IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 2, JANUARY 15, 2016 143 Mode Division Multiplexing in a Fiber Modal Interferometer for Dual-Parameters Measurement Bo Dong, Yuqi Peng, Yixin Wang, and Changyuan Yu, Senior Member, IEEE Abstract—We propose a novel scheme for simultaneous strain and temperature measurement by mode division multiplex- ing (MDM) in a dual cladding modes fiber up-taper interfer- ometer. The fiber interferometer, with a strong cladding mode and a weak cladding mode, can be constructed by fabricating two optimized adjacent fiber up-tapers along a single-mode fiber. With the MDM method, the temperature and strain measurement resolutions reach ±0.05 °C and ±3.14 με, respectively, which are much higher than those of the other proposed dual-parameters (temperature and strain) measurement solutions. Index Terms— Mode division multiplexing, dual cladding modes fiber interferometer, dual parameters measurement, temperature, strain. I. I NTRODUCTION D UAL PARAMETERS (temperature and strain) measure- ment has attracted much attention. Most of the techniques are based on two fiber devices with different sensitivities in response to strain and temperature, such as two different FBGs [1]; a fiber Bragg grating (FBG) combined with a long period grating (LPG) [2], a polarization maintaining fiber loop mirror (PMFLM) [3], a multimode fiber (MMF) modal interferometer [4], or a photonic crystal fiber (PCF) modal interferometer [5]; and an LPG combined with a Sagnac interferometer [6], or a PCF modal interferometer [7]. To realize dual parameters measurement, the modal interfer- ometers generally need to be combined with the other fiber devices [4], [5], [7]. The solutions with two fiber devices have a large footprint, which limits their practical applications since they tend to be disturbed by ambient environment. Recently, our group has proposed to excite cladding modes in the PMFLMs [8], [9] for simultaneous strain and temperature mea- surement, but to construct a PMFLM, besides a longer PMF, other components, such as a 3-dB coupler and a polarization controller, generally have to be applied, which leads to its complex structure and high cost. Hence, it is more desirable Manuscript received September 14, 2015; accepted October 2, 2015. Date of publication October 7, 2015; date of current version December 11, 2015. This work was supported in part by the project of the Singapore National Research Foundation under Grant NRF2012EWT-EIRP002-044. B. Dong is with the Institute for Infocomm Research, Agency for Science, Technology and Research, Singapore 138632, and also with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117583 (e-mail: bdong@i2r.a-star.edu.sg). Y. Peng and C. Yu are with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117583, and also with the Institute for Infocomm Research, Agency for Science, Technology and Research, Singapore 138632 (e-mail: philips0702@hotmail.com; eleyc@nus.edu.sg). Y. Wang is with the Institute for Infocomm Research, Agency for Science, Technology and Research, Singapore 138632 (e-mail: wangyx@i2r.a-star. edu.sg). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2015.2487545 to have just one small and compact sensor for dual parame- ters measurement with high measurement resolutions. As we all know, although simultaneous measurement of strain and temperature can be realized by monitoring the two resonance dips of a two-mode interferometer, the two dips generally have almost the same strain and thermal sensitivities, hence their dual parameters measurement resolutions are lower. In addition, some techniques based on one specially designed fiber device, such as an FBG written in a splice point between two different types of fibers [10], a tilted FBG [11], and a superstructure FBG [12], have been proposed. However, complicated fabrication techniques have to be adopted for fabricating them, which leads to their high cost. In this letter, for the first time to our knowledge, we propose a novel scheme for simultaneous strain and tem- perature measurement by mode division multiplexing (MDM) in a dual cladding modes (DCMs) fiber up-taper interferom- eter. Previous reports have shown that the up-taper joint can effectively excite the high order cladding modes [13]–[15]. Here, by fabricating two optimized adjacent fiber up-tapers along a single mode fiber (SMF), the fiber interferometer, with a strong cladding mode and a weak cladding mode, can be constructed. Compared to the transmission spectrum of the two-mode interferometer [15], our proposed interferometer shows one deep resonance dip and the other shallow one within C&L bands. With the MDM method, the two cladding modes in spatial frequency domain are discriminated for simultaneous temperature and strain measurement, with higher measurement resolutions of ± 0.05 °C and ± 3.14 με, which are much higher than those of the other proposed dual parameters (temperature and strain) measurement solutions. II. WORKING PRINCIPLE The interferometer is fabricated by a commercial fiber splicer (Fujikura FSM-40S). Fig.1 shows the schematic struc- ture of the interferometer and the inset shows the photograph of the fiber up-taper. To excite the DCMs, a strong cladding mode and a weak cladding mode, the “overlap” parameter of a splicer is enlarged while the other parameters, such as discharge power, duration time, gap etc, are set to be default. Repeat experiment results show that if fabricating the two fiber up-tapers with near identical maximum waist diameters and lengths of the expanded sections of about 168 μm and 245 μm, the two cladding modes can be effectively excited. When the light propagates to the first up-taper, part of the energy distributed in the fundamental mode will be coupled to the fiber cladding as strong cladding mode 1 and weak cladding mode 2. After the two cladding modes propagate to the second up-taper, they will re-couple with the core mode to construct intermodal interferences. Figs. 2 (a) and (b) show the typical transmission spectrum of the interferometer and 1041-1135 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. 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