Patent ID: 8238395

Claim:
An automatic power control loop for adjusting the light intensity of a laser diode, comprising: a laser diode, used for receiving a bias current; a photo diode, connected to a first node, and used for sensing the light intensity of said laser diode to generate a feedback current that is equal to said bias current; a controller, used for outputting a control signal; a switch selector, disposed between said first node and a plurality of predetermined currents, comprising a plurality of switches, and connected to said controller to receive said control signal, wherein each of said predetermined currents corresponds to one among said switches respectively, wherein one among said predetermined currents is selected according to said control signal, and one among said switches corresponding to said selected predetermined current is turned on, such that said selected predetermined current is electrically connected to said first node; a transducer, connected to said first node, used for receiving a current difference between said feedback current and said selected predetermined current, and transferring said current difference into a load voltage; a comparator, connected to said transducer and said controller, and comprising a first input terminal and a second input terminal, wherein said first input terminal of said comparator is connected to a reference voltage, and said second input terminal of said comparator is used for receiving said load voltage, wherein said comparator is used for generating a comparison signal according to the comparison result of said load voltage and said reference voltage, then said comparison signal is transmitted to said controller, and thereby said controller changes the signal state of said control signal according to said comparison signal so as to decide said selected predetermined current; a counter, connected to said comparator and said controller to receive said comparison signal and said control signal, and used for recording and counting a count value, wherein said count value is counted up, counted down, or maintained at an original value according to said comparison signal and said control signal; and a laser diode driver, connected to said counter and said laser diode, and used for receiving said count value from said counter so as to generate said bias current relative to said count value.