Input amplitude modulated optical signals I i (t) and Q i (t) having wavelengths » I and » Q respectively, are delivered from two multi-input interfaces (1, 2) to optical wavelength converters (4, 5), while a monochrome non-modulated continuous optical signal with a constant optical power and having an output wavelength » o is delivered from a light source (3). In the optical wavelength converters (4, 5), the input amplitude-modulated optical signals I i (t) and Q i (t) are copied onto optical signals with the output wavelength » o , and a inphase component I(t) is generated and, respectively, a quadrature component Q (t), whose phase is then moved by /2 of the output wavelength » o . After summing up the components in the adder (7) the modulated quadrature optical signal IQ (t) with an optical carrier having the output wavelength » o is obtained.
