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Optical performance monitoring is becoming an important network building block as the system becomes increasingly sophisticated and the network span becomes larger and larger. The general requirement for the optical performance monitor from the noise perspective is derived from the fundamental considerations of detector noises. Based upon this understanding, the monitor sensitivity, defined as the minimum input optical power that is required for a specified dynamic range of the signal-to-noise ratio (SNR) monitoring, for example, 30 dB of detectable SNR range, is shown related to the data rate and the monitor characteristics. It is also shown that within the optical layer, the bit-error rate as can be measured by a full receiver may not be a good measure for signal quality monitoring.