Newbie needs an intro to diffraction gratings

I am looking to acquire some diffraction gratings, but I want to be sure I understand them. Therefore I have two general questions:

1. How are specs for DGs given?
- How do I know the angle and amount of new dots? I.e how tightly spaced they are?

2. How are they usually mounted?
I understand from reading these forums that they are esentially just pieces of glass or plastic, but I wonder:
- Can I just hold one up in front of my laser pointer?
- Must it be close to the lens?

Thank you for your patience with my newbie questions.

Boulder said:

I am looking to acquire some diffraction gratings, but I want to be sure I understand them. Therefore I have two general questions:

1. How are specs for DGs given?
- How do I know the angle and amount of new dots? I.e how tightly spaced they are?

2. How are they usually mounted?
I understand from reading these forums that they are esentially just pieces of glass or plastic, but I wonder:
- Can I just hold one up in front of my laser pointer?
- Must it be close to the lens?

Thank you for your patience with my newbie questions.

Click to expand...

Well, I will just give you an answer from the experience I have with them so far. First, I got my fan diffraction gratings from Dragon Lasers, and after researching around, these seem to be the best gratings you can get for the money by far. They offer very crisp and sharp exiting beams, and you can just tell these are well made with how nice the beam quality is after exiting the grating. These are made from BK7 optical glass with a diffraction coating applied to one surface. Dragon Lasers gratings come in 2 styles, fan diffraction and matrix (dot) diffraction. I love the fan patterns so I got 2 of those. You can turn them and adjust the angle of the fan to however you like it. I use spring type clothespins to hold them and I can put them at any angle I want and on a flat surface they work well as a holder. I put them about 6-8 inches in front of the aperture and get great results. You can hold them in front of your aperture too but I photograph my lasers so I set them all up fixed. You can check these out here: Laser Beam Splitters - Diffraction Gratings

As far as the matrix (dot) gratings I did not get any of those from them but if you are looking for that pattern I am sure they are just as perfect as the fan gratings I purchased. You can see my results with the Dragon Lasers fan diffraction gratings here: http://laserpointerforums.com/f48/updated-589nm-445nm-532nm-650nm-dragon-lasers-fan-diffraction-gratings-95907.html

As far as a distance in front of the aperture, I haven't found you need to keep any particular distance, I keep it at least a few inches a way so I can keep track of the reflections coming off the back of the grating and I don't want any beams to enter back into my aperture. They are pretty straightforward optics really, easy to use in a variety of ways. Now as far as your specs and technical questions go, I will leave that for one of the higher end users here that have a lot more knowledge than I. I was not looking at any tech specs when I acquired mine, just went by review and photos people using them have taken. :beer:

Lux, Kudos I knew you would find this and help:beer:

Boulder said:

How are specs for DGs given?

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Lines per inch or lines per mm. A higher line count means fewer diffraction orders (dots) and greater spacing between them. They may be single axis (which produces a line of dots) or double axis (which produces a grid of dots).

Boulder said:

How are they usually mounted?

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Anywhere is fine, as long as the surface is roughly perpendicular to the beam.

Lux Nitida said:

...in 2 styles, fan diffraction and matrix (dot) diffraction. I love the fan patterns so I got 2 of those. You can turn them and adjust the angle of the fan to however you like it...

... I don't want any beams to enter back into my aperture...

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Thank you so much for answering Lux Nitida, and especially for providing the pictures. They tell a thousand words ;-)
Why do we not want light back into the aperture?

Cyparagon said:

Lines per inch or lines per mm. A higher line count means fewer diffraction orders (dots) and greater spacing between them. They may be single axis (which produces a line of dots) or double axis (which produces a grid of dots).

Anywhere is fine, as long as the surface is roughly perpendicular to the beam.

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Thank you too, Cyparagon.
So, just to make sure I got it correctly at Dragon Lasers the fan and the matrix is the same, only that fan type only repeats in one axis (like the pictures provided by Lux), and matrix in two (double)?
Is that it?

At Dragon Lasers I see that they give the lines/mm, which Cyparagon now has thought me indicates the diffraction orders. What are the other two? Blaze wavelength and angle?
http://www.dragonlasers.com/catalog/pdf/DL-DG-Catalog-1301.pdf

Thanks to you guys I am now able to understand a bit more about what I need, and how to express it:
I am trying to locate a DG which will split my beam on a double axis, and provide 0.5 meters between each dot at a 5 m range (ish). I'm guessing that means a rather low line/mm rating?

Boulder said:

Thank you so much for answering Lux Nitida, and especially for providing the pictures. They tell a thousand words ;-)
Why do we not want light back into the aperture?

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You are welcome! As far as not wanting any beams going back into your aperture, you never want to have a laser beam reenter the aperture because it could cause damage to the diode. There has been notable cases where laser light reflected back into the aperture caused optical damage to the diode and either killed it out right or caused it to LED. As far as any of your other counter-questions, I will leave that to the pro, Cyparagon. :beer:

Lux Nitida said:

You are welcome! As far as not wanting any beams going back into your aperture, you never want to have a laser beam reenter the aperture because it could cause damage to the diode. There has been notable cases where laser light reflected back into the aperture caused optical damage to the diode and either killed it out right or caused it to LED. As far as any of your other counter-questions, I will leave that to the pro, Cyparagon. :beer:

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Except for DPSS - don't need to worry with DPSS.

diachi said:

Except for DPSS - don't need to worry with DPSS.

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Oh yes, of course, I was having in mind only direct diodes when I wrote that.

Boulder said:

I am trying to locate a DG which will split my beam on a double axis, and provide 0.5 meters between each dot at a 5 m range (ish). I'm guessing that means a rather low line/mm rating?

Click to expand...

That depends on the color you're using. In my spectrometer thread, I used the equation d = λ / sin(arctan(S/L)). green DPSS for example:
d = 532*10^-9/sin(arctan(.5/5))
d = 5.3 micron slit spacing, when inverted, gives us 187,000 lines/m, or 187 lines per mm.

Repeating that process gives us 151 lines/mm for red, or 246 lines/mm for violet.

Another newbie question:

Does diffraction lenses require a collimated focus lens or a converging focus lens?


Basically I have a standard 12mm green laser module with a collimating focus lens and a line generator lens. I wanted to try replacing the line generator lense with a diffraction grating lens that generates 5 parallel lines instead of a single one. However, the resulting pattern is very diffracted and generates numerous lines instead of just 5 so I am thinking I need a different focus lens.

Lux Nitida said:

You are welcome! As far as not wanting any beams going back into your aperture, you never want to have a laser beam reenter the aperture because it could cause damage to the diode. There has been notable cases where laser light reflected back into the aperture caused optical damage to the diode and either killed it out right or caused it to LED. As far as any of your other counter-questions, I will leave that to the pro, Cyparagon. :beer:

Click to expand...

diachi said:

Except for DPSS - don't need to worry with DPSS.

Click to expand...

Thanks guys! So if I get a module, say a 658nm 100mw Red Laser Module, and 3D print a neat little DG holder which holds the DG close to the module it might be a problem over time? How do I solve this? What is DPSS?

Cyparagon said:

That depends on the color you're using. In my spectrometer thread, I used the equation d = λ / sin(arctan(S/L)). green DPSS for example:
d = 532*10^-9/sin(arctan(.5/5))
d = 5.3 micron slit spacing, when inverted, gives us 187,000 lines/m, or 187 lines per mm.

Repeating that process gives us 151 lines/mm for red, or 246 lines/mm for violet.

Click to expand...

Thank you sir! Especially for providing the equation