A modulating system (14) adapted to generate a multi-level quadrature amplitude modulation, includes: a first number of first optical channels (24_1), each of the first optical channels (24_1) including a modulating device (28), and a second number of first optical channels (24_1) each including a first phase shifting unit capable of introducing a phase shift of π, and a first number of second optical channels (24_2), each of the second optical channels (24_2) being associated bijectively with one of the first optical channels (24_1), each of the second optical channels (24_2) including the same elements as the first optical channel (24_1) with which the second optical channel (24_2) is associated and a second phase shifting unit able to introduce a phase shift of π/2.