A reflective fiber optic sensor based on multimode interference for the measurement of relative humidity (RH) is proposed and experimentally demonstrated. The proposed probe is fabricated by fusion-splicing, approximately 30 mm long coreless fiber section to a single mode fiber. A hydrophilic agarose gel is coated on the coreless fiber, using the dip coating technique. When the incident light comes from the SMF to the CSF, the high-order modes are excited and propagate within the CSF. These excited modes interfere with one another as they propagate along whole CSF length, giving rise to a multimode interference (MMI). Since the effective refractive index of the agarose gel changes with the ambient relative humidity, as the environmental refractive index changes, the propagation constants for each guided mode within the CSF will change too, which leads to shifts in the output spectra. The proposed sensor has a great potential in real time RH monitoring, exhibiting a large range of operation with good stability. For RH variations in the range between 60 %RH and 98.5 %RH, the sensor presents a maximum sensitivity of 44.2 pm/%RH, and taking in consideration the interrogation system, a resolution of 1.1% RH is acquired. This sensor can be of interest for applications where a control of high levels of relative humidity is required.
A hybrid sensor based on microstructured hollow core fiber is proposed for the simultaneous measurement of strain and temperature. The fiber, consisting of four silica capillaries with wall thickness of ~1 μm and a cladding with a thickness of ~26 μm, is spliced between two sections of single mode fiber. Using a low arc discharge power to splice the two fibers, a Fabry-Perot interferometer is formed. In this situation, light travels in the hollow core and the behavior of a twowave interferometer is observed. However, when the power of the arc discharge is increased, the structure near the splice area changes, generating new interferometric paths and giving rise to a different spectral response. In this work, sensors with a single degenerated area are analyzed. In such case, both Fabry-Perot and Michelson interferometers are created and different sensitivities to strain and temperature are obtained. The different spectral frequencies are analyzed, enabling the discrimination between the two parameters. For a sensor with a length of ~385 μm, strain sensitivities of 2.46 pm/με and -0.52 pm/με are obtained for the Fabry-Perot and for the Michelson interferometer, respectively. Regarding temperature, a sensitivity of 1.81 pm/°C was attained for the former, whereas for the last the sensitivity was of 42.23 pm/°C. Keywords: Hyb
There is a set of important selection criteria in the design of fiber optic sensors that determine the compromise between design complexity and performance. Optical fiber sensors not only withstand high temperatures, but they can also operate in different chemical and aqueous media allowing measurements in areas not otherwise accessible. A Fabry-Perot cavity based on an air bubble created in a multimode fiber section is proposed. The air bubble is formed using only cleaving and fusion splicing techniques. The parameters used to produce the microcavities were found empirically. Two different configurations are explored: an inline cavity formed between two sections of MMF, and a fiber tip sensor. In the last, after the air bubble is created, a cleave is made near the cavity, after which the sensor is subjected to several electrical arcs to reshape the cavity and obtain a thin diaphragm. The inline sensor, with a length of ~297 μm, was used to measure strain and presented a sensitivity of 6.48 pm/με. Regarding the fiber tip sensor, it was subjected to glycerin/water mixture variations, by immerging the sensing head in several solutions with different concentrations of water in glycerin. In this case, the sensor had a length of ~167 μm and a diaphragm thickness of ~20 μm. As expected, with the increase of the external medium refractive index, the sensor visibility decreased. Furthermore, a wavelength shift towards red was observed, with a sensitivity of 7.81 pm/%wt. Both devices exhibited low dependence to temperature (<1.8 pm/°C).
In this work, an optical fiber sensing network has been developed to assess the impact of different environmental conditions on lithium batteries performance through the real time thermal monitoring. The battery is submitted to constant current charge and different discharge C-rates, under normal and abusive operating conditions. The results show that for the discharge C-rate of 5.77C, the LiB under cold and dry climates had 32.5% and 27.2% lower temperature variations, when compared with temperate climates, respectively. The higher temperature shift detected in the temperate climate was related to the battery better performance regarding discharge capacity and power capabilities.
A Fabry-Perot interferometer based on an array of soda-lime glass microspheres is proposed for temperature sensing. The microspheres are introduced in a hollow-core silica tube using a tapered fiber tip. After the insertion of each microsphere the sensor is subjected to temperature measurements. The sensor exhibits non-linear behavior and a dependence on the number of microspheres is observed. A maximum sensitivity of 11.13 pm/°C is achieved, when there is only one microsphere inside the capillary structure.
A Fabry-Perot air bubble microcavity fabricated between a section of single mode fiber and a multimode fiber is proposed. The study of the microcavities growth with the number of applied arcs is performed. The sensors are tested for lateral load and strain, where sensitivities of 0.32 nm/N and 2.11 nm/N and of 4.49 pm/με and 9.12 pm/με are obtained for the 47 μm and 161 μm long cavities, respectively. The way of manufacturing using a standard fusion splicer and given that no oils or etching solutions are involved, emerges as an alternative to the previously developed air bubble based sensors.
A curvature sensor based on a Fabry-Perot interferometer is proposed. A capillary tube of silica is fusion spliced
between two single mode fibers, producing a Fabry-Perot cavity. The light propagates in air, when passing through
the capillary tube. Two different cavities are subjected to curvature and temperature. The cavity with shorter length
shows insensitivity to both measurands. The larger cavity shows two operating regions for curvature measurement,
where a linear response is shown, with a maximum sensitivity of 18.77pm/m<sup>-1</sup> for the high curvature radius range.
When subjected to temperature, the sensing head produces a similar response for different curvature radius, with a
sensitivity of 0.87pm/°C.
In this work, a Fabry-Perot optical fiber sensor for the measurement of strain at extreme temperatures is proposed. The cavity is formed by splicing a short section of a silica tube between two sections of single mode fiber. The tube, with a cladding ~14 μm thick and a hollow core, presents four small rods, of ~20 μm in diameter each, positioned in in diametrically opposite positions. This design ensures higher mechanical stability of the tube. Strain measurements are performed over a wide range of temperatures, until 900 °C. Some of the annealing effects are addressed in this study.
In this paper it is proposed an interrogation system based on OTDR for fiber loop mirror intensity sensors. The system has been characterized in order to obtain its maximum dynamic range. The technique demonstrated good linearity with a – 13.3 dB/mm slope. A 0.027 mm resolution was achieved. The proposed interrogation system permits multiplexing of around 10 sensors and showed to be an alternative technique for multiplexing and remote sensing.
In this work a novel optical fiber sensor based on silica microspheres array is proposed. Different sensing heads are presented and compared, differing on the number of microspheres. These structures, ranging from arrays of one to five, are spliced in series. The sensor is subjected to different physical parameters, such as strain, temperature, refractive index and bending. Depending on the number of microspheres the sensitivities to strain and bending are different. The sensor also presents a high sensitivity to temperature of 20.3 pm/°C.
A torsion active sensor based on a figure-of-eight configuration is presented. The interferometric fiber loop mirror, composed by a section of photonic crystal fiber, also acts as a sensing element. When torsion is applied over a range of 180°, a sensitivity of 7.13 pm/degree is achieved. Besides, this configuration can also be used to measure optical power variations and it presents low sensitivity to temperature.
A Fabry-Pérot microcavity tip temperature sensor based on a special design double-cladding optical fiber is proposed. The produced fiber has pure silica core and outer cladding and a silica ring doped with phosphorous. The whole ring region is removed by chemical etching post-processing. Consequently, light will be guided in the core region. In a first step, the double-cladding optical fiber is spliced to single mode fiber. Afterwards, the tip is etched in a solution of 48% hydrofluoric acid. The inner cladding will be etched faster, and the core becomes suspended and surrounded by air. The Fabry-Pérot microcavity tip sensor is subjected to temperature, and a linear sensitivity of 14.6 pm/°C is obtained.
In this work, a simple real-static nanostrain sensor based on a Bragg grating structure is presented. The setup is
constituted by a narrow linewidth laser as light source, an optical circulator and a photodetector. The sensing head is
formed by a chirped Bragg grating inscribed in a standard single mode fiber (SMF-28) by the phase technique. The fiber
face end is cleaved and coated with a silver mirror, obtaining a Fabry-Perot interferometer. It is observable that the
fringes period increases along the grating, due to the chirp spectrum (0.4 nm/cm) characteristics. The laser is fixed in one
slope region of the fringe pattern. When strain is applied, the optical power changes linearly. A sensitivity of 5.72 μW/με
in a range of 2 με . The sensing head resolution is 70 nε for a measurement step of 875 nε.
In this work, a high-birefringent Sagnac loop interferometer torsion sensor is presented. The sensing head is
inserted between the output ports of a high-birefringent coupler and it is formed by a section of standard
single mode fiber. The sensing head characterization is done for torsion, temperature and strain
measurements. The spectral response of this sensing head presents two interferometers, which are dependent
on the light polarization states. Interference occurs due to the different lengths of the coupler output arms.
This configuration allows the exclusion of a polarization controller, since it is possible to manipulate directly
the polarization of light that travels inside the coupler. When the sensing head is subjected to torsion, it is
possible to observe a beat between the two interferometers. In this case, there is a simultaneous π/4 excitation
of the two polarization states in the splices region. The torsion sensitivity is related to the sensing head length.
The sensor response is periodic and the twist range can be from -2π to 2π. The sensor is unaffected by
temperature and strain variations. This configuration is simple and when compared to the conventional
configuration, the polarization controller is suppressed. The setup can be used in specific applications, such as
in mechanical engineering.
A highly-birefringent photonic bandgap Bragg fiber loop mirror sensor is proposed. Thanks to the Bragg
fiber geometry, one can observe the group birefringence and the bandgap fiber in the transfer function.
The sensing head presented different sensitivities for strain and temperature measurements. Using the
matrix method, both the physical parameters can be discriminated. It is important to highlight that this
Bragg fiber presents sensitivity to temperature of ~5.75 nm/ºC, for the group birefringence measurand.