Abstract

When a coherent light source is modulated and coupled to a fiber, dispersion induces harmonics because the light modulators and detectors are not linear. However, when intensity modulated, the optical power varies linearly with modulating signal, and the electric field varies as the square root of the modulating signal.¹ As a result of this nonlinear dependence on the electric field, the optical spectrum contains sidebands of the modulating frequency and field, the optical spectrum contains sidebands of the modulating frequency and its harmonics. Current modulation of a semiconductor laser also causes wavelength modulation,² resulting in additional sidebands at harmonics of the modulating frequency. These nonlinear source effects, when dispersed by a fiber and detected by a square law (nonlinear) photodiode, generate harmonic distortion.

© 1982 Optical Society of America