Fourier transform ultraviolet-visible spectrometer based on a beam-folding technique

Step-scan Michelson Fourier-transform spectrometer for optical emission spectroscopy in UV-VIS spectral range

We present the design, construction, and first spectra of a step-scan Michelson Fourier-transform spectrometer for optical emission spectroscopy in the UV-VIS spectral range. The mirror motion mechanism is based on a long-travel piezo-based linear translation stage with built-in position feedback. The step-scan arrangement allows for signal integration, making the instrument suitable for measurements of less intensive radiation sources and for the photon-counting technique. The spectrometer consists of two coupled Michelson interferometers, one for the spectrometer itself and the other to provide positional reference for the mirror stepping mechanism using interference fringes of a stabilized 635 nm laser diode. Using interpolation of the laser interferogram and taking advantage of the translation stage precision in linear-piezo mode, the mechanism is capable of performing 79 nm steps, which puts the Nyquist wavelength at ∼320 nm. The spectrometer was tested by measuring the spectra of HgAr cold-cathode fluorescent lamp and electron-induced fluorescence of ambient air. Two different detectors were used, an amplified photodiode detector and a photomultiplier tube module in photon counting mode. The highest achieved experimental spectral resolving power was ∼4000 using 1 mm of total mirror travel and the highest achieved noise-free dynamic range was 10³. Test results show that the instrument is suitable for use with moderate-to-low intensity signal sources such as small gas discharges and spectroscopic measurements at astronomical telescopes.

Three-area-array coherent-dispersion stereo-imaging spectrometer

A coherent-dispersion stereo-imaging spectrometer is presented, which combines three-view stereo imaging, interferometric spectroscopy and dispersive spectroscopy. Three area-array detectors record three spectral images of each scene unit from three views. For each of the three views, each scene unit is imaged on a given column of one area-array detector, and different wavelengths are dispersed across different rows of that column. For each scene unit, multiple interferograms are simultaneously generated at each view, each interferogram covering a separate wavelength range and located in a separate pixel. The orthographic view image is used to create a two-dimensional orthophoto image. The front view and back view images are used to reconstruct the three-dimensional stereoscopic image. Preliminary theoretical calculations are given. The instrument is a unique concept to obtain three-dimensional spatial information and one-dimensional spectral information while achieving high spectral resolution measurement of an ultraviolet-visible broadband spectral range (e.g., 0.05 nm at 450 nm together with 0.1 nm at 700 nm). It will be suitable for ultraviolet-visible hyperspectral remote sensing.

Broadband high-spectral-resolution ultraviolet-visible coherent-dispersion imaging spectrometer

A coherent-dispersion imaging spectrometer combining imaging, interferometric spectroscopy and dispersive spectroscopy is presented, which is a unique concept to greatly reduce the multiplex disadvantage of existing ultraviolet-visible multiplexed spectroscopy while achieving imaging combined with high spectral resolution (e.g., 0.01 nm at 220 nm together with 0.1 nm at 700 nm, or higher) for a broadband spectral range. Each unit of the entrance slit is imaged on a given column of a detector, and different wavelengths are dispersed across different rows of that column. For each slit unit, multiple interferograms are generated simultaneously in one scan period, each interferogram with a separate wavelength range and located in a separate pixel of the detector. The expressions for the coherent-dispersion imaging are given, the preliminary design calculations are illustrated by an example, and the numerical simulations for the interferogram and spectrum are shown. This design will be suitable for broadband high-spectral-resolution ultraviolet-visible spectral imaging.

Scanning Fourier transform spectrometer in the visible range based on birefringent wedges

We introduce a spectrometer capable of measuring sample absorption spectra in the visible regime, based on a time-domain scanning Fourier transform (FT) approach. While infrared FT spectrometers typically employ a Michelson interferometer to create the two delayed light replicas, the proposed apparatus exploits a compact common-mode passive interferometer that relies on the use of birefringent wedges. This ensures excellent path-length stability (∼λ/300) and accuracy, with no need for active feedback or beam tracking. We demonstrate the robustness of the technique measuring the transmission spectrum of a colored bandpass filter over one octave of bandwidth and compare the results with those obtained with a commercial spectrophotometer.

Imaging Fourier transform endospectroscopy for in vivo and in situ multispectral imaging

We report the design and implementation of a multispectral imaging Fourier transform endospectroscopy (IFTES) system. The IFTES system employs a flexible fiber bundle catheter coupled to a home-built imaging Fourier transform spectroscope. The instrument enables the performance of non- or minimally invasive subsurface imaging and multispectral imaging at the cellular level in vivo and in situ. A maximum spectral resolution of 0.2 nm at 632.8 nm and a lateral resolution of 4.4 μm were proved. Preliminary results of a standard resolution target, ex-vivo small animal tissue, single wavelength laser, fluorescence solution, in-vivo mouse skin, microspheres mixture, and in-vivo transgenic mouse brain were given to demonstrate the potential of the technique.

Toward a UV-visible-near-infrared hyperspectral imaging platform for fast multiplex reflection spectroscopy

A reflection hyperspectral imaging system covering a 350-1000nm spectral range is realized by a UV-visible-near-IR Fourier transform imaging spectrometer. The system has a simple design and good spectral and spatial resolving performance. Accurate and fast microspectroscopic measurement results on novel colloidal crystal beads demonstrate the system has practical potential for high-throughput molecular multiplex assays.

Tests of a practical visible-NIR imaging Fourier transform spectrometer for biological and chemical fluorescence emission measurements

An imaging Fourier transform spectrometer (IFTS) designed for fluorescence emission measurements is reported. The spectral range extension from NIR to visible of the system is realized by using a simple and low-cost optical beam-folding position-tracking technique. Spectral resolution as high as 9.78cm(-1)(0.4nm at 632.8nm) and maximum image resolution up to 300x300 pixels are proved by the system tests on its optical performances. Imaging fluorescence spectra acquisition of quantum dot clusters and single 200nm diameter fluorescent beads have demonstrated the system's potential for high throughput imaging spectroscopic measurements of fluorescent biological and chemical samples.