The laser diode beam is no longer round

Laser diode beams are typically elliptical and astigmatic due to the rectangular geometry of the active layer and the ridge waveguide structure.

Beam Shape and Geometry

Laser diodes emit light from a very thin PN-junction, often only about 1 micron thick, which is the active region where light is generated. The diode's ridge waveguide confines the light, but because the waveguide is much wider in one direction than the other, the emitted beam is asymmetric, resulting in an elliptical cross-section rather than a circular one. The fast axis (narrow dimension) diverges more rapidly than the slow axis (wide dimension), producing a beam with different divergence angles in vertical and horizontal directions.

Divergence and Astigmatism

The beam divergence is also unequal along the two axes. Typical divergence ratios range from 2: 1 to 4: 1, with the fast axis diverging more strongly. This astigmatism is inherent to the diode's structure and is a key reason why the beam is not round.

Multi-mode diodes or diode bars can further complicate the beam profile, creating hot spots and uneven intensity distributions.

Optical Solutions

To achieve a more circular beam, several optical techniques are used: Cylindrical lenses: Reduce divergence along the fast axis while leaving the slow axis unaffected, partially circularizing the beam. Aspherical lenses: Can help collimate the beam more uniformly. Fiber coupling: For single-mode diodes, coupling into an optical fiber acts as a spatial filter, producing a nearly perfect TEM00 circular beam.

For multi-mode diodes, the fiber homogenizes the beam, reducing hot and dark spots. Microlens arrays: Often used for multi-emitter diodes to shape and homogenize the output.

Summary

The non-round emission of laser diodes is intrinsic to their design, caused by the rectangular active region and ridge waveguide, leading to elliptical, astigmatic, and highly divergent beams. While this is acceptable for some applications, optical elements like cylindrical lenses, aspherical lenses, or fiber coupling are commonly used to circularize and improve beam quality for precision applications.