High sensitivity high temperature sensor based on SMS structure with ...

High sensitivity high temperature sensor based on SMS structure with large-core all-solid bandgap fiber as the multimode section Marcos A. R. Francoa,b, Alice L. S. Cruza,b, Valdir A. Serrãoa, and Carmem L. Barbosaa a Instituto de Estudos Avançados – IEAv, São José dos Campos, SP, Brazil b Instituto Tecnológico de Aeronáutica – ITA, São José dos Campos, SP, Brazil ABSTRACT A fiber optic interferometric device based on a singlemode-multimode-singlemode (SMS) structure is proposed as a high sensitive high temperature sensor. The multimode section (MMF) consists of a large-core all-solid photonic bandgap fiber (AS-PBF) with silica as the background material and germanium-doped silica at the high index regions. The numerical analyses were carried out by beam propagation method. The numerical results indicate a constant high temperature sensitivity of ∼−35 pm/oC over a large temperature range from 20oC to 930oC. Keywords: Multimode interference, temperature sensor, fiber optic sensors

1. INTRODUCTION Fiber optic multimode interference devices have been investigated by many research groups as a high sensitivity sensor for temperature, strain, refractive index and curvature. The simplest structure is the singlemode-multimode-singlemode (SMS) interferometric device and leads to high sensitivities1-11. Usually, the SMS structures are based on standard singlemode fibers (SMF) and multimode fiber (MMF) in a setup that consists of a MMF spliced between two SMFs. At the input junction SMF-MMF, occurs the excitation of several MMF modes which propagate with different phase velocity forming a periodic interferometric pattern. There is a distance along the MMF where the modes, that propagate the main fraction of the energy, are in phase and strong constructive interference occurs at the center of the MMF. This position is known as self-image, because the MMF input signal is approximately reconstructed. If the MMF section has the correct length, almost all the signal couples to the output SMF section and the transmission is maximum (maximum bandpass spectral peak). The self-image position is dependent of the wavelength, MMF diameter, and can also be shifted if the MMF mode propagation is perturbed. Physical parameters as temperature, external refractive index and curvature can change the modal phase evolution and consequently also the interferometric pattern, moving the self-images positions. This is the operational principle of the SMS structures as a sensor. Interferometric temperature sensors with high sensitivity (S), narrow and large temperature range (∆T) have been extensively demonstrated in the literature. Ultrasensitive sensors operating in narrow temperature range were demonstrated based on microstructured optical fiber with liquid infiltrated12 and biconical tapered MMF13, leading to S ≈ 6600 pm/oC (∆T = 14oC) and S ≈ 3880 pm/oC (∆T = 30oC), respectively. Long period grating14 and fibers with luminescent material at the core15 were shown with S ≈ 101 pm/oC (∆T = 140oC) and S ≈ 70 pm/oC (∆T = 85oC). Simple structures based on fiber optic core-offset splicing16 allows reaching S ≈ 31 pm/oC (∆T = 60oC), and in-line fiber-optic Mach-Zehnder interferometers17 achieve S ≈ 50 pm/oC (∆T = 45oC), respectively. Sensors with larger temperature range were also subject of studies: a Mach-Zehnder interferometer18 leads to S ≈18 pm/oC (∆T = 800oC), a Michelson interferometers with abrupt junctions19-20 allow reaching S ≈ 96 pm/oC to ≈120 pm/oC (∆T from 850oC to 975 oC). FabryPerot interferometer based on micro-cavity into a fiber probe21 leads to S ≈ 20 pm/oC (∆T = 500oC). Fiber sensors based on the reflected signal of a SMF-MMF structure22-23 achieve S ≈ 11 pm/oC and 15 pm/oC (∆T from 650oC to 1050 oC), and fiber Bragg gratings inscribed in single crystalline sapphire fibers24 results S ≈ 22 pm/oC (∆T = 1100 oC). This work presents a modified SMS structure where the multimode section is replaced by an all-solid bandgap fiber (ASPBF) operating in the first bandgap. The goal of our work is the proposition of a modified design of SMS temperature sensor with high sensitivity (≈−35 pm/oC) over a large temperature range (∆T = 910oC), a low-cost and compact device (∼1,2 cm long) without the necessity of special external thermo-optic material or evanescent field interaction, and a device with simple and reproducible splicing, since the SMS is based on standard SMF and all-solid multimode fiber.

23rd International Conference on Optical Fibre Sensors, edited by José Miguel López-Higuera, Julian Jones, Manuel López-Amo, José Luis Santos, Proc. of SPIE Vol. 9157, 9157A9 © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2058077 Proc. of SPIE Vol. 9157 9157A9-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/07/2015 Terms of Use: http://spiedl.org/terms

2. RESULTS The modified SMS consists of a large-core all-solid bandgap fiber (MMF) spliced between two sections of SMF. The large-core of MMF is obtained by missing the central high-index rod and two rings of high index germanium-doped silica in the microstructure of the AS-PBF. The Figures 1(a)-1(d) present a SMS 3D model cut in the xz plane for better visualization, the transversal section of the AS-PBF, a typical interferometric pattern along the SMS, the normalized transmission of AS-PBF with four photonic bandgaps, and the only three supported MMF radial modes (LP01, LP02, and LP03). The main geometric parameters of AS-PBF are Λ=7.4 µm, d=5.0 µm, length of 12.66 mm (third self-image), and the maximum index contrast between silica and Ge-doped regions is 2.03%25. The singlemode fibers are standard commercial fibers (SMF28) and the all-solid MMF section consists of pure silica and germanium-doped silica, both with refractive index dependent of wavelength and temperature, however with different thermal expansion and thermo-optic coefficients. When the temperature changes, the modes at the MMF section experiences different index contrasts between silica and Ge-doped regions, changing its phase velocities and consequently modifying the interferometric pattern. The use of different materials inside the fiber allows obtaining temperature dependent self-images and high spectral shift of SMS band pass peak transmission. For the numerical modeling it was used a commercial software based on the beam propagation method (BeamProp RSOFT). The refractive index of silica and Ge-doped regions were evaluated as function of wavelength, temperature, and the doped molar fraction of germanium. The doped dependent thermal expansion coefficient of Ge-doped rods was also considered in the bandgap and modal propagation analyzes of AS-PBF.

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Figure 1. SMS structure with standard singlemode fiber and large-core all-solid bandgap multimode fiber (AS-PBF). (a) 3D model of SMS (cut xz plane). (b) Transversal section of AS-PBF. (c) Interference pattern along the MMF section. (d) Normalized transmission of AS-PBF, showing the four first bandgaps. (e)-(g) MMF radial modes LP01, LP02, and LP03, respectively.

The Figure 2(a)-2(b) present the results for the SMS spectral normalized transmission for some temperatures and the spectral shift of band pass transmission peak as function of temperature, respectively. The SMS operates at third selfimage. Consider high order self-image allows reducing the FWHM (full-width at half maximum) of transmission band pass what results in better quality of sensor measurements3. A linear shift of peak wavelength is obtained corresponding to a constant and high sensitivity of ∼−35 pm/oC in a temperature range from 20oC to 950oC with FWHM of ∼23 nm. This result allows temperature measurement with resolution of ∼0.05oC over ∆T=910 oC. A configuration with double identical SMS structures was also analyzed. The Figure 3(a) shows the normalized transmission peak of both the single SMS (S-SMS) and double SMS (D-SMS) (Figures 3(b) and 3(c)) for two temperatures 20oC and 930oC. The inclusion of a second SMS structure do not change the temperature sensitivity, however the bandpass width (FWHM) is reduced by ∼35%. Considering the quality factor as the ratio of the sensor

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sensitivity by the bandpass width, the D-SMS improve the sensor quality by 50%, allowing higher resolution measurement of the wavelength shift of peak transmission. We believe that optimized design of AS-PBF can substantially improve the sensitivity and sensor quality factor, preserving the large temperature range of operation. Future works will pursue this goal. (a)

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Figure 3. (a) Spectral normalized transmission of Single-SMS (S-SMS) and Double-SMS (D-SMS) structures for 20oC and 930 oC. (b) 3D model (xz cut) of S-SMF. (c) 3D model (xz cut) of D-SMF.

3. CONCLUSION This paper presents a modified SMS structure with multimode section replaced by a large-core all-solid bandgap fiber for temperature sensor purpose. High sensitivity of about −35 pm/oC for temperatures ranging from 20 oC to 930 oC was numerically demonstrated. The proposed SMS sensor has the advantages of easy fabrication, compact and low-cost, simple and reproducible splicing of the constituent parts, operation not based on evanescent field interaction with external materials, applicable to large temperature range (limited at high temperatures by the softening temperatures of Ge-doped and silica materials).

ACKNOWLEDGMENTS This work was partially supported by project Pró-Defesa CAPES/Ministério da Defesa (ref. 23038.029912/2008-05), Finep (proc. 0.1.06.1177.00, 0.1.05.0770.00, and 01.10.0628.00), CNPq and FAPESP by project INCT Fotonicom.

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