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Registered Member #151
Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153
I have an argon laser and I need to modulate it's light beam (not mechanical modulation). The problem is that it seems to be non polarized. I have an electro-optic modulator but it needs the input beam to be linearly polarized. I found here that PCAOMs also need liearly polarized light beam (I assume that it's similar with monochromatic AOMs). I know that I could put a polarizer between the laser and the modlator but then I will loose 50% of power...
I'm also not sure about the laser. The only thing I know is that the EOM doesn't work with the argon while it works with green DPSSL. I also checked the laser with polarizer from LCD display. I have a bit less than 50% of input power no matter how the polarizer is oriented, so it means that the light is non polarized or circularly polarized. The circular polarization in this case sounds strange, isn't it?
Registered Member #2372
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Posts: 62
It is surprising that the output isnt polarized, they usually are. If it is circularly polarized for some reason you can put a 1/4 waveplate in it to make it linear. I have used AOMs before and the direction of polarization didnt seem to matter, although I cant remember if I ever tried it with nonlinear polarized light. The EOM should work for any color of light, the voltage will just have to be adjusted. If it has an AR coating on it for 532nm light there will be some reflections if you use not that color light.
Registered Member #151
Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153
Thanks for the answer. The non polarized output surprised me too. The laser tube has internal mirrors so there's no brewster windows, probably that's why it's not linear polarization. I have no idea where to get a cheap 1/4 waveplate and what's more important I don't know if that would solve my problem (honestly, I doubt it because I don't think it's circularly polarized). The EOM has no coatings and the guy who sold it to me says that it should work fine with argons. And he's surprised about my nonlinear polarization too
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Indeed, it is very odd you have an unpolarized tube, unless it is an internal mirror type without Brewster stems.
One thing that is possible is to use a polarizing beam splitter to get 2 beams, 2 AOMs to modulate the vertical/horizontal beams, then recombine with another pbs (but thats a lot of optics!).
Are you sure the issue is just that the multiple wavelengths are not workign happily with your AOM, or that the AOM is tuned for the wrong frequency? It is my understanding that some AOMs are quite sensitive to wavelength, and can have a workable window of a few nm, so it might be adjusted for 532nm and just not work properly at 4xx nm. You also need to make sure that you have the aom at the right angle ('bragg angle') and that the beam is small enough to fall within the active area on the aom (which is always smaller than the clear aperture)
Registered Member #151
Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153
plazmatron wrote ... What model is this argon?
The sticker says it's National Laser Company model H61GBLc1i0. Here are some pics:
... wrote ... One thing that is possible is to use a polarizing beam splitter to get 2 beams, 2 AOMs to modulate the vertical/horizontal beams, then recombine with another pbs (but thats a lot of optics!).
Yea, too much of expensive optics ... I'd rather buy a 473nm DPSSL :P (I'm building an RGB projector)
... wrote ... Are you sure the issue is just that the multiple wavelengths are not workign happily with your AOM, or that the AOM is tuned for the wrong frequency? It is my understanding that some AOMs are quite sensitive to wavelength, and can have a workable window of a few nm, so it might be adjusted for 532nm and just not work properly at 4xx nm. You also need to make sure that you have the aom at the right angle ('bragg angle') and that the beam is small enough to fall within the active area on the aom (which is always smaller than the clear aperture)
I don't have an AOM. I have an EOM. There's no tuning besides setting proper voltages for max and min attenuaion. It depends on the wavelength indeed, but i'm changin the voltage by a potentiometer from -150 to 150V and theres absolutely no reaction with argon laser beam. I just checked the EOM with linearly polarized 532nm to check if it works. I also checked it with argon and polarizer from LCD and it works too, so the problem is laser polarization.
I disassembled the front of laser yesterday and there's nothing besides a beamsplitter for power sensor in the beam's way (I was looking for quarter waveplate).
Registered Member #2372
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If you can get access to the cavity you could put a polarizer in the cavity which would give you polarized light without a reduction in output power, if it is sealed that wouldnt really work.
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Modulation of nonpolarized laser beam - how?
