Distortion spectrum of laser intensity modulation

Abstract

The simplest and most economical method for transforming many channels of high quality video and audio analog signals into an optical signal makes use of subcarrier multiplexing. In this multiplexing scheme, a broadband analog signal consisting of the sum of many modulated subcarriers (one for each channel), is applied to amplitude modulate the intensity of a laser for transmission by optical fiber. Laser intensity modulation is also attractive for interconnecting broadband RF signals using optical fiber as, for example, between a cellular base station and its remote antenna sites, especially for implementing microcells. A fundamental, limiting transmission impairment is the nonlinear distortion of the clipped laser intensity resulting from occasional signal excursions below the laser threshold. The purpose of this paper is to apply noise theory along with numerical analysis, involving fast Fourier transforms, to determine some basic characteristics of the nonlinear distortion associated with a fully loaded system. In particular, the exact distortion spectrum and the carrier-to-nonlinear distortion ratio are determined as a function of the dynamic range and bandwidth of the multiplexed signal, and the total RMS modulation index /spl mu/. 2/spl mu//sup 2/ equals the product of the number of channels and the square of the optical modulation index per channel. The exact carrier-to-nonlinear distortion ratio per channel when all or specific orders of distortion are present is also treated. The results are presented in graphical form and they agree reasonably well with earlier theoretical and experimental works. All of the results apply to both a directly modulated laser or an externally modulated laser. >