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What is Raman scattering?

The nonlinear response of a transparent optical medium to the optical intensity of light traveling through the medium is very fast, but not instantaneous. In particular, the non-instantaneous response is caused by vibrations in the lattice (or glass). When these vibrations are associated with optical phonons, the effect is called Raman scattering, while acoustic phonons are associated with Brillouin scattering. For example, when two laser beams with different wavelengths (usually with the same direction of polarization) propagate together through a Raman-active medium, the longer wavelength beam (called a Stokes wave)) can be optically amplified at the expense of the shorter wavelength beam. In addition, the lattice vibrations are excited, leading to an increase in temperature. The Raman gain of longer wavelength beams can be utilized in Raman amplifiers and Raman lasers. The gain may be substantial if the Stokes frequency shift corresponds to a frequency difference of a few terahertz.

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 correlated with those.

During Raman scattering, a pump photon is converted into a signal photon of lower energy, and the difference in photon energy is carried away by phonons (quanta of lattice vibrations). In principle, an already existing phonon may also interact with the pump photon to produce a higher energy photon that belongs to a shorter wavelength anti-Stokes wave. However, the process is usually weak, especially at low temperatures. Note, however, that four-wave mixing also produces strong anti-Stokes light if the process is phase-matched.

When the intensity of the resulting Stokes wave becomes high enough, the wave may again act as a pump for further Raman processes. Especially in some Raman lasers, multiple Stokes orders can be observed (cascade Raman lasers).

Raman scattering is also known as inelastic scattering because the loss of photon energy involved is somehow reminiscent of the loss of kinetic energy in the collision of mechanical objects.

In addition to the above excited Raman scattering effects, which can be described in terms of classical physics, there is also spontaneous Raman scattering caused by quantum effects.

Raman scattering may also occur within the broad spectrum of, for example, an ultrashort optical pulse, thus effectively shifting the spectral envelope of the pulse to longer wavelengths (Raman self-frequency shift, also known as soliton self-frequency shift).

Some typical Raman-active media are

Certain molecular gases, such as hydrogen (H 2 ), methane (CH 4 ), and carbon dioxide (CO 2 ), used in the high-voltage cell of a Raman shifter
Solid media such as glass fibers or certain crystals such as barium nitride = Ba(NO 3)2, various tungstates such as KGd(WO 4)2 = KGW and KY(WO 4)2 = KYW, and synthetic diamonds

The Raman effect occurs simultaneously with the Kerr effect, which is due to the (almost) instantaneous response of electrons.

Figure 1: Evolution of the pulse spectrum in a fiber-optic amplifier. Near the right end, excited Raman scattering shifts a large fraction of the power into longer wavelength components. As part of the case study, simulations were performed using the software RP Fiber Power.
Figure 2: Optical power evolution in a parabolic refractive index multimode fiber, simulated as part of a case study by the digital beam propagation feature of the software RP Fiber Power. The signal wave is strongly amplified while the pump wave is severely depleted. The conversion process involves multiple modes.

In fiber optic devices such as strongly pulsed fiber amplifiers, Raman scattering can be detrimental: it diverts most of the pulse energy into wavelength ranges where no laser amplification occurs. This effect may limit the peak power achievable in such devices. Even in continuous-wave high-power fiber lasers and amplifiers, Raman scattering can be a problem. However, there are several solutions to this problem, including chi pulse amplification and the use of special fiber designs) that suppress Raman scattering by attenuating the wavelength component of the Raman shift.

In bulk media such as some nonlinear crystalline materials, if the pump intensity is quite high and the beam width is sufficiently large, undesired excited Raman scattering can occur even by noncollinear phase matching. This may occur, for example, in an optical parameter generator operating with a strong pump pulse.

Raman scattering is also used in Raman spectroscopy. In particular, it allows one to study the vibrational modes of solid materials and the vibrational/rotational states of molecules.



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