Weird Spatial Filter Effect...

[Jem]

Hi all

This is a weird one. I was adjusting my spatial filter and had a card in the expanded beam about 24" downstream. The expanded beam was about 8" diameter at the 24" point. I was bringing the objective closer to the pinhole, all the time adjusting the pinhole position whilst observing the pattern on the card. I was just about at the point where the beam becomes perfectly filtered and I noticed a sort of modulation in the beam, it was sort of phasing in and out.

I backed off the objective and the beam pattern was stable, I then moved the objective forward again to the point of 'perfection' and that was o.k. The problem only happens at one particular point as i'm moving the objective forward towards the pinhole. Iv'e checked the laser and that is as solid as a rock with a perfectly stable beam and no mode hops.

My first guess is that the pinhole is expanding and contracting with the heat generated by my 200mW Coherent 532-200 being brought to a focal point just before the actual pinhole. I'm using a 10 micron pinhole which I suspect may be a bit small for the beam diameter as when i'm adjusting the objective I am getting perfect geometric 'rings' prior to acheiving a full spatially filtered beam. If i'm correct I think that means that the pinhole is too small, please correct me on this if i'm wrong as it's only what I seem to remember reading somewhere.

Anyway, it's not causing a particular problem, it's just I had never observed this phenomenon before and wondered if anyone else had experienced it and knew for certain what it was.

Cheers

Jem

[Ed Wesly]

Yes, you are more than likely correct that your 200 mW focussed down to 10 microns or so would be heating up the pinhole and causing the very thin substrate to warp. When you take out the pinhole look for scorching around the hole with a magnifying glass.

When it comes to these higher powered lasers, it's good practice to put a shutter after the spatial filter so that you know the thing is tuned before exposure, as you could get a surprise after the pinhole cools down and goes out of alignment, and there is little if any beam coming out of the pinhole wafer until the laser makes its own new pinhole. Yes, that might mean a shutter after every spatial filter if you have a 2 or 3 beam set up.

[BobH]

Might have also been a speck of dirt in the hole being burned away, unless it still happens. The diffraction rings should disappear as you go toward the focal point, then there should be a comfortable distance before they show up again on the other side of focus. If they never quite disappear, the hole is too small. If they disappear then reappear immediately, the pinhole is at minimum diameter. I like a hole to be as big as possible to give the beam a bit of room to wander without needing to realign the spatial filters constantly. The appearance of detrimental patterns indicate a pinhole that's too large.

[Colin Kaminski]

If you go to your local high end jewler with your pinholes you

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[Colin Kaminski]

If you go to your local high end jeweler and ask them to clean your pinholes with their ultrasonic cleaner they will not charge you. Just don't let them wipe off the cleaning solution as the cotton will get in the hole.

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[Jem]

Thanks guys

Problem seems to have resolved itself now, so it was most likely a speck of dust in the pinhole.

@Ed, that's a good tip about the shutters. It's not presenting a problem with my setup as yet, but i'll bear in mind what you've said for the future.

@BobH, You said "The diffraction rings should disappear as you go toward the focal point, then there should be a comfortable distance before they show up again on the other side of focus"

That's interesting... mainly because in order to get totally clean spatially filtered light my objective is virtually sitting on the pinhole therefore there is no comfortable distance on the other side of focus. I've often thought my objectives were very close to the pinhole, so, is this unusual?, am I doing something wrong or am I using the wrong type of objectives? How far from the pinhole are your objectives?

Cheers

Jem

[BobH]

The only time my objective hits a pinhole is with a 40X. For example, with the Newport 910 spatial filter, I've removed the pinhole from its mount and taped it onto the input face of the holder instead. That gives me the space I need to be able to go through the focal point and center it on the hole. If your objective is hitting the pinhole mount, try that.

Is your beam very small diameter when it goes through the objective? That will cause you to use a shorter focal length objective, increasing the chances of the objective hitting the mount. I like the beam to almost fill the objective.

[Jem]

I like the beam to almost fill the objective.

Thanks for the reply Bob, with reference to your quote above, do you expand the beam then before it hits the objective?, is the beam collimated at that point using a lens system?

I'm using the raw (0.8mrad if memory serves) beam from my Coherent 532-200, i'm currently using a 40x objective, but don't find much difference if I use a 20x or a 60x, the distance from the pinhole to the objective is absolutely minimal in most cases.

Thanks for your advice, one of the problems is i'm pretty much self taught with all this and without a mentor it's not always easy to recognise if something is not quite as it should be.

Cheers

Jem

[BobH]

I've always used He-Ne or ion lasers at home, which usually put the laser some distance away from where the spatial filters were in the set-ups. That gives the beam time to expand a bit. Another method is to use a weak negative lens upstream to make the beams bigger going into the spatial filters. I suppose you could up-collimate properly, using two lenses, but that would really only be necessary if the spatial filters were at different distances from the laser, like in a complex set-up with multiple object beams all over the place.

[Jem]

Thanks Bob

I'll have a play around at expanding the beams before the SF when I get a chance. I've got plenty of lenses here to try.

Cheers

Jem