Un-Chul Paek

Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry–Perot interferometer

A miniature Fabry-Perot (FP) interferometric fiber-optic sensor suitable for high-temperature sensing is proposed and demonstrated. The sensor head consists of two FP cavities formed by fusion splicing a short hollow-core fiber and a piece of single-mode fiber at a photonic crystal fiber in series. The reflection spectra of an implemented sensor are measured at several temperatures and analyzed in the spatial frequency domain. The experiment shows that the thermal-optic effect of the cavity material is much more appreciable than its thermal expansion. The temperature measurements up to 1000 degrees C with a step of 50 degrees C confirm that it could be applicable as a high-temperature sensor.

Multiwavelength Raman fiber-ring laser based on tunable cascaded long-period fiber gratings

Multiwavelength Raman fiber lasers using intracavity tunable cascaded long-period fiber gratings (LPFGs) are discussed. The application of the cascaded LPFGs as a multichannel fiber filter for the multiple-wavelength generation is proposed and experimentally demonstrated. The characteristics of multiwavelength fiber-ring lasers can be controlled by changing the physical parameters of cascaded LPFGs such as the separation distance between the gratings, grating length, and number of gratings. The multiwavelength Raman fiber-ring laser with nine wavelength-division-multiplexing channels with 100-GHz spacing and 19 channels with 50-GHz spacing has been achieved by varying the physical parameters of cascaded LPFGs.

Simultaneous measurement of temperature and strain using dual long-period fiber gratings with controlled temperature and strain sensitivities

Unambiguous simultaneous measurement of strain and temperature based on dual long-period fiber gratings by controlling their thermal and strain sensitivities is proposed and experimentally demonstrated. The difference in the wavelength peak shift and the separation with the variation of strain and temperature allows discrimination between the strain and temperature effects, respectively.

Photonic crystal fiber coupler

Fiber couplers made with photonic crystal fibers (PCF) are reported. Two types of PCF were fabricated by means of stacking a group of silica tubes around a silica rod and drawing them. The fiber couplers were made by use of the fused biconical tapered method. With a fiber that had five hexagonally stacked layers of air holes, a 33/67 coupling ratio was obtained, and with a one-layer four-hole fiber, a 48/52 coupling ratio was obtained. The fabrication processes and the characteristics of the PCFs and the PCF couplers are presented.

Cross-talk free and ultra-compact fiber optic sensor for simultaneous measurement of temperature and refractive index.

We propose and demonstrate a cross-talk free simultaneous measurement system for temperature and external refractive index (ERI) implemented by dual-cavity Fabry-Perot (FP) fiber interferometer. The sensing probe consists of two cascaded FP cavities formed with a short piece of multimode fiber (MMF) and a micro-air-gap made of hollow core fiber (HOF). The fabricated sensor head was ultra-compact; the total length of the sensing part was less than 600 mum. Since the reflection spectrum of the composite FP structures is given by the superposition of each cavity spectrum, the spectrum measured in the wavelength domain was analyzed in the Fourier or spatial frequency domain. The experimental results showed that temperature could be determined independently from the spatial frequency shift without being affected by the ERI, while the ERI could be also measured solely by monitoring the intensity variation in the spatial frequency spectrum. The ERI and the temperature sensitivities were approximately 16 dB/RIU for the 1.33-1.45 index range, and 8.9 nm/ degrees C at low temperature and 14.6 nm/ degrees C at high temperature, respectively. In addition, it is also demonstrated that the proposed dual-cavity FP sensor has potential for compensating any power fluctuation that might happen in the input light source.

Fabrication of helical long-period fiber gratings by use of a CO2 laser.

We present a method of helical long-period fiber grating (H-LPFG) fabrication by use of a CO2 laser for use as an optical torque sensor. A conventional optical fiber grating has periodic vertical index changes along its fiber axis, but a H-LPFG has a screw-type index modulation. The helical index modulation is obtained with the asymmetric index change caused by a single-side laser beam exposure. The H-LPFG shows peak shifts with codirectional or contradirectional torsion to the helix. Also, the polarization-dependent loss is measured to be relatively small compared with that of a conventional long-period fiber grating.

Induction of the refractive index change in B-doped optical fibers through relaxation of the mechanical stress

The structural model for the residual stress has often been used to account for the photosensitivity in Ge-doped optical fibers, and we used this approach in this paper to analyze the eAect of the mechanical stress on the refractive index change in B-doped optical fibers. In particular, we will discuss the mechanism of refractive index modulation in long period fiber gratings induced by CO2 laser irradiation using B-doped depressed clad and matched clad fibers for the first time as far as the authors are aware. The analysis presented in this work will strengthen the structural model for description of the fiber grating formation. ” 2000 Published by Elsevier Science B.V.

Measurement of refractive-index variation with temperature by use of long-period fiber gratings

The thermo-optic coefficient of the core material of a fiber is analyzed by use of a pair of long-period fiber gratings. First the effective index difference between the core and the cladding modes is measured from the peaks of the interference fringe generated by the grating pair. The order of the cladding mode is decided by the cutoff wavelength and the numerical aperture of the fiber. The material index of the fiber core is obtained in terms of wavelength. At each wavelength the index is chosen to minimize the difference between the measured and the calculated spectra of the grating pair. Finally the thermo-optic coefficient of the fiber core is calculated by repetition of the measurement at different temperatures. With a germanosilicate-core fiber and a boron codoped germanosilicate-core fiber, the thermo-optic coefficients were 1.1x10(-5)/( degrees )C and 0.75x10(-5)/( degrees )C, respectively.

Tunable photonic crystal fiber coupler based on a side-polishing technique

A tunable photonic crystal fiber (PCF) coupler, which couples part of the optical power in one PCF with that in another PCF, has been made by side polishing. We fabricated the PCF coupler by mating two side-polished PCFs. We achieved evanescent field coupling between the core modes of the two PCFs by using side polishing to bring the cores close to each other. By adjusting the mating angle between the two side-polished PCFs we obtained as much as 90% tunability in the coupling ratio. The spectrum of the coupling ratio was almost flat, with small ripples, over a 400-nm wavelength range.

Multi-wavelength lasing oscillations in an erbium-doped fiber laser using few-mode fiber Bragg grating

We propose a simple erbium-doped fiber laser configuration for obtaining multi-wavelength oscillation at room temperature, in which a few-mode fiber Bragg grating was used as the wavelength-selective component. An amplitude variation of 1.6 dB over 120 second period was obtained for three-wavelength oscillation at room temperature, which demonstrates stability of the output power. This multi-wavelength laser can be switched between dual- and triple-wavelength operations by properly adjusting polarization controller in the cavity. This multi-wavelength laser has the advantage of simple configuration, high stability, low cost and stable operation at room temperature.