Linear Variable Filters – Technology and User Perspective
Published: October 8, 2021
Linear variable filters are special types of filters where spectral response changes along the filter physical position. The main benefit of a LVF is that a single filter can provide characteristics of many filters or can simplify mechanical design when replacing diffraction gratings. More about types and applications of LVF we described in our previous publication Linear Variable Filters – Types and Applications.
The LVFs have been growing in popularity over the last decade, due to advancements in thin film deposition technologies. The focus of this article is to familiarize the reader with linear variable filters technology – design, manufacturing, as well as with advise from user perspective. Our goal is to provide basic understanding to design engineers who would like to benefit from the advantages of linear LVFs.
Technologies of Linear Variable Filters Manufacturing
LVFs can be designed based on various types of filters, including low-pass cut-off, high-pass cut-off or narrow band as well. Spectral response, as mentioned above, changes along one dimension, typically the filter length. Nevertheless, as mentioned by Mr Zdzislaw Choromanski, Head of Thin-Film Department in Solaris Optics:
“Variable filters can be also deigned and manufactured on a circular substrate, where the spectral response varies along the sample radius.”
Mr Zdzislaw Choromanski, Head of Thin-Film Department in Solaris Optics
The linear relation between the position and wavelength transmission, defined by filter slope (in nm/mm) is the most common design of a variable filter. It shall be remembered that those characteristics are not fully linear, but linear with certain tolerances. In a more general perspective the transmission versus position function can be custom profiled to achieve e.g. Gaussian profile.
When it comes to types of the deposited thin-films, variable filters are made of dielectric layers in a number that is sufficient to achieve performance. It may take tens but also hundreds, in some cases as many as 300 layers. The thin-films are coated typically on a flat sample of needed shape, e.g. rectangular, square or circular, dimensions of which shall allow to achieve the assumed wavelength range.
We asked Mr Choromanski, also about difficulties or limitations in LVF manufacturing:
“The main limitation in custom linear variable filters manufacturing is the spectral range the variable filter shall cover. Especially UV and low areas of the visible range are difficult because of dispersion of refractive index of the applied coating materials.”
Mr Zdzislaw Choromanski, Head of Thin-Film Department in Solaris Optics
LVFs are typically deposited with techniques that allow advanced control over the deposition process. Hence a conventional Physical Vapor Deposition does not allow to produce continuously variable filters. In most cases the deposition technology of choice is ion beam sputtering (IBS, also know as ion beam deposition). It enables extremely precise thickness control and deposition of very dense, high quality films. Also Electron Beam PVD with Ion of Plasma Aid enable manufacturing of linear variable filters.
Variable Filters – Key Outcomes
Linear Variable Filters are available both as stock, catalogue products as well as custom filters. Catalogue products cover the most common ranges, types and dimensions, as mentioned in the article. Nevertheless the variable filter technology provides a wide range of possibilities, including selection of specific sample shape, wavelength ranges, slope or custom position/wavelength characteristics. According to Mr Choromanski, when ordering linear variable filters it shall be remembered that the actual beam, which goes through the filter will be slightly blurred, which is a result of beam size and the varying spectral response, related to the beam position.
Linear Variable Filters in Solaris Optics
Solaris Optics is a manufacturer of custom variable filters (linear or other profiles) within UV to NIR range with maximum sample dimensions of 300 mm in diameter (for circular substrate) or 50-70mm length and 20-30 mm width for rectangular shapes.
Please do not hesitate to contact us if questions!
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