What is meant by Laser Coherence?

What is meant by Laser Coherence?

Jan 26, 2021

Laser is a monochromatic coherent light with excellent directivity. Coherence is one of the unique characteristics of laser.


Laser coherence can be described in terms of temporal coherence and spatial coherence, which respectively represent the correlation between certain characteristics of light wave fields at different positions in space, and the correlation between light wave fields at different times in space. Temporal coherence and spatial coherence are important in producing the interference which is used to produce holograms.


Spatial coherence

The spatial coherence is closely related to the directivity of laser. The directivity of laser is generally defined by the beam divergence. When the beam divergence is less than a certain degree, the beam will have a certain spatial coherence. For example, ordinary light sources such as flashlights have very large divergence, which obviously do not have spatial coherence. If a plane wave is completely spatially coherent light, then its divergence is 0. For laser modules, the lasers produced are not ideal light sources, which will be analyzed with transverse mode and longitudinal mode. The transverse mode represents the transverse optical field distribution (represented by TEMmn), and the longitudinal mode represents the axial optical field distribution (ie, resonance frequency). The spatial coherence and directivity of the laser are directly related to the transverse mode. When there is only one TEM00 mode (fundamental mode), then the laser coherence will be good; if there are multiple transverse modes (different transverse modes are incoherent), then the degree of coherence will decrease. This shows that the spatial coherence of single-mode lasers is the best.


Temporal coherence

Temporal coherence is directly related to the monochromaticity of the light source. The longer the light source atom emits light at a time, the more fringes observed through double slit interference, we can regard that the temporal coherence is longer, and the light source atom luminescence time is called the coherence time, and the wave train length in the coherence time is called the coherence length. The longer the coherence length, the clearer the interference fringes, and the better the coherence. The longer the coherence length, the clearer the interference fringes, and the better the coherence.



ELITE provides High Stability Bios Series Laser with TEM00 mode, beam divergence (full-angle) <1.0mrad, and Industrial Laser Diode Module with 0.1mrad to 1.0mrad divergence. The samples are designed and improved by the engineering team with abundant experience, that is why we have the ability to customize laser modules and provide the best solutions.

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