| 摘要 |
PURPOSE:To prevent the deterioration in extinction ratio by providing a circuit detecting a failure of a semiconductor laser diode to an optical transmission section of the existing side to bring the bias of a semiconductor laser diode driving circuit of the optical transmission section of the spare side to the normal state when a failure is detected. CONSTITUTION:An input signal is divided into two at a distributor 1 and enters PFM modulating circuits 7, 12 of the existing optical transmission section 2 and the spare optical transmission section 3. An output of a modulating circuit drives laser diodes (LD) 9, 14 via an LD driving circuit 8. The existing optical transmission section 2 is provided with a failure detecting circuit 11, and when the LD9 is normal, a bias current of an LD drive circuit 13 is brought to a low level and when a failure is detected, the current is brought to a normal level and the bias current of the LD driving circuit 9 is stopped. Thus, the deterioration in extinction ratio is prevented and the utilizing efficiency of the LD and the extension of life are attained. |
