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In Hayes's (1993) and Williams's (1996) analyses, the photodiode nonlinearity is attributed to the space charge screening effect. In this paper, the third-order intermodulation distortions of a high frequency, large optical cavity p-i-n waveguide photodiode are characterized up to 18 GHz using a two-tone measurement. At high bias voltage, the third-order intercept point (IP3) of the waveguide photodiode is constant at low frequency and /spl sim/f/sup -3/ at high frequency. This closely agrees with our model, which is based upon variation of the photodiode impedance. The measured IP3 of the same device at low bias voltages indicates the contribution of space charge screening under low bias voltage or severe saturation.