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.