Semiconductor lasers versus external modulators: a comparison of nonlinear distortion for lightwave subcarrier CATV applications

Second- and third-order distortion measurements performed on various buried-heterostructure InGaAsP DFB (distributed feedback) lasers and on LiNbO/sub 3/ external modulators are discussed. It is shown that by optimally biasing the external modulator, the second-order distortion can be eliminated but the third-order distortion is 25 dB greater than that of direct modulation. It is also shown that these discrete two-tone measurements can be used to predict the system performance of a 42-channel CATV trunk system. The increased composite-triple beat and lower carrier-to-noise ratio, due to modulator loss, are the disadvantages of the external modulator. If it is possible to improve third-order distortion by modifying the structure of the modulator or by electronic compensation without increasing the second-order distortion, then these modulators may be useful for CATV applications.<
>