The receiver of an OTU board is an avalanche photodiode(APD).

Avalanche photodiode (APD) receivers are commonly used on OTU boards due to their high sensitivity. The input optical power range typically lies between -4 dBm (maximum) and around -28 dBm (minimum). Among the options, -4 dBm is within the acceptable input power range, ensuring the APD operates without saturation or damage. Optical powers too high can saturate the receiver, while too low powers cause poor signal-to-noise ratio. HCIP-Transmission specifies these ranges to ensure optimal receiver performance and avoid optical overload or underpower conditions.