EP1468244A2

EP1468244A2 - Interferometric optical system

Info

Publication number: EP1468244A2
Application number: EP03701534A
Authority: EP
Inventors: Thilo Weitzel
Original assignee: Scinex AG
Current assignee: CAMPUS TECHNOLOGIES AG
Priority date: 2002-01-21
Filing date: 2003-01-21
Publication date: 2004-10-20
Also published as: WO2003060422A3; JP2005515414A; AU2003202583A1; US20050117169A1; AU2003202583A8; US7161683B2; WO2003060422A2; DE10202120A1

Abstract

The invention relates to an interferometric optical device with spectral dispersion comprising a light source, elements for generating a phase shift, elements for measuring the intensity of the interference signals, elements for selectively measuring the intensity of the interference signals and elements for determining the phase angle and/or a relative phase shift of the intensity of the interference signals. According to the invention, the elements for generating a phase shift between components with a different polarisation direction in at least one of the branches of the interferometer comprise a diffraction grating. The elements for selectively measuring the intensity of the interference signal in accordance with the polarisation also permit the determination of the respective intensity for the TE and TM components of the interference signal, based on the co-ordinate system of the diffraction grating.

Description

Interferometric optical arrangement

The invention relates to dispersive interferometric optical devices which comprise a diffraction grating and measurement methods which use the special properties of these devices with respect to the relative phase positions of the two polarization components of the detected light.

The invention further encompasses the use of these devices and methods

for measuring the profile depth or the spatial modulation stroke of optical diffraction gratings,

to measure changes in an optical path length, be it by spatial change or by variation of a refractive index,

- for measuring lateral displacements of a diffraction grating,

- To measure a relative phase shift generated by a birefringent material between components of different polarization

- For measuring a rotation of the polarization (optical activity of the sample) generated by a irradiated sample and a spectroscopic measurement method for spectrally resolved determination the complex-value refractive index of a material, ie simultaneously absorption and refractive index or spectral variation of the refractive index and possibly polarization-dependent properties of a sample material introduced into an arm of the interferometer.

In addition to the actual shape of the periodic profile and the complex refractive index of the material used, the most important parameter that determines the diffraction efficiency is the profile depth of a diffraction grating working in reflection or, more generally, the modulation stroke of any periodic diffraction structure or hologram. The profile depth must therefore be controlled with great accuracy when manufacturing or testing such elements.

The following discussion refers to diffraction gratings operating at large angles in reflection without restricting generality. However, the relationships apply equally to transmission gratings and to both phase and absorption gratings and thus also to equivalent diffraction structures such as acousto-optic modulators or volume holograms.

Fig. 1 shows the surface of a sinusoidal diffraction grating (G) and the directions of an incident collimated light beam (I), the reflected beam (0) and the diffracted beam (-1) with the associated angles (θ |, θo, θ.j. ) compared to the grid normal. The designations "0" and "-1" stand for the zeroth and first diffraction order, respectively. No further diffraction orders appear in the technically advantageous geometry shown. The arrangement shown and similar arrangements, such as the autocollimation or Littrow arrangement (θι = θ.ι), show high spectral resolution and the diffraction efficiency shows no so-called anomalies (Wood's Anomaly) over a wide range of angles or wavelengths In connection with higher diffraction orders.

Of particular interest is the fact that the so-called equiva- lenz's theorem applies: the diffraction efficiency of such gratings as a function of angle and wavelength is determined solely by an effective profile depth and is largely independent of the actual shape of the profile (see Equivalence of ruled, holographic, and lamellar gratings in constant deviation mountings; M. Breidne, D. Maystre; Applied Optics 19, 1812-1821 (1980)).

The equivalence theorem can be easily understood if the respective periodic grating profile is represented as a Fourier series and the interaction of the incident light with the respective components of the Fourier series. The grating period itself defines the main component of the Fourier series and is therefore responsible for the first diffraction order, the other components generate the higher diffraction orders. If the geometry does not allow higher diffraction orders, the corresponding components of the Fourier series play no role and the grating behaves like a sinusoidal grating with an effective optical profile depth corresponding to the main component of the Fourier series. It is therefore possible to characterize such gratings largely by an effective optical profile depth.

FIG. 2 shows the diffraction efficiency to be expected for such a grating made of an electrically highly conductive material, such as a holographically produced gold-vapor-deposited grating as a function of the relative profile depth. The relative profile depth is the ratio of the actual profile depth to the spatial period of the grid.

The diffraction efficiency depending on the profile depth is very different for the two possible components of the polarization of the incident light beam: TE-polarized light (E-field component parallel to the grating lines) is influenced much less than TM-polarized light (E-field component perpendicular to the grid lines).

The effect can be explained by the fact that TE-polarized light, in contrast to TM-polarized light, only alternates with part of the actual grating profile. interaction can occur, since electrons in the upper layers of the lattice profile ie in the tips of the profile are only movable along the lattice lines.

Accordingly, FIG. 2 initially shows a rapid increase in the efficiency for TM-polarized light with increasing relative profile depth, while the efficiency for TE-polarized light increases only slowly. In the example shown, the efficiency for TM polarization reaches a maximum at a relative profile depth of approx. 0.32, then drops again as a result of the "overmodulation" to a minimum at approximately twice the profile depth and then strives for a further maximum while the TE component is still slowly approaching its first maximum.

Diffraction gratings or arrangements which show the same diffraction efficiency for TM and TE polarized light are of particular technical interest. When used in spectrometers, for example, the intensity of the incident radiation can then be measured independently of the respective polarization. In Fig. 2, such a grid is characterized by a relative profile depth at which the curves for the TM and TE efficiency intersect, that is to say in the example shown at relative profile depths of approximately 0.66 or approximately 0.83.

The determination of the effective optical profile depth of a grating is therefore of particular interest for both manufacturers and users of diffraction gratings. In addition to measuring the efficiencies for diffraction and reflection, depending on the angle and / or wavelength, by means of suitable, precise measurement of the intensities of an incident and the diffracted or reflected beams for calculating the effective optical profile depth, raster tunnel and raster force microscopes are also currently being used used to measure grid profiles directly. The determination of the profile depth based on the measurement of the different intensities is relatively imprecise, if not very high demands are made on the precision of the measuring apparatus and its calibration. Determining the effective optical profile depth with the aid of scanning force or scanning tunnel microscopy is technically demanding and requires a great deal of effort with regard to the physical-mathematical models which the measured "force surfaces" or "tunnel current surfaces" in suitable computing methods to determine the optical properties.

The present invention relates to an interferometric arrangement which relates the optical profile depth of a diffraction grating to the relative phase shift of the two measurement signals recorded for the different polarization directions, which are generated when the optical path lengths are varied.

Variants of this arrangement can therefore be used equally for measuring the profile depth of a grating or - if the properties of the grating are known - for measuring changes in the optical path length. The path length measurement also allows the determination of refractive index changes.

If the spectral selectivity of the arrangements is also used, the arrangement can be used as a spectrometer, which, depending on the wavelength, not only absorbs a sample that has been irradiated, but also the spectral variation of the refractive index and possibly polarization-dependent parameters, such as optical activity (rotation the polarization) or an anisotropy of the refractive index (birefringent material).

The fact that TM and TE polarized light beams not only "see" different profile depths of a diffraction grating is used here, since TM and TE waves interact differently with the structured surface and are therefore diffracted or reflected with different efficiencies, but that TM and TE waves perceive the diffraction grating at slightly different spatial positions. For the diffracted and reflected beams, there is a difference in the optical path lengths between the TM and TE components, which depends on the profile depth and the respective arrangement, and thus a phase shift between the TM and TE components. The exact relationship between efficiencies and phase shift can be theoretically determined on the basis of Maxwell's equations and the material properties using various mathematical methods (see Topics in Current Physics: Electromagnetic Theory of Grätings; R. Petit Editor; Springer Verlag 1980).

In the case of volume structures i.e. Layer systems or volume holograms occur with precisely corresponding phase shifts as a result of the refractive index dependency of the phase jump upon reflection at or transmission through a surface.

In general, the relationship can also be derived from the dispersion relations: Different spectral properties - as in this case with regard to the TM and TE components - must have a corresponding effect on the relative phase position of the respective reflected or diffracted rays. This connection also suggests the opposite: that with the same efficiencies i.e. same spectral properties the relative phase shift disappears.

Such relative phase shifts can be determined very precisely with the help of an interferometric setup if the possibility is provided to measure TM and TE components individually or independently of one another.

The type of interferometric arrangement is of minor importance, both the measurement of a fixed spatial interference pattern with corresponding fixed path length differences and the dependence of the intensity on a path length change in a Michelson, Mach-Zehnder or Fabry-Perot-like arrangement can be used become. Interferometric arrangements comparable to the variant shown in Figure 6a, which uses the grating itself as a beam splitter, are also particularly suitable. For TM and TE components, periodically occurring maxima and minima of the intensity depending on the path length differences in the interferometer can be measured, which show a phase shift dependent on the effective optical profile depth. This phase shift can be determined very precisely with little effort, in particular the determination of the phase shift does not require the measurement of absolute intensities or intensity ratios and it is very robust against various disturbances, such as a constant background or noise superimposed on the measurement signals.

Michelson interferometers are known for use as a path length sensor, which, with the aid of birefringent elements and polarization-dependent measurement of the interference signal, deliver two signals which are phase-shifted by 90 °. One possibility is an opposing circular polarization of the beams brought into interference from the two arms of the interferometer by means of suitable birefringent optical elements. Using a polarizing beam splitter, two linearly polarized components can be obtained, the intensity profile of which has a phase shift of 90 ° when the path length changes.

A sufficient, constant phase shift - also deviating from 90 ° - always allows both the direction of the movement to be determined using the two signals and a more precise determination of the position based on the numerical determination of the current phase angle (see Figures 5a, 5b).

In contrast to such an arrangement, an arrangement according to the invention for path length measurement, for example according to FIG. 6b, has significant advantages.

3 shows a technical arrangement according to the invention, implemented in the manner of a Michelson interferometer: a collimated, monochromatic light beam first falls through a suitable aperture (A) onto a non-polarizing beam splitter (np BS). The incident beam is either non-polarized or polarized at approximately 45 °. The incident beam shows in relation to the Dinate system of the diffraction grating (G) each TE and TM components, which are equally divided by the non-polarizing beam splitter on the arms of the interferometer. One arm of the interferometer is closed by a mirror (M), which throws the beam back to the beam splitter. The second arm of the interferometer is closed by the diffraction grating (G) to be examined in a Littrow or autocollimation arrangement, ie the beam diffracted by the grating is thrown back to the beam splitter. With the help of additional optical elements, the reflected beam or a beam diffracted at a different angle could also be detected.

In the arrangement shown, the optical path length of the first arm can be changed in a defined manner by a linear actuator (L), which can move the mirror (M) along the optical axis. The optical path length of the second arm of the interferometer to the grating remains constant, but is effectively different due to the phase effect of the diffraction grating shown above for the two components of the polarization.

The partial beams from the two arms of the interferometer are again superimposed by the non-polarizing beam splitter (np BS) and reach the polarizing beam splitter (p BS), which separates TE and TM components and supplies separate photodetectors (D1, D2). The intensities of the interference signals for TE (Sig. TE) and TM (Sig. TM) components are thus recorded independently of one another. In the technical implementation of the arrangement, further optical components may be required, such as the collimator (C) shown.

The detector signals are fed to a measuring arrangement which - for example by means of numerical methods - determines the phase shift sought.

FIG. 4 shows an example of the signals of the two detectors (Sig. TM, Sig TE) depending on the path length difference introduced by the linear actuator. The signals are sinusoidal and show a period corresponding to half the wavelength of the monochromatic light used and a phase shift Exercise of the two periodic interference signals depending on the effective profile depth of the diffraction grating. The phase shift of the signals is therefore a measure of the profile depth of the grating. This profile depth can therefore be determined with great accuracy using such measurement data, even if neither the intensity measurement nor the path length scale are calibrated to absolute values.

5 b shows a general example of the two measured signals represented as a Lissajous figure or XY diagram. The angle &, which is a measure of the current phase angle, can be recognized reliably even with signals of unequal amplitude and in the presence of noise and offsets.

6a shows a particularly advantageous technical embodiment of an arrangement according to the invention, which uses the diffraction grating at the same time as a beam splitter and for generating the phase shifts. In addition to the change in path length in one of the arms of the interferometer, this arrangement also advantageously allows the phase angle to be continuously rotated by a lateral movement of the grating, i.e. without actually changing the path length. Such an arrangement is advantageous for light sources with a short coherence length.

6b shows the arrangement supplemented by collimator and collector lenses as well as defined entrance and exit apertures. This particularly advantageous arrangement uses the spectral dispersion of the diffraction grating to select a defined wavelength. The wavelength can be adjusted precisely by changing the angle at which the beams hit the diffraction grating.

This is particularly advantageous if, instead of a laser, a broadband light source is to be used for the precise measurement of path lengths, since the wavelength of the measurement serves as a benchmark.

Examples of using arrangements according to the invention as novel spectrometers are shown in FIGS. 7a and 7b. 7a is first described: After passing through an aperture (A), the non-polarized, collimated beam from a light source (Source) reaches a non-polarizing beam splitter (np BS), which generates two partial beams. One partial beam reaches the diffraction grating (G) via a mirror (M), the other partial beam reaches the diffraction grating (G) after passage of a sample volume (sample). The diffraction grating is rotatably mounted (see illustration, grating lines and axis of rotation perpendicular to the plane of the drawing). The diffraction grating bends the partial beams back (Littrow arrangement), a spectral component of the respective partial beams that is dependent on the respective angle reaching the beam splitter exactly again. The superimposed partial beams reach an exit slit (E) via the collector lens (C) and then the polarization-sensitive detector.

In the example shown, the polarization-selective detector is implemented with the aid of a lens (L) which bundles the light coming from the aperture (E) through a polarizing beam splitter (p BS) onto two detectors (D1, D2).

A particularly interesting variant of this arrangement (FIG. 7b), by means of a pair of mirrors (M1.M2), causes the partial beams to be overlaid with opposing spectral dispersion after passage through the interferometer. This leads to a spectrally high-resolution selection of the interference signal (cf. DE 198 01 469 A), even with a wide exit gap (E).

Claims

Interferometric optical arrangement

Spectrally dispersive interferometric optical device with

A light source which radiates light into the interferometer,

Means for generating a phase shift between components of different polarization directions in at least one of the branches of the interferometer,

Means for measuring the intensity of the interference signal as a function of the relative phase position and / or optical path lengths of the partial beams brought to interference,

Means for selectively measuring the intensity of the interference signal as a function of the polarization or for separately measuring components of different polarization,

And means for determining the phase angle and / or a relative phase shift in the intensity of the interference signals between the measured components of different polarization, characterized,

that the means for generating a phase shift between components of different polarization directions in at least one of the branches of the interferometer comprise a diffraction grating and that the means for selectively measuring the intensity of the interference signal as a function of the polarization determine the respective intensity for the TE and TM components allow the interference signal based on the coordinate system of the diffraction grating.

2. Device according to claim 1, characterized in that the diffraction grating itself is used as an optical beam splitter and / or as an optical element for superimposing the partial beams.

3. Device according to claim 1, characterized in that the diffraction grating is arranged in the beam path of one of the partial beams of the interferometer.

4. The device according to claim 3, characterized in that the diffraction grating is arranged in an autocollimation or Littrow arrangement in the beam path of one of the partial beams of the interferometer.

5. The device according to one or more of the preceding claims, characterized in that the light source comprises a collimator.

6. The device according to one or more of the preceding claims, characterized in that the detector arrangement comprises a collector with an aperture in focus, in such a way that a spectral component of the incident light is selectively detected.

7. The device according to one or more of the preceding claims, characterized in that means are provided with the help of the or at least one of the angles at which the light beam or beams hit the diffraction grating can be changed. ,

8. The device according to one or more of the preceding claims, characterized in that the means for measuring the intensity of the interference signal at different relative phase positions comprise means for lateral movement of the diffraction grating.

9. The device according to one or more of the preceding claims, characterized in that the means for measuring the intensity of the interference signal at different optical path lengths of the sub-beams brought into interference comprise movable optical elements for changing the path length in the beam path of at least one of the sub-beams.

10. The device according to one or more of the preceding claims, characterized in that the means for measuring the intensity of the interference signal allow a time-dependent change in the relative phase positions and / or change in the optical path lengths of the partial beams brought to interference and the measurement of the intensity signals by suitable detectors time-dependent he follows.

11. The device according to one or more of the preceding claims, characterized in that the means for measuring the intensity of the interference signal at different optical path lengths of the sub-beams brought to interference comprise a spatially resolving detector which can detect a spatial interference pattern.

12. The device according to one or more of the preceding claims, characterized in that the means for selectively determining the intensity of the interference signal depending on the polarization, in particular the determination of the respective intensity for the TE and TM components of the interference signal with respect to the coordinate system of the diffraction grating comprise rotatable polarization filters.

13. The device according to one or more of the preceding claims, characterized in that the means for selectively determining the intensity of the interference signal depending on the polarization, in particular the determination of the respective intensity for the TE and TM components of the interference signal based on the coordinate system of the Diffraction gratings include suitable polarization sensitive detectors.

14. The device according to one or more of the preceding claims, characterized in that the means for selectively determining the intensity of the interference signal as a function of the polarization, in particular the determination of the respective intensity for the TE and TM components of the interference signal based on the coordinate system of the Diffraction gratings comprise means for determining the polarization or polarization direction of the incident light or the light source.

15. Measuring device according to one or more of claims 1-14 for accurate determination of the relative phase shift between components of different polarization generated by a birefringent optical element, based on the measurement of the relative phase shift between the TE and TM components of the measured interference signals.

16. Measuring device according to one or more of claims 1-14 for the precise determination of the effective optical profile depth of a diffraction grating, based on the measurement of the relative phase shift between the TE and TM components of the measured interference signals.

17. Measuring device according to one or more of claims 1-14 for the precise determination of optical path length changes of at least one of the Arms of the interferometer based on the determination of the phase angle of the TE and TM components of the measured interference signals.

18. Measuring device according to claim 17 for the precise determination of path length changes by coupling the path length change to be determined to a corresponding change in the optical path length of at least one of the arms of the interferometer.

19. Measuring device according to claim 17 or 18 for the precise determination of lateral displacements of the diffraction grating on the basis of the determination of the phase angle of the TE and TM components of the measured interference signals.

20. Measuring device according to claim 19 for the precise determination of path length changes by coupling the path length change to be determined to a corresponding lateral displacement of the diffraction grating.

21. Measuring device according to one or more of the preceding claims for the precise spectral determination of absorption and refractive index or spectrally dependent changes in refractive index of a sample introduced into one of the arms of the interferometer.

22. Measuring device according to one or more of the preceding claims for the precise spectral determination of polarization properties of a sample introduced into one of the arms of the interferometer.