An optical data transmission system comprising an optical transmitter having a modulation apparatus suitable for modulation of an optical signal with a data signal, wherein the modulation apparatus has a configurable digital encoding unit adapted to encode data words of said data signal received via a data interface of said modulation apparatus to provide an encoded modulation control signal (EMCS); and a signal modulation unit adapted to modulate said optical signal with respect to its signal phase and/or signal amplitude in orthogonal polarization directions in response to said encoded modulation control signal (EMCS), wherein the signal modulation unit of said modulation apparatus is adapted to generate a multi-dimensional optical signal vector transported by an optical transport medium to an optical receiver of said data transmission system; - wherein the configurable digital encoding unit of said modulation apparatus comprises: an encoding data storage for storing several encoding lookup tables (13-1, 13-2,..., 13-k), an encoding selection circuit capable of selecting an encoding scheme stored in one of the encoding lookup tables in response to a feedback modulation control signal (FMCS) fedback by the optical receiver through an embedded communication channel, wherein the encoded modulation control signal (EMCS) comprises control bits (I x, I y, Q x, Q y) forming a constellation point for the multi-dimensional optical signal vector generated by said signal modulation unit, and a logic circuit adapted to perform logic operations on data bits of a data word of the received data signal using the selected encoding scheme, wherein the encoding selection circuit is configured to switch to another encoding lookup table stored in said encoding data storage based on the feedback modulation control signal (FMCS), wherein the feedback modulation control signal (FMCS) generated by the optical receiver comprises regular time intervals adjustable to a security level, SL, of an application. - wherein the configurable digital encoding unit of said modulation apparatus comprises: an encoding data storage for storing several encoding lookup tables (13-1, 13-2,..., 13-k), an encoding selection circuit capable of selecting an encoding scheme stored in one of the encoding lookup tables in response to a feedback modulation control signal (FMCS) fedback by the optical receiver through an embedded communication channel, wherein the encoded modulation control signal (EMCS) comprises control bits (I x, I y, Q x, Q y) forming a constellation point for the multi-dimensional optical signal vector generated by said signal modulation unit, and a logic circuit adapted to perform logic operations on data bits of a data word of the received data signal using the selected encoding scheme, wherein the encoding selection circuit is configured to switch to another encoding lookup table stored in said encoding data storage based on the feedback modulation control signal (FMCS), wherein the feedback modulation control signal (FMCS) generated by the optical receiver comprises regular time intervals adjustable to a security level, SL, of an application. an encoding data storage for storing several encoding lookup tables (13-1, 13-2,..., 13-k), an encoding selection circuit capable of selecting an encoding scheme stored in one of the encoding lookup tables in response to a feedback modulation control signal (FMCS) fedback by the optical receiver through an embedded communication channel, wherein the encoded modulation control signal (EMCS) comprises control bits (I x, I y, Q x, Q y) forming a constellation point for the multi-dimensional optical signal vector generated by said signal modulation unit, and a logic circuit adapted to perform logic operations on data bits of a data word of the received data signal using the selected encoding scheme, wherein the encoding selection circuit is configured to switch to another encoding lookup table stored in said encoding data storage based on the feedback modulation control signal (FMCS), wherein the feedback modulation control signal (FMCS) generated by the optical receiver comprises regular time intervals adjustable to a security level, SL, of an application.