Optical fiber sensors based on multimode interference (MMI) have been widely used and developed into various applications. The sensing principle is mostly based on the induced wavelength shift of selected dips (or peaks) in the transmission spectrum. A simple structure to obtain MMI devices is a so-called single- mode—multimode—single-mode (SMS) fiber structure, which is composed of a short section of multimode fiber (MMF) fusion spliced between two single-mode fibers (SMFs). However, most of these MMI-based fiber sensors are related to circular core MMFs. Although the sensitivity is enhanced and the fabrication process is improved, some common problems still exist and need to be solved. To solve current challenges, such as linearity, crosstalk, and compactness, it is essential to find solutions like using a new structure or a new type of fiber. Recently, different new fibers, like hollow annular core fiber (HACF) and square no-core fiber, have been demonstrated to have advantages to overcome some of these limits. The applications of these fibers provide possibilities to study specifically shaped core multimode fibers. In this contribution, we propose a compact MMI-based fiber sensor for temperature measurement. Instead of the standard MMF used in the SMS structure, a square-core fiber (SCF) with a circular cladding is implemented as the sensing element. To the best knowledge of the authors, the SCF has not been investigated yet for sensing. Therefore, the sensing characteristics are studied experimentally. The proposed fiber sensor reaches a sensitivity of 45 pm/◦C. It proves the sensing capability of SCF is promising, provides great potential for further works.
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.
KEYWORDS: Hyperspectral imaging, Microscopy, Microscopes, Line scan image sensors, Spatial resolution, Phase shifts, Digital Light Processing, Control systems
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.
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.
Due to excellent optical performances, two-dimensional materials have emerged as promising materials for applications like optoelectronic devices, photonic devices, and optical sensors. To better study the unique optical performances of 2D materials, spectroscopy techniques such as reflectance and transmittance spectroscopy, and Raman spectroscopy have been utilized for image acquisition and optical property analysis. Hyperspectral imaging (HSI), a combination of spectroscopy and imaging technique, has been used for characterization and property analysis of new materials. A 3D datacube with the wavelength as z-axis, plus spatial axes x and y, can be acquired, and the spectral information can be extracted for characteristic analysis. With the high demand for area imaging of 2D materials, a microscopic HSI setup with a LED light source working in the visible range was proposed for 2D MoS2 imaging. The HSI imager using a reflection grating works in line-scanning mode in the range of 380-1000 nm. A 3D datacube of 2D layered MoS2 was built and processed for thickness measurement and optical property analysis, including single-band analysis of the imaging area, spectral analysis of the interesting area, and comparison with the image acquired by a white-light microscope. Finally, general performances of hyperspectral imaging of 2D MoS2 in the visible range was analyzed and discussed for further optical applications
Hyperspectral imaging is an established technique for process analysis capturing a two-dimensional spatial image and the spectral information for each pixel simultaneously. When moderate spectral resolution is sufficient, static imaging Fourier transform spectrometers (sIFTS) can offer a viable alternative to their scanning counterparts in the mid-infrared spectral range. Therefore, in this paper we present a sIFTS concept based on a single-mirror interferometer which shows no internal light losses and still works with extended light sources, achieving sufficient signal-to-noise ratios. The interferometer consists of a beam splitter, a plane mirror and a lens, which makes it both inexpensive and relatively easy to adjust. For a proof of principle we present a transmission measurement setup including a light source module, imaging optics and a single-mirror interferometer. The system achieves a spectral resolution of 12 cm−1 in a spectral range from 2700 cm−1 to 800 cm−1 , respectively from 3.7 μm to 13 μm. The spatial resolution amounts to about 10.10 lp/mm, the results for a sample containing different polymers show good agreement with a laboratory FTIR spectrometer.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.