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In this letter, we present an experimental characterization of third-order intermodulation distortion (IM3) at 5.8 GHz in a unitraveling-carrier refracting-facet photodiode (UTC-RFPD) and a p-i-n refracting-facet photodiode (pin-RFPD). The IM3 in the pin-RFPD is considerably reduced compared to the waveguide-type p-i-n photodiode. The third-order intercept point (IP3) of the UTC-RFPD does not decrease as the photocurrent increases, indicating that space-charge-induced nonlinearity is well suppressed in this photodiode. The two-tone IP3 of the UTC-RFPD estimated from the measurement reaches a very high level of 40.5 dBm.