New auroral spectrometer using an acousto-optic tunable filter

This paper reports the performance and capability of a newly developed zenith spectrometer (for measurements of airglow and aurora) that uses an acousto-optic tunable filter (AOTF). The AOTF can scan the pass-band of the spectrometer between 450 and 700 nm with a bandwidth of 2-3 nm by changing the RF driver frequency from 180 to 100 MHz. The absolute sensitivity of the spectrometer is ～0.1-1.5 counts/Rayleigh/s per spectral step. The spectrometer is fully automated. The O I (557.7 nm) airglow line can be clearly identified in test observations of midlatitude airglow performed at Shigaraki Observatory, Japan. Based on an estimate of the signal-to-noise ratio, we conclude that the full auroral spectrum (450-700) nm can be measured by the AOTF spectrometer with a time resolution of ～100 s and a signal-to-noise ratio of ～100 for an auroral emission intensity of 10 kR. An example of the auroral spectra is shown for observations made at Syowa Station in Antarctica

Hyperspectral imager with folded metasurface optics

Hyperspectral imaging is a key characterization technique used in various areas of science and technology. Almost all implementations of hyperspectral imagers rely on bulky optics including spectral filters and moving or tunable elements. Here, we propose and demonstrate a line-scanning folded metasurface hyperspectral imager (HSI) that is fabricated in a single lithographic step on a 1 mm thick glass substrate. The HSI is composed of four metasurfaces, three reflective and one transmissive, that are designed to collectively disperse and focus light of different wavelengths and incident angles on a focal plane parallel to the glass substrate. With a total volume of 8.5 mm^3, the HSI has spectral and angular resolutions of ∼1.5 nm and 0.075°, over the 750–850 nm and −15° to +15° degree ranges, respectively. Being compact, light weight, and easy to fabricate and integrate with image sensors and electronics, the metasurface HSI opens up new opportunities for utilizing hyperspectral imaging where strict volume and weight constraints exist. In addition, the demonstrated HSI exemplifies the utilization of metasurfaces as high-performance diffractive optical elements for implementation of advanced optical systems

Pure electrical, highly-efficient and sidelobe free coherent Raman spectroscopy using acousto-optics tunable filter (AOTF)

Fast and sensitive Raman spectroscopy measurements are imperative for a large number of applications in biomedical imaging, remote sensing and material characterization. Stimulated Raman spectroscopy offers a substantial improvement in the signal-to-noise ratio but is often limited to a discrete number of wavelengths. In this report, by introducing an electronically-tunable acousto-optical filter as a wavelength selector, a novel approach to a broadband stimulated Raman spectroscopy is demonstrated. The corresponding Raman shift covers the spectral range from 600 cm(−1) to 4500 cm(−1), sufficient for probing most vibrational Raman transitions. We validated the use of the new instrumentation to both coherent anti-Stokes scattering (CARS) and stimulated Raman scattering (SRS) spectroscopies

A Broadly Tunable Surface Plasmon-Coupled Wavelength Filter for Visible and Near Infrared Hyperspectral Imaging

Hyperspectral imaging is a set of techniques that has contributed to the study of advanced materials, pharmaceuticals, semiconductors, ceramics, polymers, biological specimens, and geological samples. Its use for remote sensing has advanced our understanding of agriculture, forestry, the Earth, environmental science, and the universe. The development of ultra-compact handheld hyperspectral imagers has been impeded by the scarcity of small widefield tunable wavelength filters. The widefield modality is preferred for handheld imaging applications in which image registration can be performed to counter scene shift caused by irregular user motions that would thwart scanning approaches. In the work presented here an electronically tunable widefield wavelength filter has been developed for hyperspectral imaging applications in the visible and near-infrared region. Conventional electronically tunable widefield imaging filter technologies include liquid crystal-based filters, acousto-optic tunable filters, and electronically tuned etalons; each having its own set of advantages and disadvantages. The construction of tunable filters is often complex and requires elaborate optical assemblies and electronic control circuits. I introduce in the work presented here is a novel widefield tunable filter, the surface plasmon coupled tunable filter (SPCTF), for visible and near infrared imaging. The SPCTF is based on surface plasmon coupling and has simple optical design that can be miniaturized without sacrificing performance. The SPCTF provides diffraction limited spatial resolution with a moderately narrow nominal passband (\u3c10 \u3enm) and a large spurious free spectral range (450 nm-1000 nm). The SPCTF employs surface plasmon coupling of the π-polarized component of incident light in metal films separated by a tunable dielectric layer. Acting on the π-polarized component, the device is limited to transmitting 50 percent of unpolarized incident light. This is higher than the throughput of comparable Lyot-based liquid crystal tunable filters that employ a series of linear polarizers. In addition, the SPCTF is not susceptible to the unwanted harmonic bands that lead to spurious diffraction in Bragg-based devices. Hence its spurious free spectral range covers a broad region from the blue through near infrared wavelengths. The compact design and rugged optical assembly make it suitable for hand-held hyperspectral imagers. The underlying theory and SPCTF design are presented along with a comparison of its performance to calculated estimates of transmittance, spectral resolution, and spectral range. In addition, widefield hyperspectral imaging using the SPCTF is demonstrated on model sample

Snapshot spectral imaging using image replication and birefringent interferometry : principles and applications

This thesis explores the image-replicating imaging spectrometer (IRIS). This relatively recent invention is a two-dimensional, snapshot spectral-imaging technology, capable of recording the spectral and spatial data from a scene instantaneously. Whereas conventional spectral-imaging technologies require multiple detector frames to record the entire data set, IRIS is able to record the data set in a single frame, a capability which is useful for highly dynamic scenes. The IRIS concept and the design of IRIS systems are explained in detail, and constraints on the performance of IRIS are determined. Practical issue in the use of IRIS systems are identi ed and solutions are identi ed and appraised. Some applications of IRIS are also shown, demonstrating its viability as a spectral imaging technology. Novel aspects of this work include the re nement of the IRIS design, demonstration of a registration algorithm for IRIS, designs for achromatic Wollaston prisms, a comparison of the IRIS technology with conventional spectral imaging technologies, and the application of IRIS to practical problems.Engineering and Physical Sciences Research Council (EPSRC)Selex Galile

Lenslet array tunable snapshot imaging spectrometer (LATIS) for hyperspectral fluorescence microscopy

Snapshot hyperspectral imaging augments pixel dwell time and acquisition speeds over existing scanning systems, making it a powerful tool for fluorescence microscopy. While most snapshot systems contain fixed datacube parameters (x,y,λ), our novel snapshot system, called the lenslet array tunable snapshot imaging spectrometer (LATIS), demonstrates tuning its average spectral resolution from 22.66 nm (80x80x22) to 13.94 nm (88x88x46) over 485 to 660 nm. We also describe a fixed LATIS with a datacube of 200x200x27 for larger field-of-view (FOV) imaging. We report <1 sec exposure times and high resolution fluorescence imaging with minimal artifacts

High-speed surface profilometry based on an adaptive microscope with axial chromatic encoding

An adaptive microscope with axial chromatic encoding is designed and developed, namely the AdaScope. With the ability to confocally address any locations within the measurement volume, the AdaScope provides the hardware foundation for a cascade measurement strategy to be developed, dramatically accelerating the speed of 3D confocal microscopy

Advanced Prototypes of the Aerosol Limb Imager

Over the past decades, the call for global monitoring of aerosol has amplified to better understand its role in climate change. The Canadian Space Agency has identified targeted program funding for mission development to address this call. The Aerosol Limb Imager, or ALI, is a candidate remote sensing instrument that will provide this monitoring. ALI is a Canadian developed atmospheric remote sensing instrument specifically designed to be sensitive to aerosol and clouds from the mid-troposphere through the stratosphere. An orbital-based viewing platform is necessary to realize global coverage. This work presents the development of two sub-orbital prototype instruments that inform the design of a satellite instrument. The first ALI prototype presented is a technology demonstration aimed at validating the performance of state-of-the-art optical technologies on a high-altitude balloon observatory. The instrument pairs an extended range acousto-optic tunable filter with a liquid crystal polarization rotator to capture spectrally resolved polarimetric imagery of the atmospheric limb. These technologies provide the capability to extract particle size information from the sampled radiance and to identify cloud structures. The instrument met performance expectations from a balloon platform in 2018. The ALI elegant breadboard is the latest hardware development and is designed to measure scattered sunlight from a high-altitude aircraft. An aircraft platform offers a varying spatial scene, which is analogous to the variation observed from orbit. Along-track sampling and signal-to-noise requirements are met with a state-of-the-art large-aperture acousto-optic tunable filter. The optical design surrounding the filter is equally advanced, incorporating diamond-turned mirrors and precision optical alignment. The ALI elegant breadboard is being assembled to meet a flight opportunity on the NASA ER-2 observatory in late 2022. The insight and experience gained through the development of these two prototypes are paramount to the design of a future satellite-based sensor. Teams from the Canadian Space Agency, a Canadian University consortium and industry partners have assembled to ensure that ALI is the right instrument to address a global need. If selected for a satellite mission, ALI will fuel new research into how aerosol shapes climate and the health of the planet