References: [1].

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Analog transmission of RF signals over the optical channel of a hybrid fiber coaxial (HFC) CATV system can be based on either direct laser modulation or an external modulator. The parameters of the optical channels with direct laser modulation are of poor quality due to laser chirping, nonlinearity and slightly sloping transfer characteristic etc. To eliminate such a disadvantage a laser with a constant bias current and an external modulator at its output is used.
Modern HFC CATV systems usually apply electro-optical intensity modulators based on the Mach-Zehnder interferometer and known as MZ-modulators (MZM). The linear part of the conventional MZM transfer characteristic is rather short which results in nonlinear distortion of the signals transmitted over the optical channel. With HFC CATV systems the carrier-to-composite second order (C/CSO) products and the carrier-to-composite triple beat (C/CTB) products ratio measured at the optical channel output is required to be higher than 60 dB. To provide the minimum CSO and CTB values needed the conventional MZM must be operated with a modulating RF signal of comparatively small amplitude which however causes the carrier-to-noise ratio (CNR) to decrease, hence the received information quality to worsen.
Different methods for linearization of the MZM transfer characteristic have been developed. They improve the dynamic range of the input RF signals and keep the carrier-to-intermodulation distortion ratio within the required limits. Linearization techniques most often apply several conventional MZMs (two as usual) whose mode of operation is set in a way to achieve an effective suppression of the nonlinear distortion products. The method usually applied to determine the parameters of such modulators consists in representing their transfer characteristic as a power series. A mathematical model based on Bessel functions has been used to describe the MZM. The purpose was to optimize the modulator parameters in a way to reduce the nonlinearities over a broad frequency band and to maximize the modulation efficiency, i.e. to maximize the linearity of its transfer characteristic and to minimize the optical loss.