Lasercheck Readings

[cruch]

someone who has a IR filter laying around and dont need it?
I need one

[buffo]

Do you have a prism? If so, pass the beam of your DPSS laser through the prism and get as far downrange as you can. (down a long hallway or at the far end of a long room) Now set your lasercheck for 808nm and see if you get a spike just to one side of the green (or blue) dot that the refracted beam makes on the wall. If you get a spike on the lasercheck when the sensor is just to the side of the beam, then you are reading the IR that is leaking through the optics. (IR will be refracted LESS by the prism, so it will separate from the visible portion of the beam.) If you are leaking both 808 nm and 1064 nm, then you will get TWO spikes. One very close to the visible dot, and the other slightly farther away.

If you want to "calibrate" this setup so you'll know about where to place the wand, shine a bright light through the prism first and see where the rainbow pattern falls. The IR will be just past the red end. (And yes, the rate of refraction is linear with regard to wavelength, at least for the wavelenths we're talking about.)

Adam

[JimBo]

OK no problems there, I have a prism so can do that.

But I still wont know the genuine power of the laser because by the time it gets 20 feet away it will only be a fraction of the power

Jim

BTW will the filter on a colour wheel (I have some from junktronix) help?

[cruch]

can one of you please point me to he correct ir filter at edmund optics?
Will order one or 2 from him

[buffo]

OK no problems there, I have a prism so can do that.
But I still wont know the genuine power of the laser because by the time it gets 20 feet away it will only be a fraction of the power

Nah... Think about it for a second, Jimbo. 8)

Remember that one of the primary characteristics of a laser beam is LOW DIVERGENCE. So if you are 20 feet away from the laser (ie: down the hall or on the far side of the room), the beam shoulnd't be much larger than it was when it left the aperature of the head. So the power will be nearly the same. OK, it will spread out a little bit, and the prism may make that a little worse even, but the entire beam should still be small enough that it ALL falls on the sensor of your LaserCheck wand, so you will still get an accurate reading of total power. (Minus the tiny fraction of power that is lost due to dust in the air between you and the laser head, plus whatever losses you get from the prism itself.) Your reading will be accurate to within a couple %, which is close enough.

BTW will the filter on a colour wheel (I have some from junktronix) help?

I don't know anything about the color wheel you have. If it has an IR filter section, it might work. (I doubt that it has such a section though. If it does, it will look like transparent glass - perhaps with a slight yellowish tint.)

Note that even a good IR filter will block *some* of the visible light. Not much, but some. (Why, you ask? For the same reason you can't make a perfectly transparent pane of glass, or a perfectly reflective mirror.) Still, if you only loose a couple %, you're still fine. No filter is ever going to be 100% efficient, and you really don't care about a couple mw of leaking IR anyway. (You're not looking for 5 mw of leaking IR, but 500 mw of leaking IR!)

Adam

PS: Watch this thread that aijii started a couple weeks ago. He's looking for a source of reasonably-priced panes of glass that are A/R coated for visible light. If the deal goes through, you might want to ask him if he can also get some IR filters from the same company. It's just a thought... (Edmund Scientific sells IR filters, but I'm guessing that they'll be expensive - Edmund's stuff usually is.)

[Lasermad]

mmmmmm
when i check power using my laser check even 30cm can make a huge difference ? am i doing something wrong ?
EXAMPLE
250mW viasho , decent quality I beleive modules with IR filters
Checked at about 1-5cm reads 250-280mW ....move out to say 30cm drops to maybe around 170mW
Consistent large drops over many readings so i would say it typical asnd also similiar over two 250mW modules .

Anybody gettign similiar readings ..from what i have just read seems this should not be happening
Any thoughts

[buffo]

Ok - I just performed an experiment. First, I fired up one of my single-line Argon lasers and cranked the output up to 37.5 mw, measured about 6 inches away from the aperature on the head. Then I moved back about 12 feet or so (as far as I could get while still having a small enough beam spot that would entirely fit on the lasercheck sensor) and took a second reading. I got 36.8 mw.

Fair enough. I didn't expect a significant change because my Argon laser has single line optics installed, so there should't be anything in the output beam other than 488 nm light. Ok, time to try it on a DPSS laser.

I switched the wavelength selector on the lasercheck to 532 nm, and then I fired up my Lasever DPSS green laser and cranked it up to 72.8 mw (measured less than 1 foot away from the aperature on the head). Then I checked the power level 15 feet from the head: 71 mw. Finally, I checked it a third time at ~ 36 feet from the head, and it was reading 68.6 mw.

Note that these measurements were taken with me holding the lasercheck in my hand, so there is going to be some natural variation just due to the difference in incidence angle on the sensor. And my house is a little dusty (two cats, you see). But I did wait for the lasers to warm up for ~10 min before I took the measurements. Also, I re-checked the DPSS output at the head after the last reading, and it was very close to the first reading. (72.5 mw vs 72.8 mw initially)

Bottom line - the power does not change significantly with distance.

Lasermad - If you're loosing 30% of your power after only a foot or so, I'd say you've got a problem somewhere. My first suggestion would be to check it at larger intervals (say, 1 meter, 3 meters, and 5 meters) and see if the power continues to fall off, or if it stabalizes. Also, make sure you're holding the lasercheck wand as close to perpendicular to the beam as you can. Finally, make certain that the entire beam is falling on the sensor.

But either way, it appears that you laser isn't really making 250 mw of green. Do you have a prism that you can use to test it to see what the actual output is? (See above)

Adam

[JimBo]

So if there is a large difference between readings what will cause that?
Is the divergence of IR quite high, so as you move away from the apereture the IR beam is spreading by a much greater angle?

Jim

[aijii]

So if there is a large difference between readings what will cause that?
Is the divergence of IR quite high, so as you move away from the apereture the IR beam is spreading by a much greater angle?

Jim

Sort of.. The raw 808nm from the pump diode is not really collimated, so it diverges heavily as you move away, hence the reason you'd get a lower reading is because less of it fits onto the power-meter sensor the further away you are.

The 1064nm is collimated along with the 532nm, so is the most dangerous far away. But there wouldn't be anywhere near as much of it as the 1064....

In Lasermad's case .... I'd doubt that Viasho are cheating by using an IR filter that lets through a bit of 808nm to up the readings ... but who knows... just yell at them. It usually fixes things

[Spec]

So if there is a large difference between readings what will cause that?
Is the divergence of IR quite high, so as you move away from the apereture the IR beam is spreading by a much greater angle?

Jim

The IR beam in question is usually slightly more divergent than 532nM beam, fairly common with non-folded laser cavities and optical elements that are not achromats.

Yes Lasermad, what you describe is indicative of IR present in the beam and is consistent with what the lasercheck will read.

For every laser manufacturer that produces a laser with IR filters, there are many more that do not or offer an IR filter at an added cost. I know Viasho charges for the IR filter addition.

While it is true that there is very little 808nM beam that makes it to the output coupler, the reasons are different. Most of the NLO crystal(whatever they may be using) is usually coated on one side to be AR808 and HR808/AR1064 on the other side. Best way to think of this like a filter sandwiched right on the crystal to make more passes through the cavity as that will increase output.

The 1064nM beam diverges more rapidly than the 532nM beam *unless* you have achromatic optics as the coupler. Its the basic nature of optical cavities.

The losses should be negligible (few mW at worst) with *only* a 532nM beam present.