how to focus a pulse laser

[ustink007]

Sorry, but i'm very new to lasers. So far i know that a laser beam is a Gaussian beam. I was told that it needs to be collimated before using a focusing lens. But from a thread on this forum, it seems that isn't necessary, unless there is something i'm missing. The thread that i got this information from is titled "Totaly Un-collimatable Really Divergent LD video"

Is this the same with a pulse laser? Is there a difference between the continuous beam laser and pulse laser except for the time? All i need is to focus the beam to a finer point. From the Manual i got for the laser, i think it makes a 0.5 cm circular burn at a distance of 30 cm. Can i focus it down to about 1 mm in at least 0.5 m distance without collimating and only using a focusing lens?

Another question, from the video of the previously mentioned thread, i see that with the focusing lens, it reflects, so am i correct to assume all i need is a special/expensive lens with a certain coating?

What i have is a Nd:YAG laser at 1064nm wavelength.....

 

[Aseras]

you make a line of thermal paper sheets along a path every so many feet and pulse, then check the size of the burn holes and adjust accordingly. Rinse repeat as necessary.

 

[disma]

The laser you have is different than mine. (17 watt, now dead), it was a fiber laser, yours is completely different, and has a much better beam then the fiber would ever have. You will probably need to spread the beam with a concave lens first, then focus it with a convex lens. or you might use 2 convex lenses, eather way, I dont think you will be able to focus it without spreading the beam first unless you are talking about focusing to a pinpoint dot close to the laser, in this case, its prety easy, just get a lens, and observe its focal length, your focus will be real close to this length from the lens, but slightly farther because of it bieng infrared... if anyone knows for sure, please jump in.

Max

 

[likewhat]

The fact that your laser is pulsed makes no difference. All you need is a converging lens. You can just buy one from thorlabs that has something like a 20 cm focal length and you will be fine. here is one that has a broadband antireflection coating so that it will only reflect like <2% of the light.

http://www.thorlabs.com/thorProduct.cfm?partNumber=LA1708-C



Here is the link to more regular lenses that you could use. You need a C coating for 1064nm light.
http://www.thorlabs.com/NewGroupPage9.cfm?ObjectGroup_ID=112&visNavID=165

You just need to get that lens (or whichever focal length you decide on) and a mount for it and you stick it 20 cm (or whatever focal length it is) before where you need the focus of the beam to be. The ~ size of the spot will be given by this expression

diameter of waist = 4*wavelength/PI*Focal length of lens/beam diameter when it enters the lens

So if you have a ~ 5 mm beam diameter this would give you a spot size of

4*1064e-9/PI*.2/.005 = 54 microns

So it will easily be way less than 1 mm in diameter.

if you wanna find out where exactly the focus is I would recommend just shooting something like a piece of plastic or i usually use the back of polaroid film for my excimer laser. When it is making the smallest hole that is where the focus is, make a mark on your table and measure how far away from the lens it is and that way you will know.


Always be wearing high OD goggles when you are using a pulsed laser. Pulsed lasers are many orders of magnitude more powerful than what is necessary to make you go blind. you can lose your vision in a few femtoseconds if you screwup without glasses on.

 

[wilfred1970]

great! i was looking for more info about lenses.... this could be useful.
i got now a 5" CO2 focussing lens from work (used one of course), but i think it's not very useful for focussing yag or diode lasers....