REDUCTION OF DISPERSION-INDUCED DISTORTION IN SCM TRANSMISSION-SYSTEMS BY USING PREDISTORTION-LINEARIZED MQW-EA MODULATORS

The application of low-chirp MQW (multiquantum well)-EA (electro-absor ption) modulators to subcarrier multiplexing (SCM) optical transmissio n systems is studied. The authors show that a third-order predistortio n circuit is feasible for compensating the nonlinearity of this type o f modulator. The degree of frequency chirping per optical intensity mo dulation depth of the modulator is theoretically determined to be abou t 1.4 MHz. A 42-channel AM-SCM signal transmitter with the predistorti on circuit realizes both composite second-order distortion (CSO) and c omposite triple beat distortion (CTB) values under -57 dBc after 200-k m-long transmission. The authors confirm dispersion-induced distortion of the MQW-EA modulator is as small as that of the LiNbO3 modulator. Carrier-to-noise ratios (CNR's) of 45.2 dB for channel 1 and 43.8 dS f or channel 42 were measured after 100 km transmission. It is found tha t the deterioration of a CNR is caused by the cascaded erbium doped fi ber amplifiers (EDFA's), Rayleigh backscattered power and the optical phase noise. Theoretical CNR's show that the output power of the modul ator should be higher to improve CNR.