Argon-ion laser launches into fiber

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    1 W of 514 nm Argon-ion laser light is mode-matched and launched into a single-mode polarization preserving fiber. Better wear those safety goggles!

    zumbojo, 3e, Ben Karel, donpsychote, and 13 other people added this photo to their favorites.

    1. Mr. Physics 106 months ago | reply

      Wow! Awesome!

      Where does the fiber take the laser light (what do you do with it)? What kind of losses per meter does the fiber manage? Oooh! Tell me everything!

    2. fatllama 106 months ago | reply

      In this case, the fiber went about 15 meters over to a second optical table. As with a few other high power lasers, we used this laser to tickle a single trapped atom and then studied its response in detail. Most of the losses, about 20%, occur in the input coupling (that is, not squeezing all of the free-space beam into the several micron-sized fiber core). The losses per meter are incredibly low at this color; think 0.1dB/km or so. The most important feature of this fiber was that it could maintain the polarization of the laser light around bends and loops.

    3. donpsychote 104 months ago | reply

      Excellent shot, please add more scientific laser shots with a lot of light and glass!

    4. Kaddy 104 months ago | reply

      Is this a special single-mode fibre or do all single-modes retain polarisation?

    5. fatllama 104 months ago | reply

      Yes, a special fiber is required. Two tubes of high-index glass are placed along one axis outside of the core. When pulled into fiber, these provide more stress along this axis. Here's one example, closeup:

      The tubes change the effective index of refraction of one polarization relative to the other enough so that mixing between the two modes is highly suppressed. In order to make it work, one must launch the light with its polarization either along or orthogonal to the special axis of the fiber. This is either done with a rotating chuck (hard! expensive!) or with a half-wave plate placed just in front of the fiber coupler (expensive! but easy!).

      The behavior of non-polarization maintaining single-mode fiber is complete 100% wander of the exit polarization over long timescales due to changes in temperature and a lot of short timescale noise due to microphonics on the fiber. Also, the geometrical layout of normal fiber affects the polarization of the exit beam via some interesting physics termed Berry's (or more correctly, Pancharatnam) topological phase.

      One can get away with a sleazy hack, though. By wrapping a regular SM fiber tightly around one or several cylinders one can put enough stress on the fiber to simulate the existence of the permanent stress axis in the polarization-maintaining fiber. By rotating the wrapped cylinders, one can find an orientation that works alright for several minutes until temperature changes force a bit of tweaking.

      Such "polarization controllers" sell for many hundred dollars but a few hours in the machine shop instead would be just as fruitful:

    6. Kaddy 104 months ago | reply

      Thanks for a great reply!!

    7. optick 96 months ago | reply

      A great photo and really interesting commentary!

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