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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.

The first optic is a 1D Ronchi ruling, where shortpass or longpass filters replace the traditional opaque lines in the grid pattern. The second optic is a 2D Ronchi ruling, where one set of lines is made from shortpass filters and the orthogonal set from longpass filters. By using two colors of light and a color camera in the focusing schlieren system (or a dichroic mirror with two monochrome cameras), the 1D optic enables simultaneous focusing schlieren and other co-linear techniques, while the 2D optic allows for the unambiguous measurement of two orthogonal density gradients in focusing schlieren images. Unlike standard optical filters, which typically cover an entire substrate, these Ronchi rulings feature alternating clear and filtered regions in structured 1D or 2D patterns. By leveraging color filtering and a color camera, the 1D ruling enables simultaneous focusing schlieren and complementary optical diagnostics, such as Particle Image Velocimetry (PIV), Pressure-Sensitive Paint (PSP), and Thermal-Sensitive Paint (TSP). The 2D ruling enables simultaneous and unambiguous measurement of two orthogonal density gradients, a capability not possible with conventional Ronchi rulings. This advancement significantly improves the accuracy and efficiency of schlieren-based flow measurements. The types of filters are not just limited to shortpass and longpass coatings, but could include notch, bandpass, and multiple-bandpass filter coatings as well. This design expands the utility of schlieren imaging in high-speed aerodynamics, combustion diagnostics, and other fluid dynamics applications. This Ronchi ruling methodology is at TRL 4 (component and/or breadboard validation in a lab environment) and is available for patent licensing.

This NASA technology transforms a conventional infrared (IR) imaging system into a multi-wavelength imaging pyrometer using a tunable optical filter. The actively tunable optical filter is based on an exotic phase-change material (PCM) which exhibits a large reversible refractive index shift through an applied energetic stimulus. This change is non-volatile, and no additional energy is required to maintain its state once set. The filter is placed between the scene and the imaging sensor and switched between user selected center-wavelengths to create a series of single-wavelength, monochromatic, two-dimensional images. At the pixel level, the intensity values of these monochromatic images represent the wavelength-dependent, blackbody energy emitted by the object due to its temperature. Ratioing the measured spectral irradiance for each wavelength yields emissivity-independent temperature data at each pixel. The filter’s Center Wavelength (CWL) and Full Width Half Maximum (FWHM), which are related to the quality factor (Q) of the filter, are actively tunable on the order of nanoseconds-microseconds (GHz-MHz). This behavior is electronically controlled and can be operated time-sequentially (on a nanosecond time scale) in the control electronics, a capability not possible with conventional optical filtering technologies.