| 摘要 |
PURPOSE:To improve the stability of oscillation and an S/N ratio by providing a photodetector on the monitor output side of a semiconductor laser and a lens on the front photodetection surface of the photodetector, and thus reducing the amount of feedback to a laser cavity due to scattering light. CONSTITUTION:When a diaphragm film 15 is displaced by sound pressure, a light output from one output surface of a semiconductor laser 11 is reflected by the reflecting film surface of the film 15, and fed back to the cavity of the laser 11, causing interference. Variation in light output from the other output surface of the laser 11 based upon the interference is detected by a photodetector 16, and the displacement of the film 15 is transduced into an electric signal. In this case, the center axis B of the photodetector 16 is slightly shifted from the optical axis A of the laser 11 and a lens 17 is provided on the flat photodetection surface 16a of the photodetector 16, so that light from the other light output surface of the laser 11 is refracted through the lens 17 and guided to the photodetection surface 16a. As a result, scattering light reflected by the detector 16, lens 17, etc., is reduced through the lens 17 to reduce the amount of feedback to the laser 11. |
