optical question

I would expect any matte black surface would absorb most of the light and what isn't absorbed would be diffused. If you think the beam might be too concentrated so that it would overheat the spot it is illuminating you might pass it through something like ground glass to disperse it first. I'm not sure how transparent glass is at that wavelength, but you get the idea I think. The main thing is to prevent reflections. You should be able to sample the beam directly or indirectly.

Point it directly down a hole drilled into a copper block. There are tricks to get a black enough chemical finish on copper (or iron). Dumping a couple of watts isn't a problem provided it doesn't leak.

The laser light can't hurt you if it can't get out. The dump block doesn't have to be very big to absorb a couple of watts.

The main worry with laser light you can't see is that you can sustain serious damage without knowing it. There are no pain receptors in the eyes you only find out about 8 hours later when the immune response to dead cells kicks in. Same sort of problem with looking at the last crescent of the sun during a total solar eclipse.

Dump it into a tube with a heat sink on it.

An offcut of microbore plumbing pipe squeezed flat and then turned over at the end would probably be fine for dumping just a couple of watts.

You ought to think about an interlock so the laser cannot be fired when the beam is not dumped into a safe load. I once walked in on someone with a He MIP flame with 700W of microwaves in a beautiful salmon pink flame with nothing between him and the radiation source. He defeated all the safety interlocks on the mesh faraday cage. Truly scary.

Den tirsdag den 11. november 2014 20.07.18 UTC+1 skrev John Larkin:

a bottle of water? I believe water is highly absorbing at 1500nm

or a stack of laser windows

-Lasse

That's a nice idea. A copper tube with some chemical blackening inside. Light would enter at a small angle and scatter and be absorbed along the way and not much could make it back from the end. The tube could be curved, too. Maybe put the fiber at the end to pick off what's left.

Phil will have the best approach. While you are waiting for him to come online, here's some methods from Wikipedia:

Optical beam dumps

An optical beam dump is an optical element used to absorb a beam of light. Major design concerns in a beam dump typically include the management and reduction of back reflections and scattering as well as the dissipation of heat generated by absorption. For low-power systems and less demanding applications, the device can be as simple as a piece of black velvet or flock paper glued onto a stiff backing, but higher- power beam dumps must often incorporate more elaborate features to avoid back-reflection, overheating, or excessive noise.

Dumping the beam with a simple flat surface may scatter unacceptably large amounts of light for some applications, even though the direct reflection may be effectively reduced. To minimize scattering, it is common to use deep, dark cavities lined with an absorbing material to dump the beam. A particularly simple and relatively inexpensive approach is to use a stack of razor blades with the sharp edges facing the beam, so that the spaces between the blades form very deep cavities from which little light escapes.

A commonly available type of beam dump suitable for most medium-power lasers is a cone of aluminum with greater diameter than the beam, anodized to a black color and enclosed in a canister with a black, ribbed interior. Only the point of the cone is exposed to the beam head-on; mostly, incoming light grazes the cone at an angle, which eases performance requirements. Any reflections from this black surface are then absorbed by the canister. The ribs both help to make light less likely to escape, and improve heat transfer to the surrounding air.

On Tue, 11 Nov 2014 14:21:21 -0500, rickman Gave us:

You really don't know anything about laser power levels, do you?

I wouldn't risk anything here, it's hard to verify the proper operation of a beam dump, when the light is invisible. Buy a readily available optical beam dump. Example:

(probably available cheaper from other vendors, was just the first link I found)

Klaus

On Tue, 11 Nov 2014 19:37:29 +0000, Martin Brown Gave us:

Good suggestion. Instead of glass, I would use some high purity industrial diamond thingy.

Diamond has the best heat characteristic Of anything (short of neutron star material), and can handle a lot of laser energy before getting all violent about it.

Tried razor blades?

Mark L. Fergerson

On Tue, 11 Nov 2014 12:19:32 -0800 (PST), " snipped-for-privacy@bid.nes" Gave us:

Good idea! A stack of needles might work too, eh?

The angle of incidence for X-Ray detection is very slight as well.

Gotta use all new blades. Those dull ones would fire light back at you.

There are various kinds of beam dumps, some very efficient. With that much power density, one popular beam dump is a stack of Gillette Blue Blades, bolted together through the slot in the middle. Light has to make several bounces off a dark-coloured surface before it can escape, and it comes out at all sorts of angles. So something like that, mounted inside a black metal box, might be a a good choice. I don't know how good blue blades are at absorbing 1550 nm.

Another approach is to use a couple of pieces of black glass oriented at Brewster's angle. The problem with that is the power density, so you could expand the beam with a lens or curved mirror, and then absorb it.

Krylon #1602 ultraflat black spray paint is a spectacularly good index match to fused silica, so interestingly it's a much better absorber for light coming in through the smooth glass/paint interface than through the rough air/paint side.

It's carbon-loaded, so it absorbs at all wavelengths, but I wouldn't trust it at power densities over about 100-200 mW/cm**2 (i.e. a couple of times zenith sunlight).

Cheers

Phil Hobbs

Cone dumps work at about the -40 dB level, but black anodize isn't black at 1500 nm. It's an organic dye. A shiny metal cone, stainless steel or something like that, and a carbon black-painted housing would work pretty well.

Black copper oxide probably works OK in the IR too.

Cheers

Phil Hobbs

I have a nice photograph (monochrome unfortunately) of a proton beam from a cyclotron emerging into air. I think it was taken in the 1950s when peopl e occupied the same space as such things. It looks like the flame from a l arge welding torch. My father worked on such things a long time ago.

John

For one or two watts of near to mid IR I use a carbon cone or disk inserte d in a heat-sinked aluminum tube, say 30 mm long. I typically make my carb on out of a half inch carbon welding rod. This is readily available at weld ing and compressed gas sellers. Its butter soft and easy to cut with a hack saw or Dremel tool. Keep it wet when cutting as the dust is a fine powder. I shape mine on a lathe.

-------------- The stack of razor blades works very well. I've used them to 5 Joules no pr oblems. In my opinion, the best come from Thorlabs. Who also has the most c ost effective IR viewing disks.

-----------------------------

At 1-2 watts the goal is to simply break up coherence and scatter the beam.

Diffusers made of ~1.5 to 2 mm diameter glass necklace beads from arts and crafts store work well. You just need to trap them behind a window. Windows are easily made by taking a round mirror from the craft store and strippin g the paint off the back. A dip in dilute bleach then removes the aluminizi ng from the glass.

Beads then set in a well in a aluminum block capped with a glued on widow. If I'm in my own lab, they go in clear Tic-Tak boxes, even at high power.

With a uncoated window, You still have a 8% ghost beam, but this works grea t when I need a cheap attenuator for a detector.

The beads are 2$ a bag, so this is cost effective. I've used this up to a J oule of pulsed power with no plasma formed at ~3 nS pulses.

Steve

You don't anything about anything. Hey, can you hit those urinals on your way out...

I should clarify, my bead attenuators use the small, clear, necklace beads with the hole in the center to provide scatter. These are not pure spheres, they are well suited to diffusion and scattering.

The depth of the beads controls the attenuation. You may also crush the beads for greater attenuation per a given volume. Or add silica sand.

When I use the bead scattering scheme with a window, I tilt the ghost beam into another dump.

At 1-2 Watts CW this is very feasible. At higher powers professional discretion is advised and a safety case should be made.

A good paper on Maximum Permissible Exposure calculations is here:

Steve

Stack of razor blades.

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

void _-void-_ in the obvious place

On Tue, 11 Nov 2014 20:53:19 -0800 (PST), " snipped-for-privacy@null.com" Gave us:

Thanks for that!

Sand packed in a metal tube might work, too. I could poke the probe fiber in from the other end, until I get a reasonable signal level to scope.

There might be a few picoseconds of scattering, but that wouldn't be a problem in the current application.

The blackened copper tube would work similarly; poke in the fiber until the coupling is good. Ebay sells liquid brass/copper blackening stuff, and there's a great hardware store a couple of blocks away.