An apparatus comprises an optical signal generator configured to provide a first radiation comprising a first nominal carrier frequency and a second nominal carrier frequency, and provide a second radiation comprising a third nominal carrier frequency and a fourth nominal carrier frequency; an optical to electrical converter coupled to the optical signal generator and configured to: generate a first electrical current based on the first radiation and the second radiation without the second radiation passing through the Device under Test (DUT); and generate a second electrical current based on the first radiation and the second radiation after the second radiation passes through the DUT; and a data processor configured to determine a transfer function of the DUT at the third nominal carrier frequency and the fourth nominal carrier frequency based on the first electrical current and the second electrical current.