JP3309537B2 - Fourier transform spectrophotometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Fourier transform type spectrophotometer using interference light obtained by a two-beam interferometer.

[0002]

2. Description of the Related Art A two-beam interferometer is used in a Fourier transform infrared spectrophotometer. The two-beam interferometer generates interference light by combining two beams whose optical path difference changes with time. The Fourier transform infrared spectrophotometer obtains interference infrared light by inputting infrared light to a two-beam interferometer,
The sample is irradiated with the interference infrared light to detect transmitted light or reflected light from the sample. Then, in order to obtain the spectrum of the transmitted light or the reflected light, a discrete Fourier transform is performed by numerical calculation using the detection signal. Therefore, at the time of this conversion, it is required to sample the detection signal at regular intervals of the optical path difference in the interferometer. On the other hand, conventionally, a change in the optical path difference is detected using a gas laser such as a He-Ne laser, and based on the detection result, the rate of change of the optical path difference (the amount of change in the optical path difference per unit time) is constant. The interferometer was controlled so as to become. When the changing speed of the optical path difference is constant, sampling is performed at a fixed time interval, so that the value of the detection signal can be obtained at each fixed interval of the optical path difference.

FIG. 4 is a diagram showing a configuration of a portion for detecting a change in an optical path difference in a Michelson-type two-beam interferometer. The two-beam interferometer includes a beam splitter 60, a fixed mirror 56, and a movable mirror 54, and emits laser light from a laser source 52 together with infrared light incident to generate interference infrared light. Is incident on this interferometer. This laser light is emitted from the interferometer via the same optical path as the infrared light. That is, the laser light incident on the interferometer is
First, the light is split into two light beams by the beam splitter 60,
One light beam is fixed by the fixed mirror 56 and the other light beam is
4 respectively. Both reflected light beams become one light beam again by the beam splitter 60 and exit from the interferometer. In the Michelson interferometer, the moving mirror 54
Are reciprocating linearly, whereby the optical path difference between the two light beams changes, and the intensity of the emitted laser light changes sinusoidally with respect to the optical path difference due to interference. The intensity of the emitted laser light is detected by a laser detector 58, and the reciprocating motion of the movable mirror 54 is controlled so that the frequency of the intensity change of the detection signal becomes constant. Thus, the changing speed of the optical path difference is kept constant.

On the other hand, the present applicant has proposed a two-beam interferometer that generates an optical path difference by rotating a double corner cube (Japanese Patent Laid-Open Nos. 4-190124 and 4-204334). This two-beam interferometer has a feature that the apparatus can be downsized as compared with the above-mentioned Michelson interferometer that generates an optical path difference by reciprocating a plane mirror. FIG. 3 is a diagram showing a configuration of a part for detecting a change in an optical path difference in a two-beam interferometer using such a double corner cube. In FIG. 3, a two-beam interferometer is constituted by the double corner cube 16 and the beam splitter 14. However, in FIG. 3, although the optical path of the laser beam is drawn in a planar manner for easy understanding, the two corner portions 16 a and 16 b of the actual double corner cube 16 are respectively formed by three planes orthogonal to each other. It is configured. As shown in FIG.
A laser source 12 such as a He-Ne laser and a laser detector 18 such as a photodiode are arranged with a beam splitter 14 interposed therebetween. The laser source 12 is provided together with infrared light incident to generate interference infrared light. From the laser beam enters the interferometer and exits the interferometer via the same optical path as the infrared light. That is, the incident laser light is first reflected by the beam splitter 14 (hereinafter referred to as “reflected light flux”).
And a light beam transmitted through the beam splitter 14 (hereinafter referred to as “transmitted light beam”). A double corner cube 16 is provided in front of the reflected light beam and the transmitted light beam,
This double corner cube 16 has one corner 1
The two corner portions 16a and 16b are so arranged that the reflected light beam enters the other corner portion 6a and the transmitted light beam enters the other corner portion 16b.
Are placed at positions sandwiching the surface of the beam splitter 14. The double corner cube 16 is configured to be able to rotate around a predetermined rotation axis A.

[0005] A part of the light beam reflected by one corner 16a of the double corner cube 16 is partially reflected by the beam splitter 1.
4 and a part of the transmitted light beam reflected by the other corner 16 b is reflected by the beam splitter 14, and forms one light beam toward the laser detector 18. In this way, the optical path length of the reflected light beam and the transmitted light beam toward the laser detector 18 (the optical path length from the laser source 12 to the laser detector 18) changes by rotating the double corner cube 16, and accordingly, The optical path difference between the two also changes. As a result, the detection signal output from the laser detector 18 changes sinusoidally with respect to the optical path difference. The rotation of the double corner cube 16 is controlled so that the frequency of the intensity change of the detection signal becomes constant. Thus, the changing speed of the optical path difference is kept constant.

[0006]

The resolution of a spectrum measured by a Fourier transform spectrophotometer depends on the maximum value (scanning length) of the optical path difference in the interferometer. The resolution is increased (the reciprocal of the maximum value of the optical path difference is a value representing the resolution by a wave number). On the other hand, the use of the two-beam interferometer using the above-described double-corner cube makes it possible to reduce the size of the apparatus. However, unlike the Michelson-type two-beam interferometer (see FIG. 4), the laser detector 18 has The heading laser light moves in the horizontal direction due to the change in the optical path difference (however, the direction is constant). That is, when the optical path difference is 0, the laser beam
Although the light passes through the optical path indicated by the solid line at, when the double corner cube 16 is tilted to generate an optical path difference, the laser light traveling to the laser detector 18 through the optical path indicated by the dotted line moves in the horizontal direction. This moving distance increases as the optical path difference increases. Therefore, in order to measure a spectrum with high resolution, a laser detector having a large light receiving surface is required. For example, if the required resolution of the spectrum after the Fourier transform is 0.25 cm ⁻¹ , the necessary optical path difference is 4 cm. Assuming that the distance between the two vertices of the double corner cube 16 is 88.0 mm, the double corner cube 16 must be rotated ± 8.7 degrees to generate an optical path difference of 4 cm. In this case, The optical axis of the laser beam moves 2.2 mm in the horizontal direction. That is, in this case,
A laser detector having a large light receiving surface of 2.2 mm or more is required.

However, when a laser detector having a large light receiving surface is used, the resolution in the spectrum measurement becomes high, but the response of the detection signal becomes slow, and the S / N ratio is lowered. On the other hand, the applicant of the present application disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 4-204334 that the laser light emitted from the double corner cube is reflected by a plane mirror after being reflected by each corner cube, so that A configuration is disclosed in which the light passes through the same optical axis regardless of the rotation angle. However, in this configuration, the apparatus becomes complicated, which leads to an increase in cost. In particular, in a Fourier transform spectrophotometer that measures only the mid-infrared region, such a configuration cannot be adopted from the viewpoint of cost.

Accordingly, in the present invention, a two-beam interferometer using a double corner cube is used to reduce the size, without complicating the apparatus, and using a laser detector with a small light receiving area and a high resolution spectrum. It is an object of the present invention to provide a Fourier transform type spectrophotometer which enables measurement.

[0009]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a two-beam interference device in which a double corner cube is rotated to change the optical path difference between two light beams to generate interference light. Fourier transform spectrophotometer using a meter, laser light is incident together with light incident on the two-beam interferometer to generate interference light, and the laser light emitted from the two-beam interferometer is detected by a detecting means. In the Fourier transform spectrophotometer that detects and controls the rotation of the double corner cube based on the detection signal output from the detection means,
In the optical path until the laser beam emitted from the two-beam interferometer reaches the detecting means, the laser beam covers the range in which the emitted laser beam moves when the double corner cube rotates. And a configuration in which the detecting means is arranged at or near the focal position of the condenser lens system.

[0010]

When light is incident on the two-beam interferometer in order to generate interference light for irradiating the sample, laser light is incident along with the light. The laser light incident on the two-beam interferometer is emitted through the same optical path as the incident light to generate the interference light. That is, the laser beam is split into two light beams in the two-beam interferometer, and each of the light beams passes through a different optical path to become one light beam again and is emitted from the two-beam interferometer. The difference between these optical path lengths (optical path difference) in the two-beam interferometer is changed by rotating the double corner cube to generate interference light. The laser light emitted from the two-beam interferometer passes through the condenser lens system, and then proceeds toward the detecting means.

The laser light emitted from the two-beam interferometer is
Since the laser light is reflected by the double corner cube, it is moved by the rotation of the double corner cube. However, since the laser light has a constant emission direction, it always passes through the same point (focal point) after passing through the condenser lens system. Therefore, the positional relationship between the condenser lens system and the detecting means is set such that the detecting means is placed at or near this point. Thereby, even if the light receiving area of the detecting means is small and the optical path difference is large, the laser light emitted from the two-beam interferometer is always detected by the detecting means.

The condensing lens system referred to here includes a lens system such as a single convex lens that converges parallel rays at a focal point, and a parallel section having a large cross section and condenses parallel rays with an extremely small section. The lens system is also included.

[0013]

FIG. 1 is a diagram showing a configuration of a main part of a Fourier transform type spectrophotometer for the mid-infrared region according to an embodiment of the present invention. In the analysis by the Fourier transform spectrophotometer, the infrared light applied to the sample is obtained by a two-beam interferometer that generates an optical path difference by rotating a double corner cube. The rotation of the double corner cube is controlled so that the change speed of the optical path difference is constant, and for this purpose, the change in the optical path difference is detected using laser light. FIG. 1 shows a configuration of a portion for detecting a change in an optical path difference using a laser beam, and corresponds to FIG. 3 in a conventional example. This embodiment is different from the conventional example shown in FIG. 3 in that a laser beam emitted from a two-beam interferometer (consisting of a double corner cube 16 and a beam splitter 14) reaches a laser detector 20. The point is that the single convex lens 22 is provided in the optical path. This single convex lens 22 is
Is located at a distance from the camera by the focal length. Except for the point that the single convex lens 22 is provided, the description is omitted because it is the same as the conventional example shown in FIG.

In this embodiment, as in the prior art, the double corner cube 16 is rotated to change the optical path difference, and accordingly, the laser light emitted from the interferometer and traveling to the laser detector 20 moves. . However, unlike the related art, since the single convex lens 22 is provided and the direction of the laser light is constant, the laser light is always moved to the same point (focal point) by the single convex lens 22 even if it moves. Head for. That is, when the optical path difference is 0, the light passes through the optical path indicated by the solid line in FIG. 1, and when the double corner cube 16 is inclined at a predetermined angle to generate the optical path difference, the light passes through the optical path indicated by the dotted line. After passing through the single convex lens 22, the optical path goes to the focal point together. A laser detector 20 is arranged at this focal point. Therefore, unlike the conventional laser detector 18 shown in FIG. 3, the present embodiment uses a laser detector 20 having a small light receiving area. Therefore, the response speed of the detection signal is high and the S / N ratio is large. In addition, since the laser beam always goes to one point (laser detector 20) regardless of the rotation angle of the double corner cube 16, the rotation angle of the rotation of the double corner cube 16 is increased and the maximum value of the optical path difference ( Scan length) can be increased. This allows
The spectrum of infrared light from the sample can be measured with high resolution.

In the above embodiment and the conventional examples shown in FIGS. 3 and 4, the change in the optical path difference in the two-beam interferometer can be detected, but the optical path difference itself cannot be detected. However, the detection signal cannot be sampled at the same optical path difference. Therefore, by detecting the optical path difference using the P-wave and the S-wave of the laser light, a mechanism for controlling the detection signal to be always sampled at the same optical path difference even if measurement is performed a plurality of times (“Quadrature”) Introducing a “control”) can eliminate the influence of noise on the measured data by adding the data obtained by the multiple measurements at each sampling point and taking an average.

When the above-mentioned quadrature control is incorporated in a spectrophotometer, it is necessary to separate the P wave and the S wave contained in the laser beam reflected by the double corner cube and to detect them separately. A polarizing beam splitter is provided immediately before the laser detector, and is disposed so as to form an angle of 45 degrees with the optical axis of the laser beam. Therefore, when a quadrature control is incorporated in a spectrophotometer using a conventional two-beam interferometer (see FIG. 3) using a double corner cube, the dual corner cube 16 rotates as a polarizing beam splitter. Therefore, a laser beam having a size that is √2 times the moving distance of the laser optical axis (which is even larger than the light receiving surface of the laser detector) is required, which leads to an increase in cost.

On the other hand, when the quadrature control is incorporated in the above embodiment (see FIG. 1), the configuration becomes as shown in FIG. In this case, the beam splitter 14 and the corner portion 16 of the double corner cube 16
The S wave of the laser light reflected at the corner 16a toward the laser detector by the wavelength plate (λ / 8 plate) provided between the P wave and the P wave is λ / 4 (λ is the wavelength of the laser light) more than the P wave. ), The P-wave and the S-wave are separated by the polarization beam splitter 26 provided between the single convex lens 22 and the laser detector. Then, in order to separately detect the separated P wave and S wave, a first detector 24P for detecting the P wave on the optical axis of the laser light emitted from the interferometer passes through the polarization beam splitter 26 and the light A second detector 24S for detecting the S wave on a straight line perpendicular to the axis is provided, and both detectors 24P and 24S are arranged at the focal position of the single convex lens 22. Although the intensities of the P wave and the S wave detected by these detectors 24P and 24S both change sinusoidally with respect to the optical path difference, these sine waves have a phase difference of λ / 4. Using this information, it is determined whether the change in the optical path difference is increasing or decreasing. The optical path difference is detected by counting the peak of the P wave or the S wave using an UP / DOWN counter (not shown) based on the determination result.

According to the configuration shown in FIG. 2, the polarizing beam splitter 26 can be placed at a position where the movement distance of the laser optical axis is sufficiently reduced by the single convex lens 22, so that the polarizing beam splitter 26 has a small area. Can be used. Further, since the first detector 24P for detecting the P wave and the second detector 24S for detecting the S wave are both arranged at the focal position of the single convex lens 22, even if the maximum value of the optical path difference is increased, First and second detectors 24
P and 24S having a small light receiving area can be used. Therefore, even when the quadrature control is incorporated, spectrum measurement with high resolution can be performed with a laser detector having a small light receiving area.

In the above-described embodiment shown in FIGS. 1 and 2, the light receiving area of the laser detector is reduced by providing the single convex lens 22. However, instead of the single convex lens 22, a plurality of lenses are used. Similar effects can be obtained by using other condensing lens systems such as a lens system that converges a light beam at a focal point and a lens system that converges a parallel light beam having a large cross section and converts it into a parallel light beam having a very small cross section.

[0020]

According to the present invention, it is possible to reduce the size by using a two-beam interferometer using a double corner cube and to provide a condenser lens system in the optical path of the laser beam toward the laser detector. With a simple configuration, spectrum measurement with high resolution can be performed with a laser detector having a small light receiving area. By using a laser detector having a small light receiving area, the response speed of the detection signal is increased, and the S / N ratio of the detection signal is also increased.

Also, when the quadrature control is incorporated, a polarizing beam splitter having a small area can be used, and the P-wave and S-wave of the laser beam can be used.
Both detectors for detecting waves can use those having a small light receiving area, so that spectrum measurement with high resolution can be performed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a part for detecting an optical path difference in a two-beam interferometer of a Fourier transform spectrophotometer according to one embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration of a part that detects an optical path difference in a two-beam interferometer of a Fourier transform spectrophotometer according to another embodiment of the present invention.

FIG. 3 is a diagram showing a configuration example of a part for detecting an optical path difference in a conventional two-beam interferometer using a double corner cube.

FIG. 4 is a diagram illustrating a configuration of a part that detects an optical path difference in a Michelson-type two-beam interferometer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 12 ... Laser source 14 ... Beam splitter 16 ... Double corner cube 20 ... Laser detector (detection means) 22 ... Single convex lens (condensing lens system) A ... Rotation axis

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01J 3/45 G01J 9/00-9/04 G01J 1/02-1/04 G01B 9/02 H01L 31/00-31 / 02 H01L 31/02

Claims (1)

(57) [Claims] 1. A Fourier transform spectrophotometer using a two-beam interferometer for generating interference light by changing the optical path difference between two light beams by rotating a double corner cube, wherein the interference light is generated. For this purpose, laser light is incident together with light incident on the two-beam interferometer, the laser light emitted from the two-beam interferometer is detected by the detecting means, and a double corner is formed based on a detection signal output from the detecting means. In a Fourier transform spectrophotometer for controlling the rotation of the cube, when the laser beam emitted from the two-beam interferometer is in the optical path until reaching the detection means, and the double corner cube is rotated A focusing lens system so as to cover a range in which the emitted laser light moves, and the detecting means is arranged at or near a focal position of the focusing lens system. Fourier transform spectrophotometer to.