JPH0455726A - Fourier spectroscope - Google Patents

Abstract

PURPOSE:To simplify an apparatus and to shorten the measuring time by providing an image processing device which computes the spectrum of luminous flux to be measured based on the result of the detection of an optical sensor. CONSTITUTION:Light to be measured 10 which is generated in a light source 1 is split into luminous fluxes 11 and 12 through a beam expander 2 and a half mirror 3. The luminous flux 11 is reflected from a mirror 4 and returned to the mirror 3. The luminous flux 12 is diffracted with a diffraction grating 5 and returned to the mirror 3. In the mirror 3, the synthesized luminous flux is sightly inclined so that the lateral fringes of adequate number are generated on a two-dimensional sensor 7. At this time, the distance L1 between the mirrors 3 and 4 and the distance L2 between the mirror 3 and the lower end of the grating 5 are set so that L1=L2. Then, the contrast of the interference pattern of the luminous fluxes 11 and 12 is equal to the contrast of the interference of two luminous fluxes at the light path difference of 0 in a Michelson interferometer at one end (a) of the light receiving part of the sensor 7. The contrast is equal to the contrast of the interference of two luminous fluxes at the light-path difference of D tan theta at the other end (b). Therefore, the interference fringes wherein the light-path difference l continuously change from 0 to D tan theta from one end (a) to the other end (b) on the sensor 7 are generated. The interference fringes are computed 8.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a Fourier spectrometer that measures the spectrum of light using two-beam interference.

[Prior art] A Fourier spectrometer, which is known as a device for measuring precise spectra, divides the light to be measured into two parts, causes the two lights to interfere while gradually changing the difference in optical path length, and changes the visibility of the two lights according to the optical path length. After determining the difference as a function, the spectrum is measured by Fourier transforming this function. Conventionally, in order to gradually change the optical path length difference, one of the mirrors that make up the Michelson interferometer was moved by small distances.

[Problems to be Solved by the Invention] However, in the conventional example described above, the visibility of the interference is measured every time one of the mirrors constituting the interferometer is moved minutely, so there are the following drawbacks. First, when emitting pulsed light like an excimer laser, it was not possible to measure the spectrum of one pulse. Even with continuous light, accurate measurements could not be made if the spectrum changed over time. Second, since a micro-movement mechanism for the mirror is required, the device becomes complicated.

Thirdly, measurement took a long time.

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and enables spectrum measurement by Fourier spectroscopy without moving the mirrors that make up the interferometer, thereby simplifying the device configuration and shortening the measurement time. The purpose is to provide a Fourier spectrometer with

[Means and effects for solving the problem] In order to achieve the above-mentioned object, according to the present invention, one or both of the light beams divided into two parts inside the Fourier spectrometer is divided into two parts, and the emission direction of the light beam is changed depending on the wavelength. A dispersion element such as a diffraction grating is provided, and a two-dimensional sensor is provided as a means for reading the interference fringes, and an image processing device is further provided to calculate the visibility of the interference fringes for each portion of the light beam received by the two-dimensional sensor. This makes it possible to measure spectra using Fourier spectroscopy without moving the mirror. Note that the term "dispersive element" hereinafter refers to an element having a function of dispersing the emission direction of a luminous flux depending on the wavelength.

[Embodiment] Fig. 1 is a drawing showing the configuration of an embodiment of the present invention. In the figure, 1 is a light source, 10 is the measured light emitted from the light source 1, and 2 is a light beam that is enlarged as necessary. beam expander,
3 is a half mirror that splits the light to be measured 10 into two beams; 4 is a mirror; 5 is a diffraction grating; 11 is a beam of light directed toward mirror 4 among the two beams divided by half mirror 3; and 12 is a beam directed toward diffraction grating 5. A light beam 13 is a light beam recombined by the half mirror 3, 6 is a beam condenser for adjusting the light beam 13 to the size of the light receiving part of the two-dimensional sensor 7, 7 is a two-dimensional sensor, and 8 is a signal from the two-dimensional sensor This is an image processing device that processes.

Measured light 10 emitted from a light source 1 passes through a beam expander 2 and is expanded to the radius of the beam, and then is split into a beam 11 and a beam 12 by a half mirror 3.

The light beam 11 is reflected by the mirror 4 and returns to the half mirror 3 again. On the other hand, the light beam 12 is diffracted by the diffraction grating 5 and is reflected by the half mirror 3.
Return to The diffraction grating 5 has a center wavelength λ of the light 10 to be measured. θ is adjusted so that the diffraction angle ψ and the incident angle θ are equal. As is well known, the diffraction conditions are expressed by the following equation (1), where d is the lattice constant of the diffraction grating 5, and m is an integer.
It is to satisfy the following.

d (sin θ+sin ψ)=mλ...
...(1) λ=λ. , 2dsinθ=
mλ.

(2) The half mirror 3 transmits the combined light beam 13 to the two-dimensional sensor 7
It is tilted slightly forward or backward to produce an appropriate number (several to several dozen) of horizontal stripes on the top. Here, if Ll=L2 as the distance LH between the half mirror 3 and the mirror 4, and the distance L2 between the half mirror 3 and the lower end of the diffraction grating 5, then the luminous flux 1
At one end a of the light receiving section of the two-dimensional sensor 7, the contrast of the interference pattern between the light beam 1 and the light beam 12 is equal to the contrast of the interference of two light beams with an optical path difference of 0 in a so-called Michelson interferometer, and at the other end, the contrast of the interference pattern between the two light beams 2 at difference D tanθ
Equal to the contrast of luminous flux interference. The reason for this will be explained next. The dispersion of the diffraction grating is expressed by differentiating equation (1) by λ and then eliminating m and d in equation (2), so that the light of wavelength λ0 by the diffraction grating 5 and the wavelength λ, = λ. +Δλ light differs only in the direction of the diffracted light. Therefore, at one end O of the light beam 13 combined by the half mirror, the difference Δδ in the phase difference δ between the lights λ0 and λ1
is Δδ=0, and at a point P that is away from point 0 by X, it becomes...(5). Here, the phase difference δ is the phase difference when the light beams 11 and 12 separated by the half mirror 3 are recombined by the half mirror, and the phase difference Δδ is the phase difference δ1 between the light beams with the wavelength λ1 and the phase difference Δδ. Wavelength λ. phase difference δ of light. The difference δ1−δ. It is about. On the other hand, in a so-called Michelson interferometer, if the optical path length difference between the two separated beams is k, then the phase difference δ between the lights of wavelength λ0. The phase difference δ1 of light with wavelength λ is . Therefore, Δ δ = δ 1-6° (8) Comparing equations (5) and (8) and setting 42 = x tan θ, the values of both equations become equal. In other words, the optical path difference between the contrast at point P in Figure 1 and the Michelson interferometer is x
When tan θ, the contrast of two beam interference is equal.

Therefore, from one end a to the other end on the two-dimensional sensor 5, interference fringes are generated due to two-beam interference in which the optical path difference l changes continuously from 0 to D tan θ. The light-receiving section of the two-dimensional sensor 7 is divided vertically into a large number of vertically long columns, and the visibility of the interference fringes for each column is calculated by the image processing device 8.

If the number of sections is n, then the visibility of each row is equal to the visibility of interference measured each time one mirror is moved by -tane in the Michelson interferometer. Generally known as Fourier spectroscopy, the spectrum E (v) can be determined from the relationship between the optical path length difference ρ and the visibility V (J2) using the following equation (2). However, ν is the wave number.

E(ν)−const x′)”, V(It)
C05(2πv ・Q)dΩ... (2) Here, the constant optical path 1c1 bounded by the spectral width Δν
In the above, the visibility V(Il) is v(1) ~ 0, so the formula (
The integral range of 2) may be from 0 to JZ , ,
In the case of a Gaussian distribution spectrum, j2. ,,=1/2Δ
For example, wavelength λ = 248 nm, Δλl! ! ! 1
If it is pm, Δν=Δλ・C/λ""0.16cm-
' (where C is the luminous flux). Therefore, 11 v
aa* -311J. From this, D tanθ≧3
It should be 11■.

FIG. 2 shows an embodiment in which two diffraction gratings are used to increase the difference in optical path length. In the figure, 24 is the first
This is a diffraction grating used in place of mirror 4 in the figure.

By making the above changes, it is possible to obtain a larger optical path difference with the same luminous flux diameter as in the first embodiment. For example, if the same material as the diffraction grating 5 is used as the diffraction grating 24, an optical path length difference twice that of the first embodiment can be obtained. This results in
The resolution of spectral measurements can be doubled.

FIG. 3 shows an embodiment in which a prism is used as a dispersion element in place of the diffraction grating 5 of the first embodiment.

30 is a prism, and 35 is a mirror.

In this embodiment, the same effect as in the first embodiment can be obtained when a diffraction grating with a dispersion equal to that of the prism 30 is used.

In general, the dispersion of a prism is about one to several tenths of that of a diffraction grating, making it difficult to measure light with a narrow spectral width.However, since the loss of light is small, it is better than that of a diffraction grating, which has a large loss of light. This is advantageous for measuring weak light. Furthermore, the cost can be reduced compared to the case where a diffraction grating is used.

[Effects of the Invention] As explained above, by using one or both mirrors of a Michelson interferometer, etc. that constitute a Fourier spectrometer as a dispersive element such as a diffraction grating, Fourier spectroscopy can be performed without moving the mirrors. I can do it.

This is particularly effective when measuring the spectrum of each pulse of pulsed light emission such as excimer laser light.

Furthermore, since there is no need to move the mirror, the apparatus can be simplified and the measurement time can be shortened.

[Brief explanation of drawings]

Figure 1 is a block diagram of a Fourier spectrometer embodying the present invention, Figure 2 is a partial diagram showing the interferometer part of an embodiment using two diffraction gratings, and Figure 3 is an implementation using a prism. FIG. 3 is a partial configuration diagram showing an example interferometer section. 30 Nibrism, 35: Mirror

Claims (3)

[Claims] (1) A beam splitter is provided on the optical path of the light beam to be measured to split the light beam to be measured into two, and each of the transmitted light beam and the reflected light beam divided into two by the beam splitter is placed on each optical path of the beam splitter. At least one of the reflecting means is constituted by a fixed dispersion element that disperses the emission direction of the light beam depending on the wavelength, and the light beam to be measured is recombined by the beam splitter. 1. A Fourier spectrometer, comprising: an optical sensor that detects interference fringes of the light beam to be measured on an optical path; and an image processing device that calculates a spectrum of the light beam to be measured based on the detection result of the light sensor. (2) The Fourier spectrometer according to claim 1, wherein the dispersive element comprises a diffraction grating. (3) The Fourier spectrometer according to claim 1, wherein the dispersive element comprises a combination of a prism and a mirror.