Impact of laser phase noise on binary- andM-ary-PSK coherent optical systems using multiple optical amplifiers

Abstract

%? 100.0 m. 1 so.o=/ v -120.22 START 0.045000WO CHI SrOP 0 . -GHx Figure 3 Relative phase shift between the inverter and the noninverter structuresThe use of a dual-gate transistor as part of the auxiliary amplifier of the second loop, allowing electric control of the intermodulation and avoiding the need of a cut-andtry adjustment in attenuation. Dual-gate FETs have the advantage of gain varying, without phase changes The inclusion of a lumped phase inverter (bifilar, ferrite based) and a corresponding dummy one, noninverter, just to ensure the symmetry of the insertion losses and the correct phase relation between the two loop branches. Figure 3 shows the actual phase response of these devices in the range of 0.045-2.6 GHzThe use of two capacitors (C, and C,, in Figure 2) in series with the transmission lines, in the second loop, can provide a better electric compensation of the auxiliary amplifier of this loop, resulting in an impressive performance improvement, especially at low frequen-DB[S2l] AMP 30.00 0.0000 -30.00 0.1000 0.8OOO PREP-CHZ I .so0 Figure 4The relative intermodulation improvement. The upper curve is the measured output of the system without feed forward; the difference between that curve and the lower ones is the intermodulation reduction. (a) C , nonexistent, C , = 1 nF; (b) C, = 1 nF, C , = 47 p F (c) C, = 220 pF, C , = 47 pF cies, when compared with previous results [l]. If the capacitors are replaced by varactors, electrical control of the intermodulation reduction shape can be achieved. Figure 4 shows the responses of an implemented prototype under several conditions of the compensating capacitors. As shown, it is possible to achieve an intermodulation-level reduction greater than 25 dB over more than 3 octaves. This improvement allows driving the amplifier 8 dB higher, for the same desired linearity (reducing the required backoff). CONCLUSIONSSome changes on the usual implementation of feed-forward power amplifiers have been presented, aiming at broadband operation. The simulated and measured performances have shown approximately 25-dB improvement in intermodulation canceling over more than 800-MHz bandwidth. This broadband response could be improved even further if the circuit were implemented in a monolithic form and if more advanced phase-inversion structures were used. ABSTRACTThis study evaluates the pe$ormance of binary-and M-ary-PSK coherent optical systems using cascaded optical amplifiers in the presence of noise originating from the detector, receiver, optical amplijiers, and the laser phase fluctuations. Our results on the analysis of these systems indicate that at an amplifier input power of -20 dBm and a system length of 10,000 km, the power penalties incurred due to the laser phase noise at a bit error rate of 10linewidths of 770. 31, 101.86, and 6.37 kHz for binary-PSK four-PSK, and eight-PSK systems, respectively. 0 1996 John Wiley & Sons, Inc. correspond to 0.57, 0.99, and 3.38 dB at " = LSYNCHRO-. -L