Due to bandwidth limited grating couplers with finite bandwidh we must model within our simulations the ideal flat FSR response will rather be attenuated significantly on the order of 10 dB only 20 nm away from the intended center frequency.  Since there are manufacturing variations during fabrication as well, we need to simulate and model with these variation thresholds to estimate what our fabricated design results will look like.  There is also waveguide propagation loss which can be up to 6.6 dB/cm at 1550 nm.