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Remote Gas Pressure Sensor Based on Fiber ´ Ring Laser Embedded With Fabry–Perot Interferometer and Sagnac Loop Volume 8, Number 5, October 2016 Jia Shi Yuye Wang Degang Xu Yixin He Junfeng Jiang Wei Xu Haiwei Zhang Genghua Su Chao Yan Dexian Yan Ying Lu Jianquan Yao

DOI: 10.1109/JPHOT.2016.2605460 1943-0655 © 2016 IEEE

IEEE Photonics Journal

Remote Gas Pressure Sensor Based on Fiber Ring

Remote Gas Pressure Sensor Based on Fiber Ring Laser Embedded With ´ Fabry–Perot Interferometer and Sagnac Loop Jia Shi, Yuye Wang, Degang Xu, Yixin He, Junfeng Jiang, Wei Xu, Haiwei Zhang, Genghua Su, Chao Yan, Dexian Yan, Ying Lu, and Jianquan Yao The Key Laboratory of Opto-Electronics Information Technology (Ministry of Education), Institute of Laser and Optoelectronics, College of Precision Instruments and Opto-Electronic Engineering, Tianjin University, Tianjin 300072, China.

DOI:10.1109/JPHOT.2016.2605460 C 2016 IEEE. Translations and content mining are permitted for academic research only. 1943-0655 Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received June 14, 2016; revised August 25, 2016; accepted August 30, 2016. Date of publication September 2, 2016; date of current version September 16, 2016. This work was supported in part by the National Basic Research Program of China (973) under Grant 2015CB755403 and Grant 2014CB339802; in part by the National Natural Science Foundation of China under Grant 61107086, Grant 61172010, and Grant 61471257; in part by the Natural Science Foundation of Tianjin under Grant 14JCQNJC02200; in part by the Science and Technology Support Program of Tianjin under Grant 14ZCZDGX00030; and in part by the Specialized Research Fund for the Doctoral Program of High Education under Grant 20120032110053. Corresponding author: Degang Xu ([email protected]).

Abstract: In this paper, we propose a remote gas pressure sensor based on a fiber ring laser embedded with a Fabry–Perot (F–P) interferometer and a Sagnac loop. A compact ´ optic-fiber F–P interferometer is inserted in the fiber laser and works as the sensing head for gas pressure detection. A wavelength selective filter based on optic-fiber Sagnac loop is used in the fiber laser. The sensitivity of −9.69 nm/kPa is obtained with a narrow 3-dB bandwidth less than 0.02 nm and high signal-to-noise ratio (SNR) ∼45 dB. Moreover, the excellent performance of the gas pressure sensor for remote detection is demonstrated. Index Terms: Fiber optics sensors, intracavity sensing, remote sensing.

1. Introduction Gas pressure detections play an indispensable role in the biological, chemical, and environmental safety monitoring fields. Fiber-optic gas pressure sensors have been investigated with increasing interest due to their advantages such as anti-interference, high sensitivity, compactness, and fast responses. In particular, these sensors are widely used in the detections of harmful and inflammable gas. Various structures of fiber sensors have been developed to measure gas pressure based on microstructured fiber [1]–[3], long-period fiber gratings [4], [5], F-P interferometers [6]–[8], Mach-Zehnder interferometers [9], fiber-tip sensors based on dual capillaries [10], etc. Some fiber sensors coated with sensitive films, such as Pd-Au alloy thin film and Mg-based metal hydrides, are also demonstrated to measure gas pressure [11], [12]. During these sensors, most of them have the sensitivities of 10s nm/MPa and the sensor in [1] possesses the highest sensitivity which reaches 1.4 nm/kPa. In addition, these passive sensors, employed by broadband light sources, generally have a wide operating spectrum and low average power, which limits their applications in

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Fig. 1. (a) Structure of the F-P interferometer. (b) Model of electric fields at the two reflections of the F-P interferometer.

long-distance detections. In former research, the ability of these sensors for remote gas pressure detection is rarely taken into account. Fiber lasers sensors have been investigated extensively and used widely because they can enhance the visibility of the resonant spectrum and narrowing the 3-dB bandwidth with a high output intensity [13]. Various fiber laser sensors have been proposed to measure refractive index (RI) [13], curvature [14], temperature [15], gas absorption [16], hydrostatic pressure [17], and acoustic emission [18]. There have been no reported remote gas pressure sensors based on fiber lasers to the best of our knowledge. In addition, the sensing element, which is inserted in the laser cavity, may bring extra perturbations to the output of the fiber laser which will contribute to measuring errors. Thus, output stability is an important parameter for fiber laser sensors which limits their applications. In this paper, we propose a remote gas pressure sensor based on a fiber ring laser. A fiber-optic gas pressure sensor based on F-P interferometer has been inserted in a fiber ring laser through a circulator and a 3-dB coupler. In order to suppress wavelength competition, a fiber-optic Sagnac loop filter is used in the fiber ring laser. The ability of the proposed sensor for remote gas pressure detection has been experimentally demonstrated by selecting different length of transmission optical fiber (TOF), which connects the circulator and the F-P interferometer. Experiments show that the proposed sensor has a narrow 3-dB bandwidth, high SNR, good output stability, and excellent performance in remote detection.

2. Principles ´ 2.1. Fiber-Optic Gas Pressure Sensor Based on Fabry-Perot Interferometer The structure of the fiber-optic gas pressure sensor based on F-P interferometer is depicted in Fig. 1(a). It consists of single mode fiber (SMF), glass ferrule, Pyrex glass, and silicon diaphragm.


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Fig. 2. (a) Structure of the gas pressure sensor. (b) F-P interferometer. (c) Internal structure of the gas cell. (d) Gas cell with two ports and a barometer for gas pressure controlling.

The silicon diaphragm is elastic which serves as the pressure sensing element. There are two reflective surfaces at the front and back surface of the vacuum cavity which are formed by the glass ferrule, Pyrex glass, and silicon diaphragm. The original length of the vacuum cavity is h. The model of electric fields at the two reflections is illustrated in Fig. 1(b). The total reflected electric fields E r is given approximately by the sum of the two reflected electric fields, which can be written as (1) E r = E 0 R 1 + (1 − α)(1 − R 1 ) R 2 e−j 2ϕF P where E 0 is the input field, and α is the transmission loss factors. R 1 and R 2 are the reflection coefficients of the two reflective surfaces. ϕF P is the round-trip propagation phase shift, which can be described as ϕF P =

4πh . λ

Thus, the total reflection spectra I (λ) can be expressed as 2 Er I (λ) = = A + B cos ϕF P E0

(2)

(3)

where 2 2 A = R√ 1 + (1 − α) (1 − R 1 ) R 2 B = 2 R 1 R 2 (1 − α)(1 − R 1 ) cos(2ϕF P ).

(4)

When the external gas pressure increases, the length of the vacuum cavity will decrease. The sensitivity of the sensor in response to the cavity-length change is obtained by the derivative of I (λ)

S (h ) =

dI (λ) 16π 8πh =− R 1 R 2 (1 − α)(1 − R 1 ) sin . dh λ λ

(5)

2.2. Fiber-Optic Sagnac Loop The structure of the fiber-optic Sagnac loop is shown in Fig. 2(a), which consists of a 3-dB coupler and a short length of polarization maintaining fiber (PMF). The lead-in light is split into two beams by the 3-dB coupler. These two beams transmit in opposite directions in the loop, propagate through PMF individually, and couple at the 3-dB coupler. Because


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of the birefringence of the PMF, the interference occurs, and the phase difference is expressed by 2πB L (6) λ where B is the birefringence of the PMF, L is the length of the PMF, and λ is the wavelength of the input light. The output transmission spectrum at the circulator is approximately a periodic function of the wavelength, which is expressed as [19] ϕSagnac =

T = (1 − cos ϕSagnac )/2

(7)

When ϕSagnac = 2πm (m is an integer), periodic dips are formed in the transmission spectrum. The wavelength of the mth dip is λD ip = B L /m .

(8)

Thus, the wavelength spacing between the transmission dips can be derived as λ = λ2D ip /(B L )

(9)

Fiber-optic Sagnac loops can perform as tunable filters in fiber-optic systems, and the filtering characteristic can be designed by changing the length of the PMF.

3. Experimental Setup and Results The schematic configuration of the remote gas pressure sensor based on the fiber ring laser embedded with F-P interferometer and Sagnac loop is shown in Fig. 2(a), which consists of a pump source, a section of Erbium-doped fiber (EDF), a wavelength division multiplexing (WDM) coupler, an isolator (ISO), a circulator, transmission optical fiber (TOF), a fiber-optic F-P interferometer, fiber-optic Sagnac loop, a 10:90 coupler, as well as an optical spectrum analyzer (OSA). The gain medium is a piece of EDF (Nurfen, EDFC-980-HP) with the length of 3 m, which is pumped by a diode laser centered at 976 nm via a 980/1550 nm WDM coupler (AFR Inc.). The ISO works to operate unidirectionally and prevent spatial hole-burning. After coupled in a circulator, the input light transmits through the TOF (Corning, SMF-28) and is reflected by the F-P interferometer. The F-P interferometer is used as the sensing head for gas pressure detection. The total length of the F-P interferometer is ∼1 cm, which is shown in Fig. 2(b). The original length of the F-P cavity is ∼ 25 μm. The fabrication of the F-P interferometer is under the vacuum condition, which should make the fiber cross section as perpendicular as possible to the silicon diaphragm. As shown in Fig. 2(c), the F-P interferometer is placed in a gas cell to experience different gas pressures. The gas cell has two ports for gas pressure controlling with a barometer as depicted in Fig. 2(d). Then, a wavelength selective filter based on Sagnac loop is inserted into the fiber ring laser. The output spectrum of the sensor system is measured by an OSA with the spectral resolution of 0.02 nm (YOKOGAWA, AQ6370) via a 10:90 coupler. The emission wavelength of the fiber ring laser will be at the one that has the minimum spectral transmission loss in the gain spectrum of the EDF. When there is no extra gas pressure, the transmission spectrum of the F-P interferometer is shown in Fig. 3, which is measured by a supercontinuum light source (NKT Photonics, SuperK COMPACT) and an OSA (YOKOGAWA, AQ6370). The depth of the resonant dip is obtained about 4 dB with the insertion loss about 18 dB at the peak wavelength. There are four characteristic dips from 1525 nm, to 1570 nm with the wavelength spacing about 13 nm. Among these dips, two obvious characteristic dips have the depths about 4 dB, and the other two have the weaker depths about 2.5 dB. In the experiments, when the fiber cross section is well perpendicular to the silicon diaphragm, the characteristic dips will have the same depths, but it is difficult to control in practice. There are two obvious peaks with close depths in the transmission spectrum of the F-P interferometer in the gain spectrum of the EDF. In our experiments, when the Sagnac loop is not inserted in the fiber ring laser, two emission wavelengths centered at about 1533 nm and 1548 nm are simultaneously observed accompanied by a strong competition. In order to suppress the competition, the wavelength selective filter based on Sagnac loop is designed in the fiber laser.


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Fig. 3. Spectra of the F-P interferometer, Saganc loop, and laser output. (Inset) Transmission spectrum of the Saganc loop from 1520 nm to 1640 nm.

The PMF (Nufern, PM1550-HP) with the length of 20 cm is used in the Sagnac loop to make the dip wavelength at about 1550 nm and peak wavelength at about 1525 nm, as shown in Fig. 3. The inset is the transmission spectrum of the Sagnac loop from 1520 nm to 1640 nm which shows the wavelength spacing is about 50 nm. The losses induced by Saganc loop are −6 dB at 1533 nm and −23 dB at 1548 nm. The emission wavelength centered at about 1548 nm is suppressed due to the large loss. The corresponding output spectrum of the fiber ring laser using a 2-m-long TOF is shown in Fig. 3 at the pump power 100 mW. The 3-dB bandwidth is less than 0.02 nm, which is limited by the spectral resolution of the OSA and the SNR is ∼45 dB. As the gas pressure of the gas cell increases, the output spectra of the fiber ring laser are measured by selecting the TOF with the length of 2 m, 2 km, and 5 km, respectively. As it is described in Fig. 4(a)–(c), the emission wavelength of the fiber ring laser has a blue shift along with the gas pressure increases. The responses between the gas pressure and emission wavelength are fitted in Fig. 4(d). The sensitivity of −9.69 nm/kPa is obtained when using 2-m-long TOF, and adjacent sensitivities of −10.01 nm/kPa and −11.2 nm/kPa are achieved when the length of the TOF is selected as 2 km and 5 km, respectively. When the length of the TOF increases from 2 m to 5 km, it is found out that the 3-dB bandwidth is broadened obviously. A likely reason is that, because a large loss is induced into the fiber laser with the same pump power, the gain competition effects will weaken and it will result in the broadening of the 3-dB bandwidth. Although the 3-dB bandwidth is broadened obviously when the length of the TOF increases a lot, the central wavelengths of the output nearly have no change. The results indicate that different length of TOF has little impact on the sensitivity of the gas pressure sensor, which gives have many applications in remote gas pressure detections. Output stability is an important parameter for fiber laser sensors which limits their applications. Therefore, it is necessary to measure the output stability of fiber laser sensors. In order to analyze the stability of the sensor system, output wavelength is measured over 180 minutes, as shown in Fig. 5, by fixing the gas pressures at 0.03 kPa and 0.12 kPa, respectively. The inset in Fig. 5 is the output spectra of the sensor system. The difference between the maximum and minimum values of wavelength is less than 20 pm which is limited by the resolution of the OSA. Such little perturbations reflect that the sensor possesses a good stability. The time response of the sensor is also analyzed in this paper. The traditional method of measuring the response time of intensity-modulated fiber sensors records the output power jump when the external conditions changes. Because the proposed fiber laser sensor has different wavelengths with close output power as the gas pressure changes, the method we use is to connect a narrowband wavelength filter to the output to realize the measurement of time response. When the gas


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Fig. 4. Output spectra of the gas pressure sensor system along with the gas pressure increases from 0 to 0.18 kPa when the length of the TOF is selected as (a) 2 m, (b) 2 km, and (c) 5 km, respectively. (d) Linear fitting of the gas pressure and emission wavelength when using different length of TOF.

Fig. 5. Output stability of the propose gas pressure sensor over 180 minutes. (Inset) Output spectra of the sensor over 180 minutes when the gas pressure increases at (a) 0.03 kPa and (b) 0.12 kPa, respectively.


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Fig. 6. Time response of the sensor measured by a high-speed InGaAs biased photodetector and a digital oscilloscope.

pressure of the gas cell changes, the output wavelength will have a shift which is out of the operating range of the filter, and then, there will be a power jump. A standard C-band dense wavelength division multiplexing (DWDM, AFR Inc., CH55, 100 G, central wavelength 1533.47 nm) is selected as the filter. The output of the DWDM is connected to a high-speed InGaAs biased photodetector (Thorlabs, DET01CFC/M), and the digital oscilloscope (Tektronix, DPO2024B) is used to record the time response. When the gas pressure changes from 0.03 kPa to 0.18 kPa, the result is shown in Fig. 6 when the 5-km TOF used in the system. The estimated response time of the sensor is about 144 ms.

4. Conclusion In summary, a remote gas pressure sensor based on fiber ring laser has been proposed and experimentally demonstrated. A gas pressure sensor based on F-P interferometer is embedded in the fiber laser. A wavelength selective filter based on optic-fiber Sagnac loop is inserted in the fiber laser to suppress wavelength competition. As the gas pressure of the F-P interferometer increases, the emission wavelength of the fiber ring laser has a blue shift. The sensitivity of −9.69 nm/kPa is obtained with a narrow 3-dB bandwidth less than 0.02 nm and high SNR ∼45 dB. Compared to current gas pressure sensors based on fiber devices, the sensitivity of the proposed sensor is several times higher. What is more, the excellent performance of the sensor system for remote sensing has been shown by using different length of the TOF. The stability of the sensor is also analyzed by measuring output wavelength over 180 minutes, and the results show that it has a good stability. The time response of the sensor is also analyzed. The estimated response time of the sensor is about 144 ms.

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