Measurement of microwave behavior in optical links

An overview of how S-parameter techniques can be used and misused to measure the microwave behavior of microwave photonic components and links is presented. Most high speed optical fiber links will use laser diodes in the E/O source module. The amplitude, frequency or phase of the lightwave may be modulated by microwave signals. Once modulation has been applied to the lightwave, it will be guided by the optical fiber and be subjected to attenuation, dispersion, and possible changes in polarization. Intensity modulation of the E/O source can be achieved either directly or externally. Measurement of the microwave frequency response of the various components in a fiber link is a useful tool for the digital link designer. Where there is the possibility of multiple paths for optical transmission and reflection, one must ensure that the optical source's coherence is longer than the shortest optical path difference inside the component being measured.