We studied the influence of limited bandwidth of receiver on tapping signals. The optimum sampling timing offset (STO) and filter bandwidth in 80 Gbit/s coherent optical QPSK eavesdropping system are also discussed. Different order of Bessel filters are taken into account in the presence of laser phase noise. Optimal receiver performance relies on a trade-off between additive noise and inter-symbol interference (ISI).
Optimum STO and bandwidth of eavesdropping receiver are determined by signal to noise ratio (SNR) per bit as we define. Simulation results show that the ratio of optimum STO to symbol duration are 0 and 0.25 for the 1st and 5th-order Bessel frequency response of receivers, respectively.