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Fiber ring resonators made up of single mode fibers often incorporate an optical isolator to suppress the build-up of Brillouin scattering. However, ring resonators made of multimode fibers generally do not contain an isolator.

Why is it that the adverse effects of Brillouin scattering are negligible in multimode fibers?

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  1. Stimulated Brillouin scattering is propagating "backwards", hence isolator effectively kills it while allowing laser operation in main direction.
  2. Multi-mode fiber lasers are typically not single-frequency, hence SBS is much less of a concern.

So, if you make single-mode fiber laser, but not single frequency, with large enough bandwidth (>>10GHz) - it would also not require special means for SBS suppression.

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  • $\begingroup$ Thanks a lot for your input. But could you explain the second point a little further? Why is SBS not a major issue when you have a large bandwidth? $\endgroup$
    – Merin
    May 12 '19 at 0:59
  • $\begingroup$ @Merin Because SBS has narrow gain bandwidth : "For silica fibers, the Brillouin frequency shift is of the order of 10–20 GHz, and the Brillouin gain has an intrinsic bandwidth of typically 50–100 MHz". rp-photonics.com/brillouin_scattering.html $\endgroup$ May 12 '19 at 4:59
  • $\begingroup$ Are you suggesting that for SBS to occur you need high powers at specific frequencies? That means that in multimode fibers the power is distributed between a larger number of frequencies, so often none of them have enough power to give rise to the counter-propagating Brillouin waves. Is that correct? $\endgroup$
    – Merin
    May 12 '19 at 5:39
  • $\begingroup$ @Merin That is correct. Also, it is possible that in multimode fiber different modes will have different SBS gain profile too. $\endgroup$ May 12 '19 at 5:47

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