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

Abstract

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.

Ultraviolet-induced birefringence in hydrogen-loaded optical fiber

“…In the Introduction we noted that it has recently been proposed that the characteristic curve used to measure index change, either bulk or periodic, as a function of fluence or dose can provide useful insight to this and related problems. 7 For example, in some cases hydrogen-loaded optical fiber, after a small initial nonlinear component, produces a very distinct linear response against fluence, 41 which is indicative of a purely photorefractive change in index since the response time is local and fast and proportional to the formation of a hydrogen species such as OH. On the other hand, densification involves structural relaxation in the glass and the generation of competing stresses.…”

“…28 The catalytic role of hydrogen, 3-5 now widely accepted, [37][38][39] is critical to this differentiation and has permitted a time scale separation of the two processes since one is driven by the other. 7 Practical hypersensitization, itself a catalytic process dependent on hydrogen that extends the basic index change in nonhydrogenated optical fibers and is underpinned qualitatively by a two-step photosensitive mechanism, 3-5 requires this time scale separation with the additional process at the core/ cladding interface so that it can in fact be avoided. Numerous benefits have stemmed from this but, from the time being, full hydrogen loading remains the primary method of photosensitization of optical fibers in commercial grating companies and we shall consider in this paper only fully hydrogenloaded optical fibers.…”

“…[3][4][5] There is evidence, on the other hand, that OH correlates with an initial negative index change in fully loaded phosphosilicate optical fibers which is subsequently dominated by a positive index change thought to arise from a photorefractive effect. 6 A recent approach to analyzing the characteristic curves of irradiated materials 7 indicates that the linear growth in positive index change, after the initial negative contribution to dilation is swamped, with OH formation as a function of fluence is indicative of a photorefractive process. Any other process involves local or collective glass structural relaxations that are characterized by exponential relations.…”

Rapid characterization of the ultraviolet induced fiber Bragg grating complex coupling coefficient as a function of irradiance and exposure time

“…Previous photosensitivity measurements have indicated a power law growth [23]. However it has also been suggested the growth exhibits a sigmoidal behavior and exhibits power law characteristics only over a small region of the growth curve [24,25]. It is beyond the scope of this paper to investigate the photosensitivity mechanisms contributing to the measured growth curve.…”

Induced nanoscale changes with low temperature annealing inside composite optical fibres probed by strain-insensitive long period gratings

“…Nanoscale glass milling using lasers was recently proposed as a new approach to engineering photonic components, highlighted by the use of UV inscription with thermal annealing [1][2][3]. The archetype demonstration is fibre Bragg grating (FBG) "regeneration" [28].…”