Fourier transform spectrum!), The positions of the same optical path length and thus maximum amplitude or modulation for the different wavelengths are at different locations on the detector. This has a favorable effect on the required dynamic range of the detector elements.

For special applications, for example in chemometry the detection of a substance by determining spectral "fingerprints" in certain areas of an absorption spectrum, or the simultaneous determination of certain spectral lines, special diffraction gratings can be used - as in the other arrangements according to the invention. In addition to spatially separated or spatially superimposed multiple grids and, if appropriate, an arrangement with several detectors, holographic elements can also be considered here, which can, for example, bend entire groups of different spectral lines at the same angle. This variant can be particularly favorable when using a detector that uses a mask to recognize patterns (optical correlation method).

Claims

claims

1. Apparatus for optical spectroscopy with means for generating an interference pattern and with means for coupling the light field to be examined such that only one or individual spatial modes of the field are permitted, and with a detector that spatially determines the intensity of the generated interference pattern can record different positions, the spectral dispersive or diffractive optical elements changing the wave fronts and / or the direction of propagation of at least one of the light fields involved in the interference pattern depending on the wavelength.

2. Device according to claim 1, characterized in that the means for generating the interference pattern comprise a division of the amplitude of the incident light.

3. Device according to claim 1 or 2, characterized in that the means for generating the interference pattern comprise a division of the wavefront of the incident light.

4. Device according to one of claims 1 to 3, characterized in that the means for coupling the light to be examined only allow a precisely defined spatial mode (spatial single mode).

5. The device according to one or more of claims 1 to 4, characterized in that the means for coupling the light to be examined comprise a spatial filter.

6. The device according to one or more of claims 1 to 5, characterized in that the means for coupling the light to be examined comprise a mono-mode light guide (single mode fiber).

7. The device according to one or more of claims 1 to 6, characterized in that the detector can be moved with respect to one or two spatial degrees of freedom through the interference pattern (scanning detector).

8. The device according to one or more of claims 1 to 7, characterized in that the interference pattern can be imaged onto the detector via optical elements which are movable with respect to one or two spatial degrees of freedom (scanning detector).

9. The device according to one or more of claims 1 to 8, characterized in that it has a spatially one-dimensional resolving detector (array detector).

10. The device according to one or more of claims 1 to 8, characterized in that it has a spatially two-dimensional resolving detector (array detector).

11. The device according to one or more of the preceding claims, characterized by at least one diffractive optical element which has non-periodic diffraction structures.

12. The device according to one or more of the preceding claims, characterized in that the beam splitter or beams influence the wavefront of at least one of the partial beams or light fields depending on the wavelength (spectrally dispersive beam splitter).

13. The device according to one or more of the preceding claims, characterized in that optical elements influence the wavefront and / or the optical path length of at least one of the partial beams or light fields depending on the wavelength (spectrally dispersive optical elements).

14. The device according to one or more of the preceding claims, characterized by means which allow a change or modulation of the relative phase position (phase shifter / phase modulator) of at least one of the partial beams or light fields.

15. The device according to one or more of the preceding claims, characterized by means which allow a change or modulation of the spatial position (translation and / or tilting) of at least one of the subfields and / or the incident light field.

16. The device according to one or more of the preceding claims, characterized in that the device or parts of the device form an optical resonator.

17. The apparatus according to claim 16, characterized in that one or more wavelength-dependent elements are arranged in the interior of the resonator or at least one element of the resonator is designed to be wavelength-dependent (spectrally dispersive element).

18. The device according to one or more of the preceding claims, characterized in that the device or parts of the device are designed multiple times.

19. The device according to one or more of the preceding claims, characterized in that the difference in the optical path lengths of the beams or light fields brought to interference can be changed.

20. The device according to one or more of the preceding claims, characterized in that the device has means for adjusting the path length difference of the partial beams or light fields brought to interference, whereby a selection of the light components contributing to the interference can be carried out according to their coherence properties (coherence length).

21. The device according to one or more of the preceding claims, characterized in that the interferometer comprises a retroreflector or dihedron.

22. The device according to one or more of the preceding claims, characterized in that the device has means for rotating the interferometer or means for changing or selecting the angle of incidence, which allow adjustment of the spatial frequency or the spatial frequencies of the generated interference pattern.

23. The device according to one or more of the preceding claims, characterized in that the device has means for changing the position of components of the device, in particular means for rotating the components, which allow adjustment of the spatial frequency or the spatial frequencies of the generated interference pattern.

24. The device according to one or more of the preceding claims, characterized in that the change in the relative phase position of the interfering partial beams or light fields and the change in the spatial frequency or the spatial frequencies of the generated interference pattern is carried out jointly by moving at least one component of the device.

25. The device according to one or more of the preceding claims, characterized in that the spectrally dispersive or diffractive element is a multiplex grating, a multiplex hologram, a holographic-optical element or a computer-generated hologram.

26. The device according to one or more of the preceding claims, characterized in that the resulting interference pattern or parts of the interference pattern comprise a plurality of spatial frequencies and / or comprise a continuous spectrum of spatial frequencies.

27. The device according to one or more of the preceding claims, characterized in that a diffractive optical element is used simultaneously as a beam splitter and as a wavelength-dispersive element.

28. The device according to claim 27 with a diffractive optical element used as a beam splitter, characterized in that the means for generating the interference pattern comprise precisely this or a similar element for recombining the divided beams or light fields.

29. The device according to claim 27 or 28, characterized in that the partial beams or light fields are generated by a diffraction grating under different diffraction orders and optionally including the undiffracted or reflected partial beam or light field ("0th order") by suitable means are reflected back to the diffraction grating and from there are again superimposed by diffraction of different orders.

30. The device according to claim 28 and 29, characterized in that two mirrors are provided, through which partial fields emanating from the diffraction grating or diffractive optical element are reflected back to this diffraction grating or diffractive optical element, at least one of the mirrors being arranged so as to be displaceable is that the relative phase position of the reflected light can be changed, and the coupled light field is first divided on the diffraction grating or diffractive optical element in such a way that a reflected portion reaches one of the mirrors while a diffracted portion reaches the other mirror, and that of the Mirrors of the fields reflected back to the diffraction grating or diffractive optical element are again superimposed on the detector by the diffraction grating or diffractive optical element such that a portion of the part previously reflected on the diffraction grating or diffractive optical element field by diffraction reaches the detector while a portion of the partial field previously diffracted on the diffraction grating or diffractive optical element reaches the detector by reflection.

31. The device according to one or more of the preceding claims, characterized in that an imaging optics and an aperture is provided in the image plane of the beam path.

32. Device according to one or more of the preceding claims, characterized in that the detector via a spatial mask has that correlates with at least one interference pattern to be recognized (optical correlator), wherein the mask can be designed to be fixed or changeable (spatial light modulator).

33. Device according to one or more of the preceding claims, characterized in that the ability of the detector to recognize a spatial modulation is realized in such a way that a primarily non-spatially resolving detector is combined with a suitable spatial, optionally movable, mask.

34. Device according to one or more of the preceding claims in combination with a spectrally selective filter and / or a spectrally selective detector.

35. Use of a device according to one or more of the preceding claims as an optical spectrometer.

36. Use of a device according to one or more of the preceding claims for optical spectroscopy, components of the incident light being selectively measured according to their coherence lengths or coherence properties in accordance with the respectively set path length difference of the interfering partial beams.

37. Use of a device according to one or more of the preceding claims as a chemometric sensor.

38. Use of a device according to one or more of the preceding claims as a layer thickness measuring device or distance sensor.

39. Method for determining the optical spectrum and / or measured values coded or transmitted by an optical spectrum by analysis of the interference pattern measured with a device according to one of claims 1 to 34 or using a device according to one of claims 35 to 38.

40. The method according to claim 39, characterized in that it comprises a Fourier transformation of the interference pattern or the representation of the interference pattern as a linear combination of sine and / or cosine function (e.g. Hartley transformation).

41. The method according to claim 39 or 40, characterized in that the determination of the spectrum comprises the decomposition of the measured interference pattern (s) in a set of device-dependent basic patterns, in particular the determination of a spectral component by correlating the interference pattern (s) with one for the respective device and the basic component to be determined spectral component.

42. The method according to claim 41, characterized in that the determination of the spectrally coded measurement values comprises the decomposition of the measured interference pattern (s) in a set of device-dependent basic patterns, in particular the determination of the spectrally coded measurement values by correlating the interference pattern (s) with a for the respective device and the basic pattern to be measured.

43. The method as claimed in one of claims 41 to 42, characterized in that the basic patterns required for determining the spectral components or spectrally coded measured values are obtained by a measurement.

44. The method according to one or more of claims 41 to 43, characterized in that the determination of the spectrum or the or the spectrally coded measurement values includes the recording of different interference patterns at different relative phase positions and / or starting from different spatial modes, in particular using at least one of the means for varying the generated interference patterns mentioned in claims 14, 15, 19, 20, 22, 23 or 24 ,

45. The method according to one or more of claims 41 to 44, characterized in that the determination of the basic pattern comprises the recording of different interference patterns at different relative phase positions and / or starting from different spatial modes, in particular using at least one of the claims in claims 14, 15, 19, 20, 22, 23 or 24 means for varying the interference pattern generated.

46. The method according to one or more of claims 41 to 45 such that instead of a measured interference pattern or basic pattern, numerical transformations or functions of one or more interference patterns are used.

47. A method for creating patterns in accordance with the method according to claim 46 such that the method comprises determining or measuring the difference in the optical path lengths of the sub-fields brought to interference for the individual measuring points of the patterns and sorting or indexing the individual measured values depending on the difference of the optical path lengths of the sub-fields brought into interference for the measuring point.

48. A method for creating basic patterns according to the method according to claim 46 in such a way that a Fourier or Hartley transformation is carried out as a result of a transformation according to claim 47 (orthogonalization method for basic patterns).