Astigmatism Correction

How do anamorphic optics correct astigmatism?

The optics type 5AN are cylinder lens systems and, therefore, can be additionally used to correct the astigmatic difference ∆As of the laser diode or tapered amplifier through a refocusing of the optical system. 

Astigmatism of laser diodes

Laser diodes have large aperture angles vertically (s) and smaller ­aperture angles in parallel (p) with the light-emitting layer. Additionally, some laser diodes have two virtual emission sources from the s- and p-direc­tions, i.e. astigmatism, characterized by the axial displacement, ∆As.

Optical scheme

Optical beam path through an anamorphic optics

The collimating lens produces a collimated elliptical beam with a Gaussian intensity profile . If there additionally is an astigmatic difference, ∆As, the beam is collimated in only one of the directions and is diverging in the other.

The anamorphic beam-shaping optics contains a positive and a negative ­cylinder ­lens, scaling down the longer ­elliptical axis to that of the shorter axis. To compensate for ­divergence induced in the s-direction, the distance between the elements of the cylinder lens is ­increased (astigmatism correction).

The output beam profile of the anamorphic beam-shaping ­optics is circular and the beam is collimated (if the

anamorphic form factor is chosen correctly). After astigmatism ­correction, the wave fronts are planar.

Focus with plane wave fronts

Wave front of an astigmatically corrected beam

When this beam is refocused, the spot is not only circular but also has plane wave fronts . 

Focus with curved wave fronts

Wavefront without astigmatic correction

Without astigmatism correction (e.g. when beam shaping is performed using anamorphic prism optics), the focus shows astigmatism and the wave fronts are curved.