  The distributed absorption of photons in photodiodes induces an excess noise
in continuous-wave photodetection above the transit-time roll-off frequency. We
show that it can be treated as a frequency-dependent excess optical loss in
homodyne detection. This places a limit on the bandwidth of high-accuracy
homodyne detection, even if an ideal photodetector circuit is available. We
experimentally verify the excess loss in two ways; a comparison of signal gain
and shot-noise gain of one-port homodyne detection, and a balanced homodyne
detection of squeezed light at 500 MHz sideband. These results agree with an
analytic expression we develop, where the randomness of the photoabsorption is
directly modeled by an intrusion of vacuum field. At 500 MHz, we estimate the
excess loss at 14% for a Si-PIN photodiode with 860 nm incident light, while
the numerical simulation predicts much smaller excess loss in InGaAs
photodiodes with 1550 nm light.
