Optical Interference Filters

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Optical Interference Filters

Co-deposition Adds Functionality

Co-depositing materials to produce a material with a prescribed refractive index offers a powerful technique providing thin film designers an additional optimization parameter. This ability eliminates the use of the Herpin equivalent layers with a characteristic thick/thin composition that is difficult to control optically. 

For narrow band filters, this design variable allows one to make all quarter wave designs with arbitrary bandwidths. It permits the intrinsic error correction in direct optical monitoring of turning points to obtain high performance and yield.  In very demanding short pass filters, it allows the suppression of higher order reflection bands.

Beyond multilayers, the ability to make a continuously varying index supports making filters with a sinusoidal variation that exhibit no harmonic structure at all.  These filters are known as Rugates. As a fundamental building block, they incorporate a single frequency sine wave and can be combined in a Fourier series to obtain the desired spectral performance. 

At Materion Precision Optics, the most common application of this technology is in the fabrication of precision rejection notches. A notch filter is typically employed in applications that require effective reflection or rejection of electro-magnetic radiation within a selected spectral region. They also provide high transmission at wavelengths outside of the region being rejected.

As a class of optical interference filter coatings, rugate thin films have been in existence since the early 1980s.  Typically, Materion fabricate rugates in a planetary system that has 6 ea 200 mm diameter substrates. However, we have produced rugates up to 24” in diameter.
Figure 1-Refractive Index Profile Rugate
Figure 1: Refractive index profile for a broad rugate coating.

A single sine wave structure has strong side lobes.  In order to suppress the side lobes, the sine function above is apodized.  It is also impedance-matched to the incident media and the substrate which produces a single notch.  The resulting index profile is shown in Figure 1. The optical properties of this notch are controlled by four parameters:  the average index N0, the amplitude of the index modulation dN,  the period of the modulation, and the number of periods.  The period of the modulation determines the wavelength of the notch.  The amplitude of the modulation determines the width of the notch.  The number of periods for a given index modulation determines the depth of the notch.  Finally, the average index determines the thickness of the coating and its sensitivity to angle. This structure produces a single reflection band, shown in Figure 2 
Figure 2-Comparison NIR Rugate Notch Filter
Figure 2:  Comparison of a designed with actual NIR rugate notch filter.  This filter performs like uncoated glass in the visible.

If more than one band is required, additional bands can be added via superposition - either in parallel, if the bandwidth is small, or in series, if the bandwidth is large.  Materion has fabricated filters with as many as 43 bands, although 1 to 3 is common.  Typical notch bandwidths are a few % of the center wavelength over a wavelength range of 300 to 4500 nm - the range over which our refractory metal oxides transmit. 

The potential use of co-deposition in the manufacture of optical interference filters adds some complexity to the deposition systems.  It also adds considerable capability in the pursuit of manufacturable filters with very high performance.

For more information about Materion’s optical interference filters, please contact Kevin Downing, Director of Marketing & Business Development, Kevin.Downing@Materion.com.

Sources Consulted

1. Johnson, W.E. and R.L.Crane, “Introduction to rugate filter technology,” In Homogeneous and Quasi-Inhomogeneous Optical Coatings, J.A,Dobrowolski and P.G. Verly eds. , Proc SPIE 2046 88-96 (1993)
2. Bovard, B.G. “Rugate filter design: the modified Fourier transform technique,”Appl. Opt. 29, 24-30 (1990)
3. Delano, E. “Fourier synthesis of multilayer filters,” J. Opt. Soc. Am. 57, 1529-1533 (1967)
4. Southwell, W.H. and R.L Hall, “Using apodization functions to reduce sidelobes in rugate filters,”Appl. Opt. 28 ,5091-5094 (1989)
5. Bovard, B.G.“Rugate filter theory: an overview,” Appl. Opt. 32, 5427-5441 (1993)