Sunday, March 4, 2012


A fiber Bragg grating (FBG) is a fiber-optic device that strongly reflects a narrow band of wavelengths and transmits other light, like a thin-film mirror. A Bragg grating consists of thin layers of two dielectric materials, one with high refractive index and the other with a lower index, with each layer a quarter-wave thick at the wavelength to be reflected. Reflection from layer junctions a half-wavelength apart produces constructive interference at the selected wavelength. The more layers, the more reflective the structure becomes at the selected wavelength, and the narrower the reflected band. Other wavelengths are transmitted.

Multilayer thin-film mirrors fabricated on bulk optics are Bragg gratings, but their reflective behavior depends on the angle of incidence as well as the layer thicknesses. In a fiber Bragg grating, the layers are normal (perpendicular) to light propagating along the fiber axis, fixing the angle of incidence and limiting reflection to a single narrow band. Fiber gratings are fabricated by illuminating special fibers made of light-sensitive glass with light from an ultraviolet laser, which passes through a light-scattering mask to form a series of light and dark interference fringes along the length of the fiber. The ultraviolet light breaks chemical bonds illuminated by the light fringes, changing the refractive index of the glass to create the grating within the fiber. A new process allows fabrication of fiber Bragg gratings continuously on freshly drawn fiber.

The simplest fiber Bragg gratings have uniformly spaced layers along their length so they have high reflectivity in a narrow band, making them valuable as cavity mirrors in fiber lasers. Narrow-band fiber gratings also can be used in telecommunications systems, where they select one wavelength from a signal containing several optical channels and transmit the others, as shown in the figure. Another communications application is wavelength-selective time delays, with the grating chirped in spacing so different wavelengths are reflected at different points along the grating. Fiber Bragg gratings also can be used in sensing applications such as oil-well monitoring, where the peak reflected wavelength changes with temperature and strain.

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