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A simple and compact sensor based on a hybrid interferometer, which is composed of a cascaded single-mode-multimode-single-mode (SMS) and bending single-mode fiber (BSF) structure, is proposed and investigated. By carefully designing the multimode fiber length of the SMS and the bending diameter of the BSF, the transmittance spectrum features two kinds of dips simultaneously, which show different responses to the surrounding refractive index (RI) and temperature. The SMS structure is only sensitive to the temperature, while the BSF structure is sensitive to both the RI and temperature. Therefore, the simultaneous measurement of the RI and temperature can be achieved by monitoring the two dips induced by the SMS and the BSF, respectively. According to this principle, a cascaded SMS-BSF hybrid sensor is designed, fabricated, and tested. Experimental results show that the sensor possesses an RI sensitivity of 331.71 nm/RIU (from 1.333 to 1.404) and a temperature sensitivity of −1.053 nm/°C (from 30 °C to 70 °C), which is outstanding among the reported results.

In this paper, a simplified simulation method for multimode-no-core-multimode (MNM) fiber structures is developed and demonstrated. The method is based on the mode expansion in the vectorial form, taking account of a complete set of guided modes. The simulated results are in good agreement with the experimental results. Also, it is shown that mode distribution of an input multimode fiber (MMF) is an influence on multimode interference(MMI) in the MNM fiber structure and can be controlled by using a single mode fiber(SMF) segment coupled to it. The proposed method provides an effective way for the designs of multimode-interference(MMI)-based fiber devices, which particularly contain a lot of modes.

A new concept for portable vector-magnetometer, based on smartphone platform and side-polished-fiber (SPF) integrated with magnetic fluid (MF), is proposed and demonstrated. The magnetometer is designed on a smartphone, which incorporates the functions of light source, detector, and data processing together, and is illuminated and imaged via the SPF. Moreover, thanks to the non-central symmetric structure of the SPF and the magnetic-field-induced anisotropic distribution of the MF around the SPF, the proposed magnetometer is able to detect the intensity and the orientation of magnetic field simultaneously. Experimental results show that the sensitivities of the proposed magnetometer to magnetic field intensity and orientation can reach −0.050 dB/Oe and −0.263 dB/degree, corresponding to the measurement resolutions of 0.336 Oe and 0.064 degree respectively, which are comparable to or even better than that obtained by the traditional platform. Our design can avoid the use of complex and expensive instruments that are employed in the traditional platform. Because of the features of portability, low cost, and highly-sensitive vector measurement, etc., the proposed magnetometer is advantageous for the applications in sensing magnetic vector requiring a portable size and real-time transmission of the measured results.

A novel magnetic field sensing system based on a ferrofluid-coated multimode interferometer (MI) integrated into a fiber-loop ring-down cavity is proposed and demonstrated. The MI is composed of a piece of no-core fiber (NCF) sandwiched by two sections of single-mode fiber. Through appropriately designing the length of the NCF, the ferrofluid-coated MI not only works as the sensing unit but also can help to filter the unwanted amplified spontaneous emission arising from the erbium-doped fiber amplifier in the fiber loop. The proper length of NCF can be determined by simulations. Corresponding experiments are conducted, and the experimental results demonstrate the feasibility of the sensing system. A sensitivity of $- 0.83\,\,\mu \text{s}$ /Oe in the linear region is achieved in experiments.

In this paper, an optical fiber sensor for simultaneous measurement of magnetic field and temperature is proposed and demonstrated. The sensor consists of a macro-bending single-mode-fiber structure (MSS) and a closely concatenated fiber Bragg grating (FBG), both of which are coated with magnetic fluid. The MSS and FBG induce two types of dips in the transmission spectrum of the sensor, and they have the different sensitivities to magnetic field and temperature. Consequently, the simultaneous measurement for the dual-parameter can be realized by monitoring the wavelength shifts of the two types of dips. A sensor was fabricated and experimentally characterized. The results show that the proposed sensor is simple and effective in the simultaneous measurement of temperature and magnetic field.

A novel all-fiber sensor based on cascaded singlemode-no-core-singlemode (SNS) and siglemode-multimode-singlemode (SMS) fiber structure is proposed and investigated. Simulation results show that for the given fibers, by appropriately choosing the lengths of no-core fiber and multimode fiber, the transmission spectrum will exhibit two distinguishable dips corresponding to SNS and SMS, respectively. The simultaneous measurement of liquids’ refractive index (RI) and temperature can be realized by monitoring the two dips’ central wavelength shifts. According to the simulations, a corresponding sensor is fabricated and tested. The experimental results demonstrate the feasibility of the sensor and show that the sensor has the RI and temperature sensitivities of 113.66 nm/RIU and 9.2 pm/°C, respectively.

A simple and compact sensor based on a cascaded fiber Bragg grating (FBG)/droplet-like fiber structure is proposed for simultaneous measurement of refractive index (RI) and temperature. The sensor can be easily constructed by mechanically bending a section of coating-stripped single-mode fiber at an FBG’s pigtail. In the transmission spectrum, two kinds of dips induced by FBG and droplet-like structure are observed. The central wavelength of the dip caused by droplet-like structure is susceptible to both of surrounding RI and temperature, while that caused by FBG is only dependent on temperature. A sensor with a 9.5-mm diameter of droplet-like structure is fabricated and experimentally studied. The experimental results demonstrate the feasibility of the proposed sensor and show that sensor possesses a high RI sensitivity of 157.8891 nm/RIU for the RI range from 1.3330 to 1.3785 and a temperature sensitivity of 10.3 pm/°C.

In this paper, an all-fiber optical current sensor based on ferrofluids and a singlemode-multimode-singlemode structure (SMS) with a piece of no-core fiber as the multimode section was proposed and experimentally investigated. The transmittance of the SMS that is sealed in a capillary with ferrofluids is highly sensitive to the surrounding magnetic field. Because the strength of the magnetic field is proportional to the current in the electric line, so the current can be measured. A demonstration sensor was fabricated and experimentally characterized. A sensitivity of 2.12 dB/A was achieved.