Note: Descriptions are shown in the official language in which they were submitted.
PACKAGED LASER THERMAL CONTROL SYSTEM
TECHNICAL FIELD
[0001] This invention in general relates to gas monitors used, for
example, in the
process industry. In particular this invention relates to improvements in
detection and
measurement of gas concentrations and gas emissions based on tunable diode
lasers.
BACKGROUND
[0002] Accurate monitoring of gaseous species at low concentrations is
required for a
wide range of industrial, regulatory, and academic fields. The most common
include
atmospheric chemistry, pollution monitoring, industrial process monitoring and
control,
safety, breath analysis, and agricultural research. One of the most reliable
principles for
continuous monitoring of gases is the measurement of gas absorption since most
gases have
one or more absorption lines in the ultra violet, visible or the infrared part
of the spectrum.
This technique is known as absorption spectroscopy. With this method a beam of
light such
as a laser beam that is absorbed by the gas of interest, is directed through
the gas or a
mixture of gases. The degree of absorption of the light beam is then used as
an indicator for
the concentration of the gas to be detected. Many different spectroscopic
techniques exist,
but the use of single line spectroscopy utilizing single mode tunable diode
lasers is probably
the one giving best sensitivity and selectivity due to its high spectral
resolution involving a
low risk of interference from other gases.
[0003] There are two popular spectroscopic methods of laser gas
detection. In one
the frequency of the laser is rapidly scanned across the gas absorption line
by modulation of
the laser diode current. Gas absorption results in modulation of the amplitude
of the
transmitted light and this amplitude can be measured using a photodetector and
simple
electronics. The absorption of the laser beam on- line and off- line may be
compared and the
gas absorption and concentration computed. This is method is referred to by
several names
including scanned direct absorption and rapid scan absorption. This method has
the
advantage of simplicity but it can be difficult to establish a zero absorption
baseline. The
other popular method is called modulation spectroscopy; the most commonly used
is referred
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to as wavelength modulation spectroscopy (WMS). In this method the laser
frequency and
amplitude is modulated using laser current as in the case of direct
absorption. In addition,
the laser current is also modulated at a second relatively high frequency. Gas
absorption
distorts the amplitude of the modulated laser light so that harmonics of the
high modulation
frequency appear after the beam has passed through a gas. These harmonics are
measured by
demodulating the gas signal. Sensitive tunable diode laser (TDL) absorption
measurements
have been performed for decades with wavelength modulation spectroscopy (WMS)
for a
wide variety of practical applications. With its better noise-rejection
characteristics through
laser wavelength modulation strategies, WMS has long been recognized as the
method of
choice for sensitive measurements of small values of absorption, and thus is
favored for trace
species detection.
[0004] Laser diode wavelength stability is vital in tunable diode laser
spectroscopy
(TDLS). Since both laser diode threshold current and laser emission wavelength
are
functions of temperature, laser diode temperature stability is very important
in laser
spectroscopy. For example, the commonly used 1651 nm atmospheric absorption
line of
methane has a linewidth (HWHM) of 50 pico-metres (pm). Laser spectroscopy
requires the
wavelength precision of the laser to be substantially less than this
linewidth.
[0005] TDLS gas sensing systems, the laser diode temperature is
controlled with a
thermo electric cooler (TEC). The laser die is typically mounted in close
proximity to a
Peltier element and a temperature-sensing thermistor. The TEC controlling
circuit uses
current from the thermistor in a feedback loop with the Peltier element to
regulate
temperature of the thermistor and the laser diode. It is possible and
practicable to regulate the
temperature of the Peltier element to less than 1 milli-kelvin. However, even
with good
thermal design internal temperature gradients exist between the laser die and
the thermistor
because of both ambient temperature changes and laser die heating. A change in
ambient
temperature consequently causes a change in the laser temperature and this
laser temperature
change results in a systematic error in the laser diode emission wavelength.
Typically the
laser emission wavelength changes by 5pm for each centigrade change in ambient
temperature. An ambient change of 30 C will result in a laser emission
wavelength change
of 150pm which is the equivalent to approximately three line widths of the 165
mm methane
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line. A change in ambient temperature by only one degree will typically result
in 5pm
change in laser emission wavelength which is typically 10% of a gas absorption
linewidth
and unacceptable for TDLS spectroscopy.
[0006] Thermal changes in the TEC and laser current generating circuitry
also cause
TDLS systems to drift. This drift is relatively small but is important for
applications
requiring high precision and accuracy. Electronic components are sensitive to
temperature
and dissimilar metals in a circuit board create thermo-electric voltages that
change with the
temperature of the circuitry. Even in carefully designed circuitry ambient
temperature
changes result in changes in TEC control currents and laser currents that
cause the laser
emission frequency to drift when the ambient temperature changes.
[0007] Several methods have been proposed to stabilize the emission
wavelength of
diode lasers.
[0008] { T. Ikegami, S. Sudo, Y. Sakai, "Frequency Stabilization of
semiconductor
laser diodes" Artech House,(1995) }
[0009] The commonest method is to use a sample of the target gas
contained in a
reference cell.
[0010] {Van Well, B., Murray, S., Hodgkinson, J., Pride, R., Strzoda, R.,
Gibson, G.
and Padgett, M., "An open-path,hand-held laser system for the detection of
methane gas," J.
Opt. A ¨ Pure Appl. Opt. 7, S420-S424 (2005) }
[0011] Gas reference cells are commonly used as absolute wavelength
standards.
[0012] {Gilbert, S. L., Swann, W. C. and Dennis, T., "Wavelength
standards for
optical communications," Proc. SP1E 4269, 184-191 (2001)1
[0013] When a gas reference cell is used to stabilize emission wavelength
in a TDLS
system, the system typically has an optical reference path that contains the
gas reference cell.
The system analyzer captures the spectrum of the sample gas and uses a
feedback loop to
stabilize the laser emission wavelength and prevent wavelength drift. This is
known in the art
as line centering. The system adjusts emission wavelength by changing either
the laser
temperature or the laser injection current. This method has the disadvantages
of adding
substantial opto-mechanical complexity and with even the most careful design
can introduce
optical interference effects and degrade system sensitivity. Adjustment of the
laser current or
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temperature by the system during line centering also typically causes changes
of laser light
amplitude and instrument calibration.
[0014] Other laser wavelength stabilization methods use athermalised
etalons and
electrically stabilized optical resonators as wavelength standards. These
methods share the
same disadvantages as the gas cell wavelength standard.
[0015] { Ackerman, D. A., Paget, K. M., Schneemeyer, L. F., Ketelsen, L.
J.-P.,
Warning, F. W., Sjolund, 0., Graebner, J. E., Kanan, A., Raju, V. R., Eng, L.
E., Schaeffer,
E. D. and Van Emmerik, P., "Low-cost athermal wavelength locker integrated
temperature-
tuned single-frequency laser package," J. Lightwave Technol. 22 (1), 166-171
(2004))
[0016] {Sandford, S. P. and Anti11, C. W., "Laser frequency control using
an optical
resonator locked to an electronic oscillator," IEEE J. Quantum Elect. 33 (11),
1991-1996
(1997))
[0017] A recent paper has disclosed a method of directly measuring the
laser junction
temperature by measuring the laser junction voltage. The junction voltage is
used in a
control loop to stabilize the laser temperature and emission wavelength.
[0018] {A. Asmari, J. Hodgkinson*, E. Chehura, S. E. Staines and R. P.
Tatam,
"Wavelength stabilisation of a DFB laser diode using measurement of junction
voltage"
Proc. of SPIE Vol. 9135, 91351A = C 2014 SPIE
[0019] {A. Asmari, J. Hodgkinson*, E. Chehura, S. E. Staines and R. P.
Tatam "A
new technique to stabilise the emission wavelength of laser diodes for use in
TDLS"
FLAIR, 37, Florence 2014
[0020] This method stabilizes the laser emission wavelength but the
stability is
inadequate for sensitive TDL spectroscopy. The method also requires injection
current
modulation which could compromise the modulation levels required for sensitive
WMS.
[0021] Optical interference fringes caused by reflection from optical
elements
degrade the sensitivity TDLS systems and causes thermal drift. A reflectivity
of only 0.0025
will cause interference fringes of optical depth of 1% peak to peak.
[0022] {C. R. Webster, "Brewster-plate spoiler: a novel method for
reducing the
amplitude of interference fringes that limit tunable-laser absorption
sensitivities" JOSA B,
Vol. 2, Issue 9, pp. 1464-1470 (1985)
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[0023] In most TDLS systems reflection between the laser diode and its
packaging
and other optical components cause fringes within a TDLS system. Changes in
the path
length between optical elements and the laser, usually the result of thermo-
mechanical
changes, by as little as a fraction of a wavelength will cause the fringes and
optical
interference frequency to change. For example in a typical near infrared TDLS
system only
a one degree change in temperature changes the path between the first lens and
the laser by
approximately ten wavelengths. Optical interference changes caused by ambient
temperature changes are consequently another important source of drift in TDLS
systems.
Line centering has no impact on the thermal drift caused by optical fringes in
a TDLS
system.
SUMMARY
[0024] Maintaining a TDLS system in a thermally controlled ambient
environment
with a temperature stability of a small fraction of a degree in order to avoid
system drift is
very difficult and is too large, complex, and power intensive for most TDLS
systems such as
those used for portable applications. The applicant has invented an
alternative method and
apparatus to thermally stabilize a tunable diode laser spectrometer. The
method may utilize
two thermoelectric coolers (TEC). An inner TEC may stabilize the temperature
of the laser
diode element. An outer TEC stabilizes the temperature of the laser package.
The inner TEC
uses a Peltier element, a signal indicative of a temperature of the diode,
such as from a
thermistor located adjacent to the diode, and a Peltier current driver in this
context
functioning as a controller to form a temperature control loop so as to
maintain the laser
diode at a fixed temperature and consequently fix its emission wavelength. The
outer TEC
uses another Peltier element, thermally mounted in relation to the laser
package (e.g. the
laser package is thermally mounted on the Peltier element or the Peltier
element is thermally
mounted on the laser package), a signal indicative of a temperature of the
laser package, such
as from a thermistor located adjacent to the laser package, and a Peltier
current driver to
form a temperature control loop so as to maintain the laser package at a fixed
temperature. A
Peltier element not thermally mounted in relation to the laser package but
otherwise arranged
to influence a temperature of the laser package (e.g. a Peltier element
mounted on a housing
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containing the laser package to influence the temperature of the interior of
the housing) could
also be used but would be less convenient and would likely provide a lower
precision of
temperature control. The method also includes means to stabilize the
temperature of the TEC
controlling and laser current generating circuitry as required for
applications requiring
precise and accurate gas concentration measurements.
[0025] The role of the inner TEC is to stabilize the temperature of the
laser as
practiced in the TDLS art. One role of the outer TEC is to stabilize the
temperature of the
laser package over a large environmental temperature range to improve the
thermal stability
of the inner TEC. Another role of the outer TEC is to stabilize the
temperature of the laser
package over a large environmental temperature range and limit laser emission
frequency
thermal changes and minimize system drift. Another role of the outer TEC is to
stabilize the
temperature of the laser package over a large environmental temperature range
to limit
thermal interference fringe changes and to minimize system drift.
[0026] Limiting drift by stabilizing the temperature of the electronics
that control the
inner and outer TECs and the laser current is necessary for high sensitivity
applications.
Using Peltier elements to stabilize the temperature of these electronics is
considered
impractical, but this should not be construed to limit the claims to exclude
such an
embodiment. The applicant has invented an alternative method and apparatus for
stabilizing
the TEC control and laser current generating circuitry. The method utilizes
laser current
generators and inner and outer TEC controllers all on one circuit board known
as the laser
driver. This laser driver circuit board has circuitry and heating resistors
distributed across the
surface to provide uniform resistive heating of the circuit board. The
resistor currents are
regulated by a computer controlled thermo-electric switch centrally located on
the laser
driver circuit board. Since the circuit board has no cooling means the
thermoelectrically
regulated temperature must be set above the maximum system temperature that
occurs at the
highest ambient temperature. The circuit board may be thermally insulated to
minimize the
influence of ambient temperature changes on heating.
[0027] It is therefore an object and an advantage of the present
invention to provide
an apparatus and method for thermally stabilizing a TDLS gas sensor.
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[0028] It is an advantage of the present invention to provide a thermally
stable TDLS
gas sensor that requires no frequency reference such as a sample gas
absorption cell.
[0029] It is yet another advantage of the present invention to provide a
TDLS gas
sensor that over a wide environmental temperature range does not have the
degradation of
accuracy and precision caused by line centering.
[0030] It is further advantage of the present invention to provide a TDLS
gas sensor
that over a wide environmental temperature range has very precise laser
temperature and
emission frequency stabilization.
[0031] It is a still further advantage of the present invention to
provide a TDLS gas
sensor that over a wide environmental temperature range has thermally stable
laser current
and TEC control electronics.
[0032] It is an additional advantage of the present invention to provide
a portable
TDLS gas sensor that is thermally stable over a wide environmental
temperature.
[0033] It is to be understood that, although the applicant believes that
his preferred
embodiment described herein meets the objects and provide the advantages
described, the
scope of the invention is to be determined by reference to the claims and not
necessarily by
whether the embodiment achieves all objects and advantages stated.
[0034] Accordingly, there is provided a thermal control system for a
packaged laser
diode, the packaged laser diode having a laser package and a diode within the
laser package,
the thermal control system comprising an outer Peltier element located
exterior to and
thermally mounted in relation to the laser package, and a controller connected
to the outer
Peltier element and connected to receive a signal indicative of a temperature
of the laser
package to control the outer Peltier element according to the signal
indicative of a
temperature of the laser package.
[0035] In various embodiments, there may be included any one or more of
the
following features: There may be a heat conductive element mounted between the
outer
Peltier element and the laser package. There may be an outer temperature
sensor mounted in
the heat conductive element, the outer temperature sensor connected to the
controller to
provide the signal indicative of a temperature of the laser package. There may
be a heat
conductive cap arranged around the laser package and connected thermally to
the heat
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conductive element. There may be thermal insulation arranged around the heat
conductive
cap. There may be thermal insulation arranged around the laser package. There
may be a
further controller connected to an inner Peltier element located within the
laser package, the
further controller connected to receive a signal indicative of a temperature
of the diode to
control the inner Peltier element according to the signal indicative of a
temperature of the
diode. The signal indicative of a temperature of the diode may be provided by
an inner
temperature sensor located in proximity to the diode.
[0036] In a further embodiment there is provided a thermal control system
for a
packaged laser diode, the packaged laser diode having a laser package and a
diode within the
laser package, the thermal control system comprising an outer Peltier element
located
exterior to the laser package, and arranged to influence a temperature of the
laser package, a
controller connected to the outer Peltier element and connected to receive a
signal indicative
of the temperature of the laser package to control the outer Peltier element
according to the
signal indicative of the temperature of the laser package, and a further
controller connected
to an inner Peltier element located within the laser package, the further
controller connected
to receive a signal indicative of a temperature of the diode to control the
inner Peltier
element according to the signal indicative of a temperature of the diode.
[0037] In various embodiments, there may be included any one or more of
the
following features: the signal indicative of a temperature of the diode may be
provided by an
inner temperature sensor located in proximity to the diode. There may be a
heat conductive
cap arranged around the laser package. There may be thermal insulation
arranged around the
heat conductive cap. There may be thermal insulation arranged around the laser
package.
[0038] All of the above described embodiments, unless otherwise stated,
may include
any one or more of the following features: there may be a temperature
controlled circuit
board and a laser driver mounted on the temperature controlled circuit board,
the laser driver
connected to drive the diode. In embodiments where there is a further
controller, the further
controller may be mounted on the temperature controlled circuit board. The
controller may
be located on the temperature controlled circuit board. The temperature
controlled circuit
board may be temperature controlled using heating resistors controlled
according to a
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temperature sensor mounted on the temperature controlled circuit board. The
circuit board
may be thermally insulated.
[0039] These and other aspects of the device and method are set out in
the claims.
BRIEF DESCRIPTION OF THE FIGURES
[0040] Embodiments will now be described with reference to the figures,
in which
like reference characters denote like elements, by way of example, and in
which:
[0041] Figure 1 is a functional schematic of a Butterfly packaged laser;
[0042] Figure 2 is a functional schematic of a TO packaged laser;
[0043] Figure 3 is a functional schematic of a prior art TDLS gas
detector with a
Butterfly packaged laser; and
[0044] Figure 4 is a functional schematic of the invented TDLS gas
detector with a
Butterfly packaged laser.
DETAILED DESCRIPTION
[0045] Figure 1 is a functional schematic of a prior art Butterfly
packaged laser
diode typical of those used for TDLS gas sensing. Referring to this drawing,
the laser 1 is
mounted on a ceramic submount 2. Collimating lens 3, which is mounted on the
copper
thermal header 12, collects and collimates the laser beam from the laser. Opto-
isolator 4 is
also mounted on the copper header. Focusing lens 5 is mounted in a cylindrical
tube 6
attached to the wall of the Butterfly package 7. The fibreoptic pigtail 8 is
attached to the
wall of the Butterfly package. The collimated laser beam 9 passes through the
opto-
isolator and is focused onto the fibreoptic pigtail. The copper thermal header
is attached to
a Peltier element 10 which is thermally attached to the Butterfly case.
Thermistor 11 is
attached to the ceramic submount in close proximity to the laser. The
Butterfly package is
thermally mounted on a heat sink 13.
[0046] Figure 2 is a functional schematic of a prior art TO packages
laser typical of
those used in TDLS gas sensors. Referring to this drawing, laser 14 is
attached to ceramic
submount 15. The uncollimated laser beam 16 passes through window 17 mounted
in the
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case of the TO package 18. Thermistor 19 is mounted on the ceramic submount in
close
proximity to the laser. The ceramic submount is attached to the copper thermal
header 20
which is thermally mounted on the Peltier element 21. The Peltier element is
attached to the
base 22 of the TO package and the TO package is mounted on the heat sink 23.
[0047] Figure 3 is a functional schematic of a prior art TDLS gas
detector with a
Butterfly packaged laser. Laser diode 24 is connected electrically 26 to a
current source 25
that generates the offset bias and modulation injection currents. The laser
and thermistor 27
are mounted on a ceramic submount and copper thermal header 28 and this header
is
thermally mounted on a Peltier element 29 and the Peltier element is mounted
thermally on a
heat sink 30 as shown in more detail in Figure 1. The Peltier element and the
thermistor are
connected electrically 31 to a TEC driving circuit 32. The laser beam is
focused by optics
33 onto a fibreoptic pigtail 34. The laser beam emerging from the fibreoptic
is collimated by
the collimating lens 35 and the collimated beam is split by beam splitter 36
into a reference
beam 37 and a target gas beam 38. The reference beam passes through a gas
reference cell 39
and is then focused onto the reference photodiode 40 by lens 41. The target
gas bam passes
through sample gas 42 and is then focused onto the target gas photodiode 43 by
the lens 44.
Current from the reference photodiode 45 and the current 46 from the sample
photodiode
flow to the analyzer 47 that calculates the concentration of the target gas.
Gas concentration
data from the analyzer are communicated through a user interface 48 to data
storage and
display circuitry 49. The analyzer controls the current source 50 and TEC
driver 51.
[0048] Figure 4 is a functional schematic of the preferred embodiment of
the
invented TDLS gas detector with a Butterfly packaged laser. This invention may
also be
applied to any other packaged laser including for example a TO packaged laser.
Laser 52 is
connected to the current supply 53 by conductors 54. The laser is mounted on
the ceramic
submount 55 which is in turn mounted on an inner Peltier cooler 56. The inner
Peltier cooler
is mounted on the base of a Butterfly package 57. The thermistor element 58
and the inner
Peltier cooler are connected to a TEC driving circuit 59 through conductors 60
to form a
feedback temperature control loop. This feedback loop regulates the
temperature of the
thermistor and laser with an accuracy of +/_ 1mK. The Butterfly package is
mounted on a
copper heat spreader 61 which conducts heat from the package. The butterfly
package is
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thermally insulated by insulation 80 on all surfaces other than the surface of
the base covered
by the heat spreader. This insulation 80 maintains the Butterfly package at an
essentially
uniform temperature. The heat spreader is mounted on an outer Peltier cooler
62 which in
turn is mounted on the heat sink 63. A second thermistor 64 is mounted in the
copper heat
spreader. The second thermistor and outer Peltier cooler are connected by
conductors 65 to a
second TEC driving circuit 66 to form a feedback temperature control loop.
This feedback
loop regulates the temperature of the thermistor and hence the heat spreader
with an accuracy
of +/_5mK for an ambient temperature change from -40C to 50C. Stabilization of
the
temperature of the Butterfly package by the heat spreader ensures that the
laser temperature
and hence emission frequency remain constant over an ambient temperature
change of -40C
to 50C. It would also be possible to use a temperature sensor located other
than in the heat
spreader, for example a temperature sensor inside the laser package separate
from the
temperature sensor used by the inner TEC. An alternative solution which does
not require an
additional temperature sensor would be to use the output of the inner TEC
driving circuit 59
as the signal used to control the feedback loop for the outer TEC driving
circuit 66. This
output is also a signal indicative of a temperature of the laser package as
the amount of
heating or cooling required by the inner TEC to keep the thermistor 58 at
constant
temperature depends on the temperature of the laser package. In another
embodiment, it may
be desired to omit the inner TEC, accepting a lower degree of control of the
laser diode
temperature. In such an embodiment, the outer TEC could use a signal from
thermistor 58 as
the signal used to control the feedback loop for outer TEC driving circuit 66.
In such an
embodiment in which the inner TEC is not present or not operational, the
signal from
thermistor 58 is a signal indicative of the temperature of the laser package
as well as of the
laser diode. When the inner TEC is operational and controlled according to the
signal from
thermistor 58, the outer TEC should not be controlled according to the signal
from thermistor
58 since two control circuits controlling the same signal can result in
instabilities.
[0049] The laser current source 53, first TEC control circuit 59, and
second TEC
control circuit 66 are mounted in close proximity on a laser driver circuit
board 67. This
circuit board has an array of heating resistors 68 dispersed across the
populated surface.
This array is combined with a circuit board mounted thermistor and relay to
form a
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temperature control feedback loop which stabilizes the temperature of the
laser driver
board over a range of ambient temperatures. The circuit board may also be
thermally
insulated. When the temperature of the laser driver board is set above the
highest ambient
temperature, this feedback control loop stabilizes the temperature of the
laser driver board to
an accuracy of +/_50mK.
[0050] The walls and top of a butterfly package are typically fabricated
from poor
conductivity thin steel sheet. In the preferred embodiment thermal
stabilization of the
butterfly package is enhanced with a heat conductive cap arranged around the
laser package
such as thin walled copper cap 69. This cap is connected thermally to the
copper spreader
and maintains the body of the butterfly package at the temperature of the heat
spreader with
less insulation than is required without a copper cap. In an embodiment where
the heat
conductive cap is present, the thermal insulation 80 may be arranged around
the heat
conductive cap.
[0051] In operation of the TDLS system, light from the laser is
transmitted through a
fibreoptic, through a gas to be measured, and is collect by a photodiode 70 as
in prior art
TDL gas sensors. The optical path may also be made up of a transceiver as
commonly
practiced in the TDL gas sensing art. The photodiode current passes through a
coaxial cable
71 to a transimpedance circuit 72 which converts the photodiode current to a
voltage of
several volts. This photodiode voltage is communicated to an analyzer circuit
73 through a
conductor 74. The preferred conductor is a shielded CAT6 cable typically used
for
telephone communication and may be several hundreds of metres in length, if
required. The
analyzer circuit uses the photodiode voltage to calculate the gas
concentration on the gas
measurement path as practiced in the TDL gas sensing art. Various analyzer
circuits may be
used but the preferred analyzer circuit is a digital circuit. The analyzer
sets the temperature
operating point of the two TEC controlling circuits and sets the laser
currents from the laser
current generating circuit through conductors 79. Many different types of
conductor may be
used but the preferred conductor is a ribbon cable. The operating parameters
of the analyzer
are controlled by user interface circuit 75 through conductors 76. The
analyzer
measurements are also communicated to the user interface circuit by the
conductor 75.
Many different user interface circuits may be used to both display measurement
results and
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control the TDLS gas sensor but the preferred circuit is a flat panel display.
Measurement
results may be communicated to external data collecting systems 77 through
cable or
wireless circuitry 78.
[0052] Immaterial modifications may be made to the embodiments described
here
without departing from what is covered by the claims.
[0053] In the claims, the word "comprising" is used in its inclusive
sense and does
not exclude other elements being present. The indefinite articles "a" and "an"
before a claim
feature do not exclude more than one of the feature being present. Each one of
the individual
features described here may be used in one or more embodiments and is not, by
virtue only
of being described here, to be construed as essential to all embodiments as
defined by the
claims.
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