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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

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In this situation using a laser as the light source is a good idea, as lasers emit nearly collimated light due to their working principle and geometry of the cavity. In order to focus the laser you can use a biconvex lens:

enter image description here

Then it you can place another lens further away to collimate the beam with reduced diameter, here is a reversed diagram where the diameter is expanded,

enter image description here

note that there is an inverse relation between the divergence (degree of collimation) and the radius of the beam, that's why it is clever to start with a collimated source.

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  • $\begingroup$ These cheap laser modules already have one lens inserted. With this method i’d add two more lenses. Would the same be possible by replacing the existing lens with one new lens of shorter focal length? $\endgroup$ – matthias_buehlmann Jun 2 '19 at 7:47

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