Fiber Bragg grating (FBG)

A fiber Bragg grating is the Bragg condition made permanent inside a fiber core: a periodic index modulation of period $\mathrm{\Lambda }$, written by UV interference through a phase mask (or point-by-point with femtosecond pulses), reflecting light at

${\lambda }_B=2n_{eff}\mathrm{\Lambda }$

Everything else passes through. Reflectivity and bandwidth follow from the grating's strength ($\mathrm{\Delta }n$, typically ${10}^{-5}$–${10}^{-3}$) and length: gratings of millimeters to centimeters yield reflection bands from tens of picometers to nanometers, with reflectivities from a few percent to >99.9%. Apodization (tapering the modulation envelope) suppresses the sidelobes a hard-edged grating would produce.

The Bragg wavelength moves with anything that stretches the grating or changes its index. At 1550 nm in silica the shift is about 10 to 13 pm/°C, mostly from the thermo-optic effect, and about 1.2 pm per microstrain. Tracking the reflected wavelength makes the grating a sensor: dozens of gratings, each with its own ${\lambda }_B$, can share one fiber, and they are immune to electromagnetic interference, can be embedded in composites and work down boreholes. Temperature and strain shift the wavelength in the same way, and separating them, with reference gratings or dual-parameter schemes, is the standing complication.

In component roles: FBGs serve as WDM add/drop filters (paired with circulators, since the useful signal comes back reflected), as stabilization gratings that lock pump-laser wavelengths (most 980 nm EDFA pump modules carry one), and as cavity mirrors defining fiber lasers. Chirped, with $\mathrm{\Lambda }$ varying along the length, they serve as compact dispersion compensators where each wavelength reflects from a different depth and picks up a designed delay.

Handling notes: standard UV-written gratings anneal and fade above ~300 °C (regenerated or femtosecond gratings extend the range); the writing process slightly weakens the fiber unless recoated properly; and specifying an FBG means quoting ${\lambda }_B$ at a stated temperature, bandwidth, reflectivity, and sidelobe suppression together.

The Bragg wavelength calculator converts between period and Bragg wavelength, and FBG-based demultiplexing is compared with AWGs and thin-film filters in WDM mux/demux technologies.

References: R. Kashyap, Fiber Bragg Gratings, 2nd ed. (Academic Press, 2010). T. Erdogan, "Fiber grating spectra," Journal of Lightwave Technology 15, 1277 (1997). A. D. Kersey et al., "Fiber grating sensors," Journal of Lightwave Technology 15, 1442 (1997). A. Othonos and K. Kalli, Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing (Artech House, 1999).