Infrared Physics & Technology 55 (2012) 353–356

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Infrared Physics & Technology

journal homepage: www.elsevier .com/locate / infrared

Experiment research on ellipsoidal structure methane using the absorptioncharacteristics of 3.31 lm mid-infrared spectroscopy

Zhang Yu a,b, Wang Fang-rong c,⇑, Zhao Yan-hui d, Wang Yi-ding a, Cui Tian b, Kan Ru-wen c, Wu Li-chun b,Zhang Tie-qiang b,⇑, Zhang Yuan-kun e

a State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, Chinab College of Physics, State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, Chinac College of Communication Engineering, Jilin University, Changchun 130022, Chinad Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802, USAe Faculty of Engineering, University of Wollongong, Wollongong, NSW 2522, Australia

a r t i c l e i n f o

Article history:Received 20 November 2011Available online 3 March 2012

Keywords:MethaneInfrared absorption spectraDifferential detectionTemperature measurement

1350-4495/$ - see front matter � 2012 Elsevier B.V. Adoi:10.1016/j.infrared.2012.02.006

⇑ Corresponding authors.E-mail addresses: wangfr@jlu.edu.cn (F.-r. Wang), zh

a b s t r a c t

The intensity distribution of absorption spectroscopy of methane mid-infrared fundamental absorptionbands, near-infrared combination band of v2 + 2v3 and overtone band of 2v3 were discussed in detailsin this paper. Quantitative data showed that the absorption intensities of fundamental bands are twicelarger than overtone bands, and three times larger than combination bands. Based on the methane3.31 lm (v3) fundamental absorption bands and differential signal disposal method, a rotational ellipsoi-dal light structure was designed using ordinary light source and detector to improve gas detection sen-sitivity. The experimental results of concentration detection showed that the precision of concentrationmeasurement can reach 3% and detection sensitivity is 50 ppm. Meanwhile, experiment was performedto investigate the influence of temperature on mid-infrared absorption performance of methane and theexperience curve of 3.31 lm (v3) fundamental absorption signal depending on temperature and its rate ofchange was drawn.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Methane is the major component of coal mine gas. Existingmethod for methane detection rely mostly on vector catalytic com-bustion, thermal conductivity, optical interferometry and infraredabsorption [1,2]. Among various available methods, infrared spec-troscopy absorption based methane sensors have their uniqueadvantages over the other techniques [3–5].

Infrared sensor for methane gas detection started in early 1970[5] and has been developing after that with increasing interestson it until now [6]. The detection ranges of current sophisticatedinfrared sensors are mainly targeting on the 1.3 lm and 1.66 lmnear-infrared bands [7–9], without sufficient coverage on themid-infrared bands featuring higher absorption intensity for certaincases. It was reported that the absorption intensity of methane atmid-infrared wavelength range is 160 times higher than that atthe near-infrared range, making mid-infrared an ideal candidatefor methane absorption detection [10]. However, the challengesfor effective utilization of mid-infrared bands are the weak radia-tion intensity of the light source and the insensitive response ofthe detector in the given range, therefore, haunting its application

ll rights reserved.

angtq@jlu.edu.cn (T.-q. Zhang).

in detection system. Recently, the emergence of the quantumcascade laser provides an alternate solution as mid-infrared lightsource, leading to a booming of all kinds of mid-infrared gas[11–13]. However, quantum cascade laser as light source isexpensive with complicated designs and structures, making it lessdesirable for the rising needs on portable, simple, and low-costmid-infrared gas sensors.

In this paper we have designed and characterized a new systemfor light collection to enhance mid-infrared gas absorption signals.The signal noise ratio of mid-infrared methane sensors has beenmuch improved by using inexpensive light source and commondetectors. In addition, we have analyzed the temperature charac-teristics of methane absorption, which can be applied on methanedetection in harsh conditions, like mines.

2. Principle

2.1. The characteristics of methane infrared absorption spectrum

Infrared methane gas sensors are characterized by the infraredabsorption spectroscopy of methane gas. Thus the intrinsic absorp-tion spectrum of methane should be obtained, with its infraredabsorption peak determined by engineering the structure ofdetection system.

354 Y. Zhang et al. / Infrared Physics & Technology 55 (2012) 353–356

Methane molecule has four inherent vibrations, which producefour fundamental frequencies in the mid-infrared band, respec-tively. The wave numbers of four fundamental frequencies are [14]

v1 = 2913.0 cm�1, v2 = 1533.3 cm�1, v3 = 3018.9 cm�1,v4 = 1305.9 cm�1

Each inherent vibration corresponds to a spectral absorptionwavelength. Therefore, their corresponding wavelengths can becalculated as

3.43 lm, 6.53 lm, 3.31 lm, 7.66 lm.

According to HITRAN atmospheric molecular spectroscopy data-base [15,16], the data of methane molecules absorption intensity inthe range from 1.0 lm to 3.8 lm are shown in Figs. 1 2. It can beobviously seen that methane molecules have strong absorptionbands in 3.31 lm and 7.66 lm, but have no absorption band near3.43 lm and 6.53 lm. The main reason for this is that those twocovalent bond vibrations of methane molecules do not responseto infrared, thus causing no absorption in the spectrum. Therefore,the infrared absorption spectroscopy cannot be used to detect theconcentration of methane gas at those two wavelengths.

Methane has many overtone and combination bands in the near-infrared [17], such as combination band of v2 + 2v3, which exhibits astrong absorption spectrum around 1.33 lm, and harmonic band of2v3, with its strong absorption spectrum around 1.66 lm [18].

In comparison, it was found that the absorption intensity around3.31 lm is 1300 times larger than the one around 1.3 lm, 160 timeslarger than the one around 1.66 lm, and 2.2 times larger than theone around 7.66 lm. Hence, the wavelength of 3.31 lm becomesour first choice as the central wavelength for methane detection.

Fig. 1. Absorption spectra of CH4 around 3.31 lm.

Fig. 2. Absorption spectra of CH4 around 1.66 lm and 1.33 lm.

2.2. Detection principle

In order to improve the intensity of the gas absorption signal, arotating ellipsoidal light structure was designed as shown in Fig. 3.The semi-major axis a and the semi-minor axis b satisfy e ¼ c

a < 1,

c ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2 � b2

p, and the focal point coordinates are (+c, 0) and (�c, 0).

In this design, the dual-wavelength differential absorption technol-ogy is employed. Since the signal detector A is on the focal pointand the reference detector B is not, the detector B can only receivethe radiation without reflection.

Considering the rotational symmetry of ellipsoid in U direction,the radiation flux in solid angle dx along the direction of h is

dU ¼ 2pI0 cos2 hdh ð1Þ

where I0 is the radiation intensity along the axial direction. Accord-ing to Beer–Lambert law, the relationship between emergent lightradiation flux dU0 and incident light radiation flux dU though ellip-soid can be expressed as

dU0 ¼ KdUe�aL ð2Þ

Here a is the absorption coefficient of gas; K is the reflectivity ofellipsoid; L is the light path through the absorbing gas.

When detector A receives incident radiation, the radiation fluxcan be expressed as

UA ¼ I0S

L20

e�aL0 þ 2pI0KZ h2

h1

e�aL cos2 hdh ð3Þ

where L0 is the distance between two focuses of ellipsoid and equalsto the distance between detectors and light source of ellipsoid; S isthe active area of the detectors. Considering different incident an-gles, the transmission distance L is a constant. Therefore,

UA ¼ I0S

L20

e�aL0 þ pI0KDhe�aL � 12pI0Kðsin 2h2 � sin 2h1Þe�aL ð4Þ

In Eq. (4), Dh = h2 � h1. For detector B, the receiving radiation flux is:

UB ¼ I0S

L20

e�aL0 ð5Þ

If the double wavelength differential detection structure isused, and two filters with different wavelengths are placed in frontof detectors A and B, where kA, the signal wavelength equals to3.31 lm and kB, the reference wavelength equals to 4.0 lm, thenthe output signal of the two detectors, R1 and R2, are the responseextent of detectors A and B, and a1 and a2 are gas absorption coef-ficients of two wavelengths.

x1x2

Ellipsoid reflector

Light source

Detecter A

Detecter B

Fig. 3. Schematic diagram of ellipsoid reflector, A, B are the same two detectors,detector A placed on the focus of the ellipsoid, detector B placed below the focus ofthe ellipsoid.

Fig. 4. Experiment of spectroscopy signal changing with concentration.

Fig. 5. Precision analysis curve.

Y. Zhang et al. / Infrared Physics & Technology 55 (2012) 353–356 355

VA¼R1I0S

L20

e�a1L0 þpI0R1KDhe�a1L�12pI0R1Kðsin2h2�sin2h1Þe�a1L

ð6Þ

VB ¼ R2I0S

L20

e�a2L0 ð7Þ

Dividing (6) by (7), and the differential signal can be obtained as

V 0AB ¼R1

R2eða2�a1ÞL0 þ pR1

R2

L20

SKDheða2�a1ÞL0 e�a1DL

� 12

R1

R2

L20

SpKðsin 2h2 � sin 2h1Þeða2�a1ÞL0 e�a1DL ð8Þ

However, the differential signal without ellipsoid is

VAB ¼R1

R2eða2�a1ÞL0 ð9Þ

Therefore, after adding ellipsoidal structure, the gain of the dif-ferential signal is

m ¼ V 0AB � VAB

VAB

¼ p L20

SKDhe�a1DL � 1

2L2

0

SpKðsin 2h2 � sin 2h1Þe�a1DL ð10Þ

where DL is DL = L � L0, representing the difference between re-flected optical path though ellipsoid and direct incident optical pathon the detectors.

According to the geometry of ellipsoid,

L0 ¼ 2c; L ¼ 2a; DL ¼ 2ða� cÞ ð11Þ

Apparently, after adding ellipsoid, the gain of differential detec-tion signal depends on the structure of the ellipsoid.

Considering the absorption coefficient a is very small, e�a1DL �1, then,

m � p L20

SKDh� 1

2L2

0

SpKðsin 2h2 � sin 2h1Þ

¼ L20

SpK Dh� sin 2h2 � sin 2h1

2

� �ð12Þ

3. Experiment of absorption spectroscopy characteristics andanalysis

Thermal blackbody radiation, thermopile detectors, differentialdetection structure with the double wavelength, and single lightpath in Fig. 3 are used. The central wavelength of two filters arek1 = 3.31 lm and k2 = 4.0 lm. In order to test the ellipsoid reflectorconfiguration, the concentration of methane was measured. Theellipsoid reflector is constructed using glass with a coated alumi-num film in the inner wall. The parameters of ellipsoid reflectorare a = 25.6 mm, c = 20.5 mm, x1 = 6.2 mm, and x2 = 3.6 mm.

3.1. Concentration measurement experiment

In the room condition (20 �C, 101.325 kPa), the spectrum underdifferent concentrations were tested. Fig. 4 shows the outputresponse of 22 differential signals with standard concentrations(VA/VB).

Fig. 4 shows that the differential signal decreases exponentiallywith the increasing methane concentration. The exponential indexfitting is used as follows,

VA

VB¼ 0:00428e�C=0:0569 þ 0:01371e�C=0:461

þ 0:020381e�C=14:8734 þ 0:95896 ð13Þ

The fitting curve is shown in Fig. 4. The fitting equation hasthree exponential parts, but the first part of Eq. (13) is much smal-ler than the other two parts. If the first part is ignored, Eq. (13) willbe similar to Eq. (8). The main reasons for this difference includeseveral factors in experiment such as the defocus of light sourceand detector, and the surface roughness of the inner wall of lightcollection system.

According to the calibrated fitting Eq. (13), the experiment wasrepeated to test the precision of the system and the experimentaldata are shown in Fig. 5. The measuring error is less than 3% andthe sensitivity of the system can reach 50 ppm.

3.2. Temperature effect experiment

Considering that the methane sensors are often deployed underharsh conditions and environments like in mines, here we investi-gate the temperature change affects the performance of the gas sen-sors. Figs. 6 and 7 plot the differential signal curve base ontemperature change under different concentrations of 0.3% and1.5%. Corresponding temperature changing rate curves are alsoprovided.

The noise characteristic of thermoelectric devices is propor-tional to T2. Therefore,

DT2n ¼

4kBT2

GTDf ð14Þ

The experimental curves of Figs. 6 and 7 confirmed thisassumption. The comparison between the signal curve with thechange of temperature and the corresponding temperature chang-ing rate curve proves that the changing speed of temperature has agreat effect on the change of signal. When the temperature is

Fig. 6. Signal changing with temperature curve (concentration is 0.3%).

Fig. 7. Signal changing with temperature curve (concentration is 1.5%).

356 Y. Zhang et al. / Infrared Physics & Technology 55 (2012) 353–356

changing slowly (in Figs. 6 and 7 is about 4 � 10�3 �C/s), the con-centration signal remains stable.

4. Conclusions

In this paper, a methane gas sensor device with double detec-tors of rotational ellipsoidal structure was designed. Concentrationdetection and temperature effect experiment were conducted forthe spectroscopy absorption characteristics of 3.31 lm mid-infra-red wavelength. The sensitivity of the system reached 50 ppmusing simple structure of dual-wavelength differential detection.The influence of temperature variations was investigated and itwas found that the temperature changing rate had a huge impacton the signal. When the temperature varies slowly (almost about4 � 10�3 �C/s), the concentration signal remained basically stable.The mid-infrared gas sensor with this structure can improve thesensibility of methane gas sensors.

Acknowledgements

The authors want to acknowledge the support provided by‘National High Technology Research and Development Programof China (863 Project)’ (Grant No. 2011AA050509), ‘NationalNatural Science Foundation of China’ (Grant No. 61106039),‘National Science Foundation for Post-doctoral Scientists of China’(Grant No. 20110490157) and ‘Jilin Science and TechnologyProject’ (Grant No. 201101025).

References

[1] R. Rella, P. Siciliano, L. Vasanelli, C. Gerardi, A. Licciulli, Physical properties ofosmium doped tin oxide thin films, J. Appl. Phys. 83 (1998) 2369.

[2] J. Zhang, Z. Wang, X. Li, Measurement technology of methane concentrationbased on harmonic detection and characteristic absorption of gases, J. Coal Sci.Eng. (China) 34 (2009) 24.

[3] S. Schafer, M. Mashni, J. Sherder, A. Miklos, P. Hess, H. Pitz, K.-U. Pleban, V.Ebert, Sensitive detection of methane with a 1.65 lm diode laser byphotoacoustic and absorption spectroscopy, Appl. Phys. B 66 (1998) 511.

[4] P. Werle, A review of recent advance in semiconductor laser based gasmonitors, Spectrochim. Acta A 54 (1998) 197.

[5] J. Zhao, C. Wang, T. Liu, Z. Wang, Y. Wei, Y. Li, Y. Shang, Q. Wang, Application inmethane extraction of fiber methane monitoring system based on spectralabsorption, Spectrosc. Spect. Anal. 30 (2010) 2857.

[6] H. Inaba, T. Kobayasi, M. Hiratna, M. Hamza, Optical-fibre network system forair-pollution monitoring over a wide area by optical absorption method, J.Electron. Lett. 15 (1979) 749.

[7] K. Chan, H. Ito, H. Inaba, Optical remote monitoring of CH4 gas using low-lossoptical fiber link and InGaAsP light-emitting diode in 1.33 lm region, Appl.Phys. Lett. 43 (1983) 634.

[8] H. Tai, K. Yatnatnoto, S. Osawa, Remote detection of methane using a 1.66lmdiode laser in combination with optical fibers, in: Proc. 7th OFS, vol. 8, 1990,pp. 51.

[9] T. Iseki, A portable remote methane detector using an InGaAsP DFB laser,Environ. Geol. 46 (2004) 1064.

[10] I. Armstrong, W. Johnstone, K. Duffin, M. Lengden, A.L. Chakraborty, K. Ruxton,Detection of CH4 in the mid-IR using difference frequency generation withtunable diode laser spectroscopy, J. Lightw. Technol. 28 (2010) 1435.

[11] L.W. Kornaszewski, N. Gayraud, J.M. Stone, W.N. MacPherson, A.K. George, J.C.Knight, D.P. Hand, D.T. Reid, Mid-infrared methane detection in a photonicbandgap fiber using a broadband optical parametric oscillator, Opt. Exp. 15(2007) 11219.

[12] F.K. Tittel, Y. Bakhirkin, A. Kosterev, G. Wysocki, Recent advances in trace gasdetection using quantum and interband cascade lasers, Rev. Laser Eng. 34(2006) 275.

[13] B.G. Lee, M.A. Belkin, R. Audet, J. MacArther, L. Diehl, C. Pflugl, F. Capasso, D.C.Oakley, D. Chapman, A. Napoleone, D. Bour, S. Corzine, G. Hofler, J. Faist,Widely tunable single-mode quantum cascade laser source for mid-infraredspectroscopy, Appl. Phys. Lett. 91 (2007) 231101.

[14] L.A. Kulakova, N.V. Zotova, S.A. Karandashov, B.A. Mxtveev, B.T. Melekh, N.M.Stus’, G.N. Talalakin, E.Z. Yakhkind, Si–Te acousto-optic modulator for fiberoptic gas sensor based on mid-wave InGaAsSb/InAsSbP diode laser, Proc. SPIE2895 (1996) 267.

[15] L.S. Rothman, I.E. Gordon, A. Barbe, D.C. Benner, P.F. Bernath, M. Birke, V.Boudon, et al., The HITRAN 2008 molecular spectroscopic database, J. Quant.Spectrosc. Radiat. Transfer 110 (2009) 533.

[16] The HITRAN Database. <http://www.hitran.com>.[17] W. Yan, W. Liu, D. Niu, The research of optical fiber gas sensor, Chin. J. Sens.

Actuat. 11 (1998) 22.[18] Y. Zhang, Y. Wang, L. Li, C. Zheng, Y. An, Z. Song, The principle and technical

analysis of methane detection using infrared absorption spectroscopy,Spectrosc. Spect. Anal. 28 (2008) 2515.