OPTICAL MEASUREMENT DEVICE AND MULTIPLE MIRROR

Abstract

A multiple mirror for multiplying a single incident wavefront of electromagnetic radiation into a plurality of outgoing wavefronts, including at least one first mirror, onto which the incident wavefront first falls, and a second mirror, on which the wavefront is last reflected, wherein the mirror planes are superimposed in the direction of movement of the first wavefront. The first mirror is partially transparent to the electromagnetic radiation and the second mirror is fully reflective.

Claims

1.-18. (canceled)

19. A device for interferometric, optical measurement of a surface, comprising a coherent light source emitting an electromagnetic beam; two beam splitters; a diaphragm; a multiple mirror with two distanced mirrors, comprising a first mirror to which the incoming beam hits first, and a second mirror at which the beam is reflected last, wherein the mirrors are superimposed in the direction of motion of the first wavefront and wherein the first mirror is partially transparent for the electromagnetic beam and the second mirror is fully reflective; and whereby the beam splitters and the multiple mirror are arranged in such a manner that the electromagnetic beam hits the first beam splitter, is directed from the first beam splitter to the multiple mirror and then hits the second beam splitter; and wherein the diaphragm is arranged in the beam path between the two beam splitters in such a manner that the beam passes through the diaphragm.

20. The device according to claim 19, comprising a second multiple mirror and a further diaphragm, wherein the electromagnetic beam hits the first beam splitter, is divided by the first beam splitter in two partial beams with different propagation directions and each partial beam is directed to one of the multiple mirrors and then both partial beams meet the second beam splitter, wherein one diaphragm is arranged in each of the beam paths between the two beam splitters in such a manner that each partial beam passes through one of the diaphragms.

21. The device according to claim 19, wherein a mirror of the multiple mirror is tiltable compared to the other mirror in such a manner that the mirrors enclose an angle and that a wavefront hitting the multiple mirror is divided into two outgoing wavefronts that are phase-shifted with respect to each other.

22. The device according to claim 21, wherein the mirrors of the multiple mirror are flat.

23. The device according to claim 19, wherein the first and/or second mirror of the multiple mirror are rotatable around a rotation axis.

24. The device according to claim 23, wherein the first and/or second mirror of the multiple mirror are mounted in a mounting-frame of the mirrors.

25. The device according to claim 20, wherein one multiple mirror has a mirror which is tiltable over the other mirror and the other multiple mirror has a mirror which is rotatable against the other mirror by a rotation axis.

26. The device according claim 19, wherein the first mirror polarizes the electromagnetic radiation.

27. The device according claim 26, wherein the first mirror reflects radiation of a certain polarization and transmit a different polarization.

28. The device according to claim 19, wherein a camera records the electro-magnetic beam passed through the multiple mirror.

29. The device according to claim 28, wherein the camera is a color camera.

30. A device for interferometric, optical measurement of a surface comprising a coherent light source emitting an electromagnetic beam, a beam splitter which divides the beam into a first partial beam in a first propagation direction and into a second partial beam in a second propagation direction, two mirrors, one of which is a fixed mirror and the other is a movable mirror, two diaphragms, wherein one diaphragm each is arranged be-tween the beam splitter and the mirrors and in such a manner that each partial beam passes through one diaphragm twice, the two partial beams hit the beam splitter after reflection on the mirrors and are merged again in the beam splitter, wherein the diaphragms are arranged in such a manner that after merging the two partial beams in the beam splitter the beam corresponds to a beam after passing through a double-slit.

31. The device according to claim 30, wherein the merged beam after leaving the beam splitter is directed to a camera, wherein from the direction of the camera a virtual double-slit is visible.

32. The device according to claim 19, wherein at least one diaphragm has an aperture.

33. The device according to claim 32, wherein the aperture comprises a polarization filter or a frequency filter that allows only certain wave-lengths to pass through the diaphragm.

34. The device according to claim 19, wherein at least one diaphragm has two apertures.

35. The device according to claim 34, wherein the two apertures each have a polarization filter, which are orthogonally aligned to each other.

36. The device according to claim 19, wherein one diaphragm is designed as a grating diaphragm with apertures.

37. The device according to claim 36, wherein the apertures have in two dimensions certain slit widths wherein the slit widths have the same dimensions.

Description

[0023] In the following text, the invention will be explained in more detail on the basis of an exemplary embodiment and with reference to the associated drawings.

[0024] In the drawing:

[0025] FIG. 1 shows a multiple mirror with various outgoing beams,

[0026] FIG. 2 shows a device for optical measurement of a surface by means of multiple mirrors,

[0027] FIG. 3 shows an embodiment with a beam splitter, a fixed mirror and a movable mirror, and

[0028] FIG. 4 shows an embodiment with two beam splitters, a fixed mirror and a movable mirror.

[0029] FIG. 1 shows a multiple mirror 10, which reflects electromagnetic radiation 11. The radiation 11 may consist of coherent waves, as is the case with a laser, for example. The multiple mirror 10 multiplies an incident wavefront 12 of the radiation 11, with the result that a plurality of wavefronts 14 emerge from the multiple mirror 10 after reflection. The wavefront is multiplied by two mirrors 16, 18 which are located parallel to each other and are aligned so as to overlap each other in the direction of propagation of the incident and reflected radiation 11. The mirrors 16, 18 have mirror planes on which the reflection takes place. The mirrors 16, 18 are also spaced from each other in the direction of propagation. Additionally, the first mirror 16 on which the radiation 11 hits in the direction of propagation is partially transparent for the electromagnetic radiation 11, whereas the second mirror 18 is fully reflective.

[0030] Now if an incident wavefront 12 hits on the first mirror 16, a portion thereof is reflected and sent back. Here, the angle of incidence corresponds to the angle of reflection relative to the mirror plane. The remaining portion is reflected at the mirror plane of the second mirror 18. Since the mirrors 16, 18 are spaced apart from one another, a certain period of time elapses between the first reflection and the second reflection, so the reflection on the first mirror 16 takes place before the reflection on the second mirror 18. This results in a phase shift between the two outgoing wavefronts 14 from the first and second reflection.

[0031] The incident beam 11 is polarized in different directions. The mirrors 16, 18 may be designed in such a manner that the reflected radiation is only polarized in one direction. The two mirrors 16, 18 are able to polarize the radiation 11 in two directions, which are orthogonal to each other. The respective radiation 11 reflected at each of the mirrors 16, 18 is only polarized in one direction. Alternatively, for example only the first mirror 16 may have a polarizing effect, wherein the reflected portion of the radiation 11 has a uniform polarization. The remaining component of the radiation 11 advances as far as the second mirror 18 and has an orthogonal polarization. This second component may be reflected on the second mirror 18, wherein the second mirror 18 has no polarizing property. Nevertheless, the second component emerges polarized from the multiple mirror 10.

[0032] FIG. 2 shows a device 100 for optical measurement of a surface which comprises two multiple mirrors 10, two beam splitters 102 and two diaphragms 104. The incident beam 11 is generated by irradiating the surface to be measured with a coherent light. The coherent light may be a laser, for example. When a multiple mirror or two multiple mirrors 10 are used, the phase-shifted outgoing wavefronts 14 from the individual multiple mirrors 10 are added together, and the radiation 14 reflected by the mirrors 16, 18 can interfere with each other. When one multiple mirror is used, one of each of the two polarized wavefronts (reflected by the multiple mirror) interferes with the correspondingly polarized portion of the simple wavefront which is reflected by the simple mirror.

[0033] The radiation 11 that is reflected by the surface to be measured first enters a beam splitter 102. The beam splitter 102 may have a rectangular cross-section, wherein the incident beam 11 impinges on a flat side of the beam splitter 102, which in particular does not face any of the multiple mirrors 10. After the beam 11 has entered the first beam splitter 102 and the beam 11 is split into two further beams 12, the wavefronts 12 of which each impinge on a multiple mirror 10, the beams 12 are again doubled by the multiple mirrors 10. The wavefronts 14 emerging from the respective multiple mirror 10 are directed towards the second beam splitter 102. The respective doubled and phase-shifted wavefronts 14 pass through diaphragms 104, which are each located on the flat side of the second beam splitter 102. Alternatively, the diaphragms 104 may be located anywhere in the respective beam path between the first beam splitter 102 and the second beam splitter 102. In the second beam splitter 102, the outgoing, phase-shifted beams 14 coming from the multiple mirrors 10, are doubled again, so that two wavefronts 15, each with the same phase, are directed towards a camera. The beams from the two multiple mirrors 10 are merged and split into two bundles, which each exits on one side of the beam splitter 102. The beams 14 interfere with each other during merging and form an interference image which can be spectrally decomposed, by Fourier analysis, for example.

[0034] The two beam splitters 102 are located in a row along a diagonal relative to the beam paths entering and exiting the multiple mirrors 10. The multiple mirrors 10 are located approximately between the beam splitters 102 to the left and right of the beam splitters 102. The two diaphragms 104 are located on the flat side of the second beam splitter 102, facing the first beam splitter 102. Alternatively, the diaphragms 104 may be in any position on the respective beam path between the first beam splitter 102 and the second beam splitter 102. The two diaphragms 104 are approximately perpendicular to each other. The diaphragms 104 may also be positioned at an angle not perpendicular to each other.

[0035] The multiple mirrors 10 each comprise two mirrors 16, 18, one of the mirrors 16, 18 each having an axis of rotation. For example, the first mirror 16 and/or the second mirror 18 may have an axis of rotation. One multiple mirror 10 may also have a rotatable first mirror 16 and the other multiple mirror 10 may have a rotatable second mirror. The axes of rotation can be aligned perpendicular to each other. The rotation of the mirror 16, 18 may be used to produce a phase shift of the multiplied beams 14. The phase shift is dependent on the angle 19 that is set between the two mirrors 16, 18 by the rotation. The displacement of a mirror may also cause shearing of the radiation 12. The respective interference images of the polarized beams from the first and second mirrors 16, 18 of the two multiple mirrors 10 may be shifted with respect to each other by the rotation. The axis of rotation is implemented in a frame of the multiple mirror 10, for example.

[0036] Through the second beam splitter 102, the single-slit diaphragms 104 are visible on the outgoing flat sides of the second beam splitter 102 as a virtual double-slit diaphragm, since the two phase-shifted beams from the respective multiple mirror 10 are merged again after passing through the respective diaphragm 104. The double-slit spacing requires a spatial carrier frequency for the phase shifting in space. The carrier frequency is thereby decoupled from the rotational positions of the mirrors 16, 18.

[0037] The mirrors 16, 18 polarize the radiation 12 in different directions depending on the reflection. For example, the partially transparent first mirror 16 can only reflect radiation 12 with a specific polarization, so that the radiation 12 which is able to pass through is reflected by the second mirror 18. In this situation, the radiation 12 may be polarized in the same direction by the two first mirrors 16. The reflected beams 14 with the same polarization direction are capable of interfering with each other. In the same way, the beams 14 from the second mirror can also be polarized in the same direction. Consequently, they are also able to interfere with each other. In contrast, the beams 14 from the first mirror cannot interfere with the beams 14 from the second mirror, since they are polarized in a different direction. Now if the first polarizing mirror 16 of the one multiple mirror 10 is tilted relative to the first polarizing mirror 16 of the other multiple mirror 10, an interference image is created by the reflected beams 14. This image may be understood as a sheared image. For example, it may be sheared horizontally, which allows shearographic measurement. Independently of this, the second mirror 18 of the one multiple mirror 10 may be tilted towards the second mirror 18 of the other multiple mirror 10, thereby producing another sheared image through the interference of the reflected beams 14. Shearing may take place vertically, for example.

[0038] In this way, a horizontally sheared wavefront 14 and an associated reference wavefront as well as a vertically sheared wavefront and an associated reference wavefront are generated by the two multiple mirrors 10. Thus, the use of the diaphragms 104 generates carrier frequencies for the respective wavefronts which are polarized in a manner capable of interference in the respective horizontal and vertical directions. This results in separate spectra in the Fourier space.

[0039] Both images generated by the interference between the pairs of beams 14 reflected from the first and second mirrors 16, 18 are captured by a camera. For the independent evaluation of the two sheared images, their polarizations may be used as a separation criterion. This may be achieved by the use of different spatial carrier frequencies according to polarization, for example. For this purpose, the two diaphragms 104 may have polarization-specific apertures 106. The width of the apertures 106 in the diaphragms 104 determine the frequency width of the image produced by interferometry proportionally in a Fourier analysis. The aperture 106 may be equipped with a polarization filter.

[0040] Additionally or alternatively thereto, the aperture 106 may be equipped with a frequency filter which only allows a certain wavelength of the radiation 14 to pass through. Measurement accuracy is improved by filtering certain wavelengths, since the aperture size may be adjusted to just one single wavelength, for example. Image errors are avoided thereby. A color camera enables the filtered interference images from different wavelengths to be analyzed. A frequency filter may be generated by color filters, for example.

[0041] FIG. 3 shows a device 100 comprising a beam splitter 102 which is arranged in the direction of propagation of the electromagnetic beam 11, and onto which the beam 11 enters first. Upon entering the device, the beam 11 passes into the beam splitter 102 and is split in the beam splitter 102. One portion of the beam 11 is directed in a first direction of propagation 1 towards the movable mirror 6. In a second direction of propagation 2, the remaining beam of the split beam 11 is guided onto a fixed position mirror 5. A diaphragm 104 is respectively arranged between the beam splitter 102 and each of the two mirrors 6, 5. The respective partial beam obtained through the beam splitter 102 passes through each diaphragm 104 at least twice, the partial beam passing through the diaphragm 104 once before the reflection and once after the reflection on the respective mirror 6, 5. The partial beams are merged again in the beam splitter 102 and directed to a camera. A virtual double-slit 4 is visible from the direction of the camera.

[0042] FIG. 4 discloses a further development which comprises further, second beam splitter 102. A mirror 5 and a movable mirror 6 are also part of the device 100, wherein the two beam splitters 102 are located in a diagonal alignment with respect to each other. An imaginary diagonal 3 may be laid through a part of the corners of the beam splitters, so that some of the corners of the beam splitters 102 are aligned with each other. One mirror 6, 5 is located on each of the two sides of the diagonal alignment of the beam splitters 102. The mirrors 6, 5 are aligned substantially parallel to the imaginary diagonal 3 through the beam splitters 102. The movable mirror 6 may deviate from the parallel alignment by an adjustment angle, but it may be reset to a parallel position. A diaphragm 104 is located between the beam splitter 102 the beam 11 enters second, that is to say the partial beams from the first beam splitter 102, and each of the mirrors 5, 6. The diaphragms 104 are substantially parallel to the sides of the beam splitter 102 into which the beam 11 reflected by the mirrors 5, 6 enters.

LIST OF REFERENCE NUMERALS

[0043] 1 First direction of propagation

[0044] 2 Second direction of propagation

[0045] 3 Diagonal

[0046] 4 Virtual double-slit

[0047] 5 Fixed mirror

[0048] 6 Movable mirror

[0049] 10 Multiple mirror

[0050] 11 Electromagnetic radiation

[0051] 12 Incident wavefront

[0052] 14 Outgoing wavefronts

[0053] 15 Wavefronts towards camera

[0054] 16 First mirror

[0055] 18 Second Mirror

[0056] 19 Angle

[0057] 20 Axis of rotation

[0058] 100 Device

[0059] 102 Beam splitter

[0060] 104 Diaphragms

[0061] 106 Aperture