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A temperature and strain insensitive refractive index (RI) sensor based on a multimode fiber (MMF) embedded in a few-mode fiber (FMF) structure constructing a long-period fiber grating (LPFG) is proposed and investigated. Due to the RI difference between the MMF and the FMF and the mismatch of their fiber core diameters, an RI modulation region is provided in the combined region of the MMF and the FMF. The periodic splicing can be regarded as the introduction of an LPFG, which generates a strong evanescent field, and the RI sensing can be achieved due to the LPFG-excited cladding modes. The experimental results show that the RI sensitivity of the proposed MFM-LPFG fiber structure is 779.00 nm/RIU, while it exhibits low strain response and temperature response with an overall size of 3.2 mm. The MFM-LPFG sensor with resistance to interference from other parameters is expected to have a wide range of roles in chemical and biochemical sensing.

A hybrid interferometer sensor based on the enhanced Vernier effect was proposed for the simultaneous measurement of strain and temperature. The sensor was composed of a Fabry-Perot interferometer (FPI) made of hollow core fiber (HCF) and a Michelson interferometer (MI) made of few-mode fiber. Unlike the traditional Vernier effect, the reference arm of enhanced Vernier effect does not need to be isolated but participates in sensing along with the sensing arm, and its response to strain is opposite that of the sensing arm, thereby improving the sensitivity of the traditional Vernier effect. Given that the two interferometers participate in sensing, a coefficient matrix was used for the simultaneous measurement of strain and temperature, which overcame the cross-sensitivity problem of temperature. First, we theoretically analyzed the relationship between the mode excited in a few-mode fiber and the offset and the relationship between the coupling efficiency and the offset of different modes when a few-mode fiber was coupled with a single-mode fiber (SMF). Then, we conducted experimental verification and measured the strain and temperature responses of the sensor. The sensor had a strain sensitivity of −13.7 pm/ $\mu \varepsilon $ , which was 4.28 and 1.36 times the sensitivity of the traditional Vernier effect using MI or FPI as the sensing arm, and the temperature sensitivity was −97.7 pm/°C. The proposed sensing device exhibits the benefits of straightforward fabrication, an extensive measurement range, and low-temperature interference, holding promising prospects for strain assessment.

A fiber-optic strain sensor with antiresonance suppression was proposed, and sensor sensitization was achieved by constructing a simulated reference interferometer (SRI) using the Vernier effect. In the capillary-based Fabry–Perot interferometer (FPI), antiresonance resulted in uneven contrast of the reflection spectrum, which could not meet the conditions for the use of the Vernier effect. In addition to controlling the length of the capillary to below the critical length to overcome this problem, antiresonance could be suppressed by increasing the thickness. A strain sensitivity of 14.58 pm/ $\mu \varepsilon $ , which is 7.15 times higher than the strain sensitivity of a single sensing interferometer, was achieved using the antiresonance-suppressed FPI as a sensing interferometer and modulating it with the constructed SRI. In addition, tunable sensitivities of 22.36 and 32.83 pm/ $\mu \varepsilon $ were obtained by only changing the parameters of SRI instead of remaking the reference interferometer. The sensor realized the Vernier effect with only one interferometer. It has the advantages of online control sensitivity, simple structure, and low cost and has potentially important applications in the field of strain measurement.