Method for characterizing an optical device, which comprises: - a 90°optical hybrid unit having a first and a second optical input (30E1, 30E2) and at least two optical outputs (30A1-30A4) wherein optical output signals (So1-So4) leaving the optical outputs have optical phase differences between each other of 90° or multiple thereof; - a first photodetector connected to a first optical output (30A1) and a second photodetector connected to a second optical output (30A2), wherein the first optical output emits a first optical output signal and the second optical output emits a second optical output signal, said second optical output signal having an optical phase difference of 180° relative to the first optical output signal; - a first transimpedance amplifier connected to the first and second optical outputs of the 90°optical hybrid unit; - a third photodetector connected to a third optical output (30A3) of the 90°optical hybrid unit, and - a fourth photodetector connected to a fourth optical output (30A4) of the 90°optical hybrid unit, the third optical output emitting a third optical output signal and the fourth optical output emitting a fourth optical output signal, said fourth optical output signal having an optical phase difference of 180° relative to the third optical output signal; characterized in that - a first intensity-modulated optical input signal and a second intensity-modulated optical input signal are generated; - the first intensity-modulated optical input signal is inputted into the first optical input and the second intensity-modulated optical input signal is inputted into the second optical input; - the optical phase of the second intensity-modulated optical input signal is phase-shifted relative to the first intensity-modulated optical input signal; and - the electrical output signal of the first transimpedance amplifier is used to characterize the optical device; - wherein the electrical output signals (I3, I4) of the third and fourth photodetector are low-pass filtered and the low-pass filtered signals (I3', I4') are inputted into an operational amplifier, and - wherein the output signal of the operational amplifier is used to control an optical phase shifter which controls the optical phase-difference (φ) between the first and second intensity-modulated optical input signals in order to provide that the low-pass filtered signals have the same amplitude. - a 90°optical hybrid unit having a first and a second optical input (30E1, 30E2) and at least two optical outputs (30A1-30A4) wherein optical output signals (So1-So4) leaving the optical outputs have optical phase differences between each other of 90° or multiple thereof; - a first photodetector connected to a first optical output (30A1) and a second photodetector connected to a second optical output (30A2), wherein the first optical output emits a first optical output signal and the second optical output emits a second optical output signal, said second optical output signal having an optical phase difference of 180° relative to the first optical output signal; - a first transimpedance amplifier connected to the first and second optical outputs of the 90°optical hybrid unit; - a third photodetector connected to a third optical output (30A3) of the 90°optical hybrid unit, and - a fourth photodetector connected to a fourth optical output (30A4) of the 90°optical hybrid unit, the third optical output emitting a third optical output signal and the fourth optical output emitting a fourth optical output signal, said fourth optical output signal having an optical phase difference of 180° relative to the third optical output signal; - a first intensity-modulated optical input signal and a second intensity-modulated optical input signal are generated; - the first intensity-modulated optical input signal is inputted into the first optical input and the second intensity-modulated optical input signal is inputted into the second optical input; - the optical phase of the second intensity-modulated optical input signal is phase-shifted relative to the first intensity-modulated optical input signal; and - the electrical output signal of the first transimpedance amplifier is used to characterize the optical device; - wherein the electrical output signals (I3, I4) of the third and fourth photodetector are low-pass filtered and the low-pass filtered signals (I3', I4') are inputted into an operational amplifier, and - wherein the output signal of the operational amplifier is used to control an optical phase shifter which controls the optical phase-difference (φ) between the first and second intensity-modulated optical input signals in order to provide that the low-pass filtered signals have the same amplitude.