OPTICAL DEVICE, SYSTEM AND METHOD FOR DISPERSION INTERFEROMETRY

Abstract

An optical device, a system and a method for dispersion interferometry includes a frequency doubling device and an optical modulation device, a transmission beam path which is configured to emit a first and second measurement beam on an optical element and a reception beam path which is configured to receive a first and second measuring beam returned by the optical element.

Claims

1. An optical device, comprising a laser beam source for emitting a first laser beam having a fundamental frequency of the laser beam source; a frequency doubling device and an optical modulation device for generating a second laser beam at a frequency of a second harmonic of the fundamental frequency, and a first diffracted laser beam having an intensity of the first diffraction order of a modulated fundamental frequency, and a second diffracted laser beam having an intensity of the second diffraction order of the second harmonic; a reference beam path which is designed to image a first and second reference beam onto a receiving unit; a transmission beam path which is designed to emit a first and second measurement beam onto an optical element; a reception beam path which is designed to receive a first and second measuring beam returned by the optical element; a superposition unit which is used for superimposing the first and second reference beams on the returned first and second measurement beams and for imaging the superimposed returned first and second measurement beams and first and second reference beams onto the receiving unit, wherein the receiving unit is configured for receiving the superimposed returned first and second measuring beams and first and second reference beams and for converting them into an electrical reception signal; and an evaluation unit which is configured to at least evaluate the electrical reception signal.

2. The optical device according to claim 1, wherein the first and second measurement beams are configured as first and second laser beams and the first and second reference beams are configured as first and second diffracted laser beams or wherein the first and second measurement beams are configured as first and second diffracted laser beams and the first and second reference beams are configured as first and second laser beams.

3. The optical device according to claim 1, wherein the frequency doubling device is arranged in the transmission beam path before the optical modulation device.

4. The optical device according to claim 1, wherein the frequency doubling device is arranged in the transmission beam path after the optical modulation device.

5. The optical device according to claim 1, wherein the frequency doubling device has at least one wedge, by means of which an offset angle between the first laser beam and the second laser beam can be compensated by at least one wedge.

6. The optical device according to claim 1, wherein the first and second diffracted laser beams are emitted collinearly at an angle greater than 0 mrad relative to the first and second laser beams.

7. The optical device according to claim 1, wherein the superposition unit is designed as a beamsplitter, wherein the returned first and second measuring beams pass through the superposition unit and the first and second reference beams are reflected by the superposition unit, or wherein the returned first and second measurement beams are reflected by the superposition unit and the first and second measurement beam and the first and second reference beam pass through the superposition unit

8. The optical device according to claim 7, wherein a normal vector of the beamsplitter surface lies on a bisector between the reference beams and the returned measurement beams

9. The optical device according to claim 1, wherein an optical system for beam adjustment is arranged in the reference beam path and/or in the transmission beam path and/or in the reception beam path.

10. The optical device according to claim 1, wherein a beam deflection unit is arranged in the transmission beam path and/or in the reception beam path, which beam deflection unit splits the transmission beam path and/or the reception beam path into at least one first beam path and a second beam path.

11. The optical device according to claim 1, wherein at least one mirror for deflecting the first and second reference beams and/or the first and second measurement beams and/or the returned first and second measurement beams is arranged in the reference beam path (and/or in the transmission beam path and/or in the reception beam path.

12. A system for dispersion interferometry, at least comprising an optical device according to claim 1.

13. A method for dispersion interferometry with an optical device, comprising: emitting a first laser beam by means of a laser beam source having a fundamental frequency of the laser beam source; generating a second laser beam at a frequency of a second harmonic of the fundamental frequency, and a first diffracted laser beam having an intensity of the first diffraction order of a modulated fundamental frequency, and a second diffracted laser beam having an intensity of the second diffraction order of the second harmonic frequency by means of a frequency doubling device and an optical modulation device; imaging a first and second reference beam in a reference beam path onto a receiving unit; emitting a first and second measurement beam in a transmission beam path onto an optical element; receiving a first and second measuring beam returned by the optical element in a reception beam path; superimposing the returned first and second measurement beams on the first and second reference beams by a superposition unit and imaging onto the receiving unit; receiving the superimposed returned first and second measurement beams and the first and second reference beams by the receiving unit and converting into an electrical reception signal; evaluating the received electrical signal by an evaluation unit.

14. The method according to claim 13, wherein the first and second laser beams are used as the first and second measuring beams and the first and second diffracted laser beams are used as the first and second reference beams, or wherein the first and second diffracted laser beams are used as the first and second measurement beams and the first and second laser beams are used as the first and second reference beams.

15. The method according to claim 13, wherein the second laser beam is generated from the first laser beam by the frequency doubling device and the first and second diffracted laser beams are generated from the first and second laser beams by frequency modulation with a modulation frequency in the optical modulation device.

16. The method according to claim 13, wherein the first diffracted laser beam is generated from the first laser beam by frequency modulation with a modulation frequency in the optical modulation device and the second laser beam and the second diffracted laser beam are generated from the first laser beam and the first diffracted laser beam by the frequency doubling device.

17. The method according to claim 13, wherein an offset angle between the first laser beam and the second laser beam is compensated for by at least one wedge.

18. The method according to claim 13, wherein the first and second diffracted laser beams are emitted collinearly at an angle relative to the first and second laser beams.

19. The method according to claim 13, wherein the returned first measuring beam is superimposed with the first reference beam and the returned second measuring beam is superimposed with the second reference beam by the superposition unit

20. The method according to claim 13, wherein the electrical reception signal is demodulated and/or filtered in the evaluation unit, in particular wherein the electrical reception signal (44) is demodulated in the evaluation unit (24) by means of quadrature demodulation and is filtered using a low-pass filter, and/or wherein the received electrical signal is mixed in the evaluation unit by a two-tone signal offset from the fundamental frequency and is demodulated by quadrature demodulation and filtered with a low-pass filter.

21. The method according to claim 13, wherein phase differences between the first reference beam and the returned first measurement beam as well as the second reference beam and the returned second measurement beam are determined in the receiving unit separately in separable frequency ranges for the fundamental frequency and the second harmonic

22. The method according to claim 21, wherein a relative dispersion between the fundamental frequency and the second harmonic is determined by difference formation of the phase differences

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Further advantages will be apparent from the following description of the drawings. Exemplary embodiments of the invention are shown in the figures. The figures, the description, and the claims contain numerous features in combination. A person skilled in the art will expediently also consider the features individually and combine them into further meaningful combinations.

[0071] In the Exemplary Figures:

[0072] FIG. 1 shows a system for dispersion interferometry with an optical device according to an exemplary embodiment of the invention in a schematic representation;

[0073] FIG. 2 shows a system for dispersion interferometry with an optical device according to a further exemplary embodiment of the invention with an additional optical system in a schematic representation;

[0074] FIG. 3 shows a system for dispersion interferometry with an optical device according to a further exemplary embodiment of the invention with swapped frequency doubling device and modulation device in a schematic representation;

[0075] FIG. 4 shows a system for dispersion interferometry with an optical device according to a further exemplary embodiment of the invention, in which the returned measurement beams are deflected, in a schematic representation; and

[0076] FIG. 5 shows a splitting of the transmission beam path and/or reception beam path into different beam paths according to a further exemplary embodiment.

DETAILED DESCRIPTION

[0077] In the figures, identical or identically acting components are identified by the same reference signs. The figures only show examples and are not to be understood as restrictive.

[0078] Directional terminology used in the following with terms such as left, right, above, below, in front of, behind, after, and the like only serves for better comprehension of the figures and is in no way intended to restrict the generality. The components and elements shown, their design and use can vary according to the considerations of a person skilled in the art and can be adapted to the respective applications.

[0079] FIG. 1 shows a system 200 for dispersion interferometry with an optical device 100 according to an exemplary embodiment of the invention in a schematic representation;

[0080] The optical device 100 has a laser beam source 10 for emitting a first laser beam 31 with a fundamental frequency of the laser beam source 10 and a frequency doubling device 12 and an optical modulation device 16 which is controlled by an oscillator 14 with a modulation frequency. The frequency doubling device 12 is arranged in the transmission beam path 50 before the optical modulation device 16.

[0081] A second laser beam 33 is generated by the frequency doubling device 12. First and second diffracted laser beams 34, 35 are generated from the first and second laser beams 31, 33 by frequency modulation with a modulation frequency in the optical modulation device 16.

[0082] The first diffracted laser beam 34 has an intensity of the first diffraction order of the modulated fundamental frequency, while the second diffracted laser beam 35 has an intensity of the second diffraction order of the second harmonic.

[0083] The second laser beam 33 is generated from the first laser beam 31 in the frequency doubling device 12 at a frequency of a second harmonic of the fundamental frequency. The frequency of the first laser beam 31 of the laser beam source 10, for example a continuous wave laser, is thus doubled in the frequency doubling device 12, for example a non-linear optical crystal. Except for a small offset angle, which can optionally be compensated with at least one wedge, the optical beams of the fundamental frequency and the second harmonic run collinearly through the transmission beam path 50 of the optical device 100.

[0084] Both laser beams 31, 33 pass through the optical modulation device 16, in which the first diffracted laser beam 34 and the second diffracted laser beam 35 are generated therefrom by frequency modulation with a modulation frequency. An acousto-optic modulator or frequency shifter 16, by applying a high-frequency signal from the oscillator 14 to the Bragg crystal contained in the frequency shifter 16, diffracts part of the light as a diffracted laser beam 34, 35 at a small angle away from the first and second laser beams 31, 33. The first diffracted laser beam 34 has an intensity of the first diffraction order of the fundamental frequency and the second diffracted laser beam 35 has an intensity of the second diffraction order of the second harmonic.

[0085] The Bragg crystal can advantageously be rotated in such a way that the first diffraction order for the fundamental frequency and the second diffraction order for the second harmonic fulfill the diffraction condition or Bragg condition. In contrast to the prior art, the beams diffracted in this way are still collinear.

[0086] The first and second diffracted laser beams 34, 35 are emitted collinearly at an angle 28 greater than 0 mrad relative to the first and second laser beams 31, 33. In particular, the angle 28 can be at most 50 mrad, preferably at most 10 mrad.

[0087] The first and second laser beams 31, 33 are emitted as a measurement beam 30, 32 in a transmission beam path 50 onto an optical element 46. The optical element 46 can reflect and/or scatter the first and second measurement beam 30, 32. A first and second measurement beam 36, 38 returned by the optical element 46, are received in a reception beam path 52. The transmission beam path 50 and the reception beam path 52 can be identical.

[0088] The first and second measurement beam 30, 32 and the first and second measurement beam 30, 32 returned by the optical element 46 pass through the medium, the dispersion of which is to be determined, on their way from the optical device 100 to the optical element 46 and back again.

[0089] The first and second diffracted laser beams 34, 35 are imaged as a reference beam 40, 42 in a reference beam path 54 by deflection via a mirror 18 and a superposition unit 20, which is implemented, for example, by a beamsplitter in the form of a semi-transparent mirror, onto a receiving unit 22.

[0090] Alternatively, the first and second diffracted laser beam 34, 35 can also be used as a first and second measurement beam 30, 32 and the first and second laser beam 31, 33 can be used as the first and second reference beam 40, 42.

[0091] After passing through the superposition unit 20, the returned first and second measurement beams 36, 38 are superimposed with the first and second reference beams 40, 42 and imaged on the receiving unit 22.

[0092] The superposition unit 20 superimposes the first reference beam 40 on the returned first measurement beam 36 and the second reference beam 42 on the returned second measurement beam 38.

[0093] The sum of the length of the transmission beam path 50 and the reception beam path 52 is referred to as the measurement arm length 56 of the interferometer. The optical device 100 can advantageously be operated with a variable measurement arm length 56, for example by folding a further reflector into the reception beam path 52. In this way, an absolute calibration of the intensity of the optical beams 30, 32, 36, 38, 40, 42 can be carried out.

[0094] The returned measurement beam 36, 38 is superimposed at the beamsplitter 20 with the reference beam 40, 42 and is imaged on the receiving unit 22, which can be a photodiode, for example.

[0095] In the exemplary embodiment in FIG. 1, the superposition unit 20 is embodied as a beamsplitter, wherein the returned first and second measurement beams 36, 38 pass through the superposition unit 20 and the first and second reference beams 40, 42 are reflected by the superposition unit 20.

[0096] The interference arising there contains a beat signal at the modulation frequency of the acousto-optic modulation device 16 and the second harmonic thereof. These beat signals can be assigned directly to the respective returned measurement signals 36, 38 and separated into separate frequency bands.

[0097] The receiving unit 22 receives the superimposed returned first and second measurement beams 36, 38 and first and second reference beams 40, 42 and converts them into an electrical reception signal 44.

[0098] An evaluation unit 24 receives, digitizes and evaluates the electrical reception signal 44.

[0099] The electrical reception signal 44 can advantageously be digitized in the evaluation unit 24 using an analog/digital converter. The two band-limited signals are demodulated, for example quadrature demodulated, and low-pass filtered using digital signal processing. The relative dispersion between the fundamental frequency and the second harmonic can thus be determined by numerical subtraction. The relative dispersion is directly dependent on the environmental parameters, such as pressure, temperature, air humidity, CO.sub.2 content and thus enables these parameters to be measured. The purely digital processing simplifies the measurement setup considerably.

[0100] For this purpose, the electrical reception signal 44 is mixed in the evaluation unit 24 by a two-tone signal (+)+2(+) with a deviation from the fundamental frequency , in particular with the modulation frequency Q, to the frequencies and 2 .

[0101] Phase differences between the first reference beam 40 and the returned first measurement beam 36 as well as the second reference beam 42 and the returned second measurement beam 38 are determined in the receiving unit 22 separately in separable frequency ranges for the fundamental frequency and the second harmonic.

[0102] A relative dispersion between the fundamental frequency and the second harmonic can be determined by, in particular numerical, subtraction of the phase differences.

[0103] According to the method according to the invention for dispersion interferometry using the optical device 100, a first laser beam 31 is emitted by the laser beam source 10 with the fundamental frequency of the laser beam source 10. The wave equation of laser light is as follows:

[00001]$\sin \left(t-\mathrm{kx}\right)$$k=\frac{n}{c}=\text{?}$$\text{?}\text{indicates text missing or illegible when filed}$ [0104] wherein is the angular frequency of the laser radiation (2*fundamental frequency of the laser light), t is the time, k is the wave vector, x is the location along the transmission beam path 50, n is the (frequency-dependent) refractive index of the medium passed through, c is the speed of light, .sub.vacuum is the wavelength of laser radiation in a vacuum.

[0105] A second laser beam 33 is generated at the frequency 2e of the second harmonic of the fundamental frequency by means of the frequency doubling device 12. A first diffracted laser beam 34 and a second diffracted laser beam 35 are generated by the optical modulation device 16 with an offset to the fundamental frequency to or an offset 2 to twice the fundamental frequency 2.

[0106] The first diffracted laser beam 34 has an intensity of the first diffraction order of the fundamental frequency, while the second diffracted laser beam 35 has an intensity of the second diffraction order of the second harmonic. The first and second diffracted laser beams 34, 35 are thus emitted collinearly at an angle 28 relative to the first and second optical beam 32, 34. In particular, the angle 28 can be at most 50 mrad, preferably at most 10 mrad.

[0107] The first and second diffracted laser beams 34, 35 are imaged as first and second reference beams 40, 42 in a reference beam path 54 onto the receiving unit 22. The first and second laser beams 31, 33 are emitted as a first and second measurement beam 30, 32 in a transmission beam path 50 onto an optical element 46.

[0108] Alternatively, the first and second diffracted laser beam 34, 35 can also be emitted as a first and second measurement beam 30, 32 and the first and second laser beam 31, 33 can be emitted as a first and second reference beam 40, 42.

[0109] A first and second measurement beam 36, 38 returned by the optical element 46 is received in a reception beam path 52 and superimposed with the first and second reference beam 40, 42 by a superposition unit 20 and imaged on the receiving unit 22.

[0110] The receiving unit 22 receives the superimposed returned first and second measurement beams 36, 38 and first and second reference beams 40, 42 and converts them into an electrical reception signal 44. The electrical reception signal 44 is received and digitized by an evaluation unit 24.

[0111] After passing through the medium, the dispersion of which is to be determined, on the way along the transmission beam path 50 and the reception beam path 52, the phases .sub., .sub.2 of the laser light of the returned first and second measurement beams 36, 38 result at the receiving unit 22:

[00002]${}_{}=t-\frac{\text{?}\left(\right)\left(l+l_{\mathrm{vibration}}\right)}{}$${}_2=2t-\frac{\text{?}\left(\right)\left(l+l_{\mathrm{vibration}}\right)}{\text{?}}$$\text{?}\text{indicates text missing or illegible when filed}$ [0112] l represents the measuring arm length 56 of the interferometer, namely the sum of the lengths of the transmission beam path 50 and the reception beam path 52. l.sub.vibration represents a change in the measuring arm length 56 of the interferometer due to mechanical vibrations.

[0113] The phases of the laser light of the two reference beams 40, 42 result in:

.sub.reference=(+)t+.sub.NCP

.sub.2reference=2(+)t+.sub.NCP2 [0114] wherein represents the modulation frequency of the modulation device 16, predetermined by the oscillator 14, and .sub.NCP, .sub.NCP2 represent the phases of a so-called non common path (NCP). The non-common path represents the part of the beam path on which the first and second reference beam 40, 42 are not superimposed with the returned first and second measurement beam 36, 38, namely the beam path between the optical modulation device 16 via the mirror 18 to the superposition unit 20.

[0115] If the phases .sub., .sub.2 of the returned measurement beams 36, 38 are subtracted from the phases .sub.reference, .sub.2reference of the respective reference beams 40, 42, phase differences .sub., .sub.2 measured in the receiving unit 22 result:

[00003]${}_{}=t+{}_{\mathrm{NCP}}+\frac{2n\left(\right)\left(l+l_{\mathrm{vibration}}\right)}{}$${}_2=2t+{}_{\mathrm{NCP}2}+\frac{2n\left(2\right)\left(l+l_{\mathrm{vibration}}\right)}{\text{?}}$$\text{?}\text{indicates text missing or illegible when filed}$

[0116] The evaluation of the phase differences in the evaluation unit 24 thus yields:

[00004]${}_2=2{}_{}+{}_{\mathrm{NCP}2}-2{}_{\mathrm{NCP}}+\left(n\left(2\right)-n\left(\right)\right)\text{?}{}_{\mathrm{NCP}2}-2{}_{\mathrm{NCP}}+\left(n\left(2\right)-n\left(\right)\right)\text{?}\mathrm{with}l{l}_{\mathrm{vibration}}.$$\text{?}\text{indicates text missing or illegible when filed}$

[0117] Thus, the refractive index difference n(2)-n() of the dispersion of the medium can be determined.

[0118] FIG. 2 shows a system 200 for dispersion interferometry with an optical device 100 according to a further exemplary embodiment of the invention with an additional optical system 26 in a schematic representation.

[0119] The optical system 26 can be arranged in the reference beam path 54 and/or in the transmission beam path 50 and/or in the reception beam path 52 for beam adjustment. The optical system 26 also has beam shaping optics, which can optionally be implemented in the beam path 50, 52, 54.

[0120] The optics are used to adjust the beam propagation, which is different in the reference beam path 54 with respect to the transmission beam path 50 and reception beam path 52, so that the interference of the beams 36, 38, 40, 42 is maximized. In the exemplary embodiment illustrated in FIG. 2, one optic of the optical system 26 is respectively arranged in the transmission beam path 50, in the reception beam path 52 and in the reference beam path 54 between the mirror 18 and the superposition unit 20. The optical system 26 can be a lens system, for example, as shown in FIG. 2. Alternatively, the optical system 26 can also comprise mirrors and/or holographic elements.

[0121] FIG. 3 shows a system 200 for dispersion interferometry with an optical device 100 according to a further exemplary embodiment of the invention with swapped frequency doubling device 12 and modulation device 16 in a schematic representation.

[0122] The first diffracted laser beam 34 is generated by frequency modulation with the modulation frequency in the optical modulation device 16 and the second laser beam 33 and the second diffracted laser beam 35 are generated by the frequency doubling device 12.

[0123] The frequency doubling device 12 is arranged in the transmission beam path 50 after the optical modulation device 16. In this alternative embodiment, the first diffracted laser beam 34 is first generated from the first laser beam 31 at the fundamental frequency by means of the modulation device 16, before the second laser beam 33 of the second harmonic is then generated from the first laser beam 31 by frequency doubling and the second diffracted laser beam 35 is generated from the first diffracted laser beam 34. For reasons of arrangement, two separate frequency doubling devices 12 for the first and second laser beams 31, 33 and for the two diffracted laser beams 34, 35 may be required.

[0124] FIG. 4 shows a system 200 for dispersion interferometry with an optical device 100 according to a further exemplary embodiment of the invention, in which the returned measurement beams 36, 38 are deflected, in a schematic representation.

[0125] In this exemplary embodiment, the transmission beam path 50 and the reception beam path 52 are designed to be collinear. These can be split again into separate beam paths using conventional methods, such as beamsplitters, and can therefore be further processed as before.

[0126] Alternatively, as shown in FIG. 4, the superposition unit 20 designed as a beamsplitter can be arranged directly after the modulation device 16. For example, the beamsplitter can be formed by an equivalently angled and coated facet of the modulation device 16.

[0127] The returned first and second measurement beams 36, 38 are reflected by the superposition unit 20, while the first and second measurement beams 30, 32 and the first and second reference beams 40, 42 pass through the superposition unit 20.

[0128] The normal vector 48 of the beamsplitter surface 58 lies on the bisector of the angle between the reflected, returned measurement beam 36, 38 or reference beam 40, 42 and the measurement beam 30, 32. In this way, the returned measurement beam 36, 38 is reflected onto the reference beam 40, 42 and made to interfere.

[0129] In addition, to compensate for beam changes due to propagation, compensation optics can be mounted in both or one of the two beam paths 50, 52; 54. The first and second measurement beams 30, 32 as well as the returned first and second measurement beams 36, 38 can also, as shown in FIG. 4, be imaged by elements of an optical system 26, such as lenses or objectives, onto the optical element 46, as well as the reception unit 22.

[0130] FIG. 5 shows a splitting of the transmission beam path 50 and/or the reception beam path 52 into different beam paths 62, 64 according to a further exemplary embodiment.

[0131] FIG. 5 shows how it is possible to switch between different beam paths 62, 64 in the beam path 50, 52. Typically, the measurement beam 30, 32 can be guided differently through a measuring volume by beam offset or beam deflection, for example by a scanner, so that a suitable optical element 46 in the form of a reflector or scatterer is impinged upon. By varying the measurement distance, free parameters in the evaluation of the measured phases can be calibrated with respect to pressure or gas concentration without having to resort to additional measuring systems. Typical methods to manipulate the beam are polarizing beamsplitters and a polarization rotating element such as a rotatable lambda/2 plate, as well as rotatable prisms or mirrors, liquid crystals and the like.

[0132] For this purpose, a beam deflection unit 60 can be arranged in the transmission beam path 50 and/or in the reception beam path 52, which unit splits the transmission beam path 50 and/or the reception beam path 52 into a first beam path 62 and a second beam path 64. In particular, the first beam path 62 or the second beam path 64 can be selected as the transmission beam path 50 and/or as the reception beam path 52 by means of the beam deflection unit 60. Alternatively, a second receiving unit 22 can be added in order to evaluate both beam paths 62, 64 in parallel.

LIST OF REFERENCE NUMERALS

[0133] 10 laser beam source [0134] 12 frequency doubling device [0135] 14 oscillator [0136] 16 optical modulation device [0137] 18 mirror [0138] 20 superposition unit [0139] 22 receiving unit [0140] 24 analysis unit [0141] 26 optical system [0142] 28 angle [0143] 30 first measurement beam [0144] 31 first laser beam [0145] 32 second measurement beam [0146] 33 second laser beam [0147] 34 first diffracted laser beam [0148] 35 second diffracted laser beam [0149] 36 returned first measurement beam [0150] 38 returned second measurement beam [0151] 40 first reference beam [0152] 42 second reference beam [0153] 44 electrical reception signal [0154] 46 optical element [0155] 48 normal vector [0156] 50 transmission beam path [0157] 52 reception beam path [0158] 54 reference beam path [0159] 56 arm length of interferometer [0160] 58 beamsplitter surface [0161] 60 beam deflection unit [0162] 62 first beam path [0163] 64 second beam path