Optics lesson please

[dnar]

Tonight I was fiddling with my latest 445nm lab build. I removed the prisms and focused the laser for the smallest target at 10 meters. The LD shape at aperture is "|" yet at 10 meters the target is "-". I discovered that the beam flips several meters from the diode.

What is going on here?

[dave]

This spells it out pretty nicely

http://www.rp-photonics.com/broad_ar...er_diodes.html

What you are seeing is the fast axis diverging a lot quicker than the slow axis.

[dnar]

Thanks Dave. Well explained indeed.

So do I have it correct, my LD beam profile is "I" at aperture (5x1mm) for Simon's prisms?

[laser_freak]

I have always been confused by this too. When using corrective optics (such as simon's prisms or drlava's lenses), should the shape of the beam at the aperture be a "I" or a "-"?

[dave]

The diode spot at the aperture needs to be vertical. You use the prism set, or cylindrical lenses to expand the fast axis, and therefore slow the divergence of that axis.

[dnar]

Yeah thanks Dave, I played around a fair bit last night with both V and H, also various prism angles. I had it almost ideal from the 1st build, lol. It was an interesting night of trying various things.

I now have 3x3mm at aperture due mostly to the prisms, but also by virtue of the exit aperture of this housing. The final beam is pretty clean!!! I have an LPM on the way, I'll measure the final beam when it arrives.

[Things]

I noticed this too while taking divergence measurements. The dot at around 1.2 metres is almost round

[Eidetic]

I noticed this too while taking divergence measurements. The dot at around 1.2 metres is almost round

That's called the "circle of least confusion" in an anamorphic beam.