Optically Corrective Microprobe for White Light Interferometry

• US 20130188195A1
• Filed: 12/03/2012
• Published: 07/25/2013
• Est. Priority Date: 12/02/2011
• Status: Active Grant

• Current Assignee: Grintech GmbH
• Original Assignee: Grintech GmbH
• Inventors: MESSERSCHMIDT, Bernhard
• Granted Patent: US 9,074,862 B2
• Time in Patent Office: Days
• US Class Current: 356/479
• CPC Class Codes: G01B 9/02007 Two or more frequencies or sources used for interferometric measurement using only beat G01B9/02003
• CPC Class Codes: G01B 9/02049 characterised by particular mechanical design details
• CPC Class Codes: G01B 9/0205 of probe head
• CPC Class Codes: G01B 9/02057 by using common path configuration, i.e. reference and object path almost entirely overlapping
• CPC Class Codes: G01B 9/02058 by particular optical compensation or alignment elements, e.g. dispersion compensation
• CPC Class Codes: G01B 9/0209 Low-coherence interferometers
• CPC Class Codes: G01B 9/02091 Tomographic interferometers, e.g. based on optical coherence
• CPC Class Codes: G02B 23/2423 of the distal end
• CPC Class Codes: G02B 27/005 for correction of secondary colour or higher-order chromatic aberrations
• CPC Class Codes: G02B 27/1006 for splitting or combining different wavelengths G02B27/1086, G02B27/141 take precedence
• CPC Class Codes: G02B 27/141 using dichroic mirrors
• CPC Class Codes: G02B 3/0087 with index gradient
• CPC Class Codes: G02B 6/26 Optical coupling means G02B6/36, G02B6/42 take precedence
• CPC Class Codes: G02B 6/262 Optical details of coupling light into, or out of, or between fibre ends, e.g. special fibre end shapes or associated optical elements
• CPC Class Codes: G02B 6/29364 Cascading by a light guide path between filters or filtering operations, e.g. fibre interconnected single filter modules

Abstract

An optically corrective microprobe for white light interferometry is disclosed. In white light interferometry which uses different spectral ranges, the dispersion-induced dependency of optical path differences on the wavelength between measurement light bundle and reference light bundle is compensated by an efficient, easily miniaturized arrangement. A reference beamsplitter with partially reflecting filters in at least two different spectral ranges for generating at least two transmitted measurement light bundles and associated reflected reference light bundles of nonoverlapping spectral ranges is arranged between the light output surface of the light-conducting fiber and the focusing optics. An axial distance between two partially reflecting filters is adjusted in such a way that a spectrally induced optical path difference to which the respective measurement light bundles are subjected when passing through dispersive elements in the light path to the object surface and back is present to the same magnitude between the respective reference light bundles.

Claims

1. An optical microprobe for focusing a light bundle on an object surface for interferometric measurements comprising:

a light-conducting fiber which transmits a light bundle and has a light output surface opening into the microprobe, the light bundle comprising white light of at least two different spectral ranges;

focusing optics for focusing the light bundle on the object surface,

a light path adjoining the light output surface of the light-conducting fiber for generating divergent light for exploiting the aperture of the focusing optics,

at least one measurement light bundle being released from the light output surface is directed by the focusing optics to the object surface and reflected from the object surface back through the microprobe into the light-conducting fiber;

at least one reference light bundle generated by partial reflection at the light output surface of the light-conducting fiber and reflected back into the light-conducting fiber;

a reference beamsplitter disposed between the light output surface of the light-conducting fiber and the focusing optics, the reference beamsplitter having filters which are partially reflecting in at least two different spectral ranges for generating at least two transmitted measurement light bundles (M₁, M₂) and associated reflected reference light bundles (R₁, R₂) of nonoverlapping spectral ranges, wherein

an axial distance between two partially reflecting filters is adjusted in such a way that a spectrally induced optical path difference to which the respective measurement light bundles (M₁, M₂) are subjected when passing through dispersive elements in the light path to the object surface and back is present in the same magnitude between the respective reference light bundles (R₁, R₂), so that the dispersion-induced path difference is compensated when the measurement light bundles (M₁, M₂) and associated reference light bundles (R₁, R₂) are superposed, and

the focusing optics has such a slight chromatic aberration that foci of the spectrally different measurement light bundles at least partially overlap at least in a desired measurement range for the object surface.

2. The optical microprobe according to claim 1, wherein the reference beamsplitter has a layer construction comprising at least two partially reflecting filters adapted to the spectral ranges of the spectrally separated measurement light bundles and reference light bundles (M₁, M₂, R₁, R₂) and, in each instance, a transparent spacer layer which axially separates the partially reflecting filters, wherein the dispersion-compensating effect with respect to the measurement light bundles (M₁, M₂) can be adjusted by adapting the layer thickness and refractive index of the spacer layer .

3. The optical microprobe according to claim 2, wherein the partially reflecting filters are formed as interference filters with a sharp cut-off filter characteristic for generating measurement light bundles and reference light bundles (M₁, M₂, R₁, R₂) with no spectral overlapping.

4. The optical microprobe according to claim 2, wherein the partially reflecting filters are formed as special color filters for generating spectrally separated measurement light bundles and reference light bundles (M₁, M₂, R₁, R₂), wherein the light source has spectrally separated spectral ranges for generating measurement light bundles (M₁, M₂) and reference light bundles (R₁, R₂) with no spectral overlapping.

5. The optical microprobe according to claim 1, wherein the reference beamsplitter has a plurality of partially reflecting filters in a filter layer system of filter layers passing into one another, wherein layers of the filter layer system are so selected with respect to thickness and refractive index that separate spacer layers are omitted and a quasi-continuous dispersion compensation can accordingly be achieved.

6. The optical microprobe according to claim 1, wherein the reference beamsplitter is arranged on a surface of a transparent support material, which surface is arranged opposite the light output surface of the light-conducting fiber.

7. The optical microprobe according to claim 1, wherein the reference beamsplitter is arranged directly on the light output surface of the light-conducting fiber.

8. The optical microprobe according to claim 1, wherein that the focusing optics comprise conventional refractive optics and diffractive optics for correcting the chromatic aberration.

9. The optical microprobe according to claim 1, wherein the focusing optics comprise a GRIN lens which is combined with diffractive optics for correcting the chromatic aberration.

10. The optical microprobe according to claim 9, wherein the focusing optics comprise a GRIN lens and additional refractive optics for increasing the object-side numerical aperture and are combined with diffractive optics for correcting the chromatic aberration.

11. The optical microprobe according to claim 1, wherein the focusing optics have diffractive optics for correcting the chromatic aberration which are arranged as layer system on an optical surface of the focusing optics .

12. The optical microprobe according to claim 1, characterized in that the focusing optics are formed as an achromat for correcting the chromatic aberration.

13. A method of using a microprobe in an arrangement according to claim 1 for coherence tomography in which, as a result of the compensation of the dispersion-induced path difference and the correction of the chromatic aberration in coherence tomography, an absolute interferometric path length measurement is achieved in a nanometer range also for sharply inclined measurement surfaces at base distances between the microprobe and measurement object of a hundred micrometers or more.

14. An arrangement for white light interferometry, comprising:

an interferometer,

a microprobe for interferometric measurements;

a light source characterized by a white light spectrum or by at least two different spectral ranges;

a light-conducting fiber for transporting the light source light into the microprobe and for transmitting the light reflected back from the microprobe and the object surface into the interferometer;

a light coupler for producing an optical connection between the light source and the light-conducting fiber; and

focusing optics located in the microprobe and serving to generate a divergent light bundle for exploiting the aperture of the focusing optics to generate at least two spectrally different measurement light bundles;

wherein a light path is provided in the microprobe between a light output surface of the light-conducting fiber and the focusing optics , the light path having a size that is adapted to the numerical aperture of the light-conducting fiber and the aperture of the focusing optics;

a reference beamsplitter for generating at least two spectrally different reference light bundles, the reference beamsplitter comprising at least two spectrally differently partially reflecting filters for generating two transmitted measurement light bundles (M₁, M₂) and associated reflected reference light bundles (R₁, R₂) of spectral ranges which do not overlap, wherein;

the partially reflecting filters have an axial distance with respect to one another such that a spectrally induced optical path difference resulting for center wavelengths (λ

₁,λ

₂) of the different spectral ranges of the measurement bundles when passing through dispersive optics in the light path to the object surface and back is adjusted to the same magnitude between the associated reference light bundles (R₁, R₂) so that when the measurement light bundles and reference light bundles (M₁, M₂, R₁, R₂) are superposed the dispersion-induced path difference is compensated, and

the chromatic aberration of the focusing optics is sufficiently slight for the different spectral ranges used so that the foci of the spectrally different measurement light bundles (M₁, M₂) at least partially overlap over an adjustable measurement range to the object surface.

15. The method of using an arrangement according to claim 14 for coherence tomography in which, as a result of the compensation of the dispersion-induced path difference and the correction of the chromatic aberration in coherence tomography, an absolute interferometric path length measurement is achieved in a nanometer range also for sharply inclined measurement surfaces at base distances between the microprobe and measurement object of a hundred micrometers or more.

16. A method for producing a dispersion-compensating reference beamsplitter for at least two spectral ranges of a light bundle the method comprising:

a) arranging a first layer system on a transparent support material with precision-optical surface quality, the first layer system forming a first filter that is designed to reflect a minor portion of a shorter-wavelength spectral component of the light bundle into a first reference light bundle (R₁) and to transmit all remaining portions of said shorter-wavelength spectral component into a first measurement light bundle (M₁);

b) arranging a transparent spacer layer having a layer thickness dependent on an optical path difference of two spectrally adjacent measurement light bundles (M₁, M₂) undergone inside dispersive optical elements along a given (pre-defined?) measurement light path;

c) arranging a second layer system on the spacer layer, the second layer system forming a second filter that is designed to reflect a minor portion of a longer-wavelength spectral component of the light bundle into a second reference light bundle (R₂) and to transmit all remaining portions of said longer-wavelength spectral component into a second measurement light bundle (M₂); and

d) singulating the support material into individual reference beamsplitters.