A dual side hole fiber (DSHF) with a pair of large air holes is proposed for free transducer temperature sensing in the infrared region (1500–1600 nm). The results of birefringence measurement based on a sagnac interferometer showed the linear group birefringence is from 4.35×10<sup>-5</sup> at 1500 nm, to 1.14×10<sup>-4</sup> at 1600 nm, respectively. The group birefringence of DSHF leads to a group temperature sensitivity from 3.9nm/°c at 1501nm to 1nm/°c at 1595nm, respectively.
A high-birefringent fiber (HBF) was tapered as adiabatic in sequence steps by utilizing a CO<sub>2</sub> laser and its birefringence
was measured in fiber loop mirror (FLM) setup. The birefringence of tapered section and total sensor was obtained to be
-8.02×10<sup>-2</sup>, and 2.46×10<sup>-4</sup>, respectively. Then, refractive index (RI) sensitivity increased and temperature sensitivity of
the tapered Hi-Bi fiber (THBF) decreased. The sensitivity of the proposed FLM interferometer for RI changes in the
range from 1.3380 to 1.3470 was measured to be 389.85 nm/RIU. The temperature sensitivity in the range from 50°C to
90 °C was measured to be -1.19nm/°C.
In this work, fiber in-line Mach-Zehnder Interferometer (MZI) based on triangular-shape suspended core fibers (SCFs) is
investigated. The sensitivity of the sensing head was determined for pressure and temperature, respectively. The sensitivities
are 0.4 pm/psi and 13 pm/psi for longitudinal and radial pressure, respectively. The sensing head was also subjected to
temperature and presented very low sensitivity.
A simple refractive index sensor based on a fat long period fiber grating (FLPFG) in a single-mode fiber is constructed
and demonstrated. The sensor consists of periodical fattening region in a short piece of single-mode fiber. In this method,
the fiber fattening is realized by applying a standard fusion splicing procedure on single mode optical fiber. The
sensitivity of the sensor is 7.5 x 10<sup>-5</sup> which the sensitivity is enhanced to the other kinds of long period fiber grating
A hybrid long period fiber grating (HLPFG) sensor was utilized for sensing the refractive index and pressure for downhole
applications. The HLPFG is fabricated by fattening and tapering on a single mode fiber, utilizing a standard fusion
splicing and a CO2 laser, respectively. The limit of detection (LOD) of the HLPFG for the RI measurement in the range
from 1.3150 to 1.3559 is 4x10<sup>-5</sup> as a refractometer sensor which can be used for analysis of multicomponent in native
petroleum. Pressure sensitivity of the HLPFG sensors in the range from 500 to 6000 psi is -0.6 pm/psi. With a 10 pm
resolution for the wavelength shift detection our OSA, the LOD of the device at room temperature for pressure
measurement is calculated to be 17 psi. This sensor can be used as a high pressure sensor in downhole application.
We report fabrication of a high pressure optical fiber sensor by using a fat long period fiber grating (FLPFG) for
downhole applications. The pressure sensitivity of the bare optical fiber is low so we have designed a mechanical
transducer for increasing the pressure sensitivity and possibility of installation the sensor in downhole. The pressure
along the longitudinal direction changes the physical characteristic of the FLPFG and result in shifting the resonance
wavelength. The FLPFG sensor has been installed on transducer and the pressure sensitivity of the fiber sensor has been
measured. Since the temperature changes can affect the FLPFG output, the high pressure vessel was isolated and the
temperature was kept constant during the experiment. Pressure sensitivity of the FLPFG sensors has been measured by
increasing the pressure from 1500 psi to 10000 psi in steps of 700 psi which is equal to -1.04 pm/psi. With a 10 pm
resolution for the wavelength shift detection our OSA, detection limit of our device at room temperature for pressure
measurement is calculated to be 10 psi.