Automatic bias control for electro-optic modulators

An electro-optic modulator system with automatic bias control comprising:

• a pilot signal generator for generating a pilot signal and for generating a coherency reference signal;
• an electro-optic modulator having an electrical input, an optical input and an optical output, the electrical input coupled to the pilot signal generator, the electro-optic modulator for modulating an optical carrier received at the optical input by the pilot signal to produce a modulated optical signal at the optical output, wherein the modulated signal includes a pilot component;
• an optical sampler coupled to the optical output of the electro-optic modulator, for sampling the optical output;
• a coherent feedback circuit coupled to the optical sampler and to the pilot signal generator for coherently generating an error signal based on the coherency reference signal and on the pilot component;
• a bias generator coupled to the coherent feedback circuit and to the electrical input of the electro-optic modulator, for generating a bias signal responsive to the error signal and applying the bias signal to the electrical input of the electro-optic modulator for controlling a bias point of the electro-optic modulator.

In a system comprising an electro-optic modulator having an electrical input, an optical input and an optical output, for modulating an optical carrier received at the optical input by an electrical signal received at the electrical input to produce a modulated optical signal at the optical output, a method for automatically controlling a bias point of the electro-optic modulator comprising the steps of:

• applying a pilot signal to the electrical input of the electro-optic modulator;
• generating a coherency reference signal based on the pilot signal;
• sampling the optical output of the electro-optic modulator, wherein the sampled optical output includes a pilot component resulting from the pilot signal;
• coherently generating an error signal based on the coherency reference signal and on the pilot component;
• generating a bias signal responsive to the error signal;
• and
• applying the bias signal to the electrical input of the electro-optic modulator.

A bias control system for automatically controlling a bias point of an electro-optic modulator, wherein the electro-optic modulator has an electrical input, an optical input and an optical output, for modulating an optical carrier received at the optical input by an electrical signal received at the electrical input to produce a modulated optical signal at the optical output, the bias control system comprising:

• a dual pilot tone generator for generating first and second pilot tones applicable to the electrical input of the electro-optic modulator, the first and second pilot tones characterized by first and second frequencies, respectively;
• an optical sampler for sampling the optical output of the electro-optic modulator;
• a difference frequency feedback circuit coupled to the optical sampler for generating an error signal based on a difference component in the sampled optical output, the difference component resulting from the first and second pilot tones and characterized by a difference frequency equal to the difference between the first and second frequencies;
• and
• a bias generator coupled to the feedback circuit for generating a bias signal responsive to the error signal, the bias signal applicable to the electrical input of the electro-optic modulator for controlling the bias point of the electro-optic modulator.

In a system comprising an electro-optic modulator having an electrical input, an optical input and an optical output, for modulating an optical carrier received at the optical input by an electrical signal received at the electrical input to produce a modulated optical signal at the optical output, a method for automatically controlling a bias point of the electro-optic modulator comprising the steps of:

• applying first and second pilot tones to the electrical input of the electro-optic modulator, the first and second pilot tones characterized by first and second frequencies, respectively;
• sampling the optical output of the electro-optic modulator, wherein the sampled optical output includes a difference component resulting from the first and second pilot tones and characterized by a difference frequency equal to the difference between the first and second frequencies;
• generating an error signal based on the difference pilot component;
• generating a bias signal responsive to the error signal;
• and
• applying the bias signal to the electrical input of the electro-optic modulator.