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Raman Scattering

Definition: a nonlinear scattering process involving optical phonons

More general term: inelastic scattering

German: Raman-Streuung

Categories: fiber optics and waveguidesfiber optics and waveguides, nonlinear opticsnonlinear optics, physical foundationsphysical foundations


Cite the article using its DOI: https://doi.org/10.61835/l9l

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The nonlinear <$\chi^{(3)}$> 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.

optical powers along the fiber
Figure 1: Evolution of optical powers in a parabolic-index multimode fiber, simulated with the numerical beam propagation feature of the software RP Fiber Power as part of a case study. A signal wave is strongly amplified, while the pump wave is strongly depleted. Multiple modes are involved in the conversion process.

Certain Raman crystals are most often used as Raman gain media, although various glasses can be considered (particularly in the form of fibers). Raman scattering can occur not only in solid materials, but also in liquids or gases. For example, molecular gases 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 <$\chi^{(3)}$> 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.

Raman-active Media

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.

evolution of the pulse spectrum in the fiber
Figure 2: Evolution of the pulse spectrum in a fiber amplifier. Near the right end, a significant part of the power is shifted into longer-wavelength components by stimulated Raman scattering. The simulation has been made with the software RP Fiber Power.
case study Raman scattering in fiber amplifier

Case Studies

Case Study: Raman Scattering in a Fiber Amplifier

We investigate the effects of stimulated Raman scattering in an ytterbium-doped fiber amplifier for ultrashort pulses, considering three very different input pulse duration regimes. Surprisingly, the effect of Raman scattering always gets substantial only on the last meter, although the input peak powers vary by two orders of magnitude.

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. [9]) 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 Spectroscopy

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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[1]V. V. Raman and K. S. Krishnan, “A new type of secondary radiation”, Nature 121, 501 (1928); https://doi.org/10.1038/121501c0
[2]G. Eckhardt et al., “Stimulated emission of Stokes and anti-Stokes Raman lines from diamond, calcite and alpha-sulfur single crystals”, Appl. Phys. Lett. 3, 137 (1963); https://doi.org/10.1063/1.1753903 (first report of stimulated Raman scattering in insulating crystals)
[3]N. Bloembergen, “The stimulated Raman effect”, Am. J. Phys. 35 (11), 989 (1967); https://doi.org/10.1119/1.1973774
[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); https://doi.org/10.1364/AO.11.002489
[5]K. J. Blow and D. Wood, “Theoretical description of transient stimulated Raman scattering in optical fibers”, IEEE J. Quantum Electron. 25 (12), 2665 (1989); https://doi.org/10.1109/3.40655
[6]R. H. Stolen et al., “Raman response function of silica-core fibers”, J. Opt. Soc. Am. B 6 (6), 1159 (1989); https://doi.org/10.1364/JOSAB.6.001159
[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); https://doi.org/10.1364/JOSAB.19.002886
[8]J. Santhanam and G. P. Agrawal, “Raman-induced spectral shifts in optical fibers: general theory based on the moment method”, Opt. Commun. 222, 413 (2003); https://doi.org/10.1016/S0030-4018(03)01561-X
[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); https://doi.org/10.1364/OL.31.002550
[10]Q. Lin and G. P. Agrawal, “Raman response function for silica fibers”, Opt. Lett. 31 (21), 3086 (2006); https://doi.org/10.1364/OL.31.003086
[11]D. J. Spence and R. P. Mildren, “Mode locking using stimulated Raman scattering”, Opt. Express 15 (13), 8170 (2007); https://doi.org/10.1364/OE.15.008170
[12]X. Ma et al., “Propagation-length independent SRS threshold in chirally-coupled-core fibers”, Opt. Express 19 (23), 22575 (2011); https://doi.org/10.1364/OE.19.022575
[13]R. Paschotta, "Dependence on Raman conversion on the optical bandwidth" (2018)
[14]G. P. Agrawal, Nonlinear Fiber Optics, 4th edn., Academic Press, New York (2007)

(Suggest additional literature!)

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