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Figure 1. Schematic of the simulation setup

The dynamically tracking receiver uses a training sequence and has digital signal processing options for estimation of phase and counting of errors (excluding training bits). A benefit of plotting Q² is that Q² (dB) penalty is equivalent to OSNR nonlinear interference (NLI) penalty, measuring which is otherwise computationally expensive since it would require adjusting OSNR until you match the initial BER.

An alternate approach to the above, to save on simulation time, is to employ what¡¯s widely known in literature as ¡°noise-loading.¡± In this approach, we use a noiseless amplifier in the span, and add an equivalent noise after all the spans. Figure 2 shows conceptual schematic of the noise loading mechanism.

Figure 2. Using noise loading and VOA (bottom) to model an equivalent span on the top

By introducing additional span loss and a reduction in OSNR by equivalent additional loss of the span, we mimic simulation of longer transmission without actually simulating a longer fiber thereby saving on simulation time. As will be seen later the penalty from noise-loading is minimal at optimal channel power for our case where fiber core size is larger compared to the standard single-mode fiber. For more treatment of this subject, please refer to References 1 to 3.

A further reduction in simulation time is possible using the Gaussian noise (GN) emulator instead of the fiber-amplifier spans of Fig. 1. It is shown experimentally [4], via OptSim-based modeling [5], and analytically [Ref. 6 and Chapter 7 of Ref. 3] that for uncompensated, long-haul coherent systems such as in this application note, nonlinear transmission impairments in fiber manifest as nonlinear interference (NLI) with Gaussian noise power spectral density. Using the GN model approach, the simulation time reduces by one-third compared to simulating actual fiber-amplifier spans. As will be seen later, the penalty from the GN emulator model is negligible at optimal channel power values.

Finisar Waveshaper type filters are used in practice to implement Nyquist filtering, and are modeled as super-Gaussian filters of orders 2 to 4 [8] as is also done in this application note.

Figure 3 shows BER vs. per channel transmitted power (top) comparing all three cases. 

Figure 3. log(BER) vs. per channel transmitted power (dBm) plots

For all three cases, the inverted bell curves show that the optimal per channel power is around 2dBm for the current design.

Figure 4 shows bell curves for the Q²(dB) for all three cases.