Why getting a good focus of a laser beam can be an issue
The most commonly used laser diodes are called Edge Emitting Laser diodes (EEL) , as the laser beam is issued from the side surface of the LD chip.
The structure of the chip has an influence on the shape of the emitted beam, which is not circular and has more of an elliptic pattern causing an astigmatic shift.
The laser beam shows a “Slow Axis” and a “Fast Axis”. This can be an issue, as the astigmatism would make the focus of the full laser beam into a single point physically impossible.
In addition, the Slow Axis and Fast Axis rotate along the direction of propagation of the laser beam, due to its electromagnetic wave properties with transverse modes.
In order to correct these shifts of focus, the laser beam emitted from an EEL diode must be shaped with a collimating lens. The collimating lens will help the beam converge at one defined focus point but only for a specific divergence angle. The divergence angle is then limited by the Numerical Aperture (NA) of the lens, which is determined by its dimensions and geometry.
Another way to avoid this focus issue is to use a different kind of Laser Diodes called VCSEL, which stands for Vertical Cavity Surface Emitting Laser. This time the laser beam is emitted through the top of the LD chip, and such a structure reduces the astigmatic shift.
Hence a VCSEL with a larger aperture can create a beam with a more circular shape, along with a lower divergence.
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