PBS confusion

[buffo]

A PBS cube will combine two beams that have opposite polarization. That is, if you have one beam that is vertically polarized, and another that is horizontally polarized, the cube will combine the two beams into a single output beam of roughly the same diameter as the input beams, but with roughly twice the output power - assuming both input beams are the same size and power to start with. (You loose a few percent power as the beams pass through the cube, and the optical face inside the cube can give you a little splash, which may increase the beam diameter slightly.) Of course, the output beam will have random polarization.

Also note that, while a PBS cube is normally used to mix two beams of the same wavelength, it can also combine two beams of different wavelengths. Now, I know you're asking, "Why would you want to do this with an expensive PBS cube when a cheap dichro would work just as well?" Here's an example:

Say you had a pair of red lasers, one at 660 nm and one at 650 nm, that you wanted to combine. You'd be hard pressed to find a dichro that would combine two lasers that are so close together in the spectrum - even if you had several hundred dollars to spend. But a PBS cube will work just fine, so long as both lasers are polarized. You simply rotate the mounts on the lasers so one beam is horizontally polarized and the other is vertically polarized, and you're all set!

Adam

[frank_1257]

Are PBS cubes like dichroic mirrors in that they vary considerably in quality of the reflective materials and are some very bad about losses of energy or disproportionate splitting combing efficiencies? My problems is trying to split very high power 808nm beams to pump YVO4 crystals from multimode fibers all the wrong polarization energy hitting my crystals turns into wasted heat so why not split the randomly polarized beam in to two polarizations and pump 2 crystals and then pbs combine both doubled lasers into one! The problem is that the 808nm pbs cubes are only taking about 12 watts before catching on fire not the 40 watts I need. I guess I will have to find better cubes or I will have to cool mine with a peltier?.

[frank_1257]

Guess your saying hitting a pbs with randomised energy will not result in two distinct polarizations ? Correct ? Shite?

Hooking up with some of the local fellows is a good suggestion.!

I don't thing there's going to be any "standard" configuration you can test for a catch-all guideline, since there's a LOT of variation in optics. You'll need to test the specific lasers you plan on using with specific PBS cubes designed for their wavelengths.

Also, my (limited) understanding of the theory behind the use of PBS cubes is you can't take just any 2 lasers of the same wavelength for it to work - they have to have very specific polarized output beam characteristics. The details of THAT, unfortunately, I can't be much help with, but hopefully some of the more knowledgeable folks will chime in with some more info...

[Stuka]

I'll defer to someone more knowledgeable -

you pretty much exceeded my current knowledge base at "high power 808nm beams to pump YVO4 crystals from multimode fibers"

[buffo]

Guess your saying hitting a pbs with randomised energy will not result in two distinct polarizations ? Correct ? Shite?

A PBS cube is designed to take a randomly polarized beam and separate it into a horizontally polarized beam and a vertically polarized beam.

Laserists typically use one in reverse to combine the polarized beams from two separate lasers in order to get a single beam of greater power but roughly the same beam diameter. (The fact that the ouput beam has random polarization is irrelevant to us. We don't really care about polarization...)

Adam

[Loonatic]

You loose a few percent power as the beams pass through the cube, and the optical face inside the cube can give you a little splash, which may increase the beam diameter slightly.

Adam[/quote]

I gets this but what I want to know is what % of power is lost.

[Stuka]

You just answered it yourself!

See # 10 & # 11 of this thread - LOTS of variations possible!!

[buffo]

As Stuka said - lots of variables. Figure on losing 5% power from each beam and you'll be in the ballpark. If you have a really good cube you might do better. If you have a lousy cube, or the polarization ratio of your laser is lousy, then you'll loose more.

Adam