Patent ID: 7418018

Claim:
A driving circuit for a semiconductor laser diode for controlling an output power and an extinction ratio of light emitted from the laser diode by supplying a modulation current and a bias current responding to an external signal, the driving circuit comprising: a first current-to-voltage converter for converting a photocurrent output from a photodiode that monitors the light emitted from the laser diode into a monitored signal; a current generator for outputting a reference current generated by superposing a reference bias current on a reference modulation current modulated by the external signal; a second current-to-voltage converter for converting the reference current into a reference signal; a first error amplifier configured to compare a first level of the monitored signal with a first reference level of the reference signal and to output a first error signal; a second error amplifier configured to compare a second level of the monitored signal with a second reference level of the reference signal and to output a second error signal; a bias driver for controlling the bias current by receiving the first error signal; a modulation driver for controlling the modulation current by receiving the second error signal; and a variable capacitor provided between the current generator and the second current-to-voltage converter, wherein the first error amplifier outputs the first error signal so as to equalize the first level of the monitored signal to the first level of the reference signal, and the second error amplifier outputs the second error signal so as to equalize the second level of the monitored signal to the second level of the reference signal, wherein the variable capacitor compensates for a junction capacitance of the photodiode.