The nonlinear χ(3) response of a transparent optical medium to the optical intensity of light propagating through the medium is very fast, but not instantaneous. In particular, a delayed nonlinear response is caused by vibrations of the crystal (or glass) lattice. When these vibrations are associated with optical phonons, the effect is called Raman scattering, whereas acoustical phonons are associated with Brillouin scattering. When e.g. two laser beams with different wavelengths (and normally with the same polarization direction) propagate together through a Raman-active medium, the longer wavelength beam (called the Stokes wave) can experience optical amplification at the expense of the shorter wavelength beam. In addition, lattice vibrations are excited, leading to a temperature rise. The Raman gain for the longer wavelength beam can be exploited in Raman amplifiers and Raman lasers. That gain can be substantial if the Stokes shift corresponds to a frequency difference of several terahertz.
Figure 1 shows a numerical simulation for Raman scattering in an optical fiber, exploited for signal amplification of nanosecond pulses. It demonstrates that under suitable conditions the Raman conversion can be highly efficient.
Raman scattering can occur not only in solid materials, but also in liquids or gases. For example, molecular glasses have vibrational/rotational excitations, and the observed Stokes shifts are related to those.
The Raman effect occurs together with the Kerr effect, which results from the (nearly) instantaneous χ(3) response of the electrons.
Interaction of Photons and Phonons
In the Raman scattering process, one pump photon is converted into one lower-energy signal photon, and the difference of photon energies is carried away by a phonon (a quantum of the lattice vibrations).
In principle, it is also possible that an already existing phonon interacts with a pump photon to generate one higher-energy photon, belonging to an anti-Stokes wave at a shorter wavelength. That process, however, is usually weak, particularly at low temperatures. Note, however, that strong anti-Stokes light can also arise from four wave mixing if that process is phase-matched.
Raman scattering can be spontaneous or stimulated. Spontaneous Raman scattering occurs when there is a pump wave but no signal input wave (i.e., no signal input photons). It can be considered as a quantum effect – amplification of zero-point oscillations of the signal field.
Raman scattering is also called inelastic scattering, because the involved loss of photon energy is somewhat reminiscent of losses of kinetic energy in collisions of mechanical objects.
Some typical Raman-active media are
- certain molecular gases, e.g. hydrogen (H2), methane (CH4) and carbon dioxide (CO2), used in, e.g., high pressure cells for Raman shifters
- solid-state media such as glass fibers or certain optical crystals, e.g. barium nitride = Ba(NO3)2, various tungstates such as KGd(WO4)2 = KGW and KY(WO4)2 = KYW, and synthetic diamond
Cascaded Raman Scattering
When the intensity of the generated Stokes wave becomes sufficiently high, that wave may again act as the pump for a further Raman process. Particularly in some Raman lasers, it is possible to observe several Stokes orders (cascaded Raman lasers).
Raman Scattering with Ultrashort Pulses
Raman scattering can also occur within the broad optical spectrum of, e.g., an ultrashort optical pulse, effectively shifting the spectral envelope of the pulse towards longer wavelengths (Raman self-frequency shift, also called soliton self-frequency shift when soliton pulses are involved).
In optical fiber devices such as fiber amplifiers for intense pulses, Raman scattering can be detrimental: it can transfer much of the pulse energy into a wavelength range where laser amplification does not occur. Figure 2 shows a simulation for an example case. That effect can limit the peak power achievable with such devices.
More on Unwanted Raman Scattering
Even in continuous-wave high-power fiber lasers and amplifiers, Raman scattering can be a problem. There are, however, various kinds of solutions to such problems, including chirped-pulse amplification and the use of special fiber designs (see e.g. ) which suppress Raman scattering by attenuating the Raman-shifted wavelength component.
In bulk media, such as certain nonlinear crystal materials, unwanted stimulated Raman scattering can occur even via non-collinear phase matching, if the pump intensity is rather high and the beam width is large enough. This can occur e.g. in optical parametric generators operated with intense pump pulses.
Raman scattering can be exploited for Raman spectroscopy. In particular, it allows one to investigate the vibrational modes of solid materials and vibrational/rotational states of molecules.
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See also: Raman gain, Raman amplifiers, Raman lasers, delayed nonlinear response, Stokes shift, Brillouin scattering, Kerr effect, hyper Raman scattering, Raman spectroscopy, The Photonics Spotlight 2010-03-03
and other articles in the categories fiber optics and waveguides, nonlinear optics, physical foundations