sensors Article
A Robust Distributed Multipoint Fiber Optic Gas Sensor System Based on AGC Amplifier Structure Cunguang Zhu, Rende Wang, Xuechen Tao, Guangwei Wang and Pengpeng Wang * School of Physics and Technology, University of Jinan, Jinan 250022, China; [email protected] (C.Z.); [email protected] (R.W.); [email protected] (X.T.); [email protected] (G.W.) * Correspondence: [email protected]; Tel.: +86-187-6443-8579 Academic Editor: Vittorio M. N. Passaro Received: 12 June 2016; Accepted: 21 July 2016; Published: 28 July 2016
Abstract: A harsh environment-oriented distributed multipoint fiber optic gas sensor system realized by automatic gain control (AGC) technology is proposed. To improve the photoelectric signal reliability, the electronic variable gain can be modified in real time by an AGC closed-loop feedback structure to compensate for optical transmission loss which is caused by the fiber bend loss or other reasons. The deviation of the system based on AGC structure is below 4.02% when photoelectric signal decays due to fiber bending loss for bending radius of 5 mm, which is 20 times lower than the ordinary differential system. In addition, the AGC circuit with the same electric parameters can keep the baseline intensity of signals in different channels of the distributed multipoint sensor system at the same level. This avoids repetitive calibrations and streamlines the installation process. Keywords: gas sensor; AGC; distributed multipoint sensor; optical transmission loss
1. Introduction One of the most important actual problems in the gas detection field is that there are strong demands for environmentally hazardous gas detection to prevent explosions or poisoning accidents [1–3]. Optical gas sensors play an important role in the sensing field for the measurement of combustible and explosive gas under harsh environmental conditions [4–7]. As a kind of optical sensing technology, tunable diode laser absorption spectroscopy (TDLAS) is widely used because of its advantages such as excellent insulation properties, high sensitivity, wide range, long life, etc. Moreover, matching with a fiber-optic network technology has added the prospect of broad application in the field of distributed multipoint detection [8–11]. Direct absorption spectroscopy is the simplest and most direct detection technique in TDLAS [12,13]. It often employs the self-balanced technology for demodulating absorption signals, which has been described in detail in previous research [14]. A distributed feedback laser diode (DFB-LD) emission scanning across a gas absorption wavelength is split into two beams by a 1 ˆ 2 fiber coupler. One is directly coupled on a photoelectric detector (PD), which serves as the reference signal input of the self-balanced circuit. The other beam is coupled on a similar PD after being passed through the gas cell, which is received by the second input of the self-balanced circuit. Then, the absorption peak can be obtained by self-balanced demodulation such as a differential or balanced ratiometric detector (BRD) circuit. Intuitive and original absorption line shapes can be used to determine the gas properties [15,16]. However, besides gas absorption, there could be many other factors such as temperature change, fiber bending loss and the laser aging process, which can cause the transmission intensity to drop in real-world applications [17,18]. By contrast, the variation of light intensity induced by these factors can be more obvious. Undoubtedly, this is a principal reason for accuracy deterioration of TDLAS sensor system. Two inputs of the self-balanced demodulation circuit in direct absorption spectroscopy Sensors 2016, 16, 1187; doi:10.3390/s16081187
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factors can be more obvious. Undoubtedly, this is a principal reason for accuracy deterioration of 2 of 8 TDLAS sensor system. Two inputs of the self-balanced demodulation circuit in direct absorption spectroscopy require a strict balance to be met in order to obtain the absorption spectrum with a require a strict balance to be metthe in order to obtain absorption spectrum withofa the horizontal baseline. horizontal baseline. Otherwise, baseline tilt willthe lead to a serious distortion absorption line, Otherwise, the baseline tilt will lead to a serious distortion of the absorption line, which could result in which could result in further obvious measurement errors [19]. further measurement [19]. multipoint fiber optic gas sensor system based on an In obvious this paper, a robust errors distributed In thisgain paper, a robust distributed multipoint optic gasThe sensor system based on anits automatic automatic control (AGC) amplifier structure fiber is proposed. AGC amplifier adopts internal gain control (AGC) amplifier structure is proposed. The AGC amplifier adopts its internal self-gain self-gain amplification function for adjustment and stabilization of the baselines of the two inputs of amplification function for adjustment of the baselines the two of the the self-balanced demodulation circuitand to stabilization the same magnitude. In thisofway, not inputs only can self-balanced circuitby to the the same magnitude. way, notcompensated, only can the attenuation attenuation ofdemodulation the signal caused transmission loss In bethis effectively but also a of the signalsimultaneous caused by the transmission be effectively compensated, but also multi-point multi-point detection can beloss easily realized without the affection of ainsertion loss simultaneous can be easily realizedchannels. without the affection of insertion loss differences of gas differences of detection gas cells in different detection cells in different detection channels. 2. Theory of AGC Amplifier 2. Theory of AGC Amplifier
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2.1. Beer-Lambert Law 2.1. Beer-Lambert Law The measurement technique is based on the absorption of monochromatic near-IR radiation. The measurement technique is based on the absorption of monochromatic near-IR radiation. The transmission of the probe beam through a uniform absorbing medium follows the Beer-Lambert The transmission of the probe beam through a uniform absorbing medium follows the law: Beer-Lambert law: (1) − pυq CLs I pυq “= I0 pυq exp exp r´α (1) where CCis the is the target gas concentration, L is the length of the absorption path. The where target gas concentration, and L isand the length of the absorption path. The concentration C concentration C by canidentifying be obtained identifying emergent lightthe intensity I(υ) and the incident can be obtained theby emergent lightthe intensity I(υ) and incident light intensity I0 (υ), light represents intensity I0the (υ),absorption α(υ) represents the absorption at the number as: υ, it can be α(υ) coefficient at the lightcoefficient wave number υ, light it canwave be expressed expressed as: α pυq “ PS pTq φ pυq (2) (2) = P(Pa) is is the the total total pressure, pressure,S(T) S(T) (cm¨mol (cm∙mol´−11))isisthe where P(Pa) theline line strength strength at at an an arbitrary arbitrary temperature temperature (T), ϕ(υ) (cm) (cm) is is the the linear linear function function of of absorption absorption spectra. spectra. φ(υ) 2.2. Fiber Optic Optic Gas Gas Sensor Sensor System System based 2.2. Fiber based on on AGC AGC Amplifier Amplifier Structure Structure A thethe fiber optic gas sensor system based based on AGC structurestructure is shown in A block blockdiagram diagramofof fiber optic gas sensor system onamplifier AGC amplifier is Figure AGC module is added between the pre-amplifier andpre-amplifier the self-balanced (differential shown1.inAn Figure 1. An AGC module is added between the and circuit the self-balanced circuit). The baselinecircuit). amplitudes of the detection signal reference signal the differential circuit (differential The baseline amplitudes ofand the the detection signal andbefore the reference signal circuit can be amplified to the same level through the AGC circuit no matter how the transmission before the differential circuit can be amplified to the same level through the AGC circuit no matter loss howchanges. the transmission loss changes.
Figure 1. Schematic of the automatic gain control (AGC) amplifier detection system. Figure 1. Schematic of the automatic gain control (AGC) amplifier detection system.
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The intensities of the two signals (the reference signal Iref (υ) and the detection signal Idet3(υ)) Sensors 2016, 16, x FOR PEER REVIEW of 9 coupled into the AGC circuits are described below:
The intensities of the two signals (the reference signal Iref(υ) and the detection signal Idet(υ)) (3) Iref pυqbelow: “ γ1 ptq I0 pυq coupled into the AGC circuits are described γ2 ptq 𝛾1(𝑡)𝐼0 (𝜐) (3) exp r´α pυq CLs (4) Idet pυq “ 𝐼ref (𝜐) I=0 pυq ε 𝛾2(𝑡) 𝐼det (𝜐) = transmission I (υ)exp[ − 𝛼(𝜐)𝐶𝐿] where and γ2(t) is the time-varying optical loss coefficient of the two beams due(4) to 𝜀 0 various non-absorption losses, and ε is the coupler splitting ratio. The AGC circuit can achieve a timely where and γ2(t) is the time-varying optical transmission loss coefficient of the two beams due to gain adjustment to maintain a stable baseline according to the reset thresholds even if the γ1(t) and various non-absorption losses, and ε is the coupler splitting ratio. The AGC circuit can achieve a γ2(t) are continually changing. The two signal outputs by the AGC circuits are given by: timely gain adjustment to maintain a stable baseline according to the reset thresholds even if the γ1(t) and γ2(t) are continually changing. 1The two signal outputs by the AGC circuits are given by: Iref pυq “ I0 pυq (5) γ1 ptq 1 𝐼 (𝜐) = 𝐼0 (𝜐) (5) 𝛾1(𝑡) ref ε (6) I pυq “ I0 pυq exp r´α pυq CLs 𝜀 det γ2 ptq 𝐼det (𝜐) = 𝐼0 (𝜐)exp[ − 𝛼(𝜐)𝐶𝐿] (6) The final signal processed by 𝛾2(𝑡) the differential circuit is given by:
The final signal processed by the differential circuit is given by: 1 ε Iout “ Iref pυq ´ 1 𝜀 Idet pυq “ I0 pυq ´ I0 pυq exp r´α pυq CLs γ1 ptq 𝐼 (𝜐) −γ2 ptq 𝐼out = 𝐼 (𝜐) = 𝐼0 (𝜐) − 𝐼0 (𝜐)exp[ − 𝛼(𝜐)𝐶𝐿] ref 𝛾1(𝑡) 𝛾2(𝑡) det When exp [´α(υ)CL] is much less than 1, When exp [-α(υ)CL] is much less than 1, exp r´α CLs «≈ 11´−α pυq CL exp[ − pυq 𝛼(𝜐)𝐶𝐿] 𝛼(𝜐)𝐶𝐿
(7) (7)
(8) (8)
WhenII0(υ) (υ) did did not notchange, change,the thefinal finalsignal signal(I(Iout(υ)) (υ)) will will be beproportional proportionalto tothe theconcentration concentrationof of When out 0 the gas to be measured, so the gas concentration (C) can be derived from the absorption peak. the gas to be measured, so the gas concentration (C) can be derived from the absorption peak. 2.3.AGC AGCAmplifier Amplifier 2.3. TheAGC AGCamplifier amplifier is automatic an automatic amplification gain adjustment device. the AGC The is an amplification gain adjustment device. Using theUsing AGC amplifier amplifier can guarantee relatively stable output case of an unstable input. can guarantee a relativelyastable output in the case in of the an unstable input. closedloop loopAGC AGCamplifier amplifieras asshown shownininFigure Figure22isisused usedininthe thesystem. system.ItItisisactually actuallyaanegative negative AAclosed feedback system, which is composed of a variable gain amplifier and a feedback circuit. The variable feedback system, which is composed of a variable gain amplifier and a feedback circuit. The variable gainamplifier amplifier contains contains aa controlled controlled and and aa non-controlled non-controlled amplifier gain amplifier circuit circuit unit. unit. AAcomparator comparatorand anda detection circuit form a feedback circuit. The effect of the feedback circuit is to convert the output a detection circuit form a feedback circuit. The effect of the feedback circuit is to convert the signal (V out) into a )direct (DC) voltage achieve control and detection output signal (Vout into acurrent direct current (DC) to voltage to gain achieve gain(Comparator control (Comparator and circuit). circuit). detection
Variable gain amplifier Signal input(Vin)
Controlled circuit
Non-controlled circuit
Signal output(Vout)
Control voltage(Vc) Comparator Geophone
Feedback circuit
Reference voltage(Vr)
Figure Figure2.2.AGC AGCamplifier amplifierstructure structurediagram. diagram.
The input signal (Vin) is time-variable due to transmission loss and other interferences, which is introduced by the input port of the controlled circuit unit and finally output by the non-controlled circuit unit. The Vout can remain steady and be fed to the next stage circuit (differential circuit).
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The input signal (Vin ) is time-variable due to transmission loss and other interferences, which is introduced by the input port of the controlled circuit unit and finally output by the non-controlled SensorsThe 2016, 16,out 1187 4 of 8 circuit unit. Sensors 2016, 16,V 1187 can remain steady and be fed to the next stage circuit (differential circuit). 4 of 8 When Vin is small, Vout is also smaller than the reference voltage (Vr ). The output of the feedback When Vin is small, Vout is also smaller than the reference voltage (Vr). The output of the feedback WhenThus, Vin is small, Vout is also smaller than the reference voltage (Vr). The full-scale output of the feedback circuit is zero. gain regulation does not occur. The system maintains amplification. circuit is zero. Thus, gain regulation does not occur. The system maintains full-scale amplification. circuit is zero. Thus, gain regulation does not occur. The system maintains full-scale amplification. When the input signal reaches a certain ), and a result thus V , the> Vr , the When the input signal reaches a certainvalue value(V (V11 Vr, the difference between Vout signal Vreaches can be served as the of the feedback circuit. At this point, the difference between Vand out and V r can be served as the input of the feedback circuit. At this point, the r difference between Vout and Vr can be served as the input of the feedback circuit. At this point, the gain control voltage (V c ) of the controlled circuit unit will change and result in a gain adjustment to gain control voltage (Vc ) of the controlled circuit unit will change and result in a gain adjustment to gain control voltage (Vc) of the controlled circuit unit will change and result in a gain adjustment to keep the V out in the vicinity of the Vr. keep the Vout in the vicinity of the Vr . keep the Vout in vicinity r. When Vinthe is large (Vinof>the V2),VAGC amplifier no longer works, the system is amplified with a When Vin Visin large (V(V AGC amplifier no longer works, the system is amplified with in in>>VV22), When is large ), AGC minimum gain, as shown in Figure 3. amplifier no longer works, the system is amplified with a a minimum gain, asasshown 3. minimum gain, shownin inFigure Figure 3.
Vout
Vout
without AGC
with AGC
without AGC
with AGC
Vr
Vr
V1
V2
Vin
Vin
V1 ofofAGC V Among Figure 3. Static regulation characteristic them, the threshold 1 is Figure 3. Static regulation characteristic AGC amplifier. amplifier.2 Among them, V1 isVthe threshold voltage,voltage, and theand V2 the is out control voltage, they keyperformance performance indicators the amplifier. AGC amplifier. V2 isofout of control voltage, theyare aretwo two key indicators of theofAGC Figure 3. Static regulation characteristic of AGC amplifier. Among them, V1 is the threshold voltage, 3. Experimental Apparatus and Results and the V2Apparatus is out of control voltage, they are two key performance indicators of the AGC amplifier. 3. Experimental and Results
Stress-induced fiber bending will inevitably occur and cause a transmission loss in the
Stress-induced bending inevitably occur and cause a transmission loss in the long-haul 3. Experimental fiber Apparatus and will Results long-haul optical fiber transmission. In a multi-point detection system, the amount of loss between optical different fiber transmission. In a multi-point detection system, the amount loss between different monitoring points must will be distinct and unsynchronized. willofaffect theloss TDLAS Stress-induced fiber bending inevitably occur and causeThis a transmission in the monitoring points must distinct andIn unsynchronized. This will affect TDLAS detection system detection system farbe more than some random high frequency noise. long-haul optical fiber transmission. a multi-point detection system, thethe amount of loss between far more than some random high frequency noise. A fiber bending test is used to evaluate the reliability of the system. Acetylene is selected as the different monitoring points must be distinct and unsynchronized. This will affect the TDLAS gas tobending be detected. single-mode DFB-LD (WSLS-137010C1424-20, Agilecom, FiberisSolution, A fiber testmore isA used evaluate the reliability of noise. the system. Acetylene selected as the detection system far thanto some random high frequency San Jose, CA, USA) is used as the light source, in which the output wavelength is modulated bySan A fiber bending test is usedDFB-LD to evaluate the reliability of the system. Acetylene is selected as the Jose, gas to be detected. A single-mode (WSLS-137010C1424-20, Agilecom, Fiber Solution, adjusting the laser temperature and the injection current. The injection current is scanned with a gas to isbeused detected. single-mode DFB-LD Agilecom, Fiber Solution, CA, USA) as theAlight source, in which(WSLS-137010C1424-20, the output wavelength is modulated by adjusting saw-tooth function, which ensures that the light emitted from the laser source can cover the entire San Jose, CA, USA)and is used as the lightcurrent. source, in which the output wavelength is modulated by the laser temperature the injection The injection current is scanned a saw-tooth wavelength range of acetylene absorption lines near 1532 nm. Laser temperature is set towith a suitable adjusting the laser temperature and the injection current. The injection current is scanned with a function, whichpoint ensures that the light emitted fromchip the(LTC laser1923, source canTechnology, cover the Milpitas, entire wavelength working by a micro-temperature controlling Linear CA, saw-tooth function, which ensures that the light emitted from the laser source can cover the entire The LDabsorption beam is splitlines into reference detection by a 1 × is 2 coupler. range ofUSA). acetylene near 1532and nm. Laser beams temperature set to a suitable working point wavelength range of acetylene absorptionalines near 1532 nm.ofLaser temperature is set to gas a suitable The detection beam passes through gas1923, cell consisting a stainless-steel tube.CA, The USA). cell The LD by a micro-temperature controlling chipcontrolling (LTC Linear Technology, Milpitas, working point by a micro-temperature chip (LTC 1923, Linear Technology, Milpitas, shown in Figure 4 is filled with 1000 ppm acetylene in which the absorption path is 3 cm. A goodCA, beamUSA). is split into andinto detection beams by a 1 ˆbeams 2 coupler. The LDreference beamensures is split detection a 1 × 2 coupler. air seal structure that reference we do notand need to consider theby measurement errors caused by an The detection beam passes through a gas cell consisting stainless-steel tube. gas cell The detection beam passes through a gas cell consisting ofofa astainless-steel tube. The The gas cell unstable air concentration. shown in Figure 4 is 4filled with 1000 ppm acetylene pathisis33cm. cm.AAgood good air shown in Figure is filled with 1000 ppm acetyleneininwhich whichthe the absorption absorption path air seal structure we do nottoneed to consider the measurement caused byunstable an seal structure ensuresensures that wethat do not need consider the measurement errorserrors caused by an unstable air concentration. air concentration.
Figure 4. The gas cell structure.
Figure 4. The gas cell structure. Figure 4. The gas cell structure.
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An ordinary differential demodulation system is employed to compare with the AGC AnAn ordinary differential demodulation system is choose employed compare with thewith AGC differential differential demodulation system in the test. They thetosame andthe differential ordinary differential demodulation system is employed topre-amplifier compare AGC demodulation system in the test. They choose the same pre-amplifier and differential circuit. The only circuit. The only difference system is whether to test. join They the AGC module. In the AGC system, controlled differential demodulation in the choose the same pre-amplifier and the differential difference isiswhether to join module. themodule. AGC system, the controlled unit is circuit unit constructed with theAGC AD603 chipthe asIn shown in Figure 5. AGC circuit. The only difference isthe whether to join AGC In the system, thecircuit controlled constructed the AD603with chipthe asAD603 shownchip in Figure 5. in Figure 5. circuit unitwith is constructed as shown
Figure diagram of the AGC amplifier. Figure 5. The circuit diagram diagramof ofthe theAGC AGCamplifier. amplifier. Figure5. 5. The The circuit circuit
InIn the ordinary system, the baseline of the the two twosignals signals(the (thedetection detectionsignal signal and the reference the ordinarysystem, system,the thebaseline baseline of of the and thethe reference In the ordinary two signals (the detection signal and reference signal) canachieve achievethe thesame samemagnitude magnitude by by adjusting amplitude of of thethe final signal) can achieve the same magnitude by adjusting adjustingthe thepreamplifier. preamplifier.The The amplitude of the final signal) can the preamplifier. The amplitude final demodulatedsignal signalisis1.668 1.668 V. V. Bending Bending with aa bending radii ofof5 5mm causes propagation losses in in demodulated with bending radii mm causes propagation losses demodulated signal is 1.668 V. Bending with a bending radii of 5 mm causes propagation losses opticalfiber fiber between the the DFB-LD and coupler, and magnitude ofofthe signal anan optical thedemodulated demodulated signal in an optical fiberbetween between theDFB-LD DFB-LDand and coupler, coupler, and and magnitude magnitude of the demodulated signal changes to 0.321 V. The measurement error is 80.76%. As shown in Figure 6a, although there is ais a changes to 0.321 V. The measurement error is 80.76%. As shown in Figure 6a, although there changes to 0.321 V. The measurement error is 80.76%. As shown in Figure 6a, although there is a large largeamplitude amplitudeattenuation, attenuation, aa relatively relatively significant absorption profile can bebe still detected because large significant absorption profile can still detected because amplitude attenuation, a relatively significant absorption profile can be still detected because the the amounts of light loss of the two signals are the almost same. However, the macro bending lossloss the amounts of light loss of the two signals are the almost same. However, the macro bending amounts of light loss of the two signals are the almost same. However, the macro bending loss occurs occursononthe thetransmission transmission fiber fiber of of the the detection signal or fiber of of thethe reference occurs detection signal oron onthe thetransmission transmission reference on the transmission fiber of the detection signal or on the transmission fiber offiber the6b. reference signal. signal. This situation is especially serious in profile measurement as shown in Figure signal. This situation is especially serious in profile measurement as shown in Figure 6b. This situation is especially serious in profile measurement as shown in Figure 6b. We have replicated this experiment on the AGC differential demodulation system. Without a Wehave havereplicated replicatedthis this experiment the AGC differential demodulation system. Without We experiment on on the AGC differential Without fibera fiber bending loss, the magnitude of the demodulated signal demodulation is 1.527 V. As system. shown in Figurea6c, fiber bending loss, the magnitude of the demodulated signal is 1.527 V. As shown in Figure 6c, bending the magnitude theoccurs demodulated is 1.527 the V. As shown in signal Figurechanges 6c, when when aloss, 5 mm macro bendingofloss before thesignal fiber coupler, demodulated amacro 5 mm bending macro bending lossbefore occursthe before the fiberthe coupler, the demodulated signal changes awhen 5tomm loss occurs fiber demodulated signal changes 1.491 V. 1.491 V. The measurement error is reduced tocoupler, 2.36%. As shown in Figure 6d, when thetomacro to 1.491 V. The measurement error is reduced to 2.36%. As shown in Figure 6d, when the macro The measurement error reduced to 2.36%. in Figure 6d,orwhen macro bending loss bending loss occurs on isthe transmission fiberAs of shown the reference signal on thethe detection signal, the bending loss occurs on the transmission fiber of the reference signal or on the detection signal, occurs on the transmission fiber(1.527–1.469 of the reference signal on the detection signal, theReliability measurement measurement errors are 3.80% V) and 4.06%or(1.527–1.465 V), respectively. ofthe measurement errors 3.80% V) and 4.06% respectively. of the system has beenare enhanced. errors are 3.80% (1.527–1.469 V)(1.527–1.469 and 4.06% (1.527–1.465 V),(1.527–1.465 respectively.V), Reliability of theReliability system has the system has been enhanced. been enhanced.
Figure 6. Cont.
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Figure Figure 6.6. Macro Macro bending bending loss loss test test results results of ofthe thedifferent differentsystems. systems. (a) (a) The The change change of of the the absorption absorption Figure 6.obtained Macro bending loss test results of thedemodulation different systems. (a) The change the absorption waveform by differential system under fiber bending before waveform obtained by the the ordinary ordinary differential demodulation system under fiberof bending before the the waveform obtained byof the ordinary demodulation system under bendingdemodulation before the coupler; (b) The change the waveform obtained by the ordinary differential coupler; (b) The change ofabsorption the differential absorption waveform obtained by fiber the ordinary differential coupler; (b) fiber The bending change the absorption waveform the reference ordinary differential system under on thebending detection or theobtained reference (c) The signal; change of The the demodulation system underof fiber onsignal the detection signal by orsignal; the (c) demodulation system under fiber bending on the detection signal or the reference signal; (c) The absorption waveform obtained by the AGC differential demodulation system under fiber bending change of the absorption waveform obtained by the AGC differential demodulation system under change thebefore absorption waveform by the AGC differential demodulation system under before theofsplitter; (d)the The change thechange absorption waveform obtained by obtained the AGC differential fiber bending splitter; (d)ofobtained The of the absorption waveform by the AGC fiber bending before the splitter; (d) The change of the absorption waveform obtained by the AGC demodulation system under fiber bending on the detection signal or thesignal reference signal. differential demodulation system under fiber bending on the detection or the reference signal. differential demodulation system under fiber bending on the detection signal or the reference signal.
Acetylene concentration concentration measurement in in different different mixtures mixtures of acetylene acetylene and and nitrogen nitrogen is is Acetylene Acetylene concentration measurement measurement in different mixtures ofofacetylene and nitrogen is performed. Absorption line shape under different acetylene concentrations concentrations in aa 20 20 cm gas gas cell is is performed. under different differentacetylene acetylene performed.Absorption Absorption line line shape shape under concentrations in in a 20 cmcm gas cellcell is shown in Figure 7a. As shown in Figure 7b, the demodulated signal is well proportional to shown in in Figure 7a. 7a. As shown in Figure 7b, the signalsignal is wellisproportional to acetylene shown Figure As shown in Figure 7b,demodulated the demodulated well proportional to acetylene concentration ranging from
A Robust Distributed Multipoint Fiber Optic Gas Sensor System Based on AGC Amplifier Structure Cunguang Zhu, Rende Wang, Xuechen Tao, Guangwei Wang and Pengpeng Wang * School of Physics and Technology, University of Jinan, Jinan 250022, China; [email protected] (C.Z.); [email protected] (R.W.); [email protected] (X.T.); [email protected] (G.W.) * Correspondence: [email protected]; Tel.: +86-187-6443-8579 Academic Editor: Vittorio M. N. Passaro Received: 12 June 2016; Accepted: 21 July 2016; Published: 28 July 2016
Abstract: A harsh environment-oriented distributed multipoint fiber optic gas sensor system realized by automatic gain control (AGC) technology is proposed. To improve the photoelectric signal reliability, the electronic variable gain can be modified in real time by an AGC closed-loop feedback structure to compensate for optical transmission loss which is caused by the fiber bend loss or other reasons. The deviation of the system based on AGC structure is below 4.02% when photoelectric signal decays due to fiber bending loss for bending radius of 5 mm, which is 20 times lower than the ordinary differential system. In addition, the AGC circuit with the same electric parameters can keep the baseline intensity of signals in different channels of the distributed multipoint sensor system at the same level. This avoids repetitive calibrations and streamlines the installation process. Keywords: gas sensor; AGC; distributed multipoint sensor; optical transmission loss
1. Introduction One of the most important actual problems in the gas detection field is that there are strong demands for environmentally hazardous gas detection to prevent explosions or poisoning accidents [1–3]. Optical gas sensors play an important role in the sensing field for the measurement of combustible and explosive gas under harsh environmental conditions [4–7]. As a kind of optical sensing technology, tunable diode laser absorption spectroscopy (TDLAS) is widely used because of its advantages such as excellent insulation properties, high sensitivity, wide range, long life, etc. Moreover, matching with a fiber-optic network technology has added the prospect of broad application in the field of distributed multipoint detection [8–11]. Direct absorption spectroscopy is the simplest and most direct detection technique in TDLAS [12,13]. It often employs the self-balanced technology for demodulating absorption signals, which has been described in detail in previous research [14]. A distributed feedback laser diode (DFB-LD) emission scanning across a gas absorption wavelength is split into two beams by a 1 ˆ 2 fiber coupler. One is directly coupled on a photoelectric detector (PD), which serves as the reference signal input of the self-balanced circuit. The other beam is coupled on a similar PD after being passed through the gas cell, which is received by the second input of the self-balanced circuit. Then, the absorption peak can be obtained by self-balanced demodulation such as a differential or balanced ratiometric detector (BRD) circuit. Intuitive and original absorption line shapes can be used to determine the gas properties [15,16]. However, besides gas absorption, there could be many other factors such as temperature change, fiber bending loss and the laser aging process, which can cause the transmission intensity to drop in real-world applications [17,18]. By contrast, the variation of light intensity induced by these factors can be more obvious. Undoubtedly, this is a principal reason for accuracy deterioration of TDLAS sensor system. Two inputs of the self-balanced demodulation circuit in direct absorption spectroscopy Sensors 2016, 16, 1187; doi:10.3390/s16081187
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factors can be more obvious. Undoubtedly, this is a principal reason for accuracy deterioration of 2 of 8 TDLAS sensor system. Two inputs of the self-balanced demodulation circuit in direct absorption spectroscopy require a strict balance to be met in order to obtain the absorption spectrum with a require a strict balance to be metthe in order to obtain absorption spectrum withofa the horizontal baseline. horizontal baseline. Otherwise, baseline tilt willthe lead to a serious distortion absorption line, Otherwise, the baseline tilt will lead to a serious distortion of the absorption line, which could result in which could result in further obvious measurement errors [19]. further measurement [19]. multipoint fiber optic gas sensor system based on an In obvious this paper, a robust errors distributed In thisgain paper, a robust distributed multipoint optic gasThe sensor system based on anits automatic automatic control (AGC) amplifier structure fiber is proposed. AGC amplifier adopts internal gain control (AGC) amplifier structure is proposed. The AGC amplifier adopts its internal self-gain self-gain amplification function for adjustment and stabilization of the baselines of the two inputs of amplification function for adjustment of the baselines the two of the the self-balanced demodulation circuitand to stabilization the same magnitude. In thisofway, not inputs only can self-balanced circuitby to the the same magnitude. way, notcompensated, only can the attenuation attenuation ofdemodulation the signal caused transmission loss In bethis effectively but also a of the signalsimultaneous caused by the transmission be effectively compensated, but also multi-point multi-point detection can beloss easily realized without the affection of ainsertion loss simultaneous can be easily realizedchannels. without the affection of insertion loss differences of gas differences of detection gas cells in different detection cells in different detection channels. 2. Theory of AGC Amplifier 2. Theory of AGC Amplifier
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2.1. Beer-Lambert Law 2.1. Beer-Lambert Law The measurement technique is based on the absorption of monochromatic near-IR radiation. The measurement technique is based on the absorption of monochromatic near-IR radiation. The transmission of the probe beam through a uniform absorbing medium follows the Beer-Lambert The transmission of the probe beam through a uniform absorbing medium follows the law: Beer-Lambert law: (1) − pυq CLs I pυq “= I0 pυq exp exp r´α (1) where CCis the is the target gas concentration, L is the length of the absorption path. The where target gas concentration, and L isand the length of the absorption path. The concentration C concentration C by canidentifying be obtained identifying emergent lightthe intensity I(υ) and the incident can be obtained theby emergent lightthe intensity I(υ) and incident light intensity I0 (υ), light represents intensity I0the (υ),absorption α(υ) represents the absorption at the number as: υ, it can be α(υ) coefficient at the lightcoefficient wave number υ, light it canwave be expressed expressed as: α pυq “ PS pTq φ pυq (2) (2) = P(Pa) is is the the total total pressure, pressure,S(T) S(T) (cm¨mol (cm∙mol´−11))isisthe where P(Pa) theline line strength strength at at an an arbitrary arbitrary temperature temperature (T), ϕ(υ) (cm) (cm) is is the the linear linear function function of of absorption absorption spectra. spectra. φ(υ) 2.2. Fiber Optic Optic Gas Gas Sensor Sensor System System based 2.2. Fiber based on on AGC AGC Amplifier Amplifier Structure Structure A thethe fiber optic gas sensor system based based on AGC structurestructure is shown in A block blockdiagram diagramofof fiber optic gas sensor system onamplifier AGC amplifier is Figure AGC module is added between the pre-amplifier andpre-amplifier the self-balanced (differential shown1.inAn Figure 1. An AGC module is added between the and circuit the self-balanced circuit). The baselinecircuit). amplitudes of the detection signal reference signal the differential circuit (differential The baseline amplitudes ofand the the detection signal andbefore the reference signal circuit can be amplified to the same level through the AGC circuit no matter how the transmission before the differential circuit can be amplified to the same level through the AGC circuit no matter loss howchanges. the transmission loss changes.
Figure 1. Schematic of the automatic gain control (AGC) amplifier detection system. Figure 1. Schematic of the automatic gain control (AGC) amplifier detection system.
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The intensities of the two signals (the reference signal Iref (υ) and the detection signal Idet3(υ)) Sensors 2016, 16, x FOR PEER REVIEW of 9 coupled into the AGC circuits are described below:
The intensities of the two signals (the reference signal Iref(υ) and the detection signal Idet(υ)) (3) Iref pυqbelow: “ γ1 ptq I0 pυq coupled into the AGC circuits are described γ2 ptq 𝛾1(𝑡)𝐼0 (𝜐) (3) exp r´α pυq CLs (4) Idet pυq “ 𝐼ref (𝜐) I=0 pυq ε 𝛾2(𝑡) 𝐼det (𝜐) = transmission I (υ)exp[ − 𝛼(𝜐)𝐶𝐿] where and γ2(t) is the time-varying optical loss coefficient of the two beams due(4) to 𝜀 0 various non-absorption losses, and ε is the coupler splitting ratio. The AGC circuit can achieve a timely where and γ2(t) is the time-varying optical transmission loss coefficient of the two beams due to gain adjustment to maintain a stable baseline according to the reset thresholds even if the γ1(t) and various non-absorption losses, and ε is the coupler splitting ratio. The AGC circuit can achieve a γ2(t) are continually changing. The two signal outputs by the AGC circuits are given by: timely gain adjustment to maintain a stable baseline according to the reset thresholds even if the γ1(t) and γ2(t) are continually changing. 1The two signal outputs by the AGC circuits are given by: Iref pυq “ I0 pυq (5) γ1 ptq 1 𝐼 (𝜐) = 𝐼0 (𝜐) (5) 𝛾1(𝑡) ref ε (6) I pυq “ I0 pυq exp r´α pυq CLs 𝜀 det γ2 ptq 𝐼det (𝜐) = 𝐼0 (𝜐)exp[ − 𝛼(𝜐)𝐶𝐿] (6) The final signal processed by 𝛾2(𝑡) the differential circuit is given by:
The final signal processed by the differential circuit is given by: 1 ε Iout “ Iref pυq ´ 1 𝜀 Idet pυq “ I0 pυq ´ I0 pυq exp r´α pυq CLs γ1 ptq 𝐼 (𝜐) −γ2 ptq 𝐼out = 𝐼 (𝜐) = 𝐼0 (𝜐) − 𝐼0 (𝜐)exp[ − 𝛼(𝜐)𝐶𝐿] ref 𝛾1(𝑡) 𝛾2(𝑡) det When exp [´α(υ)CL] is much less than 1, When exp [-α(υ)CL] is much less than 1, exp r´α CLs «≈ 11´−α pυq CL exp[ − pυq 𝛼(𝜐)𝐶𝐿] 𝛼(𝜐)𝐶𝐿
(7) (7)
(8) (8)
WhenII0(υ) (υ) did did not notchange, change,the thefinal finalsignal signal(I(Iout(υ)) (υ)) will will be beproportional proportionalto tothe theconcentration concentrationof of When out 0 the gas to be measured, so the gas concentration (C) can be derived from the absorption peak. the gas to be measured, so the gas concentration (C) can be derived from the absorption peak. 2.3.AGC AGCAmplifier Amplifier 2.3. TheAGC AGCamplifier amplifier is automatic an automatic amplification gain adjustment device. the AGC The is an amplification gain adjustment device. Using theUsing AGC amplifier amplifier can guarantee relatively stable output case of an unstable input. can guarantee a relativelyastable output in the case in of the an unstable input. closedloop loopAGC AGCamplifier amplifieras asshown shownininFigure Figure22isisused usedininthe thesystem. system.ItItisisactually actuallyaanegative negative AAclosed feedback system, which is composed of a variable gain amplifier and a feedback circuit. The variable feedback system, which is composed of a variable gain amplifier and a feedback circuit. The variable gainamplifier amplifier contains contains aa controlled controlled and and aa non-controlled non-controlled amplifier gain amplifier circuit circuit unit. unit. AAcomparator comparatorand anda detection circuit form a feedback circuit. The effect of the feedback circuit is to convert the output a detection circuit form a feedback circuit. The effect of the feedback circuit is to convert the signal (V out) into a )direct (DC) voltage achieve control and detection output signal (Vout into acurrent direct current (DC) to voltage to gain achieve gain(Comparator control (Comparator and circuit). circuit). detection
Variable gain amplifier Signal input(Vin)
Controlled circuit
Non-controlled circuit
Signal output(Vout)
Control voltage(Vc) Comparator Geophone
Feedback circuit
Reference voltage(Vr)
Figure Figure2.2.AGC AGCamplifier amplifierstructure structurediagram. diagram.
The input signal (Vin) is time-variable due to transmission loss and other interferences, which is introduced by the input port of the controlled circuit unit and finally output by the non-controlled circuit unit. The Vout can remain steady and be fed to the next stage circuit (differential circuit).
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The input signal (Vin ) is time-variable due to transmission loss and other interferences, which is introduced by the input port of the controlled circuit unit and finally output by the non-controlled SensorsThe 2016, 16,out 1187 4 of 8 circuit unit. Sensors 2016, 16,V 1187 can remain steady and be fed to the next stage circuit (differential circuit). 4 of 8 When Vin is small, Vout is also smaller than the reference voltage (Vr ). The output of the feedback When Vin is small, Vout is also smaller than the reference voltage (Vr). The output of the feedback WhenThus, Vin is small, Vout is also smaller than the reference voltage (Vr). The full-scale output of the feedback circuit is zero. gain regulation does not occur. The system maintains amplification. circuit is zero. Thus, gain regulation does not occur. The system maintains full-scale amplification. circuit is zero. Thus, gain regulation does not occur. The system maintains full-scale amplification. When the input signal reaches a certain ), and a result thus V , the> Vr , the When the input signal reaches a certainvalue value(V (V11 Vr, the difference between Vout signal Vreaches can be served as the of the feedback circuit. At this point, the difference between Vand out and V r can be served as the input of the feedback circuit. At this point, the r difference between Vout and Vr can be served as the input of the feedback circuit. At this point, the gain control voltage (V c ) of the controlled circuit unit will change and result in a gain adjustment to gain control voltage (Vc ) of the controlled circuit unit will change and result in a gain adjustment to gain control voltage (Vc) of the controlled circuit unit will change and result in a gain adjustment to keep the V out in the vicinity of the Vr. keep the Vout in the vicinity of the Vr . keep the Vout in vicinity r. When Vinthe is large (Vinof>the V2),VAGC amplifier no longer works, the system is amplified with a When Vin Visin large (V(V AGC amplifier no longer works, the system is amplified with in in>>VV22), When is large ), AGC minimum gain, as shown in Figure 3. amplifier no longer works, the system is amplified with a a minimum gain, asasshown 3. minimum gain, shownin inFigure Figure 3.
Vout
Vout
without AGC
with AGC
without AGC
with AGC
Vr
Vr
V1
V2
Vin
Vin
V1 ofofAGC V Among Figure 3. Static regulation characteristic them, the threshold 1 is Figure 3. Static regulation characteristic AGC amplifier. amplifier.2 Among them, V1 isVthe threshold voltage,voltage, and theand V2 the is out control voltage, they keyperformance performance indicators the amplifier. AGC amplifier. V2 isofout of control voltage, theyare aretwo two key indicators of theofAGC Figure 3. Static regulation characteristic of AGC amplifier. Among them, V1 is the threshold voltage, 3. Experimental Apparatus and Results and the V2Apparatus is out of control voltage, they are two key performance indicators of the AGC amplifier. 3. Experimental and Results
Stress-induced fiber bending will inevitably occur and cause a transmission loss in the
Stress-induced bending inevitably occur and cause a transmission loss in the long-haul 3. Experimental fiber Apparatus and will Results long-haul optical fiber transmission. In a multi-point detection system, the amount of loss between optical different fiber transmission. In a multi-point detection system, the amount loss between different monitoring points must will be distinct and unsynchronized. willofaffect theloss TDLAS Stress-induced fiber bending inevitably occur and causeThis a transmission in the monitoring points must distinct andIn unsynchronized. This will affect TDLAS detection system detection system farbe more than some random high frequency noise. long-haul optical fiber transmission. a multi-point detection system, thethe amount of loss between far more than some random high frequency noise. A fiber bending test is used to evaluate the reliability of the system. Acetylene is selected as the different monitoring points must be distinct and unsynchronized. This will affect the TDLAS gas tobending be detected. single-mode DFB-LD (WSLS-137010C1424-20, Agilecom, FiberisSolution, A fiber testmore isA used evaluate the reliability of noise. the system. Acetylene selected as the detection system far thanto some random high frequency San Jose, CA, USA) is used as the light source, in which the output wavelength is modulated bySan A fiber bending test is usedDFB-LD to evaluate the reliability of the system. Acetylene is selected as the Jose, gas to be detected. A single-mode (WSLS-137010C1424-20, Agilecom, Fiber Solution, adjusting the laser temperature and the injection current. The injection current is scanned with a gas to isbeused detected. single-mode DFB-LD Agilecom, Fiber Solution, CA, USA) as theAlight source, in which(WSLS-137010C1424-20, the output wavelength is modulated by adjusting saw-tooth function, which ensures that the light emitted from the laser source can cover the entire San Jose, CA, USA)and is used as the lightcurrent. source, in which the output wavelength is modulated by the laser temperature the injection The injection current is scanned a saw-tooth wavelength range of acetylene absorption lines near 1532 nm. Laser temperature is set towith a suitable adjusting the laser temperature and the injection current. The injection current is scanned with a function, whichpoint ensures that the light emitted fromchip the(LTC laser1923, source canTechnology, cover the Milpitas, entire wavelength working by a micro-temperature controlling Linear CA, saw-tooth function, which ensures that the light emitted from the laser source can cover the entire The LDabsorption beam is splitlines into reference detection by a 1 × is 2 coupler. range ofUSA). acetylene near 1532and nm. Laser beams temperature set to a suitable working point wavelength range of acetylene absorptionalines near 1532 nm.ofLaser temperature is set to gas a suitable The detection beam passes through gas1923, cell consisting a stainless-steel tube.CA, The USA). cell The LD by a micro-temperature controlling chipcontrolling (LTC Linear Technology, Milpitas, working point by a micro-temperature chip (LTC 1923, Linear Technology, Milpitas, shown in Figure 4 is filled with 1000 ppm acetylene in which the absorption path is 3 cm. A goodCA, beamUSA). is split into andinto detection beams by a 1 ˆbeams 2 coupler. The LDreference beamensures is split detection a 1 × 2 coupler. air seal structure that reference we do notand need to consider theby measurement errors caused by an The detection beam passes through a gas cell consisting stainless-steel tube. gas cell The detection beam passes through a gas cell consisting ofofa astainless-steel tube. The The gas cell unstable air concentration. shown in Figure 4 is 4filled with 1000 ppm acetylene pathisis33cm. cm.AAgood good air shown in Figure is filled with 1000 ppm acetyleneininwhich whichthe the absorption absorption path air seal structure we do nottoneed to consider the measurement caused byunstable an seal structure ensuresensures that wethat do not need consider the measurement errorserrors caused by an unstable air concentration. air concentration.
Figure 4. The gas cell structure.
Figure 4. The gas cell structure. Figure 4. The gas cell structure.
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An ordinary differential demodulation system is employed to compare with the AGC AnAn ordinary differential demodulation system is choose employed compare with thewith AGC differential differential demodulation system in the test. They thetosame andthe differential ordinary differential demodulation system is employed topre-amplifier compare AGC demodulation system in the test. They choose the same pre-amplifier and differential circuit. The only circuit. The only difference system is whether to test. join They the AGC module. In the AGC system, controlled differential demodulation in the choose the same pre-amplifier and the differential difference isiswhether to join module. themodule. AGC system, the controlled unit is circuit unit constructed with theAGC AD603 chipthe asIn shown in Figure 5. AGC circuit. The only difference isthe whether to join AGC In the system, thecircuit controlled constructed the AD603with chipthe asAD603 shownchip in Figure 5. in Figure 5. circuit unitwith is constructed as shown
Figure diagram of the AGC amplifier. Figure 5. The circuit diagram diagramof ofthe theAGC AGCamplifier. amplifier. Figure5. 5. The The circuit circuit
InIn the ordinary system, the baseline of the the two twosignals signals(the (thedetection detectionsignal signal and the reference the ordinarysystem, system,the thebaseline baseline of of the and thethe reference In the ordinary two signals (the detection signal and reference signal) canachieve achievethe thesame samemagnitude magnitude by by adjusting amplitude of of thethe final signal) can achieve the same magnitude by adjusting adjustingthe thepreamplifier. preamplifier.The The amplitude of the final signal) can the preamplifier. The amplitude final demodulatedsignal signalisis1.668 1.668 V. V. Bending Bending with aa bending radii ofof5 5mm causes propagation losses in in demodulated with bending radii mm causes propagation losses demodulated signal is 1.668 V. Bending with a bending radii of 5 mm causes propagation losses opticalfiber fiber between the the DFB-LD and coupler, and magnitude ofofthe signal anan optical thedemodulated demodulated signal in an optical fiberbetween between theDFB-LD DFB-LDand and coupler, coupler, and and magnitude magnitude of the demodulated signal changes to 0.321 V. The measurement error is 80.76%. As shown in Figure 6a, although there is ais a changes to 0.321 V. The measurement error is 80.76%. As shown in Figure 6a, although there changes to 0.321 V. The measurement error is 80.76%. As shown in Figure 6a, although there is a large largeamplitude amplitudeattenuation, attenuation, aa relatively relatively significant absorption profile can bebe still detected because large significant absorption profile can still detected because amplitude attenuation, a relatively significant absorption profile can be still detected because the the amounts of light loss of the two signals are the almost same. However, the macro bending lossloss the amounts of light loss of the two signals are the almost same. However, the macro bending amounts of light loss of the two signals are the almost same. However, the macro bending loss occurs occursononthe thetransmission transmission fiber fiber of of the the detection signal or fiber of of thethe reference occurs detection signal oron onthe thetransmission transmission reference on the transmission fiber of the detection signal or on the transmission fiber offiber the6b. reference signal. signal. This situation is especially serious in profile measurement as shown in Figure signal. This situation is especially serious in profile measurement as shown in Figure 6b. This situation is especially serious in profile measurement as shown in Figure 6b. We have replicated this experiment on the AGC differential demodulation system. Without a Wehave havereplicated replicatedthis this experiment the AGC differential demodulation system. Without We experiment on on the AGC differential Without fibera fiber bending loss, the magnitude of the demodulated signal demodulation is 1.527 V. As system. shown in Figurea6c, fiber bending loss, the magnitude of the demodulated signal is 1.527 V. As shown in Figure 6c, bending the magnitude theoccurs demodulated is 1.527 the V. As shown in signal Figurechanges 6c, when when aloss, 5 mm macro bendingofloss before thesignal fiber coupler, demodulated amacro 5 mm bending macro bending lossbefore occursthe before the fiberthe coupler, the demodulated signal changes awhen 5tomm loss occurs fiber demodulated signal changes 1.491 V. 1.491 V. The measurement error is reduced tocoupler, 2.36%. As shown in Figure 6d, when thetomacro to 1.491 V. The measurement error is reduced to 2.36%. As shown in Figure 6d, when the macro The measurement error reduced to 2.36%. in Figure 6d,orwhen macro bending loss bending loss occurs on isthe transmission fiberAs of shown the reference signal on thethe detection signal, the bending loss occurs on the transmission fiber of the reference signal or on the detection signal, occurs on the transmission fiber(1.527–1.469 of the reference signal on the detection signal, theReliability measurement measurement errors are 3.80% V) and 4.06%or(1.527–1.465 V), respectively. ofthe measurement errors 3.80% V) and 4.06% respectively. of the system has beenare enhanced. errors are 3.80% (1.527–1.469 V)(1.527–1.469 and 4.06% (1.527–1.465 V),(1.527–1.465 respectively.V), Reliability of theReliability system has the system has been enhanced. been enhanced.
Figure 6. Cont.
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Figure Figure 6.6. Macro Macro bending bending loss loss test test results results of ofthe thedifferent differentsystems. systems. (a) (a) The The change change of of the the absorption absorption Figure 6.obtained Macro bending loss test results of thedemodulation different systems. (a) The change the absorption waveform by differential system under fiber bending before waveform obtained by the the ordinary ordinary differential demodulation system under fiberof bending before the the waveform obtained byof the ordinary demodulation system under bendingdemodulation before the coupler; (b) The change the waveform obtained by the ordinary differential coupler; (b) The change ofabsorption the differential absorption waveform obtained by fiber the ordinary differential coupler; (b) fiber The bending change the absorption waveform the reference ordinary differential system under on thebending detection or theobtained reference (c) The signal; change of The the demodulation system underof fiber onsignal the detection signal by orsignal; the (c) demodulation system under fiber bending on the detection signal or the reference signal; (c) The absorption waveform obtained by the AGC differential demodulation system under fiber bending change of the absorption waveform obtained by the AGC differential demodulation system under change thebefore absorption waveform by the AGC differential demodulation system under before theofsplitter; (d)the The change thechange absorption waveform obtained by obtained the AGC differential fiber bending splitter; (d)ofobtained The of the absorption waveform by the AGC fiber bending before the splitter; (d) The change of the absorption waveform obtained by the AGC demodulation system under fiber bending on the detection signal or thesignal reference signal. differential demodulation system under fiber bending on the detection or the reference signal. differential demodulation system under fiber bending on the detection signal or the reference signal.
Acetylene concentration concentration measurement in in different different mixtures mixtures of acetylene acetylene and and nitrogen nitrogen is is Acetylene Acetylene concentration measurement measurement in different mixtures ofofacetylene and nitrogen is performed. Absorption line shape under different acetylene concentrations concentrations in aa 20 20 cm gas gas cell is is performed. under different differentacetylene acetylene performed.Absorption Absorption line line shape shape under concentrations in in a 20 cmcm gas cellcell is shown in Figure 7a. As shown in Figure 7b, the demodulated signal is well proportional to shown in in Figure 7a. 7a. As shown in Figure 7b, the signalsignal is wellisproportional to acetylene shown Figure As shown in Figure 7b,demodulated the demodulated well proportional to acetylene concentration ranging from
