t noise contribution. In order to evaluate the ideal error probability it is convenient to express (6.93) via the field quadratures. Assuming as reference a polarization unit vector parallel to the instan- taneous received field polarization, the photocurrent can be written as (6.94) where Ej(t)ej(t)+njt)(j=1,2,3,4) are the quadratures of the received field composed by a signal contribution and an ASE contribution. The signal part of the quadrature of the field, that is, e'(t), is related to the transmitted quadratures via a rotation of the quadrature space induced by fiber propagation (see Chapter 4). Since (6.94) is a quadratic form of Gaussian variables plus a Gaussian term, the method of the characteristic function can be used. Delay T signal Low pass filter Sampler Decision device FIGURE 6.33 Block scheme of a DPSK receiver. Transmission Systems Architectures and Performances Neglecting as usual when amplifiers are used the receiver noise, the integral relating the characteristic function of the decision variable and the error probability can be solved exactly in this case providing (6.95) This is a very simple expression of the error probability and the required SN for an error probability of 10-12 is 14.3dB. This represents a gain of almost 3 dB with respect to IM, essentially, this gain is due to the fact that the average power along the line is doubled for the same peak power. This advantage, however, is not without a price. Not only phase modulation is affected by phase noise [8,42] that directly superimposes to the information bringing phase, but it is also more sensible to SPM, essentially for the same reason. The draft design in case DPSK is used can be carried out with a procedure similar to that already used in the previous section. The system parameters are reported in Table 6.14. Here it is a key to have a narrow linewidth; thus, a high emission power external cavity laser is selected as transmitter. Moreover, all the blocks constituted by splitters and delay line are supposed to be inte- grated 