In-Line Mach-Zehnder Interferometers Based on a Capillary Hollow-Core Fiber Using Vernier Effect for a Highly Sensitive Temperature Sensor.

In this paper, we propose a highly sensitive temperature sensor based on two cascaded Mach-Zehnder interferometers (MZIs) that work using the Vernier effect. The all-fiber MZIs were assembled by splicing a segment of capillary hollow-core fiber (CHCF) between two sections of multimode fibers (MMFs). This cascaded configuration exhibits a temperature sensitivity of 1.964 nm/°C in a range from 10 to 70 °C, which is ~67.03 times higher than the sensitivity of the single MZI. Moreover, this device exhibits a high-temperature resolution of 0.0153 °C. A numerical analysis was carried out to estimate the devices' temperature sensitivity and calculate the magnification of the sensitivity produced by the Vernier effect. The numerical results have an excellent agreement with the experimental results and provide a better insight into the working principle of the MZI devices. The sensor's performance, small size, and easy fabrication make us believe that it is an attractive candidate for temperature measurement in biological applications.

Sensitivity Enhancement of Curvature Fiber Sensor Based on Polymer-Coated Capillary Hollow-Core Fiber.

In this paper, we propose and experimentally demonstrate a simple technique to enhance the curvature sensitivity of a bending fiber optic sensor based on anti-resonant reflecting optical waveguide (ARROW) guidance. The sensing structure is assembled by splicing a segment of capillary hollow-core fiber (CHCF) between two single-mode fibers (SMF), and the device is set on a steel sheet for measuring different curvatures. Without any surface treatment, the ARROW sensor exhibits a curvature sensitivity of 1.6 dB/m-1 in a curvature range from 0 to 2.14 m-1. By carefully coating half of the CHCF length with polydimethylsiloxane (PDMS), the curvature sensitivity of the ARROW sensor is enhanced to -5.62 dB/m-1, as well as an increment in the curvature range (from 0 to 2.68 m-1). Moreover, the covered device exhibits a low-temperature sensitivity (0.038 dB/°C), meaning that temperature fluctuations do not compromise the bending fiber optic sensor operation. The ARROW sensor fabricated with this technique has high sensitivity and a wide range for curvature measurements, with the advantage that the technique is cost-effective and easy to implement. All these features make this technique appealing for real sensing applications, such as structural health monitoring.

Polarimetric parameters associated to special microstructured fibers.

The Mueller matrices associated to six homemade special index-guiding microstructured fibers are determined at a transmission wavelength of 1550 nm. Diattenuation, polarizance, and polarization-dependent loss, among other polarimetric parameters, are determined from the Mueller matrix associated to a 2 m length of each fiber studied here. Results show that the fiber fabrication parameters have a clear effect on the output degree of polarization (DoP), providing an inverse relationship between the DoP output and the relative hole diameter, the d/Λ value.

Photonic crystal fiber interferometric vector bending sensor.

A compact and highly sensitive interferometric bending sensor (inclinometer) capable of distinguishing the bending or inclination orientation is demonstrated. The device operates in reflection mode and consists of a short segment of photonic crystal fiber (PCF) inserted in conventional single-mode optical fiber (SMF). A microscopic collapsed zone in the PCF-SMF junction allows the excitation and recombination of core modes, hence, to build a mode interferometer. Bending on the device induces asymmetric refractive index changes in the PCF core as well as losses. As a result, the effective indices and intensities of the interfering modes are altered, which makes the interference pattern shift and shrink. The asymmetric index changes in the PCF make our device capable of distinguishing the bending orientation. The sensitivity of our sensor is up to 1225 pm/degree and it can be used to monitor small bending angles (±2°). We believe that the attributes of our sensor make it appealing in a number of applications.

Locally pressed photonic crystal fiber interferometer for multiparameter sensing.

A mode interferometer consisting of a short section of photonic crystal fiber (PCF) fusion spliced to a standard single-mode optical fiber with localized perturbations is proposed for multiparameter sensing. In this sensing configuration, the parameter being sensed changes the visibility (an absolute parameter) of the interference pattern and also causes a shift (a relative parameter) to the interference pattern. To achieve this dual effect, a portion of the PCF is squeezed on localized regions with a serrated mechanical piece. In this manner, we introduce attenuation losses and effective refractive index changes to the interfering modes, hence, visibility changes and a shift to the interference pattern. Our device is suitable for monitoring diverse physical parameters, such as weight, lateral force, pressure, load, etc., with the advantage that compensation to temperature or power fluctuations is not required. Moreover, the sensor sensitivity can be adjusted in a simple manner.

Photonic crystal fiber interferometer for chemical vapor detection with high sensitivity.

We report an in-reflection photonic crystal fiber (PCF) interferometer which exhibits high sensitivity to different volatile organic compounds (VOCs), without the need of any permeable material. The interferometer is compact, robust, and consists of a stub of PCF spliced to standard optical fiber. In the splice the voids of the PCF are fully collapsed, thus allowing the excitation and recombination of two core modes. The device reflection spectrum exhibits sinusoidal interference pattern which shifts differently when the voids of the PCF are infiltrated with VOC molecules. The volume of voids responsible for the shift is less than 600 picoliters whereas the detectable levels are in the nanomole range.

Capillary refractometer integrated in a microfluidic configuration.

We propose a microfluidic method to measure the refractive index of liquids. This method is based on the dynamic focusing by a capillary when liquids with different refractive indexes are inserted into it. Fabrication of such a refractometer has been done by encapsulating two fibers and a capillary. A calibration method is proposed.

Simple all-microstructured-optical-fiber interferometer built via fusion splicing.

We report a compact and stable all-microstructured-optical-fiber interferometer built with two fusion splices separated a few centimeters from each other. The air-holes of the fiber are intentionally collapsed in the vicinity of the splices. This broadens the propagating optical mode, allowing coupling of two modes in the section between the splices. A truly sinusoidal interference pattern was observed from 800 nm to 1600 nm with fringe visibility reaching 80%. The fringe spacing was inversely proportional to the distance between the splices. The potential of the device for sensing applications is demonstrated.

Microlenses in the end of capillary tubes and their application to photonic crystal fibers.

We show that it is possible to make biconvex microlenses at the end of capillary tubes. These lenses can be fixed to the flat end of photonic crystal fibers and can collimate their light.

Temperature-independent strain sensor made from tapered holey optical fiber.

A large-mode-area holey fiber was tapered to a point in which the airholes collapsed, and its dependence on temperature and strain was studied. The transmission spectrum of such a fiber exhibits a series of peaks owing to the interference between the modes of the solid taper waist. We found that the interference peaks shifted to shorter wavelengths as the taper was elongated. However, the peaks were insensitive to temperature. The fabrication and advantages of our novel wavelength-encoded temperature-independent strain sensor compared with other optical fiber strain sensors are discussed.

Microstructured optical fiber coated with thin films for gas and chemical sensing.

We propose the use of tapered microstructured fibers with collapsed air-holes coated with thin layers for gas sensing. The collapsing of the holes allows having access to the evanescent fields which can be absorbed or attenuated with gas-permeable thin films. On the other hand, a section of the holey fiber is transformed into a solid multimode fiber. The beating between the multiple modes of the latter makes the transmission spectra of the device to exhibit an oscillatory pattern. This evanescent-fields-plus-modal-interferometer structure may offer interesting properties for gas and chemical sensing. As an example we demonstrate a hydrogen sensor.