Coherent and IM-DD data signals, including time transfer, fiber sensing, and QKD, transmitted in one ULL fiber

The simultaneous transmission of coherent signals with intensity-modulated direct-detection (IM-DD) signals within a single optical fiber offers economic benefits to network providers. However, this coexistence can pose challenges, of particular concern are parasitic nonlinear effects such as cross-phase modulation (XPM). In a poorly implemented system, these effects have the potential to degrading the bit error rate (BER) of the coherent signal. Although DWDM system vendors are pushing for the exclusive deployment of coherent modulation formats, IM-DD signals—often referred to as legacy formats—remain relevant due to emerging applications such as high-precision time transfer and distributed fiber sensing. QKD and stable frequency transfer are examples of other new applications, but they are not as challenging because of the different modulation and spectral characteristics. Our findings quantify the extent of this degradation of coherent signals and show how frequency guardbands can be used to mitigate or even eliminate the problem.