Quadrature laser interferometer for in-line thickness measurement of glass panels using a current modulation technique

A thickness measurement system is proposed for in-line inspection of thickness variation of flat glass panels. Multi-reflection on the surfaces of glass panel generates an interference signal whose phase is proportional to the thickness of the glass panel. For accurate and stable calculation of the phase value, we obtain quadrature interference signals using a current modulation technique. The proposed system can measure a thickness profile with high speed and nanometric resolution, and obtain higher accuracy through real-time nonlinear error compensation. The thickness profile, measured by a transmissive-type experimental setup, coincided with a comparative result obtained using a contact-type thickness measurement system within the range of ±40 nm. The standard deviations of the measured thickness profiles and their waviness components were less than 3 nm with a scanning speed of 300 mm/s.