In this work, we introduce a new concept for an in-line optical fiber temperature and force sensor, which are based on two cascaded Fabry-Perot cavities and the Vernier effect. The sensing cavity is composed of the UV glue, while the reference cavity is composed of the silica fiber. Due to the high thermo-optic coefficient and low Young's modulus of the UV glue in the sensing cavity, this sensor can be used for temperature and force measurements. The obtained temperature and force sensitivity are 8.0 nm/°C and ‒5.44×104 nm/N, respectively.
This paper demonstrates a portable optical sensor for human gait monitoring. The device is based on a smartphone and POF sensor specifically designed for use in shoe insoles. The monitoring of multiple sensors by a single smart device is achieved by image segmentation based on Voronoi tessellation, as this work describes in detail. Experimental tests performed with the sensor have demonstrated its ability to provide information on spatial and temporal parameters of gait as well as pressure at different plantar loci.
We propose and experimentally demonstrate a fiber refractometer based on a C-shaped fiber and the Vernier effect. The sensor is fabricated by cascading a single mode fiber (SMF) pigtail together with a C-shaped fiber segment and another SMF segment. Thus, the C-shaped fiber would constitute an open cavity (sensing cavity) in which test analytes could be filled, while the SMF segment would constitute another reference cavity. Due to the similar optical path length of these two cavities, Vernier effect would be activated, thus forming spectral envelops in the reflection spectrum of the sensor. Variations in the refractive index (RI) of analytes would result in the shifts of the spectral envelops. Experiments are carried out in the characterization of the sensor measuring gaseous analytes. The sensitivity of the sensor is found to be ~37238 nm/RIU for gas RI measurement. The proposed sensor features the advantages such as ease of fabrication, extremely high sensitivity, capability of sensing of both gaseous and liquid analytes, small footprint, and good mechanical strength.
In this paper, we proposed an in-fiber Mach-Zehnder temperature sensor based on a dual-core fiber (DCF) with one core as the sensing arm suspended in the centre fluidic channel of the DCF and the other core as the reference arm running eccentrically along the fiber. The fluidic channel was infiltrated with silicone oil. Temperature variations would change the refractive index of silicone oil and thus the effective index of the guided mode in the suspended core, thus shifting the interference spectra. The sensitivity of the sensor using a DCF infiltrated with ∼20 cm-long silicone oil was found to be as high as −1.42 nm/°C, comparable to those of the SPR fiber sensors and other interferometric sensors. The measuring range of the sensor was more than 120°C. The proposed sensor could be easily fabricated with good robustness and stability, which makes the sensor suitable for applications such as environment and architecture monitoring.
In this work, we reported highly reflective polymer optical fiber Bragg gratings (POFBGs) in poly(methyl methacrylate) (PMMA)-based polymer optical fibers (POFs) by means of a 266 nm pulsed laser and phase mask technique. Fiber Bragg gratings (FBGs) were inscribed with a single pulse up to 3.7 mJ. After post-annealing, a stable refractive index change up to 4.2×10-4 was obtained. The behavior may mainly be attributed to the movement of initiating radicals arising from BDK under UV irradiation.
In this work, we reported 2-mm-long fiber Bragg gratings (FBGs) in benzyl dimethyl ketal (BDK)-doped poly(methyl methacrylate) (PMMA) optical fibers by means of a 520 nm femtosecond laser and point-by-point FBG inscription technique. The highest reflectivity of ~99% is obtained with a pulse energy of 11.2 nJ, showing a large refractive index modulation amplitude of 7.2×10-4 .
In this paper, we report photo-inscription of uniform Bragg gratings in trans-4-stilbenemethanol-doped photosensitive step-index polymer optical fiber. Gratings were produced at ~1575 nm by the phase mask technique with a femtosecond laser emitting at 400 nm with different average optical powers (8 mW, 13 mW and 20 mW). The grating growth dynamics in transmission were monitored during the manufacturing process, showing that the grating grows faster with higher power. Using 20 mW laser beam power, the reflectivity reaches 94 % (8 dB transmission loss) in 70 seconds. Finally, the gratings were characterized in temperature in the range 20 - 45 °C. The thermal sensitivity has been computed equal to – 86.6 pm/°C.
In this paper, both non-annealed and annealed trans-4-stilbenemethanol-doped step-index polymer optical fibers were photo-inscribed using a 325 nm HeCd laser with two different beam power densities reaching the fiber core. In the high density regime where 637 mW/mm2 are used, the grating reflectivity is stable over time after the photo-writing process but the reflected spectrum is of limited quality, as the grating physical length is limited to 1.2 mm. To produce longer gratings exhibiting more interesting spectral features, the beam is enlarged to 6 mm, decreasing the power density to 127 mW/mm2. In this second regime, the grating reflectivity is not stable after the inscription process but tends to decay for both kinds of fibers. A fortunate property in this case results from the possibility to fully recover the initial reflectivity using a post-inscription thermal annealing, where the gratings are annealed at 80 °C during 2 days. The observed evolutions for both regimes are attributed to the behavior of the excited intermediate states between the excited singlet and the ground singlet state of trans- and cis-isomers as well as the temperature-dependent glassy polymer matrix.
Step-index polymer optical fiber Bragg gratings (POFBGs) and microstructured polymer optical fiber Bragg gratings (mPOFBGs) present several attractive features, especially for sensing purposes. In comparison to FBGs written in silica fibers, they are more sensitive to temperature and pressure because of the larger thermo-optic coefficient and smaller Young’s modulus of polymer materials. (M)POFBGs are most often photowritten in poly(methylmethacrylate) (PMMA) materials using a continuous-wave 325 nm HeCd laser. For the first time to the best of our knowledge, we study photoinduced birefringence effects in (m)POFBGs. To achieve this, highly reflective gratings were inscribed with the phase mask technique. They were then monitored in transmission with polarized light. For this, (m)POF sections a few cm in length containing the gratings were glued to angled silica fibers. Polarization dependent loss (PDL) and differential group delay (DGD) were computed from the Jones matrix eigenanalysis using an optical vector analyser. Maximum values exceeding several dB and a few picoseconds were obtained for the PDL and DGD, respectively. The response to lateral force was finally investigated. As it induces birefringence in addition to the photo-induced one, an increase of the PDL and DGD values were noticed.
In this paper, we report photo-inscription of uniform Bragg gratings in Trans-4-stilbenemethanol-doped photosensitive step-index polymer optical fiber characterized by a core diameter of 8.2 μm. Single-mode gratings were produced at ~1550 nm by the phase mask technique with a Helium-Cadmium emitting at 325 nm with an average power of 30 mW. The grating growth was monitored during the manufacturing process, showing that the reflected band is blue shifted by a few hundreds of picometers. Finally, the gratings were characterized in temperature in the range 25 − 50 °C. Their sensitivity has been computed equal to − 47 pm/°C.
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