# Line width of green DPSS laser

Could someone suggest what is the line width/mode stability/coherence length of generic cheapo green (532nm) DPSS laser?

I've found lots of nice info here: http://redlum.xohp.pagesperso-orange.fr/laser/modeanalysis.html but there are no any DPSS lasers.

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You may build an interferometer as you split it through a mirror and your optical pathlengths differ < 1 cm and you want to do no serious science. However cheap green laser pointers are not ideal:

• It is no single-mode light.
• Extensive mode hopping and power modulation drifting with temperature
• awfull beam profile, you need a tiny hole in a foil to get an acceptable profile and be able to focus it with optics (the rest is lost)
• the infrared light is allways a risk to your eye if it is not correctly filterd out

I suggest using a HeNe laser tube (ebay 20€) instead of a chinese laser pointer (ebay 30€?)

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wait wait wait, mode hopping in DPSS laser? This should be the case only due to thermal expansion of the Nd:YAG crystal itself, and it it hopefully easily controlled on lower power and stabilized cooling... IR light is out of focus, so not a critical issue... As for cheap NeHe - they might easily be not tuned into single mode too... –  BarsMonster Feb 8 '12 at 21:20
I connected my chinese laser pointer labeled <8mW to a laser power meter with a photo diode and got heavy spikes on the oscilloscope. So I can confirm no stable single-mode operation. But I do not connected it to the wavemeter. However several sources states that this pointers are not suitable for e.g. holographic imaging. For your application of an interferometer single-mode or not isn't relevant. It's spatial and temporal coherence. You may use an interferometer with white light from Xenon lamps if the paths are within few microns. So my guess on the < few centimeters for laserpointers. –  Alex1167623 Feb 8 '12 at 21:54
HeNe lasers with a tube length < 50 cm are single-mode by geometry. Warning! : Get a green laserpointer with proper IR filter. I used a prism to diffract it and an IR Viewer to get the laser power meter on the IR spot. I meassured 80mW, so 10times the green output power. This level is DANGEROUS to your eyes. You can not see 1064 nm radiation! –  Alex1167623 Feb 8 '12 at 22:01
yeah, I used a camera without IR filter to see 1064 & 808nm beams from green laser ) –  BarsMonster Feb 10 '12 at 6:28
The emitted beam from an Nd:YAG or Nd:YVO crystal is very pure 1064 nm light and the nonlinear crystal, typically potassium titanyl phosphate (or KTP for short), doubles it precisely to 532 nm ($\pm1$ nm I'd guess). You can find several diagrams of the internals of green laser pointers in the link above. My guess is that main effect of variations in the performance of the 808 nm pump diode would be to reduce the coupling efficiency and thus the output power, but I believe that Nd:YVO is relatively tolerant to such variations.