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liquid detection system based on intra-cavity absorption spectroscopy and tilted fiber Bragg grating
Optical Fiber Technology 53 (2019) 102068
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Gas/liquid detection system based on intra-cavity absorption spectroscopy and tilted fiber Bragg grating
T
⁎
Kun Liua,b,c, , Yuanhao Zhaoa,b,c, Junfeng Jianga,b,c, Tianhua Xua,b,c, Pengxiang Changa,b,c, Zhao Zhanga,b,c, Jinying Maa,b,c, Yu Lianga,b,c, Tiegen Liua,b,c a
School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China Key Laboratory of Opto-electronics Information Technology, Ministry of Education, Tianjin 300072, China c Tianjin Optical Fiber Sensing Engineering Center, Institute of Optical Fiber Sensing of Tianjin University, Tianjin 300072, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fiber ring laser Intra-cavity absorption Gas/liquid detection Tilted fiber Bragg grating (TFBG)
A gas/liquid detection system is developed based on the intra-cavity absorption spectroscopy (ICAS) and a tilted fiber Bragg grating (TFBG). Multiple gases such as acetylene, carbon monoxide and ammonia gas with absorption lines within C-band can be detected using the proposed scheme. The wavelength sweeping range can be controlled to obtain a specific number of absorption lines to meet the requirements on different measurement accuracy. In this work, a TFBG structure is introduced into the ICAS technique to realize the sensing of liquid refractive index. The TFBG works around the wavelength of 1550 nm, which allows a long-distance signal transmission and a flexible system adjustment. The proposed system has been applied to detect the concentration of acetylene with an absolute error of less than 46 ppm. In addition, a set of glycerol-water solutions with refractive indices from 1.3959 to 1.4430 have been measured using the proposed scheme with a sensitivity of 528.2 nm/RIU.
1. Introduction The detection of the concentration and the composition of gases and liquids is always significant in the areas of energy exploitation and environmental monitoring [1,2]. In some cases, gases or liquids are flammable, explosive and corrosive, which are difficult to detect using conventional electronic sensors. Therefore, all-fiber sensors are widely employed in various gases and liquids detection due to their advantages such as the anti-electromagnetic interference, the corrosion resistance, the high sensitivity and safety in severe environments [3,4]. Fiber laser intra-cavity absorption spectroscopy (ICAS) is a popular gas detection technique, where the gas cell is placed in a fiber ring cavity and the laser beam passes through the gas cell multiple times during its oscillation process in the cavity. Consequently, the effective absorption length will be increased, and this will lead to a significant improvement of the sensing sensitivity [5,6]. In 2011, Liu et al. introduced the wavelength modulation technique and the wavelength sweeping technique into ICAS, and developed a system for the detection of gas concentration and absorption wavelength [7]. In 2016, Yu et al. utilized the C + L band erbium-doped fiber amplifier (EDFA) to achieve the detection of multiple gases including acetylene, carbon monoxide and carbon dioxide [8]. ⁎
In all these reported works, ICAS was developed for the gas detection only. If a liquid sensing is required, one more separate detection system has to be built, which greatly increases the cost. However, in the energy exploitation, environmental monitoring and biosensing industries, there are frequent demands on detecting both gases and liquids simultaneously. In order to meet this request, we developed a new sensing scheme by introducing a tilted fiber Bragg grating (TFBG) into the ICAS technique to achieve both gas and liquid sensing. The FBG, as a passive optical device, has many advantages such as the compact size and high stability, and can be used for the sensing of various physical quantities. Changes in the external environments such as the strain, temperature, and refractive index will affect the transmission spectrum of the fiber grating. With these features, the measurement of external environment parameters can be realized by detecting corresponding changes in the transmission spectra. The structure of the TFBG is different from that of the conventional FBG. The index modulation pattern of the TFBG is tilted against the fiber axis by a specific angle. There is only one core mode transmission peak existing in the FBG, while the TFBG transmission spectrum includes a series of cladding mode resonance peaks [9]. The cladding modes can be used to measure the refractive index of liquids. In the past few years, the TFBG has attracted widespread attention [9–11]. In 2007, C.-F. Chan et al. developed a
Corresponding author at: School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China. E-mail address: [email protected] (K. Liu).
https://doi.org/10.1016/j.yofte.2019.102068 Received 18 July 2019; Received in revised form 27 September 2019; Accepted 30 October 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 53 (2019) 102068
K. Liu, et al.
convert the short chamber length into the long effective absorption length, and thereby the sensitivity of the gas sensing can be largely improved. The gas absorption follows Lambert-Beer law:
refractometer based on the narrowband cladding-mode resonance shifts from the short-period fiber Bragg gratings with weakly tilted grating planes [12]. In 2009, Y.-P. Miao et al. proposed a scheme of the refractive index sensor through measuring the transmission power of the tilted fiber Bragg grating [13]. In 2016, B. Jiang et al. employed a tilted fiber Bragg grating (TFBG) deposited with carbon nanotubes to measure the refractive index and temperature simultaneously [14]. However, all these reported TFBG work focused on the measurement of the liquid refractive index only. In this paper, a gas/liquid sensor based on fiber laser ICAS and a TFBG is developed and presented. The TFBG is operated around the wavelength of 1550 nm, which is in the same range as the gas absorption wavelength. Therefore, the combination of the TFBG and the ICAS technique can be implemented to achieve a simultaneous detection of gas and liquid, and the sensing signal can be transmitted over long distances. In addition, the tunable optical filter and the photodetector are used to replace the optical spectrum analyzer to obtain the transmission spectrum of the TFBG. The wavelength positioning error is less than 52 pm. Similar wavelength accuracy can be achieved with lower cost compared to methods using a broadband source and a spectrum analyzer. Furthermore, acetylene of different concentrations and a set of solutions with various refractive indices are detected to demonstrate the performance of the developed sensor system.
I (ν ) = I0 (ν ) exp[−α (ν ) CL]
where I denotes the intensity of transmitted light after the gas absorption, I0 represents the intensity of the incident light, ν is the light frequency, α represents the absorption cross section, C represents the gas concentration, and L represents the effective absorption length. Then the absorbance parameter K can be described as
K (ν ) = ln
The measurement of the liquid refractive index is realized based on the TFBG. TFBGs are short-period fiber gratings and have all the characteristics of FBGs. They also have other advantages because of their specific structures. The index modulation pattern of the TFBG is tilted against the fiber axis by a specific angle, and this enhances the coupling of the light from the forward-propagated core mode to the backward-propagated cladding modes and reduces the coupling to the backward core mode. Therefore, besides the core mode resonance peak, there are several cladding modes in the transmission spectrum of the TFBG. Compared to the core mode, the cladding modes of the TFBG is in contact with the outer surroundings due to the light transmission in the cladding. Therefore, the effective refractive indices of the cladding modes are affected by the external environment, hence the TFBG can be used for detecting the refractive index of the liquid. The Bragg reflection wavelength λBragg and the cladding-mode resonance wavelength λ coupling, i of the TFBG are described by the following equations:
2.1. Output power of the erbium-doped fiber ring laser The fiber ring laser uses erbium-doped fiber (EDF) as the gain medium and a 980 nm laser diode as the pumping source, and the output power of the fiber ring laser can be derived by solving the rate equations. When the spontaneous emission is neglected, the output power I of the laser is proportional to the photon number q in the intracavity and can be expressed as [15]:
λBragg =
2neff , core Λ
λ coupling, i =
(5)
cos θ (neff , core + neff , clad, i )Λ cos θ
(6)
where neff , core and neff , clad, i are the effective refractive indices of the core mode and the i th cladding mode, respectively. Λ represents the nominal period, and its relationship with the actual grating period Λ g along the axis of the fiber is expressed as Λ = Λ g cosθ , where θ refers to the tilt angle. As the surrounding refractive index (SRI) increases, some of the high-order cladding modes will leak out, but the low-order cladding modes can still be transmitted. The cladding mode between the leaky modes and the guided modes is called the cut-off mode. When the SRI changes, the cut-off mode position changes accordingly. The refractive index of the liquid can be obtained by tracking the cut-off mode position correspondingly.
(1)
where we have Sτ l (W τ σ − σ ) Nc
c a p 2 e a ⎧ ⎪A = τ2 (σe + σa) ⎨ B = Sτc (1 + Wp τ2) ⎪ ηs τ2 (σe + σa) ⎩
(4)
2.3. Liquid refractive index measurement
The basic principle of this proposed sensing system is built on the intra-cavity absorption spectroscopy technique. The gas sensing is implemented based on the measurement of the specific spectral extinction of the light transmitted through the gas to be detected. The liquid sensing is realized through the effect that the position of the cut-off cladding mode varies with the refractive index of the liquid. The gas cell and TFBG share a same system that uses an optical switch to control the selection between the gas sensing and the liquid sensing functions. A computer program is designed to alternate the status of the optical switch automatically during the measurement process to achieve the simultaneous gas and liquid sensing.
A − Bδ δ
I0 (ν ) = α (ν ) CL I (ν )
It is seen from Eq. (4) that the absorbance K is proportional to the gas concentration, so this characteristic can be applied to achieve the measurement of the gas concentration. A set of gases of which the concentrations are known have been used to calibrate the relationship between gas concentration and the absorbance, and then the concentration of the test gas can be derived accordingly based on Eq. (4) and the calibration results.
2. Principle
I∝q=
(3)
(2)
A and B are the constants determined by physical parameters of the sensing system. δ represents the total loss of the cavity, S represents the fiber core area, τc is the cavity round time, la is the length of the EDF, Wp denotes the pump probability, τ2 denotes the life time of the stable level, σa and σe are the absorption and emission cross-section of the mode, respectively, Nc is the dopant concentration of the EDF, and ηs is the proportion of the signal power within the fiber core.
3. System configuration The gas and liquid sensing system based on the ICAS and TFBG is illustrated in Fig. 1. The solid line represents the optical signal path and the dotted line represents the electrical signal path. This sensing system including the following components: an EDFA, an isolator, two fiber couplers, an optical circulator, an electronic variable optical attenuator (EVOA), a tunable optical filter, an etalon, a gas cell, a TFBG, a Faraday rotator mirror (FRM), two InGaAs PIN photodetectors (PDs), two
2.2. Gas absorption The ICAS technique allows the light to oscillate in the cavity to 2
Optical Fiber Technology 53 (2019) 102068
K. Liu, et al.
Fig. 1. Schematic diagram of the gas/liquid detection system.
Time ≤ 10 ms) are used to switch the optical path. The DAQ card(NI, USB-6251) is used to input and output signals. And the computer is used to process signals. After passing through the EVOA, the laser beam enters the tunable filter for the frequency selection. Since the spectral absorption line of the gas is narrow and the shift of the cut-off mode position is small, a tunable filter with a fineness of 3829 and a free spectral range (FSR) of 107 nm is employed. The wavelength sweeping is realized by adjusting the driving voltage of the tunable filter through the computer. The light enters the circulator and the fiber Coupler 1 and it is further split into two parts. When the gas detection part is used, 99% of the light enters the gas cell and is reflected back by the FRM and passes through the Coupler 2 and the circulator again and then enters the Coupler 1. When the liquid detection part is used, 99% of the light passes through the TFBG and enters the coupler 1. The other 1% of the light enters the etalon, and then the signal is sent to the computer via PD 1 and the DAQ card. The etalon is used to realize a real-time calibration of the tunable filter to reduce the wavelength positioning error introduced due to the nonlinearity of the tunable filter. Coupler 1 splits the laser beam into two parts with the ratios of 90% and 10%, respectively. 10% of the laser beam will enter the computer via PD 1 and DAQ card to acquire the gas absorption spectra. The other 90% of the laser beam continues to oscillate and to be amplified in the cavity. The DAQ card is used for the data acquisition and device control. It is managed by the computer to acquire the output voltages of the PDs and to provide a driving signal for the tunable filter and the EVOA. The sensing system is divided into two parts: the gas sensing and the liquid sensing, and the function is controlled by two optical switches. A 12 cm long steel pipe is employed as the gas cell, combined with two flanges through the interference fit. Two calibrated pigtailed fiber collimators are mounted in the center of the flanges to achieve the input and output ports for the light. A Faraday rotator mirror is placed at the rear end of the gas cell to return the light to increase the effective absorption length. The liquid sensing component is implemented using a TFBG with a tilt angle of 8°.
optical switches and a data acquisition (DAQ) card controlled by a computer. The C-band EDFA (Hoyatek, HY-EDFA-C-21) is used for the light amplification. It is pumped by a 980 nm power-tunable laser diode with pump powers of up to 400 mW. And the saturated output power is 21 dBm. The operating wavelength is 1525–1570 nm. The EVOA (Sercalo, VXP1) is used to adjust the laser gain to obtain a higher sensitivity and protect the tunable filter. The attenuation is controlled by the input voltage. The EVOA has a maximum attenuation of 30 dB and a response time of 20 ms. The tunable optical filter (Micron Optics, FFP-TF2) is the core component of the system. It is used to select the output wavelength of the fiber ring laser to achieve wavelength sweep. The fineness of the tunable filter is 3829 and the free spectral range (FSR) is 107 nm. The etalon (Primanex, 100GHZMag) is used to provide reference wavelengths for wavelength calibration because of the nonlinearity of the tunable filter. The FSR is 100 GHz and the 3 dB bandwidth is 0.06 nm. The isolator (Shconnet, Isolation ≥ 46 dB) is used to ensure the oneway transmission of light to prevent the laser from entering the EDFA backward and causing the damage to the device. The couplers (Shconnet, Type: Fused biconical taper (FBT), 2 * 2) are used to split a beam of light into two beams. The circulator (Shconet, Insertion loss ≤ 1.2 dB, Isolation ≥ 38 dB, Return loss ≥ 50 dB) is used to connect the gas cell and the TFBG to the sensing system. The coupling ratio of coupler 1 is 10:90 and insertion loss is 10.8/0.6 dB. The coupling ratio of coupler 2 is 1:99 and insertion loss is 21.5/0.2 dB. The specific coupling ratios are selected to balance the output spectral range, output power, and the SNR of the fiber ring laser. For Coupler1, 90% of the light is used to continue the oscillation in the intra-cavity to be amplified to form a stable laser output, and 10% of the light is used to output to PD1 to detect the optical power. Optical power detection does not require much light, and 10% can meet the demand, so most of the light continues to oscillate in the intra-cavity. For Coupler2, 99% of the light is used for gas and liquid sensing, and 1% of the light is used for wavelength calibration. Wavelength calibration also does not require much optical power, while gas and liquid sensing are based on spectral absorption, so most light is used for sensor absorption in order to improve the signal-to-noise ratio for detection. The gas cell made with metal tube and two fiber collimators acts as a place where light interacts with the gas. The TFBG is used to achieve liquid refractive index sensing. The FRM (Shconnet, Return loss ≥ 50 dB, Insertion loss ≤ 0.5 dB) is used to reflect the light back. PDs (Qnoptics, LSIPDA75) are used to convert the optical signal into the electrical signal. The spectral range is 800–1700 nm. The responsivity at 1550 nm is 0.9 mA/ mW and response time is 140 ps. Optical switches (Sercalo, SXLA2x2SMF, Insertion loss ≤ 0.4 dB, Return Loss ≥ 50 dB, Switching
4. Experiment and discussion 4.1. Gas concentration calibration and detection The wavelength tuning range of the EDFA used in the system is from 1525 nm to 1565 nm. Multiple types of gases can be detected using this system, such as acetylene, carbon monoxide and other gases with absorption lines within this range. Gas absorption data can be obtained from the HITRAN database [16]. The concentration of acetylene was 3
Optical Fiber Technology 53 (2019) 102068
K. Liu, et al.
Fig. 3. Concentration calibration results of three absorption lines. Fig. 2. Absorption spectra of acetylene at normal temperature and pressure. Table 1 Results of concentration detection.
measured in our experiments. The single-trip scanning time of the system becomes longer as the wavelength range increases. Therefore, it is necessary to ensure that the wavelength sweeping range can be as small as possible and there are enough strong absorption lines at the meantime. For this reason, the wavelength sweeping range is set from 1528 nm to 1535 nm, where the acetylene has several strong absorption lines and the output intensity of the light source is relatively stable. A mixture of the acetylene and the nitrogen (0.3% acetylene, 99.7% nitrogen) was filled into the gas cell to obtain the absorption spectra at normal temperatures and pressures. Corresponding results are shown in Fig. 2. The gas cell is 12 cm in length. It is expected that a number of strong absorption lines can be obtained within the spectral range. The gas concentration detection can be achieved using a single absorption line, but the use of multiple absorption lines can reduce the measurement error. Therefore, we selected three of them for the acetylene detection, and have averaged the three sets of data to reduce the analysis error. Three strong absorption lines centered at 1529.18 nm, 1530.37 nm, and 1531.59 nm were selected for the concentration calibration. Acetylene and nitrogen were mixed using a mass flow controller. Acetylene with concentrations of 1000, 2000, 3000, 4000 and 5000 ppm were prepared for the concentration calibration and another group with concentrations of 1500, 2500, 3500, 4500 ppm were prepared for the sensing test. We measured the acetylene with different concentrations in the first group and the results are shown in Fig. 3. It is noted that the absorbance is slightly different from the HITTRAN database since the effective absorption length is different. When the absorption line is closer to the laser threshold, the effective absorption length will become longer. The experimental data are further polynomial fitted and the slopes of three absorption lines are 2.41/%, 2.17/ % and 1.96/%, respectively. Absorption line 1 has the largest slope, indicating that it has the highest measurement sensitivity. And the R2 of the three lines are 0.9996, 0.9993 and 0.9995 respectively. Different concentrations of acetylene were measured based on the calibration results above. The tested acetylene has the concentration varying from 1500 ppm to 4500 ppm with interval of 1000 ppm. The detection results are described in Table 1. It can be seen that the maximum error is 53 ppm when the gas measurement is performed with a single absorption line. The maximum error is reduced to 46 ppm and the relative error becomes less than 1.3% using the average measurement results of three absorption lines. More absorption lines can be employed for improving the detection if there is a higher requirement for the measurement accuracy.
Practical Concentration (ppm) Detected concentration (ppm)
Absolute error (ppm) Relative error (%)
Line1 Line2 Line3 Mean
1500
2500
3500
4500
1496 1481 1504 1494 6 0.40
2505 2536 2537 2526 26 1.04
3544 3545 3550 3546 46 1.3
4520 4486 4447 4484 16 0.35
4.2. Wavelength calibration Before the measurement, it is necessary to calibrate the wavelengthvoltage relationship of the tunable filter to improve the accuracy of wavelength positioning. An F-P etalon is used as the wavelength reference to solve this problem introduced by the nonlinearity of the tunable filter. The distance between two adjacent transmission peaks in the etalon is 0.8 nm. The etalon is blocked at a wavelength of 1533.504 nm and has no transmission there. The central wavelength of the transmission peak around the blocked position can be derived accordingly. The transmission spectrum of the etalon is shown in Fig. 4(a). The blocked wavelength is marked with a black star. It can be expected that the transmission wavelength has a one-to-one correspondence with the driving voltage during each period of sweeping. Therefore, the relationship between the wavelength and the driving voltage can be calculated in each sweeping period. The wavelength of each peak of the etalon is known, and the corresponding driving voltage is recorded for each sweep period, and then the relationship between wavelength and the driving voltage is obtained by second-order polynomial fitting. During each sweep, the program sends a sawtooth waveform to the tunable filter as the drive voltage signal. Then on the falling edge of the sawtooth waveform, the driving voltage of the tunable filter and the output of the PD change synchronously with time and these data are recorded by the program. Wavelength-demodulation is then achieved using the fitting curve of the drive voltage and wavelength of the tunable filter. One of the fitting curves is shown in the Fig. 4(b). To test the accuracy of wavelength calibration with the tunable filter and the etalon, an FBG(Technica, single FBG) is used as a wavelength reference. The center wavelength of the FBG is 1546.76 nm and the FWHM is less than 0.3 nm. The FBG is connected to the system shown in Fig. 1 and its center wavelength is measured in each sweep. The transmission spectrum of the FBG is shown in Fig. 5(a). Five hundred sets of experiments were performed and the results are shown in 4
Optical Fiber Technology 53 (2019) 102068
K. Liu, et al.
Fig. 4. (a) Transmission spectrum of the etalon. (b) One of the fitting results of the transmission wavelength of tunable filter and the driving voltage of the tunable filter.
Fig. 5. The maximum error is 52 pm and the minimum error is 16 pm, and the standard deviation is 0.0141. The wavelength positioning results is smaller than the nominal center wavelength of the FBG as a whole, that is because the etalon is affected by external temperature and other factors, and the actual transmission wavelength is different from the data provided by the manufacturer.
4.3. Liquid refractive index sensing In the liquid sensing, a TFBG with 8° tilt angle is used to measure the refractive index of the liquid. The center wavelength of the TFBG is around 1550 nm. This makes it easy be combined with the fiber ring laser system, and the signal can be transmitted over long distances. A set of refractive index solutions with refractive indices from 1.3959 to 1.4430 are measured at the temperature of 25 °C. The solutions used in the experiment are composed of glycerin and deionized water at different concentrations and were calibrated with an Abbe refractometer. In order to prevent the TFBG from being damaged and to keep the strain stable during the experiments, the grating region is taped to a microscope slide. When the refractive index of the liquid is measured, the solution is dropped into the grating region with a plastic dropper. After each measurement, the grating region is fully cleaned with absolute ethanol. The evolution of the TFBG spectrum with the SRI is illustrated in Fig. 6, and the positions of the cut-off modes are marked with black stars. Fig. 7 demonstrates the relation between cut-off mode
Fig. 6. Evolution of the TFBG spectrum with different SRI. (The different curves are shifted vertically for clarity.)
wavelength and SRI. The polynomial fitting of the experimental results is carried out and the R2 is 0.9996. The sensitivity of the TFBG is 528.2 nm/RIU for the refractive indices ranging from 1.3959 to 1.4430.
Fig. 5. (a) Transmission spectrum of the FBG. (b) Results of the wavelength positioning accuracy tests. 5
Optical Fiber Technology 53 (2019) 102068
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interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported in parts by the National Natural Science Foundation of China under Grants 61922061, 61775161 and 61775011, the National Instrument Program under Grant 2013YQ030915 and the Marine Geological Survey Program under Grant DD20190231. References [1] S. Li, Y. Zhang, T. Koscica, H.-L. Cui, Near-infrared fiber optics gas sensor for remote sensing of CH4 gas in coal mines, Proc. SPIE (2006) 6299. [2] A. Pasic, H. Koehler, L. Schaupp, T.R. Pieber, I. Klimant, Fiber-optic flow-through sensor for online monitoring of glucose, Anal. Bioanal. Chem. 386 (2006) 1293–1302. [3] A. Banerjee, S. Mukherjee, R.K. Verma, B. Jana, T.K. Khan, M. Chakroborty, et al., Fiber optic sensing of liquid refractive index, Sens. Actuat. B Chem. 123 (2007) 594–605. [4] Y. Zhao, W. Jin, Y. Lin, F. Yang, H.L. Ho, All-fiber gas sensor with intracavity photothermal spectroscopy, Opt. Lett. 43 (2018) 1566–1569. [5] V.M. Baev, T. Latz, P.E. Toschek, Laser intracavity absorption spectroscopy, Appl. Phys. B 69 (1999) 171–202. [6] Y. Zhang, M. Zhang, W. Jin, Sensitivity enhancement in erbium-doped fiber laser intra-cavity absorption sensor, Sens. Actuat. A Phys. 104 (2003) 183–187. [7] K. Liu, T. Liu, J. Jiang, G. Peng, H. Zhang, D. Jia, et al., Investigation of wavelength modulation and wavelength sweep techniques in intracavity fiber laser for gas detection, J. Light. Technol. 29 (2011) 15–21. [8] L. Yu, T. Liu, K. Liu, J. Jiang, T. Wang, Intracavity multigas detection based on multiband fiber ring laser, Sens. Actuat. B Chem. 226 (2016) 170–175. [9] J. Albert, L.Y. Shao, C. Caucheteur, Tilted fiber Bragg grating sensors, Laser Photon. Rev. 7 (2013) 83–108. [10] Y.X. Jin, C.C. Chan, X.Y. Dong, Y.F. Zhang, Temperature-independent bending sensor with tilted fiber Bragg grating interacting with multimode fiber, Opt. Commun. 282 (2009) 3905–3907. [11] X. Dong, H. Zhang, B. Liu, Y. Miao, Tilted fiber Bragg gratings: Principle and sensing applications, Phot. Sens. 1 (2011) 6–30. [12] C.-F. Chan, C. Chen, A. Jafari, A. Laronche, D.J. Thomson, J. Albert, Optical fiber refractometer using narrowband cladding-mode resonance shifts, Appl. Opt. 46 (2007) 1142–1149. [13] Y.-P. Miao, B. Liu, Q.-D. Zhao, Refractive index sensor based on measuring the transmission power of tilted fiber Bragg grating, Opt. Fiber Technol. 15 (2009) 233–236. [14] B. Jiang, X. Lu, D. Mao, Y. Wang, W. Zhang, X. Gan, et al., Carbon nanotube-deposited tilted fiber bragg grating for refractive index and temperature sensing, IEEE Phot. Technol. Lett. 28 (2016) 994–997. [15] K. Liu, T.G. Liu, G.D. Peng, J.F. Jiang, H.X. Zhang, D.G. Jia, et al., Theoretical investigation of an optical fiber amplifier loop for intra-cavity and ring-down cavity gas sensing, Sens. Actuat. B Chem. 146 (2010) 116–121. [16] L.S. Rothman, I.E. Gordon, Y. Babikov, A. Barbe, D.C. Benner, P.F. Bernath, et al., The HITRAN2012 molecular spectroscopic database, J. Quant. Spectrosc. Radiat. Transf. 130 (2017) 4–50.
Fig. 7. Relation between cut-off mode wavelength and SRI.
5. Conclusion In this paper, we developed a gas and liquid detection system based on ICAS technique and the TFBG, where the TFBG is introduced to the fiber ring laser system to measure the refractive index of liquid. Compared to both conventional intra-cavity and TFBG methods, this system, for the first time to our knowledge, can be used to simultaneously detect the gas and liquid. The gas cell and the liquid refractive index sensor are alternated automatically by two optical switches controlled by the computer. With a reference wavelength provided by an F-P etalon, the voltage-wavelength relationship of the tunable filter is established in each sweeping period to overcome the nonlinear characteristic of the filter. And the wavelength positioning error is less than 52 pm. When the system is applied to detect the concentration of acetylene, the absolute error of the measurement results can go down to 46 ppm. Multiple gases with absorption lines within the gain bandwidth of the EDFA can be detected using this sensing system. When the system is used to measure the refractive index of glycerol-water solution with refractive indices from 1.3959 to 1.443, the sensitivity of the detection is 528.2 nm/RIU. The varieties of the detectable gases and the range of the liquid refractive indices can be further expanded if light sources with different gain bands and TFBGs with corresponding center wavelengths are employed. Declaration of Competing Interest The authors declare that they have no known competing financial
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Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Gas/liquid detection system based on intra-cavity absorption spectroscopy and tilted fiber Bragg grating
T
⁎
Kun Liua,b,c, , Yuanhao Zhaoa,b,c, Junfeng Jianga,b,c, Tianhua Xua,b,c, Pengxiang Changa,b,c, Zhao Zhanga,b,c, Jinying Maa,b,c, Yu Lianga,b,c, Tiegen Liua,b,c a
School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China Key Laboratory of Opto-electronics Information Technology, Ministry of Education, Tianjin 300072, China c Tianjin Optical Fiber Sensing Engineering Center, Institute of Optical Fiber Sensing of Tianjin University, Tianjin 300072, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fiber ring laser Intra-cavity absorption Gas/liquid detection Tilted fiber Bragg grating (TFBG)
A gas/liquid detection system is developed based on the intra-cavity absorption spectroscopy (ICAS) and a tilted fiber Bragg grating (TFBG). Multiple gases such as acetylene, carbon monoxide and ammonia gas with absorption lines within C-band can be detected using the proposed scheme. The wavelength sweeping range can be controlled to obtain a specific number of absorption lines to meet the requirements on different measurement accuracy. In this work, a TFBG structure is introduced into the ICAS technique to realize the sensing of liquid refractive index. The TFBG works around the wavelength of 1550 nm, which allows a long-distance signal transmission and a flexible system adjustment. The proposed system has been applied to detect the concentration of acetylene with an absolute error of less than 46 ppm. In addition, a set of glycerol-water solutions with refractive indices from 1.3959 to 1.4430 have been measured using the proposed scheme with a sensitivity of 528.2 nm/RIU.
1. Introduction The detection of the concentration and the composition of gases and liquids is always significant in the areas of energy exploitation and environmental monitoring [1,2]. In some cases, gases or liquids are flammable, explosive and corrosive, which are difficult to detect using conventional electronic sensors. Therefore, all-fiber sensors are widely employed in various gases and liquids detection due to their advantages such as the anti-electromagnetic interference, the corrosion resistance, the high sensitivity and safety in severe environments [3,4]. Fiber laser intra-cavity absorption spectroscopy (ICAS) is a popular gas detection technique, where the gas cell is placed in a fiber ring cavity and the laser beam passes through the gas cell multiple times during its oscillation process in the cavity. Consequently, the effective absorption length will be increased, and this will lead to a significant improvement of the sensing sensitivity [5,6]. In 2011, Liu et al. introduced the wavelength modulation technique and the wavelength sweeping technique into ICAS, and developed a system for the detection of gas concentration and absorption wavelength [7]. In 2016, Yu et al. utilized the C + L band erbium-doped fiber amplifier (EDFA) to achieve the detection of multiple gases including acetylene, carbon monoxide and carbon dioxide [8]. ⁎
In all these reported works, ICAS was developed for the gas detection only. If a liquid sensing is required, one more separate detection system has to be built, which greatly increases the cost. However, in the energy exploitation, environmental monitoring and biosensing industries, there are frequent demands on detecting both gases and liquids simultaneously. In order to meet this request, we developed a new sensing scheme by introducing a tilted fiber Bragg grating (TFBG) into the ICAS technique to achieve both gas and liquid sensing. The FBG, as a passive optical device, has many advantages such as the compact size and high stability, and can be used for the sensing of various physical quantities. Changes in the external environments such as the strain, temperature, and refractive index will affect the transmission spectrum of the fiber grating. With these features, the measurement of external environment parameters can be realized by detecting corresponding changes in the transmission spectra. The structure of the TFBG is different from that of the conventional FBG. The index modulation pattern of the TFBG is tilted against the fiber axis by a specific angle. There is only one core mode transmission peak existing in the FBG, while the TFBG transmission spectrum includes a series of cladding mode resonance peaks [9]. The cladding modes can be used to measure the refractive index of liquids. In the past few years, the TFBG has attracted widespread attention [9–11]. In 2007, C.-F. Chan et al. developed a
Corresponding author at: School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China. E-mail address: [email protected] (K. Liu).
https://doi.org/10.1016/j.yofte.2019.102068 Received 18 July 2019; Received in revised form 27 September 2019; Accepted 30 October 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
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convert the short chamber length into the long effective absorption length, and thereby the sensitivity of the gas sensing can be largely improved. The gas absorption follows Lambert-Beer law:
refractometer based on the narrowband cladding-mode resonance shifts from the short-period fiber Bragg gratings with weakly tilted grating planes [12]. In 2009, Y.-P. Miao et al. proposed a scheme of the refractive index sensor through measuring the transmission power of the tilted fiber Bragg grating [13]. In 2016, B. Jiang et al. employed a tilted fiber Bragg grating (TFBG) deposited with carbon nanotubes to measure the refractive index and temperature simultaneously [14]. However, all these reported TFBG work focused on the measurement of the liquid refractive index only. In this paper, a gas/liquid sensor based on fiber laser ICAS and a TFBG is developed and presented. The TFBG is operated around the wavelength of 1550 nm, which is in the same range as the gas absorption wavelength. Therefore, the combination of the TFBG and the ICAS technique can be implemented to achieve a simultaneous detection of gas and liquid, and the sensing signal can be transmitted over long distances. In addition, the tunable optical filter and the photodetector are used to replace the optical spectrum analyzer to obtain the transmission spectrum of the TFBG. The wavelength positioning error is less than 52 pm. Similar wavelength accuracy can be achieved with lower cost compared to methods using a broadband source and a spectrum analyzer. Furthermore, acetylene of different concentrations and a set of solutions with various refractive indices are detected to demonstrate the performance of the developed sensor system.
I (ν ) = I0 (ν ) exp[−α (ν ) CL]
where I denotes the intensity of transmitted light after the gas absorption, I0 represents the intensity of the incident light, ν is the light frequency, α represents the absorption cross section, C represents the gas concentration, and L represents the effective absorption length. Then the absorbance parameter K can be described as
K (ν ) = ln
The measurement of the liquid refractive index is realized based on the TFBG. TFBGs are short-period fiber gratings and have all the characteristics of FBGs. They also have other advantages because of their specific structures. The index modulation pattern of the TFBG is tilted against the fiber axis by a specific angle, and this enhances the coupling of the light from the forward-propagated core mode to the backward-propagated cladding modes and reduces the coupling to the backward core mode. Therefore, besides the core mode resonance peak, there are several cladding modes in the transmission spectrum of the TFBG. Compared to the core mode, the cladding modes of the TFBG is in contact with the outer surroundings due to the light transmission in the cladding. Therefore, the effective refractive indices of the cladding modes are affected by the external environment, hence the TFBG can be used for detecting the refractive index of the liquid. The Bragg reflection wavelength λBragg and the cladding-mode resonance wavelength λ coupling, i of the TFBG are described by the following equations:
2.1. Output power of the erbium-doped fiber ring laser The fiber ring laser uses erbium-doped fiber (EDF) as the gain medium and a 980 nm laser diode as the pumping source, and the output power of the fiber ring laser can be derived by solving the rate equations. When the spontaneous emission is neglected, the output power I of the laser is proportional to the photon number q in the intracavity and can be expressed as [15]:
λBragg =
2neff , core Λ
λ coupling, i =
(5)
cos θ (neff , core + neff , clad, i )Λ cos θ
(6)
where neff , core and neff , clad, i are the effective refractive indices of the core mode and the i th cladding mode, respectively. Λ represents the nominal period, and its relationship with the actual grating period Λ g along the axis of the fiber is expressed as Λ = Λ g cosθ , where θ refers to the tilt angle. As the surrounding refractive index (SRI) increases, some of the high-order cladding modes will leak out, but the low-order cladding modes can still be transmitted. The cladding mode between the leaky modes and the guided modes is called the cut-off mode. When the SRI changes, the cut-off mode position changes accordingly. The refractive index of the liquid can be obtained by tracking the cut-off mode position correspondingly.
(1)
where we have Sτ l (W τ σ − σ ) Nc
c a p 2 e a ⎧ ⎪A = τ2 (σe + σa) ⎨ B = Sτc (1 + Wp τ2) ⎪ ηs τ2 (σe + σa) ⎩
(4)
2.3. Liquid refractive index measurement
The basic principle of this proposed sensing system is built on the intra-cavity absorption spectroscopy technique. The gas sensing is implemented based on the measurement of the specific spectral extinction of the light transmitted through the gas to be detected. The liquid sensing is realized through the effect that the position of the cut-off cladding mode varies with the refractive index of the liquid. The gas cell and TFBG share a same system that uses an optical switch to control the selection between the gas sensing and the liquid sensing functions. A computer program is designed to alternate the status of the optical switch automatically during the measurement process to achieve the simultaneous gas and liquid sensing.
A − Bδ δ
I0 (ν ) = α (ν ) CL I (ν )
It is seen from Eq. (4) that the absorbance K is proportional to the gas concentration, so this characteristic can be applied to achieve the measurement of the gas concentration. A set of gases of which the concentrations are known have been used to calibrate the relationship between gas concentration and the absorbance, and then the concentration of the test gas can be derived accordingly based on Eq. (4) and the calibration results.
2. Principle
I∝q=
(3)
(2)
A and B are the constants determined by physical parameters of the sensing system. δ represents the total loss of the cavity, S represents the fiber core area, τc is the cavity round time, la is the length of the EDF, Wp denotes the pump probability, τ2 denotes the life time of the stable level, σa and σe are the absorption and emission cross-section of the mode, respectively, Nc is the dopant concentration of the EDF, and ηs is the proportion of the signal power within the fiber core.
3. System configuration The gas and liquid sensing system based on the ICAS and TFBG is illustrated in Fig. 1. The solid line represents the optical signal path and the dotted line represents the electrical signal path. This sensing system including the following components: an EDFA, an isolator, two fiber couplers, an optical circulator, an electronic variable optical attenuator (EVOA), a tunable optical filter, an etalon, a gas cell, a TFBG, a Faraday rotator mirror (FRM), two InGaAs PIN photodetectors (PDs), two
2.2. Gas absorption The ICAS technique allows the light to oscillate in the cavity to 2
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Fig. 1. Schematic diagram of the gas/liquid detection system.
Time ≤ 10 ms) are used to switch the optical path. The DAQ card(NI, USB-6251) is used to input and output signals. And the computer is used to process signals. After passing through the EVOA, the laser beam enters the tunable filter for the frequency selection. Since the spectral absorption line of the gas is narrow and the shift of the cut-off mode position is small, a tunable filter with a fineness of 3829 and a free spectral range (FSR) of 107 nm is employed. The wavelength sweeping is realized by adjusting the driving voltage of the tunable filter through the computer. The light enters the circulator and the fiber Coupler 1 and it is further split into two parts. When the gas detection part is used, 99% of the light enters the gas cell and is reflected back by the FRM and passes through the Coupler 2 and the circulator again and then enters the Coupler 1. When the liquid detection part is used, 99% of the light passes through the TFBG and enters the coupler 1. The other 1% of the light enters the etalon, and then the signal is sent to the computer via PD 1 and the DAQ card. The etalon is used to realize a real-time calibration of the tunable filter to reduce the wavelength positioning error introduced due to the nonlinearity of the tunable filter. Coupler 1 splits the laser beam into two parts with the ratios of 90% and 10%, respectively. 10% of the laser beam will enter the computer via PD 1 and DAQ card to acquire the gas absorption spectra. The other 90% of the laser beam continues to oscillate and to be amplified in the cavity. The DAQ card is used for the data acquisition and device control. It is managed by the computer to acquire the output voltages of the PDs and to provide a driving signal for the tunable filter and the EVOA. The sensing system is divided into two parts: the gas sensing and the liquid sensing, and the function is controlled by two optical switches. A 12 cm long steel pipe is employed as the gas cell, combined with two flanges through the interference fit. Two calibrated pigtailed fiber collimators are mounted in the center of the flanges to achieve the input and output ports for the light. A Faraday rotator mirror is placed at the rear end of the gas cell to return the light to increase the effective absorption length. The liquid sensing component is implemented using a TFBG with a tilt angle of 8°.
optical switches and a data acquisition (DAQ) card controlled by a computer. The C-band EDFA (Hoyatek, HY-EDFA-C-21) is used for the light amplification. It is pumped by a 980 nm power-tunable laser diode with pump powers of up to 400 mW. And the saturated output power is 21 dBm. The operating wavelength is 1525–1570 nm. The EVOA (Sercalo, VXP1) is used to adjust the laser gain to obtain a higher sensitivity and protect the tunable filter. The attenuation is controlled by the input voltage. The EVOA has a maximum attenuation of 30 dB and a response time of 20 ms. The tunable optical filter (Micron Optics, FFP-TF2) is the core component of the system. It is used to select the output wavelength of the fiber ring laser to achieve wavelength sweep. The fineness of the tunable filter is 3829 and the free spectral range (FSR) is 107 nm. The etalon (Primanex, 100GHZMag) is used to provide reference wavelengths for wavelength calibration because of the nonlinearity of the tunable filter. The FSR is 100 GHz and the 3 dB bandwidth is 0.06 nm. The isolator (Shconnet, Isolation ≥ 46 dB) is used to ensure the oneway transmission of light to prevent the laser from entering the EDFA backward and causing the damage to the device. The couplers (Shconnet, Type: Fused biconical taper (FBT), 2 * 2) are used to split a beam of light into two beams. The circulator (Shconet, Insertion loss ≤ 1.2 dB, Isolation ≥ 38 dB, Return loss ≥ 50 dB) is used to connect the gas cell and the TFBG to the sensing system. The coupling ratio of coupler 1 is 10:90 and insertion loss is 10.8/0.6 dB. The coupling ratio of coupler 2 is 1:99 and insertion loss is 21.5/0.2 dB. The specific coupling ratios are selected to balance the output spectral range, output power, and the SNR of the fiber ring laser. For Coupler1, 90% of the light is used to continue the oscillation in the intra-cavity to be amplified to form a stable laser output, and 10% of the light is used to output to PD1 to detect the optical power. Optical power detection does not require much light, and 10% can meet the demand, so most of the light continues to oscillate in the intra-cavity. For Coupler2, 99% of the light is used for gas and liquid sensing, and 1% of the light is used for wavelength calibration. Wavelength calibration also does not require much optical power, while gas and liquid sensing are based on spectral absorption, so most light is used for sensor absorption in order to improve the signal-to-noise ratio for detection. The gas cell made with metal tube and two fiber collimators acts as a place where light interacts with the gas. The TFBG is used to achieve liquid refractive index sensing. The FRM (Shconnet, Return loss ≥ 50 dB, Insertion loss ≤ 0.5 dB) is used to reflect the light back. PDs (Qnoptics, LSIPDA75) are used to convert the optical signal into the electrical signal. The spectral range is 800–1700 nm. The responsivity at 1550 nm is 0.9 mA/ mW and response time is 140 ps. Optical switches (Sercalo, SXLA2x2SMF, Insertion loss ≤ 0.4 dB, Return Loss ≥ 50 dB, Switching
4. Experiment and discussion 4.1. Gas concentration calibration and detection The wavelength tuning range of the EDFA used in the system is from 1525 nm to 1565 nm. Multiple types of gases can be detected using this system, such as acetylene, carbon monoxide and other gases with absorption lines within this range. Gas absorption data can be obtained from the HITRAN database [16]. The concentration of acetylene was 3
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Fig. 3. Concentration calibration results of three absorption lines. Fig. 2. Absorption spectra of acetylene at normal temperature and pressure. Table 1 Results of concentration detection.
measured in our experiments. The single-trip scanning time of the system becomes longer as the wavelength range increases. Therefore, it is necessary to ensure that the wavelength sweeping range can be as small as possible and there are enough strong absorption lines at the meantime. For this reason, the wavelength sweeping range is set from 1528 nm to 1535 nm, where the acetylene has several strong absorption lines and the output intensity of the light source is relatively stable. A mixture of the acetylene and the nitrogen (0.3% acetylene, 99.7% nitrogen) was filled into the gas cell to obtain the absorption spectra at normal temperatures and pressures. Corresponding results are shown in Fig. 2. The gas cell is 12 cm in length. It is expected that a number of strong absorption lines can be obtained within the spectral range. The gas concentration detection can be achieved using a single absorption line, but the use of multiple absorption lines can reduce the measurement error. Therefore, we selected three of them for the acetylene detection, and have averaged the three sets of data to reduce the analysis error. Three strong absorption lines centered at 1529.18 nm, 1530.37 nm, and 1531.59 nm were selected for the concentration calibration. Acetylene and nitrogen were mixed using a mass flow controller. Acetylene with concentrations of 1000, 2000, 3000, 4000 and 5000 ppm were prepared for the concentration calibration and another group with concentrations of 1500, 2500, 3500, 4500 ppm were prepared for the sensing test. We measured the acetylene with different concentrations in the first group and the results are shown in Fig. 3. It is noted that the absorbance is slightly different from the HITTRAN database since the effective absorption length is different. When the absorption line is closer to the laser threshold, the effective absorption length will become longer. The experimental data are further polynomial fitted and the slopes of three absorption lines are 2.41/%, 2.17/ % and 1.96/%, respectively. Absorption line 1 has the largest slope, indicating that it has the highest measurement sensitivity. And the R2 of the three lines are 0.9996, 0.9993 and 0.9995 respectively. Different concentrations of acetylene were measured based on the calibration results above. The tested acetylene has the concentration varying from 1500 ppm to 4500 ppm with interval of 1000 ppm. The detection results are described in Table 1. It can be seen that the maximum error is 53 ppm when the gas measurement is performed with a single absorption line. The maximum error is reduced to 46 ppm and the relative error becomes less than 1.3% using the average measurement results of three absorption lines. More absorption lines can be employed for improving the detection if there is a higher requirement for the measurement accuracy.
Practical Concentration (ppm) Detected concentration (ppm)
Absolute error (ppm) Relative error (%)
Line1 Line2 Line3 Mean
1500
2500
3500
4500
1496 1481 1504 1494 6 0.40
2505 2536 2537 2526 26 1.04
3544 3545 3550 3546 46 1.3
4520 4486 4447 4484 16 0.35
4.2. Wavelength calibration Before the measurement, it is necessary to calibrate the wavelengthvoltage relationship of the tunable filter to improve the accuracy of wavelength positioning. An F-P etalon is used as the wavelength reference to solve this problem introduced by the nonlinearity of the tunable filter. The distance between two adjacent transmission peaks in the etalon is 0.8 nm. The etalon is blocked at a wavelength of 1533.504 nm and has no transmission there. The central wavelength of the transmission peak around the blocked position can be derived accordingly. The transmission spectrum of the etalon is shown in Fig. 4(a). The blocked wavelength is marked with a black star. It can be expected that the transmission wavelength has a one-to-one correspondence with the driving voltage during each period of sweeping. Therefore, the relationship between the wavelength and the driving voltage can be calculated in each sweeping period. The wavelength of each peak of the etalon is known, and the corresponding driving voltage is recorded for each sweep period, and then the relationship between wavelength and the driving voltage is obtained by second-order polynomial fitting. During each sweep, the program sends a sawtooth waveform to the tunable filter as the drive voltage signal. Then on the falling edge of the sawtooth waveform, the driving voltage of the tunable filter and the output of the PD change synchronously with time and these data are recorded by the program. Wavelength-demodulation is then achieved using the fitting curve of the drive voltage and wavelength of the tunable filter. One of the fitting curves is shown in the Fig. 4(b). To test the accuracy of wavelength calibration with the tunable filter and the etalon, an FBG(Technica, single FBG) is used as a wavelength reference. The center wavelength of the FBG is 1546.76 nm and the FWHM is less than 0.3 nm. The FBG is connected to the system shown in Fig. 1 and its center wavelength is measured in each sweep. The transmission spectrum of the FBG is shown in Fig. 5(a). Five hundred sets of experiments were performed and the results are shown in 4
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Fig. 4. (a) Transmission spectrum of the etalon. (b) One of the fitting results of the transmission wavelength of tunable filter and the driving voltage of the tunable filter.
Fig. 5. The maximum error is 52 pm and the minimum error is 16 pm, and the standard deviation is 0.0141. The wavelength positioning results is smaller than the nominal center wavelength of the FBG as a whole, that is because the etalon is affected by external temperature and other factors, and the actual transmission wavelength is different from the data provided by the manufacturer.
4.3. Liquid refractive index sensing In the liquid sensing, a TFBG with 8° tilt angle is used to measure the refractive index of the liquid. The center wavelength of the TFBG is around 1550 nm. This makes it easy be combined with the fiber ring laser system, and the signal can be transmitted over long distances. A set of refractive index solutions with refractive indices from 1.3959 to 1.4430 are measured at the temperature of 25 °C. The solutions used in the experiment are composed of glycerin and deionized water at different concentrations and were calibrated with an Abbe refractometer. In order to prevent the TFBG from being damaged and to keep the strain stable during the experiments, the grating region is taped to a microscope slide. When the refractive index of the liquid is measured, the solution is dropped into the grating region with a plastic dropper. After each measurement, the grating region is fully cleaned with absolute ethanol. The evolution of the TFBG spectrum with the SRI is illustrated in Fig. 6, and the positions of the cut-off modes are marked with black stars. Fig. 7 demonstrates the relation between cut-off mode
Fig. 6. Evolution of the TFBG spectrum with different SRI. (The different curves are shifted vertically for clarity.)
wavelength and SRI. The polynomial fitting of the experimental results is carried out and the R2 is 0.9996. The sensitivity of the TFBG is 528.2 nm/RIU for the refractive indices ranging from 1.3959 to 1.4430.
Fig. 5. (a) Transmission spectrum of the FBG. (b) Results of the wavelength positioning accuracy tests. 5
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interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported in parts by the National Natural Science Foundation of China under Grants 61922061, 61775161 and 61775011, the National Instrument Program under Grant 2013YQ030915 and the Marine Geological Survey Program under Grant DD20190231. References [1] S. Li, Y. Zhang, T. Koscica, H.-L. Cui, Near-infrared fiber optics gas sensor for remote sensing of CH4 gas in coal mines, Proc. SPIE (2006) 6299. [2] A. Pasic, H. Koehler, L. Schaupp, T.R. Pieber, I. Klimant, Fiber-optic flow-through sensor for online monitoring of glucose, Anal. Bioanal. Chem. 386 (2006) 1293–1302. [3] A. Banerjee, S. Mukherjee, R.K. Verma, B. Jana, T.K. Khan, M. Chakroborty, et al., Fiber optic sensing of liquid refractive index, Sens. Actuat. B Chem. 123 (2007) 594–605. [4] Y. Zhao, W. Jin, Y. Lin, F. Yang, H.L. Ho, All-fiber gas sensor with intracavity photothermal spectroscopy, Opt. Lett. 43 (2018) 1566–1569. [5] V.M. Baev, T. Latz, P.E. Toschek, Laser intracavity absorption spectroscopy, Appl. Phys. B 69 (1999) 171–202. [6] Y. Zhang, M. Zhang, W. Jin, Sensitivity enhancement in erbium-doped fiber laser intra-cavity absorption sensor, Sens. Actuat. A Phys. 104 (2003) 183–187. [7] K. Liu, T. Liu, J. Jiang, G. Peng, H. Zhang, D. Jia, et al., Investigation of wavelength modulation and wavelength sweep techniques in intracavity fiber laser for gas detection, J. Light. Technol. 29 (2011) 15–21. [8] L. Yu, T. Liu, K. Liu, J. Jiang, T. Wang, Intracavity multigas detection based on multiband fiber ring laser, Sens. Actuat. B Chem. 226 (2016) 170–175. [9] J. Albert, L.Y. Shao, C. Caucheteur, Tilted fiber Bragg grating sensors, Laser Photon. Rev. 7 (2013) 83–108. [10] Y.X. Jin, C.C. Chan, X.Y. Dong, Y.F. Zhang, Temperature-independent bending sensor with tilted fiber Bragg grating interacting with multimode fiber, Opt. Commun. 282 (2009) 3905–3907. [11] X. Dong, H. Zhang, B. Liu, Y. Miao, Tilted fiber Bragg gratings: Principle and sensing applications, Phot. Sens. 1 (2011) 6–30. [12] C.-F. Chan, C. Chen, A. Jafari, A. Laronche, D.J. Thomson, J. Albert, Optical fiber refractometer using narrowband cladding-mode resonance shifts, Appl. Opt. 46 (2007) 1142–1149. [13] Y.-P. Miao, B. Liu, Q.-D. Zhao, Refractive index sensor based on measuring the transmission power of tilted fiber Bragg grating, Opt. Fiber Technol. 15 (2009) 233–236. [14] B. Jiang, X. Lu, D. Mao, Y. Wang, W. Zhang, X. Gan, et al., Carbon nanotube-deposited tilted fiber bragg grating for refractive index and temperature sensing, IEEE Phot. Technol. Lett. 28 (2016) 994–997. [15] K. Liu, T.G. Liu, G.D. Peng, J.F. Jiang, H.X. Zhang, D.G. Jia, et al., Theoretical investigation of an optical fiber amplifier loop for intra-cavity and ring-down cavity gas sensing, Sens. Actuat. B Chem. 146 (2010) 116–121. [16] L.S. Rothman, I.E. Gordon, Y. Babikov, A. Barbe, D.C. Benner, P.F. Bernath, et al., The HITRAN2012 molecular spectroscopic database, J. Quant. Spectrosc. Radiat. Transf. 130 (2017) 4–50.
Fig. 7. Relation between cut-off mode wavelength and SRI.
5. Conclusion In this paper, we developed a gas and liquid detection system based on ICAS technique and the TFBG, where the TFBG is introduced to the fiber ring laser system to measure the refractive index of liquid. Compared to both conventional intra-cavity and TFBG methods, this system, for the first time to our knowledge, can be used to simultaneously detect the gas and liquid. The gas cell and the liquid refractive index sensor are alternated automatically by two optical switches controlled by the computer. With a reference wavelength provided by an F-P etalon, the voltage-wavelength relationship of the tunable filter is established in each sweeping period to overcome the nonlinear characteristic of the filter. And the wavelength positioning error is less than 52 pm. When the system is applied to detect the concentration of acetylene, the absolute error of the measurement results can go down to 46 ppm. Multiple gases with absorption lines within the gain bandwidth of the EDFA can be detected using this sensing system. When the system is used to measure the refractive index of glycerol-water solution with refractive indices from 1.3959 to 1.443, the sensitivity of the detection is 528.2 nm/RIU. The varieties of the detectable gases and the range of the liquid refractive indices can be further expanded if light sources with different gain bands and TFBGs with corresponding center wavelengths are employed. Declaration of Competing Interest The authors declare that they have no known competing financial
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