As optical technology continues to advance toward higher precision and higher power, coating technologies for complex spectral optics are playing an increasingly critical role in numerous cutting-edge fields such as laser processing, optical communications, and biomedical applications. These optical components primarily include filters, beam splitters, and beam combiners, and their performance bottlenecks are mainly concentrated in two areas: efficiency optimization and high-power reliability.

I. Key Factors Limiting Coating Efficiency

1. Depolarization Treatment under 45° Incidence

For any optical thin film (single-layer or multi-layer), its spectral characteristics are determined by the effective refractive index (optical admittance) of the materials. Under non-normal incidence, the effective refractive indices for s-polarized and p-polarized light are respectively:

ηs = n·cosθ
ηp = n/cosθ

where θ is the angle of incidence and n is the intrinsic refractive index of the thin-film material. Only at normal incidence (θ = 0°) are the effective refractive indices of the two polarization states equal. Once an incident angle is introduced, the effective refractive indices of s- and p-polarizations separate. Furthermore, the equivalent phase thickness experienced by the two polarization states within the film layers also differs. Together, these effects constitute the physical origin of thin-film polarization effects and represent the core challenge in the design of tilted-incidence beam-splitting components.

To eliminate polarization effects, depolarization design methods such as equivalent layer design theory and global optimization algorithms for non-regular coating systems are typically employed. In addition, schemes combining metal-dielectric coating systems with asymmetric equivalent admittance matching layers have been validated in engineering projects such as spaceborne remote sensing, capable of controlling polarization sensitivity to low levels across a wide spectral band. These matching layers often have irregular thicknesses and are highly sensitive to polarization separation, placing stringent demands on thickness control precision.

2. Steepness of the Transition from Reflection to Transmission Zone

The greater the steepness of the spectral transition zone, the more significant the challenges posed to coating system design and fabrication:

• The complexity of coating design and processing increases sharply with steepness, imposing higher requirements on process stability and systematic error control. Notably, recent research has demonstrated that introducing a third thin-film material (e.g., the three-material Nb₂O₅/Al₂O₃/SiO₂ system) can effectively improve steepness without increasing the total number of layers. Under the same layer thickness conditions, the transition zone width can be narrowed by over 3 nm compared to conventional two-material solutions.

• The demand for wavelength positioning precision correspondingly rises, elevating the risk of scrap and rework during manufacturing and prolonging process trial cycles.

• Sensitivity to thickness uniformity increases significantly, making uniformity control critical when coating large-area or curved substrates.

II. Thermal Deformation and Absorption Control under High-Power Conditions

The issue of thermal deformation in high-power optical components primarily stems from the absorption of the component itself, which can be categorized into three sources:

• Substrate Absorption. Fused silica can be selected as the substrate material, but the appropriate material grade should be chosen based on the application scenario. For conventional high-power applications, Corning 7980 fused silica with an absorption rate of approximately 1–2 ppm/cm can be used; for ultra-high-power applications, grades such as Heraeus Suprasil 3001 with absorption below 1 ppm/cm are preferred, to minimize thermal effects arising from substrate absorption to the greatest extent.

• Coating Absorption. Employing electron-beam evaporation with plasma/ion-assisted deposition (PIAD) processes, along with high-purity coating materials, can produce dense, low-absorption films at relatively low temperatures. In scenarios demanding extremely high film density and ultra-low loss, ion beam sputtering (IBS) is a superior alternative, producing films with higher durability and higher laser damage thresholds. Additionally, atomic layer deposition (ALD) technology shows promising prospects for highly uniform thin-film deposition on complex curved optical elements.

• In-Service Environmental Contamination. Inadequate cleaning of the component surface can leave behind residual impurities, which, under high-energy laser irradiation, trigger photo-induced absorption, leading to localized overheating or even damage. The industry-standard surface quality specifications, referenced to MIL-PRF-13830B or GB/T 1185-2006, recommend a scratch-dig grade of no worse than 60-40 for general applications. For high-power laser systems, a grade of 20-10 or better is advisable to minimize the risk of contaminant adhesion from the source.

Furthermore, the laser-induced damage threshold (LIDT) is a critical parameter for evaluating the anti-damage capability of high-power optical thin-film components, influenced by multiple factors such as substrate defects, splatter defects from coating materials, and impurity contamination. LIDT must be incorporated as an essential performance parameter during the design and fabrication of high-power optical components.

The above outlines the core considerations in the design, fabrication, and application of complex spectral beam-splitting coatings. For more technical details or product solutions, please feel free to contact us.

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