We propose an in-line modal coupler based on a multicore fiber (MCF) which can be readily fabricated by using the adiabatic tapering method. The intermodal coupling of the in-line modal coupler apparently generated the transmission oscillation of the center core and the multiple side core modes depending on the waist diameter. The reduction of the waist diameter of the adiabatically tapered MCF could dramatically change its sensitivities to strain, temperature, and ambient index. We believe that experimental results are very useful to fabricate the in-line modal coupler based on the MCF and to improve the performance of the fiber-optic sensors by controlling the waist diameter of the adiabatically tapered MCF.
A highly sensitive RH sensor based on the MKR with the PVA overlay is investigated. After making a tie using the microfiber with a diameter of 2 μm, the MRK with a loop diameter of 90 μm is fabricated and coated by using a PVA which can absorb humidity. The optical spectra of the MRK with the PVA overlay are converted to the spatial frequency spectra by using the FFT for precise measurement. The absorption of humidity in the proposed MKR-based sensing probe effectively changes the spatial frequency of the MKR. The RH sensitivities of the proposed MKR-based RH sensing prober with higher order modes can be dramatically improved.
A simple technique to effectively enhance the temperature sensitivity of the Mach-Zehnder interferometer (MZI) by using a microfiber with low index polymer coating is theoretically and experimentally investigated. We successfully improve the temperature sensitivity of the polymer-overlaid microfiber MZI to be -6.68 nm/°C around a temperature of 25°C, which is 13 times higher than that of the microfiber MZI without polymer coating. A linear temperature sensitivity of 10.44 pm-1/°C was achieved in the spatial frequency domain after Fourier transform.
A microfiber-based Mach-Zehnder interferometer (MZI) for measurement of absolute strain is proposed and demonstrated experimentally. The diameter of the microfiber is optimized to induce the negative thermo-optic effect in the microfiber MZI and the temperature sensitivity of the microfiber MZI is successfully suppressed. When the diameter of the microfiber is ~5 μm, the temperature sensitivity of the microfiber-MZI is dramatically mitigated. We apply the proposed microfiber MZI for absolute strain measurement and its strain sensitivity is measured to be ~7.13×10-2 nm/ μЄ.
We investigate the effect of a waist diameter of a polarization-maintaining fiber (PMF) on ambient index and temperature sensitivities by configuring a Sagnac loop interferometer. To make the PMF sensitive to external index change, a micro-tapering technique is exploited to fabricate the tapered PMF. The Sagnac loop interfoermeter is fabricated by using the tapered PMF with various waist diameters. The reduction of the PMF diameter results in the enhancement of the ambient index sensitivity of the tapered-PMF-based Sagnac interferometer. However, the temperature sensitivities of the proposed Sagnac interferometers are not changed by reducing the waist diameters of the PMFs.
A highly sensitive current sensor based on an optical microfiber loop resonator (MLR) incorporating low index polymer is proposed and experimentally demonstrated. The microfiber with a waist diameter of 1 μm is wrapped around the nicrhrome wire with low index polymer coating and the optical MLR is realized. The use of the microfiber and low index polymer with high thermal property can effectively improve the current sensitivity of the proposed MLR-based sensing probe to be 437.9 pm/A2, which is ~10 times higher than the previous result.
KEYWORDS: Interferometers, Polarization, Temperature metrology, Cladding, Photonic crystal fibers, Reflectivity, Single mode fibers, Chemical fiber sensors, Scanning electron microscopy, Sensors
A reflective in-line modal interferometer based on a polarization-maintaining photonic crystal fiber (PM-PCF) with two exterior air holes is proposed for simultaneous measurement of chemical vapor and temperature. After fusion-splicing the PM-PCF with a standard single-mode fiber, we collapse all of air holes in the PM-PCF resulting in two types of interference patterns between the core and the cladding modes in the PM-PCF depending on two polarization states. Since two large air holes at the facet of the proposed modal interferometer are left open, a chemical vapor can be infiltrated into the voids. Different sensitivities corresponding to input polarization states are utilized for discrimination between chemical vapor and temperature sensitivities.
Micro-tapered long-period fiber gratings (MT-LPFGs) written in tapered fibers with different diameters of 125, 100, 75,
and 50 μm are fabricated and their transmission characteristics with variations in strain and temperature are investigated.
Since the variation of the refractive index with the applied strain change is inversely proportional to the cross section
area of the MT-LPFGs, the strain sensitivity of the MT-LPFGs is further improved. The temperature sensitivity is also
enhanced by reducing the diameter of the tapered fiber. The experimental results are very useful for discrimination of
strain and temperature sensitivities.
A new fabrication of micro-tapered long-period fiber grating based on a micro-tapering technique is proposed for
simultaneous measurement of strain and temperature. The transmission characteristics of the periodically tapered
long-period fiber gratings with variations in strain and temperature are measured.
We propose the temperature-insensitive bending sensor based on a hole-assisted single polarization fiber (SPF). Without
fiber grating structures, the SPF-based sensing probe can provide the sensing technique to measure the bending change.
If bending is applied to the SPF, two cutoff wavelengths are shifted to shorter wavelengths and the transmission power is
diminished because the structural deformation of the SPF induced by bending changes birefringence depending on
principle axes of the SPF. However, the applied temperature variation has no effect on the birefringence change severely
and two cutoff wavelengths are not shifted by changing temperature. Therefore, the proposed SPF-based sensing probe
with temperature insensitivity can measure the bending change effectively.
A simple scheme for simultaneous measurement of bending and temperature based on a dualwavelength Raman
fiber laser at a long distance of more than 50 km is investigated without additional light sources.
A simple and flexible sensing configuration for discrimination of temperature and strain is investigated by implementing
a PM-PCF-based Sagnac fiber loop mirror incorporating an EDF. The integration of an optical source and the sensing
probe can obviously simplify the overall system configuration without requiring any additional broadband light source.
Since the ASE of the EDF was reduced by the increase of temperature, the transmission peak power of the proposed
sensor decreased as the temperature increased. The temperature sensitivity of the transmission peak power was estimated
to be -0.04 dB/°C. When the strain was applied to the proposed sensing head, the transmission peak power was not
changed by the applied strain because the ASE power of the EDF was independent of the strain. However, the peak
wavelength shifted into the longer wavelength because the variation of the peak wavelength was directly proportional to
the applied strain. The strain sensitivity was measured to be 1.3 pm/με
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