In this paper, a fiber Mach Zehnder (MZ) interferometer based on sawtooth structure was proposed. It not only has the ability of bending sensing and bending direction recognition, but also can be applied to multi parameter sensing. Here we established the fiber hetero core structure with single mode fiber-multimode fiber-single mode fiber (MSM), and then fabricated the sawtooth structure on the single mode fiber in the middle by CO ₂ laser. Since part of the cladding on the single-mode fiber was removed, the fiber core was no longer at the neutral plane. When the sensor is bent in the 0° direction or 180° direction, the effective refractive index and optical path length of the core mode will increase or decrease, resulting in the increase or decrease of the optical path difference between the core mode and the cladding mode. And then the interference wavelength will show a red shift or blue shift, so as to realize the curvature sensing with bending direction recognition. Meanwhile, due to the local stress of the sawtooth structure in the bending, the sensitivity of the bending sensor is improved. The testing results indicate that the sensitivity of 0° bending and 180° bending are −15.16 nm/m ⁻¹ and 8.32 nm/m ⁻¹ with the curvature range of 0–1 m ⁻¹ . Moreover, the sawtooth structure sensor has excellent performance in strain, temperature and torsion sensing. The torsional sensitivity of the sensor reaches −0.30 nm/(rad/m) with torsional range of 0–13.96 rad/m, the temperature sensitivity reaches 58.90 pm/°C, and the strain sensitivity reaches −3.62 pm/μϵ. In strain sensing, there are some interference valleys insensitive to strain, which can realize the simultaneous measurement of other parameters and strain.

By using a seven-core fiber (SCF), we propose and demonstrate a novel segmented detection SPR sensor, which solves two bottlenecks about the fiber SPR sensor of low sensitivity and the difficulty in the multichannel detection. The proposed sensor has ultra high sensitivity and wide detection range because of employing the segmented detection technology. Besides that, the proposed sensor employs reflection-type time division multiplexing (TDM) technology in fiber multichannel detection for the first time. We couple light into and out of the six circularly symmetric distributed cores of the seven-core fiber to realize the three channel SPR sensing and testing. This three-channel SPR sensor has the advantages of detecting biochemical or multi analytes reactions and eliminating the distractions due to temperature fluctuations or sample composition variations and adsorption of non-target molecules to the sensor surface. This SPR sensor also has the advantages of online monitoring by inserting into the blood vessel because of its small size. Furthermore, this paper has deeply researched the relationship between the refractive index of the solution to be measured, the grinding angle of the sensing channel, the sensitivity and the detection range. In this paper, we propose a novel segmented detection method which realizes the wide detection range with the wider refractive index range of 1.333~1.395 and the narrower working bandwidth of 250nm compared with the common SPR sensor, the average sensitivity and the maximum sensitivity of the sensor reach 7387.1nm/RIU and 8502.5nm/RIU respectively.

The heavy metal ions in food usually exist in the form of mixtures and have low concentrations. It is crucial for human health to achieve highly sensitive and specific detection of heavy metal ions in food. In this paper, we propose a fiber SPR sensor that adjusts sensitivity by adjusting the incident angle through fiber microstructure, and uses it for the specific detection of heavy metal nickel ions. By fabricating an S-taper microstructure on a single-mode fiber, adjusting its axial offset and taper area diameter, the sensitivity of the sensor was increased from 2294.50nm/RIU to 3244.62nm/RIU. By modifying double stranded DNA on the surface of the sensing region of the fiber S-taper structure, specific recognition of heavy metal nickel ions was achieved, with a detection sensitivity of 3.587 nm/[lg (mg/mL)] and a detection limit as low as 8.65 pg/mL. The sensitivity of the sensor for detecting other heavy metal ions is lower than 0.44nm/(lg (mg/mL)). The Sensitization specific fiber SPR sensor proposed in this paper provides a new solution for the highly sensitive and specific detection of various heavy metal ions in food.

Micro-displacement measurements play a crucial role in many industrial applications. Aiming to address the defects of existing optical-fiber displacement sensors, such as low sensitivity and temperature interference, we propose and demonstrate a novel surface plasmon resonance (SPR)-based optical-fiber micro-displacement sensor with temperature compensation. The sensor consists of a displacement-sensing region (DSR) and a temperature-sensing region (TSR). We employed a graded-index multimode fiber (GI-MMF) to fabricate the DSR and a hetero-core structure fiber to fabricate the TSR. For the DSR, we employed a single-mode fiber (SMF) to change the radial position of the incident beam as displacement. The resonance angle in the DSR is highly sensitive to displacement; thus, the resonance wavelength of the DSR shifts. For the TSR, we employed polydimethylsiloxane (PDMS) as a temperature-sensitive medium, whose refractive index is highly sensitive to temperature; thus, the resonance wavelength of the TSR shifts. The displacement and temperature detection ranges are 0–25 μm and 20–60 °C; the displacement and temperature sensitivities of the DSR are 4.24 nm/μm and −0.19 nm/°C, and those of the TSR are 0.46 nm/μm and −2.485 nm/°C, respectively. Finally, by means of a sensing matrix, the temperature compensation was realized.

Fiber surface plasmon resonance (SPR) curvature sensor has the advantages of being insensitive to axial strain and temperature, but there are still difficulties in multi-channel cascade and curvature direction identification. In this paper, a multi-channel directional recognition SPR curvature sensor based on D-type double clad multimode fiber was proposed. The asymmetric structure of D-type multimode fiber can well realize directional curvature sensing. When the light in the multimode core leaks to the silica clad, due to the existence of plastic clad, it can still continue to transmit. When the refractive index of UV curable adhesive coated in different sensing areas is different, wavelength division multiplexing multi-channel sensing can be realized. When the sensing probe is bent to be convex, the maximum intensity sensitivity is 0.0057a.u./m ^-1 , the resonance wavelength is red-shift, and the wavelength sensitivity is 0.6114nm/m ^-1 . When the sensor probe is bent to be concave, the maximum intensity sensitivity is 0.0043a.u./m ^-1 , the wavelength is blue-shift, and its highest sensitivity is 0.5831nm/m ^-1 . In addition, this paper also realizes three-channel directional recognition SPR curvature sensor based on D-type double clad multimode fiber, which provides a new scheme for multi-channel directional curvature measurement.