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Highly sensitive dual-wavelength fiber ring laser sensor for the low concentration gas detection
Sensors & Actuators: B. Chemical 296 (2019) 126637
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly sensitive dual-wavelength fiber ring laser sensor for the low concentration gas detection
T
⁎
Xianchao Yanga, Liangcheng Duana, Haiwei Zhangb, Ying Lua, , Guangyao Wanga, Jianquan Yaoa a
College of Precision Instrument and Opto-electronics Engineering, Key Laboratory of Opto-electronics Information Technology (Ministry of Education), Tianjin University, Tianjin, 300072, PR China b School of Electrical and Electronic Engineering, Tianjin University of Technology, Tianjin, 300384, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Gas sensor Low concentration acetylene detection Dualwavelength Fiber ring laser sensor Highly-Sensitive
A highly sensitive low concentration gas sensor based on dual-wavelength Erbium-doped fiber ring laser (EDFRL) is designed and demonstrated. The dual-wavelength lasing output is introduced by a tunable FabryPerot (F-P) filter and a fiber Bragg grating (FBG) due to the mode competition as they both share a same EDF gain medium. One wavelength at 1530.37 nm serving as the sensing element is inserted by a hollow core photonic crystal fiber (HCPCF) gas cell which can bring in much more power change when the cavity loss slightly varies while the other wavelength at 1532.38 nm acting as a reference. Experimental results show that the sensor can achieve a good linear response (R2 = 0.9864) to acetylene concentration variation and a minimum detection limit (MDL) of 10.42 ppmV at 20 s response time, which is the lowest to our best knowledge and improving 6.44 times than that of single-wavelength EDFRL based. Moreover, the absolute detected error induced by the power fluctuation (< 0.1 dB) is less than 1.78% over more than 300 s of observation.
1. Introduction Detection of pollutant or inflammable low concentration gases, such as methane, acetylene and carbon monoxide, is important for safety monitoring in industrial, biomedical and environmental applications [1]. Intra-cavity absorption gas sensors using Erbium-doped fiber lasers (EDFL) have attracted much research interest due to the enhanced sensitivity by the increment of interaction time which has been reported early in 1992 [2], also with the advantages of optical sensors including immunity to electromagnetic interference, remote sensing without amplifier, low cost components provided by the telecommunication industry, multipoint sensing and networking capability, etc [3]. By placing the gas cell into the cavity of EDFL, the absorption sensitivity can improve 91 times higher than that of the single pass absorption measurement [4]. Compared with the conventional gas cells which need costly integration to increase the effective absorption length of the gas [4–8], hollow core photonic crystal fiber (HCPCF) is demonstrated to be an excellent alternative as the compact structure making it much easier to increase the interaction time by changing the fiber length simply using coiling method [9]. To realize high sensitivity for extremely low concentration gas sensing, the EDFL can work near the threshold but limited by the spontaneous emission noise [4,5]. Wavelength modulation technique
⁎
(WMT) [6] and wavelength sweep technique (WST) [7] are introduced to eliminate the limitation introduced by the noise and the minimum detectable gas (C2H2) concentration can reach 75 ppmV [8]. Using HCPCF as the gas cell in EDFL, all-fiber dual-point [10] and multi-point sensor network [11] are designed with the minimum detectable concentration less than 100 ppmV. Other methods are reported such as using quartz-enhanced photoacoustic spectroscopy for ultra-sensitive calibration-free trace-gas detection and fast spectral scan applications [12], as well as exploiting photothermal effect in a HCPCF for ultrasensitive gas detection by probing the gas-absorption-induced phase change via fibre-optic interferometry [13]. The minimum detectable concentration can be further decreased to about 2 ppbV compromises to the more complex and costly structures. Moreover, almost all of the intra-cavity gas sensors previously reported are single-wavelength fiber lasers based, the only way to improve sensitivity with simple structure seems to increase the HCPCF length as described in Ref. [14], which means additional costs as the HCPCF is so expensive. The intra-cavity gas absorption obeys the Beer-Lambert law [15]:
I (v ) = I0 (v )exp[−κ (v ) L]
(1)
Where I(v) is the intensity of the transmitted light, I0(v) is the intensity of the incident light. I(v) = I0(v) – ΔI, ΔI is the reduction of the output power caused by the cavity loss variation Δδ. κ(v) is the absorption
Corresponding author. E-mail address: [email protected] (Y. Lu).
https://doi.org/10.1016/j.snb.2019.126637 Received 14 December 2018; Received in revised form 14 May 2019; Accepted 30 May 2019 Available online 07 June 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Sensors & Actuators: B. Chemical 296 (2019) 126637
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the sensing wavelength further. As shown in Fig. 2, in the gas cell, one end of HCPCF (HC-1550-02 produced by NKT Photonics Inc.) with the length of 1 m is connected with F-P filter via bare fiber adapter and single mode fiber (SMF), while the other end is connected with DWDM via bare fiber adapter and SMF placed outside of the gas cell. The inflation and exhaust of detected gas can be realized rapidly through two values on the top of the aluminum cylindrical container by the pressure difference between two ends of the HCPCF. The acetylene gases we used are all custom-made with standard concentrations. For exhaust, the gas cell is firstly filled with pure nitrogen and sealed till there is no absorption at the absorption line of acetylene. The absorption losses of different acetylene concentrations are marked when the maximum value of absorption loss is reached and then saturated with time going on. The sensitivity can be improved significantly by inserting the gas cell into the EDFRL cavity due to the multi-pass increased interaction length between the laser and detected gas. The common port of the DWDM is spliced to a 10: 90 coupler where 10% port is used to extract the laser to the OSA and the rest 90% is fed back to the cavity. In Fig. 1, the condition to realize stable oscillation of dual wavelengths can be expressed as [21,26]:
coefficient of absorber and L is the optical path length of the absorber which decided by the HCPCF length in this paper. From Eq. (1) we can know:
κ (v ) L = ln
I0 (v ) I (v )
(2)
For a small absorption signal, ΔI ≪ I0(v),
κ (v ) L =
ΔI I0 (v )
(3)
Then the reduction of the output power ΔI can be expressed as:
ΔI = I0 (v ) κ (v ) L
(4)
Eq. (4) demonstrates that the reduction of the output power ΔI caused by the intra-cavity gas absorption is proportion to the optical path length of the absorber decided by the HCPCF length. Dual-wavelength or multi-wavelength EDFLs have attracted considerable interest for their potential applications in optical fiber sensing regions due to the advantages like simple structure, multiple narrow bandwidth, good compatibility with other components, etc [16–18]. To achieve stable multi-wavelength lasing output, several approaches have been used such as polarization hole burning effect [16], deeply saturated spectral hole-burning effect [17], frequency-shifted feedback technique [19], four-wave mixing effect [20] and so on. Compared with single-wavelength fiber laser based sensor, dual-wavelength EDFL based sensor with good stability of power fluctuation less than 0.1 dB [16–18] can provide extremely high sensitivity and realize multiparameter measurement, opens new ways for the fiber sensing applications, especially for the low concentration gas detection. Many kinds of sensing schemes have been reported and optimized to measure refractive index [21], liquid-level [22], gas concentration [23], temperature and strain [24], etc. But to our best knowledge, dual-wavelength EDFL used in the intra-cavity gas absorption sensing regions is rare. In this paper, we design and demonstrate a highly sensitive low concentration gas sensor based on dual-wavelength Erbium-doped fiber ring laser (EDFRL). One wavelength selected by a tunable Fabry-Perot (F-P) filter at 1530.37 nm is served as the sensing element by inserting a HCPCF gas cell while the other wavelength selected by a fiber Bragg grating (FBG) at 1532.38 nm acted as a reference. Due to the mode competition phenomenon, the variation of absorption loss caused by the acetylene concentration can bring in much more power change in the sensing laser. WST is applied to ensure high accuracy. The minimum detection limit (MDL) of 10.42 ppmV at 20 s response time is obtained by experiment with good linear response (R2 = 0.9864), improving 6.44 times than that of single-wavelength EDFRL based. Moreover, the absolute detected error induced by the power fluctuation (< 0.1 dB) is less than 1.78% over more than 300 s of observation.
g λi × L = Gth (λi ) = δ λi, i = 1, 2
(5)
λ1 and λ2 represent the sensing wavelength and the reference wavelength, respectively. gλi is the gain coefficient. L is the EDF length. Gth (λi) is the single-path threshold gain and δλi is the single-path loss. When L =2 m, Gth (λ1) = Gth (λ2) ≈ 6 × 2 ≈ 12 dB. By adjusting the VOA, δF–P filter + δgas cell = δcircular + δVOA + δFBG can be satisfied to realize δλ1 = δλ2. When δλ1 ≈ δλ2 = Gth (λ1) = Gth (λ2) is satisfied, the stable oscillation of dual wavelengths EDFRL can be realized. The output power of dual-wavelength oscillations for EDFRL is investigated numerically before experiment. The behavior of the dualwavelength EDFRL is described by two-level pumping system [27,28] and the propagation equations are as follows:
dPs± (λi ) = +Γs (σes N2 − σas N1) Ps± − αs Ps± (λi ), i = 1, 2 dz
dPp± dz
(6)
= +Γp (σep N2 − σap N1) Pp± − αp Pp±
(7) 2
± dPase ± = +2hγs⋅Δγ⋅σes Γs N2 + Γs (σes N2 − σas N1) Pase − αs ∑ Ps± (λi ) dz i=1
(8)
0 = (R12 + W12) N1 − (R21 + W21 + A21 ) N2
(9) (10)
Nt = N1 + N2
The light of backward is ignored in Eq. (6)–(10) as the undesired reflection is suppressed by ISO. The transition rates of R12, R21, W12, W21 and A21 are defined as:
2. Experimental setup and principle
R12 = The schematic configuration of dual-wavelength EDFRL gas sensor is shown in Fig. 1. A 2 m long Erbium-doped fiber (EDF, Nufern EDFC980-HP, gain coefficient is 6 ± 1 dB/m) is pumped by 976 nm laser diode via a filter wavelength division multiplexing (FWDM). The isolator (ISO) is used to guarantee the unidirectional operation of EDFRL and prevent spatial hole-burning [25]. The sensing wavelength is selected by a tunable F-P filter controlled by NI-PXI platform spliced to one HCPCF gas cell. The reference wavelength is selected by a fiber Bragg grating (FBG) (center wavelength @1532.38 nm) spliced to a variable optical attenuator (VOA) and circulator. The reference cavity loss is precisely controlled by the VOA to achieve stable oscillation of dual-wavelength lasing output. Two laser cavities are connected with a 50: 50 coupler which can realize similar output power and a dense wavelength division multiplexing (DWDM) filter (the spacing of 200 GHz at 1530.33 nm) is worked as the narrow band filter to identify
R21 =
σap Γp hγp Aeff σep Γp hγp Aeff
Pp
Pp
(11)
(12)
W12 =
σas Γs + (Pp + Pase ) hγs Aeff
(13)
W21 =
σes Γs + (Pp + Pase ) hγs Aeff
(14)
A21 =
1 τ21
(15)
Where Aeff is the effective core area of the erbium-doped optical fiber, Γs and ΓP are the overlap of the optical modes with the erbium distribution. τ21 is the fluorescence lifetime of the metastable level of the 2
Sensors & Actuators: B. Chemical 296 (2019) 126637
X. Yang, et al.
Fig. 1. The schematic of the dual-wavelength EDFRL gas sensor configuration.
Fig. 2. The schematic of HCPCF gas cell configuration for acetylene filling.
two-level system. The differential equations of first-order listed above are calculated by finite-difference method using MATLAB software and the detailed process refer to Ref. [27]. The EDF with the optimized length of 2 m is divided into finite segments averagely then calculated by steady-state solutions at different wavelengths. When the pump light is absorbed by EDF, the amplified spontaneous emission (ASE) is firstly generated. The sensing and reference wavelengths can be selected by the F-P filter and FBG by balancing the cavity losses added to two cavities. Part of the light is extracted as the output while the others are fed back to the EDF as input signal. In simulation, the sensing wavelength is set at λ1 = 1530.37 nm where the acetylene exhibits highest absorption peak according to the Hitran database [29] as shown in Fig. 3. The reference wavelength is set at λ2 = 1532.38 nm with initial cavity loss of 0.75 dB higher than the cavity loss of the sensing laser. In Fig. 4, we can see that the EDFRL exhibits single-wavelength at λ1 when the sensing cavity loss is lower than that of the reference. With the increment of the sensing laser cavity loss which can be regarded as the acetylene concentration increasing in experiment, the output power of the sensing laser damps gradually while the reference increases. Dual-wavelength lasers with equal output powers appear when two cavity losses are balanced. However, when the sensing cavity loss is too high, the sensing laser will disappear quickly and only leave the reference laser at λ2. The sharply power decrement of the sensing laser away from the stable dual-wavelength lasing point, which caused by the variation of gas concentration due to the mode competition, can improve the sensitivity significantly compared with single-wavelength mode. When the central wavelengths of dual-wavelength lasers far from each other (at 1528.01 nm and 1532.38 nm), the output powers are reduced due to the relative weaker mode competition which is in consisted with Ref. [30]. Considering the sensing laser is extremely sensitive to the cavity loss
Fig. 3. Absorption spectrum of acetylene from 1528 nm to 1539 nm.
caused by the different gas concentrations, a low power of dual-wavelength lasers will reduce the gas detection range. However, when the central wavelengths are too close, the large power fluctuation will break the balance of dual-wavelength lasers [17]. 3. Results and discussions WST is used in experiment to ensure high accuracy and the spectra of single-wavelength and dual-wavelength EDFRL gas sensors filled with acetylene with concentration of 1000 ppmV are shown in Fig. 5. The F-P filter we used to sweep the sensing wavelength linearly has a 3
Sensors & Actuators: B. Chemical 296 (2019) 126637
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Fig. 6. Output spectra of dual-wavelength EDFRL gas sensor filled with acetylene with concentrations of 0, 100 ppmV, 500 ppmV, 1000 ppmV and 1500 ppmV.
Fig. 4. Simulated output powers of dual-wavelength lasers with central wavelengths 2.01 nm and 4.37 nm apart when sensing cavity loss increases from 0 to 1.5 dB.
more sharply away from the stable dual-wavelength lasing point compared with the increment of reference. The power changes can be explained by the mode competition mechanism caused by the variation of sensing cavity loss. The increment of absorption loss with high acetylene concentration will lead to a mode contention by the reference laser. Fig. 7 shows the absorption losses of dual-wavelength EDFRL gas sensor when the acetylene concentration c increases from 0 to 1500 ppmV and then reduced to 0. The power variations at λ1 and λ2 with different acetylene concentrations are nearly the same at c increasing and decreasing processes, indicating good repeatability of the designed sensor. The acetylene absorptions of the sensing and reference lasers all exhibit good linear fitting along with the concentration varying which can be expressed as P1 = 0.0096c + 0.0154 for λ1 and P2 = 0.0044c + 0.0233 for λ2, with R2 = 0.9864 and 0.9820, respectively. Compared with the simulation results in Fig. 4, the experimental datas we obtained at λ1 exhibit the same trend but lower slope along with the acetylene concentration increasing as shown in Fig. 8. The reasons are mainly caused by the differences of the cavity loss at λ1 between simulation and experiment or the power fluctuations of the EDFRL. The MDL used to evaluate the sensing performance in similar works is defined as [14]:
Fig. 5. Output spectra of single-wavelength and dual-wavelength EDFRL gas sensors filled with 1000 ppmV acetylene using WST.
free spectral range of 108 nm and fineness of 10,090 controlled by the NI PXIe-4319 system source measure unit, with the resolution of 100 nV and sampling rate of 1.8MS/s. When the drive voltage applied to the FP filter increases from 1.28 V to 1.313 V linearly, at the scanning waveband around 1530 nm, the absorption dip in spectrum (blue solid line) we observed is in consistent with that of acetylene in Fig. 3. The absorption line of acetylene at 1530.37 nm can be recognized automatically when the selected voltage range applied to the F-P filter and the whole sweeping time is about 8 s. When acetylene with the concentration of 1000 ppmV is filled into the gas cell, the output powers of dual-wavelength lasers have a decrement of 5.6 dB at the sensing wavelength and an increment of 2.17 dB at the reference wavelength, about 6.44 times higher than that of the single-wavelength mode of 0.87 dB. The sensing performance of the gas sensor is investigated by filling acetylene with different concentrations from 0 to 1500 ppmV in experiment as shown in Fig. 6. With the acetylene concentration increasing, the output power of the sensing laser at λ1 = 1530.37 nm decreases from -18.43 dB to -32.76 dB while that of the reference at λ2 = 1532.38 nm increases from -24.62 dB to -17.89 dB. Moreover, the power change of 14.33 dB at λ1 is much larger than that of 6.73 dB at λ2. The results we obtained in experiment are in consistent with the simulations in Fig. 4, which show the power decrement of sensing laser
Fig. 7. Absorption losses of dual-wavelength EDFRL gas sensor as a function of the acetylene concentration. 4
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Table 1 Performances of Intra-cavity gas sensors. Characteristics
MDL(ppmV)
Ref.
Gas cell made of a pair of collimated graded index WST technique WMT technique WST and WMT Dual-point automatic switching with 1 meter HCPCF Multiplexed sensor network with 1 meter HCPCF Single-wavelength EDFRL with 8 meters HCPCF Ring-down cavity Mode-locked fiber laser Our work with 1 meter HCPCF
2253 1000 200 75 398 100 5.4 1000 781 10.42
[4] [6] [7] [8] [10] [11] [14] [31] [32] N/A
Fig. 8. Comparison between the simulated and the experimental results.
MDL =
fluctuation × concentration absorption
(16)
According to Eq. (16), the power stability of dual-wavelength EDFRL sensing system has great influence on MDL. The output power and wavelength stabilities are measured over 300 s by fixing the operating wavelength at λ1 and λ2 as shown in Fig. 9. The fluctuations of the peak intensities are measured to be less than 0.1 dB, corresponding to the MDL of 10.42 ppmV. Compared with other similar works listed in Table 1, the MDL of dual-wavelength EDFRL gas sensor is improved about 7–216 times. However, due to the limitation of HCPCF length, the MDL we obtained is lower than that in Ref. [14] (8 m long HCPCF is used by Zhao et al. and in this work the HCPCF is only 1 m). According to Eq. (4) and Eq. (16), the MDL is inversely proportional to the HCPCF length L, which means a larger L leading to a smaller MDL, thus a higher sensitivity. So in the following works, the sensitivity can be improved further by increasing the HCPCF length with the same stability of EDFRL, or improving the stability of EDFRL with the same HCPCF length. The detection efficiency is not only related to the sweeping time of F-P filter but also with the filling time of the gas cell. As mentioned above, the whole sweeping time is about 8 s when selected voltage range from 1.28 V to 1.313 V applied to the F-P filter to recognize the absorption line of acetylene at 1530.37 nm exactly. To ensure high accuracy, pure nitrogen is full filled into the gas cell before acetylene filling until there is no absorption line in spectrum for exhaust. As shown in Fig. 10, when the filling pressure is 0.3 MPa, the maximum
Fig. 10. Filling time of gas cell with 1 m long HCPCF under 0.3 MPa pressure.
value of absorption loss reached at λ1 using about 20 s and then saturated with time going on. Therefore, the fast response time of dualwavelength EDFRL gas sensors is only 20 s with 1 m long HCPCF due to the pressure difference between the gas cell and outside environment. By increasing the filling pressure, the detection efficiency can be improved further as the larger the pressure, the shorter the filling time. 4. Conclusion In this paper, we demonstrated a highly sensitive low concentration gas sensor based on dual-wavelength EDFRL both theoretically and experimentally. The relationship between cavity losses and output powers are calculated for the sensing laser at 1530.37 nm and the reference at 1532.38 nm. Due to the mode competition phenomenon, the sensing laser power shows extremely sensitive to the variations of acetylene concentration in experiment which can achieve a good linear response (R2 = 0.9864) and a MDL of 10.42 ppmV at 20 s response time. Compared with other similar works, the MDL we obtained is the lowest to our best knowledge and improving 6.44 times than that of single-wavelength EDFRL. The gas sensor exhibits good stability with the absolute detected error induced by the power fluctuation (< 0.1 dB) less than 1.78% over more than 300 s of observation. Moreover, the sensitivity can be improved further by increasing the HCPCF length or improving the stability of EDFRL. Acknowledgments This work was supported by the National Basic Research Program of China (973 Program) (Grant number: 2010CB327801) and Scientific Research Project of Tianjin Municipal Education Commission (Grant No. 2018KJ136).
Fig. 9. The output power and wavelength stabilities of dual-wavelength EDFRL gas sensor. 5
Sensors & Actuators: B. Chemical 296 (2019) 126637
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Xianchao Yang received the Bachelor’s degree in Electronic and Information Engineering from the Tianjin Polytechnic University, Tianjin, China, in 2014. He is currently working toward the pH.D. degree at the Institute of Laser and Optoelectronics, Tianjin University. Liangcheng Duan received his B. S. degree in Optical Information Science and Technology in 2011 from Tianjin University of Technology, Tianjin, China. He is currently working towards the Ph. D degree in opto-electronic science and technology in Tianjin University. His current research focuses on optical fiber sensors. Haiwei Zhang received his Ph. D degree in Opto-electronic Science and Technology in Tianjin University. He is currently working in Optical Information Science and Technology in Tianjin University of Technology, Tianjin, China. His current research interests include fiber lasers and amplifiers, and optical fiber sensors. Ying Lu is a professor of the Key Laboratory of Opto-electronics Information Technology in Tianjin University. She received her PhD degree from Nankai University, China, in 2000. Her current research focuses on photonics microstructure, fiber sensing and nonlinear optics. Guangyao Wang received the Master’s degree from Yanshan University, Qinhuangdao, China, in 2017. Now he is working toward the pH.D. degree in Tianjin University, Tianjin, China. Jianquan Yao is currently a Professor and the Director of the Institute of Laser and Optoelectronics of Tianjin University. From 1980–1982, he joined the Department of Applied Physics, Stanford University, USA, as a Visiting Scholar. He has long been engaged in the research of all-solid laser, nonlinear optics frequency conversion and Terahertz science & technology. His theory of precise calculation of optimum phase matching of biaxial crystal has been called the “Yao and Fahlen Technology”. He received the World Gold Medal of Invention, Eureka, Brussels, and National Invention Award Class II of China. In 1997, he was elected as Academician of the Chinese Academy of Science.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly sensitive dual-wavelength fiber ring laser sensor for the low concentration gas detection
T
⁎
Xianchao Yanga, Liangcheng Duana, Haiwei Zhangb, Ying Lua, , Guangyao Wanga, Jianquan Yaoa a
College of Precision Instrument and Opto-electronics Engineering, Key Laboratory of Opto-electronics Information Technology (Ministry of Education), Tianjin University, Tianjin, 300072, PR China b School of Electrical and Electronic Engineering, Tianjin University of Technology, Tianjin, 300384, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Gas sensor Low concentration acetylene detection Dualwavelength Fiber ring laser sensor Highly-Sensitive
A highly sensitive low concentration gas sensor based on dual-wavelength Erbium-doped fiber ring laser (EDFRL) is designed and demonstrated. The dual-wavelength lasing output is introduced by a tunable FabryPerot (F-P) filter and a fiber Bragg grating (FBG) due to the mode competition as they both share a same EDF gain medium. One wavelength at 1530.37 nm serving as the sensing element is inserted by a hollow core photonic crystal fiber (HCPCF) gas cell which can bring in much more power change when the cavity loss slightly varies while the other wavelength at 1532.38 nm acting as a reference. Experimental results show that the sensor can achieve a good linear response (R2 = 0.9864) to acetylene concentration variation and a minimum detection limit (MDL) of 10.42 ppmV at 20 s response time, which is the lowest to our best knowledge and improving 6.44 times than that of single-wavelength EDFRL based. Moreover, the absolute detected error induced by the power fluctuation (< 0.1 dB) is less than 1.78% over more than 300 s of observation.
1. Introduction Detection of pollutant or inflammable low concentration gases, such as methane, acetylene and carbon monoxide, is important for safety monitoring in industrial, biomedical and environmental applications [1]. Intra-cavity absorption gas sensors using Erbium-doped fiber lasers (EDFL) have attracted much research interest due to the enhanced sensitivity by the increment of interaction time which has been reported early in 1992 [2], also with the advantages of optical sensors including immunity to electromagnetic interference, remote sensing without amplifier, low cost components provided by the telecommunication industry, multipoint sensing and networking capability, etc [3]. By placing the gas cell into the cavity of EDFL, the absorption sensitivity can improve 91 times higher than that of the single pass absorption measurement [4]. Compared with the conventional gas cells which need costly integration to increase the effective absorption length of the gas [4–8], hollow core photonic crystal fiber (HCPCF) is demonstrated to be an excellent alternative as the compact structure making it much easier to increase the interaction time by changing the fiber length simply using coiling method [9]. To realize high sensitivity for extremely low concentration gas sensing, the EDFL can work near the threshold but limited by the spontaneous emission noise [4,5]. Wavelength modulation technique
⁎
(WMT) [6] and wavelength sweep technique (WST) [7] are introduced to eliminate the limitation introduced by the noise and the minimum detectable gas (C2H2) concentration can reach 75 ppmV [8]. Using HCPCF as the gas cell in EDFL, all-fiber dual-point [10] and multi-point sensor network [11] are designed with the minimum detectable concentration less than 100 ppmV. Other methods are reported such as using quartz-enhanced photoacoustic spectroscopy for ultra-sensitive calibration-free trace-gas detection and fast spectral scan applications [12], as well as exploiting photothermal effect in a HCPCF for ultrasensitive gas detection by probing the gas-absorption-induced phase change via fibre-optic interferometry [13]. The minimum detectable concentration can be further decreased to about 2 ppbV compromises to the more complex and costly structures. Moreover, almost all of the intra-cavity gas sensors previously reported are single-wavelength fiber lasers based, the only way to improve sensitivity with simple structure seems to increase the HCPCF length as described in Ref. [14], which means additional costs as the HCPCF is so expensive. The intra-cavity gas absorption obeys the Beer-Lambert law [15]:
I (v ) = I0 (v )exp[−κ (v ) L]
(1)
Where I(v) is the intensity of the transmitted light, I0(v) is the intensity of the incident light. I(v) = I0(v) – ΔI, ΔI is the reduction of the output power caused by the cavity loss variation Δδ. κ(v) is the absorption
Corresponding author. E-mail address: [email protected] (Y. Lu).
https://doi.org/10.1016/j.snb.2019.126637 Received 14 December 2018; Received in revised form 14 May 2019; Accepted 30 May 2019 Available online 07 June 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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the sensing wavelength further. As shown in Fig. 2, in the gas cell, one end of HCPCF (HC-1550-02 produced by NKT Photonics Inc.) with the length of 1 m is connected with F-P filter via bare fiber adapter and single mode fiber (SMF), while the other end is connected with DWDM via bare fiber adapter and SMF placed outside of the gas cell. The inflation and exhaust of detected gas can be realized rapidly through two values on the top of the aluminum cylindrical container by the pressure difference between two ends of the HCPCF. The acetylene gases we used are all custom-made with standard concentrations. For exhaust, the gas cell is firstly filled with pure nitrogen and sealed till there is no absorption at the absorption line of acetylene. The absorption losses of different acetylene concentrations are marked when the maximum value of absorption loss is reached and then saturated with time going on. The sensitivity can be improved significantly by inserting the gas cell into the EDFRL cavity due to the multi-pass increased interaction length between the laser and detected gas. The common port of the DWDM is spliced to a 10: 90 coupler where 10% port is used to extract the laser to the OSA and the rest 90% is fed back to the cavity. In Fig. 1, the condition to realize stable oscillation of dual wavelengths can be expressed as [21,26]:
coefficient of absorber and L is the optical path length of the absorber which decided by the HCPCF length in this paper. From Eq. (1) we can know:
κ (v ) L = ln
I0 (v ) I (v )
(2)
For a small absorption signal, ΔI ≪ I0(v),
κ (v ) L =
ΔI I0 (v )
(3)
Then the reduction of the output power ΔI can be expressed as:
ΔI = I0 (v ) κ (v ) L
(4)
Eq. (4) demonstrates that the reduction of the output power ΔI caused by the intra-cavity gas absorption is proportion to the optical path length of the absorber decided by the HCPCF length. Dual-wavelength or multi-wavelength EDFLs have attracted considerable interest for their potential applications in optical fiber sensing regions due to the advantages like simple structure, multiple narrow bandwidth, good compatibility with other components, etc [16–18]. To achieve stable multi-wavelength lasing output, several approaches have been used such as polarization hole burning effect [16], deeply saturated spectral hole-burning effect [17], frequency-shifted feedback technique [19], four-wave mixing effect [20] and so on. Compared with single-wavelength fiber laser based sensor, dual-wavelength EDFL based sensor with good stability of power fluctuation less than 0.1 dB [16–18] can provide extremely high sensitivity and realize multiparameter measurement, opens new ways for the fiber sensing applications, especially for the low concentration gas detection. Many kinds of sensing schemes have been reported and optimized to measure refractive index [21], liquid-level [22], gas concentration [23], temperature and strain [24], etc. But to our best knowledge, dual-wavelength EDFL used in the intra-cavity gas absorption sensing regions is rare. In this paper, we design and demonstrate a highly sensitive low concentration gas sensor based on dual-wavelength Erbium-doped fiber ring laser (EDFRL). One wavelength selected by a tunable Fabry-Perot (F-P) filter at 1530.37 nm is served as the sensing element by inserting a HCPCF gas cell while the other wavelength selected by a fiber Bragg grating (FBG) at 1532.38 nm acted as a reference. Due to the mode competition phenomenon, the variation of absorption loss caused by the acetylene concentration can bring in much more power change in the sensing laser. WST is applied to ensure high accuracy. The minimum detection limit (MDL) of 10.42 ppmV at 20 s response time is obtained by experiment with good linear response (R2 = 0.9864), improving 6.44 times than that of single-wavelength EDFRL based. Moreover, the absolute detected error induced by the power fluctuation (< 0.1 dB) is less than 1.78% over more than 300 s of observation.
g λi × L = Gth (λi ) = δ λi, i = 1, 2
(5)
λ1 and λ2 represent the sensing wavelength and the reference wavelength, respectively. gλi is the gain coefficient. L is the EDF length. Gth (λi) is the single-path threshold gain and δλi is the single-path loss. When L =2 m, Gth (λ1) = Gth (λ2) ≈ 6 × 2 ≈ 12 dB. By adjusting the VOA, δF–P filter + δgas cell = δcircular + δVOA + δFBG can be satisfied to realize δλ1 = δλ2. When δλ1 ≈ δλ2 = Gth (λ1) = Gth (λ2) is satisfied, the stable oscillation of dual wavelengths EDFRL can be realized. The output power of dual-wavelength oscillations for EDFRL is investigated numerically before experiment. The behavior of the dualwavelength EDFRL is described by two-level pumping system [27,28] and the propagation equations are as follows:
dPs± (λi ) = +Γs (σes N2 − σas N1) Ps± − αs Ps± (λi ), i = 1, 2 dz
dPp± dz
(6)
= +Γp (σep N2 − σap N1) Pp± − αp Pp±
(7) 2
± dPase ± = +2hγs⋅Δγ⋅σes Γs N2 + Γs (σes N2 − σas N1) Pase − αs ∑ Ps± (λi ) dz i=1
(8)
0 = (R12 + W12) N1 − (R21 + W21 + A21 ) N2
(9) (10)
Nt = N1 + N2
The light of backward is ignored in Eq. (6)–(10) as the undesired reflection is suppressed by ISO. The transition rates of R12, R21, W12, W21 and A21 are defined as:
2. Experimental setup and principle
R12 = The schematic configuration of dual-wavelength EDFRL gas sensor is shown in Fig. 1. A 2 m long Erbium-doped fiber (EDF, Nufern EDFC980-HP, gain coefficient is 6 ± 1 dB/m) is pumped by 976 nm laser diode via a filter wavelength division multiplexing (FWDM). The isolator (ISO) is used to guarantee the unidirectional operation of EDFRL and prevent spatial hole-burning [25]. The sensing wavelength is selected by a tunable F-P filter controlled by NI-PXI platform spliced to one HCPCF gas cell. The reference wavelength is selected by a fiber Bragg grating (FBG) (center wavelength @1532.38 nm) spliced to a variable optical attenuator (VOA) and circulator. The reference cavity loss is precisely controlled by the VOA to achieve stable oscillation of dual-wavelength lasing output. Two laser cavities are connected with a 50: 50 coupler which can realize similar output power and a dense wavelength division multiplexing (DWDM) filter (the spacing of 200 GHz at 1530.33 nm) is worked as the narrow band filter to identify
R21 =
σap Γp hγp Aeff σep Γp hγp Aeff
Pp
Pp
(11)
(12)
W12 =
σas Γs + (Pp + Pase ) hγs Aeff
(13)
W21 =
σes Γs + (Pp + Pase ) hγs Aeff
(14)
A21 =
1 τ21
(15)
Where Aeff is the effective core area of the erbium-doped optical fiber, Γs and ΓP are the overlap of the optical modes with the erbium distribution. τ21 is the fluorescence lifetime of the metastable level of the 2
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Fig. 1. The schematic of the dual-wavelength EDFRL gas sensor configuration.
Fig. 2. The schematic of HCPCF gas cell configuration for acetylene filling.
two-level system. The differential equations of first-order listed above are calculated by finite-difference method using MATLAB software and the detailed process refer to Ref. [27]. The EDF with the optimized length of 2 m is divided into finite segments averagely then calculated by steady-state solutions at different wavelengths. When the pump light is absorbed by EDF, the amplified spontaneous emission (ASE) is firstly generated. The sensing and reference wavelengths can be selected by the F-P filter and FBG by balancing the cavity losses added to two cavities. Part of the light is extracted as the output while the others are fed back to the EDF as input signal. In simulation, the sensing wavelength is set at λ1 = 1530.37 nm where the acetylene exhibits highest absorption peak according to the Hitran database [29] as shown in Fig. 3. The reference wavelength is set at λ2 = 1532.38 nm with initial cavity loss of 0.75 dB higher than the cavity loss of the sensing laser. In Fig. 4, we can see that the EDFRL exhibits single-wavelength at λ1 when the sensing cavity loss is lower than that of the reference. With the increment of the sensing laser cavity loss which can be regarded as the acetylene concentration increasing in experiment, the output power of the sensing laser damps gradually while the reference increases. Dual-wavelength lasers with equal output powers appear when two cavity losses are balanced. However, when the sensing cavity loss is too high, the sensing laser will disappear quickly and only leave the reference laser at λ2. The sharply power decrement of the sensing laser away from the stable dual-wavelength lasing point, which caused by the variation of gas concentration due to the mode competition, can improve the sensitivity significantly compared with single-wavelength mode. When the central wavelengths of dual-wavelength lasers far from each other (at 1528.01 nm and 1532.38 nm), the output powers are reduced due to the relative weaker mode competition which is in consisted with Ref. [30]. Considering the sensing laser is extremely sensitive to the cavity loss
Fig. 3. Absorption spectrum of acetylene from 1528 nm to 1539 nm.
caused by the different gas concentrations, a low power of dual-wavelength lasers will reduce the gas detection range. However, when the central wavelengths are too close, the large power fluctuation will break the balance of dual-wavelength lasers [17]. 3. Results and discussions WST is used in experiment to ensure high accuracy and the spectra of single-wavelength and dual-wavelength EDFRL gas sensors filled with acetylene with concentration of 1000 ppmV are shown in Fig. 5. The F-P filter we used to sweep the sensing wavelength linearly has a 3
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Fig. 6. Output spectra of dual-wavelength EDFRL gas sensor filled with acetylene with concentrations of 0, 100 ppmV, 500 ppmV, 1000 ppmV and 1500 ppmV.
Fig. 4. Simulated output powers of dual-wavelength lasers with central wavelengths 2.01 nm and 4.37 nm apart when sensing cavity loss increases from 0 to 1.5 dB.
more sharply away from the stable dual-wavelength lasing point compared with the increment of reference. The power changes can be explained by the mode competition mechanism caused by the variation of sensing cavity loss. The increment of absorption loss with high acetylene concentration will lead to a mode contention by the reference laser. Fig. 7 shows the absorption losses of dual-wavelength EDFRL gas sensor when the acetylene concentration c increases from 0 to 1500 ppmV and then reduced to 0. The power variations at λ1 and λ2 with different acetylene concentrations are nearly the same at c increasing and decreasing processes, indicating good repeatability of the designed sensor. The acetylene absorptions of the sensing and reference lasers all exhibit good linear fitting along with the concentration varying which can be expressed as P1 = 0.0096c + 0.0154 for λ1 and P2 = 0.0044c + 0.0233 for λ2, with R2 = 0.9864 and 0.9820, respectively. Compared with the simulation results in Fig. 4, the experimental datas we obtained at λ1 exhibit the same trend but lower slope along with the acetylene concentration increasing as shown in Fig. 8. The reasons are mainly caused by the differences of the cavity loss at λ1 between simulation and experiment or the power fluctuations of the EDFRL. The MDL used to evaluate the sensing performance in similar works is defined as [14]:
Fig. 5. Output spectra of single-wavelength and dual-wavelength EDFRL gas sensors filled with 1000 ppmV acetylene using WST.
free spectral range of 108 nm and fineness of 10,090 controlled by the NI PXIe-4319 system source measure unit, with the resolution of 100 nV and sampling rate of 1.8MS/s. When the drive voltage applied to the FP filter increases from 1.28 V to 1.313 V linearly, at the scanning waveband around 1530 nm, the absorption dip in spectrum (blue solid line) we observed is in consistent with that of acetylene in Fig. 3. The absorption line of acetylene at 1530.37 nm can be recognized automatically when the selected voltage range applied to the F-P filter and the whole sweeping time is about 8 s. When acetylene with the concentration of 1000 ppmV is filled into the gas cell, the output powers of dual-wavelength lasers have a decrement of 5.6 dB at the sensing wavelength and an increment of 2.17 dB at the reference wavelength, about 6.44 times higher than that of the single-wavelength mode of 0.87 dB. The sensing performance of the gas sensor is investigated by filling acetylene with different concentrations from 0 to 1500 ppmV in experiment as shown in Fig. 6. With the acetylene concentration increasing, the output power of the sensing laser at λ1 = 1530.37 nm decreases from -18.43 dB to -32.76 dB while that of the reference at λ2 = 1532.38 nm increases from -24.62 dB to -17.89 dB. Moreover, the power change of 14.33 dB at λ1 is much larger than that of 6.73 dB at λ2. The results we obtained in experiment are in consistent with the simulations in Fig. 4, which show the power decrement of sensing laser
Fig. 7. Absorption losses of dual-wavelength EDFRL gas sensor as a function of the acetylene concentration. 4
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Table 1 Performances of Intra-cavity gas sensors. Characteristics
MDL(ppmV)
Ref.
Gas cell made of a pair of collimated graded index WST technique WMT technique WST and WMT Dual-point automatic switching with 1 meter HCPCF Multiplexed sensor network with 1 meter HCPCF Single-wavelength EDFRL with 8 meters HCPCF Ring-down cavity Mode-locked fiber laser Our work with 1 meter HCPCF
2253 1000 200 75 398 100 5.4 1000 781 10.42
[4] [6] [7] [8] [10] [11] [14] [31] [32] N/A
Fig. 8. Comparison between the simulated and the experimental results.
MDL =
fluctuation × concentration absorption
(16)
According to Eq. (16), the power stability of dual-wavelength EDFRL sensing system has great influence on MDL. The output power and wavelength stabilities are measured over 300 s by fixing the operating wavelength at λ1 and λ2 as shown in Fig. 9. The fluctuations of the peak intensities are measured to be less than 0.1 dB, corresponding to the MDL of 10.42 ppmV. Compared with other similar works listed in Table 1, the MDL of dual-wavelength EDFRL gas sensor is improved about 7–216 times. However, due to the limitation of HCPCF length, the MDL we obtained is lower than that in Ref. [14] (8 m long HCPCF is used by Zhao et al. and in this work the HCPCF is only 1 m). According to Eq. (4) and Eq. (16), the MDL is inversely proportional to the HCPCF length L, which means a larger L leading to a smaller MDL, thus a higher sensitivity. So in the following works, the sensitivity can be improved further by increasing the HCPCF length with the same stability of EDFRL, or improving the stability of EDFRL with the same HCPCF length. The detection efficiency is not only related to the sweeping time of F-P filter but also with the filling time of the gas cell. As mentioned above, the whole sweeping time is about 8 s when selected voltage range from 1.28 V to 1.313 V applied to the F-P filter to recognize the absorption line of acetylene at 1530.37 nm exactly. To ensure high accuracy, pure nitrogen is full filled into the gas cell before acetylene filling until there is no absorption line in spectrum for exhaust. As shown in Fig. 10, when the filling pressure is 0.3 MPa, the maximum
Fig. 10. Filling time of gas cell with 1 m long HCPCF under 0.3 MPa pressure.
value of absorption loss reached at λ1 using about 20 s and then saturated with time going on. Therefore, the fast response time of dualwavelength EDFRL gas sensors is only 20 s with 1 m long HCPCF due to the pressure difference between the gas cell and outside environment. By increasing the filling pressure, the detection efficiency can be improved further as the larger the pressure, the shorter the filling time. 4. Conclusion In this paper, we demonstrated a highly sensitive low concentration gas sensor based on dual-wavelength EDFRL both theoretically and experimentally. The relationship between cavity losses and output powers are calculated for the sensing laser at 1530.37 nm and the reference at 1532.38 nm. Due to the mode competition phenomenon, the sensing laser power shows extremely sensitive to the variations of acetylene concentration in experiment which can achieve a good linear response (R2 = 0.9864) and a MDL of 10.42 ppmV at 20 s response time. Compared with other similar works, the MDL we obtained is the lowest to our best knowledge and improving 6.44 times than that of single-wavelength EDFRL. The gas sensor exhibits good stability with the absolute detected error induced by the power fluctuation (< 0.1 dB) less than 1.78% over more than 300 s of observation. Moreover, the sensitivity can be improved further by increasing the HCPCF length or improving the stability of EDFRL. Acknowledgments This work was supported by the National Basic Research Program of China (973 Program) (Grant number: 2010CB327801) and Scientific Research Project of Tianjin Municipal Education Commission (Grant No. 2018KJ136).
Fig. 9. The output power and wavelength stabilities of dual-wavelength EDFRL gas sensor. 5
Sensors & Actuators: B. Chemical 296 (2019) 126637
X. Yang, et al.
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Xianchao Yang received the Bachelor’s degree in Electronic and Information Engineering from the Tianjin Polytechnic University, Tianjin, China, in 2014. He is currently working toward the pH.D. degree at the Institute of Laser and Optoelectronics, Tianjin University. Liangcheng Duan received his B. S. degree in Optical Information Science and Technology in 2011 from Tianjin University of Technology, Tianjin, China. He is currently working towards the Ph. D degree in opto-electronic science and technology in Tianjin University. His current research focuses on optical fiber sensors. Haiwei Zhang received his Ph. D degree in Opto-electronic Science and Technology in Tianjin University. He is currently working in Optical Information Science and Technology in Tianjin University of Technology, Tianjin, China. His current research interests include fiber lasers and amplifiers, and optical fiber sensors. Ying Lu is a professor of the Key Laboratory of Opto-electronics Information Technology in Tianjin University. She received her PhD degree from Nankai University, China, in 2000. Her current research focuses on photonics microstructure, fiber sensing and nonlinear optics. Guangyao Wang received the Master’s degree from Yanshan University, Qinhuangdao, China, in 2017. Now he is working toward the pH.D. degree in Tianjin University, Tianjin, China. Jianquan Yao is currently a Professor and the Director of the Institute of Laser and Optoelectronics of Tianjin University. From 1980–1982, he joined the Department of Applied Physics, Stanford University, USA, as a Visiting Scholar. He has long been engaged in the research of all-solid laser, nonlinear optics frequency conversion and Terahertz science & technology. His theory of precise calculation of optimum phase matching of biaxial crystal has been called the “Yao and Fahlen Technology”. He received the World Gold Medal of Invention, Eureka, Brussels, and National Invention Award Class II of China. In 1997, he was elected as Academician of the Chinese Academy of Science.
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