Development of an intra-cavity gas detection system based on L-band erbium-doped fiber ring laser

Sensors and Actuators B 193 (2014) 356–362

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General

Development of an intra-cavity gas detection system based on L-band erbium-doped fiber ring laser Lin Yu a,b , Tiegen Liu a,b , Kun Liu a,b,∗ , Junfeng Jiang a,b , Lei Zhang c , Yunwei Jia a,b , Tao Wang a,b a

College of Precision Instrument & Opto-electronics Engineering, Tianjin University, Tianjin 300072, China Key Laboratory of Opto-electronics Information and Technical Science, Tianjin University, Ministry of Education, Tianjin 300072, China c College of Computer Science and Technology, Tianjin University, Tianjin 300072, China b

a r t i c l e

i n f o

Article history: Received 20 August 2013 Received in revised form 21 November 2013 Accepted 23 November 2013 Available online 11 December 2013 Keywords: L-band Fiber laser Intra-cavity absorption Gas detection Temperature compensation

a b s t r a c t In this study, a gas detection system based on intra-cavity absorption spectroscopy technique and L-band erbium-doped fiber ring laser is constructed. With a fiber Fabry–Perot tunable filter used for spectral scanning, the system can acquire multiple absorption lines of the detected gas. In order to improve the detection accuracy, reference wavelengths are introduced to calibrate the nonlinearity of the filter, and polynomial fitting is proposed to compensate the impact of temperature variation on gas absorption. When the system is applied to detect the concentration of CO2 , the absolute error is less than 400 ppm, corresponding to a relative error under 0.52%. The monitoring error of absorption wavelength is no more than 30 pm. The system also has the ability to detect other gas species which have absorption lines in L-band. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction Near-infrared absorption spectroscopy techniques used for gas detection have attracted much attention in recent years due to the advantages brought by fiber optic systems, such as the availability of low cost components, the capability of multiplexing and safety in hazardous conditions [1,2]. Many kinds of pollutant or inflammable gases, including C2 H2 , NH3 , CO, CO2 and H2 S, have absorption lines in this region [3]. However, the optical absorption strengths of the near-infrared overtone absorption lines are relatively weak [4], so the key problem is to improve the sensitivity for trace gas detection. With a gas cell inserted into the laser cavity, intra-cavity absorption spectroscopy technique (ICAST) can greatly increase the effective optical absorption length in the course of laser oscillation, thus a high sensitivity can be obtained [5,6]. Compared with other kinds of lasers such as dye laser, Nd:glass laser, Ti:sapphire laser and color center laser, rare-earth-doped fiber laser is more suitable for an economic and compact system [5,7]. ICAST based on erbium-doped fiber amplifier (EDFA) has attracted much attention in the past ten years. In 2001, Stewart et al. conducted the initial investigation of fiber loop cavity for intra-cavity gas detection

∗ Corresponding author. Tel.: +86 22 27409621. E-mail address: [email protected] (K. Liu).

[8]. In 2003, Zhang et al. demonstrated a multi-point, fiber-optic intra-cavity gas detection system [9], and proposed the sensitivity enhancement method that sets the laser work close to the threshold [10]. In 2008, Liu et al. applied wavelength sweep technique to ICAST, and realized gas detection at low concentrations [11]. In 2011, Liu et al. introduced both wavelength modulation technique and wavelength sweep technique into ICAST. The method improved the precision of concentration detection significantly and made gas-type recognition possible [12]. However, previous works were mainly limited to the C-band of erbium-doped fiber laser (EDFL) and the detected gas was C2 H2 or NH3 . Since gases such as CO, CO2 and H2 S all have absorption lines in L-band, it is essential to extend the operating wavelength range of EDFL into this gain band to make more types of gases detectable. Earlier reports on L-band EDFL for application to gas sensing were mainly focused on the multi-wavelength operation [13,14], but the optical components for lasing wavelength selection had to be replaced when different gases were detected. Single-wavelength EDFL with continuous tuning ability over a wide wavelength range may be an alternative. Fiber Fabry–Perot tunable filter is a common used device for frequency selection in EDFL [15]. The transmission wavelength of the filter can be tuned to match the absorption line of the detected gas [16]. Due to the piezoelectric nonlinearity [17], the wavelength–voltage relationship of the filter has to be real time calibrated to get precise absorption wavelengths. Thus wavelength

0925-4005/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.11.073

L. Yu et al. / Sensors and Actuators B 193 (2014) 356–362

references [18] should be introduced into the gas detection system to provide stable wavelengths in L-band for laser wavelength calibration. In this paper, we constructed a tunable L-band erbium-doped fiber ring laser to detect gas concentration. Multiple absorption lines of the detected gas were acquired with a fiber Fabry–Perot tunable filter. In order to calibrate the nonlinearity of the filter, a Fabry–Perot etalon (F–P etalon) and a fiber Bragg grating (FBG) were used cooperatively to provide reference wavelengths. The performance of the system was demonstrated by the detection of CO2 . Besides, the impact of temperature variation on gas absorption was investigated and the compensation method was proposed to avoid repeated concentration calibration at each temperature. 2. Principle of the system The behavior of the near-infrared spectral extinction caused by gas absorption can be described by the Lambert–Beer Law and the absorbance K can be written as K() = ln

 I ()  0 I()

= ˛()CL

(1)

where I0 is the intensity of incident light, I is the intensity of transmitted light after absorption, ˛ and C represent the absorption cross-section and number density of the detected gas molecule, respectively, L denotes the effective optical absorption length and  is the light frequency. The absorption cross-section can be further expressed as the product of the absorption line strength (S) and the normalized line shape function (g): ˛() = S(T, 0 )g(, 0 )

(2)

The absorption line strength S varies with its center absorption frequency 0 . For a specific absorption line, S is only the function of temperature T and the relationship between them can be inquired from the HITRAN molecular spectroscopic database [19]. Considering the influence of collision broadening and Doppler broadening, the practical absorption line shape of gas molecule is a Voigt function, which is the convolution of Lorentzian function (gL ) and Gaussian function (gD ):



+∞

gL ( , 0 )gD ( −  , 0 )d

g(, 0 ) =

vL 2

(v − v0 ) +

 2 gD (, v0 ) = vG vG = 7.16 × 10



vL 2



n 2 vL = 2air (296/T ) P



ln2  − v0 exp −4 ln2  vG



−7

v0

T M

C=

PCV PCV NA = RT kT

(6)

where CV is the volume fraction of the detected gas, R is the ideal gas constant, NA is the Avogadro constant and k is the Boltzmann’s constant. For a specific absorption line, the absorbance is proportional to the volume fraction CV and this characteristic can be used to calibrate gas concentration. In order to get a higher sensitivity, the peak absorbance at center absorption frequency is usually chosen for calibration. When Lorentzian function is used to fit absorption line shape, the peak absorbance can be derived from Eqs. (1), (2), (4) and (6) as K(0 ) =

LCV T n−1 S(T, 0 ) kair 296n

(7)

According to Eq. (7), the peak absorbance is pressureindependent but temperature-dependent in the condition of Lorentzian fitting. The relationship between the peak absorbance and temperature can be simulated theoretically. In the following example, three absorption lines of CO2 with center wavelengths 1571.406 nm, 1572.018 nm and 1572.660 nm are selected. L and CV used in the simulation are set to be 100 cm and 10%. According to the HITRAN database, the air-broadened linewidth  air of three lines are 0.072 cm−1 /atm, 0.0736 cm−1 /atm and 0.0765 cm−1 /atm, and their corresponding temperature coefficient n are 0.7, 0.67 and 0.69, respectively. When the temperature is changed from 173 K to 473 K, the peak absorbance curves are shown in Fig. 1(a), and a zoomed-in view of the absorbance curves in the general air temperature range from 273 K to 313 K are shown in Fig. 1(b). Fig. 1(a) shows that the relationships between temperature and peak absorbance of three absorption lines are different from each other. When temperature rises, the absorbance values of line 2 and line 3 decrease monotonously while the value of line 1 first increases then decreases. Since the temperature drift can cause errors in gas detection [21], its influence on peak absorbance should be considered and compensated to improve the measurement accuracy. 3. Establishment of the system

where 1 2

According to the ideal gas law, the molecule density of the absorber can be given by

(3)

−∞

gL (, v0 ) =

357

(4)

2

(5)

and ␯L , ␯G are the full width at half maximum for Lorentzian and Gaussian function, respectively. In Eq. (4),  air is the air-broadened linewidth at 296 K, n is the air-broadening temperature coefficient, and P represents the total pressure of the gas. In Eq. (5), M is the molecule molar mass. Due to the complexity of numerical calculation, it is difficult to get an analytical expression for Voigt function. Instead, the Gaussian or Lorentzian function is usually used as an alternative. It has been shown that when the pressure is greater than 100 Torr, the Voigt shape can be well approximated by the Lorentzian shape [20].

The intra-cavity gas detection system based on L-band erbiumdoped fiber ring laser consists of four parts shown in Fig. 2. The solid line represents optical signal while the dot line represents electric signal. The first part is used for light amplification. The EDFA is pumped by a 980 nm laser diode with adjustable pump power up to 400 mW. An electronic variable optical attenuator (EVOA) is employed to adjust the laser gain finely so as to get high sensitivity and signal to noise ratio for L-band gas sensing. 10% of the laser output is sent to the detector PIN1 to acquire the gas absorption spectra. The other 90% is reinjected into the resonant cavity for laser re-amplification. An isolator is employed to ensure the unidirectional traveling wave operation. The second part acts a role of frequency selection and calibration. It consists of a fiber Fabry–Perot tunable filter, an F–P etalon, an FBG, two couplers, two circulators, and two PIN detectors. Due to the narrow linewidth of gas absorption line, the filter has an FSR of 120 nm and a finesse of 3243 to get a narrow laser beam for high-resolution measurement. The laser wavelength can be tuned continuously by variable driven voltages applied to the filter. In order to reduce the wavelength positioning error brought by the nonlinearity of the filter, an F–P etalon and an FBG are

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used cooperatively to provide reference wavelengths for real-time wavelength calibration. Detectors PIN2 and PIN3 are set to detect the transmission spectra of etalon and the reflection spectra of FBG, respectively. The third part is designed for gas absorption. The gas cell with a length of 50 cm is made of a steel pipe and two flanges through interference fit. The laser is collimated by a pair of fiber pigtailed C-lens in the center of the flanges. Compared with GRIN lens, Clens has a longer working distance and a spherical rear end which can eliminate the unwanted interference fringes effectively. The Faraday reflection mirror (FRM) can avoid the interference between the incident light and the reflected light, and extend the effective absorption length twice of a single trip. The last part is data acquisition and device control. A computer controlled data acquisition (DAQ) card is used to get the output voltages of detectors for signal processing, and supply driven voltages for EVOA and tunable filter.

4. Experiments and discussion 4.1. Concentration calibration and detection

Fig. 1. Theoretical temperature dependence of peak absorbance in the temperature range (a) from 173 K to 473 K and (b) from 273 K to 313 K for zoomed-in view.

The constructed L-band erbium-doped fiber ring laser has a wide wavelength tuning range covering 1565–1600 nm. When the system is applied to measure the concentration of CO2 , the laser wavelength is set to scan from 1570 nm to 1574 nm, because CO2 has multiple strong absorption lines in this range. When the gas cell was filled with 10% of CO2 at normal temperature and pressure, the absorption spectra shown in Fig. 3 were acquired. Twelve strong absorption lines were obtained, which were sufficient for multi-wavelength detection. Besides, when a narrower wavelength range is scanned, a shorter time will be spent at the same sampling rate but fewer absorption lines can be acquired.

Fig. 2. Schematic diagram of the L-band intra-cavity gas detection system.

L. Yu et al. / Sensors and Actuators B 193 (2014) 356–362

Fig. 3. Absorption spectra of CO2 at normal temperature and pressure.

In order to get the peak absorbance of each absorption line, baseline extraction and line shape fitting are essential. Baseline extraction, as shown in Fig. 4(a), is realized by linear interpolation to acquire the background light intensity without the absorber in the gas cell [22]. The starting point for interpolation should be chosen in the non-absorption region. Due to the oblique baseline caused by gain unflatness of the laser, the absorbance curve instead of the initial absorption dip is adopted for line shape fitting. The fitting results using Lorentzian function are shown in Fig. 4(b). The center position and the peak value of the fitted curve represent the absorption wavelength and peak absorbance, respectively.

359

Fig. 5. Concentration calibration results of three absorption lines. (Markers: experimental results; lines: polynomial fitting of the experimental results.).

Three strong absorption lines with center wavelength 1571.406 nm, 1572.018 nm and 1572.660 nm are chosen for gas concentration calibration. Their line strengths are 1.51 × 10−23 cm−1 /(molecule cm−2 ), 1.71 × 10−23 cm−1 / (molecule cm−2 ) and 1.73 × 10−23 cm−1 /(molecule cm−2 ), respectively at 296 K. When CO2 concentration was varied from 2% to 10% with interval of 1% by mixing CO2 and N2 in the mass flow controller, the absorbance curves of these three lines were acquired after averaging 50 spectral scanning results. The results are shown in Fig. 5. Compared with the theoretical results shown in Fig. 1, the peak absorbance values of three lines get enhanced differently, because the increments of effective absorption length are different. If the position of an absorption line is closer to the laser threshold, the absorption length is longer. Linear fitting is adopted and the slopes of three lines are 0.0037/%, 0.0043/% and 0.0047/%, respectively. Absorption line 3 has the largest slope, which means the detection sensitivity is the highest. By contrast, absorption line 1 has the smallest slope and the lowest sensitivity. The tendency of slope variation is in accordance with that of absorption line strength and it is important to select strong absorption lines for trace gas detection. In order to reduce the random error, when 3.5%, 4.5%, 5.5%, 6.5%, 7.5% and 8.5% of CO2 were filled in the gas cell, the detected concentrations were calculated by linear calibration coefficients and the average peak absorbance of 50 spectral scans. The results are given in Table 1. Three absorption lines mentioned above can be used for gas concentration detection individually. However, the maximum error is as high as 820 ppm. If averaging the measured results, the error can be reduced significantly. The minimum absolute error is 7 ppm while the maximum is 388 ppm. The relative error is less than 0.52%. Besides, the error can be further decreased if more absorption lines are used for detection.

4.2. Wavelength monitoring

Fig. 4. (a) Baseline extraction and (b) Lorentzian line shape fitting of a single absorption line.

In order to calibrate the wavelength–voltage relationship of the tunable filter, a commercial athermal F–P etalon, which has thermal stable transmission wavelengths coinciding with the ITU channel grid, is used to provide comb reference wavelengths. The FSR and finesse of the F–P etalon are 100 GHz and 14, respectively. An FBG with center wavelength at 1573.5 nm is adopted as a wavelength mark. The FBG can help the system to distinguish the wavelength corresponding to each transmission peak of F–P etalon.

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Table 1 Results of concentration detection and error analysis. Practical Concentration (%)

3.5 4.5 5.5 6.5 7.5 8.5

Measured Concentration (%) Line 2

Line 3

Mean

Absolute

Relative

3.5285 4.5480 5.5363 6.5210 7.5273 8.4628

3.4709 4.5305 5.5256 6.5203 7.5353 8.4800

3.4985 4.4798 5.4901 6.5116 7.5537 8.5820

3.4993 4.5194 5.5173 6.5176 7.5388 8.5083

0.0007 0.0194 0.0173 0.0176 0.0388 0.0083

0.020 0.431 0.315 0.271 0.517 0.098

The nonlinearity of the tunable filter can be described by polynomial fitting as



Error (%)

Line 1

describe the consistence quantitatively, which can be expressed as



n

k =

ai Vk

(k = 1, 2, . . .)

(8)

i=0

where {k } and {Vk } are the transmitted wavelengths of the F–P etalon and their corresponding driven voltages, respectively, {ai } are the fitting coefficients and n is the polynomial order. The selected {k } for calibration should not match the strong absorption lines of the detected gas, otherwise the etalon’s wavelength referencing accuracy would be affected due to the modulation induced by gas absorption. The absorption wavelengths can be calculated by the polynomial fitting coefficients and the driven voltages corresponding to the center of absorption lines. Two order fitting is adopted because of the smallest standard deviation. When the concentration of CO2 was varied from 2% to 10%, the detected wavelengths of three absorption lines shown in Fig. 6 were acquired after averaging 50 spectral scanning results. Compared with the absorption wavelengths in HITRAN database, the maximum deviation of these three lines are 28 pm, 7 pm and 18 pm, respectively. The deviation is caused by the peak searching of gas absorption lines and other minor errors such as polynomial fitting. If the average wavelength deviation of multiple absorption lines is calculated and added to the measured results, the precision of wavelength detection can be further increased.

n

(xi −¯x)2 CV =

 = x

i=1 n

x¯

(9)

where x¯ and  are the mean and standard deviation, respectively. When different concentrations of CO2 are filled in the gas cell, the coefficients of variation after 50 consecutive spectral scans are shown in Fig. 7. The center wavelengths of absorption line 1 to absorption line 3 are 1571.406 nm, 1572.018 nm and 1572.660 nm, respectively. The CV is high at low concentrations because of the low signal to noise ratio. When only one absorption line is used for concentration measurement, the maximum CV can be as high as 7.9%. However, if the average concentration demodulated by three absorption lines is taken as the result, the CV is less than 4.2%. As a result, the system stability is improved and the concentration detection error is decreased significantly. 4.4. Temperature compensation

The system stability can be evaluated by the consistence of the detected concentrations after multiple measurements under the same conditions. Coefficient of variation (CV) is generally used to

According to the theoretical analysis, the peak absorbance of gas absorption line varies with the environment temperature. Temperature influence on gas absorption should be considered in the concentration detection. The gas cell filled with 10% of CO2 was placed in the thermostat and the temperature was changed from 273 K to 313 K with interval of 5 K to simulate the general air temperature. The absorbance values of three absorption lines are shown in Fig. 8. Their center absorption wavelengths are 1571.40 nm, 1572.02 nm and 1572.66 nm, respectively. Compared with Fig. 1(b), Fig. 8 shows that gas absorption gets enhanced in the intra-cavity detection system. When temperature rises, absorption becomes weaker in the given temperature range and the variation tendency coincides with the simulation

Fig. 6. Measured absorption wavelengths under different concentrations. (Markers: experimental results; lines: wavelengths in HITRAN database.).

Fig. 7. Coefficients of variation calculated under different concentrations.

4.3. System stability analysis

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tunable filter is real-time calibrated with the reference wavelengths provided by an F–P etalon. Temperature influence on gas absorption is simulated and the compensation method is proposed. When the system is applied to detect the concentration of CO2 , twelve strong absorption lines are acquired within a 4 nm tuning range. Linear interpolation and Lorentzian fitting are used to get the peak absorbance of absorption lines for concentration calibration and detection. The absolute and relative error can be reduced to less than 400 ppm and 0.52%, respectively by averaging the demodulated concentrations of three absorption lines. The monitoring error of absorption wavelength is no more than 30 pm. Besides, other kinds of gases with absorption lines in L-band can also be detected by the system proposed in this paper. Acknowledgments Fig. 8. Relationship between gas absorbance and temperature. (Markers: experimental results; lines: polynomial fitting of the experimental results.).

result. However, the experimental absorbance of absorption line 3 is larger than absorption line 2, because the laser works closer to the threshold at the wavelength of line 3 and the increment of the effective absorption length is more obvious. In order to realize temperature compensation, two order polynomial fitting is used to calibrate the absorbance-temperature curve. The maximum relative error between the sampling points and the fitting points of three absorption lines are 0.61%, 0.63% and 0.76%, respectively. When a sensitized FBG used as the temperature sensor is introduced into the gas detection system, the current temperature T can be acquired. We assume that the peak absorbance of the detected CO2 is K(T), the absorbance-concentration curves are calibrated at reference temperature T0 , the peak absorbance values corresponding to CO2 of 10% at T and T0 are K0 (T) and K0 (T0 ), respectively. Both K0 (T) and K0 (T0 ) can be got from the absorbance-temperature calibration curve in Fig. 8 with polynomial interpolation algorithm. The absorbance ratio at different temperatures is concentration-independent. The compensated concentration of the detected gas can be required by substituting the value K(T)K0 (T0 )/K0 (T) into the concentration calibration curve in Fig. 5. The method can improve the accuracy of concentration measurements effectively and avoid repeated calibrations at every environmental temperature. If gas detection is implemented far away from the room temperature, the temperature calibration range should be extended correspondingly since the interpolation algorithm is usually more precise than extrapolation algorithm. 4.5. Discussion Compared with other methods, ICAST has several advantages. Although wide wavelength coverage in spectrophotometry and very high resolution in TDLAS are hard to realize at the same time in optical gas sensing [23], intra-cavity system based on EDFL can close the gap. The fiber configuration also has a better immunity to environmental interference than the space structure used in mid-infrared non-dispersive and cavity enhanced systems. Due to the weak gas absorption in L-band, noise factors such as the amplified spontaneous emission (ASE) and the residual etalon effect in the gas cell become the main limitations on the system performances. The improvements in the stability of pump power and laser gain, as well as the configuration of the gas cell, should be focused in future study. 5. Conclusion We have constructed an intra-cavity gas detection system based on L-band erbium doped fiber ring laser. The nonlinearity of the

This work was supported by the National Basic Research Program of China under Grant 2010CB327802, the National Natural Science Foundation of China under Grant 61108070, Grant 11004150 and Grant 61201081, the Tianjin Science and Technology Support Key Project under Grant 11ZCKFGX01900 and the China Postdoctoral Science Foundation under Grant 201003298. References [1] G. Stewart, P. Shields, B. Culshaw, Development of fibre laser systems for ringdown and intracavity gas spectroscopy in the near-IR, Meas. Sci. Technol. 15 (2004) 1621–1628. [2] Y. Jiang, C.J. Tang, An optical fiber methane sensing system employing a twostep reference measuring method, Sens. Actuators, B 133 (2008) 174–179. [3] B. Löhden, S. Kuznetsova, K. Sengstock, V.M. Baev, A. Goldman, S. Cheskis, B. Pálsdóttir, Fiber laser intracavity absorption spectroscopy for in situ multicomponent gas analysis in the atmosphere and combustion environments, Appl. Phys. B 102 (2011) 331–344. [4] N. Arsad, M. Li, G. Stewart, W. Johnstone, Intra-cavity spectroscopy using amplified spontaneous emission in fiber lasers, J. Lightwave Technol. 29 (2011) 782–788. [5] V.M. Baev, T. Latz, P.E. Toschek, Laser intracavity absorption spectroscopy, Appl. Phys. B 69 (1999) 171–202. [6] J. Hernandez-Cordero, T.F. Morse, Fiber laser intra-cavity spectroscopy (FLICS), IEICE Trans. Electron. E Ser. C 83 (2000) 371–377. [7] A. Stark, L. Correia, M. Teichmann, S. Salewski, C. Larsen, V.M. Baev, P.E. Toschek, Intracavity absorption spectroscopy with thulium-doped fibre laser, Opt. Commun. 215 (2003) 113–123. [8] G. Stewart, K. Atherton, H.B. Yu, B. Culshaw, An investigation of an optical fibre amplifier loop for intra-cavity and ring-down cavity loss measurements, Meas. Sci. Technol. 12 (2001) 843–849. [9] Y. Zhang, M. Zhang, W. Jin, Multi-point, fiber-optic gas detection with intracavity spectroscopy, Opt. Commun. 220 (2003) 361–364. [10] Y. Zhang, M. Zhang, W. Jin, Sensitivity enhancement in erbium-doped fiber laser intra-cavity absorption sensor, Sens. Actuators, A 104 (2003) 183–187. [11] K. Liu, W.C. Jing, G.D. Peng, J.Z. Zhang, Y. Wang, T.G. Liu, D.G. Jia, H.X. Zhang, Y.M. Zhang, Wavelength sweep of intracavity fiber laser for low concentration gas detection, IEEE Photonics Technol. Lett. 20 (2008) 1515–1517. [12] K. Liu, T.G. Liu, J.F. Jiang, G.D. Peng, H.X. Zhang, D.G. Jia, Y. Wang, W.C. Jing, Y.M. Zhang, Investigation of wavelength modulation and wavelength sweep techniques in intracavity fiber laser for gas detection, J. Lightwave Technol. 29 (2011) 15–21. [13] J. Marshall, G. Stewart, G. Whitenett, Design of a tunable L-band multiwavelength laser system for application to gas spectroscopy, Meas. Sci. Technol. 17 (2006) 1023–1031. [14] R.A. Perez-Herrera, A. Ullan, D. Leandro, M. Fernandez-Vallejo, M.A. Quintela, A. Loayssa, J.M. Lopez-Higuera, M. Lopez-Amo, L-band multiwavelength singlelongitudinal mode fiber laser for sensing applications, J. Lightwave Technol. 30 (2012) 1173–1177. [15] S. Yamashita, M. Nishihara, Widely tunable erbium-doped fiber ring laser covering both C-band and L-band, IEEE J. Sel. Top. Quantum Electron. 7 (2001) 41–43. [16] E. Vargas-Rodríguez, H.N. Rutt, Design of CO, CO2 and CH4 gas sensors based on correlation spectroscopy using a Fabry–Perot interferometer, Sens. Actuators, B 137 (2009) 410–419. [17] D.A. Hall, Nonlinearity in piezoelectric ceramics, J. Mater. Sci. 36 (2001) 4575–4601. [18] J. Tuominen, T. Ritari, H. Ludvigsen, J.C. Petersen, Gas filled photonic bandgap fibers as wavelength references, Opt. Commun. 255 (2005) 272–277. [19] L.S. Rothman, I.E. Gordon, A. Barbe, D.C. Benner, P.F. Bernath, M. Birk, V. Boudon, et al., The HITRAN 2008 molecular spectroscopic database, J. Quant. Spectrosc. Radiat. Transfer 110 (2009) 533–572.

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Liu is a Chief Scientist of the National Basic Research Program of China under Grant 2010CB327802. Kun Liu received the B.Eng., M.Eng. and Ph.D. degrees in 2004, 2006 and 2009, respectively, from Tianjin University, China. He is currently an associate professor in Tianjin University. His research interests are fiber physics and chemistry sensing systems. Junfeng Jiang received his B.S. degree in 1998 from Southwest Institute of Technology, China. He obtained the M.S. degree and Ph.D. degree from Tianjin University, China, in 2001 and 2004, respectively. He is currently an associate professor in Tianjin University. His research interests include fiber sensors and optical communication performance measurement.

Biographies

Lei Zhang received her M.S and Ph.D degrees in 2005 and 2008, respectively, from Auburn University, USA. She is currently an assistant professor in Tianjin University. Her research interest includes algorithm design and networking.

Lin Yu received her B.Eng. in electronic science and technology (optoelectron) in 2011 from Tianjin University, China. She is currently working toward the Ph.D. degree in optical engineering at the same university. Her research interests are fiber gas sensing systems.

Yunwei Jia received her B.Eng. and M.Eng. degrees in 2002 and 2005, respectively, from China University of Mining and Technology. Now she is pursuing her Ph.D. degree in Tianjin University. Her research interests include fiber gas sensing and digital image processing.

Tiegen Liu received the B.Eng., M.Eng. and Ph.D. degrees in 1982, 1987 and 1999, respectively, from Tianjin University, China. Now he is a professor in Tianjin University. His research interests involve photoelectric detection and fiber sensing. Prof.

Tao Wang received his B.Eng. in optoelectronic technology and science in 2013 from Tianjin University, China. He is ready prepared to pursue the Ph.D. degree at the same institute. His research interests are focused on fiber gas sensors.