Flattening Less than Zero Point 1 Waves RMS with Dichroic and Polychroic Thin Film Filters

A new line of ultra-flat dichroic and polychroic high performance thin film filters featuring a flatness of <0.1 wave RMS have been introduced by Alluxa. The new filters are designed for use in imaging applications that need flatness levels beyond 0.1 waves RMS per inch.

The filters are unique as flatness is achieved by them through the elimination of high stresses of as-deposited standard ion-based coating process, such as ion assisted deposition (IAD) and ion beam sputtering (IBS).

The new technique developed by Alluxa uses a novel plasma coating process to create low loss, completely dense dielectric films with essentially net zero stress on the primary coated side. Therefore, the coatings eliminate the need for industry standard backside compensation or the use of extremely thick and expensive substrates to guarantee flat surface figures.

A simple, cost-effective and high performance anti-reflection (AR) coating is used to coat the backside. Alluxa has achieved flatness levels of <0.1 waves RMS at 632.8 nm per inch with substrates as thin as 0.5 mm.

Several benefits can be obtained by eliminating backside compensation. It reduces the total cost by decreasing the coating thickness. It enhances transmission levels by lowering optical scatter and also improves transmitted wave-front properties by lowering overall thickness.

Eliminating stress in thin film coating also provides the thin film engineer with much more design options to develop filters with enhanced slopes and rejection levels by using an increased number of coating layers where earlier this may have been unfeasible.

Alluxa’s new polychroic and dichroic filters are made up of fully dense, hard coated refractory oxides using a proprietary high speed plasma deposition technology designed to address the challenges presented by the recent optical instruments of the industrial markets and life sciences. Reliability has been illustrated against stringent Telcordia standards.

Flat Filters: Dichroic and Polychroic Overview

Dichroic filters have been used to split one spectral band of light from another for several decades. The advent of hard coating technologies like ion beam sputtering and energy sputtering has resulted advances in coating durability and spectral performance.

While these processes made and continue to make filters with good spectral performance, the finished product is physically distorted by curving the substrate due to the compressive stress levels in the resulting coatings. Many of these filters work in laser or imaging applications or both, and because of this, there has been increasing pressure on these filters to meet stringent flatness requirements.

A technique called backside compensation has been traditionally used to correct these filters in order to obtain the level of flatness or reflected wave-front distortion required in several of these applications. Put simply, this compensation method involves coating both sides of the substrate with a coating that will help to balance out the stress of the filter and flatten the surfaces.

The drawback of this method is also straightforward: it can almost double the amount of coating needed, causing a significant increase in the manufacturing cost of the filters.

The need for more complex filters has emerged over the past two decades following the widespread adoption of dichroic filters. Currently, applications such as Pinkel and Sedat style fluorescence systems, patterned filters, DNA sequencing, STED, and confocal microscopy have all demonstrated strong demand for flat filters that separate multiple spectral bands from each other simultaneously.

The performance of the instruments is optimized by bandpass, notch, and multi-band polychroic filters. The added complexity of these filters also brings about the need for significantly more coating layers than simple dichroic filters. This results in making the compensation of these parts a lot more cumbersome for manufacturing processes utilizing conventional compensation techniques to balance coating stress.

Figure 1 displays representative flatness measurements on a dichroic coated with a standard hard coating process and Figure 2 shows a filter coated using Alluxa’s extremely low stress coating process. Figure 3 and Figure 4 show two examples of very low stress polychroic filters used for confocal microscopy.

Shows a flatness measurement of a dichroic coated with a standard hard coating process.

Figure 1. Shows a flatness measurement of a dichroic coated with a standard hard coating process.

Shows the same dichroic coated using Alluxa’s very low stress process.

Figure 2. Shows the same dichroic coated using Alluxa’s very low stress process.

riple bandpass polychroic on a 1.0 mm thick fused silica substrate with measured flatness of <0.25 Waves / inch RMS @ 632.8 nm

Figure 3. Triple bandpass polychroic on a 1.0 mm thick fused silica substrate with measured flatness of <0.25 Waves / inch RMS @ 632.8 nm

Dual notch polychroic on a 0.5 mm thick fused silica substrate with measured flatness of <0.25 Waves / inch RMS @ 632.8 nm.

Figure 4. Dual notch polychroic on a 0.5 mm thick fused silica substrate with measured flatness of <0.25 Waves / inch RMS @ 632.8 nm.

Specifying Dichroic and Polychroic Filters

Specifying polychroics and dichroics filters primarily involves selecting the desired pass bands, blocking bands, substrate type, and thickness. The spectral gaps between the bands are the key drivers of coating cost and complexity.

The standard angle is 45°, but performance of slope, blocking and transmission can be easily achieved when the angle is lower. Likewise, better performance can be achieved when the angular range of the beam is lower and more collimated. A summary of recommended specifications for dichroic and polychroic filters is displayed in Table 1.

Table 1. Summary of specifications for dichroics and polychroics

Term/Parameter Description High Performance Standard Lowest Cost
Center wavelength Center of pass band. This should be used only as a nominal value +/- 1% (nominal) +/- 2% (nominal) +/- 3% (nominal)
Pass band Transmission Transmission average across pass band >95% >90% >85%
Pass band width Range of wavelengths required to transmit >1% wide 310 – 1100 nm >2% wide 310 nm – 1100 nm >4% wide 310 – 1100 nm
Blocking band Range of wavelengths required to suppress >1% wide 310 – 1100 nm 300 nm - 1100 nm Optimized for detector and light source
Passband to blocking band delta Spectral gap between the passband and blocking band giving tolerance for slope, centering, etc. <1% of wavelength >2% wide 310 nm – 1100 nm Optimized for detector and light source
Blocking levels Blocking suppression levels in log units average over the band 1% average across blocking band 5% average across blocking band 10% average across blocking band
Pass band to blocking band gaps Spectral gap between the pass band and blocking band giving tolerance for slope, centering, etc. <1% of wavelength 1% to 3% of wavelength >3% of wavelength
Substrate Thickness and type 1 mm or less on fused silica 2 mm on fused silica or polished borofloat Borofloat
Flatness RMS flatness measured at 632 nm using interferometer <0.1 wave RMS/inch 0.5 wave RMS/inch 1 wave RMS/inch

Summary

The filters used by fluorescence detection systems are arguably the most vital element defining the system performance. The new line of thin substrate ultra-flat dichroics and polychroic filters from Alluxa provide enhanced optical performance, require only a simple AR coating on the back, and achieve near zero net stress on the primary coated side.

Eliminating backside compensation improves transmitted wave-front, reduces scatter, improves transmission levels, and lowers costs. Flatness levels of <0.1 waves RMS at 632 for a one inch part can be achieved at thicknesses ranging from 0.5 mm and above.

These new polychroic filters are made up of fully dense, hard coated refractory oxides using a proprietary high speed plasma deposition technology that is specifically designed to handle the challenges presented by the advanced optical instruments of the life sciences industry.

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

For more information on this source, please visit Alluxa.

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