Research on optical path optimization design and signal enhancement technology for direct optical film thickness control systems

Film thickness is a critical parameter that determines optical performance, and the accuracy of its monitoring system directly affects spectral characteristics. To mitigate the significant thickness control errors in conventional direct monitoring systems—caused by light source divergence and weak detector response signals—this paper proposes an externalized optical configuration. In this design, both the optical transmitter and receiver are placed outside the vacuum chamber, thereby avoiding interference from chamber’s vibration, temperature variations, and assembly inconsistencies. Additionally, an optical signal modulation scheme based on fiber coupling and collimation-focusing is introduced. By adopting an external integrated light source combined with multimode optical fibers and a composite optical path, and by optimizing component parameters through optical simulation to improve spot quality and energy density, the stability of both optical and electrical signals is enhanced. After optimization, irradiance at the fiber receiving end increased by 222.7%, signal strength by 156.6%, and the signal-to-noise ratio by 70.38%. The system’s performance was validated by preparing a narrowband filter film with a center wavelength of 2400 nm and a bandwidth of 40 nm, achieving a wavelength deviation within 1 nm over three repeated tests while consistently maintaining the 40 nm bandwidth. These results confirm that the system enables high-precision and stable film thickness monitoring even in spectral bands with weak detector response.