Yes, there is too many people who are just too lazy to spent few minutes to background research on their own.. :beer:
Let's be honest - information is scattered all over the forum as verbal descriptions, which does not make that advice very helpful.
There is very little organization for the information we gather and the attempts that have been made are often difficult to read (e.g. images of spreadsheets with tiny text), do not have good photographic documentation, and other issues.
While I totally understand the desire to berate newcomers and advise them to "use the f**king search," there is a limit to the usefulness of that advice because of the nebulous organization of information here.
I'd welcome anyone to prove me wrong - show me how a 30 minute scan of the forum can pull up information on beam characteristics for at least 25% these diodes with
measurements,
photos, and
clear standardized comparisons.
Gaining this knowledge, if it exists here, requires A) sifting through a ton of often irrelevant conversation to find the nuggets of information in every relevant thread or B) a long history on the forum to accumulate bits of information from conversation and experience. This is not easy for newcomers.
"Read the stickies" and "use the search" are not always helpful comments especially with specific questions like this one.
My advice to Coonie:
If available, look at the data sheets for the diodes you are interested in.
Where the tables provide information of divergence angles, there will be two fields, one number for the parallel-axis (we can call it the x-axis for simplicity) divergence angle and one for the perpendicular "y-axis".
If you divide one by the other, you will get a ratio of the divergence angles for the x- and y-axes. The further that ratio is from 1, the more elliptical or bar-shaped the beam will be.
When the beam is highly elliptical (i.e the ratio is far from 1), it will be much more difficult to obtain a collimated beam with the convex lenses we typically use.
This is because the two axes diverge at different angles and so require different focal lengths to collimate them simultaneously.
So, for multimode diodes, your best bet is to go with one that has a divergence angle ratio as close to 1 as possible.
When such data is not available, you will have to try to fill in the gaps by searching for that particular diode and seeing if anyone has reported anything on the beam characteristics.