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Thread: birefringence in fiber optic cables

  1. #1
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    Default birefringence in fiber optic cables

    Basic question: will a fiber scramble polarization to a random polarization even if only one polarization is present to start?


    Does anyone know if birefringence plays a significant role in a basic single mode fiber optic cable? Reason I am asking is I am curious if I need to polarize my laser ie filter it first before fiber launching it and if I do will the fiber then scramble that polarization. i was thinking to minimize the effects of birefringence by eliminating one of the polarizations before entering the fiber thinking this might minimize mixing and hence the scrambling. i was also thinking this might let me be clever and mismatch phase so I can minimize other non linear effects and susceptibility caused by the fiber including optical rectification induced fluorescence or SHG. This is for research and I might even consider heating the fiber to push that phase matching out of the extraordinary plane by heating. Not sure how sensitive to temperature a fiber is however will have to look that up. I am pretty sure that my coil is short enough that group velocity will not affect the pulses and stretch them out so its just these factors I have to consider. I will be using a lowpass filter on the beam just before use to grab any SHG that might be created. Maybe that will be enough.

    I always complain tell us what you are doing so here goes.

    I have a 3ps dye laser set to 38 mhz 589nm that I use to stimulate a platinum electrode single crystal in an aqueous solution. In the solution I have bromide ions. Using a potentiostat I selectively run through cyclic voltammetry. As the potential goes positive or negative the bromide adsorbs or desorbs from the surface. When the bromide is attached to the surface, the platinum surface and just the surface changes to a new packing order that supports second harmonic generation. By monitoring this SHG signal we get information about the surface. Since the laser and photon counter are fast I can time resolve the absorption and desorption and thus map the surface of the crystal. That's part one. Then I am going to impinge two lasers on the crystal to induce sum and difference mixing as well as optical rectification. This tells me more about the way the atoms are packed on the surface. If I get it all right I will be able to make movies of the atoms coming and going from the surface. So why the above question? Part of this setup involves very precise timing of the wavefronts so I have to be able to control this. I am thinking to use the fiber as a very precise delay line but I can't do that if the polarization gets scrambled because the signal I get off the crystal will be hard to interpret. I guess I could use a couple mirrors to do the delay line too. I was thinking by controlling the birefringence and refractive index I could more precisely control the phase delay and group delay through applied heat to the fiber coil. But because of birefringence I'd have two signals overlapped. Thinking polarization might eliminate one of them and still let me use refractive changes to form the delay line. So will a fiber scramble this anyway even if I first polarize? The alternative is a very touchy interferometer setup.

    If i gave you a headache I am sorry maybe I should have posted this to a physics board. I think that much of all you that I know you can understand this and maybe help.

    Basic question through all this is will a fiber scramble polarization to a random polarization even if only one polarization is present to start?

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    ... with my fibecoupled IR-laserdiodes and after some tests with 445nm-diodes and external fiber-coupling, the output beam is "perfectly round" after some ten mm fiber length -- so seems to scramble or "harmonize" or average all eventual polarizations ...

    Viktor
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    Damn. That sucks. I tried it but was not sure if the laser was multipolarized or the fibers scrambled. I tried a polarizer and still see some other polarization’s after the fiber. I loose a ton of power if I filter after the fiber so I think yea it scrambles 50/50

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    ... I think, the "scrambling" is normal with multimode fibers -- but could be different with monomode or GRIN fibers ... especially with GRIN fibers - there is no deflection, so could be transferring the polarization without changing orientation ...

    Viktor
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    Call for the project "garbage-free seas" - https://reprap.org/forum/list.php?425

  5. #5
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    polarization preserving fiber exists and has a "batwing" shaped core. You have to rotate the fiber when installing it.

    Steve
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    Do you thing I can modify the refractive index enough by heating to move a wavefront phase one w forward or back? 589nm 5.89e-5cm peak to peak seems possible. I just wonder if bending the fiber slightly might do the same job changing the path length.

    https://www.thorlabs.com/newgrouppag...tgroup_ID=1596 ordered thank you

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    ... how long is the fiber? -- simply stretching by some ten Nanometers sounds plausible too ...

    Viktor
    Aufruf zum Projekt "Müll-freie Meere" - https://reprap.org/forum/list.php?426
    Call for the project "garbage-free seas" - https://reprap.org/forum/list.php?425

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