| 摘要 |
PURPOSE: To quickly recover a normal oscillation state and enable accurate alignment, if a mode hop state is generated when a laser diode is used as a light source. CONSTITUTION: Frequency difference is given to laser beams by acoustooptic modulator elements 14, 15 which beams are obtained by dividing a laser light outputted from a semiconductor light source 11 into two beams. An alignment mark is irradiated with the two laser beams via a beam splitter 26 or the like. A heterodyne beam from the alignment mark is converted into a beat signal with a photoelectric transducer 30. When a mode hop state is detected on the basis of a detection signal from a photodetector in the laser light source 11, via a high-pass filter or the like, the light emission intensity of a laser diode in the semiconductor laser light source 11 is a little shifted via an objective light intensity determining circuit 47 and a laser drive circuit 42. |
