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I recently started learning silicon photonics. After reading a few books, I understand that a single-mode fiber (especially non-polarization-maintaining fiber) supports two degenerate guided modes. Hence, the actual polarization state at the output of fiber is unknown. Due to this problem, optical tables typically have a polarization paddle with a fixed fiber shape to maximize the optical fiber's SNR.

(good reference: https://www.youtube.com/watch?v=kuht5Nv3Iio in this video at 3:30, professor Ezekiel moves around the fiber and shows that the output polarization of a fiber changes by external physical stress applied to it)

However, in the pluggable optical transceiver modules used in the data center, these optical connectors do not seem capable of polarization-maintaining features. I understand that some people use polarization-maintaining fiber to avoid this issue. However, without such a special fiber, I wonder how one can set the correct polarization for the best light coupling for a typical non-polarization-maintaining single-mode fiber.

Acronyms used:

  • QSFP-DD: Quad Small Form-factor Pluggable - Double Density
  • SNR: Signal-to-Noise Ratio
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  1. Many of the systems that use transceivers such as QSFP-DD are designed to be insensitive to polarization. If the propagation delay through the fiber is independent of the polarization (because the modes are degenerate) and the receiver is equally sensitive to both polarizations, and we are not trying to maximize the system bandwidth by carrying different signals on the two polarizations, then we can simply ignore the problem and allow the polarization to drift as it will.

  2. Any fiber inside a QSFP-DD module is likely to be only a few millimeters long, and held rigidly in place. Therefore it is not subject to changes in stress that could cause the polarization to drift. In the case where a longer fiber is needed (I once worked on an X2 form-factor module that required this) then if polarization control is required polarization-maintaining fiber can be used.

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  • $\begingroup$ Thanks a lot for the quick and detailed response. I have a follow-up question about the 1st comment: From my understanding, an on-chip optical waveguide (rectangular waveguide) is typically designed to support only a single-mode & single polarization. So the optical transmitter sends a signal from the on-chip, then it gets coupled to the fiber (mostly in TE mode, unless dual polarization is used). Within the fiber, only one polarization contains meaningful signals. This is the most common situation, but I am not quite convinced how this kind of system can be polarization insensitive. $\endgroup$
    – Emm386
    Sep 12, 2022 at 21:36
  • $\begingroup$ The laser might be strongly polarized (most edge-emitting lasers) or not (some VCSELs). You must design your output coupling to support whatever laser you use. But then the signal goes through 100's of meters or 10's of kilometers of external fiber that the module designer has no control of. When it gets to the receiver it might be in any polarization state. The receiver must be designed to not be sensitive to polarization. $\endgroup$
    – The Photon
    Sep 12, 2022 at 21:51
  • $\begingroup$ RE: "Within the fiber, only one polarization contains meaningful signals. " This is not true if you are not using polarization maintaining fiber. If polarization isn't controlled, your signal could be in any polarization, and its out of your control. And if your receiver is sensitive to both polarizations that's okay it just treats signals from both polarizations as meaningful. $\endgroup$
    – The Photon
    Sep 12, 2022 at 21:54
  • $\begingroup$ This makes a lot more sense. Thanks a lot. I have a quick follow-up question: What is the common technique to make the receiver insensitive to polarization? Is this going to be governed by the grating coupler? If so, is there any common technique that makes the grating coupler insensitive to the incoming polarization? $\endgroup$
    – Emm386
    Sep 12, 2022 at 22:06
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    $\begingroup$ @Emm386, there are a dozen different flavors ("PMD types") of QSFP-DD modules, depending on the link reach required. It might help to narrow down your question to some particular PMD to focus on one particular design. $\endgroup$
    – The Photon
    Sep 12, 2022 at 22:09

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