As noted in the previous chapter on all-dielectric filters, blocking unwanted radiation requires many layers and substantial design time. An alternate solution used throughout the industry today is to block low-wavelength leaks with either colored glass or a layer of epoxy containing dissolved dyes, and then block the high-side leaks with a metallic interference filter (M filter). Here, I will briefly review the history of silver filters and describe ways to integrate the metal filters with all-dielectric filters.
In 1942, Geffcken was issued a patent for the first silver filters. They were constructed by depositing two layers of silver separated by a fluoride layer. Schott North America, Inc. manufactures filters this way with two or three silver layers. When the dielectric layer(s) that separates the silver layers is ~one-halfwave thick, the films allow light to pass through; otherwise the light is reflected.
9.2 1M Filters
Adding pairs of dielectric layers to a silver film can either increase or decrease reflection of the layer, depending on the layer sequence and thickness. A single layer added to a thin metal layer reduces reflection and increases transmission. For silver, a dielectric layer of about three-fourths quarterwaves has the maximum effect. A filter designed for a wavelength at 500 nm consists of a stack-metal-stack (see Fig. 9.1).
Design: Glass (HL)3 [0:77L 41nmAg 0:75L] (LH)3 air.
The thickness of the layers between the brackets is selected to produce ~0% reflection at the design wavelength (band) from each direction for the complete filter; the indices used are 2.0 and 1.45. The dispersive optical constants of silver are derived from those reported by Johnson and Christy. The thickness of the low-index odd-numbered layers next to the silver layer phase match the stacks to the silver layer. All of the designs shown in this chapter are deposited on BK7 glass and matched to air. 1M filter characteristics: %T is very high at ~90%; blocking is not deep at ~1%; bandwidth is narrow; and peak is not symmetrical about the center design wavelength.