The dynamics of grating growth in both H2-loaded and non-H2-loaded GeB-doped fiber is investigated with cw 244 nm light and pulsed 266 nm light. The growth function is found to be the same in both cases, and both the growth rate and the saturation levels are proportional to the writing power density. Such a dynamics can be explained by a two- photon defect creation mechanism counterbalanced by a one- photon destruction mechanism, the saturation level being determined not by depletion of available defects, but by an equilibrium between the two mechanisms. We discuss the implications of such a model.
We report on the fabrication f comb filters which operate on the principle of linear polarization rotation through the twisting of the birefringent axes of photosensitive elliptical-core fibers by UV radiation. The polarization coupling spectrum of these filters consist of a series of narrow resonances which are roughly equally spaced in frequency and which extend over the entire single-mode frequency range of the fiber. Some applications of these structures are also discussed.
Phosphosilicate and germanosilicate optical fibers doped with Tb3+ are darkened rapidly by exposure to 488 nm light, resonant with the 7F6 yields 5D4 transition of Tb3+. The induced absorption decreases monotonically throughout the visible, becoming negligible at around 750 nm, and can be bleached out by exposure to 514 nm light. In phosphosilicate fibers the bleaching goes virtually to completion, but in germanosilicate fibers it is hindered by the gradual formation of Ge-based color centers. The darkening and bleaching processes occur over comparable timescales, and require three and two photons respectively. For the optical intensities used, the bleaching of the induced loss is photonic, not thermal in nature, but thermal processes may become important at higher intensities. Other argon laser lines are able to induce darkening to a certain extent, dependent on the overlap with the 7F6 yields 5D4 band, and the equilibrium absorption state of the fiber depends on the competition between the darkening and bleaching processes. The absorption state, once established, is stable at room temperature over a timescale of months. The darkening is believed to occur by photo-ionization of Tb3+, with trapping of the released electron at sites in the network; the absorption may be due either to these electron traps or to charge transfer bands between Tb3+ and Tb4+. Refractive index changes associated with the absorption changes may be useful for side-writing fiber gratings with blue-green light, and the reversibility of the absorption changes may have application in optical memories.
The luminescence of a Ge-doped fiber preform excited by 240 nm light was studied. Luminescence at 290 nm, 400 nm, and 650 nm was observed, and lifetimes were measured, as well as the spatial profiles. For the first two components, the spatial profile is different from the index profile and the absorption profile, while the 650 nm luminescence follows the index profile. We also measured different lifetimes at different positions across the core.
The index change induced by two-photon absorption of green light in Ge-doped optical fiber is partly anisotropic. We review the experimental facts that form the basis for a physical model of bleachable oriented defects. We also describe how a density matrix formalism, based on a simili three-level system, can be used to determine the tensorial properties of the photoinduced index change.
We describe a simple technique for aligning optical fibers prior to fusion splicing. The technique relies on the fact that well-cleaved fiber ends have extremely sharp edges. By making the narrow pencil of light emerging from one fiber scan laterally over the entrance face of a second fiber, and by monitoring the light diffracted past its sharp edges, we can locate precisely the geometric center of the output fiber. With this technique, we have aligned fiber cores with a mean lateral offset of 0.81 μm, the major part of this offset caused by the eccentricity of the core relative to the cladding's circular perimeter.
This panel discussion asks the question: is a high conversion efficiency possible with optical fibers. This is an important question to ask since it will determine if this phenomenon will have practical applications or not.
The different phenomena related to photoinduced self-organization in optical fiber are reviewed, from photoinduced absorption and refraction changes, to index grating formation and harmonic generation. Some pertinent questions regarding the origin and the physics of these effects are raised.
We propose devices for wavelength-division muliplexing based on coupling between different modes in optical fiber. A mode-selective directional coupler (MSDC) couples the LP01 mode of one fiber to the LP02 mode of a second fiber with a bandwidth. -length products of about 4 nmcm an improvement of a factor of 10 over LP01 + LP01 coupling. Refractive index gratings written with UV light can also couple light from the LP01 to the LP02 mode either in a counterpropagating way (Bragg grating) or a codirectional way. Combined with the MSDC they can make efficient channel-drop filters.