a modulation signal input of the second modulator 5 the outputs of both modulators 4 and 5 are connected to a first combiner 3 db optical coupler 6 one output 7 is chosen as transmitter output the laser sources 1 and 2 emit a first carrier signals l 1 with a first carrier frequency f 1 and second carrier signal l 2 with a second carrier frequency f 2 having a phase difference compared to f 1 the carrier signals l 1 and l 2 are fed to the inputs of the first coupler 3 3 db coupler splitter the output signals of the first coupler 3 are the input signals of the modulators 4 and 5 neglecting constant factors can be derived as the mixed carrier signals l 1 jl 2 jl 1 l 2 can also be derived by modulation a single laser source e g as described in an article by a sano proceedings of ecec 2007 incorporated by reference the first mixed carrier signal l 1 jl 2 is modulated by the first modulation signal a t and the second mixed carrier signal jl 1 l 2 is modulated by the second modulation signal b t l 1 j u2062 u2062 l 2 j u2062 u2062 l 1 l 2 u2062 u2062 modulation u2062 u2062 by u2062 u2062 a u2061 t b u2061 t u2192 a 1 j u2062 u2062 a 2 j u2062 u2062 b 1 b 2 2 the indices 1 and 2 are still indicating the carrier frequencies f 1.