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Fiber in-line Mach–Zehnder interferometer based on near-elliptical core photonic crystal fiber for temperature and strain sensing Hu Liang, Weigang Zhang,* Huayu Wang, Pengcheng Geng, Shanshan Zhang, Shecheng Gao, Chunxue Yang, and Jieliang Li Key Laboratory of Optical Information Science and Technology, Ministry of Education, Institute of Modern Optics, Nankai University, Tianjin 300071, China *Corresponding author: [email protected] Received May 13, 2013; revised July 19, 2013; accepted August 28, 2013; posted September 5, 2013 (Doc. ID 190407); published October 4, 2013 A fiber in-line Mach–Zehnder interferometer is fabricated by selectively filling liquid into one air hole of the innermost layer of a photonic crystal fiber (PCF). The refractive index of the liquid is so close to that of the background silica in the wavelength range of 1300–1600 nm that the two-mode PCF evolves into multimode PCF with an elliptically shaped core. Due to the different propagation constants, interference can occur between the fundamental mode and higher-order modes of the liquid-filled PCF. Such a device is applied in temperature and strain measurements with high sensitivities of 16.49 nm∕°C and −14.595 nm∕N, respectively. © 2013 Optical Society of America OCIS codes: (060.2300) Fiber measurements; (060.2370) Fiber optics sensors; (060.5295) Photonic crystal fibers; (230.1150) All-optical devices. http://dx.doi.org/10.1364/OL.38.004019

Optical fiber Mach–Zehnder interferometer (MZI) sensors have attracted tremendous research interests for their compactness, immunity to electromagnetism, and measurement of various parameters, including monitoring temperature [1,2], strain [3], refractive index (RI) [4], bending [5], pressure [6], etc. Recently, a number of techniques have been proposed to fabricate new types of inline fiber MZIs, such as fiber tapering [7,8], microcavity imbedded in fiber [9] or fiber Bragg grating [10], hollow optical fiber spliced between single mode fibers [11], in-line single-mode/multimode/thinned-single-mode-fiber structure [12], etc. For most of the above MZIs, the interference is produced by the fundamental core mode and higher-order cladding modes since they have different propagation constants which can result in optical path difference. However, it is not easy to find out the exact cladding modes of these MZIs, and this may limit their applications in some cases where the RI variation of cladding modes needs to be precisely located. In recent years, selectively filling liquids into photonic crystal fiber (PCF) [13] has become a focus of study in the field of fiber optics as an effective approach applied in many tunable devices such as filters, polarizers, and certainly MZIs as well. Compared with the aforementioned MZIs, it would be easier to analyze the modes involved in the interference pattern of the MZI constructed by liquid-filled PCF [14]. Besides, this kind of interferometer exhibits extremely high temperature sensitivity and flexible operation capability. To some extent, the modes which generate interference can be controlled by changing the filling liquids, so there is huge potential to design liquid-filled PCF-based MZIs with excellent properties. In this Letter, a novel fiber optic MZI based on nearelliptic core PCF is fabricated by selectively filling index-matching liquid into one air hole of the innermost layer of the PCF. The solid core of the fiber will be greatly influenced by the liquid rod and a modified (elliptically shaped) core is effectively formed to support a certain 0146-9592/13/204019-04$15.00/0

number of modes similar to those excited by elliptic-core PCF. The liquid-filled PCF-based MZI can be employed as a high-sensitivity temperature and strain sensor with advantages of compactness, high firmness, high visibility, and wide dynamic range. In our experiment, a section of PCF (fabricated by Yangtze Optical Fiber and Cable Company Ltd. of China, with a hole diameter of 3.5 μm and a hole-to-hole spacing of 5.8 μm) is selectively filled with a liquid (Cargille Labs, n  1.454 at 25°C) that has a thermal coefficient of −3.9 × 10−4 RIU∕°C using the direct manual gluing method described in [15]. It should be noted that all the PCFs we used are filled with the RI liquid. Figures 1(a) and 1(b) illustrate the cross sections of the unfilled and the selectively filled PCF, respectively. In addition, a super-continuum light source with a spectral range of 600–1700 nm, a temperature oven, and an optical spectrum analyzer (OSA) are employed for sensing experiment. The schematic of the experimental arrangement is shown in Fig. 1(c). The dotted black line in the

Fig. 1. Cross-sectional images of (a) the unfilled PCFs and (b) the liquid-filled PCFs. (c) Schematic diagram of the experimental setup. © 2013 Optical Society of America

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liquid-filled PCF represents the fiber core, and the solid red line represents the liquid rod. Considering the material dispersion, the RI of the liquid is in close proximity to that of the background silica for the wavelength range of 1300–1600 nm. Therefore, the core of the PCF evolves into a near-elliptic core and the liquid-filled PCF will play a role similar to the elliptical-core PCF. When light propagates into the filled PCF, it couples to the near-elliptic core, including the solid core, the liquid rod and the silica around it, via the excitation of a number of modes. Compared with the unfilled PCF, the number and quality of the excited modes change much. Due to the overlap of mode fields and different propagation constants, intermodal interference can be easily obtained. In this Letter, the liquid-filled PCF is named as the near-elliptic core PCF. Unlike the solid elliptical-core PCF, the mode field distribution of each mode of the near-elliptic core PCF is wavelengthdependent as a result of material dispersion. By using the full-vector finite-element method (FEM), we have simulated the first six modes of the near-elliptic core PCF, as shown in Fig. 2, and only the x-polarization state is plotted for convenience. The mode in Fig. 2(a) is the fundamental mode while the others belong to higher-order modes. We numerically calculated the effective refractive indices of the modes appearing in Fig. 2, and the dispersion curves of the modes are shown in Fig. 3, in which the insets give the mode field distribution of fundamental mode for different wavelengths of 1300, 1450, and 1600 nm. The transmission spectra of the near-elliptic core PCFs with different liquid-filled lengths are shown in Fig. 4. In a certain temperature range the difference between the refractive indices of the filled liquid and the background silica is quite trivial in the wavelength range of 1300– 1600 nm, and therefore the modes excited by the modified core have a good stability and the interference fringes can be quite stable. As shown in Fig. 4, the free

Fig. 2. Simulation of the first six mode fields of the nearelliptic core PCF; only the x-polarization state is plotted. (a) The fundamental mode and (b)–(f) the higher-order modes.

Fig. 3. Dispersion curves for different modes shown in Fig. 2. The insets are the mode field distributions of the fundamental mode at wavelengths of 1300, 1450, and 1600 nm, respectively.

spectral range (FSR) of the interferometer is dependent on the liquid-filled length. The FSR around 1520 nm is 19.75, 35.64, and 43.26 nm for the liquid-filled lengths of 4.1, 2.5, and 1.8 cm, respectively. In order to determine the number and power distribution of the modes involved in the interference pattern, the spectrum of the MZI with liquid-filled length of 2.5 cm is Fourier-transformed to obtain the spatial frequency spectrum, as shown in Fig. 5, four modes including the fundamental mode and three higher-order modes construct the interference pattern. Obviously, the power is primarily distributed in the fundamental mode and one of the higher-order modes, while the other two higher-order modes have very small contribution to the interference pattern. To further investigate the main higher-order mode involved in the interference pattern, the temperature sensing property of the MZIs has been studied. The wavelength shifts of the fringe dip around 1520 nm corresponding to different liquid-filled lengths are shown in Fig. 6, where the coefficients of the temperaturedependent wavelength shift obtained are 15.74, 16.49, and 11.87 nm∕°C. The sensitivity variation may come

Fig. 4. Transmission spectrum of the near-elliptic core PCF with different liquid-filled lengths.

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Fig. 5. Spatial frequency spectrum of the MZI with liquid-filled length of 2.5 cm.

Fig. 7. Simulated thermo coefficients of the modes shown in Fig. 2. Lines, linear fit of the simulation results.

from the temperature fluctuation during the measurement process, and the same issue was described in the [14]. In theory, the temperature sensitivity of a certain fringe dip for a MZI should be [2]

calculated values is mainly due to the errors of the standard RI liquid (nominal value of 1.454  0.0002) and the geometrical structural parameters of the PCF used in the theoretical calculations. Besides temperature sensing, many articles have focused on the measurement of strain with high sensitivity [16]. The MZI we proposed can also be used in high-sensitivity axial strain measurements. The straindependent spectral response has been characterized for the MZI with a liquid-filled length of 2.5 cm. Figure 8 shows the wavelength shifts of the fringe dip around 1520 nm as the load varies at 21.5°C, and the inset shows the spectral responses of the fringe dip. The transmission spectrum of the dip shifts toward a shorter wavelength region with a strain sensitivity of −14.595 nm∕N. Compared with the MZIs reported in [1–3], the temperature and strain sensitivities of our proposed device are much higher. However, one of the most important points that must be addressed in practical applications is the temperature–strain cross sensitivity issue. In order to solve this problem, the temperature and strain sensitivities of the fringe dip around 1450 nm of the MZI with liquid-filled

  dλ λ dneff1 dneff2 − ;  dT dT neff1 − neff2 dT

(1)

where neff1 and neff2 are the effective RI of the two modes involved in interference. The thermo coefficients of the six modes of the near-elliptic core PCF are simulated by FEM, as shown in Fig. 7. According to the simulated effective RI value of each mode and corresponding thermo coefficient, the most possible modes involved in the interference pattern should be the fundamental mode (a) and the higher-order mode (b). The theoretical calculated temperature sensitivity of the dip around 1520 nm is approximately 13.86 nm∕°C, according to Eq. (1), which is consistent with our experimental results. The difference between the measured and the

Fig. 6. Fringe wavelength shifts for different liquid-filled lengths. Squares, 4.1 cm; circles, 1.8 cm; triangles, 2.5 cm; lines, linear fitted.

Fig. 8. Wavelength shift of the fringe dip around 1520 nm of the MZI as the load varies. The inset represents the variation of the fringe dip as the load increases.

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length of 2.5 cm have been experimentally studied, and the corresponding values are 12.36 nm∕°C and −13.309 nm∕N, respectively. Hence, the temperature and strain variations could be independently determined by using a sensitivity matrix as below 

ΔT Δε





12.36  16.49

−13.309 −14.595

−1 

 ΔλA ; ΔλB

(2)

where ΔT and Δε are the temperature and strain variations, and ΔλA and ΔλB represent the wavelength shifts of the two fringe dips. In conclusion, a novel fiber in-line MZI has been proposed and experimentally demonstrated based on selectively filling RI liquid into one of the innermost air holes of the solid-core PCF. Intermodal interference occurs between the fundamental mode and higher-order modes excited by the near-elliptical core PCF. The device exhibits extremely high temperature sensitivity of 16.49 nm∕°C and strain sensitivity of −14.595 nm∕N. In addition, such a fiber in-line MZI has many other merits, such as compactness, high firmness, high visibility, and wide dynamic range. Finally, compared with the MZI reported in [14], it is obvious that the main modes involved in the liquid-filled PCF-based MZI could be flexibly adjusted and controlled by filling liquids with different RI. Therefore, this type of MZIs will find useful applications in the field of fiber optics. This work was supported Science Foundation under 10974100, and 10674075; the Project of the Ministry of No. 20120031110033; and by of Application Foundations

by the National Natural Grant Nos. 11274181, Doctoral Scientific Fund Education under Grant the Tianjin Key Program and Future Technology

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Fiber in-line Mach-Zehnder interferometer based on near-elliptical core photonic crystal fiber for temperature and strain sensing.

A fiber in-line Mach-Zehnder interferometer is fabricated by selectively filling liquid into one air hole of the innermost layer of a photonic crystal...
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