Nonlinear distortion evaluation in a directly modulated distributed feedback laser diode-based fiber-optic cable television transport system

Abstract

Abstract. This study evaluated a directly modulated distributed feedback (DFB) laser diode (LD) for cable TV systems with respect to carrier-to-nonlinear distortion of LDs. The second-order distortion-to-carrier ratio is found to be proportional to that of the second-order coefficient-to-first-order coefficient of the DFB laser diode driving current and to the optical modulation index (OMI). Furthermore, the third-order distortion-to-carrier ratio is proportional to that of the third-order coefficient-to-first-order coefficient of the DFB laser diode driving current, and to the OMI IntroductionWith the popularity of optical fiber networks and the demand for high-quality videos, the development of changing TV signals from traditional cables to optic fibers has been promoted globally; thus, fiber-optic cable television (CATV) has been widely discussed. 1-5 Therefore, the transmission quality of such a system has been defined and regulated, which basically determines the signal-receiving quality of the system based on the following three parameters: carrier-to-noise ratio (CNR), composite second order (CSO), and composite triple beat (CTB).6 Both noise and nonlinear effects in the fiber-optic CATV transmission system are mostly caused by active components but rarely by passive components. Such noise severely affects both system performance and transmission distance; thus, CNR is usually adopted in the parameter measurement of CATV systems to analyze the characteristics of the transmission signal quality of the system. Given that CNR is proportional to the optical modulation index (OMI), a higher CNR could be obtained by increasing the OMI. However, an extremely high OMI could worsen the nonlinear distortion of the system, causing the system to generate second and third harmonic distortions, as well as second and third intermodulation distortions. These nonlinear distortions result from the accumulation of signals that fall on other channels, which decreases both CSO and CTB of the system. Under ordinary conditions, designing a CATV system to obtain high CNR, CSO, and CTB values altogether is impossible, but an optimum balance range between noise and the systematic nonlinear effect could be determined. Such a nonlinear effect has become an important research topic to directly modulate distributed feedback (DFB) laser diode (LD) in fiber-optic CATV systems.7-10 The present study theoretically deduces the relationship among the direct modulation, driving current,