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Polarization expertise please

Morgan

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Hi All,

I have a couple of questions regards polarization please. I will try to be clear in text as I have limited access to pics but I hope you will all be able to understand my queries and offer some insights...

Firstly, the aim here is use PBS cubes to combine two magenta beams. Each single magenta beam will be achieved by using dichros to combine red and blue, (or BR), and then run the two magentas through the cube.

My questions are:

Is polarization directly related to die orientation? I.e. will two dies, of similar diodes, in the same orientation have the same polarization angle and is that angle perpendicular to fast and slow axes?

Illustration -

20zntar.jpg
20zntar.jpg


If the pins show this layout for two diodes, will the polarization angle be the same?

20zntar.jpg
rwp1sp.jpg


Will this layout show a 90 degree difference in polarization angle for diodes of different wavelengths?



The next question is, (depending on the answer to the first, this may be a moot point), Will different diodes show the same polarization when the pins are are in the same configuration? If we look at the first illustration and assume one is a red LD and one a BR.


Next, Looking at most sleds, if we take the line between the two outer pins, they seem to be aligned in either the x or y axiz with relation to the PBS cubes inside. This leads me to think I could simply align my diodes in this fashion and expect reasonable results without having to, 'tune', their orientation using an LPM before fixing their position. Is this correct?


I hope that's clear enough but if I need to clarify anything then please ask.


Many thanks all,

M
:)
 
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I have been wondering the same thing here too....
:bumpit: to see if we get an answer. It was my understanding that the orientation of the diode gives you the difference in polarization. One turned 90 degree from the other.
I'm sure someone has the answer here;)
 

anselm

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Talking out of my ass here, but(t) :D :
I'd assume all diodes (at least of the same wavelength, maybe even every TO56)
are built in the same way, have their optic cavity oriented the same way, etc.
So among themselves they would be polarized in just the same orientation.

Then if you'd want one passing through and one reflected, they'd have to be
turned 90 degress apart from each other.

Hope that makes some sense. I'm sure someone who actually knows what he's talking
about will correct me on this.:D
 

HIMNL9

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The polarization depend from the chip placement, and in the case of LDs, it's the same ..... also, is for this reason, mainly, that there are indentations on the surrounding of the case, so they can be placed always in the same sense in their heatsinks (industrially speaking, ofcourse, not for aixiz modules :p)

Put the cases in the same orientation, and you will end with the same polarization for all the beams.


EDIT: i realized i gave incomplete informations, sorry.

I suppose you are referring only about light polarization, that is usually indicated also as electrilcal field polarization, and it's parallel to the junction of the chip (there is also a magnetical polarization that is perpendicular to the chip junction, and coplanar with the fast axis).

This polarization is coplanar with the slow axis, and for standard 5,6mm cases, it's normally indicated from the two "V" shaped dents on the borders of the case (the red line in the image is the polarization plane), where usually the "squared" dent indicate the base-plate direction where the chip is soldered (not all the times, anyway, cause some manufacturers places 2 squared dents around the case, instead 1) .

So, yes, "V" shaped dents are more precises, but considering that the pins are usually aligned with enough precision, you can use also them (if they are not too much bent or displaced), for align polarizations.
 

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Benm

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Indeed, the polarization will always be the same relative to the markings on the diode casing for a give type of diode.

For your project you should therefor make 2 magenta beams, one beam with both diodes orientated straight up, and one with both diodes oriented sideways.

Direction of polarization is not incredibly critial though. You will obviously lose power when you get further away from the optimal position. Even if you are off 5 degrees, the loss would be only 1 percent.
 

Morgan

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@ HIMNL9 and Benm - Perfect expanation guys, +Rep to you both on that one. It is as I suspected but confirmation from those that know is always confidence building and now I know what those, "V", shape indentations indicate too. I would have done the experiments myself but simply didn't have the kit or time to do a proper test and log the outcomes.

One more question that didn't occur to me. Is the same true of DPSS modules? As the 808nm output is converted and, frequency doubled, does the polarization change in the crystal cavities? I know that crystal orientation is crucial for efficient conversion but would a poorly aligned set show a deflection as compared to another module and it's, "V", indentations?

Thanks again,

M
:)
 

Benm

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I disagree with the above... all green dpps lasers i have are polarized (easily checked with a polarizer). I don't know for sure if there is a defined relationship between the position of the module and the output polarization, but i doubt that would hold true for all dpss modules anyway.

And yes: there is a disticnt relationship between the polarization of the 1064 nm laser and the 532 doubled output, they are orthogonal.

I'm not entirely sure if there is also a relationship between the polarization of the 808 pump diode, and the orientation of the solid state laser. My gut feeling says there must be since the polarization of the pump light is important for performance, but i'm unsure what it is exactly.
 

Morgan

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Hmmm, okay, I'm taking the reps away! Hehe!

Not so clear cut on the greens then? No problem, I guess I'll have to check that one then. I don't know what Prototype said but I have seen polarisation on all the greens I've had but, of course, never put any figures to what I saw.

Thanks guys, any more input welcomed.

M
:)
 
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I erroneously stated that unless you paid for a certain polarization DPSS green, you were probably going to get a randomly polarized module.
 

Morgan

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^Ahh, I see. Thanks.

What I was trying to establish was whether I can assume that if I line up the, "V", indents, (or the pins), at the back of a 12mm standard 532nm DPSS modules with a red, blue or BR, whether the polarization would line up in the same way as discussed above.

It would be so convenient if it did work the same but somehow I suspect that just because it would be convenient, it won't be the case!!!

M
:)
 

HIMNL9

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^ Also, if you use a PBS for add the beams, you will forcedly end with one color polarized in one sense, and the other color polarized in the other sense (or, if you add 2 diodes of the same color, with a beam polarized on both the planes), cause is this the way in which PBSs works .....

In the example of the 2 magenta beams, you must use the same polarization for both the couples of red/BR diodes (one couple horizontal and the other couple vertical), checking that the dichros accepts both the polarization planes, and also be sure to use a broadband PBS (not like the ones in PHR sleds, as example, that are polarized dichroic combiners), and then you will end with a magenta polarized on both the planes .....

About the green modules, is difficult to get the polarization plane without make a test with a polarizer filter, cause the polarization depend from the crystal orientation, and the manufacturers don't care to indicate this, on the modules bodies.
 

Morgan

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Yes, I had looked into broadband PBS cubes for this reason. Edmund Optics do some. Expensive but maybe I can find them somewhere else too.

I guess the green will be more tricky then. I will put it all into the, "Thought lab", and see what comes out.

M
:)
 
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About the greens, some are polarised, some aren't. Most times they're randomly polarised, and you don't get a certain polarisation unless you ask the manufacturer.

In order to achieve maximum efficiency, the crystal must be polarisation-matched with the diode. This may or may not mean that the orientation of the diode will be completely random in respect to the crystal. Pump beam polarisation has no effect on output beam polarisation.

However, the output of the SHG optic will always be the same polarisation of the beam to be SHG'd.

You'll have to manually test for output polarisation, each module will be different.

That's why you see the crystal sets epoxied to a mount in cheap modules. Break the crystal set off, and begin to rotate it (wearing 808nm eye protection, of course!). You'll notice at one critical point there'll be a lot more green output. That's the optimal point for the Nd:YVO4, and at this stage the crystal set would be epoxied down at the factory.
 

Benm

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I think we should clarify the meaning of 'randomly polarized': It's often used in the same way as 'natural polarization', i.e. the light is a combination of waves of various polarization directions, like the light from the sun or a lightbulb.

But i can see how this can be confusing with 'polarized, but in a random direction' which is, afaik, the case with most green laser modules. Perhaps it is possible to purchase laser modules (i'm thinking lab style here) with a predefined direction of polarization at a premium.
 

HIMNL9

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Well, i'm not 100% sure (memory fades when peoples becomes olds :p), but, as far as i remember, vanadate does not alter incoming polarization, where instead KTP rotates it in the doubling process ..... i think it was around 45 degrees (but this for labby cavities with separated crystals, i don't know for sure the angles for "glued" crystals for cheap modules) ..... so, basically there's a little possibility to guess how much the polarization plane is coming out, looking at the diode pins ..... but, cause there's no way for know what working modality the crystal have in a commercial cheap module, and cause for some crystals the polarization changes also changing temperatures (it switch from, say, horizontal, to circular, to vertical, changing the temperature), the only way is to get a polarizer filter (also a cheap photographic one, or an old lens from a pair of polarized sunglasses, can work), and rotate it in front of the module, once it's stabilized in themperature, for know what polarization you're getting from it .....
 




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