Laser theory question

[Ashton]

What's the longest (theoretical) focus for a laser? provided optics are 100% flawless an perfect and it is used in a universe of a absolute and total vaccume with nothing to disrupt the beam.

Just curious to this since, IIRC the smalles posible focus is the wavelength tiself (i.e. 532nm can only be focused down to a point of 532nm in size)

 

[pseudonomen137]

Well, it actually depends on how wide the initial diameter is. All beams must diverge unfortunately, no optics can escape that fact. The relationship is that divergence is proportional to wavelength and inversely proportional to minimum beam diameter. Therefore, the larger the beam is, the better collimation, and thus the further you could try to focus it down. If a far focus is your goal, the size the focus is your minimum beam diameter, so you just need to figure out what initial beam size is needed so the laser can converge to that point without defying physics.

For a 532nm beam, the best specs you can have are ~0.678 mm*mRad, if you're multiplying the full-angle divergence by the minimum spot diameter. So if your initial diameter was 10mm, and you wanted to focus down to a spot of 0.1mm, that would mean your minimum rate of convergence would be 6.78mRad, which works out to a distance of ~1.475 meters.

So mathematically, with an infinitely large spot size, you could focus to a pinpoint at any distance you want  

 

[Justin]

So a beam the size of the Universe could create a singularity? Finally, we know where the Big Bang originated! A previous Universe must have produced such a laser and extinguished their entire existence in a horrible balloon-popping accident, giving rise to our Universe in the process.

HA!

 

[Ashton]

so that's why CO2 lasers ahve such a huge beam... (comparitivly) thanks for the info, I'd been playing with trying to get measured focal lengths from my laser and it wasn't working as well as I had planned, so I was wondering how the physics worked for that one. (I really need to find a good light/physics book to read so I dont keep bugging people on here.....)