Unlike the helium-neon and other common gas lasers, the raw output beam from an edge emitting (also called Fabry-Perot or FP) laser diode is highly divergent and suffers from two asymmetries: astigmatism and an elliptical beam profile. The beam is also inherently linearly polarized.

*Low power edge emitting diodes usually operate in a single spatial mode for both axes. Edge emitting diodes more than about 100 mW are almost always multiple spatial mode in the slow (horizontal) axis, single mode in the fast (vertical) axis. Although it may take some effort (see below), a single mode diode can be focused or collimated with diffraction limited performance in both axes. This isn't possible even theoretically for a the slow axis of a multimode diode.

*The dimensions of the end-facet for a typical small index guided laser diode are around 1µm x 3µm. However, for a high power gain guided laser diode, the long dimension may be 10 or even 100 µm or more.

Note that since at least one of the dimensions of the end-facet is close to the wavelength that the laser diode emits - it may even be smaller - this simple equation is not very precise but typical low power laser diodes do produce beams with a divergence of around 10 x 30 degrees.

Laser diode divergence will generally be given in terms of the Full Width Half Maximum (FWHM) in terms of output power. At the 10% level, this may be more like 70 or 80 degrees than the 30 degrees in the specifications.

*This asymmetry in the X and Y dimensions of the emitting end-facet results in a beam with an elliptical profile. Sometimes this may be described as wedge-shaped. Instead of a point or circular source, it is a line or rectangle. If you focus a laser diode (or laser pointer), the result will be elongated. Due to slight differences in manufacture, the precise angles will not be exactly the same even for various samples from the same batch of laser diodes!

*The beam from edge emitting laser diode is generally linearly polarized. Typical values for the polarization ratio of index guided laser diodes are 50:1 to 100:1 and 30:1 for wide stripe laser diode arrays (compared to at least 500:1 for a common linearly polarized HeNe laser) with the electric field vector oriented parallel to the laser diode junction. The polarization ratio is somewhat lower near the lasing threshold where there is significant contribution from spontaneously emitted light which is randomly polarized.