Precision Infrared Bandpass and Dual Bandpass Filter Technology

Alluxa’s latest series of infrared (IR) bandpass and dual band filters brings new levels of performance with steeper slopes, higher transmissions, and flatter pass bands than conventional evaporated coatings.

The new precision infrared bandpass and dual bandpass filter technology brings the precision of dielectric filters to the IR between 2 and 5 microns. These filters are manufactured with an improved, plasma enhanced PVD process using long-lasting, hard metal-oxide, and robust semiconductor front surface thin films.

The films have exceptional environmental stability. The OH (water) band at 2.7 microns shows loss below the measurement limit even after 10 days of severe Mil-810G temperature and humidity cycling.

Applications

The dual band and narrowband IR filters are the most recent innovation in the Ultra product line to come from Alluxa’s new plasma hard coating process. The filters offer new design options and performance improvements in a variety of applications from night vision, chemical monitoring, and gas sensing, to FTIR Spectroscopy and LIDAR.

Bandpass Filter Review

The fundamental design principles of resonant cavity bandpass filters are well known and relatively simple. The filters are made up of stacked, resonant Fabry-Perot cavities where quarter-wave stack reflectors are spaced apart by a cavity layer. The subject is further described by an excellent book titled “Thin Film Optical Filters” by Dr. Angus Macleod.

Designing filters to have improved squareness, or steeper slopes, is a fairly simple way of increasing the number of cavities in the filter design. The designer increases the reflectivity of the quarter-wave stack mirrors or increases the optical thickness of the spacer layer in order to make a filter narrower. This is similar to increasing the finesse in any other resonant cavity.

An alternative design approach commonly used in this industry is to incorporate a Long Wave Pass filter (LWP) with a Short Wave Pass filter (SWP). This design approach is preferred for wider filters, and for filters that may have asymmetric slope requirements. Adding layers to the SWP or the LWP can increase the slopes.

The deposition process control system is challenged when cavities are added to the resonant cavity or layers to the edge filters. This invariably leads to undesirable ripple and loss to the pass band of the filter.

Alluxa has developed a new low noise IR optical monitoring system that provides the ultimate in layer thickness control, which enables many more cavities and layers to be added to designs that are still practical to manufacture. The advanced computer control system consistently measures the filter function and compensates for the associated thickness errors.

Narrowband Filter with Low OH

Traditional narrowband filters have traces of water incorporated in them during the deposition process, or adsorbed from the atmosphere after deposition due to the porous, low density film structure feature of low energy evaporated films.

The new plasma deposition process developed by Alluxa incorporates essentially zero OH during deposition and the fully dense films lack the required porosity to adsorb water. To present a demonstration, a fully blocked narrow bandpass filter was developed with a wavelength centered in the water band, as shown in Figure 1.

This filter comprises of a multi-cavity bandpass deposited on side 1 with a blocking filter that blocks from the UV to 6 microns on side 2. Silicon is the substrate material used. The filter is positioned to transmit at the center of the OH stretch band in order to demonstrate the low levels of OH bonds incorporated in the filter.

Figure 1. Fully blocked narrowband filter centered on the OH-stretch absorption band with transmission > 90% on a silicon substrate.

Dual Band Filters

Optical filters that transmit dual bandpass, or two pass bands, present varied and complex challenges when manufactured. These can be built using resonant cavity techniques, but very often they are combinations of notch filters, edge filters, and/or wide band filters.

Two examples of fully blocked dual band filters on silicon substrates are shown n Figures 2 and 3. The designer can easily adjust the random pass band center wavelengths, filter slopes, and the pass band widths.

Figure 2. Fully blocked dual band filter to OD4 level with two independent pass bands feature square filter shape and high transmission.

Figure 3. Fully blocked dual band filter to OD4 average level, with two independent pass bands centered at 2.4 microns and 4.7 microns that feature a square filter shape and high transmission.

Environmental Stability

A wide range of filters such as Anti-Reflection coatings (AR’s), dual bandpass filters, and bandpass filters, all on silicon substrates, were subjected to severe environmental stability testing. All of these filters successfully passed 10 cycles of the MIL – 810G,

Section 507.4 Humidity Testing Procedure, which is generally considered to be the toughest environmental test for IR filters. Within normal measurement variation, the filters do not show any change in performance due to the dense film structure created by Alluxa’s proprietary energetic plasma deposition. The filters also passed the snap tape adhesion test.

The filters continued to show low OH levels in the film as there was no increase in loss or absorption in the transmission measurements. The results indicate no change within the measurement limit of the spectrophotometers. Shown below are the results of the AR and dual bandpass measurements made before and after using an FTIR spectrophotometer.

Figure 4. FTIR measurements of AR coated Silicon substrate covering full water absorption wavelength band before and after MIL-810G testing.

Figure 5. Chart of the transmission of the dual bandpass sample shown in Figure 2 before and after 10 cycles of MIL-810G testing.

Conclusion

Alluxa’s IR bandpass and dual band filters were launched to deliver new high performance filter benefits to the IR spectrum between 2 microns and 5 microns.

Alluxa’s proprietary hard coating deposition process allows these filters to offer several improvements to designers working in the IR including very high transmission in the water bands between 2.5 microns and 3 microns, multi-band capability, full MIL 810 Humidity acceptance, and Alluxa’s extremely high transmission and steep edge slopes.

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

For more information on this source, please visit Alluxa