Method and a control circuit for controlling the extinction ratio of a laser diode

摘要

A control circuit (10) controls the operation of a laser diode (1) for controlling the average power output (Pav) and the extinction ratio. A state machine (21) controls the control circuit (10) which reads the current from a monitor photo diode (2) which is coupled to the laser diode (1). An amplifier (20) determines the average power output of the laser diode (1) which is fed to a first comparator (23). The first comparator (23) compares the average power output with a reference value set by a resistor (R3). The output from the comparator (23) is fed to the up/down pin of a first counter (25) which is clocked by the state machine (21). In the event that the average power output is too high the first counter (25) decreases the bias current to the laser diode (1) outputted by a constant current source (5), and vice versa. To determine the extinction ratio of the laser diode (1) the slope of the operating portion of the power/current characteristic curve is determined by applying an incremental increase (DELTAIm) to the modulation current (Im) of the laser diode (1), which is a small percentage of the modulation current (Im). The increase (DELTAPav) to the average power output (Pav) of the laser diode (1) resulting from the increase DELTAIm to the modulation current Im is determined through a latch (29) and an adder (33) and is fed to a second comparator (35) which compares the increase DELTAPav in the average power output with a reference value (DELTAPavref) set by the resistor (R4). A second up/down counter (28) increases the modulation current (Im) applied to the laser diode (1) by a constant current source (6) if the value of (DELTAPav) is less than (DELTAPavref) and vice versa.