Youngjoo Chung

High temperature fiber sensor with high sensitivity based on core diameter mismatch

We report a simple fiber sensor for measurement of high temperature with high sensitivity. The sensing head is a multimode-single mode-multimode (MM-SM-MM) fiber configuration formed by splicing a section of uncoated single mode fiber (SMF) with two short sections of multimode fibers (MMF) whose core is composed of pure silica. Because of the mode-field mismatch at the splicing points of the SMF with 2 sections of MMFs, as well as index matching between the core of the MMF and the cladding of the SMF, optical power from the lead-in fiber can be partly coupled to the cladding modes of the SMF through the MMF. The cladding modes of the SMF then re-coupled to the lead-out fiber, in the same fashion. Due to the effective index difference between the core and cladding modes, an interference pattern in the transmission spectrum of the proposed device was obtained. The interference pattern was found to shift to the longer wavelength region with respect to temperature variation. The temperature sensor can measure temperature stably up to more than 900 degrees C with sensitivity of 0.088 nm/ degrees C.

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.

Residual stress relaxation in the core of optical fiber by CO 2 laser irradiation

We observed residual stress relaxation by CO(2) laser irradiation in the cores of optical fibers by direct stress measurement. It was demonstrated that the mechanical stress was fully relaxed by CO(2) laser irradiation and that the remaining stress in the core was thermal stress that was due to a mismatch of the thermal expansion coefficients of the fiber core and cladding. The net core stresses after relaxation were 17, 68, and 203 MPa in Ge-B-codoped fibers drawn at 0.53, 1.38, and 3.48 N, respectively. Changes in the refractive indices of the cores as a result of residual stress relaxation were also estimated.

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.

Temperature-Insensitive Torsion Sensor With Enhanced Sensitivity by Use of a Highly Birefringent Photonic Crystal Fiber

We report on enhanced torsion sensitivity by using a highly birefringent photonic crystal fiber (HB-PCF)-based Sagnac interferometer. In order to increase the torsion sensitivity, we introduced an anisotropic microstructure into the cross section of an HB-PCF by enlarging the size of air holes of one row. This can result in a high birefringence of the order of 10-3 and low sensitivities to bending and temperature. The torsion sensitivity was measured to be high with ~0.06 nm/°.

Simultaneous Temperature and Strain Measurement Using Two Types of High-Birefringence Fibers in Sagnac Loop Mirror

A fiber sensor configuration capable of simultaneous measurement of temperature and strain is newly presented and investigated. Two different types of high-birefringence fibers (HBFs) spliced together are inserted in a Sagnac loop mirror to act as the sensing head for temperature and strain discrimination. Unlike an individual HBF, the two wavelength dips used as detected parameters in the transmission spectrum of the Sagnac loop mirror may have opposite sensitivity responses to temperature and/or strain variation, which depends on the relative angle between the primary axes of the two sections of HBFs and rotation angle of the polarization controller in the Sagnac loop mirror.

Discrimination of temperature and strain with a single FBG based on the birefringence effect

We will demonstrate a new technique to discriminate the temperature and strain effects using a single fiber Bragg grating (FBG). The birefringence is typically induced during FBG inscription, and it is manifested as polarization-dependent loss (PDL), and it is defined as the maximum change in the transmitted power for polarizations. Two independent measurements of the resonance wavelength shift and the changes of PDL can discriminate those effects.

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.