Dual parallel modulation schemes for low-distortion analog optical transmission

S.K. Korotky, R.M. de Ridder
1990 IEEE Journal on Selected Areas in Communications  
We examine the use of two integrated-optical modulators operated in parallel to attain low-distortion analog optical transmission. This static feedforward approach achieves significantly improved linearity at the expense of small increases in the required optical power and moderate increases in the required drive voltage. A novel variation that uses the unique characteristics of an integrated-optical modulator to correct the dominant quadratic distortion of a directly modulated laser is
more » ... . NALOG optical fiber transmission can provide an ef- A ficient means of delivering high-frequency signals over long distances. This can be accomplished, for example, by directly modulating the injection current of a semiconductor laser diode and using direct detection. In cases where the signal consists of many frequency multiplexed channels like the composite cable television (CATV) format, the requirements on linearity are particularly stringent. At present, the second harmonic distortion of directly modulated lasers can be a limitation for CATV applications when the number of frequency multiplexed channels is 40 or more [l]. Thus, feedback and feedforward techniques, as well as alternative modulation schemes, are of interest to attain the necessary optical signal level with low distortion [2]. Here we examine the use of integratedoptical modulators external to the laser source to modulate the optical wave. In particular, we generalize the twopolarization scheme introduced by Johnson and Roussell [3] to the case of a pair of optical transmitters operated in parallel, and analyze the tradeoffs involved. We find that dual parallel modulation schemes provide improved linearity for a given optical signal level at the expense of a small increase of the required optical power and a moderate increase in the required drive voltage. The tolerances on the design parameters are such that practical implementations may require adjustment of the electrical drive signal levels or provisions for trimming of the optical circuit splitters and combiners. The functional representation of the optical circuit we consider is illustrated in Fig. 1 . It consists of two intensity modulators, designated MA and M" that are fed from a single optical source, the outputs of which are combined Manuscript
doi:10.1109/49.59140 fatcat:42osguxm2fgqjnc4qt6vejojhi