Narrow line-width filters based on rugate structure and antireflection coating

Filters that have high transmittance over the passband region of the reflectance spectrum are designed by combining rugate structures and antireflection (AR) coatings. It is found that, under certain conditions, the refractive index of the substrate could be “converted” to the air-side refractive index of the graded-refractive-index coating. A method for the fabrication of graded-index coatings by rapidly alternating deposition of low (SiO₂) and high (Nb₂O₅) refractive index materials is introduced, and this technology was used to fabricate a rugate structure. An AR coating with a refractive index of 1.23 was grown onto the rugate structure by glancing angle deposition technology. Optical measurements of the combined structure show excellent transmittances over the wavelength regions around the reflection band and high reflectances in the stopband.