Patent ID: 8576886

Claim:
A laser diode drive circuit to drive multiple laser diodes under automatic power control (APC), the laser diode drive circuit comprising: a first photo diode to detect light emission amounts of the multiple laser diodes; multiple second photo diodes corresponding to the respective multiple laser diodes, to detect light emission amounts of the corresponding laser diodes, a number of the second photo diodes being the same as a number of the multiple laser diodes; a first photo diode connection terminal to connect the first photo diode; multiple second photo diode connection terminals to connect the multiple second photo diodes; multiple APC controllers provided for the respective multiple laser diodes, each of which controlling the light emission amount of one of the multiple laser diodes automatically based on a first monitor current from the first photo diode connected to the first photo diode connection terminal or a second monitor current from a respective one of the multiple second photo diodes connected to the corresponding multiple second photo diode connection terminals; a monitor current path, formed between the first photo diode connection terminal and the respective multiple APC controllers, through which the first monitor current flows from the first photo diode connection terminal to the corresponding multiple APC controller; multiple switches provided for the respective multiple laser diodes, disposed between the monitor current path and the respective multiple APC controllers, to connect and disconnect the monitor current path with the respective APC controllers; and a detector to detect whether or not the first photo diode is connected to the first photo diode connection terminal, and cause all of the switches to disconnect the monitor current path from the respective APC controllers when the first photo diode is not connected to the first photo diode connection terminal.