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When laser source is launched into two 1x2 50/50 fiber optic couplers connected as below the output power constantly fluctuates in range of 70 uW. The fluctuation happen roughly one to two times per second. Why does this happen?

            -------
Laser -----|       |----- Power Meter
            -------

However, when the output power of each coupler is measured individually (as below) on each leg the output power is stable on every leg of every coupler.

            ------- Power Meter
Laser -----|       
            ------- Power Meter

I initially suspected that this may be caused by laser source instability due to back reflections, however after I used a source with an optical isolator, the output power still fluctuated as before. I tried several laser sources, and several couplers with the same fluctuating result.

I tried with 1,3,4 and 5 mW laser input power and 1310nm and 1550nm wavelengths. I also tried DFB abd FP laser diodes with the same result. The legs of the couplers are 1m each.

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  • $\begingroup$ Can you specify the type of laser(s) you're using? $\endgroup$
    – A Nejati
    Commented Jul 2 at 18:26
  • $\begingroup$ I tried mostly with DFB lasers and one test was done with FP 3mW laser diode. The fluctuations occure with both types of lasers, although with the FP laser the range of fluctuations was 15% grater. $\endgroup$
    – Jimski
    Commented Jul 2 at 18:42
  • $\begingroup$ Have you tried with a low-coherence source like an LED to see if there are still fluctuations? $\endgroup$
    – A Nejati
    Commented Jul 2 at 19:22
  • $\begingroup$ And how long are the fiber optic legs? $\endgroup$
    – A Nejati
    Commented Jul 2 at 19:23
  • $\begingroup$ also do you have a waveform showing the shape of the fluctuations? i.e. are they smooth or sharp fluctuations $\endgroup$
    – A Nejati
    Commented Jul 2 at 19:24

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Also, What is the wavelength and line width of the laser? You might be seeing interference effects and as the relative length of the fibers change due to temperature/etc you'll get a change in interference.

Comment by Jimski: It was indeed the interference inside the second coupler caused by microphonic effect. To determine this, I connected only one leg of the couplers (as pictured below) measured the power using a photoamplifier and an oscilloscope, and the power was stable, other than some minor microphonic effects. But when both legs were connected the power began to fluctuate and the fluctuation could be excited to a large degree by tapping into the optical table. Looks like the microphonic effect was strong enough to alter the polarization and/or cause a phase shift and this in turn caused fluctuations in the intensity of the interference.

            ---------------------------
Laser -----|                           |----- Power Meter
            ---  Disconnected Leg  ----

Additionally, the fluctuations which were initially observed one to two times per second were recorded using an optical power meter which was to slow to show real time microphonic fluctuations. However, when a photoamplifier and an oscilloscope were connected the microphonic effect could be observed in real time.

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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

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