Objective The scattering loss of optical thin film is an important index to evaluate the performance of optical thin films. With the development of sophisticated laser test systems and high-precision optical systems, the optical industry has established high requirements for the performance of low-loss optical thin films. The scattering of thin films mainly arises from two aspects: bulk scattering and interface scattering, and the amount of interface scattering is much greater than that of bulk scattering. The scattering of single-layer films has been studied in detail by many scholars in China and other countries, but the scattering of multilayer films is more complex and difficult to study. The 1064 nm Nd∶YAG solid laser has been used widely in laser medical treatment, laser beauty, and distance measurement. Because the 1064 nm bandpass filter is an important part of the laser, it is particularly important to study the scattering of a 1064 nm multilayer film bandpass filter. Alternating evaporation of high- and low-refractive-index materials is usually accompanied by a change in vacuum degree, and this change may aggravate the scattering of thin films and affect the spectral characteristics of multilayer films. In this study, we deposited a 1064 nm bandpass filter film by two different vacuum control methods, tested the effects of two different processes on the film scattering and spectrum, and analyzed the influence mechanism of scattering under two different processes. By analyzing the process and scattering, we hope to present a manufacturing method for depositing low-scattering-loss bandpass filter films.
Methods First, we designed a 1064 nm bandpass filter by using the theory of film design with scattering. Second, on K9 substrate with the same roughness, the 1064 nm bandpass filter film was deposited via ion-assisted deposition with TiO2 and SiO2, which were deposited under the same oxygen partial pressures and different oxygen partial pressures. Third, the film's spectrum was characterized using a spectrophotometer, which showed that the transmittance of the film deposited at the same oxygen partial pressure was better. Then, the effects of material parameters and random errors were eliminated by fitting. After that, by profilometer, scanning electron microscope, and integrating sphere, we concluded that the spectral difference mainly comes from scattering. Finally, the scattering formulas of the multilayer films under the model of complete and incomplete correlation were derived from scalar scattering theory, and the formulas were used to fit the transmittance curve with scattering under the two manufacturing methods, thus verifying the conclusion.
Results and Discussions The influences of various manufacturing factors on the scattering of multilayer films of 1064 nm bandpass filters and their spectral properties are the main focus of this work. First, we calculated the angular resolution scattering of two films with different permutation stacks (Fig. 5), which can provide guidance for the design of the less-scattering film (Fig. 6). Second, a 1064 nm bandpass filter film was deposited by two different methods of oxygen partial pressure control (Table 4), leading to the conclusion that the spectrum of films at the same oxygen partial pressure is better than that of films at different oxygen partial pressures (Fig. 7). Then, we analyzed the influence factors and mechanism of transmittance under two manufacturing methods, excluding the influences of the material's optical constant (Figs. 3--4) and film thickness control error (Figs. 8--10) and regarding the interface roughness and light scattering as the main causes of the transmittance reduction, and verified the accuracy of judgment through a roughness test (Fig. 11, Table 5), scanning electron microscope test (Fig. 12) and integral scattering test (Fig. 13). Third, based on the single-interface scalar scattering theory, we derived a model to calculate the total scattering loss and transmittance of multilayer films with complete and incomplete correlation, and used the roughness and spectral data of the films prepared under two different processes to verify the accuracy of the calculation model (Figs. 14--15).
Conclusions Based on the scalar scattering theory, we designed a low-scattering-loss 1064 nm bandpass filter film, which was deposited using fixed and alternating oxygen partial pressures for TiO2 and SiO2 materials on K9 substrates with the same roughness. The spectra of the films deposited under a fixed oxygen partial pressure were better than those of films deposited under alternating oxygen partial pressures. According to the profilometer test, scanning electron microscope test, and integrating sphere test, the main reason for this situation was that the change of the oxygen partial pressure caused instability of the gas distribution within the ionization chamber of the Kaufman ion source, causing an unstable ion beam. The unstable ion beam produced an uneven structure in the initial thin-film deposition and increased the film-interface roughness, both of which induced the scattering that affected the film's transmittance. Finally, the transmittance curves of the films deposited under the two manufacturing methods were fitted by the spectral calculation formula with scattering, which fit well with the actual transmittance test curves.
