Remote sensing in the thermal infrared (TIR) band (3-14 μm) is an increasingly attractive tool for environmental sciences and security due to its chemical specificity and the large spectral superposition with the atmospheric transparency window. Currently, the technique is largely dominated by dispersive-type hyperspectral imagers, which usually require expensive, cumbersome and cooled quantum detectors setups to compensate for their low optical throughput. Here, we present a compact and low-cost hyperspectral camera based on the Fourier-transform approach. Its main element is a common-path birefringent interferometer made of TIR-transparent crystal calomel (Hg2Cl2). We characterize it with both coherent and incoherent sources in the TIR, determining high optical throughput, adjustable spectral resolution up to 4.5 cm-1, interferometric contrast higher than 90% even for incoherent radiation and a robust and long-term interferometric stability. By coupling the interferometer to an uncooled microbolometer detector with 640x480 pixels, we demonstrate hyperspectral imaging in the 8-14 μm spectral range for transmission and emission remote measurements.
We introduce a compact hyperspectral camera based on the time-domain Fourier-transform approach, equipped with an ultrastable birefringent interferometer. The time-domain approach enables hyperspectral imaging with shorter acquisition times and higher spectral accuracy compared to standard dispersive optics. We provide experimental proofs of the camera capability by performing remote-sensing measurements in the visible and near-infrared range. Recently we extended the spectral range to the thermal infrared, where vibrational transitions associated with chemical bonds have their absorption. Due to its compactness, lightweight and extreme stability even in harsh environments, the camera is a unique enabling technology for remote unambiguous chemical identification.
Hyperspectral imaging is rapidly advancing and transforming industries such as agriculture, medicine, and defense. The introduction of HERA, a new hyperspectral camera, is a notable development in this field. Utilizing a Fourier-transform approach, HERA consists of a monochromatic camera, camera lens, and an innovative common-path birefringent interferometer. By scanning the interferometer's position and capturing a sequence of monochromatic images, the hyperspectral data-cube is acquired without any camera or sample movement. This approach offers advantages like high light throughput, signal-to-noise ratio, and wavelength accuracy. HERA's exceptional light throughput enables high-quality data even in low-illuminance conditions and fluorescence studies. Additionally, its flexibility and stability allow for integration with commercial microscopes, expanding the applications of hyperspectral imaging to the microscopy field. This work focuses on showcasing HERA's significant applications in microbiology.
Spectral imaging is a method to acquire the spectrum of the light for each point in the image of a scene. By combining classical imaging with Fourier-transform spectrometry it is possible to acquire hyperspectral images with higher spectral accuracy and lower times compared to standard dispersive optical systems. The technique is based on interferometry and is hence technically challenging as it requires to generate field replicas with delay controlled within a small fraction (1/100 or better) of the optical cycle. Standard FT spectrometers are heavy, cumbersome and too sensitive to mechanical and thermal perturbations for use in portable devices or for deployment in space applications. Here we propose and experimentally validate a compact FT-based hyperspectral camera, in which the FT module is an innovative ultra-stable birefringent common-path interferometer (the Translating-Wedge-Based Identical Pulses eNcoding System, TWINS). TWINS has intrinsic interferometric stability, it is lightweight and is ultracompact, making our FT-based hyperspectral camera an ideal device for portable on-field and spaceborne applications. Our prototype camera is able to measure absolute reflectance and fluorescence with very high spectral accuracy in the visible and near-infrared spectral range and can be extended to the spectroscopically rich thermal infrared range (3 mu;m to 14 μm) using suitable birefringent materials and detectors. We present some examples of application in the visible and TIR ranges.
We present a novel hyperspectral imaging system working in the visible and in the short-wave infrared (SWIR) spectral region based on a Fourier-transform (FT) approach. The technology presents high light throughput and spatial resolution, a software adjustable spectral resolution, and a wide versatility of use. It employs a common-path interferometer, generating two replicas of the image with controllable delay and remarkable accuracy and stability. The monochromatic camera (CMOS, CCD, or InGaAs bidimensional sensor) does not require any relative movement with respect to the sample. The absence of gratings and slits guarantees an exceptional throughput that ensures high-quality data even at the lowest light dose, making this technology particularly suitable in fluorescence studies, or where low-illuminance conditions are recommended in order not to damage the samples. We will show several examples of the use in remote sensing and microscopy.
We present a novel hyperspectral imaging system working in the visible and in the short-wave infrared (SWIR) spectral region based on a Fourier-transform approach. The technology presents an exceptional light throughput, a high spatial resolution, a software adjustable spectral resolution, and a wide versatility of use. In this work, we illustrate a broad portfolio of applications both in the visible and in the SWIR regions, with particular focus on microscopy and biology, cultural heritage, and quality control for the agri-food sector, in collaboration with a vertical farm.
Optical activity spectroscopy such as circular dichroism (CD) and optical rotatory dispersion (ORD) is frequently employed to investigate (bio)-molecular structures and chiroptical responses of materials. Here we present an innovative, simple configuration for the quick and sensitive measurements of broadband optical activity spanning the visible and nearinfrared. A linearly polarized light illuminates a chiral sample to create a chiral free-induction decay field (CFID), along with an orthogonally polarized achiral transmitted field which serves as the phase-locked local oscillator for heterodyne amplifications. A common-path birefringent interferometer varies the relative delay between the chiral and achiral components and a balanced photodetector records their delay-dependent interferogram from which broadband CD and ORD spectra are obtained by the Fourier transform. Using an incoherent thermal light source, we achieve state-of-the-art sensitivity for broadband CD and ORD spectra, with a measurement time of just a few seconds. The setup allows highly sensitive measurements of glucose concentration and real-time monitoring of fast asymmetric chemical reactions. In comparison to standard spectropolarimeters, our setup is considerably faster, more compact, and cost-effective, as it does not require any monochromator, photo-elastic modulator, or lock-in amplifier. The setup also accepts ultrashort pulses, thus paving the way towards broadband transient optical activity spectroscopy and broadband CD imaging.
Hyperspectral imaging (HSI) is a novel non-invasive analytical technique based on spectroscopy. It measures the light spectrum from each point of a scene of interest. It generates an accurate digital record for documentation and archiving in art conservation. This information can be used for diagnostics by monitoring changes, degradation or damages to paintings, for supporting in artwork restoration, for artist's material identification and pigment mapping, as well as for discovering the history of a piece of art by e.g. revealing underdrawings and retouchings. HSI measures the continuous spectrum of the light as a function of the wavelength λ for each pixel of the scene at coordinates (x, y) with fine spectral resolution, thus building the so-called 3-dimensional hyperspectral image. This data cube contains an extensive amount of information. Therefore, many numerical methods and algorithms have been developed to enable the extraction of quantitative parameters related to the physicochemical properties of the imaged objects from congested scenes. Here we present HERA, a novel and compact hyperspectral camera by NIREOS. The device, based on the Fourier-transform approach, combines high delay precision, long-term stability, insensitivity to vibrations and broad spectral coverage. It can measure absolute reflectance and fluorescence with very high spectral accuracy and resolution (≈3-nm, comparable to best commercial push-room systems) in the visible and near-infrared. HERA does not require any scanning of the object under analysis. This feature makes it extremely flexible and applicable to the study of works of art of different nature and size. Potential applications range from the on-site study of museum paintings, to the assessment of the conservation status of large monuments and historic buildings, to the documentation of small objects using standard macro photography lenses. In this paper, we will present a series of application examples in the field of conservation science to demonstrate its versatility.
We present "HERA", a novel hyperspectral camera based on a Fourier-transform approach. Two delayed replicas of the optical field entering an imaging system are produced by an ultra-stable common-path birefringent interferometer that combines compactness, high delay precision, long-term stability and insensitivity to vibrations. Each pixel of a bidimensional sensor records a high contrast interferogram as a function of the relative delay between the two replicas. An appropriate Fourier transformation of the dataset produces spectrally resolved images that exhibit high contrast and spatial resolution, only limited by the lens and the sensor. Examples of application in cultural heritage will be reported.
We propose and experimentally validate a new hyperspectral camera based on the Fourier-transform approach. Two delayed replicas of the optical field entering an imaging system are produced by the Translating-Wedge-Based Identical Pulses eNcoding System (TWINS).1 This device is a common-path birefringent interferometer that combines compactness, high delay precision, long-term stability and insensitivity to vibrations. The TWINS scans the phase delay between the two replicates along many optical cycles. The replicas produce, upon recombination, a high contrast interferogram in each pixel of the detector. An appropriate Fourier transformation of the dataset produces spectrally resolved images that exhibit high contrast and spatial resolution only limited by the lens and the sensor. The main features of the system in terms of spectral resolution, contrast and bandwidth will be presented. Examples of application in conservation science will be also reported.
Stimulated Raman scattering spectroscopy is a powerful nonlinear optical technique for label-free identification of molecules, based on their characteristic vibrational fingerprint. Current implementations of SRS, while achieving extremely high acquisition speeds up to the video rate, mostly work at a single frequency, thus providing limited chemical information. Broadband implementation of SRS is technically challenging, as for imaging dilute species in biological microscopy applications one must detect very small (approx. 10^-5) signals sitting on a large background via modulation transfer technique. We introduce and experimentally demonstrate a novel approach to broadband SRS spectroscopy based on photonic time stretch (PTS). The broadband femtosecond Stokes pulse, after interacting with the sample, is stretched by a telecom fiber to 15ns duration, mapping its spectrum in time. The signal is sampled through a fast analog-to-digital converter, providing single-shot spectra at 80-kHz rate. We demonstrate 10^-5 sensitivity, over 500 cm-1 bandwidth in the C-H region with high resolution. These performances are already suitable for a number of applications, such as monitoring microfluidic flows, the onset of chemical reactions or solid-state samples such as pharmaceutical products. As the acquisition speed of PTS does not depend upon the covered spectral region, we are planning to extend the spectral coverage of SRS to the fingerprint region. Furthermore, using commercially available lasers with higher rep-rates, we could shorten the acquisition time considerably. This will pave the way to high-speed broadband vibrational imaging for materials science and biophotonics.
Single molecule (SM) fluorescence spectroscopy has proven to be a powerful, noninvasive tool in life science, materials science, and photophysics. Here we present an innovative approach to SM fluorescence spectroscopy, able to collect two-dimensional excitation-emission (2D-EEM) maps rapidly and under ambient conditions. If emission occurs from the initially excited state, excitation spectra are equivalent to absorption spectra and are sensitive to couplings of the SM with the local environment or other molecules. The high signal to noise ratio of the measurements presented in this work allow for a characterization of molecular properties on electronic ground and excited states. Among such properties are reorganization energies, the strength of system-bath interaction as well as vibrational anharmonicity constants. As a result, excitation/emission spectra provide unique insight into SMs, beyond effects related to inhomogeneity which are unavoidable in ensemble measurements.
Our approach to SM 2D-EEM is based on Fourier-transform spectroscopy. We employ an innovative, compact, fast, versatile and highly stable common-path interferometer based on birefringent crystals. It generates two phase-locked replicas of the excitation light without the need for active stabilization or auxiliary tracking beams. It provides adjustable excitation wavelength resolution (down to the sub-nm range). We collected sixty SM 2D-EEM maps from terrylene diimide dye with data quality equal to bulk spectra obtained with commercial absorption spectrometers. Based on statistical analysis, we discuss the distribution of spectral shapes of individual molecules due to a combination of intrinsic molecular variety and different interactions of the molecules with their local environment.
We introduce a home-built laser-scanning nonlinear optical microscope, combining two-photon excitation fluorescence (TPEF), stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS). Narrowband pump and tunable Stokes pulses at 40-MHz are delivered by an Erbium-fiber laser source, thus greatly simplifying the excitation scheme and reducing the costs and maintenance with respect to standard bulk free-space lasers. After the sample, a dichroic beam splitter transmits the Stokes beam for SRS imaging and reflects the CARS or TPEF (at shorter wavelengths). Signal-to-noise ratio in SRS imaging is greatly enhanced (by up to 30 dB, reaching shot-noise-limited detection without the need of any electronic auto-balancing) thanks to the use of an innovative scheme that we call In-line Balanced Detection (IBD). IBD-SRS not only completely removes high-frequency laser fluctuations but also passively and automatically balances the low-frequency signal variation due to spatially varying sample transmission. We record SRS/CARS spectra in the 2800-3100 cm-1 Raman vibrational spectrum, thus providing a detailed chemical information on the sample in the C-H stretching region. We report various bioimaging applications of our instrument: the study of breast tumour cells using CARS, three-dimensional visualization of lipid distribution in HuH7 and in HepaRG hepatic cells using SRS and a combined TPEF/SRS study of plant cells. Microscopy in scattering media such as a bovine liver tissue is as well demonstrated.
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