Laser Beams

In most cases, a laser emits light in the form of a well directed light beam, which is called a laser beam. This means that the light dominantly propagates in a certain direction, typically with most of the optical power concentrated to a small area of the order of a square millimeter.

Laser beams are often close to Gaussian beams, where the transverse profile of the optical intensity can be described with a Gaussian function, the width of which varies along the propagation direction.

Generally, laser beams exhibit a high degree of spatial coherence, which is related to a high beam quality. As a result, one obtains good focusability and the potential to form collimated beams with very low beam divergence.

When a laser beam hits some object (for example a workpiece in laser material processing), the arriving optical intensity is also called the irradiance.

The propagation of Gaussian beams can be calculated with a set of relatively simple equations. In cases with non-ideal beam quality, one can use a generalized set of equations which also involves the so-called beam quality factor M2. In this case, the equations cannot predict the detailed evolution of beam profile, but only of the beam radius based on the second moment of the intensity profile (D4σ method).

Laser light often have a small optical bandwidth, so that the temporal coherence is also high. An often unwanted consequence of the high level of coherence is the tendency to form laser speckle patterns.

The optical power of a laser beam may hardly change during propagation in a transparent medium, or quickly decay in an absorbing or scattering medium. Inhomogeneous media (i.e., media with a locally varying refractive index) can also distort the shapes of laser beams. This can happen due to e.g. thermal effects such as thermal lensing in a gain medium. In gases or liquids, thermal blooming effects may occur at high optical powers.

Some lasers emit continuously, but a laser beam can also consist of a fast sequence of pulses, with many millions or even billions of pulses per second (→ pulse repetition rate). The light distribution may then be described as a regular sequence of a kind of “light bullets”.

Laser light is often linearly polarized, i.e., the electric field oscillates in a certain direction perpendicular to the propagation direction. Some lasers, however, emit light with an undefined, fluctuating polarization state.

Is a laser beam visible from the side?

A laser beam of visible light with sufficiently high power may be visible when propagating in air, particularly in a relatively dark room. This is because a tiny portion of the optical power is scattered by dust particles and/or density fluctuations in the air and can therefore reach the observing eye. When the laser beam hits some diffusely scattering object, such as a white screen, a much brighter spot is seen on that screen, since most of the optical power is scattered at this point.