Birefringence | previous | next | feedback |
Definition: the phenomenon of double refraction, or the polarization dependence of the refractive index in a medium
In the literature, the term birefringence occurs with two different meanings. In classical optics, it is normally considered to have the same meaning as double refraction, as explained below. In nonlinear optics and laser technology, however, birefringence is usually meant to be the property of some non-isotropic transparent material that the refractive index depends on the polarization direction (direction of the electric field). The latter property makes this material (then called birefringent) capable of exhibiting double refraction, when being hit by an unpolarized light beam.
Consequences of a Polarization-dependent Refractive Index
The polarization dependence of the refractive index can have a variety of effects:
- When a beam is refracted at the surface of a birefringent crystal, the refraction angle depends on the polarization direction. An unpolarized light beam can then be split into two polarized beams, when hitting surfaces of the material with non-normal incidence (→ double refraction). When some object, which is illuminated with unpolarized light, is viewed through a birefringent crystal (e.g. made of calcite), two images occur which are slightly displaced.
- If a linearly polarized laser beam propagates through a birefringent medium, with the polarization direction not being aligned with one of the birefringent axes, there are two polarization components with different wavenumbers. Therefore, the polarization state changes during propagation due to the change in relative phase of the two linearly polarized components. This effect can be applied, for example, in birefringent tuners, because it is wavelength-dependent (even if the difference in refractive indices is not wavelength-dependent). It can also be power-dependent (→ nonlinear polarization rotation) through self- and cross-phase modulation, e.g. in an optical fiber, and this effect is sometimes used for passive mode locking of fiber lasers.
- Similarly, the polarization state of a laser beam in a laser crystal with thermally induced birefringence is distorted. The kind of distortion depends on the position, since the birefringent axis has a varying (e.g. always radial) orientation. This effect (combined with a polarizing optical element in the laser resonator) is the origin of depolarization loss.
- The birefringence of nonlinear crystal materials allows for birefringent phase matching of nonlinear interactions.
Examples of Birefringence
In laser technology and nonlinear optics, the phenomenon of birefringence occurs mainly in the context of non-isotropic crystals, such as some laser crystals (e.g. vanadate or tungstate crystals), and nonlinear crystals as used for nonlinear frequency conversion.
Even in a naturally isotropic medium, birefringence can be induced e.g. by inhomogeneous mechanical stress. This can be observed e.g. by placing a piece of acrylic between two crossed polarizers: when stress is applied to the acrylic, one observes colored patterns resulting from the wavelength-dependent effect of stress-induced birefringence. Similar effects occur in bent optical fibers, and also due to thermal effects in laser crystals, which can lead to depolarization loss.
Straight optical fibers usually exhibit only a small degree of random birefringence, which can however scramble the polarization state of guided light over some propagation distance, e.g. 1 m. There are polarization-maintaining fibers, where a strong artificial birefringence can be used for suppressing such effects.
See also: birefringent tuners, polarization-maintaining fibers, polarization of laser emission, spatial walk-off, Spotlight article 2007-05-26
