JPH07190858A - Fourier transform spectrometer - Google Patents

Abstract

(57) [Summary] [Purpose] To obtain higher wavelength resolution while maintaining spatial resolution. [Structure] A light splitting means (BS) for splitting incident light from a measurement object into a reference light path and a measurement light path, and a fixture fixed in the reference light path for reflecting light from the light splitting means toward the light splitting means A reflecting mirror (M ₁ ), a movable reflecting mirror (M ₂ ) that is movably provided in the measurement light path and reflects the light from the light splitting means toward the light splitting means, and a reference light path through the light splitting means. In the Fourier transform spectroscope having a light receiving means (L _C , Dec) for receiving the light and the light from the measurement optical path through the light splitting means,
An afocal optical system (20) having a magnifying power is arranged in the optical path between the measurement target and the light splitting means.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Fourier transform spectrometer suitable for observing the atmosphere and analyzing a sample.

[0002]

2. Description of the Related Art Conventionally, as a method for observing the state of the atmosphere by remote sensing, a Fourier transform spectroscope has been used to detect the attenuation of visible rays, infrared rays, ultraviolet rays, etc. by the atmosphere. Measuring the composition of air is for example SPIE vol.1129 p42
~ 51 (R. Beer and T. Glvaich). The structure of this Fourier transform spectrometer is shown in FIG.

A conventional Fourier transform spectroscope 100 shown in FIGS. 4 (a) and 4 (b) is a so-called Michelson type interferometer, which disperses light from an observation point P. Figure 4 (b)
In, the Fourier transform spectroscope 100 has a beam splitter BS, a fixed mirror M ₁ , a movable mirror M ₂ , a condenser lens L _C, and a light receiving element Dec. Here, the light from the observation point P reaches the observation point P in the atmosphere from the sun,
The light scattered to the Fourier transform spectroscope 100 side at this observation point P is split by the beam splitter BS into a reference optical path and a measurement optical path. A fixed mirror M ₁ as a reference mirror is arranged on the reflection side of the beam splitter BS, and the light reflected by the beam splitter BS and traveling on the reference optical path is reflected by the fixed mirror M ₁ .
It reaches the beam splitter BS again.

On the transmission side of the beam splitter BS, there is provided a moving mirror M ₂ which moves so as to make the optical path length with the beam splitter BS variable, and the movable mirror M ₂ passes through the beam splitter BS to advance the measurement optical path. The light is reflected by this moving mirror M ₂ and reaches the beam splitter again. The light passing through the reference optical path is reflected by the beam splitter B.
The light passing through S and passing through the measurement optical path is reflected by the beam splitter BS and reaches the condenser lens L _C , respectively. At the rear focal position of the condenser lens L _C , for example, 1
A light receiving element Dec composed of a three-dimensional CCD is arranged.

On the light receiving element Dec, interference fringes are formed by the light passing through the reference light path and the measurement light path. Here, the light from the observation point P has a predetermined wavelength distribution, and the light receiving element Dec according to the distance between the movable mirror M ₂ and the beam splitter BS, that is, the optical path length between the reference optical path and the measurement optical path.
The constructive wavelength changes above. As a result, the light receiving element D
The wavelength distribution of the light from the observation point P can be measured by detecting the change in the amount of light on ec and the amount of movement of the movable mirror M ₂ .

Specifically, the control unit 110 controls the movable mirror M
₂The movable mirror M is controlled by controlling the drive unit 120 that drives the₂To
To move. At this time, use an interferometer (not shown) to
Ra M₂The amount of movement of is transmitted to the control unit 110. Also,
The control unit 110 includes a movable mirror M. ₂Light receiving element Dec while moving
Receives photoelectric conversion output from. Here, the control unit 110
Movable mirror M₂Is the light receiving element Dec related to the distance moved?
Fourier transform of the change in photoelectric conversion output from the observation point P
Calculate the wavelength distribution of these lights.

Here, the discriminable wavelength in the Fourier transform spectrometer, that is, the wavelength resolution, is expressed by the following equation.

[0008]

## EQU1 ## δν ≧ ν _max δL ² / 8R ² (1) where δν: wavelength resolution, ν _max : maximum frequency observed, δL: spatial resolution, R: distance from observation point to Fourier transform spectrometer ,.

In the above equation (1), the spatial resolution δL is the size of the object on the observation point P in which one pixel of the light receiving element Dec is expected.

[0010]

However, in the Fourier transform spectroscope as described above, it is required to discriminate wavelengths more finely, that is, a wavelength resolution with higher accuracy. From the above formula (1), it is understood that it is necessary to increase the spatial resolution δL in order to increase the wavelength resolution δν. Here, when the spatial resolution δL is improved, the distance on the observation point P at which one pixel of the light receiving element Dec of the Fourier transform spectroscope is expected becomes small, so that the light intensity incident on the light receiving element Dec decreases. That is, the sensitivity of the light receiving element Dec determines the upper limit of the spatial resolution δL, and thus the accuracy of the wavelength resolution δν. Therefore, in the conventional Fourier transform spectroscope as shown in FIGS. 4A and 4B, it is difficult to further improve the wavelength resolution.

Therefore, an object of the present invention is to provide a Fourier transform spectroscope capable of achieving higher wavelength resolution while maintaining the spatial resolution.

[0012]

In order to achieve the above-mentioned object, the Fourier transform spectrometer according to the present invention has a structure as shown in FIG.
As shown in, a light splitting means (BS) for splitting the incident light from the measurement object into a reference light path and a measurement light path, and light reflected from the light splitting means fixed to the reference light path toward the light splitting means. A fixed reflecting mirror (M ₁ ), a movable reflecting mirror (M ₂ ) movably provided in the measurement light path to reflect light from the light splitting means toward the light splitting means, and a reference light path via the light splitting means. Light receiving means (L _C , Dec) for receiving the light from and the light from the measurement optical path via the light splitting means.

An afocal optical system (20) having a magnifying power is arranged in the optical path between the object to be measured and the light splitting means. Incidentally, the term “magnification” in the present invention means that the lateral magnification β of the afocal optical system is | β |> 1.
It means that.

[0014]

The operation of the present invention will be described below with reference to FIGS. In the following description, the afocal optical system 2
Consider a case where measurement is performed on a measurement target Y at a position separated by 0 and a distance R. Here, when the magnification of the afocal optical system 20 is β (| β |> 1), the Fourier transform spectrometer 10 observes the virtual image Yi of the measurement object formed by the afocal optical system 20. Become. here,
When the spatial resolution of the measuring object Y is δL and the spatial resolution of the image Yi of the measuring object is δLi,

[0015]

## EQU00002 ## .delta.Li = .beta..delta.L (2) holds. Further, if the distance between the image Yi of the measurement object and the afocal optical system 20 is Ri,

[0016]

## EQU3 ## Ri = β ² · R (3) holds. By substituting these equations (2) and (3) into the above equation (1), the following equation is obtained.

[0017]

## EQU4 ## δν i = δν / β ² (4) Here, in the present invention, the magnification β of the afocal system 20 is the enlargement magnification, that is, | β |> 1, therefore, from the above formula (4),

[0018]

## EQU5 ## δν i <δν (5) That is, by providing the afocal system 20 having a magnifying power, it is possible to improve only the wavelength resolution while maintaining the spatial resolution. Next, as shown in FIG. 3, consider the wavelength resolution at a predetermined image height on the peripheral portion of the object having a predetermined spread in the direction perpendicular to the optical axis, that is, on the light receiving element.
Here, the above SPIE vol.1129 p42-51 (R. Beer a
nd T. Glvaich), the wavelength resolution δν f is L, where L is the measurement range on the object.

[0019]

[6] represented by _{δνf ≧ ν max · L · δL} / 2R 2 ... (6). Even in this case, by providing the afocal system 20 (| β |> 1) of the enlargement magnification, the Fourier transform spectroscope 10 observes the virtual image Yi of the measurement object.

Here, the range L of the object to be measured by the afocal system 20 is as shown in the following equation.

[0021]

## EQU7 ## Li = βL (7) Converted to the range Li of the virtual image Yi. Formula (2) above,
From equations (3) and (7), the wavelength resolution δνfi at a given image height is

[0022]

[Equation 8] indicated by _{δνfi ≧ ν max · Li · δLi} / 2Ri 2 = δν / β 2 ... (8), | than> 1, | β

[0023]

## EQU9 ## δν fi <δν (9) That is, by providing the afocal system 20 having a magnifying power, only the wavelength resolution at a predetermined image height can be improved while maintaining the spatial resolution. As is clear from the equation (8), the observation range L is widened and the spatial resolution δL is increased.
Although it is possible to improve the wavelength resolution by reducing the value of, it is not preferable because the number of light receiving elements must be increased in that case.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram for explaining the configuration of an embodiment according to the present invention. In FIG. 1, light from an observation point (not shown) enters the Fourier transform spectroscope 10 via the afocal system 20. The light incident on the Fourier transform spectroscope 10 is split into a reference optical path and a measurement optical path by the beam splitter BS.

The light reflected by the beam splitter BS and traveling in the reference optical path is reflected by the fixed mirror M ₁ arranged in the reference optical path and reaches the beam splitter BS again. The light that has passed through the beam splitter BS and travels in the measurement optical path is reflected by the movable mirror M ₂ provided in the measurement optical path and reaches the beam splitter BS again.

The light that has reached the beam splitter BS via the reference optical path passes through the beam splitter BS and goes to the condenser lens L _C. On the other hand, the light that has reached the beam splitter BS via the measurement optical path is reflected by the beam splitter BS and heads for the condenser lens L _C. The condenser lens L _C condenses the light having passed through the reference light path and the measurement light path on the light receiving element Dec. As a result, interference fringes are formed on the light receiving element Dec due to the interference between the light passing through the reference light path and the light passing through the measurement light path. The light receiving element Dec is composed of a photoelectric conversion element such as a one-dimensional line sensor or a two-dimensional CCD,
The light intensity of the interference fringes is photoelectrically converted and output to the control unit 11.

The control unit 11 controls the drive unit 12 that drives the movable mirror M _{2 so} that the distance between the movable mirror M ₂ and the beam splitter BS changes. Further, the interferometer 13 detects the position of the movable mirror M ₂ and outputs the position information to the control unit 11. Here, as the interferometer 13, the fixed mirror M
_{A configuration in which 1} is used as a reference mirror and the moving mirror M ₂ is used as a moving mirror, and a configuration in which the amount of movement of the moving mirror M ₂ is detected from the back surface side can be considered.

The control unit 11 calculates the wavelength distribution of the light emitted from the observation point based on the photoelectric conversion output from the light receiving element Dec and the position information of the movable mirror M ₂ by the interferometer. The calculation method will be briefly described below. The photoelectric conversion output I (x) from the light receiving element Dec is expressed by the following equation.

[0029]

I (x) = ∫I (k) coskxdk (10) where k = 2π / λ, x is the moving amount of the movable mirror M ₂ , and λ is the wavelength.

Here, the control unit 11 performs the Fourier transform of this photoelectric conversion output I (x) to obtain the spectral function I
Get (k). This spectral function I (k) is expressed by the following equation.

[0031]

I (k) = ∫I (x) coskxdx (11) That is, the wavelength distribution at the observation point is obtained from the photoelectric conversion output I (x) regarding the movement amount x of the movable mirror M ₂ .

Specifically, in FIG. 1, the control unit 11
Controls the drive unit 12 to move the movable mirror M ₂ continuously or discretely at predetermined intervals. At this time, the control unit 11 controls the photoelectric conversion output from the light receiving element Dec and the interferometer 13
(11), the photoelectric conversion output related to the moving amount of the movable mirror M ₂ is Fourier transformed to obtain the wavelength distribution.

Here, in the present embodiment, the afocal system 20 having a magnifying power is arranged between the Fourier transform spectroscope 10 and the observation point, more precisely, in the optical path between the beam splitter BS and the observation point. Has been done. Since the magnification β of this afocal system 20 is | β |> 1, the above (5)
As shown in the formula, in the central portion of the light receiving element Dec, only the wavelength resolution can be improved while maintaining the spatial resolution. Also,
As shown in the above equation (9), only the wavelength resolution can be improved while maintaining the spatial resolution even in the peripheral portion of the light receiving element Dec. Therefore, since the wavelength resolution can be improved without changing the number of pixels of the light receiving element Dec, there is no fear that the light amount becomes insufficient.

In this embodiment, since the afocal system 20 has a magnifying power, the size of one pixel of the light receiving element Dec is reduced in order to keep the spatial resolution constant. Here, if the focal length of the condenser lens L _C is extended by the angular magnification of the afocal system 20, it is not necessary to change the size of one pixel of the light receiving element Dec. As the afocal system 20, in order from the observation point side,
A so-called Kepler type optical system having a positive / positive refractive power arrangement or a so-called Galileo type optical system having a negative / positive refractive power arrangement may be used.

The structure of the Fourier transform spectroscope according to the present invention can be applied to, for example, a film thickness measuring instrument or a Fourier transform spectroscopic microscope.

[0036]

As described above, according to the present invention, it is possible to provide a Fourier transform spectroscope capable of obtaining a higher wavelength resolution while maintaining the spatial resolution.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a configuration of an example according to the present invention.

FIG. 2 is a diagram illustrating the operation of the present invention.

FIG. 3 is a diagram illustrating the operation of the present invention.

FIG. 4 is a diagram showing a configuration of a conventional Fourier transform spectrometer.

[Explanation of symbols]

10 ... Fourier transform spectrometer, 20 ... afocal system, M ₁ ... fixed mirror, M ₂ ... movable mirror, BS ... beam splitter, L _C ... condenser lens, Dec ... light-

Claims (1)

[Claims] 1. A light splitting means for splitting an incident light from a measurement object into a reference light path and a measurement light path, and light reflected from the light splitting means fixed to the reference light path toward the light splitting means. A fixed reflecting mirror, a movable reflecting mirror that is movably provided in the measurement light path and reflects light from the light splitting means toward the light splitting means, and a reference light path through the light splitting means. In a Fourier transform spectroscope having light receiving means for receiving light and light from the measurement optical path through the light splitting means, in the optical path between the measurement target and the light splitting means, a magnifying power is set. A Fourier transform spectroscope, in which an afocal optical system is disposed.