An optical signal processing apparatus comprising: an optical switch (2; 11); means for generating control light (E P; E P1; E P2), said means being coupled to a first input of the optical switch (2; 11); means for generating a subcarrier modulated data signal (E S; E S1; E S2), the means for generating the subcarrier modulated data signal (E S; E S1; E S2) being coupled to a second input of the optical switch (2; 11); an optical combiner (3; 12) configured to receive a carrier light (E 0; E 01; E 02) at a first input of the optical combiner (3; 12) and the output of the optical switch (2; 11) at a second input of the optical combiner (3; 12), the optical combiner being configured to optically combine the received carrier light (E 0; E 01; E 02) and the received output of the optical switch to generate an output light; a nonlinear optical medium configured to receive, at an input of the nonlinear optical medium, the output light generated by the optical combiner (3; 12); an optical splitter (4; 13) whose input is coupled to an output of the nonlinear optical medium; and a receiver coupled to an output of the optical splitter (4; 13), wherein the apparatus is adapted to: (i) generate an idler light when the carrier light (E 0; E 01; E 02) is combined with the control light (E P; E P1; E P2) into the nonlinear optical medium, and to acquire by the receiver a signal multiplexed on the carrier light (E 0; E 01; E 02) from the idler light split by the optical splitter (4; 13); and (ii) multiplex a data signal on the carrier light (E 0; E 01; E 02) through cross phase modulation in the nonlinear optical medium, when the carrier light (E 0) is combined with the subcarrier modulated data signal (E S; E S1; E S2). an optical switch (2; 11); means for generating control light (E P; E P1; E P2), said means being coupled to a first input of the optical switch (2; 11); means for generating a subcarrier modulated data signal (E S; E S1; E S2), the means for generating the subcarrier modulated data signal (E S; E S1; E S2) being coupled to a second input of the optical switch (2; 11); an optical combiner (3; 12) configured to receive a carrier light (E 0; E 01; E 02) at a first input of the optical combiner (3; 12) and the output of the optical switch (2; 11) at a second input of the optical combiner (3; 12), the optical combiner being configured to optically combine the received carrier light (E 0; E 01; E 02) and the received output of the optical switch to generate an output light; a nonlinear optical medium configured to receive, at an input of the nonlinear optical medium, the output light generated by the optical combiner (3; 12); an optical splitter (4; 13) whose input is coupled to an output of the nonlinear optical medium; and a receiver coupled to an output of the optical splitter (4; 13), wherein the apparatus is adapted to: (i) generate an idler light when the carrier light (E 0; E 01; E 02) is combined with the control light (E P; E P1; E P2) into the nonlinear optical medium, and to acquire by the receiver a signal multiplexed on the carrier light (E 0; E 01; E 02) from the idler light split by the optical splitter (4; 13); and (ii) multiplex a data signal on the carrier light (E 0; E 01; E 02) through cross phase modulation in the nonlinear optical medium, when the carrier light (E 0) is combined with the subcarrier modulated data signal (E S; E S1; E S2). (i) generate an idler light when the carrier light (E 0; E 01; E 02) is combined with the control light (E P; E P1; E P2) into the nonlinear optical medium, and to acquire by the receiver a signal multiplexed on the carrier light (E 0; E 01; E 02) from the idler light split by the optical splitter (4; 13); and (ii) multiplex a data signal on the carrier light (E 0; E 01; E 02) through cross phase modulation in the nonlinear optical medium, when the carrier light (E 0) is combined with the subcarrier modulated data signal (E S; E S1; E S2).