METHOD AND APPARATUS FOR INTERFEROMETRY

• US 20110235045A1
• Filed: 10/12/2009
• Published: 09/29/2011
• Est. Priority Date: 10/10/2008
• Status: Active Grant

• Current Assignee: Universitaet Stuttgart
• Original Assignee: Universitaet Stuttgart
• Inventors: Koerner, Klaus, Osten, Wolfgang
• Granted Patent: US 8,605,289 B2
• Time in Patent Office: Days
• US Class Current: 356/451
• CPC Class Codes: G01B 2210/50 Using chromatic effects to achieve wavelength-dependent depth resolution
• CPC Class Codes: G01B 2290/25 Fabry-Perot in interferometer, e.g. etalon, cavity
• CPC Class Codes: G01B 2290/30 Grating as beam-splitter
• CPC Class Codes: G01B 9/02004 using frequency scans
• CPC Class Codes: G01B 9/02008 by using a frequency comb
• CPC Class Codes: G01B 9/02036 by using chromatic effects, e.g. a wavelength dependent focal point
• CPC Class Codes: G01B 9/02042 Confocal imaging
• CPC Class Codes: G01B 9/02057 by using common path configuration, i.e. reference and object path almost entirely overlapping
• CPC Class Codes: G01B 9/02091 Tomographic interferometers, e.g. based on optical coherence
• CPC Class Codes: G02B 21/0056 based on optical coherence, e.g. phase-contrast arrangements, interference arrangements

Abstract

A method and an arrangement are provided for scalable confocal interferometry for distance measurement, for 3-D detection of an object, for OC tomography with an object imaging interferometer and at least one light source. The interferometer has an optical path difference not equal to zero at each optically detected object element. Thus, the maxima of a sinusoidal frequency wavelet, associated with each detected object element, each have a frequency difference Δf_Objekt. At least one spectrally integrally detecting, rastered detector is arranged to record the object. The light source preferably has a frequency comb, and the frequency comb differences Δf_Quelle are changed in a predefined manner over time in a scan during measuring. In the process, the frequency differences Δf_Quelle are made equal to the frequency difference Δf_Objekt or equal to an integer multiple of the frequency differences Δf_Objekt at least once for each object element.

Claims

1. A method for interferometry comprising:

production of an electromagnetic measurement signal;

splitting of the measurement signal into a scanning beam component and a reference beam component;

illumination of at least one object point with at least a portion of the scanning beam component;

production of an interference signal by superimposition of a component of the scanning beam component that has been reflected from the at least one object point with the reference beam component, wherein that portion of the scanning beam component in the interference signal which has been reflected from the at least one object point has an optical path-length difference x_O, which depends on the position of the object point, relative to the reference beam component,

wherein the measurement signal is produced with a frequency comb spectrum with the same frequency comb intervals Δ

f_Signal of the individual frequency components, and/or wherein the interference signal is filtered by means of a frequency comb filter such that the filtered interference signal now has only a frequency comb spectrum with the same frequency comb intervals Δ

f_Signal of the individual frequency components; and

wherein the method furthermore comprises;

variation of the frequency comb intervals Δ

f_Signal in the frequency comb spectrum and/or variation of the optical path-length difference x_O over time such that the frequency comb intervals Δ

f_Signal correspond at least at times to an integer multiple of the quotient c/x_O of the speed of light c and the optical path-length difference x_O; and

detection of an intensity and/or of an intensity change in the interference signal for a multiplicity of frequency comb intervals Δ

f_Signal and/or for a multiplicity of optical path-length differences x_O.

2. The method of claim 1, wherein a section of a position-resolving detector, in particular of a detector array, having a multiplicity of optical detector elements is used as an optical detector element.

3. The method of claim 1, wherein the production of the electromagnetic measurement signal comprises:

production of an electromagnetic output signal with a continuous spectrum; and

filtering of the output signal by means of a tunable multi-beam interferometer in order to produce the electromagnetic measurement signal with a frequency comb spectrum such that the frequency comb intervals Δ

f_Signal of the measurement signal are varied over time in a modulation interval ([Δ

f_Signal_min;

Δ

f_Signal_max]) of the frequency comb intervals.

4. The method of claim 1, furthermore comprising determination of a frequency comb modulation interval Δ

f_Source_Mod from the detected values of the intensity and/or intensity changes of the interference signal, wherein the frequency comb modulation interval Δ

f_Source_Mod is determined

as the frequency comb interval Δ

f_Signal for the maximum modulation of the detected signal profile of the interference signal; and

/or

as the frequency comb interval Δ

f_Signal for the signal maximum in the detected signal profile of the interference signal, and/or;

as the frequency comb interval Δ

f_Signal at the signal centroid of the detected signal profile of the interference signal.

5. The method of claim 4, furthermore comprising determination of a value of the optical path-length difference x_O from the frequency comb modulation interval Δ

f_Source_mod using x_O=c/Δ

f_Source_Mod with the speed of light c.

6. The method of claim 1, which comprises:

a first scanning process such that the frequency comb intervals Δ

f_Signal are in this case varied continuously; and

a second scanning process, which is carried out repeatedly during the first scanning process, such that the optical path-length difference x_O is in this case varied continuously such that the continuous change in the quotient c/x_O resulting from this corresponds to the mathematical sign after the continuous change, at least at times, in the frequency comb intervals Δ

f_Signal which occurred in the first scanning process,

wherein an intensity and/or an intensity change of the interference signal are/is in each case detected during the second scanning process.

7. The method of claim 1, which comprises:

a first scanning process such that the optical path-length difference x_O is in this case varied continuously; and

a second scanning process, which is carried out repeatedly during the first scanning process, such that the frequency comb intervals Δ

f_Signal are in this case varied continuously such that the change in the frequency comb intervals Δ

f_Signal corresponds to the mathematical sign after the continuous change, at least at times, in the quotient c/x_O which resulted from the first scanning process,

wherein an intensity and/or an intensity change of the interference signal are/is in each case detected during the second scanning process.

8. The method of claim 1, which comprises:

a first scanning process, in that an optical delay length or path length Y of the signal path in the measurement signal source is varied continuously; and

a second scanning process, which is carried out repeatedly during the first scanning process, such that the optical delay length or path length Y of the signal path in the measurement signal source is in this case varied continuously such that the change in the optical delay length resulting from the second scanning process is opposite the mathematical sign after the change, at least at times, in the optical delay length resulting from the first scanning process,

wherein an intensity and/or an intensity change of the interference signal are/is in each case detected during the second scanning process.

9. The method of claim 8, wherein an intensity and/or intensity change is detected during a detector integration time period Δ

tD, during which the magnitude of the phase in the interference signal changes through no more than 180 degrees.

10. The method of claim 8, wherein the second scanning process has a sawtooth profile of the optical path-length difference x_O or of the reciprocal 1/Δ

f_Signal of the frequency comb intervals Δ

f_Signal or of the optical delay length Y in the measurement signal source over time, and wherein an intensity and/or intensity change of the interference signal is detected during the long flank of the sawtooth profile.

11. The method of claim 8, wherein the second scanning process is in the form of a harmonic oscillation of the optical path-length difference x_O or of the reciprocal 1/Δ

f_Signal of the frequency comb intervals Δ

f_Signal over time, and wherein an intensity and/or intensity change of the interference signal is detected in a time period which includes the harmonic oscillation passing through that point of inflection of the spatial oscillation movement at which the change in the frequency comb intervals Δ

f_Signal corresponds to the mathematical sign after the change of the quotient c/x_O.

12. The method of claim 9, wherein the second scanning process is in the form of a harmonic oscillation of the optical delay length Y in the measurement signal source over time, and wherein an intensity and/or intensity change of the interference signal is detected in a time period which includes the harmonic oscillation passing through that point of inflection of the spatial oscillation movement at which the change in the optical delay length Y resulting from the second scanning process is opposite the mathematical sign after the change in the optical delay length Y resulting from the first scanning process.

13. An apparatus for interferometry comprising:

a measurement signal source for production of an electromagnetic measurement signal;

an interferometer arrangement which is designed

to split the measurement signal into a scanning beam component and a reference beam component;

to illuminate at least one object point with at least one portion of the scanning beam component; and

to produce an interference signal by superimposition of a component of the scanning beam component which has been reflected from the at least one object point with the reference beam component, wherein that portion of the scanning beam component in the interference signal which has been reflected from the at least one object point has an optical path-length difference x_O, which depends on the position of the object point, relative to the reference beam component;

wherein the measurement signal source is designed to produce the measurement signal with a frequency comb spectrum with the same frequency comb intervals Δ

f_Signal of the individual frequency components, and/or wherein the apparatus also comprises a frequency comb filter which is designed to filter the interference signal such that the filtered interference signal now has only a frequency comb spectrum with the same frequency comb intervals Δ

f_Signal of the individual frequency components; and

wherein the apparatus also comprises;

a control device for varying the frequency comb intervals Δ

f_Signal in the frequency comb spectrum and/or for varying the optical path-length difference x_O over time such that the frequency comb intervals Δ

f_Signal correspond at least at times to an integer multiple of the quotient c/x_O of the speed of light c and the optical path-length difference x_O; and

at least one detector element for detection of an intensity and/or intensity change in the interference signal for a multiplicity of frequency comb intervals Δ

f_Signal and/or for a multiplicity of optical path-length differences x_O.

14. The apparatus of claim 13, wherein the interferometer arrangement comprises a Fizeau interferometer and/or a Michelson interferometer and/or a Twyman-Green interferometer, and/or a Mirau interferometer and/or a Linnik interferometer and/or a Mach-Zehnder interferometer.

15. The apparatus of claim 13, wherein the measurement signal source comprises a tunable frequency comb laser.

16. The apparatus as claimed in claim 13, wherein the measurement signal source comprises:

a radiation source for production of an electromagnetic output signal with a continuous spectrum; and

a frequency comb filter for filtering the output signal in order to produce the electromagnetic measurement signal with a frequency comb spectrum such that the frequency comb intervals Δ

f_Signal of the measurement signal can be varied over time in a modulation interval ([Δ

f_Signal_min;

Δ

f_Signal_max]) of the frequency comb intervals.

17. The apparatus as claimed in claim 13, comprising an optical waveguide for transmission of the measurement signal from the measurement signal source to the interferometer arrangement.

18. The apparatus as claimed in claim 13, wherein the control device is designed to control a first and a second scanning process synchronously such that

in a first scanning process, the frequency comb intervals Δ

f_Signal are varied continuously; and

in the second scanning process, which is carried out repeatedly during the first scanning process, the optical path-length difference x_O is varied continuously such that the continuous change in the quotient c/x_O resulting from this corresponds to the mathematical sign after the continuous change, at least at times, in the frequency comb intervals Δ

f_Signal which occurred in the first scanning process,

and wherein the control device is designed to control the at least one detector element such that an intensity and/or intensity change of the interference signal are/is in each case detected during the second scanning process.

19. The apparatus as claimed in claim 13, wherein the control device is designed to control a first and a second scanning process synchronously such that

in the first scanning process, the optical path-length difference x_O is varied continuously;

and

in the second scanning process, which is carried out repeatedly during the first scanning process, the frequency comb intervals Δ

f_Signal are varied continuously such that the change in the frequency comb intervals Δ

f_Signal corresponds to the mathematical sign after the continuous change, at least at times, in the quotient c/x_O which resulted from the first scanning process,

and wherein the control device is designed to control the at least one detector element such that an intensity and/or intensity change of the interference signal are/is in each case detected during the second scanning process.

20. The apparatus of claim 13, wherein the measurement signal source comprises:

a first signal scanning device for carrying out a first scanning process such that an optical delay length or path length Y of the signal path in the measurement signal source is in this case varied continuously; and

a second signal scanning device for carrying out a second scanning process, which is carried out repeatedly during the first scanning process, such that the optical delay length or path length Y of the signal path in the measurement signal source is in this case varied continuously such that the change in the optical delay length resulting from the second signal scanning process is opposite the mathematical sign after the change, at least at times, in the optical delay length resulting from the first signal scanning process,

and wherein the control device is designed to control the at least one detector element such that an intensity and/or intensity change of the interference signal are/is in each case detected during the second scanning process.