Re-writable fiber optic Bragg gratings for multiple parameter sensing

Abstract

While fiber optic Bragg gratings sensors have emerged as a viable commercial product they possess the inherent inflexibility that once written, the nonstressed gratin spacing is fixed. We have begun to explore an alternative method of fabricating Bragg sensors - one where you have a single fiber optic Bragg grating sensor, but where you are able to write your own grating prior to parameter measuring with the broadband light source. In this alternative, a photosensitive fiber core material exists at the FBGS sensing 'site' along the fiber. THe diffraction grating is written via a deliberate intensity variation in the light which is injected into the fiber. The slight difference in refractive index between the photogrey section of the core and the 'regular' fiber causes an internal Fabry-Perot resonator cavity to be established. The intensity-modulated high power laser bit stream reflects back and forth within this cavity establishing a standing wave pattern. This pattern may be varied, and hence the grating spacing is variable, by changing the high power laser's bit pattern. This standing wave pattern effectively illuminates the photogrey section nonuniformly with the high power portions of the standing wave pattern causing more darkening - thereby in essence creating the Bragg diffraction grating.