JP3329898B2 - Multi-channel Fourier transform spectrometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-channel Fourier transform spectrometer using a light receiving element.

[0002]

2. Description of the Related Art Spectral analysis for measuring the components of a substance by analyzing the spectrum of the reflected light or transmitted light from the substance has been known for a long time, and prisms and diffraction gratings were used in the early days. It has been said that these require a long time for analysis and that the S / N ratio is extremely poor because only a part of the incident light is used. Since the calculation by a computer became possible thereafter, the Fourier transform of interference fringes (interferogram) became easy, and a Fourier transform spectrometer for obtaining a spectrum using the Fourier transform has been widely used. The advantages of Fourier exchange spectroscopy are that it is bright, has a high S / N ratio, and has high wavenumber accuracy.

[0003]

[0005] By the way, in the final stage, CC
In a multi-channel Fourier transform spectrometer that is provided with a solid-state imaging device such as a D image sensor and Fourier-transforms an output signal from the solid-state imaging device, in order to increase the wavenumber resolution, it is necessary to increase the number of solid-state imaging devices. However, it is not easy to obtain such an element because it causes a cost problem and an increase in complexity and size of the device.

Accordingly, the present inventors have paid attention to the fact that an interferometer can obtain an interferogram that is spread symmetrically with a multi-channel Fourier transform spectrometer by using an interferometer. It is an object to improve the wave number resolution of the device.

[0005]

In order to achieve the above object, the present invention provides a polarizer that polarizes incident light along an optical path of the incident light, and amplitude-divides the polarized light into two light waves. A birefringent polarizing element and a plurality of birefringent crystal plates having different refractive indices for the two light waves, having different thicknesses with respect to the traveling direction of light, and being laminated substantially parallel to the traveling direction of light. A birefringent crystal plate structure, an analyzer for aligning the polarization directions of the two light waves, a lens for performing Fourier transform, and a two-dimensional light receiving element for receiving the light converged by the lens. Thus, the optical system of the multi-channel Fourier transform spectrometer was constructed.

[0006]

According to the present invention, the light which has been polarized by the above-described structure and is amplitude-divided into two light waves by the birefringent polarizing element according to the polarization state is further given a phase difference according to the polarization state by the birefringent crystal plate. Therefore, after the interference, the zero phase difference position comes close to the end of the light receiving element, so that a one-sided interferogram can be obtained, and the density of the interferogram on the light receiving element can be increased, whereby the wave number resolution is reduced. It is improved to about twice the wave number resolution. Further, if the birefringent crystal plate is formed by stacking a plurality of birefringent crystal plates having different thicknesses in the light traveling direction and a two-dimensional light receiving element is used, the wave number resolution is further improved.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 shows an optical system of a multi-channel Fourier transform spectrometer according to the present invention. The illustrated example uses a birefringent polarizing element using a Savart plate.
Is a light source as reflected light or transmitted light from a substance to be measured, 2 is an image forming lens, 3 is a polarizer, 4 is a polarized light wave, and two light waves having the same amplitude according to the polarization state (usually two light waves). A birefringent polarizing element (hereinafter, referred to as a “Savart plate”) that divides a light beam into an extraordinary light beam), a birefringent crystal plate having a different refractive index for two light waves having two polarization directions orthogonal to each other, and An analyzer for aligning the polarization directions of the light beam and the extraordinary light beam, a Fourier transform lens 7 for interfering the two light beams, and a solid-state image pickup device 8 such as a CCD image sensor (e.g., P
tSi element). In addition, Pb is used as
When an image sensor using S, Ge, InGaAs or the like is used, the vacuum bell jar 9 is unnecessary. The light (one light wave) from the light source 1 is combined by the imaging lens 2 and then polarized by the polarizer 3.
It is amplitude-divided into light waves. At this time, the polarization state of the light is adjusted by the polarizer so that the amplitudes of the two light waves become equal. Since the two light waves that have exited the Savart plate 4 have polarization directions orthogonal to each other, the two light waves are given a phase difference by the birefringent crystal plate 5 and passed through the analyzer 6. Then, the two light waves having the phase difference interfere with each other by the Fourier transform lens 7 and form an image on the solid-state imaging device 8. As a result, the interferogram obtained from the solid-state imaging device 8 has the zero phase difference position at the center in FIG.
Unlike the conventional interferogram shown in (a),
FIG. 2 in which the zero phase difference position is biased near the end of the solid-state imaging device 8
The shape becomes asymmetric as shown in FIG. Note that FIG.
The amplitudes of the waveforms in FIG. 2A and FIG. 2B indicate the density of interference fringes. The interferogram obtained by the present invention uses a signal processing technique called phase correction,
A spectrum similar to that of a conventional interferogram can be obtained. Sometimes you can.

The Savart plate 4 used in the present invention includes titanium dioxide (TiO ₂ ) and calcium carbonate (calcite).
Etc. can be used. In this embodiment, titanium dioxide is used.

On the other hand, in the embodiment, the birefringent crystal plate 5 has a refractive index for ordinary light of 2.62 and a refractive index for extraordinary light of 2.90, which is a difference between the refractive index for ordinary light and the refractive index for extraordinary light. titanium dioxide (TiO ₂₎ (called a rutile) is used large. The direction of the crystal axis is, for example, 45 ° as shown by arrow A in FIG.

The thickness of the crystal which determines the amount of phase difference between the two light rays was determined by simulation. As a result, it was found that a 0.7 mm crystal plate was required when applied to the experimental system of the inventors. Incidentally, in the experimental system of the inventors, the number of pixels of the solid-state imaging device 8 is 4096, the pixel interval is 10 μm, the sharing width of two light waves by the Savart plate 4 is about 1 mm, and the focal length of the Fourier transform lens 7 is 60 mm. .

By using the birefringent crystal plate 5 in this manner, a one-sided interferogram,
To be more precise, as shown in FIG. 2B, an interferogram including all of one side of the zero phase difference position and a very small part of the opposite side can be formed, so that the solid-state imaging device having the same wave number resolution can be formed. 8 is about 1.8 times that of the conventional two-sided interferogram shown in FIG. Note that if the optical aberration and the like are made small while paying attention to the design of the optical system, the resolution will slightly decrease, but practical resolution can be secured without taking the interferogram of the other side and performing phase correction. In some cases.

Meanwhile, the wave number resolution of a multi-channel type spectroscopic device is limited by the number of solid-state imaging devices.
With the current technology, the number of one-dimensional solid-state imaging devices that can be used is at most several thousand, but a two-dimensional solid-state imaging device has, for example, 250,000 elements (512 × 512). If can be used for the detector of the Fourier transform spectrometer, the wave number resolution is greatly improved.

Therefore, as a second embodiment of the present invention, an apparatus using a two-dimensional solid-state image sensor as a detector of a Fourier transform spectrometer will be described.

FIG. 3 shows a birefringent crystal used in a second embodiment of the multichannel spectroscopic device according to the present invention. This birefringent crystal is formed by laminating the birefringent crystal plates 5 used in the first embodiment described above, and the illustrated one has a four-layer structure. As described above, interference is caused by making the dimensions of each crystal plate of the laminated structure different.
The phase difference given to the light wave can be made different for each crystal plate. Therefore, two-dimensional CCDs such as two-
By using a two-dimensional solid-state imaging device, a one-sided interferogram obtained from a birefringent crystal can be received by dividing it into four layers for each layer as shown in FIG. In FIG. 4, the amplitude of the waveform shown on each row of the solid-state imaging device indicates the density of the interference fringes. In order to smoothly connect the interferograms on the two-dimensional solid-state imaging device, the dimensions of each birefringent crystal plate constituting the birefringent crystal having a four-layer structure are applied to the above-described experimental system of the present inventors. The values obtained by simulation were as shown in FIG. Actually, the interferograms obtained by the respective crystal plates are designed to slightly overlap as shown in FIG.
Combine at the time of operation. If a birefringent crystal having an n-layer structure is used, the wave number resolution is ideally up to 2n times that of the conventional case in which a symmetrical interferogram is taken. Thus, when the interferogram is captured with an intensity equal to or greater than the quantization error of the AD converter, an improvement in wavenumber resolution of about six times can be expected.

In the above embodiment, titanium dioxide is used as the material of the birefringent crystal plate. However, an element having a strong birefringence such as calcite may be used. In addition to a natural material, an artificial material can be used as the serval plate or the birefringent crystal plate. Further, in the above-described embodiment, a solid-state imaging device such as a CCD image sensor is used as the light-receiving device, but a photosensitive film or an imaging tube may be used instead.

[0017]

As described above, according to the present invention,
By using a birefringent crystal structure with a structure in which a plurality of birefringent crystal plates with different thicknesses in the light traveling direction in the optical path are used together with a two-dimensional light receiving element, a one-sided interferogram is formed on the light receiving element with a simple structure. Can be formed, and a higher wavenumber resolution can be obtained than in the case of forming a conventional bilaterally symmetric interferogram.

[Brief description of the drawings]

FIG. 1 shows an optical system of a multi-channel Fourier transform spectrometer according to the present invention.

FIG. 2A is an interferogram obtained by a conventional multi-channel Fourier transform spectrometer,
(B) is an interferogram obtained by the multi-channel Fourier transform spectrometer according to the present invention.

FIG. 3 is a perspective view of a birefringent crystal having a four-layer structure used in a multi-channel Fourier transform spectrometer according to a second embodiment of the present invention.

FIG. 4 shows a state of receiving light of a one-sided interferogram by a two-dimensional solid-state imaging device of a multi-channel Fourier transform spectrometer according to a second embodiment of the present invention.

FIG. 5 shows a correspondence between a two-dimensional solid-state imaging device and a one-sided interferogram of a multi-channel Fourier transform spectrometer according to a second embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 light source 2 condenser lens 3 polarizer 4 Savart plate 5 birefringent crystal plate 6 analyzer 7 Fourier transform lens 8 solid-state imaging device 9 vacuum bell jar

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hiroomi Uehara Examiner of Nisshin Flour Milling Co., Ltd. 19-12 Nihonbashi Koami-cho, Chuo-ku, Tokyo Ayako Yokoi (56) References JP-A-2-268234 (JP, A) JP-A-51-8958 (JP, A) JP-A-55-99034 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01J 3/00-3/52 G01J 9 / 00-9/04 G01N 21/00-21/61

Claims (6)

(57) [Claims] 1. A polarizer for polarizing incident light, a birefringent polarizing element for amplitude-dividing the polarized light into two light waves, and a different refractive index for the two light waves along an optical path of the incident light. A plurality of birefringent crystal plates having different thicknesses with respect to the traveling direction of light , and having a flat laminated surface with respect to the traveling direction of light.
A birefringent crystal plate structure stacked in rows, an analyzer for aligning the polarization directions of the two light waves, a lens for performing Fourier transform, and a two-dimensional light receiving element for receiving light converged by the lens. A multi-channel type Fourier transform spectrometer characterized by sequentially disposing elements. 2. The multi-channel Fourier transform spectrometer according to claim 1, wherein said light receiving element is a solid-state image sensor. 3. The multi-channel Fourier transform spectrometer according to claim 2, wherein the fixed image pickup device is a CCD image sensor. 4. The multi-channel Fourier transform spectrometer according to claim 2, wherein the fixed imaging element outputs one side interferogram and a part of the other side interferogram. 5. The multi-channel Fourier transform spectrometer according to claim 1, wherein the birefringent crystal plate is rutin. 6. The multi-channel Fourier transform spectrometer according to claim 1, wherein the birefringent element for dividing the amplitude is a Savart plate.