Optics: measuring and testing – By dispersed light spectroscopy – With monochromator structure
Reexamination Certificate
2001-07-17
2004-01-13
Chang, Audrey (Department: 2872)
Optics: measuring and testing
By dispersed light spectroscopy
With monochromator structure
C356S331000, C356S332000
Reexamination Certificate
active
06678044
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a monochromator capable of picking up light of a desired wavelength in high precision without being influenced by changes in ambient temperature. The invention also relates to an optical spectrum analyzer equipped with the monochromator.
2. Description of the Related Art
FIG. 3
shows an example of the conventional optical spectrum analyzer. The optical spectrum analyzer generally indicated at
100
comprises a monochromator
110
of the Czerny-Turner dispersion type, an analysis and control section
120
which not only controls the monochromator
110
but also analyzes the result of spectroscopy with the monochromator
110
, and a display section
130
for displaying the result of measurement.
The monochromator
110
basically comprises an entrance slit
111
through which the incident light is passed from a light source
200
, a concave mirror
112
by which the light passing through the entrance slit
111
is converted to parallel light and reflected, a plane diffraction grating
113
for diffracting the parallel light from the concave mirror
112
, a concave mirror
114
for reflecting and condensing part of the diffracted light from the plane diffraction grating
113
, an exit slit
115
through which the reflected light from the concave mirror
114
passes to emerge, and a photo detector
116
which measures the intensity of the emerging light from the exit slit
115
and which outputs it to the analysis and control section
120
.
Thus, the monochromator
110
condenses that part of the diffracted light from the plane diffraction grating
113
which is directed toward the concave mirror
114
and then measures the intensity of that light.
The plane diffraction grating
113
is driven to rotate by a motor
113
a
so that it alters the wavelength of the light being diffracted toward the concave mirror
114
. The motor
113
a
is controlled by the analysis and control section
120
. Thus, the analysis and control section
120
controls the angle of the plane diffraction grating
113
via the motor
113
a
, thereby controlling the wavelength of the light being received by the photo detector
116
. In other words, the analysis and control section
120
measures the intensity of light of a specified wavelength by setting the angle of the plane diffraction grating
113
to a specified value.
All members of the monochromator
110
including the substrate will expand and shrink thermally. Hence, the position of the concave mirror
114
relative to the plane diffraction grating
113
changes subtly with the ambient temperature and there has been the potential for the failure to separate light of a specified wavelength even if the angle of the plane diffraction grating
113
is set to a specified value.
SUMMARY OF THE INVENTION
An object, therefore, of the present invention is to ensure that light of a specified wavelength is separated in high precision despite changes in the ambient temperature.
To achieve the above object of the invention, there is provided a monochromator (
10
) comprising a diffraction grating (e.g. plane diffraction grating
13
) for diffracting incident light and a selection means (e.g. concave mirror
14
) for selecting light of a desired wavelength from the light diffracted by said diffraction grating, characterized in that said diffraction grating is formed of a material selected such that a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of members other than said diffraction grating is cancelled out or reduced by a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of said diffraction grating.
The diffraction grating also expands and shrinks thermally as temperature changes. Upon expansion or shrinking, the grating constant of the diffraction grating changes and so does the diffraction angle of light of the same wavelength. Hence, thermal expansion or shrinkage of the diffraction grating causes variations in the light selected by the selection means. The amount or direction of such variations can be adjusted by altering the constituent material of the diffraction grating.
Hence, by choosing an appropriate constituent material for the diffraction grating a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of members other than the diffraction grating can be cancelled out or reduced by a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of the diffraction grating. As a result, there is provided a monochromator that can separate light of a specified wavelength in high precision despite changes in ambient temperature.
According to the present invention, there is provided a monochromator (
10
) comprising a diffraction grating (e.g. plane diffraction grating
13
) for diffracting incident light and a selection means (e.g. concave mirror
14
) for selecting light of a desired wavelength from the light diffracted by said diffraction grating, characterized in that said diffraction grating is formed of a material selected such that a variation in the wavelength of the selected light that occurs under the same selection conditions on account of a change in the layout of individual members due to thermal expansion or shrinkage of the substrate for fixing the individual members is cancelled out or reduced by a variation in the wavelength of the selected light that occurs under the same selection conditions on account of thermal expansion or shrinkage of said diffraction grating.
According to invention, there is provided a monochromator that relies upon the same principle to separate light of a specified wavelength in high precision despite changes in ambient temperature.
The monochromator may be of the Czerny-Turner dispersion type. The constituent material of the diffraction grating may be PYREX™-brand borosilicate glass.
According to the present invention, there is provided an optical spectrum analyzer (
1
) equipped with the monochromator above mentioned.
According to the invention, light of a specified wavelength is separated and its intensity measured in high precision irrespective of changes in ambient temperature.
REFERENCES:
patent: 5192981 (1993-03-01), Slutter et al.
patent: 5521995 (1996-05-01), Brazas
patent: 6134039 (2000-10-01), Rudeen
Allen Denise S.
Ando Electric Co. Ltd.
Chang Audrey
Fish & Richardson P.C.
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