Next Generation Thin Film Optical Filters for Life Sciences
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White Paper Series fk AllUX3 Next Generation Thin Film Optical Filters for Life Alluxa Engineering Staff

Next generation in thin film optical filter enhances excitation/emission fluorescence systems. Alluxa offers a new line of high performance optical filters designed to improve fluorescence detection systems by offering improvements in throughput, blocking, wavefront accuracy and manufacturing repeatability, all at competitive price points. These filters are built with fully dense, hard coated refractory oxides using a proprietary high speed plasma deposition technology purposely built for the challenges presented by the latest optical instruments of the life sciences industry. Bandpass...

Fig. 1. Filter “squareness” is a direct and simple function of the number of resonant Fabry-Perot cavities. However, adding cavities challenges the deposition process control system, and invariably introduces undesirable ripple and loss to the passband. Alluxa has created a novel optical thickness monitoring system that enables an increase in optical cavity count to as many as 20 while maintaining low passband ripple, well in excess of industry limits of approximately 10 cavities. This system works by constantly measuring the filter function and compensating for thickness errors associated...

Optical filters for life sciences generally reside between 340nm and 900nm. The chart of Fig. 3 shows typical transmission response for a variety of common filter wavelengths and bandwidths. These filters are fully blocked to OD6 or greater levels to ensure required signal to noise ratio. Filters in the blue and UV side of the spectrum may be a few percent lower in transmission than those in the Red and NIR. Fig.3 Typical transmission levels of fully blocked to OD6 levels all-dielectric filters. Fluorescence detection filters are defined by blocking levels, in particular with respect to the...

BP 612 Blocking (4 consecutive runs) Fig. 4. Blocking levels ofAlluxa's high cavity count filters. The noise floor on blocking measurements generally is between 60 and 80dB. Multi-Bandpass Filters The latest systems require the flexibility of multiple bands illumination as both excitation and emission. Alluxa's advanced plasma coated hard coatings are ideal suited for this demanding application and consistently deliver high transmission and low ripple in multiband applications. A typical multiband emission filters is shown in Fig. 5 below. Multiband filters are typically specified as...

Multibnnd emission filter Fig. 5 Typical measured multiband emission filter in log scale. Note that the noise floor of this measurement is approximately 50D. Dichroic and Polychroic Filters Dichroic filters present a new set of challenges to the fluorescence system. Their function is to separate the excitation and emission spectral bands with minimum loss and with minimum spectral gap between bands. They must possess low angle shift, low polarization splitting, and excellent flatness of both the reflected and transmitted regions. Fig. 6 shows a typical dichroic filter response. Fig. 6...

Multiband system push the performance envelope even further and require a comb-like performance where multiple excitation bands are separated from multiple emission bands. Fig. 7 shows a polychroic (multiple wavelength) beamsplitter. Fig. 7 and Fig. 8 Multiband polychroic beamsplitters used at 45 degrees vs. theory.

Coating Stress The deposition of a coating typically results in stress within the thin film which can often result in a change in the flatness of the optical substrate. The type of stress, tensile or compressive, depends on the type of coating process used as well as details of the coating conditions. Typically the effect of stress-induced curvature is to add a roughly spherical component, i.e. a hill or bowl shape, to the initial shape of the substrate. This curvature can be measured using an optical interferometer as shown in Fig. 6. The effect of this curvature on an optical beam or...

instrumentation design have driven the levels of TWE performance such that measureable amounts of TWE due to the coating itself need to be carefully managed. Fig. 10. TWE or change in optical path length in transmission. The figure on the left shows the measured phase using an interferometer while the right shows the variation in optical path length through the part. Cost Sensitivity Many factors directly affect the cost of the filter, particularly in volume production. While selection of many of these product attributes is common and straightforward, sometimes unanticipated costs and...

Summary The filters used by fluorescence detection systems are arguably the most important element defining the system performance. Alluxa now offers filters with improved transmission, blocking and both transmitted and reflected wavefront properties. These filters are shipping in production volume at lower price points than previously available. Alluxa also offers a lower performance version that competes favorably in cost with “soft coatings”. See related white paper on the subject, available on our website at Alluxa.com. Contact Alluxa for further information. Glossary Angle of Incidence...

Optical Density (OD): The log (base 10) of transmittance (T). (T on a zero to unity scale) Passband: The range of wavelengths that are desired to be transmitted. Ripple: Variations in transmission in the passband. Shortwave Pass (SWP): An edge-type filter that transmits the shorter wavelengths and blocks higher wavelengths. Spectral Slope: Rate of change of the spectrum from a passband to a blocking band. Transmitted Wavefront Error (TWE): Distortion of the wavefront of a plane wave as it transmits through a filter, generally given in units of the HeNe wavelength of 633nm. Alluxa Alluxa...