Ultra-Narrowband Optical Filters Pushing Boundaries from the ...
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Developments in Ultra-Narrowband Manufacturing Capabilities
Advancements in optical sensors and imaging technologies are rapidly being integrated into human interactions and our understanding of the world. The inquiry scope for optical devices is broad, enabling transformational technologies such as implanted transdermal bioMEMS devices and space-flight surveyors deployed for near and deep-space research. Central to the functionality of modern optical devices, ultra-narrow bandpass (UNBP) thin-film optical filters facilitate discrimination of sub-nanometer bands within broad spectra. Originally developed as NIR DWDM filters for telecommunications, these filters are now essential in extracting meaningful signals from imaging and sensing devices operating across the deep ultraviolet to mid-infrared bands.
Innovative deposition techniques, such as SIRRUS®, developed at Alluxa, have transformed the fabrication of UNBP filters from a low throughput, high-cost operation into a highly deterministic manufacturing process. Key filter features such as form factor, spatial uniformity, and spectral resolution have significantly improved compared to recent years. Below are some of the latest applications utilizing UNBPs.
Heliophysics Applications
Heliophysics has gained attention as a social interest and as a study venue for various sun-earth interactions. Research into magnetic field and plasma phenomena, such as coronal mass ejections (CMEs), remains highly valuable. Investigations over the past two decades have spanned from high atmospheric flights to instrumentation aboard multiple space missions. Solar surveyors have utilized multi-wavelength UNBP filters for high-resolution scans of different heliosphere segments. As next-generation instrumentation is developed, the demand for finer resolution imaging of plasma velocity and temperature profiling within the heliosphere has increased. To address this, Alluxa is developing high finesse multi-cavity UNBPs in the UV region.
Figure 1. Interface Region Imaging Spectrograph (IRIS) explorer visualization with windowed SJI imagery.
Alluxa has produced filters with over 500 layers for imaging spectrographs and other sensing applications. Utilizing an array of internally developed instrumentation, Alluxa filters demonstrate FWHM < 0.5 nm and peak transmittance of 10-50%, while blocking 200-nm at OD6-12 as depicted in Figure 2. This impressive spectral profile results from optimized deposition system configurations and methodologies to minimize losses (scatter + absorption) critical for high-performance UV filters.
Figure 2. UV Ultra-Narrow Solar Filter.
Atmospheric Observation and Modeling
Atmospheric observation and modeling are vital to global environmental concerns, impacting individual lives and geo-economic constructs like the greenhouse emissions market. Specifically, filters at 532 nm will enhance our ability to measure backscatter and volume depolarization in diurnal cloud and aerosol profiles within the troposphere. Figures 3 and 4 highlight the performance of the 532-0.127 and -0.25 OD6 Ultra-Narrow Bandpass filters, which closely match the intended theoretical simulations. These filters are set to be implemented in a flight mission for atmospheric Lidar remote sensing.
Figure 3. VIS Ultra-Narrow Bandpass Filter, CWL @ 532 nm.
Figure 4. NIR Ultra-Narrow Bandpass Filter, CWL @ nm.
Gas Monitoring and Analysis
Another growing area is gas monitoring and analysis, particularly at LWIR wavelengths. Laser applications within this domain range from experimental tattoo removal to medical equipment production. Controlling laser bandwidth and attenuating out-of-band light are crucial, hence ultra-narrow filters are becoming essential components. Figure 5 illustrates Alluxa’s capability to design and manufacture a 10.6 micron filter with FWHM < 0.1 micron in the LWIR range.
Figure 5. 10.6 micron LWIR Ultra-Narrow Bandpass.
The diverse applications requiring ultra-narrow bandpass filters create an ongoing demand for innovation and improvement. This spans minimizing loss or scatter in the UV, achieving greater repeatability in the VIS and NIR, to enhancing in-band transmission with narrower bandwidths in LWIR. Alluxa continues to rise to the challenge, leveraging the SIRRUS® deposition platform along with engineering expertise to push the boundaries of UNBPs.
References:
[1] SVS. NASA Scientific Visualization Studio | a Slice of Light: How IRIS Observes the Sun. SVS, 26 June, svs.gsfc.nasa.gov//.
[2] EUVST NASA Science. NASA, science.nasa.gov/mission/euvst/.
[3] Cloud-Aerosol Transport System (CATS). Cats.gsfc.nasa.gov.
[4] NASA AOS the ALICAT Lidar for AOS-Inclined Orbit: Instrument Overview and Projected Performance. Aos.gsfc.nasa.gov.
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