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  • $\begingroup$ I updated the question. I used 1,3,4 and 5 mW laser input power and 1310 and 1550nm wavelengths. Interference could cause the power to drop or increase but it fluctuates one to two times per second continuously without touching or vibrating the fiber. $\endgroup$
    – Jimski
    Commented Jul 2 at 18:31
  • $\begingroup$ @Jimski the fluctuations can be caused by micro phonics, or thermal changes. You don’t need to touch anything. You could verify this by placing the fiber coupler in a thermal controlled environment free from air currents. $\endgroup$
    – JQK
    Commented Jul 2 at 22:53
  • $\begingroup$ I think your initial idea about interference is more likely and I will investigate it further and post an update tomorrow. The microphonic and thermal changes are unlikely as they would cause power fluctuation irrespective of whether the power is measured after two couplers or on each leg of a single coupler independently. Also, if I connect a longer fiber of 20m (without couplers) on the same optical table, the power output is stable and does not fluctuate. $\endgroup$
    – Jimski
    Commented Jul 3 at 0:47
  • $\begingroup$ The longer fiber could be a statement of the coherence length of the lasers. Once the 2 paths have a delay greater than the coherence length, once wouldn't expect these fluctuations. $\endgroup$
    – JQK
    Commented Jul 3 at 17:20
  • $\begingroup$ Also, I wouldn't expect the fluctuations to be present when you're just measuring a single leg. $\endgroup$
    – JQK
    Commented Jul 3 at 17:20