Given a cheap laser pointer diode module with a lens that creates a beam which is about 3-4mm wide when it leaves the laser, i would like to create a beam significantly thinner (in one dimension at least and over the ray length of about 30cm). Just putting a slit in front of the laser obviously doesn’t work and has the opposite effect. How can I go about creating a really thin laser beam?
Edit: I don’t necessarily need the source of the beam to be a laser. It can be any kind of light that can excite a photoelectrode