定义：涉及光学声子的非线性散射过程。

一个透明的光学介质，通过介质传播的光的光强度的非线性响应很快，但不是瞬时的。特别是，非瞬时响应所造成的振动的晶体（或玻璃）晶格。当这些振动与光学声子，效果被称为拉曼散射，而声学声子与Brillouin散射。当两个具有不同波长（通常具有相同偏振方向）的激光束通过拉曼活性介质一起传播时，较长波长的光束（称为斯托克斯波）可以以较短波长的光束为代价进行光放大.。此外，晶格振动被激发，导致温度上升。拉曼放大器和拉曼激光器可以利用较长的波长的拉曼增益。如果斯托克斯位移对应于几个太赫兹的频率差，增益可以是可观的。

在拉曼散射过程中，一个泵浦光子被转换成一个较低的能量信号光子，光子能量的差被声子（晶格振动的量子）带走.。原则上，也有可能是一个已经存在的声子与泵光子相互作用产生一个更高的能量光子，属于一个反斯托克斯波在较短的波长。然而，这个过程通常是弱的，特别是在低温下。然而，注意到，强反斯托克斯光也可以出现四波混频，如果该过程是相位匹配。

当生成的斯托克斯波的强度变得足够高，该波可能再次作为进一步拉曼过程的泵。特别是在一些拉曼激光器，它是可以观察到几个斯托克斯订单（级联拉曼激光器）。

除了上述受激拉曼散射效应，可以用经典物理描述，也有自发的拉曼散射，量子效应引起的。

拉曼散射也可以发生在宽的光谱中，例如，超短光脉冲，有效地移动的频谱包络的脉冲朝向较长的波长（拉曼自频移，也被称为孤子自频移）。
一些典型的拉曼活性介质:

• 某些气体，如氢气（H2），甲烷（CH4）和二氧化碳（CO2），用于，例如，为拉曼频移器的高压细胞
• 固态介质如玻璃纤维或某些晶体，如氮化钡=巴（NO3）2，各种tungstatessuch KGd（WO4）2 = KGW和KY（WO4）2 = KYW，及合成金刚石

拉曼效应发生在一起的克尔效应，这导致从（几乎）的瞬时响应的电子。

图1：光纤放大器中脉冲频谱的演化。在右端附近，一个重要的部分的功率被转移到较长的波长分量的受激拉曼散射。仿真已经取得了与软件RP光纤功率作为案例研究的一部分。

在诸如强脉冲光纤放大器之类的光纤器件中，拉曼散射是有害的：它可以将许多脉冲能量传递到波长范围内，不产生激光放大.。这种效果可以限制峰值功率实现这样的设备。即使在连续波高功率光纤激光器和放大器，拉曼散射可能是一个问题。有，然而，各种类问题的解决方案，包括啁啾脉冲放大和特殊设计的光纤的使用抑制散射衰减的拉曼频移波长成分的拉曼光谱。

散装的媒体，如某些非线性晶体材料，多余的受激拉曼散射甚至可以通过非共线相位匹配时，如果泵的强度较高，波束宽度足够大。这可能会发生例如在光学参数发生器与强烈的泵脉冲操作。

拉曼散射也用于光谱学。特别是，它允许一个调查材料的振动模式。

Definition: a nonlinear scattering process involving optical phonons

More general terms: scattering

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.

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.

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.

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.

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 as part of a case study.

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.

Bibliography