The 2nd-order nonlinear distortion of avalanche photodiodes (TIXL-55, C-20817) was found to be less than −60 dB for an incident average optical power of −20 dBm modulated at a peak depth of 75%. p-i-n photodiodes (HP 5082-4207) were found to have 2nd-order-distortion levels below the noise level. A method using three independent optical signal sources was employed in the measurement.