Abstract

Interest in AM-VSB cable TV (CATV) systems operating at 1.55 μm with erbium-doped fiber amplifiers (EDFAs) is increasing rapidly. These systems provide high optical powers and enable large splitting ratios. The chirp produced by directly modulated DFB lasers at 1.55 μm leads to distortion in nondispersion-shifted fiber and in EDFAs with gain slope.¹ Integrated electroabsorption modulatd lasers (EMLs) have low chirp, Transmission through nondispersionshifted fiber, without dispersion compensation and with negligible penalty, has recently been demonstrated, both using a linearized EML² and a linearized discrete electroabsorption modulator.³ Simply biasing the modulator at the inflection point of its transmission-vs.-voltage curve produces low composite second-order distortion (CSO). The odd-order nonlinearity, parameterized by the composite-triple-beat (CTB), is high, however, and must be corrected by predistortion. In Ref. 2, 11.4 dB of third-order cancellation was achieved, yielding a CTB of −54.1 dBc. Typical specifications call for CTB s −65 to −60 dBc. Here we report an improved predistorter, which cancels fifth-order (IMD5) as well as third-order (IMD3) intermodulation distortion and yields a CTB of −60 dBc. The uncorrected CSO is −61 dBc. The reduction in CTB (17.5 dB) is 6 dB better than the largest previously reported for an electroabsorption modulator. Low distortion is preserved after fiber amplification and propagation through 13 km of nondis persion-shifted fiber with a transmission-fiber launch power of +13.4 dBm.

© 1996 Optical Society of America