Laser gas analyzer

Abstract

According to the present invention, it is possible to provide a laser gas analyzer that can achieve a simplification of the routing performed by a user and a reduction in the size of a device. The present invention is directed to a laser gas analyzer 1 that performs gas analysis of a gas to be measured that is present in a space to be measured, the laser gas analyzer 1 including: a light emitting unit 10 including a laser element 12 configured to emit a laser light 30 in a wavelength band that includes a light absorption wavelength of an absorption line spectrum of the gas to be measured, and a modulated light generation unit 11 configured to supply a drive current to the laser element such that a wavelength of the drive current is swept and modulated in the wavelength band that includes the light absorption wavelength of the absorption line spectrum of the gas to be measured; and a light receiving unit 20 including a light receiving element 22 configured to receive the laser light that passes through the space to be measured, and a light reception signal processing unit 21 configured to analyze the gas to be measured based on a detection signal outputted from the light receiving element 22, wherein communication between the light emitting unit 10 and the light receiving unit 20 is performed through the laser light 30. Fig. 3

Description

Field of the Invention

[0001] The present invention relates to a laser gasanalyzer that analyzes the presence or absence and theconcentration of various kinds of gas to be measured in aspace.Description of the Related Art

[0002] A laser gas analyzer uses a laser element of a lightemitting unit to emit laser light of a light absorptionwavelength at which the laser light is absorbed by a gasto be measured, which is made up of gaseous gasmolecules, causing the gas to be measured to absorb thelaser light, and performs a gas analysis of the gas to bemeasured by the light receiving unit based on the amount of absorption of the laser light at the light absorptionwavelength.

[0003] As described in Japanese Patent Laid-Open No. 2017-106742 (Patent Literature 1), for example, wavelength modulation spectroscopy is generally used for a gasanalysis of the gas to be measured. That is, accordingto Patent Literature 1, laser light that has a wavelengthswept by a drive current and that is modulated at aspecific frequency is emitted by a wavelength variablelaser light source, the laser light is detected by aphotodetector, and a lock-in amplifier lock-in detects asignal with a frequency that is the integral multiple ofthe modulated frequency. Based on correspondence(proportional relationship or the like) between the gasconcentration of the gas to be measured and the amplitudeof the lock-in detection waveform, it is possible toperform an arithmetic operation for the concentration of the gas.

[0004] The light emitting unit and the light receiving unitare connected with each other by a communication line forsynchronization of signals or for exchanging set values.However, at an actual installation site, there may be thecase where the diameter of a flue is several meters. Forthis reason, an extremely long cable is required for thecommunication line that connects the light emitting unitand the light receiving unit with each other. Further,the laser gas analyzer is installed outdoors and hence,it is necessary to eliminate the influence of surge or anindirect lightning stroke. Therefore, an insulatedcommunication module is mounted on each of the lightemitting unit and the light receiving unit that form thelaser gas analyzer, and these modules occupy a relativelylarge area even for simple serial communication. As aresult, it is difficult to reduce the size of the lasergas analyzer. An increase in the size of the laser gasanalyzer also causes a problem in explosion proofingproportional to the volume of the laser gas analyzer, forexample.

[0005] The present invention, which has been made to solvethe above-mentioned problem, provides a laser gasanalyzer that can exchange required information betweenthe light emitting unit and the light receiving unitwithout connecting the light emitting unit and the light receiving unit by the communication line.SUMMARY OF THE INVENTION

[0006] The present invention is directed to a laser gasanalyzer that performs a gas analysis of a gas to bemeasured that is present in a space to be measured, thelaser gas analyzer including: a light emitting unitincluding a laser element configured to emit laser lightin a wavelength band that includes a light absorption wavelength of an absorption line spectrum of the gas tobe measured, and a modulated light generation unitconfigured to supply a drive current to the laser elementsuch that a wavelength of the drive current is swept andmodulated in the wavelength band that includes the lightabsorption wavelength of the absorption line spectrum ofthe gas to be measured; and a light receiving unitincluding a light receiving element configured to receivethe laser light that passes through the space to bemeasured, and a light reception signal processing unitconfigured to analyze the gas to be measured based on adetection signal outputted from the light receivingelement, wherein communication between the light emittingunit and the light receiving unit is performed throughthe laser light.

[0007] One aspect of the present invention is characterizedin that the light emitting unit generates, by themodulated light generation unit, information on the lightemitting unit in a form of a light modulation signal, theinformation being required by the light receiving unit,and superimposes the light modulation signal on the laserlight, and the light reception signal processing unitobtains a light modulation signal for gas analysis and alight modulation signal for communication from adetection signal outputted from the light receivingelement.

[0008] One aspect of the present invention is characterizedin that the information on the light emitting unit is ameasurement start timing at which the gas analysis isperformed, the information being required by the lightreceiving unit.

[0009] One aspect of the present invention is characterizedin that the light emitting unit digital-analog convertsthe light modulation signal for gas analysis and thelight modulation signal for communication, which aregenerated by the modulated light generation unit, and thelight emitting unit superimposes the light modulationsignal for gas analysis and the light modulation signalfor communication on the laser light.

[0010] One aspect of the present invention is characterizedin that the light reception signal processing unitanalog-digital convers the detection signal outputtedfrom the light receiving element to obtain the lightmodulation signal for gas analysis and the lightmodulation signal for communication.

[0011] According to the present invention, it is possibleto perform communication for exchanging variouscommunication information without through thecommunication line, so that it becomes unnecessary toconnect the light emitting unit and the light receivingunit by the communication line. Accordingly, it ispossible to provide a laser gas analyzer that can achievea simplification of the routing of wiring or the likeperformed by a worker and a reduction in size.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a diagram of the overall configuration ofa laser gas analyzer according to this embodiment; FIG. 2 is a diagram of the waveform of a lock-indetection signal; FIG. 3 is a diagram of a signal processing block ofthe laser gas analyzer according to this embodiment;FIG. 4 is a diagram of the overall configuration ofa conventional laser gas analyzer; and FIG. 5 is a diagram of the waveform of a drivecurrent that includes a light modulation signal for gasanalysis and a light modulation signal for communication.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] Hereinafter, a laser gas analyzer according to anembodiment of the present invention will be described indetail with reference to attached drawings. The presentinvention is not limited to the following embodiments,and can be carried out by suitably modifying withoutdeparting from the gist of the present invention.

[0014] <Diagram of overall configuration of laser gas analyzer> FIG. 1 is a diagram of the overall configuration ofa laser gas analyzer according to an embodiment of thepresent invention. As shown in FIG. 1, a laser gasanalyzer 1 includes a light emitting unit 10 and a lightreceiving unit 20.

[0015] The laser gas analyzer 1 analyzes a gas to bemeasured that is present in a space to be measured. Inthe laser gas analyzer 1, the gas to be measured thatflows through a space inside walls 50a, 50b (a space tobe measured) is irradiated with a laser light 30 emittedfrom the light emitting unit 10, the walls 50a, 50bforming a gas pipe. The laser light 30 that transmitsthrough the gas to be measured is incident on the lightreceiving unit 20, so that it is possible to obtain theconcentration of a specific gas based on the amount ofdetected light. Further, when the concentration of the gas is zero or equal to or less than a predeterminedvalue, it is possible to detect that the gas is notpresent. Accordingly, it is also possible to detect thepresence or absence of the gas.

[0016] The light emitting unit 10 and the light receivingunit 20 are detachably attached to the walls 50a, 50bforming the gas pipe. The walls 50a, 50b are the wallsof pipes or the like in which a specific gas is present,and each of the walls 50a, 50b has a hole. Flanges 51a,51b are fixed to these holes by welding or the like.Optical axis adjustment flanges 52a, 52b, provided to thelight emitting unit 10 and the light receiving unit 20,are mechanically detachably attached to these flanges 51a,51b. The light emitting unit 10 and the light receiving unit 20 are disposed at positions that face each otherwith the walls 50a, 50b interposed therebetween. However,the positions of the light emitting unit 10 and the lightreceiving unit 20 can be adjusted by the optical axisadjustment flanges 52a, 52b.

[0017] The optical axis adjustment flange 52a can adjustthe emission angle of the laser light 30. The opticalaxis adjustment flange 52b can adjust the incident angleof the laser light 30. Due to the optical axisadjustment flanges 52a, 52b, a maximum amount of thelaser light 30 emitted from the light emitting unit 10 isreceived by the light receiving unit 20.

[0018] [Light emitting unit 10] The light emitting unit 10 will be described. Asshown in FIG. 1, the light emitting unit 10 is configuredto include a modulated light generation unit 11, a laserelement 12, a collimating lens 13, a light emitting unitwindow plate 14, a light emitting unit container 15, andthe optical axis adjustment flange 52a. As shown in FIG.1, the modulated light generation unit 11, the laserelement 12, and the collimating lens 13 are disposed inthe light emitting unit container 15. The light emittingunit container 15 isolates the respective componentsincorporated in the light emitting unit container 15 fromoutside air, thus protecting the respective componentsfrom wind and rain, dust and dirt, contamination, or thelike.

[0019] The modulated light generation unit 11 generates adrive current that is generated such that the wavelengthof the drive current is repeatedly swept and modulated ina wavelength band that includes the light absorptionwavelength of the absorption line spectrum of the gas tobe measured. The modulated light generation unit 11supplies, to the laser element 12, the drive current foremitting modulated laser light. With such aconfiguration, to analyze the concentration of a gas, itis possible to perform irradiation with wavelengthmodulated modulated light corresponding to lightabsorption characteristic of the gas to be measured.

[0020] The laser element 12 emits light at a centerwavelength λ1 of a specific absorption line spectrum andwavelengths around the center wavelength, the gas to bemeasured absorbing the light. The laser element 12variably controls an emission wavelength by controlling adrive current and a temperature.

[0021] The laser element 12 is temperature-controlled suchthat an emission center wavelength takes the centerwavelength λ1 of the absorption line spectrum of the gasto be measured. Further, the laser light 30 emitted from the laser element 12 is controlled such that wavelengthsaround the center wavelength of the absorption linespectrum of the gas to be measured are swept in terms oftime by the drive current supplied from the modulatedlight generation unit 11. Further, the laser light 30 ismodulated by superimposing an appropriate sine wave so asto allow measurement with high sensitivity by wavelengthmodulation spectroscopy (WMS). Wavelength modulationspectroscopy is also referred to as 2f detection.

[0022] The laser element 12 to be used is not particularlylimited. However, the laser element 12 may be a DFBlaser diode (Distributed Feedback Laser Diode), a VCSEL(Vertical Cavity Surface Emitting Laser), or a DBR laserdiode (Distributed Bragg Reflector Laser Diode), forexample.

[0023] The collimating lens 13 is made of a material havinghigh transmittance at the center wavelength λ1 of theabsorption line spectrum of the gas to be measured and atwavelengths around the center wavelength λ1. The laserlight 30 is converted to substantially parallel light bythe collimating lens 13, and can be transmitted to thelight receiving unit 20 while loss caused by diffusion issuppressed.

[0024] The light emitting point of the laser element 12 isdisposed at a position in the vicinity of the focal pointof the collimating lens 13. The light emitted from thelaser element 12 is incident on the collimating lens 13while diffusing, thus being converted to the laser light30, which is substantially parallel light. In thepresent embodiment, the description is made assuming thatthe collimating lens 13 is used as a parallel lightconversion unit. However, it should not be construedthat the parallel light conversion unit is limited to acollimating lens. For example, it is also possible touse a parabolic mirror as the parallel light conversionunit in place of the collimating lens 13.

[0025] The laser light 30, which is substantially parallellight, transmits through the light emitting unit windowplate 14, and propagates through a space inside the walls50a, 50b, that is, a space in which gases including thegas to be measured are present. A hole is formed at aportion of the light emitting unit container 15, and thelight emitting unit window plate 14 is provided to closethis hole. The light emitting unit window plate 14 isdisposed in the optical path of the laser light 30, andprevents the gases including the specific gas to bemeasured from intruding into the light emitting unit 10while allowing the laser light 30 to transmit through thelight emitting unit window plate 14. With such aconfiguration, the respective components disposed in thelight emitting unit container 15 are prevented fromdirectly coming into contact with the gas and hence, itis possible to protect the respective components in thelight emitting unit container 15.

[0026] [Light receiving unit 20] The light receiving unit 20 will be described. Thelight receiving unit 20 is configured to include a lightreception signal processing unit 21, a light receivingelement 22, a condenser lens 23, a light receiving unitwindow plate 24, and a light receiving unit container 25.The light receiving unit container 25 incorporates thelight receiving element 22, optical components, and anelectrical and electronic circuit therein. The lightreceiving unit container 25 isolates these componentsfrom outside air, thus protecting these components fromwind and rain, dust and dirt, contamination, or the like.

[0027] The light receiving unit 20 receives the laser light30 that transmits through the light receiving unit windowplate 24, and analyzes light that is absorbed by the gasto be measured due to light absorption characteristic ofthe gas to be measured. A hole is formed at a portion ofthe light receiving unit container 25, and the lightreceiving unit window plate 24 is provided to close thishole. The light receiving unit window plate 24 isdisposed in the optical path of the laser light 30, andprevents the gases including the specific gas to bemeasured from intruding into the light receiving unit 20while allowing the laser light 30 to transmit through thelight receiving unit window plate 24. With such aconfiguration, the respective components disposed in thelight receiving unit 20 are prevented from directlycoming into contact with the gas and hence, it ispossible to protect the respective components in thelight receiving unit 20. The laser light 30 is condensedby the condenser lens 23, and is incident on the lightreceiving element 22. In the present embodiment, thecondenser lens 23 is used. However, it is also possibleto adopt a parabolic mirror, a doublet lens, adiffractive lens, or the like in place of the condenserlens 23.

[0028] The light receiving element 22 receives the laserlight 30 that passes through the gas to be measured. Forthe light receiving element 22, it is possible to selecta light receiving element having sensitivity to thecenter wavelength λ1 of the absorption line spectrum ofthe gas to be measured and to wavelengths around thecenter wavelength λ1. A light reception signal outputtedfrom the light receiving element 22 is transmitted to thelight reception signal processing unit 21 as an electricsignal.

[0029] The condenser lens 23 is made of a material havinghigh transmittance at the center wavelength λ1 of theabsorption line spectrum of the gas to be measured and atwavelengths around the center wavelength λ1. The laserlight 30 is caused to be condensed on the light receivingelement 22 by the condenser lens 23 and hence, it ispossible to obtain high signal intensity.

[0030] The light reception signal processing unit 21performs processing on the electric signal received bythe light receiving element 22 to calculate theconcentration of the gas. The harmonic of the modulatedfrequency of the wavelength-modulated laser light 30 islock-in detected, and amplitude information for thedetected waveform is calculated and hence, it is possibleto detect the gas with high sensitivity.

[0031] In the case where a plurality of gases that arepresent in the space to be measured have a fixedcomposition, the amplitude of a lock-in detectionwavelength obtained by absorption of light by the gas tobe measured is a function of the wavelength modulationamplitude, and has a maximum value. Accordingly, incalibrating a standard gas, it is possible to maximize asignal-to-noise ratio by adjusting the wavelengthmodulation amplitude such that the amplitude of a lock-indetection waveform takes a maximum value.

[0032] FIG. 2 is a diagram of the waveform of a lock-indetection signal. As shown in FIG. 2, the lock-indetection signal has a waveform having extreme valuesbased on the absorption line of the component of the gasto be measured. The range with a width W shows the lockin detection signal caused by absorption by the gas.

[0033] A difference D in signal intensity of the lock-indetection signal shown in FIG. 2 between the bottom andthe peak has a correlation with the concentration of thegas. Accordingly, by performing calibration with astandard gas set to each concentration in advance, it ispossible to measure the concentration of the gas bydetecting the difference D.

[0034] The width W of the lock-in detection signal can beused for performing an arithmetic operation forcorrection according to the kind of gas, for example.

[0035] <Description of block forming laser gas analyzer ofpresent embodiment>FIG. 3 is a diagram of the signal processing blockof the laser gas analyzer according to this embodiment.Signal processing performed by the laser gas analyzeraccording to the present embodiment will be describedwith reference to FIG. 3. Of the light emitting unit 10and the light receiving unit 20 of the laser gas analyzer1 shown in FIG. 1, the modulated light generation unit 11and the light reception signal processing unit 21 will bedescribed particularly in detail with reference to thediagram of the block shown in FIG. 3. Even whencomponents that the laser gas analyzer 1 usually includesare not shown in FIG. 3, it is assumed that suchcomponents are included.

[0036] As shown in FIG. 3, the light emitting unit 10 isconfigured to include the laser element 12 and themodulated light generation unit 11, and the modulatedlight generation unit 11 is configured to include a laserelement temperature control circuit 112, a wavelengthswept and modulated current setting unit 113, and a DAconverter 114.

[0037] The wavelength swept and modulated current settingunit 113 controls a drive current for the laser element12 such that the wavelength of the laser light 30 emittedfrom the laser element 12 is swept in the vicinity of anabsorption line at the center wavelength λ1 of theabsorption line spectrum of the gas to be measured and ismodulated by a predetermined signal. Further, as will bedescribed later, the wavelength swept and modulatedcurrent setting unit 113 generates information on thelight emitting unit 10 in the form of a light modulationsignal, the information on the light emitting unit 10being required by the light receiving unit 20.

[0038] The DA converter 114 converts a digital signal to ananalog signal. The DA converter 114 DA converts a lightmodulation signal transmitted from the wavelength sweptand modulated current setting unit 113, and transmits thelight modulation signal to the laser element 12.

[0039] The laser element temperature control circuit 112controls an output from the laser element 12 and awavelength to fixed values, thus stabilizing the outputfrom the laser element 12 and the wavelength. The outputfrom the laser element 12 and the wavelength fluctuatedepending on a temperature. Therefore, to prevent anoutput and a wavelength from fluctuating due to a changein ambient temperature, the laser element 12 iscontrolled to a fixed temperature by the laser elementtemperature control circuit 112.

[0040] The light receiving unit 20 is configured to includethe light receiving element 22 and the light receptionsignal processing unit 21, and the light reception signalprocessing unit 21 is configured to include an IVconversion circuit 122, a gas absorption measuringcircuit 123, an AD converter 125, a gas concentrationarithmetic correction unit 124, a demodulator / decoder 127,and a control unit 126.

[0041] The light receiving element 22 is an element havingsensitivity to the wavelength of the laser light 30. Forthe light receiving element 22, it is possible tosuitably select a photodiode, for example, according tothe wavelength of the laser light 30 or intensity of asignal.

[0042] The IV conversion circuit 122 is a circuit thatconverts a current signal outputted from the lightreceiving element 22 to a voltage signal. For example,when the light receiving element 22 is a photodiode, itis possible to select a transimpedance amplifier thatamplifies a current signal outputted from the photodiodewhile converting the current signal to a voltage signal.In this case, under the condition that the laser light 30is least attenuated, that is, under the condition thatthere is no dust or the like in the optical path, asignal may be suitably amplified by an amplifier circuitnot shown in the drawing to an extent that the signal isnot saturated.

[0043] The gas absorption measuring circuit 123 performsvarious kinds of processing for measuring gas absorption.Although not shown in the drawing, the gas absorptionmeasuring circuit 123 is provided with various kinds offilters, an amplifier circuit, and a lock-in detectionunit, for example. In lock-in detection, phase detectionis performed at a frequency that is an integral multiple(two times, for example) of the modulated frequencyincluded in the detection signal outputted from the lightreceiving element 22, the modulated frequency being setin the wavelength swept and modulated current settingunit 113. The lock-in detected lock-in detection signalis transmitted to the AD converter 125, and is analogdigital converted. The gas concentration arithmeticcorrection unit 124 performs arithmetic operation for theconcentration of the gas based on correspondence(proportional relationship or the like) between the gasconcentration of the gas to be measured and the amplitudeof the lock-in detection waveform. In FIG. 3, the ADconverter 125 is connected to the downstream of the gasabsorption measuring circuit 123. However, the ADconverter 125 may be connected to the upstream of the gasabsorption measuring circuit 123.

[0044] The gas absorption measuring circuit 123 can extracta light modulation signal for gas analysis and a lightmodulation signal for communication from the detectionsignal outputted from the light receiving element 22. Ofthese signals, the light modulation signal for gasanalysis is subjected to gas analysis in the abovementioned lock-in detection. In contrast, as will bedescribed later, the light modulation signal forcommunication is a synchronizing signal for a measurementstart timing in the gas analysis, for example.

[0045] The demodulator / decoder 127 demodulates / decodes asignal of communication information into a form that cancorrespond with the analyzer of the present embodiment.

[0046] The control unit 126 controls the respectiveprocessing units (the gas absorption measuring circuit123, the gas concentration arithmetic correction unit124) of the light receiving unit 20. The control unit126 can control the respective processing units based oninformation on the gas to be measured and thecommunication information transmitted from the lightemitting unit 10.

[0047] <Process to achieve present embodiment> FIG. 4 is a diagram showing the overallconfiguration of a conventional laser gas analyzer 2. Inthe conventional configuration shown in FIG. 4, variouscommunication information, such as synchronization of aconcentration measurement start timing, is exchangedbetween the light emitting unit 10 and the lightreceiving unit 20 through the communication line 40.

[0048] As shown in FIG. 4, the communication line 40connects a modulated light generation unit 31 and a lightreception signal processing unit 32 with each other. Inthe conventional configuration, the communication line 40,the modulated light generation unit 31, and the lightreception signal processing unit 32 are formed based onPatent Literature 1, for example.

[0049] As described above, in the conventionalconfiguration, the light emitting unit 10 and the lightreceiving unit 20 are connected with each other by thecommunication line 40 for synchronization of signals orfor exchanging set values. However, the conventionalconfiguration causes an increase in the size of the lasergas analyzer 2, and has a problem that the routing ofwiring becomes complicated, for example.

[0050] In view of the above, in the laser gas analyzer 1 ofthe present embodiment, as shown in FIG. 1, communicationbetween the light emitting unit 10 and the lightreceiving unit 20 is performed through the laser light 30without using the communication line 40 between the lightemitting unit 10 and the light receiving unit 20. Asdescribed above, in the present embodiment, it isunnecessary to connect the light emitting unit 10 and thelight receiving unit 20 by the communication line 40 andhence, the routing of wiring or the like performed by aworker is simplified, and it is also possible to achievea reduction in the size of the laser gas analyzer 1.

[0051] <Communication between light emitting unit 10 and lightreceiving unit 20>In the laser gas analyzer 1 of the presentembodiment, a communication signal between the lightemitting unit 10 and the light receiving unit 20 can begenerated by the wavelength swept and modulated currentsetting unit 113 shown in FIG. 3 in the form of a lightmodulation signal. It is preferable that thecommunication signal be information on the light emittingunit 10, the information being required by the lightreceiving unit 20.

[0052] The wavelength swept and modulated current settingunit 113 can generate a light modulation signal for gasanalysis and a light modulation signal for communication.For example, the light modulation signal forcommunication is a synchronizing signal for aligningphases in performing lock-in detection. Thesynchronizing signal may be a pulse-like intensitymodulation signal, for example, and can cause the modulated light generation unit 11 and the lightreception signal processing unit 21 to be synchronizedwith each other at the clock time at which a pulse isreceived.

[0053] When the concentration of the gas is detected bylock-in detection, if a measurement start timing is notsynchronized between the modulated light generation unit11 and the light reception signal processing unit 21, aphase shift is generated in the detected waveform. Inview of the above, in the present embodiment,communication between the modulated light generation unit11 and the light reception signal processing unit 21 isperformed by superimposing a synchronizing signal on thelaser light to align phases in place of using theconventional communication line 40. With such aconfiguration, it is possible to synchronize themeasurement start timing between the modulated lightgeneration unit 11 and the light reception signalprocessing unit 21 and hence, the generation of a phaseshift can be suppressed.

[0054] FIG. 5 is a diagram of the waveform of a drivecurrent generated by the wavelength swept and modulatedcurrent setting unit 113. A synchronization / measurementID signal, in the form of a light modulation signal (I)for communication, is provided prior to each lightmodulation signal (II) for gas analysis. The lightmodulation signal (II) is a light modulation signal thatsweeps a wavelength in a wavelength band that includesthe light absorption wavelength of the absorption linespectrum of the gas to be measured.

[0055] A light modulation signal (III) shown in FIG. 5 isdifferent from the light modulation signal (I), and isinformation on the light emitting unit 10, theinformation being required by the light receiving unit 20.Although not limited, the light modulation signal (III)may be information on characteristic change, such aslaser element temperature information on the laserelement 12, deterioration information, or information onthe gas to be measured, for example.

[0056] The wavelength swept and modulated current settingunit 113 shown in FIG. 3 can generate a light modulationsignal in the form of a digital signal. The lightmodulation signal is DA converted by the DA converter 114,and is transmitted to the laser element 12. In thepresent embodiment, the light modulation signals (I),(III) for communication and the light modulation signal(II) for gas analysis are superimposed on the laser light30.

[0057] In the light receiving unit 20, the gas absorptionmeasuring circuit 123 obtains a light modulation signalfor gas analysis and a light modulation signal forcommunication from the detection signal outputted fromthe light receiving element 22 to synchronize ameasurement start timing in the lock-in detection. Withsuch a configuration, it is possible to correct the phaseshift of the lock-in detection waveform in the gasabsorption measuring circuit and hence, the concentrationof the gas can be detected with high accuracy. Further,synchronization is performed by the light modulationsignal (I) for each gas measurement and, thereafter, theconcentration of the gas is detected by the lightmodulation signal (II) and hence, the concentration ofthe gas can be detected without generating a phase shiftfor each measurement. Accordingly, such a configurationis preferable.

[0058] When a synchronization / measurement ID signalgenerated by the wavelength swept and modulated currentsetting unit 113 is given as a simple single pulse, thereis a possibility that the correct timing cannot beobtained in the case where the laser light 30 isinterrupted by dust or the like. In view of the above, aplurality of pulse patterns are determined in advance,and only a signal in the measurement when the lightemitting unit 10 and the light receiving unit 20 cancorrectly receive such a pattern is used for measuringconcentration. With such a configuration, it is possibleto stably measure the concentration.

[0059] For the pulse pattern, exactly the same pulsepattern may be used. However, such a case rests on apremise that measurement is continuously performed underthe same condition. In the case where a plurality of gascomponents are measured, such as the case where a gascomponent to be measured is changed, to allow the lightreceiving unit 20 to identify which gas component isunder measurement, pulse patterns are set such that thenext pulse pattern can be predicted based on the pulsepattern received by the light receiving unit 20 from thelight emitting unit 10. Such a configuration allowsmeasurement to be performed under a plurality ofconditions. The gas concentration arithmetic correctionunit 124 is notified by the control unit 126 that the gasconcentration arithmetic correction unit 124 shouldcalculate the concentration of which component.

[0060] For example, when two patterns of 10001 and 10101are provided as pulse patterns, measurement can beperformed under two conditions. By adopting sequentialnumbers, such as 10001, 10010, and 10011, it is possibleto predict a next pulse pattern.

[0061] FIG. 5 shows an example in which communication isperformed with a pulse pattern, that is, communication isperformed by intensity modulation. However, it ispossible to use a general optical communication systemthat uses a single laser. It is also possible to usebinary communication, such as Amplitude Shift Keying(ASK), Frequency Shift Keying (FSK), or Phase ShiftKeying (PSK), or to use multi-value processing (QASK,QPSK, or the like) that uses amplitude or the like. Insuch a case, the demodulator / decoder 127 shown in FIG. 4causes a signal to correspond. Alternatively, byintroducing the mechanism of optical heterodyne detectionto the light receiving unit 20, it is possible to cause asignal to correspond.

[0062] In the present embodiment, the light modulationsignal (III) for communication between the light emittingunit and the light receiving unit is transmitted from themodulated light generation unit 11 to the light receptionsignal processing unit 21. It is possible to eitherperiodically or non-periodically transmit the lightmodulation signal (III). However, it is preferable thatcommunication be periodically performed between the lightemitting unit 10 and the light receiving unit 20 toexchange set values or the like. In the presentembodiment, concentration is detected a plurality oftimes by the light modulation signal (I) and the lightmodulation signal (II) and, thereafter, the lightmodulation signal (III) is periodically transmitted. Atthis point of operation, one-way communication isperformed from the light emitting unit 10 to the lightreceiving unit 20. Accordingly, it is possible handlethis case by a method where information that is requiredby the light receiving unit 20 is determined in advance,and the light emitting unit 10 periodically transmits(broadcasts) this information.

[0063] Further, as described above, it is anticipated thata pulse pattern is broken due to a disturbance factor,such as dust. Accordingly, by adding redundancy, such aschecksum, it is possible to achieve communication withhigher reliability.

[0064] To simultaneously achieve the above-mentionedfunction of measuring concentration and communicationfunction, it is desirable that a low distortion DAconverter 114 be provided between the modulated lightgeneration unit 11 and the laser element 12. Theconventional laser gas analyzer has the property that, byutilizing the fact that distortion is generated inoptically modulated laser light due to absorption by thegas to be measured, concentration is detected bymeasuring a harmonic component (second harmonic ingeneral) of a modulated frequency. For this reason, theconventional laser gas analyzer often uses a lowdistortion analog oscillator that oscillates at aspecific frequency. Even when such an analog oscillatoris used, intensity modulation (ASK) allows communicationto be relatively easily performed by using amixer / multiplexer (AND) or the like. In contrast, whencommunication is performed by modulation, such as PSK orFSK, or by multi-value processing, there is a concernthat peripheral circuits become complicated, leading toan increase in circuit scale. In view of the above, inthe present embodiment, a DA converter is used that has alow distortion factor well below the total harmonic distortion factor (approximately -90 to -80 dB) of theconventional low distortion analog oscillator and hence,the laser can be driven with a desired waveform whilehaving low distortion modulation performance that isrequired to measure the concentration of the gas. Accordingly, it is possible to prevent an increase inscale of a circuit even for communication that includesmulti-value processing between the light emitting unit 10and the light receiving unit 20.

[0065] The laser gas analyzer of the present invention isoptimal for measuring combustion exhaust gases or forcontrolling combustion for a boiler or refuseincineration, for example. In addition to the above, thelaser gas analyzer of the present invention is alsopreferably used as an analyzer for analyzing gases usedfor iron and steel [blast furnaces, converter furnaces,heat treatment furnaces, sintering (pellet facility),coke furnaces], for storing and ripening fruit andvegetables, for biochemistry (microorganism)[fermentation], for air pollution [incinerators, flue gasdesulfurization / denitrification], for exhaust gases fromthe internal combustion engine of automobiles, ships orthe like (removal of a tester), for disaster prevention[detection of explosive gases, detection of toxic gases,analysis of combustion gases from new building materials],for growing plants, for chemical analysis [petroleumrefining plants, petroleum chemical plants, gasgeneration plants], for the environment [ground-levelconcentration, concentration in tunnels, parking lots,building management], and for various experiments forphysics and chemistry, for example.[Reference Signs List]

[0066] 1 laser gas analyzer2 laser gas analyzer10 light emitting unit11 modulated light generation unit12 laser element13 collimating lens14 light emitting unit window plate15 light emitting unit container20 light receiving unit21 light reception signal processing unit22 light receiving element23 condenser lens24 light receiving unit window plate25 light receiving unit container30 laser light40 communication line50a, 50b wall51a, 51b flange52a, 52b optical axis adjustment flange112 laser element temperature control circuit113 wavelength swept and modulated current setting unit114 DA converter122 IV conversion circuit123 gas absorption measuring circuit124 gas concentration arithmetic correction unit125 AD converter126 control unit127 demodulator / decoder

Claims

1. A laser gas analyzer that performs a gas analysis of a gas to be measured that is present in a space to be measured, the laser gas analyzer comprising: a light emitting unit including a laser element configured to emit laser light in a wavelength band that includes a light absorption wavelength of an absorption line spectrum of the gas to be measured, and a modulated light generation unit configured to supply a drive current to the laser element such that a wavelength of the drive current is swept and modulated in the wavelength band that includes the light absorption wavelength of the absorption line spectrum of the gas to be measured; and a light receiving unit including a light receiving element configured to receive the laser light that passes through the space to be measured, and a light reception signal processing unit configured to analyze the gas to be measured based on a detection signal outputted from the light receiving element, wherein communication between the light emitting unit and the light receiving unit is performed through the laser light.

2. The laser gas analyzer according to claim 1, wherein the light emitting unit generates, by the modulated light generation unit, information on the light emitting unit in a form of a light modulation signal, the information being required by the light receiving unit, and superimposes the light modulation signal on the laser light, and the light reception signal processing unit obtains a light modulation signal for gas analysis and a light modulation signal for communication from a detection signal outputted from the light receiving element.

3. The laser gas analyzer according to claim 2, wherein the information on the light emitting unit is a measurement start timing at which the gas analysis is performed, the information being required by the light receiving unit.

4. The laser gas analyzer according to any one of claims 1 to 3, wherein the light emitting unit digitalanalog converts the light modulation signal for gas analysis and the light modulation signal for communication, which are generated by the modulated light generation unit, and the light emitting unit superimposes the light modulation signal for gas analysis and the light modulation signal for communication on the laser light.

5. The laser gas analyzer according to any one of claims 1 to 3, wherein the light reception signal processing unit analog-digital convers the detection signal outputted from the light receiving element to obtain the light modulation signal for gas analysis and the light modulation signal for communication.