Optics: measuring and testing – By dispersed light spectroscopy – With monochromator structure
Reexamination Certificate
2001-04-10
2003-11-11
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
With monochromator structure
Reexamination Certificate
active
06646739
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a four-stage type monochromator whose wavelength resolving power is improved with a small number of component parts and which has a high dynamic range.
2. Description of the Related Art
FIG. 6
shows an example of a conventional four-stage type monochromator. Reference numerals
24
a
to
24
d
denote monochromator units, and a single pass monochromator unit is formed by an entrance slit plate, a plane diffraction grating, an exit slit plate, and two concave mirrors. The incident light from a light source
1
is made incident through a slit in an entrance slit plate
25
a,
is converted to parallel light by a concave mirror
26
a,
and is made incident on a plane diffraction grating
27
a.
Of the incident light, a specific wavelength component, which is determined by an angle of rotation of the plane diffraction grating
27
a
having an axis parallel to the grating as its axis of rotation, is made emergent toward a concave mirror
26
b
as emergent light.
The reflected light from the concave mirror mirror
26
b
is made emergent through a slit in an exit slip plate
28
a.
The emergent light from the slit in the exit slit plate
28
a
is made incident on the double pass monochromator unit through a slit in an entrance slit plate
25
b,
is reflected by a concave mirror
26
c
toward a plane diffraction grating
27
b,
and is used as second diffracted emergent light. In the same way as the monochromator unit
24
a,
the plane diffraction grating
27
b
reflects the light at an angle which differs depending on the wavelength of the incident light, and a specific wavelength component, which is determined by the angle of rotation of the plane diffraction grating
27
b
having an axis parallel to the grating as its axis of rotation, is made emergent toward a concave mirror
26
d
as emergent light.
The reflected light from the concave mirror
26
d
is made emergent through a slit in an exit slit plate
28
b,
and is used as third-stage incident light. In the triple pass monochromator unit as well, diffraction similar to the second stage occurs, and its emergent light is used as incident light for fourth-stage incident light. In the fourth stage as well, diffraction occurs in a similar manner, and reflected light from a concave mirror
26
h
is made emergent from a slit in an exit slit plate
28
d.
In such a configuration, if the angles of the plane diffraction gratings
27
a
to
27
d
are set such that the wavelengths passing through the respective monochromator units
24
a
to
24
d
become equal, of the incident light from the light source
1
, only a specific wavelength component can be led out through the slit in the exit slit plate
28
d
of the monochromator unit
24
d.
In such a configuration, the deviation of the angle of reflection at the wavelength occurring in the first diffraction becomes the deviation of the second incident light, and angular dispersion which is indicated by the angle of deviation of the angle of reflection per unit wavelength is added. As the angular dispersion is similarly added in the third and fourth diffraction as well, the amount of movement of an image per unit wavelength in the fourth-stage slit becomes large, so that it is possible to improve the wavelength resolving power at a position where the wavelength dropped by a predetermined level (3 dB) from a peak value of a mean wavelength &lgr;. In addition, the exit slit plates are provided for the respective monochromator units
24
a
to
24
d,
and since passing wavelengths are selected, it is possible to obtain a high dynamic range.
FIG. 7
shows another example of a conventional quadruple pass monochromator. This quadruple pass monochromator is comprised of an entrance slitplate, a plane diffraction grating, a concave mirror, and a plane mirror. The incident light from the light source is made incident through a slit in an entrance slit plate
29
, is converted to parallel light by a concave mirror
30
, and is made incident on a plane diffraction grating
31
. Of the incident light, a specific wavelength component, which is determined by an angle of rotation of the plane diffraction grating
31
having an axis parallel to the grating as its axis of rotation, is made emergent toward a plane mirror
32
as emergent light. The emergent light is reflected by the plane mirror
32
, and is made incident again on the plane diffraction grating
31
. The second diffracted light from the plane diffraction grating
31
is reflected again by the concave mirror
30
, and is made incident on a returning reflecting means
33
.
The returning reflecting means in this example is formed by two plane mirrors and an intermediate slit plate, and emergent light from the returning reflecting means, from which only a specific wavelength component has been led out through a slit in an intermediate slit plate
36
, is made incident on the concave mirror
30
. The reflected light from the returning reflecting means is converted to parallel light by the concave mirror
30
, and is made incident on the plane diffraction grating
31
as third incident light.
The third diffracted light from the plane diffraction grating
31
is reflected by the plane mirror
32
, is made incident again on the plane diffraction grating
31
so as to be reflected as fourth diffracted light. The fourth diffracted light is reflected again by the concave mirror
30
, and is focused onto a final slit plate
37
so as to select a passing wavelength.
In this example, since the wavelength of the emergent light is selected by causing the light to pass through two slits including the intermediate slit and the exit slit, it is possible to obtain a high dynamic range. However, although the angular dispersion which is indicated by the deviation of the angle of reflection per unit wavelength is added in the first and second diffraction, the angular dispersion is subtracted in the third and fourth diffraction in which the reflected light returning from the returning reflecting means is used as incident light. In the final slit, the dispersed light selected by the intermediate slit is recombined.
With the above-described first conventional example of the quadruple pass monochromator, four sets each comprised of two concave mirrors, two slit plates, and one plane diffraction grating which make up each monochromator unit are required, so that the number of component parts increases, and the size of the apparatus becomes large. In addition, since all the wavelengths of light transmitted through the monochromator units
24
a
to
24
d
must completely match, the plane diffraction gratings
27
a
to
27
d
must be rotated to such angles at which the light with the same wavelength is transmitted, a complicated synchronizing mechanism is necessary.
In addition, with the above-described second example of the quadruple pass monochromator, the deviation of the angle of reflection at the wavelength occurring in the first diffraction becomes the deviation of the second incident light, and the angular dispersion which is indicated by the angle of deviation of the angle of reflection per unit wavelength is added. However, in the third and fourth diffraction in which the reflected light returning from the returning reflecting means is used as incident light, since the deviation of the angle at the wavelength of the incident light is reversed, the angular dispersion of the emergent light from the diffraction grating is subtracted. In the final slit, the dispersed light selected by the intermediate slit is recombined, and in this aspect there has been a limit to the wavelength resolving power.
SUMMARY OF THE INVENTION
An object of the invention is to provide a four-stage type monochromator whose wavelength resolving power is improved with a small number of component parts and which has a high dynamic range.
To overcome the above-described problems, in accordance with the invention there is provided a four-stage type monochromator including a concave mirror for converting incident li
Ando Electric Co. Ltd.
Nguyen Tu T.
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