Comparison of alternative modulation techniques for microwave optical links

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Abstract
Directly- and externally-modulated fiber optic links have been compared in terms of gain, noise, and dynamic range. Data obtained indicate that direct modulation has gain and noise figures relatively insensitive to the optical bias level, provided the increased dynamic range at the higher laser bias level; excess optical noise caused by relaxation oscillation in the semiconductor laser limited the noise figure and dynamic range. For the externally-modulated link, gain and noise figures appeared to significantly depend on the optical input power and RF switching power, but the dynamic range is primarily determined by the optical input power. Photodetector linearity is a significant factor limiting the dynamic range of both of these links, especially at lower microwave frequencies.

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