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Thread: Polarisation

  1. #1
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    Default Polarisation

    Are all lasers polarised?

    Jim

  2. #2
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    Yes and no... It depends on whether you mean "polarized" or "stable polarization" (There is a big difference.)

    Nearly all lasers emit polarized light in some form, but that doesn't mean that the polarization is always the same. If the laser is rated as having "random polarization", that means that the polarization changes as the laser operates, and thus you can't know what the polarization will be at any given moment. Still, at any given moment there is a majority of the power to be found in one of the polarization states, so the light is technically still polarized. (Very few lasers emit 100% unpolarized light, though lots of randomly polarized lasers will rapidly switch between polarization states many times per second!)

    Lasers that exibit fixed or stable polarization do not change the polarization angle as they operate. Ion lasers, and the direct-injection red diode lasers that are often combined in pairs using a polarizing beam-splitting cube are good examples. Conversly, most DPSS lasers exhibit random polarization.

    Here is a link to a short article explaining the basics.

    Adam

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    So I wont be able to combine 2 random reds?
    If I want 400mW will I have to buy a specific type that can be combined ith a cube?

    Jim

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    If you buy two maxyzmods they will give you two different polarities...
    Love, peace, and grease,

    allthat... aka: aaron@pangolin

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    But they'll be stationary polarities. I started to do this with 150mw red modules from Wicked Lasers and a polarizing beam splitter cube. By hand everything was going well, but I burnt up one of the diodes while mounting everything :cry: stupid static

    Anyway, you just literally rotate the laser until you get either reflection or transmission. If you have a power meter you can get optimized results. But yea alignment will be a big pain.

    As for 2 random modules... I guess it could be possible, but it all depends on how polarized they are (some are 100:1, some are 50:1, some are close to 1:1 which is pretty much unpolarized if I'm not mistaken).

    Also remember beamsplitters are not 100%, so there will always be some loss. For instance, I wouldn't get a 300mw laser, but it would definitely be more powerful than either of the 150mw

  6. #6
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    Quote Originally Posted by JimBo
    So I wont be able to combine 2 random reds?
    If I want 400mW will I have to buy a specific type that can be combined ith a cube?
    If the two reds you buy exhibit randompolarization, then you will NOT be able to combine them with a PBS cube. (In order to combine them, the polarizations need to be 90 degrees out of phase from each other. If the polarization is constantly changing you can't combine them.)

    As allthat pointed out, the Maxyz modules have fixed polarization. If you tell them you want two of them to combine into a single beam, they will mount one so the beam is horizontally polarized, and the other so it's vertically polarized. (Just a matter of twisting the diode in the mount...)

    Adam

  7. #7
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    I always wondered if there are no beam combiners, working partly like polarizing beam splitters, that you can use to combine 4 beams.
    In other words: vertical, 45, horizontal and -45.
    That sould be usefull for combining upto higher powers.

  8. #8
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    jeejeeder;

    Polarized light needs to be 90 degrees out of phase for a polarizing filter to work. If you are only 45 degrees out of phase, half the light will pass through and the other half won't. So, using your example, you'd get half the output of all 4 lasers, which would be the same as 100% from just two lasers. (Actually, you'd end up with a lot less than 1/2 of the four once you factored in the optical losses and so fourth.)

    The bottom line is that you can't combine more than two beams of the same wavelength using a polarizing beam splitting cube and end up with more power than you would get from just using two beams.

    You can test this phenomenon yourself with a cheap pair of polarizing sunglasses. Take the lenses out and stack them on top of one another. Now, while you are looking through both lenses, slowly rotate one lens through 90 degrees. You'll find that the amount of light transmitted won't change very much until right before you get the first lens perpendicular to the other one. Once they are perpendicular, the amount of transmitted light will drop off dramatically. That's because only vertically polarized light is getting through the first lens, and since the second one is oriented horizontally now, it blocks nearly all the light. (subject to the limits of the polarizing filter itself...)

    Adam

  9. #9
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    :lol:

    I'll whip out the dumbass twice in ten minutes... could you tilt two of your lasers at a 45 degree angle and mixem that way, or do we have the 50% loss problem? Or maybe we make a light node and it all cancels it's self out...

  10. #10
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    Quote Originally Posted by Buffo
    jeejeeder;

    Polarized light needs to be 90 degrees out of phase for a polarizing filter to work. If you are only 45 degrees out of phase, half the light will pass through and the other half won't. So, using your example, you'd get half the output of all 4 lasers, which would be the same as 100% from just two lasers. (Actually, you'd end up with a lot less than 1/2 of the four once you factored in the optical losses and so fourth.)

    The bottom line is that you can't combine more than two beams of the same wavelength using a polarizing beam splitting cube and end up with more power than you would get from just using two beams.

    You can test this phenomenon yourself with a cheap pair of polarizing sunglasses. Take the lenses out and stack them on top of one another. Now, while you are looking through both lenses, slowly rotate one lens through 90 degrees. You'll find that the amount of light transmitted won't change very much until right before you get the first lens perpendicular to the other one. Once they are perpendicular, the amount of transmitted light will drop off dramatically. That's because only vertically polarized light is getting through the first lens, and since the second one is oriented horizontally now, it blocks nearly all the light. (subject to the limits of the polarizing filter itself...)

    Adam
    The story you tell is corerct. What I ment however is the following:
    A normal polarizing beam splitter splits the incoming beam into a vertical and horizontal polarized beam. 90 apart.
    Maybe some kind of christal exists with 4 faces that splits an incoming beam into 4 sperate beam with a polarisation 45 apart (* as input and |, -, / and \ polarisation as output). Horizontal or vertical polarisation is just a name we give to the two planes 90 from each other. So maybe that exists. So combining would then be that the polarisation plane is 45 apart each time.

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