Comparison of beam divergence between laser diode and DPSS lasers I'm currently comparing a pulsed laser diode and a pulsed DPSS laser. For the DPSS laser, the beam divergence is listed as "beam divergence (80% of total energy): $<4 \text{mr}$". The laser diode is said to already be collimated, and has a beam divergence angle of "$\theta_\perp$: $\le 10^\circ$" and "$\theta_\parallel \le 12^\circ$". I know that plain laser diodes have more divergent beams than DPSS lasers, but I want to understand the differences here and whether I can lower the beam divergence of the laser diode to be closer to the DPSS laser. What is meant by "beam divergence (80% of total energy)"? And what is meant by a beam divergence angle of "$\theta_\perp$: $\le 10^\circ$" and "$\theta_\parallel \le 12^\circ$"? Furthermore, how do I compare "beam divergence (80% of total energy): $<4 \text{mr}$" to a divergence angle of "$\theta_\perp$: $\le 10^\circ$" and "$\theta_\parallel \le 12^\circ$"? And, lastly, how can I decrease the beam divergence of the LD so that it is closer to the DPSSL? The experiment is over relatively short distances (varying between 10-100cm).

EDIT
This datasheet seems to have the same parameters.
 A: DPSS lasers typically have much better $M^2$ than laser diodes, hence the findings you have (0.2° for the DPSS laser and 10/12° for the laser diode).
If you expand the beam, then you will intrinsically have lower divergence, so in principle yes, it is possible to have a laser diode have a divergence <0.2°, but at the cost of fluence, ie, your beam will be extremely big.
The 80% of energy is kinda close enough to the ISO standard of beam divergence (but not exactly), all it means is that 80% of the energy of the beam will always be contained in a cone with a half-angle of 4mrad.
I would imagine the values for the diodes are either given in FWHM or ISO standard values. You need 2 because diodes have 2 emission axis and due to the confinement in the gain region, each has its own divergence. I point you to this website to read more and understand what beam divergence is and how to compare the two.
For an experiment with less than a meter I don't think that the divergence will be your problem. As I seen you want to do interferometry, maybe your limiting factor would be coherence length instead. In any case, all parameters will probably be better with a DPSS laser.
