Sophisticated FSS Design for Excellent Results

BPF Series filters offer excellent precision and high spectral resolution. The essence of these sophisticated filter designs are frequency selective surfaces (FSS), the same technology the military uses in some stealth applications. BPF Series filters are designed with special cross mesh patterns to transmit the pass frequency and reject unwanted out-of-band radiation largely by reflection and destructive interference. The Gaussian-shaped pass band is capable of up to 90% max transmission and full width half max (FWHM) bandwidths between 7 and 25% of center wavelength. Out-of-band rejection levels are typically under 4.6%, and for even greater selectivity, two filters can be placed in series to reduce out-of-band levels under 1%. Each filter is individually characterized using a high quality THz or IR spectrometer.

Good as Gold, Hot or Cold

Lake Shore’s unique construction makes BPF Series filters ideal for use in cryostats and adjacent to cooled detectors. The pure gold filter element behaves extremely well at cryogenic temperatures and over wide thermal cycles, which are often present in astronomy and materials research. The gold filter requires no polymer support layer, eliminating issues of darkening due to radiation, delamination due to thermal cycling, and unwanted vibronic bands associated with polymer. These compact filters fit wherever needed and have a high ratio of usable clear aperture area to overall filter size. The ultra-thin gold filter layer makes for extreme light weight in payload situations, especially important when multiple filters spanning a range of CWLs are deployed. The filter surface is held flat and taut at all times by the nickel trim ring—assuring precise, repeatable performance under all operating conditions.

Conventional band pass filters with multiple layers can become delaminated with cryogenic use and darkened by gamma radiation. Lake Shore filters use a single lightweight layer of thin gold, with no delamination or radiation issues.

A Key to Better Detection

Advanced FSS Technology

Band pass filters allow energy at a specific range of frequencies to pass through, while rejecting unwanted energy at other frequencies. As a result, the electro-optic device detecting the transmitted energies (be it a thermal detector such as a bolometer, a broad frequency range detector such as a terahertz photoconductive switch, or an imaging focal plane array) sees more signal and less noise. Metal mesh frequency-selective surfaces (FSS) are formed by creating very thin metal films with specifically designed patterns of geometric shaped holes, with features sized according to the wavelengths of the radiation to be passed. These carefully engineered designs create so-called “metamaterials”, artificial materials that interact with electromagnetic waves in ways engineered to resonate with some frequencies while rejecting others. Altering the size, shape, and pattern of the holes enables designs with different pass band maximums or CWLs and other filtering characteristics.

While FSS technology has been studied for a number of years, especially in antenna and stealth surface designs, the more recent development of surface plasmonic and trapped-mode effects have advanced application of FSS-based structures using metamaterial principles. In the case of band pass filters, as electromagnetic waves pass through the filter metamaterial they generate plasmon polaritons (resonating electrons) at the holes, producing interference effects that block unwanted frequencies.

FSS-based filters are very efficient and have low intrinsic loss. A pronounced transmission peak of over 80% amplitude is quite common in the pass band, while transmission rejection can be down to the noise level (under 5% amplitude) in the out-of-band frequencies. These designs are optimized to operate when incoming energy waves are perpendicular to the filter surface and are mostly independent of field polarization. Advanced cross mesh FSS designs typically have greater feature precision and resolution than simpler wire grid devices, enabling filtering at higher frequencies. Wire grid devices frequently resemble simple screen door mesh, but Lake Shore’s gold cross mesh filters are designed and simulated using sophisticated FSS modeling software, resulting in patterns that are then manufactured using thin film metal deposition and photolithography processes, techniques similar to those in semiconductor microfabrication.

Simple wire grid filter	Lake Shore's advanced cross mesh design