It seems that the period of the oscillations is about dlambda = 4nm.

We may think that comes from some spatial interference ( e.g. gap in the connector)

It would corresponds to interference effect over distance order of

d = lambda^2 /dl = 0.18 mm

If VL's assumption (above) is correct, I don't have an explanation but more likely, NP (above) is right about a weak reflection occurring between connectors or elsewhere in your system. Here's an article that uses such spectral features (and actually, the Fourier transform of such features) to uncover various Fabry-Perot resonant cavities that are visible, as you point out, as 'oscillations' in a spectrum. "Characterizing residual reflections within semiconductor lasers, integrated sources, and coupling optics," IEEE Journal of Quantum Electronics, vol. 34, 7, pp 1224-1230, Jul, 1998. As a guess -- when your measurement system is free of residual reflections, the spectrum of the laser or other source you are examining will be free of these artifacts.

Vincent Lecoeuche

Dear Vincent,

Thanks for your reply.

I reduced the signal power in order to increase the integration time in the spectrometer. Those small oscillations are gone now, but still there are bigger oscillations on top if the spectrum profile.(see the picture below please)

David A. Ackerman

Nikolay Pavlov

Dear Nikolay and David,

Thank you for your suggestions.

This oscillation looks like interference pattern to me as well. But what I don't understand is that where are they coming from. Ideally when I use angled fiber connectors there should be no back reflections. Even when I apply some index matching gel where the fibers are connected, I see no change in the oscillations.

The peaks of these oscillations are 1.2nm apart, which corresponds to almost 0.2mm distance.

If I touch the fibers (that are connecting the light source, the circulator and the spectrometer together), the frequency of these oscillations does not change but their phase and amplitude are different.

Possibilities include the gap between emitter and coupling optics, index discontinuities within the source itself or a 0.3mm air gap in the measurement device. However, your evidence of change in the spectrum when you touch the fibers is a good clue. I'm sure that you have checked and double checked your fiber connections (since you have even added index matching gel) but maybe there is a bad connector? You are on the right track to get rid of this annoying problem. But if it is not in the source or the spectrometer and it changes when the fiber is disturbed, it looks like the fiber connectors are involved.

I realized that when I change one of the fibers in the setup and instead of normal single mode fiber, put polarization maintaining fiber (PM fiber, which is also singe mode but highly birefringence ) the oscillation gets stronger.

Somehow polarization is making trouble. As far as I know the relationship between the polarization and the wavelength is not periodic.

The frequency of these oscillations changes when I add another PM fiber or a polarization sensitive circulator.

Samane -- perhaps the best way to get answers to your questions related to different types of fibers and get rid of the unwanted oscillations in your spectra is to try using all normal (non-PM) single mode fiber and make sure that all connectors are clean, undamaged, angled properly and well connected. Then remove the circulator to form a single path from your source to your spectrometer. Best case -- use a single fiber, i.e., no extra connectors. Hopefully, you will be able to suppress all the parasitic Fabry-Perot oscillations. (Perhaps you have already tried this.) If you are successful, then add the other components back. But be careful coupling unlike fibers that might cause the reflections that you are working to eliminate. Good luck -- these problems can be frustrating.

Thanks David. I have already tried to check the spectrum profile when there is only one fiber between the light source and the spectrometer. These oscillations get stronger and less noisy when I put the circulator, which has PM fibers channels, in the setup.

It might be that there is some longitudinal mode mixing similar to the bound states in mode-locked lasers. But I haven't found the connection yet.

Hi Vincent,

The 0.3nm oscillations were indeed due to the low integration time (systematic error of the spectrometer). The peak to peak distance of the oscillations in the second picture (posted 7 days ago) is around 1.2nm. Unfortunately I don't have a higher resolution spectrometer but I can try the lower resolution version.

Running low on candidate explanations beyond Vincent's other hypothesis above.

Is there an optical isolator between your source and the fiber? If not, perhaps your spectral artifacts are not due to passive cavities but instead due to interaction of the coupling optics, including the first surface of the fiber, with your source, i.e., active cavity effects. Not knowing the details of your source, it is hard to speculate on this explanation. But the FSR of the unwanted oscillations suggests something close to the source if the source itself is involved.

Of course, if your source is well isolated, active cavity effects are unlikely.

Dear Davide,

The source has no isolator, but they have an specific fiber to be connected to it.

My guess is that the fiber has an AR coating, but I will ask them. thanks

Hi Samane -- from your answer, I will assume that the source and it's out-coupler (optics and fiber) is integrated rather than a source like a semiconductor laser on a submount with a lensed fiber or microscope objective to couple to a fiber. In the latter case, an isolator would be required. But in your case, I will assume that the source and out-coupler are properly coupled so that an isolator is not required. My original question about isolators was aimed at cases in which the source can be strongly influenced by small reflections, as in the case of semiconductor lasers.

Vincent Lecoeuche, David A. Ackerman

Dear David and Vincent,

I tested the spectrum profile of the source without any fiber connections in between. I set the power of the source to its max and aligned it with the spectrometer. There is still some small oscillations. I contacted the company and it turned out the source has some internal problem which is causing the high frequency oscillations(2.5nm).

I also found out that the output of the light source is partially polarized but in unknown direction as it goes to a standard fiber and not PM. So this PMD that you guys mentioned could actually be the reason of 1.2nm oscillations I saw after the circulator.

I will order a polarized light source to somehow avoid this PDL.

The only thing that concerns me is that even if I put PM fibers all over my setup, still my fiber sensor is in standard fiber and as I need to study the reflected signal from my sensor it will pass trough the circulator again. So part of the PDL will still affect the measurement.

I just hope that in won't be that big.

Thank you again for your help

Samaneh