Precision Infrared Narrow Bandpass and Dual Bandpass Filters Featuring Low OH-Band Absorption
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White Paper Series Precision Infrared Narrow Bandpass and Dual Bandpass Filters Featuring Low OH-Band Alluxa Engineering Staff

Alluxa’s new precision infrared bandpass and dual bandpass filter technology brings new levels of performance with higher transmissions, steeper slopes, and flatter pass bands than traditional evaporated coatings. Alluxa’s new class of Infrared (IR) bandpass and dual band filters bring the precision of dielectric filters to the IR between 2 and 5 microns. They are manufactured with an advanced, plasma enhanced PVD process using durable, hard metal-oxide, and durable semiconductor front surface thin films. The films have excellent environmental stability. The OH (water) band at 2.7 microns...

with prior layers. The system produces filters with remarkably low ripple and steep slopes that consistently match theory and are remarkably repeatable. Narrowband Filter with Low OH Traditional narrowband filters have traces of water incorporated in them during the deposition process, or adsorbed from the atmosphere post deposition due to the porous, low density film structure characteristic of low energy evaporated films. Alluxa’s new plasma deposition process incorporates essentially zero OH during deposition and the fully dense films have no porosity to adsorb water. As a demonstration,...

Dual Band Filters Optical filters that transmit two pass bands, or dual bandpass, offer different and complex challenges when manufactured. These can be built using resonant cavity techniques, but more often they are combinations of edge filters, notch filters, and/or wide band filters. Figure 2 and Figure 3 below are two examples of fully blocked dual band filters on silicon substrates. The pass band center wavelengths, pass band widths, and the filter slopes are arbitrary and relatively easy for the designer to adjust. …… Fig. 2. Fully blocked dual band filter to OD4 level with two...

…….. Fig. 3. Fully blocked dual band filter to OD4 average level, with two independent pass bands centered at 2.4microns and 4.7 microns that feature a square filter shape and high transmission. Environmental Stability A variety of filters including Anti-Reflection coatings (AR’s), bandpass filters, and dual bandpass filters, all on silicon substrates, were subjected to harsh environmental stability testing. These filters all successfully passed 10 cycles of the MIL – 810G, Section 507.4 Humidity Testing Procedure, generally considered the toughest environmental test for IR filters. The...

Fig. 4. FTIR measurements ofAR coated Silicon substrate covering full water absorption wavelength band before and after MIL-810G testing. Fig. 5. Chart of the transmission of the dual bandpass sample shown in Figure 2 before and after 10 cycles of MIL-810G testing.

Summary Alluxa’s introduction of IR bandpass and dual band filters are intended to deliver new high performance filter advantages to the IR spectrum between 2 microns and 5 microns. Using Alluxa’s proprietary hard coating deposition process, these filters offer many improvements to designers working in the IR including very high transmission in the water bands between 2.5 microns and 3 microns, full MIL 810 Humidity acceptance, multi-band capability, and Alluxa’s very high transmission and steep edge slopes. Alluxa Alluxa designs and manufactures next generation hard coated optical filters...