JPS6011123A - Fourier transform type spectrophotometer - Google Patents

Abstract

PURPOSE:To enable exact data sampling by inserting a layer of a medium of which the refractive index varies with wavelength to one side of bisected two optical paths more than into the other optical path and making the two optical path asymmetrical. CONSTITUTION:A medium layer of which the refractive index varies slightly with wavelength is inserted into either of optical paths for two light beams (a), (b) asymetrically from the other. A laser light source N emits a visible light beam which is made incident to an interferometer and is detected by a detector D'. As a movable mirror M2 moves, the quantity of the visible light incident to the detector D' fluctuates periodically and therefore the output from the D' is made by waveform shaping into the sampling pulses to operate a sumple holding circuit SH. The pulses are at the same time counted by an arithmetic control circuit C. The start point for scanning of the mirror M2 is taken at the position of suitable -X and the photometric outputs in the positions x1, x2-xn for the count values 1, 2, 3-n of the sampling pulses are stored in a memory Me. The circuit C reads out the data from the memory Me and calculates power spectra after one time of scanning with the mirror M2.

Description

[Detailed description of the invention] b) Industrial application field The present invention relates to a Fourier transform spectrophotometer.

(0) In conventional Fourier transform spectrophotometers, the light of all wavelengths overlaps in phase at the optical zero point, so the interferogram signal becomes very sharp at the optical zero point, and the signal at other parts There was a problem in that the dynamic range of the photometry system could not be used effectively because it was too thick compared to the above. This point will be explained in more detail with reference to FIG.

Figure 1 shows a Michelson type interferometer, where l is a light source, B is a semi-transparent mirror beam splitter, Ml is a fixed mirror, and M2 is a movable mirror that can be moved left and right. D is a photodetector. Considering light of wavelength λ, when movable mirror M2 is moved in one direction, the amount of light incident on the photodetector is expressed as When light is incident, the output signal from the photodetector changes in the form of overlapping cosine waves of various periods with X as a variable. That is, the output D(X) of the photodetector is 1
/λ−ν (wave number), D(x)=r”?(ν) c
os 4 rc v x * d v (1) Here, F(ν) is the spectrum of the incident light, and D(x) is the Fourier transform of the spectrum of the incident light.

This D(x) is an interferogram. If the function obtained by Fourier transformation is inversely Fourier transformed, it becomes the original function, so F'(v) can be found by inversely Fourier transforming D(X). This is the principle of the Fourier transform spectrophotometer that is the object of the present invention, but as a method of measuring the amount of movement X of the movable mirror M2, Lx = Ll in Figure 1.
Let the position of the movable mirror M2 be the origin of X. Above (1)
The formula is the one when the origin is determined in this way. The optical zero point mentioned at the beginning is the origin of X where Lx=L1. At this point, COB 4πνXa is l for all ν, so D (x) at this point has a maximum value, and since the interferogram at this point determines the dynamic range of the photometric system, other parts Accurate data sampling becomes impossible.

In FIG. 1, only the semi-transparent mirror layer of beam splitter B is shown, but conventional beam splitters had a configuration as shown in FIG. 1 is the board, for example KB
r (for infrared spectroscopy) is used, with a semi-transparent mirror layer 2 on one side,
For example, a vapor-deposited layer of Ge is provided, and a compensator 3 made of the same material and of the same thickness as the substrate 1 is laminated together. The compensator 3 is provided so that the rays a and b in FIG. 1 meet at the optical zero point through the same optical path length, and by doing so, the interferogram is It has the form shown by the formula, and is symmetrical with respect to the origin of X. Therefore, as mentioned above, there was a problem with the dynamic range of the photometric system.

(c) Purpose The present invention aims to solve the above-mentioned problems in the prior art.

(d) Structure In the present invention, in FIG. 1, a medium layer having a refractive index slightly different depending on the wavelength is inserted into one of the optical paths of the a and b2 rays asymmetrically with respect to the other. In this way, one ray passes through the medium layer more than the other ray, and the refractive index of the medium layer differs depending on the wavelength, so at any position of the movable mirror M2, a, b
The difference in optical path length between the two light beams varies depending on the wavelength.

This point is significantly different from conventional Fourier transform spectrophotometers. Even if the optical path lengths of both the a and b rays are equal for all light of a specific wavelength, the optical path lengths are not equal for light of other wavelengths. Therefore, there is no such thing as the optical zero point in the conventional example, and although the interferogram has an asymmetrical shape, there is no point where the signal strength is extremely high.

(E) Embodiment FIG. 3 shows an embodiment of the present invention. Ml is a fixed mirror, and M2 is a movable mirror. In the beam splitter B, 1 is a substrate and 2 is a semi-transparent mirror. This embodiment is characterized in that the beam splitter has a structure in which the compensator 3 is removed from the conventional beam splitter shown in FIG.

In FIG. 1, the compensator 3 is connected to the ray a.

It was used to make the optical path of
Also, the a and b2 optical paths can be made symmetrical by eliminating the substrate l and using a translucent mirror layer spanning the mirror layer. Therefore, in this embodiment, the beam splitter
Since there is no compensator and only the substrate 1 is present, the substrate itself becomes a medium inserted into one optical path asymmetrically with respect to the other optical path a, as described in the configuration section. This embodiment is for infrared spectroscopy from 2.5 μm to 25 μm, the material of the substrate 1 is KI3r, and the semi-transparent mirror 2 is a vapor deposited layer of Ge. The table below shows the relationship between wavelength and refractive index of KBr.

Wavelength (μm) Refractive index 2.44 1.53'i'38 1LO351,52403 25,141,46322 The incident ray is now split into two rays a and b, and the point where they meet again is P, and each dimension and angle Decide the code as shown,
Optical path length OA of light ray a going back and forth between point P and fixed mirror Ml
The optical path length OB of the light beam traveling back and forth between point P and the movable mirror is calculated.

C) A = 2HIV 1 OB- or 12+2nd/cos θ where n is the refractive index of the substrate 1. The optical path difference Δ between the two rays is determined by the above formula (the variable part of the maggot is 12, and the position of the movable mirror M2 where 12=11 is the origin of the position variable X of the movable mirror M2,
In Figure 2 of M2, if the movement to the left is the positive direction, then n and cos θ are functions of wavelength, and using the wave number ν, it is written as ]ko ``−〇(ν). Wave number When converting the optical path difference Δ to the phase angle ψ for the light of ν, ψ=2πΔQ s22πν(2x−C(ν)...
...(3) In the equation (1) above, in X==O, the lights of all wavelengths meet at a phase angle of 0, but in the case of the present invention (
1), and D(X) has no symmetry at all, so it does not occur that the signal intensity at the optical zero point in a conventional symmetrical interferogram becomes much stronger than other parts. No.

In this example, the substrate of the beam splitter is used as a medium layer having a different refractive index depending on the wavelength in order to make the two optical paths asymmetrical, but this medium layer constitutes the beam splitter. It does not need to be a structural material, and may just be a medium layer inserted into one optical path.

In Figure 3, D is a photodetector, SH is a sample and hold circuit,
AD is an A/D converter, Me is a memory, and C is an arithmetic control circuit. A visible light beam is emitted by an N-type laser light source, and since the amount of interference light in this visible light varies periodically, the output of the DI is waveform-shaped and used as a sampling pulse to operate the sample-and-hold circuit SH, and this pulse is counted by the arithmetic control circuit C, and this counted value becomes the value of the variable X in the above equation (5). In the present invention, the origin of the position of the movable mirror has no special meaning, and the scanning start point of the movable mirror M2 is set at an appropriate position of -X, and the count value of the sampling pulse is 1. 2. 3...Position 'XI corresponding to n.

x2. ...The photometric output of xn is stored in the memory Me. After one scan of the movable mirror M2, the arithmetic control circuit C reads the data from the memory Me, performs Fourier transform, and calculates the power spectrum.

(f) Effects According to the present invention, although the moving distance of the movable mirror for interferogram measurement is larger than that of the conventional example, the dynamic range required for the interferogram measurement circuit is reduced to about 1/5 of that of the conventional example. This allows the interferogram to provide more accurate data sampling. Note that when the compensator is eliminated from the beam splitter as in the embodiment, there is an advantage that the number of optical elements can be reduced by one.

[Brief explanation of drawings]

Fig. 1 is a plan view of the interferometer, Fig. 2 is a side view with the thickness of a conventional beam splitter enlarged, and Fig. 3 is a plan view of the optical system and a block diagram of the measurement system according to an embodiment of the present invention. It is. L...Light source, B...Beam splitter, MAL・
... Fixed mirror, M2... Movable mirror, l... Beam splitter substrate, 2... Semi-transparent mirror layer. Agent Patent Attorney Kosuke Agata

Claims (1)

[Claims] In an interferometer, a Fourier interferometer is characterized in that a layer of a medium whose refractive index changes depending on the wavelength is inserted on one side of two divided optical paths, more than the other optical path, to make the two optical paths asymmetrical. Conversion spectrophotometer.