Progress toward the development of practical fiber Bragg grating instrumentation systems

Fiber Bragg gratings (FBGs) are simple intrinsic sensing elements which can be 'photo-imprinted' into fiber and represent one of the most exciting developments in the area of fiber optic sensing, probably since the inception of the all-fiber interferometer. These devices are based on the photosensitivity of optical fibers: Conventional tele- communications grade germanium-doped (Ge) optical fiber has been shown to exhibit a significant photosensitive response when illuminated with UV light in the region of 248 nm, a wavelength which corresponds to an absorption band, or 'color center,' in the glass associated with the Ge/SiO2 bonding. Absorption of the UV light in the glass breaks bonds creating changes in color centers which modify the absorption characteristics of the glass. This change in absorption results in a shift in the index of the glass at wavelengths removed from the absorption region through the Krammers-Kronig relationship. As the Ge dopant is usually confined only to the core (light guiding) region of the fiber, the effect is observed only in the core. The required UV light can be readily produced by various sources: KrF excimer lasers, dye lasers, frequency doubled Ar lasers and quadrupled Nd:YAG lasers. The optical power levels produced by these sources vary, and the most common source used is the KrF laser which can produce intense pulses at 10 to 50 Hz repetition rates. Changes in index are generally on the order of 10-3 or less, although recent work on fibers with enhanced photosensitive response (hydrogen- loaded fiber), index shifts of approximately 10-2 have been reported. The advent of the holographic side- exposure and phase-mask techniques for writing gratings has made the devices readily available for widespread usage in fiber optic systems. Although the primary application area for Bragg gratings appears to be in the fiber communications field, there has been strong interest in FBG-based sensing, and progress in this area has been rapid with many significant developments over the past 5 years.