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In this paper, a narrow spectral linewidth and wavelength-tunable erbium-doped fiber laser based on a Mach-Zehnder interferometer (MZI) as a filter is proposed and experimentally demonstrated. The filter is fabricated by splicing a short length of capillary hollow-core fiber (CHCF) between two small sections of multimode fibers (MMF). The interference pattern generated by the filter has a free spectral range of 10.5 nm and a maximum extinction ratio of 36 dB at 1560 nm. Then, by incorporating the filter inside a ring cavity laser, a single laser emission centered at 1566.46 nm with a spectral width around 0.03 nm and a high SNR of 66 dB (also has a SMSR of 53 dB) is obtained. By changing the temperature of the filter, the wavelength emission of the laser can be tuned from 1566.46 to 1570.09 nm. Considering that the fabrication process of the filter is relatively simple and can be easily implemented in an EDFL, we strongly believe that it can have a great deal of interest in telecom applications.

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).

A fiber optic Fabry-Perot interferometer (FPI) was proposed and demonstrated. The sensor was constructed by splicing a single mode fiber (SMF) and a hollow core fiber (HCF), cleaving the HCF to a length of 15 ± 1 μm and filling it with polydimethylsiloxane (PDMS). The sensitivity of the sensor tip was 0.61 nm/°C and was caused by a combined effect of the thermo-optical and expansion coefficients.

We compare the behavior of three temperature sensors based on fiber optic Fabry-Perot interferometers (FPI). The length of the cavities were 30 ± 2 µm and they were filled with polydimethylsiloxane (PDMS), poly(methyl methacrylate) (PMMA), and the optical adhesive NOA61. The sensitivities for each sensor were 2.20 nm/°C, 0.42 nm/°C, and 0.57 nm/°C, respectively. The refractive index (RI) sensitivity showed by these devices was caused by an integrated consequence of the thermo-optical and expansion coefficients.

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.