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A Is laser light polarised? In which direction?

  1. Nov 1, 2016 #1
    Hello. I want to know about the polarisation of lasers and I've been reading basically two kinds of answers for the question 'Is laser light polarised?' The first one is 'yes' and the second one 'not necessarily'.

    Let's consider a gaussian laser. Ideally its light is monochromatic and coherent. Doesn't this already establish that it is polarised? When people say that laser light is not polarised are they referring to how distant from an ideal laser the real laser is, meaning that is not perfectly coherent or monochromatic?

    Finally... if it is polarised... when light leaves the laser device, in which direction it is polarised?

    Thank you.
     
  2. jcsd
  3. Nov 1, 2016 #2

    mfb

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    It depends on the laser.

    Typically laser light is polarized but there are some exceptions.
    A perfect laser cannot exist because it has to operate for infinite time to be perfectly monochromatic with infinite coherence time.
     
  4. Nov 1, 2016 #3

    The exceptions are exceptions on technical grounds? I mean, they have poorer coherence and monochromaticity than the best lasers?

    In an ideal world, do perfect monochromaticity and perfect coherence imply a perfect polarisation state?
     
  5. Nov 1, 2016 #4

    mfb

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    Yes. But the polarization could still be random.

    Real lasers typically have a slightly better amplification or lower losses for some polarization - that is sufficient to produce a polarized beam. If the difference is small, even small changes in factors like the temperature can change the polarization quickly.
     
  6. Nov 2, 2016 #5

    Andy Resnick

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    A lot depends on the specific type of laser. Many lasers have a Brewster angle window incorporated into the cavity; this causes the emitted light to have a high degree of linear polarization. Pulsed lasers may use a Pockels cell within the cavity, this forces the field to be polarized. There's no intrinsic reason why laser light *must* be polarized, AFAIK. Some solid-state lasers emit beams with 'radial' or 'tangential' polarization states, which are quite unlike the more familiar linear/circular/elliptical states.

    Having a high degree of polarization does increase the coherence and also is better for many applications- launching into a polarization-maintaining fiber, for example.
     
  7. Nov 2, 2016 #6

    Andy Resnick

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    If the field is randomly polarized, the degree of temporal coherence can be low.
     
  8. Nov 2, 2016 #7

    mfb

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    I meant random as in "you cannot look at the laser material and tell which way the laser is polarized" - with infinite coherence the polarization is constant over infinite time. Which is an ill-defined case anyway.
     
  9. Nov 3, 2016 #8
    I think these answers may have wandered away from the intent of the question. Yes many lasers are polarized, and yes there are many reasons different kinds of lasers are polarized.

    However, there is nothing that fundamentally requires that a laser must be polarized. Unpolarized lasers are not a trick of less than perfect coherence or anything like that. Lack of polarization does not violate any of the descriptions of how lasers work. A laser can be tremendously coherent. It can run in a single transverse mode. It can be made to run in a single longitudinal mode so that it approaches in every way the simple textbook description of a laser mode and yet it can still be unpolarized. I'll mention as a very obvious example that unless intentionally forced to be polarized all gas lasers including all the ubiquitous Helium Neon lasers are unpolarized.

    Polarization is an extra degree of freedom in the description of light. The frequency, the phase, the amplitude, and completely independently, the plane of the oscillation. The principle of laser operation can be described completely with the first three without ever noting or making mention of the polarization.

    It's kind of like a pendulum made from a ball hanging on a string. Does the ball spinning on the string affect how the system behaves as a pendulum? Well there may be some interesting connections, but you can pretty much describe everything about the pendulum motion without knowing or caring whether or not the ball is spinning. Polarization is like that. It is an extra degree of freedom that we can describe and manipulate without regard to the other interesting parameters.
     
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