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The nonlinearities generated by wide-band, high-current photodetectors are discussed in the context of the performance of an externally modulated analog radio-frequency fiber-optic link. The quantitative impart of the detector's third-order intermodulation product intercept (IP3) on the link's spur-free dynamic range is analyzed. Waveguide p-i-n photodiodes were developed to specifically address these stringent IP3 goals. The IP3 was measured using a four laser heterodyne system resulting in an IP3 of 27 dBm measured at a center frequency of 20 GHz with 20 mA of de photocurrent. The authors believe that this is the highest photodetector IP3 reported to date at these operating conditions.