Chirped diffraction grating optical switching

While fiber optic Bragg grating sensors have emerged as a viable commercial product and principle component within numerous fiber optic components, they possess the inherent inflexibility that once written, the nonstressed grating spacing is fixed. An alternative method of fabricating Bragg gratings where a photosensitive fiber core material (similar to that used in "photogrey sunglasses") exists at the grating "site" along the fiber has been examined. As opposed to a conventional grating writing method using lateral illumination of the fiber core, in this instance 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. By using an intensity-modulated high power laser, the illuminating modulation pattern reflects back and forth within this cavity establishing a standing wave pattern. Different patterns may be launched into the fiber resulting in a grating spacing which is variable. 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. Removal of the illumination source results in a grating that fades away yielding a re-writable component. An examination of this type of re-writable component will be reported along with its suitability for 100- and 50-Ghz DWDM applications.