optical receivers and a method for providing a diagnostic measurement of optical modulation amplitude at other than a signal output of an optical receiver are invented and disclosed . the method includes the steps of applying a representation of an output of an optical detector to a circuit that determines a difference between a first signal level and a second signal level that is different from the first signal level and buffering the difference between the first signal level and the second signal level received from the circuit . the optical receivers include an optical detector , a transimpedance amplifier that applies automatic gain control , a circuit that determines a difference between two signal values and a limiting post amplifier . the receivers provide a diagnostic measure that can be compared to certain thresholds relating to known bit error rates or applied in a calculation to generate an absolute optical modulation amplitude value .