How are notch filters designed and manufactured?

A notch filter is designed to block a pre-selected wavelength region or bandwidth, while transmitting all other wavelengths within the design range of the filter. Different methods exist to design and manufacture notch filters, however, the two most common are the dielectric stack method and the Rugate method.

Dielectric Stack Method

Dielectric Stack notch filters are simple filters fabricated using a series of thin layers (discrete layers) of dielectric materials, of alternating refractive index. While relatively inexpensive to make, dielectric stack filters suffer from the presence of harmonic structure, which can severely limit the transmission band.

Rugate Method

Edmund Optics® offers both Dielectric Stack Notch Filters and Rugate Notch Filters. A Rugate notch filter is constructed using a single layer thin film in which the refractive index varies continuously with position in a direction perpendicular to the substrate plane. This design eliminates the harmonic structure problems of Dielectric Stack filters, yielding high transmission across a broad wavelength range. The wavelength range for transmission of the rugate notch filter is limited only by the materials used in its construction. In addition to offering a very broad transmission range free from harmonic structure, the rugate notch can provide deep blocking (high optical density) as well as a high degree of reflectivity at wavelengths within the notch.

The figure below shows the comparative transmission spectrum for a notch filter made with a Rugate design and a notch filter made with a simple, non-optimized Dielectric Stack design. The largely harmonic-free transmission band associated with the Rugate design provides high out-of-band transmission over a very broad range of wavelength. Rugate notch filters are used for many laser-based applications including medical/surgical, spectroscopy, laser-based fluorescence, astronomy, and in display applications.