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Old 02-17-2012, 11:28 AM #1
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Default determining wave lengths

along with my never ending quest for quantification, rhd's posts on using his spectrometer have me wanting to know just what wl's my lasers are putting out. i've gotten some "close" results with diffraction grating. using a (i hope) standard 532nm source; after repeated tries, using varied distance the mean results were near 532. so with careful linear measurement i can get near to the true wl using this method.
this isn't good enough tho....
is there another method to measure wl that gives more precision? or a way to refine the diffraction grating tec?
buying a laserbee to measure power was one thing, a spectrometer isn't in my budget, tho i haven't really priced them. but i'd like to be able to measure, say, how the wl changes with temperature changes etc. along with just what my lasers are emitting.


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Old 02-17-2012, 12:01 PM #2
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Default Re: determining wave lengths

There was a Thread not too long ago discussing alternative
ways to test Laser Wavelengths... Ask RHD he was involved
with that Thread... IIRC.


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Old 02-17-2012, 05:20 PM #3
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Default Re: determining wave lengths

We tried a few things with gratings and cameras, but it didn't work out (not linearly, anyway). Spectrometers themselves use diffraction gratings and the spacing of the colors to determine wavelength, so you're doing everything you can. All you can really do is make your measurements more precise.
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Old 02-17-2012, 08:38 PM #4
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Default Re: determining wave lengths

I don't think that callibrating a DIY spectrometer with one laser works. Or any spectrometer.

You'd need at least a blue, green and red laser of known wavelength. And from what I've seen '532' handhelds can be found anywhere from 530 - 540nm.

Your best bet would be to use single line gas lasers for the most accurate callibration.

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Old 02-17-2012, 09:42 PM #5
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Default Re: determining wave lengths

532nm handhelds are 532.05nm. Always. The Nd line is locked there. It does not drift with temperature or current like in the case of laser diodes.

You don't need calibration if you know the math. The angle of diffraction has physics behind it, and is therefore predictable. I made a thread explaining the details here.
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Old 02-17-2012, 10:19 PM #6
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Default Re: determining wave lengths

Arent HeNe reds right at 632.8nm, and quite stable?
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Old 02-17-2012, 10:43 PM #7
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Default Re: determining wave lengths

Yep, HeNes are great for calibration, and if you've got the $$$ there are many wavelengths.
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Old 02-17-2012, 11:54 PM #8
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Default Re: determining wave lengths

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Originally Posted by Cyparagon View Post
532nm handhelds are 532.05nm. Always. The Nd line is locked there. It does not drift with temperature or current like in the case of laser diodes.

You don't need calibration if you know the math. The angle of diffraction has physics behind it, and is therefore predictable. I made a thread explaining the details here.
Hmmm I could have sworn I saw a Spectro graph showing otherwise, but I can't seem to remember where so thanks for pointing that out.

Thanks for that link too, physics was never my strong point so I'll have to learn a bit more.

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Old 02-18-2012, 01:18 AM #9
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Default Re: determining wave lengths

LED museum measured a green at "531.880nm" recently, but that just means his calibration is off by a hair. He needs to learn about significant digits, too.
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Old 02-18-2012, 02:14 AM #10
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Default Re: determining wave lengths

Cool..

I just ordered a 10 pack of the 1000L/mm diffraction gratings.. I can pull a baseline off my 2-3mw HeNe I built (going on 17 years now) back in highschool.

I'm actually really curious to see just how much current increase changes the wavelength of solid state laser diodes. I have a half dozen A-140 445s left, a couple PHR 805 (or something like that ) 405nm ~100mw, and a few open can reds (Can't remember the name / series, but theyre common, ~150-200mw peak, and like $10 each)

I'm a total measurbating geek, and want to see just how much the wl of these LDs vary over current input. And see (if I can) just how close their wl specs are to their published wl specs at Imax.

I have an oldschool WickedLaser green "Executive" that peaks at like 60mw that I can use as another baseline, if the green DPSS are as clamped on output wl as the OP says.
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Old 02-21-2012, 07:25 AM #11
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Default Re: determining wave lengths

Green DPSS lasers don't vary a lot. Nd:YAG has a gain bandwidth of roughly 0.5nm, so it definately can't vary more than that. In practice I think the variation was around 0.1nm, but I'm not sure.

HeNe's are 632.9 or 632.8, depending if you take the vacuum wavelength or the wavelength in air.
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Old 02-22-2012, 10:23 PM #12
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Default Re: determining wave lengths

why does the wl get shorter in air? would other gas combo's (media the hene beam passes thru) do the same?
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Old 02-22-2012, 11:12 PM #13
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Default Re: determining wave lengths

Notwithstanding the fact that Cyparagon and I failed on our first few attempts, I'm fairly certain that it's doable to mathematically calculate wavelength based on a digital photo of dots going through a spectrometer.

Something that may have thrown off my earlier attempts, was assuming that I owned a 556nm DPSS unit. It ended up that this was a 561 (which I found out by using 4 other wavelengths that were rock solid to calibrate my spectrometer).

I'm going to retry the DIY spectrometer-less spectrometering tonight. I'm actually quite certain that it requires zero funky math, and basically just a copy of Excel. It will likely require 4 known wavelengths though - potentially just 3.
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Old 02-23-2012, 01:35 AM #14
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Default Re: determining wave lengths

Quote:
Originally Posted by cheech226 View Post
why does the wl get shorter in air?
Refraction - Wikipedia, the free encyclopedia

The wavelength of an 830nm beam while in glass is ~553nm. It doesn't actually turn it green though. Frequency remains constant.

Here's something you can try. Measure the distance between laser dots when set throught a grating at distance X from the grating. Then immerse the grating in water and repeat the measurement. Because the light is traveling through water instead of air this time, the wavelength is shortened greatly. Since the angle of diffraction is based on wavelength, not frequency, you will find the angle is much less than it was before.
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Old 02-23-2012, 09:31 AM #15
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Default Re: determining wave lengths

That's a pretty neat experiment. The index of refraction of water is closer to that of a plastic or glass transmission diffraction grating, that could affect the efficiency. And you should compensate for the refraction when a beam leaves the water. Still, I'll try this out when I get home.
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Old 02-23-2012, 07:07 PM #16
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Default Re: determining wave lengths

Here's what I didn't get around to doing, but should work:

1) Setup a DSLR in a fixed position pointing at a wall
2) Setup a grating in a fixed position near the DSLR
3) Shine a known wavelength through the center of the grating at a point on the wall that you label with a dot.
4) Make sure this point on the wall, as well as the second point generated by the grating, are within frame of the DSLR.
5) Shoot a photo.
6) Setup another known wavelength, point it at the same dot marked on the wall, and take another photo (do this for 3 or 4 total different known wavelengths, ie 473, 532, 589, 593.5)
7) Run through the same setup for your UNKNOWN wavelength.
8) In software, count the pixels between the two dots for each photograph, and plot them as X and Y values in Excel, like this:


PIXELS WAVELENGTH
313 473
731 532
1221 589
1271 593.5

8) Graph those as a line graph, and add a "Trendline" with Polynomial Order "3".
9) Display the equation on your chart, and then copy it down. You'll get something like "y = 2E-09x3 - 3E-05x2 + 0.1715x + 422.44"
10 Take the pixel count of your UNKNOWN wavelength as "x" and sub it into this equation.
11) Solve the equation, and "y" will be the wavelength of your unknown.

My feeling is that with a 3rd order polynomial, you're completely compensating for the tilt angle of the wall, the perspective skew of your camera's lensing, and the non-perfect angle of the actual diffraction grating projection. I probably wouldn't do it with 3 wavelengths, and in fact I'd probably try to avoid using both 589 and 593.5 as separate ones. If you had a HeNe, that would be a more appropriate 4th wavelength.
__________________
CURRENT WAVELENGTHS:
405|441|450|467|473|495|505|520|532|556|PGLRG|589|594|632|638|660


SAFETY & IMPORTANT READING (threads authored and contributed to by various members of LPF):
- Federal Aviation Administration - Laser Safety Initiative (link)
- Info release from Laserglow - Default Canadian restrictions on portable lasers (link)
- The Necessity of Safety Goggles (link)


Last edited by rhd; 02-23-2012 at 07:08 PM.
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