Glossary entry

Brillouin scattering

Inelastic scattering of light from acoustic phonons or material density waves, producing a small frequency shift (10s of GHz). Used in distributed strain sensing and a power limit in narrow-linewidth fiber transmission.

Optics fundamentals Fiber & telecomUpdated October 2026

Brillouin scattering arises from light interacting with acoustic waves (phonons) in the medium. The incident photon scatters off a density modulation traveling at the acoustic velocity, exchanging frequency with the phonon:

ν_{Stokes} = ν_{pump} - ν_{B}, ν_{aS} = ν_{pump} + ν_{B} .

The Brillouin frequency shift is:

ν_{B} = \frac{2 n v_{a}}{λ_{0}},

where $n$ is refractive index, $v_{a}$ is acoustic velocity, and $λ_{0}$ is the free-space optical wavelength.

For silica fiber at 1550 nm:

Parameter	Value
Acoustic velocity	$\sim$ 5,960 m/s
Brillouin shift $ν_{B}$	$\sim$ 11.0 GHz
Brillouin linewidth $Δ ν_{B}$	10 – 30 MHz
Brillouin gain coefficient $g_{B}$	$\sim 5 \times 10^{- 11}$ m/W

The shift is much smaller than Raman ( $\sim$ 13 THz) and the linewidth is also much narrower (10 MHz vs 5 THz). This makes Brillouin scattering sensitive to small effects that Raman cannot resolve.

Stimulated Brillouin scattering (SBS) threshold. Above a threshold pump power, SBS produces a strong backward-propagating Stokes wave that limits forward signal transmission:

P_{th} \approx 21 \frac{A_{eff}}{g_{B} L_{eff}} \cdot \frac{Δ ν_{pump} + Δ ν_{B}}{Δ ν_{B}},

For a long span of SMF ( $A_{eff}$ = 80 µm², $L_{eff}$ near 20 km) and a pump of 10 MHz linewidth, $P_{th}$ is about 2 to 5 mW, the upper end allowing for the random polarization of ordinary fiber. This is the main reason that narrow-linewidth fiber transmission systems (analog CATV, fiber sensing) use phase-modulated, dithered, or otherwise spectrally-broadened sources to suppress SBS by reducing the effective gain over the narrow Brillouin linewidth.

Applications.

Application	Mechanism
Brillouin optical time-domain reflectometry (BOTDR)	Distributed strain and temperature sensing; spatial resolution from OTDR time gating, sensitivity from Brillouin shift dependence on $n$ and $v_{a}$
Brillouin fiber lasers	Narrow-linewidth laser source pumped by a narrow CW laser; sub-Hz linewidth has been demonstrated
Frequency offset	The shift, about 11 GHz in silica fiber, sets the spacing between pump and Stokes waves, used for microwave generation; it moves by about 1 MHz/K with temperature and with strain, so it is not an absolute reference
Slow light	Brillouin gain near resonance produces strong dispersion; group velocity can be reduced by orders of magnitude over narrow bands
Hypersonic spectroscopy	Brillouin spectra reveal elastic and viscoelastic properties of materials

The Brillouin shift depends on temperature (via $n (T)$ , $v_{a} (T)$ ) and strain (via mechanical effects on $v_{a}$ ), making distributed Brillouin sensing the technique of choice for long-haul structural health monitoring of fiber-instrumented bridges, pipelines, and tunnels.

Pump linewidth dependence. A narrow-linewidth pump (1 MHz) produces strong SBS at the standard threshold. Broadening the pump linewidth to exceed $Δ ν_{B}$ (10–30 MHz) raises the SBS threshold roughly linearly with $Δ ν_{pump} / Δ ν_{B}$ .