Modification of Level Dependent ASE-Signal Beat Noise by Optical and Electrical Filtering in Optically Preamplified Direct Detection Receivers

We derive compact equations describing the modification of amplified spontaneous emission signal beat noise arising from optical and electrical filtering in optically preamplified direct detection receivers. In particular, we show that this modification typically results in a further decrease of the signal quality factor. This is particularly pronounced in the presence of electrical filters with steep transfer functions such as, e.g., occurring when feeding the signal through an antialiasing filter prior to analog-to-digital conversion or in a real-time oscilloscope, in the latter case leading to counter-intuitive dependencies of the measured signal quality on the characteristics of the test setup. Predictions are exemplified in concrete system models and verified with experiments. While the modeling assumptions and the accuracy of the predictions are in line with models previously reported in the literature, derived expressions allow straightforwardly tying the modification of the level dependent noise to signal levels, baud rate, signal spectrum, and filter transfer functions.