In this contribution we introduce a compact version of a broadband static Fourier transform spectrometer (bs- FTS) for the mid-infrared spectral range. The bsFTS covers a spectral range from about 4.5 µm to 14 µm, respectively 2220 cm−1 to 700 cm−1 at a spectral resolution of 8 cm−1. As, in contrast to scanning Fourier trans- form spectrometers, the interferogram is modulated not over time but in the spatial domain, the measurement speed is only limited by the detector. This allows for infrared spectroscopy at 25 Hz to 200 Hz using uncooled microbolometer arrays. Besides liquid measurements in attenuated total reflection (ATR), demonstrating the accuracy and linearity of the bsFTS, we show time-resolved analyses of 1,1,1,2-Tetrafluoroethane (R134a) and carbon monoxide test gases to prove the suitability of the system for high-speed spectroscopy.
Hyperspectral imaging microscopy is a powerful analytical tool for spatial identification and spectral feature extraction in chemical and biological complex systems. Inspired by super-resolution microscopy, structured programmable projection coupled with spectral image reconstruction techniques is employed to improve the spatial resolution of spectroscopic imaging microscopy. In this work, a line-scan hyperspectral imaging microscope implemented with a digital light projector (DLP) was demonstrated. The DLP with a digital micromirror device (DMD) was used to project sinusoidal fringes with three angular orientations and three phase shifts. After synchronization of fringe projection, stage movement, and image acquisition, hyperspectral data sets were acquired, and image reconstruction was conducted using the nine-frame images for improved spatial resolution over the full wavelength range. This work contributes to the progress in microscale and nanoscale imaging using line-scan hyperspectral microscopy.
Laser triangulation is an optical method for measuring distances to an object. A laser beam is directed towards a measurement surface, and the diffusely reflected light is collected by an imaging system onto a detector. The absolute distance can be obtained by using known geometric relations of the system and the position of the laser spot on the detector. Therefore, a change in the measurement distance results in a corresponding movement of the imaged laser spot, defining the sensitivity of the system in pixels per millimeter. This value depends on the geometrical and optical design of the laser triangulation setup, especially the base distance between the laser and the imaging lens, as well as its focal length. As those parameters also influence the geometric dimensions and the possible measurement range of the device, the sensitivity cannot be increased arbitrarily. Thus, the sensitivity of a standard laser triangulation system is limited to a certain value. In this contribution, structured optical surfaces are applied onto the measurement surface to further increase the sensitivity. Through the spatial modulation of the imaged laser spot intensity distribution, the calculated laser spot displacement is larger than its actual geometrical displacement. This effect is examined through simulations with a bar structure, which leads to an improvement of the sensitivity by a factor of up to 5.7 at a distance of 1 m and a measurement range of 2 mm. Eventually, the concept is proven in measurements and feasible implementations of such a structure are considered.
Two-dimensional van der Waals materials are attractive for photonics and optoelectronics due to distinctive layerdependent optical properties. Optical properties based on light-matter interactions have been revealed by modern imaging and spectroscopy techniques. Hyperspectral imaging microscopy working in line-scan mode (push-broom microspectroscopy) can provide abundant spectral information covering a large area compared to conventional spectroscopy techniques, with a higher acquisition speed than point-scan techniques such as atomic force microscopy and Raman imaging microscopy. This contribution studies in-depth the reconstruction of 3D datacubes and the extraction of optical responses of the sample. Monolayer MoS2, a subclass of semiconducting two-dimensional materials, is fabricated by the mechanical exfoliation method on the SiO2/Si substrate with an oxide thickness of 285 nm. The isolated monolayer MoS2 is observed and identified by a conventional optical microscope. The custom-built push-broom microspectroscope is utilized to scan the region of interest, with the whole spectrum of every line recorded at each frame. The spectral information of every point is collected and 3D spectral data sets are reconstructed for feature extraction and property analysis. To realize the thickness mapping of flakes, linear unmixing is employed to calculate the abundance of isolated monolayer MoS2 on the SiO2/Si substrate, improving flake identification performances. The characteristic spectrum of monolayer MoS2 is acquired by averaging the spectrum from the monolayer MoS2 flake. Furthermore, the optical dielectric response is further analyzed by Kramers-Kronig constrained analysis and Fresnel-law-based analysis. The optical dielectric function is calculated and compared based on the refractive index and medium thickness. This detailed analysis of optical dielectric responses highlights the feasibility of push-broom microspectroscopy for two-dimensional materials characterization.
In this contribution we present a broadband static Fourier transform spectrometer (bsFTS) based on a single- mirror interferometer containing only off-the-shelf optical components and an uncooled microbolometer detector
array. The system uses concave mirrors instead of lenses and therefore covers a wide spectral range from 3.6 μm to 17 μm at a spectral resolution of 12 cm-1. Furthermore, dispersion effects can be minimized and the system can thus be designed with increased temperature stability. We demonstrate the optical and mechanical design of
the current laboratory prototype and compare the instrument to a scanning Fourier transform infrared (FTIR) spectrometer. Additionally, we present a technique for simultaneously acquiring the sample spectrum and the background spectrum. Thereby, a variation of the background over time can be compensated continuously and hence the bsFTS presented in this contribution offers significant potential with regard to long-term stability.
In this contribution, the extent to which the Nyquist criterion can be violated in optical imaging systems with a digital sensor, e.g., a digital microscope, is investigated. In detail, we analyze the subpixel uncertainty of the detected position of a step edge, the edge of a stripe with a varying width, and that of a periodic rectangular pattern for varying pixel pitches of the sensor, thus also in aliased conditions. The analysis includes the investigation of different algorithms of edge localization based on direct fitting or based on the derivative of the edge profile, such as the common centroid method. In addition to the systematic error of these algorithms, the influence of the photon noise (PN) is included in the investigation. A simplified closed form solution for the uncertainty of the edge position caused by the PN is derived. The presented results show that, in the vast majority of cases, the pixel pitch can exceed the Nyquist sampling distance by about 50% without an increase of the uncertainty of edge localization. This allows one to increase the field-of-view without increasing the resolution of the sensor and to decrease the size of the setup by reducing the magnification. Experimental results confirm the simulation results.