Method for Generating Reference Currents in Proportion to Enable Control of Average Power and Extinction Ratio of Laser Modulation in an Optical Transmitter

A system for controlling an optical data transmitter comprising:

• current driver circuitry configured to supply a drive current to a laser diode;
• and
• monitor circuitry, said monitor circuitry comprising:
• a photodiode;
• and
• a first transimpedance amplifier coupled to said photodiode, wherein said monitor circuitry is configured to provide an output current related to an optical intensity of said laser diode;
• a second transimpedance amplifier, said second transimpedance amplifier configured to be substantially identical in construction to said first transimpedance amplifier;
• control circuitry configured to control said current driver circuitry configured to compare at least one property of outputs of said first and second transimpedance amplifiers, and generate at least one control signal to control said current driver circuitry based on the comparison, wherein
• the control circuitry comprises a reference current generator which comprises:
• a defined current source;
• a voltage controlled current source configured to generate said reference current configured to generate at least two currents whose magnitudes are in a fixed ratio wherein at least one of said currents is coupled to said second transimpedance amplifier and whose output is used by said control circuitry to control said current driver circuitry;
• a first plurality of serially coupled resistors, each resistor from the first plurality of serially coupled resistors being a unit resistor and configured such that one end of the first plurality of serially coupled resistors is coupled to a first reference voltage;
• and
• a first plurality of switches, configured such that at least one switch of the first plurality of switches is associated with one of the first plurality of serially coupled resistors, and configured to couple the defined current source and a first controlling input terminal of the voltage controlled current source to a same resistor from the first plurality of serially coupled resistors;
• a second plurality of serially coupled resistors, each resistor of the second plurality of resistors being a unit resistor whose resistance is substantially identical to the unit resistors in the said first plurality of serially coupled resistors and configured such that one end of the second plurality of serially coupled resistors is coupled to the said reference voltage;
• a second plurality of switches, configured such that at least one switch of the second plurality of switches is associated with one of the second plurality of serially coupled resistors, and configured to couple a first current from the voltage controlled current source and a second controlling input terminal of the voltage controlled current source to a same resistor from the second plurality of serially coupled resistors;
• and
• a look-up-table configured to control the first plurality of switches and the second plurality of switches based on an input to control a ratio between the defined current source current and at least one of the voltage controlled current source current.