A novel angle-tuned thin film filter with low angle sensitivity

• Angle sensitivity and polarization sensitivity of the filter have been analyzed.
• Using the α-Siα-Si and the SiO2 as the high and low refractive index materials, the filter with low angle sensitivity was obtained.
• The de-polarization structure is fabricated.
• The incident angle range can be enlarged to 32°.

An angle-tuned thin film narrowband filter is widely used in the dense wavelength division multiplexing (DWDM) system. With increase of incident angle of the thin film filter, the central wavelengths of both S-polarization and P-polarization will separate obviously and shift to short wavelength much faster, which will cause serious polarization sensitivity and angle sensitivity. In conventional angle-tuned thin film filters, the research works usually focus on the polarization sensitivity. However, their angle sensitivity is very high because the effective refractive indexes of their spacer are very low. Their precision of the angle controlling system is very rigorous (less than 0.005°) and their incident angles are usually less than 20°, which will limit their wavelength tuning range. In the present paper, we propose and fabricate a novel 100 GHz angle-tuned thin film filter stack with low angle sensitivity which uses the high refractive index material α-Siα-Si as the spacer and its incident angle can be expanded to 32°. Using the polarization beam-splitters and the half wave plates, this angle-tuned thin film filter can also eliminate the polarization sensitivity. The simulation results and the experiments show that the angle-tuned thin film filter with low angle sensitivity has a effective tuning range of 40 nm, which can cover the whole C-band and its precision of the angle control is relatively easy (more than 0.05°).