Advanced Plasma Deposited Hard Coated Optical Bandpass and Dichroic Filters

Thin film optical filters have always been a challenge to specify because of their complex nature and varied end-use applications. This article describes the main types of filters and provides guidelines on how to specify key attributes, while also optimizing the cost.

The following filter types are discussed:

• Band pass types of ultra-square high cavity count, ultra-narrow and soft coating replacements
• Polychroic and Dichroic tilted beamsplitter filters

Substrates

Borofloat 33, NBK7, and fused silica (or equivalents) are the most standard substrates for thin film filters. Alluxa strongly discourages using “green” float glass and color glass except for specialty or extremely low cost applications. For most applications, the thin film equivalent of a color glass is lower cost and higher performance.

Color glass is manufactured in limited shapes and sizes, which restricts the effective load sizes of each coating lot. Green glass suffers from absorption, scatter, and auto-fluorescence and other “low quality” faults. Laminated filters that sandwich a number of green glass substrates between environmentally sensitive and fragile coatings should be avoided other than for laboratory scale experiments.

Precision filters have to be deposited on flat substrates. Although curved substrates can be used, they tend to cause thickness variations which degrade performance.

Background: Light Sources, Detectors

A filter does not have to be specified in wavelength for transmission range, blocking range, or reflection range beyond either the detector range or the light source range. For example, photomultipliers and silicon detectors have very clear response curves, and there is no gain other than the extra cost caused by specifying past its response curve.

Similarly, for LED and laser diode “clean up” filter applications, there is no benefit to specifying the blocking range beyond the relatively narrow emission spectrum.

Blocking and transmission has to be specified as average levels unless a discrete light source such as a laser has to be blocked or transmitted. Often, thin films filters display uneven blocking levels and very narrow spikes that can rise 1 or even 2 OD above the average blocking levels, but as spikes have a very narrow spectral bandwidth they do not have a major impact on signal to noise ratio or blocking level performance.

Bandpass Filters

A spectral band is selected by bandpass filters to transmit while rejecting light outside that band. Traditionally, these filters were specified with half-power bandwidth (HPBW), center wavelength (CWL), and peak transmission. A more advanced and precise way to specify is to provide a variety of wavelengths to transmit, the level of transmission, and a range of wavelengths to reject.

The gap between these ranges is used as a single specification to capture the tolerance of the filter for centering, uniformity, filter slope, and manufacturing margin.

Generally, bandpass filters are used at near zero Angle of Incidence (AOI). Angles larger than zero degrade performance because of the filter’s angle sensitivity. Filters shift with angle that is roughly proportional to the square of the angle and the inverse of the square of the effective index.

When the angle of incidence is larger, the angle creates more issues. Angles also cause polarization splitting where the S & P polarization states have varying performances as a function of angle within the filter design. Since each polarization state fundamentally produces a different filter function as angle increases, polarization splitting degrades the filter’s performance.

Bandpass filters are available in two types, a long pass/short pass structure or a resonant cavity Fabry-Perot structure. ‘Squareness’ of a bandpass filter is a function of the number of layers for a SWP and LWP type filter and the number of cavities for resonant cavity type filters.

The effect of increasing the number of cavities is illustrated in Figure 1; the typical performance of a broad band high cavity count filter is shown in Figure 2; and a flat top narrow band cavity filter is depicted in Figure 3.

Figure 1. Squareness of narrow band pass filter is a function of number of cavities.

Figure 2. Typical high cavity count thin film cavity band pass filter.

Table 1. Bandpass filter specification guidelines

Ultra-Narrow Bandpass Filters

Ultra-narrow bandpass filters normally refer to the class of bandpass filters with bandwidths lower than 1% of wavelength. The main features are transmission, blocking levels, and bandwidth. Most applications for narrow bandpass filters are engineered to transmit a single wavelength such as an emission line or a laser.

Figure 3. Ultra-narrow and flat top band pass filter with bandwidth <1 nm at 532 nm.

Table 2. Ultra-narrow band pass filter specification guidelines.

Multi-Bandpass Filter

A multi-bandpass filter is a band pass filter that transmits a number of pass bands or a number of spectral regions, and has blocking regions in between the transmitted regions (Figure 2). To specify these filters, the same guidelines given for single bandpass filters should be followed, however the tolerances have to be more carefully considered to reduce cost. A multi-bandpass filter is more challenging to deposit than producing two single bandpass filters individually.

Figure 4. Multi-bandpass filter theory and measured.

Table 3. Multi-band pass filter specification guidelines

Soft Coating Replacement Filters

“Soft coatings”, also known as “laminated coatings” are widely known to possess poor environmental resilience, high optical scatter, low transmission, poor temperature stability, and low blocking levels, yet they are still extensively used due to their ready availability, low cost, and legacy status.

For the first time, Alluxa’s proprietary high speed plasma deposition technology delivers the durability and optical performance of hard coated thin film optical filters at laminated soft coating pricing.

Figure 5 shows the typical performance of soft coating replacement filters for a variety ofdifferent wavelengths from the blue to the NIR. These are normally deposited on borofloat for cost purposes.

Price is usually the most vital parameter for these filters and Table 1 price sensitive column is most fitting.

Figure 5. A variety of soft coating replacement filters. The bandwidth is typically between 10 and 50 nm with blocking > OD4. Substrates are borofloat to reduce cost.

Tilted Filters – Dichroic and Trichroic Filter

Dichroic filters refer to the group of filter that are tilted and reflect and transmit. Specifying Polychroics and Dichroics filters is quite similar to multi-bandpass filters, and primarily involves choosing the desired pass bands and blocking bands and type and thickness of substrates.

The spectral gaps between the bands are the key driver of cost and coating complexity. Given the complexity of suppressing polarization splitting as well as other effects, the performance of tilted filters is less than the filters used at normal incidence.

Figure 6. Multiband dichroic beamsplitter used at 45 degrees vs. theory.

Mounting can be done at a standard angle of 45 degrees; however, the lower the angle the easier it is to accomplish blocking, performance of slope, and transmission. Similarly, the lower the angular range of the beam, and more collimated, the better the overall performance. A summary of recommended specifications for dichroic and polychroic filters is shown in Table 4.

Table 4. Summary of specifications for Dichroics and Polychroics

Conclusion

Contemporary hard coated optical thin film filters are almost infinitely flexible in capability. Dichroic tilted, single band, and multiband filters are all available with exceptional blocking and high transmission. However, thin film optical filters have always been a challenge to specify because of their complex nature and varied end-use applications.

Guidelines provided in this article on how to specify main features, while also optimizing the cost, are intended to help the designer with selecting specifications.

This information has been sourced, reviewed and adapted from materials provided by Alluxa

For more information on this source, please visit Alluxa