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The output signal of Mach-Zehnder curvature sensor was Fourier transformed to increase its curvature range to high value of 1.69 m-1, relative to the traditional fringe contrast measurement 1.09 m-1.

A multimode interference (MMI) sensor was designed and experimentally demonstrated for simultaneous measurement of curvature and temperature. A typical fiber structure of single-mode fiber – multimode fiber – single-mode fiber (SMS), mounted in a long and thin carbon steel sheet, and then coated with polydimethylsiloxane (PDMS) was manufactured and tested in curvature and temperature. Bending laboratory results showed that the proffered sensor has a curvature sensitivity of -0.9835 dB/m-1 over a range from 0 m-1 to 1.3652 m-1 , measurements were taken by keeping a constant temperature of 30°C. The laboratory temperature response was -119 pm/°C at a temperature range from 30°C to 60°C, showing an improvement in temperature response, temperature measurements were taken by keeping a constant bending of 0 m-1 . The results show that PDMS coatings are a good way to improve multimode interferometer sensitivity during temperature measurement while keeping a good curvature measuring response, moreover the device shows a linear response within the curvature and temperature ranges. Another advantage of the PDMS coating is that it makes the sensor insensitive to refractive index changes, it gives the sensor robustness and protection against dust.

An easy to construct curvature sensor is proposed and experimentally demonstrated. The device was fabricated splicing a segment of hollow core fiber (HCF) between two segments of coreless fiber (CLF) forming a Mach-Zehnder interferometer (MZI).

An in-fiber Mach–Zehnder Interferometer (MZI), based on hollow core fiber (HCF), for measuring curvature is fabricated and experimentally demonstrated. The sensing part was fabricated by splicing a section of HCF between two sections of multimode fiber (MMF), and different HCF lengths were investigated in order to achieve the highest curvature sensitivity. These devices were tested in a transmission configuration using lead-in and lead-out single mode fibers (SMF). The sensor was attached to a steel sheet by using the polymer Polydimethylsiloxane (PDMS) for accurate control of the applied curvature. The modal analysis was carried out using a commercial software based on finite element method (FEM) and by incorporating some of the experimental data, it was feasible to determine the two dominant modes that interfere in this sensor. The devices were characterized by measuring the fringe contrast variations due to the curvature changes. The interferometer fabricated with a HCF 2.5 mm in length HCF showed the highest curvature sensitivity, −17.28 ± 2.30 dB/m−1, in a range from 1.84 m−1 to 2.94 m−1. Moreover, the sensor that exhibited a better performance was fabricated with a HCF length of 1 mm, combining the most extensive curvature range (from 0.95 m−1 to 2.68 m−1) and an adequate sensitivity (−11.80 ±1.30 dB/m−1). The analysis of the interferometric signal of this device in Fourier domain, allows us to establish a one to one relationship between the contrast and the curvature in a broader range (from 0 m−1 to 2.94 m−1). Moreover, the fringe contrast showed a very low dependency on temperature (from 30 °C to 90 °C), depicting that this device was not affected by temperate fluctuation.

A Mach-Zehnder curvature sensor with enhanced contrast based on lateral-offset splicing is proposed. The highest contrast was obtained at 2 µm lateral-offset. Using the Fourier Transform, amplitude changes were traced from 0 m-1 to 2.68 m-1.