In this paper we discuss results obtained with an in-line Fabry-Perot interferometer (FPI) built by splicing a small section of capillary fiber between two pieces of standard single mode fiber, resulting in a rectangular air cavity. The FPIs were characterized regarding sensitivity to temperature and longitudinal strain. The FPIs were bonded to pieces of Terfenol-D, a magnetostrictive alloy, to be used as magnetic field sensors. Fiber Bragg Gratings were also bonded to Terfenol-D for comparison. The FPI based on capillary optical fiber and Terfenol-D showed a higher sensitivity to an applied magnetic field when compared to an FBG.
In the present work, the use of a single mode fiber (SMF) with high Germanium doped core as temperature sensor is studied. The fiber core consists of a 1.1 μm highly germanium doped step index waveguide surrounded by a pedestal of 3.5 μm diameter. The outer diameter of the fiber is 125 μm. A short stub of ~2 mm is used in the fabrication of the interferometer. The highly germanium doped fiber is spliced between two standard SMF. In one of the splices both fibers are ideally aligned, in the other splice a small misalignment between the fibers is done. An annealing process is made for 5 hours at 850°C which results in a good operation stability up to 700°C. A wavelength shift as a function of temperature of 76 pm/°C is reported. To demonstrate the interferometer efficiency, a fiber Bragg grating is written in the highly germanium doped core and tested for temperature response. A temperature sensitivity of 13pm/°C was demonstrated. The interferometer fabrication requires only a few and easy steps. Due to the standard splices made between the fibers, the device is robust. We believe that the sensor may be used under harsh environmental conditions, since it shows a high sensitivity and a small size in combination with great robustness.
In this work, the use of a photonic crystal fiber (PCF) with a highly Germanium (Ge) doped core is exploited as temperature sensor for the first time (to our knowledge). The PCF has an outer diameter of 125 μm and consists of a microstructured cladding with an average pitch and hole diameter of Λ=4.6 μm and d=1.0 μm, respectively. A short PCF stub (~2.0 mm) is used for the preparation of an interferometer. The PCF is spliced between single mode fibers (SMF), meaning that the PCF holes are fully collapsed in the splicing region while the Ge-doped core is still present. The splice parameters were changed to make a short collapse region of (200±30) μm. The first splice is used to excite the fundamental core mode and multiple higher order cladding modes by applying a core-to-core offset. The second splice acts as spatial filter to detect only the light which is guided in and near the core. The interferometer is heated up to 500°C and the total wavelength shift with the temperature variation found to be 74 pm/°C which is more than 5 times higher than a fiber Bragg grating at 1550 nm (13 pm/°C). The PCF interferometer preparation requires only a few steps, cleaving and splicing the fibers. The short length, the high thermal sensitivity and stability of the structure make the device attractive for many sensing applications including high temperature ranges.
In this paper a simple photonic crystal fiber (PCF) interferometric breathing sensor is introduced. The interferometer consists of a section of PCF fusion spliced at the distal end of a standard telecommunications optical fiber. Two collapsed regions in the PCF caused by the splicing process allow the excitation and recombination of a core and a cladding PCF mode. As a result, the reflection spectrum of the device exhibits a sinusoidal interference pattern that instantly shifts when water molecules, present in exhaled air, are adsorbed on or desorbed from the PCF surface. The device can be used to monitor a person's breathing whatever the respiration rate. The device here proposed could be particularly important in applications where electronic sensors fail or are not recommended. It may also be useful in the evaluation of a person's health and even in the diagnosis and study of the progression of serious illnesses such as sleep apnea syndrome.
A simple and compact photonic crystal fiber (PCF) interferometer that operates in reflection mode is proposed for
refractive index (RI) sensing. The device consists of a ~12mm-long stub of commercially available PCF (LMA-10)
fusion spliced to standard optical fiber (SMF-28). The device reflection spectrum exhibits interference patterns with
fringe contrast up to 40 dB. One of the excited modes in the PCF is sensitive to external RI therefore the device can
be useful for refractrometry. The shift of the interference pattern can be monitored as a function of the external
index. In the operating range, from 1.33 to 1.43, the maximum shift is less than the interferometer period, so there is
no-ambiguity in the measurements. The maximum sensitivity and resolution achieved were 735 nm per RI units and
7×10-5, respectively. Another approach to measure the external RI consists of monitoring the reflection power
located at the quadrature point of the inference pattern in a properly selected wavelength. Consequently the
measuring range is narrower but the resolution is higher, up ~7×10-6, thanks to the high fringe contrast.
A high sensitivity optical fiber pressure sensor based on a modal interferometer with high birefringence photonic fiber is
proposed and demonstrated. The sensor dependence with hydrostatic pressure is evaluated both numerically and
experimentally. The measured pressure sensitivity at room temperature is found to be 3.36 nmMPa-1.