RSoft Enewsletter

March 2015

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Modeling Multipath Interference (MPI) in OptSim Circuit

Multipath interference (MPI) plays a major role in optical communication links, especially in fiber-to-the-x passive optical network (FTTx PON) architecture, where a splitter-based distribution causes reflections from each splitter. Isolators can help mitigate this problem, but they are not very suitable for passive networks due to their insertion loss.

Accurate modeling of MPI is crucial in order to understand system penalties. However, MPI modeling is complex because of bidirectional interactions between a large number of reflections from multiple sources, which often propagate through nonlinear components, such as fibers. A frequency-domain approach, like the one described in Ref. [1], calculates power spectral density of MPI to be used in a way similar to the relative intensity noise (RIN). A time-domain approach, like the one described in Ref. [2], models MPI as “sum of signals” with Monte Carlo estimations on amplitude and phase to calculate the signal at the receiver.

OptSim Circuit™ uses time- and frequency-domain methods to model bidirectional optical and electrical signal propagation and feedback. It can model MPI in a natural and intuitive way without any linear filter type assumptions. The OptSim Circuit’s bidirectional fiber model includes both bidirectional optical signal propagation and Rayleigh backscattering, the two main causes of MPI.

Here is an example showing how to take advantage of OptSim Circuit’s bi-directional propagation capabilities to model MPI.

Figure 1: Layout of the topology to study multipath interference effects 
(Click for a larger image)

The layout consists of a link of two spans of fibers of 20-km lengths each. The bi-directional fiber model includes bidirectional optical signal propagation, a main cause of the MPI. Three bi-directional connectors, between the transmitter, the receiver and the fiber spans, create two closed loops where the optical signal is enclosed and circulates back and forth. The bidirectional connectors enable adjusting the optical insertion loss and return loss, thereby physically creating a backward propagating optical signal.

The received eye diagrams show worsening performance with stronger MPI.

Impact of MPI

Impact of MPI: Received eye diagrams for connector loss values 
of 10 dB (left) and 3 dB (right)

Ref. [3] presents an interesting and more complex study using OptSim Circuit on the effect of interplay of MPI with jitter and noise in a directly modulated PAM-4 system. 

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  1. Gimlett, J.L, and Cheung, N. K., “Effects of phase-to-intensity noise conversion by multiple reflections on Gigabit-per-second DFB laser transmission systems,” Journal of Lightwave Technology, vol. 7, no. 6, pp. 888-895, June 1989.
  2. Fishman, D.A., Duff, D.G., and Nagel, J.A., “Measurements and simulations of multipath Interference for 1.7 Gbit/s lightwave system utilizing single- and multifrequency lasers,” Journal of Lightwave Technology, vol. 8, no. 6, pp. 894-905, June 1990.
  3. Ghiasi, A., Patel, J., Ghillino, E., and Richards, D., “Tool for simulating 400GbE optical PMDs,” IEEE 802.3bs Task Force meeting, July 2014,