Optics: measuring and testing – For light transmission or absorption – Of fluent material
Reexamination Certificate
1999-11-12
2003-02-11
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
Of fluent material
Reexamination Certificate
active
06519039
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and device for analyzing trace components present in a gas with high sensitivity and high accuracy by means of spectroscopic analysis using a diode laser as the light source. More particularly, the present invention is designed to facilitate optimization of the conditions for measurement.
BACKGROUND ART
A spectroscopic analysis method in which the degree of light absorption by a gas is measured using a diode laser as the light source is widely used as a method for analyzing trace impurities in gas, since it offers good measurement accuracy and sensitivity.
FIG. 10
is a schematic structural diagram showing an example of a conventional gas spectroscopic analysis device. In this device, the laser light oscillated from diode laser
11
, the light source, is collimated at collimating lens system
12
, and then split into three lines, i.e., first through third lines, by two beam splitters
13
,
13
. The laser light in the first line is projected on measuring gas-cell
14
. The intensity of the outgoing transmitted light which has passed through measuring gas-cell
14
is then detected at first light detector
15
. The laser light in the second line is projected on reference gas-cell
16
. The intensity of the outgoing transmitted light which has passed through reference gas-cell
16
is then detected at second light detector
17
. The intensity of the laser light in the third line is detected at third light detector
18
.
A sample gas supply system
23
is provided to measuring gas-cell
14
, by means of which the sample gas is introduced into cell
14
at a suitable reduced pressure and constant flow rate. The impurities to be measured that are included in the sample gas are supplied to reference gas-cell
16
, and the absorption peaks due to these impurities are detected.
An InGaAsP, InGaAs, GaInAsSb, GaInSbP, AlInSb, AlInAs, AlGaSb or the like may be suitably employed for diode laser
11
. Diode laser
11
is not limited to these however. In addition, a tunable diode laser which can oscillate laser light of a wavelength suitable for analysis may be used.
A device having a sensitivity to the oscillation wavelength band of diode laser
11
, the light source, is employed for the first through third light detectors
15
,
17
,
18
. A light sensor such as a Ge photo diode may be employed, for example. The respective outputs from these first through third light detectors
15
,
17
,
18
undergo signal processing at first through third lock-in amplifiers
19
,
20
,
21
, are relayed to computer
22
, and then subject to data processing as necessary.
A temperature controller
24
for controlling the temperature of the laser element, an LD driver
25
for supplying current to and driving laser
11
, and a function generator
26
for serving as a frequency modulating device for modulating the oscillation frequency of laser
11
based on a frequency modulation method, are provided to diode laser
11
. Temperature controller
24
, LD driver
25
, and function generator
26
are connected to computer
22
. By adjusting the temperature of the laser element using temperature controller
24
, the oscillation wavelength of laser
11
is changed to approach the central wavelength of the absorption peak for the impurities being measured, after which the temperature of the laser element is maintained at a constant value. In addition, by continuously changing the injection current (direct current component) to laser
11
, the oscillation wavelength of laser
11
is continuously changed. In addition, by introducing a modulation signal (alternating current component) to LD driver
25
that is based on the frequency modulating method from function generator
26
, and superimposing this modulation signal (alternating current component) on the injection current (direct current component) to laser
11
, frequency modulation can be applied directly to the laser light oscillated from laser
11
.
The phrase “oscillation wavelength of laser
11
” as used in this specification means the wavelength which is not in a frequency modulated state, i.e., the central wavelength.
In this example, frequency modulation is applied to the laser light, and only the twice component of the modulated frequency is extracted using first through third lock-in amplifiers
19
,
20
, and
21
. Specific data processing is then performed by computer
22
to obtain the second derivative spectrum. It is known that good measurement sensitivity can be obtained by this method (Japanese Patent Application, First Publication No. Hei 5-99845). In addition, it is known that the peak intensity of the second derivative spectrum that is obtained can be increased by placing the sample gas in a reduced pressure state (International Publication Number WO 95/26497).
In the aforementioned frequency modulating method, the current i introduced into the diode laser can be expressed as
i=I
0
+a
·sin(&ohgr;
t
)
Here, I
0
is the direct current component, a·sin(&ohgr;t) is the alternating current component (modulation signal). &agr; is the modulation amplitude (amplitude of the modulation signal), and &ohgr; is the modulation angular frequency. As a result of this type of frequency modulation, the frequency (wavelength) of the laser light varies cyclically at a fixed amplitude around the central frequency (central wavelength) when there is no modulation. The amplitude by which the frequency (wavelength) of the laser light varies becomes greater as modulation amplitude &agr; becomes bigger. The cycle by which the frequency (wavelength) varies is determined based on the frequency of the modulation signal (modulation frequency).
If the modulation amplitude of the laser light is made large in the measurement, then the spectrum width becomes bigger. However, the variation in output power also becomes greater, so that there is an increase in noise as a result.
FIG. 11
shows an example of the second derivative spectrum obtained using a gas spectroscopic analysis method employing this type of device. In this figure, the oscillation wavelength is shown on the horizontal axis, while the second derivative value (optional units) of the light absorption intensity is shown on the vertical axis. The average of the respective differences between peak value P in the second derivative spectrum and minimum values A and B on the left and right hand of peak value P, i.e., differences I
SL
and I
SR
(I
SL
=P−A, I
SR
=P−B), is the absorption intensity (absorption intensity =(I
SL
+I
SR
)/2). The ratio of the absorption intensity and the standard deviation of the background noise (indicated by n in the figure) is the S/N ratio. The symbol Win the figure indicates the wavelength interval between the minimum values on the left and right hand of the peak.
The modulation amplitude of the laser light and the measurement pressure effect the S/N ratio in this type of spectroscopic analysis method. Accordingly, it is necessary to optimize these conditions in order to perform highly sensitive measurements. In order to optimize the modulation amplitude and measurement pressure, it has been the practice to employ a method in which a highly pure base gas and a sample gas containing a trace component which is to be measured in the base gas are each measured while gradually varying the measurement pressure and the modulation amplitude, and the modulation amplitude and measurement pressure at which the S/N ratio is maximal are obtained. The optimization of the modulation amplitude and the measurement pressure must be carried out each time there is a change in the sample gas. Accordingly, this requires much effort and time, and has been a cause of increased cost.
The present invention was conceived in view of the above-described circumstances, and is intended to facilitate the optimization of measurement conditions in a method for analyzing trace impurities in a sample gas by obtaining the second derivative spectrum of the light absorption intensity by passing frequency modulated diode
Ishihara Yoshio
Morishita Jun-ichi
Wu Shang-Qian
Nippon Sanso Corporation
Rosenberger Richard A.
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