Min-Seok Yoon

Simultaneous Measurement of Strain and Temperature by Using a Micro-Tapered Fiber Grating

We investigate a periodically micro-tapered fiber grating for simultaneous measurement of strain and temperature. The fundamental core mode can be coupled to the cladding mode by tapering a single-mode fiber periodically, which results to the resonant wavelength. The coupling strength is directly proportional to the number of micro-tapered regions in the cladding. The transmission characteristics of the periodically micro-tapered fiber grating are measured as strain or temperature is changed. The resonant wavelength is shifted to shorter wavelengths and the transmission is reduced as the applied strain increases. However, as the applied temperature increases, the resonant wavelength is shifted to longer wavelengths and the transmission is also diminished. Consequently, it is possible to effectively discriminate concurrent sensitivities between strain and temperature by measuring the resonant wavelength shift and the transmission variation of the periodically micro-tapered fiber grating.

Influence of the waist diameters on transmission characteristics and strain sensitivity of microtapered long-period fiber gratings

Transmission characteristics of microtapered long-period fiber gratings (MTLPGs) and their strain and temperature sensitivities with variations in the waist diameters are investigated theoretically and experimentally. Transmission characteristics of MTLPGs strongly depend on the waist diameter of the tapered optical fiber (TOF) because of the modification of the effective index difference between the core and the cladding modes. Based on the photoelastic effect, the resonant wavelengths of MTLPGs with variations in strain shift to shorter wavelengths. The strain sensitivity of the MTLPG with a waist diameter of 25 μm is improved by a factor of 20 compared with that of a 125 μm long-period fiber grating. The temperature sensitivities of MTLPGs are also enhanced by reducing the waist diameter of the TOF.

Highly Sensitive Current Sensor Based on an Optical Microfiber Loop Resonator Incorporating Low Index Polymer Overlay

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 2 μm is wrapped around the nichrome wire with low index polymer overlay to realize the MLR as the highly sensitive current sensor. Low index polymer overlay can successfully reduce round-trip fractional loss resulting in the improvement of the Q-factor (Q) and the finesse (F) of the MLR around the nichrome wire. The use of the microfiber with a waist diameter of 2 μm and low index polymer with high thermal property can effectively improve the current sensitivity of the proposed MLR to be 437.9 pm/A2.

Relative Humidity Sensor Based on an Optical Microfiber Knot Resonator With a Polyvinyl Alcohol Overlay

A highly sensitive relative humidity (RH) sensor based on a microfiber knot resonator (MKR) with a polyvinyl alcohol (PVA) overlay is investigated. To realize a miniature fiberoptic RH sensing probe, we designed a MKR with a PVA coating. After making a tie using the microfiber with a diameter of 2 μm, the MKR with a loop diameter of 70 μm is realized and coated with PVA. The MKR with the PVA overlay effectively responds to variations in the concentration of RH because of a strong evanescent field from the microfiber. For precise measurement of the concentration of RH, the optical spectra of the MKR-based RH sensor are converted to spatial frequency spectra by using the fast Fourier transform algorithm. The absorption of humidity in the proposed MKR-based sensing probe effectively changes the spatial frequency. The RH sensitivity of the proposed MKR-based RH sensing probe with a higher order mode dramatically improved by a factor of four compared to that of lower order mode.

Development of micro-tapered long-period fiber gratings written in tapered fibers with different diameters for enhancement of strain sensitivity

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.

In-line interferometer based on intermodal coupling of a multicore fiber

An in-line interferometer based on the intermodal coupling of a multicore fiber (MCF) is proposed and experimentally demonstrated. The in-line interferometer is fabricated by adiabatically tapering the MCF. The intermodal coupling of the in-line interferometer is strongly affected by the waist diameter of the MCF, which changes the evanescent field and the pitch size. The effect of the waist diameters of the MCF on the intermodal coupling in the in-line interferometer is theoretically and experimentally investigated. The transmission oscillations of the multiple core modes resulting from the intermodal coupling and interference substantially become stronger as the waist diameter decreases. The extinction ratio and the oscillation periodicity of the transmissions oscillations are changed by the waist diameter.

Influence of the Thickness and Doping Concentration in p- and n-Type Poly-Si Layers on the Efficiency of a Solar Cell Based on a Carbon Fiber

We investigated the effects of the thickness and doping concentration in p- and n-type poly-Si layers on the performance of a solar cell based on a carbon fiber in order to improve the energy conversion efficiency of the cell. The short-circuit current density and open-circuit voltage of the carbon fiber-based solar cell were significantly influenced by the thickness and doping concentration in the p- and n-type poly-Si layers. The solar cell efficiency was successfully enhanced to ~10.5%.

Effect of a Waist Diameter of a Microtapered Polarization-Maintaining Fiber on Temperature and Ambient Index Sensitivities

The effect of a waist diameter of a microtapered polarization-maintaining fiber (PMF) on the temperature and the ambient index sensitivities is investigated theoretically and experimentally. Stress-induced and geometrical birefringences strongly depend on the waist diameter of the microtapered PMF, which plays a key role in temperature and ambient index sensitivities of the Sagnac interferometer. By reducing the waist diameter to ~5 μm, we successfully control the ambient index and the temperature sensitivities of the Sagnac interferometer to be -98.7 nm/RIU and -0.16 nm/°C, respectively, which are ~23 times higher and 1.7 times lower than those with a waist diameter of 25 μm.

Fabrication of in-line modal couplers based on multicore fibers and their applications to fiber optic sensors

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.

Enhancement of temperature sensitivity of a Mach-Zehnder interferometer based on a polymer-overlaid microfiber

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.