The possibility of making new SMF-NCF-SMF- long period fiber grating (SNS-LPG) by periodically embedding no-core fiber (NCF) is demonstrated. It is shown that an 11-period SNS-LPG can exhibit an extremely high loss resonant dip (>20 dB). The theoretical analysis shows good agreement with the measured transmission spectrum. We investigated the refractive index and temperature sensitivity of SNS-LPG, and achieved the average wavelength sensitivity of 141.837 nm/RIU in the range of 1.333-1.400 RIU. The sensor also exhibits a lower temperature sensitivity (-6.43 pm/°C and 0.005 dB/°C).

A novel sensor structure has been proposed and experimentally investigated for simultaneous strain and temperature measurement. The structure is fabricated by weak power modulation of CO2 laser exposure on tapered long period fiber grating (LPFG). Compared with the transmission spectrum of the tapered LPFG, two peaks appear in the transmission spectrum of the novel structure. These resonance peaks exhibit different sensitivity responses; thus, simultaneous measurement of strain and temperature is realized by monitoring the wavelength shift of the two peaks. Experiment results indicate that strain sensitivities of the two peaks are 1.82 pm/μE and 8.17 pm/μE, and temperature sensitivities are 47.9 pm/°C and 65 pm/°C, respectively.

This review focuses on the preparation methods and wide applications of optic fiber sensors based on core-offset structure. Firstly, the classification and preparation methods of core-offset structures are introduced. Then, the applications of different types of sensors based on the core-offset structure are investigated, including the measurement of physical parameters such as temperature, strain, refractive index (RI), bending, magnetic field by Mach-Zehnder interferometer (MZI), cascaded sensor, long period fiber grating (LPFG), fiber Bragg grating (FBG), Fabry-Perot interferometer (FPI) and Surface plasmon resonance (SPR). Compared with the traditional fiber sensor, core-offset structure sensor with a non-axisymmetric characteristic has excellent sensing characteristics, especially in the recognition of bending direction and high RI sensitivity. Furthermore, the design scheme, working principle and sensing characteristics of different core-offset sensors are also briefly described. Finally, the existing challenges and development direction of optic fiber sensors based on core-offset structure is discussed and prospected.

We investigated and prepared a chirped long-period grating for high sensitivity bending measurement. The novel fiber structure is composed of the multimode fiber (MMF) with fixed length and gradually longer single-mode fiber using the continuous splicing method. The powerful ability of refractive index modulation in the MMF renders the miniaturization of the sensor. The total length of the sensor is 3.45 mm. Chirped period arrangement is adopted to improve the bending sensitivity. Through numerical calculation, the chirp coefficient is determined, and the clear resonance peaks are obtained using the wavelength scanning. The experimental results of several samples show that the maximum bending sensitivity is 53.68 n m / m − 1 at 0 − 1.803 m − 1 . With the advantages of small size and high sensitivity, the sensor is especially suitable for bending sensing with a micro-structure.

A multimode fiber chirped long-period grating (MCLPG) is proposed and studied as a sensor for simultaneous measurement of strain and temperature. This sensor consists of linear chirped multimode fiber and fixed-length single-mode fiber. The numerical analyses of the transmission spectrum and coupling mode are carried out to obtain appropriate period arrangement. The experimental results are in good agreement with the theoretical results. The chirped period arrangement induces a transmission spectrum with double peaks. Therefore, the simultaneous measurement of strain and temperature is realized by observing the wavelength shift of the two peaks. In addition, the high sensitivity of two resonance peaks provides a good resolution for the sensor.

The CLP Publishing collects CLP journals and partnered optics and photonics journals, and provides readers an optical publishing platform with global influence. SPIE, OSA, CUP. Chinese Laser Press (CLP), established by Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS) and Chinese Optical Society (COS) in 2009, nowadays publishes eight journals and manages three online platforms.