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A mirror for exactly
one wavelength,
written inside a fiber.

A fiber Bragg grating (FBG) is a short section of optical fiber whose core carries a permanent, periodic variation in refractive index. Broadband light passes straight through, except for one narrow wavelength that is reflected back. Stretch or heat the fiber and that wavelength shifts. Measure the shift, and the fiber becomes a precision sensor.

One fiber, many sensors

Each grating along a fiber can be written at its own Bragg wavelength. The interrogator sees a comb of peaks and tracks them all simultaneously, so a single hair-thin fiber replaces a whole loom of copper strain gauge wiring. Channel spacing is chosen so that neighbouring peaks never collide at the extremes of their strain and temperature range.

Strain shifts the Bragg peak

λB = 2 · neff · Λ
CLADDING Ø125 µm CORE Ø9 µm GRATING PERIOD Λ ≈ 530 nm · LENGTH ~5 mm REFLECTED SPECTRUM 1550.000 nm 1548 nm 1552 nm
1550.000 nm
Δλ = +0.000 nm · 0 µε

Thousands of faint reflections, adding up in phase

The grating is written into the photosensitive core with UV laser light, typically through a phase mask. The result is a stack of thousands of index modulation planes, each reflecting a tiny fraction of the passing light. For most wavelengths these micro-reflections are out of phase and cancel. At one specific wavelength they interfere constructively, and the grating behaves as a highly selective mirror.

That wavelength is the Bragg wavelength, set entirely by two physical quantities: the effective refractive index of the core and the spacing of the planes.

λB = 2 · neff · Λ
λB  Bragg wavelength, the reflected wavelength (typically near 1550 nm)
neff  effective refractive index of the fiber core (≈ 1.447 for standard SMF)
Λ  grating period, the spacing between index planes (≈ 535 nm)
01 · Principle

Anything that changes Λ or neff moves the peak

Strain stretches the grating period and alters the index through the photo-elastic effect. Temperature changes the index and thermally expands the glass. Both show up as a clean, repeatable shift of the reflected peak that an interrogator reads out at kilohertz rates with picometer resolution.

εStrain sensitivity

Around 1550 nm, axial strain shifts the peak by roughly 1.2 pm per microstrain. A 1 pm resolution interrogator therefore resolves better than 1 µε.

~1.2 pm / µε

TTemperature sensitivity

Temperature moves the same peak at roughly 10 to 13 pm per kelvin, dominated by the thermo-optic effect rather than expansion of the glass itself.

~10–13 pm / K

ΔCompensation

Because both effects act on one peak, practical designs pair a strain-coupled FBG with a strain-isolated reference FBG to separate mechanical load from temperature.

ε = f(Δλ1, Δλ2)
02 · From mirror to sensor

One fiber, many sensors

Each grating along a fiber can be written at its own Bragg wavelength. The interrogator sees a comb of peaks and tracks them all simultaneously, so a single hair-thin fiber replaces a whole loom of copper strain gauge wiring. Channel spacing is chosen so that neighbouring peaks never collide at the extremes of their strain and temperature range.

FBG 1
1530
FBG 2
1533
FBG 3
1536
FBG 4
1539
FBG 5
1542
FBG 6
1545
…
FBG n
1560+
Wavelength division multiplexing: 10 to 40+ gratings per fiber is routine, and multi-channel interrogators scale this to hundreds of measurement points per instrument.

+Immune to EMI

The signal is light in glass. Electromagnetic interference, lightning, high-voltage environments and long cable runs do not corrupt the measurement.

+Embeddable and durable

At 125 to 250 µm diameter, fibers embed inside composites, adhesive bondlines and welded or machined structures, surviving millions of load cycles without drift-prone electronics at the measurement point.

03 · The killer feature

Proving that structures perform as designed

Simulation predicts behaviour; FBGs verify it in the field. That gap between "designed to spec" and "proven in service" is where fiber sensing has displaced conventional instrumentation.

AUTO

Vehicle and trailer load sensing

Instrumented components such as kingpins, axles and couplings measure real forces in service, enabling load monitoring, overload detection and smarter energy management.

CIVIL

Bridges, tunnels and wind turbines

Long-term structural health monitoring with dozens of points per fiber, no electronics on the structure, and decades of stable operation.

MARINE

Ships and offshore structures

Hull girder loads, shaft torque and mooring line tension measured in salt water and heavy EMI environments where copper instrumentation struggles.

AERO

Composites and aerospace

Fibers laminated inside carbon structures monitor cure, residual strain and in-flight loads from within the material itself.

ENERGY

Power and hydrogen systems

Intrinsically safe sensing in transformers, fuel cells and battery packs, where sparks and electrical pickup rule out conventional gauges.

MED

Medical instruments

Shape sensing and force feedback in catheters and surgical robots, exploiting the fiber's small diameter and MRI compatibility.

04 · Where FBGs earn their keep