br Conflict of interest br Acknowledgments

Conflict of interest
Acknowledgments Authors greatly acknowledge Misses N. Watanabe and A. Misao for their helps in experiments. This work was financially supported by Grant-in-Aid for Scientific and Research (B) (No. 21350045 and No. 2541049) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Financial support from a research grant from the College of Humanities and Sciences at Nihon University is also acknowledged.
Introduction Twist is an important sensing physical parameter that needs to be monitored in multi-area applications, such as detecting the health condition of engineering structure and mechanical equipment. And optical fiber sensors have the advantages of possessing a small size, easy to embed into the structures and anti-electromagnetic interference, et al. [1]. So far, there have been many works to study fiber sensor characteristic of twist/torsion, for example, twist sensors by using the birefringence of photonic crystal fiber [2–4] and long pteryxin Supplier fiber gratings (LPFGs) [5–7]. In 2011, a fiber ring laser incorporating a pair of rotary long-period grating was used for torsion sensing with a torsion sensitivity of 0.084nm/(rad/m) in the torsion range ±100rad/m [8]. In the same year, Chen WG reported a highly sensitive torsion sensor based on Sagnac interferometer using side-leakage photonic crystal fiber [9]. The achieved maximum torsion sensitivity is about 0.9354nm/degree. Twist sensors, such as special fibers composed by carbon nanotube [10], fiber-optic polarimetric twist sensor [4], or special fluid filled in photonic crystal fiber [11], have been evolving to diversified applications. To our best acknowledge, twist fiber sensors with a low temperature cross-sensitivity have been reported above, while simultaneous sensing of twist and temperature based on fiber has not been reported, which may be have a potential twist applications for the real-time monitoring of the components working in harsh and complex conditions. Since hollow core photonic bandgap fiber (HC-PBGF) was proved of light guiding in low index air core in 1999 [12], its unique merits [13–15] have been attracting scholars’ attention to investigate for sensor applications, such as the employ of original hollow-core trait structure as Fabry-Pérot-type strain sensor [16–18], fabricating LPFG sensor by collapsing part of surrounding air-core [19], in-fiber polarimeters and Sagnac interferometer are reported [20,21], offset-splicing a single-mode fiber (SMF) with HC-PBGF as a inclinometer for detecting direction was also reported as a novel application [22].
Experimental setup Scheme of experimental setup for simultaneous measurement of twist and temperature is shown in Fig. 1(a). A broadband light source, generated by a superluminescent light-emitting diode (SLED) (LIGHT COMM Inc. China) and an optical spectrum analyzer (OSA, YOKOGAWA AQ6370B) are connected to the sensing component to monitor the transmission spectra as applied physics parameters vary. The sensing component is composed of a section of EHC-PBGF spliced with SMFs. EHC-PBGF is fabricated and supported by Yangtze Optical Fiber and Cable Corporation in China. Scanning electron micrograph (SEM) image of its cross section is shown in Fig. 1(b) and its local enlarged image is shown in Fig. 1(c). The dark areas are the air holes and the bright areas are solid silica. The maximum dark area in the center is air core which is a low elliptical geometry with a long axis of 7.5μm and a short axis of 6.9μm, which indicates that it is a low-birefringence fiber, while six solid silica rods distributing around the air core are high birefringence due to their asymmetric structures, as shown in Fig. 1(c) by six red circle lines. The cladding is with a diameter of 125μm. And the pitch of air holes surrounding the air core is about 4μm. Fig. 1(d) shows the photograph of the segment of EHC-PBGF with a length of 651.8μm that captured by optical microscope (Olympus BX51, Olympus Inc.). It can be seen that there is no obvious collapse in the air holes. Both ends of EHC-PBGF were spliced with SMFs by using a FETEL S178A fusion splicer; and all the splicing process was done manually by controlling the appropriate arc intensity and arc duration to protect collapse of air-core and realize a high splicing strength. We firstly spliced one end of EHC-PBGF with a SMF and cut off the other end under the optical microscope by remaining a needed length and then spliced the remaining EHC-PBGF with a SMF.