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Definition: nonlinear scattering process involving optical phonons
The nonlinear 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 non-instantaneous response is caused by vibrations of the crystal (or glass) lattice. As far as 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 somewhat different wavelengths (and normally with the same polarization direction) propagate together through a Raman-active medium, the longer-wavelength beam 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.
When the intensity of the generated wave (called the 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).
Apart from the mentioned stimulated Raman scattering effect, there is also spontaneous Raman scattering, caused by quantum noise effects. This may be interpreted as Raman amplification of vacuum noise.
Raman scattering can also occur within the broad spectrum e.g. of 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).
Some typical Raman-active media are
- certain gases, e.g. hydrogen (H2), methane (CH4), or carbon dioxide (CO2), used e.g. in high pressure cells for Raman shifters
- solid-state media such as glass fibers or certain crystals, e.g. barium nitride = Ba(NO3)2 or various tungstates such as KGd(WO4)2 = KGW and KY(WO4)2 = KYW.
In solid-state media, the Raman effect occurs together with the Kerr effect, which results from the (nearly) instantaneous response of the electrons.
Figure 1: Evolution of the optical spectrum of a 5-ps pulse with 18 kW peak power in an optical fiber, shown with a logarithmic color scale for the power spectral density. The spectrum is first broadened mainly due to self-phase modulation, but after ∼300 mm of fiber, some Raman-shifted light is observed, which is then further amplified.
In optical fiber devices such as fiber amplifiers for intense pulses, Raman scattering can be quite detrimental: it can transfer much of the pulse energy into a wavelength range where laser amplification does not occur. This effect can limit the achievable peak power from such devices. 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. [9]) which suppress Raman scattering by attenuating the Raman-shifted wavelength component.
Bibliography
| [1] | V. V. Raman and K. S. Krishnan, "A new type of secondary radiation", Nature 121, 501-2 (1928) |
| [2] | G. Eckhardt et al., "Stimulated emission of Stokes and anti-Stokes Raman lines from diamond, calcite and alpha-sulfur single crystals" (first report of stimulated Raman scattering in an insulating crystal), Appl. Phys. Lett. 3, 137 (1963) |
| [3] | N. Bloembergen, "The stimulated Raman effect", Am. J. Phys. 35 (11), 989 (1967) |
| [4] | R. G. Smith, "Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and Brillouin scattering", Appl. Opt. 11 (11), 2489 (1972) |
| [5] | K. J. Blow and D. Wood, "Theoretical description of transient stimulated Raman scattering in optical fibers", IEEE J. Quantum Electron. 25, 2665 (1989) |
| [6] | R. H. Stolen et al., "Raman response function of silica-core fibers", J. Opt. Soc. Am. B 6 (6), 1159 (1989) |
| [7] | D. Hollenbeck and C. D. Cantrell, "Multiple-vibrational-mode model for fiber-optic Raman gain spectrum and response function", J. Opt. Soc. Am. B 19 (12), 2886 (2002) |
| [8] | J. Santhanam and G. P. Agrawal, "Raman-induced spectral shifts in optical fibers: general theory based on the moment method", Opt. Comm. 222, 413 (2003) |
| [9] | J. M. Fini et al., "Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier", Opt. Lett. 31 (17), 2550 (2006) |
| [10] | D. J. Spence and R. P. Mildren, "Mode locking using stimulated Raman scattering", Opt. Express 15 (13), 8170 (2007) |
| [11] | G. P. Agrawal, "Nonlinear fiber optics", 3rd edition, Academic Press |
See also: Raman amplifiers, Raman lasers, nonlinearities, Brillouin scattering, Kerr effect, hyper Raman scattering
Categories: fibers and other waveguides, nonlinear optics, physical foundations
