A method to operate a semiconductor laser diode (LD) in a differential configuration is disclosed. The method first obtains the threshold current ITH in a bared LD under at least one temperature. Then, a linear relation with coefficients of α and β between the bias current IB and the modulation current IM independent of temperatures is evaluated by, under the operation of the APC circuit to set the bias current and under the at least one temperature, measuring at least two extinction ratios, ER1 and ER2, as varying the modulation current at two levels, IM1 and IM2. Two coefficients of α and β are estimated by a mathematical comparison.