Signal fluctuation suppression in confocal measurements by laser modulation and power monitoring

Signal fluctuation is an important issue in confocal measurements because minor intensity variation would cause a distinct deviation in the measurement result. In this work, it is proposed to use laser modulation and power monitoring to suppress the signal fluctuation in confocal measurements. The power monitoring is utilized to obtain the reference signal for correcting detection intensities, and the laser was operated at a modulated frequency to avoid unexpected vibrations from the ambient environment. The results indicated that the detection stability was well improved at least 3 times in the proposed way. Based on that, the precise measurement was successfully realized to resolve the nanometer-scaled structure of a blazed grating, and the surface topography was also achieved for a SIM card in a range of 40 μm. The proposed way is practical in confocal measurements, and it is hoped to be applied in more applications.