| 摘要 |
PROBLEM TO BE SOLVED: To facilitate the setting of a threshold and to prevent the malfunction of alarm output enlarging identification potential when a mark rate is low through the use of a limitter amplifier. SOLUTION: A photo current Ip proportional to the level of light which an LD element 1 emits is detected through a PD(photodiode) element 2 monitoring light which the LD (semiconductor laser) element 1 emits. The photo current Ip is converted into voltage Vp proportional to the photo current Ip by an I/V conversion circuit 3. A limitter amplifier 4 operates so that the potential difference of output voltage V1 becomes the largest in the mark rates 1/(m+1) and 1/m, in which the threshold is set. When the mark rate is not less than 1/(m-1), limited voltage is outputted. A comparator 5 is set so that it gives the output voltage V1 of a limitter amplifier 4 when the mark rate is not more than 1/(m+1) being a first threshold, and it is released when it is not less than 1/m. |
