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Definition: light beams propagating dominantly in one direction
In most cases, a laser emits light in the form of 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 of the beam can be described with a Gaussian function, the width of which varies along the propagation direction. This variation of beam size can be very small for beams with large width or very fast for tightly focused beams.
Generally, laser beams exhibit a high degree of spatial coherence, which is related to a high beam quality. As a result, such beams exhibit good focusability and the potential to form collimated beams with very low beam divergence.
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.
Most laser beams are 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.
A laser beam of visible light with sufficiently high power may be visible when propagating in air. 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.
Near Field and Far Field of Laser Beams
The near field of a laser beam is understood to be the region around the beam waist (focus). The far field concerns the beam profile far from the beam waist, i.e., in a distance from the focus which is large compared with the effective Rayleigh length. The far field intensity profile reveals details of the beam divergence, which in the near field can be obtained only with wavefront measurements. As it is often not practical to access the far field directly, one may use a focusing lens (or mirror) to obtain an intensity profile in its focal plane which reveals a scaled-down version of the far field pattern.
Experimental Characterization of Laser Beams
There are various devices and techniques for characterizing a laser beam in various respects:
- The total optical power carried by a laser beam can be measured with a powermeter. There are also energy meters for measuring pulse energies of pulsed lasers.
- The intensity profile (irradiance profile) can be measured with some kind of beam profiler, which may be based on a camera. Some laser beams exhibit “hot spots”, i.e., regions with higher intensity, which may play role in damage phenomena. There can also be satellite structures, halos and other deviations from beam uniformity. Note that the shape of the beam intensity profile may change during propagation, and is also not always stable over time.
- Some devices do not measure the whole beam profile, but only a beam radius or diameter. Note that the beam radius is usually not defined as a half width at half maximum (HWHM) value, but as the radial position where the intensity falls to 1/e2 times the peak intensity. This comes from the context of Gaussian beams.
- Wavefront sensors, for example of the Shack–Hartmann type, can be used to measure the shape of the optical wavefronts.
- The beam quality (M2 factor) can be calculated from multiple intensity profiles or from intensity and wavefront measurements at one location only.
- A laser beam may exhibit beam pointing fluctuations, which can be measured by monitoring the position (center of gravity of the intensity distribution) at a suitable location.
- The polarization state maybe roughly characterized based on the power transmission of a polarizer for different orientations, or more completely with various techniques for polarimetry.
For many laser applications, it is essential to have proper means for beam diagnostics, as many possible problems can be identified with such instruments.
|||H. Kogelnik and T. Li, “Laser beams and resonators”, Appl. Opt. 5 (10), 1550 (1966)|
|||A. E. Siegman, “Defining, measuring, and optimizing laser beam quality”, Proc. SPIE 1868, 2 (1993)|
|||A. E. Siegman, Lasers, University Science Books, Mill Valley, CA (1986)|
See also: laser light, collimated beams, coherence, beam quality, Gaussian beams, beam divergence, beam pointing fluctuations, polarization of laser emission, speckle, beam profilers, powermeters, Spotlight article 2010-04-08