Correlation of ultraviolet-induced stress changes and negative index growth in type IIa germanosilicate waveguide gratings

Response of in-fiber Bragg gratings to hydrogen loading and subsequent heat treatment in H2 ambience

The evolution of in-fiber Bragg gratings' contrast and resonance wavelengths caused by molecular hydrogen loading and subsequent heat treatment in a hydrogen atmosphere at a pressure of 8 MPa is experimentally studied. Changes in the gratings' transmission spectra brought about by room-temperature hydrogen loading followed by isochronal annealing cycles at temperatures up to 700 degrees C and at invariable hydrogen pressure are recorded in situ. The gratings' thermal decay dynamics in hydrogen ambience and in air are found to be different and dependent on the gratings' type and on the type of glass in the fiber core. In light of the data obtained, the origin of type IIa gratings is anew under discussion.

The characteristic curve and site-selective laser excitation of local relaxation in glass

The so-called characteristic curve describing photosensitivity change is elaborated and shown to be a powerful tool for understanding and characterizing photosensitive growth both at a fundamental and practical level. It has been used successfully to diagnose when optimal hypersensitization has been achieved and the physical basis for this is explained. By way of example, previous results using 355 nm hypersensitization are re-examined. Evidence of single site-selective glass relaxation through direct laser excitation offers a new approach to accessing and studying induced relaxations.

Temperature-induced reversible changes in the spectral characteristics of fiber Bragg gratings

It is reported that reversible changes in the reflectivity of Bragg gratings can be induced by a change in the temperature of the grating (77 K < T < K). The changes have proved to be greater in highly doped Ge fibers than in standard fibers, whereas they could hardly be detected in hydrogenated fibers. The sign of the change for type I gratings was opposite that for type IIA gratings. The changes are likely due to a temperature-induced increase (or a decrease) in the amplitude of the refractive-index modulation. Possible mechanisms for these changes in modulation are discussed. Interestingly for the purpose of correcting data of isothermal accelerated aging experiments, a numerical relation that accounts for the temperature-induced changes in type I grating reflectivity is given.

Engineering large anisotropy in amorphous glass

By exploiting the strain-related rollover thresholds that occur between UV-induced positive and negative index regimes in germanosilica glass it is possible to engineer large anisotropy into the structure. This is qualitatively analyzed for a planar waveguide and has been confirmed experimentally. However, the technique described here is not confined to waveguide devices and can in principle be applied to any glass structure that is capable of undergoing strain-sensitive transitions.

Analysis of photoinduced stress distribution in fiber Bragg gratings

We discuss the analytical expressions that describe the displacement field resulting from the inscription of a Bragg grating in a Ge-doped optical fiber. The equations stem from a phenomenological approach and allow calculation of the induced stresses. Our model provides an efficient tool to analyze the effects of radiation-induced density variations on the properties of fiber Bragg gratings.