| 摘要 |
PURPOSE:To prevent an accident that a laser diode is damaged by a method wherein, when the output of an external pin photodiode does not exist, the input of a laser-beam power control circuit is changed over to the output of an internal pin photodiode by using a changeover means and an electric current flowing to the laser diode is controlled by means of the output. CONSTITUTION:A protective circuit which is constituted of the following is installed additionally to an automatic laser-beam power control circuit: an I/V amplifier 11; a TTL inverter 12; an AND circuit 13; a low-pass filter 14; and a changeover device 15. In a normal operation, the influence of a beam of returned light is reduced by using an external pin photodiode 10, and an electric current flowing in a laser diode 1-1 can be conrolled. When a laser beam is turned aside and cannot be detected by means of the external pin photodiode 10, the input of the laser-beam power control circuit is instantaneously changed over to the output of an internal pin photodiode 1-2 by means of the protective circuit such as the changeover device 15 or the like. Thereby, it is controlled that an overcurrent does not flow to the laser diode 1-1 so as to prevent the accident of the damage of the laser diode. |
