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Design and characteristics of quantum cascade laser-based CO detection system
Sensors and Actuators B 142 (2009) 33–38
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Design and characteristics of quantum cascade laser-based CO detection system Lei Li a , Feng Cao a , Yiding Wang a,∗ , Menglong Cong a , Li Li a , Yupeng An a , Zhenyu Song a , Shuxu Guo a , Fengqi Liu b , Lijun Wang b a b
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 1 September 2008 Received in revised form 31 July 2009 Accepted 14 August 2009 Available online 22 August 2009
a b s t r a c t The authors report the design and characteristics of a mid-infrared quantum cascade laser-based CO detection system using the absorption in the P (14) line, at 2086.32 cm−1 . Measurements are performed through line scanning and signal processing to give sensitivities down to 2 ppm with the response time of 7 s. © 2009 Elsevier B.V. All rights reserved.
Keywords: CO QC laser Optical detection systems Absorption spectroscopy Chemical sensors
1. Introduction The detection and quantification of toxic and flammable gases in ambient air are of considerable importance for a number of applications, such as environmental monitoring, combustion diagnostics, and atmospheric chemistry [1–4]. Recently, optical detection systems have attracted increasing interest for gas detection applications due to their potential advantages of (i) intrinsically safe, (ii) ability to detect a specific gas by selection of appropriate wavelengths, and (iii) able to operate in zero-oxygen environment [5–8]. In 1998, Zahniser et al. have presented a new pulsed quantum cascade laser (QCL) system to measure CO with a dual thermoelectric-cooled DFB QCLs, astigmatic mirror multipass cell (76-m pathlength) based spectrometer. This instrument successfully detected the limit of concentration of CO about 0.5 ppb in 1 s [9]. Hence, extensive studies have been put on improving the system properties, such as fabricating high performance optical source, optimizing signal processing, controlling optical path, etc. [10–13]. From the beginning of putting forward the concept of quantum cascade lasers up to now, the QCL whose structural conformations, operating conditions such as operating temperature, threshold current density are being continuously promoted
∗ Corresponding author. Tel.: +86 431 85168240; fax: +86 431 85168270. E-mail addresses: lilei [email protected] (L. Li), [email protected] (Y. Wang). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.08.025
is used as the high performance optical source [14–16]. At the same time, many optical detection systems have been successfully obtained, just like, the White-type multipass cell, which can provide sufficient optical absorption path length for high sensitivity measurement in a small volume of gas and thus can enhance the detection limit effectively [5,19], but the properties still need to be improved for the increasing demands and potential applications. Since their birth, in 1994 [14], quantum cascade (QC) lasers have shown tremendous performance improvements and technological progress. These lasers are based on one type of carrier (electrons) jumping between energy levels created by quantum confinement [17–21]. At present, they are the only semiconductor lasers operating at room temperature in the 4–24 m wavelength range, in pulsed and continuous mode, with output power from tens of mW to several Watts [17–21]. The availability of high optical power, in addition to their high stability and monochromaticity, makes these lasers very well fit for high sensitivity spectroscopy in the mid-infrared (MIR) spectral range. Up to now, QC lasers have already been used for numerous applications: non-invasive medical sensors [22], pollutant or industrial chemical probes [23], collision-avoidance radar [24] and so on. However, the CO detection system based on QC laser has rarely been reported. Herein, we demonstrate a CO detection system based on a QC laser using a differential absorption measurement. By simply adjusting the operating current, the wavelength of our laser can achieve the P(14) absorption line, at 2086.32 cm−1 [25], and the concentration of CO can be precisely measured by comparing the
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L. Li et al. / Sensors and Actuators B 142 (2009) 33–38
Fig. 1. Experimental schematic image of QC laser CO detection system.
output signals from the sample cell and the reference cell. These experimental results provide a good basis for the development of practical gas sensors.
2. Experimental 2.1. Instrument details The experimental apparatus developed in our CO detection system is schematically shown in Fig. 1. Target ambiences were obtained using a static test system. Saturated target vapor was injected into a test chamber by a micro-injector through a rubber plug. The light generated by a strain-compensated InGaAs/InAlAs QC laser was collected and coupled out of the enclosure by an antireflection-coated germanium aspheric lens [26]. The collimated signal from the QC laser was then split into signal and reference beams (with a power ratio of 1:1) by using of an antireflectioncoated Ge beam splitter. The signal beam propagated through a 200 mm electropolished, stainless-steel absorption cell that had antireflection-coated sapphire windows. After exiting the cell, the beam was refocused onto a thermoelectric-cooled HgCdTe detector 1 (SITP TE2). Similarly, the reference beam was propagated through a 40 mm reference cell with pure N2 (99.99%), and then refocused onto the detector 2. The photoconductive HgCdTe IR detectors were used in the experiment with a peak wavelength at 4.5 m and response spectral range 2–5 m. The response time was less than 1 s. The dimension of detecting head was 1 mm × 1 mm. The output impedance of the IR detector was 1 k, and its detectivity is about (0.5–1) × 109 cm Hz1/2 /W. The bias voltage applied to the detectors was set in the range of 12–15 V in the experiment. These electrical signals produced by the detectors were amplified by 80 MHz bandwidth AC coupled amplifiers, amplifying about 1000 times around. During this procedure, the high and low-pass filters were applied on the signals output from the two detectors to reduce noise, and then sent to an ARM LPC 2136 for data acquisition. 100 scans were averaged for each single concentration measurement in our sensor. The operation of the detection system relied on the local heating induced in the QC laser by the current pulse generating an optical pulse with a frequency down-chirped spectrum. To quantify the frequency-swept output from the laser, a temperature stabilize circuit was developed based on the thermoelectric cooler (Tes1 1702L Hainan). The QC laser was driven by a 1 A ampli-
tude, 5 kHz train of 2 s wide current pulse current with ripple noise small than 1 A (∼1% duty factor), thus the frequency fluctuations of the laser line that due to current noise can be neglected (<1 MHz). The laser source could output 12 mW optical powers, in a single mode and without any mode hop. We fine tuned the laser by superimposing upon the pulsed waveform a 100 Hz saw-toothed current ramp (the average current is 500 mA), which could cause a temperature modulation on the laser sufficient to sweep the lasing frequency by approximately 0.2 cm−1 .
2.2. Signal processing The signal processing in our system performs two divisions and can be used to measure accurately the distortion of any modulated signal multiplied by an unknown gain (Fig. 2). For this, a reference, having the same modulation as the signal but a different gain is used. Basically a first division is used to cancel the modulation effects and a second one is used to cancel the unknown gain effects. In this method the details of the signal processing are shown below. Laser absorption spectroscopy (LAS) of target gas species, which is based on the Beer–Lambert absorption law, effectively deter-
Fig. 2. Schematic image of signal processing.
L. Li et al. / Sensors and Actuators B 142 (2009) 33–38
mines real-time gas concentrations. Beer–Lambert law states I(v) = I0 (v)e[−˛(v)CL+ˇ(v)]
(1)
where I(v) is the intensity of light passing through the absorbing medium, I0 (v) is the input intensity, L is the optical path length, v is the radiation frequency, and ˛(v) is the molecular line intensity, ˇ(v) is a constant, depending on the experimental equipments. I(v) can be obtained on a detector receiving an optical beam that has encountered target gas absorption on the optical path (propagated through the sample cell), which will be called Is(v), and on the other detector receiving the signal propagated through the reference cell, which will be called Ir(v). As the optical frequency is also a function of time, we can write Is[v(t)] and Ir[v(t)] without loss of generality. Is(v(t)) = mI0 (v(t))e[−˛(v)CL+ˇ(v)]
(2)
Ir(v(t)) = nI0 (v(t))
(3)
where m is a factor of proportionality between intensities of the signal beam and the source beam, and n is that between reference beam and the source beam. For gas detection, the most interesting region is the low absorption one. In the limit of low absorption, the above expression can be approximated as: Is(v(t)) = me[ˇ(v)] I0 (v(t))[1 − ˛(v)CL] = mkI0 (v(t))[1 − ˛(v)CL]
(4)
Ir(v(t)) = nI0 (v(t))
(3 )
where k represents e[ˇ(v)] . The signal processing then consists in a first divider stage where Is[v(t)] is divided by Ir[(v(t))] to suppress the laser modulation. The output of this divider stage is Is(v(t)) mk[1 − ˛(v(t))CL] = n Ir(v(t))
(5)
The d.c. and a.c. parts of this signal are then extracted, which can be done easily with low-pass and high-pass filters: d.c. =
mk n
(6)
a.c. =
mk[−˛(v(t))CL] n
(7)
Then the a.c. signal is divided by the d.c. signal in a second divider stage to cancel the factor mk/n, which contains the differences of the static absorption between both optical paths: O = −a(v(t))CL
(8)
35
The output of the second divider (O) is thus proportional to the absolute optical absorption, gas concentration and optical path length. 3. Results and discussion As one can tune the lasing wavelength of a QC laser by changing the device’s temperature, we preliminarily investigated the temperature dependence of laser emission in the range of 200–293 K. The substrate temperature of the QC laser was maintained via the thermoelectric cooler, and the fluctuation was less than 1 K. The laser spectrum was analyzed with a Fourier-transform infrared spectrometer (FTIR, THERMO 4700) with a resolution of 0.125 cm−1 . The results shown in Fig. 3(a) indicate that the single lasing mode tunes to longer wavenumber with increase the operating temperature. In particular, the single lasing mode tunes linearly with temperature from 2113.60 cm−1 at 200 K to 2086.41 cm−1 at 293 K. A linear fit describes the tuning rate of the QC laser as a function of temperature with a coefficient of −0.292 cm−1 /K. At an operating temperature of 293 K, the QC laser has a total tuning range of about 0.2 cm−1 from 2086.41 cm−1 to 2086.21 cm−1 , which is achievable by adjusting the operating current from 900 mA to 2.0 A (Fig. 3(b)). The wavenumber of 2086.32 cm−1 of our laser can be achieved at an operating current of 1480 mA. In Fig. 4(a) the P branch of the fundamental rho-vibrational band is displayed (HITRAN 2003) [26]. The best spectral location in terms of CO absorbance is P(7) in this region, at 2115.63 cm−1 . However, the QC laser could not be operated at this wavelength, so we selected a shorter operating wavelength that would still permit sensitive detection of CO, as marked in Fig. 4. This line is also free of interference from other species (Fig. 4(b)), which make our laser an excellent light source suitable for high resolution spectroscopic applications. Comparing the measured HWHM of carbon monoxide absorption lines with the HITRAN database, we found the former was larger than the values given by the later. From comparison of these data the laser linewidth was found to be less than 100 MHz. The possible reasons of errors are the poor fineness of the germanium etalon and minor nonlinearities in frequency versus current, which affect the frequency calibration of experimental spectra [29]. The corresponding absorption cross-sections of 1% CO in air were calculated on the basis of a molecular spectroscopic database from HITRAN 2003. The absorption coefficient for this line is 45.12 cm−1 . Before proceeding with absorption studies, we investigated the electrical and the optical pulse waveforms for our system. The pulse generator was set to produce pulses of 2 s widths at a repetition rate of 5 kHz. The optical pulse was measured with the fast, highbandwidth HgCdTe detector (1 MHz) and a preamplifier (80 MHz).
Fig. 3. (a) QC laser spectrum versus temperature curves, the insert is the lasing mode temperature tuning, and (b) the lasing mode current tuning.
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L. Li et al. / Sensors and Actuators B 142 (2009) 33–38
Fig. 4. (a) CO absorption lines accessible by a 4.79 m QC laser, and (b) CO and H2 O adsorption lines.
Fig. 5. (a) Temporal response of the current pulse used to drive the QC laser, and (b) temporal output from the QC laser measured using the HgCdTe detector.
The current and the optical waveforms of our system are shown in Fig. 5(a) and (b), respectively. Both the rise and fall times of these two waveforms are less than 50 ns, and no obvious different can be observed between the pulse widths. These results indicate an excellent agreement in the signal and optical pulses. The example absorption spectrum shown in Fig. 6 was acquired for 200 ppm CO and is an average of 100 scans. The peak absorption is 0.088, and the signal-to-noise ratio (SNR) is about 120, which implies a minimum detectable absorption of 7.3 × 10−4 and concentration of 1.7 ppm. It is important to note that these results are typical detectivity based on our system, which can further improved by noise-reduction techniques [27].
A time response trace of the QC laser-based detection system output for 50 ppm and 200 ppm CO is shown in Fig. 7. The magnitude of change for 50 ppm CO is about 48 V, and the change in signal for 200 ppm CO is about 200 V. The response time is observed to be about 7 s, this value includes the response delay time and disposal time of the circuits of the whole instrument mostly, such as 100 scans acquisition and calculating times, related data reading and displaying times, etc. Fig. 8 shows the sensor signal as a function of CO concentration reduced from 200 ppm to 2 ppm in steps [28]. These measurements were performed after accurate purging of the cell to prevent interference from adsorbed–desorbed molecules. The solid line is the linear fit of the background-divided data and shows a strong linear relation-
Fig. 6. Spectrum of the P(14) line of 200 ppm CO.
Fig. 7. Time response of the QC laser detection system to 50 ppm and 200 ppm CO.
L. Li et al. / Sensors and Actuators B 142 (2009) 33–38
Fig. 8. QC laser signal versus CO concentration.
ship between the output signal and the CO concentration. The ultimate sensitivity of our system is determined to be 2 ppm, which approximately agrees with the theoretical results from SNR (Fig. 6). 4. Conclusion Basically, the realization of a CO detection system using the absorption in the P(14) line, at 2086.32 cm−1 with the differential signal processing is reported. The system exhibits quick response and high sensitivity to CO, and its performances can be further improved by using noise-reduction, optical path-optimization and other techniques. The results demonstrate the utility of QC laser for sensor applications in the area of toxic gas monitoring and detecting. Acknowledgements This research was financially supported by the National High Technology Research and Development Program of China, No. 2007AA03Z112, by the Program of Ministry of Education of China, No. 20060183030, by the Program of Jilin Provincial Science and Technology Department of China, No. 20070709, and by the Program of Bureau of Science and Technology of Changchun City, No. 2007107. References [1] J. Janata, M. Josowicz, D.M. DeVaney, Chemical sensors, Anal. Chem. 66 (1994) 207–228. [2] H. Dacres, R. Narayanaswamy, A new optical sensing reaction for nitric oxide, Sens. Actuators B: Chem. 90 (2003) 222–229. [3] N. Carmona, E. Herrero, J. Llopis, M.A. Villegas, Chemical sol–gel-based sensors for evaluation of environmental humidity, Sens. Actuators B: Chem. 126 (2007) 455–460. [4] N. Rajabbeigi, B. Elyassi, A.A. Khodadadi, S. Mohajerzadeh, Y. Mortazavi, M. Sahimi, Oxygen sensor with solid-state CeO2 –ZrO2 –TiO2 reference, Sens. Actuators B: Chem. 108 (2005) 341–345. [5] L. Gianfrani, A. Sasso, G.M. Tino, Monitoring of O2 and NO2 using tunable diode lasers in the near-infrared region, Sens. Actuators B: Chem. 38–39 (1997) 283–285. [6] S. Svanberg, Chemical sensing with laser spectroscopy, Sens. Actuators B: Chem. 33 (1996) 1–4. [7] C. Massie, G. Stewart, G. McGregor, J.R. Gilchrist, Design of a portable optical sensor for methane gas detection, Sens. Actuators B: Chem. 113 (2006) 830–836. [8] M.B. Filho, M.G. da Silva, M.S. Sthel, D.U. Schramm, H. Vargas, A. Miklós, P. Hess, Ammonia detection by using quantum-cascade laser photoacoustic spectroscopy, Appl. Opt. 45 (2006) 4966–4971. [9] A. Fried, G. Diskin, P. Weibring, D. Richter, J.G. Walega, G. Sachse, T. Slate, M. Rana, J. Podolske, Tunable infrared laser instruments for airborne atmospheric studies, Appl. Phys. B 92 (2008) 409–417.
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Biographies Lei Li received his MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2008. He entered the PhD course in 2008, majored in circuits and system. Feng Cao received his MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2006. He entered the PhD course in 2006, majored in microelectronics and solid-state electronics. Yiding Wang received her MS degree in major of physics in 1991 from Jilin University. He was appointed a full professor in College of Electronics Science and Engineering, Jilin University in 2001. Now, he is interested in the field of optical sensors. Menglong Cong received her MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2008. She entered the PhD course in 2008, majored in circuits and system.
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Li Li received her MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2006. She entered the PhD course in 2006, majored in microelectronics and solid-state electronics. Yupeng An received his MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2005. He entered the PhD course in 2005, majored in microelectronics and solid state electronics. Zhenyu Song received his MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2005. He entered the PhD course in 2005, majored in microelectronics and solid state electronics.
Shuxu Guo received her BS degree from the College of Electronics Science and Engineering, Jilin University, China in 1987. He was appointed a full professor in College of Electronics Science and Engineering, Jilin University in 2001. Now, he is interested in the field of digital image processing and wavelet technology. Fengqi Liu received his PhD degree in the field of physics in 2004 from Nanjing University. Now, he is interested in the field of QC laser, nanomaterials and nanodevices. Lijun Wang received his PhD degree in the field of microelectronics in 2004 from West Virginia University. Now, he is interested in the field of optical sensors.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Design and characteristics of quantum cascade laser-based CO detection system Lei Li a , Feng Cao a , Yiding Wang a,∗ , Menglong Cong a , Li Li a , Yupeng An a , Zhenyu Song a , Shuxu Guo a , Fengqi Liu b , Lijun Wang b a b
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 1 September 2008 Received in revised form 31 July 2009 Accepted 14 August 2009 Available online 22 August 2009
a b s t r a c t The authors report the design and characteristics of a mid-infrared quantum cascade laser-based CO detection system using the absorption in the P (14) line, at 2086.32 cm−1 . Measurements are performed through line scanning and signal processing to give sensitivities down to 2 ppm with the response time of 7 s. © 2009 Elsevier B.V. All rights reserved.
Keywords: CO QC laser Optical detection systems Absorption spectroscopy Chemical sensors
1. Introduction The detection and quantification of toxic and flammable gases in ambient air are of considerable importance for a number of applications, such as environmental monitoring, combustion diagnostics, and atmospheric chemistry [1–4]. Recently, optical detection systems have attracted increasing interest for gas detection applications due to their potential advantages of (i) intrinsically safe, (ii) ability to detect a specific gas by selection of appropriate wavelengths, and (iii) able to operate in zero-oxygen environment [5–8]. In 1998, Zahniser et al. have presented a new pulsed quantum cascade laser (QCL) system to measure CO with a dual thermoelectric-cooled DFB QCLs, astigmatic mirror multipass cell (76-m pathlength) based spectrometer. This instrument successfully detected the limit of concentration of CO about 0.5 ppb in 1 s [9]. Hence, extensive studies have been put on improving the system properties, such as fabricating high performance optical source, optimizing signal processing, controlling optical path, etc. [10–13]. From the beginning of putting forward the concept of quantum cascade lasers up to now, the QCL whose structural conformations, operating conditions such as operating temperature, threshold current density are being continuously promoted
∗ Corresponding author. Tel.: +86 431 85168240; fax: +86 431 85168270. E-mail addresses: lilei [email protected] (L. Li), [email protected] (Y. Wang). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.08.025
is used as the high performance optical source [14–16]. At the same time, many optical detection systems have been successfully obtained, just like, the White-type multipass cell, which can provide sufficient optical absorption path length for high sensitivity measurement in a small volume of gas and thus can enhance the detection limit effectively [5,19], but the properties still need to be improved for the increasing demands and potential applications. Since their birth, in 1994 [14], quantum cascade (QC) lasers have shown tremendous performance improvements and technological progress. These lasers are based on one type of carrier (electrons) jumping between energy levels created by quantum confinement [17–21]. At present, they are the only semiconductor lasers operating at room temperature in the 4–24 m wavelength range, in pulsed and continuous mode, with output power from tens of mW to several Watts [17–21]. The availability of high optical power, in addition to their high stability and monochromaticity, makes these lasers very well fit for high sensitivity spectroscopy in the mid-infrared (MIR) spectral range. Up to now, QC lasers have already been used for numerous applications: non-invasive medical sensors [22], pollutant or industrial chemical probes [23], collision-avoidance radar [24] and so on. However, the CO detection system based on QC laser has rarely been reported. Herein, we demonstrate a CO detection system based on a QC laser using a differential absorption measurement. By simply adjusting the operating current, the wavelength of our laser can achieve the P(14) absorption line, at 2086.32 cm−1 [25], and the concentration of CO can be precisely measured by comparing the
34
L. Li et al. / Sensors and Actuators B 142 (2009) 33–38
Fig. 1. Experimental schematic image of QC laser CO detection system.
output signals from the sample cell and the reference cell. These experimental results provide a good basis for the development of practical gas sensors.
2. Experimental 2.1. Instrument details The experimental apparatus developed in our CO detection system is schematically shown in Fig. 1. Target ambiences were obtained using a static test system. Saturated target vapor was injected into a test chamber by a micro-injector through a rubber plug. The light generated by a strain-compensated InGaAs/InAlAs QC laser was collected and coupled out of the enclosure by an antireflection-coated germanium aspheric lens [26]. The collimated signal from the QC laser was then split into signal and reference beams (with a power ratio of 1:1) by using of an antireflectioncoated Ge beam splitter. The signal beam propagated through a 200 mm electropolished, stainless-steel absorption cell that had antireflection-coated sapphire windows. After exiting the cell, the beam was refocused onto a thermoelectric-cooled HgCdTe detector 1 (SITP TE2). Similarly, the reference beam was propagated through a 40 mm reference cell with pure N2 (99.99%), and then refocused onto the detector 2. The photoconductive HgCdTe IR detectors were used in the experiment with a peak wavelength at 4.5 m and response spectral range 2–5 m. The response time was less than 1 s. The dimension of detecting head was 1 mm × 1 mm. The output impedance of the IR detector was 1 k, and its detectivity is about (0.5–1) × 109 cm Hz1/2 /W. The bias voltage applied to the detectors was set in the range of 12–15 V in the experiment. These electrical signals produced by the detectors were amplified by 80 MHz bandwidth AC coupled amplifiers, amplifying about 1000 times around. During this procedure, the high and low-pass filters were applied on the signals output from the two detectors to reduce noise, and then sent to an ARM LPC 2136 for data acquisition. 100 scans were averaged for each single concentration measurement in our sensor. The operation of the detection system relied on the local heating induced in the QC laser by the current pulse generating an optical pulse with a frequency down-chirped spectrum. To quantify the frequency-swept output from the laser, a temperature stabilize circuit was developed based on the thermoelectric cooler (Tes1 1702L Hainan). The QC laser was driven by a 1 A ampli-
tude, 5 kHz train of 2 s wide current pulse current with ripple noise small than 1 A (∼1% duty factor), thus the frequency fluctuations of the laser line that due to current noise can be neglected (<1 MHz). The laser source could output 12 mW optical powers, in a single mode and without any mode hop. We fine tuned the laser by superimposing upon the pulsed waveform a 100 Hz saw-toothed current ramp (the average current is 500 mA), which could cause a temperature modulation on the laser sufficient to sweep the lasing frequency by approximately 0.2 cm−1 .
2.2. Signal processing The signal processing in our system performs two divisions and can be used to measure accurately the distortion of any modulated signal multiplied by an unknown gain (Fig. 2). For this, a reference, having the same modulation as the signal but a different gain is used. Basically a first division is used to cancel the modulation effects and a second one is used to cancel the unknown gain effects. In this method the details of the signal processing are shown below. Laser absorption spectroscopy (LAS) of target gas species, which is based on the Beer–Lambert absorption law, effectively deter-
Fig. 2. Schematic image of signal processing.
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mines real-time gas concentrations. Beer–Lambert law states I(v) = I0 (v)e[−˛(v)CL+ˇ(v)]
(1)
where I(v) is the intensity of light passing through the absorbing medium, I0 (v) is the input intensity, L is the optical path length, v is the radiation frequency, and ˛(v) is the molecular line intensity, ˇ(v) is a constant, depending on the experimental equipments. I(v) can be obtained on a detector receiving an optical beam that has encountered target gas absorption on the optical path (propagated through the sample cell), which will be called Is(v), and on the other detector receiving the signal propagated through the reference cell, which will be called Ir(v). As the optical frequency is also a function of time, we can write Is[v(t)] and Ir[v(t)] without loss of generality. Is(v(t)) = mI0 (v(t))e[−˛(v)CL+ˇ(v)]
(2)
Ir(v(t)) = nI0 (v(t))
(3)
where m is a factor of proportionality between intensities of the signal beam and the source beam, and n is that between reference beam and the source beam. For gas detection, the most interesting region is the low absorption one. In the limit of low absorption, the above expression can be approximated as: Is(v(t)) = me[ˇ(v)] I0 (v(t))[1 − ˛(v)CL] = mkI0 (v(t))[1 − ˛(v)CL]
(4)
Ir(v(t)) = nI0 (v(t))
(3 )
where k represents e[ˇ(v)] . The signal processing then consists in a first divider stage where Is[v(t)] is divided by Ir[(v(t))] to suppress the laser modulation. The output of this divider stage is Is(v(t)) mk[1 − ˛(v(t))CL] = n Ir(v(t))
(5)
The d.c. and a.c. parts of this signal are then extracted, which can be done easily with low-pass and high-pass filters: d.c. =
mk n
(6)
a.c. =
mk[−˛(v(t))CL] n
(7)
Then the a.c. signal is divided by the d.c. signal in a second divider stage to cancel the factor mk/n, which contains the differences of the static absorption between both optical paths: O = −a(v(t))CL
(8)
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The output of the second divider (O) is thus proportional to the absolute optical absorption, gas concentration and optical path length. 3. Results and discussion As one can tune the lasing wavelength of a QC laser by changing the device’s temperature, we preliminarily investigated the temperature dependence of laser emission in the range of 200–293 K. The substrate temperature of the QC laser was maintained via the thermoelectric cooler, and the fluctuation was less than 1 K. The laser spectrum was analyzed with a Fourier-transform infrared spectrometer (FTIR, THERMO 4700) with a resolution of 0.125 cm−1 . The results shown in Fig. 3(a) indicate that the single lasing mode tunes to longer wavenumber with increase the operating temperature. In particular, the single lasing mode tunes linearly with temperature from 2113.60 cm−1 at 200 K to 2086.41 cm−1 at 293 K. A linear fit describes the tuning rate of the QC laser as a function of temperature with a coefficient of −0.292 cm−1 /K. At an operating temperature of 293 K, the QC laser has a total tuning range of about 0.2 cm−1 from 2086.41 cm−1 to 2086.21 cm−1 , which is achievable by adjusting the operating current from 900 mA to 2.0 A (Fig. 3(b)). The wavenumber of 2086.32 cm−1 of our laser can be achieved at an operating current of 1480 mA. In Fig. 4(a) the P branch of the fundamental rho-vibrational band is displayed (HITRAN 2003) [26]. The best spectral location in terms of CO absorbance is P(7) in this region, at 2115.63 cm−1 . However, the QC laser could not be operated at this wavelength, so we selected a shorter operating wavelength that would still permit sensitive detection of CO, as marked in Fig. 4. This line is also free of interference from other species (Fig. 4(b)), which make our laser an excellent light source suitable for high resolution spectroscopic applications. Comparing the measured HWHM of carbon monoxide absorption lines with the HITRAN database, we found the former was larger than the values given by the later. From comparison of these data the laser linewidth was found to be less than 100 MHz. The possible reasons of errors are the poor fineness of the germanium etalon and minor nonlinearities in frequency versus current, which affect the frequency calibration of experimental spectra [29]. The corresponding absorption cross-sections of 1% CO in air were calculated on the basis of a molecular spectroscopic database from HITRAN 2003. The absorption coefficient for this line is 45.12 cm−1 . Before proceeding with absorption studies, we investigated the electrical and the optical pulse waveforms for our system. The pulse generator was set to produce pulses of 2 s widths at a repetition rate of 5 kHz. The optical pulse was measured with the fast, highbandwidth HgCdTe detector (1 MHz) and a preamplifier (80 MHz).
Fig. 3. (a) QC laser spectrum versus temperature curves, the insert is the lasing mode temperature tuning, and (b) the lasing mode current tuning.
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L. Li et al. / Sensors and Actuators B 142 (2009) 33–38
Fig. 4. (a) CO absorption lines accessible by a 4.79 m QC laser, and (b) CO and H2 O adsorption lines.
Fig. 5. (a) Temporal response of the current pulse used to drive the QC laser, and (b) temporal output from the QC laser measured using the HgCdTe detector.
The current and the optical waveforms of our system are shown in Fig. 5(a) and (b), respectively. Both the rise and fall times of these two waveforms are less than 50 ns, and no obvious different can be observed between the pulse widths. These results indicate an excellent agreement in the signal and optical pulses. The example absorption spectrum shown in Fig. 6 was acquired for 200 ppm CO and is an average of 100 scans. The peak absorption is 0.088, and the signal-to-noise ratio (SNR) is about 120, which implies a minimum detectable absorption of 7.3 × 10−4 and concentration of 1.7 ppm. It is important to note that these results are typical detectivity based on our system, which can further improved by noise-reduction techniques [27].
A time response trace of the QC laser-based detection system output for 50 ppm and 200 ppm CO is shown in Fig. 7. The magnitude of change for 50 ppm CO is about 48 V, and the change in signal for 200 ppm CO is about 200 V. The response time is observed to be about 7 s, this value includes the response delay time and disposal time of the circuits of the whole instrument mostly, such as 100 scans acquisition and calculating times, related data reading and displaying times, etc. Fig. 8 shows the sensor signal as a function of CO concentration reduced from 200 ppm to 2 ppm in steps [28]. These measurements were performed after accurate purging of the cell to prevent interference from adsorbed–desorbed molecules. The solid line is the linear fit of the background-divided data and shows a strong linear relation-
Fig. 6. Spectrum of the P(14) line of 200 ppm CO.
Fig. 7. Time response of the QC laser detection system to 50 ppm and 200 ppm CO.
L. Li et al. / Sensors and Actuators B 142 (2009) 33–38
Fig. 8. QC laser signal versus CO concentration.
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Biographies Lei Li received his MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2008. He entered the PhD course in 2008, majored in circuits and system. Feng Cao received his MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2006. He entered the PhD course in 2006, majored in microelectronics and solid-state electronics. Yiding Wang received her MS degree in major of physics in 1991 from Jilin University. He was appointed a full professor in College of Electronics Science and Engineering, Jilin University in 2001. Now, he is interested in the field of optical sensors. Menglong Cong received her MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2008. She entered the PhD course in 2008, majored in circuits and system.
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Li Li received her MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2006. She entered the PhD course in 2006, majored in microelectronics and solid-state electronics. Yupeng An received his MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2005. He entered the PhD course in 2005, majored in microelectronics and solid state electronics. Zhenyu Song received his MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2005. He entered the PhD course in 2005, majored in microelectronics and solid state electronics.
Shuxu Guo received her BS degree from the College of Electronics Science and Engineering, Jilin University, China in 1987. He was appointed a full professor in College of Electronics Science and Engineering, Jilin University in 2001. Now, he is interested in the field of digital image processing and wavelet technology. Fengqi Liu received his PhD degree in the field of physics in 2004 from Nanjing University. Now, he is interested in the field of QC laser, nanomaterials and nanodevices. Lijun Wang received his PhD degree in the field of microelectronics in 2004 from West Virginia University. Now, he is interested in the field of optical sensors.