With the advent of the information age, optical communication has been widely used in the communication field owing to its high transmission rate, significant information capacity, good security performance, and strong anti-interference capability. Narrowband filter films, which are crucial components of multiplexers and demultiplexers, are the key components of optical communication. With the development of optical communication, the demand for narrowband filter films are increasing. Currently, research on narrowband filter films is relatively well-established both domestically and internationally. However, there a scarcity of literature regarding the preparation of fine-wavelength division multiplexing (LWDM) filter films via thermal evaporation. Moreover, the LWDM filter film comprises a substantial number of layers, making it vulnerable to monitoring errors and spectrum distortion. Consequently, the development of narrowband filter films that possess a narrow bandwidth, small passband ripple, low insertion loss, and a large cutoff transmission isolation, and that meet the technical requirements of modern optical communication is of great research significance and has immense application value.

High-quality filter films for optical communication were deposited on K9 substrates using electron-beam thermal evaporation and ion-assisted deposition techniques. The film was designed using Ta₂O₅ and SiO₂ selected as high- and low-refractive-index materials, respectively, owing to their stable properties. The factors affecting the half-width of the passband of the F-P film system were theoretically calculated and analyzed, and the film system was adjusted according to Baumeister’s theory. Multiple single-cavity F-P film systems were connected in series to improve the spectral rectangularity, and a matching layer was added to reduce the passband ripple, resulting in the completion of the LWDM narrowband filter film design. The design of the LWDM narrowband filter film was completed. A SPOC-1300TCI vacuum coater was used to prepare the filter film. The effects of deposition rate and ion source energy on the surface roughness of Ta₂O₅ and SiO₂ were investigated. The surface roughness of Ta₂O₅ was smaller than that of the substrate, while that of SiO₂ was larger. The surface roughness of both materials gradually decreased and leveled off as the deposition rate increased. The effect of the ion source energy on the surface morphology of the film layer was minimal. Through the above single-layer experiments, the deposition rates of Ta₂O₅ and SiO₂ were chosen to be 0.4 and 0.8 nm/s, respectively. These rates were chosen while considering the influence of the deposition rate on the surface roughness of the film layer and challenges in monitoring the thickness of the film layer.

Owing to the numerous film layers and long coating time required for the narrow-band filter film, mechanical structures such as the vacuum degree of the vacuum chamber, material evaporation characteristics, correction plate alterations under long-term high-temperature environments, and the actual thickness of the film layer are inconsistent with the design. Moreover, there can be spectral distortions due to the unbalanced optical thickness ratio of Ta₂O₅. To address this issue, we propose a debugging method for achieving high-precision film thickness uniformity and spectral consistency (Fig. 9). According to the equivalent layer theory of the membrane system, the F-P membrane system with two cavities is equivalent to a single-cavity F-P membrane system; that is, the first and second cavity layers are equivalent to the reflection layer on both sides of the single-cavity F-P membrane system, and the original coupling layer is equivalent to the spacer layer on both sides of the single-cavity F-P membrane system. When the coupling layer in the 44 L double-cavity membrane system is changed from L to 2 L, it increases the thickness of the spacer layer and the interference order of the membrane system. As a result, transmission peaks appeared at the central wavelength. The wavelength spacing between adjacent transmittance peaks is not equal and changes with the material type and film thickness. The error of the optical thickness of Ta₂O₅ and SiO₂ can be quickly analyzed by inverse analysis of the coating results of special film systems (Figs. 10 and 11). The correction plate is adjusted according to the analysis results to adjust the ratio of the optical thickness of Ta₂O₅ and SiO₂ to improve the narrow-band filter film spectra. The film thickness monitoring method is studied, and a new monitoring method is proposed to monitor the accuracy of photoelectric polarization method. The optical direct monitoring method is used to monitor the film thickness during the plating process (Fig. 5); the substrate real-time measurement curve is fitted, and the film thickness is monitored according to the fitting results. Moreover, the coupling layer and non-regular film layer are monitored by the crystal-controlled average thickness method, which improves the monitoring accuracy.

Based on the above debugging, the resulting filter film has bandwidths of 4.1 and 6.0 nm at -0.2 dB and -27 dB, respectively, a maximum insertion loss of 0.14 dB in the passband, and a passband ripple of 0.04 dB, and a bandwidth of 6.0 nm at -27 dB, thereby satisfying the technical requirements of narrowband filter film for LWDM.