Optical: systems and elements – Light interference – Electrically or mechanically variable
Reexamination Certificate
2001-02-05
2003-07-08
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Light interference
Electrically or mechanically variable
C359S578000, C359S586000, C359S589000, C356S454000, C356S519000
Reexamination Certificate
active
06590710
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a Fabry-Perot filter device used as a wavelength selective filter for transmitting rays of light, such as infrared radiation, in a wavelength selective manner; a wavelength selective infrared detector comprising the Fabry-Perot filter device and an infrared detector; and a non-dispersive infrared gas analyzer for measuring concentration of a gas or gases by detecting infrared radiation transmitted through the Fabry-Perot filter device.
2. Description of the Prior Art
A non-dispersive infrared gas analyzer, hereinafter called “NDIR gas analyzer”, is used to analyze a gas, wherein the analyzer detects the amount of infrared absorption to measure the concentration of the gas,utilizing the principle that the wavelength bands of infrared radiation to be absorbed depends on the type of gas.
FIGS. 1
to
5
show examples of prior art NDIR gas analyzers. In the following, the gas being measured is assumed to be carbon dioxide whose peak wavelength of infrared radiation absorption is approximately 4.25 &mgr;m.
FIG. 1
shows a single ray, one wavelength NDIR gas analyzer comprising a sample cell
110
, whereinto a gas is supplied; a light source
111
; a filter
112
; and an infrared detector
113
. In this example, filter
112
is tuned to the absorption bands of carbon dioxide shown in
FIG. 6
to select and pass a band of infrared radiation having wavelengths in the vicinity of 4.25 &mgr;m. Infrared detector
113
determines the concentration of the gas being measured by detecting infrared radiation transmitted through filter
112
.
FIG. 2
shows a single ray two wavelength comparative NDIR gas analyzer, wherein the analyzer selects two wavelength bands using filter
112
, tuned to the absorption band of carbon dioxide, and a filter
114
for transmitting an infrared radiation band of wavelengths in the vicinity of approximately 3.9 &mgr;m, as a reference light. The bands of infrared radiation thus selected are detected respectively with infrared detectors
113
and
115
. In this example, it is possible to correct a change in the output signal with time due to, for example, deterioration in light source
111
or contamination in sample cell
110
by comparing the carbon dioxide absorption band with the measured absorption band of the reference light.
FIG. 3
shows another single ray two wavelength comparative NDIR gas analyzer, wherein the analyzer selects two wavelength bands using filter
112
formed on a disc
116
and tuned to the absorption band of carbon dioxide and the filter
114
for reference light. The bands of infrared radiation selected by the filters as disc
116
is rotated are then detected with infrared detector
113
. In this example, it is also possible to correct a change in the output signal with time due to, for example, deterioration in light source
111
or contamination in sample cell
110
, by comparing the carbon dioxide absorption band with the measured absorption band of the reference light.
FIG. 4
shows a single ray two wavelength Fabry-Perot NDIR gas analyzer, wherein a gap between two parallel mirrors, comprising a Fabry-Perot filter
117
, is made variable so that two bands are selected, one being a band tuned to the absorption band of the gas being measured and the other being a band of reference light. The bands of infrared radiation thus selected are respectively detected with infrared detector
113
. In this example, it is also possible to correct a change in the output signal with time due to, for example, deterioration in light source
111
or contamination in sample cell
110
, by comparing the carbon dioxide absorption band with the measured absorption band of the reference light.
As shown in
FIG. 8
, the Fabry-Perot filter is an optical filter using a device (e.g. Fabry-Perot plates) wherein a pair of high reflectance mirrors
120
and
121
are placed in parallel and opposite to each other with a gap formed therebetween. Assuming that the width (i.e. distance between) of the gap in the Fabry-Perot filter is “d” and the refractive index within the gap is “n”, then the rays, among the rays of incident light, that satisfy the phase relationship represented by below equation (1) will enhance each other by interference, thus becoming transmitted light. In equation (1), &dgr; is the phase reference, &phgr; is the angle of light incident on the device, and &lgr; is the wavelength of the light.
FIG. 9
shows how the rays are transmitted.
&dgr;=4¶
nd
cos&phgr;/&lgr; (1)
The Fabry-Perot filter is designed so that the wavelength bands of the transmitted light passing through the device can be varied by making the gap width “d” variable. An example of a variable wavelength Fabry-Perot filter comprises a fixed mirror, and a movable mirror arranged opposite to the fixed mirror with a gap formed therebetween, with a fixed electrode on the fixed mirror and a movable electrode on the movable mirror, and means are provided for applying a voltage to the electrodes so that the movable mirror is moved accordingly and the gap is thereby adjusted in width. For example, such a Fabry-Perot filter can be used as a wavelength selective filter for a non-dispersive infrared carbon dioxide sensor and selectively pass the wavelength band of carbon dioxide absorption, of approximately 4.25 &mgr;m, and the wavelength band of reference light, of approximately 3.9 &mgr;m.
FIG. 5
shows a double ray, one wavelength NDIR gas analyzer, wherein two light sources
111
and
118
are arranged so that the optical paths thereof differ from each other within sample cell
110
. In this example, a change in the output signal with time due to, for example, contamination of sample cell
110
is corrected according to the ratio between two output signals of infrared detector
113
provided by a band of infrared radiation that is transmitted through filter
112
tuned to the absorption band of the gas being measured. The concentration of the gas being measured is then determined.
In the foregoing type of NDIR gas analyzer, however, the number of filters must be increased to be able to measure concentrations of a plurality of gases or a plurality of constituents of a gas. Hence, the prior art gas analyzers all have such problems as high cost and large sizes.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to overcome the aforementioned and other problems, deficiencies and disadvantages of the prior art.
Another object is to provide an infrared gas analyzer capable of simultaneously determining concentrations of at least two constituents of a gas being measured by using a Fabry-Perot filter device for selectively transmitting at least three wavelength bands of infrared radiation which includes a reference light.
The foregoing and other objects are attained in the invention which encompasses a Fabry-Perot filter device for passing infrared radiation from a light source in a wavelength selective manner, and comprising a fixed mirror formed on a substrate; a movable mirror arranged opposite to the fixed mirror with a gap formed therebetween so that the movable mirror is displaced with respect to the fixed mirror by applying an external force; a fixed electrode formed on the fixed mirror; and a movable electrode formed on the movable mirror and arranged opposite to the fixed electrode; wherein the movable mirror is displaced by applying a potential difference across the fixed and movable electrodes so that the width of the gap is varied in at least three steps, whereby at least three wavelength bands of the infrared radiation are selectively transmitted through the filter device.
A feature of the invention is the fixed mirror and the movable mirror both comprising silicon.
Another feature is the fixed electrode and the movable electrode both comprise silicon with a high impurity concentration.
Another aspect of the invention is an infrared gas analyzer for determining concentration of a gas being measured according to output of an infrared detector, end comprising a light sourc
Hara Hitosh
Iwaoka Hideto
Kishi Naoki
Noro Makoto
Suzuki Kentaro
Lavarias Arnel C.
Nguyen Thong
Yokogawa Electric Corporation
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