Biasing system for an optical modulator with double output

Method of modulating an optical signal in response to a signal with a frequency in a predetermined input band, comprising the following steps:

• supplying an optical signal to an input of an electro-optical modulator;
• supplying an external electrical signal with a frequency in said band to an input of said electro-optical modulator;
• modulating said input optical signal with said external electrical signal;
• dividing said modulated optical signal into a first and a second modulated optical signal, which are complementary to each other;
• supplying an electrical bias signal to an input of said electro-optical modulator,
• in which said stage of supplying an electrical bias signal comprises:
• generating an electrical signal at a predetermined frequency outside said predetermined band, this signal being superimposed on said external electrical signal;
• generating an electrical control signal in response to said modulated optical signal;
• characterized in that said step of generating an electrical control signal in response to said modulated optical signal comprises:
• separating a fraction of said first modulated optical signal and detecting a first electrical feedback signal at a frequency which is twice said predetermined frequency;
• separating a fraction of said second modulated optical signal and detecting a second electrical feedback signal at a frequency which is twice said predetermined frequency;
• subtracting said first electrical feedback signal from said second electrical feedback signal.

Method of modulating an optical signal in response to a signal whose frequency is in a predetermined input band, comprising the following steps:

• supplying an optical signal to an input of an electro-optical modulator;
• supplying an external electrical signal whose frequency is in said band to an input of said electro-optical modulator;
• modulating said input optical signal with said external electrical signal;
• dividing said modulated optical signal into a first and a second modulated optical signal which are complementary to each other;
• supplying an electrical bias signal to the input of said electro-optical modulator;
• measuring the amplitude of the odd-order distortions of said first and second modulated optical signals,
• characterized in that said electrical bias signal is generated in such a way as to balance the amplitude of the second-order distortions of said first and second modulated optical signals.

Optical emitter with external analog modulation, comprising an optical source, an electro-optical modulator and a driving circuit for said electro-optical modulator,

• said electro-optical modulator having:
• an optical input for an optical signal, connected to said optical source;
• a first electrical input for a modulating signal;
• a second electrical input for a bias voltage;
• and
• two optical outputs, supplying a first and a second analog modulated signal which are complementary to each other;
• each of said two optical outputs of said modulator being associated with an optical receiver capable of receiving a portion of modulated optical signal in which said optical receivers associated with said outputs are capable of generating, respectively, a first and a second electrical control signal in response to a second frequency of said first and second analog modulated optical signals,
• in which said driving circuit comprises:
• an input for an external analog electrical signal;
• an input for a predetermined bias voltage;
• two inputs for said first and second electrical control signals;
• a circuit generating an electrical pilot signal at a first frequency;
• an algebraic adder capable of producing at its output the difference between said first electrical control signal and said second electrical control signal;
• a demodulator capable of producing at its output a demodulated signal of said difference between said first electrical control signal and said second electrical control signal;
• an algebraic adder capable of producing at its output a continuous signal equal to the difference between said demodulated signal and said predetermined bias voltage;
• an algebraic adder capable of producing at its output said bias voltage which is equal to the sum of said continuous signal and said electrical pilot signal at a first frequency.