FM4H.6.pdf
FiO/LS Technical Digest © OSA 2012
Salinity Sensor using a Two-Core Optical Fiber J. R. Guzman-Sepulveda1*, J. A. Arredondo-Lucio1, W. Margulis2, and D. A. May-Arrioja1 1
Departamento de Electrónica, UAM Reynosa-Rodhe, Universidad Autónoma de Tamaulipas, Carr. Reynosa-San Fernando S/N, Reynosa, Tamaulipas 88779, México. 2 Dept. Fiber Photonics, ACREO, Electrum 236, 16440 Stockholm, Sweden *
[email protected]
Abstract: A salinity sensor based on a Two-Core optical fiber is demonstrated. The sensor response and sensitivity can be easily adjusted by simply controlling an etching process to expose the off-axis core. The proposed RI sensor exhibits a sensitivity exceeding 1,400 nm/RIU for salinity concentrations below 1 M. OCIS codes: (060.2370) Fiber optics sensors; (120.0280) remote sensing and sensors; (280.4788) optical sensing and sensors.
1. Introduction Salinity is an important parameter for process control in manufacturing industries, prevention of global warming, seasonal climate prediction, study of oceanic conditions, and protection of the ecosystems [1]. Although the traditional way to determine the degree of salinity of water is by measuring the electrical conductivity, the measurement of the refractive index of the solution is also a suitable alternative. In terms of fiber optic sensors for refractive index measurements, a growing interest has arisen due to their advantages such as immunity to external electromagnetic interference, compactness, high sensitivity, and in situ measurement. In this regard, several techniques can be found in the literature, being the most widely studied those based on fiber gratings, specialty fibers, and in-line fiber interferometers. Fiber grating based sensors has been reported with a maximum sensitivity of 340 nm/refractive index unit (RIU) [2], while those based on photonic crystal fiber (PCF) exhibit a maximum sensitivity of 191 nm/RIU [3]. Although refractive index sensors based on in-line fiber interferometers exhibit the highest sensitivities, on the order of -9,370 nm/RIU [4], they also require complex fabrication processes and special equipment to achieve the micromachining requirements. In this work we propose a novel and quite simple all-fiber sensor based on a Two-Core fiber (TCF). This configuration allows us to adjust the sensitivity by simply controlling an etching process on the TCF. A sensitivity greater than 1,400 nm/RIU is demonstrated for aqueous solutions with salinity concentration below 1M. 2. Principle of Operation and Experimental Setup The cross section of the TCF is shown in Fig. 1(a). The diameter of the cladding is 125µm and the two cores, both with diameter of 8.6µm, are asymmetrically located: one of the cores is located at the center of the fiber while the other is located 15µm away from the central core. Given the separation between the cores, the TCF is basically a directional coupler. Therefore, light coupled into one of the cores will eventually couple into the other core and then back to the first one, and so on, along the length of the TCF. Since one of the cores is located at the center of the fiber, the TCF can be easily spliced to single mode fibers (SMF). Therefore, when a section of TCF is spliced between two SMF, the expected spectral response is as shown in Fig. 1(b). The response exhibits a sinusoidal behavior as a function of wavelength with the peak to peak separation being directly proportional to the TCF length.
Fig. 1. (a) Cross view of the TCF. (b) Theoretical response of the sensor for several concentrations. (c) Schematic of the TCF salinity sensor.
FM4H.6.pdf
FiO/LS Technical Digest © OSA 2012
Complete crossover occurs when the guides are phase matched and the interaction length is an exact odd multiple of the coupling length [5]. The coupler response is highly dependent on the coupling coefficient which is also related to the cladding RI. Therefore, if the effective RI of the cladding is modified, then the phase match condition will be achieved at a different wavelength and the coupling process is modified. This is reflected as a shift of the wavelength response of the TCF, and this is the effect that we use to detect RI changes. The proposed TCF RI sensor is schematically shown in Fig. 1(c). The sensor consists of a super luminescent diode (SLD), a 50 mm long section of TCF spliced between two single-mode fibers (SMF), and an optical spectrum analyzer (OSA). The interaction length (i.e. 50 mm of TCF) was fixed to the bottom of a channel and carefully aligned to have the external core facing up. The TCF cladding was slowly etched using buffered oxide etchant (BOE), which allows the external core to interact more with the liquid surrounding the TCF. The etching time thus allows us to control the sensitivity of the sensor. 3. Experimental Results The salinity sensor was tested on NaCl aqueous solutions with concentration from 0 M to 1.069 M (i.e. refractive index ranging from 1.3333 to 1.3435) at room temperature [6]. The response for the concentrations mentioned above and etching times of 293 min and 299 min, are shown in Fig. 2(a) and 2(b), respectively. The deviation from a constant intensity sinusoidal response is attributed to the fiber dispersion and also slight non-uniformity during the etching process. As the etching time increases, a larger amount of material is removed thus causing a stronger interaction between the external core and the surrounding liquid. This interaction manifests by changes in the effective RI of the cladding of the external core in the TCF, which modifies the sensor response according to the concentration of the solution, as shown in Fig. 2(a) and 2(b). Refractive Index (RIU)
0.65
a) Normalized Power (A.U.)
Normalized Power (A.U.)
0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10
Water NaCl 0.172 M NaCl 0.346 M NaCl 0.523 M NaCl 0.703 M NaCl 0.885 M NaCl 1.069 M
0.05 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660
Wavelength (nm)
0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
b)
Water NaCl 0.172 M NaCl 0.346 M NaCl 0.523 M NaCl 0.703 M NaCl 0.885 M NaCl 1.069 M
1480 1500 1520 1540 1560 1580 1600 1620 1640 1660
Wavelength (nm)
Absolute Wavelength Shift (nm)
1.3333 0.70
14
1.3347
c)
12
1.3365
1.3383
1.3400
293min 296min 299min 302min
7 1,4
10 8 6
1.3418
n 0.6
IU m/R 2n 40. 2 , 1 /RIU nm .69 965
4
5 612.7
2
1.3435
RIU m/
nm/R
IU
0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
-1
NaCl Concentration (mol L )
Fig. 2. Sensor response after etching times of (a) 293 min, and (b) 299 min. (c) Absolute wavelength shifts for several etching times.
Figure 2(c) shows the absolute wavelength shifts as a function of the NaCl concentration for different etching times, as well as the corresponding sensitivity. The sensor exhibits linear response for the tested refractive index range and a sensitivity of 1,470.6 nm/RIU can be achieved. Since the fabrication process is relatively simple and does not require any expensive equipment, the proposed configuration provides a simple, cost-effective, and highly sensitive salinity sensor. 4. Conclusions In summary, a novel, simple, and cost-effective salinity sensor based on a TCF has been demonstrated. The proposed sensor exhibits a sensitivity of 1,470.6 nm/RIU for a concentration range from 0M to 1.069M. The interaction between the TCF and the surrounding liquid is determined by the amount of material removed around the external core of the TCF. Therefore, adjusting the sensitivity of the TCF salinity sensor is possible by controlling the etching thickness. 5. References [1] [2] [3] [4] [5] [6]
Liqiu Men, Ping Lu, and Qiying Chen, “A multiplexed fiber Bragg grating sensor for simultaneous salinity and temperature measurement”, Journal of Applied Physics, 103, pp. 053107-1 - 053107-7 (2008). K. Zhou, L. Zhang X. Chen, and I. Bennion, “Optic sensors of high refractive-index responsivity a d low thermal cross sensitivity that use fiber Bragg gratings of >80° tilted structures”, Optics Letters, 31, pp. 1193-1195 (2006). R. Jha, J. Villatoro, G. Badenes, and V. Pruneri, “Refractometry based on a photonic crystal fiber interferometer”, Optics Letters, 34, pp. 617-619 (2009). Y. Wang, M. Yang, D. N. Wang, S. Liu, and P. Lu, “Fiber in-line Mach-Zehnder interferometer fabricated by femtosecond laser micromachining for refractive index measurement with high sensitivity”, J. Opt. Soc. Am. B, 27, pp. 370 - 374 (2010). Herwig Kogelnik and Ronald V. Schmidt, “Switched directional couplers with alternating Δβ”, IEEE Journal of Quantum Electronics, QE12 (7), pp. 396-401 (1976). CRC Handbook of Chemistry and Physics, 90th Edition, p. 8-71 (2010).
