| 摘要 |
PURPOSE:To prevent a mistake of the deterioration of the mark ratio for the deterioration of output of a light emitting element by offsetting the variance value of the smoothed voltage due to the change of the mark ratio and the variable value of the reference voltage due to the change of the mark ratio each other. CONSTITUTION:A laser diode driving circuit 12 inputs a data signal X and outputs a drive current Id to drive a laser diode 11. A photodiode 13 sends an optical current Ip proportional to the light emitting output of the diode 11 to a current/voltage converter 14 to convert the current into the voltage Vp. This voltage is outputted to a smoothing circuit 15. The circuit 15 smoothes the voltage Vp into the voltage Vd. A reference voltage generating part 2 inputs the signal X to detect the data mark ratio (m) and varies the reference voltage Vt with the detection output of the ratio (m). A comparator 3 inputs the voltage Vd and Vt and monitors the voltage Vd with the voltage Vt defined as the threshold value. Then the comparator 3 outputs an alarm signal (y) when the voltage Vd exceeds the voltage Vt. Thus it is possible to prevent the mistake of the deterioration of the mark ratio the signal X for the deterioration of the light emitting output of the diode 11. |
