Laser spectroscopy system

Details Laser spectroscopy system Laser spectroscopy system

2000-07-06

2002-11-19

Dang, Hung Xuan (Department: 2873)

Optics: measuring and testing

For light transmission or absorption

Of fluent material

C356S436000

Reexamination Certificate

active

06483589

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a laser spectroscopy system, particularly to a spectroscopy system using a tunable laser diode as the optical source for analyzing a very small amount of ingredients included in a gas through infrared spectroscopy.
BACKGROUND OF THE INVENTION
Conventionally, an infrared spectroscopy system is widely used as an analyzer for analyzing ingredients in the gaseous sample, for example analyzing impurities included in a gas sample. The conventional infrared spectroscopy is the technique to measure an absorption spectrum by transmitting an infrared beam into the sample gas and to analyze this absorption spectrum, so that it is possible to identify the molecules (impurities) to be measured in the sample gas from the wavelength of the absorbed beam in the absorption spectrum and to determine the amount of the molecules from the amount of absorbed beam in the absorption spectrum. Particularly, since it is known that the conventional spectroscopy which uses a near-infrared laser diode as the optical source has high sensitivity and precision, the conventional spectroscopy is used for determining the small amount of water molecules in the semiconductor material gases manufactured or used in the field of semiconductor industry and the related materials industry, or diagnosing diseases by analyzing the stable isotopes in the patients' exhalation.
FIG. 3
is a diagram which shows a general configuration of a conventional spectroscopy system using a laser diode as the optical source. The spectroscopy system shown in
FIG. 3
includes an optical system which has a tunable laser diode source
10
for generating a laser beam for measurement, a sample cell
11
where a sample gas is introduced, the first photo detector
12
for measuring an intensity of a laser beam transmitted through the sample cell
11
, two beam splitters
13
and
14
for splitting a portion of the laser beam from the laser source
10
, the second photo detector
15
for measuring an intensity of a laser beam splitted (reflected) by the first beam splitter
13
, a reference cell
16
where an object to be measured is introduced under depressurized condition, and the third photo detector
17
for measuring an intensity of a laser beam splitted (reflected) by the second beam splitter
14
and transmitted through the reference cell
16
. Generally, this optical system is contained in a purge box
18
. Further, the laser source
10
has driving means
10
a
and
10
b
for controlling driving current and operating temperature. The photo detectors
12
,
15
and
17
respectively have pre-amplifiers
20
for converting the detected amount of laser beams into electrical signals, amplifying the signals and outputting them to lock-in amplifier
19
.
According to the conventional laser spectroscopy system, the object gas to be measured is introduced in the reference cell
16
at a predetermined pressure, for example about 100 Torr and the sample gas flows through the sample cell
11
at a predetermined pressure, for example about 100 Torr. Under this condition, a laser beam of a predetermined wavelength is generated by the laser source
10
via the driving means
10
a
and
10
b
under the control of the control means
21
, such as a personal computer. The amount of detected laser beams by the respective photo detectors
12
,
15
and
17
are inputted to the control means
21
through the lock-in amplifier
19
, and the amount of ingredients to be measured in the sample gas is acquired by calculations. The laser beam from the laser source
10
is irradiated as dispersion is removed by adjusting the diameter of the beam while passing the lens
22
or slit, pinhole or the like.
FIG. 4
is a diagram of an example of second derivative absorption spectra for measuring concentration of water molecules in hydrogen chloride by using the conventional laser spectroscopy system. The uppermost second derivative absorption spectrum X shows an absorption intensity of the laser beam detected by the first photo detector
12
, wherein the laser beam is transmitted through the beam splitters
13
and
14
and the sample cell
11
. The middle second derivative absorption spectrum Y shows an absorption intensity of the laser beam reflected by the beam splitter
13
and detected by the second photo detector
15
. The lowermost second derivative absorption spectrum Z is acquired by subtracting the absorption intensity detected by the second photo detector
15
from the absorption intensity detected by the first photo detector
12
, and is an absorption intensity of the water molecules in the sample gas flowing through the sample cell
11
. According to what is described above, it is possible to cancel the absorption intensity of the beam other than that in the sample cell
11
line and to acquire only the absorption intensity of the water molecules in the sample gas in the sample cell
11
by subtracting the absorption intensity detected by the second photo detector
15
of the cancel line from the absorption intensity detected by the first photo detector
12
of the so called sample line. Therefore, it is possible to calculate the concentration of the water molecules in the hydrogen chloride by reading values of peak and valleys of the second derivative absorption spectrum Z.
In the real measurement, however, since it is rare to get such a clear second derivative absorption spectrum as shown in FIG.
4
and there is an undulation called “fringe noise” in the ordinary second derivative absorption spectrum, it is very difficult to measure a very small amount of ingredient with high precision. For example,
FIG. 5
is a diagram of an example of second derivative absorption spectra of a refined and dehydrated hydrogen chloride flowing through the sample cell
11
. As before, the lowermost second derivative absorption spectrum Z is acquired by subtracting the middle second derivative absorption spectrum Y from the uppermost second derivative absorption spectrum X. As shown in
FIG. 5
, though there is no water molecule in the sample gas, there is a large undulation by fringe noise in the second derivative absorption spectrum Z, so that there is a peak at the wavelength of water molecule's line.
This fringe noise is generated when the laser beam is transmitted or reflected through/by the inside wall and windows of the sample cell
11
and/or the beam splitters
13
and
14
. When this fringe noise is generated, the measuring precision is deteriorated because a large distortion is generated in the valley area. For example, as shown in
FIG. 6
, if the fringe noise becomes larger, the peak P of water molecules, which originally would be represented as the upper spectrum of
FIG. 6
, is buried by the fringe noise Q, so that the measurement becomes difficult. Further, when other ingredient, such as carbon dioxide or hydrogen bromide in case of water molecule, of which the absorption wavelength is similar to that of the water molecule, exists, the peak R of the hydrogen bromide is located near the peak P of the water molecule, and it becomes difficult to distinguish the peaks and to perform precise measurement. These above described problems become much more serious particularly when a very small amount of impurities in a highly purified gas is analyzed.
Therefore, when analyzing water molecules, a 100% of water moisture is installed in the reference cell
16
with a prescribed pressure and the absorption wavelength of water molecule is identified by detecting the laser beam transmitted through the reference cell
16
by the third photo detector
17
. In other words, even when the peak of the second derivative absorption spectrum Z is as small as the fringe noise, it is possible to clearly grasp the peak of absorption spectrum of the laser beam transmitted through the sample cell
11
by referencing the peak of the laser beam transmitted through the reference cell
16
. As a result, it is possible to measure the amount of the water molecules with high precision. Further, by providing the reference cel

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