JP2018009909A - Fourier transform type spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Fourier transform type spectrometer which can suppress reduction in the SNR if the mechanical accuracy of an interferometer is low.SOLUTION: A Fourier transform type spectrometer 1 includes: a beam splitter 14; a movable mirror 15 for reflecting a light flux which propagates through a variable-length optical path to the direction of the beam splitter 14; a fixed mirror 16 for reflecting a light flux which propagates through a fixed-length optical path to the direction of the beam splitter 14; a mirror driving part 24 for moving the movable mirror 15 and changing the length of the variable length optical path; a correction light receiving element 32 for photoelectrically converting an interference light output from the beam splitter 14 and generating a correction detection signal; a modulation degree measuring unit 34 for measuring the degree of modulation of an interferogram based on the correction detection signal; an optical deflector 30 having a variable deviation angle mechanism for deflecting a propagation light flux in the variable-length optical path; and a light deflection control unit 31 for deflecting the propagation light flux to a direction in which the degree of modulation is increased, by controlling the optical deflector 30.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a spectroscopic device, and more particularly to a Fourier transform spectroscopic device having a Michelson interferometer.

  In recent years, Fourier Transform Spectrometer (FTS) having a Michelson interferometer has been widely used. This type of Fourier transform spectrometer includes a beam splitter that divides observation light into two light beams, a fixed mirror that is fixed to the beam splitter, and a movement that moves along the optical axis with respect to the beam splitter. With a mirror. The beam splitter emits one of the two light beams toward the fixed mirror, and simultaneously emits the other of the two light beams toward the moving mirror. The beam splitter combines the return light beam reflected by the mirror surface of the fixed mirror and the return light beam reflected by the mirror surface of the movable mirror to generate interference light, and causes the light detection element to observe the interference light. Since the optical path length between the beam splitter and the fixed mirror does not change, and the optical path length between the beam splitter and the movable mirror changes with time, the optical path difference between the two return beams incident on the beam splitter is also Change over time. For this reason, the observation intensity of the interference light changes in a wave shape according to the change in the optical path difference. By subtracting the DC component from the observed intensity of the interference light, an intensity distribution called an interferogram is obtained. The spectrum of the observation light can be obtained by Fourier transforming the interferogram. For example, Patent Document 1 (Japanese Patent Laid-Open No. 7-243806) discloses a conventional technique related to such a Fourier transform spectrometer.

JP-A-7-243806

  In the Fourier transform spectrometer described above, the higher the degree of modulation of the interferogram, the higher the signal-to-noise ratio (Signal-to-Noise Ratio, SNR) of the spectrum, and a high-accuracy spectrum can be obtained. However, if the mechanical accuracy of the Michelson interferometer including the beam splitter, fixed mirror, and moving mirror is low, the SNR decreases. For example, the mirror surface of the movable mirror or the mirror surface of the fixed mirror may be inclined without being orthogonal to the optical axis due to a disturbance such as a temperature change or the movement of the movable mirror. In such a case, the SNR decreases.

  In view of the above, an object of the present invention is to provide a Fourier transform spectroscopic device that can suppress a decrease in spectral SNR even if the mechanical accuracy of the Michelson interferometer is low.

  In the Fourier transform type spectroscopic device according to one aspect of the present invention, an incident light beam is demultiplexed into a first light beam and a second light beam, and the first light beam and the second light beam are respectively supplied to a variable length optical path and a fixed length optical path. A beam splitter that outputs and combines the return light flux from the fixed length optical path and the return light flux from the variable length optical path to output interference light, and the first light flux that propagates through the variable length optical path to the beam splitter. A movable mirror that reflects in the direction of, a fixed mirror that reflects the second light beam propagating through the fixed length optical path in the direction of the beam splitter, and the movable mirror is moved to change the optical path length of the variable length optical path. A mirror driving unit; a light receiving element that photoelectrically converts an incident light beam to generate a detection signal; a condensing optical system that causes the interference light output from the beam splitter to enter the light receiving element; and the detection signal A spectroscopic measurement unit that calculates a spectrum based on; a correction light-receiving element that photoelectrically converts the interference light output from the beam splitter to generate a correction detection signal; and an interferogram based on the correction detection signal A modulation degree measuring unit for measuring the modulation degree of the optical path, an optical deflector having a variable deflection mechanism for deflecting a light beam propagating in the variable length optical path or the fixed length optical path, and controlling the optical deflector to obtain the modulation degree. And an optical deflection controller that deflects the propagating light beam in a direction of increasing.

  According to the present invention, even if the mechanical accuracy of the Michelson interferometer including the beam splitter, the movable mirror, and the fixed mirror is lowered, the propagating light beam is deflected in the direction in which the degree of modulation of the interferogram is increased. As a result, a decrease in the SNR of the spectrum can be suppressed.

It is a figure which shows schematically the structure of the Fourier-transform type | mold spectrometer of Embodiment 1 which concerns on this invention. 3 is a diagram illustrating a configuration example of an optical deflector in Embodiment 1. FIG. FIG. 6 is a schematic diagram illustrating an operation example of the Fourier transform spectrometer according to the first embodiment. 4A is a graph showing the intensity distribution of the interferogram, and FIG. 4B is a graph showing the spectrum intensity distribution corresponding to the interferogram of FIG. 4A. FIG. 6 is a schematic diagram illustrating another operation example of the Fourier transform spectrometer according to the first embodiment. 6A and 6B are graphs showing the intensity distribution of the interferogram before correction. 7A and 7B are graphs showing the intensity distribution of the interferogram after correction. 8A and 8B are diagrams for explaining the function of the optical deflector according to the first embodiment. FIG. 6 is a diagram for explaining a function of the optical deflector according to the first embodiment. 3 is a flowchart schematically illustrating an example of a correction processing procedure according to the first embodiment. It is a figure which shows roughly the structure of the Fourier-transform type | mold spectrometer of Embodiment 2 which concerns on this invention. 12A and 12B are graphs showing intensity distributions of various interferograms. It is a figure which shows schematically the structure of the Fourier-transform type | mold spectrometer of Embodiment 3 which concerns on this invention.

  Hereinafter, various embodiments according to the present invention will be described in detail with reference to the drawings. In addition, the component to which the same code | symbol was attached | subjected in the whole drawing shall have the same function and the same structure.

Embodiment 1 FIG.
FIG. 1 is a diagram schematically showing a configuration of a Fourier transform spectrometer 1 (hereinafter also referred to as “Fourier spectrometer 1”) according to Embodiment 1 of the present invention. As shown in FIG. 1, the Fourier spectroscopic device 1 outputs an observation window 11 that outputs a parallel light beam to be measured as an observation light, and a correction light beam for correcting an optical axis angle shift in the Fourier spectroscopic device 1. A correction light source 12, a plane mirror 13 that bends the propagation direction of the observation light or the correction light beam by 90 °, a beam splitter 14 that splits the light beam (observation light or correction light beam) incident from the plane mirror 13 into two light beams, and The fixed mirror 16 disposed at a fixed position with respect to the beam splitter 14, the movable mirror 15 movable relative to the beam splitter 14 in the direction along the optical axis A1, the beam splitter 14 and the movable mirror 15 And an optical deflector 30 disposed in the optical path between the two. The beam splitter 14, the movable mirror 15, and the fixed mirror 16 are components of a Michelson interferometer (hereinafter also simply referred to as “interferometer”).

  Hereinafter, in this specification, spectrum measurement of observation light is also simply referred to as “measurement”, and correction of optical axis angle deviation using a correction light beam is also simply referred to as “correction”. Here, the optical axis angle deviation means an angle difference between the propagation directions of two light beams (principal rays) combined in the Fourier spectroscopic device 1.

  Further, the Fourier spectroscopic device 1 includes a condensing optical system 17 that condenses the incident light beam from the beam splitter 14, a measurement light receiving element 18 that receives the light beam condensed by the condensing optical system 17, An A / D converter (ADC) 19 that converts the analog output of the light receiving element 18 into a digital detection signal for measurement, a signal processing unit 20 that performs signal processing on the digital detection signal for measurement, and incidence from the beam splitter 14 A correction light receiving element 32 for receiving a light beam, an A / D converter (ADC) 33 for converting an analog output of the light receiving element 32 into a correction digital detection signal, and an interpolator based on the correction digital detection signal. Modulation degree measurement unit 34 that measures the modulation degree of the ferrogram, optical deflection control unit 31 that controls the operation of the optical deflector 30, and mirror drive that moves the movable mirror 15 along the optical axis A1. 24, is configured to include a drive control unit 25 that controls the operation of the mirror driving unit 24.

  The fixed mirror 16 has a reflecting mirror surface orthogonal to the optical axis A 2, and a fixed length optical path having a fixed optical path length is formed between the reflecting mirror surface and the beam splitter 14. On the other hand, the movable mirror 15 has a reflecting mirror surface orthogonal to the optical axis A1, and a variable length optical path having a variable optical path length is formed between the reflecting mirror surface and the beam splitter 14.

  The beam splitter 14 splits the incident light beam from the plane mirror 13 into a first light beam and a second light beam, and reflects the first light beam in the direction of the variable length optical path. The movable mirror 15 reflects the first light beam incident through the variable length optical path in the direction of the beam splitter 14. Thereby, the propagation direction of the first light flux is also folded. On the other hand, the beam splitter 14 transmits the second light flux to the fixed length optical path. The fixed mirror 16 reflects the second light beam incident through the fixed length optical path in the direction of the beam splitter 14. As a result, the propagation direction of the second light flux is folded. The beam splitter 14 reflects the return light beam from the fixed mirror 16 in the direction of the condensing optical system 17 or the light receiving element 32, and combines the reflected light beam with the return light beam from the movable mirror 15 to generate interference light. As will be described later, the beam splitter 14 is configured to output the interference light generated based on the observation light to the condensing optical system 17 and output the interference light generated based on the correction light beam to the light receiving element 32. Has been.

  The optical deflector 30 has a variable deflection mechanism that deflects a propagating light beam in a variable length optical path between the beam splitter 14 and the movable mirror 15. The optical deflector 30 includes, for example, one or a plurality of wedge prisms arranged in a variable length optical path, and a rotation driving unit that rotates each wedge prism in accordance with control by the optical deflection control unit 31. Can do. As will be described later, the optical deflector 30 and the optical deflection controller 31 can correct the optical axis angle deviation in the interferometer.

  FIG. 2 is a diagram schematically illustrating a configuration example of the optical deflector 30. The optical deflector 30 shown in FIG. 2 includes a pair of wedge prisms 41 and 42 and a prism drive unit (rotation drive unit) that rotates each of the wedge prisms 41 and 42 by a rotation angle specified by the light deflection control unit 31. 40. The configuration of the prism driving unit 40 includes, for example, a rotation holder that holds the wedge prisms 41 and 42, an electric motor that corresponds to each of the rotation holders, and a rotation driving force generated by these electric motors to the corresponding rotation holder. Although it is realizable with the transmission mechanism which transmits, it is not limited to this. The wedge prism 41 disposed on the beam splitter 14 side has a prism surface 41a that forms a flat surface perpendicular to the optical axis A1, and a prism surface 41b that is inclined with respect to the prism surface 41a. On the other hand, the wedge prism 42 disposed on the movable mirror 15 side includes a prism surface 42b that forms a flat surface perpendicular to the optical axis A1, and a prism surface 42a that is inclined with respect to the prism surface 42b. Yes. The wedge prisms 41 and 42 are arranged so that the prism surfaces 41a and 42b are parallel to each other.

  Next, the operation of the Fourier spectrometer 1 at the time of spectrum measurement will be described in detail. FIG. 3 is a diagram schematically showing a propagation path of observation light in the Fourier spectroscopic device 1 at the time of spectrum measurement. In the example of FIG. 3, for convenience of explanation, the reflecting mirror surface of the movable mirror 15 is inclined by an angle φ.

  The signal processing unit 20 includes a measurement control unit 21 and a spectroscopic measurement unit 22. At the time of spectrum measurement, the measurement control unit 21 supplies a drive start signal to the drive control unit 25. The drive control unit 25 controls the operation of the mirror drive unit 24 according to the drive start signal to reciprocate the movable mirror 15 at a constant speed in a direction parallel to the optical axis A1. At this time, the mirror driving unit 24 reciprocates the movable mirror 15 from a predetermined center position within a designated movable range in a direction parallel to the optical axis A1. As a result, the optical path length of the variable length optical path between the beam splitter 14 and the reflecting mirror surface of the movable mirror 15 changes with time. Here, the drive control unit 25 supplies distance data indicating the movement distance of the mirror drive unit 24 from the reference position to the signal processing unit 20 and the modulation degree measurement unit 34 in real time.

As shown in FIG. 3, the observation light L ₀ output from the observation window 11 is reflected by the plane mirror 13 in the direction of the beam splitter 14 and propagates along the optical axis A 2, and then enters the beam splitter 14. The beam splitter 14 splits the observation light L ₀ into a first light beam L ₁ propagating in the direction of the movable mirror 15 and a second light beam L ₂ propagating in the direction of the fixed mirror 16.

The first light beam L ₁ enters the movable mirror 15 via the optical deflector 30, and is reflected toward the beam splitter 14 by the movable mirror 15. The return light beam R ₁ reflected by the movable mirror 15 enters the beam splitter 14 through the optical deflector 30. On the other hand, the second light beam L ₂ generated by the beam splitter 14 is reflected by the fixed mirror 16 toward the beam splitter 14. The return light beam R ₂ reflected by the fixed mirror 16 is reflected by the beam splitter 14 toward the condensing optical system 17. The beam splitter 14 combines the reflected light beam generated by the reflection of the return light beam R ₂ and the return light beam R ₁ to generate interference light R _3, and outputs this interference light R ₃ to the condensing optical system 17. . The optical path length of the fixed-length optical path does not change with time, and the optical path length of the variable-length optical path changes with time due to the movement of the movable mirror 15, so that it changes with time between the return beams R ₁ and R _2. An optical path difference θ is generated. Therefore, the intensity of the interference light R ₃ changes in a wave shape according to the change in the optical path difference θ.

The condensing optical system 17 condenses the interference light R ₃ incident from the beam splitter 14 on the light receiving surface of the light receiving element 18. The light receiving element 18 photoelectrically converts the interference light collected by the condensing optical system 17 into an analog detection signal, and outputs the analog detection signal to the ADC 19. The light receiving element 18 may be composed of an optical semiconductor element such as a photodiode or a phototransistor. The ADC 19 converts the analog detection signal into a digital detection signal and supplies the digital detection signal to the signal processing unit 20.

In the signal processing unit 20, the spectroscopic measurement unit 22 detects the interferogram (interferogram for measurement) of the interference light R ₃ based on the digital detection signal using the distance data supplied from the drive control unit 25. To do. The spectroscopic measurement unit 22 can measure the spectrum (wave number spectrum, wavelength spectrum, or frequency spectrum) of the observation light L ₀ by Fourier-transforming the interferogram.

4A is a graph showing an example of the intensity distribution of the interferogram, and FIG. 4B is a graph showing a wave number spectrum generated by Fourier transform of the interferogram of FIG. 4A. In FIG. 4A, a waveform F1 indicated by a solid line is a waveform when the tilt angle θ of the movable mirror 15 is zero, and a waveform F2 indicated by a broken line is when the tilt angle θ of the movable mirror 15 is non-zero. It is a waveform. In FIG. 4B, distribution S1 indicated by a solid line corresponds to waveform F1 in FIG. 4A, and distribution S2 indicated by a broken line corresponds to waveform F2 in FIG. 4A. The point at which the intensity of the interferogram in FIG. 4A is maximum is a position where the optical path difference θ between the return light beams R ₁ and R ₂ after demultiplexing by the beam splitter 14 is substantially zero, that is, substantially all of the observation light L ₀ . It occurs at a position where the optical path difference θ becomes substantially zero with respect to the wavelength.

  Of the optical elements 13, 14, 15, and 16 that change the traveling direction of the propagation light beam in the Fourier spectrometer 1, the movable optical element is the movable mirror 15. Since the other optical elements 13, 14, and 16 are fixed in a robust configuration, there is a low possibility of causing an optical axis angle shift. On the other hand, compared to the other optical elements 13, 14, 16, it is difficult to make the moving movable mirror 15 robust. Therefore, the movable mirror 15 may cause, for example, an optical axis angle shift accompanying a change with time. This optical axis angle shift becomes a factor of a decrease in the degree of modulation of the interferogram. If the movable mirror 15 is tilted, an optical axis angle shift occurs, and as a result, the SNR of the wave number spectrum decreases as shown by the distribution S2 in FIG.

Next, the operation of the Fourier spectroscopic apparatus 1 at the time of correcting the optical axis angle deviation will be described in detail. FIG. 5 is a diagram schematically showing a propagation path of the correction light beam in the Fourier spectroscopic device 1 at the time of correction. Note that the Fourier spectroscopic apparatus 1 can also execute the spectrum measurement of the observation light L ₀ and the correction of the optical axis angle deviation simultaneously in parallel.

  At the time of correction, the measurement controller 21 supplies a drive start signal to the drive controller 25 and controls the correction light source 12 to emit light. The correction light source 12 may be formed of a monochromatic light source such as a laser diode that outputs a light beam having a single center wavelength with a constant output intensity.

As shown in FIG. 5, the correction light beam M ₀ output from the correction light source 12 is reflected by the plane mirror 13 toward the beam splitter 14 and propagates along the optical axis A 2, and then enters the beam splitter 14. To do. The beam splitter 14 splits the correction light beam M ₀ into a first light beam M ₁ propagating in the direction of the movable mirror 15 and a second light beam M ₂ propagating in the direction of the fixed mirror 16.

The first light beam M ₁ enters the movable mirror 15 through the optical deflector 30, and is reflected toward the beam splitter 14 by the movable mirror 15. The return light beam K ₁ reflected by the movable mirror 15 enters the beam splitter 14 through the optical deflector 30. On the other hand, the second light beam M ₂ generated by the beam splitter 14 is reflected by the fixed mirror 16 toward the beam splitter 14. The return light beam K ₂ reflected by the fixed mirror 16 is reflected by the beam splitter 14 toward the condensing optical system 17. The beam splitter 14 combines the reflected light beam generated by the reflection of the return light beam K ₂ and the return light beam K ₁ to generate interference light K _3, and outputs this interference light K ₃ to the light receiving element 32. As in the case of spectrum measurement, the optical path length of the fixed-length optical path does not change with time, and the optical path length of the variable-length optical path changes with time due to the movement of the movable mirror 15, so that the return beam K _{1 is} between R ₁ and R _2. Causes an optical path difference θ that varies with time. Therefore, the intensity of the interference light K ₃ changes in a wave shape according to the change in the optical path difference θ.

The light receiving element 32 photoelectrically converts the interference light K ₃ incident from the beam splitter 14 into a correction analog detection signal and outputs the correction analog detection signal to the ADC 33. The light receiving element 32 may be composed of an optical semiconductor element such as a photodiode or a phototransistor. The ADC 33 converts the correction analog detection signal into a correction digital detection signal, and supplies the correction digital detection signal to the modulation degree measurement unit 34.

The modulation degree measurement unit 34 uses the distance data supplied from the drive control unit 25 to detect the interferogram (correction interferogram) of the interference light K ₃ based on the correction digital detection signal. The modulation degree measurement unit 34 measures the modulation degree of the interferogram, and supplies the modulation degree data indicating the modulation degree to the light deflection control unit 31. When the modulation degree is represented by M _d , the modulation degree measurement unit 34 calculates, for example, the maximum value D _max , the minimum value D _min, and the average value D _mean (= (D _max + D _min ) / 2) of the correction digital detection signal. By using the following equation (1), the modulation degree M _d can be calculated.
M _d = (D _max −D _min ) / (2 × D _mean ) (1)

The SNR of the spectrum of the interference light K ₃ depends on the modulation degree of the correction interferogram, and the SNR of the spectrum decreases due to the decrease of the modulation degree of the interferogram. When the intensity of the interferogram with respect to the single wave number σ is expressed by I (θ, σ), the intensity I (θ, σ) is given by the following equation (2), for example.
I (θ, σ) = M _d × cos (2πσθ) (2)

Here, θ represents an optical path difference. Further, in this equation (2), the offset component (DC component) of the interference wave intensity is removed. The intensity of the interferogram of the observation light having a wide wavelength range can be obtained by integrating I (θ, σ) with respect to the wave number σ. Since SNR of the spectrum is proportional to the degree of modulation M _d, the case of the correction for the interferogram, when the modulation degree M _d is the maximum, the SNR of the spectrum also becomes maximum. Note that the SNR of the spectrum in this case is reduced by a component that does not contribute to the modulation degree M _d of the correction interferogram (for example, the SNR due to the transmittance of the beam splitter 14 and the reflectance of the movable mirror 15 and the fixed mirror 16). Including decrease).

The optical deflection control unit 31 monitors the modulation degree supplied from the modulation degree measurement unit 34 and controls the operation of the optical deflector 30 to deflect the propagation light beam in the fixed length path in the deflection direction in which the modulation degree increases. Specifically, the light deflection control unit 31 finds a deflection direction that maximizes the degree of modulation by continuously or stepwise changing the rotation angles of the wedge prisms 41 and 42 shown in FIG. The wedge prisms 41 and 42 are rotated so that the propagating light beam is deflected in the direction. Thereby, at the time of spectrum measurement, even if the mechanical accuracy in the interferometer is lowered (for example, when the reflecting mirror surface of the movable mirror 15 or the reflecting mirror surface of the fixed mirror 16 is inclined), the observation light L ₀ A decrease in the SNR of the spectrum can be suppressed. For example, the optical deflector control unit 31 monitors the modulation factor supplied from the modulation degree measuring unit 34, and executes control only to increase the degree of modulation when the modulation factor is equal to or less than the threshold value eta _t, the degree of modulation When the threshold value η _t is exceeded, the control can be terminated.

When the mechanical accuracy in the interferometer is lowered to cause an optical axis angle shift, the SNR of the spectrum of the observation light L ₀ is lowered. FIG. 6A is a diagram illustrating an example of the signal intensity of the interferogram of the measurement interference light R ₃ in such a case, and FIG. 6B illustrates the signal intensity of the interferogram of the correction interference light K ₃ . It is a figure which shows an example. Since the correction light source 12 is composed of a monochromatic light source, the waveform of the interferogram detected by the modulation degree measurement unit 34 is a sine waveform having a substantially single period.

The optical deflection control unit 31 constantly monitors the change in the modulation factor calculated by the modulation factor measurement unit 34 and corrects the optical axis angle deviation by controlling the operation of the optical deflector 30 so as to increase the modulation factor. . FIG. 7A is a diagram illustrating an example of the signal intensity of the interferogram of the measurement interference light R ₃ after correction, and FIG. 7B is an example of the signal intensity of the interferogram of the interference light K ₃ after correction. FIG. 7A and 7B, the waveform indicated by the dotted line indicates the waveform shown in FIGS. 6A and 6B. As shown in FIG. 7A, it can be seen that the amplitude (modulation degree) of the interferogram waveform of the measurement interference wave R ₃ is improved by the correction. For this reason, the SNR of the spectrum of the observation light derived by Fourier transforming the interferogram is improved compared to the SNR of the spectrum before correction, and substantially converges to the SNR that can be acquired without any optical axis angle deviation. To do.

  Next, the principle of correcting the optical axis angle shift when the movable mirror 15 is tilted will be described with reference to FIGS. 8A, 8B, and 9. FIG.

If the resulting inclination angle φ to the reflecting mirror surface of the movable mirror 15 as shown in Figure 8A, during the first light beam L ₁ and the return beam R ₁ which is substantially parallel to the incident to the optical axis A1, the drawings An optical axis angle shift corresponding to an angle 2φ occurs in a plane parallel to the. The rotation of the wedge prisms 41 and 42 of the optical deflector 30 corrects the optical axis angular deviation corresponding to this angle 2φ. FIG. 8B is a diagram schematically showing the corrected first light flux L ₁ and return light flux R ₁ .

Now, the wedge angle, which is the inclination angle of the prism surfaces 41b, 42a of the two wedge prisms 41, 42, is α, the deviation angle per wedge prism 41, 42 is δ, and the wedge prisms 41, 42 are refracted. The rate is n, and the rotation angles of the wedge prisms 41 and 42 are ψ ₁ and ψ ₂ , respectively. At this time, as shown in FIG. 9, light rays passing through the two wedge prisms 41 and 42 are deflected in the deflection direction indicated by the deflection angle [delta] _t and declination direction [psi _t. In FIG. 9, the directions of the X axis and the Y axis orthogonal to each other are parallel to a plane perpendicular to the optical axis A1. Specifically, the deflection angle δ _t and the deflection angle direction ψ _t are given by the following equations (3) and (4), respectively.
δ _t = [(δ cos Ψ ₁ + δ cos Ψ ₂ ) ² + (δ sin Ψ ₁ + δ sin Ψ ₂ ) ² ] ^1/2
(3)
Ψ _t = tan ⁻¹ [(δ sin Ψ ₁ + δ sin Ψ ₂ ) / (δ cos Ψ ₁ + δ cos Ψ ₂ )]
(4)

Here, assuming that the wedge angle α is sufficiently small, δ is given by the following equation (5).
δ = (n−1) α (5)

As described above, by rotating the two wedge prisms 41 and 42, the light beam passes through the wedge prism 41 and 42, are deflected by the deflection angle [delta] _t and the deflection direction [psi _t corresponding to their angle of rotation.

Note that the minimum value δ _t — min (corresponding to the accuracy of optical axis angle deviation correction) of the deflection angle δ _t that can be _generated by the optical deflector 30 is the rotation angle Ψ ₁ , Ψ _{2 of the} wedge prisms 41, 42. It can be seen that it depends on the resolution of the wedge prisms 41 and 42, the wedge angle α of the wedge prisms 41 and 42, and the refractive index n of the wedge prisms 41 and 42.

For example, when correcting an angle 2φ as shown in FIG. 8A, the optical deflector control unit 31, a and [psi _t the argument [delta] _t and 2φ as 0, the above equation (3), solving the simultaneous equations (4) operation By executing the above, it is possible to uniquely obtain Ψ ₁ and Ψ ₂ .

  Next, details of the correction processing according to the first embodiment will be described with reference to FIG. FIG. 10 is a flowchart schematically showing an example of the procedure of the correction process according to the first embodiment.

  Referring to FIG. 10, first, the measurement control unit 21 causes the correction light source 12 to emit light (step ST10). Next, the light deflection control unit 31 selects a wedge prism to be rotated out of the two wedge prisms 41 and 42 in the light deflector 30 (step ST11). Specifically, since prism numbers i = 1 and 2 are assigned to the wedge prisms 41 and 42, the light deflection control unit 31 sets the prism number i to an initial value (for example, 1).

Subsequently, the light deflection control unit 31 initializes the modulation degree measurement count m to 0 (step ST12), and rotates and fixes the i-th wedge prism by the rotation angle Φ _{i, m} from the reference angle (step ST13). The rotation angle Φ _{i, m} is a preset value and is stored in an internal memory (not shown) of the light deflection control unit 31.

Next, the measurement control unit 21 supplies a drive start signal to the drive control unit 25, thereby moving the movable mirror 15 within a predetermined movable range (step ST14). As a result, the correction light receiving element 32 receives the interference light K ₃ generated based on the correction light beam M ₀ . At this time, the modulation degree measurement unit 34 receives a digital detection signal for correction from the ADC 33. The modulation degree measurement unit 34 acquires an interferogram of the interference light K ₃ based on the digital detection signal for correction (step ST15), and measures the modulation degree η (i, Φ _{i, m} ) (step ST16). ). The modulation degree η (i, Φ _{i, m} ) is supplied to the light deflection control unit 31 and stored in the internal memory.

  Thereafter, the modulation degree measurement unit 34 determines whether or not the modulation degree measurement count m has reached the upper limit value M (step ST17). If the modulation degree measurement number m has not reached the upper limit M (NO in step ST17), the modulation degree measurement unit 34 increments the modulation degree measurement number m by 1 (step ST18), and the process proceeds to step ST13. Transition.

Thereafter, when it is determined that the number m of modulation degree measurements has reached the upper limit M (YES in step ST17), the light deflection control unit 31 adds a total of M + 1 modulation degrees η (i, Φ) for the i-th wedge prism. _{i, 0} ), η (i, Φ _{i, 1} ),..., η (i, Φ _{i, M} ) are acquired. For example, when a total of three modulation degrees η (i, Φ _{i, 0} ), η (i, Φ _{i, 1} ), η (i, Φ _{i, 2} ) are acquired, the rotation is as follows: The angles Φ _{i, 0} , Φ _{i, 1} , Φ _{i, 2} can be set (ψ ₁ is a predetermined amount of rotation).
Φ _{i, 0} = 0, Φ _{i, 1} = + ψ ₁ , Φ _{i, 2} = −ψ ₁ .

Next, the light deflection control unit 31 selects the maximum from the calculated modulation degrees η (i, Φ _{i, 0} ), η (i, Φ _{i, 1} ),..., Η (i, Φ _{i, M} ). The modulation degree η (i, Φ _p ) is detected (step ST21), and the i-th wedge prism is rotated from the reference angle by the rotation angle Φ _p corresponding to the maximum modulation degree η (i, Φ _p ) (step ST21). ST21).

Next, when the maximum modulation degree η (i, Φ _p ) does not exceed the threshold value η _t (NO in step ST22), the light deflection control unit 31 switches the wedge prism, that is, changes the prism number i. (Step ST23), the process proceeds to Step ST12. In this case, steps ST12 to ST21 are executed for the other wedge prisms.

Finally, when the maximum modulation degree η (i, Φ _p ) exceeds the threshold η _t (YES in step ST22), the light deflection control unit 31 ends the correction process. By executing the correction process according to the procedure shown in FIG. 10, the state of the wedge prisms 41 and 42 of the optical deflector 30 can be changed to a state in which the degree of modulation is maximized. Therefore, the degree of modulation gradually recovers, and the degree of modulation gradually approaches a constant value when the optical axis angle deviation between the two combined light beams converges to substantially zero.

  The hardware configuration of a part or all of the combination of the signal processing unit 20, the drive control unit 25, the light deflection control unit 31, and the modulation degree measurement unit 34 includes, for example, a CPU (Central Processing Unit) such as a microcomputer. It can be realized by a computer and a digital input / output interface or an analog input / output interface. Alternatively, the hardware configuration of a part or all of the combination of the signal processing unit 20, the drive control unit 25, the light deflection control unit 31, and the modulation degree measurement unit 34 may be a DSP (Digital Signal Processor) or an ASIC (Application Specific Integrated Circuit). Alternatively, it may be realized by a signal processing circuit such as a field-programmable gate array (FPGA) or a combination thereof, and a digital input / output interface or an analog input / output interface. In the present embodiment, the signal processing unit 20 and the modulation degree measurement unit 34 are configured to be separated from each other, but are not limited to this configuration. The modulation degree measurement unit 34 may be incorporated in the signal processing unit 20.

As described above, in the Fourier spectrometer 1 of the first embodiment, when the mechanical accuracy of the interferometer including the beam splitter 14, the movable mirror 15, and the fixed mirror 16 is lowered (for example, the movable mirror 15 or the fixed mirror 16). Even when the reflecting mirror surface of the interferometer is inclined), the propagation light beam in the interferometer is deflected in the direction in which the degree of modulation of the interferogram of the interference light K ₃ generated based on the correction light beam M ₀ is increased. Can do. As a result, the SNR of the spectrum of the observation light L ₀ can be restored to the original good value.

Further, the light deflection control unit 31 of the present embodiment can constantly monitor the modulation degree supplied from the modulation degree measurement unit 34 and control the optical deflector 30 so as to maximize this modulation degree. . For this reason, in comparison with a method in which the tilt of the movable mirror 15 or the fixed mirror 16 is directly monitored and the tilt is adjusted so as to correct the optical axis angle deviation, this embodiment more directly increases the SNR of the spectrum. Can be improved. Even if the inclination of the movable mirror 15 is adjusted so that the reflecting mirror surface of the movable mirror 15 is perpendicular to the optical axis A1 to correct the optical axis angle deviation, the change with time is caused by the fixed mirror 16 side. When the accompanying optical axis angle deviation occurs, the optical axis angle deviation cannot be easily corrected. In other words, even if the tilt of the movable mirror 15 is corrected so that the movable mirror 15 is perpendicular to the optical axis A1, light is transmitted between the return beams R ₁ and R ₂ from the movable mirror 15 and the fixed mirror 16. There may be a case where the axial angle deviation remains. On the other hand, in the present embodiment, the optical axis angle deviation caused by the fixed mirror 16 side can also be corrected.

  In addition, when the optical axis angle deviation caused by the movable mirror 15 is directly monitored to correct the optical axis angle deviation, a function for adjusting the angle of the reflecting mirror surface is added to the fixed mirror 16 side, and then after demultiplexing. It is possible to correct the optical axis between the two light beams. However, in this case, there is a disadvantage that the apparatus becomes complicated or large. On the other hand, since this embodiment employs a configuration in which the wedge prisms 41 and 42 are arranged on the optical path, it is possible to avoid complication or enlargement of the apparatus.

Further, in the present embodiment, since the correction light source 12 that outputs the correction light beam M ₀ with a constant output intensity is employed, the degree of modulation that depends only on the change in the interference state due to the rotation of the wedge prisms 41 and 42. There is an advantage that you can get. If correction of the optical axis angle deviation is performed using only the observation light L ₀ without using the correction light beam M ₀ , the degree of modulation depends on the intensity of the light source. It is difficult to distinguish between the change in the modulation degree and the change in the modulation degree due to the intensity of the light source.

  In the present embodiment, wedge prisms 41 and 42 are used, but the present invention is not limited to this. Instead of the web prisms 41 and 42, an optical deflector may be configured by a reflection optical system having one or more mirrors. Further, although the refractive indexes n of the two wedge prisms 41 and 42 of the present embodiment are equal, they are not limited to this. The two wedge prisms 41 and 42 may be made of different refractive index materials.

  In the present embodiment, the optical deflector 30 is disposed on the variable length optical path between the movable mirror 15 and the beam splitter 14, but the present invention is not limited to this. Instead of the configuration shown in FIG. 1, a configuration in which the optical deflector 30 is arranged in a fixed length optical path between the fixed mirror 16 and the beam splitter 14 may be adopted.

Further, when the correction and the spectral measurements of the optical axis angle displacement is performed simultaneously, the light shielding member may be provided for preventing interference between the correction beam M ₀ and the observation light L _0. For example, a light shielding member may be disposed between the plane mirror 13 and the observation window 11 or between the plane mirror 13 and the beam splitter 14.

  Further, the reflecting mirror surface of the fixed mirror 16 and the reflecting mirror surface of the movable mirror 15 are not necessarily plane mirror surfaces. Furthermore, the number of light receiving elements 18 for measurement is not limited to one, and a plurality of light receiving elements may be provided.

  Further, the correction light source 12 may not be a monochromatic light source. In this case, the waveform of the correction interference light does not become a single-cycle sine wave, and beats are generated. In the measurement of the modulation degree in this case, the maximum value and the minimum value of the modulation degree at a specific optical path difference can be used. Further, correction control for comparing three kinds of modulation degrees obtained by control for correcting the optical axis angle deviation is also possible.

Regarding the rotation angles of the wedge prisms 41 and 42, the rotation angle resolutions of the wedge prisms 41 and 42 are Ψ ₁ and Ψ ₂ , but Ψ ₁ and Ψ ₂ have a desired matching accuracy of the optical axis between the two light beams. It may be set accordingly.

Moreover, in this Embodiment, although the plane mirror 13 is arrange | positioned, it is not limited to this. A configuration in which the correction light beam M ₀ and the observation light L ₀ are directly input to the beam splitter 14 without using the plane mirror 13 may be employed.

  Further, there is a conventional technique for avoiding the optical axis angle deviation by configuring an interferometer using a corner cube (for example, Japanese Patent Laid-Open No. 10-62250). However, when a corner cube is used, it is difficult to increase the diameter for the purpose of improving the SNR of the spectrum. On the other hand, in this embodiment, there is an advantage that the optical axis angle deviation can be corrected without using a corner cube, and the configuration can be easily increased.

Embodiment 2. FIG.
Next, a second embodiment according to the present invention will be described. FIG. 11 is a diagram schematically showing a configuration of a Fourier transform spectrometer 1A (hereinafter also referred to as “Fourier spectrometer 1A”) according to the second embodiment of the present invention. The configuration of the Fourier spectroscopic apparatus 1A of the present embodiment is the same as the configuration of the Fourier spectroscopic apparatus 1 of the above-described first embodiment, except that the drive control section 25 of FIG. 11 is provided instead of the drive control section 25 of FIG. The same.

  In the drive control unit 25A of the present embodiment, after the optical deflection control unit 31 controls the optical deflector 30 to deflect the propagating light beam in the direction of increasing the modulation degree, the waveform distribution of the measurement interferogram is This has a function of adjusting the moving range of the movable mirror 15 so as to be symmetrical.

  In the first embodiment, in the correction process, the light deflection control unit 31 uses the two wedge prisms 41 and 42 of the light deflector 30. The wedge prisms 41 and 42 are disposed between the beam splitter 14 and the movable mirror 15. However, since the wedge prisms 41 and 42 made of a refractive index material are arranged on the optical path, the peak position (maximum intensity position) of the waveform distribution of the measurement interferogram, that is, two light fluxes after the demultiplexing is performed by the correction process. In some cases, the position where the optical path difference becomes zero changes.

  12A and 12B are graphs showing examples of waveform distributions of measurement interferograms before and after correction. In the example of FIG. 12A, the waveform of the measurement interferogram before correction is indicated by a dotted line, and the waveform of the measurement interferogram after correction is indicated by a solid line. The peak position of the waveform distribution of the measurement interferogram after correction is shifted by Δ from the peak position of the waveform distribution of the measurement interferogram before correction. The corrected waveform distribution of the measurement interferogram is not symmetric with respect to the point Cp corresponding to the center position of the movable range of the movable mirror 15. When the peak position is shifted in this way, the waveform distribution of the measurement interferogram is not symmetrical. For this reason, the spectroscopic measurement unit 22 extracts an interferogram within a symmetric range from the peak position, and measures a spectrum based on the extracted interferogram, thereby obtaining a desired resolution (for example, wave number resolution). May not be obtained. In order to avoid this, the drive control unit 25A has a function of adjusting the moving range of the movable mirror 15 so that the waveform distribution of the measurement interferogram is symmetric after the correction process.

  Specifically, the drive control unit 25A acquires from the spectroscopic measurement unit 22 the waveform data of the measurement interferogram corresponding to the maximum modulation degree among the measurement interferograms calculated in the course of the correction process. Here, the maximum modulation degree is the maximum value of the modulation degrees measured a plurality of times by the modulation degree measurement unit 34 in the course of the correction process, and was used to determine the rotation angle of the wedge prisms 41 and 42. Value. Then, the drive control unit 25A detects the peak position of the waveform distribution of the measurement interferogram acquired from the spectroscopic measurement unit 22 (that is, the position where the optical path difference is zero).

  Then, the drive control unit 25A adjusts (offsets) the movable range of the movable mirror 15 so that the detected peak position coincides with the corresponding point Cp. FIG. 12B is a graph schematically showing the waveform distribution of the measurement interferogram after adjustment of the movable range of the movable mirror 15 in this way. In FIG. 12B, the waveform of the measurement interferogram after adjustment of the movable range is indicated by a solid line, and the waveform of the measurement interferogram before adjustment of the movable range is indicated by an alternate long and short dash line. The peak position of the waveform distribution of the measurement interferogram after adjustment of the movable range coincides with the corresponding point Cp. Therefore, the waveform distribution (profile) of the measurement interferogram after adjustment of the movable range is symmetric with respect to the corresponding point Cp.

  As described above, the Fourier spectroscopic apparatus 1A according to the second embodiment can offset the center position of the movable range of the movable mirror 15, so that it is measured at the observation time corresponding to the center position of the movable range of the movable mirror 15. The peak position of the interferogram can be detected. Thereby, an interferogram having a symmetric waveform distribution with respect to the peak position can be acquired. Therefore, a spectrum having a desired resolution (for example, wave number resolution) can be obtained by a simple control method.

  In the present embodiment, control is performed to offset the center position of the movable mirror 15 so that the peak position of the measurement interferogram is shifted to the point Cp corresponding to the center position of the movable mirror 15. doing. Instead, when the desired movement range of the movable mirror 15 is ensured, the drive control unit 25A sets the movement range of the movable mirror 15 to an asymmetric range with respect to the center position, thereby increasing the measurement range. The waveform distribution of the interferogram may be symmetric.

  A hardware configuration of a part or all of the combination of the signal processing unit 20, the drive control unit 25A, the light deflection control unit 31, and the modulation degree measurement unit 34 in the second embodiment is a computer with a built-in CPU such as a microcomputer, digital It can be realized with an input / output interface or an analog input / output interface. Alternatively, the hardware configuration of a part or all of the combination of the signal processing unit 20, the drive control unit 25, the light deflection control unit 31, and the modulation degree measurement unit 34 is a signal processing circuit such as a DSP, ASIC, FPGA, or a combination thereof. And a digital input / output interface or an analog input / output interface. In the present embodiment, the signal processing unit 20 and the modulation degree measurement unit 34 are configured to be separated from each other, but are not limited to this configuration. The modulation degree measurement unit 34 may be incorporated in the signal processing unit 20.

Embodiment 3 FIG.
Next, a third embodiment according to the present invention will be described. FIG. 13 is a diagram schematically showing the configuration of a Fourier transform spectrometer 1B (hereinafter also referred to as “Fourier spectrometer 1B”) according to the third embodiment of the present invention. The configuration of the Fourier spectroscopic device 1B of the present embodiment is the condensing optical system 17B of FIG. 13 instead of the condensing optical system 17, the light receiving element 32, the ADC 33, the modulation degree measuring unit 34, and the signal processing unit 20 of FIG. The configuration is the same as that of the Fourier spectroscopic apparatus 1A of the second embodiment except that the signal processing unit 20B is included.

The condensing optical system 17B of the present embodiment is generated based on the function of condensing the interference light R ₃ generated based on the observation light L ₀ on the light receiving element 18 and the correction light beam M ₀ . A function of condensing the interference light K ₃ on the light receiving element 18. The light receiving element 18 photoelectrically converts the observation light or the correction light beam collected by the condensing optical system 17 and outputs an analog detection signal. The ADC 19 photoelectrically converts the analog detection signal and outputs a digital detection signal. The signal processing unit 20 </ b> B includes a modulation degree measurement unit 23 in addition to the measurement control unit 21 and the spectroscopic measurement unit 22. The modulation degree measurement unit 23 has the same configuration and function as the modulation degree measurement unit 34 of the first embodiment. For example, the modulation degree measurement unit 23 can calculate the modulation degree of the interferogram based on the digital detection signal. Here, the wavelength of the correction light source 12 is adjusted to the detection band of the light receiving element 18.

As described above, the Fourier spectroscopic apparatus 1B according to the third embodiment does not require the light receiving element for correction and the ADC. For this reason, the whole size of the Fourier spectrometer 1B can be reduced.
The modulation degree measuring unit 23 may be arranged separately from the signal processing unit 20B.

  The hardware configuration of a part or all of the combination of the signal processing unit 20B, the drive control unit 25A, and the light deflection control unit 31 in the third embodiment includes a computer with a CPU such as a microcomputer, a digital input / output interface or an analog input. It can be realized with an output interface. Alternatively, the hardware configuration of a part or all of the combination of the signal processing unit 20B, the drive control unit 25A, and the optical deflection control unit 31 includes a signal processing circuit such as a DSP, ASIC, FPGA, or a combination thereof, and a digital input / output interface. Alternatively, it may be realized with an analog input / output interface.

  Although various embodiments according to the present invention have been described above with reference to the drawings, these embodiments are examples of the present invention, and various forms other than these embodiments can be adopted. In the present invention, within the scope of the invention, the above-described first to third embodiments can be freely combined, any constituent element of each embodiment can be modified, or any constituent element of each embodiment can be omitted. It is.

  A1, A2 Optical axis, 1 Fourier transform spectrometer, 11 Observation window, 12 Correction light source, 13 Plane mirror, 14 Beam splitter, 15 Movable mirror, 16 Fixed mirror, 17, 17B Condensing optical system, 18 Light receiving element, 19 A / D converter (ADC), 20, 20B Signal processing unit, 21 Measurement control unit, 22 Spectroscopic measurement unit, 23 Modulation degree measurement unit, 24 Mirror drive unit, 25, 25A Drive control unit, 30 Optical deflector, 31 Light deflection control unit, 32 light receiving element, 33 A / D converter (ADC), 34 modulation degree measuring unit, 40 prism driving unit, 41, 42 wedge prism, 41a, 41b, 42a, 42b prism surface.

Claims (11)

The incident light beam is split into a first light beam and a second light beam, and the first light beam and the second light beam are output to a variable-length optical path and a fixed-length optical path, respectively. A beam splitter that combines the return light flux from the variable-length optical path and outputs interference light;
A movable mirror that reflects the first light flux propagating in the variable-length optical path toward the beam splitter;
A fixed mirror that reflects the second light flux propagating in the fixed length optical path toward the beam splitter;
A mirror driving unit that moves the movable mirror to change the optical path length of the variable-length optical path;
A light receiving element that photoelectrically converts an incident light beam to generate a detection signal;
A condensing optical system for causing the interference light output from the beam splitter to enter the light receiving element;
A spectroscopic measurement unit that calculates a spectrum based on the detection signal;
A correction light receiving element that photoelectrically converts the interference light output from the beam splitter to generate a correction detection signal;
A modulation degree measurement unit that measures the modulation degree of the interferogram based on the correction detection signal;
An optical deflector having a variable declination mechanism for deflecting a propagation light beam in the variable length optical path or the fixed length optical path;
A Fourier transform type spectroscopic apparatus comprising: an optical deflection control unit that controls the optical deflector to deflect the propagating light beam in a direction in which the degree of modulation increases. The Fourier transform spectrometer according to claim 1, further comprising a correction light source that outputs a correction light beam to be incident on the beam splitter,
The Fourier transform type spectroscopic device, wherein the correction light receiving element generates the correction detection signal by photoelectrically converting the interference light output from the beam splitter in response to incidence of the correction light beam.   3. The Fourier transform spectroscopic apparatus according to claim 2, wherein the correction light source is a light source that outputs a light beam having a single center wavelength with a constant output intensity.   4. The Fourier transform spectroscopic device according to claim 1, wherein the condensing optical system receives incident observation light when the observation light is incident on the beam splitter. 5. In response, the interference light output from the beam splitter is incident on the light receiving element. The incident light beam is split into a first light beam and a second light beam, and the first light beam and the second light beam are output to a variable-length optical path and a fixed-length optical path, respectively. A beam splitter that combines the return light flux from the variable-length optical path and outputs interference light;
A movable mirror that reflects the first light flux propagating in the variable-length optical path toward the beam splitter;
A fixed mirror that reflects the second light flux propagating in the fixed length optical path toward the beam splitter;
A mirror driving unit that moves the movable mirror to change the optical path length of the variable-length optical path;
A light receiving element that photoelectrically converts an incident light beam to generate a detection signal;
A condensing optical system for causing the interference light output from the beam splitter to enter the light receiving element;
A spectroscopic measurement unit that calculates a spectrum based on the detection signal;
A modulation degree measurement unit that measures the modulation degree of the interferogram based on the detection signal;
An optical deflector having a variable declination mechanism for deflecting a propagation light beam in the variable length optical path or the fixed length optical path;
A Fourier transform type spectroscopic apparatus comprising: an optical deflection control unit that controls the optical deflector to deflect the propagating light beam in a direction in which the degree of modulation increases. The Fourier transform type spectroscopic device according to claim 5, further comprising a correction light source that outputs a correction light beam to be incident on the beam splitter,
The Fourier transform type spectroscopic device, wherein the modulation degree measurement unit measures the modulation degree of the interferogram based on the detection signal when the correction light beam is incident on a liter.   7. The Fourier transform spectroscopic apparatus according to claim 6, wherein the correction light source is a light source that outputs a light beam having a single center wavelength with a constant output intensity.   8. The Fourier transform type spectroscopic device according to claim 1, wherein the light deflection control unit controls the light deflector to maximize the modulation degree. A Fourier transform spectroscopic device characterized by deflecting a propagating light beam.   9. The Fourier transform spectroscopic device according to claim 1, wherein the optical deflector is disposed in the variable length optical path between the beam splitter and the movable mirror. 10. A Fourier transform type spectroscopic device characterized in that: A Fourier transform spectrometer according to any one of claims 1 to 9,
The optical deflector is
One or more wedge prisms;
A Fourier transform spectroscopic device, comprising: a rotation driving unit configured to rotate each wedge prism in accordance with control by the light deflection control unit. The Fourier transform type spectroscopic device according to any one of claims 1 to 10, further comprising a drive control unit that controls the mirror driving unit,
The drive control unit controls the optical deflector to control the optical deflector so as to deflect the propagating light beam in a direction in which the degree of modulation increases, and then the drive control unit corresponds to the observation light incident on the beam splitter. A Fourier transform type spectroscopic device, wherein the moving range of the movable mirror is adjusted so that the waveform distribution of the ferrogram is symmetric.

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