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Thread: four to one?

  1. #11
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    Never heard of a PBC. How do they work? What are the losses, price etc.?

    Sounds like a complicated setup and it would probably be both cheaper and easier to just buy a more powerful laser, at least at this power level. You can get a TTL laser modules at these level fairly cheap. They have better cooling and will most likely live longer than cheap laser pointers as well.

  2. #12
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    Cool

    Sorry guys, there is no free lunch. Once you combine a vertically-polarized beam with a horizontally polarized beam (using a PBS cube), you get a RANDOMLY POLARIZED beam.

    Now, if you sent that randomly polarized beam into the horizontally polarized face of a second PBS cube, half the energy in the beam (the vertically polarized component) will be reflected back. The same thing goes for a randomly polarized beam that is sent into the vertically polarized face of the PBS cube.

    So you see, you only get 1 shot with the PBS cube. Once you have a randomly polarized beam, further polarization-sensitive optics are useless because they will always reject half the beam's power.

    If you try to use a 1/4 wave plate or some other device to re-polarize your randomly polarized beam, you can certainly do so. However, you will loose 50% of the power. So now you're back to where you started from. (Actually, you're worse off, because of the optical losses.)

    Bottom line: you can use a BPS cube to mix 2 beams (or tight clusters of beams) ONE time, and one time only.

    What some people have done in an effort to get more power is to position a pair of beams very close together, so that you essentially have a single TEM01 or TEM10 beam. Do that with two horizontally polarized lasers, and do the same thing with a pair of vertically polarized lasers, and you can then combine 4 diodes into a single beam. But it will still be a TEM01 beam.

    For example, Arctos has lasers with 24 or more diodes combined into a matrix of beams. The 24-beam unit has a pair of TEM34 beams that are combined using a PBS cube, and the resulting output beam is then passed through several different optical elements to clean up the beam profile as best as possible.

    Finally, there is *one* exception to this rule that I should point out, even if it's totally impractical: If you have a pair of single longitudinal mode, frequency stabilized lasers, and you can control the frequency and the position of each laser with *exceptional* precision, then it is possible to combine the two beams using a PBS cube such that all the waves interfere with each other constructively inside the PBS cube and you'll get a vertically or horizontally polarized beam coming out of the cube.

    This is *incredibly* difficult and requires considerable effort, expertise, and expense. (Figure a solid 5 figures, minimum) But if you can precisely control the interference pattern at the cube face where the beams combine, you *can* get a vertically or horizontally polarized beam coming out of the PBS cube, which means you can do the combination more than once. More of that creepy quantum weirdness going on...

    But this is an *exceptional* case that is absolutely impractical for anything other than a major laboratory. You'll need a firm understanding of the quantum effects that control a lasers output frequency, as well as a way to mitigate those effects. I only mention it because it technically *is* possible, however impractical it may be, and because the topic has come up in discussions before.

    If you're really curious, here's a link to the USNET discussion that started it all. (Thanks to Rick, aka LaserLover, for posting the original information last October.)

    And Tocket, you hit the nail on the head. Most times it's cheaper to just buy a larger laser in the first place than to even try to fool around with a PBS cube. Unless you're right at the limit of what you can get out of a single laser (such as the case with the Maxyz modules), and you need to add a second one to get you to the power you need, a PBS cube is a waste of money and space. Buy a bigger laser instead.

    Adam

  3. #13
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    Ha, I completely agree... but I don't think I would ever call a PBS cube useless

    EDIT - Ooops Adam, I misread your post---

    In the case that we are discussing in the thread, yea, it would be more work than its worth. In this case buying a 200-300mw Greenie would probably be cheaper once you add the value your time is worth.

    Tocket, PBS cubes are fairly cheap and they're also nice to have a few around -AND- (shameless group buy ad here) they are available in the group buy!

  4. #14
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    Thanks for an answers!

    So, what would be smartest (=cheapest) place to buy a new laser diode? E.g. 150mW green laser 532nm costs in MediaLas about 550€ + vat...

  5. #15
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    Bart;

    Have a look at lasershowparts.com. They've got very competitive prices, they stand behind their products, and they are also members here on the forums and are quite active. (Remember that the prices on the website are quoted in Australian dollars...)

    550 Euros for a 150 mw green laser? That's insane! Expect to pay about half that price for most brands, or maybe just over half that price if you go with a laser from CNI.

    Adam

  6. #16
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    Just a new question without starting a new topic..

    I have one 1W Viasho laser, and there will be a second one in some time. I would like to try to combine the 2 beams to create a 2W green source. Is this possible? Can someone do me a children-style-explanation of PBS-cubes and other toys?

  7. #17
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    Cool

    JeroenVDV;

    To combine two lasers of the same wavelength, you need to start with a pair of lasers that produce a linearly polarized beam. You mount one laser so the beam is horizontally polarized and mount the other laser so it's beam is vertically polarized. Then you use a polarizing beam-splitting cube to combine the the two beams into a single beam. (This output beam will be randomly polarized.)

    How do you use a beam splitter to combine beams, you ask? Easy. You reverse the "splitting" process. A polarizing beam-splitting cube is designed to split an incoming beam into two separate beams. The reflective face inside the cube is designed so that only the horizontally-polarized waves will be reflected off 90 degrees to the side. So if you send a randomly polarized beam into the front of cube, you get a horizontally polarized beam coming out the side, and a vertically polarized beam coming out the back. Each beam will have roughly 50% of the input power.

    We simply reverse this process. We send a horizontally polarized beam into the side, and a vertically polarized beam into the back, and we end up with a randomly polarized beam coming out the front that has twice the power. (Well, almost twice the power. You do have some loss through the cube, of course.)

    So, to answer your question, yes - it will be possible to combine two 1 watt lasers to get nearly 2 watts of output. All you need is a PBS cube. (Well, to be perfectly accurate, you'll also need to be sure that both lasers output a linearly polarized beam, but just about all DPSS green lasers are linearly polarized, so this isn't a big deal.) The only other problem you might run into is how to mount the lasers so that one has a vertically polarized beam, while the other is horizontally polarized. (You may have to mount one laser sideways.)

    Adam

  8. #18
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    Quote Originally Posted by buffo View Post
    JeroenVDV;
    Just call me Jeroen

    We simply reverse this process. We send a horizontally polarized beam into the side, and a vertically polarized beam into the back, and we end up with a randomly polarized beam coming out the front that has twice the power. (Well, almost twice the power. You do have some loss through the cube, of course.)
    So, it works just like a dischro, except that it filters on polarity instead of wavelength?

    Well, to be perfectly accurate, you'll also need to be sure that both lasers output a linearly polarized beam, but just about all DPSS green lasers are linearly polarized, so this isn't a big deal.)
    That's the problem.. Let's assume I build a black box. 2 "inputs", 1 "output". So, basically, I don't know how the laserbeams are polarized when they enter the box. Is is possible to use waveplates or other magic parts to make sure they're polarized the same way? Is it possible to "rotate" the polarization by an optical part?

    I'll explain the whole concept later..

  9. #19
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    Smile

    Quote Originally Posted by JeroenVDV View Post
    Just call me Jeroen
    Hehe! OK Jeroen!
    So, it works just like a dichro, except that it filters on polarity instead of wavelength?
    Yeah, that's a good analogy. Dichro's filter based on wavelength, while the PBS cube (or more accurately, the polarizing diagonal face inside the cube) filters based on polarization.
    Let's assume I build a black box. 2 "inputs", 1 "output". So, basically, I don't know how the laserbeams are polarized when they enter the box. Is is possible to use waveplates or other magic parts to make sure they're polarized the same way? Is it possible to "rotate" the polarization by an optical part?
    You *can* rotate the polarization of a linearly polarized beam slightly by using a 1/4 wave plate. However, you will loose power each time you do it, so this isn't really practical. If your goal is to combine a pair of beams to get more output power, then you need to figure out the polarization of each laser first, and then mount them so that the horizontally polarized beam is striking the correct side of the PBS cube, and ditto for the vertically polarized beam. Trying to rotate the polarization after the beam exits the laser will waste power.

    To measure the polarization angle of the laser, all you need is the PBS cube and a power meter. (Assuming that the cube is marked as to which face is horizontal and which one is vertical.) Shine the laser at the horizontal face and measure the power in the output beam. Rotate the laser until you get the maximum power in the output beam. That is the position that will give you a horizontally-polarized beam.

    Now do the same thing with the other laser and use the vertically-polarized face of the cube. When you have both lasers mounted correctly, you should have nearly double the power in the output beam from the cube. (You will lose 1-2 % power through the cube due to optical losses in the cube.)

    In fact, even if the cube *isn't* marked, you can still do the experiment above. The only thing is that you won't really know which one is horizontal and which one is vertical. But you really don't care, so long as you've adjusted the mounts of both lasers so that you have maximum output through the cube.

    I hope this makes sense. I haven't had my morning coffee yet!

    Adam

  10. #20
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    OK, thanks for the great explanation.

    How precisely do I have to rotate the laser 90 degrees? What if it's 91 degrees? Will it completely NOT work or do I loose some percents of power?

    I'll post some renders/drawings of my idea later this day.

    It has already been lunch-time here, so I've had my cups of coffee already

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