Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2002-04-23
2004-03-23
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S226000, C250S573000, C356S328000, C356S334000
Reexamination Certificate
active
06710332
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system for detecting rotation angles of a diffracting portion, and more particularly to a system including an apparatus and method for detecting rotation angles of a diffracting portion using an absorption cell sealed with a plurality of gases of different kinds having mutually different absorption line wavelengths, as the technology for achieving a correct correspondence of relation between the angle of an optical element of the diffracting portion to be detected in the apparatus for detecting rotation angles of a diffracting portion used in an optical appliance, and the wavelength of the light to be diffracted in a predetermined direction by the diffracting portion.
2. Description of the Related Art
Hitherto, in optical appliances such as an optical spectrum analyzer and a tunable wavelength light source, a diffracting portion composed of an optical element including a diffraction grating is used in order to detect the light to be measured or light of desired wavelength from light of wide band.
The diffraction grating is an element for diffracting the incident light in a direction depending on its wavelength by means of multiple grooves disposed parallel at predetermined intervals on its surface.
In the optical appliances such as an optical spectrum analyzer and a tunable wavelength light source, it is designed to vary the wavelength of the light diffracted in a desired direction, by varying the angle of the diffraction grating of the diffracting portion according to the incident light, or varying the angle of a reflector for reflecting the diffracted light from the diffraction grating to the diffraction grating with respect to the diffraction grating.
Therefore, an optical appliance having such a diffracting portion comprises a driving device for rotating the diffraction grating or reflector, and an angle detector for detecting its angle, and the corresponding relation between the angle detected by the angle detector and the wavelength of the light diffracted in a predetermined direction (hereinafter called angle-wavelength relation) is stored preliminarily.
In such optical appliances, for example, when a specific wavelength is designated, the diffraction grating or reflector for composing the diffracting portion is set at an angle corresponding to the designated wavelength so that the light of the designated wavelength may be diffracted in a predetermined direction, or the wavelength of the light diffracted in a predetermined direction is determined from the angle detected by the angle detector.
In this case, however, the characteristic of the optical system including the diffracting portion changes slightly according to environmental conditions, such as ambient temperature, humidity or atmospheric pressure, and this characteristic varies the angle-wavelength relation.
To solve this problem, the present applicants etc. disclosed an apparatus for detecting rotation angles of a diffracting portion intended to determine the angle-wavelength relation by using an absorption cell extremely small in change in wavelength due to changes in environmental conditions in U.S. Pat. No. 5,828,061.
The absorption cell used in this apparatus for detecting rotation angles of a diffracting portion is sealed with a single specific gas, and it absorbs the light of a wavelength (absorption line wavelength) determined by this gas from the incident light, and emits the light of the absorption wavelength.
This absorption line wavelength is a known value determined by the kind of the single gas sealed inside, and is hardly changed by changes in environmental conditions such as ambient temperature, humidity or atmospheric pressure, and the precision of the absorption line wavelength is extremely high.
Accordingly, this apparatus for detecting rotation angles of a diffracting portion is capable of determining accurately the angle-wavelength relation of the diffraction grating in the present environments by determining the angle detected by the angle detector when the diffraction grating diffracts the light of this absorption line wavelength in a predetermined direction.
In this case, the absorption lines of the specific single gas sealed in the absorption cell are concentrated in a relatively narrow wavelength region.
Accordingly, this apparatus for detecting rotation angles of a diffracting portion can accurately determine the angle-wavelength relation near the wavelength region, but cannot accurately determine the angle-wavelength relation in a wavelength region far apart from this wavelength region.
Therefore, in optical appliances required to vary the wavelength of measuring light or output light in a wide range such as the optical spectrum analyzer or the tunable wavelength light source, high wavelength precision cannot be maintained in a full wide range.
To solve this problem, simply, it may be considered to determine each specific rotation angle by selectively changing over a plurality of absorption cells sealed with a single gas of different kinds, but in such a configuration the rotation angle between specific rotation angles cannot be determined, and the structure of the entire apparatus becomes complicated.
BRIEF SUMMARY OF THE INVENTION
It is hence an object of the invention to solve these problems and present an apparatus and method for detecting rotation angles of a diffracting portion capable of accurately determining the angle-wavelength relation in a wide wavelength region in a simple configuration.
In order to achieve the above-described object, according to a first aspect of the present invention, there is provided an apparatus for detecting rotation angles of a diffracting portion comprising:
a diffracting portion which is composed of optical elements including a predetermined optical element and diffracts light;
a drive unit which rotates the predetermined optical element of the diffracting portion;
an angle detector which detects the rotation angle of the predetermined optical element;
a reference wavelength light source which includes a light source and an absorption cell and emits the reference light of a specific wavelength determined depending on the absorption cell toward the diffracting portion;
a reference photodetector which receives a diffracted light emitted in a predetermined direction from the diffracting portion receiving the reference light from the reference wavelength light source, and converts the diffracted light into an electric signal; and
a signal processor which receives a rotation angle to be issued by the angle detector when detecting a predetermined value of the electric signal from the reference photodetector by rotating the predetermined optical element by the drive unit, and determines the predetermined rotation angle corresponding to the specific wavelength,
wherein the absorption cell is sealed with a plurality of gases of different kinds having mutually different in the wavelength region in which absorption lines exist, and
the signal processor is designed to determine plural specific rotation angles corresponding to the specific wavelength determined depending on the plurality of gases of different kinds sealed in the absorption cell, and also determine the rotation angle between plural specific rotation angles by predetermined complementary calculation including linear complement.
In order to achieve the above-described object, according to a second aspect of the present invention, there is provided the apparatus for detecting rotation angles of a diffracting portion according to the first aspect, wherein the plurality of gases of different kinds sealed in the absorption cell are at least two gases selected from carbon dioxide (CO
2
) of which absorption line wavelengths are concentrated around 1.43 &mgr;m, acetylene gas (C
2
H
2
) of which absorption line wavelengths are concentrated around 1.53 &mgr;m, and methane gas (CH
4
) of which absorption line wavelengths are concentrated around 1.65 &mgr;m.
In order to achieve the above-described object, according
Joukawa Hideya
Kakimoto Tatsuki
Anritsu Corporation
Frishauf Holtz Goodman & Chick P.C.
Lee Patrick J.
Porta David
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