JP2015064228A - Fourier interference spectrometer and spectral intensity measurement method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Fourier interference spectrometer capable of precisely measuring the spectral intensity of an incoming beam even when the intensity of the incoming beam changes in a time-variant manner.SOLUTION: The Fourier interference spectrometer includes: an intensity monitoring section 3 which has an optical axis in a direction parallel to the optical axis of a beam interference section 1 for measuring the intensity of an incoming beam; and a gain adjustment section 4 that sets an inverse number of an intensity of the incoming beam measured by the intensity monitoring section 3 as a gain coefficient and multiplies the intensity of a synthetic light flux detected by a beam intensity detection section 2 by the gain coefficient. With this, the spectral intensity of an incoming beam can be precisely measured even when the intensity of the incoming beam changes in a time-variant manner.

Description

  The present invention relates to a Fourier interferometer that measures a spectral spectrum of an incident light beam by utilizing the interference action of light, and more particularly, a Fourier interferometer that measures a light beam whose intensity changes with time and a spectral intensity measurement. It is about the method.

In Patent Document 1 below, an incident light beam is branched into two optical paths whose length difference changes with time, and a spectral transmittance is calculated from the intensity of an interference signal obtained by combining the light beams that have passed through the two optical paths. A Fourier interferometer spectrometer that measures the above is disclosed.
The intensity of the interference signal repeatedly increases and decreases with respect to the optical path length difference between the two optical paths at a period equal to the wavelength of the light beam. it can.

Japanese Unexamined Patent Publication No. 7-286902 (paragraph number [0016], FIG. 1)

Since the conventional Fourier interferometer is configured as described above, if the intensity of the incident light beam is always constant, the spectral transmittance and the like can be measured from the intensity of the interference signal. However, if the intensity of the incident light beam changes with time, it appears as an intensity change of the interference signal, and this intensity change of the interference signal is erroneously analyzed as a change in the optical path length difference between the two optical paths. As a result, there is a problem that a change in the intensity of the incident light beam appears as an error and the measurement accuracy is lowered.
In particular, measurement of spectral characteristics of measurement objects that are not firmly fixed, measurement using a light source whose intensity is not controlled outdoors, etc., measurement in a state where the visual axis changes slightly by observing from a moving object, etc. When measuring with a change in the distance to the measurement object or a change in transmittance in the area in between, the spectral characteristics do not change with time, but the intensity easily changes with time, so the measurement accuracy is There was a problem that would decrease.

  The present invention has been made to solve the above-described problems. A Fourier interferometer that can measure the spectral intensity of an incident light beam with high accuracy even when the intensity of the incident light beam changes with time. And to obtain a spectral intensity measurement method.

  A Fourier interferometer according to the present invention includes a light beam interference unit that splits an incident light beam into two optical paths whose length difference changes with time, and combines the light beams passing through the two optical paths, and the light beam interference unit. Luminous intensity detecting means for detecting the intensity of the synthesized luminous flux, luminous intensity measuring means for measuring the intensity of the incident luminous flux having an optical axis in a direction parallel to the optical axis of the luminous flux interference means, and luminous intensity measuring means A gain adjusting means for setting the reciprocal of the intensity of the incident light beam measured by the gain coefficient, multiplying the gain coefficient by the intensity detected by the light beam intensity detecting means, and the intensity multiplied by the gain coefficient by the gain adjusting means. The average intensity subtracting means for subtracting the average value of the intensity is provided, and the spectral intensity converting means converts the subtraction result of the average value subtracting means to the spectral intensity of the incident light beam by Fourier transform. A.

  According to the present invention, the light beam intensity measuring means having the optical axis in the direction parallel to the optical axis of the light beam interference means and measuring the intensity of the incident light flux, and the reciprocal of the intensity of the incident light flux measured by the light flux intensity measuring means. Is set as a gain coefficient, and gain adjustment means for multiplying the gain coefficient detected by the light flux intensity detection means is provided, so that even when the intensity of the incident light flux changes with time, high accuracy There is an effect that the spectral intensity of the incident light beam can be measured.

It is a block diagram which shows the Fourier interferometer-type spectrometer by Embodiment 1 of this invention. It is a flowchart which shows the processing content (spectral-intensity measuring method) of the Fourier interferometer-type spectrometer by Embodiment 1 of this invention. It is a block diagram which shows the Fourier interferometer-type spectrometer by Embodiment 2 of this invention. It is a block diagram which shows the Fourier interferometer-type spectrometer by Embodiment 3 of this invention.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a Fourier interferometer according to Embodiment 1 of the present invention.
In FIG. 1, a light beam interference unit 1 includes two optical paths whose length difference changes with time. The incident light beam is branched into two optical paths, and the optical path length is temporally continuous with the two light beams. After giving the difference, a process of combining the two light beams is performed. The light beam interference section 1 constitutes a light beam interference means.
The light beam intensity detection unit 2 is composed of, for example, a photodetector, detects the intensity of the light beam synthesized by the light beam interference unit 1, and outputs a combined light beam intensity signal I _obs (x) indicating the intensity to the gain adjustment unit 4. Execute the process to output to.
However, when the intensity of the incident light beam changes with time, the combined light beam intensity signal output from the light beam intensity detector 2 changes from I _obs (x) to I ′ _obs (x).
The light intensity detector 2 constitutes light intensity detecting means.

The intensity monitor unit 3 is composed of, for example, a luminance sensor having an optical axis in a direction parallel to the optical axis of the light beam interference unit 1, measures the intensity of the incident light beam, and measures the incident light beam intensity signal I indicating the intensity. A process of outputting _rad (x) to the gain adjusting unit 4 is performed. The intensity monitor unit 3 constitutes a light beam intensity measuring unit.
The gain adjusting unit 4 is composed of, for example, a semiconductor integrated circuit mounted with a CPU or a one-chip microcomputer, and gains the reciprocal of the incident light beam measurement intensity signal I _rad (x) output from the intensity monitor unit 3. Is set to a coefficient, and the gain coefficient is multiplied by the combined light intensity signal I ′ _obs (x) output from the light intensity detector 2 to correct the intensity fluctuation of the incident light, and the combined light intensity after the gain coefficient is multiplied. A process of outputting the signal I _sup (x) to the high-pass filter unit 5 is performed. The gain adjusting unit 4 constitutes a gain adjusting unit.

The high-pass filter unit 5 is composed of, for example, a semiconductor integrated circuit on which a CPU is mounted, a one-chip microcomputer or the like, and a combined luminous flux intensity signal I _sup (x) output from the gain adjustment unit 4 and multiplied by a gain coefficient. _Is calculated, and the average value is subtracted from the combined luminous intensity signal I _sup (x). The high-pass filter unit 5 constitutes an average value subtraction unit.
The Fourier transform unit 6 is composed of, for example, a semiconductor integrated circuit mounted with a CPU or a one-chip microcomputer, and converts the subtraction result of the high-pass filter unit 5 into the spectral intensity B (σ) of the incident light beam by Fourier transform. Perform the process. The Fourier transform unit 6 constitutes spectral intensity conversion means.

In the example of FIG. 1, each of the light beam interference unit 1, the light beam intensity detection unit 2, the intensity monitor unit 3, the gain adjustment unit 4, the high-pass filter unit 5, and the Fourier transform unit 6, which are components of the Fourier interference spectrometer. However, all or part of the Fourier interferometer may be configured with a computer.
For example, when a part of the Fourier interferometer (for example, the gain adjustment unit 4, the high pass filter unit 5, and the Fourier transform unit 6) is configured by a computer, the gain adjustment unit 4, the high pass filter unit 5, and the Fourier transform unit. 6 may be stored in a memory of a computer, and the CPU of the computer may execute the program stored in the memory.
FIG. 2 is a flowchart showing the processing contents (spectral intensity measurement method) of the Fourier interferometer spectrometer according to the first embodiment of the present invention.

Next, the operation will be described.
When a light beam is incident, the light beam interference unit 1 branches the incident light beam into two optical paths, thereby giving a temporally continuous optical path length difference between the two light beams.
When the two light beams pass through the respective optical paths, the light beam interference unit 1 combines the light beams and outputs the combined light beam to the light beam intensity detection unit 2 (step ST1).

式（１）において、σは光線波数であり、Ｂ（σ）は光線波数σにおける光束の分光強度である。 Upon receiving the combined light beam from the light beam interference unit 1, the light beam intensity detection unit 2 detects the intensity of the light beam and outputs a combined light beam intensity signal I _obs (x) indicating the intensity to the gain adjustment unit 4 ( Step ST2).
The combined luminous intensity signal I _obs (x) indicating the intensity of the luminous flux is expressed by the following equation (1) by the interference action of light.

In equation (1), σ is the ray wave number, and B (σ) is the spectral intensity of the light beam at the ray wave number σ.

Here, the average value I _obs (x) bar of the intensity of the incident luminous flux (in the document of the specification, “−” cannot be written on the character because of the electronic application, so I _obs (x) (Expressed as a bar) can be expressed as the following equation (2).

Then, the average value I _obs (x) bar is subtracted from the combined luminous intensity signal I _obs (x) (see equation (3)), and the subtraction result is doubled to obtain a Fourier cosine transform (or to a negative space). When extended and normal Fourier transform), the spectral intensity B (σ) of the incident light beam is obtained.

式（５）において、Ｓ（ｘ）は、下記の式（６）に示すように、光路長差がｘである場合の入射光束の強度比（光路長差が０である場合の入射光束強度と、光路長差がｘである場合の入射光束強度との比）であり、光路長差が０である場合の入射光束強度で正規化している。即ち、Ｓ（０）＝１としている。

However, even if the spectral characteristic of the measurement object does not change, if only the intensity changes with time, the combined light intensity signal I _obs (x) output from the light intensity detector 2 is expressed by the following equation ( It changes to I ′ _obs (x) as shown in 5).

In equation (5), S (x) is the intensity ratio of the incident light beam when the optical path length difference is x (incident light beam intensity when the optical path length difference is 0), as shown in the following equation (6). And the ratio of the incident light beam intensity when the optical path length difference is x), and is normalized by the incident light beam intensity when the optical path length difference is zero. That is, S (0) = 1.

As described above, when the combined light intensity signal output from the light intensity detector 2 changes from I _obs (x) to I ′ _obs (x), the accurate spectral intensity B (σ of the incident light is calculated from the equation (4). ) Cannot be obtained.
Therefore, in the first embodiment, in order to obtain an accurate spectral intensity B (σ) of the incident light beam even if the intensity of the incident light beam changes with time, the intensity monitor unit 3 and the gain adjustment unit 4 is provided.

式（６）において、Ｉ_rad（０）は光路長差が０である場合の入射光束の強度であり、式（６）は光路長差が０である場合を基準にしている。 When the same light beam as the light beam interference unit 1 is incident, the intensity monitor unit 3 measures the intensity of the incident light beam, and outputs an incident light beam measurement intensity signal I _rad (x) indicating the intensity to the gain adjustment unit 4 ( Step ST3).
Upon receiving the incident light beam measurement intensity signal I _rad (x) from the intensity monitor unit 3, the gain adjustment unit 4 sets the inverse of the incident light beam measurement intensity signal I _rad (x) to the gain coefficient 1 / I _rad (x). To do.
When the gain adjustment unit 4 sets the gain coefficient 1 / I _rad (x), the gain coefficient 1 / I _rad (x) is output from the beam intensity detection unit 2 as shown in the following equation (7). Multiplying the combined luminous intensity signal I ′ _obs (x) corrects the intensity fluctuation of the incident luminous flux, and outputs the combined luminous intensity signal I _sup (x) after multiplication of the gain coefficient to the high-pass filter unit 5 (step ST4).

In equation (6), I _rad (0) is the intensity of the incident light beam when the optical path length difference is zero, and equation (6) is based on the case where the optical path length difference is zero.

これにより、ゲイン調整部４から出力される合成光束強度信号Ｉ_sup（ｘ）は、時間的に変化していない場合の入射光束の強度として取り扱うことができる。 Here, as described above, S (x) is the intensity ratio of the incident light beam and is expressed by Expression (6). Therefore, when Expression (5) and Expression (6) are substituted into Expression (7), The combined light intensity signal I _sup (x) after multiplication by the gain coefficient coincides with the combined light intensity signal I _obs (x) as shown in the following equation (8).

Thereby, the synthetic light beam intensity signal I _sup (x) output from the gain adjusting unit 4 can be handled as the intensity of the incident light beam when it does not change with time.

When the high-pass filter unit 5 receives the combined light beam intensity signal I _sup (x) after gain coefficient multiplication from the gain adjustment unit 4, as shown in the following equation (9), the combined light beam intensity signal I _sup (x) The average value of I _sup (x) bar (in the document of the specification, “−” cannot be written on the letter because of the electronic application, so it is expressed as I _sup (x) bar. Calculated).

When the high pass filter unit 5 calculates the average value I _sup (x) bar of the combined luminous flux intensity signal I _sup (x), as shown in the following equation (10), from the combined luminous flux intensity signal I _sup (x), The average value I _sup (x) bar is subtracted (step ST5).

When the high-pass filter unit 5 subtracts the average value I _sup (x) bar from the combined light intensity signal I _sup (x), the Fourier transform unit 6 doubles the subtraction result as shown in the following equation (11). Then, the spectral intensity B (σ) of the incident light beam is calculated by performing Fourier cosine transform (or normal Fourier transform expanded to a negative space) (step ST6).

  As is apparent from the above, according to the first embodiment, the intensity monitor unit 3 having an optical axis in a direction parallel to the optical axis of the light beam interference unit 1 and measuring the intensity of the incident light beam, and the intensity monitor unit And a gain adjustment unit 4 that sets the inverse of the intensity of the incident light beam measured by 3 as a gain coefficient, and multiplies the gain coefficient by the intensity of the combined light beam detected by the light beam intensity detection unit 2. Even when the intensity of the incident light beam changes with time, the spectral intensity of the incident light beam can be measured with high accuracy.

Embodiment 2. FIG.
FIG. 3 is a block diagram showing a Fourier interferometer according to Embodiment 2 of the present invention. In the figure, the same reference numerals as those in FIG.
The semiconductor integrated circuit signal correction unit 7 implements the CPU for example, or, are composed of such one-chip microcomputer, the average value I _sup highpass filter unit 5 from the combined light beam intensity signal I _sup (x) (x) When the process of subtracting the bar is performed N times, the average value of the nth subtraction result from the beginning (n = 1, 2,..., N) and the nth subtraction result from the end is calculated. The nth subtraction result from the beginning and the nth subtraction result from the end are replaced with the average value. The signal correction unit 7 constitutes correction means.

In the first embodiment, the intensity monitor unit 3 and the gain adjustment unit 4 are provided in order to obtain an accurate spectral intensity B (σ) of the incident light beam even if the intensity of the incident light beam changes with time. The gain adjustment unit 4 corrects the intensity fluctuation of the incident light beam. However, when a correction residual occurs, the correction residual leads to a measurement error.
That is, in a Fourier interferometer that changes the optical path length difference in both positive and negative directions, when observed when the absolute values are at the same position, the intensity of the incident light beam synthesized by the light beam interference unit 1 becomes equal, and time is required for output. However, if there is a correction residual, the temporal symmetry is lost.
Therefore, in the second embodiment, a signal correction unit 7 is mounted between the high-pass filter unit 5 and the Fourier transform unit 6 to reduce the correction residual by the gain adjustment unit 4.

When the high-pass filter unit 5 performs the process of subtracting the average value I _sup (x) bar from the combined light flux intensity signal I _sup (x) N times, the signal correction unit 7 calculates the nth subtraction result from the beginning. Then, an average value with the nth subtraction result from the end is calculated, and a process of replacing the nth subtraction result from the beginning and the nth subtraction result from the end with the average value is performed.
Thereby, the n-th subtraction result from the beginning and the n-th subtraction result from the end are replaced with the same average value.
As a result, the output signal becomes completely symmetric, and the asymmetric component generated by the correction residual is removed.

Embodiment 3 FIG.
4 is a block diagram showing a Fourier interferometer according to Embodiment 3 of the present invention. In the figure, the same reference numerals as those in FIG.
The signal integrating unit 8 is constituted by, for example, a semiconductor integrated circuit on which a CPU is mounted, a one-chip microcomputer, or the like, and integrates the combined luminous intensity signal I ′ _obs (x) output from the luminous intensity detector 2. Then, a process of outputting the integrated combined luminous flux intensity signal I ′ _obs (x) to the gain adjusting unit 4 is performed. The signal integrating unit 8 constitutes intensity integrating means.
The Fourier interferometer of FIG. 4 shows an example in which the signal integrator 8 is applied to the Fourier interferometer of FIG. 1, but the signal integrator 8 is applied to the Fourier interferometer of FIG. You may make it do.

The light intensity detector 2 determines a dynamic range that can be handled as a significant signal range depending on the sensitivity as a detector, the presence or absence of noise, the band, and the like.
The signal integrating unit 8 integrates the combined luminous intensity signal I ′ _obs (x) for an arbitrary time output from the luminous intensity detector 2 and adjusts the gain of the integrated synthetic luminous intensity signal I ′ _obs (x). Although output to the unit 4, the signal amount included in the output signal of the signal integrating unit 8 increases in accordance with the integrated amount, while an increase in noise is suppressed from an increase in the signal amount. For example, in the case of white noise, the increase is 1 / 1.41 times the increase in signal amount.
For this reason, the signal integration unit 8 integrates the combined luminous intensity signal I ′ _obs (x), so that the signal-to-noise ratio of the signal increases.
An increase in the S / N ratio makes it possible to detect even a minute signal, so that the range that can be handled as a significant signal is expanded and the dynamic range is expanded.

Embodiment 4 FIG.
In the first to third embodiments, the intensity monitor unit 3 has the optical axis in the direction parallel to the optical axis of the light beam interference unit 1, but the optical axis of the intensity monitor unit 3 is the light beam interference unit. It may be the same as one optical axis.
When the optical axis of the intensity monitor unit 3 is the same as the optical axis of the light beam interference unit 1, it is necessary to distribute the incident light beam to the light beam interference unit 1 and the intensity monitor unit 3.
As a method for distributing the incident light beam to the light beam interference unit 1 and the intensity monitor unit 3, for example, the incident light beam in the wavelength band to be measured is transmitted on the optical axis, while the incident light beam in the wavelength band to be measured is reflected. A method is conceivable in which a dichroic mirror is arranged, an incident light beam that passes through the dichroic mirror is given to the light beam interference unit 1, and an incident light beam that reflects the dichroic mirror is given to the intensity monitor unit 3.
With such a method, the intensity change can be acquired without decreasing the signal in the wavelength band to be measured.
At this time, for example, by providing the intensity monitor unit 3 with a light beam having a wavelength with a relatively high SN ratio, the intensity change can be acquired without decreasing the signal with a wavelength with a low SN ratio.

  In the fourth embodiment, since the optical axes of the intensity monitor unit 3 and the light beam interference unit 1 are the same optical axis, the intensity monitor unit 3 measures the intensity of the light beam incident on the light beam interference unit 1 itself. As a result, there is an effect that the accuracy of signal gain correction in the gain adjusting unit 4 can be improved.

  In the present invention, within the scope of the invention, any combination of the embodiments, or any modification of any component in each embodiment, or omission of any component in each embodiment is possible. .

  DESCRIPTION OF SYMBOLS 1 Light beam interference part (light beam interference means), 2 Light beam intensity detection part (light beam intensity detection means), 3 Intensity monitor part (light beam intensity measurement means), 4 Gain adjustment part (gain adjustment means), 5 High pass filter part (Average value) Subtraction unit), 6 Fourier transform unit (spectral intensity conversion unit), 7 signal correction unit (correction unit), 8 signal integration unit (intensity integration unit).

Claims (5)

A light beam interference means for branching an incident light beam into two optical paths in which the difference in length changes with time, and combining the light beams passing through the two optical paths;
Luminous flux intensity detection means for detecting the intensity of the luminous flux synthesized by the luminous flux interference means;
A luminous intensity measuring means having an optical axis in a direction parallel to the optical axis of the luminous flux interference means and measuring the intensity of the incident luminous flux;
A gain adjusting unit that sets a reciprocal of the intensity of the incident light beam measured by the light beam intensity measuring unit as a gain coefficient, and multiplies the gain coefficient detected by the light beam intensity detecting unit;
Average value subtracting means for subtracting the average value of the multiplied intensity from the intensity multiplied by the gain coefficient by the gain adjusting means;
Spectral intensity converting means for converting the subtraction result of the average value subtracting means into the spectral intensity of the incident light beam by Fourier transform. Of the subtraction results of the average value subtracting means, the average value of the n (n = 1, 2,..., N) -th subtraction result from the beginning and the n-th subtraction result from the end is calculated. correction means for replacing the n-th subtraction result and the n-th subtraction result from the end with the average value of the calculated subtraction result;
2. The Fourier interference type spectroscope according to claim 1, wherein the spectral intensity converting means converts the subtraction result replaced by the average value by the correcting means into the spectral intensity of the incident light beam by Fourier transform.   3. The Fourier interference spectroscopy according to claim 1, further comprising intensity integrating means for integrating the intensity detected by the light beam intensity detecting means and outputting the integrated intensity to the gain adjusting means. vessel.   The optical axis of the light flux intensity measuring means is the same as the optical axis of the light flux interference means, and the incident light flux is distributed to the light flux interference means and the light flux intensity measuring means by a mirror. The Fourier interferometer of any one of claims 1 to 3. A light beam interference processing step in which the light beam interference means branches the incident light beam into two optical paths in which the difference in length changes with time, and combines the light beams passing through the two optical paths;
A light beam intensity detecting means for detecting the intensity of the light beam synthesized in the light beam interference processing step;
A light beam intensity measuring unit having a light beam intensity measuring unit having an optical axis in a direction parallel to the optical axis of the light beam interfering unit, and measuring the intensity of the incident light beam; and
A gain adjustment processing step in which the gain adjustment means sets a reciprocal of the intensity of the incident light beam measured in the light beam intensity measurement processing step as a gain coefficient, and multiplies the gain coefficient detected by the light beam intensity detection processing step. When,
An average value subtracting means for subtracting an average value of the multiplied intensity from the intensity multiplied by the gain coefficient in the gain adjustment process step;
A spectral intensity measurement method comprising: a spectral intensity conversion step in which spectral intensity conversion means converts the subtraction result in the average value subtraction processing step into a spectral intensity of the incident light beam by Fourier transform.

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