Thermal blooming is a physical phenomenon associated with the propagation of high-power laser beams through a medium, typically a gas (e.g. air) or a liquid (e.g. seawater). It means that there is a distortion of the beam profile, including a slight change in beam direction, and all this typically with some irregular temporal variation.
This effect is particularly significant in laser applications involving high-power lasers and long propagation distances, such as laser weapons and laser-based atmospheric research. Here, the focusing or collimating of a beam over a long distance can be significantly hindered by the beam distortions and motion.
Physical Mechanisms Behind Thermal Blooming
The following aspects are involved in thermal blooming:
- An intense laser beam will exhibit some absorption in the medium, resulting in local heating. Even if one does not expect absorption in the relevant wavelength range, e.g. in air, there may be some weak absorption of molecules (especially water vapor) related to their overtones. There may also be some absorption by impurities, such as dust particles in air. For pulsed beams with very high optical intensity, there may also be nonlinear absorption effects.
- The heating effect can be strong even at moderate power levels if the wavelength of the laser is in a spectral region with substantial absorption in the medium, i.e. in the mid infrared.
- The heat deposition causes an inhomogeneous temperature profile and also a density profile. Gases in particular expand significantly when heated. In addition, especially for horizontally propagating beams, the heat profile will induce some convection (with heated material moving upward), which further modifies the temperature profile in an asymmetric manner.
- The density and temperature variation is accompanied by a variation in the refractive index, which in turn causes locally varying changes in the optical phase. As the phase retardation is reduced inside the heated area, a defocusing effect occurs, tentatively increasing the beam diameter. Irregularities of the profile are the origin of the observed beam distortions, which also imply a reduction of the spatial coherence. In addition, the aforementioned asymmetry of the refractive index profile (even for an originally perfectly symmetric beam intensity profile) leads to a change in the propagation direction.
- As long as these effects are weak, they tend to be relatively constant. When they become strong, however, there can be irregular temporal behavior as the temperature profile affects the beam propagation, while the latter affects the temperature profile.
- When thermal blooming becomes strong, there is also a significant effect of external influences such as wind, which affects the temperature distribution. Wind alone (i.e., without laser heating of the medium) would not have such a strong effect on small-diameter beams, where the density variations are on a longer length scale.
Relevance of Thermal Blooming and Mitigation Strategies
The strength of thermal blooming effects on laser beams depends mainly on the following factors
- the optical power
- the beam radius (with smaller beam radii generally leading to stronger effects)
- the absorption coefficient of the medium at the wavelength of the laser
- other material parameters, such as the sensitivity of density and refractive index to temperature variations, thermal conductivity, specific heat capacity, and viscosity (which affect the strength of the induced convection)
- the propagation distance
- external influences like wind
The strongest effects are seen in cases of very high optical powers propagating over large distances (e.g. for laser weapons or sometimes in laser-based remote sensing) and in cases of substantial absorption of the medium. In laser materials processing, blooming effects are usually weak despite high optical powers because the propagation distances are not very long.
Thermal blooming can be mitigated by optimizing the above factors. In addition, adaptive optics can be used to pre-compensate for beam distortion.
Thermal Effects for Beams in Solids
When a laser beam passes through a transparent solid medium, thermal effects can also occur, but two aspects are different:
- In most cases, there will be an increase in the refractive index, rather than a decrease, as in gases and liquids, leading to a reduction in density. (There are, however, some solids that have a negative <$\partial n/\partial T$>).
- The solid medium cannot move as a result of heating, except for small movements due to thermal expansion.
In this context, the observed effect is usually called thermal lensing rather than thermal blooming.
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