Apparatus configured to control a guided wave electro-optical switch, wherein the guided wave electro-optical switch comprises a port for receiving a control voltage signal and has a transfer function such that its optical power output P varies with the level of control voltage applied to its port between a minimum value P(V min) representing the OFF state, when the applied control voltage level is equal to a first value V min, and a maximum value P(V max) representing the ON state, when the applied control voltage level is equal to a second value V max, the apparatus comprising: signal processing means comprising a voltage generator configured to generate the control voltage signal, wherein the signal processing means is configured to perform a calibration procedure to determine a third value V a and a fourth value V b of the control voltage signal, the third value V a and the fourth value V b being voltages in the vicinity of the second value V max that are respectively smaller and larger than V max and determined such that the optical power output P(V a) and the optical power output P(V b) are identical and equal to a predetermined fraction of the maximum value P(V max) of the optical power output P, wherein the control voltage signal is a pulse train whose pulses have a voltage level alternating between the first value V min and either the third value V a or the fourth value V b, the voltage generator being arranged to apply the control voltage signal to the port of the electro-optical switch, wherein the signal processing means is configured to receive an electrical signal representative of the optical power P output from the electro-optical switch and, subsequent to the calibration, to continuously monitor the electrical signals P(V a) and P(V b) representative of the output optical power P at the calibration voltage values V a and V b, the signal processing means being further configured to determine, on the basis of the monitored electrical signals P(V a) and P(V b), the value of a visibility function vis(V 0, V a, V b) expressed as vis(V 0, V a, V b) = [P(V a,V 0) - P(V b,V 0)]/[P(Va,V 0) + P(V b,V 0)], wherein V 0 is the optical bias voltage for an optical bias drift of zero, the signal processing means being further configured to derive a corrective voltage as a predetermined function of the value of the visibility function, and to apply the thus derived corrective voltage to the control voltage signal in order to compensate for a voltage drift caused by optical bias variation of the electro-optical switch over time. signal processing means comprising a voltage generator configured to generate the control voltage signal, wherein the signal processing means is configured to perform a calibration procedure to determine a third value V a and a fourth value V b of the control voltage signal, the third value V a and the fourth value V b being voltages in the vicinity of the second value V max that are respectively smaller and larger than V max and determined such that the optical power output P(V a) and the optical power output P(V b) are identical and equal to a predetermined fraction of the maximum value P(V max) of the optical power output P, wherein the control voltage signal is a pulse train whose pulses have a voltage level alternating between the first value V min and either the third value V a or the fourth value V b, the voltage generator being arranged to apply the control voltage signal to the port of the electro-optical switch, wherein the signal processing means is configured to receive an electrical signal representative of the optical power P output from the electro-optical switch and, subsequent to the calibration, to continuously monitor the electrical signals P(V a) and P(V b) representative of the output optical power P at the calibration voltage values V a and V b, the signal processing means being further configured to determine, on the basis of the monitored electrical signals P(V a) and P(V b), the value of a visibility function vis(V 0, V a, V b) expressed as vis(V 0, V a, V b) = [P(V a,V 0) - P(V b,V 0)]/[P(Va,V 0) + P(V b,V 0)], wherein V 0 is the optical bias voltage for an optical bias drift of zero, the signal processing means being further configured to derive a corrective voltage as a predetermined function of the value of the visibility function, and to apply the thus derived corrective voltage to the control voltage signal in order to compensate for a voltage drift caused by optical bias variation of the electro-optical switch over time.