Laser modulation control method and apparatus

A method of controlling a semiconductor laser having an average laser drive modulated by a laser drive signal to provide an optical signal comprising:

• providing a first controllable current through the laser;
• providing a second controllable current through the laser, the second current being modulated between on and off conditions by an input signal;
• providing a third current proportional to the second current through the laser, the third current also being modulated between on and off conditions by the input signal, the third current also being modulated between on and off conditions by a pilot tone, the pilot tone having a frequency which is less than the frequency of the input signal;
• sensing part of the optical signal from the laser to provide a measure of the average optical output of the laser and to provide a measure of the variations in the optical output of the laser caused by the pilot tone;
• controlling the first current to provide a predetermined average optical signal from the laser;
• and,
• controlling the second current to provide a predetermined variation in the optical signal from the laser caused by the pilot tone.

A semiconductor laser and laser control system comprising:

• a semiconductor laser;
• a first controllable current source biasing the semiconductor laser;
• a second controllable current source selectively switchable across the semiconductor laser responsive to an input signal;
• a third current source proportional to the second current source and selectively switchable across the semiconductor laser responsive to a pilot tone of a frequency substantially lower than the frequency of the input signal and responsive to an input signal;
• a sensor sensing part of the laser emission and providing a sensor output responsive thereto;
• a first circuit responsive to the average sensor output controlling the first current source to maintain a predetermined average laser emission;
• and,
• a second circuit responsive to the variations in the sensor output at the pilot tone frequency controlling the second current source to maintain a predetermined laser emission at the pilot tone frequency.

A semiconductor laser control system comprising:

• a first controllable current source for biasing a semiconductor laser;
• a second controllable current source selectively switchable across a semiconductor laser responsive to an input signal;
• a third current source proportional to the second current source and selectively switchable across a semiconductor laser responsive to a pilot tone of a frequency substantially lower than the frequency of the input signal and responsive to an input signal;
• a sensor for sensing part of the emission of a semiconductor laser and providing a sensor output responsive thereto;
• a first circuit responsive to the average sensor output controlling the first current source to maintain an average sensor output;
• and,
• a second circuit responsive to the variations in the sensor output at the pilot tone frequency controlling the second current source to maintain a predetermined sensor output at the pilot tone frequency.