Saturation of silicon photodiodes at high modulation frequency

We have used the constant-amplitude, 600-MHz beat note of a 2-mW He-Ne laser to measure the rms current delivered by silicon photodiodes at 600 MHz. We find that this current first rises with increasing radiant power, saturates, and then begins to decrease at average irradiances greater, similar 100 W cm(-2), even though the direct (or average) current remains linear with radiant power. We have also scanned across the faces of the diodes; we find that uniform dc responsivity need not imply uniform 600-MHz responsivity and that saturation of rf responsivity takes place first in regions of high dc responsivity. The response of photodiode at 600 MHz may drop an order of magnitude at an average irradiance of ~1000 W cm(-2); we have observed that the impulse response at roughly the same irradiance is accordingly lengthened.