Hydrogen-enhanced UV photosensitivity of optical fibers: Mechanisms and reliability

Abstract

The use of optical fiber gratings1 has rapidly increased in recent years, due to the invention of the side-writing technique,2 and to the availability of fibers with enhanced photosensitivity. Enhanced photosensitivity has been achieved in various ways, most of which serve to increase the concentration of “native” defects in the fibers.3,4,5,6 More recently the UV photosensitivity of optical fibers has been greatly enhanced by “loading” the fibers with molecular H2 or D2 at high pressure.7 Subsequent exposure of the sensitized fiber to intense UV light at wavelengths less than about 248nm causes the physically dissolved H2 to react with cation dopants resulting in the formation of defects which cause large increases in the refractive index of the glass. This high pressure H2 sensitization technique has been used to great advantage in the UV writing of gratings in optical fibers, and has also been used to sensitize planar waveguides for the UV patterning of waveguide devices,8 and in bulk glasses for demonstration of holographic data storage.9 The UV induced index changes can readily exceed the initial core-to- cladding index difference in GeO2 doped fibers, permitting the UV writing of strong gratings in virtually any GeO2 doped optical fiber. The enhanced photosensitivity is sufficient to allow strong gratings to be written in several minutes using pulsed laser systems (~10-30Hz) at typical irradiances of several 100's mJ/cm2. For instance, in standard single mode fiber (3.5% GeO2) index changes of Δn=5x10-3 can be easily achieved and Δn's of 0.011 have been attained with longer exposures. Recent results have shown that H2 sensitization can also be used to advantage in P2O5 doped waveguides and fibers, either by using 193nm excimer irradiation10 or by using simultaneous heating and UV exposure at 248nm.11 One advantage of using H2 sensitization is that any existing fiber (that is either Ge or P doped) can be sensitized after it has been drawn and coated. Hydrogen molecules readily diffuse through polymer coatings and silica claddings at low temperatures, allowing the loading to be done at temperatures as low as 22-75°C, without degradation of polymer coatings. Figure 1 shows the transmission spectrum for a strong grating written in a standard (3.5% GeO2) single mode fiber that was H2 sensitized with 3.3% H2.