Growth Kinetics Of Photoinduced Gratings And Paramagnetic Centers In High NA, Heavily Ge-Doped Silica Optical Fibers

Abstract

The achievement of UV photoinduced refractive index changes in germanosilicate fibers paved the way for the development of fiber Bragg grating (FBG) based optical devices (e.g., filters, fiber lasers, sensors). Although the mechanisms underlying the UV photosensitivity are not fully understood, it is now well established that the refractive index changes are associated with a partial bleaching of an absorption band around 5 eV, arising from the Germanium Oxygen Defect Center (GODC). Several mechanisms have been proposed to explain the photosensitivity in germanosilicate fibers: color centers [1], stress relief [2] and compaction [3]. Depending on the Ge concentration and the experimental conditions used to write the gratings, three types of photosensitivity, type 1, type IIa and type 2 have been identified in unloaded Ge-doped silica fibers. The most commonly observed, type 1, arises in standard fibers ([GeO2] < 20 mol%) under either pulsed or cw UV exposure in the low fluence regime. Type 1 photosensitivity is characterized by a monotonie increase of both the mean refractive index <Δneff> and the induced refractive index modulation Δnmod under UV exposure. Unlike type 1, the most relevant feature of type IIa photosensitivity ([GeO2] > 25 mol%, ϕc ≈ 2µm) is the complex behavior of <Δneff> and Δnmod as a function of exposure time [4]. Indeed, with increasing UV exposure time, a partial or total bleaching of the saturated type I grating is followed by the formation of a new grating spectrum. At the time of the second grating growth, the shift experienced by the Bragg grating wavelength is nulled or slightly negative, indicating a negative refractive index change. Moreover, a noticeable difference between type 1 and type IIa photosensitivities is their markedly different thermal behavior. Type 1 gratings begin to anneal at temperatures as low as 300 C and are no longer observed at 550 C while type IIa gratings experience no annealing at temperatures below 450-500 C; a rapid decay is then observed at annealing temperatures ~600-700 C. Recent experiments have shown that the formation dynamics of type 1/IIa grating spectra are strongly affected when the gratings are written in strained fibers [5]. Using the “Photoelastic densification” model and stress dependence kinetics, it has been demonstrated that, under the assumption of a decrease in densification rate, stretching the fiber will in turn decrease the type 1 index modulation [3], [6]. Nevertheless the origin of type IIa photosensitivity in high NA Ge-doped silica fibers still remains unresolved.