comb filter-based fiber-optic methane sensor system with mitigation of cross gas sensitivity

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 19, OCTOBER 1, 2012 3103

Comb Filter-Based Fiber-Optic Methane SensorSystem With Mitigation of Cross Gas Sensitivity

Duan Liu, Songnian Fu, Ming Tang, Senior Member, IEEE, Perry Shum, Senior Member, IEEE, and Deming Liu

Abstract—A remote fiber-optic methane gas sensor system isproposed and demonstrated with accurate gas concentration mea-surement and good mitigation of cross gas sensitivity. We use apolarization-maintaining photonic crystal fiber (PM-PCF)-basedSagnac loop filter to slice the spectrum of a broadband light sourceso as to precisely match several absorption lines of the methanegas within the near-infrared band. Meanwhile, a compact andcost-effective gas cell with multiple reflections is designed toenhance the interaction between the light beam and the methanegas to be detected, which also subsequently increase the systemsensitivity. Due to the insensitive temperature dependence ofthe PM-PCF-based comb filter, we can obtain gas concentrationmeasurement with a sensitivity of ppm. Moreover, byintentionally pumping the acetylene gas into the gas cell duringthe methane gas concentration measurement, the power variationcaused by the interfering gas with 100% concentration is onlyequals to 0.7% of the power variation induced by the 100%concentration methane gas. Thus, effective mitigation of cross gassensitivity is experimentally verified. The proposed fiber-opticmethane gas sensor system is verified with low cost, compact size,potential capability of multipoint detection, and high sensitivity.

Index Terms—Comb filter, cross gas sensitivity, fiber opticalsensor, methane detection, photonic crystal fiber.

I. INTRODUCTION

U NDERGROUND coal mine safety has been a majorsocial and economic issue during past few years. Pre-

caution measures in coal mine safety monitoring, pollutioncontrol, or industrial process control are becoming more andmore crucial [1]. The major causes of those serious accidentsinclude coal-bed gas outburst induced explosion, undergroundmine fire, roof collapsing, and underground water flooding.Generally, underground coal-bed gas is composed of harmful

Manuscript received March 25, 2012; revised June 28, 2012; acceptedJuly 25, 2012. Date of publication August 20, 2012; date of current versionSeptember 19, 2012. This work was jointly supported by the National BasicResearch Program of China (2010CB328302), National Natural Science Foun-dation of China (61007044 and 60937002), Fundamental Research Funds forthe Central Universities (HUST 2011TS008), Natural Science Foundation ofHubei Province (2009CDA129), and China Postdoctoral Science Foundation(20090451044 and 201104466).D. Liu, S. Fu, M. Tang, and D. Liu are with the National Engineering Labo-

ratory of Next Generation Internet Access Networks and the School of Opticaland Electronic Information, Huazhong University of Science and Technology,Wuhan 430074, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]).P. Shum is with the Wuhan National Laboratory of Optoelectronics, and

School of Optical and Electronic Information, Huazhong University of Scienceand Technology, Wuhan 430074, China, and also with the School of Electricaland Electronic Engineering, Nanyang Technological University, 637798Singapore (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JLT.2012.2211073

gases, particular methane gas and other gases such as heavy hy-drocarbons, hydrogen, carbon dioxide, and nitrogen. Methanegas is not toxic. However, it is extremely flammable and mayform explosive mixtures with air. It is verified that, when themethane gas is exposed into the atmosphere, the lower explo-sive limit of methane gas concentration is 4.4–5% and the upperexplosive limit is 15–17%. Thus, for the purpose of risk alarm,the detection resolution of the methane gas for undergroundcoal mine is normally around %. High sensitivity andremote detection is extremely important for underground coalmines outburst prediction.With the development of fiber-optic communication tech-

nique, fiber-optic gas sensor becomes a promising alternativedue to the unique advantages such as the inherent immunityto electromagnetic interference, the capability of fast, in situ,remote detection, safe in dangerous or hazardous environments,low cost, and compact size [2]. So far, many fiber-optic gassensor systems have been proposed and demonstrated [3]–[7].Among those fiber-optic gas sensors, the infrared (IR) absorp-tion spectroscopy is the most promising one [3], [4], as it hasmany advantages including high sensitivity, gas selectivity, fastresponse time, robust to the environment disturbance, simplestructure, and easily to form a sensor network by time-divisionmultiplexing [5], wavelength-division multiplexing [6], or spa-tial division multiplexing scheme with an electrical controlled1 optical switch [7], etc. Fiber-optic evanescent wavespectroscopy based on the attenuated total-internal-reflectioneffect has also become a popular choice in IR absorbancespectroscopy schemes for the environment monitoring [8].Though they can achieve fast measurement response, theysuffered from low system sensitivity and poor mechanicalstability. Fiber Bragg gratings (FBGs) were also introduced forfiber-optic gas sensing applications [9]. However, packagingof those FBGs into the gas cell was quite challenging and itstill suffered from mechanical vulnerability. Recently, photoniccrystal fibers (PCFs) were proposed as the gas cells as they canguide light over a long distance within either the cladding airholes [10] or the central air holes [11]–[14]. Moreover, FBGsfabricated in microstructured optical fibers were also reportedfor gas sensing applications [15]. However, most of thoseexperiments are still in the laboratory stage and are far fromreal implementation. On the other hand, with different lengthsof optical path, sensor heads, and data-processing procedures,various optical gas sensors were reported with consequentlydifferent methane detection levels in [16]–[19]. Rather highdetection levels were achieved in [16]—100 ppb with the helpof White multipass cell (base length 2 m and optical path length80 m), in [17]—0.3 ppm in laboratory system with low methane

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3104 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 19, OCTOBER 1, 2012

pressure, in [18]—2 ppm in a 10 m cell, and in [19]—55 ppbin laboratory system. It is worth noting that, most fiber-opticmethane sensors based on broadband light source (BLS) ab-sorption spectroscopy are basically nonselective and they mayproduce false alarms under a domestic environment. In orderto mitigate cross gas sensitivity, the laser diode (LD)-basedabsorption spectroscopy are commonly used for methanegas concentration measurement. However, environmentalspectral fluctuations demand the implementation of activewavelength-locking schemes in order to ensure the stabilitynecessary and to provide proper gas selectivity. Moreover,those LD-based fiber-optic gas sensors have disadvantages inthat they are gas specific, with limited tuning capabilities, andtoo expensive when using LDs with a typical spectral widthless than 1 MHz. Although BLS-based methane gas detectionsystem is theoretically found to be approximately 40 timesweaker than those using a narrow-linewidth LD anchoredon the absorption line at 1665 nm, the effective absorptioncoefficient of BLS-based fiber-optic methane sensors can beimproved with much lower cost and easy implementationthrough spectrum splicing technique [20].In this paper, we propose and experimentally demonstrate a

fiber-optic methane gas sensor system with an enhanced sensi-tivity based on a matched comb filter and novel gas cell design.The comb filter, consisted of a section of polarization main-taining (PM)-PCF, is designed to match a few absorption linesof methane gas within the 1650 nm band. Moreover, a novel gascell with many good merits such as improved reliability, easymaintenance, high efficiency, and compact size is realized. Dueto the temperature insensitiveness of the PM-PCF-based combfilter, we can experimentally achieve precise methane concen-tration measurement with the proposed fiber-optic gas sensor.After performing the methane gas concentration measurement,it is found that both the stability and cross gas sensitivity arewell resolved. The rest of the paper is organized as follows.Section II describes the design and implementation of the PCF-based comb filter capable of matching several absorption linesin the 1650 nm band. Section III describes the design of a com-pact and reliable gas cell. The fiber-optic gas sensor system withenhanced sensitivity and mitigation of interfering g gas is ex-perimentally demonstrated in Section IV. The conclusions aresummarized in Section V.

II. MATCHED COMB FILTER

Most gasses to be monitored have absorption lines within thelow-loss transmission window of silica fiber [21]. Fiber-opticgas sensors for the air pollution, noxious, explosive gas moni-toring are mostly based on monitoring the optical power attenu-ation induced by the gas molecule’s absorption. When the inputlight passes through the gas under test, a portion of light is ab-sorbed by the gas and another portion is scattered by the gas.And this can be described by the Beer–Lambert Law:

(1)

where is the output optical power when the methane gas con-centration is zero, is the optical power of the light passingthrough the gas, is the gas concentration, is the length of

the gas cell, and is the absorption coefficient of the gas atfrequency ( : ranging from 0.035 to 0.3 cm in the 1650 nmband). The absorption coefficient can be rewritten as [22]

(2)

(3)

(4)

where is the molecule absorption line intensity, isthe normalized linear function, is the total particle numberper unit pressure per unit volume, is the wave number of thecentral absorption frequency, is the collision spread coeffi-cient, is the pressure spread coefficient, is the air pressureof the gas cell when the methane gas concentration is zero, isthe environmental temperature, and is the temperature coeffi-cient (here n is approximately 0.5). At the standard atmosphereand room temperature condition, we have(mol cm ).

(5)

From (1)–(5), in case the gas cell length and the gas absorp-tion coefficient are determined, the gas concentration can beestimated by measuring the ratio between and . When wechoose a BLS to cover the whole absorption lines, the gas con-centration can be well determined. However, cross gas sensi-tivity is an issue for different types of methane gas sensors, be-cause there exist lots of other gas absorption lines within theoperation wavelength range of the BLS. Thus, we need to slicethe spectrum of the BLS according to a specific gas to be de-tected. Therefore, it is desirable to have a light source with apredefined selectivity within the operation spectrum to matchthe gas absorption lines. Second, current research for fiber-opticgas sensors concentrated at the near-IR band because of theavailability of suitable semiconductor devices operating in thiswavelength region. The most commonly used BLS in currentmethane gas detection systems has a typical spectral width of20–100 nm. However, the absorption line of methane gas witha 3 dB linewidth of pm, is much less than this, which resultsin a very small optical power variation even for relative largegas concentrations. Some fiber-optic methane sensors have beendemonstrated using a BLS and a filter with its transmissionpeaks matched to the absorption lines of the methane gas ateither 1.33 m with an FBG [23], or 1.64 m [2], [24] witha sinusoidal signal modulated Fabry–Pérot cavity as a combfilter. But the traditional filter suffered a lot from the environ-mental changes, especially the temperature variations. Here, wepropose a temperature-insensitive comb filter using a PM-PCFbased Sagnac loop filter to slice the BLS’s optical spectrum, andsubsequently match multiple absorption lines of the methanegas. As a result, the dynamic range of the power variation forthe proposed fiber-optic gas sensor is substantially enhanced.A polarization-maintaining fiber (PMF)-based Sagnac loop

interferometer has some favorable characteristics such as lowinsertion loss, polarization independence to the input light, anduseful broad spectral bandwidth [25]. These unique character-istics make it a promising wavelength-selective comb filter for

LIU et al.: COMB FILTER-BASED FIBER-OPTIC METHANE SENSOR SYSTEM 3105

sensing applications [26]. Nevertheless, the PMF is influencedby the environmental changes such as temperature, which re-sults in a slow drift of the peak transmission wavelengths. Re-cently, PCF-based Sagnac filters were found to be extremely lowsensitive to temperature variations, compared with those tradi-tional PMF based Sagnac filters. For a Sagnac loop filter with1 m PMF, one can expect a full period shift with a temperaturedrift of only C; while for a PM-PCF-based Sagnac loopfilter, over the full temperature range (0–200 C), the maximumpeak shifts was less than 10% of the filter period [27]. Sincethe methane gas absorption lines have almost uniform wave-length spacing and are particularly insensitive to environmentalperturbation [28], we can design a comb filter with a free spec-tral range equal to the wavelength spacing of the methane gasabsorption lines, and match the transmission peaks to as manyabsorption lines as possible. The proposed Sagnac loop interfer-ometer is made up of a single-mode 50:50 fiber coupler, oper-ating in 1650 nm band, and a segment of PM-PCF. The transfermatrix of the comb filter can be calculated as

(6)

where is the optical power transmission coefficient, is thelength of PM-PCF, is the center wavelength of the light source,and is the birefringence of the PM-PCF. The wavelengthspacing of the peaks , is inversely proportional to the length, and birefringence of PMF :

(7)

A typical methane gas absorption spectrum is measuredusing a superluminescent diode (SLED) BLS (Denselight,DL-CS65M5A) by an optical spectrum analyzer with a resolu-tion of 0.06 nm, as shown in Fig. 1. The wavelength spacingamong multiple methane gas absorption lines is nm.Based on (7), we can calculate the length of the PM-PCFneeded to match several absorption lines. One can see that byproperly choosing the PMF length and birefringence, a fiberSagnac loop filter with multiple transmission peaks exactlymatching the methane absorption lines around 1650 nm bandis possible. In our design, we choose two specific absorptionlines (1645.2 and 1642.6 nm) as the best matched wavelengths.The PM-PCF from Blaze photonics (PM-1550-01) used herehas a beat length of 4 mm@1550 nm, and a loss of less than 1.5dB/km. Thus, 2 m long PM-PCF is enough to match such twoabsorption lines very well. The insertion loss of the proposedcomb filter is around 4 dB. Fig. 2 shows that the calculatedtransmission spectrum of the comb filter matches very wellwith most of the absorption lines within the 1635–1665 nmwavelength range. Hence, the comb filter’s low sensitivityto the temperature variation makes it vigorous to the envi-ronmental changes. Furthermore, the more absorption linesare matched by the comb filter, the more sensitive the sensorwill be due to the removal of background light. Consideringthe uniform wavelength spacing of the methane absorptionlines at the near-IR band, we can only match most absorptionlines within this wavelength region but sacrifice the maximumabsorption wavelength at 1665 nm in our design. Note that

Fig. 1. Measured absorption lines of the methane gas characterized by a 1650nm SLED.

Fig. 2. Calculated Sagnac loop filter spectrum and the corresponding methanegas absorption lines.

our configuration of comb filter can also be applied to monitorother gases after varying the length of PM-PCF according tothe wavelength spacing of a specific gas absorption lines.

III. GAS CELL IMPLEMENTATION

From (1), we can see that the variation of the detected op-tical power after gas absorption is proportional to the length ofthe gas cell and the gas concentration. In order to enhance theminimal detectable gas concentration, the most straightforwardway is to use a gas cell with a long interactive cavity length.However, there is a tradeoff between the real implementationand the measurement accuracy for the gas sensor system. Analternative solution is to guide the light beam traveling multipletimes within one single compact gas cell cavity. For the gas cell,basically, there are two types: transmission gas cells and reflec-tion gas cells. In order to achieve gas concentration measure-ment with high sensitivity, the transmission gas cell are alwayscascaded for longer interaction length, as the received opticalpower variation are much larger for a longer gas cell. However,this will make the gas cell complex, bulky, and only suitablefor the laboratory applications. Moreover, in order to make thissensor system practical, the gas cell must be dust-proof, mois-ture-proof, and indestructible. Thus, the out layer of the pro-posed gas cell was made of stainless steel. Around the cornerof the gas cell, dust-proof, and moisture-proof material waswrapped to make the gas cell air proof. Inside the cell, we used a


Fig. 3. Schematic diagram and prototype of the multiple reflection cavity de-sign of the gas cell.

pair of gradient-index lens optical fiber collimators and a pair offlat-bottomed glasses coated with high reflection film on the sideface (the fiber collimators are set at an angle of 3 to the tiltedside of the glasses) to form the sensor head. The light from thecomb filter’s output port was guided by the two fiber collimatorsand reflected a number of times (in our case, 15 times) betweenthe two glasses. The gas under test interacted with the input lightin between the two glass blocks. And the length between thosetwo glass blocks is 2 cm long. The two fiber collimators werefixed on the V-grooves in the tilted slop of the glasses, as shownin Fig. 3. The effective optical length of the sensor head is now30 cm after 15 times reflections within the gas cell, and the in-sertion loss of such gas cell is dB. Our gas cell designhas many advantages such as low insertion loss, compact size,and easy maintenance. Since temperature stability of gas cellis important to minimize measurement error and reduce falsealarms, here we theoretically evaluate the thermal stability ofthe proposed gas cell. The glass block used here has a thermalexpansion coefficient of K , while the unit metalmaterial has a thermal expansion coefficient ofK . When the environmental temperature changes 1 C, thethermally induced gas cell expansion is m. Based on (5),the concentration measurement error due to thermally inducedlight-path change is calculated to be approximately 1.67% for atemperature variation from 10 C to 45 C.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

The schematic diagram of the experimental setup is shownin Fig. 4. The light from another SLED (Photonik BLS-1590/SLED/200) was first sliced by a PM-PCF-based comb filter,and then split into two parts by a 3 dB fiber coupler, referred asthe signal light and the reference light, respectively. The signallight was subsequently introduced into the gas cell to interactwith the methane gas and then complete the optic-to-electricalconversion by a p-i-n photodetector (PD), while the referencelight was directly sent to another PD for the purpose of differ-ential detection. As a result, the power fluctuation of the SLEDis solved by differential detection technique. The two PDs usedhere were HTOE’s FC photodiode (HITECH PIN7-10-12) witha response wavelength range of 0.5–1.7 m and a responsibilityof 0.8 A/W @1550 nm. Since the optical spectrum of the combfilter has matched at least six absorption lines of methane gasin our scheme, the signal light was effectively absorbed by the

Fig. 4. Schematic diagram of the proposed methane gas sensor system. APC:automatic power control, TEC: thermoelectric cooler.

Fig. 5. Prototype of the proposed fiber-optic methane gas sensor system.

gas under test. Hence, at the receiver end, the optical intensi-ties of the two light beams were different. Then the electric sig-nals were converted into digital signal after two analog-to-dig-ital converter modules, and processed by a digital signal pro-cessing module to calculate and display the concentration ofthe methane gas. As shown in Fig. 5, the proposed methane gassensor system mainly has three parts: the host machine with allsignal processing functions, the gas cell, and the alarm device.Optical fiber cable was used to link the host machine and the re-mote gas cell. The length of the fiber cable was scalable, whilethe host machine was put in the supervision room for the pur-posed of remote monitoring.The proposed sensor system has been applied to measure dif-

ferent concentrations of methane gas, contained in several gasbottles. The SLED’s bias current was fixed at 300 mA, and itsoutput power was mW, and 3 km single-mode fiber cablewas used to communicate between the supervision room and thesensor head. An alarm device was also equipped when the con-centration ofmethane reaches the dangerous explosive level. Allthe equipment of the proposed system is located at the supervi-sion room except for the sensor head within a gas cell, so that itcan keep the electronic device far away from the coal mine. Weuse our fiber-optic methane sensor to measure the gas concen-tration and do a comparison with that of commercial IR sensor(Dynament TDS0034) for the purpose of calibration. The IRmethane gas sensor has a measurement sensitivity of 100 ppmfor a methane gas concentration of 0–10%. Fig. 6 shows thecomparison of the standard gas concentration and measured re-sults. It can be clearly observed that the measured results agree


Fig. 6. Measurement results of the proposed methane gas sensor system underdifferent methane gas concentration.

well with the standard results calibrated by the IR sensor. As thetwo PDs both have a minimal voltage value of 1 mV due to thevariation of the output optical power, the proposed fiber-opticmethane sensor system has a sensitivity of ppm. Mean-while, the ratio between the signal and reference light inducedvoltage has a standard deviation of ; thus, this willcause a gas concentration measurement error of %. Then,we characterize the advantage owing to the matched comb filter.Without the comb filter, when the concentration of methane gasis 100%, only 0.21 V dynamic changes of output voltage canbe obtained. After using the designed comb filter, the outputpower varies 0.33 V, when measuring the same concentrationof methane gas. Thus, we can easily achieve 57% sensitivityimprovement due to the removal of background light by usinga matched comb filter. In particular, such sensitivity improve-ment keeps stable, with a temperature range of 10 C to 60 C,as a result of the insensitive temperature dependence of combfilter.In order to further verify our effective solution of cross gas

sensitivity from the use of a BLS, we intentionally pump theacetylene gas into the gas cell during the concentration mea-surement of the methane gas. Since there are also some absorp-tion lines of acetylene gas within such SLED spectrum, whichhas a typical 3 dB bandwidth of nm. Then, for the pur-pose of laboratory safety, we can treat the acetylene gas as apotential interfering gas for methane gas detection, and henceinvestigate the effectiveness of our proposed gas sensor in termof cross gas sensitivity. Finally, nitrogen gas is used as the emp-tying gas to pump the gas to be detected out of the gas cell, asit is nontoxic and has no evident absorption lines within suchSLED’s spectrum. Since the pressure of nitrogen gas from thegas bottle is much higher than that in the gas cell, the instan-taneous large absorption peaks clearly observed in the border-lines of the five segments in Fig. 7, correspond to the large ab-sorption induced by beam wander within the turbulent gas. Itis found that the absorption coefficient changes due to theinhomogeneity of the mixed gas in the gas cell along with thepressure-induced fast variation of the gas concentration [29].Thus, this instantaneous large absorption peaks can be treated as

Fig. 7. Cross gas selectivity results of the proposed methane gas sensor system.

the corresponding pumping or emptying moments. The wholemonitoring curve can be divided into five segments by thosefour peaks. The segment I shows the starting point of the mea-surement, when the SLED is connected after the system cali-bration. The output voltage is quite stable in the first segment.And the segment II of the curve shows the introducing of theinterfering gas, the acetylene gas, while at this segment 100%concentration of acetylene gas is inflated into the gas cell. Wecan observe that no obvious power variation occurs. And thesegment III shows the process of emptying the acetylene gasby pumping the nitrogen gas into the gas cell. The segment IVshows the process of pumping the 100% concentration of themethane gas into the gas cell, when we can observe a strongabsorption line in Fig. 7. Around 0.33 V power variation canbe observed due to the absorption of methane gas. The seg-ment V shows the emptying process of the methane gas bypumping the nitrogen gas again, and ends the measurement.For the purpose of evaluation, we can use the peak-to-peakpower variation as the criterion of the gas selectivity evalua-tion of the gas sensor. Then, the acetylene gas has a powervariation of only 0.1%, while the measured methane-gas-in-duced power variation is 14%. Therefore, the 100% concentra-tion acetylene-gas-induced signal variation equals to only 0.7%of the 100% concentration methane-gas-induced signal varia-tion. We can conclude that the cross gas sensitivity is well re-solved. The experimental results show the proposed gas sensor’seffectiveness in the mitigation of the cross gas sensitivity duringmethane gas concentration measurement. However, the inter-fering gas’ influence is not totally removed, which is probablybecause, first, additional optical filter is helpful to further at-tenuate the optical power beyond the absorption band and onlytake in the optical spectrum within the methane gas’ absorptionpeaks. Second, to thoroughly eliminate the acetylene gas’ influ-ence, initially, we need to use an extra LD with a lasing wave-length anchored at the acetylene gas’ absorption lines to monitorthe acetylene gas’ concentration of the mixed gas in the gas cell.Then, the system source is switched to the BLS for the methanegas concentration measurement. As a result, the cross gas sen-sitivity can be well resolved in the electrical signal processingunit of the sensor system. This will increase the system com-plexity and cost; thus it is not considered.


V. CONCLUSION

We have proposed and experimentally demonstrated a fiber-optic methane gas sensor system with an enhanced sensitivityby using a matched comb filter and novel gas cell design. Acomb filter, consisted of a section of PM-PCF, is designed tomatch a number of absorption lines of methane gas within thenear-IR band, in case a low-cost SLED is used as system lightsource. As a result, success removal of background light andcross gas sensitivity can be experimentally achieved. Mean-while, the gas cell in the proposed gas sensor adopts a novelmultiple reflection cavity structure, which is superior in someaspects such as improved reliability, easy maintenance, high ef-ficiency, and compact in size. After measurement, the proposedmethane gas sensor has shown precise methane concentrationmeasurement with an accuracy of ppm, due to the tem-perature insensitivity of the PM-PCF-based comb filter. Afterintentionally pumping the acetylene gas into the gas cell duringthe gas concentration measurement, the acetylene-gas-inducedsignal variation only equals to 0.7% of the methane-gas-inducedsignal variation. Thus, the cross gas sensitivity due to the use ofBLS has been well solved. The proposed methane gas sensorsystem with a capability of low cost, compact size, potentialcapability of multipoint detection, and high sensitivity has suc-cessfully installed and operated under the coal mine condition.

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Duan Liu was born in Wuhan, Hubei, China, in 1978. He received the B.Eng.degree from the Huazhong University of Science and Technology (HUST),Wuhan, China, in 2001, and the Ph.D. degree from Nanyang TechnologicalUniversity, Singapore, in 2007.He then joined the Wuhan National Laboratory for Optoelectronics and

the School of Optoelectronics Science and Engineering, HUST, in 2009. Hisresearch interests include fiber lasers, fiber Bragg gratings, and fiber-opticsensors.

Songnian Fu received the B.Sc. and M.Sc. degrees from Xiamen University,Xiamen, China, in 1998 and 2001, respectively, and the Ph.D. degree from Bei-jing Jiaotong University, Beijing, China, in 2004.From 2005 to 2010, he was at the Network Technology Research Center,

Nanyang Technological University, Singapore, as a Research Fellow. In Feb-ruary 2011, he jointed the Wuhan National Laboratory for Optoelectronics andthe School of Optoelectronics Science and Engineering, Huazhong Universityof Science and Technology, Wuhan, China, as Professor. His current researchinterests include all-optical signal processing, microwave photonic, and fiberlaser.


Ming Tang (SM’11) received the B.Eng. degree from the Huazhong Universityof Science and Technology (HUST), Wuhan, China, in 2001, and the Ph.D. de-gree from Nanyang Technological University, Singapore, in 2005.His postdoctoral research at the Network Technology Research Centre was

focused on the optical fiber amplifiers, high-power fiber lasers, nonlinear fiberoptics, and all-optical signal processing. In February 2009, he joined the Tera-photonics group led by Prof. H. Ito in RIKEN, Wako, Japan, as a Research Sci-entist conducting research on terahertz-wave generation, detection, and appli-cation using nonlinear optical technologies. Since March 2011, he has been aProfessor in the School of Optoelectronics Science and Engineering, WuhanNational Laboratory for Optoelectronics, HUST. He has published more than70 technical papers in the international recognized journals and conferences.Dr. Tang serves as the Regular Reviewer for the IEEE JOURNAL OF

QUANTUM ELECTRONICS, the JOURNAL OF LIGHTWAVE TECHNOLOGY, theIEEE PHOTONICS TECHNOLOGY LETTERS, and Optical Society of Americapublications. He has been a member of the Lasers and Electro-Optics Society(now IEEE Photonics Society) since 2001.

Perry Shum (SM’05) received the B.Eng. and Ph.D. degrees in electronic andelectrical engineering from the University of Birmingham, Birmingham, U.K.,in 1991 and 1995, respectively.He was an Honorary Postdoctoral Research Fellow in the University of Birm-

ingham. In 1996, he was involved in research of semiconductor laser and high-speed optical laser communication in the Department of Electrical and Elec-tronic Engineering, Hong Kong University, as a Visiting Research Fellow. InJuly 1997, he joined the Department of Electronic Engineering, OptoelectronicsResearch Centre, City University of Hong Kong, Hong Kong. In 1999, he joinedthe Network Technology Research Center, Nanyang Technology University,Singapore, and served successively as Assistant Professor, associate professor,Professor and the Director of the Research Center of Network Technology. In2010, he joined theWuhan National Laboratory for Optoelectronics, and Schoolof Optoelectronics Science and Engineering, Huazhong University of Scienceand Technology,Wuhan, China, as an Adjunct Professor. He has publishedmorethan 400 international journal and conference papers. His research interests in-clude optical communications, nonlinear waveguide modeling, and fiber-basedtechnology.Dr. Shum received the Singapore National Academy of Science Young Sci-

entist Award in 2002 for his contributions on next-generation optical communi-cation technology.

Deming Liu was born in Hubei Province, China. He received the Master’sdegree from the Chengdu Institute of Telecommunication (now University ofElectronic Science and Technology of China), Chengdu, China, in 1984, andthe Ph.D. degree from the Huazhong University of Science and Technology(HUST), Wuhan, China, in 1999.He is currently a Professor at HUST. His recent research interests include

optical access network, optical communication devices, and fiber-optic sensors.

comb filter-based fiber-optic methane sensor system with mitigation of cross gas sensitivity

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