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Could you help me please, I think I will be crazy !
I own two 473nm Dpss, one with the diode polarised vertical and the other horizontal...but the two are polarised as the same on output !!
when I put a pbs cube the spot goes at 90° with the two lasers !
A friends give me a polarised filter, I've try, they have the same polarisation...
use a nice 10-15mm thick alum plate and mount one module sideways, very easy and cheap.
Oh no !
I want to understand first !
But sure, it's a soluce..
Maybe there's a halfwave plate on the output end of the optical train on each of the lasers? If this was the case it wouldn't matter what orientation the diodes were as the halfwave plate would rotate the polarisation on output to whatever orientation the plates were set to.
Jem
Only an opinion from what I have learned...mostly from Sam's Faq and hands on........
The actual polorisation factor lies within the lasing medium...IE the BiBo crystal. As long as the pump diode, polarity of the beam if you will, is 45 deg to the optimal lasing axis of the Nd:yag....then the lasing polorisation is set in stone there. The propogation of the waves generated from there are absorbed inside the second harmonic crystal. This is where the true output standing wave of polarity is set , all while the 946 and 473 are travelling back and forth through the whole cavity and the yag. So the diode will pump in 4 places 0 deg - 90 deg- 180 deg - 270 deg. without losing phase. They usually set the optimal position for the SHG and tune them as a marked set as installed from the manu. Usually an arrow on the top of the crystal. The crystal {BiBo} is controlling the output polarity.... They look like CNI's...should be polorised to better or near 100:1. With out rotation of the cavity it will have to be mounted on it's side to employ a PBS cube to combine them. Waveplates are lossy here....unless you spend big $ on a really nice piece.
thank you for your replies,now I've understand, and now......I need to mount it sideway :mad:
I'd say this way:
1. Nd:YAG absorbtion is not pump wavelength polarisation sensitive.
2. No matter what the pump LD position is Nd:YAG will lase at any polarisation.
3. In this case Nd:YAG lasing at 946nm is most likely lasing at random polarisation.
4. The LBO or BiBO SHG crystal IS polarisation sensitive, thus performs SHG only on fraction of 946nm available intracavity.
This implies that intracavity SHG crystal axes position determines output 473nm polarisation.
Hmmm, does this also mean that if one put two SHG crystals in single 946nm cavity but set for different polarisations, would get extra 473nm power ? :confused:
Piotr.K
and you need to re-fill your head with an inert gas. The crystals in there are hygroscopic ... :(
It should be fine the crystals are in a slead conatiner to keep things away from them like water , there is a lens at each end to let the light in and out , i thinkQuote:
Originally Posted by FourDee
You sure there isnt a brewster plate in there to force the YAG to lase with a polarisation that matches the SHG (May be machined right onto one of the crystals faces)?Quote:
Originally Posted by LesioQ
I would expect that without something to cause losses to the wrong polarisation, most of the 946 power would be produced in whatever polarisation the SHG does NOT use (more loop gain available here), so there must surely be somthing to force the issue intracavity.
Regards, Dan.
Not any one that I know of. At least not a brewster-inclined one :(
Piotr.K
It might be the case on more expensive models...
However, I've seen DPSS greens with *lousy* polarization ratios (we're talking 5:1 here). So it wouldn't surprise me to discover that those grossly inefficient cavities were simply sending the randomly-polarized 1064 into the KTP and just hoping for the best. And thus it follows that they'd do the same thing with cheaper blue models as well...
Adam
Remember that a blue cavity needs something to suppress the 1064 anyway or the yag will lase there instead of on the much lower gain 946 line, It might be possible to make that component (probably one or both cavity mirrors) polarisation sensitive.
946 is MUCH less gainy then 1064, so you will tend to get lower intracavity powers and hence MUCH lower conversion efficiency for a given non linear effect.
I strongly suspect that doubled 946 takes polarisation quite seriously even in the 'cheap' units.
Regards, Dan.
You would be surprised....the polorisation is random out of the yag to some extent with regards to the pump polarity. The issue I have seen is that some crystals are just a better cut than others. BiBo that is. Some crystals I have dropped into a cavity and no matter where it was it made blue photons. Others, most of them, only had a very small position that would peak in the 473/946 conversion. I mean small enough to make a 1/16 turn on the micro positioner and loose it totally. The crystal itself just regards 946 more so than 1064..... So getting a quality crystal was the big thing to me. But that is what you get with bulk lasers. They die...they get replaced.
Hum, Nd:vandate is supposed to be inherently 400:1, maybe not all blues use the more effective vandate? Nd:Yag does not have a fast and slow axis. Probably some are thus using nd:yag.
Steve
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Your right, I just looked it up. Its there, but much weaker then in YAG. Still,depending on if its type one or type two conversion, you still may have strong polarization.Quote:
Originally Posted by ElektroFreak
Steve