All-Solid-State Frequency Agile Filter (AF2)
Optics
All-Solid-State Frequency Agile Filter (AF2) (LAR-TOPS-393)
A multilayer optical filter offering broad and fast tuning for multispectral imaging
Overview
A thin-film interference filter (TFIF) is an optical device that uses light interference to selectively transmit, reflect, or block specific wavelengths. TFIFs are used in imaging systems like LiDAR to control the light reaching the detector and are easily mass-produced with excellent optical performance. However, TFIFs typically target specific wavelengths or narrow bands. Thus, multispectral applications require multiple TFIFs. Traditionally, this is done with motorized filter wheels, which add bulk, cause slower response, limit spectral resolution, and introduce moving parts subject to wear and tear. Alternatively, liquid crystal tunable filters (LCTFs) enable rapid electronic tuning without mechanical movement, improving speed and compactness, though they still have limitations including polarization sensitivity, low throughput, narrow spectral range, slow switching times (milliseconds), and high cost.
In an effort to improve upon the limitations of motorized filter wheels and available tunable filters, engineers at NASA’s Langley Research Center have developed an all-solid-state frequency agile filter (AF2) based on exotic phase change materials (PCMs). This innovative technology is capable of fast (MHz to GHz in nanoseconds) and wide (500nm – ~15 micrometer) spectral tuning. Furthermore, the filter has no moving parts and is polarization insensitive, making it an excellent solution for applications requiring multispectral filtering.
The Technology
NASA engineers and innovators have developed an all-solid-state frequency agile filter (AF2) with an exotic phase change material (PCM) and Fabry-Perot (FP) multilayer optical design for space-based observation of our atmosphere to assist in climate data acquisition.
At the heart of the AF2 design is an exotic PCM, such as GST, GSST, or SbS, and multilayer optical design. The PCM has a large reversible refractive index shift based on an energetic change which provides a non-volatile system where no additional energy is required to maintain its state. Additionally, during transitions between phases (switching), PCMs maintain their structural state and only require energy during the switching process.
The design of the system includes the PCM as a phase change cavity embedded between multiple Bragg reflectors, used to steer, or modulate, a light wave. This enables the spectrally-tunable solid state fields to operate across visible mid-wave infrared wavebands. The multilayer optical design includes a single filter and a single detector that are not dependent on the number of wavelengths needed to transmit for sampling.
NASA’s AF2 system was developed to provide improved spectral filtering capabilities for the NASA Differential Absorption Lidar (DIAL), a LiDAR with four lasers operating at three different wavelength bands that measures ozone and aerosols in the atmosphere. However, the performance benefits offered by AF2 may prove valuable for a broad range of commercial hyperspectral imaging applications, including in the agriculture, food and beverage, forensics, healthcare, mining, pharmaceutical, and automotive industries. Notably, the system’s 20pm bandwidth is especially useful for gas detection, enabling precise isolation of specific absorption lines in target gases. In the geological sector, aircraft fitted with an AF2-enhanced hyperspectral sensor could identify mineral-rich areas of interest with greater accuracy.
Benefits
- Broad spectral tunability: capable of wide spectral tunability (~500nm - 15 micrometer) and dual spectral tunability (~200 - 400 pm)
- Narrow band filtering: ultra-narrow passband bandwidth (
- Fast tunability: nanosecond-fast tunability (i.e., MHz to GHz) that can be electronically controlled
- Polarization insensitive: improved glare, contrast, and identification properties
- Inexpensive and easily scalable: can be fabricated using conventional thin film deposition techniques
- No moving parts: robust design minimizing risk of failure
- Low power requirements: only requires power to change between the crystalline and amorphous phases (i.e., does not require energy to maintain a particular state)
- Instrument systems simplification: enables single filter, single detector spectrometers decreasing the cost, risk, and systematic bias that results from the use of multiple filters or detectors
Applications
- Hyperspectral imaging: precise and fast spectral filtering across a wide range of wavelengths
- Agriculture: crop health monitoring, soil analysis, or pest control
- Food and Beverage: quality control and contaminant detection, sorting and grading
- Healthcare and Biomedical: disease diagnosis, tissue analysis, non-invasive blood analysis
- Mining and Geology: mineral exploration, ore assessment
- Pharmaceuticals: drug composition analysis, counterfeit drug detection
- Forensics: crime scene investigation, trace evidence analysis
- Manufacturing: paint inspection, defect analysis
Technology Details
Optics
LAR-TOPS-393
LAR-19870-1
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