Fiber Bragg grating (FBG)

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

$\lambda_B = 2\, n_{eff}\, \Lambda$

Everything else passes through. Reflectivity and bandwidth follow from the grating's strength ($\Delta n$, 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, and that sensitivity is a product, not a defect. At 1550 nm in silica: ~10 pm/°C (thermo-optic dominated) and ~1.2 pm/µε (strain). Interrogate the reflected wavelength and the fiber becomes a sensor: multiplexable dozens-deep along one strand by giving each grating its own $\lambda_B$, immune to EMI, embeddable in composites, deployable down boreholes. Temperature–strain cross-sensitivity is the standing complication, handled with reference gratings or dual-parameter schemes.

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 (the 980 nm pump behind every EDFA carries one), and as cavity mirrors defining fiber lasers. Chirped, with $\Lambda$ 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.