Limiting distortion of CATV lasers

Recent research indicates that carrier heating effects resulting from coherent radiation and input current inherently limits the linearity of CATV lasers. This limitation on the linearity was estimated to limit the carrier-to-second order distortion ratio per channel to about 62 dB for an 80 channel CATV system operating in the frequency band 60-540 MHz with a total root-mean-square (rms) modulation index μ=0.19. Since this level of distortion is near the tolerance limit of present subcarrier multiplexing CATV systems, this new source of nonlinearity is of some concern. Prevailing practice now attributes the limiting distortion to laser clipping rather than to the linearity of the laser. However, it is known that laser clipping alone cannot account for this border-line level of distortion. The limiting distortion also applies to optical fiber interconnections of RF signals as, for example, between a cellular base station and its remote antenna sites, especially for implementing microcells. The purpose of this paper is to apply an exact method based on noise theory applied to a fully loaded system in order to evaluate the joint distortion resulting from both laser clipping and the nonlinearity introduced by the carrier heating effects. All orders of distortion are treated and compared with the corresponding cases when the carrier heating effects are suppressed. The resulting carrier-to-nonlinear distortion ratios per channel are presented in graphical form as a function of μ and I_b, the laser bias current. The results also confirm an earlier finding that in the vicinity of μ=0.19, second-order distortion limits the performance per channel rather than laser clipping. However, when second order distortion is suppressed, laser clipping again limits the performance per channel. Based on the composite second order (CSO) and composite triple beat (CTB) distortions, it is shown that the optimal total rms modulation index is about μ=0.25 for subcarrier multiplexed CATV applications