GRIN Lens

I have a 12 Watt FAP 800 module. The output is a multi mode, circular, 19 fiber mess which can't be focused with a normal lens. Will a GRIN lens pull the mess together to get good focus??
HMike

Hemlock_Mike said:

...can't be focused with a normal lens...

Click to expand...

I doubt it. Sounds to me like your lens is too small and/or has a focal length that is too long.

A GRIN lens would work too, I think, but you
would run into the same problems as above.

Not sure what you are looking to do, but I use one of these for my FAP lasers:



High Power 808nm 50W Coherent FAP Laser Diode Collimator Lens SMA | eBay

The lens is meant to produce a close in focal point for cutting..... (Edit: I was wrong, recently found this lens does not focus to a fine point, instead just collimates the output), ...but beyond the focal point the beam spread isn't too bad, considering. I measured a 4 inch spot on the wall about 25 feet away (using a camera) and tried to figure the divergence, if I remember right, it was somewhere below 20 mRad but I'm not sure this applies when the focal point is so close in and the beam has not been focused to infinity.

This has been something I have wanted the answer to for a long time and the reason I joined this forum trying to find an answer to whether I can reduce the divergence from my 808nm FAP laser down to under 1 mRad through optics or not. Since the beam is obviously expanding, couldn't I put a simple PCX lens in front of the beam to reduce the divergence that way, like a regular beam expander and the further out the lens is placed, the greater the amount of expansion to reduce the divergence?

I gave up on trying to find the answer and an appropriate lens set up to do that for while, became more interested in beam expanders for visible light instead, although I'm hoping to figure this out at some point and apply it to my IR laser. Perhaps I have enough understanding now, that all I need is to place a large enough PCX lens at the appropriate distance and bam, divergence reduced.

.............................
Edit 11/08/2015. IR lasers collimate and act exactly the same way visible lasers do, I don't know why I thought working with IR was such a difficult thing except I needed a modified CCD camera with the IR filter removed to see for myself. See the pix here http://laserpointerforums.com/f49/coherent-fap800-expanded-808nm-beam-shot-95501.html#post1387366

LP --
I'd sure like to know what's in that optic!!
I have my holder for the GRIN lens almost finished BUT
The face of the module output looks corrupted. I will ask the local
phone Co how they polish fiber optics tomorrow if globule warming
permits here in SNOW.
HMike

Here's a pix of the module and GRIN mount I machined.

The lens assembly is definitely just one lens, the back end open because I can pass a straw through it to the front lens. I don't know what the back side of that lens is for sure, but it appears to be flat and thus a PCX. I can loan it to you, if you wish, through the mail.

I'm interested in the outcome with the grin lens.

You seem to have missed the stalker's comment and my comment, Mr Mike.

What is the raw divergence of the fiber output?
What is the focal length and type of lens you have tried?
What is the diameter of the lens you have tried?

Mr Ragon !!!
I didn't see the questions you have sent----
1. The output is 19 fibers smooshed into a round connector housing. The output opening is about 1 mm dia. Divergence ??? LOL -- at 5" the beam is 1.5" across.
2. I have tried different lenses but the last one I tried is 13 mm dia x 25 mm FL. I believe I got this lens from a camera. It is glass PCX.

I'm hoping the GRIN will provide a better beam profile for an expander.

HMike

Put your 13mm lens 25mm in front of the fiber output. That's the best you're going to get with that lens.

Beam expander is not necessary. In fact, it's a total waste of time and resources. It means you're to take a diverging beam, collimate it, diverge it again, and then collimate it again. You can get the same effect with much lower losses by using a single large lens. Just match up the focal length with whatever size you want. I post this one a lot, but it's a good example:

I was afraid of that but hope springs
eternal !!!! Everything I have tried
supports your observations.
Thanks -- HM

Big thanks to Laser Project for sending me an actual lens housing for a FAP module. It appears to have a single lens of almost the same diameter as mine but has about a 30 mm FL. Beam output is about 5/8" dia from 1" to 6". As with my original lens, I can get some focus with an additional lens. I will still try the GRIN rod.
Thank you Chris for your generous contribution.
HMike

Welcome, my project to use eight of them turned into just one, so I am glad you have some use for it.

Well, I machined the holder for the GRIN lens and tried it tonight.
I was expecting a tighter beam..
What I saw was a large divergent beam of 19 dots .... The b@$tard
got hot at half power. The $55 GRIN lens now has 19 tiny dots burned into
the input side of the glass --- FAIL ;-(
Back to LP's lens. This is a nasty beam.
HMike

Can you get close enough with a camera to take a photo of it showing what it did? I'm curious if you wouldn't mind doing so. I guess the fused fiber output of your FAP laser didn't burn too, that is one thing I'd hate to see happen. You have me getting interested in working with my FAP 808nm laser again now to check and see what the mRad is with that kind of lens, I never did get a very accurate measurement before, I thought it wasn't too bad for what it was at 20 some feet, considering it was the output beyond its close in focus.

LP --
I may have a problem here. The output end of my FAP get hot quickly at medium power. I have cleaned this with acetone and it looks pretty good with a magnifier lens --- BUT I may have damaged it long ago by trying to get a lens too close. Is there a way to polish it? I Also am considering a spherical lens for getting its $h1t together !!!
HMike

I just tried a clear 1/2" glass marble.... that might have some possibilities with a good sapphire.

It's done all the time in the telecom industry.
I'm not familiar with the procedure myself,
but it can probably be found somewhere on
the web.