METHOD AND ARRANGEMENT FOR ROBUST INTERFEROMETRY

• US 20120307258A1
• Filed: 01/21/2011
• Published: 12/06/2012
• Est. Priority Date: 01/22/2010
• Status: Active Grant

• Current Assignee: Universitaet Stuttgart
• Original Assignee: Universitaet Stuttgart
• Inventors: Koerner, Klaus, Osten, Wolfgang, Berger, Reinhard
• Granted Patent: US 8,934,104 B2
• Time in Patent Office: Days
• US Class Current: 356/497
• CPC Class Codes: G01B 2290/15 Cat eye, i.e. reflection always parallel to incoming beam
• CPC Class Codes: G01B 9/02032 generating a spatial carrier frequency, e.g. by creating lateral or angular offset between reference and object beam shearing interferometers G01B9/02098
• CPC Class Codes: G01B 9/02061 Reduction or prevention of effects of tilts or misalignment
• CPC Class Codes: G01B 9/0209 Low-coherence interferometers

Abstract

An arrangement and a method are provided for robust interferometry for detecting distance, depth, profile, form, undulation, flatness deviation and/or roughness or the optical path length in or on technical or biological objects, including in layered form, or else for optical coherence tomography (OCT), with a source of electromagnetic radiation and with an interferometer, in particular also in the form of an interference microscope, having an object beam path and having a reference beam path, in which an end reflector is arranged, and a line-scan detector for detecting electromagnetic radiation in the form of at least one spatial interferogram.

Claims

1. An arrangement for robust interferometry for detecting distance, depth, profile, form, undulation and/or roughness or the optical path length and/or flatness deviation in or on technical or biological objects, including in layered form, or else for optical coherence tomography (OCT), the arrangement comprising:

a monochromatic or quasi-monochromatic source of electromagnetic radiation and/or a source of short-coherent electromagnetic radiation (1, 101, 110, 111) for illuminating the object,

an interferometer, in particular also in the form of an interference microscope, having an object beam path (O) and having at least one reference beam path (R), in which at least one end reflector is arranged, and a measurement plane (ME) in the object beam path, in which measurement plane the surface or volume elements of the object (7, 70, 700) which are to be optically measured are at least approximately situated,

and at least one line-scan detector (12, 102) for detecting electromagnetic radiation in the form of at least one spatial interferogram,

at least one end reflector (8, 8i) having three plane mirrors (9, 10, 11) is arranged as reference reflector in the reference beam path of the interferometer, the three plane mirrors (9, 10, 11) each being at least approximately perpendicular to a common reference plane BE, and

three trace lines of planes represented by the surfaces of the three plane mirrors (9, 10, 11) forming a triangle ABC in the reference plane BE, in order that there is a lateral shear of the absolute value delta_g between reference beam (RS) and object beam (OS) in said interferometer.

2. The arrangement for robust interferometry of claim 1, characterized in that the three trace lines (AB, BC, CA) form an obtuse triangle ABC in the reference plane BE.

3. The arrangement for robust of claim 1, characterized in that a multiplicity of miniaturized three-plane-mirror reference end reflectors (8, 8i) define an array (18) arranged in the reference beam path.

4. for robust interferometry of claim 1, characterized in that the source of short-coherent electromagnetic radiation (1, 101, 110, 111) is designed for laterally finely structured illumination of the object (7, 70, 700) by a switchable spatial light modulator array (115).

5. The arrangement for robust interferometry of claim 1, characterized in that further comprising at least one double diaphragm (16, 116a, 116b) with slits or pinholes is arranged in a jointly used imaging stage (15, 17) for reference radiation and object radiation, wherein a first opening of the double diaphragm (16, 116a, 116b) is reserved for the reference radiation and a second opening is reserved for the object radiation.

6. The arrangement for robust interferometry of claim 1, characterized in that means (19, 23) for chromatic depth splitting of foci are arranged in the object beam path.

7. The arrangement for robust interferometry of claim 1, characterized in that at least one confocally discriminating diagram (16, 116a, 116b) is arranged in the object beam path.

8. The arrangement for robust interferometry of claim 1, characterized in that the interferometer a Michelson, Linnik, Mirau or Schulz-Minor interferometer and at least one three-mirror reference end reflector (8, 8i) is arranged in the reference beam path.

9. The arrangement for robust interferometry of claim 1, characterized in that the arrangement comprises:

a monochromatic or quasi-monochromatic source of electromagnetic radiation and also a source (1, 101) of short-coherent electromagnetic radiation for illuminating the object (7, 70, 700),

wherein means (19, 23) for spectral splitting are arranged in the detection beam path.

10. A method for robust interferometry for detecting distance, depth, profile, form, undulation and/or roughness or the optical path length in or on technical or biological objects, including in layered form, or else for optical coherence tomography (OCT), comprising the following method steps:

generating at least one object beam of rays of electromagnetic radiation for illuminating the object (7, 70, 700),

generating at least one reference beam of rays by means of beam splitting in an interferometer,

generating spatial short-coherence interferences of object and reference rays,

detecting spatial short-coherence interferences on a line-scan detector (12, 102) of electromagnetic radiation,

directly carrying out successive reflections at three respective plane mirrors (9, 10, 11) in the reference beam path,

surfaces of said three plane mirrors (9, 10, 11) each being at least approximately perpendicular to a common reference plane BE thereby formed a three-plane-mirror reference end reflector (8, 8i),

the introducing a lateral shear having an absolute value delta_q between object beam of rays and reference beam of rays in the interferometer, thereby generating at least one spatial short-coherence interferogram on the line-scan detector (12, 102), and

evaluating at least one spatial interferogram to obtain information about distance, depth, profile, form, undulation and/or roughness or the optical path length of an object (7, 70, 700), including in optical coherence tomography.

11. The method for robust interferometry of claim 10, characterized in that a point illumination or a laterally structured illumination of the object (7, 70, 700) is effected.

12. The method for robust interferometry of claim 10, characterized in that at least one three-plane-mirror reference end reflector (8, 8i) is produced in a metal or plastics body (18) by means of a single-point diamond processing technique.

13. The method for robust interferometry of claim 10, characterized in that a spectral splitting of object and reference radiation is carried out in the detection beam path, wherein the at least one spatial interferogram with spectral splitting is detected and evaluated in order to obtain information about distance, depth, profile, form, undulation and/or roughness or the optical path length of an object (7, 70, 700).

14. The method of claim 10, for robust, wide-scale, interferometry for detecting distance, depth, profile, form, undulation and/or roughness or the optical path length in or on technical or biological objects, in particular including in layered form, or else for optical coherence tomography (OCT), which comprises the following means:

at least one source of short-coherent electromagnetic radiation for illuminating the object, in particular also in the form of a frequency comb laser cavity (111) disposed upstream of the interferometer and having the single optical length L or/and at least one multi-beam interference cavity disposed downstream of a spectrally broadband source in the light path and having the single optical length L,

an interferometer, in particular also in the form of an interference microscope, having an object beam path and having at least one reference beam path, in which at least one end reflector having three plane mirrors as reference reflector is arranged, wherein the surfaces of said three plane mirrors lie in each case at least approximately perpendicular to a common reference plane BE and wherein the three trace lines of the planes which are represented by the surfaces of the three plane mirrors form a triangle ABC in the reference plane BE, in order that there is a lateral shear of the absolute value delta_q between reference beam (RS) and object beam (OS) in said interferometer, and having a measurement plane (ME1, ME2) or a measurement volume in the object beam path, in which the surface or volume elements of the object (7, 70, 700) which are to be optically measured are at least approximately situated,

and at least one line-scan detector (12, 102) for detecting electromagnetic radiation,

and the following method steps;

generating at least one object beam of rays (O) of electromagnetic radiation by means of beam splitting in an interferometer for illuminating the object (7, 70, 700),

generating at least one reference beam of rays (R) by means of beam splitting in an interferometer,

generating a lateral offset delta_q for the reference beam of rays,

generating spatial superimpositions of object and reference rays by beam combining,

detecting spatial superimpositions of object and reference rays on a line-scan detector (12, 102) of electromagnetic radiation,

characterized in that

at the instant t_i a first superimposition of object radiation and reference radiation and a first detection of superimposed object radiation and reference radiation by means of line-scan detector (12, 102) are carried out and subsequently

either a relative movement in the direction of light propagation between the interferometer and the object (7, 70, 700)

or a movement of the end reflector (8, 18, 118) in the reference beam path in the direction of light propagation

or an optical length change delta_L of at least one frequency comb laser cavity (111) disposed upstream of the interferometer or/and of at least one multi-beam interference cavity disposed downstream of the source in the light path is carried out in the time duration delta_tz, wherein

the adjusting distance s of the relative movement between the interferometer and the object (7, 70, 700)

or the adjusting distance s of the movement of the end reflector (8) in the reference beam path

or the optical length change delta_L of at least one frequency comb laser cavity (111) disposed upstream of the interferometer or/and of at least one multi-beam interference cavity disposed downstream of the source in the light path

in the time duration delta_tz is at least half the absolute value of the centroid wavelength lambda_cent

and the spatial change delta_xr_max in the optical path difference xr in the superimposition region of object radiation and reference radiation on the line-scan detector (12, 102) has at least the whole absolute value of the centroid wavelength lambda_cent of the interfering radiation

and after the time duration delta_tz at the instant t_i+1 at least one second superimposition of object radiation and reference radiation and at least one second detection of superimposed object radiation and reference radiation by means of line-scan detector (12, 102) are carried out.

15. The method for robust, in particular wide-scale interferometry of claim 14, characterized in that

the adjusting distance s or the optical length change delta_L in the time duration delta_tz has at least approximately in each case the absolute value of one quarter of the maximum spatial change delta_xr_max of the optical path difference xr of the spatial interferogram recorded on the line-scan detector (12, 102).

16. The method for robust wide-scale, interferometry of claim 14, characterized in that

the relative movement between the interferometer and the object (7, 70, 700) in the time duration delta_tz is carried out continuously, quasi-continuously or in a stepwise manner.

17. The method for robust, wide-scale, interferometry of claim 14, characterized in that

the movement of the end reflector (8, 18, 118) in the time duration delta_tz is carried out continuously, quasi-continuously or in a stepwise manner.

18. The method for robust, wide-scale, interferometry according to at least one of claims 14 and 15, characterized in that

the optical length change delta_L in the time duration delta_tz is carried out continuously, quasi-continuously or in a stepwise manner.

19. An arrangement for robust, wide-scale interferometry for detecting distance, depth, profile, form, undulation and/or roughness or the optical path length in or on technical or biological objects, including in layered form, or else for optical coherence tomography (OCT), which comprises the following means:

at least one source of short-coherent electromagnetic radiation for illuminating the object, comprising in particular a frequency comb laser cavity (111) disposed upstream of the interferometer and having the single optical length L or/and at least one multi-beam interference cavity disposed downstream of a spectrally broadband source in the light path and having the single optical length L,

an interferometer, having an object beam path and having at least one reference beam path, in which at least one end reflector having three plane mirrors as reference reflector is arranged, wherein the surfaces of said three plane mirrors lie in each case at least approximately perpendicular to a common reference plane BE and wherein the three trace lines of the planes which are represented by the surfaces of the three plane mirrors form a triangle ABC in the reference plane BE, in order that there is a lateral shear of the absolute value delta_q between reference beam (RS) and object beam (OS) in said interferometer, and having a measurement plane (ME1, ME2) or a measurement volume in the object beam path, in which the surface or volume elements of the object (7, 70, 700) which are to be optically measured are at least approximately situated,

and at least one line-scan detector (12, 102) for detecting electromagnetic radiation, and

means for tuning the frequency comb laser cavity (111) or/and the multi-beam interference cavity.

20. The arrangement for robust interferometry of claim 19, characterized in that there are arranged means for temporal synchronization for carrying out an optical length change delta_L of at least one frequency comb laser cavity (111) disposed upstream of the interferometer or/and of at least one multi-beam interference cavity disposed downstream of the source in the light path with the image recording by means of line-scan detector (12, 102).