An apparatus comprises an optical detecting signal generator configured to provide an optical spectrum comprising two frequency carriers, the two frequency carriers having two different nominal carrier frequencies, and the output port of the optical detecting signal generator being further configured to be coupled to a device under test (DUT); an optical to electrical converter configured to generate a first electrical current based on the optical spectrum without the optical spectrum passing through the DUT; and generate a second electrical current based on the optical spectrum after the optical spectrum passes through the DUT; and a data processor coupled to the optical to electrical converter, the data processor being configured to determine a transfer function of the DUT at an average of the two different nominal carrier frequencies based on the first electrical current and the second electrical current.