Measuring Wavelength?

What is the best/cheapest way to accurately measure the wavelength of a laser?

Cappernicus said:

What is the best/cheapest way to accurately measure the wavelength of a laser?

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you could find someone who owns a calibrated monochromoter.

If you have a Calibrated Thermal LPM and a Coherent LaserCheck LPM...
Take a reading of the (lets say red 660nm) laser with the Thermal LPM
and note it... (Let's say 100mW)
Now take the LaserCheck and set it for 660nm and take a reading
if it is not 100mW then change the LC Wavelength up or down until
you get a reading of 100mW...
That wavelength selected to get 100mW on the LaserCheck should
be the wavelength of that laser...

I'm not sure how accurate this is but my Dilda was 655nm not 660nm..
measured this way...

BTW... Best... Cheapest... Accurate... should not be used in the
same sentence... IMHO


Jerry

If you have a simple diffraction grating, just simple slits in it of even spacing, that you have all the numbers for, all the spacings and such, and a measuring stick, then you can find it that way with a little math.

You could also theoretically do it with a prism, but that's tougher, and you need more data for it (you need the exact fit for wavelength vs. index of refraction for that).

But you can buy a diffraction grating, and do the rest with a ruler/measuring tape and a calculator.

i would like to ask the OP...Why would you want to measure the wavelength? Define accuarcy?

because these words rarely work together well when talking about lasers.

"best/cheapest accurately"

Depends on how accurate you want to get, but a diffraction grating is the way to go.  You should be able to get something for around $10.  

This Hand-Held Spectroscope http://www.teachersource.com/LightAndColor/Spectra/HandHeldSpectroscope.aspx for $10.95 contains a diffraction grating.  With this instrument you may be able to get within 50 or 75 nm.   You put the laser spot on the wall, look at the spot (not into the beam) with the spectroscope, and read the wavelength off the scale.

To get more accurate, you can pull the diffraction grating out of the above or just buy a whole sheet of the stuff for $6.75, cut out a little piece and sell the rest.  http://www.teachersource.com/LightAndColor/Spectra/SingleAxisDiffractionGrating.aspx

The idea is long base line.  Shoot your laser beam through the grating at a wall 10 ft. away.  You get a spot on the wall that is straight on, and spots to the left and right that are 4 feet away.  This distance is proportional to your wavelength, and you can read changes of less than 1 nm this way using a tape measure.  Easy, cheap.  To get accurate absolute measurements you will need to calibrate it and watch for temperature drifts, but very doable. There is a lot of info on the web about this.  You can start here:  http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/gratcal.html

Thanks for the replies.

I guess I should have been a little more specific. I would like to measure the wavelength of a given laser to within 1 or 2 nm. I know Ocean Optics produces a spectrometer that is excellent, and even graphs it on the computer. However, at $2600 it's WAY out of my price range: http://www.oceanoptics.com/Products/usb2000.asp If you know of any others for cheaper let me know.

I am well aware that those three words shouldn't be used together but I figured I'd throw them out there just in case, and to get different ideas that favor one or the other so I can compare my options. I'm still new to this hobby so I don't know much, but I would like to learn.

Warske, thanks for the info. It sounds great! I may be understanding it wrong, but the problem I see would be calibrating it so I can tell what wavelength it actually is, not just the changes.

Warske said:

To get accurate absolute measurements you will need to calibrate it and watch for temperature drifts, but very doable. There is a lot of info on the web about this.

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Cappernicus said:

I may be understanding it wrong, but the problem I see would be calibrating it so I can tell what wavelength it actually is, not just the changes.

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As I mentioned, this is easy to find on the web.  Google: [highlight]calibrating diffraction grating[/highlight]
and you get 80,700 hits.  A good one is http://it.stlawu.edu/~physics/labs/152_lab/h_spectrum.pdf

Calibrating the diffraction grating:

1. You will first calibrate your diffraction grating using the light from a He–Ne laser (w = 632.8 nm). Set up the laser and grating as shown below (the grating side of the glass slide should face the laser). Align the laser perpendicular to the wall by holding a mirror against the wall and making sure that the light beam retraces its path back to the laser. Aim the laser near the center of the grating, and align the grating perpendicular to the laser beam by making sure that the diffracting maxima are located at equal distances from the central bright spot.

2. Use the grating equation nw = d sin t to find d, the spacing between each slit of the grating. Use both first– (n = 1) and second–order (n = 2) maxima (don’t use the small angle approximation here!). Calculate an average value for d.

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The point is:  the He–Ne laser has a wavelength of exactly 632.8 nm.  That gives you a calibration point, and you only need one.

There have been He–Ne lasers on ebay selling for $30 or less.  Total cost: about $40.  If you don't want to spend that much, there are obviously other ways to do it.  You could probably use the spectral lines from the sun, for example, or a sodium vapor lamp, or a neon lamp.  You could try a Google search.

Be aware that what you will be measuring, the distance between two dots on a wall, is proportional to the line spacing on the grating.  If the grating expands, the line spacing will change.  The grating will expand in a predictable way with increases in temperature.  So you want to watch that your room temperature is relatively constant, and that your laser power isn't so high that you heat the grating (its basically transparent, so it normally wouldn't be an issue).  If the laser power is so high that it heats the grating, attenuate the beam by reflecting it off a scrap of glass.  Added cost:  ten cents.

It is a myth that this sort of thing needs to be expensive for the hobbyist.  The cheap way usually does  take more time and creativity, though.  If you are willing to invest that, you can often get MORE accurate results than with the expensive equipment.

Finally, if you do make progress with this, please report back!  Others will be interested, including myself.

You could also probably use green DPSS for calibration if you want 1nm precision and you don't have a HeNe. :-/

Yeah, all DPSS and gas lasers have a consistent wavelength that can be used for calibrating.

I've got one of those hand-held diffraction-based spectrometers, and it's utter Horse sh**. The value you get is +/- 70nm on mine, and I can do better just judging from the color. Maybe they just built mine wrong, but I have thus far been unsuccessful at any disassembly attempts. I think the plastic is melted together.

im still looking for a reasonable reason why he wants to measure it. If it is for work and precision is important it has to be done one way. If it is ballpark it is done another way. if it is just for grins it can be done the ways listed here. It is all about the desired result.

Cyparagon said:

I think the plastic is melted together.

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That's what saws are for.

Again, thanks for the info. I think one of the hardest parts for me will be the math. I'm not that great with math. And what does it mean about the slits in the grating?

Laserman532 said:

im still looking for a reasonable reason why he wants to measure it.  If it is for work and precision is important it has to be done one way.  If it is ballpark it is done another way.  if it is just for grins it can be done the ways listed here.   It is all about the desired result.

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Just for grins.

I just want it to measure my own lasers, so it's no biggie. I guess you could attribute it to my perfectionism.

Laserman532 said:

If it is for work and precision is important it has to be done one way.  If it is ballpark it is done another way.  if it is just for grins it can be done the ways listed here.

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Why do you think the ways listed here would be less accurate than using a piece of pre-built lab equipment?

Cappernicus said:

[quote author=Laserman532 link=1239067307/0#10 date=1239122061]im still looking for a reasonable reason why he wants to measure it.  If it is for work and precision is important it has to be done one way.  If it is ballpark it is done another way.  if it is just for grins it can be done the ways listed here.   It is all about the desired result.

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Just for grins.

I just want it to measure my own lasers, so it's no biggie. I guess you could attribute it to my perfectionism.[/quote]

well if you are not trying to be exact, you should try one of the suggestions, it would be a great learning experience.

Or you could use my method... i turn it on and say that looks red, or that looks green, or that looks blue ;D (JK of course)

Warske said:

[quote author=Laserman532 link=1239067307/0#10 date=1239122061]If it is for work and precision is important it has to be done one way.  If it is ballpark it is done another way.  if it is just for grins it can be done the ways listed here.

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Why do you think the ways listed here would be less accurate than using a piece of pre-built lab equipment?  [/quote]

I will assure you, for a fact they will be less accurate. It is all about resolution, how much resolution do you need to "resolve" your question. read my first point...how many decimal points do you want to figure out pi? You are limited to the resolution of the measurement device, or the precision of your technique. I worked on single frequency scanning ring dye lasers. We needed $25,000.00 worth of equipment just to see what the laser was actually doing...it is all about resolution. But this post goes light years beyond the scope of the resolution the OP is desiring.