JP5915737B2 - Fourier transform spectrometer and Fourier transform spectroscopic method - Google Patents

Description

  The present invention relates to a Fourier transform spectrometer and a Fourier transform spectroscopic method, and in particular, a Fourier transform spectrometer and a Fourier transform type using a moving mirror that reciprocally vibrates in the optical axis direction as an optical path difference forming optical element of an interferometer. It relates to a spectroscopic method.

  A spectrometer is a device that measures a spectrum representing a component (light intensity) of each wavelength (each wave number) in light to be measured. As one of the spectrometers, there is a Fourier transform spectrometer that measures the interference light of the light to be measured with an interferometer and obtains the spectrum of the light to be measured by Fourier transforming the measurement result.

  In this Fourier transform spectrometer, the output of the interferometer is a composite waveform in which light of a plurality of wavelengths included in the light to be measured is interfered at once by the interferometer, and is called an interferogram. The spectrum of the light to be measured is obtained by Fourier transforming the interferogram. This interferogram has a profile that has one or a plurality of steep peaks in a predetermined range and a substantially zero level in the remaining range, and the center peak of the one or more steep peaks has a center burst. Called.

  The interferometer of such a Fourier transform type spectrometer is configured to include a plurality of optical elements that receive predetermined light and form two optical paths between the incident position of the predetermined light and the interference position, The plurality of optical elements include an optical path difference forming optical element that generates an optical path difference between the two optical paths by moving in the optical axis direction. Examples of the optical path difference forming optical element include a moving mirror that scans a scanning range along the optical axis direction at a constant speed. However, such a movable mirror usually uses a gas bearing or a voice coil motor for driving the movable mirror, so that the interferometer becomes relatively large. For this reason, in order to achieve further miniaturization, for example, moving mirrors using parallel movement mechanisms disclosed in Patent Document 1 and Patent Document 2 have been proposed.

  The movable mirror of the parallel movement mechanism disclosed in Patent Document 1 and Patent Document 2 is between the first and second leaf springs arranged opposite to each other and the first and second leaf springs, and both ends thereof. First and second supports respectively disposed on the first and second plate springs, and provided on a surface of one end of the first plate spring, and the first and second plates. A piezoelectric element that translates one of the first and second support bodies in the opposing direction of the first and second leaf springs by bending one of the springs, and the first plate On the surface of the other end of the spring, a mirror surface region for reflecting light is provided. When the piezoelectric element is extended, the movable mirror of the parallel movement mechanism having such a configuration is deformed so that the first leaf spring is convex upward, and as a result, the mirror surface area is displaced downward in the facing direction. When the piezoelectric element is reduced, the first leaf spring is deformed so as to protrude downward, and as a result, the mirror surface area is displaced upward in the facing direction. The movable mirror of the parallel movement mechanism repeats the displacement by resonance in order to obtain a large amount of displacement.

  By the way, since the movable mirror of the parallel movement mechanism disclosed in Patent Document 1 and Patent Document 2 vibrates due to resonance, vibration is applied to the movable mirror of the parallel movement mechanism from the outside of the movable mirror of the parallel movement mechanism. In this case, when the frequency of the external vibration is close to the resonance frequency of the movable mirror of the parallel movement mechanism, a so-called undulation occurs in the amplitude of the movable mirror of the parallel movement mechanism. For this reason, while the amplitude in the movable mirror of the parallel movement mechanism changes, the time required for one reciprocation by resonance driving does not change, so the moving speed of the movable mirror changes. As a result, the amplitude of the interferogram is modulated by the influence of the frequency characteristics of the electric circuit that receives the interference light from the interferometer of the measured light, particularly the amplifier, and the amplitude of the spectrum obtained by the Fourier transform of the interferogram Will also change. That is, an error occurs in the spectral intensity of the spectral result.

JP 2011-80854 A JP 2012-42257 A

  The present invention has been made in view of the above-described circumstances, and its purpose is to obtain a more accurate measurement result even when external vibration having a frequency close to the resonance frequency causing the swell is applied. It is to provide a Fourier transform spectrometer and a Fourier transform spectrometer that can be used.

  A Fourier transform spectrometer and a Fourier transform spectroscopic method according to the present invention include a time based on an amplitude variation period in an optical path difference forming optical element that generates an optical path difference between two optical paths of an interferometer. A plurality of interferograms of predetermined light are continuously measured, and a spectrum is obtained based on the plurality of interferograms. For this reason, such a Fourier transform spectrometer and the method can obtain a more accurate measurement result even when an external vibration having a frequency close to the resonance frequency causing the swell is applied.

  The above and other objects, features and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings.

It is a block diagram which shows the structure of the Fourier-transform type spectrometer in one Embodiment of this invention. It is a figure which mainly shows the structure of the interferometer in the said Fourier-transform type spectrometer. It is a block diagram which shows the structure of the light reception process part in the said Fourier-transform spectrometer. As an example, it is a figure which shows the interference waveform of the laser beam in the said Fourier-transform spectrometer. It is a perspective view which shows the structure of the movement mirror in the interferometer of the said Fourier-transform type spectrometer. It is sectional drawing which shows the mode of the reciprocating vibration of the movable mirror in the said Fourier-transform spectrometer. In the Fourier transform spectrometer, as an example, it is a diagram showing a waveform (interferogram) of interference light of measured light to be measured. It is a figure which shows the relationship between the said interferogram and a window function. It is a figure which shows the frequency characteristic of the amplification part of the light reception process part in the said Fourier-transform type spectrometer. In the Fourier transform type spectrometer, it is a schematic diagram showing the relationship between the frequency characteristics of the amplification unit in the light reception processing unit and the frequency band of the signal included in the interferogram. It is a figure which shows the relationship between each amplitude of the movable mirror in an interferometer, and the intensity ratio of the spectrum obtained based on each interferogram in the amplitude. It is a figure which shows the intensity ratio of the spectrum which averaged the spectrum measured by the amplitude before and behind that centering on the case where the amplitude of a movable mirror is 5500 (A / D conversion count value). FIG. 5 is a diagram for explaining a calculation method of a first mode of a vibration period in a moving mirror in the Fourier transform spectrometer. FIG. 5 is a diagram for explaining a calculation method of a second aspect of a vibration period in a moving mirror in the Fourier transform spectrometer. It is a figure for demonstrating the mode of writing and reading of the amplitude of the moving mirror by the circular buffer in the said Fourier-transform type spectrometer.

  Hereinafter, an embodiment according to the present invention will be described with reference to the drawings. In addition, the structure which attached | subjected the same code | symbol in each figure shows that it is the same structure, The description is abbreviate | omitted suitably.

  FIG. 1 is a block diagram illustrating a configuration of a Fourier transform spectrometer according to the embodiment. FIG. 2 is a diagram mainly illustrating a configuration of an interferometer in the Fourier transform spectrometer according to the embodiment. FIG. 3 is a block diagram illustrating a configuration of the light receiving processing unit 20 in the Fourier transform spectrometer according to the embodiment. FIG. 4 is a diagram illustrating an interference waveform of laser light in the Fourier transform spectrometer according to the embodiment as an example. The horizontal axis in FIG. 4 is the optical path difference, and the vertical axis is the intensity of the interference waveform. FIG. 5 is a perspective view illustrating a configuration of a movable mirror in the interferometer of the Fourier transform spectrometer according to the embodiment. FIG. 6 is a cross-sectional view showing a state of reciprocal vibration of the movable mirror in the Fourier transform spectrometer of the embodiment. 6A shows a state in the case of displacing downward in the drawing indicated by an arrow, and FIG. 6B shows a state in the case of displacing upward in the drawing indicated by an arrow. FIG. 6C shows a state of reciprocal vibration at one end of the movable mirror 115.

  A Fourier transform spectrometer (hereinafter abbreviated as “FT spectrometer”) D in the embodiment is an apparatus for measuring a spectrum of light to be measured, and measures the light to be measured with an interferometer. This is a device for obtaining the spectrum of the light to be measured by Fourier transforming the waveform (interferogram) of the measured interference light of the light to be measured. Then, in the FT spectrometer D of the present embodiment, in order to improve the S / N ratio and obtain a good accuracy result, the transformation target to be Fourier-transformed to obtain the spectrum of the light to be measured includes the interference. An integrated interferogram obtained by integrating a plurality of interferograms of the light to be measured generated by a meter is used. Such an FT spectrometer D, for example, as shown in FIGS. 1 to 6, is reflected by the measurement light source unit 50 for irradiating the measurement light to the sample SM, which is an object to be measured, and the sample SM. The reflected light of the measurement light is incident as the measurement light, and the interferometer 11 that emits the interference light of the measurement light, and the interference light of the measurement light obtained by the interferometer 11 are received and photoelectrically converted. A light reception processing unit 20 that outputs an electrical signal related to the waveform of the interference light of the light to be measured (an electrical signal representing a light intensity change in the interference light of the light to be measured), and a sampling timing at which measurement data is sampled by the light reception processing unit 20 The timing generation part 30 to produce | generate, the control calculating part 41, the input part 42, and the output part 43 are provided.

  The measurement light source unit 50 is a device that irradiates the sample SM with measurement light with a predetermined geometry, and includes, for example, a measurement light source 51 (see FIG. 2) and its peripheral circuits. The measurement light source 51 is a device that emits measurement light and irradiates the sample SM with the measurement light with a geometry of 45: 0 degrees, for example. The measurement light is used to measure the spectrum of the reflected light in the sample SM, and is light having a continuous spectrum in a predetermined wavelength band set in advance. In this embodiment, for example, a halogen lamp is used as such a measurement light source 51.

  In the FT spectrometer D of the present embodiment, the measurement light emitted from the measurement light source 51 is incident on the surface (measurement surface SF) of the sample SM at an incident angle of 45 degrees as shown in FIG. Reflected by the SM, the reflected light of the reflected measurement light is measured from the 0 degree direction. That is, the component of the reflected light reflected in the normal direction (0 degree) of the measurement surface enters the interferometer 11 as the light to be measured. As described above, the FT spectrometer D of the present embodiment is a reflection type in which the reflected light of the measurement light reflected by the sample SM is the light to be measured.

  In this example, the light to be measured is reflected light of the measurement light reflected by the sample SM. However, the light to be measured may be light re-radiated from the sample SM (for example, fluorescence emission) by irradiating the measurement light. In addition, the light may be light emitted by the sample SM without being irradiated with the measurement light. The reflective FT spectrometer D can measure not only the reflected light but also such re-radiated light and self-luminous light. The FT spectrometer D may be a transmission type that measures measurement light that has passed through the sample, and the light to be measured may be measurement light that has passed through the sample.

  The interferometer 11 receives the light to be measured, branches the incident light to be measured into two first and second light to be measured, and each of the branched first and second light to be measured, It travels (propagates) to each of the first and second optical paths, which are two different paths, and merges again. Between this branch point (branch position) and a merge point (merging position, interference position) If there is an optical path difference between the first and second optical paths, a phase difference is generated at the time of the merging, so that the light is shaded by the merging. As the interferometer 11, for example, an interferometer having various types of first and second optical paths such as a Mach-Zehnder interferometer can be used. In this embodiment, as shown in FIG. It is constituted by.

  More specifically, as shown in FIG. 2, the interferometer 11 includes a semi-transparent mirror (half mirror) 112, a fixed mirror 114, and a moving mirror 115 whose light reflecting surface moves in the optical axis direction as a plurality of optical elements. The fixed mirror 114 and the movable mirror 115 are arranged so that the normals of the mirror surfaces are orthogonal to each other, and the semi-transparent mirror 112 has a normal line corresponding to each of the normal lines of the fixed mirror 114 and the movable mirror 115. Are arranged so as to cross each normal line at an angle of 45 degrees. In the interferometer 11, the light to be measured incident on the interferometer 11 is branched into two first and second light to be measured by the semi-transparent mirror 112. The branched first first measured light is reflected by the semi-transparent mirror 112 and enters the fixed mirror 114. The first light to be measured is reflected by the fixed mirror 114 and returns to the semi-transparent mirror 112 again following the optical path that has come. On the other hand, the other branched second measured light passes through the semi-transparent mirror 112 and enters the movable mirror 115. This second light to be measured is reflected by the movable mirror 115, and reversely follows the optical path that has come to return to the semi-transparent mirror 112 again. The first light to be measured reflected by the fixed mirror 114 and the second light to be measured reflected by the moving mirror 115 are merged with each other by the semi-transparent mirror 112 and interfere with each other. In the Michelson interferometer 11 having such a configuration, the light to be measured is incident on the interferometer 11 along the normal direction on the mirror surface of the movable mirror 115, and the interference light of the light to be measured is reflected on the mirror surface of the fixed mirror 114. The light is emitted from the interferometer 11 along the normal direction.

  In this embodiment, the interferometer 11 is arranged on the reflection side of the semi-transparent mirror 112 reflected by the semi-transparent mirror 112 when the light to be measured is branched into two first and second measured light beams by the semi-transparent mirror 112. A phase compensation plate 113 is further provided. That is, in the present embodiment, the first measured light reflected by the semi-transparent mirror 112 is incident on the fixed mirror 114 via the phase compensation plate 113, and the first measured light reflected by the fixed mirror 114 is phase compensated. The light enters the semi-transparent mirror 112 again through the plate 113. The phase compensation plate 113 is a phase difference between the first measured light and the second measured light, which is caused by the difference in the number of times the first measured light is transmitted through the semi-transparent mirror 112 and the number of times the second measured light is transmitted through the semi-transmissive mirror 112. Is used to compensate for the phase difference.

  Therefore, in the present embodiment, the first measured light is transmitted from the incident position of the measured light through the semi-transparent mirror 112, the phase compensation plate 113, the fixed mirror 114, and the phase compensation plate 113 in this order. Follow the first optical path that leads to again. The second measured light follows a second optical path from the incident position of the measured light to reach the semi-transmissive mirror 112 again through the semi-transmissive mirror 112 and the movable mirror 115 in this order. In the interferometer 11 of the FT type spectrometer D, the intensity of light caused by the optical path difference generated by the moving mirror 115 is generated.

  In the present embodiment, for example, a collimator lens 111 is disposed as an incident optical system at an appropriate position between the sample surface SF and the semi-transparent mirror 112 in order to cause the light to be measured to enter the semi-transparent mirror 112 with parallel light. In order to collect the interference light of the light to be measured generated by combining the first and second light to be measured with the semi-transparent mirror 112 and causing them to interfere with each other, For example, a condensing lens 116 is further disposed as an emission optical system at an appropriate position between the light receiving unit 21 and the light receiving unit 21.

  Here, the movable mirror 115 of the interferometer 11 will be further described. In the present embodiment, the movable mirror 115 is an example of an optical path difference forming optical element, and an optical element that generates an optical path difference between the two first and second optical paths by using resonance vibration is used. . The movable mirror 115 reciprocates twice or more in the optical axis direction in order to generate a plurality of interferograms of the light to be measured.

  As such a movable mirror 115, for example, a movable mirror of a parallel movement mechanism disclosed in Patent Document 1 and Patent Document 2 can be used. More specifically, as shown in FIG. 5, the movable mirror 115 includes first and second leaf springs 151 and 152 arranged in parallel to face each other, and the first and second leaf springs 151 and 152. Between the first and second leaf springs 151 and 152 and the first support member 153 arranged to be connected to the first and second leaf springs 151 and 152 at one end between the first and second leaf springs 151 and 152. A second support 154 that is connected to the first and second leaf springs 151 and 152 at the other end separated from the first support 153, and an upper surface of the other end of the first leaf spring 151; The piezoelectric element 155 is disposed, and a mirror surface region 156 formed on the upper surface at one end of the first leaf spring 151 is provided. That is, the piezoelectric element 155 is disposed above the second support 154 in the first plate spring 151 and on the surface opposite to the second support 154. The mirror surface region 156 is disposed above the first support 153 in the first leaf spring 151 and on the surface (main outer surface) opposite to the second support 154. The piezoelectric element 155 has a structure in which a piezoelectric material 155a such as PZT (lead zirconate titanate), which is a piezoelectric material, is sandwiched between a pair of electrodes 155b and 155c (see FIG. 5). The mirror surface region 156 is formed by attaching a mirror or forming a thin film of metal such as aluminum. The movable mirror 115 is manufactured by, for example, a MEMS (Micro Electro Mechanical Systems) technique.

  In the movable mirror 115 having such a structure, as shown in FIG. 6A, when the piezoelectric element 155 expands, the first leaf spring 151 is deformed so as to be convex upward. When the piezoelectric element 155 shrinks while the first leaf spring 151 and 152 are displaced downward in the opposing direction, the first leaf spring 151 is deformed to be convex downward as shown in FIG. 6B. As a result, the mirror surface region 16 is displaced upward in the facing direction. The movable mirror 115 repeats the displacement by resonance in order to obtain a large amount of displacement, and reciprocates along the optical axis direction. More specifically, when there is no distortion in the reciprocating movement, the movable mirror 15 has an origin position X0 (center position of vibration amplitude) at which the displacement amount becomes 0 (zero) and a displacement amount from the origin position X0. Is displaced in a substantially sinusoidal shape with the passage of time, and vibrates accurately at a constant frequency and at a constant period.

  As described above, the movable mirror 115 includes a driving unit configured by a parallel leaf spring structure including the first and second leaf springs 151 and 152 arranged in parallel to face each other, and the first and second leaf springs 151 and 152. And a mirror surface 156 serving as a reflection surface formed on one main outer surface of the. The optical path difference is caused by the reflection surface moving in parallel along the optical axis by the above-described resonance driving of the driving unit.

  Returning to FIG. 1, for example, as shown in FIG. 3, the light reception processing unit 20 includes a first light reception unit 21, an amplification unit 22, a high pass filter (High Pass Filter) 23, and a low pass filter (Low Pass Filter). 24 and an analog-digital converter (hereinafter referred to as “AD converter”) 25.

  The first light receiving unit 21 is a circuit that outputs an electric signal corresponding to the light intensity in the interference light of the measurement light by receiving and photoelectrically converting the interference light of the measurement light obtained by the interferometer 11. . The FT spectrometer D of the present embodiment is, for example, a specification whose measurement object is light in the infrared region with a wavelength of 1200 nm or more, more specifically, light in the infrared region with a wavelength of 1200 nm or more to 2500 nm or less. The first light receiving unit 21 is, for example, an infrared sensor configured to include such an InGaAs photodiode and its peripheral circuit that can receive such infrared light. The first light receiving unit 21 outputs the light reception result to the amplification unit 22.

  The amplification unit 22 is a circuit that amplifies the output (amplification result) of the first light receiving unit 21 with a predetermined amplification factor set in advance, and includes an amplifier such as an operational amplifier and its peripheral circuits. The amplification unit 22 outputs the amplification result to the high pass filter 23.

  The high-pass filter 23 is a circuit for passing a signal having a frequency equal to or higher than a predetermined cutoff frequency and cutting low-frequency noise, and outputs the filtered result to the low-pass filter 24. The low-pass filter 24 is a circuit for passing a signal having a frequency equal to or lower than a predetermined cutoff frequency and cutting high-frequency noise, and outputs the filtered result to the AD conversion unit 25. The high-pass filter 23 and the low-pass filter 24 constitute a band-pass filter that passes only a desired frequency band in order to cut noise.

  The AD conversion unit 25 is a circuit that converts the output of the first light receiving unit 21 input via the amplification unit 22, the high pass filter 23, and the low pass filter 24 from an analog signal to a digital signal (AD conversion). The AD conversion timing (sampling timing) is executed at the zero cross timing input from the zero cross detector 37 described later. The AD conversion unit 25 outputs a digital signal as a result of the conversion to the control calculation unit 41.

  The timing generation unit 30 includes, for example, a position measurement light source 31, a second light receiving unit 36, and a zero cross detection unit 37. Then, in order to obtain the interference light of the laser light emitted from the position measuring light source 31 with the interferometer 11, the timing generation unit 30 has a collimator lens 32, an optical multiplexer 33, as shown in FIG. An optical demultiplexer 34 and a condenser lens 35 are further provided.

  The position measuring light source 31 is a light source device that emits monochromatic laser light, and includes, for example, a semiconductor laser that emits red laser light having a wavelength of 680 nm. In FIG. 2, a collimator lens 32 and an optical multiplexer 33 are incident optical systems for causing the laser light emitted from the position measuring light source 31 to enter the interferometer 11 as parallel light. The optical multiplexer 33 is, for example, a dichroic mirror that reflects laser light and transmits measured light, and a collimator so that the normal line intersects the normal line (optical axis) of the movable mirror 115 at 45 degrees. It is disposed between the lens 111 and the semi-transparent mirror 112. The collimator lens 32 is, for example, a biconvex lens, and the laser beam emitted from the position measuring light source 31 is incident on the optical multiplexer 33 arranged in this manner at an incident angle of 45 degrees as appropriate. It is arranged in the position. The optical demultiplexer 34 and the condensing lens 35 are emission optical systems for taking out the interference light of the laser light generated by the interferometer 11 from the interferometer 11. The optical demultiplexer 34 is, for example, a dichroic mirror that reflects the interference light of the laser light and transmits the interference light of the light to be measured, and its normal line is 45 degrees with respect to the normal line (optical axis) of the fixed mirror 114. Are arranged between the semi-transparent mirror 112 and the condenser lens 116 so as to intersect each other. The condensing lens 35 is, for example, a biconvex lens, and condenses the interference light of the laser beam emitted at an emission angle of 45 degrees in the optical demultiplexer 34 arranged in this manner, thereby the second light receiving unit 36. To enter.

  When the optical elements such as the collimator lens 32, the optical multiplexer 33, the optical demultiplexer 34, and the condenser lens 35 are arranged in this way, the monochromatic laser light emitted from the position measuring light source 31 is converted into the collimator lens 32. The optical path is bent by about 90 degrees by the dichroic mirror 33 of the optical multiplexer 33 and travels along the optical axis of the interferometer 11 (normal direction on the mirror surface of the movable mirror 115). . Therefore, this laser light travels in the interferometer 11 as with the light to be measured, and the interferometer 11 generates the interference light. Then, the interference light of this laser light is bent by about 90 degrees by the dichroic mirror 34 of the optical demultiplexer 34, taken out from the interferometer 11, collected by the condenser lens 35, and condensed by the second light receiving unit 36. Is received.

  Returning to FIG. 1, the second light receiving unit 36 receives the interference light of the laser light obtained by the interferometer 11 and photoelectrically converts it, thereby outputting an electric signal corresponding to the light intensity of the interference light of the laser light. It is a circuit to do. The second light receiving unit 36 is, for example, a light receiving sensor including a silicon photodiode (SPD) and its peripheral circuit. The second light receiving unit 36 outputs an electrical signal corresponding to the light intensity of the interference light of the laser light to the zero cross detection unit 37.

  The zero cross detection unit 37 is a circuit that detects a timing (zero cross timing) at which the electric signal corresponding to the light intensity of the interference light of the laser beam input from the second light receiving unit 36 becomes zero. The zero cross timing is a position on the time axis at which the electric signal becomes a predetermined zero. When the movable mirror 115 of the interferometer 11 is moved in the optical axis direction, the movable mirror 115 is moved from the semi-transparent mirror 112 to the phase of the laser light that has returned from the semi-transparent mirror 112 to the semi-transparent mirror via the fixed mirror 114. Since the phase of the laser light that has returned to the semi-transparent mirror is shifted again, the interference light of the laser light becomes strong and weak in a sine wave shape according to the amount of movement. When the movable mirror 115 of the interferometer 11 moves by a length that is ½ of the wavelength of the laser light, the phase of the laser light that has returned from the semi-transparent mirror 112 to the semi-transparent mirror through the movable mirror 115 is There is a 2π shift before and after. For this reason, for example, as shown in FIG. 4, the interference light of the laser light repeatedly increases and decreases in a sine wave shape as the moving mirror 115 moves. The zero cross detector 37 detects the zero cross of the electrical signal that repeats the strength in a sine wave form. The zero-cross detection unit 37 outputs the detected zero-cross timing to the AD conversion unit 23, and the AD conversion unit 23 outputs the interference light of the measured light input from the first light receiving unit 21 at the zero-cross timing. An electrical signal corresponding to the light intensity is sampled and AD converted.

  The movable mirror operation detection unit 200 is a sensor device that detects the movement of the movable mirror 115 in order to detect one scan of the movable mirror 115 in order to obtain the amplitude of the movable mirror 115. In the example illustrated in FIG. 2, the moving mirror operation detection unit 200 includes a photo reflector as a detection sensor. This photoreflector includes a light emitting element that irradiates light on the back surface of the movable mirror 115 and a light receiving element that receives light reflected on the back surface of the movable mirror 115, and the amount of reflected light that changes according to the movement of the movable mirror 115. The movement of the movable mirror 115 is detected by detection, and this photo reflector outputs a signal synchronized with the movement of the movable mirror 115. Therefore, one cycle of the output of the photo reflector corresponds to one reciprocation of the movable mirror 115, and one scan of the movable mirror 115 is detected from the output of the photo reflector.

  Returning to FIG. 1, the control calculation unit 41 controls each part of the FT spectrometer D according to the function of each part in order to obtain the spectrum of the light to be measured. The control arithmetic unit 41 is, for example, a CPU (Central Processing Unit), a ROM (Read Only Memory) or an EEPROM (Electrically Erasable Programmable Read Only) that stores various programs executed by the CPU and data necessary for the execution in advance. A non-volatile memory element such as Memory), a volatile memory element such as a RAM (Random Access Memory) serving as a so-called working memory of the CPU, and a microcomputer including peripheral circuits thereof. The control calculation unit 41 may further include a relatively large capacity storage device such as a hard disk, for example, in order to store data output from the AD conversion unit 23. Then, the control calculation unit 41 is functionally executed by executing a program, such as a sampling data storage unit 411, a center burst position calculation unit 412, an integrated interferogram calculation unit 413, a spectrum calculation unit 414, and a moving mirror amplitude storage. The unit 415 and the amplitude fluctuation calculation unit 416 are configured.

  The sampling data storage unit 411 stores measurement data related to the interference light of the light to be measured output from the AD conversion unit 23. As described above, the measurement data is obtained by sampling the electrical signal corresponding to the light intensity in the interference light of the light to be measured by the AD conversion unit 23 at the zero cross timing detected by the zero cross detection unit 37.

  The center burst position calculation unit 412 obtains the position of the center burst from the measurement data stored in the sampling data storage unit 411 by a known conventional method.

  The integrated interferogram calculation unit 413 aligns a plurality of interferograms obtained by continuously measuring the measured light at a plurality of times at each center burst position obtained by the center burst position calculation unit 412. While performing, the integrated interferogram is obtained by integrating.

  The spectrum calculation unit 414 obtains a spectrum by performing Fourier transform on the integrated interferogram obtained by integrating a plurality of interferograms by the integrated interferogram calculating unit 413.

  The movable mirror amplitude storage unit 415 and the amplitude fluctuation calculation unit 416 will be described later.

  The input unit 42, for example, various commands such as a command for instructing the measurement start of the sample SM, and a spectrum such as an input of an identifier in the sample SM to be measured and a selection input of a window function used for Fourier transform, for example. A device that inputs various data necessary for measurement to the FT spectrometer D, such as a keyboard and a mouse. The output unit 43 is a device that outputs the command and data input from the input unit 42 and the spectrum of the light to be measured measured by the FT spectrometer D. For example, the output unit 43 is a CRT display, LCD, organic EL display, and plasma. A display device such as a display, or a printing device such as a printer.

  Next, the operation of this embodiment will be described. FIG. 7 is a diagram illustrating a waveform (interferogram) of actually measured interference light of the measured light in the Fourier transform spectrometer according to the embodiment. The horizontal axis in FIG. 7 is the optical path difference x between the first optical path and the second optical path, and the vertical axis is the amplitude Fm (x) of the interferogram. FIG. 8 is a diagram showing the relationship between the interferogram and the window function. The horizontal axis in FIG. 8 is the optical path difference x between the first optical path and the second optical path, and the vertical axis is the amplitude. A solid line is an interferogram, and a broken line is a window function.

  In the FT spectrometer D having such a configuration, when measuring the sample SM to be measured, first, the sample SM is set in the FT spectrometer D, and measurement is started. When the measurement is started, the measurement light source 51 emits measurement light and irradiates the sample SM with the measurement light at an incident angle of 45 degrees, for example. Then, the reflected light of the measurement light reflected by the sample SM is measured from the 0 degree direction as light to be measured, and is incident on the interferometer 11.

  The light to be measured incident on the interferometer 11 is received by the first light receiving unit 21 of the light receiving processing unit 20 as interference light of the light to be measured by the interferometer 11. More specifically, the light to be measured is converted into parallel light by the collimator lens 111, and is reflected and transmitted by the semi-transparent mirror 112 through the optical multiplexer 33, thereby being branched into the first and second light to be measured. The first light to be measured branched by being reflected by the semi-transparent mirror 112 is incident on the fixed mirror 114 via the phase compensation plate 113, reflected by the fixed mirror 114, and traces the optical path that has come in reverse, and again the semi-transparent mirror 112. Return to. On the other hand, the second light to be measured branched by passing through the semi-transparent mirror 112 is incident on the movable mirror 115, reflected by the movable mirror 115, and returns to the semi-transparent mirror 112 by tracing back the optical path that has come. The first light to be measured reflected by the fixed mirror 114 and the second light to be measured reflected by the moving mirror 115 are merged with each other by the semi-transparent mirror 112 and interfere with each other. The interference light of the light to be measured is emitted from the interferometer 11 to the first light receiving unit 21. The first light receiving unit 21 photoelectrically converts the incident interference light of the measurement light, and outputs an electrical signal corresponding to the light intensity in the interference light of the measurement light to the amplification unit 22. The amplifying unit 22 amplifies the electric signal corresponding to the interference light of the light to be measured with a predetermined amplification factor, and outputs it to the AD converting unit 23.

  On the other hand, the FT spectrometer D also captures monochromatic laser light emitted from the position measurement light source 31. This laser light is incident on the interferometer 11 via the optical multiplexer 33, interferes with the interferometer 11 in the same manner as described above, becomes interference light of the laser light, and passes through the optical demultiplexer 34 to the second light receiving unit. Light is received at 36. The second light receiving unit 36 photoelectrically converts the incident interference light of the laser beam, and outputs an electrical signal corresponding to the light intensity in the interference light of the laser beam to the zero cross detection unit 37. The zero cross detection unit 37 detects a timing at which the electric signal corresponding to the interference light of the laser beam becomes zero as a zero cross timing, and outputs the zero cross timing to the AD conversion unit 23 as a sampling timing (AD conversion timing).

  While such measured light and laser light are respectively taken into the interferometer 11, the movable mirror 115 of the interferometer 11 is moved along the optical axis direction according to the control of the control calculation unit 41 by resonance vibration. Yes.

  The AD conversion unit 23 samples the electrical signal output from the amplification unit 22 according to the light intensity in the interference light of the light to be measured at the zero cross timing input from the zero cross detection unit 37, and converts the electrical signal from an analog signal to a digital signal. AD conversion is performed, and the electric signal of the digital signal subjected to the AD conversion is output to the control calculation unit 41.

  By operating in this way, measurement data in the interferogram of the light to be measured is output from the AD conversion unit 23 to the control calculation unit 41, and this measurement data is stored in the sampling data storage unit 411. An example of the interferogram of the light to be measured thus measured is shown in FIG. Then, in order to improve the S / N ratio and obtain a result with good accuracy, the interferogram of such light to be measured is measured in a similar manner continuously several times in accordance with the reciprocation of the movable mirror 115, and these Each measurement data of each interferogram is stored in the sampling data storage unit 411. When the movable mirror 115 reciprocates once, one scan is completed, and one interferogram measurement data is obtained for each of the forward path and the backward path. That is, one interferogram is data from the maximum amplitude position at one end to the maximum amplitude position at the other end via the vibration center (optical path difference 0).

  Next, the center burst position calculation unit 412 obtains the position of the center burst in the interferogram of the measured light for each measurement data of each interferogram stored in the sampling data storage unit 411.

  Next, the integrated interferogram calculation unit 413 obtains a plurality of interferograms of the measured light obtained by measuring a plurality of times by the center burst position calculation unit 412 in each of the forward path and the return path. An integrated interferogram for the measured light is obtained by performing integration while performing alignment at each center burst position.

  Next, the spectrum calculation unit 414 obtains the spectrum of the light to be measured by Fourier-transforming the accumulated interferogram in each of the forward path and the return path obtained by the accumulated interferogram calculation unit 413. Then, the spectrum calculation unit 414 obtains the spectrum of the final measured light output to the output unit 43 as the measurement result by obtaining the average of the obtained spectra for each of the forward path and the return path.

The calculation of this spectrum will be described more specifically. First, the interferogram F _m (x _i ) in the m-th measurement has an optical path difference x _i , a wave number ν _j , and a wave number ν _j spectrum. When the amplitude is B (ν _j ), the position of the optical path difference 0 is X _0, and the phase at the position of the optical path difference 0 of the wave number ν _j is φ (ν _j ), it is expressed by Expression 1. Note that m represents the measurement result of the mth measurement.

Therefore, the integrated interferogram F (x _i ) is expressed by Equation 2.

  When the accumulated interferogram is obtained by the accumulated interferogram calculating unit 413 in this way, the spectrum calculating unit 414 obtains the spectrum of the light to be measured by, for example, fast Fourier transform (FFT) of the accumulated interferogram.

More specifically, in the case of fast Fourier transform, in order to reduce the occurrence of side lobes, as shown in FIG. 8, a window function A _{window that is} symmetric about the optical path difference 0 (center burst position) is used. After multiplying (x _i ) (Equation 3), fast Fourier transform is performed, and the amplitude | B _window (ν _j ) | of the spectrum of the measured light is obtained (Equation 4).

Examples of the window function A _window (x _i ) can include various appropriate functions. For example, the window function A _window (x _i ) is a function represented by Expression 5-1 to Expression 5-3. Equation 5-1 is called a Hanning Window function, Equation 5-2 is called a Hamming Window function, and Equation 5-3 is called a Blackman Window function. .

  As described above, when the spectrum is obtained, the control calculation unit 41 outputs the obtained spectrum to the output unit 43.

  Here, in the measurement of the spectrum of the light to be measured, as described above, when external vibration having a frequency close to the resonance frequency of the movable mirror 115 is applied to the FT spectrometer D, so-called undulation occurs, and the light receiving processing unit Since the 20 amplification units 22 have frequency characteristics, an error occurs in the spectrum.

  FIG. 9 is a diagram illustrating frequency characteristics of the amplification unit of the light receiving processing unit in the Fourier transform spectrometer according to the embodiment. FIG. 9A is an overall view, and FIG. 9B is a partially enlarged view thereof. The horizontal axis in FIGS. 9A and 9B is the frequency expressed in kHz, and the vertical axis is the amplification factor (gain). FIG. 10 is a schematic diagram showing the relationship between the frequency characteristic of the amplification unit in the light receiving processing unit and the frequency band of the signal included in the interferogram. The horizontal axis in FIG. 10 is the frequency, and the vertical axis is the amplification factor (gain). FIG. 11 is a diagram showing the relationship between each amplitude of the movable mirror in the interferometer and the intensity ratio of the spectrum obtained based on each interferogram at that amplitude. FIG. 12 shows an average of spectra measured at amplitudes before and after the case where the amplitude of the moving mirror is 5500 (the number of sampling in one interferogram measurement (one scan), the same applies hereinafter). It is a figure which shows the intensity ratio of a spectrum. Each horizontal axis in FIGS. 11 and 12 is a wavelength expressed in nm units, and each vertical axis represents a spectral ratio of each amplitude to a spectrum when the amplitude of the moving mirror is 5500 (the amplitude of the starting mirror is 5500). The spectrum in each amplitude is normalized with respect to the spectrum at each amplitude).

  More specifically, as shown in FIG. 9, the amplifying unit 22 has a frequency at which the amplification factor gradually decreases from the frequency 0 (zero) as the frequency increases (see FIG. 9B), and then decreases more rapidly. Has characteristics.

  On the other hand, since the movable mirror 115 is resonantly driven, the driving frequency is constant. Therefore, while the movable mirror 115 is driven to resonate, the time required for the movable mirror 115 to reciprocate once by vibration is constant regardless of the amplitude. For this reason, when the undulation occurs in the movable mirror 115 and the amplitude varies, the speed of the movable mirror 115 changes, and the frequency band of the signal included in the interferogram varies. For example, if the amplitude of the moving mirror 115 that is resonating when there is no external vibration and no undulation is set as the target amplitude, as shown in FIG. 10, the undulation is generated by the external vibration and resonance occurs at an amplitude smaller than the target amplitude. In the frequency band BNs of the signal included in the interferogram obtained by the moving mirror 115, the speed of the moving mirror 115 decreases, so that the interferogram obtained by the moving mirror 115 resonating at the target amplitude ( Hereinafter, it is abbreviated as “interferogram of target amplitude”), and changes from the frequency band BN0 of the signal included in the signal to the low frequency side. On the other hand, the frequency band BNw of the signal included in the interferogram obtained by the moving mirror 115 that is swelled by external vibration and resonates with an amplitude larger than the target amplitude increases the speed of the moving mirror 115. It fluctuates to the high frequency side from the frequency band BN0 of the signal included in the interferogram of the target amplitude.

  If the frequency band BN fluctuates in this way, it is included in the interferogram obtained by the movable mirror 115 that resonates at an amplitude smaller than the target amplitude, as shown in FIG. The signal to be transmitted is amplified with a larger amplification factor than the amplification factor of the signal included in the interferogram of the target amplitude. On the other hand, the signal included in the interferogram obtained by the movable mirror 115 that resonates with an amplitude larger than the target amplitude is amplified with a smaller amplification factor than the amplification factor of the signal included in the target amplitude interferogram. Will be.

  As described above, when the amplitude of the movable mirror 115 varies due to the undulation, the amplification factor when the signal included in the interferogram is amplified by the amplification unit 22 varies. As a result, the intensity of the spectrum obtained by the Fourier transform of the interferogram changes and an error occurs in the spectrum.

  Note that the signal included in the interferogram is a signal extracted as a spectrum of light to be measured by Fourier transform of the interferogram.

  Therefore, with a spectrum obtained by Fourier transform from the interferogram of the target amplitude (hereinafter abbreviated as “reference spectrum”) as a reference, the movable mirror 115 resonates at an amplitude of the magnitude before and after the target amplitude. A spectrum obtained by Fourier transform from the obtained interferogram was simulated (numerical experiment). For example, when the target amplitude is the count value 5500, each spectrum is numerically calculated by Fourier transform from each interferogram when the amplitude is the count value 4000, 4500, 5000, 6000, 6500, 7000. The ratio is shown in FIG. In order to convert the count value into a length, an equation of (count value) × (wavelength of position measurement light source 31; 680 nm) / 4) is used. For example, the count value 5500 is 0.935 mm. is there.

In FIG. 11, the solid line “4000” is the ratio r ₀ (λ) of the spectrum B ₄₀₀₀ (λ) having an amplitude of 4000 to the reference spectrum B ₅₅₀₀ (λ), and the solid line “4500” is the reference spectrum B ₅₅₀₀ ( The ratio r ₁ (λ) of the spectrum B ₄₅₀₀ (λ) with the amplitude 4500 to λ), and the solid line “5000” is the ratio r ₂ of the spectrum B ₅₀₀₀ (λ) with the amplitude 5000 to the reference spectrum B ₅₅₀₀ (λ). (Λ), and the solid line “5500” is the ratio r ₃ (λ) of the spectrum B ₅₅₀₀ (λ) (ie, the reference spectrum) having the amplitude 5500 to the reference spectrum B ₅₅₀₀ (λ), and the solid line “6000” the spectrum _{B 6000} of the amplitude 6000 to the reference spectrum _{B 5500} (λ) (λ) is the ratio _{r 4} of (lambda), "6 The solid line 00 "is a spectrum _{B 6500} of the amplitude 6500 to the reference spectrum _{B 5500} (λ) (λ) ratio _{r 5} of (lambda), and, the solid line" 7000 "is the reference spectrum _{B 5500} (lambda) This is the ratio r ₆ (λ) of the spectrum B ₇₀₀₀ (λ) having an amplitude of 7000. Each expression of these ratios r ₀ (λ) to r ₆ (λ) is summarized below.

As can be seen from FIG. 11, the ratios r ₀ (λ) to r ₆ (λ) are divided into upper and lower portions approximately symmetrically about the ratio r ₃ (λ). That is, the ratio r ₀ (λ) to ratio r ₂ (λ) is greater than the ratio r ₃ (λ), while the ratio r ₄ (λ) to ratio r ₆ (λ) is greater than the ratio r ₃ (λ). small.

  Thus, the spectra at the amplitudes before and after the target amplitude are averaged, and the relationship between the averaged spectrum and the reference spectrum is simulated, and the result is shown in FIG.

In FIG. 12, the solid line “5000” is the average <r ₀ (λ)> of the reference spectrum, that is, the ratio r ₃ (λ) itself, and the solid line “5000-6000” is the spectrum at the amplitude 5000. The average <r ₁ (λ)> of the spectrum at the target amplitude 5500 and the spectrum at the amplitude 6000, and the solid line “4500-6500” indicates the spectrum at the amplitude 4500, the spectrum at the amplitude 5000, and the target amplitude 5500. The average of the spectrum and the spectrum at amplitude 6000 and the spectrum at amplitude 6500 <r ₂ (λ)>, and the solid line “4000-7000” is the spectrum at amplitude 4000 and the spectrum and amplitude at amplitude 4500 The spectrum at 5000, the spectrum at target amplitude 5500, the spectrum at amplitude 6000, and the spectrum at amplitude 6500. It is the average <r ₃ (λ)> between the spectrum at the spectrum and the amplitude at 7000.

That is, the average <r ₀ (λ)> is the average of the spectrum at the target amplitude (reference spectrum) and each spectrum at the zero amplitude before and after the target amplitude (in other words, as described above) In addition, the ratio r ₃ (λ) itself) and the average <r ₁ (λ)> are the average of the spectrum at the target amplitude (reference spectrum) and each spectrum at one amplitude before and after the target amplitude. The average <r ₂ (λ)> is the average of the spectrum at the target amplitude (reference spectrum) and each spectrum at the two amplitudes before and after the target amplitude, and the average <r ₃ (λ)> is the average of the spectrum at the target amplitude (reference spectrum) and each spectrum at the three amplitudes before and after the target amplitude. Each expression of these averages <r ₀ (λ)> to <r ₃ (λ)> is summarized below.

  As can be seen from FIG. 12, by adding the spectrum at the amplitude before and after the target amplitude of the moving mirror 115 to the spectrum at the target amplitude and averaging the spectrum, the spectrum of this average result is The reproducibility of the spectral intensity is improved by approaching the spectrum in amplitude.

  For this reason, in the above-described FT spectrometer D of the present embodiment, the measurement time (total measurement time, total scan time) of the interferogram that is continuously measured is an integer of the amplitude fluctuation period of the movable mirror 115. It is set to be doubled. That is, when the measurement time of a plurality of interferograms measured continuously is not an integral multiple of the amplitude fluctuation period of the movable mirror 115, the measurement time of the interferogram measured continuously is set. Accordingly, when the integrated interferogram is obtained, the interferogram when the amplitude of the movable mirror 115 is large is included in the accumulated interferogram, or conversely, when the amplitude of the movable mirror 115 is small. Many interferograms are included in the integrated interferogram. In other words, when the measurement time of the interferogram ends at a time when the undulation is large, when the integrated interferogram is obtained, the interferogram is accumulated including many interferograms that are amplified with a relatively small amplification factor. In other words, the intensity of the spectrum obtained by Fourier transform of the integrated interferogram is smaller than the true value. On the other hand, when the measurement time of the interferogram ends at a point when the undulation is small, when the interferogram is obtained, the interferogram is accumulated including many interferograms that are amplified with a relatively large amplification factor. In other words, the intensity of the spectrum obtained by Fourier transform of the integrated interferogram becomes a value larger than the true value. Therefore, as described above, when the measurement time of the interferogram is set to an integral multiple of the amplitude fluctuation period of the movable mirror 115, the interferogram measured at the time when the swell is large when obtaining the integrated interferogram. The interferograms measured at the time when the undulation is small are almost evenly included, so that they work to cancel each other, and as described with reference to FIG. Integrated interferogram closer to true results with reduced impact. Therefore, the intensity of the spectrum obtained by Fourier transform of such an integrated interferogram becomes a value closer to the true value, the measurement accuracy is improved, and the reproducibility is improved.

  More specifically, as shown in FIG. 1, the control calculation unit 41 includes a movable mirror amplitude storage unit 415 and an amplitude variation calculation unit 416, and the amplitude variation calculation unit 416 is an integral multiple of the amplitude variation period. The sampling data storage unit 411 stores measurement data of a plurality of interferograms obtained by measuring a plurality of interferograms of the light to be measured continuously by the interferometer 11 over time.

For example, if the measurement time set by the user is Tmeas and the scanning period of the moving mirror 115 (= reciprocal of the resonance driving frequency) is Tscan, if there is no change in the amplitude of the moving mirror 115, the default RepeatNum0 is ( Interferograms are measured and stored in each of the outbound and return paths of the number of times Tmeas / Tscan), and the integrated interferogram calculation unit 413 individually accumulates these interferograms in each of the outbound and inbound paths. The measurement time is appropriately set according to the desired number of integrations. When the amplitude of the movable mirror 115 varies, the number of interferogram measurements RepeatNum is an integral multiple of the amplitude variation period ΔI of the movable mirror 115 and is set to be equal to or greater than the default number. It is represented by the formula FC.
RepeatNum
= Round (ceil (RepeatNum0 / ΔI) × ΔI) (FC)
Here, round (z) is a function that rounds off z, and ceil (z) is a function that rounds z up to the nearest integer of z.

  The moving mirror amplitude storage unit 415 stores each amplitude of the moving mirror 115 in each of a plurality of scans (measurement of a plurality of interferograms) executed continuously. Therefore, the movable mirror amplitude storage unit 415 includes a plurality of storage areas for storing the amplitude of the movable mirror 115 in one scan (measurement of the interferogram in one round trip of the movable mirror 115, the forward path and the return path), In this embodiment, the movable mirror amplitude storage unit 415 sequentially stores the amplitudes of the movable mirror 115 in each scan from the storage area of the first address to the storage area of the final address, for example, as shown in FIG. When the amplitude is stored in the storage area of the final address, the circular buffer circuit returns to the storage area of the top address and stores the amplitude again in order from the storage area of the top address to the storage area of the final address. Therefore, the moving mirror amplitude storage unit 415 of the circular buffer circuit stores past amplitude data that has been traced back to the size of the circular buffer circuit (the number of storage areas) from the latest amplitude data.

  The amplitude fluctuation calculation unit 416 obtains the amplitude of the movable mirror 115 in one scan, stores the obtained amplitude of the movable mirror 115 in the movable mirror amplitude storage unit 415, and stores it in the movable mirror amplitude storage unit 415. From the amplitude of the movable mirror 115, it is determined whether or not there is a variation in the amplitude of the movable mirror 115. If it is determined that there is a variation in the amplitude of the movable mirror 115 as a result of the determination, the variation in the amplitude is determined. The period is obtained. As described above, the amplitude fluctuation calculation unit 416 is an example of the amplitude calculation unit and the amplitude fluctuation period calculation unit, and further functions as an amplitude fluctuation determination unit that determines whether or not the amplitude of the movable mirror 115 fluctuates. It is also an example of a period determination unit.

  The operation of detecting the amplitude of the movable mirror 115 in one scan and storing the detected amplitude of the movable mirror 115 in the movable mirror amplitude storage unit 415 is executed for each scan after the scan is completed, for example. If it is not determined that the amplitude of the movable mirror 115 is varied as a result of the determination, the variation period of the amplitude cannot be obtained. In this case, the amplitude fluctuation calculation unit 416 calculates a plurality of interferograms obtained by continuously measuring a plurality of interferograms of the light to be measured by the interferometer 11 during the measurement time set by the user. The measurement data is stored in the sampling data storage unit 411.

  More specifically, in detecting the amplitude of the movable mirror 115, the amplitude fluctuation calculating unit 416 performs the forward and return periods (moving mirror 115) in one scan of the movable mirror 115 detected by the movable mirror operation detecting unit 200. 1), the number of measurement data output from the AD conversion unit 25 (in other words, the number of zero cross timings output from the zero cross detection unit 37) is counted. Store in the amplitude storage unit 415. As described above, since the movable mirror 115 is driven to resonate, the time required for one reciprocation is the same (constant) even if the amplitude varies. Accordingly, the amplitude fluctuation calculation unit 416 counts the number of measurement data output from the AD conversion unit 25 (output from the zero cross detection unit 37 during the time from the start of the forward path to the end of the return path in one scan. The number of zero cross timings) is counted, and each count value is stored in the movable mirror amplitude storage unit 415. As described above, the sampling timing is zero-cross timing, and the sampling interval is determined by the wavelength of the laser light of the position measurement light source 31. Therefore, (count value) × (wavelength of the position measurement light source 31; 680 nm). / 4), the amplitude of the movable mirror 115 is obtained in actual length.

  Further, in the determination of the presence or absence of amplitude fluctuation of the movable mirror 115, more specifically, when the target amplitude of the movable mirror 115 is MirrorAmpTarget, the amplitude fluctuation calculation unit 416 determines that the detected amplitude of the movable mirror 115 is The presence / absence is determined based on whether the target amplitude MirrorAmpTarget is within a predetermined range ± ΔN. If the detected amplitude of the movable mirror 115 is within a predetermined range ± ΔN centered on the target amplitude MirrorAmpTarget, it is determined that the amplitude of the movable mirror 115 does not vary, while the detected movement When the amplitude of the mirror 115 is not within a predetermined range ± ΔN centering on the target amplitude MirrorAmpTarget, it is determined that the amplitude of the movable mirror 115 is varied. In other words, the amplitude fluctuation calculation unit 416 determines the presence / absence based on whether or not the detected amplitude of the moving mirror 115 is not less than the lower limit target amplitude MirrorAmpTarget-ΔN and not more than the upper limit target amplitude MirrorAmpTarget + ΔN. To do. When the detected amplitude of the movable mirror 115 is not less than the lower limit value and not more than the upper limit value, it is determined that there is no variation in the amplitude of the movable mirror 115, while the detected amplitude of the movable mirror 115 is Is less than the lower limit value or exceeds the upper limit value, it is determined that there is a variation in the amplitude of the movable mirror 115.

  The ΔN is appropriately set according to the spectrum tolerance determined by the specifications of the FT spectrometer D. For example, when the target amplitude MirrorAmpTarget = 5500 (count value), ΔN = 100 may be set.

  In the detection of the amplitude fluctuation period of the movable mirror 115, more specifically, the amplitude fluctuation calculation unit 416 is stored in the movable mirror amplitude storage unit 415 by the calculation method of the following first aspect or second aspect. From the amplitude of the movable mirror 115, the amplitude fluctuation period (swell period) is obtained.

  FIG. 13 is a diagram for explaining a calculation method of the first mode of the vibration period in the movable mirror. FIG. 14 is a diagram for explaining a calculation method of the second mode of the vibration period in the movable mirror. Each horizontal axis in FIGS. 13 and 14 represents the number of scans of the movable mirror, and each vertical axis represents the amplitude of the movable mirror.

  First, the calculation method of a 1st aspect is demonstrated. The target amplitude of the movable mirror 115 is set to MirrorAmpTarget, the amplitude of the movable mirror 115 in the i-th scanning (measurement) is set to MirrorAmp (i), and the number of storage areas of the movable mirror amplitude storage unit 415 is set to Imax. . The amplitude fluctuation calculation unit 416 searches the storage area of the moving mirror amplitude storage unit 415 in order from the amplitude MirrorAmp (i) from i = 0 indicating the latest data to i = Imax−1 indicating the most past data, The amplitude fluctuation calculation unit 416 first finds an amplitude MirrorAmp (i) that intersects the target amplitude MirrorAmpTarget. For example, as shown in FIG. 13, the amplitude fluctuation calculation unit 416 first finds an amplitude MirrorAmp (I1) exceeding the target amplitude MirrorAmpTarget by the search from a state of an amplitude smaller than the target amplitude MirrorAmpTarget. The amplitude fluctuation calculation unit 416 sets the first intersecting position as I01. The amplitude fluctuation calculation unit 416 further searches the storage area of the movable mirror amplitude storage unit 415 in order, and the amplitude fluctuation calculation unit 416 performs the same method (the same crossing mode) as the first crossing method (the first crossing mode) next. ) To find the amplitude MirrorAmp (I2) that intersects the target amplitude MirrorAmpTarget. In other words, in the above example, the amplitude MirrorAmp (I1) is data that intersects in a manner exceeding the target amplitude MirrorAmpTarget from a state where the amplitude is smaller than the target amplitude MirrorAmpTarget. From the state of the amplitude smaller than MirrorAmpTarget, the amplitude MirrorAmp (i) exceeding the target amplitude MirrorAmpTarget is found as the amplitude MirrorAmp (I2). The amplitude fluctuation calculation unit 416 sets the next intersecting position as I02.

  The position I01 is between the amplitude MirrorAmp (I1-1) and the amplitude MirrorAmp (I1), and is obtained by the following linear interpolation 8-1. Similarly, the position I02 is between the amplitude MirrorAmp (I2-1) and the amplitude MirrorAmp (I2), and is obtained by the following linear interpolation 8-2.

  Then, the amplitude fluctuation calculation unit 416 obtains the amplitude fluctuation period ΔI of the movable mirror 115 as abs (I02−I01) from the obtained positions I01 and I02. Note that abs (x) is an absolute value function for obtaining the absolute value of x.

  In the example shown in FIG. 13, the amplitude MirrorAmp (i) exceeding the target amplitude MirrorAmpTarget is found from the state of the amplitude smaller than the target amplitude MirrorAmpTarget. Conversely, from the state of the amplitude larger than the target amplitude MirrorAmpTarget, An amplitude MirrorAmp (i) below the target amplitude MirrorAmpTarget may be found and similarly the amplitude variation period is determined.

  Next, the calculation method of a 2nd aspect is demonstrated. The calculation method according to the first aspect is a calculation method that directly obtains the amplitude fluctuation period. However, the calculation method according to the second aspect obtains half of the amplitude fluctuation period and doubles it to obtain the amplitude fluctuation period. Is what you want. By calculating in this way, when the moving mirror amplitude storage unit 415 has the same capacity (the number of the storage areas of the moving mirror amplitude storage unit 415 is the same), the second method compared to the calculation method of the first mode. The aspect calculation method can detect a longer amplitude fluctuation period. That is, the calculation method of the second aspect can detect an amplitude fluctuation period having a period twice as long as the period that can be calculated by the calculation method of the first aspect.

  In the calculation method of the second mode, the amplitude fluctuation calculation unit 416 sequentially stores the amplitude MirrorAmp (i) from i = 0 indicating the latest data to i = Imax−1 indicating the past data in order. The storage area of the unit 415 is searched, and the amplitude fluctuation calculation unit 416 first finds an amplitude MirrorAmp (i) that intersects the target amplitude MirrorAmpTarget. For example, as shown in FIG. 14A, the amplitude fluctuation calculation unit 416 first finds an amplitude MirrorAmp (I1) exceeding the target amplitude MirrorAmpTarget by the search from a state of an amplitude smaller than the target amplitude MirrorAmpTarget. The amplitude fluctuation calculation unit 416 sets the first intersecting position as I01. The amplitude fluctuation calculation unit 416 further searches the storage area of the movable mirror amplitude storage unit 415 in order, and the amplitude fluctuation calculation unit 416 then reverses the method of the first crossing (first crossing mode) (reverse) The amplitude MirrorAmp (I2) that intersects with the target amplitude MirrorAmpTarget is found. That is, in the example shown in FIG. 14A, the amplitude MirrorAmp (I1) is data that intersects in a manner that exceeds the target amplitude MirrorAmpTarget from a state where the amplitude is smaller than the target amplitude MirrorAmpTarget. Then, an amplitude MirrorAmp (i) lower than the target amplitude MirrorAmpTarget is found as an amplitude MirrorAmp (I2) from the state of the amplitude larger than the target amplitude MirrorAmpTarget. The amplitude fluctuation calculation unit 416 sets the next intersecting position as I02.

  Further, for example, as illustrated in FIG. 14B, the amplitude variation calculation unit 416 first finds an amplitude MirrorAmp (I2) lower than the target amplitude MirrorAmpTarget by the search from a state of an amplitude larger than the target amplitude MirrorAmpTarget. The amplitude fluctuation calculation unit 416 sets the first intersecting position as I02. The amplitude fluctuation calculation unit 416 further searches the storage area of the movable mirror amplitude storage unit 415 in order, and the amplitude fluctuation calculation unit 416 then reverses the method of the first crossing (first crossing mode) (reverse) The amplitude MirrorAmp (I1) that intersects the target amplitude MirrorAmpTarget is found. That is, in the example shown in FIG. 14B, the amplitude MirrorAmp (I2) is data that intersects in a manner that is lower than the target amplitude MirrorAmpTarget from a state where the amplitude is larger than the target amplitude MirrorAmpTarget. Then, from the state of the amplitude smaller than the target amplitude MirrorAmpTarget, the amplitude MirrorAmp (i) exceeding the target amplitude MirrorAmpTarget is found as the amplitude MirrorAmp (I1). The amplitude fluctuation calculation unit 416 sets the next intersecting position as I01.

  The position I01 is between the amplitude MirrorAmp (I1-1) and the amplitude MirrorAmp (I1), and is obtained by the following linear interpolation 9-1. Similarly, the position I02 is between the amplitude MirrorAmp (I2-1) and the amplitude MirrorAmp (I2), and is obtained by the following linear interpolation 9-2.

  Then, the amplitude fluctuation calculation unit 416 obtains the amplitude fluctuation period ΔI of the movable mirror 115 as 2 × abs (I02−I01) from the obtained position I01 and position I02.

  By such an operation, the amplitude fluctuation calculation unit 416 can obtain the amplitude fluctuation period of the movable mirror 115.

  As described above, in the FT spectrometer D of this embodiment and the FD spectrometer method mounted thereon, as described above, the measurement time of the interferogram that is continuously measured is the amplitude of the movable mirror 115. Since the time is set based on the fluctuation cycle, the fluctuation cycle of the amplitude of the movable mirror 115 is taken into consideration. Therefore, such an FT spectrometer D and the method can obtain a more accurate measurement result even when an external vibration having a frequency close to the resonance frequency causing the swell is applied.

  Further, in the FT spectrometer D and the method of the present embodiment, the measurement time of the interferograms that are continuously measured is set to a time that is an integral multiple of the amplitude fluctuation period of the movable mirror 115. When integrating the ferroframs, the interferogram obtained when the amplitude of the movable mirror 115 is relatively small and the interferogram obtained when the amplitude of the movable mirror 115 is relatively large are substantially equal. Are included in the accumulated interferogram. Therefore, such an FT spectrometer D and the method can obtain an interferogram closer to the true interferogram, and can obtain a more accurate measurement result.

  Further, in the FT spectrometer D and the method according to the present embodiment, when it is determined that the amplitude of the moving mirror 115 does not vary, a spectrum is obtained by a normal calculation process, and the amplitude of the moving mirror 115 varies. Only when it is determined that there is an interferogram, the measurement time of an interferogram that is continuously measured is set to a time that is an integral multiple of the amplitude variation period. Therefore, such an FT spectrometer D and the method do not require the influence of external vibration, and the measurement time of the interferogram that is continuously measured is not required in the normal measurement. Since it is possible to omit the process of removing the influence of external vibration such as setting the time based on the fluctuation cycle of the amplitude of the movable mirror 115, the processing load and the measurement time can be reduced as a whole.

  In the present embodiment, the zero-cross detection unit 37, the movable mirror operation detection unit 200, the movable mirror amplitude storage unit 415, and the amplitude variation calculation unit 416 are a first detection unit that detects the amplitude variation period of the optical path difference forming optical element. The zero cross detector 37, the moving mirror operation detector 200, and the amplitude fluctuation calculator 416 correspond to an example of an (amplitude fluctuation period detector), and correspond to an example of an amplitude calculator that calculates the amplitude of the optical path difference forming optical element. The movable mirror amplitude storage unit 415 corresponds to an example of an amplitude storage unit that stores the amplitude obtained by the amplitude calculation unit, and the amplitude variation calculation unit 416 obtains an amplitude variation period from the amplitude stored in the amplitude storage unit. This corresponds to an example of an amplitude fluctuation period calculation unit.

  In the present embodiment, the light reception processing unit 20, the timing generation unit 30, and the sampling data storage unit 411 of the control calculation unit 41 are time based on the amplitude variation period detected by the first detection unit (amplitude variation period detection unit). The interferometer 11 corresponds to an example of an interferogram measuring unit that continuously measures a plurality of interferograms of predetermined light, and includes a center burst position calculating unit 412, an integrated interferogram calculating unit 413, and a spectrum calculating unit 414. Corresponds to an example of a spectrum processing unit that obtains a spectrum using Fourier transform based on a plurality of interferograms measured by the interferogram measuring unit.

  In the present embodiment, the FT spectrometer D measures wavelengths from 1200 nm to 2500 nm. However, the FT spectrometer D may be a device that measures light in the near infrared region, and may also be an FT spectrometer. The meter D may be a device that measures light in the infrared region, and the FT spectrometer D may be a device that measures light from the near infrared region to the infrared region.

  The present specification discloses various aspects of the technology as described above, and the main technologies are summarized below.

  A Fourier transform spectrometer according to one aspect includes a plurality of optical elements that receive predetermined light and form two optical paths between an incident position of the predetermined light and an interference position. Includes an interferometer including an optical path difference forming optical element that generates an optical path difference between the two optical paths by reciprocatingly oscillating in the optical axis direction, and a first detecting period of amplitude fluctuation of the optical path difference forming optical element. A plurality of interferograms of the predetermined light are continuously measured by the interferometer at a time based on the amplitude variation period detected by one detection unit (for example, an amplitude variation period detection unit) and the first detection unit. An interferogram measuring unit; and a spectrum processing unit that obtains a spectrum using Fourier transform based on a plurality of interferograms measured by the interferogram measuring unit.

  In such a Fourier transform spectrometer, the measurement time of a plurality of interferograms that are continuously measured is set to a time based on the amplitude variation period of the optical path difference forming optical element. The amplitude variation period is taken into account. Accordingly, such a Fourier transform spectrometer can obtain a more accurate measurement result even when external vibration having a frequency close to the resonance frequency that causes the swell is applied.

  According to another aspect, in the Fourier transform spectrometer described above, the interferogram measurement unit is a time that is an integral multiple of the fluctuation period of the amplitude detected by the first detection unit. A plurality of interferograms of predetermined light are continuously measured.

  In such a Fourier transform spectrometer, the measurement time of a plurality of interferograms that are continuously measured is set to a time that is an integral multiple of the amplitude fluctuation period detected by the first detector. When integrating the ferroframs, the interferogram obtained when the optical path difference forming optical element has a relatively small amplitude is also the interferogram obtained when the optical path difference forming optical element has a relatively large amplitude. Are also included in the integrated interferogram approximately evenly. Accordingly, such a Fourier transform spectrometer can obtain an interferogram closer to the true interferogram, and can obtain a more accurate measurement result.

  According to another aspect, in the above-described Fourier transform spectrometer, an amplitude variation determination unit that determines presence / absence of variation in the amplitude of the optical path difference forming optical element based on a detection result of the first detection unit. The interferogram measuring unit further includes the interference in a time that is an integral multiple of the amplitude variation period only when the amplitude variation determination unit determines that there is a variation in the amplitude of the optical path difference forming optical element. A plurality of interferograms of the predetermined light are continuously measured by a meter.

  In such a Fourier transform spectrometer, when it is determined that there is no fluctuation in the amplitude of the optical path difference forming optical element, a spectrum is obtained by a normal calculation process, and there is a fluctuation in the amplitude of the optical path difference forming optical element. Only when it is determined that the measurement time of the interferograms that are continuously measured is set to a time that is an integral multiple of the amplitude fluctuation period detected by the first detector. Therefore, such a Fourier transform spectrometer does not require the removal of the influence of external vibration, and it is not necessary for normal measurement. Since the process of removing the influence of external vibration such as setting the time based on the fluctuation cycle of the amplitude of the optical element can be omitted, the processing load can be reduced as a whole and the measurement time can be shortened.

  According to another aspect, in the above-described Fourier transform spectrometer, the first detection unit includes an amplitude calculation unit that calculates an amplitude of the optical path difference forming optical element, and the amplitude calculated by the amplitude calculation unit. An amplitude storage unit that stores the amplitude, and an amplitude variation period calculation unit that obtains the variation period of the amplitude from the amplitude stored in the amplitude storage unit.

  According to this configuration, there is provided a Fourier transform spectrometer in which the first detection unit includes an amplitude calculation unit, an amplitude storage unit, and an amplitude variation period calculation unit.

  In another aspect, in the above-described Fourier transform spectrometer, the predetermined light is near-infrared and / or infrared light incident as a measurement target.

  According to this configuration, a Fourier transform spectrometer capable of measuring any of light in the near infrared region, light in the infrared region, and light from the near infrared region to the infrared region is provided.

  Here, the near infrared region has a wavelength of 700 nm to 2500 nm, and the infrared region has a wavelength of 2500 nm to 4000 nm. A and / or B means at least one of A and B.

  In another aspect, in the above-described Fourier transform spectrometer, the optical path difference forming optical element is configured by a parallel leaf spring structure including first and second leaf springs arranged in parallel to face each other. And a reflection surface formed on one main outer surface of the first and second leaf springs, and the optical path difference is determined by the resonance drive of the drive unit so that the reflection surface is along the optical axis. Caused by moving in parallel.

  According to this, a Fourier transform type spectrometer provided with the drive part of a parallel leaf | plate spring structure is provided.

  The Fourier transform type spectroscopic method according to another aspect includes a plurality of optical elements that form two optical paths of predetermined light between an incident position of the predetermined light and an interference position. The element includes an incident step of entering an interferometer including an optical path difference forming optical element that generates an optical path difference between the two optical paths by moving in the optical axis direction, and an amplitude of the optical path difference forming optical element. A plurality of interferograms of the predetermined light are continuously measured by the interferometer at a time based on an amplitude fluctuation period detection step for detecting a fluctuation period and the amplitude fluctuation period detected in the amplitude fluctuation period detection step. Interferogram measurement process and spectrum processing for obtaining a spectrum using Fourier transform based on the interferogram measured in the interferogram measurement process And a degree.

  In such a Fourier transform type spectroscopic method, the measurement time of a plurality of interferograms that are continuously measured is set as the time based on the fluctuation period of the amplitude of the optical path difference forming optical element. The amplitude variation period is taken into account. Therefore, such a Fourier transform type spectroscopic method can obtain a more accurate measurement result even when an external vibration having a frequency close to the resonance frequency causing the swell is applied.

  This application is based on Japanese Patent Application No. 2012-122224 filed on May 29, 2012, the contents of which are included in the present application.

  In order to express the present invention, the present invention has been properly and fully described through the embodiments with reference to the drawings. However, those skilled in the art can easily change and / or improve the above-described embodiments. It should be recognized that this is possible. Therefore, unless the modifications or improvements implemented by those skilled in the art are at a level that departs from the scope of the claims recited in the claims, the modifications or improvements are not covered by the claims. To be construed as inclusive.

  According to the present invention, a Fourier transform spectrometer and a Fourier transform spectrometer can be provided.

Claims (7)

Predetermined light is incident, comprising a plurality of optical elements forming the two optical paths until the interference position from the incident position of the predetermined light, wherein the plurality of optical elements, by reciprocating in the direction of the optical axis An interferometer including an optical path difference forming optical element that generates an optical path difference between the two optical paths;
A first detector that detects a fluctuation period of an amplitude that is a period of undulation between a plurality of reciprocations of the optical path difference forming optical element;
An interferogram measuring unit that continuously measures a plurality of interferograms of the predetermined light by the interferometer at a time based on the fluctuation period of the amplitude detected by the first detecting unit;
A Fourier transform spectrometer, comprising: a spectrum processing unit that obtains a spectrum using Fourier transform based on a plurality of interferograms measured by the interferogram measuring unit. The interferogram measuring unit continuously measures a plurality of interferograms of the predetermined light by the interferometer for a time that is an integral multiple of the amplitude fluctuation period detected by the first detecting unit. The Fourier transform spectrometer according to claim 1. An amplitude variation determination unit that determines whether or not there is a variation in the amplitude of the optical path difference forming optical element based on the detection result of the first detection unit;
The interferogram measurement unit, the only case where the amplitude variation of the optical path difference forming optical element by amplitude change determination unit determines that there, in the integer multiple of the fluctuation period of the amplitude time, by the interferometer The Fourier transform spectrometer according to claim 2, wherein a plurality of interferograms of the predetermined light are continuously measured. The first detection unit includes:
An amplitude calculation unit for obtaining the amplitude of the optical path difference forming optical element;
An amplitude storage unit for storing the amplitude obtained by the amplitude calculation unit;
The Fourier transform type according to any one of claims 1 to 3, further comprising: an amplitude variation period calculation unit that obtains a variation period of the amplitude from the amplitude stored in the amplitude storage unit. Spectrometer. The Fourier transform spectrometer according to any one of claims 1 to 4, wherein the predetermined light is near-infrared and / or infrared light incident as a measurement target. The optical path difference forming optical element is:
A drive unit configured by a parallel leaf spring structure composed of first and second leaf springs arranged in parallel to face each other;
A reflective surface formed on one main outer surface of the first and second leaf springs,
5. The Fourier according to claim 1, wherein the optical path difference is caused by the reflection surface moving in parallel along an optical axis by resonance driving of the driving unit. Conversion spectrometer. A plurality of optical elements that form two optical paths between predetermined light incident positions and interference positions are provided for the predetermined light, and the two optical elements reciprocate in the optical axis direction to move the 2 An incident step of entering an interferometer including an optical path difference forming optical element that generates an optical path difference between the optical paths;
An amplitude fluctuation period detection step of detecting a fluctuation period of amplitude which is a period of waviness between a plurality of reciprocations of the optical path difference forming optical element;
An interferogram measuring step of continuously measuring a plurality of interferograms of the predetermined light by the interferometer at a time based on the amplitude fluctuation cycle detected in the amplitude fluctuation cycle detection step;
A spectrum processing step of obtaining a spectrum using Fourier transform based on the interferogram measured in the interferogram measurement step. A Fourier transform type spectroscopic method, comprising:

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