ral as (5.50) where PASE is the ASE power in the optical bandwidth. On the other hand, the SN has to be evaluated taking into account the quadratic detec- tion law of the photodiode and neglecting shot noise and receiver noise. In evaluating SN we do not neglect the quadratic term in the noise, due to the fact that in the presence of FEC sometimes it is possible to work with a small SN and the quadratic term in the noise can be relevant in situations in which errors are possible. The quadratic term power, after electrical filtering, is evaluated assuming a flat optical noise spectral density in the band Bo; thus after square law detection the power spectral Technology for Telecommunications: Integrated Optics and Microelectronics density of the term containing the square of the noise is a triangle with base extending from -2B to 2B, 0 This triangular power spectral density is filtered by an almost ideal elec- trical filter of unilateral bandwidth Be The receiver model with the definition of the relevant wavelength and frequency band- widths is represented for clarity in Figure 5.70. The conventions in terms of bandwidths and SN definition that are shown in the figure will be maintained for the remaining part of the book. The ASE noise is considered a white Gaussian noise with power spectral density SO that the ASE power in the optical bandwidth, with the definitions of Figure 5.61, could be defined as PASE = 2B. Since, in order to have a parameter tailored for the application we will study in Chapter 6, we have defined Bo as unilateral bandwidth, it is convenient to define a unilateral ASE spectrum; let us call it So = 2 SO that the ASE power can be writ- ten as PASE=SOBo. The fact that this definition is useful to write simply the equations immediately results from the case of a receiver using a single pre-amplifier. In the case of a single amplifier, considering the expression of the ASE spectral density deduced in Chapter 4, we have, B being unilateral, yields So = hwo((G-1)NF (5.51) where WC is the carrier an