Diode lasers usually have elliptical beam profiles. In most cases, though, the shape of the circular beam is required, such as pattern matching with the external resonator to make the beam size adapt to the small aperture through the isolator. A deformed pair of prisms is used to transform an elliptical laser diode beam into an approximately circular beam to amplify an elliptical beam in one dimension.
|Dimension Tolerances||+0.0, -0.2 mm|
|Surface Flatness||<λ/8 @ 632.8 nm|
|Surface Quality||60-40 scratch & dig|
|Theta Angle||29°27' ± 3"|
|Clear Aperture||> 85% in central circular dimension|
|Coating||MgF2 single layer on perpendicular surface|
Anamorphic prisms are used to expand one beam axis while keeping the other unchanged. High power laser diode like 405nm, 445nm, 520nm or 638nm have a high divergent axis. In far field it gives an asymmetrical beam profile and excludes these laser diodes from many applications like show lasers etc. Using our anamorphic prisms allows to reduce the far field beam divergence dramatically.
An anamorphic prism. The output beam is narrower than the input beam.
The principle of beam shaping with anamorphic prism pairs is not based on focusing effects (i.e., changes of wavefront curvature), but rather on changes of the beam radius for refraction at flat prism interfaces. Such changes occur at the interfaces of any prism (except for normal incidence), because the angle of the beam against the surface-normal direction is different inside and outside according to Snell's law (see the article on refraction). However, for a symmetric beam path, where the beam angles against input and output face of the prism are identical, the two changes in beam radius cancel each other. Therefore, one has to use an asymmetric configuration. It can be convenient, for example, to have normal incidence (or some small angle) at one interface and Brewster's angle at the other one; that requires an angle between the prism surfaces which equals the internal angle of refraction. Only the former interface (with normal incidence) then requires an anti-reflection coating; the losses on the Brewster interface are minimized for p polarization. In the described configuration, the demagnification factor (ratio of output to input beam radius) of a single prism is equal to the inverse refractive index of the prism material, or the inverse of that for the other orientation. If a prism material with suitable refractive index for the wanted magnification cannot be found, one may arrange the prism for different input and output angles.
A single prism is sufficient for changing the beam radius in one direction, but it also changes the beam direction. By using an anamorphic prism pair, one can obtain an output beam with an unchanged direction, only a position offset. The two prisms are of course oriented such that they change the beam radius in the same direction. The overall magnification is then the square of the refractive index, or the inverse of that. If the mentioned beam offset also needs to be avoided, one may use a combination of four prisms.