sensors Article
Optical Fiber Temperature and Torsion Sensor Based on Lyot-Sagnac Interferometer Li-Yang Shao *, Xinpu Zhang *, Haijun He, Zhiyong Zhang, Xihua Zou, Bin Luo, Wei Pan and Lianshan Yan Center for Information Photonics & Communications, School of Information Science and Technology, Southwest Jiaotong University, Chengdu 611756, China; [email protected] (H.H.); [email protected] (Z.Z.); [email protected] (X.Z.); [email protected] (B.L.); [email protected] (W.P.); [email protected] (L.Y.) * Correspondances: [email protected] (L.-Y.S.); [email protected] (X.Z.); Tel./Fax: +86-28-6636-7465 (L.-Y.S.) Academic Editor: Vittorio M. N. Passaro Received: 25 August 2016; Accepted: 20 October 2016; Published: 24 October 2016
Abstract: An optical fiber temperature and torsion sensor has been proposed by employing the Lyot-Sagnac interferometer, which was composed by inserting two sections of high-birefringence (HiBi) fiber into the Sagnac loop. The two inserted sections of HiBi fiber have different functions; while one section acts as the temperature sensitive region, the other can be used as reference fiber. The temperature and twist sensor based on the proposed interferometer structure have been experimentally demonstrated. The experimental results show that the envelope of the output spectrum will shift with the temperature evolution. The temperature sensitivity is calculated to be −17.99 nm/◦ C, which is enlarged over 12 times compared to that of the single Sagnac interferometer. Additionally, the fringe visibility of the spectrum will change due to the fiber twist, and the test results reveal that the fringe visibility and twist angle perfectly conform to a Sine relationship over a 360◦ twist angle. Consequently, simultaneous torsion and temperature measurement could be realized by detecting the envelope shift and fringe visibility of the spectrum. Keywords: fiber sensor; high-birefringence fiber
temperature and torsion sensor;
Lyot-Sagnac interferometer;
1. Introduction Fiber sensors based on a Sagnac interferometer have been widely used in parameter monitoring due to their potential applications and their distinctive advantages over traditional sensors, such as immunity to electromagnetic radiation, resistance to chemical corrosion, miniature size, high accuracy, and remote operation [1,2]. Normally, when a section of high-birefringence (HiBi) Fiber is inserted in the Sagnac loop, this will introduce optical path difference between two counter-propagating waves and cause an interference spectrum, which can be used as investigation signal for sensing [3,4]. Recently, the applications of fiber Sagnac interferometer-based sensors are the subject of considerable research, such as temperature sensors [5,6], pressure sensors [7,8], strain sensors [9,10], and biochemical sensors [11,12]. Among them, temperature sensors based on Sagnac interferometers with conventional HiBi fiber have been demonstrated and used in high-sensitivity temperature sensing (about 1.46 nm/◦ C) [13]. Especially, Qian et al. proposed an alcohol-filled HiBi-PCF (photonic crystal fiber) Sagnac sensor to improve the sensitivity up to 6.6 nm/◦ C [14]. Dong proposed a sensitivity-enhanced temperature sensor based on the cladding mode interference by splicing a mode mis-matched dispersion compensation fiber between single mode fibers [15]. Yan et al. demonstrated highly-sensitive temperature and strain sensors based on an all-fiber Lyot filter structure,
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45◦ -tilted
which is formed by concatenating two fiber gratings with a polarization-maintaining fiber formed by concatenating two 45°-tilted fiber gratings with a polarization-maintaining fiber cavity cavity [16,17]. However, the fabrication of this sensor requires complicated and time-consuming [16,17]. However, the fabrication of this sensor requires complicated and time-consuming fabricating fabricating procedures, which increase the cost and complexity of fabrication and operation. procedures, which increase the cost and complexity of fabrication and operation. Although this Although this method can provide high-sensitivity measurement, the sensor fabrication procedures method can provide high-sensitivity measurement, the sensor fabrication procedures are are complicated, which restricts the practical applications of this sensor. Besides, torsion sensors are complicated, which restricts the practical applications of this sensor. Besides, torsion sensors are an an important application of fiber Sagnac interferometers [18]. However, the Sagnac interferometer important application of fiber Sagnac interferometers [18]. However, the Sagnac interferometer based based on low-birefringence photonic crystal fiber is temperature-insensitive, making it difficult on low-birefringence photonic crystal fiber is temperature-insensitive, making it difficult to obtain to obtain real-time temperature variation and realize simultaneous measurement of temperature real-time temperature variation and realize simultaneous measurement of temperature and torsion. and torsion. In this paper, we present a Lyot-Sagnac interferometer by inserting two sections of highIn this paper, we present a Lyot-Sagnac interferometer by inserting two sections of birefringence fibers and two polarization controllers (PCs) into the fiber loop, and experimentally high-birefringence fibers and two polarization controllers (PCs) into the fiber loop, and experimentally demonstrate its high sensitivity temperature and twist-sensing capability. The experimental results demonstrate its high sensitivity temperature and twist-sensing capability. The experimental results show that the proposed sensor has high sensitivity to temperature, and a 17.99 nm/°C temperature show that the proposed sensor has high sensitivity to temperature, and a 17.99 nm/◦ C temperature sensitivity is obtained. Furthermore, we calibrated and tested the twist responses of the proposed sensitivity is obtained. Furthermore, we calibrated and tested the twist responses of the proposed sensor, and the test results reveal that the fringe visibility and twist angle perfectly conform to Sine sensor, and the test results reveal that the fringe visibility and twist angle perfectly conform to Sine relationship. Therefore, the twist angle can be detected and evaluated by measuring the evolution of relationship. Therefore, the twist angle can be detected and evaluated by measuring the evolution of the fringe visibility. the fringe visibility.
2. Operation 2. OperationPrinciple Principle The schematic schematicdiagram diagramofofthe theproposed proposed Lyot-Sagnac interferometer is shown in Figure The The Lyot-Sagnac interferometer is shown in Figure 1. The1.light light source used in the experiment is a broadband light source, and an optical spectrum analyzer source used in the experiment is a broadband light source, and an optical spectrum analyzer (OSA) was (OSA) wastoemployed to measure the spectra. interference The Sagnac interferometer consists of a employed measure the interference The spectra. Sagnac interferometer consists of a conventional conventional 3-dB fiber single-mode fiber (SMF) and two inserted sections HiBi fiber the 3-dB single-mode (SMF) coupler and coupler two inserted sections of HiBi fiberofwithin the within fiber loop. fiber loop. As shown in Figure 1, the 3 dB coupler splits the input signal equally into two signals. The As shown in Figure 1, the 3 dB coupler splits the input signal equally into two signals. The two signals two signals counter-propagate through fiberthey loopinterfere before they interfere again atWith the coupler. With counter-propagate through the fiber loopthe before again at the coupler. a broadband a broadband light source at the input, the output transmission spectrum is approximately a periodic light source at the input, the output transmission spectrum is approximately a periodic function of function of the wavelength. the wavelength.
Figure setup of the interferometer. ASE: Amplified spontaneous emission; Figure 1.1.Experimental Experimental setup of Lyot-Sagnac the Lyot-Sagnac interferometer. ASE: Amplified spontaneous HiBi: High-birefringence; OSA: Optical spectrum analyzer; PC: Polarization controller. emission; HiBi: High-birefringence; OSA: Optical spectrum analyzer; PC: Polarization controller.
HiBi fiber have different functions; whilewhile one section is used the The two two inserted insertedsections sectionsofof HiBi fiber have different functions; one section isas used temperature sensitivesensitive element, element, the other the can act as reference fiber. The temperature as the temperature other can act as reference fiber. The perturbationtemperature induced birefringence birefringence change of HiBichange fiber 1ofresults a phase difference between the twobetween signals perturbation-induced HiBi in fiber 1 results in a phase difference andtwo thensignals causesand a shift ofcauses the output interference spectrum. Temperature variation can bevariation determined the then a shift of the output interference spectrum. Temperature can by determined measuring the shift of the interference spectrum. To verify the torsion characteristics be by wavelength measuring the wavelength shift of the interference spectrum. To verify the torsion of the Sagnac interferometers, we induced a torsion device aontorsion the single mode between thefiber two characteristics of the Sagnac interferometers, we induced device on fiber the single mode sections of HiBi fiber. One end of the SMF was fixed firmly, and the other end was twisted by the between the two sections of HiBi fiber. One end of the SMF was fixed firmly, and the other end was torsion device. twisted by the torsion device. As mentioned above, the transmission spectrum of the Lyot-Sagnac interferometer can be given by [19]
T cos 1 2 sin(1 3 ) cos 2 cos 1 2 cos(1 3 )sin 2 2 2
2
(1)
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As mentioned above, the transmission spectrum of the Lyot-Sagnac interferometer can be given by [19] 2 ϕ1 + ϕ2 ϕ1 − ϕ2 T = cos sin(θ1 + θ3 )cosθ2 + cos cos(θ1 + θ3 )sinθ2 2 2
(1)
where ϕ1 = 2πBL1 /λ and ϕ2 = 2πBL2 /λ are the phase differences between the two orthogonal modes caused by HiBi fiber 1 (length L1 ) and HiBi fiber 2 (length L2 ), respectively. θ1 and θ3 are the angles between the polarization directions of counter propagating beams and the corresponding fast axes of the two sections of HiBi fiber. θ2 is the angle between the fast axes of the two sections of HiBi fiber. In the Sagnac interferometer, HiBi fiber 1 functions as the temperature sensing fiber; when HiBi fiber 2 is isolated to external temperature perturbations, ϕ1 can be expressed as ϕ1 = ϕ10 + ∆ϕ10 = 2π ( B + Kt ∆T ) L1 /λ
(2)
where Kt and ∆T are the birefringence-temperature coefficient and the variations of temperature, respectively. Additionally, θ2 (the angle between the fast axes of the two sections of HiBi fiber) can be modulated by torsion, and the ∆θ2 can be given by [18] ∆θ2 = gφ
(3)
where φ and g are the twist angle and a constant of SMF, respectively, and g depends on the photoelastic coefficients of the material; for a standard SMF, g = 0.08. Therefore, Equation (1) can be written as h i2 ϕ +∆ϕ + ϕ ϕ +∆ϕ − ϕ T = cos 1 2 1 2 sin(θ1 + θ3 )cos(θ2 + gφ) + cos 1 2 1 2 cos(θ1 + θ3 )sin(θ2 + gφ)
(4)
where the change of ∆ϕ1 and ∆θ2 are completely independent of one another, ∆ϕ1 is caused by temperature perturbations, and ∆θ2 is determined only by torsion, so this interferometer can be used for simultaneous temperature and torsion measurement. 3. Experiment and Results In the experiment, a broadband amplified spontaneous emission (ASE) source with 60 nm wavelength range was used as a light source. The transmission spectrum of the Lyot-Sagnac interferometer was measured by an optical spectrum analyzer with a wavelength resolution of 0.02 nm. The HiBi fiber used here was Panda fiber (PM1550 125-18/250) fabricated by Yangtze Optical Fiber and Cable Company Ltd.; the phase modal birefringence B is about 6.9 × 10−4 . HiBi fibers (2.2 m and 2 m) were inserted into the fiber loop and labeled as HiBi fiber 1 and HiBi fiber 2, respectively. Figure 2 illustrates the spectra of the conventional Sagnac and Lyot-Sagnac interferometers at the 1550 nm wavelength band (at 20 ◦ C, without torsion). From Figure 2, we can clearly observe that the Lyot-Sagnac interferometer has a more dense free spectral range and sharper resonance peaks than the conventional Sagnac interferometer, which is beneficial for the sensor resolution. The extinction ratio of the Lyot-Sagnac interferometer between the transmission maxima and the transmission minima is about 26 dB around 1550 nm. The spacing between two adjacent transmission minima is 1.73 nm at 1550 nm.
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Figure 2. Spectra theconventional conventional Sagnac Sagnac and interferometers. Figure 2. Spectra of of the andLyot-Sagnac Lyot-Sagnac interferometers.
During the individual temperature sensing process, HiBi fiber 1 was put into a programmable
During the individual temperaturein sensing process, HiBi fiber 1 wasHiBi putfiber into1a(temperature programmable tubular furnace for temperature order toSagnac realizeand easy measurement, Figure 2. Spectra oftesting; the conventional Lyot-Sagnac interferometers. tubular furnace for temperature testing; in order to realize easy measurement, HiBi 1 sensing fiber) was coiled into a small circle to put into the programmable tubular furnace, andfiber a (temperature sensing fiber) was coiled into a small circle to put into the programmable tubular thermocouple was used for temperature standard temperature calibration fiber 1. into We characterized the During the individual sensing process, HiBinear fiberHiBi 1 was put a programmable furnace, and a thermocouple used for standard temperature calibration near fiber 1. temperature performance bywas tracking peak of the easy transmission spectrum from theHiBi Sagnac tubular furnace for temperature testing; inaorder to realize measurement, HiBi fiber 1 (temperature interferometer. Thetemperature temperature range was increased from 41 °Cofwith atransmission step furnace, size of 0.1 °C. sensing fiber) was coiled into a small circle to put the 40 programmable tubular and a We characterized the performance by into tracking atopeak the spectrum Figure 3a presents thefortransmission spectra ofcalibration this was fibernear Lyot-Sagnac versus was used standard temperature HiBi fiber 1.interferometer We thea step from thermocouple the Sagnac interferometer. The temperature range increased from 40 characterized to 41 ◦ C with temperature variations at 40byand 40.4transmission °C.aAs the temperature there are obvious spectrum temperature performance tracking peak of spectra the transmission spectrum from the Sagnac size of 0.1 ◦ C. Figure 3a presents the ofincreases, this fiber Lyot-Sagnac interferometer shifts. As shownThe in Figure 3a, therange spectrum the Lyot-Sagnac interferometer significant blueinterferometer. temperature was of increased from 40 to 41 °C with ahas step size of 0.1 °C. ◦ versus temperature variations at 40 and 40.4 C. As the temperature increases, there are obvious shifts with temperature. Figure 3b illustrates temperature characteristics of this fiber Figure 3a increasing presents the transmission spectra of this the fiber Lyot-Sagnac interferometer versus spectrum shifts. As shown in Figure 3a, the spectrum of the Lyot-Sagnac interferometer has significant Lyot-Sagnac variations interferometer; spectra shifted linearly as there the temperature temperature at 40 the andtransmission 40.4 °C. As the temperature increases, are obviousincreased. spectrum blue-shifts with temperature. Figure 3b Lyot-Sagnac illustrates temperature characteristics of this As shown inincreasing Figure 3b, when betweenthe 40 and 41 °C, from the linear fit, the shifts. As shown in Figure 3a,the thetemperature spectrum ofchanges the interferometer has significant bluefiber Lyot-Sagnac interferometer; the transmission spectra shifted linearly as the temperature increased. temperature sensitivitytemperature. is −17.99 nm/°C for this fiber Lyot-Sagnac interferometer. To demonstrate the shifts with increasing Figure 3b illustrates the temperature characteristics of this fiber As shown in Figure 3b, when the thetransmission temperature changes between 41 ◦ C, we from the linear high temperature sensitivity characteristics of spectra this fibershifted Lyot-Sagnac interferometer, compared it fit, Lyot-Sagnac interferometer; linearly40 asand the temperature increased. the temperature is −17.99 nm/◦ C forFrom this fiber Lyot-Sagnac interferometer. To demonstrate with the conventional interferometer. Figure 3b, the interferometer has As shown insensitivity Figure 3b,Sagnac when the temperature changes between 40 Lyot-Sagnac and 41 °C, from the linear fit, thea much highersensitivity temperature sensitivity that of Lyot-Sagnac the conventional one (1.46To nm/°C). Thecompared high temperature is −17.99 nm/°C than for this fiber interferometer. demonstrate the the high temperature sensitivity characteristics of this fiber Lyot-Sagnac interferometer, we temperature response is induced by the Vernier effect, which is an efficient method for the high temperature sensitivity characteristics of this fiber Lyot-Sagnac interferometer, we compared it has it with the conventional Sagnac interferometer. From Figure 3b, the Lyot-Sagnac interferometer improvement of temperature sensitivity and enhancing measurement accuracy [13]; when the with the conventional Sagnac interferometer. From Figure 3b, the Lyot-Sagnac interferometer has ◦ a much higher temperature sensitivity than that of the conventional one (1.46 nm/ C). Thea high temperature the transmission of thethat single loop sensorone will(1.46 shift,nm/°C). and thenThe the high shift much higherchanges, temperature sensitivity than of Sagnac the conventional temperature response is induced by the Vernier effect, which is an efficient method for the improvement of Lyot-Sagnac envelope be magnified by a certain of its high temperature temperature response is will induced by the Vernier effect, factor. which Because is an efficient method for the of temperature sensitivity and enhancing measurement accuracy [13]; when the temperature changes, sensitivity, thisof interferometer can meet and highenhancing temperature sensitivity accuracy requirements various improvement temperature sensitivity measurement [13]; in when the the transmission of the single Sagnac sensor will shift, and thewill shift ofresolution. Lyot-Sagnac envelope applications, changes, especially in transmission the fieldsloop thatofrequire highSagnac temperature measurement temperature the the single loopthen sensor shift, and then the shift will be a certainwill factor. Because of high temperature sensitivity, thistemperature interferometer of magnified Lyot-Sagnacbyenvelope be magnified byits a certain factor. Because of its high can meet high temperature sensitivity requirements in various applications, especially the fields sensitivity, this interferometer can meet high temperature sensitivity requirements in in various applications, in the fields that require high temperature measurement resolution. that require high especially temperature measurement resolution.
Figure 3. (a) Spectra of Lyot-Sagnac interferometer at 40.0 °C and 40.4 °C; (b) Temperature responses of the Lyot-Sagnac interferometer. Figure 3. (a) Spectra of Lyot-Sagnac interferometer at 40.0 °C and 40.4 °C; (b) Temperature responses
Figure 3. (a) Spectra of Lyot-Sagnac interferometer at 40.0 ◦ C and 40.4 ◦ C; (b) Temperature responses of the Lyot-Sagnac interferometer. of the Lyot-Sagnac interferometer.
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To thethe torsion-sensing capability and torsion of the Lyot-Sagnac To verify verify torsion-sensing capability and characteristics torsion characteristics of theinterferometer, Lyot-Sagnac we tested the torsion responses of the Lyot-Sagnac interferometer at room temperature. of interferometer, we tested the torsion responses of the Lyot-Sagnac interferometerOne at end room the SMF in between sections of HiBi the fiber was fixed firmly, other end was twisted temperature. One endthe of two the SMF in between two sections of HiBiand fiberthe was fixed firmly, and the by the torsion device. When torsion is applied to the SMF, the angle between the fast axes of the other end was twisted by the torsion device. When torsion is applied to the SMF, the angle between the two sections of HiBi fiber will change, resulting in changes in the fringe visibility of the spectrum fast axes of the two sections of HiBi fiber will change, resulting in changes in the fringe visibility of the in the interferometer. Consequently, the fringe visibility the applied torsion. Therefore, spectrum in the interferometer. Consequently, the fringechanges visibilitywith changes with the applied torsion. the applied torsion can be detected and evaluated by measuring the evolution of fringe Therefore, the applied torsion can be detected and evaluated by measuring the evolution visibility. of fringe In order toIn clearly the change fringe visibility, we performed fastperformed Fourier-transform on the visibility. orderobserve to clearly observeinthe change in fringe visibility, awe a fast Fourierspectra of the interferometer to obtain the spatial frequency spectra. Figure 4a illustrates the spatial transform on the spectra of the interferometer to obtain the spatial frequency spectra. Figure 4a ◦ . Figure 4b shows the response spectra of the interferometer versus twist angle variations at 0 angle and 180 illustrates the spatial spectra of the interferometer versus twist variations at 0 and 180°. Figure 4b as a function of the twist angle. As shown in Figure 4b, the intensity and twist angle perfectlyand conform shows the response as a function of the twist angle. As shown in Figure 4b, the intensity twist to Sine relationship. The experimental results are in good agreement with the theoretical analysis angle perfectly conform to Sine relationship. The experimental results are in good agreement from with Equation (4). the theoretical analysis from Equation (4).
Figure 4. (a) Fast-Fourier transform (FFT) of the spectra; (b) Torsion responses of the Figure 4. (a) Fast-Fourier transform (FFT) of the spectra; (b) Torsion responses of the Lyot-Sagnac Lyot-Sagnac interferometer. interferometer.
In practical applications, multi-parameter simultaneous measurement is an important subject for many fiber sensors; in order to understand the cross influence between temperature and torsion measurements, interferometer-based sensor by measurements, we we investigated investigatedthe theperformances performancesofofthe theLyot-Sagnac Lyot-Sagnac interferometer-based sensor introducing the alternative variation of temperature and torsion. Firstly,Firstly, in orderintoorder studytothe effectthe of by introducing the alternative variation of temperature and torsion. study torsion on temperature characteristic, the temperature responses were measured under different twist effect of torsion on temperature characteristic, the temperature responses were measured under angles. Figure shows the transmission spectra of the Lyot-Sagnac at 20.0 and 20.2 ◦ C different twist5angles. Figure 5 shows the transmission spectra ofinterferometer the Lyot-Sagnac interferometer ◦ when twist is set the at 100 . Figure the temperature increases, spectrum at 20.0the and 20.2angle °C when twist angle 5isshows set at that, 100°.asFigure 5 shows that, as the the temperature showed blue shift and the total wavelength shift was up to 3.56 nm, and the fringe visibility increases, the spectrum showed blue shift and the total wavelength shift was up to 3.56 nm, was and largely unchanged. 6a presents the wavelength versusthe thewavelength temperatureshift when the twist the fringe visibility Figure was largely unchanged. Figure 6ashift presents versus the ◦ , 20◦ , 40◦ , 60◦ , 80◦ , and 100◦ . Under different twist angles, the temperature angle is maintained at 0twist temperature when the angle is maintained at 0°, 20°, 40°, 60°, 80°, and 100°. Under different sensitivities the same. From the linear fit, thethe temperature arethe approximately twist angles,are thealmost temperature sensitivities are almost same. Fromsensitivities the linear fit, temperature ◦ − 18 nm/ C.are Figure 6b shows the of shows the Lyot-Sagnac versus 100◦Lyot-Sagnac twist angle sensitivities approximately −18torsion nm/°C.response Figure 6b the torsionsensor response of the variations. Within this twist angle range, there is a linear relation between the intensity and twist angle. sensor versus 100° twist angle variations. Within this twist angle range, there is a linear relation ◦ The intensity increasedand linearly the twist changed by 100 , and the measurement can by be between the intensity twistasangle. The angle intensity increased linearly as the twist anglerange changed seen thethe linear area of therange measurement range in Figure 100°, as and measurement can be seen as shown the linear area of4b. the measurement range shown in
Figure 4b.
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◦°C ◦°C. Figure5. Spectraof Lyot-Sagnac sensor at Figure 5.5.Spectra Spectra ofofLyot-Sagnac Lyot-Sagnac sensor at at the the20.0 20.0°C and20.2 20.2°C. Figure the 20.0 Cand and 20.2 C.
Figure Temperatureresponses responsesof ofthe theLyot-Sagnac Lyot-Sagnac interferometer twist angles; Figure 6. 6. (a)(a) Temperature interferometerunder underdifferent different twist angles; FigureTorsion 6. (a) Temperature responses of theinterferometer-based Lyot-Sagnac interferometer responses the Lyot-Sagnac sensor. under different twist angles; (b)(b) Torsion responses ofof the Lyot-Sagnac interferometer-based sensor. (b) Torsion responses of the Lyot-Sagnac interferometer-based sensor.
Conclusions 4. 4. Conclusions 4. Conclusions We propose a Lyot-Sagnac interferometer by inserting two sections of HiBi fibers into the fiber We propose a Lyot-Sagnac interferometer by inserting two sections of HiBi fibers into the fiber We propose a Lyot-Sagnac interferometer by inserting and two twist-sensing sections of HiBi fibers into thetwo fiber loop and experimentally demonstrating its temperature capability. The loop and experimentally demonstrating its temperature and twist-sensing capability. The two inserted loop and experimentally its temperature and twist-sensing capability. The two inserted sections of HiBi demonstrating fiber have different functions; while one section acts as the temperaturesections of HiBi fiber have different functions; while one section acts as the temperature-sensitive sensitive region,ofthe other canhave be used as a reference fiber. Theone experimental results show that the inserted sections HiBi fiber different functions; while section acts as the temperatureregion, the other can be used as a reference fiber. The experimental results show that the proposed proposed sensor highcan sensitivity 17.99experimental nm/°C temperature sensitive region, thehas other be usedtoastemperature, a reference and fiber.a The results sensitivity show thatisthe ◦ C temperature sensitivity sensor has high sensitivity tocalibrated temperature, and athe 17.99 nm/ is and obtained. obtained. Furthermore, we and tested twist responses of the proposed sensor, the is proposed sensor has high sensitivity to temperature, and a 17.99 nm/°C temperature sensitivity Furthermore, we calibrated and tested the twist responses of the proposed sensor, and the test results test results reveal thatwe thecalibrated fringe visibility and twist angle perfectlyof conform to Sinesensor, relationship. obtained. Furthermore, and tested the twist responses the proposed and the reveal that the fringe visibility and twist angle perfectlyby conform to Sine relationship. Therefore, Therefore, the twist angle can be detected and evaluated measuring the evolution of the fringe test results reveal that the fringe visibility and twist angle perfectly conform to Sine relationship. thevisibility. twist angle can beofdetected and evaluatedsensitivity, by measuring the evolution ofand the low fringe visibility. Because itscan high of fabrication, this Therefore, the twist angle be temperature detected and evaluated ease by measuring the evolution ofcost, the fringe Because of its high temperature sensitivity, ease in of different fabrication, andFurther low cost, this interferometer could interferometer could find various applications areas. investigation will focus visibility. Because of its high temperature sensitivity, ease of fabrication, and low cost, on this find various applications in different areas. Further investigation will focus on the optimization of the the optimization of the structure parameters for different practical applications. interferometer could find various applications in different areas. Further investigation will focus on structure parametersThe forresearch differentsupported practical in applications. part by the National Natural Science Foundation of China (No. theAcknowledgments: optimization of the structureis parameters for different practical applications. 61475128), the International Science and Technology Cooperation Program of China (2014DFA11170), the Key Acknowledgments: The research researchisissupported supported in part National Natural Science Foundation of China Acknowledgments: The part byby thethe National Natural Science Foundation (No. Grant Project of Chinese Ministry of Educationin(No.313049) and the Fundamental Fund for of theChina Central (No. 61475128), the International Science and Technology Cooperation Program of Research China (2014DFA11170), the Key 61475128), the International Science and Technology Cooperation Program of China (2014DFA11170), the Key Universities Grant Project of(2682014RC22). Chinese Ministry of Education (No.313049) and the Fundamental Research Fund for the Central Grant Project of Chinese Ministry of Education (No.313049) and the Fundamental Research Fund for the Central Universities (2682014RC22). Author Contributions: Li-yang Shao conceived and supervised the study and wrote the manuscript; Xinpu Universities (2682014RC22). Zhang designed and Li-yang performed theconceived experiments wrote the Haijun and Zhiyong Zhang Author Contributions: Shao and and supervised the manuscript; study and wrote theHe manuscript; Xinpu Zhang designed andthe performed the experiments andLuo, wrote the manuscript; and Zhiyong Zhang supported Author Contributions: Li-yang Shao conceived and supervised theHaijun study and wrote the Xinpu supported experiments; Xihua Zou, Bin Wei Pan and Lianshan YanHe supervised the manuscript; study and wrote the experiments; Xihua Zou, Binthe Luo, Wei Pan and supervised the study andZhiyong wrote part of part of the manuscript. Zhang designed and performed experiments and Lianshan wrote theYan manuscript; Haijun He and Zhang the manuscript. supported the experiments; Xihua Zou, Bin Luo, Wei Pan and Lianshan Yan supervised the study and wrote Conflicts of Interest: The authors declare no conflict of interest. Conflicts Interest: The authors declare no conflict of interest. part of theofmanuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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Liu, Y.; Liu, B.; Feng, X.; Zhang, W.; Zhou, G.; Yuan, S.; Kai, G.; Dong, X. High-birefringence fiber loop mirrors and their applications as sensors. Appl. Opt. 2005, 44, 2382–2390. [CrossRef] [PubMed] Andre, M.; Marques, M.; Roy, P.; Frazao, O. Fiber loop mirror using a small core microstructured fiber for strain and temperature discrimination. IEEE Photonics Technol. Lett. 2010, 22, 1120–1122. [CrossRef] Silva, R.; Layeghi, A.; Zibaii, M.; Latifi, H.; Santos, J.; Frazao, O. Theoretical and experimental results of high-birefringent fiber loop mirror with an output port probe. J. Lightwave Technol. 2012, 30, 1032–1036. [CrossRef] Dong, X.; Tam, H.; Shum, P. Temperature-insensitive strain sensor with polarization-maintaining photonic crystal fiber based Sagnac interferometer. Appl. Phys. Lett. 2007, 90. [CrossRef] Wo, J.; Sun, Q.; Liu, H.; Li, X.; Zhang, J.; Liu, D.; Shum, P. Sensitivity-enhanced fiber optic temperature sensor with strain response suppression. Opt. Fiber Technol. 2013, 19, 289–292. [CrossRef] Favero, F.; Spittel, R.; Just, F.; Kobelke, J.; Rothhardt, M.; Bartelt, H. A miniature temperature high germanium doped PCF interferometer sensor. Opt. Express 2013, 21, 30266–30274. [CrossRef] [PubMed] Fu, H.; Wu, C.; Tse, M.; Zhang, L.; Cheng, K.; Tam, H.; Guan, B.; Lu, C. High pressure sensor based on photonic crystal fiber for downhole application. Appl. Opt. 2010, 49, 2639–2643. [CrossRef] Cho, L.; Wu, C.; Lu, C.; Tam, H. A Highly Sensitive and Low-Cost Sagnac Loop Based Pressure Sensor. IEEE Sens. J. 2013, 13, 3073–3078. [CrossRef] Gong, H.; Chan, C.; Chen, L.; Dong, X. Strain Sensor Realized by Using Low-Birefringence Photonic-Crystal-Fiber-Based Sagnac Loop. IEEE Photonics Technol. Lett. 2010, 22, 1238–1240. [CrossRef] Sun, G.; Moon, D.; Chung, Y. Simultaneous Temperature and Strain Measurement Using Two Types of High-Birefringence Fibers in Sagnac Loop Mirror. IEEE Photonics Technol. Lett. 2007, 19, 2027–2029. [CrossRef] Tom, C.; Wim, B.; Peter, B. Experimental characterization of a silicon photonic biosensor consisting of two cascaded ring resonators based on the Vernier-effect and introduction of a curve fitting method for an improved detection limit. Opt. Express 2010, 18, 22747–22761. Wu, C.; Tse, M.; Liu, Z.; Guan, B.; Lu, C.; Tam, H. In-line microfluidic refractometer based on C-shaped fiber assisted photonic crystal fiber Sagnac interferometer. Opt. Lett. 2013, 38, 3283–3286. [CrossRef] [PubMed] Shao, L.; Luo, Y.; Zhang, Z.; Zou, X.; Luo, B.; Pan, W.; Yan, L. Sensitivity-enhanced temperature sensor with cascaded fiber optic Sagnac interferometers based on Vernier-effect. Opt. Commun. 2015, 336, 73–76. [CrossRef] Qian, W.; Zhao, C.; He, S.; Dong, X.; Zhang, S.; Zhang, Z.; Jin, S.; Guo, J.; Wei, H. High-sensitivity temperature sensor based on an alcohol-filled photonic crystal fiber loop mirror. Opt. Lett. 2011, 36, 1548–1550. [CrossRef] [PubMed] Dong, B.; Wei, L.; Zhou, D. Miniature high-sensitivity high-temperature fiber sensor with a dispersion compensation fiber-based interferometer. Appl. Opt. 2009, 48, 6466–6469. [CrossRef] [PubMed] Yan, Z.; Wang, H.; Zhou, K.; Wang, Y.; Zhao, W.; Zhang, L. Broadband tunable all-fiber polarization interference filter based on 45◦ tilted fiber gratings. J. Lightw. Technol. 2013, 31, 94–98. [CrossRef] Yan, Z.; Mou, C.; Wang, H.; Zhou, K.; Wang, Y.; Zhao, W.; Zhang, L. All-fiber polarization interference filters based on 45◦ -tilted fiber gratings. Opt. Lett. 2012, 37, 353–355. [CrossRef] [PubMed] Zu, P.; Xiang, W.; Jin, Y. Fabrication of temperature-insensitive twist sensor using low birefringent photonic crystal fiber based sagnac interferometer. Acta. Opt. Sin. 2011, 40, 1433–1437. Zu, P.; Chan, C.C.; Koh, G.W.; Lew, W.S.; Jin, Y.; Liew, H.F.; Wong, W.C.; Dong, X. Enhancement of the sensitivity of magneto-optical fiber sensor by magnifying the birefringence of magnetic fluid film with Lyot–Sagnac interferometer. Sens. Actuators B Chem. 2014, 191, 19–23. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
Optical Fiber Temperature and Torsion Sensor Based on Lyot-Sagnac Interferometer Li-Yang Shao *, Xinpu Zhang *, Haijun He, Zhiyong Zhang, Xihua Zou, Bin Luo, Wei Pan and Lianshan Yan Center for Information Photonics & Communications, School of Information Science and Technology, Southwest Jiaotong University, Chengdu 611756, China; [email protected] (H.H.); [email protected] (Z.Z.); [email protected] (X.Z.); [email protected] (B.L.); [email protected] (W.P.); [email protected] (L.Y.) * Correspondances: [email protected] (L.-Y.S.); [email protected] (X.Z.); Tel./Fax: +86-28-6636-7465 (L.-Y.S.) Academic Editor: Vittorio M. N. Passaro Received: 25 August 2016; Accepted: 20 October 2016; Published: 24 October 2016
Abstract: An optical fiber temperature and torsion sensor has been proposed by employing the Lyot-Sagnac interferometer, which was composed by inserting two sections of high-birefringence (HiBi) fiber into the Sagnac loop. The two inserted sections of HiBi fiber have different functions; while one section acts as the temperature sensitive region, the other can be used as reference fiber. The temperature and twist sensor based on the proposed interferometer structure have been experimentally demonstrated. The experimental results show that the envelope of the output spectrum will shift with the temperature evolution. The temperature sensitivity is calculated to be −17.99 nm/◦ C, which is enlarged over 12 times compared to that of the single Sagnac interferometer. Additionally, the fringe visibility of the spectrum will change due to the fiber twist, and the test results reveal that the fringe visibility and twist angle perfectly conform to a Sine relationship over a 360◦ twist angle. Consequently, simultaneous torsion and temperature measurement could be realized by detecting the envelope shift and fringe visibility of the spectrum. Keywords: fiber sensor; high-birefringence fiber
temperature and torsion sensor;
Lyot-Sagnac interferometer;
1. Introduction Fiber sensors based on a Sagnac interferometer have been widely used in parameter monitoring due to their potential applications and their distinctive advantages over traditional sensors, such as immunity to electromagnetic radiation, resistance to chemical corrosion, miniature size, high accuracy, and remote operation [1,2]. Normally, when a section of high-birefringence (HiBi) Fiber is inserted in the Sagnac loop, this will introduce optical path difference between two counter-propagating waves and cause an interference spectrum, which can be used as investigation signal for sensing [3,4]. Recently, the applications of fiber Sagnac interferometer-based sensors are the subject of considerable research, such as temperature sensors [5,6], pressure sensors [7,8], strain sensors [9,10], and biochemical sensors [11,12]. Among them, temperature sensors based on Sagnac interferometers with conventional HiBi fiber have been demonstrated and used in high-sensitivity temperature sensing (about 1.46 nm/◦ C) [13]. Especially, Qian et al. proposed an alcohol-filled HiBi-PCF (photonic crystal fiber) Sagnac sensor to improve the sensitivity up to 6.6 nm/◦ C [14]. Dong proposed a sensitivity-enhanced temperature sensor based on the cladding mode interference by splicing a mode mis-matched dispersion compensation fiber between single mode fibers [15]. Yan et al. demonstrated highly-sensitive temperature and strain sensors based on an all-fiber Lyot filter structure,
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45◦ -tilted
which is formed by concatenating two fiber gratings with a polarization-maintaining fiber formed by concatenating two 45°-tilted fiber gratings with a polarization-maintaining fiber cavity cavity [16,17]. However, the fabrication of this sensor requires complicated and time-consuming [16,17]. However, the fabrication of this sensor requires complicated and time-consuming fabricating fabricating procedures, which increase the cost and complexity of fabrication and operation. procedures, which increase the cost and complexity of fabrication and operation. Although this Although this method can provide high-sensitivity measurement, the sensor fabrication procedures method can provide high-sensitivity measurement, the sensor fabrication procedures are are complicated, which restricts the practical applications of this sensor. Besides, torsion sensors are complicated, which restricts the practical applications of this sensor. Besides, torsion sensors are an an important application of fiber Sagnac interferometers [18]. However, the Sagnac interferometer important application of fiber Sagnac interferometers [18]. However, the Sagnac interferometer based based on low-birefringence photonic crystal fiber is temperature-insensitive, making it difficult on low-birefringence photonic crystal fiber is temperature-insensitive, making it difficult to obtain to obtain real-time temperature variation and realize simultaneous measurement of temperature real-time temperature variation and realize simultaneous measurement of temperature and torsion. and torsion. In this paper, we present a Lyot-Sagnac interferometer by inserting two sections of highIn this paper, we present a Lyot-Sagnac interferometer by inserting two sections of birefringence fibers and two polarization controllers (PCs) into the fiber loop, and experimentally high-birefringence fibers and two polarization controllers (PCs) into the fiber loop, and experimentally demonstrate its high sensitivity temperature and twist-sensing capability. The experimental results demonstrate its high sensitivity temperature and twist-sensing capability. The experimental results show that the proposed sensor has high sensitivity to temperature, and a 17.99 nm/°C temperature show that the proposed sensor has high sensitivity to temperature, and a 17.99 nm/◦ C temperature sensitivity is obtained. Furthermore, we calibrated and tested the twist responses of the proposed sensitivity is obtained. Furthermore, we calibrated and tested the twist responses of the proposed sensor, and the test results reveal that the fringe visibility and twist angle perfectly conform to Sine sensor, and the test results reveal that the fringe visibility and twist angle perfectly conform to Sine relationship. Therefore, the twist angle can be detected and evaluated by measuring the evolution of relationship. Therefore, the twist angle can be detected and evaluated by measuring the evolution of the fringe visibility. the fringe visibility.
2. Operation 2. OperationPrinciple Principle The schematic schematicdiagram diagramofofthe theproposed proposed Lyot-Sagnac interferometer is shown in Figure The The Lyot-Sagnac interferometer is shown in Figure 1. The1.light light source used in the experiment is a broadband light source, and an optical spectrum analyzer source used in the experiment is a broadband light source, and an optical spectrum analyzer (OSA) was (OSA) wastoemployed to measure the spectra. interference The Sagnac interferometer consists of a employed measure the interference The spectra. Sagnac interferometer consists of a conventional conventional 3-dB fiber single-mode fiber (SMF) and two inserted sections HiBi fiber the 3-dB single-mode (SMF) coupler and coupler two inserted sections of HiBi fiberofwithin the within fiber loop. fiber loop. As shown in Figure 1, the 3 dB coupler splits the input signal equally into two signals. The As shown in Figure 1, the 3 dB coupler splits the input signal equally into two signals. The two signals two signals counter-propagate through fiberthey loopinterfere before they interfere again atWith the coupler. With counter-propagate through the fiber loopthe before again at the coupler. a broadband a broadband light source at the input, the output transmission spectrum is approximately a periodic light source at the input, the output transmission spectrum is approximately a periodic function of function of the wavelength. the wavelength.
Figure setup of the interferometer. ASE: Amplified spontaneous emission; Figure 1.1.Experimental Experimental setup of Lyot-Sagnac the Lyot-Sagnac interferometer. ASE: Amplified spontaneous HiBi: High-birefringence; OSA: Optical spectrum analyzer; PC: Polarization controller. emission; HiBi: High-birefringence; OSA: Optical spectrum analyzer; PC: Polarization controller.
HiBi fiber have different functions; whilewhile one section is used the The two two inserted insertedsections sectionsofof HiBi fiber have different functions; one section isas used temperature sensitivesensitive element, element, the other the can act as reference fiber. The temperature as the temperature other can act as reference fiber. The perturbationtemperature induced birefringence birefringence change of HiBichange fiber 1ofresults a phase difference between the twobetween signals perturbation-induced HiBi in fiber 1 results in a phase difference andtwo thensignals causesand a shift ofcauses the output interference spectrum. Temperature variation can bevariation determined the then a shift of the output interference spectrum. Temperature can by determined measuring the shift of the interference spectrum. To verify the torsion characteristics be by wavelength measuring the wavelength shift of the interference spectrum. To verify the torsion of the Sagnac interferometers, we induced a torsion device aontorsion the single mode between thefiber two characteristics of the Sagnac interferometers, we induced device on fiber the single mode sections of HiBi fiber. One end of the SMF was fixed firmly, and the other end was twisted by the between the two sections of HiBi fiber. One end of the SMF was fixed firmly, and the other end was torsion device. twisted by the torsion device. As mentioned above, the transmission spectrum of the Lyot-Sagnac interferometer can be given by [19]
T cos 1 2 sin(1 3 ) cos 2 cos 1 2 cos(1 3 )sin 2 2 2
2
(1)
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As mentioned above, the transmission spectrum of the Lyot-Sagnac interferometer can be given by [19] 2 ϕ1 + ϕ2 ϕ1 − ϕ2 T = cos sin(θ1 + θ3 )cosθ2 + cos cos(θ1 + θ3 )sinθ2 2 2
(1)
where ϕ1 = 2πBL1 /λ and ϕ2 = 2πBL2 /λ are the phase differences between the two orthogonal modes caused by HiBi fiber 1 (length L1 ) and HiBi fiber 2 (length L2 ), respectively. θ1 and θ3 are the angles between the polarization directions of counter propagating beams and the corresponding fast axes of the two sections of HiBi fiber. θ2 is the angle between the fast axes of the two sections of HiBi fiber. In the Sagnac interferometer, HiBi fiber 1 functions as the temperature sensing fiber; when HiBi fiber 2 is isolated to external temperature perturbations, ϕ1 can be expressed as ϕ1 = ϕ10 + ∆ϕ10 = 2π ( B + Kt ∆T ) L1 /λ
(2)
where Kt and ∆T are the birefringence-temperature coefficient and the variations of temperature, respectively. Additionally, θ2 (the angle between the fast axes of the two sections of HiBi fiber) can be modulated by torsion, and the ∆θ2 can be given by [18] ∆θ2 = gφ
(3)
where φ and g are the twist angle and a constant of SMF, respectively, and g depends on the photoelastic coefficients of the material; for a standard SMF, g = 0.08. Therefore, Equation (1) can be written as h i2 ϕ +∆ϕ + ϕ ϕ +∆ϕ − ϕ T = cos 1 2 1 2 sin(θ1 + θ3 )cos(θ2 + gφ) + cos 1 2 1 2 cos(θ1 + θ3 )sin(θ2 + gφ)
(4)
where the change of ∆ϕ1 and ∆θ2 are completely independent of one another, ∆ϕ1 is caused by temperature perturbations, and ∆θ2 is determined only by torsion, so this interferometer can be used for simultaneous temperature and torsion measurement. 3. Experiment and Results In the experiment, a broadband amplified spontaneous emission (ASE) source with 60 nm wavelength range was used as a light source. The transmission spectrum of the Lyot-Sagnac interferometer was measured by an optical spectrum analyzer with a wavelength resolution of 0.02 nm. The HiBi fiber used here was Panda fiber (PM1550 125-18/250) fabricated by Yangtze Optical Fiber and Cable Company Ltd.; the phase modal birefringence B is about 6.9 × 10−4 . HiBi fibers (2.2 m and 2 m) were inserted into the fiber loop and labeled as HiBi fiber 1 and HiBi fiber 2, respectively. Figure 2 illustrates the spectra of the conventional Sagnac and Lyot-Sagnac interferometers at the 1550 nm wavelength band (at 20 ◦ C, without torsion). From Figure 2, we can clearly observe that the Lyot-Sagnac interferometer has a more dense free spectral range and sharper resonance peaks than the conventional Sagnac interferometer, which is beneficial for the sensor resolution. The extinction ratio of the Lyot-Sagnac interferometer between the transmission maxima and the transmission minima is about 26 dB around 1550 nm. The spacing between two adjacent transmission minima is 1.73 nm at 1550 nm.
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Figure 2. Spectra theconventional conventional Sagnac Sagnac and interferometers. Figure 2. Spectra of of the andLyot-Sagnac Lyot-Sagnac interferometers.
During the individual temperature sensing process, HiBi fiber 1 was put into a programmable
During the individual temperaturein sensing process, HiBi fiber 1 wasHiBi putfiber into1a(temperature programmable tubular furnace for temperature order toSagnac realizeand easy measurement, Figure 2. Spectra oftesting; the conventional Lyot-Sagnac interferometers. tubular furnace for temperature testing; in order to realize easy measurement, HiBi 1 sensing fiber) was coiled into a small circle to put into the programmable tubular furnace, andfiber a (temperature sensing fiber) was coiled into a small circle to put into the programmable tubular thermocouple was used for temperature standard temperature calibration fiber 1. into We characterized the During the individual sensing process, HiBinear fiberHiBi 1 was put a programmable furnace, and a thermocouple used for standard temperature calibration near fiber 1. temperature performance bywas tracking peak of the easy transmission spectrum from theHiBi Sagnac tubular furnace for temperature testing; inaorder to realize measurement, HiBi fiber 1 (temperature interferometer. Thetemperature temperature range was increased from 41 °Cofwith atransmission step furnace, size of 0.1 °C. sensing fiber) was coiled into a small circle to put the 40 programmable tubular and a We characterized the performance by into tracking atopeak the spectrum Figure 3a presents thefortransmission spectra ofcalibration this was fibernear Lyot-Sagnac versus was used standard temperature HiBi fiber 1.interferometer We thea step from thermocouple the Sagnac interferometer. The temperature range increased from 40 characterized to 41 ◦ C with temperature variations at 40byand 40.4transmission °C.aAs the temperature there are obvious spectrum temperature performance tracking peak of spectra the transmission spectrum from the Sagnac size of 0.1 ◦ C. Figure 3a presents the ofincreases, this fiber Lyot-Sagnac interferometer shifts. As shownThe in Figure 3a, therange spectrum the Lyot-Sagnac interferometer significant blueinterferometer. temperature was of increased from 40 to 41 °C with ahas step size of 0.1 °C. ◦ versus temperature variations at 40 and 40.4 C. As the temperature increases, there are obvious shifts with temperature. Figure 3b illustrates temperature characteristics of this fiber Figure 3a increasing presents the transmission spectra of this the fiber Lyot-Sagnac interferometer versus spectrum shifts. As shown in Figure 3a, the spectrum of the Lyot-Sagnac interferometer has significant Lyot-Sagnac variations interferometer; spectra shifted linearly as there the temperature temperature at 40 the andtransmission 40.4 °C. As the temperature increases, are obviousincreased. spectrum blue-shifts with temperature. Figure 3b Lyot-Sagnac illustrates temperature characteristics of this As shown inincreasing Figure 3b, when betweenthe 40 and 41 °C, from the linear fit, the shifts. As shown in Figure 3a,the thetemperature spectrum ofchanges the interferometer has significant bluefiber Lyot-Sagnac interferometer; the transmission spectra shifted linearly as the temperature increased. temperature sensitivitytemperature. is −17.99 nm/°C for this fiber Lyot-Sagnac interferometer. To demonstrate the shifts with increasing Figure 3b illustrates the temperature characteristics of this fiber As shown in Figure 3b, when the thetransmission temperature changes between 41 ◦ C, we from the linear high temperature sensitivity characteristics of spectra this fibershifted Lyot-Sagnac interferometer, compared it fit, Lyot-Sagnac interferometer; linearly40 asand the temperature increased. the temperature is −17.99 nm/◦ C forFrom this fiber Lyot-Sagnac interferometer. To demonstrate with the conventional interferometer. Figure 3b, the interferometer has As shown insensitivity Figure 3b,Sagnac when the temperature changes between 40 Lyot-Sagnac and 41 °C, from the linear fit, thea much highersensitivity temperature sensitivity that of Lyot-Sagnac the conventional one (1.46To nm/°C). Thecompared high temperature is −17.99 nm/°C than for this fiber interferometer. demonstrate the the high temperature sensitivity characteristics of this fiber Lyot-Sagnac interferometer, we temperature response is induced by the Vernier effect, which is an efficient method for the high temperature sensitivity characteristics of this fiber Lyot-Sagnac interferometer, we compared it has it with the conventional Sagnac interferometer. From Figure 3b, the Lyot-Sagnac interferometer improvement of temperature sensitivity and enhancing measurement accuracy [13]; when the with the conventional Sagnac interferometer. From Figure 3b, the Lyot-Sagnac interferometer has ◦ a much higher temperature sensitivity than that of the conventional one (1.46 nm/ C). Thea high temperature the transmission of thethat single loop sensorone will(1.46 shift,nm/°C). and thenThe the high shift much higherchanges, temperature sensitivity than of Sagnac the conventional temperature response is induced by the Vernier effect, which is an efficient method for the improvement of Lyot-Sagnac envelope be magnified by a certain of its high temperature temperature response is will induced by the Vernier effect, factor. which Because is an efficient method for the of temperature sensitivity and enhancing measurement accuracy [13]; when the temperature changes, sensitivity, thisof interferometer can meet and highenhancing temperature sensitivity accuracy requirements various improvement temperature sensitivity measurement [13]; in when the the transmission of the single Sagnac sensor will shift, and thewill shift ofresolution. Lyot-Sagnac envelope applications, changes, especially in transmission the fieldsloop thatofrequire highSagnac temperature measurement temperature the the single loopthen sensor shift, and then the shift will be a certainwill factor. Because of high temperature sensitivity, thistemperature interferometer of magnified Lyot-Sagnacbyenvelope be magnified byits a certain factor. Because of its high can meet high temperature sensitivity requirements in various applications, especially the fields sensitivity, this interferometer can meet high temperature sensitivity requirements in in various applications, in the fields that require high temperature measurement resolution. that require high especially temperature measurement resolution.
Figure 3. (a) Spectra of Lyot-Sagnac interferometer at 40.0 °C and 40.4 °C; (b) Temperature responses of the Lyot-Sagnac interferometer. Figure 3. (a) Spectra of Lyot-Sagnac interferometer at 40.0 °C and 40.4 °C; (b) Temperature responses
Figure 3. (a) Spectra of Lyot-Sagnac interferometer at 40.0 ◦ C and 40.4 ◦ C; (b) Temperature responses of the Lyot-Sagnac interferometer. of the Lyot-Sagnac interferometer.
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To thethe torsion-sensing capability and torsion of the Lyot-Sagnac To verify verify torsion-sensing capability and characteristics torsion characteristics of theinterferometer, Lyot-Sagnac we tested the torsion responses of the Lyot-Sagnac interferometer at room temperature. of interferometer, we tested the torsion responses of the Lyot-Sagnac interferometerOne at end room the SMF in between sections of HiBi the fiber was fixed firmly, other end was twisted temperature. One endthe of two the SMF in between two sections of HiBiand fiberthe was fixed firmly, and the by the torsion device. When torsion is applied to the SMF, the angle between the fast axes of the other end was twisted by the torsion device. When torsion is applied to the SMF, the angle between the two sections of HiBi fiber will change, resulting in changes in the fringe visibility of the spectrum fast axes of the two sections of HiBi fiber will change, resulting in changes in the fringe visibility of the in the interferometer. Consequently, the fringe visibility the applied torsion. Therefore, spectrum in the interferometer. Consequently, the fringechanges visibilitywith changes with the applied torsion. the applied torsion can be detected and evaluated by measuring the evolution of fringe Therefore, the applied torsion can be detected and evaluated by measuring the evolution visibility. of fringe In order toIn clearly the change fringe visibility, we performed fastperformed Fourier-transform on the visibility. orderobserve to clearly observeinthe change in fringe visibility, awe a fast Fourierspectra of the interferometer to obtain the spatial frequency spectra. Figure 4a illustrates the spatial transform on the spectra of the interferometer to obtain the spatial frequency spectra. Figure 4a ◦ . Figure 4b shows the response spectra of the interferometer versus twist angle variations at 0 angle and 180 illustrates the spatial spectra of the interferometer versus twist variations at 0 and 180°. Figure 4b as a function of the twist angle. As shown in Figure 4b, the intensity and twist angle perfectlyand conform shows the response as a function of the twist angle. As shown in Figure 4b, the intensity twist to Sine relationship. The experimental results are in good agreement with the theoretical analysis angle perfectly conform to Sine relationship. The experimental results are in good agreement from with Equation (4). the theoretical analysis from Equation (4).
Figure 4. (a) Fast-Fourier transform (FFT) of the spectra; (b) Torsion responses of the Figure 4. (a) Fast-Fourier transform (FFT) of the spectra; (b) Torsion responses of the Lyot-Sagnac Lyot-Sagnac interferometer. interferometer.
In practical applications, multi-parameter simultaneous measurement is an important subject for many fiber sensors; in order to understand the cross influence between temperature and torsion measurements, interferometer-based sensor by measurements, we we investigated investigatedthe theperformances performancesofofthe theLyot-Sagnac Lyot-Sagnac interferometer-based sensor introducing the alternative variation of temperature and torsion. Firstly,Firstly, in orderintoorder studytothe effectthe of by introducing the alternative variation of temperature and torsion. study torsion on temperature characteristic, the temperature responses were measured under different twist effect of torsion on temperature characteristic, the temperature responses were measured under angles. Figure shows the transmission spectra of the Lyot-Sagnac at 20.0 and 20.2 ◦ C different twist5angles. Figure 5 shows the transmission spectra ofinterferometer the Lyot-Sagnac interferometer ◦ when twist is set the at 100 . Figure the temperature increases, spectrum at 20.0the and 20.2angle °C when twist angle 5isshows set at that, 100°.asFigure 5 shows that, as the the temperature showed blue shift and the total wavelength shift was up to 3.56 nm, and the fringe visibility increases, the spectrum showed blue shift and the total wavelength shift was up to 3.56 nm, was and largely unchanged. 6a presents the wavelength versusthe thewavelength temperatureshift when the twist the fringe visibility Figure was largely unchanged. Figure 6ashift presents versus the ◦ , 20◦ , 40◦ , 60◦ , 80◦ , and 100◦ . Under different twist angles, the temperature angle is maintained at 0twist temperature when the angle is maintained at 0°, 20°, 40°, 60°, 80°, and 100°. Under different sensitivities the same. From the linear fit, thethe temperature arethe approximately twist angles,are thealmost temperature sensitivities are almost same. Fromsensitivities the linear fit, temperature ◦ − 18 nm/ C.are Figure 6b shows the of shows the Lyot-Sagnac versus 100◦Lyot-Sagnac twist angle sensitivities approximately −18torsion nm/°C.response Figure 6b the torsionsensor response of the variations. Within this twist angle range, there is a linear relation between the intensity and twist angle. sensor versus 100° twist angle variations. Within this twist angle range, there is a linear relation ◦ The intensity increasedand linearly the twist changed by 100 , and the measurement can by be between the intensity twistasangle. The angle intensity increased linearly as the twist anglerange changed seen thethe linear area of therange measurement range in Figure 100°, as and measurement can be seen as shown the linear area of4b. the measurement range shown in
Figure 4b.
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◦°C ◦°C. Figure5. Spectraof Lyot-Sagnac sensor at Figure 5.5.Spectra Spectra ofofLyot-Sagnac Lyot-Sagnac sensor at at the the20.0 20.0°C and20.2 20.2°C. Figure the 20.0 Cand and 20.2 C.
Figure Temperatureresponses responsesof ofthe theLyot-Sagnac Lyot-Sagnac interferometer twist angles; Figure 6. 6. (a)(a) Temperature interferometerunder underdifferent different twist angles; FigureTorsion 6. (a) Temperature responses of theinterferometer-based Lyot-Sagnac interferometer responses the Lyot-Sagnac sensor. under different twist angles; (b)(b) Torsion responses ofof the Lyot-Sagnac interferometer-based sensor. (b) Torsion responses of the Lyot-Sagnac interferometer-based sensor.
Conclusions 4. 4. Conclusions 4. Conclusions We propose a Lyot-Sagnac interferometer by inserting two sections of HiBi fibers into the fiber We propose a Lyot-Sagnac interferometer by inserting two sections of HiBi fibers into the fiber We propose a Lyot-Sagnac interferometer by inserting and two twist-sensing sections of HiBi fibers into thetwo fiber loop and experimentally demonstrating its temperature capability. The loop and experimentally demonstrating its temperature and twist-sensing capability. The two inserted loop and experimentally its temperature and twist-sensing capability. The two inserted sections of HiBi demonstrating fiber have different functions; while one section acts as the temperaturesections of HiBi fiber have different functions; while one section acts as the temperature-sensitive sensitive region,ofthe other canhave be used as a reference fiber. Theone experimental results show that the inserted sections HiBi fiber different functions; while section acts as the temperatureregion, the other can be used as a reference fiber. The experimental results show that the proposed proposed sensor highcan sensitivity 17.99experimental nm/°C temperature sensitive region, thehas other be usedtoastemperature, a reference and fiber.a The results sensitivity show thatisthe ◦ C temperature sensitivity sensor has high sensitivity tocalibrated temperature, and athe 17.99 nm/ is and obtained. obtained. Furthermore, we and tested twist responses of the proposed sensor, the is proposed sensor has high sensitivity to temperature, and a 17.99 nm/°C temperature sensitivity Furthermore, we calibrated and tested the twist responses of the proposed sensor, and the test results test results reveal thatwe thecalibrated fringe visibility and twist angle perfectlyof conform to Sinesensor, relationship. obtained. Furthermore, and tested the twist responses the proposed and the reveal that the fringe visibility and twist angle perfectlyby conform to Sine relationship. Therefore, Therefore, the twist angle can be detected and evaluated measuring the evolution of the fringe test results reveal that the fringe visibility and twist angle perfectly conform to Sine relationship. thevisibility. twist angle can beofdetected and evaluatedsensitivity, by measuring the evolution ofand the low fringe visibility. Because itscan high of fabrication, this Therefore, the twist angle be temperature detected and evaluated ease by measuring the evolution ofcost, the fringe Because of its high temperature sensitivity, ease in of different fabrication, andFurther low cost, this interferometer could interferometer could find various applications areas. investigation will focus visibility. Because of its high temperature sensitivity, ease of fabrication, and low cost, on this find various applications in different areas. Further investigation will focus on the optimization of the the optimization of the structure parameters for different practical applications. interferometer could find various applications in different areas. Further investigation will focus on structure parametersThe forresearch differentsupported practical in applications. part by the National Natural Science Foundation of China (No. theAcknowledgments: optimization of the structureis parameters for different practical applications. 61475128), the International Science and Technology Cooperation Program of China (2014DFA11170), the Key Acknowledgments: The research researchisissupported supported in part National Natural Science Foundation of China Acknowledgments: The part byby thethe National Natural Science Foundation (No. Grant Project of Chinese Ministry of Educationin(No.313049) and the Fundamental Fund for of theChina Central (No. 61475128), the International Science and Technology Cooperation Program of Research China (2014DFA11170), the Key 61475128), the International Science and Technology Cooperation Program of China (2014DFA11170), the Key Universities Grant Project of(2682014RC22). Chinese Ministry of Education (No.313049) and the Fundamental Research Fund for the Central Grant Project of Chinese Ministry of Education (No.313049) and the Fundamental Research Fund for the Central Universities (2682014RC22). Author Contributions: Li-yang Shao conceived and supervised the study and wrote the manuscript; Xinpu Universities (2682014RC22). Zhang designed and Li-yang performed theconceived experiments wrote the Haijun and Zhiyong Zhang Author Contributions: Shao and and supervised the manuscript; study and wrote theHe manuscript; Xinpu Zhang designed andthe performed the experiments andLuo, wrote the manuscript; and Zhiyong Zhang supported Author Contributions: Li-yang Shao conceived and supervised theHaijun study and wrote the Xinpu supported experiments; Xihua Zou, Bin Wei Pan and Lianshan YanHe supervised the manuscript; study and wrote the experiments; Xihua Zou, Binthe Luo, Wei Pan and supervised the study andZhiyong wrote part of part of the manuscript. Zhang designed and performed experiments and Lianshan wrote theYan manuscript; Haijun He and Zhang the manuscript. supported the experiments; Xihua Zou, Bin Luo, Wei Pan and Lianshan Yan supervised the study and wrote Conflicts of Interest: The authors declare no conflict of interest. Conflicts Interest: The authors declare no conflict of interest. part of theofmanuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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