thanks for sharing your discoveries with us. The pattern you see looks more like a broader emission spectrum, than a one that consists of multiple lines.
Usually the emission width (FWHM) of a GaN laser diode in the + 450nm region is around 2nm. The width of the gain profile though is around 10nm, so
in general every frequency in these area can lase if it has enough gain. Your pictures look for me more broadened than two distinct lines (the human eye
detects light with a logarithmic scale, so it is hard to tell without a camera or s.e. similar if the brightness is half as strong or just a few %..)
Have you moved your grating (without changing the angle in respect to the laser beam) to cancel out spatial deformations of grating? A measurement
with a spectrometer can resolve your question but also a picture of a direct caparison with a laser diode, that you know is operating in a single mode
Yeah I'm thinking that it's having a wider emmission spectrum. The 485 I have however has a second distinct line.
And I can see my 589 doing the extra lines as well
And yeah it's tough saying much about the brightness, I have no clue how to measure the power across that spot with the equipment I have.
And yes, this pattern isn't a defect in the grating, as it only exists with these 480nm diodes (to this degree - the 502nm I have does this too but to a far lesser extent). I'll take some comparisons with my 450nm, 520nm, 635nm, and 650nm sometime today. I can with the 405 as well, but I'm worried about burning the grating since it's so powerful
450 at same position
So I guess they do kind of start to spread a bit, but not nearly as much as the 480nm diodes
What Singlemode was saying is the center line spectrum of these diodes is limited to 2nm, but the FWHM allows a 10nm wide area that can be caused to lase with diffraction tuning. Since that is not what you are doing here, I would expect the line width to be limited to 2nm.