U. C. Paek

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.

Residual stresses in a doubly clad fiber with depressed inner cladding (DIC)

Thermal and mechanical stresses developed in concentric three-layered optical fiber-core, and inner and outer cladding, have been thoroughly studied for various concentrations of dopants and geometric structures. In order to examine the parametric results of thermal stresses in preforms, the stresses were measured with a polariscope. The results agreed well with the theoretical calculations. The thermal stresses were calculated for three temperature ranges in which the glass in each layer has a different thermal expansion coefficient. The mechanical stresses were studied considering the normal stress in the molten neck down region and its development with time. In order to include the time dependence of the stress below softening point, Maxwells one dimensional viscoelasticity was applied. In a parametric study, the analyzes were carried out based on the fiber parameters such as relative index difference, ratio of clad to core, and depressed relative index difference. With an increase of core index above the silica, the thermal stresses in core increased linearly, but the depressed inner clad does not affect the stresses in core. From the parametric studies and modeling it was found that when the depressed inner cladding (DIC) layer has a large cross-section or high dopant concentration, the mechanical stress in core change from compression to tension.

Novel mode converter based on hollow optical fiber for Gigabit LAN communication

A novel mode converter based on a hollow optical fiber is proposed to reduce the differential modal delay penalty in optical transmission over multimode fibers (MMFs). The device adiabatically converts a fundamental mode in a single-mode fiber to a ring-shaped mode in order to excite selectively a set of higher order modes with a similar group velocity in MMF, maintaining center-launching configuration. The mode converter is composed of serially concatenated concentric segments of a single-mode, hollow, and multimode optical fiber. For 2.5-Gb/s transmission over 500-m-long MMF using a laser diode transmitter, the proposed mode converter shows improvement in bit-error-rate performance at both wavelengths of 1.31 and 1.55 /spl mu/m.

Melt coating of tin on silica optical fiber

The coating of a silica optical fiber with molten tin metal is analyzed rigorously by developing a numerical method based on two- and three-dimensional (2- and 3-D) conduction models. In the analysis, the axial temperature distribution in both the fiber and coating is obtained in terms of the depth the melt and the fiber draw speed. A coating applicator has been designed for tin (Sn) coating and fibers with a coating thickness from 5 to 20 /spl mu/m were fabricated with draw speeds ranging from 50-150 cm/s. The numerical model was found to be in agreement with the experimentally obtained results for various coating conditions and fiber drawing parameters. It is shown that for tin, a low-melting-point metal, the freezing takes place primarily within the coating applicator. As a result of the presence of this subcoating, additional coating occurs as the fiber leaves the applicator. Since this tin coating is hermetic, a mean failure strength of 8 GPa is measured for these tin-coated fibers by the two-point bending technique rather than the 5.5 GPa normally found for polymer-coated fibers.

Compensation of Raman-induced crosstalk using a lumped germanosilicate fiber Raman amplifier in the 1.571-1.591-μm region

A new method to equalize power imbalance caused by Raman-induced crosstalk among optical channels is proposed using a lumped germanosilicate fiber Raman amplifier. Evolution of optical channels through the Raman amplifier was simulated using Raman frequency modeling, which theoretically predicted simultaneous amplification and power equalization. Experimentally, a gain band with negative slope in the range of 1.571-1.591 /spl mu/m was achieved in a lumped Raman amplifier pumped by a broad-band laser diode centered at 1.467 /spl mu/m. We demonstrated compensation of the Raman-induced crosstalk of 5 dB accumulated along 330 km of conventional single-mode fiber.

Simultaneous amplification and channel equalization using Raman amplifier for 30 channels in 1.3-/spl mu/m band

A new technique is proposed to simultaneously amplify signals and equalize power unbalance caused by Raman-induced crosstalk among optical channels in the 1.3-/spl mu/m band. The crosstalk is induced in GeO/sub 2/-doped silica fibers due to the Raman-gain coefficient with a positive slope below the peak-frequency shift of 440 cm/sup -1/. Highly doped GeO/sub 2/ silica fiber, however, shows a negative slope across the band between 440 cm/sup -1/ and 490 cm/sup -1/. We propose expansion of the Raman-gain band with a negative slope over the bandwidth of 150 cm/sup -1/ using multiple pumps. Utilizing the negative gain slope of the proposed band, simultaneous amplification and power equalization was theoretically demonstrated for 30 channels in the 1.310-1.345 /spl mu/m region without external filters.

New defect design in index guiding holey fiber for uniform birefringence and negative flat dispersion over a wide spectral range

A novel silica index guiding holey fiber (IGHF) design is proposed utilizing a new defect structure that is composed of an elliptic high index ring structure and an elliptic air-hole at the center with triangular lattice structure. The proposed IGHF showed unique modal properties such as uniform and high birefringence over a wide spectral range and single polarization single mode (SPSM) guidance along with a flat negative chromatic dispersion. Optical waveguide properties were numerically analyzed using the plane wave expansion method in terms of mode intensity distribution, modal birefringence, chromatic dispersion for the new defect structural parameters.

Reduction of birefringence and polarization-dependent loss of long-period fiber gratings fabricated with a KrF excimer laser

We discuss the effect of UV laser exposure time, repetition rate, and heating by a heating coil on the resonance peak depth and polarization-dependent loss (PDL) of long-period fiber gratings (LPFGs). The LPFGs were fabricated with a KrF excimer laser and an amplitude mask. We observed an initial increase of the resonance peak depth and PDL as the UV exposure time was increased, which eventually decreased in response to over-coupling. With the total UV fluence kept constant, the peak depth continued to increase as the repetition rate was increased beyond 10 Hz, whereas the maximum PDL decreased when the repetition rate was higher than 17 Hz. This is believed to be a thermal effect caused by the rapid delivery of UV laser pulses. We observed a similar reduction of the maximum PDL from 1.35 to 0.25 dB when the fiber was heated by an adjacent heating coil.

Characterization of elliptic core fiber acousto-optic tunable filters operated in the single mode and the multi-mode range

Summary form only given. We have fabricated an elliptical-core fiber that can be applied to broadband acousto-optic devices operated in both 1.5 and 1.3 /spl mu/m communication windows. We believe the interaction of acoustic wave with the optical modes to have a strong potential in fiber characterization.

Inherent enhancement of gain flatness and achievement of broad gain bandwidth in erbium-doped silica fiber amplifiers

We report new methods to inherently increase the flatness and bandwidth of erbium-doped silica fiber amplifiers from three perspectives: fiber design, pump-signal WDM coupler optimization, and amplifier structure. First, to achieve inherent control of the gain spectrum, a new type of composite fiber structure with an Er-doped core and a Sm-doped cladding ring is proposed and experimentally demonstrated. Interaction of the optical field with the Sm-doped cladding to produce evanescent wave filtering is modeled, which provides an in-line control of gain fluctuation in the erbium-doped flier amplifier (EDFA) C band, 1530-1560 nm. Second, the effect of the spectral characteristics of WDM couplers over the L band of an EDFA is explored. A fused taper fiber coupler for a 1480-nm pump is optimized for signals in the wavelength range of 1570-1610 nm by measuring the small-signal gain, gain tilt, and noise figure in an L-band EDFA. Finally, a new all-fiber structure for a wide-band EDFA, where the L and C bands were coupled serially, is demonstrated with optimized pump-signal couplers. Further optimization of the new composite fiber structure and the transient effects in the serially coupled EDFAs are also discussed.