White Paper Series Specifying Plasma Deposited Hard Coated Optical Thin Film Filters. Alluxa Engineering Staff

Specifying Advanced Plasma Deposited Hard Coated Optical Bandpass and Dichroic Filters. Introduction Due to their complex nature and diverse end-use applications, thin film optical filters have always been a challenge to specify. This paper defines the major types of filters and offers guidelines on how to specify key attributes, while also optimizing the cost. The filter types discussed include • • Band pass types of ultra-square high cavity count, ultra-narrow and soft coating replacements Dichroic and Polychroic tilted beamsplitter filters Substrates The most common substrates for thin film...

Bandpass Filters Bandpass filters select a spectral band to transmit while rejecting light outside that band. Historically, these were specified with half-power bandwidth (HPBW), peak transmission, and center wavelength. (CWL). A more precise and modern way to specify is to simply provide a range 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, filter slope, uniformity and manufacturing margin. Bandpass filters are generally used at near...

Fig. 1 – Squareness of narrow band pass filter is a function of number of cavities. Fig. 2 – Typical high cavity count thin film cavity band pass filter Table 1 – Bandpass filter specification guidelines Term / Parameter Center wavelength Transmission Blocking ranges Passband to blocking band delta Blocking levels AOI and cone angle Description Center of passband. This should be used only as a nominal value Average transmission over desired band Range of wavelengths required to suppress Spectral gap between the passband and blocking band giving tolerance for slope, centering, etc. Blocking suppression...

Ultra-Narrow Bandpass Filters Ultra-narrow bandpass filters generally refer to the class of bandpass filters with bandwidths less than 1% of wavelength. Key attributes are blocking levels, transmission and bandwith. Most applications for narrow bandpass filters are designed to transmit a single wavelength such a laser or an emission line. Ultra-narrow and flat top band pass filter with bandwidth <1nm at 532nm Table 2 – Ultra-narrow band pass filter specification guidelines Term / Parameter Center wavelength Transmission Bandwidth Blocking ranges Passband to blocking band delta Blocking levels...

Multi-Bandpass Filter A multi-bandpass filter is simply a band pass filter that transmits multiple pass bands or multiple spectral regions and has blocking regions in between the transmitted regions as shown in Fig. 2. To specify these filters follow the same guidelines as for single bandpass filters above; however, the tolerances should be more carefully considered to minimize cost. A multibandpass filter is generally more challenging to deposit than creating two single bandpass filters separately. Table 3 – Multi-band pass filter specification guidelines Term / Parameter Center wavelength Transmission...

Multi-bandpass filter theory and measured. Soft Coating Replacement Filters “Soft coatings”, also known as “laminated coatings” are well known to possess poor environmental durability, poor temperature stability, low transmission, high optical scatter and low blocking levels, yet they are still widely used due to their low cost, ready availability and legacy status. Alluxa’s proprietary high speed plasma deposition technology for the first time delivers the optical performance and durability of hard coated thin film optical filters at laminated soft coating pricing. Typical performance of soft...

Fig. 5 A variety of soft coating replacement filters. The bandwidth is typically between 10 and 50nm with blocking > OD4. Substrates are borofloat to reduce cost. Tilted Filters – Dichroic and Trichroic Filter Table 4 – Summary of specifications for Dichroics and Polychroics Term / Parameter Center wavelength Pass band Transmission Pass band width Blocking band Description Center of pass band. This should be used only as a nominal value Transmission average across pass band Range of wavelengths required to transmit Range of wavelengths required to suppress Blocking levels Blocking suppression...

Dichroic filters refer to the family of filter that are tilted and reflect and transmit. Specifying Dichroics and Polychroics filters is much like multi-bandpass filters, and primarily involves selecting the desired pass bands and blocking bands and substrate type and thickness. The spectral gaps between the bands are the primary driver of coating complexity and cost. Due to complexity of suppressing polarization splitting as well as other effects, the performance of tilted filters is lower than the filters used at normal incidence. 45 degrees is a standard angle for mounting, however, the lower...

Alluxa Alluxa designs and manufactures next generation hard coated optical filters using a proprietary plasma deposition process in Santa Rosa, CA. Alluxa’s unique, purpose-built deposition platform and control systems were designed, developed, and built by our team in Santa Rosa, CA to address the demanding requirements of the next generation of systems and instruments. This unique technology allows Alluxa to create the world’s most challenging filters at breakthrough price points. Contact Alluxa for more information at [email protected] or visit our website at http://www.alluxa.com All content...