Lasers are coherent, which means there is a fixed relationship between the phase values of the electric field at different times or locations. This is because laser light is produced by stimulated emission, unlike most other types of light sources. Coherence gradually weakens throughout propagation, and the coherence length of a laser defines the distance over which its temporal coherence maintains a certain quality.
Polarization:
Polarization defines the direction of the electric field of a light wave, which is always perpendicular to the direction of propagation. In most cases, laser light is linearly polarized, meaning that the emitted electric field always points in the same direction. Unpolarized light produces electric fields that point in many different directions. The degree of polarization is usually expressed as the ratio of the optical power of two orthogonal polarization states, such as 100:1 or 500:1.
Beam diameter (common units: mm to cm):
The beam diameter of a laser represents the lateral extension of the beam, or the physical size perpendicular to the direction of propagation. It is usually defined at 1/e2 width, that is, the point at which the beam intensity reaches 1/e2 (≈ 13.5%) of its maximum value. At the 1/e2 point, the electric field strength drops to 1/e (≈ 37%) of its maximum value. The larger the beam diameter, the larger the optics and overall system required to avoid beam clipping, resulting in increased cost. However, reducing the beam diameter increases the power/energy density, which can also have detrimental effects.
Power or energy density (common units: W/cm2 to MW/cm2 or µJ/cm2 to J/cm2):
The beam diameter is related to the power/energy density of the laser beam (that is, the optical power/energy per unit area). When the power or energy of the beam is constant, the larger the beam diameter, the smaller the power/energy density. High power/energy density lasers are usually the ideal final output of the system (such as in laser cutting or laser welding applications), but low The power/energy density of the laser is often beneficial within the system, preventing laser-induced damage. This also prevents the high power/high energy density regions of the beam from ionizing the air. For these reasons, beam expanders are often used to increase the diameter, thus reducing the power/energy density inside the laser system. Care must be taken, however, not to expand the beam so much that it gets clipped within the system‘s aperture, resulting in wasted energy and possible damage.
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