Do optical filters exhibit polarization dependency, and does it cause leakage at blocked wavelengths?

The short answer is yes. Most optical filters exhibit some level of polarization dependency, especially when used at an Angle of Incidence (AOI) other than 0°. This dependency can indeed lead to performance degradation, including leakage in the blocking range.

1. Why Polarization Dependency Occurs

Optical filters are typically constructed from thin layers of dielectric materials with alternating refractive indices. When light hits these layers at an angle, the reflection and transmission coefficients for s-polarized light (perpendicular to the plane of incidence) and p-polarized light (parallel to the plane of incidence) diverge.

This phenomenon, governed by the Fresnel equations, causes the filter’s spectral response to "split":

• S-polarization: Generally exhibits higher reflectivity and a narrower bandwidth.
• P-polarization: Generally exhibits lower reflectivity and a broader bandwidth.

2. Shift in Center Wavelength (CWL)

As the AOI increases, the filter's passband shifts toward shorter wavelengths (blue-shift). However, the p-polarization shifts further and faster than the s-polarization.

λ_θ=λ₀√1−(n_effsinθ)²

Because of this differential shift, a filter used at a high angle will have two distinct transmission peaks. If you are using unpolarized light, the resulting "average" peak will appear broader and have lower peak transmission.

3. Leakage at Blocked Wavelengths

Polarization dependency can significantly impact the "rejection" or "blocking" capabilities of a filter (its Optical Density, or OD).

• The "Leakage" Mechanism: In the transition regions (the "edges" of the filter), one polarization state might be blocked while the other is partially transmitted. If your application requires a specific OD (e.g., OD6) at a laser line, polarization splitting might cause the p-polarization to shift into the "leakage" zone, effectively "opening" the filter to unwanted light.
• Broadband Blocking: In deep-blocking regions, p-polarization often has lower reflection efficiency at high angles. This can result in a higher "noise floor," where the filter fails to maintain its rated blocking level across all polarization states.

Summary Table: Polarization Effects