User:Andy Maloney/Notebook/Lab Notebook of Andy Maloney/2009/02/22/Optical tweezers/Beam shaping
The shape of the beam from a laser diode is not spherical. It is more elliptical. While I'm not a fan of Newport's equipment, they do have a brief tutorial on laser diodes that is decent.
If we want the shape of our laser diode beam to be spherical, we will have to introduce optics in our system to do so. The easiest way to change an elliptical beam into a spherical beam is to use an anamorphic prism pair. What these prisms do is magnify the elliptical beam on it's semiminor axis and demagnify the beam on its semimajor axis. The end result is a quasi spherical beam.
Alternatively one can use cylindrical lenses to do the same thing as the prism pair. If you have a collimated elliptical beam, you can use one cylindrical lens to squish the semimajor axis and another one to recollimate it.
Either way, changing the shape of the beam can be done. It just depends on which route your optical system needs to take. Changing the shape of the beam does nothing for the modality of it. We have not decided if we want to do this yet but typical applications of optical tweezers use single mode TEM00 light sources. We can take our multimode laser diode and convert it to a single mode light source at the expense of loosing power. See my section on spatial filtering.