Optical transmitter comprising at least a first and a second optical line cards, an optical line card (10, 10', 10") comprising: - a first tunable laser, - a second tunable laser, - a first optical switch comprising a first input linked to the first tunable laser, a second input linked to the second tunable laser, a first output which can be linked to the first or to the second input, a second output which can be linked to the first or to the second input, - a dual modulation unit comprising: - a first modulator configured for modulating a first signal received on a first input linked to the first output of the first optical switch, the first modulated signal being transmitted to a first output, - a second modulator configured for modulating a second signal received on a second input (16'a) linked to the second output of the first optical switch, the second modulated signal being transmitted to a second output (16'b), - a first optical circulator comprising a first input/output port linked to the first output of the dual modulation unit and a second input/output port, - a second optical circulator comprising a first input/output port (18'a) linked to the second output (16'b) of the dual modulation unit and a second input/output port (18'b), - a second optical switch comprising a first input linked to the second input/output port of the first optical circulator and a second input linked to the second input/output port (18'b) of the second optical circulator, wherein the first and the second optical circulators comprise a third input/output port (18c, 18'c), the third input/output port (18'c) of the second optical circulator of an optical line card is configured to be linked to the third input/output port of the first optical circulator of the next optical line card. - a first tunable laser, - a second tunable laser, - a first optical switch comprising a first input linked to the first tunable laser, a second input linked to the second tunable laser, a first output which can be linked to the first or to the second input, a second output which can be linked to the first or to the second input, - a dual modulation unit comprising: - a first modulator configured for modulating a first signal received on a first input linked to the first output of the first optical switch, the first modulated signal being transmitted to a first output, - a second modulator configured for modulating a second signal received on a second input (16'a) linked to the second output of the first optical switch, the second modulated signal being transmitted to a second output (16'b), - a first modulator configured for modulating a first signal received on a first input linked to the first output of the first optical switch, the first modulated signal being transmitted to a first output, - a second modulator configured for modulating a second signal received on a second input (16'a) linked to the second output of the first optical switch, the second modulated signal being transmitted to a second output (16'b), - a first optical circulator comprising a first input/output port linked to the first output of the dual modulation unit and a second input/output port, - a second optical circulator comprising a first input/output port (18'a) linked to the second output (16'b) of the dual modulation unit and a second input/output port (18'b), - a second optical switch comprising a first input linked to the second input/output port of the first optical circulator and a second input linked to the second input/output port (18'b) of the second optical circulator,