# Making a 50 um collimated beam on a budget

1. Jun 15, 2014

Hello again,

My handy optics engineer has left the country leaving me with a problem that's over my head. I need a low cost (i.e. 500.00) solution to produce a circular, collimated, 405nm beam. My beam needs to be within the target diameter (+/-20%) from 0 - 4" from the output lens and have an output power of about 6.5 mW +/-2mw. My first thought is to take a 405nm laser with single-mode pigtail coupled into a lens tube with an aspheric lens to collimate it and a couple of plano- convex lenses to reduce the collimation to .002". This method is more expensive than I like, but it avoids having a pinhole / alignment fixture, which is also expensive. Any ideas? Thanks in advance, Mike 2. Jun 16, 2014 ### Andy Resnick It's been a while since I tried something similar, but if the laser is already fiber coupled, there should be a reasonably simple way to do this (but to be fair, your spec may be unrealistic). Maybe a ball or GRIN lens at the fiber output? 3. Jun 16, 2014 ### Mike_In_Plano Conceptual adjustment. It appears that any efforts to re-collimate down to 50um is unrealistic. Thus, I perceive my best option as focusing a circular image at some distance from my optics. This is unfortunate in that I'm attempting to expose 3D objects, and my current mechanical system lacks a translation axis to keep the image in focus as the object's distance varies from about 1-4 inches. Given the need still achieve a 100um spot with roughly 6.5mw at 405nm, do I have any suggestions on how to achieve this on a500.00 budget and what optical configuration would be most appropriate?

- Mike

4. Jun 17, 2014

### UltrafastPED

Use a really long focal length lens; but then the focal spot will be bigger.

Also note that the nominal focal length is usually based on green light (~530 nm); it will be in the lens specification. So you will have to recompute the focal length for the 405 nm light Be sure to buy a lens that has good transmission for that wavelength! Fused silica has higher transmission as you near the UV than does BK7; but it costs a bit more.

Also a long focal length will increase the focal spot size, as well as the Rayleigh range. For a 2 meter focal length with UV light I got a 200 um spot size.

PPS: Or call tech sales support at one of the optics supply houses - they are very knowledgeable, and know all of the gadgets and their costs
PPPS: And the cheapest way is to borrow the 405 nm laser ...

5. Jul 11, 2014

### Opticsforsale

Hi!
I´m not a laser guy, but I do have a collimator and other laser stuff for sale. Maybe this
Collimator: https://www.dropbox.com/sh/hkwvox1ok71bsjf/AAAxPe-eanOPPXA53uoHd0ppa
Other stuff: https://www.dropbox.com/sh/l71hg5ww2x55z9p/AAAtYXH3O8QEeAr1Cx0CcxGja
I will upload pics of more gear soon.
If there is something of interest, let me know and leave a reasonable bid.
/Klas

6. Jul 19, 2014

### Claude Bile

Provided your beam is collimated to begin with, a standard 4f lens configuration should be able to telescope the beam to whatever diameter you require, within the bounds allowed by diffraction.

Note that the narrower you attempt to focus/collimate a spot, the more it will diffract. This is a standard trade off that has no cheap solution unfortunately.

Claude.