Raman scattering
Inelastic scattering of light by optical phonons or molecular vibrations, producing frequency-shifted output. Used for spectroscopy, Raman amplification, and distributed sensing.
Raman scattering occurs when an incident photon excites a molecular vibration (or optical phonon, in a solid), losing energy equal to the vibrational quantum. The scattered photon emerges at a lower frequency, the Stokes Raman line:
If the molecule was already in an excited vibrational state, the photon can gain energy and scatter at higher frequency, the anti-Stokes Raman line:
At thermal equilibrium the anti-Stokes/Stokes intensity ratio is the Boltzmann ratio of vibrational populations, , multiplied by for the stronger scattering at the higher frequency (the exponent is 3 when photons are counted rather than power).
Spontaneous Raman scattering is weak: cross-sections are typically to cm²/molecule, roughly three orders of magnitude weaker than Rayleigh scattering. Spontaneous Raman spectra reveal molecular vibrational fingerprints and are the basis of Raman spectroscopy for chemical identification.
Stimulated Raman scattering (SRS) becomes dominant at high optical intensity. The Raman gain coefficient is
for silica pumped near 1 µm, scaling inversely with pump wavelength to about m/W at 1550 nm.
SRS in silica fiber produces gain at a Stokes shift of 13.2 THz (about 100 nm at 1550 nm), with the gain peak nearly Lorentzian-shaped and 5 THz wide.
Applications.
| Application | Mechanism |
|---|---|
| Raman spectroscopy | Spontaneous Raman lines identify chemical species |
| Distributed temperature sensing (DTS) | Anti-Stokes / Stokes ratio gives temperature; spatial resolution from OTDR-style time gating |
| Distributed Raman amplification | Pump light at 1450 nm produces gain at 1550 nm in the transmission fiber itself |
| Cascaded Raman lasers | Multiple Stokes shifts cascade to reach long wavelengths (e.g., pumping fiber lasers at 1700–2000 nm) |
| Coherent anti-Stokes Raman (CARS) | Pump and Stokes beams together produce coherent anti-Stokes output for vibrational imaging |
Raman amplification in telecom. A 1450 nm pump pumped into a span of transmission fiber produces gain centered at 1550 nm. Effective noise figure can be negative (in dB) because gain is distributed along the fiber. The signal experiences amplification continuously rather than just at the span end, partially canceling propagation loss. Distributed Raman has been used to extend ultra-long-haul reach.
Brillouin vs Raman. Both are inelastic scattering, but Raman couples to optical phonons (10s of THz shift) while Brillouin couples to acoustic phonons (10s of GHz shift). Raman is broadband (THz-class gain bandwidth); Brillouin is narrow (sub-GHz). Different applications, different dispersion engineering required.
How a Raman spectrometer is built, filtered and calibrated is set out in Raman Spectroscopy Setup.