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optical question

Started by John Larkin November 11, 2014
On Tue, 11 Nov 2014 12:19:32 -0800 (PST), "nuny@bid.nes"
<Alien8752@gmail.com> Gave us:

>On Tuesday, November 11, 2014 11:07:18 AM UTC-8, John Larkin wrote: >> We will be firing up a semiconductor laser, at a couple of watts >> optical output, around 1500 nm. The output will be small diameter and >> well collimated, so it will be dangerous. >> >> We can bolt our driver and the tiny laser to a metal block, and put >> that into a metal box with a tight-fitting cover. >> >> What should we do with the light? If we just hit an anodized surface, >> a lot of it will bounce around. >> >> What do people do to dump laser power safely? > > Tried razor blades? > >http://www.mazepath.com/uncleal/blade.htm >
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.
On 11/11/2014 02:07 PM, John Larkin wrote:
> > > We will be firing up a semiconductor laser, at a couple of watts > optical output, around 1500 nm. The output will be small diameter and > well collimated, so it will be dangerous. > > We can bolt our driver and the tiny laser to a metal block, and put > that into a metal box with a tight-fitting cover. > > What should we do with the light? If we just hit an anodized surface, > a lot of it will bounce around. > > What do people do to dump laser power safely? > > We'll probably want to sneak in a multimode fiber somewhere, to sample > the optical waveform, too. Maybe a milliwatt there. > > Maybe a black version of an integrating sphere, with a tiny inlet? > Maybe a tapered tunnel, black anodized, an optical cul-de-sac? > > Maybe use a tiny ball lens, or some diffuser, to scatter the light > some? > >
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. http://tinyurl.com/l6ak9ny 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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On 11/11/2014 03:11 PM, Tom Swift wrote:
> John Larkin <jlarkin@highlandtechnology.com> wrote: > > >> We will be firing up a semiconductor laser, at a couple of watts >> optical output, around 1500 nm. The output will be small diameter and >> well collimated, so it will be dangerous. > >> We can bolt our driver and the tiny laser to a metal block, and put >> that into a metal box with a tight-fitting cover. > >> What should we do with the light? If we just hit an anodized surface, >> a lot of it will bounce around. > >> What do people do to dump laser power safely? > >> We'll probably want to sneak in a multimode fiber somewhere, to sample >> the optical waveform, too. Maybe a milliwatt there. > >> Maybe a black version of an integrating sphere, with a tiny inlet? >> Maybe a tapered tunnel, black anodized, an optical cul-de-sac? > >> Maybe use a tiny ball lens, or some diffuser, to scatter the light >> some? > > 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.
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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Tuesday, November 11, 2014 7:37:37 PM UTC, Martin Brown wrote:

> You ought to think about an interlock so the laser cannot be fired when=
=20
> the beam is not dumped into a safe load. I once walked in on someone=20 > with a He MIP flame with 700W of microwaves in a beautiful salmon pink=20 > flame with nothing between him and the radiation source. He defeated all=
=20
> the safety interlocks on the mesh faraday cage. Truly scary. >=20
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.=
=20
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.=20

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
On Tuesday, November 11, 2014 3:15:50 PM UTC-5, DecadentLinuxUserNumeroUno wrote:
> On Tue, 11 Nov 2014 14:21:21 -0500, rickman <gnuarm@gmail.com> Gave us: > > >I would expect any matte black surface would absorb most of the light > >and what isn't absorbed would be diffused. > > You really don't know anything about laser power levels, do you?
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:
http://safety.uchicago.edu/files/Laser%20MPE%20and%20NHZ%20Calculations.pdf

Steve 
On Tue, 11 Nov 2014 11:07:12 -0800, John Larkin
<jlarkin@highlandtechnology.com> wrote:

> > >We will be firing up a semiconductor laser, at a couple of watts >optical output, around 1500 nm. The output will be small diameter and >well collimated, so it will be dangerous. >
Stack of razor blades. Regards, Boris Mohar Got Knock? - see: Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca void _-void-_ in the obvious place --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com
On Tue, 11 Nov 2014 20:53:19 -0800 (PST), "duedilligence@null.com"
<sroberts6328@gmail.com> Gave us:

> A good paper on Maximum Permissible Exposure calculations is here: >http://safety.uchicago.edu/files/Laser%20MPE%20and%20NHZ%20Calculations.pdf > >Steve
Thanks for that!
On Tue, 11 Nov 2014 20:53:19 -0800 (PST), "duedilligence@null.com"
<sroberts6328@gmail.com> wrote:

>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: >http://safety.uchicago.edu/files/Laser%20MPE%20and%20NHZ%20Calculations.pdf > >Steve
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. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com