- Email: [email protected]
Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure
G Model
ARTICLE IN PRESS
SNB-23093; No. of Pages 5
Sensors and Actuators B xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Research Paper
Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure WenChuan Yan a , Qun Han a,∗ , Yaofei Chen a,c , Huiling Song a , Xiaoyun Tang a , Tiegen Liu a,b a
College of Precision Instrument and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China Key Laboratory of Opto-electronics Information Technology (Tianjin University) Ministry of Education, Tianjin 300072, China c Department of Mechanical Engineering, Columbia University, New York 10027, USA b
a r t i c l e
i n f o
Article history: Received 23 May 2017 Received in revised form 23 August 2017 Accepted 1 September 2017 Available online xxx Keywords: Optical fiber sensors Fiber loop ring-down Refractive index Refractometer
a b s t r a c t In this paper, a fiber-loop ring-down interrogated refractive index sensor based on a single-mode-nocore-single-mode (SNS) fiber structure is reported. The influence of the wavelength and pulsewidth on the performance of the sensor are systematically investigated. Results show that the performance of the sensing system is strongly dependent on the interrogation wavelength and a narrower pulsewidth is helpful to avoid the disturbance of the relaxation oscillation. A method to determine the optimal interrogation wavelength was proposed. After determining the interrogation wavelength and pulsewidth, refractive index (RI) measurement was experimentally performed in the RI range of 1.3330–1.3539. And a cubic polynomial fitting was established between RI and ring-down time. In the linear region, a sensitivity of −3271 s/RIU was achieved. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Refractive index (RI) measurement is important to many biological and chemical applications [1,2]. In the past several years, fiber RI sensors based on several different structures, such as a long period grating [3,4], an etched fiber Bragg grating [5], a photonic crystal fiber [6], a Mach-Zehnder interferometer [7], and singlemode-multimode-single-mode fiber structures [2,8], have been proposed. Demodulation of these sensors are either wavelengthbased or intensity-based. However, wavelength demodulation requires an expensive and bulky optical spectrum analyzer (OSA) to track the spectrum change. Whereas the accuracy of intensity demodulation is prone to the interference of the power fluctuation of the light source. Fiber-loop ring-down (FLRD) interrogation provides an effective way to overcome these problems by measuring the ring-down time of a short pulse cycling in a fiber loop instead of the spectral or intensity change [9]. This method has been used with several different fiber structures to realize measurement of the number of mammalian cancer cells [10], concentration of chemical solutions [11] or gas [12], electrical current [13], magnetic field [14], pressure [15], and RI [1,15–18], etc.
∗ Corresponding author. E-mail address: [email protected] (Q. Han).
It is well-known that the detection sensitivity of a FLRD system is proportional to the number of pulses within the ring-down time [15]. To compensate the cavity loss thus increasing the number of pulses, an erbium-doped fiber amplifier (EDFA) is usually inserted in the fiber loop. However, besides the beneficial gain, the EDFA also introduces amplified spontaneous emission (ASE) noise in the same time [19]. To reduce noise and increase the pulse number, a narrow-band fiber Brag grating (FBG) is commonly used to filter the ASE through wavelength selective reflection [20,21]. So the wavelength of the interrogation pulse has to be precisely matched with the central wavelength of the FBG. However, the center wavelength of the FBG is temperature-sensitive. A mismatch with the wavelength of the interrogation pulse will lead to unstable loss in the ring cavity [20]. In this paper, an FLRD-interrogated RI sensing system based on a single-mode-no-core-single-mode (SNS) fiber structure is proposed and experimentally demonstrated, for the first time to the best of our knowledge. Instead of using a bandpass filter, we show that by properly selecting the wavelength and width of the interrogation pulse the system performance can be optimized. The influence of the wavelength and the width of the interrogation pulse on the performance of the sensing system is systematically investigated. The results show that because both the gain of the EDFA and the loss of the SNS structure are wavelengthdependent, the performance of the sensing system depends greatly on the interrogation wavelength. An optimal wavelength can be
http://dx.doi.org/10.1016/j.snb.2017.09.002 0925-4005/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002
G Model SNB-23093; No. of Pages 5
ARTICLE IN PRESS W. Yan et al. / Sensors and Actuators B xxx (2017) xxx–xxx
2
Fig. 2. Schematic diagram of the (a) the SNS structure and (b) the experimental arrangement.
Fig. 1. Fiber loop ring-down system with an SNS fiber structure.
determined by examining the lasing wavelength of the cavity. We also found that a narrower pulsewidth is helpful to reduce the disturbance of relaxation oscillation. A demonstration sensor was fabricated and characterized. The results show that, the relationship between the ring-down time and RI can be well fitted by a cubic function. In the linear range, a sensitivity of −3271 s/RIU was achieved. Compared with the FLRD interrogated fiber sensors based on other structures [14–18], the SNS structure features an easy fabrication, low-cost, and good stability. 2. working principles The schematic diagram of the sensing system is shown in Fig. 1. The fiber loop consists of a 3 dB coupler, a fiber delay line (DL), an EDFA, and an SNS structure. The interrogation pulses are generated by switching the output of a tunable laser (TLS, AQ8460) with an ultra-fast ceramic optical switch [22] which is pulse-triggered by a function generator (Agilent 33220A). The laser pulses are injected into the fiber-loop cavity via one arm of the coupler. A trail of decayed pulse train is detected by a photodiode (PD, DET01CFC, Thorlabs) and recorded by an oscilloscope (TDS 2022B, Tektronix). Finally, the data are sent to a computer for processing. The peak intensity of the output pulses I decays exponentially with time t, and can be expressed as [14,15] t I(t) = I0 exp(− ), t = (N − 1)T, N = 1, 2, 3, · · ·
(1)
where I0 is the intensity of the first ring-down pulse, is the ringdown time of the system which is defined as the time when I ¯ decreases to I0 /e, T = nl/cis the trip time which is defined as the ¯ time for the pulse to cycling the whole fiber loop, nand l are the average refractive index and the total length of the loop, respectively, c is the speed of light in vacuum, and N is a sequence number of the out pulses. For a given fiber loop, the ring-down time is determined by the net loss of the cavity and can be expressed as [14]: =
T A−G+V
(2)
where A represents the intrinsic loss of the cavity including the transmission loss, splicing loss, and output loss of the coupler, G is the gain of the EDFA, and V is the loss introduced by the SNS structure. These three parameters are all wavelength dependent. And G is also dependent on the energy (intensity × pulsewidth) of the interrogation pulse. For a given fiber-loop and a given interrogation pulse, A and G are constant while V changes with the measurand. Fig. 2a shows the structure of the SNS. It consists of a section of no-core fiber (NCF) sandwiched between two sections of conven-
tional single-mode fibers (SMFs). The NCF is made of fused silica. Its RI at 1550 nm is about 1.4440. RI values at other wavelengths can be calculated by the Sellmeier equation [23]. Assuming that the SMFs and NCF are ideally aligned, only the symmetric LP0m modes will be excited when the fundamental mode of the input SMF propagates into the NCF. Multimode interference will occur when these modes are coupled back to the output SMF. The transmittance of the SNS at wavelength can be calculated by [2,24] T () =
M
ci2 cj2 · cos[(ˇi − ˇj )L]
(3)
i,j=1
where M is the total number of modes excited in the NCF, ci and ˇi are the excitation coefficient and propagation constant of the LP0m mode, respectively, and L is the length of the NCF. When the RI of the medium surrounding the NCF changes, ci and ˇi will change. So T() in Eq. (3) or V in Eq. (2) will change accordingly. This in turn changes the ring-down time when interrogated with a pulse at a specific wavelength. So by measuring the corresponding RI change can be achieved. 3. Experiments and discussions As Eq.(2) reveals that the ring-down time of a FLRD system is dependent on the wavelength and width of the interrogation pulse, so to ensure the sensing system operate properly and optimize its performance, the influence of the wavelength and width of the pulse must be characterized. In the experiment, an SNS was fabricated by fusion splicing a ∼4.92 cm NCF (Prime Optical Fiber Co.) with a diameter of 80 m between two standard SMFs (SMF28, Corning Inc.). Two ends of the SNS were fixed to a holder to keep the SNS straight and slightly above the plate as shown in Fig. 2b. The sensing medium was dripped on the plate by an injector until the SNS section was well immersed. A coil of SMF with a length of ∼1.42 km was inserted into the cavity to serve as a delay line. A C-band tunable EDFA with a relative gain profile as shown in Fig. 3a was used to compensate the cavity loss. The length of the NCF was determined by numerical simulation, as shown in Fig. 3a, to ensure the transmission peak of the resulted SNS is near the gain peak of the EDFA. To ensure a monotonic change of the transmission loss with the increase of the RI, the transmission peak of the SNS should be at or slightly to the right of the gain peak of the EDFA when the SNS is immersed in a liquid with the lowest RI of the measurement range. The influence of the wavelength and pulsewidth of the interrogation pulse was experimentally investigated by immersing the SNS in distilled water with n = 1.3330 (starting RI of the measurement range). And then with the optimized wavelength and pulsewidth, the performance of the sensor was demonstrated. A. Influence of the interrogation wavelength The simulated and measured spectral responses of the SNS to RI are shown in Fig. 3a and b respectively. The relative gain profile of
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002
G Model SNB-23093; No. of Pages 5
ARTICLE IN PRESS W. Yan et al. / Sensors and Actuators B xxx (2017) xxx–xxx
3
Fig. 4. Lasing spectrum. The lasing wavelength is 1527.3 nm.
Table 1 Number of pulses N within the ring-down time when interrogated with pulses at different wavelength .
Fig. 3. (a) Simulated transmission spectra change with RI and relative gain of the EDFA, and (b) measured transmission spectra change with RI of the fabricated SNS.
(nm)
1525
1526
1527.3
1528
1529
1530
1535
1540
N
1
1
7
6
3
2
1
1
the EDFA is also show in Fig. 3a. From the figure we can see that both the gain spectrum of the EDFA and the transmission spectrum of the SNS are wavelength dependent. In our previous work [14], to assess the influence of the wavelength on the ring-down time, we introduce a quality factor Q, which is defined as Q () = |
G() | ı()
(4)
where ı() is the loss of the fiber loop cavity, including A and V in Eq. (2), and G() is the gain of the EDFA. A higher Q means that more loss can be compensated by the EDFA with lower pump power (because with a lower Q, pump power has to be increased to reach higher ringdown pulse number). And thus a higher pulses number, or ring-down time can be attained before severe ASE and relaxation oscillation [14]. In this paper, we proposed a simple experimental method to determine the optimal wavelength which leads to the highest Q. Compared with checking each wavelength and making comparison, this method is more precise and effective because it omits comparison and calculation. The optimal wavelength can be determined as following: without input of the interrogation pulse, we increase the gain of the EDFA. As the gain increases over the lasing threshold of the ring-down cavity, lasing oscillation will be observed at the optimal wavelength 0 . Because based on the principle of lasers Q(0 ) is greater than Q() at any other wavelength. So when interrogating at wavelength 0 , the transmission loss V can be best compensated by the EDFA and the maximum ring-down time can be expected. In a word, the optimal interrogation wavelength can be determined as the lasing wavelength of the idle cavity when we increase the gain of the EDFA slightly above the threshold. Now we experimentally verify that 0 is indeed the optimal interrogation wavelength that brings maximum number of pulses within the ring-down time. The lasing spectrum of the idle cavity with n = 1.3330 is shown in Fig. 4. We can see that the lasing wavelength 0 appears at 1527.3 nm. To verify that 0 is the optimal wavelength, wavelength of the interrogating pluses (1.5 s pulsewidth) was changed around 0 . At each wavelength, the gain of the EDFA was gradually increased from 0 dB until just before obvious relaxation oscillation was observed. Table 1 shows the relationship between interrogation wavelength and the pulse number within the ring-down time, which is proportional to the ring-down time in the decay train. As an example, the measured decay train when the interrogation wavelengths are 1527.3 nm and 1529 nm are show in Fig. 5a and b respectively. The red vertical dash line indicates the corresponding ring-down time. From Table 1 we can
Fig. 5. Influence of the interrogation wavelength on the decay train. (a) = 1527.3 nm and (b) = 1529.0 nm.
see that the pulse number changes dramatically with the interrogation wavelength, and the maximum pulse number is indeed obtained at 1527.3 nm. So in practice, in order to attain maximum ring-down time and so to increase the sensitivity of the sensing system, the interrogating wavelength should be set at the optimal wavelength. And for a given sensor, the optimal wavelength can be obtained by observing the lasing wavelength of the idle cavity under the starting RI of the measurement range. B. Influence of the pulsewidth As discussed above the gain of the EDFA should be properly set to achieve the most pulses in the decay train without the occurrence of relaxation oscillation. Besides the wavelength, the gain characteristic of the EDFA is also influenced by the width of the input pulse. We experimentally found that the pulsewidth could influence the amplitude of the relaxation oscillation. To demonstrate this phenomenon, we kept the gain of the EDFA constant, and tuned the TLS to the optimal wavelength, i.e. 1527.3 nm. Then set the width of the injected light pulse to 1.5 s, 3.5 s, and 5.5 s, respectively. The measured output signals are shown in Fig. 6. We can see that with the increase of the pulsewidth, the deviation of the bottom of the pulses from the base line becomes more and more severe. This phenomenon can be explained as following. A wider light pulse being amplified means more inverted population in the EDFA are engaged, and thus the continuous wave (CW) ASE power
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002
G Model SNB-23093; No. of Pages 5 4
ARTICLE IN PRESS W. Yan et al. / Sensors and Actuators B xxx (2017) xxx–xxx
Fig. 8. Decayed ring-down pulse train of the sensor as the surrounding RI is 1.3330.
Fig. 9. Ring-down time as a function of the surrounding RI of the sensor.
Fig. 6. Observation of the influence of the pulse width on the relaxation oscillation.
Fig. 7. Influence of the pule width on the ring-down time of the sensor.
or CW lasing power determined by the inverted population are disturbed more severely [19]. So the severity of relaxation oscillation increases with the increase of the pulsewidth. To relive relaxation oscillation, the gain of the EDFA must be properly decreased with the increase of the pulsewidth. This in return, will decrease the ring-down time and so influence the detection sensitivity of the sensor. To avoid relaxation oscillation and obtain more ring-down pulses, the pulsewidth must also be properly set. Another experiment was conducted to investigate the influence of the pulsewidth on the sensing system. Light signals with different pulsewidths at 1527.3 nm were injected into the fiber ring cavity under a RI of 1.3330. At each pulsewidth, the gain of the EDFA was gradually increased until the maximum number of pulses was recorded just before obvious relaxation oscillation. We calculated the ring-down time of each output decay train and the experimental results are shown in Fig. 7. From Fig. 7 we can see that with the increase of the pulsewidth the ring-down time decreases. This is because with a narrower pulse, more gain of the EDFA is allowable, while with a wider pulse the gain of the EDFA must be properly decreased to
avoid relaxation oscillation. So in practice a narrower pulsewidth is preferable because it can increase ring-down time of the decay train and increase the detection sensitivity of the sensor. However, in practice, the pulsewidth can’t be too narrow that the pulse peak would not be detected due to the limited sample rate of the detection instruments. So it is also important to carefully choose the pulsewidth depending on the detection limit of the instruments. C. Measurement result and discussion After determined the wavelength and width of the interrogation pulse, the sensor system depicted in Fig. 1 was used to measure RI of sucrose solution with different concentration (The experiment RIs are 1.3330, 1.3360, 1.3389, 1.3420, 1.3450, 1.3480, 1.3509, 1.3539. And the related sucrose concentration can be obtained in this reference [25]). The measurement was conducted at a room temperature of 20 ◦ C. The SNS was initially covered by distilled water with a RI of 1.3330. The TLS was tuned to the optimal wavelength, 1527.3 nm. The pulsewidth was set to 1.5 s. The gain of the EDFA was tuned to obtain maximum ring-down pulses just before obvious relaxation oscillation. The total cavity net loss is estimated to be about 0.56 dB (coupling loss 3.28 dB, delay line loss 0.29 dB, SNS insertion loss in water 8.41 dB, and EDFA gain 11.42 dB).The decayed ring-down pulse train is shown in Fig. 8. By fitting the peaks of the pulses with an exponential function similar to Eq. (1), we can get the ring-down time . For this case is 51.93 s and the pulse number within the ringdown time is 7, which is comparable to that achieved using FBG filters [20,21]. So by optimizing the wavelength and width of the interrogation pulse the system complicity can be reduced by omitting the FBG filters. As shown in Fig. 3, the transmittance of the SNS structure will decrease with the increase of the surrounding RI at 1527.3 nm, so the ring-down time will decrease accordingly. Fig. 9 shows the experimental ring-down time as a function of the surrounding RI as it was being changed from 1.3330 to 1.3539 during our measurement. For comparison, simulation results with parameters similar to those used in the experiment are also shown in Fig. 9. From Fig. 9 we can see that the experimental results agree fairly well with the simulation results. Because with the increase of the RI, the loss introduced by the SNS increases and the number of pulses in the decay train decreases, so there is an upper limit to the working range of the sensor. From Fig. 9 we can see that the RI and ring-down time generally follows a nonlinear relationship.
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002
G Model SNB-23093; No. of Pages 5
ARTICLE IN PRESS W. Yan et al. / Sensors and Actuators B xxx (2017) xxx–xxx
They can be fitted very well with a cubic polynomial. In the linear region from 1.3330 to 1.3420, the sensitivity is about −3271 s/RIU as revealed by the linear fitting in Fig. 9. 4. Conclusions In conclusion, a novel FLRD interrogated RI sensor based on SNS fiber structure is proposed in this paper for the first time to the best of our knowledge. An EDFA was introduced into the fiber ring cavity to maximize the ring-down time and enhance the detection sensitivity of the sensing system. The influence of the wavelength and width of the interrogation pulse on the performance of the sensor was systematically investigated. We found that an optimal wavelength can be determined by examining the lasing wavelength of the idle cavity. We also found that a narrower input light pulse is helpful to reduce the influence of the relaxation oscillation. A demonstration sensor was fabricated and experimentally demonstrated by measuring the RI of sucrose solution with different concentration in the RI range of 1.3330–1.3539. The results show that the RI and ring-down time follows a nonlinear relationship and can be very well fitted by a cubic polynomial. In the relatively linear region, a sensitivity of −3271 s/RIU was obtained. The sensor is easy to be fabricated and immune to the power fluctuation of the light source in nature. It can be used in chemical or bio-chemical applications to measure the RI change. Acknowledgement This work is supported by the National Natural Science Foundation of China (No. 61107035), the National Key Scientific Instrument and Equipment Development Project of China(No. 2013YQ03091502), and the National Basic Research Program of China (973 Program, grant 2014CB340104). References [1] Xu Liu, Qi Wang, Chunyue Li, Chengwu Zhao, Yong Zhao, Haifeng Hu, Jin Li, Refractive index sensor based on fiber loop ring-down spectroscopy, Instrumentation Science and Technology 44 (2016) 241–248. [2] Yaofei Chen, Qun Han, Tiegen Liu, Xiaoying Lü, Self-temperature-compensative refractometer based on singlemode–multimode–singlemode fiber structure, Sensors and Actuators B: Chemical 212 (2015) 107–111. [3] Qun Han, Xinwei Lan, Jie Huang, Amardeep Kaur, Long-Period Grating Inscribed on Concatenated Double-Clad and Single-Clad Fiber for Simultaneous Measurement of Temperature and Refractive Index, IEEE Photonics Technology Letters 24 (2012) 1130–1132. [4] L. Coelho, D. Viegas, J.L. Santos, J.M.M.M. De Almeida, Enhanced refractive index sensing characteristics of optical fibre long period grating coated with titanium dioxide thin films, Sensors and Actuators B Chemical 202 (2014) 929–934. [5] A. Iadicicco, S. Campopiano, A. Cutolo, M. Giordano, A. Cusano, Self temperature referenced refractive index sensor by non-uniform thinned fiber Bragg gratings, Sensors and Actuators B Chemical 120 (2006) 231–237. [6] Yong Zhao, Xue Gang Li, Lu Cai, Yang Yang, Refractive index sensing based on photonic crystal fiber interferometer structure with up-tapered joints, Sensors & Actuators B Chemical 221 (2015) 406–410. [7] Zhaobing Tian, S.H. Yam, J. Barnes, W. Bock, Refractive Index Sensing With Mach–Zehnder Interferometer Based on Concatenating Two Single-Mode Fiber Tapers, IEEE Photonics Technology Letters 20 (2008) 626–628. [8] Y. Wu, P. Wang, G. Farrell, High sensitivity SMS fiber structure based refractometer-analysis and experiment, Optics Express 19 (2011) 7937–7944. [9] George Stewart, Kathryn Atherton, Hongbo Yu, Brian Culshaw, An investigation of an optical fibre amplifier loop for intra-cavity and ring-down cavity loss measurements, Measurement Science and Technology 12 (2001) 843. [10] Peter B. Tarsa, Aislyn D. Wist, Paul Rabinowitz, Kevin K. Lehmann, Single-cell detection by cavity ring-down spectroscopy, Applied Physics Letters 85 (2004) 4523–4525. [11] Cathy M. Rushworth, Dean James, Jason W.L. Lee, Claire Vallance, Top notch design for fiber-loop cavity ring-down spectroscopy, Analytical Chemistry 83 (2011) 8492–8500.
5
[12] Shimizu Hiromasa, Noriyasu Hiroshi, Measurement of carbon dioxide concentration by fiber-loop ring-down spectroscopy for continuous remote measurement, Japanese Journal of Applied Physics 53 (2014) 116601. [13] Qi Wang, Xu Liu, Ji Xia, Yong Zhao, A Novel Long-Tail Fiber Current Sensor Based on Fiber Loop Ring-Down Spectroscopy and Fabry-Perot Cavity Filled With Magnetic Fluid, IEEE Transactions on Instrumentation and Measurement 64 (2015) 2005–2011. [14] Yaofei Chen, Tiegen Liu, Qun Han, Wenchuan Yan, Lin Yu, Fiber loop ring-down cavity integrated U-bent single-mode-fiber for magnetic field sensing, Photonics Research 4 (2016) 322–326. [15] Chuji Wang, Chamini Herath, High-sensitivity fiber-loop ringdown evanescent-field index sensors using single-mode fiber, Optics letters 35 (2010) 1629–1631. [16] K. Zhou, D.J. Webb, C. Mou, M. Farries, N. Hayes, I. Bennion, Optical Fiber Cavity Ring Down Measurement of Refractive Index With a Microchannel Drilled by Femtosecond Laser, IEEE Photonics Technology Letters 21 (2009) 1653–1655. [17] Y.N. Zhang, Y. Zhao, D. Wu, Q. Wang, Fiber Loop Ring-Down Refractive Index Sensor Based on High-Q Photonic Crystal Cavity, IEEE Sensors Journal 14 (2014) 1878–1885. [18] D. Wu, Y. Zhao, Q. Wang, SMF Taper Evanescent Field-Based RI Sensor Combined With Fiber Loop Ring Down Technology, IEEE Photonics Technology Letters 27 (2015) 1802–1805. [19] George Stewart, Peter Shields, Brian Culshaw, Development of fibre laser systems for ring-down and intracavity gas spectroscopy in the near-IR, Measurement Science and Technology 15 (2004) 1621. [20] Wei Chang Wong, Wenjun Zhou, Chi Chiu Chan, Xinyong Dong, Kam Chew Leong, Cavity ringdown refractive index sensor using photonic crystal fiber interferometer, Sensors and Actuators B: Chemical 161 (2012) 108–113. [21] N. Ni, C.C. Chan, L. Xia, P. Shum, Fiber cavity ring-down refractive index sensor, IEEE Photonics Technology Letters 16 (2008) 1351–1353. [22] Lü Xiaoying, Han Qun, Liu Tiegen, Chen Yaofei, Ren Kun, Actively Q-switched erbium-doped fiber ring laser with a nanosecond ceramic optical switch, Laser Physics 24 (2014) 115102. [23] H. Murata, Handbook of Optical Fibers and Cables, 2nd ed., CRC, USA, 1996. [24] Yaofei Chen, Qun Han, Tiegen Liu, Xinwei Lan, Hai Xiao, Optical fiber magnetic field sensor based on single-mode-multimode-single-mode structure and magnetic fluid, Optics Letters 38 (2013) 3999–4001. [25] United States Department of Agriculture, USDA Technical Procedures Manual, United States Department of Agriculture, USA, 2011, pp. 32–56.
Biographies Wenchuan Yan was born in Tianjin, China, in 1991. He received the B.E. degree in optoelectronics from Tianjin University, Tianjin, China, in 2015. He is currently pursuing his M.E degree at the School of Precision Instrument and Opto-electronics Engineering (SPIOE), Tianjin University, Tianjin, China. His research interest is focused on optical fiber sensors and demodulation methods. Qun Han was born in Zibo, Shandong Province, China, in 1977. He received the M.S. degree in optics from Nankai University, Tianjin, China, in 2003, and the Ph.D. degree in physical electronics from Tianjin University, Tianjin, China, in 2006. Since 2006, he has been with the SPIOE of Tianjin University, where he is now a tenured Associated Professor. From 2011 to 2012 he was a Visiting Scholar at Missouri University of Science and Technology, Missouri, USA. His current research interests include fiber sensors, fiber lasers, and high-power fiber amplifiers. He is the author or coauthor of more than 100 journal papers. Dr. Han is a member of the Optical Society of America. Yaofei Chen was born in Luoyang, Henan Province, China,in 1989. He received the B.S. degree in optoelectronics from Changchun University of Science and Technology, Changchun, China, in 2012. He is currently pursuing his Ph.D. degree at SPIOE, Tianjin University, Tianjin, China. His research interest is focused on optical fiber sensors. Huiling Song was born in Xingtai, Hebei Province, China,in 1994. She received the B.S. degree in physics from Heibei University, Baoding, China, in 2016. She is now pursuing her M.E. degree at SPIOE, Tianjin University, Tianjin, China. Her research interest is focused on optical materials for sensing. Xiaoyun Tang was born in Guangan, Sichuan Province, China,in 1995. She received the B.E. degree in optoelectronics from Changchun University of Science and Technology, Changchun, China, in 2016. She is now pursuing her M.E. degree at SPIOE, Tianjin University, Tianjin, China. Her research interest is focused on fiber amplifiers and lasers. Tiegen Liu was born in Tianjin, China, in 1955. He received the B.E. degree, the M.E. degree, and the Ph.D. degree in optical engineering from Tianjin University, China, in 1982, 1987, and 1999, respectively. He joined Tianjin University in 1982, where he is currently a Professor at the College of Precision Instrument and Opto-electronics Engineering. His research interests include optical fiber sensors, optical polarization technology, and optoelectronics measurement.
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002
ARTICLE IN PRESS
SNB-23093; No. of Pages 5
Sensors and Actuators B xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Research Paper
Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure WenChuan Yan a , Qun Han a,∗ , Yaofei Chen a,c , Huiling Song a , Xiaoyun Tang a , Tiegen Liu a,b a
College of Precision Instrument and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China Key Laboratory of Opto-electronics Information Technology (Tianjin University) Ministry of Education, Tianjin 300072, China c Department of Mechanical Engineering, Columbia University, New York 10027, USA b
a r t i c l e
i n f o
Article history: Received 23 May 2017 Received in revised form 23 August 2017 Accepted 1 September 2017 Available online xxx Keywords: Optical fiber sensors Fiber loop ring-down Refractive index Refractometer
a b s t r a c t In this paper, a fiber-loop ring-down interrogated refractive index sensor based on a single-mode-nocore-single-mode (SNS) fiber structure is reported. The influence of the wavelength and pulsewidth on the performance of the sensor are systematically investigated. Results show that the performance of the sensing system is strongly dependent on the interrogation wavelength and a narrower pulsewidth is helpful to avoid the disturbance of the relaxation oscillation. A method to determine the optimal interrogation wavelength was proposed. After determining the interrogation wavelength and pulsewidth, refractive index (RI) measurement was experimentally performed in the RI range of 1.3330–1.3539. And a cubic polynomial fitting was established between RI and ring-down time. In the linear region, a sensitivity of −3271 s/RIU was achieved. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Refractive index (RI) measurement is important to many biological and chemical applications [1,2]. In the past several years, fiber RI sensors based on several different structures, such as a long period grating [3,4], an etched fiber Bragg grating [5], a photonic crystal fiber [6], a Mach-Zehnder interferometer [7], and singlemode-multimode-single-mode fiber structures [2,8], have been proposed. Demodulation of these sensors are either wavelengthbased or intensity-based. However, wavelength demodulation requires an expensive and bulky optical spectrum analyzer (OSA) to track the spectrum change. Whereas the accuracy of intensity demodulation is prone to the interference of the power fluctuation of the light source. Fiber-loop ring-down (FLRD) interrogation provides an effective way to overcome these problems by measuring the ring-down time of a short pulse cycling in a fiber loop instead of the spectral or intensity change [9]. This method has been used with several different fiber structures to realize measurement of the number of mammalian cancer cells [10], concentration of chemical solutions [11] or gas [12], electrical current [13], magnetic field [14], pressure [15], and RI [1,15–18], etc.
∗ Corresponding author. E-mail address: [email protected] (Q. Han).
It is well-known that the detection sensitivity of a FLRD system is proportional to the number of pulses within the ring-down time [15]. To compensate the cavity loss thus increasing the number of pulses, an erbium-doped fiber amplifier (EDFA) is usually inserted in the fiber loop. However, besides the beneficial gain, the EDFA also introduces amplified spontaneous emission (ASE) noise in the same time [19]. To reduce noise and increase the pulse number, a narrow-band fiber Brag grating (FBG) is commonly used to filter the ASE through wavelength selective reflection [20,21]. So the wavelength of the interrogation pulse has to be precisely matched with the central wavelength of the FBG. However, the center wavelength of the FBG is temperature-sensitive. A mismatch with the wavelength of the interrogation pulse will lead to unstable loss in the ring cavity [20]. In this paper, an FLRD-interrogated RI sensing system based on a single-mode-no-core-single-mode (SNS) fiber structure is proposed and experimentally demonstrated, for the first time to the best of our knowledge. Instead of using a bandpass filter, we show that by properly selecting the wavelength and width of the interrogation pulse the system performance can be optimized. The influence of the wavelength and the width of the interrogation pulse on the performance of the sensing system is systematically investigated. The results show that because both the gain of the EDFA and the loss of the SNS structure are wavelengthdependent, the performance of the sensing system depends greatly on the interrogation wavelength. An optimal wavelength can be
http://dx.doi.org/10.1016/j.snb.2017.09.002 0925-4005/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002
G Model SNB-23093; No. of Pages 5
ARTICLE IN PRESS W. Yan et al. / Sensors and Actuators B xxx (2017) xxx–xxx
2
Fig. 2. Schematic diagram of the (a) the SNS structure and (b) the experimental arrangement.
Fig. 1. Fiber loop ring-down system with an SNS fiber structure.
determined by examining the lasing wavelength of the cavity. We also found that a narrower pulsewidth is helpful to reduce the disturbance of relaxation oscillation. A demonstration sensor was fabricated and characterized. The results show that, the relationship between the ring-down time and RI can be well fitted by a cubic function. In the linear range, a sensitivity of −3271 s/RIU was achieved. Compared with the FLRD interrogated fiber sensors based on other structures [14–18], the SNS structure features an easy fabrication, low-cost, and good stability. 2. working principles The schematic diagram of the sensing system is shown in Fig. 1. The fiber loop consists of a 3 dB coupler, a fiber delay line (DL), an EDFA, and an SNS structure. The interrogation pulses are generated by switching the output of a tunable laser (TLS, AQ8460) with an ultra-fast ceramic optical switch [22] which is pulse-triggered by a function generator (Agilent 33220A). The laser pulses are injected into the fiber-loop cavity via one arm of the coupler. A trail of decayed pulse train is detected by a photodiode (PD, DET01CFC, Thorlabs) and recorded by an oscilloscope (TDS 2022B, Tektronix). Finally, the data are sent to a computer for processing. The peak intensity of the output pulses I decays exponentially with time t, and can be expressed as [14,15] t I(t) = I0 exp(− ), t = (N − 1)T, N = 1, 2, 3, · · ·
(1)
where I0 is the intensity of the first ring-down pulse, is the ringdown time of the system which is defined as the time when I ¯ decreases to I0 /e, T = nl/cis the trip time which is defined as the ¯ time for the pulse to cycling the whole fiber loop, nand l are the average refractive index and the total length of the loop, respectively, c is the speed of light in vacuum, and N is a sequence number of the out pulses. For a given fiber loop, the ring-down time is determined by the net loss of the cavity and can be expressed as [14]: =
T A−G+V
(2)
where A represents the intrinsic loss of the cavity including the transmission loss, splicing loss, and output loss of the coupler, G is the gain of the EDFA, and V is the loss introduced by the SNS structure. These three parameters are all wavelength dependent. And G is also dependent on the energy (intensity × pulsewidth) of the interrogation pulse. For a given fiber-loop and a given interrogation pulse, A and G are constant while V changes with the measurand. Fig. 2a shows the structure of the SNS. It consists of a section of no-core fiber (NCF) sandwiched between two sections of conven-
tional single-mode fibers (SMFs). The NCF is made of fused silica. Its RI at 1550 nm is about 1.4440. RI values at other wavelengths can be calculated by the Sellmeier equation [23]. Assuming that the SMFs and NCF are ideally aligned, only the symmetric LP0m modes will be excited when the fundamental mode of the input SMF propagates into the NCF. Multimode interference will occur when these modes are coupled back to the output SMF. The transmittance of the SNS at wavelength can be calculated by [2,24] T () =
M
ci2 cj2 · cos[(ˇi − ˇj )L]
(3)
i,j=1
where M is the total number of modes excited in the NCF, ci and ˇi are the excitation coefficient and propagation constant of the LP0m mode, respectively, and L is the length of the NCF. When the RI of the medium surrounding the NCF changes, ci and ˇi will change. So T() in Eq. (3) or V in Eq. (2) will change accordingly. This in turn changes the ring-down time when interrogated with a pulse at a specific wavelength. So by measuring the corresponding RI change can be achieved. 3. Experiments and discussions As Eq.(2) reveals that the ring-down time of a FLRD system is dependent on the wavelength and width of the interrogation pulse, so to ensure the sensing system operate properly and optimize its performance, the influence of the wavelength and width of the pulse must be characterized. In the experiment, an SNS was fabricated by fusion splicing a ∼4.92 cm NCF (Prime Optical Fiber Co.) with a diameter of 80 m between two standard SMFs (SMF28, Corning Inc.). Two ends of the SNS were fixed to a holder to keep the SNS straight and slightly above the plate as shown in Fig. 2b. The sensing medium was dripped on the plate by an injector until the SNS section was well immersed. A coil of SMF with a length of ∼1.42 km was inserted into the cavity to serve as a delay line. A C-band tunable EDFA with a relative gain profile as shown in Fig. 3a was used to compensate the cavity loss. The length of the NCF was determined by numerical simulation, as shown in Fig. 3a, to ensure the transmission peak of the resulted SNS is near the gain peak of the EDFA. To ensure a monotonic change of the transmission loss with the increase of the RI, the transmission peak of the SNS should be at or slightly to the right of the gain peak of the EDFA when the SNS is immersed in a liquid with the lowest RI of the measurement range. The influence of the wavelength and pulsewidth of the interrogation pulse was experimentally investigated by immersing the SNS in distilled water with n = 1.3330 (starting RI of the measurement range). And then with the optimized wavelength and pulsewidth, the performance of the sensor was demonstrated. A. Influence of the interrogation wavelength The simulated and measured spectral responses of the SNS to RI are shown in Fig. 3a and b respectively. The relative gain profile of
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002
G Model SNB-23093; No. of Pages 5
ARTICLE IN PRESS W. Yan et al. / Sensors and Actuators B xxx (2017) xxx–xxx
3
Fig. 4. Lasing spectrum. The lasing wavelength is 1527.3 nm.
Table 1 Number of pulses N within the ring-down time when interrogated with pulses at different wavelength .
Fig. 3. (a) Simulated transmission spectra change with RI and relative gain of the EDFA, and (b) measured transmission spectra change with RI of the fabricated SNS.
(nm)
1525
1526
1527.3
1528
1529
1530
1535
1540
N
1
1
7
6
3
2
1
1
the EDFA is also show in Fig. 3a. From the figure we can see that both the gain spectrum of the EDFA and the transmission spectrum of the SNS are wavelength dependent. In our previous work [14], to assess the influence of the wavelength on the ring-down time, we introduce a quality factor Q, which is defined as Q () = |
G() | ı()
(4)
where ı() is the loss of the fiber loop cavity, including A and V in Eq. (2), and G() is the gain of the EDFA. A higher Q means that more loss can be compensated by the EDFA with lower pump power (because with a lower Q, pump power has to be increased to reach higher ringdown pulse number). And thus a higher pulses number, or ring-down time can be attained before severe ASE and relaxation oscillation [14]. In this paper, we proposed a simple experimental method to determine the optimal wavelength which leads to the highest Q. Compared with checking each wavelength and making comparison, this method is more precise and effective because it omits comparison and calculation. The optimal wavelength can be determined as following: without input of the interrogation pulse, we increase the gain of the EDFA. As the gain increases over the lasing threshold of the ring-down cavity, lasing oscillation will be observed at the optimal wavelength 0 . Because based on the principle of lasers Q(0 ) is greater than Q() at any other wavelength. So when interrogating at wavelength 0 , the transmission loss V can be best compensated by the EDFA and the maximum ring-down time can be expected. In a word, the optimal interrogation wavelength can be determined as the lasing wavelength of the idle cavity when we increase the gain of the EDFA slightly above the threshold. Now we experimentally verify that 0 is indeed the optimal interrogation wavelength that brings maximum number of pulses within the ring-down time. The lasing spectrum of the idle cavity with n = 1.3330 is shown in Fig. 4. We can see that the lasing wavelength 0 appears at 1527.3 nm. To verify that 0 is the optimal wavelength, wavelength of the interrogating pluses (1.5 s pulsewidth) was changed around 0 . At each wavelength, the gain of the EDFA was gradually increased from 0 dB until just before obvious relaxation oscillation was observed. Table 1 shows the relationship between interrogation wavelength and the pulse number within the ring-down time, which is proportional to the ring-down time in the decay train. As an example, the measured decay train when the interrogation wavelengths are 1527.3 nm and 1529 nm are show in Fig. 5a and b respectively. The red vertical dash line indicates the corresponding ring-down time. From Table 1 we can
Fig. 5. Influence of the interrogation wavelength on the decay train. (a) = 1527.3 nm and (b) = 1529.0 nm.
see that the pulse number changes dramatically with the interrogation wavelength, and the maximum pulse number is indeed obtained at 1527.3 nm. So in practice, in order to attain maximum ring-down time and so to increase the sensitivity of the sensing system, the interrogating wavelength should be set at the optimal wavelength. And for a given sensor, the optimal wavelength can be obtained by observing the lasing wavelength of the idle cavity under the starting RI of the measurement range. B. Influence of the pulsewidth As discussed above the gain of the EDFA should be properly set to achieve the most pulses in the decay train without the occurrence of relaxation oscillation. Besides the wavelength, the gain characteristic of the EDFA is also influenced by the width of the input pulse. We experimentally found that the pulsewidth could influence the amplitude of the relaxation oscillation. To demonstrate this phenomenon, we kept the gain of the EDFA constant, and tuned the TLS to the optimal wavelength, i.e. 1527.3 nm. Then set the width of the injected light pulse to 1.5 s, 3.5 s, and 5.5 s, respectively. The measured output signals are shown in Fig. 6. We can see that with the increase of the pulsewidth, the deviation of the bottom of the pulses from the base line becomes more and more severe. This phenomenon can be explained as following. A wider light pulse being amplified means more inverted population in the EDFA are engaged, and thus the continuous wave (CW) ASE power
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002
G Model SNB-23093; No. of Pages 5 4
ARTICLE IN PRESS W. Yan et al. / Sensors and Actuators B xxx (2017) xxx–xxx
Fig. 8. Decayed ring-down pulse train of the sensor as the surrounding RI is 1.3330.
Fig. 9. Ring-down time as a function of the surrounding RI of the sensor.
Fig. 6. Observation of the influence of the pulse width on the relaxation oscillation.
Fig. 7. Influence of the pule width on the ring-down time of the sensor.
or CW lasing power determined by the inverted population are disturbed more severely [19]. So the severity of relaxation oscillation increases with the increase of the pulsewidth. To relive relaxation oscillation, the gain of the EDFA must be properly decreased with the increase of the pulsewidth. This in return, will decrease the ring-down time and so influence the detection sensitivity of the sensor. To avoid relaxation oscillation and obtain more ring-down pulses, the pulsewidth must also be properly set. Another experiment was conducted to investigate the influence of the pulsewidth on the sensing system. Light signals with different pulsewidths at 1527.3 nm were injected into the fiber ring cavity under a RI of 1.3330. At each pulsewidth, the gain of the EDFA was gradually increased until the maximum number of pulses was recorded just before obvious relaxation oscillation. We calculated the ring-down time of each output decay train and the experimental results are shown in Fig. 7. From Fig. 7 we can see that with the increase of the pulsewidth the ring-down time decreases. This is because with a narrower pulse, more gain of the EDFA is allowable, while with a wider pulse the gain of the EDFA must be properly decreased to
avoid relaxation oscillation. So in practice a narrower pulsewidth is preferable because it can increase ring-down time of the decay train and increase the detection sensitivity of the sensor. However, in practice, the pulsewidth can’t be too narrow that the pulse peak would not be detected due to the limited sample rate of the detection instruments. So it is also important to carefully choose the pulsewidth depending on the detection limit of the instruments. C. Measurement result and discussion After determined the wavelength and width of the interrogation pulse, the sensor system depicted in Fig. 1 was used to measure RI of sucrose solution with different concentration (The experiment RIs are 1.3330, 1.3360, 1.3389, 1.3420, 1.3450, 1.3480, 1.3509, 1.3539. And the related sucrose concentration can be obtained in this reference [25]). The measurement was conducted at a room temperature of 20 ◦ C. The SNS was initially covered by distilled water with a RI of 1.3330. The TLS was tuned to the optimal wavelength, 1527.3 nm. The pulsewidth was set to 1.5 s. The gain of the EDFA was tuned to obtain maximum ring-down pulses just before obvious relaxation oscillation. The total cavity net loss is estimated to be about 0.56 dB (coupling loss 3.28 dB, delay line loss 0.29 dB, SNS insertion loss in water 8.41 dB, and EDFA gain 11.42 dB).The decayed ring-down pulse train is shown in Fig. 8. By fitting the peaks of the pulses with an exponential function similar to Eq. (1), we can get the ring-down time . For this case is 51.93 s and the pulse number within the ringdown time is 7, which is comparable to that achieved using FBG filters [20,21]. So by optimizing the wavelength and width of the interrogation pulse the system complicity can be reduced by omitting the FBG filters. As shown in Fig. 3, the transmittance of the SNS structure will decrease with the increase of the surrounding RI at 1527.3 nm, so the ring-down time will decrease accordingly. Fig. 9 shows the experimental ring-down time as a function of the surrounding RI as it was being changed from 1.3330 to 1.3539 during our measurement. For comparison, simulation results with parameters similar to those used in the experiment are also shown in Fig. 9. From Fig. 9 we can see that the experimental results agree fairly well with the simulation results. Because with the increase of the RI, the loss introduced by the SNS increases and the number of pulses in the decay train decreases, so there is an upper limit to the working range of the sensor. From Fig. 9 we can see that the RI and ring-down time generally follows a nonlinear relationship.
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002
G Model SNB-23093; No. of Pages 5
ARTICLE IN PRESS W. Yan et al. / Sensors and Actuators B xxx (2017) xxx–xxx
They can be fitted very well with a cubic polynomial. In the linear region from 1.3330 to 1.3420, the sensitivity is about −3271 s/RIU as revealed by the linear fitting in Fig. 9. 4. Conclusions In conclusion, a novel FLRD interrogated RI sensor based on SNS fiber structure is proposed in this paper for the first time to the best of our knowledge. An EDFA was introduced into the fiber ring cavity to maximize the ring-down time and enhance the detection sensitivity of the sensing system. The influence of the wavelength and width of the interrogation pulse on the performance of the sensor was systematically investigated. We found that an optimal wavelength can be determined by examining the lasing wavelength of the idle cavity. We also found that a narrower input light pulse is helpful to reduce the influence of the relaxation oscillation. A demonstration sensor was fabricated and experimentally demonstrated by measuring the RI of sucrose solution with different concentration in the RI range of 1.3330–1.3539. The results show that the RI and ring-down time follows a nonlinear relationship and can be very well fitted by a cubic polynomial. In the relatively linear region, a sensitivity of −3271 s/RIU was obtained. The sensor is easy to be fabricated and immune to the power fluctuation of the light source in nature. It can be used in chemical or bio-chemical applications to measure the RI change. Acknowledgement This work is supported by the National Natural Science Foundation of China (No. 61107035), the National Key Scientific Instrument and Equipment Development Project of China(No. 2013YQ03091502), and the National Basic Research Program of China (973 Program, grant 2014CB340104). References [1] Xu Liu, Qi Wang, Chunyue Li, Chengwu Zhao, Yong Zhao, Haifeng Hu, Jin Li, Refractive index sensor based on fiber loop ring-down spectroscopy, Instrumentation Science and Technology 44 (2016) 241–248. [2] Yaofei Chen, Qun Han, Tiegen Liu, Xiaoying Lü, Self-temperature-compensative refractometer based on singlemode–multimode–singlemode fiber structure, Sensors and Actuators B: Chemical 212 (2015) 107–111. [3] Qun Han, Xinwei Lan, Jie Huang, Amardeep Kaur, Long-Period Grating Inscribed on Concatenated Double-Clad and Single-Clad Fiber for Simultaneous Measurement of Temperature and Refractive Index, IEEE Photonics Technology Letters 24 (2012) 1130–1132. [4] L. Coelho, D. Viegas, J.L. Santos, J.M.M.M. De Almeida, Enhanced refractive index sensing characteristics of optical fibre long period grating coated with titanium dioxide thin films, Sensors and Actuators B Chemical 202 (2014) 929–934. [5] A. Iadicicco, S. Campopiano, A. Cutolo, M. Giordano, A. Cusano, Self temperature referenced refractive index sensor by non-uniform thinned fiber Bragg gratings, Sensors and Actuators B Chemical 120 (2006) 231–237. [6] Yong Zhao, Xue Gang Li, Lu Cai, Yang Yang, Refractive index sensing based on photonic crystal fiber interferometer structure with up-tapered joints, Sensors & Actuators B Chemical 221 (2015) 406–410. [7] Zhaobing Tian, S.H. Yam, J. Barnes, W. Bock, Refractive Index Sensing With Mach–Zehnder Interferometer Based on Concatenating Two Single-Mode Fiber Tapers, IEEE Photonics Technology Letters 20 (2008) 626–628. [8] Y. Wu, P. Wang, G. Farrell, High sensitivity SMS fiber structure based refractometer-analysis and experiment, Optics Express 19 (2011) 7937–7944. [9] George Stewart, Kathryn Atherton, Hongbo Yu, Brian Culshaw, An investigation of an optical fibre amplifier loop for intra-cavity and ring-down cavity loss measurements, Measurement Science and Technology 12 (2001) 843. [10] Peter B. Tarsa, Aislyn D. Wist, Paul Rabinowitz, Kevin K. Lehmann, Single-cell detection by cavity ring-down spectroscopy, Applied Physics Letters 85 (2004) 4523–4525. [11] Cathy M. Rushworth, Dean James, Jason W.L. Lee, Claire Vallance, Top notch design for fiber-loop cavity ring-down spectroscopy, Analytical Chemistry 83 (2011) 8492–8500.
5
[12] Shimizu Hiromasa, Noriyasu Hiroshi, Measurement of carbon dioxide concentration by fiber-loop ring-down spectroscopy for continuous remote measurement, Japanese Journal of Applied Physics 53 (2014) 116601. [13] Qi Wang, Xu Liu, Ji Xia, Yong Zhao, A Novel Long-Tail Fiber Current Sensor Based on Fiber Loop Ring-Down Spectroscopy and Fabry-Perot Cavity Filled With Magnetic Fluid, IEEE Transactions on Instrumentation and Measurement 64 (2015) 2005–2011. [14] Yaofei Chen, Tiegen Liu, Qun Han, Wenchuan Yan, Lin Yu, Fiber loop ring-down cavity integrated U-bent single-mode-fiber for magnetic field sensing, Photonics Research 4 (2016) 322–326. [15] Chuji Wang, Chamini Herath, High-sensitivity fiber-loop ringdown evanescent-field index sensors using single-mode fiber, Optics letters 35 (2010) 1629–1631. [16] K. Zhou, D.J. Webb, C. Mou, M. Farries, N. Hayes, I. Bennion, Optical Fiber Cavity Ring Down Measurement of Refractive Index With a Microchannel Drilled by Femtosecond Laser, IEEE Photonics Technology Letters 21 (2009) 1653–1655. [17] Y.N. Zhang, Y. Zhao, D. Wu, Q. Wang, Fiber Loop Ring-Down Refractive Index Sensor Based on High-Q Photonic Crystal Cavity, IEEE Sensors Journal 14 (2014) 1878–1885. [18] D. Wu, Y. Zhao, Q. Wang, SMF Taper Evanescent Field-Based RI Sensor Combined With Fiber Loop Ring Down Technology, IEEE Photonics Technology Letters 27 (2015) 1802–1805. [19] George Stewart, Peter Shields, Brian Culshaw, Development of fibre laser systems for ring-down and intracavity gas spectroscopy in the near-IR, Measurement Science and Technology 15 (2004) 1621. [20] Wei Chang Wong, Wenjun Zhou, Chi Chiu Chan, Xinyong Dong, Kam Chew Leong, Cavity ringdown refractive index sensor using photonic crystal fiber interferometer, Sensors and Actuators B: Chemical 161 (2012) 108–113. [21] N. Ni, C.C. Chan, L. Xia, P. Shum, Fiber cavity ring-down refractive index sensor, IEEE Photonics Technology Letters 16 (2008) 1351–1353. [22] Lü Xiaoying, Han Qun, Liu Tiegen, Chen Yaofei, Ren Kun, Actively Q-switched erbium-doped fiber ring laser with a nanosecond ceramic optical switch, Laser Physics 24 (2014) 115102. [23] H. Murata, Handbook of Optical Fibers and Cables, 2nd ed., CRC, USA, 1996. [24] Yaofei Chen, Qun Han, Tiegen Liu, Xinwei Lan, Hai Xiao, Optical fiber magnetic field sensor based on single-mode-multimode-single-mode structure and magnetic fluid, Optics Letters 38 (2013) 3999–4001. [25] United States Department of Agriculture, USDA Technical Procedures Manual, United States Department of Agriculture, USA, 2011, pp. 32–56.
Biographies Wenchuan Yan was born in Tianjin, China, in 1991. He received the B.E. degree in optoelectronics from Tianjin University, Tianjin, China, in 2015. He is currently pursuing his M.E degree at the School of Precision Instrument and Opto-electronics Engineering (SPIOE), Tianjin University, Tianjin, China. His research interest is focused on optical fiber sensors and demodulation methods. Qun Han was born in Zibo, Shandong Province, China, in 1977. He received the M.S. degree in optics from Nankai University, Tianjin, China, in 2003, and the Ph.D. degree in physical electronics from Tianjin University, Tianjin, China, in 2006. Since 2006, he has been with the SPIOE of Tianjin University, where he is now a tenured Associated Professor. From 2011 to 2012 he was a Visiting Scholar at Missouri University of Science and Technology, Missouri, USA. His current research interests include fiber sensors, fiber lasers, and high-power fiber amplifiers. He is the author or coauthor of more than 100 journal papers. Dr. Han is a member of the Optical Society of America. Yaofei Chen was born in Luoyang, Henan Province, China,in 1989. He received the B.S. degree in optoelectronics from Changchun University of Science and Technology, Changchun, China, in 2012. He is currently pursuing his Ph.D. degree at SPIOE, Tianjin University, Tianjin, China. His research interest is focused on optical fiber sensors. Huiling Song was born in Xingtai, Hebei Province, China,in 1994. She received the B.S. degree in physics from Heibei University, Baoding, China, in 2016. She is now pursuing her M.E. degree at SPIOE, Tianjin University, Tianjin, China. Her research interest is focused on optical materials for sensing. Xiaoyun Tang was born in Guangan, Sichuan Province, China,in 1995. She received the B.E. degree in optoelectronics from Changchun University of Science and Technology, Changchun, China, in 2016. She is now pursuing her M.E. degree at SPIOE, Tianjin University, Tianjin, China. Her research interest is focused on fiber amplifiers and lasers. Tiegen Liu was born in Tianjin, China, in 1955. He received the B.E. degree, the M.E. degree, and the Ph.D. degree in optical engineering from Tianjin University, China, in 1982, 1987, and 1999, respectively. He joined Tianjin University in 1982, where he is currently a Professor at the College of Precision Instrument and Opto-electronics Engineering. His research interests include optical fiber sensors, optical polarization technology, and optoelectronics measurement.
Please cite this article in press as: W. Yan, et al., Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure, Sens. Actuators B: Chem. (2017), http://dx.doi.org/10.1016/j.snb.2017.09.002