CA2007240A1 - Method and apparatus for spectroscopic measurement of the concentration of a gas - Google Patents
Method and apparatus for spectroscopic measurement of the concentration of a gasInfo
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- CA2007240A1 CA2007240A1 CA 2007240 CA2007240A CA2007240A1 CA 2007240 A1 CA2007240 A1 CA 2007240A1 CA 2007240 CA2007240 CA 2007240 CA 2007240 A CA2007240 A CA 2007240A CA 2007240 A1 CA2007240 A1 CA 2007240A1
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Abstract
Abstract Method and apparatus for spectroscopic measurement of the concentration of a gas in a sample where you detect the intensity of light from a light source (1) passed through the sample (4) and through a reference cell (5) and gene-rates a signal which represents the concentration of the gas. A laser diode (1) constitutes the light source which is locked to the absorption line of the gas at known con-centration and pressure contained in a reference cell (5) The laser light with the properly selected wavelength is distributed via optical fibers (2,3,6) and/or glass prisms (19,20) to the reference cell (5) and the sample (4).
Nongasrelated transmission variations in the measurement path or in the optics is automatically compensated for by a special laser modulation which generates a time multiplexed reference. The measurement can be performed in a measurement cell incorporating several reflective mirrors (30) to reflect the laser beam several times inside the measurement cell thereby increasing the effective length of the measure-ment path. The measurement can also be performed using an optical fiber designed so that the surrounding gas or fluid via the evanescent field effects the laser light so that the concentration of the substance in question in the surround-ing fluid or gas can be determined.
Nongasrelated transmission variations in the measurement path or in the optics is automatically compensated for by a special laser modulation which generates a time multiplexed reference. The measurement can be performed in a measurement cell incorporating several reflective mirrors (30) to reflect the laser beam several times inside the measurement cell thereby increasing the effective length of the measure-ment path. The measurement can also be performed using an optical fiber designed so that the surrounding gas or fluid via the evanescent field effects the laser light so that the concentration of the substance in question in the surround-ing fluid or gas can be determined.
Description
u~ CI~ I Li ~gO72AO
ME;THOD AND APPARArrUS FVR SPEC~OSCOPIC MEASURE1~1ENT OF THE
CONCENTRATION 0~ A G~S
Thi~ lnventlon concerns a method and a~ apparatus to spec-trosoopiaally measu~e the concentration of a gas ln a sample, whereby the intenslty of llyht from a light souroe is passed through the ~ample and through a reference cell and is detected, and that a signal is generated which rep~e~ents the oonoentration of the gas. The li~ht source 10 consists of a laser diode whlch i~ locked onto the absorp-tion llne of the ~as at known preQ~ure and ooncentration contained in ~he refe~ence cell. In order to avold inter-ference from the atmosphere the laser light of the seleoted wavelength ls distrlbuted ~long optical fibers and/ or glass prisms to ~he referenoe cell and to the ~ample. ~he in-vention al80 compri~es an apparatus fo~ spectroscopic measurement of the eoncQntratlon of a gas ~ e . ~ . oxygen ln a sample. ~n the apparatu~ the modulated lntenslty of ll~ht transmitted through the sample and reference cell i5 ~0 dete¢ted and n slgnal proportional to the ratio of ths demodulated int~nBities i8 ~enerated which repre~ents the cQncentratlon of the g~.
Background of the lnvention ~5 Tunable diode laser~ have quickly become lmportant for h~h resolu~lon in~trument~ for analysls of molecule ~pectra.
~o~t of the sppllcatlons for alode laser spect~oscopy have so ~ar been in baslc research. Lead salt diode la~ers emitting ln the wavelength range of 3-~0 ~m have been used to study absorptlon lins parameters and for measurement of molecule constants (1,2~. The po*sibllity to lndirectly determine different parameters such as pres~ure, tempera-ture, or elec~ric~l field strength has ~1B~ been successful-ly demonstrated (3,4). ~here ha* ~een a relatlvely ~mall interest for the overtone band~ around 1 um ~ince these donot signlflcantly contri~ute to any new lnformatlon on the 4~ ~1 4~ MIB ~ 01~4 15'~ 05 26:)07240 analyzed molecules. The main llmitation for pr~ctlcal use of lead salt dlods lasers ln measurement systems have been the need for cooling of to temper~ture~ below that of liquld nitrogen. AlG~As and InGaAsP diode lasers emit in the 0.68 - 1.7 ym wavelength ran~e and can operate at temPeratures up to 5~ degree~ centigrade. A characteristlc of these lasers compared to lead salt laserB 18 ~heir hlgh inten~ity modulation lndex which requlre~ ~peclal sign~l analysls tO
obtaln optimum ~ensitlvlty. A large number of molecule~ of interest ln proceQQ control and environmental me~urement applicatlons have ab-aorptlon bands in the 0.7 - l. 7 ~m range, although, the~e ~re relatlvely we~k overtone band~.
The superior performance cf AlGaAs and InGaAsP l~erY
compar~d wlth lead salt lasers however, more than outweigh~
this difference and they have grea~ potentl~l ~o bs used in instrumental application~ (5).
A very interestlng applic~tlon 18 for measurement of oxygen concentratlon, whlch ha~ m~ny practlcal appllcatlons e.g. in the medlcal are~. Today, oxygen is usually measured uslng dlffe~ent electrochemlcal methoda. For ex~mple qemlconduotor sensors or the transformat~on of 2 to secondary products for later detectlon ia freguently used. These methods hsve several llmltations. They don't perform real-~1me measure-ments and are usually not sultable ln exploslve envlronments~nd moreover, they often lnfluence the me~urAnd slnce ~
~m~ll amount of the moasured (oxygen) gas 19 consumed by the measurem~nt instrument. These meQsurement in~truments ~re Qlso often sensltlve to other types of gases or organic ~0 pollutlons whlch results ln ~ conslderably limlted lifetlme.
From EP-Al-0 015 170 lt 1~ known that by uQing spectrometers with reference ~nd measurQmen~s cells and by u~lng optical flbers or glas~ priSm and gas-tight boxe~ interference with the ~urroundlng envlronment 18 avolded. ~hls ls true concernlng the ellmlnation of unwanted contrlbutions to the measurement result from other path~ than the ~ea~urement ~ :
J~ y ~ 'UItlllt~ 4 1~6yo7~40 ~6 path. However, the problem of elimin~tlng the lnfluence of nonga~related ~ran~mi~8ion varlatlons ln the measurement path or optlc~1 probe i~ not addre~sed at all.
5 In DE-A1-3.633.93 the la~er i~ not locked onto the frequency of the ab~o~ption llne. In~-tend, the laser current i~
modulated and the absorption spectrum is studied. The absorption line i8 then sampled ~s the current 18 modul~ted to determlne the gas absorption. ~ur~hermore, the speotrum ls ~ampled on 40th ~ldes of the line to measure the non~as-related a~tenuAtion. ~he slgnal conditioning ls performed direotly ~rom the ~ampled spect~um which re~ults ln poor accuracy ~ince the meagurement value is th~ result of the substractlon of two, nearly equal, numbers. ThiY method i~
the most commonl~ used to avoid measurement errors from attenuation caused b~ fog, rain etc.
Purpose and slgnl~loant characterlstlc~ of the inventlon The purpose of the pro~ent lnvention ig to ogtablish a measurement method which ls not lnfluenced by the environ-ment outslde the meaQurement volume or cell (the claimed optical probe) or b~ the envlronmental properties in the measurement path and ln addltion has a short response time.
~5 This has been ob~aln~d by looking a laser diode onto the absorptlon llne of the sas in questlon in a ~eference cell containlng this gas at known conCentration and pre~sure and thereafter deteotlng the harmonios at 2f~ that are generated from the lnteraotion betweQn the absorptlon lin~ and a trlangular modulatlon current of ~requency fO superlmposed on the laser blas current and ~urthermore, to avo~d interferen-ce from the a~mosphere optical flbers and/or glass pri~m~
are used for the distribution of the modulated laser llght with the selected wavel~ngth tc the reference cell and the 3S optlcal pro4e or measurement path, and that the si~n~l ~ondition$ng compensate~ for nongasrelated transml~sion vari~tions ln the measurement pat~ or the opti~al probe by 4~ ~l 4~ 'hlENl~ "3~ 01/~4 15~ 7 2 ~0724 0 using a tlme-multiplexed dual-beam technique. The pre~ent ~nvention also lncludes an apparatus for the method descrl-bed where optical components of with low reflectivity (e.g.
AR-coating o~ optic~l ~urfaces) are used to mlnimize interference problemY. Moreo~er, multip~exing technique oan ~e used to cover ~everal meaYurement paths with Ju~t one laser or to msasure ~everal gases with ~everal lasers utlllzlng the same mea~urement path and distributlon nst.
De~criptlon of the drawlngs The invention wlll in the followlng be d~cribed with referenc~ to the encl~ed drawings.
~ig. 1 shows a ~chematic of thè electrooptlcal g~s ~ensor accordin~ to the ln~ention.
Fig. 2 shows the measurement method with the used modulatlon ~ignals and the corresponding detected waveforms afte~
ab~orption in the mess~re~ ga~. S~ecifically, the lntonsit~
modulation o~ the la~er, the ~re~uen¢y modulatlon of the laser, and the deteated lntensity var$ation after pas~age of th~ measurement volume ~re lllu~trated.
Fig~ 3a ~hows computed ~lgnals ~fter compensation for the intensity modulatlon of the laser and the followlng lock-in detection.
Flg. 3b i~ the same a~ fig 3a but ln thlo ca~s for an incomplete lntenslty compensation resultlng in a remalning lntensity modulation of l~.
Flg. 3O ~hows the mea~urement ~i~nal as a functlon of the gas concentratlon after ~lgnal conditioning.
Fig. 3d shows how the output from the lassr ~pectr~meter varles with phase error ln the lock-in detectlon.
.
" ~1 J ~ Ul.l l I H I ~l~ l b ~ Jt ~
Flg. 3e shows ~e ou~put Qlgn31 from the la~er diode spectrometer as a fwlction o~ the frequency modulation of t~e laser.
5 Fi~. 3P Qhows the output from the lager speotrometer as function of the total presBure in the measurement volume.
Flg. 4 shows the ba~ic structure of the lnstrument uslng optlaal fi~ers accordiny to one embodlment of the invention.
Flg. 5 ehows a laser clrcuit ~oard for the laeer wlth the reference ~ell.
Flg . 6 shows a -Qystem with several laser clrouits who' 8 lS output iY multlplexed l~to o~e fiber for slmultaneou~
measurem~nt of eeveral gases.
Flg~ 7 shows a ey~tem wlth ane laser cirouit ~hich i8 multiplexed to ~everal fibers for simultaneous mea~urements of one gas over ~everal ~ifferent measurement path4.
Fig. ~ ehows a schematlc of the basic apparatus deslgn Ueing yl~s~ pri~ms for llght dis~ribution.
25 Fig. 9 ~how~ a echematic of the ha~lo Hpparatus deslgn uslng both glass prlsms and optlcal flbers.
Fig. lO shows a ~lock dlagram of the measure~ent s~stem lncluding ~ansmlssion compeneation an~ lock-in detectlon.
Fig. 11 ehow~ ~ block diagram o~ t~e measurement ~y~tem u~ing dig~tal ~odulatlon and mea~urement control.
Fig. 1~ shows, in per~pectl~e, an exploded view of the optical measurement probe.
F~ g . 13a showQ a hori~ontal sectlonal view throu~h the iUI ~U~ 07240 mea~urement probe and 13b a vertical ~ectional view through it~ lower part.
Fig. 14 shows, ln per~pective, a reference cell.
Flg. 15 ~how~, ln a lar~er ~cale, a ~ection through the reference cell.
Descriptlon of the lnventioA
The invention concerns a method to analyze gases, e.g.
oxygen, u~lng a la~er dlode ~pectrometer. A special signal conditloning method for diode laser spectromet~rs, based on diodq la~ers ~lth high inten~lty modulation index, has ~een lS developed. Interferenoe from nonga~related transmisslon ~arlations s~ch as fo~, rain turbulence ~nd v~rylng trans-ml~slon through the optlcs of the apparatu~ caused hy du~t particle~, condense etc. ls taken care of by the intenslty modulstion compen~atlon ~n a unlque manner. Inter~erence 2~ ~rom gases not contained ln ~he measur~ment volume i8 eliminated by u~lng gla~s w~veguides in the in~trument. The mea~urement~ can be per~ormed over an atmospheric path, in an with the lnstr~ment lntegrated meaqurement volume to w~lch the gas 18 transferred uslng e.g. tubes, uslng the evane~cent fleld of an opticAl ~lber surrounded by a medium contalning the gas to be measured, or in a ~peclal measure-ment cell, a unl~uely de~igned optical probe to whloh the light i~ pas8ed ln opt~cal flbers.
A meaYurement cell, a ~elf-allgned optical probe that 1~
sufflclently small to be lnte~rated ln e.g. the mouth pleoe of a patlent snd ~111 gi~es a measurement path of suffici-Qnt length to perform aocurate measurements of oxygen, has been developed specifically to be used wlth the spectrome-ter.
~he in~trument 1~ deYlgned ~o that all measurements of 41~ FIRIEN~ 4 15:1~ 0 dlfferent parameters iQ au~omatically callbr~ted if you knowthe correspondlng parameter~ for the gas ln guestlon in a reference cell. ThiY al~o lncludes the elimination of lnfluences from fog, raln, du~t vr oondenRe in the measure-ment volume or in the optlcY of ~he instrument itself. ~hlsmeans that you are independent of publiQhed n~mbers on line strength etc. A ~peclal compaot referenoe ~ell that oan be included on e . g . a ~lmple standard europe-circult board has been developed. ~he measurement re~ul~s from the measuremen~
~olume ( atmo~pherlc path, optlcal probe or evanescent flbe~
probe) are compared wlth the correspondlng rQ~ults from the reference cell~ The gas concentra~lon is computed from two length~ (tho~e of the reference ~ell and the measurement path) and the known concentratlon in the reerence cell.
Thls results in a ver~ slmple calibration procedure and high mea~urement accurac~.
~elow i~ a 118t 0~ a number of ~pplication areas for th~
in~ented dlode la~er spectrometer.
~edlcal appllcatlons a)~Measure~ent of the oxygen concentratlon ln breathlng alr.
Here a ~peclal optio~l measurement probe has been ~eveloped th~t can be lncorporated in e. g. a re~plrator. The optic~l probe le connected to the mo~th piece a~ the patlent ~nd ~he light is pas8ed to the probe u~lng optical waveguides (optical flbHr~).
b) Control of ane~thetlo gase~ ~urlng oper~tlon.
c) Control of oxygen or humldlt~ in an lncubator.
d) Studle8 of metabollsm. Here an isotope ga~ can be used ~ince the hlgh re~olutlon of the ln~trument 1~ sufficlen~ to dlscriminate thl~ from the nat~ral abu~dance of ~he gas.
~1 4~ 'HI~ Y~ 15ll3 e) Pharma~eutieal productlon. Inert packaging of medlcal~.
Indu~tri~l applioation~
S ~) Safe mea~urement~ of oxygen or ot~er gase~ in explo~ive ~nvironments ~uch a~ pe~rol supplles, oll tankers etc. ~he light i~ passed to and from the optlcal measurement probe using optlcal ~veguide~. The pro~e can al~o be located in environm~nts wl~h ~trong electromagnstlc lnterference effects such a~ ln a radar ~et.
g) ~easurement of gas concentr~tion in environments with strong radar or radio interferenc~. Thls means that the laser diode ~pectrometer o~n be incorporated in a levelmeter based on microwa~e teohnology.
h) Control the presence of oxygen in lnert envlronments.
i~ Control of combu~tlon proce~e~.
J) Automotlve lndustry an~lysi~ of ~xh~ust yase~, con~rol of air/fuel mlxtures.
k) Heatlng And electriclt~ produclng plantss con~rol of combu~tlon, minlml~ation of pollutlon.
1) ¢~s productlon.
m) Lamp manufact~rlng.
n) Semloonduotor lndugtry: oontrol of ga~ concentrat~on in dif~erent prooe~se~, ln o~nY, glo~eboxe~ etc.
o~ Procn~ control ln unnealing and oxldation own~ .
p) ~ontrol of fermentation pro~esse~.
. --4~ lllt~ 4 1521~o724 q) ~ood 1ndustry; beer production, packaging.
r ) Mea~urement of temperature pro~iles ln oxygen contalning atmosphere in proces~ oven~.
s) MeaYuremen~ of pressure ln gase~
t) Measurement of pre~ure ln prooes~e~, t~nk~ etc.
u) Galvanlc insulated measurements of electrlcal ~ield streng~h.
v) Me~urement of g~ ooncentratlon~ in ~luids, ~.~. gase~
ln a b~ttery or the oxy~en contents in a lake or sea using an evane~cent flber probe.
x) MeaBuremont of le~kage from ga~ tubes b,v using an evane~cent fiber probe that ls dsployed in p~rallel wlth the gas tube.
Dlode laser spectrosqopy has ~raditlonal~y been used to an~lyze the Pundamental IR-band at 3 -30 ,um. ~a~e~ llght is selectively ab~orbed at frequencie~ ~ vz V3 ... whlch ~orre~ponds to fundsmental rot~tional and vibr~tional tr~nsitlonY. ~he l~ser~ thnt h~vo been u~ed in thls WBV~-length r~n~e are co~plica~ed and expensive. ~ large number of molecule8 have oombination and overtone band~ o~ the 2v, ~ V2 type ln the w~volength range 0-7 - 1-7 ~um- These bands u~ually don't ~ontribute any new information about molecul~r const~nt~ but 4~ well ~ulted for mea~urement~ of the concentration of tho flbsorbing gas. Thi~ ls espeoially true since in thi8 ~avoleng~h range there exi~ts a large number of inexpensive hlgh quallty l~sers.
In the following, moa~urement of oxyg~n ls used to illu~tra-te the prin~lple of operatlon of the inventlon.
4~ ~1 4~1513~ bOI P~TEI~T~ 4 15'14 Z40 ~13 ~he oxygen molecule ha~ a number of weak ab~orption bands yenerated by magnetic dlpole transitions. ~hese bands are looated arount 630, 690, and 760 nm. The A-band of oxygen covers the ~avelength ran~e of 759 - 771 nm and 1B relatlve-1~ free from interferlng molecules why it i~ suitable formeasurement of ox~gen uQlng electrooptic techni~ues. AlG~Ao or InGaAsP lasers a~e sulta~le light source~ for msa~ure-ments ba~ed on selectl~e absorption o~ l~ght by dlscrete mol~cule line~ slnce they have yood spatial and temporal coherence. By using oxygen at kno~n concen~ration and pre~sure in a reference cell a normall~ation ~ignal i8 obtalned and al~o the laser can be looked to the ab~orptlon llne o$ oxygen. ~hls enables very ~oourate ~easursment~ of the oxygen concentration in the mea~urement volume.
AlGa~s, InGaAsP and other semlconduator laserY can ea~ily be frequency modulated by means of the inJectlon current and can therefore be u~ed $n differential speotroscopy to obtaln a good eignal-to-nolse ~atlo. Another ohar~cteri~tic of 2~ the~e laser~, compared with e.g. lead salt lasQrs, 18 that apart from the frequenoy modulatlon a con~ldersblo intonsity modulatlon i~ obt~ined at the re~uency modul~tion that i~
optimum fo~ speotro~oopla me~urements. The intQnnlty modulation ls used to elim~nate the influenoe of nonga~-related ab~orptlon in the me~urement ~olum~ or ln theinstrument itself w~thout h~vlng to use the basellne of the nb~orptlon spectrum. Thl~ pro~ides superior stabillty and accuracy. When ono me~sure atmospheric gases it 1~ importan~
to ~llminate absorptlo~ ~ air path~ to ands from the me~urement volume. By u~ing gla~ prism~ or optical fibers thi~ unw~nted absorption can be avolded.
Fig. 1 show9 a 8chomatlc of the ele~trooptical ~a~ yQn~or accordlng to the inventlon where 1 i~ a ll~h~ 80urce, la8er diode , from ~hich th~ light i~ di~tributed in optlcal waveguide~ 2 and 3 to a measurement cell 4, whlch oan be an optical probe or sn atmospherlc path, and to a reference J ~ j IJ U I I 1 l i L I ~ I Ll .! ~ 14 cell 5. The ~ea~urement cell 4 ~nd the reference cell 5 ls conne~ted with a control unit 8 u~lng optlcal waveguldes 6 or electrlcal cables 7.
5 The re~erenoe cell 5, whioh contalns oxyg~n at known concentratlon, pre~sure, and temperature, provides the p~rameter~ neces~ary to lock the wavelenyth of the llght ~ouro~ to the absorption line, and the para~eters neoessary to caloulate the gas concentration, the temperature, pressure, and electrlc field stren~th. Thls 1~ pe~formed acco~dlng to the following. A trlangular AC-¢urrent with frequency f~ is added to the laser bias curr~nt. The bia~
current 1~ adJu~ted so that the average wavelenyth of the laser co$nclde-4 with that of the ~bsorption line. The llght source 1 wlll, due to the added AC-current, ohange itR
wavelength (frequency modulated) with the frequenoy fO around the averaged value ~et by thQ bla~ current. The light, now fr~quency modulated ~round the oenter of the ab~orption line of the ga~ p~sed through the measurement ~ell ~an st-mospherlo path, an optloal probe, or an evanescent flber)and is thereafter detected. By feedback of th~ slgnal of 3b vla an analog ~egulator to the laser bias current the wavel~ngth of the llght source 1 iY stabill~ed to the center of the absorption lino~ The llght from the llght souroe 1 is divided lnto to b~anches, one which ls pas~ed to th~
re$erence oell 5, nnd one which ls pa~8ed uslng optlcal wa~eguldes 2 to the mea~urement cell. The measurement signal i8 ~hQn ya~8sd back and lock-in detectsd at frHquenoy 2fo.
~hen the llght 80uroe 18 modulated wlth a frequenoy fO and the slgnal from the detecta~ 1~ demodulated with the frequenoy 2fo~ thls ~l~nal lookQ accordlng to flg. 3a. The signal 2x~ ln flg. 3a 1B proportionsl to the gas concentra-tion.
Apart fro~ the vPri~tlon ln ab~orptlon cau~ed by variatlonQ
ln the conce~t~atlon of the measured gas there wlll be variatlons ln the transmission caused by ~hanges in ~e-4~ :~1 4~151~ 130T P~lEi`lTB "~ 4 1521~o7Z4O B15 tran mls~lon o~ the optlc~ due to rain, dust, or humidlty or~hen the received ~ignal after the measurement path varies d~e to turbulence, raln, and fo~ etc. Normally, this 1~
compen~ated for by me~suring the basellne of the absorption spectr~ ~nd ma~e a normallzatlon. ~hl~ ba~ellne value however, ls a slo~ly varyiny signal with a very low lower frequency llmlt whlah lmpose~ drlft problema. Furthermore, preamplifler~ des~gned to accommoda~e these low frequencle~
have much le~s sen~ltivity th~n AC-ooupled ~mpllflers oper~tlng with a ~mall bandwldth.
In medsurements of gas ~oncentratlon~ it ls v~ry lmportant to compen~te for absorption that can arise in alr paths that exlst ln the dlstrlbution of the ltght to and from the mea~urement area. ~y u~ng glas~ prlsm~ or optlcal f~bers thl~ unw~nted absorptlon can be ellminated. ~urthermore, it ls also lmportant to compensate for varyln~ trans~lsslon ln the instrument ltsel~ or in the mea~urement path th~t i8 not rel~d ~o the measured ya~. Thls o~n be sohieved usln~ a slgnal condltlonlng whl~h identifle~ this componont and compen~ate for it wlthout ha~ing to use the bs~Qline of the absorption spectrum. Thi~ technique pro~ldes ~uperlor ~tablllty and measurement accuracy.
25 In the Ry~tem accordlny to the lnventlon a ~lgnal pro-portlonal to the atmospherlo ~ran~ml~slon 1~ generated by sampling at the peak of ths recelved trlangular~ly shaped optlcal slgnal. Thu peak value ~ compared with the oignal ~rom the monltor photodlode. The peaks of the trlangular signal 18 ~ell separated from the absorption peak a~
lllustrated in fig. 2. ~he relatlve size of the ~bsorptlon peak i~ usually much smaller than w~at ~s shown in the figu~e. ~hls method of m~asur~ny both on the peak~ o~ the trlangular a~gnal ~nd on the ab~orption lln~ g~ne~ates a m~a~urement referen~e that autom~tleolly experlence~ the ~ame transml~ion losse~ a~ the ll~ht ab~orbed by the ab~orptlon line ~lnce by deflnitlon the ~eamB are coali~ned.
4~ ~1 4U5 1;~ 3 Ijll l F'~ I EN I B 5;y~07Z40 A "dual-beam spectrometer" has been re~ll2ed by time-multlplexlng of one mea~urement beam. In re~. 7 the absorp-tlon 1~ determlned ~rom ~ampling direo~l~ in the ab~orpt~on Rpectrum around the llne. Ho~ever, no modulatian technique ls u~ed why the drlft problems of thi~ type of in~trument 1~
considerably large. In our method lt 1~ very ea~y to calculate the gas coneentration u~ing a normall~atlon procedure con~ldering that we know the gas ~onoentra~lon ln the rererenae oell. The con~entr~tlon i~ computed ~rom th~
length~ of the pa~h in the measurement and reference ~ell and f~om the meaQured de~ector signals from the measuremen~
and re~erewe oh~nnel-Q a~ well as tha ga~ concentrat~on in the reference cell.
ThlS method dlffer~ consider~bly from those used in ref. 6, 7, and ~, where ~he la~e~ light, which is unmodul~ted, is ~lowly varied around th- ~bsorption line and the re3ulting speotrum 1~ sampled relatl~e to the b~seline. Normally, this iB the method u~ed ~o compensate for nongasrelated tr~n~mls-sion variation~. Tho ~ign~l c~nditloning method accordlng tothe inventlon, whlch ha~ been do~cribed here, p~rformg all ~ignMl conditloning ~t hlgh ~udio ~requencle~, res~ltlng ln superlor stability, messurement aocuracy and sen~itlv$ty.
To determlne the pressu~o ln tho mea~urement volum~ we procned aacordlng to the followiny. Slnce the width of the absorption llne depends on pressure Qnd temperatur~ one parameter ~as to be i301~ted lf we are going to be ~ble to determine the other. We let the wavelongth of t~ light source sweep ove~ the ~bsorption llne. The light 1B th~n detected aftor pa~sage through the ga~ and the ~idth and the lntegr~l of the a~sorptlon line ls measured slmultaneou~ly.
The reQ~l~ of the inte~ratlon gives the in~egrated l~ne strength ~hich 18 ~ par~meter whos temp~rature dopendence ls known 2n~ ~s ava~lsble ln the litersture. S~nce the pre~sure and temperature in the referenco cell 5 1~ known, the presgure ln the me~oùrement volume can be calcul~ted.
~t~ Jl~ UI ~HI~ 26~07240 1~
The temperature can be me~sured with high ac~uracy by u~lng two la~er olrcuits where the laser~ are locked to two different rotatlonal transition~ in e.g. oxygen. ~he laser llght from the cardY are multiplexed to~ether ln an op~lcal fiber a~ shown ln flg. 7 and the combined llght i8 passed to the measurement volume. The temperature i~ then calculated from the difference in ab-~orptlon in th~ two wavelengths.
Thi~ is a new wa~ to 8pply Mason's method. In thls way the temperature can be meas~ed locally and int~in~lc~lly safe u~$ng the optical probe or alternatively, the average temperature over an a~mospherlo path can be determined.
An electrlcal field ~treng~h 18 measured in a slmilar way by sweeping the la~er wavelength ove~ the a~sorption llne. In the pre~enoe o~ an electrical fleld ln the gas the absorp-tion m~xlmum will be changed or di~plaaed in wavel~ngth.
Sinoe we know the posltion and st~ength o f the ab~orptlon line in the reference cell 5, thls dlsplace~ent oan be determined whlch l~ ~ msasure o~ t~e elsotrical fleld s~rQngth .
Strong magnetlo flelds reqult~ ln a dlrect modulatlon of the gaQ ooncentration in a paramagnetic ga~ as oxygen. ~h~
~5 v~rlation in ga8 concentratlon glve~ a direot measure of the magnetic fleld v~rlatlons. With the Qpeotro~e~er con~i~ured for a free measurement path ~ield varl~tion~ batwoen di~ferent path~ can be analyzed.
It i8 u~ually advantageous ~o a large axtent u~e dlgital ~ectronlc~ ~o generate t~e modulatlon and reference signal~
in the la-~er dlode spectrometer. For lnst~nce it iq ~a~ier to obtain a high degree o~ pha~e ~tabillty co~pared to an analo~ solution and moreover, lt is easler to d~gitally oontrol ~he frequenay and phase.
It is al80 advant~geou~ to use a (digltal) square-wave 4 ~ 3 1 4 ~ 5 ~ H I E ~ 3 F~ ~ I J ~ 4 1 5 ~
1 F;
signal to demodu~te ~he mea~urement and refQrence sl~nal~
ln the ~pectrometer. This ~voiclQ a squ~re-wa~e to ~inu~-wave transformatlon ~hlch if performed wl~h analog technique can lead to drlft ln the phase and decreased meaSurement ~ccur~cy . Tradi~lonally, the modulation slgnal ln laser d~ode speotrometer~ ha~ been ~ sine-wave but lt would ~e a clear advantaye lf ~ ~impler waveform could be used. An ideal alternative would be a triangular wave ~hlch ls easy to generate from ~ square-wave wlth an AC-coupled integra-tor Flgure 2 illustrates the slgnal condltloning used ln the lnventl~n. ~he laser iy modulated wlth the current l~t) ~ io~ 1 + g(t) ~ where g(t) ls a trlangular wave ~ith the amplitude a~
The la~er wlll be modulated both ln intensity ~flg. 2~
PL =PO [1 + mg(t~] and ln fre~uency (flg 2:2), VL - VO ~ 1 t Bg(t)~ with thn frequ~ncy fO.
~ere, m ~ intensity modulation lndex oP the la~er ~nd B i~
th~ freguency modulation index.
When the optical signal sweepq over th~ abso~ption line of the measured gas lts frequency modulation wi~l transform lnto an amplitude modulatlon accordlng to $ C. NL/n * ~7~7~*~v"/~(Bg(t))2 ~ ~,v2]
herH S, NL and ~v~ are confitants oP the moleaule llne and o~ is the ga~ conaent~tlon.
A compo~ed ~i~nal ~ith the Pundamental frequenc~ fO ls ` obtained according to flg 2:3 ~(t) ~ G~ PO ~ (l+m ~t)) (1 - ~(t~
The AG-c~mponent of this ~lgnal ls v,(t) Gl PO ~ [m g(t~ - (1 + m g(t)~ ~(t~L~ ]
4~ ~1 4~513~ ~OT P~TE~IT~ 4 1~707Z40 ~1~
1~
The slgnal ln the measurement ch~nnel 1~ now sub-straoted wlth the monltor ~lgnal whlch after AC-coupling osn be expre~ed a~
v~on(t) G2 P0 m ~(t) The detec~ed measurement ~ignal now has the form ~e~(t)~ GiPo m g(~) - GIPO~e~m g(t) - (1 ~ mg(t))~m(t)L~]
This ~ignal ls ~ampled at the peaks of the triangular wave and thereafter the analog slgn~l oondltioning unit ad~ust~
the gain ~1 so that ~e~(nT)~ G2Po m g(nT)-G~P0 ~ mg(nT)-(l+mg(t))~(nT)~] . o whlch re~ults in Gz m g(n~) G, -~ m g(n~ 1 + m g(nT)} ~[n~]L.~
In all practlcal applicationo tho residu~l term du0 to the eXtension of the ab~orptlon llne out to tho peak of the triangula~ wave will be neylect~ble and the expression for Gl becomes Gl = G2 / ~
¢onseq~entl~, wlth this method the system wlll automatically compen4ate for variations in tran~mlssion in the me~surement path. ~he m~surement signal ~fter thls compen3ation i8 v~ G~ P0 (1 ~ m g(t)) ~[t] ~e~
The mea~urement sl~nal i~ demodulated ln two ste~Y accordlng to flg. 10. The ~lr~t step ls the analog slgnal condltionlng unit with the transmi~sion oompensation de~cribe~ above. ~n ~l~ JI 't'l~ IJI~ lLl~ U Ul~ J-Io step two the compen~ated measurement.~gnal i~ ~nchrono~sly demodulated usin~ a ~quare-wave. ~he oompensated mea~urement slgnal i~ dlvided accord~ng to lts frequency components V~(t)-G2po~n~{l~m8/n2~ 3-l)/2~/J2 * c ~.1,3.5 * H~(v)~ c~9 tiwt]
By multiplying wlth a sgua~e-wave of frequency 2fo and perfor~ a lo~-paæs fllterlng the flnal ~ignal læ obtained Vme~ = ~;2 Po Lm~{H2(~)~ ~ m8/Tt~ [(H,(v), + H3(v)") ~coY[~
- Hl( v )"/18 cos ~3~] ~ H3( v )",/50 coY ~ 5~3~ ] }
~he phase ~ læ ad~u~ted to mlnimi~e the residual t~rm. In the viclnity of the line center v ~ vO ~ Hl(v)" H3(v)~ h 0 and thu6 the measurement signal a~ter lock-in det~ctlon becomes - v",,. - G2 Po ~ H~ ~ v ) al In the ~ame manner, ~he slynal from ~he re~eren~e detector finally becomes .
~r~t ~G~ P0 Lr~ H2(v)r These two slgnals are Yamplod, A/D-converted and dlvld~d ln the ~omp~ter v,~.~vr.~ ~ G2 Po L~., H2(v)~/ G2 Po Lre~ H2(V)r ~he ~ignal HltV), 18 proportional to the ga~ concentratlon o~
and the laYer d~ode ~pectrometer calculateR the ga~ con-centratlon ln the measurement path aæ
~ ~1 4~ 1..s~ bIJ I ~'hI~ 2~)0724~) C~ V~,~a/V,~t ~ ~ Lr.~ Cre~/ L,~.~ ]
The constants Lr~s and cr~ ar~ known w~lle L~e~ either 19 meaæured aeparately or using the spectrometer. This can be achieved by meaYuring the phase difference between the referenoe and mea~urement ~lgnal from which a si~nal proportlonal to L~e~ o~n be obtalned. ~hls means that the instrument after calibratlon by itself can mea~ure all dsta neceasary to caloulate the yas conoentratlon.
A computer ~lmulatlon has been made to verify the function of the la~er diode apectrometer. ~he followln~ pargmsters have been used.
4~ ~1 4~1513'~ PRIEIITEI ~07Z4 1~
Param~ter Typlcal Rang~ Unit value Modulation f requenGy~ ~/2~ 10 k Laser modulatlon IM, m 0 Laser modul~tlon F~, 2BL~ 5 1-20 ~Hzp-p ~hermal ~utoff fre~uency 10.1 k~z 10 Absorption at vO, ~(Vo)L ~ 1-3 %
Pre~s~re broad.coeff.(HWHM), V3 1 ~Hz/atm Pressure, p 1 0.5-l~S atm Temperature, ~ 273 ~elvin Ab~orptlon wavelength 765 n m Laser aenter wavelength 765 ~-0.02 n m Phase er~or, referenoe ~ignal 0 0-90 dogrees A triangular wave 1~ ~enerated direc~l~ from a dlgital control unlt ~ing an integrating ~P-amplifier. The laser blas c~rrent ls adJu~te~ YO t~at the laser light ia a~orbed in the gas molecule~ ~t the ze~o crossings of th~ trlangul~r wave. Thi~ triangular wave modulates the l~ser lntenslty ac~ording 4round P0 acaord~ng to flg. ~, c~rve 1 and the laser frequency around vO aocordin~ to ourve 2 at a frequen-~5 cy fO. After passage through the gas volume a mea~urement si~nal accordlng to ourve 3 ln fig. 2 1~ obtalned. ~he gain : Gl iB autom~tloally ad~usted a3 previou~ly descrlbed and the resulting measurement ~lgnal i~ ob~alned after sub~trac~lon of the monltor ~lgnal. Lock-ln detectlon at 2fo and 3fO wlth digltal reference signals provldes the iinal measurement signal and feedback 41gnal. Thi~ leads to a stralght forward instrumental deQlgn and provide~ ~igher ~ensitivity co~pared w$th the conventlonal app~oaoh u~ing s~ne-w~ve~. The de~e~ted olgnals ~ro~ an absorptlon line at the dlfferent lock-in ~ete~tlon frequencies are ~hown in fig. 3a. From flg. 3b it iq apparont th~t only 3fO i9 suitable for looking of the laqer, ~ince thl~ signal i~ insenRitive ~o remainlng intensity modulatlon a~ter the first demodulatlon step.
J~ IJ ~ C ~ i 26)07Z4~) Figure 3c ehows ~hat by ugirlg thls signal conditlonlng method a llnear re~ponse for yas concentrotlon measurements is obt~ined.
The measurement lt~elf ls performed ln A ~easurement volume that might be an atmospherio path, an ev~nescent opticel flber, or an optical probe th~t in a small volume creates A
suitable ~bsorptlon path. The di~ribution of llght both to and from the probe can be performed wlth optl~al wave~ul~es.
Thls means tha~ the sensor part of the in~trument i~
entlrely optlcal and iB very suitable ln potentially exploslve or electrically dl~tur~ed envi~onment~. Moreo~er, ---the attenuatlon ln optlcal flbers is very low why the sensor can be locHted several kilometers from the control electro-nics.
Using optical flbers provides 8 flexible lnstrumen~ which may be of modul~r de~i~n ac~ordin~ to ~ig. 4 whlch shows a measurement cell 4 lnteyrated ln the diode la~er spec-trometer. Laser light source l, mea~urement cell 4, andreferenoe C811 5 can be placed in an lnstrument box. The dlfferent module~ oan be constructed on standard europe circuit board~ in a standard 1~ n rack. The measuremen~ cell ls located inside the ln~trument ~nd the gas ~o be ~nalyzed i8 led to lt u~lny flexible hoYes 9 and 10. Other parts of the system includes a modulatlon ~nd oontrol unlt 11, ~
laser and pel~ier dri~er 12, an interface l~ and pow~r upply 14.
Figure 5 shows an alternatlve lnstrumental deslgn where a comp~ct measurement cell 4 ls connected to the dlode laser spectromete~ with optlcal ~iber~ 2. The me~Yurement cell 4 i~ deslgned to be integrated in e.g. ~ patient mouth pieCQ
to measure the oxy~en ~ontent~. The ga~ flow thro~gh the me~urement cell 4 ls indlc~ted with arrows. The measurement cell 4 is then connected to the recelver card with optic~l f~bers or electrical wire~ ~.
4 6 ~1 4 U ~ i U I ~ H I ~ 01~' ~ 4 15 ' ~ 4 The instrument ls lntended to operate completely automatic and therefor~ ~e adJu~tmQnt of laRer temperature, blas current, looklng to the a~Qorptlon line, and me~surem~nt of the abQorptlon ls controlled ~rom a computer, standard PC or equivalent, via an lnterface 13.
On the laser clrault board lfi (fi~. ~) thoRe part~ are lntegr~ed that determlne~ ~hich ~as that i8 to be measured.
10 A molecule 1~ selected b~ matchlng a sultable laQer 1 and a referen~e ga-Q oontained in the reference cell S integr~ted on the card. A la~er module 1 with fiberplgtall, an optlc~l power dlvlde~, referen~e cell, and electronio~ 1~ mounted on - the laser clr~ult board 16. The reference cell 5 i~ used to lS stabillze the laser to the sb~orption line and to provide normalizatlon valuQs for the calculation of the gas con-centratlon ~y locatlng the la~er and the referQnoe cell on the ~ame card spliclng of the fibe~ to the ref~rence cell 1Q
avolded which results ln a lower lnstrumental nolse.
~20 Moreo~er, a modular deslgn of the instrument i~ obtalned.
- The colllmated light out ~rom the measurement fl~er is centered on wavelength Of the a~sorptlon llne of the gss in the reference cell. This mean8 that mea~rement~ ~an be performed in Yeveral dlf~erent measurement sltuations, o.~.
over an ~tmo~pherlc path, ln ~n evane~cent fiber probe, ~
speclally designed optlcal probe, or in a oonventlonal mQasur~ment cell.
The modular deslgn o~ the speatromnter m~an~ that sev~r~l ~0 la~ circuits oan ~e multlplexed on the same fl4er, alt~er wlth wavelenyth mul~lplexln~ or alternAtively wlth time multiplexing there one l~er clrult ~t a ~me 1~ used . In th~ wa~ it i8 po~sl~le to measure sev~ral ~asn8 over th~
same me~surement p~th. You can also choo~e to multiplex one la~r clr~ult to several m~asurement fibers and in thls way measure one g8~ on Be~eral dl~erent measurement paths that oan be located kilometer~ ap~rt. Several componen~s ~uitable 46 31 4~513~ l~oT PRTEt~T~ 2~07240 for this multlplexin~ ha~ been de~eloped ln the area of optical fiber communioatlon.
Flgure 7 ~how~ a device where laser ~ircuits 16a-d, each S tuned to an ab~orp~lon llne, are multlplexed lnto a common mea~urement flbe~ 18 by ~lng a f~ber multiplexer 17.
~nother way of avolding ~ree alr paths ln the ln~trument ~nd obtain a eomp~ct de~lgn ls to use ylas~ pri~ms for l~ ght distrlbutlon be~ween laser and reference cell and for splltting the llght to a me~urement and a reference path.
Su~h an arrangement i~ shown in flg. 8 where the laser llght form the laser l 18 ~lstributed to mea~urement cell 4 and t~e reference cell 5 by ~ean~ of bQamsplitter 19 and a gla~
prism Z0, ~nd p~sed on ~o the d~ectors of ~h~ measurement 4' and refarenoe 5' cells.
Other parts of ths ~rr~n~ement ~re in and outgoiny ~lexlble tubes, 9 and 10, to the me~ur~ment cell 4 and a pel~ler cooler ~1.
Fi~ure 9 show ~ modlfled arran~ement aocording t~ which the lnstrument aan be ~onneotea to ~ compaot optical pro~e of the type sown i flg. 4 u~lng optlcal fibers 2 ~nd 6. A
seal-off val~e to the reference cell is marked wlth 22.
A ~inyle-mode fiber ~ ~8 connected to ~he optical probe to malntaln yood b~am quallty whlle a multimode flber 6 ls prefe~able to pass the llght back from the optlcal probe to collect ~8 much llght a~ po~sible.
~wo dlfferent methods csn be usQd for ~he slgnal condltlo-ning depending on what degree of ~n~trumental complexlty ~h~t can be accepted.
In the flr~t method, tran~mls~lon compensation followed by 46 .~1 4~51~ 1311~ P~TE~IT~ 26)07240 lock-ln detection, whleh i~ ~chematically lllu~trated ln fi~. 10, the la~er 1~ modulated wit~ a triangular wave which ls o~t~ned directly from ~e dlgital unlt and the received slgnal 1~ demodulated ln two ~tep~. Before the phase S sen~itive lock-in detection an analog Qlgnal condltloning i8 performed 32, where the si~nal from t~e monitor photodiode of the laser ls used to COmpenQate the meagurement slgnal for unwanted at~enuatlon caused ~y environmental ln1uences ln the measurement path and ln the optlc~ of the instrument.
The harmonics of the reoeived inten~ity, whlch 18 modulated by the ab~orp~ion line, 1~ thereafter detected with phase sensltlve demodulator~ (lock-in detection). ~he intenslty variatlons of the re~ulting normalized ~ignal wlll then only b~ depend~nt of molecular absorption as de~oribed in the invented slynal ~onditionin~.
The roference slgnals to the phase ~ensitive demodulator~
can be elther ~qua~e-wave or slne-wave, preferably square-wave. The phase difference betweon the recelved modulation Z0 signals and the reference signals i8 optlmized automatically by a computer. Tho pha~e dlfference between the~e slgnal~
can al~o be use~ to determlne the length of the mea~urement path, a value needed for the yas concentrat~on cAlculation.
The inorm~tion from the slgnal on the se~ond ha~monic 2fo is fed to an analog multlplexer and an A/~-converter fo~
caloulatlon of the ~a8 concentration. Informatlon from the third harmonio 3fO lB u~ed to stabllize the laser to the center of the a~sorptlon line via a PID-regulator. Thls slgn~l is also transferred to the computer to verlfy that the lase~ operate~ at the proper wavelength.
The seaond measurement method, ~chematically ~hown ln fig.
11, uses digltal modulation and measurement and i~ based on that the laser i~ modulated wlth a speci~l waveform and that the amplitude of the measurement and referenc~ si~n~ls are sampled with an AJD-converter. The gas concentratlcn i~ then calculated ln the computer. Information a~ou~ the la~er t t~ .~ 1 4 ~ 7 26)07240 wavelength oompared to the absorption llne i~ ex~racted from t~ese sampl~d value~ and the la~er blas current and tempera-ture 1~ adJusted qocordlngly. The gi~n~l analysl~ 18 of the same type a~ ~hat used in ref ~, but with the lmportant dlfference that we ~se a preprocesQlng with snalog signal condltlonlng to compen~at~ for nongasrelated attenhation, which result~ in hlgher m~asurement accuracy. The ~lgnal from the measu~ement cell 4 18 fed back to the electronic unlt using a multl-mode fiber or alternatlvely the detoctor can be located in the me~urement cell and the ~i~nal led back uslng ordlnary electrlcal cables~ In AlGaA~ and InGaAsP
lasers a conslderable ~ntensity modulation i9 obt~ined toge~her with the frequen~y modulation. In the inventlon thi~ i~ u~d to automatically comPensate for nonya~related variatlons in the transmis~lon ln the measurement path or ln the optlcs of the instrument.
Mea~urement ~ell In certaln measurement applications it i~ important to have 8 vory compact moasurement oell. Thls can be realized by "foldlny" tho cell a large number of tlmes. Each tlme thls foldin~ ocaur~ mean~ tha~ the beam iY refleoted again~t a mlrror whlch ~dd~ 108~8~ in to the beam path. ~hese los~es mus~ be kept low and moreover, the are not allowed to be strongly dependent of the optical wavelength. The mlnlmum c~ll acceptablQ oell tran~m1~g1on 1~ ~et by the Yignal-to-n-018~ ratio (SNR) requlred.
~0 One type o~ mirror thqt can be used here ls dielectr~a mirror~. Th~y have ~ limltod w~velen~th range but ~part from that they havQ very ~ood ~ectral chsracteri8tics lncludlng low los~es.
Fi~ure 12 shows ln por~pectlvo the measurement optlcal probe and fig. 13 ~hows a sea~lonal vlew through it to illustrate the beam path.
4~ ~l 4~ IjU~ t~ 0724 0 ~he measurement optlcal probe con~ists of a oylindrlcal ~ea~urement volume, wlth dim~nsions e.g. 20x20 mm. The oyllnder 1~ dlvlded in~o 12 segmentY each of 30 degre~s which each contaln~ a dielectrlc mirror havln~ a refla~tanoe o~ at lea~t 98~. The beam will be reflected 36 tlme~ in the probe which corre~ponds to a maxlmum attenuatlon of o.g836 ~
0.48 (3.~ dB~. ~he probe i9 fed from a slngel-mode flber (typlcal 5/125 ~m) and a lens which create~ a parallel beam with a dlameter of approxlma~ly 0.8 mm. ~he effeotlve path length in the probe ~B approximately 70 cm. The detectlon of the outcomlng be~ either performed dlr~c~ly on ~
photodiode mounted in the probe or via a flber with lnrge core diameter in which case the photodiode can be located far away fro~ the mes~urement volume.
A unlt 24 for coupllng of the light into the optical probe enables th~ all~n~en~ of the beam with a ~ufftclent number o~ degrees of freedom.
The materlal of the mea~urement optl~al probe can be aluminum, ~tainlQ~s ~teel, or a ceromic.
Another type of optloal probe 18 an optlcal fiber where the evanes~ent flel~ which propagat~Y out~ide the core i~ u~ed.
When llght prop~gates ln an optical fiber the main part of lt i~ confined to the core but a certaln part also pro-pag~te~ ln the claddin~ (the evane~Cent fleld). ~y using a sp~ciall~ de~igned ~iber that only consi~ts o~ a core and letting this fi4er be ~urrounded by a fluid that constltute~
the claddlng ~or olternati~ely let a Yurrounding gas constltute the oladdin~) th~ evanes~ent field and thereby al~o the total optlcal fleld will b~ lnfluenced in a slmllar way a~ w~en light propa~ates over an atmo~pherlc path. In thi~ ~ay it is po~sl~le to determinQ ths concentration of a ~peclfic g~Q ln ~ surrounding otmo~phere or fluid. Further-more, OTDR ( Optical Tlme Domain Reflectomet~y) can be usea .
' 4~ ~1 4~51~3 130T PRTEHTg 26)07240 2~
to locallze point emi~slons along an evanescent ~lber probe.
Fiber adapter ~t must ~e pos~ible to allgn the fiber tip so th~t the llght beam after passage through the mea~urement cell 4 or th~
reference cell 5 lmpinys up~n the photodetector (or out-coupllng fiber)~ Thls i~ obtained ~ing a fiber Lip equipp~d with a ~pherical adapter. T~lS arranyement provides sufflci-ent number of degrees of freedom in the ollgnment. The flberis allgned once and for all and then locked with a clamping sorew.
Reference cell ~igure 14 show~ in perspectlve the reference cell 5 whloh coneist~ of ~ rlgld plate 23, an lnooupllng unlt 24 and the measurement volume 25. Ac~ordln~ to one de~ign lt has en effectlv~ length o~ 10 cm and i8 ~onstructed for 8 p~Y~ages, i.~. a total mea~ur~men~ path of 80 cm. A ~eal-of valve ~6 and a quartz window 27 be~ween the incoupling unit 24 ~nd the cell lq also inoluded in the referen~e cell.
-Figure 15 shows in a larg~r soale ~ 4ection throu~h the~5 re~erence oell. ~wo dielectric m1rrors 27 and 28 wlth high reflectivlty ~typically b~tter than 0.98) i4 mount~d at both ~nds of the coll. In thls example the refe~ence beam i~
reflected 7 times co~re~ponding to o totol attenuatlon Of O . ~87 ~ O. 87 ( O. 6d~). The bose pl~te 23 ls deslgned to flt onto a standord europe clrcult ~oard.
Reference~
l. R.S. ~ng, ~.F. ~utler, and K.~. Linden, "~unable diode laxer~pectro~copy: an lnvited revlQw,~ Optical En~lneerlng, vol. 19, p. 945, 1980.
J 1 i .' U 1~1 1 1 H I L I S li j~ 1 J; ~ ;J
26)07240 2. S. Lundqvi~t, ~. Margolis, and J. Reld, "Measurement~ of pressure broadenlng coefficient3 of nitrio oxide~nd oonsing a computerlzsd tunable diode la~er spectrometer," Applied Optlc~, vol. 21, p. 310g, 1982.
ME;THOD AND APPARArrUS FVR SPEC~OSCOPIC MEASURE1~1ENT OF THE
CONCENTRATION 0~ A G~S
Thi~ lnventlon concerns a method and a~ apparatus to spec-trosoopiaally measu~e the concentration of a gas ln a sample, whereby the intenslty of llyht from a light souroe is passed through the ~ample and through a reference cell and is detected, and that a signal is generated which rep~e~ents the oonoentration of the gas. The li~ht source 10 consists of a laser diode whlch i~ locked onto the absorp-tion llne of the ~as at known preQ~ure and ooncentration contained in ~he refe~ence cell. In order to avold inter-ference from the atmosphere the laser light of the seleoted wavelength ls distrlbuted ~long optical fibers and/ or glass prisms to ~he referenoe cell and to the ~ample. ~he in-vention al80 compri~es an apparatus fo~ spectroscopic measurement of the eoncQntratlon of a gas ~ e . ~ . oxygen ln a sample. ~n the apparatu~ the modulated lntenslty of ll~ht transmitted through the sample and reference cell i5 ~0 dete¢ted and n slgnal proportional to the ratio of ths demodulated int~nBities i8 ~enerated which repre~ents the cQncentratlon of the g~.
Background of the lnvention ~5 Tunable diode laser~ have quickly become lmportant for h~h resolu~lon in~trument~ for analysls of molecule ~pectra.
~o~t of the sppllcatlons for alode laser spect~oscopy have so ~ar been in baslc research. Lead salt diode la~ers emitting ln the wavelength range of 3-~0 ~m have been used to study absorptlon lins parameters and for measurement of molecule constants (1,2~. The po*sibllity to lndirectly determine different parameters such as pres~ure, tempera-ture, or elec~ric~l field strength has ~1B~ been successful-ly demonstrated (3,4). ~here ha* ~een a relatlvely ~mall interest for the overtone band~ around 1 um ~ince these donot signlflcantly contri~ute to any new lnformatlon on the 4~ ~1 4~ MIB ~ 01~4 15'~ 05 26:)07240 analyzed molecules. The main llmitation for pr~ctlcal use of lead salt dlods lasers ln measurement systems have been the need for cooling of to temper~ture~ below that of liquld nitrogen. AlG~As and InGaAsP diode lasers emit in the 0.68 - 1.7 ym wavelength ran~e and can operate at temPeratures up to 5~ degree~ centigrade. A characteristlc of these lasers compared to lead salt laserB 18 ~heir hlgh inten~ity modulation lndex which requlre~ ~peclal sign~l analysls tO
obtaln optimum ~ensitlvlty. A large number of molecule~ of interest ln proceQQ control and environmental me~urement applicatlons have ab-aorptlon bands in the 0.7 - l. 7 ~m range, although, the~e ~re relatlvely we~k overtone band~.
The superior performance cf AlGaAs and InGaAsP l~erY
compar~d wlth lead salt lasers however, more than outweigh~
this difference and they have grea~ potentl~l ~o bs used in instrumental application~ (5).
A very interestlng applic~tlon 18 for measurement of oxygen concentratlon, whlch ha~ m~ny practlcal appllcatlons e.g. in the medlcal are~. Today, oxygen is usually measured uslng dlffe~ent electrochemlcal methoda. For ex~mple qemlconduotor sensors or the transformat~on of 2 to secondary products for later detectlon ia freguently used. These methods hsve several llmltations. They don't perform real-~1me measure-ments and are usually not sultable ln exploslve envlronments~nd moreover, they often lnfluence the me~urAnd slnce ~
~m~ll amount of the moasured (oxygen) gas 19 consumed by the measurem~nt instrument. These meQsurement in~truments ~re Qlso often sensltlve to other types of gases or organic ~0 pollutlons whlch results ln ~ conslderably limlted lifetlme.
From EP-Al-0 015 170 lt 1~ known that by uQing spectrometers with reference ~nd measurQmen~s cells and by u~lng optical flbers or glas~ priSm and gas-tight boxe~ interference with the ~urroundlng envlronment 18 avolded. ~hls ls true concernlng the ellmlnation of unwanted contrlbutions to the measurement result from other path~ than the ~ea~urement ~ :
J~ y ~ 'UItlllt~ 4 1~6yo7~40 ~6 path. However, the problem of elimin~tlng the lnfluence of nonga~related ~ran~mi~8ion varlatlons ln the measurement path or optlc~1 probe i~ not addre~sed at all.
5 In DE-A1-3.633.93 the la~er i~ not locked onto the frequency of the ab~o~ption llne. In~-tend, the laser current i~
modulated and the absorption spectrum is studied. The absorption line i8 then sampled ~s the current 18 modul~ted to determlne the gas absorption. ~ur~hermore, the speotrum ls ~ampled on 40th ~ldes of the line to measure the non~as-related a~tenuAtion. ~he slgnal conditioning ls performed direotly ~rom the ~ampled spect~um which re~ults ln poor accuracy ~ince the meagurement value is th~ result of the substractlon of two, nearly equal, numbers. ThiY method i~
the most commonl~ used to avoid measurement errors from attenuation caused b~ fog, rain etc.
Purpose and slgnl~loant characterlstlc~ of the inventlon The purpose of the pro~ent lnvention ig to ogtablish a measurement method which ls not lnfluenced by the environ-ment outslde the meaQurement volume or cell (the claimed optical probe) or b~ the envlronmental properties in the measurement path and ln addltion has a short response time.
~5 This has been ob~aln~d by looking a laser diode onto the absorptlon llne of the sas in questlon in a ~eference cell containlng this gas at known conCentration and pre~sure and thereafter deteotlng the harmonios at 2f~ that are generated from the lnteraotion betweQn the absorptlon lin~ and a trlangular modulatlon current of ~requency fO superlmposed on the laser blas current and ~urthermore, to avo~d interferen-ce from the a~mosphere optical flbers and/or glass pri~m~
are used for the distribution of the modulated laser llght with the selected wavel~ngth tc the reference cell and the 3S optlcal pro4e or measurement path, and that the si~n~l ~ondition$ng compensate~ for nongasrelated transml~sion vari~tions ln the measurement pat~ or the opti~al probe by 4~ ~l 4~ 'hlENl~ "3~ 01/~4 15~ 7 2 ~0724 0 using a tlme-multiplexed dual-beam technique. The pre~ent ~nvention also lncludes an apparatus for the method descrl-bed where optical components of with low reflectivity (e.g.
AR-coating o~ optic~l ~urfaces) are used to mlnimize interference problemY. Moreo~er, multip~exing technique oan ~e used to cover ~everal meaYurement paths with Ju~t one laser or to msasure ~everal gases with ~everal lasers utlllzlng the same mea~urement path and distributlon nst.
De~criptlon of the drawlngs The invention wlll in the followlng be d~cribed with referenc~ to the encl~ed drawings.
~ig. 1 shows a ~chematic of thè electrooptlcal g~s ~ensor accordin~ to the ln~ention.
Fig. 2 shows the measurement method with the used modulatlon ~ignals and the corresponding detected waveforms afte~
ab~orption in the mess~re~ ga~. S~ecifically, the lntonsit~
modulation o~ the la~er, the ~re~uen¢y modulatlon of the laser, and the deteated lntensity var$ation after pas~age of th~ measurement volume ~re lllu~trated.
Fig~ 3a ~hows computed ~lgnals ~fter compensation for the intensity modulatlon of the laser and the followlng lock-in detection.
Flg. 3b i~ the same a~ fig 3a but ln thlo ca~s for an incomplete lntenslty compensation resultlng in a remalning lntensity modulation of l~.
Flg. 3O ~hows the mea~urement ~i~nal as a functlon of the gas concentratlon after ~lgnal conditioning.
Fig. 3d shows how the output from the lassr ~pectr~meter varles with phase error ln the lock-in detectlon.
.
" ~1 J ~ Ul.l l I H I ~l~ l b ~ Jt ~
Flg. 3e shows ~e ou~put Qlgn31 from the la~er diode spectrometer as a fwlction o~ the frequency modulation of t~e laser.
5 Fi~. 3P Qhows the output from the lager speotrometer as function of the total presBure in the measurement volume.
Flg. 4 shows the ba~ic structure of the lnstrument uslng optlaal fi~ers accordiny to one embodlment of the invention.
Flg. 5 ehows a laser clrcuit ~oard for the laeer wlth the reference ~ell.
Flg . 6 shows a -Qystem with several laser clrouits who' 8 lS output iY multlplexed l~to o~e fiber for slmultaneou~
measurem~nt of eeveral gases.
Flg~ 7 shows a ey~tem wlth ane laser cirouit ~hich i8 multiplexed to ~everal fibers for simultaneous mea~urements of one gas over ~everal ~ifferent measurement path4.
Fig. ~ ehows a schematlc of the basic apparatus deslgn Ueing yl~s~ pri~ms for llght dis~ribution.
25 Fig. 9 ~how~ a echematic of the ha~lo Hpparatus deslgn uslng both glass prlsms and optlcal flbers.
Fig. lO shows a ~lock dlagram of the measure~ent s~stem lncluding ~ansmlssion compeneation an~ lock-in detectlon.
Fig. 11 ehow~ ~ block diagram o~ t~e measurement ~y~tem u~ing dig~tal ~odulatlon and mea~urement control.
Fig. 1~ shows, in per~pectl~e, an exploded view of the optical measurement probe.
F~ g . 13a showQ a hori~ontal sectlonal view throu~h the iUI ~U~ 07240 mea~urement probe and 13b a vertical ~ectional view through it~ lower part.
Fig. 14 shows, ln per~pective, a reference cell.
Flg. 15 ~how~, ln a lar~er ~cale, a ~ection through the reference cell.
Descriptlon of the lnventioA
The invention concerns a method to analyze gases, e.g.
oxygen, u~lng a la~er dlode ~pectrometer. A special signal conditloning method for diode laser spectromet~rs, based on diodq la~ers ~lth high inten~lty modulation index, has ~een lS developed. Interferenoe from nonga~related transmisslon ~arlations s~ch as fo~, rain turbulence ~nd v~rylng trans-ml~slon through the optlcs of the apparatu~ caused hy du~t particle~, condense etc. ls taken care of by the intenslty modulstion compen~atlon ~n a unlque manner. Inter~erence 2~ ~rom gases not contained ln ~he measur~ment volume i8 eliminated by u~lng gla~s w~veguides in the in~trument. The mea~urement~ can be per~ormed over an atmospheric path, in an with the lnstr~ment lntegrated meaqurement volume to w~lch the gas 18 transferred uslng e.g. tubes, uslng the evane~cent fleld of an opticAl ~lber surrounded by a medium contalning the gas to be measured, or in a ~peclal measure-ment cell, a unl~uely de~igned optical probe to whloh the light i~ pas8ed ln opt~cal flbers.
A meaYurement cell, a ~elf-allgned optical probe that 1~
sufflclently small to be lnte~rated ln e.g. the mouth pleoe of a patlent snd ~111 gi~es a measurement path of suffici-Qnt length to perform aocurate measurements of oxygen, has been developed specifically to be used wlth the spectrome-ter.
~he in~trument 1~ deYlgned ~o that all measurements of 41~ FIRIEN~ 4 15:1~ 0 dlfferent parameters iQ au~omatically callbr~ted if you knowthe correspondlng parameter~ for the gas ln guestlon in a reference cell. ThiY al~o lncludes the elimination of lnfluences from fog, raln, du~t vr oondenRe in the measure-ment volume or in the optlcY of ~he instrument itself. ~hlsmeans that you are independent of publiQhed n~mbers on line strength etc. A ~peclal compaot referenoe ~ell that oan be included on e . g . a ~lmple standard europe-circult board has been developed. ~he measurement re~ul~s from the measuremen~
~olume ( atmo~pherlc path, optlcal probe or evanescent flbe~
probe) are compared wlth the correspondlng rQ~ults from the reference cell~ The gas concentra~lon is computed from two length~ (tho~e of the reference ~ell and the measurement path) and the known concentratlon in the reerence cell.
Thls results in a ver~ slmple calibration procedure and high mea~urement accurac~.
~elow i~ a 118t 0~ a number of ~pplication areas for th~
in~ented dlode la~er spectrometer.
~edlcal appllcatlons a)~Measure~ent of the oxygen concentratlon ln breathlng alr.
Here a ~peclal optio~l measurement probe has been ~eveloped th~t can be lncorporated in e. g. a re~plrator. The optic~l probe le connected to the mo~th piece a~ the patlent ~nd ~he light is pas8ed to the probe u~lng optical waveguides (optical flbHr~).
b) Control of ane~thetlo gase~ ~urlng oper~tlon.
c) Control of oxygen or humldlt~ in an lncubator.
d) Studle8 of metabollsm. Here an isotope ga~ can be used ~ince the hlgh re~olutlon of the ln~trument 1~ sufficlen~ to dlscriminate thl~ from the nat~ral abu~dance of ~he gas.
~1 4~ 'HI~ Y~ 15ll3 e) Pharma~eutieal productlon. Inert packaging of medlcal~.
Indu~tri~l applioation~
S ~) Safe mea~urement~ of oxygen or ot~er gase~ in explo~ive ~nvironments ~uch a~ pe~rol supplles, oll tankers etc. ~he light i~ passed to and from the optlcal measurement probe using optlcal ~veguide~. The pro~e can al~o be located in environm~nts wl~h ~trong electromagnstlc lnterference effects such a~ ln a radar ~et.
g) ~easurement of gas concentr~tion in environments with strong radar or radio interferenc~. Thls means that the laser diode ~pectrometer o~n be incorporated in a levelmeter based on microwa~e teohnology.
h) Control the presence of oxygen in lnert envlronments.
i~ Control of combu~tlon proce~e~.
J) Automotlve lndustry an~lysi~ of ~xh~ust yase~, con~rol of air/fuel mlxtures.
k) Heatlng And electriclt~ produclng plantss con~rol of combu~tlon, minlml~ation of pollutlon.
1) ¢~s productlon.
m) Lamp manufact~rlng.
n) Semloonduotor lndugtry: oontrol of ga~ concentrat~on in dif~erent prooe~se~, ln o~nY, glo~eboxe~ etc.
o~ Procn~ control ln unnealing and oxldation own~ .
p) ~ontrol of fermentation pro~esse~.
. --4~ lllt~ 4 1521~o724 q) ~ood 1ndustry; beer production, packaging.
r ) Mea~urement of temperature pro~iles ln oxygen contalning atmosphere in proces~ oven~.
s) MeaYuremen~ of pressure ln gase~
t) Measurement of pre~ure ln prooes~e~, t~nk~ etc.
u) Galvanlc insulated measurements of electrlcal ~ield streng~h.
v) Me~urement of g~ ooncentratlon~ in ~luids, ~.~. gase~
ln a b~ttery or the oxy~en contents in a lake or sea using an evane~cent flber probe.
x) MeaBuremont of le~kage from ga~ tubes b,v using an evane~cent fiber probe that ls dsployed in p~rallel wlth the gas tube.
Dlode laser spectrosqopy has ~raditlonal~y been used to an~lyze the Pundamental IR-band at 3 -30 ,um. ~a~e~ llght is selectively ab~orbed at frequencie~ ~ vz V3 ... whlch ~orre~ponds to fundsmental rot~tional and vibr~tional tr~nsitlonY. ~he l~ser~ thnt h~vo been u~ed in thls WBV~-length r~n~e are co~plica~ed and expensive. ~ large number of molecule8 have oombination and overtone band~ o~ the 2v, ~ V2 type ln the w~volength range 0-7 - 1-7 ~um- These bands u~ually don't ~ontribute any new information about molecul~r const~nt~ but 4~ well ~ulted for mea~urement~ of the concentration of tho flbsorbing gas. Thi~ ls espeoially true since in thi8 ~avoleng~h range there exi~ts a large number of inexpensive hlgh quallty l~sers.
In the following, moa~urement of oxyg~n ls used to illu~tra-te the prin~lple of operatlon of the inventlon.
4~ ~1 4~1513~ bOI P~TEI~T~ 4 15'14 Z40 ~13 ~he oxygen molecule ha~ a number of weak ab~orption bands yenerated by magnetic dlpole transitions. ~hese bands are looated arount 630, 690, and 760 nm. The A-band of oxygen covers the ~avelength ran~e of 759 - 771 nm and 1B relatlve-1~ free from interferlng molecules why it i~ suitable formeasurement of ox~gen uQlng electrooptic techni~ues. AlG~Ao or InGaAsP lasers a~e sulta~le light source~ for msa~ure-ments ba~ed on selectl~e absorption o~ l~ght by dlscrete mol~cule line~ slnce they have yood spatial and temporal coherence. By using oxygen at kno~n concen~ration and pre~sure in a reference cell a normall~ation ~ignal i8 obtalned and al~o the laser can be looked to the ab~orptlon llne o$ oxygen. ~hls enables very ~oourate ~easursment~ of the oxygen concentration in the mea~urement volume.
AlGa~s, InGaAsP and other semlconduator laserY can ea~ily be frequency modulated by means of the inJectlon current and can therefore be u~ed $n differential speotroscopy to obtaln a good eignal-to-nolse ~atlo. Another ohar~cteri~tic of 2~ the~e laser~, compared with e.g. lead salt lasQrs, 18 that apart from the frequenoy modulatlon a con~ldersblo intonsity modulatlon i~ obt~ined at the re~uency modul~tion that i~
optimum fo~ speotro~oopla me~urements. The intQnnlty modulation ls used to elim~nate the influenoe of nonga~-related ab~orptlon in the me~urement ~olum~ or ln theinstrument itself w~thout h~vlng to use the basellne of the nb~orptlon spectrum. Thl~ pro~ides superior stabillty and accuracy. When ono me~sure atmospheric gases it 1~ importan~
to ~llminate absorptlo~ ~ air path~ to ands from the me~urement volume. By u~ing gla~ prism~ or optical fibers thi~ unw~nted absorption can be avolded.
Fig. 1 show9 a 8chomatlc of the ele~trooptical ~a~ yQn~or accordlng to the inventlon where 1 i~ a ll~h~ 80urce, la8er diode , from ~hich th~ light i~ di~tributed in optlcal waveguide~ 2 and 3 to a measurement cell 4, whlch oan be an optical probe or sn atmospherlc path, and to a reference J ~ j IJ U I I 1 l i L I ~ I Ll .! ~ 14 cell 5. The ~ea~urement cell 4 ~nd the reference cell 5 ls conne~ted with a control unit 8 u~lng optlcal waveguldes 6 or electrlcal cables 7.
5 The re~erenoe cell 5, whioh contalns oxyg~n at known concentratlon, pre~sure, and temperature, provides the p~rameter~ neces~ary to lock the wavelenyth of the llght ~ouro~ to the absorption line, and the para~eters neoessary to caloulate the gas concentration, the temperature, pressure, and electrlc field stren~th. Thls 1~ pe~formed acco~dlng to the following. A trlangular AC-¢urrent with frequency f~ is added to the laser bias curr~nt. The bia~
current 1~ adJu~ted so that the average wavelenyth of the laser co$nclde-4 with that of the ~bsorption line. The llght source 1 wlll, due to the added AC-current, ohange itR
wavelength (frequency modulated) with the frequenoy fO around the averaged value ~et by thQ bla~ current. The light, now fr~quency modulated ~round the oenter of the ab~orption line of the ga~ p~sed through the measurement ~ell ~an st-mospherlo path, an optloal probe, or an evanescent flber)and is thereafter detected. By feedback of th~ slgnal of 3b vla an analog ~egulator to the laser bias current the wavel~ngth of the llght source 1 iY stabill~ed to the center of the absorption lino~ The llght from the llght souroe 1 is divided lnto to b~anches, one which ls pas~ed to th~
re$erence oell 5, nnd one which ls pa~8ed uslng optlcal wa~eguldes 2 to the mea~urement cell. The measurement signal i8 ~hQn ya~8sd back and lock-in detectsd at frHquenoy 2fo.
~hen the llght 80uroe 18 modulated wlth a frequenoy fO and the slgnal from the detecta~ 1~ demodulated with the frequenoy 2fo~ thls ~l~nal lookQ accordlng to flg. 3a. The signal 2x~ ln flg. 3a 1B proportionsl to the gas concentra-tion.
Apart fro~ the vPri~tlon ln ab~orptlon cau~ed by variatlonQ
ln the conce~t~atlon of the measured gas there wlll be variatlons ln the transmission caused by ~hanges in ~e-4~ :~1 4~151~ 130T P~lEi`lTB "~ 4 1521~o7Z4O B15 tran mls~lon o~ the optlc~ due to rain, dust, or humidlty or~hen the received ~ignal after the measurement path varies d~e to turbulence, raln, and fo~ etc. Normally, this 1~
compen~ated for by me~suring the basellne of the absorption spectr~ ~nd ma~e a normallzatlon. ~hl~ ba~ellne value however, ls a slo~ly varyiny signal with a very low lower frequency llmlt whlah lmpose~ drlft problema. Furthermore, preamplifler~ des~gned to accommoda~e these low frequencle~
have much le~s sen~ltivity th~n AC-ooupled ~mpllflers oper~tlng with a ~mall bandwldth.
In medsurements of gas ~oncentratlon~ it ls v~ry lmportant to compen~te for absorption that can arise in alr paths that exlst ln the dlstrlbution of the ltght to and from the mea~urement area. ~y u~ng glas~ prlsm~ or optlcal f~bers thl~ unw~nted absorptlon can be ellminated. ~urthermore, it ls also lmportant to compensate for varyln~ trans~lsslon ln the instrument ltsel~ or in the mea~urement path th~t i8 not rel~d ~o the measured ya~. Thls o~n be sohieved usln~ a slgnal condltlonlng whl~h identifle~ this componont and compen~ate for it wlthout ha~ing to use the bs~Qline of the absorption spectrum. Thi~ technique pro~ldes ~uperlor ~tablllty and measurement accuracy.
25 In the Ry~tem accordlny to the lnventlon a ~lgnal pro-portlonal to the atmospherlo ~ran~ml~slon 1~ generated by sampling at the peak of ths recelved trlangular~ly shaped optlcal slgnal. Thu peak value ~ compared with the oignal ~rom the monltor photodlode. The peaks of the trlangular signal 18 ~ell separated from the absorption peak a~
lllustrated in fig. 2. ~he relatlve size of the ~bsorptlon peak i~ usually much smaller than w~at ~s shown in the figu~e. ~hls method of m~asur~ny both on the peak~ o~ the trlangular a~gnal ~nd on the ab~orption lln~ g~ne~ates a m~a~urement referen~e that autom~tleolly experlence~ the ~ame transml~ion losse~ a~ the ll~ht ab~orbed by the ab~orptlon line ~lnce by deflnitlon the ~eamB are coali~ned.
4~ ~1 4U5 1;~ 3 Ijll l F'~ I EN I B 5;y~07Z40 A "dual-beam spectrometer" has been re~ll2ed by time-multlplexlng of one mea~urement beam. In re~. 7 the absorp-tlon 1~ determlned ~rom ~ampling direo~l~ in the ab~orpt~on Rpectrum around the llne. Ho~ever, no modulatian technique ls u~ed why the drlft problems of thi~ type of in~trument 1~
considerably large. In our method lt 1~ very ea~y to calculate the gas coneentration u~ing a normall~atlon procedure con~ldering that we know the gas ~onoentra~lon ln the rererenae oell. The con~entr~tlon i~ computed ~rom th~
length~ of the pa~h in the measurement and reference ~ell and f~om the meaQured de~ector signals from the measuremen~
and re~erewe oh~nnel-Q a~ well as tha ga~ concentrat~on in the reference cell.
ThlS method dlffer~ consider~bly from those used in ref. 6, 7, and ~, where ~he la~e~ light, which is unmodul~ted, is ~lowly varied around th- ~bsorption line and the re3ulting speotrum 1~ sampled relatl~e to the b~seline. Normally, this iB the method u~ed ~o compensate for nongasrelated tr~n~mls-sion variation~. Tho ~ign~l c~nditloning method accordlng tothe inventlon, whlch ha~ been do~cribed here, p~rformg all ~ignMl conditloning ~t hlgh ~udio ~requencle~, res~ltlng ln superlor stability, messurement aocuracy and sen~itlv$ty.
To determlne the pressu~o ln tho mea~urement volum~ we procned aacordlng to the followiny. Slnce the width of the absorption llne depends on pressure Qnd temperatur~ one parameter ~as to be i301~ted lf we are going to be ~ble to determine the other. We let the wavelongth of t~ light source sweep ove~ the ~bsorption llne. The light 1B th~n detected aftor pa~sage through the ga~ and the ~idth and the lntegr~l of the a~sorptlon line ls measured slmultaneou~ly.
The reQ~l~ of the inte~ratlon gives the in~egrated l~ne strength ~hich 18 ~ par~meter whos temp~rature dopendence ls known 2n~ ~s ava~lsble ln the litersture. S~nce the pre~sure and temperature in the referenco cell 5 1~ known, the presgure ln the me~oùrement volume can be calcul~ted.
~t~ Jl~ UI ~HI~ 26~07240 1~
The temperature can be me~sured with high ac~uracy by u~lng two la~er olrcuits where the laser~ are locked to two different rotatlonal transition~ in e.g. oxygen. ~he laser llght from the cardY are multiplexed to~ether ln an op~lcal fiber a~ shown ln flg. 7 and the combined llght i8 passed to the measurement volume. The temperature i~ then calculated from the difference in ab-~orptlon in th~ two wavelengths.
Thi~ is a new wa~ to 8pply Mason's method. In thls way the temperature can be meas~ed locally and int~in~lc~lly safe u~$ng the optical probe or alternatively, the average temperature over an a~mospherlo path can be determined.
An electrlcal field ~treng~h 18 measured in a slmilar way by sweeping the la~er wavelength ove~ the a~sorption llne. In the pre~enoe o~ an electrical fleld ln the gas the absorp-tion m~xlmum will be changed or di~plaaed in wavel~ngth.
Sinoe we know the posltion and st~ength o f the ab~orptlon line in the reference cell 5, thls dlsplace~ent oan be determined whlch l~ ~ msasure o~ t~e elsotrical fleld s~rQngth .
Strong magnetlo flelds reqult~ ln a dlrect modulatlon of the gaQ ooncentration in a paramagnetic ga~ as oxygen. ~h~
~5 v~rlation in ga8 concentratlon glve~ a direot measure of the magnetic fleld v~rlatlons. With the Qpeotro~e~er con~i~ured for a free measurement path ~ield varl~tion~ batwoen di~ferent path~ can be analyzed.
It i8 u~ually advantageous ~o a large axtent u~e dlgital ~ectronlc~ ~o generate t~e modulatlon and reference signal~
in the la-~er dlode spectrometer. For lnst~nce it iq ~a~ier to obtain a high degree o~ pha~e ~tabillty co~pared to an analo~ solution and moreover, lt is easler to d~gitally oontrol ~he frequenay and phase.
It is al80 advant~geou~ to use a (digltal) square-wave 4 ~ 3 1 4 ~ 5 ~ H I E ~ 3 F~ ~ I J ~ 4 1 5 ~
1 F;
signal to demodu~te ~he mea~urement and refQrence sl~nal~
ln the ~pectrometer. This ~voiclQ a squ~re-wa~e to ~inu~-wave transformatlon ~hlch if performed wl~h analog technique can lead to drlft ln the phase and decreased meaSurement ~ccur~cy . Tradi~lonally, the modulation slgnal ln laser d~ode speotrometer~ ha~ been ~ sine-wave but lt would ~e a clear advantaye lf ~ ~impler waveform could be used. An ideal alternative would be a triangular wave ~hlch ls easy to generate from ~ square-wave wlth an AC-coupled integra-tor Flgure 2 illustrates the slgnal condltloning used ln the lnventl~n. ~he laser iy modulated wlth the current l~t) ~ io~ 1 + g(t) ~ where g(t) ls a trlangular wave ~ith the amplitude a~
The la~er wlll be modulated both ln intensity ~flg. 2~
PL =PO [1 + mg(t~] and ln fre~uency (flg 2:2), VL - VO ~ 1 t Bg(t)~ with thn frequ~ncy fO.
~ere, m ~ intensity modulation lndex oP the la~er ~nd B i~
th~ freguency modulation index.
When the optical signal sweepq over th~ abso~ption line of the measured gas lts frequency modulation wi~l transform lnto an amplitude modulatlon accordlng to $ C. NL/n * ~7~7~*~v"/~(Bg(t))2 ~ ~,v2]
herH S, NL and ~v~ are confitants oP the moleaule llne and o~ is the ga~ conaent~tlon.
A compo~ed ~i~nal ~ith the Pundamental frequenc~ fO ls ` obtained according to flg 2:3 ~(t) ~ G~ PO ~ (l+m ~t)) (1 - ~(t~
The AG-c~mponent of this ~lgnal ls v,(t) Gl PO ~ [m g(t~ - (1 + m g(t)~ ~(t~L~ ]
4~ ~1 4~513~ ~OT P~TE~IT~ 4 1~707Z40 ~1~
1~
The slgnal ln the measurement ch~nnel 1~ now sub-straoted wlth the monltor ~lgnal whlch after AC-coupling osn be expre~ed a~
v~on(t) G2 P0 m ~(t) The detec~ed measurement ~ignal now has the form ~e~(t)~ GiPo m g(~) - GIPO~e~m g(t) - (1 ~ mg(t))~m(t)L~]
This ~ignal ls ~ampled at the peaks of the triangular wave and thereafter the analog slgn~l oondltioning unit ad~ust~
the gain ~1 so that ~e~(nT)~ G2Po m g(nT)-G~P0 ~ mg(nT)-(l+mg(t))~(nT)~] . o whlch re~ults in Gz m g(n~) G, -~ m g(n~ 1 + m g(nT)} ~[n~]L.~
In all practlcal applicationo tho residu~l term du0 to the eXtension of the ab~orptlon llne out to tho peak of the triangula~ wave will be neylect~ble and the expression for Gl becomes Gl = G2 / ~
¢onseq~entl~, wlth this method the system wlll automatically compen4ate for variations in tran~mlssion in the me~surement path. ~he m~surement signal ~fter thls compen3ation i8 v~ G~ P0 (1 ~ m g(t)) ~[t] ~e~
The mea~urement sl~nal i~ demodulated ln two ste~Y accordlng to flg. 10. The ~lr~t step ls the analog slgnal condltionlng unit with the transmi~sion oompensation de~cribe~ above. ~n ~l~ JI 't'l~ IJI~ lLl~ U Ul~ J-Io step two the compen~ated measurement.~gnal i~ ~nchrono~sly demodulated usin~ a ~quare-wave. ~he oompensated mea~urement slgnal i~ dlvided accord~ng to lts frequency components V~(t)-G2po~n~{l~m8/n2~ 3-l)/2~/J2 * c ~.1,3.5 * H~(v)~ c~9 tiwt]
By multiplying wlth a sgua~e-wave of frequency 2fo and perfor~ a lo~-paæs fllterlng the flnal ~ignal læ obtained Vme~ = ~;2 Po Lm~{H2(~)~ ~ m8/Tt~ [(H,(v), + H3(v)") ~coY[~
- Hl( v )"/18 cos ~3~] ~ H3( v )",/50 coY ~ 5~3~ ] }
~he phase ~ læ ad~u~ted to mlnimi~e the residual t~rm. In the viclnity of the line center v ~ vO ~ Hl(v)" H3(v)~ h 0 and thu6 the measurement signal a~ter lock-in det~ctlon becomes - v",,. - G2 Po ~ H~ ~ v ) al In the ~ame manner, ~he slynal from ~he re~eren~e detector finally becomes .
~r~t ~G~ P0 Lr~ H2(v)r These two slgnals are Yamplod, A/D-converted and dlvld~d ln the ~omp~ter v,~.~vr.~ ~ G2 Po L~., H2(v)~/ G2 Po Lre~ H2(V)r ~he ~ignal HltV), 18 proportional to the ga~ concentratlon o~
and the laYer d~ode ~pectrometer calculateR the ga~ con-centratlon ln the measurement path aæ
~ ~1 4~ 1..s~ bIJ I ~'hI~ 2~)0724~) C~ V~,~a/V,~t ~ ~ Lr.~ Cre~/ L,~.~ ]
The constants Lr~s and cr~ ar~ known w~lle L~e~ either 19 meaæured aeparately or using the spectrometer. This can be achieved by meaYuring the phase difference between the referenoe and mea~urement ~lgnal from which a si~nal proportlonal to L~e~ o~n be obtalned. ~hls means that the instrument after calibratlon by itself can mea~ure all dsta neceasary to caloulate the yas conoentratlon.
A computer ~lmulatlon has been made to verify the function of the la~er diode apectrometer. ~he followln~ pargmsters have been used.
4~ ~1 4~1513'~ PRIEIITEI ~07Z4 1~
Param~ter Typlcal Rang~ Unit value Modulation f requenGy~ ~/2~ 10 k Laser modulatlon IM, m 0 Laser modul~tlon F~, 2BL~ 5 1-20 ~Hzp-p ~hermal ~utoff fre~uency 10.1 k~z 10 Absorption at vO, ~(Vo)L ~ 1-3 %
Pre~s~re broad.coeff.(HWHM), V3 1 ~Hz/atm Pressure, p 1 0.5-l~S atm Temperature, ~ 273 ~elvin Ab~orptlon wavelength 765 n m Laser aenter wavelength 765 ~-0.02 n m Phase er~or, referenoe ~ignal 0 0-90 dogrees A triangular wave 1~ ~enerated direc~l~ from a dlgital control unlt ~ing an integrating ~P-amplifier. The laser blas c~rrent ls adJu~te~ YO t~at the laser light ia a~orbed in the gas molecule~ ~t the ze~o crossings of th~ trlangul~r wave. Thi~ triangular wave modulates the l~ser lntenslty ac~ording 4round P0 acaord~ng to flg. ~, c~rve 1 and the laser frequency around vO aocordin~ to ourve 2 at a frequen-~5 cy fO. After passage through the gas volume a mea~urement si~nal accordlng to ourve 3 ln fig. 2 1~ obtalned. ~he gain : Gl iB autom~tloally ad~usted a3 previou~ly descrlbed and the resulting measurement ~lgnal i~ ob~alned after sub~trac~lon of the monltor ~lgnal. Lock-ln detectlon at 2fo and 3fO wlth digltal reference signals provldes the iinal measurement signal and feedback 41gnal. Thi~ leads to a stralght forward instrumental deQlgn and provide~ ~igher ~ensitivity co~pared w$th the conventlonal app~oaoh u~ing s~ne-w~ve~. The de~e~ted olgnals ~ro~ an absorptlon line at the dlfferent lock-in ~ete~tlon frequencies are ~hown in fig. 3a. From flg. 3b it iq apparont th~t only 3fO i9 suitable for looking of the laqer, ~ince thl~ signal i~ insenRitive ~o remainlng intensity modulatlon a~ter the first demodulatlon step.
J~ IJ ~ C ~ i 26)07Z4~) Figure 3c ehows ~hat by ugirlg thls signal conditlonlng method a llnear re~ponse for yas concentrotlon measurements is obt~ined.
The measurement lt~elf ls performed ln A ~easurement volume that might be an atmospherio path, an ev~nescent opticel flber, or an optical probe th~t in a small volume creates A
suitable ~bsorptlon path. The di~ribution of llght both to and from the probe can be performed wlth optl~al wave~ul~es.
Thls means tha~ the sensor part of the in~trument i~
entlrely optlcal and iB very suitable ln potentially exploslve or electrically dl~tur~ed envi~onment~. Moreo~er, ---the attenuatlon ln optlcal flbers is very low why the sensor can be locHted several kilometers from the control electro-nics.
Using optical flbers provides 8 flexible lnstrumen~ which may be of modul~r de~i~n ac~ordin~ to ~ig. 4 whlch shows a measurement cell 4 lnteyrated ln the diode la~er spec-trometer. Laser light source l, mea~urement cell 4, andreferenoe C811 5 can be placed in an lnstrument box. The dlfferent module~ oan be constructed on standard europe circuit board~ in a standard 1~ n rack. The measuremen~ cell ls located inside the ln~trument ~nd the gas ~o be ~nalyzed i8 led to lt u~lny flexible hoYes 9 and 10. Other parts of the system includes a modulatlon ~nd oontrol unlt 11, ~
laser and pel~ier dri~er 12, an interface l~ and pow~r upply 14.
Figure 5 shows an alternatlve lnstrumental deslgn where a comp~ct measurement cell 4 ls connected to the dlode laser spectromete~ with optlcal ~iber~ 2. The me~Yurement cell 4 i~ deslgned to be integrated in e.g. ~ patient mouth pieCQ
to measure the oxy~en ~ontent~. The ga~ flow thro~gh the me~urement cell 4 ls indlc~ted with arrows. The measurement cell 4 is then connected to the recelver card with optic~l f~bers or electrical wire~ ~.
4 6 ~1 4 U ~ i U I ~ H I ~ 01~' ~ 4 15 ' ~ 4 The instrument ls lntended to operate completely automatic and therefor~ ~e adJu~tmQnt of laRer temperature, blas current, looklng to the a~Qorptlon line, and me~surem~nt of the abQorptlon ls controlled ~rom a computer, standard PC or equivalent, via an lnterface 13.
On the laser clrault board lfi (fi~. ~) thoRe part~ are lntegr~ed that determlne~ ~hich ~as that i8 to be measured.
10 A molecule 1~ selected b~ matchlng a sultable laQer 1 and a referen~e ga-Q oontained in the reference cell S integr~ted on the card. A la~er module 1 with fiberplgtall, an optlc~l power dlvlde~, referen~e cell, and electronio~ 1~ mounted on - the laser clr~ult board 16. The reference cell 5 i~ used to lS stabillze the laser to the sb~orption line and to provide normalizatlon valuQs for the calculation of the gas con-centratlon ~y locatlng the la~er and the referQnoe cell on the ~ame card spliclng of the fibe~ to the ref~rence cell 1Q
avolded which results ln a lower lnstrumental nolse.
~20 Moreo~er, a modular deslgn of the instrument i~ obtalned.
- The colllmated light out ~rom the measurement fl~er is centered on wavelength Of the a~sorptlon llne of the gss in the reference cell. This mean8 that mea~rement~ ~an be performed in Yeveral dlf~erent measurement sltuations, o.~.
over an ~tmo~pherlc path, ln ~n evane~cent fiber probe, ~
speclally designed optlcal probe, or in a oonventlonal mQasur~ment cell.
The modular deslgn o~ the speatromnter m~an~ that sev~r~l ~0 la~ circuits oan ~e multlplexed on the same fl4er, alt~er wlth wavelenyth mul~lplexln~ or alternAtively wlth time multiplexing there one l~er clrult ~t a ~me 1~ used . In th~ wa~ it i8 po~sl~le to measure sev~ral ~asn8 over th~
same me~surement p~th. You can also choo~e to multiplex one la~r clr~ult to several m~asurement fibers and in thls way measure one g8~ on Be~eral dl~erent measurement paths that oan be located kilometer~ ap~rt. Several componen~s ~uitable 46 31 4~513~ l~oT PRTEt~T~ 2~07240 for this multlplexin~ ha~ been de~eloped ln the area of optical fiber communioatlon.
Flgure 7 ~how~ a device where laser ~ircuits 16a-d, each S tuned to an ab~orp~lon llne, are multlplexed lnto a common mea~urement flbe~ 18 by ~lng a f~ber multiplexer 17.
~nother way of avolding ~ree alr paths ln the ln~trument ~nd obtain a eomp~ct de~lgn ls to use ylas~ pri~ms for l~ ght distrlbutlon be~ween laser and reference cell and for splltting the llght to a me~urement and a reference path.
Su~h an arrangement i~ shown in flg. 8 where the laser llght form the laser l 18 ~lstributed to mea~urement cell 4 and t~e reference cell 5 by ~ean~ of bQamsplitter 19 and a gla~
prism Z0, ~nd p~sed on ~o the d~ectors of ~h~ measurement 4' and refarenoe 5' cells.
Other parts of ths ~rr~n~ement ~re in and outgoiny ~lexlble tubes, 9 and 10, to the me~ur~ment cell 4 and a pel~ler cooler ~1.
Fi~ure 9 show ~ modlfled arran~ement aocording t~ which the lnstrument aan be ~onneotea to ~ compaot optical pro~e of the type sown i flg. 4 u~lng optlcal fibers 2 ~nd 6. A
seal-off val~e to the reference cell is marked wlth 22.
A ~inyle-mode fiber ~ ~8 connected to ~he optical probe to malntaln yood b~am quallty whlle a multimode flber 6 ls prefe~able to pass the llght back from the optlcal probe to collect ~8 much llght a~ po~sible.
~wo dlfferent methods csn be usQd for ~he slgnal condltlo-ning depending on what degree of ~n~trumental complexlty ~h~t can be accepted.
In the flr~t method, tran~mls~lon compensation followed by 46 .~1 4~51~ 1311~ P~TE~IT~ 26)07240 lock-ln detection, whleh i~ ~chematically lllu~trated ln fi~. 10, the la~er 1~ modulated wit~ a triangular wave which ls o~t~ned directly from ~e dlgital unlt and the received slgnal 1~ demodulated ln two ~tep~. Before the phase S sen~itive lock-in detection an analog Qlgnal condltloning i8 performed 32, where the si~nal from t~e monitor photodiode of the laser ls used to COmpenQate the meagurement slgnal for unwanted at~enuatlon caused ~y environmental ln1uences ln the measurement path and ln the optlc~ of the instrument.
The harmonics of the reoeived inten~ity, whlch 18 modulated by the ab~orp~ion line, 1~ thereafter detected with phase sensltlve demodulator~ (lock-in detection). ~he intenslty variatlons of the re~ulting normalized ~ignal wlll then only b~ depend~nt of molecular absorption as de~oribed in the invented slynal ~onditionin~.
The roference slgnals to the phase ~ensitive demodulator~
can be elther ~qua~e-wave or slne-wave, preferably square-wave. The phase difference betweon the recelved modulation Z0 signals and the reference signals i8 optlmized automatically by a computer. Tho pha~e dlfference between the~e slgnal~
can al~o be use~ to determlne the length of the mea~urement path, a value needed for the yas concentrat~on cAlculation.
The inorm~tion from the slgnal on the se~ond ha~monic 2fo is fed to an analog multlplexer and an A/~-converter fo~
caloulatlon of the ~a8 concentration. Informatlon from the third harmonio 3fO lB u~ed to stabllize the laser to the center of the a~sorptlon line via a PID-regulator. Thls slgn~l is also transferred to the computer to verlfy that the lase~ operate~ at the proper wavelength.
The seaond measurement method, ~chematically ~hown ln fig.
11, uses digltal modulation and measurement and i~ based on that the laser i~ modulated wlth a speci~l waveform and that the amplitude of the measurement and referenc~ si~n~ls are sampled with an AJD-converter. The gas concentratlcn i~ then calculated ln the computer. Information a~ou~ the la~er t t~ .~ 1 4 ~ 7 26)07240 wavelength oompared to the absorption llne i~ ex~racted from t~ese sampl~d value~ and the la~er blas current and tempera-ture 1~ adJusted qocordlngly. The gi~n~l analysl~ 18 of the same type a~ ~hat used in ref ~, but with the lmportant dlfference that we ~se a preprocesQlng with snalog signal condltlonlng to compen~at~ for nongasrelated attenhation, which result~ in hlgher m~asurement accuracy. The ~lgnal from the measu~ement cell 4 18 fed back to the electronic unlt using a multl-mode fiber or alternatlvely the detoctor can be located in the me~urement cell and the ~i~nal led back uslng ordlnary electrlcal cables~ In AlGaA~ and InGaAsP
lasers a conslderable ~ntensity modulation i9 obt~ined toge~her with the frequen~y modulation. In the inventlon thi~ i~ u~d to automatically comPensate for nonya~related variatlons in the transmis~lon ln the measurement path or ln the optlcs of the instrument.
Mea~urement ~ell In certaln measurement applications it i~ important to have 8 vory compact moasurement oell. Thls can be realized by "foldlny" tho cell a large number of tlmes. Each tlme thls foldin~ ocaur~ mean~ tha~ the beam iY refleoted again~t a mlrror whlch ~dd~ 108~8~ in to the beam path. ~hese los~es mus~ be kept low and moreover, the are not allowed to be strongly dependent of the optical wavelength. The mlnlmum c~ll acceptablQ oell tran~m1~g1on 1~ ~et by the Yignal-to-n-018~ ratio (SNR) requlred.
~0 One type o~ mirror thqt can be used here ls dielectr~a mirror~. Th~y have ~ limltod w~velen~th range but ~part from that they havQ very ~ood ~ectral chsracteri8tics lncludlng low los~es.
Fi~ure 12 shows ln por~pectlvo the measurement optlcal probe and fig. 13 ~hows a sea~lonal vlew through it to illustrate the beam path.
4~ ~l 4~ IjU~ t~ 0724 0 ~he measurement optlcal probe con~ists of a oylindrlcal ~ea~urement volume, wlth dim~nsions e.g. 20x20 mm. The oyllnder 1~ dlvlded in~o 12 segmentY each of 30 degre~s which each contaln~ a dielectrlc mirror havln~ a refla~tanoe o~ at lea~t 98~. The beam will be reflected 36 tlme~ in the probe which corre~ponds to a maxlmum attenuatlon of o.g836 ~
0.48 (3.~ dB~. ~he probe i9 fed from a slngel-mode flber (typlcal 5/125 ~m) and a lens which create~ a parallel beam with a dlameter of approxlma~ly 0.8 mm. ~he effeotlve path length in the probe ~B approximately 70 cm. The detectlon of the outcomlng be~ either performed dlr~c~ly on ~
photodiode mounted in the probe or via a flber with lnrge core diameter in which case the photodiode can be located far away fro~ the mes~urement volume.
A unlt 24 for coupllng of the light into the optical probe enables th~ all~n~en~ of the beam with a ~ufftclent number o~ degrees of freedom.
The materlal of the mea~urement optl~al probe can be aluminum, ~tainlQ~s ~teel, or a ceromic.
Another type of optloal probe 18 an optlcal fiber where the evanes~ent flel~ which propagat~Y out~ide the core i~ u~ed.
When llght prop~gates ln an optical fiber the main part of lt i~ confined to the core but a certaln part also pro-pag~te~ ln the claddin~ (the evane~Cent fleld). ~y using a sp~ciall~ de~igned ~iber that only consi~ts o~ a core and letting this fi4er be ~urrounded by a fluid that constltute~
the claddlng ~or olternati~ely let a Yurrounding gas constltute the oladdin~) th~ evanes~ent field and thereby al~o the total optlcal fleld will b~ lnfluenced in a slmllar way a~ w~en light propa~ates over an atmo~pherlc path. In thi~ ~ay it is po~sl~le to determinQ ths concentration of a ~peclfic g~Q ln ~ surrounding otmo~phere or fluid. Further-more, OTDR ( Optical Tlme Domain Reflectomet~y) can be usea .
' 4~ ~1 4~51~3 130T PRTEHTg 26)07240 2~
to locallze point emi~slons along an evanescent ~lber probe.
Fiber adapter ~t must ~e pos~ible to allgn the fiber tip so th~t the llght beam after passage through the mea~urement cell 4 or th~
reference cell 5 lmpinys up~n the photodetector (or out-coupllng fiber)~ Thls i~ obtained ~ing a fiber Lip equipp~d with a ~pherical adapter. T~lS arranyement provides sufflci-ent number of degrees of freedom in the ollgnment. The flberis allgned once and for all and then locked with a clamping sorew.
Reference cell ~igure 14 show~ in perspectlve the reference cell 5 whloh coneist~ of ~ rlgld plate 23, an lnooupllng unlt 24 and the measurement volume 25. Ac~ordln~ to one de~ign lt has en effectlv~ length o~ 10 cm and i8 ~onstructed for 8 p~Y~ages, i.~. a total mea~ur~men~ path of 80 cm. A ~eal-of valve ~6 and a quartz window 27 be~ween the incoupling unit 24 ~nd the cell lq also inoluded in the referen~e cell.
-Figure 15 shows in a larg~r soale ~ 4ection throu~h the~5 re~erence oell. ~wo dielectric m1rrors 27 and 28 wlth high reflectivlty ~typically b~tter than 0.98) i4 mount~d at both ~nds of the coll. In thls example the refe~ence beam i~
reflected 7 times co~re~ponding to o totol attenuatlon Of O . ~87 ~ O. 87 ( O. 6d~). The bose pl~te 23 ls deslgned to flt onto a standord europe clrcult ~oard.
Reference~
l. R.S. ~ng, ~.F. ~utler, and K.~. Linden, "~unable diode laxer~pectro~copy: an lnvited revlQw,~ Optical En~lneerlng, vol. 19, p. 945, 1980.
J 1 i .' U 1~1 1 1 H I L I S li j~ 1 J; ~ ;J
26)07240 2. S. Lundqvi~t, ~. Margolis, and J. Reld, "Measurement~ of pressure broadenlng coefficient3 of nitrio oxide~nd oonsing a computerlzsd tunable diode la~er spectrometer," Applied Optlc~, vol. 21, p. 310g, 1982.
3. H. Ahlberg ~nd S. Lundqvi~t, "IR-lager bpectro~copy for measurement Rppli~atlons ln the lndu~trial envlronment," 7th Inter~atlonal Conference on laser Speotro~copy, SEICOLS 85, M~Ul, Rawali, USA, 24-28 ~uni 1985. In Laser Spectro~copy ~II, Sprlnger series ln optlqal sc~ences; Sprin~er Verlag Berlin, ~ol. 49, 1985.
4. S. HJer, H. A~lber~, and S. ~undqvl~t, "Mea~urements of electrlc field~ in ga~ in~ulated high voltage component~
using infrared dlode la~er ~peotroscopy," Applied Optic~, vol. 2S, p. 2984, 1986.
S. D.~. Ca~idy, "~race gas detection using 1.3 m InGaA~P
diode laYer transmitter modu~e8," Applied Optl~, vol. 27,p.
610, 19~6.
6. G. ~olsde, G. ~he~alier, and ~.-J. Perez, Dem~nde de 3revot Europeen 0 015 170 Al, ~ur~ean Patent Offiee, 18 Januari 1980.
7. K. Cerff, H. Giraud, and G. Krieg, offenlegunqss~hrift DE 3633931 Al, Deut~he4 Pate~tamt, ~undesrepubllk ~eutsch-l~nd, 4 oktober 198fi.
8. ~. D. Pltt, D.N. Batohelder, and R.E. Jone~, UK P~tent Application GB 2 165 640 A, 13 oktober 19~4.
using infrared dlode la~er ~peotroscopy," Applied Optic~, vol. 2S, p. 2984, 1986.
S. D.~. Ca~idy, "~race gas detection using 1.3 m InGaA~P
diode laYer transmitter modu~e8," Applied Optl~, vol. 27,p.
610, 19~6.
6. G. ~olsde, G. ~he~alier, and ~.-J. Perez, Dem~nde de 3revot Europeen 0 015 170 Al, ~ur~ean Patent Offiee, 18 Januari 1980.
7. K. Cerff, H. Giraud, and G. Krieg, offenlegunqss~hrift DE 3633931 Al, Deut~he4 Pate~tamt, ~undesrepubllk ~eutsch-l~nd, 4 oktober 198fi.
8. ~. D. Pltt, D.N. Batohelder, and R.E. Jone~, UK P~tent Application GB 2 165 640 A, 13 oktober 19~4.
Claims (14)
1. A spectrosoopic method to measure the concentration of a gas in a sample in which the intensity of light from a light source (1) passed through the sample (4) and through a reference cell (5) is detected and a signal is generated which represents the concentration of the gas wherein a laser diode (1) constitutes the light source which is looked to the absorption line of the gas at known pressure and concentration contained in a reference cell (5) , and that to avoid interference from outside the measurement path the laser light is distributed via optical fibers (2,3,6) and/or glass prisms ( 19,20) to the reference cell (5) and the sample (4) c h a r a c t e r i z e d b y , that nongasrelated transmission variations automatically is compensated for by the laser being intensity modulated with a preferably triangular wave audio frequency signal of the period T whereby in a first demodulation step the intensity modulation is used to measure the transmission of the measuring path by forming a first difference signal between a monitoring signal from a monitor diode (31) and a detected measuring signal from a detector (41) and a second differen-ce signal between the monitoring signal and a reference signal from a detector (51) whereby the gain level of (G1) and (G3) respectively of the recieved signals automatically is adjusted in an analog signal conditioning unit (32) such that first and second difference signals are respectively equal to zero when sampling in a maximum point at the times n * T whereupon in a second demodulation step the harmonics of the intensity which is modulated by the absorption line is detected by phase sensitive demodulators (42, 52, 53).
2. Method as claimed in 1, c h a r a c t e r i z e d b y that the laser diode (1) is current modulated with a signal in the audio frequency range, preferably triangular in shape, obtained from a digital modulation and control unit (11).
3. Method as claimed in 2, c h a r a c t e r i z e d b y , that the detected signals from the sample (4) and reference cell ( 5 ) is demodulated in two steps, where the first step (32) uses the intensity modulation of the laser diode and the signal from the monitor photodiode (31) of the laser to automatically compensate for nongasrelated transmission variations in the instrument without having to use the baseline of the spectrum, and thereafter in stop two performs lock-in detection with digital reference signals obtained from the modulation and control unit (11 ), whereafter the demodulated signal from the sample (4) is divided with the demodulated signal from the reference cell (5).
4. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d b y , that the phase difference between the signals in the measurement and reference channel is used to determine the length of the measurement path which is necessary for the gas concentration calculation.
5. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d b y , that the laser diode (1) is modulated with the frequency (f), that the measurement signal from the sample (4) is demodulated with the frequency (f ), double or quadruple frequency ( 2f ,4f ) and that a signal demodulated with the triple frequency (3f) provides the feedback signal for locking of the laser diode, while the signal at f gives an automatic compensation for nongasrelated transmission variations.
6. Method as claimed in 1 , c h a r a c t e r i z e d b y , that the current modulation of the laser diode (1) is performed using a stepwise modulation whereafter the corresponding points on the absorption spectrum of the gas after compensation for the intensity modulation are measured with an A/D-converter which calculates derivatives and measuremwnt value and also locks the laser diode to the absorption line.
7. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d b y , that the pressure, temperature and/or magnetic and electric field strength is determined by msasuring the corresponding relationship of these parameters to the shape, position and strength of the absorption line.
8. Method as claimed in any of the preceding claims, c h a r a a t e r i z e d b y , that a long effective measurement path is created in a measurement optical probe (4) and/or reference cell (5) by reflecting the light beam several times within the cell.
9. Apparatus for spectroscopic measurement of the concentra-tion of a gas in a sample according to the method claimed in 1, incorporating a light source (1) to provide the light beam through the sample (4) and a reference cell (5), and incorporating means for detection of intensity of the light transmitted through the sample and through the reference cell, and also means for generating a signal proportional to the ratio of the demodulated intensities, where the referen-ce cell (5) contains the gas to be measured with known concentration and pressure, and the light source (1) is a laser diode locked to the absorption line of the gas in the reference cell (5), and also including means for distribu-tion of the laser light to the reference cell (5) and to the sample (4) consisting of optical fibers (2,3,6) and/or glass prisms to avoid unwanted interference from the atmosphere, c h a r a c t e r i z e d b y, a laser current modulator which modulates the intensity of the laser with a periodic preferably triangular audio frequency signal, an analogue signal conditioning unit (32) which is provided with inputs for a monitoring signal from a monitor diode and for a measuring signal from a detec-tor/amplifier (41) and for a reference signal from a detector/amplifier (51) whereby the signal conditioning unit (32) contains controllable amplifiers (G1) and (G3) the outputs of which are connected to difference amplifiers (D1) and (D2) respectively, the output of said difference amplifiers are connected respectively to sample and hold circuits (SH1) and (SH2) which are triggered from a peak detector (PT) which detects peak values of said monitor signal whereby the signal from respectively sample and hold circuit are filtered in the circuit (PI1) and (PI2) to form a control signal for the setting of the gain of the amplifiers (G1) and (G3) respectively.
10. Apparatus as claimed in 9 , c h a r a c t e r i z e d b y , that the measurement is performed in a measurement cell (4) to which the laser light is distributed, and that in the measurement cell reflecting means (30) are provided to reflect the laser beam several times thereby increasing the effective length of the measurement path inside the measure-ment cell.
11. Apparatus as claimed in 9 or 10, c h a r a c t e r i z e d b y that the measurement is performed in a measurement cell (4) to which the laser light is distributed, and that coupling means (24) are provided at the measurement cell to align the laser beam with a sufficient number of degrees of freedom.
12. Apparatus as claimed in 10 or 11, c h a r a c t e r i z e d b y , that the measurement cell (4) is formed as a tube, prefe-rably cylindrical, and is divided in several segments each of which containing reflective elements, e.g. a dielectric mirror (30), arranged to reflect the lager beam in several planes inside the measurement cell in such a way that the beam pattern automatically repeats itself in consecutive planes.
13. Apparatus as claimed in 9-12, c h a r a c t e r i z e d b y , that the reference cell (5) incorporates reflective ele-ments, e.g. dielectric mirrors (28,29), to reflect the laser beam several times inside the reference cell.
14. Apparatus as claimed in 9, c h a r a c t e r i z e d b y , that the measurement is performed in an optical fiber designed so that the surrounding fluid or gas via the evanescent field affects the laser light so that the concentration of the substance in question in the surroun-ding fluid or gas can be determined.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2007240 CA2007240A1 (en) | 1990-01-05 | 1990-01-05 | Method and apparatus for spectroscopic measurement of the concentration of a gas |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2007240 CA2007240A1 (en) | 1990-01-05 | 1990-01-05 | Method and apparatus for spectroscopic measurement of the concentration of a gas |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2007240A1 true CA2007240A1 (en) | 1991-07-05 |
Family
ID=4143943
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2007240 Abandoned CA2007240A1 (en) | 1990-01-05 | 1990-01-05 | Method and apparatus for spectroscopic measurement of the concentration of a gas |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2007240A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110186852A (en) * | 2019-06-26 | 2019-08-30 | 南京无书化工有限公司 | A kind of double ponds series connection optoacoustic spectroscopy gas-detecting device and method |
-
1990
- 1990-01-05 CA CA 2007240 patent/CA2007240A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110186852A (en) * | 2019-06-26 | 2019-08-30 | 南京无书化工有限公司 | A kind of double ponds series connection optoacoustic spectroscopy gas-detecting device and method |
| CN110186852B (en) * | 2019-06-26 | 2023-12-01 | 南京无书化工有限公司 | Double-cell series photoacoustic spectrum gas detection device and method |
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