To address the limitations of current direct optical film thickness monitoring systems， including the dispersion of halogen lamp light sources， weak detector signal amplitudes， and poor monitoring accuracy in the short-wave band， as well as the spectral non-rectangularity-induced errors in the fabrication of narrow-band filter films （bandwidth <5 nm）， this study proposes an innovative optical system design. By analyzing the intensity distribution of the HLX64623 halogen lamp and the principles of optical fiber coupling for signal reception， an initial structure for a collimated focusing coupled optical system was developed based on the structural parameters of the coating machine. The design incorporates an aspheric U-type reflector and free-form surface lenses to achieve collimation of the dispersive light source， focusing of optical signals， and efficient optical fiber coupling. Using Zemax software， the system was iteratively optimized with the irradiance and spot size on the substrate and the optical fiber receiving end as key objectives. After optimization， the irradiance intensity on the substrate increased by 147%， while the irradiance intensity at the optical fiber receiving end improved from 0 to 1.53%， compared to the independent light source configuration. When installed on a coating machine， the collimated focusing coupled optical system demonstrated a 362.3% improvement in signal intensity and a 91.9% enhancement in the signal-to-noise ratio compared to the independent light source. Experimental validation involved the fabrication of narrow-band filter films with a central wavelength of 365 nm and a bandwidth of 10 nm. Continuous deposition across three coating cycles achieved a central wavelength shift of less than 1 nm and a full width at half maximum of 10 nm. These results confirm that the proposed system enables high-precision monitoring of narrow-band filter films across wavelengths ranging from the near-ultraviolet to the visible and near-infrared regions.

optical film;film thickness monitoring;optical path design;fiber optic coupling;optical film thickness