定义：激光光束的电场振动方向。
大多数情况下激光器的出射光都是偏振的。通常是线偏振的，即电场在某一特定的与激光光束传播方向垂直的方向上振荡
。有的激光器（例如，光纤激光器）产生的不是线偏振光，而是其它的稳定的偏振态，可以采用合适的波片组合在一起将其转化为线偏振光。
如果是宽带辐射，且偏振态与波长有关，则不能采用以上方法。

在一些特殊情形，可以产生径向偏振光束，即光束截面中的偏振方向是径向的。
通常，径向偏振辐射是先偏振光经过一些光学元件得到的，也可以直接由激光器得到。
这种方法的好处在于可以避免去极化损耗，可以应用于固态体激光器中。

在许多应用中需要采用偏振的激光辐射。例如：

1. 非线性频率转换，其中只在一个偏振方向上可以满足相位匹配
2. 需要两激光光束进行偏振耦合（参阅偏振合束）
3. 在偏振相关装置中处理激光光束，例如，干涉仪，半导体光放大器和光调制器

也有一些激光器（许多光纤激光器）出射光是不是偏振的。这并不是值激光器输出的是非偏振光，在任意时刻两偏振分量的功率都是相等的，二者的振幅之间完全无关。
只是偏振态非常不稳定，例如，由于温度涨落，或者在不同方向间变化。为了得到完全非偏振光，则需要采用一些消除偏振的光学装置。

图一：激光辐射不同的偏振态，几个脉冲从左往右传播。
线偏振光的偏振度由偏振光消光比（PER）来表征，定义为两个偏振方向功率的比值，单位为分贝，可以测量光通过偏振片后随方向而变化的功率来得到。偏振片的消光比必须要大于激光光束的消光比。

偏振或非偏振辐射机制
不同的机制都对激光器的辐射线偏振光有贡献：

• 激光器增益与偏振有关。激光晶体为非线性各向同性时就是这种情况（例如，Nd:YVO4或Nd:YLF），还有一些半导体光放大器也是。
• 谐振腔损耗也可能与偏振有关，例如，谐振腔中包含一个布儒斯特盘或者只有一个稍微倾斜的光学元件。

在两偏振方向上增益或损耗的很小差别都能得到稳定的线偏振，前提是在激光器谐振腔中，偏振模式之间不会发生显著的耦合。
另外，激光器输出的偏振态会受随机双折射的影响，例如光纤中（如果不是保偏或者单模光纤）和激光晶体或玻璃中的热效应（参阅去极化损耗）。
如果激光器增益是各向同性的，很小的双折射都会极大改变偏振态，并且光束截面上的偏振态也会发生显著变化。

Definition: direction of the electric field oscillation of a laser beam

In many, although not all, cases the output of a laser is polarized. This normally means a linear polarization state, where the electric field oscillates in a certain (stable) direction perpendicular to the propagation direction of the laser beam. There are cases (occurring e.g. with fiber lasers) where a different, e.g. elliptical but also more or less stable polarization state is generated, which can be transformed into a linearly polarized beam, e.g. by using an appropriate combination of waveplates. This is not possible, however, for broadband emission with wavelength-dependent polarization state.

Figure 1: Different polarization states of laser emission, illustrated for a few-cycle pulse propagating from left to right.

Under special circumstances, radially polarized beams can be generated, where the polarization direction within the beam profile is radially oriented. Usually, a radially polarized beam is generated from a linearly polarized beam with some optical element, but it is also possible to obtain radially polarized emission directly from a laser. The advantage of this approach, applied in a solid-state bulk laser, is that depolarization loss may be avoided [4].

Polarized laser emission is important for a range of applications. Some examples are:

• nonlinear frequency conversion, where phase matching in a nonlinear crystal is normally obtained only for one polarization direction
• cases where two laser beams need to be polarization-coupled (→ polarization beam combining)
• processing of laser beams in polarization-dependent devices, such as interferometers, semiconductor optical amplifiers, and optical modulators

However, some lasers (e.g. many fiber lasers) do not generate a polarized output. This does not necessarily mean that the laser output is truly unpolarized, containing equal optical powers in two polarization components at any time, without any correlation of the corresponding amplitudes. The polarization state may simply be unstable, e.g. due to temperature drifts, or randomly switch between different directions. For generating a truly unpolarized laser beam, some kind of polarization-scrambling optical device is usually required.

The degree of linear polarization is often quantified with the polarization extinction ratio (PER), defined as the ratio of optical powers in the two polarization directions, often specified in decibels, and measured by recording the orientation-dependent power transmission of a polarizer. Of course, the extinction ratio of the polarizer itself must be higher than that of the laser beam.

Mechanisms for Polarized or Unpolarized Emission

Different mechanisms can be responsible for linearly polarized emission of a laser:

• The laser gain may be polarization dependent. This is the case in some anisotropic laser crystals (e.g. Nd:YVO₄ or Nd:YLF), but also in some semiconductor optical amplifiers.
• The resonator losses may be polarization dependent, e.g. when the resonator contains a Brewster plate or even only a slightly tilted optical component.

Note that a very small gain or loss difference for the two polarization directions can be sufficient for obtaining a stable linear polarization, provided that there is no significant coupling of polarization modes within the laser resonator.

On the other hand, the polarization state of the laser output can be disturbed e.g. by random (and temperature-dependent) birefringence, such as occurs e.g. in optical fibers (if they are not polarization-maintaining or single-polarization fibers) and also in laser crystals or glasses as a result of thermal effects (→ depolarization loss). If the laser gain is isotropic, small drifts of the birefringence may lead to large changes of the polarization state, and also a significant variation in the polarization state across the beam profile.

Bibliography