U.S. patent application number 11/227477 was filed with the patent office on 2007-02-08 for gas detection method and gas detection device.
Invention is credited to Markus Kohli, Andreas Seifert, Bert Willing.
Application Number | 20070030487 11/227477 |
Document ID | / |
Family ID | 35500599 |
Filed Date | 2007-02-08 |
United States Patent
Application |
20070030487 |
Kind Code |
A1 |
Willing; Bert ; et
al. |
February 8, 2007 |
Gas detection method and gas detection device
Abstract
The gas detector device comprises at least a VCSEL source and at
least a light sensor for detecting a light beam having passed
through a sample chamber containing a given gas to be detected. The
detection signal of the sensor directly provided to or is time
derivated by an electronic derivator and then provided to
respective lock-in amplifiers in order to generate a two different
2f-detection, f being the frequency of a wavelength modulation of
the source, and thus to provide two corresponding measuring signals
the division of which gives a precise value of the gas
concentration. The invention uses at least a first modulation
reference signal at twice and a second modulation reference signal
at twice of the modulation frequency of the laser source. Providing
at least a first 2f modulation reference signal has advantages over
the prior art, because by using such a reference modulation signal
it is possible measure the absolute intensity and therefore to
receive the same result at different temperatures or at mode
hopping of the laser. A further advantage is that the measurement
accuracy is independent from the gas concentration.
Inventors: |
Willing; Bert; (Blonay,
CH) ; Kohli; Markus; (Grandson, CH) ; Seifert;
Andreas; (Denens, CH) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510
US
|
Family ID: |
35500599 |
Appl. No.: |
11/227477 |
Filed: |
September 15, 2005 |
Current U.S.
Class: |
356/437 |
Current CPC
Class: |
G01N 21/3504 20130101;
G01N 21/39 20130101; G01N 2201/0691 20130101; G01J 3/433
20130101 |
Class at
Publication: |
356/437 |
International
Class: |
G01N 21/00 20060101
G01N021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 4, 2005 |
EP |
05 016 948.1 |
Claims
1. Gas detection method comprising the following steps of providing
an initial light signal (S.sub.0), by a wavelength modulated laser
source (1); providing an AC modulation signal at an initial
frequency for wavelength modulation of said initial light signal
(S.sub.0) at said initial frequency (f) symmetrically around an
absorption line of a gas the concentration or presence of which is
to be determined; passing said initial light signal (S.sub.0)
having intensity variations over the time resulting from an
alternative scanning around said gas absorption line through a gas
detection region (4) intended for receiving at least one of said
gases; receiving a resulting light signal (S.sub.G) exciting said
gas detection region (4) by detection means (8), said resulting
light signal (S.sub.G) comprises changes in the intensity of the
initial light signal (S.sub.0) due to the gas concentration in the
detection region (4); generating a first modulation reference
signal (S.sub.2f0) at twice of said initial frequency and then
integrated over time, whereby said first modulation reference
signal (S.sub.2f0) has a 45.degree. phase angle to said initial
light signal (S.sub.0), and oscillates at an amplitude level being
different from the amplitude level of said second modulation
reference signal (S.sub.2f1) between amplitude levels 1 and 0;
generating a second modulation reference signal (S.sub.2f,
S.sub.2f1) at twice of said initial frequency (f) and then
integrated over time, whereby the second modulation reference
signal (S.sub.2f, S.sub.2f1) has a defined amplitude level and a
defined phase relationship with the intensity variations of said
initial light signal (S.sub.0); generating a first measuring signal
(S.sub.MI), which is a function of intensity of said initial light
signal (S.sub.0), said first measuring signal (S.sub.MI) is
generated by multiplying a detection signal (S.sub.D0) directly
received from said resulting light signal (S.sub.G) with said first
modulation reference signal (S.sub.2f0); generating a second
measuring signal (S.sub.MA), which is a function of the gas
absorption and substantially independent of an intensity modulation
of said initial light signal at said initial frequency (f), said
second measuring signal (S.sub.MA) is generated by multiplying a
second detection signal (S.sub.D0, S.sub.D1) received from said
resulting light signal (S.sub.G) with said second modulation
reference signal (S.sub.2f, S.sub.2f1); providing a final measuring
signal being independent from the intensity of light incident onto
the detection means (8) by dividing said second measuring signal
(S.sub.MA) by said first measuring signal (S.sub.MI) and thereby
providing a signal relative to the presence or the concentration of
a given gas.
2. A method according to claim 1, further defined by generating
said second modulation reference signal (S.sub.2f1) at twice of
said initial frequency (f), and oscillating said second modulation
reference signal (S.sub.2f1) between amplitude levels 1 and -1,
whereby said first and second modulation reference signals
(S.sub.2f0, S.sub.2f1) have the same phase correlation to said
initial light signal (S.sub.0); and multiplying said second
detection signal (S.sub.D0) directly received from said resulting
light signal (S.sub.G) with said second modulation reference signal
(S.sub.2f1).
3. A method according to claim 1, further defined generating said
second modulation reference signal (S.sub.2f) at twice of said
initial frequency (f), whereby said second modulation reference
signal (S.sub.2f) is exactly in phase with the intensity variations
of said initial light signal (S.sub.0); generating a second
detection signal (S.sub.D1) being substantially proportional to the
time derivate of said resulting light signal (S.sub.G); and
generating said second measuring signal (S.sub.MA) by multiplying
said detection signal (S.sub.D1) with said second modulation
reference signal (S.sub.2f).
4. A method according to claim 1, further defined generating a
second detection signal (S.sub.D1) being substantially proportional
to the time derivate of said resulting light signal (S.sub.G);
generating a third modulation reference signal (S.sub.f) at the
initial frequency (f) and then integrated over time; whereby the
third modulation reference signal (S.sub.f) is exactly defined in
phase with the intensity variations of said initial light signal
(S.sub.0); generating a third measuring signal (S.sub.MI1) from
said detection signal (S.sub.D1), which is a function of intensity
of said initial light signal (S.sub.0), said third measuring signal
(S.sub.MI1) is generated by multiplying said detection signal
(S.sub.D1) with said third modulation reference signal (S.sub.f);
generating said second modulation reference signal (S.sub.2f) at
twice of said initial frequency (f) and then integrated over time,
whereby the second modulation reference signal (S.sub.2f) is
exactly defined in phase with the intensity variations of said
initial light signal (S.sub.0); generating a second measuring
signal (S.sub.MA) from said detection signal (S.sub.D1), which is a
function of the gas absorption and substantially independent of an
intensity modulation of said initial light signal at said initial
frequency (F), said second measuring signal (S.sub.MA) is generated
by multiplying said second detection signal (S.sub.D1) with said
second modulation reference signal (S.sub.2f).
5. A gas detector device comprising a least one wavelength
modulated laser source (1) providing an initial light signal
(S.sub.0); a detection region (4) intended for receiving at least
one gas the concentration or presence of which is to be determined;
supply control means (13, 15) for wavelength modulating said
initial light signal (S.sub.0) at a initial frequency (f)
symmetrically around an absorption line of one of said gases and
providing said initial light signal (S.sub.0) having intensity
variation over the time, said supply control means comprise DC
supply control means (13) for defining a DC current signal and AC
supply control means (15) for defining an AC current signal at said
given initial frequency (f) for generating an alternative scanning
of light intensity of said initial light signal (S.sub.0) around
said gas absorption line; a light sensor (8) respectively arranged
at the periphery of said detection region (4), said sensor (8) is
intended for receiving a resulting light signal (S.sub.G)
comprising changes in the intensity of the initial light signal
(S.sub.0) having passed through said detection region (4) and
providing a detection signal (S.sub.D0, S.sub.D1) proportional to
the light intensity variation of said resulting light signal
(S.sub.G); processing means (19-26) for providing from said
detection signal (S.sub.D0, S.sub.D1) a signal relative to the
presence or the concentration of a given gas in said detection
region (4), said processing means comprise first generating means
(17, 18) for generating a first modulation reference signal
(S.sub.2f0, S.sub.f) at a defined first frequency and second
generating means (16) for generating a second modulation reference
signal (S.sub.2f1, S.sub.2f) at twice of said initial frequency
(F); first means (20, 21) for multiplying said first modulation
reference signal (S.sub.2f0, S.sub.f) with said detection signal
(S.sub.D0, S.sub.D1) and then integrating over time the resulting
signal in order to provide a first measuring signal (S.sub.MI,
S.sub.MC) which is a function of the intensity of said initial
light signal (S.sub.0) and substantially independent of the
concentration of said gas; second means (19) for multiplying said
second modulation reference signal (S.sub.2f1, S.sub.2f) with said
detection signal (S.sub.D0, S.sub.D1) and then integrating over
time in order to provide a second measuring signal (S.sub.MA) which
is a function of the gas absorption and substantially independent
of an intensity modulation of said initial light signal (S.sub.0)
at said initial frequency (f); a processing unit (22) for dividing
said second measuring signal (S.sub.MA) by the first measuring
signal (S.sub.MI, S.sub.MIC) for providing the signal relative to
the presence of a given gas or to its concentration; whereby said
light sensor (8) is connected to said first means (20) for
multiplying said first modulation reference signal (S.sub.2f0) and
thus providing said resulting light signal (S.sub.G) as detection
signal (S.sub.D0); said first generating means (17) generate a
first modulation reference signal (S.sub.2f0) at twice of said
initial frequency (f) with a 45.degree. phase angle to said initial
light signal (S.sub.0) and oscillate said first modulation
reference signal (S.sub.2f0) between amplitude levels 1 and 0.
6. The gas detector device according claim 5, whereby said second
generating means (16) generate said second modulation reference
signal (S.sub.2f1) at twice of said initial frequency (f) with a
45.degree. phase angle to said initial light signal (S.sub.0) and
oscillate said second modulation reference signal (S.sub.2f1)
between amplitude levels 1 and -1.
7. The gas detector device according claim 5 whereby said
processing means comprise means (25) for providing a detection
signal (S.sub.D1) substantially proportional to the time derivate
of said resulting light signal (S.sub.G); said second generating
means (16) generate a second modulation reference signal (S.sub.2f)
at twice of said initial frequency (f) exactly in phase with the
intensity variations of said initial light signal (S.sub.0); and
wherein said first means (20) for multiplying said first modulation
reference signal (S.sub.2f0) with said detection signal (S.sub.D0,
S.sub.D1) receive said resulting light signal (S.sub.G) as
detection signal (S.sub.D0), and wherein said second means (19) for
multiplying said second modulation reference signal (S.sub.2f) with
said detection signal (S.sub.D0, S.sub.D1) receive the detection
signal (S.sub.D1) substantially proportional to the time derivate
of said resulting light signal (S.sub.G).
8. The gas detector device according claim 7 whereby third means
(18) for generating a third modulation reference signal (S.sub.f)
at said initial frequency (f) exactly in phase with the intensity
variations of said initial light signal (S.sub.0), third means (21)
for multiplying said third modulation reference signal (S.sub.f)
with said detection signal (S.sub.D1), substantially proportional
to the time derivate of said resulting light signal (S.sub.G), and
then integrating over time the resulting signal in order to provide
a third measuring signal (S.sub.MI1) which is a function of the
intensity of said initial light signal (S.sub.0) and substantially
independent of the concentration of said gas, a processing unit
(22, 26) for correlating said first measuring signal (S.sub.MI)
with said third measuring signal (S.sub.MI1) and for dividing said
second measuring signal (S.sub.MA) by the correlated measuring
signal (S.sub.MIC) for providing the signal relative to the
presence of a given gas or to its concentration.
Description
[0001] The present invention concerns in particular low-cost
infrared (IR) gas detection as disclosed in WO 2005/026705 A1.
[0002] The gas detection method and gas detector device as
described in this prior art publication is based on a source formed
by a wavelength modulated Vertical Cavity Surface Emitting Laser
(VCSEL) or Distributed FeedBack (DFB) laser and uses the fact that
the modulation of the wavelength is directly connected to a
modulation of the laser source output intensity. The intensity of
the light having passed the gas volume and being incident to the
detector therefore shows a first modulation related to the laser
source intensity and a second modulation related to the gas
absorption as the wavelength is scanned across the gas absorption
line. Accordingly, the known detection method and device provides
an initial light signal by a wavelength modulated laser source.
[0003] The source provides an initial light signal, which is
wavelength modulated with an AC modulation signal at a given
initial frequency (f) at the absorption line around the gas to be
determined. A light sensor respectively is arranged at the
periphery of a detection region intended for receiving at least a
gas the concentration of which is to be determined. The light
sensor receives a resulting light signal formed by the initial
light signal having passed through the detection region. In the
following a detection signal is formed which is substantially
proportional to the time derivate of the resulting light signal.
Further disclosed are first means for generating a first modulation
reference signal at the given frequency (f) and second means for
generating a second modulation reference signal at twice this
frequency (2f). The detection signal is multiplied by the first
modulation reference signal and then integrated over time in order
to provide a first measuring signal which is a function of the
intensity of said initial light signal and substantially
independent of the concentration of said gas. The detection signal
is further multiplied by said second modulation reference signal
and then integrated over time in order to provide a second
measuring signal which is a function of the gas absorption and
substantially independent of an intensity modulation of the initial
light signal at the given initial frequency. The final measuring
signal is then received by dividing the second measuring signal by
the first measuring signal, thereby providing a signal relative to
the concentration or the presence of a given gas. This gas detector
method and device have the advantage that only a single sensor unit
is needed for one laser source. All necessary information for
determining a precise gas concentration value is given by the
processing of the generated detection signal which is proportional
to the derivate of the light signal received by the sensor unit
after having passed through a sample of the defined gas.
[0004] The first and second reference modulation signal both are in
phase with the intensity variations of the initial light signal.
With this known measurement technique the detector signal is time
derivated, and the derivated signal is fed into a two-channel
lock-in amplifier. The first channel operates on the modulation
frequency f, and the output signal is proportional to the slope of
the optical power as function of the laser current. The second
channel operates of twice the modulation frequency and its output
gives a signal, which is proportional to the gas concentration
encountered by the laser beam. The ratio of the measuring signal at
the frequency 2f to the measuring signal at the frequency f gives
the absolute concentration of the gas independent of the laser
output as the measuring signal at the frequency f contains
information about the laser intensity under the assumption that
variations of the laser intensity stem from optical degradations in
the light path, such as dust, condensation, speckles. This
assumption only holds for two conditions:
[0005] 1. The laser does not show mode hopping, i.e. sudden changes
of wavelength. If such a mode hopping occurs, the wavelength has to
be re-adjusted by a change of the DC laser current, which in turn
changes the laser output power. With a VCSEL the slope, which is
measured by the signal at the frequency f does not necessarily
change accordingly. In the case of a DFB laser, the output power is
strictly proportional to the DC current which gives the same signal
at the frequency f for different output powers.
[0006] 2. The temperature of the laser is exactly stabilized. For a
change of the laser temperature, the wavelength changes, which in
turn leads to a re-adjustment of the DC laser current to stay
centered on the wavelength of the gas absorption line. Such a
change of the current means an intensity change as described in
item 1.
[0007] With the method described in the prior art patent
application, the signal based on a modulation reference signal at
the frequency f shows a slope around the center of the gas
absorption line, which is proportional to the gas concentration. At
high gas concentrations, the accuracy of the measurement is limited
by the accuracy of the DC laser current of which the error
influences the modulation reference signal at the frequency f.
Variations of the current will cause variation of the laser signal,
and this effect increases with concentration. This shows, that for
some applications the prior art method and device is quite
demanding in terms of temperature control of the laser, and depends
very much on the thermal mounting of the latter. DFB lasers and
VCSEL's differ very much in their thermal budget so that the
tracking of the gas absorption line, which is always necessary in
term of DC current, has to include a temperature tracking as
well.
[0008] In view of this, it is the object of the present invention
to provide further possibilities for gas detection, which are less
dependent from the temperature and sudden wavelength changes.
[0009] This problem is solved by the gas detection method and the
detector device as claimed. Further advantageous features are
described in the respective subclaims.
[0010] According to the invention, a first modulation reference
signal at twice of said initial frequency is generated by
respective means, whereby said first modulation reference signal
has a 45.degree. phase angle to said initial light signal. This
first modulation reference signal oscillates at an amplitude level
between amplitude levels 1 and 0 and is different from the
amplitude level of the second modulation reference signal. Finally
the detection signal directly received from the resulting light
signal is multiplied with the first modulation reference
signal.
[0011] Thus, the first modulation reference signal is not measured
on the frequency f, but on the frequency 2f with a slight
modification of the 2f modulation reference signal in the amplitude
levels and a phase shifting of 45.degree. between the first
modulation reference signal and the initial frequency, which is
necessary to provide the same phase which is obtained by a derivate
over time. Further, the detector signal is no longer derivated but
directly fed to the lock-in amplifier for generating a first
measuring signal, which is a function of the intensity of the
initial light signal. The resulting signal is directly proportional
to the light intensity of the laser as seen by the detector without
gas absorption (i.e. including any degradations of the light beam
between laser and detector).
[0012] Providing a first 2f modulation reference signal has
advantages over the prior art, because by using such a reference
modulation signal it is possible to measure the absolute intensity
and therefore to receive the same result at different temperatures
or at mode hopping of the laser. A further advantage is that the
measurement accuracy is independent from the gas concentration.
[0013] According to the invention, it is possible to combine this
first 2f modulation reference signal and its signal treatment with
other treatments is order to obtain stable final measuring signals
dependent on the special application of gas detection. In a further
embodiment of the invention, the second modulation reference signal
is generated at twice of said initial frequency f, whereby the
first and second modulation reference signals have the same phase
correlation to the initial light signal; therefore both signals
have 45.degree. phase angle to the AC modulation signal for the
laser source. Further, the second modulation reference signal
oscillates between amplitude levels 1 and -1. For generating the
second measuring signal the detection signal directly received from
the resulting light signal is multiplied via lock-in amplifier with
said second modulation reference signal. The final measuring signal
is obtained by the above-mentioned ratio. In this embodiment the
final measuring signal is obtained by a first and a second
measuring signal based on a 2f modulation reference signal, both
obtained with a detection signal directly received from the
resulting light signal.
[0014] In a preferred embodiment of the invention the second
modulation reference signal is generated at twice of said initial
frequency f, whereby said second modulation reference signal is
exactly in phase with the intensity variations of said initial
light signal. The detection signal is generated by said detection
means is substantially proportional to the time derivate of said
resulting light signal and the second measuring signal is generated
by multiplying said detection signal with said second modulation
reference signal. This signal treatment shows the best result,
which is independent from the laser temperature and sudden
wavelength changes. In this embodiment also the final measuring
signal is obtained by a first and a second measuring signal based
on a 2f modulation reference signal, but the second measuring
signal, which is a function of the absorption is obtained with a
derivated detection signal.
[0015] In a further embodiment, which needs more electronic parts,
two reference modulation signals at a frequency f and 2f are used
for generating two measuring signals, which are a function of
intensity of the initial light signal. This is realised by
generating, additionally to the first measuring signal based on the
first 2f modulation reference signal, a third measuring signal,
which is also a function of intensity of said initial light signal.
This third measuring signal is generated from a detection signal by
multiplying the detection signal with a third modulation reference
signal at the initial frequency f and then integrated over time.
Further the second measuring signal is generated from said
detection signal, by multiplying said detection signal with a
second 2f modulation reference signal at twice of said initial
frequency f and then integrated over time. The third and second
modulation reference signals are exactly defined in phase with the
intensity variations of said initial light signal and the detection
signal for both measuring signals are substantially proportional to
the time derivate of the resulting light signal. The final
measuring signal is obtained by correlating the first and third
measuring signal and generating the ratio between the second
measuring signal and the correlated signal of the first and second
measuring signal.
[0016] In the following other particular features and advantages of
the present invention will be described by way of non limiting
embodiments with reference to the annexed drawings, in which:
[0017] FIG. 1 shows the intensity of the laser light beam entering
the sample chamber;
[0018] FIG. 2 shows the intensity of the light beam incident on the
detector after gas absorption;
[0019] FIG. 3 shows the AC modulation signal and the 2f reference
modulation signal as a function of time;
[0020] FIG. 4 shows the multiplication of the detection signal
directly proportional to the resulting light signal with the first
modulation reference signal;
[0021] FIG. 5 shows the multiplication of the detection signal
directly proportional to the resulting light signal with the second
modulation reference signal;
[0022] FIG. 6 is a schematic principle view of a first embodiment
of the gas detector device according to the present invention using
only a detection signal directly proportional to the resulting
light signal;
[0023] FIG. 7 is a schematic principle view of a second embodiment
of the gas detector device according to the present invention using
a detection signal directly proportional to the resulting light
signal and a detection signal directly proportional to the derivate
of the resulting light signal; and
[0024] FIG. 8 is a schematic principle view of a third embodiment
of the gas detector device according to the present invention using
a detection signal directly proportional to the resulting light
signal and a detection signal directly proportional to the derivate
of the resulting light signal, thereby providing a first and second
2f modulation reference signal and a third f modulation reference
signal.
[0025] In the following, the signal treatment is described in
detail as far as it differs from the prior art mentioned in WO
2005/026705 A1. The content of this document is incorporated by
reference as far as signal treatment is concerned, which might not
be described in this description.
[0026] As described previously and already mentioned in WO
2005/026705 A1, the laser source is operated with a DC current so
that its wavelength corresponds exactly to the center of the gas
absorption line. This current is constantly modulated at a
frequency f and amplitude such that the wavelength of the laser
scans the gas absorption line completely during each cycle by a
respective AC modulation signal. FIG. 1 shows the laser output
reflected in the initial light signal S.sub.0 as a function of time
which receives a detection region with the gas to be determined,
and FIG. 2 shows the light intensity as a function of time which is
incident on the detector in the presence of a given gas
concentration, and to which the detector signal S.sub.G is
proportional. The waveform of the modulation is chosen here to be
triangular; however, the waveform is not of importance to the
measurement technique and a sinoidal modulation is actually easier
to handle electronically.
[0027] The FIGS. 6 to 8 show three embodiments of a gas detector
device of the invention. The common parts of these embodiments are
a laser source 1 (it can be also more laser sources and respective
sensors) arranged in a laser head of a housing 6. This head further
might comprise a sealed cell filled with at least one gas for
precisely determined the electrical current value to be furnished
the source 1 so that the central wavelength of the provided light
peak corresponds to the center of the absorption line of the
respective gas, as explained here-before and generally known.
Finally the head comprises a temperature sensor 12 electrically
connected to temperature means 11. The housing has a sample chamber
or gas detection region 4 with gas inlet 5 for the gas to be
detected through which the laser beam provided by the laser source
1 pass through. The light sensor 8 receive the laser beam and
provides a resulting signal S.sub.G comprising changes in the
intensity of the initial light signal S.sub.0 due to the gas
concentration in the detection region 4 being direct proportional
to the intensity. In general, this detection signal S.sub.G as
detection signal S.sub.D0 is directed to at least one lock-in
amplifier for generating at least one measuring signal.
[0028] The gas detector device of the FIGS. 6 to 8 further comprise
electrical supply means 3 for the laser source 1 and DC supply
control means 13 for defining a DC current signal for controlling
the laser source 1. AC processing means 12 comprise AC supply
control means 15 for defining an AC modulation signal at a given
reference frequency f generating an alternative scanning around the
gas absorption line as explained before. From the AC modulation
signal, as known from the prior art, reference modulation signals
are generated. The AC processing means further comprise generating
means 17 to generate a first reference modulation signal S.sub.2f0
at twice of said initial frequency, whereby said first modulation
reference signal has a 45.degree. phase angle to said initial light
signal and oscillates at an amplitude level being different from
the amplitude level of the second modulation reference signal
between amplitude levels 1 and 0.
[0029] In the embodiment of FIG. 6 two modulation reference
signals, a first modulation reference signal S.sub.2f0 and a second
modulation reference signal S.sub.2f1 on twice the initial
modulation frequency f are generated. Latter by the generating
means 16. Both reference signals have the same phase correlation to
the AC modulation signal as shown in FIG. 3 only for the second
modulation signal. The difference between the two reference signals
is only their amplitude levels: The modulation reference signal
S.sub.2f1 is a rectangular oscillation between the levels 1 and -1,
whereas the reference signal S.sub.2f0 is a rectangular oscillation
between the levels 1 and 0.
[0030] According to the present invention, these first and second
modulation reference signals S.sub.2f0 and S.sub.2f1 are
respectively provided to two lock-in amplifiers 20 and 19 in which
these reference signals are respectively multiplied with the
detection signal S.sub.D0 provided by the light sensor 8 to these
two lock-in amplifiers 19, 20 through the preamplifier means 23,
and then integrated over several time periods of the AC modulation
signal.
[0031] The first lock-in amplifier 20 provides a first measuring
signal S.sub.MI, which is independent from the gas absorption. As
seen in FIG. 4, the multiplication with the first modulation
reference signal S.sub.2f0 has the simple effect of cutting out the
parts of the detector signal S.sub.D0 (=S.sub.G) containing
information on the gas absorption. In this way, the integration
over time does not cancel the information on the DC laser
intensity, and the output signal is actually the time average of
S.sub.0 as seen by the light sensor and equals S.sub.G at the
center of the gas absorption peak divided by 2. This channel does
not correspond to a lock-in detection but rather to a time
averaging of a part of the detector signal. At the second lock-in
amplifier 19, this corresponds to a 2f-lock-in detection at a phase
angle of 45.degree. where the DC part of the light intensity as
well as the oscillation on the modulation frequency are cancelled.
The result is a measuring signal S.sub.MA (FIG. 5), which is
proportional to the gas concentration, and implicitly proportional
to the laser intensity S.sub.G at the center of the gas absorption
peak as seen by the light sensor 8. In the prior art patent
application, the same result is obtained with a time derivated
detector signal and therefore at a different phase angle.
[0032] The final measuring signal is then given as
S.sub.MA/S.sub.MI and is independent of the laser light
intensity.
[0033] In a preliminary step, the second measuring signal S.sub.2f1
can be used to define the DC current signal by detecting the
maximum of this second measuring signal S.sub.2f1, when the DC
current level is linearly varied. It is to be noted that this
preliminary step can be avoided if the device is equipped with a
very precise temperature control for the laser source.
[0034] The main advantages of this method are that changes of the
laser output through temperature variations are compensated and
mode hopping of the laser is compensated as long as the gas
absorption peak can be tracked. With respect to the prior art, the
accuracy of the measurement is independent of the gas
concentration. Therefore it is not further necessary to provide a
temperature tracking as well, which leads to less cost for a gas
detector device.
[0035] In the embodiment depicted in FIG. 7, the generating means
16 generate a second modulation reference signal S.sub.2f, which is
exactly in phase with the intensity variations of said initial
light signal S.sub.0 and a derivator 25 generates a detection
signal S.sub.D, which is substantially proportional to the time
derivate of said resulting light signal generated by the light
sensor 8. The derivator 25 is connected with the preamplifier means
23, whereas additional preamplifier means 24 are provided for the
detector signal S.sub.G comprising changes in the intensity of the
initial light signal S.sub.0. This embodiment also provides
reasonable results, because the difference between the second
measuring signal S.sub.MA of the embodiment of FIG. 7 (which is
multiplied with a derivated detection signal) differs from the
second measuring signal S.sub.MA of the embodiment of FIG. 6 (which
is multiplied with a not derivated detection signal) only in the
fact, that the derivated second measuring signal S.sub.MA is
especially larger than the non-derivated second measuring signal
S.sub.MA at small gas concentrations. The different phase results
from that a derivation takes place or not.
[0036] According to the embodiment of FIG. 8 it is also possible to
generate a third measuring signal S.sub.MI1 additional to the first
and second measuring signals S.sub.MI and S.sub.MA by using three
lock-in amplifiers 19, 20, 21 and additional generating means 18
within the AC processing means 14. The generating means 18 generate
a third modulation reference signal S.sub.f at the initial
frequency f and then integrated over time, which is exactly defined
in phase with the intensity variations of said initial light signal
S.sub.0. The third measuring signal S.sub.MI1 is generated by
multiplying the derivated detection signal S.sub.D1, which is
derivated by the derivator 25, with the third modulation reference
signal S.sub.f. The third measuring signal S.sub.MI1 is a function
of intensity of the initial light signal S.sub.0 as described in
the prior art, thus dependent on the temperature of the laser
source. The two measuring signals S.sub.MI and S.sub.MI1 provide
more information, because the first measuring signal S.sub.MI
represents the absolute intensity, whereas the third measuring
signal S.sub.MI1 represents the slope of the intensity of the
initial light signal. In order to generate a final measuring signal
with the processing unit 22 as described before, the first and
third measuring signals S.sub.MI and S.sub.MI1 are correlated by
correlation means 26, connected with the processing means 22 to use
the additional information provided by the two measuring signals
S.sub.MI and S.sub.MI1 for the resulting signal.
* * * * *