How does a waveplate work?

[LaserLover]

This is misleading at best, if not outright false. "Some form" can include random polarization, which means no polarization, strictly speaking. There are several different types of laser that have randomly-polarized output, making them unsuitable for use with PCAOM's or PBS cubes (at least for the purposes of combining two beams).

If we're talking an ion laser, then if you see brewster windows you can be certain that the output is polarized. If we're talking about a solid state laser, then the answer is "probably".

Nearly all single-emitter diodes are polarized, but with DPSS designs, the polarization efficiency is all over the map. You might have one that has a 1000:1 ratio (meaning that for every watt of light output that is polarized one way, you only have 1 milliwatt of light polarized the other way), or you might have one that has only a 5:1 ratio. Obviously the 1000:1 is the preferable case, if you plan to mix a pair of beams using a PBS.

Adam

I totally agree that not all lasers are created equal when it comes to polarization,but random polarization is a form of polarization (only it's all over the map) and unsuitable for combining using a PBS cube or any AOM. The point to take away from this discussion is to know the polarization specs of your laser in order to attain maximum efficiency. The higher the polarization ratio, the better.

Rick

[taggalucci]

So given the title of this thread is "How does a waveplate work?", it seems the answer is clear.

With regards to is a waveplate required "to get the best beamspecs for a quad setup", I'm starting to think the answer is "no, not really", unless you can't directly rotate the polarised laser source and would then need to use a waveplate to achieve this rotation seperately. I.e. I do not see how a waveplate could improve the polarisation ratio, unless I'm completely off the mark.

Also, with regards to the beamspecs of profile, diameter and divergence, am I also correct in thinking that waveplates would have little or no effect on these?

And on an associated but slightly off-topic query, are there ways to align multiple differing polarisations? E.g. a vertically and a horizontally polarised component within a single (prbably pre-combined) beam having one axis rotated to match the other? ...Or even unify random polarisations?

Assuming that having two linear polarised laser sources are required for PBS-combining and this seems a popular method for achieving increased power in specific violet, blue and red wavelengths, why aren't the polarisation specifications generally published in lasershow-oriented supplier data?

[LaserLover]

So given the title of this thread is "How does a waveplate work?", it seems the answer is clear.

With regards to is a waveplate required "to get the best beamspecs for a quad setup", I'm starting to think the answer is "no, not really", unless you can't directly rotate the polarised laser source and would then need to use a waveplate to achieve this rotation seperately. I.e. I do not see how a waveplate could improve the polarisation ratio, unless I'm completely off the mark.

Also, with regards to the beamspecs of profile, diameter and divergence, am I also correct in thinking that waveplates would have little or no effect on these?

And on an associated but slightly off-topic query, are there ways to align multiple differing polarisations? E.g. a vertically and a horizontally polarised component within a single (prbably pre-combined) beam having one axis rotated to match the other? ...Or even unify random polarisations?

Assuming that having two linear polarised laser sources are required for PBS-combining and this seems a popular method for achieving increased power in specific violet, blue and red wavelengths, why aren't the polarisation specifications generally published in lasershow-oriented supplier data?

You are correct. A waveplate is not required to combine polarized laser beams in a PBS cube unless you cannot physically rotate the lasers to match the polarization plane of each laser beam entry point on the PBS cube.

One case where I used a waveplate was with my Melles-Griot 457nm blue laser, which had 2 fixed beams output. I bought a anti-reflection coated waveplate and a AR coated PBS cube and dichroic mirror to combine the 2 beams. I found that I had to rotate the waveplate in order to get the maximum power output of the combined beams. The result was almost 400 mw output of beautiful 457nm blue. You can see the pictures here http://www.laserlover.zoomshare.com/...20DPSS%20Laser

A waveplate cannot affect the laser beam divergence If it's plano/plano flat on both sides but it would If it had other lens configurations (ie plano/convex, etc...)

There are too many laser suppliers who don't bother to publish all the specifications. If you don't ask, they won't volunteer the information. If they cannot provide an answer to your queries then don't buy the product.

[buffo]

I totally agree that not all lasers are created equal when it comes to polarization,but random polarization is a form of polarization (only it's all over the map) and unsuitable for combining using a PBS cube or any AOM.

This really should have been in your initial post. It wasn't, which is why I added it.

Saying "all lasers have some form of polarization" is easily interpreted by a new user to mean "all lasers are polarized". This is wrong, and will lead to endless frustration.

The fact that "random polarization" is considered a "type" of polarization is a linguistic oddity. In fact, randomly polarized is the same thing as un-polarized. This fact is not clear in your post, but it's quite important to understand.

The point to take away from this discussion is to know the polarization specs of your laser in order to attain maximum efficiency. The higher the polarization ratio, the better.

I completely agree.

Also, with regard to 1/4 wave plates, while it is *possible* to use them to adjust the polarization angle of an otherwise polarized laser, it's not an optimal solution by any means. More importantly, it's completely impractical if the laser is randomly polarized to start with, as the losses will be roughly 50% of the total output even with very expensive optics.

As a general rule, 1/4 wave plates are expensive and lossy. It's much easier to rotate one of the lasers by 90 degrees when setting up a pair to be used with a PBS cube.

And yes, if you spend a small fortune, you can get losses under 2% (assuming you start with a polarized laser), but if you have that kind of money for optics you'd be better off just buying a more powerful laser to start with rather than trying to combine two smaller (and cheaper) ones with a PBS cube.

Adam

[LaserLover]

This really should have been in your initial post. It wasn't, which is why I added it.

Saying "all lasers have some form of polarization" is easily interpreted by a new user to mean "all lasers are polarized". This is wrong, and will lead to endless frustration.

The fact that "random polarization" is considered a "type" of polarization is a linguistic oddity. In fact, randomly polarized is the same thing as un-polarized. This fact is not clear in your post, but it's quite important to understand.
I completely agree.

Also, with regard to 1/4 wave plates, while it is *possible* to use them to adjust the polarization angle of an otherwise polarized laser, it's not an optimal solution by any means. More importantly, it's completely impractical if the laser is randomly polarized to start with, as the losses will be roughly 50% of the total output even with very expensive optics.

As a general rule, 1/4 wave plates are expensive and lossy. It's much easier to rotate one of the lasers by 90 degrees when setting up a pair to be used with a PBS cube.

And yes, if you spend a small fortune, you can get losses under 2% (assuming you start with a polarized laser), but if you have that kind of money for optics you'd be better off just buying a more powerful laser to start with rather than trying to combine two smaller (and cheaper) ones with a PBS cube.

Adam

Mea culpa !
One more thing that I should have included regarding combining 2 linearly polarized lasers using a PBS cube. When shopping around for a good quality PBS cube, look at the specifications to make sure the peak wavelength closely matches the lasers you are combining and it has anti-reflection coatings. You should look for the maximum transmitted power at the wavelength you are using. A transmission value of 95% or greater is desired.
A high quality PBS cube can be very expensive . You may find some good quality PBS cubes in first generation laser video players (surplus market) that used Helium-Neon lasers as a source (to combine red lasers at 635,650,660nm).

Rick

[mixedgas]

Um, guys, lets fix something in this thread. A Quarter wave converts linear to circular polarization and vice versa. A Half Wave plate is what rotates linear polarization to a new angle. For a combining application you want half waves , NOT quarter waves.

And only cheap half wave plates are lossy.


Steve

[LaserLover]

Um, guys, lets fix something in this thread. A Quarter wave converts linear to circular polarization and vice versa. A Half Wave plate is what rotates linear polarization to a new angle. For a combining application you want half waves , NOT quarter waves.

And only cheap half wave plates are lossy.


Steve

The above statement regarding half wave plates is correct if the angle between the plane of linear polarization and the crystalline optic axis is exactly 45 degrees. For other angles the transformation is from linear to elliptical ( or vice versa).

Rick

[andythemechanic]

The above statement regarding half wave plates is correct if the angle between the plane of linear polarization and the crystalline optic axis is exactly 45 degrees. For other angles the transformation is from linear to elliptical ( or vice versa).

Rick

No, a half wave plate always rotates the polarization angle double the angle between the polarization angle of the incident wave and the fast axis of the wave plate. The linear polarization state is conserved.
Regarding losses, in most case not the losses from the waveplate itself are important but from it's alignment. When you hit the waveplate not exactly normal it will not act as an half wave plate but turn the incoming beam to an elliptical polarization state. On the next polarizing optical element after the waveplate (usually the PBS) you will lose than the part of the light which has not the proper linear polarization.

Andreas

[LaserLover]

No, a half wave plate always rotates the polarization angle double the angle between the polarization angle of the incident wave and the fast axis of the wave plate. The linear polarization state is conserved.
Regarding losses, in most case not the losses from the waveplate itself are important but from it's alignment. When you hit the waveplate not exactly normal it will not act as an half wave plate but turn the incoming beam to an elliptical polarization state. On the next polarizing optical element after the waveplate (usually the PBS) you will lose than the part of the light which has not the proper linear polarization.

Andreas

Sorry , that last statement I made should apply to quarter wave plates instead of half wave plates. Not enough sleep.

Rick