Decay of ultraviolet-induced fiber Bragg gratings

Abstract

We report measurements of thermally induced decay of fiber Bragg gratings patterned by ultraviolet irradiation in germanium-doped silica fiber. The decay is well characterized by a ‘‘power-law’’ function of time with a small exponent, which is consistent with the rapid initial decay followed by a substantially decreasing rate of decay. We propose a decay mechanism in which carriers excited during writing are trapped in a broad distribution of trap states, and the rate of thermal depopulation is an activated function of the trap depth. This model is consistent with the observed power-law behavior. An important consequence of this mechanism is that the decay of the induced index change can be accelerated by increasing temperature. A decelerated-aging experiment verifies this prediction. This result demonstrates that it is possible to preanneal a device incorporating ultraviolet-induced refractive-index changes, wiping out the portion of the index change that would decay over the lifetime of the device, and keeping only the very stable portion of the index change.

Ultraviolet-light-induced processes in germanium-doped silica

“…The thermal rates and activation energies are expected to be comparable to the average of values determined for presumed broad energy distribution. 12 This is indeed the case. It is expected that the higher-order coefficients (⑀␣ 2,0 and ⑀␣ 3,0 ) are very small but still somewhat larger than (3) for pure silica, which is confirmed by the actual values.…”

“…Most of the results are analyzed using a model that assumes the index changes are due to a very broad spectrum of defects with activation energies from 0.5 eV to 3.5 eV. 12,13 However, to the best of my knowledge no articles discuss whether such a broad smooth spectrum is physically reasonable and only limited work has been done to relate the annealing results to the models for UV-induced processes. This paper is devoted to the development of a model which describes UV writing near 242 nm, the influence of hydrogen loading, and the thermal erasure in germanium-doped silica.…”

Continuous anneal method for characterizing the thermal stability of ultraviolet Bragg gratings

“…During the experiment we record transmission spectra of the grating with an optical spectrum analyzer ͑OSA͒. We use this to monitor the temperature through the wavelength shift of the Bragg resonance, which we have calibrated from room temperature to 1125 K. Further, the transmission spectra are used to calculate the grating strength L determined from the transmission depth using the formula 9 Lϭarctanh͑ͱR ͒, ͑7͒ where R is the fraction of the light that is reflected, L is the grating length, and the coupling constant. When annealing the grating it is important to avoid interference between core and cladding modes that have passed through the grating.…”

“…By using step isochronal anneal on four gratings and varying the time interval for each grating the long-term stability has been derived. 8 Erdogan et al 9 proposed an aging curve model based on an energy distribution of UV-induced defects. The gratings used were written in nonloaded fiber and the actual energy distribution was derived from a power law fit of the isothermal decay.…”

Tritium locked in silica using 248nm KrF laser irradiation

“…4 The permeated hydrogen molecule reacts with silica under deep ultraviolet ͑UV͒ laser irradiation to form a stable -OH bond. [5][6][7] Hydrogen occluded through this photochemical process increases the UV-induced refractive index change in the silica fiber by four orders of magnitudes. 8 Owing to the increased optical loss in the 1550 nm telecommunication window induced by the -OH bond, deuterium has also been used to red shift the -OH absorption tails beyond 1900 nm.…”