Laser intensity modulation as a clipped Gaussian process

Abstract

An attractive application of lightwave technology is cable television (CATV) transmission. In an important lightwave application known as subcarrier multiplexing, a broadband analog signal, consisting of the sum of amplitude modulated subcarriers, is used to intensity modulate a laser to produce an optical signal for transmission by optical fiber. It has been determined that the large negative excursions of the broadband signal lead to nonlinear distortion of the optical signal which also distorts the information bearing modulation on the subcarriers. The purpose of this paper is to present a general result concerning the average power of a clipped Gaussian process and apply this result to analyze the nonlinear distortion resulting from intensity modulation of a laser. In particular, a general formula for computing the average power of a stationary Gaussian process that is clipped at two arbitrary levels is applied to develop an approximation for the carrier-to-nonlinear distortion ratio (C/NLD) per channel as a function of the dynamic range of the clipping levels of the broadband analog signal, and the total RMS modulation index /spl mu/. 2/spl mu//sup 2/ equals the product of the number of channels being multiplexed and the square of the optical modulation index per channel. The approximation stems from an exact result and is especially useful because the conservative version of the approximation is independent of the bandwidth or spectral location of the broadband analog signal. Results are presented in graphical form and they are in reasonable agreement with earlier theoretical and experimental works. >