Fiber Bragg Grating

A microscopic barcode burned into a glass thread that hears how hard you squeeze it.

A fiber Bragg grating (FBG) is a 1–20 mm long region inside a single-mode fiber's germanium-doped silica core where the refractive index has been periodically modulated, creating a wavelength-selective mirror at the Bragg wavelength λ_B = 2 n_eff Λ. The grating reflects a narrow band (~0.1–1 nm wide) and transmits all other wavelengths essentially unattenuated. Because λ_B shifts ~10 pm/°C with temperature and ~1.2 pm/µε with strain, FBGs are deployed as embedded distributed sensors in aircraft wings, wind-turbine blades, bridges, oil pipelines, and inside surgical catheters — and as low-loss wavelength-selective mirrors in fiber lasers and dispersion-compensation modules.

FBGs are typically inscribed by exposing photosensitive Ge-doped fiber to a periodic UV interference pattern from a 248 nm KrF or 193 nm ArF excimer laser, usually via a phase mask whose ±1 diffraction orders interfere on the fiber core. The UV-induced photochemistry breaks Ge–Si bonds and forms color centers, raising the refractive index by Δn ≈ 10⁻⁵–10⁻³ in the bright fringes. A 5 mm grating with Δn ≈ 10⁻⁴ achieves >90 % peak reflectivity. Reading is by an interrogator — a swept-wavelength laser or spectrometer with sub-picometer resolution. The Bragg shift versus measurand is Δλ_B/λ_B = (1−p_e)ε + (α+ξ)ΔT, where p_e ≈ 0.22 is the effective photoelastic coefficient, α ≈ 0.55 ppm/°C the thermal-expansion coefficient, and ξ ≈ 6.7 ppm/°C the thermo-optic coefficient. Hundreds of FBGs at distinct λ_B values can share a single fiber, addressed by wavelength-division multiplexing.

In plain terms

Imagine a glass straw with tiny rings etched inside at perfectly even intervals — like a microscopic xylophone. Blow one note in (one color of light) and only that note bounces back; everything else passes through. Now squeeze the straw: the rings get closer, so a slightly different note bounces back. By listening to which note returns, you can tell exactly how hard the squeeze was, or how hot the straw is, without touching it. That's a fiber Bragg grating.

Also called: FBG, in-fiber Bragg reflector, fiber grating sensor.

Why it matters

Where it lives

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A short history

Photosensitivity in Ge-doped silica was discovered in 1978 by Hill, Fujii, Johnson, and Kawasaki at the Communications Research Centre Canada — they observed that a standing-wave Ar-ion laser pattern wrote a permanent grating in fiber. Phase-mask inscription (Hill et al. 1993) made FBGs reproducibly manufacturable. Erdogan's 1997 J. Lightwave Tech. paper standardized the analytical framework. By the 2000s FBGs had become standard components in fiber lasers, telecom dispersion compensation, and structural-health monitoring.

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