Aberration-corrected Czerny-Turner imaging spectrometer with a wide spectral region

Designing an imaging spectrometer with high resolution using manufacturable gradient-index (MGRIN) lenses with a linear distribution of refractive index

In this work, an imaging spectrometer with a diffraction-limited resolution of 0.043 nm is designed by using two manufacturable gradient-index (MGRIN) lenses with a linear refractive index distribution to suppress the aberrations. The MGRIN lenses are made from a combination of two separate base materials that their refractive indices are determined by the fractional composition of each base material at any point. The volume fraction of each base material in both lenses changes linearly from the front edge to the rear vertex of the lens. The input light to the spectrometer originates from a single-mode fiber with a core diameter of 9 μm and a numerical aperture of 0.1. The parallel rays after passing through the collimator are diffracted by a diffraction grating with a number of grooves of 1200 g/mm. The criteria determining the quality of the image show that the aberrations of the image have been optimally controlled. Comparing the results of this design with some similar studies done by other researchers shows that the root-mean-square radius of the spot diagram and Airy disk radius are significantly smaller than those designs. In addition, the modulation transfer function diagram has a better fit with the diffraction limit curve. These results make our proposed spectrometer have a stronger resolution than those in previous studies.

TDI-like multi-slit hyperspectral imaging for enhanced throughput via the Kalman filter

The time-delay integration (TDI) technique is increasingly used to improve the signal-to-noise ratio (SNR) of remote sensing and imaging by exposing the scene multiple times. Inspired by the principle of TDI, we propose a TDI-like pushbroom multi-slit hyperspectral imaging (MSHSI) approach. In our system, multiple slits are used to significantly improve the throughput of the system, thereby enhancing the sensitivity and SNR through multiple exposures of the same scene during pushbroom scan. Meanwhile, a linear dynamic model for the pushbroom MSHSI is established, where the Kalman filter (KF) is employed to reconstruct the time-varying overlapped spectral images on a single conventional image sensor. Further, we designed and fabricated a customized optical system that can operate in both multi-slit and single slit modes to experimentally verify the feasibility of the proposed method. Experimental results indicate that the developed system improved SNR by a factor of about 7 compared to that of the single slit mode, while demonstrating excellent resolution in both spatial and spectral dimensions.

High-Performance Ultra-Thin Spectrometer Optical Design Based on Coddington's Equations

A unique method to design a high-throughput and high-resolution ultrathin Czerny-Turner (UTCT) spectrometer is proposed. This paper reveals an infrequent design process of spectrometers based on Coddington's equations, which will lead us to develop a high-performance spectrometer from scratch. The spectrometer is composed of cylindrical elements except a planar grating. In the simulation design, spot radius is sub-pixel size, which means that almost all of the energy is collected by the detector. The spectral resolution is 0.4 nm at central wavelength and 0.75 nm at edge wavelength when the width of slit is chosen to be 25 μm and the groove density is 900 lines/mm.

Hadamard transform-based hyperspectral imaging using a single-pixel detector

In this paper, a single-pixel hyperspectral imager is developed based on the Hadamard transformation. The imager's design, fabrication, signal processing method, and experimental results are discussed. The single-pixel hyperspectral imager works in pushbroom mode and employs both spatial encoding and spectral encoding to acquire the hyperspectral data cube. Hadamard encoding patterns, which are known for their multiplexing advantage to achieve high signal-to-noise ratio (SNR), are used in both encoding schemes. A digital micromirror device (DMD) from Texas Instruments (TI) is used for slow spatial encoding and a resonant scanning mirror in combination with a fixed Hadamard mask is used for fast spectral encoding. In addition, the pushbroom operation can be achieved internally by spatially shifting the location of the Hadamard encoded slit on the DMD, thus the imager is able to acquire 3D data cubes without the need to scan it across the object. Although our experimental results demonstrate the hyperspectral data cubes of various objects over a 450 nm ∼ 750 nm visible spectral range, the proposed imager can be easily configured to be used at other wavelengths due to the single-pixel detection mechanism used.

Optical design of a crossed Czerny-Turner spectrometer with a linear array photomultiplier tube

Conventional Czerny-Turner spectrometers exhibit keystone distortion due to astigmatism, which affects their performance when a linear array photomultiplier tube (LaPMT) is used as the detector. We propose a novel optical design of a crossed Czerny-Turner spectrometer with a 32-channel LaPMT detector. We use convergent illumination of the grating to modify the astigmatism of the off-axis spherical mirrors, and the image spot radius at the central wavelength in the sagittal plane is matched with the width of the LaPMT. A first-order equation is derived for the elimination of the broadband keystone distortion. After optimization, the image size over the entire bandwidth matches well with the LaPMT size, which is consistent with the theory, and the spectral resolution is ∼4  nm per channel.

Construction method through multiple off-axis parabolic surfaces expansion and mixing to design an easy-aligned freeform spectrometer

The classic Czerny-Turner spectrometer consists of a plane grating and two spherical mirrors. The optical path geometry adopted for incident and grating dispersed light is off-axis reflection, so the spherical collimating and focusing mirrors introduce coma and astigmatism. The conventional configuration is asymmetrical for coma automatic compensation, but suffers from astigmatism. We substitute the off-axis parabolic (OAP) surfaces for spherical surfaces of the collimating mirror and each sub-region of the focusing mirror, to achieve an aberration free configuration. The multiple OAP surfaces are then expanded and mixed, to construct a freeform surface integrating the collimating and focusing mirrors into a single element. Results show that a 0.1 nm spectral resolution is achieved over a bandwidth of 400 nm centered at 800 nm, in the designed spectrometer comprised of a plane grating and one freeform mirror. The construction method is advantageous to integrated optic design, and the resulting freeform mirror spectrometer is compact, and simplifies manufacture and alignment.

Atmospheric Aerosol Multiband Synthesis Imaging Spectrometer

According to the characteristics of the spectrum distribution for atmospheric aerosol detection, a multiband synthesis imaging spectrometer system based on Czerny-Turner configuration is designed and proposed in this paper. Using a grating array instead of a traditional single grating, and together with a filter array, the proposed configuration can achieve hyperspectral imaging with the spectral resolution of 0.16 nm, 0.24 nm, 0.29 nm, and 2.05 nm in the spectral bands of 370-430 nm, 640-680 nm, 840-880 nm, and 1560-1660 nm, respectively. First, the system aberration caused by the spectral change was eliminated based on Rowland circle theory; then, Zemax software was used to optimize and analyze the optical design. The analysis results show that the root mean square (RMS) of the spot diagram is < 9 µm in all the working spectral bands, which demonstrates that the aberration has been corrected and a good imaging quality can be achieved. This design of multiband synthesis imaging spectrometer configuration proves to be not only feasible, but also simple and compact, which lays a solid foundation for the practical application in the field of atmospheric aerosol remote sensing spectroscopy.

Optical design of double-grating and double wave band spectrometers using a common CCD

A new type of optical system comprising double-grating and double wave band spectrometers is designed for atmospheric detection. The optical system can bring oxygen A band (758-778 nm) and water vapor absorption band (758-880 nm) on a charge-coupled device (CCD) at the same time for ultrahigh resolution spectrum measurement. Each absorbed band with three observation directions of atmospheric radiation is imaged in different positions of a common CCD. The spectral resolution is less than 0.07 nm in oxygen A band (758-778 nm), and the spectral resolution is less than 0.28 nm in water vapor absorption band (758-880 nm). Three end faces of the optical fiber are on the slit plane for each wave band, and each end face corresponds to an observation angle. The optical fiber core diameter is 600 μm, the slit width is 25 μm, and the slit length is 18.4 mm. The principle of smile correction is analyzed. The smile of the Czerny-Turner double-grating spectrometer can be compensated by using the tilt field lens in front of the focal plane. The design results corroborate that the maximum smile of the double-grating spectrometer is 5 μm and that the approach of correcting smile is effective. The stray light is analyzed, and the approaches of suppressing the stray light are proposed.

Astigmatism-free Czerny-Turner compact spectrometer with cylindrical mirrors

A modified optical design for a broadband, high resolution, astigmatism-free Czerny-Turner spectrometer is proposed. Astigmatism is corrected by using cylindrical mirrors over a broad spectral range. The theory and method for astigmatism correction are thoroughly analyzed. The comparison between the modified Czerny-Turner spectrometer and the traditional Czerny-Turner spectrometer is also described in detail. The ray-tracing results show that the RMS spot radius has decreased to 4.2 μm at the central wavelength and 17 μm at the wedge wavelength.

Astigmatism-corrected echelle spectrometer using an off-the-shelf cylindrical lens

As a special kind of spectrometer with the Czerny-Turner structure, the echelle spectrometer features two-dimensional dispersion, which leads to a complex astigmatic condition. In this work, we propose an optical design of astigmatism-corrected echelle spectrometer using an off-the-shelf cylindrical lens. The mathematical model considering astigmatism introduced by the off-axis mirrors, the echelle grating, and the prism is established. Our solution features simplified calculation and low-cost construction, which is capable of overall compensation of the astigmatism in a wide spectral range (200-600 nm). An optical simulation utilizing ZEMAX software, astigmatism assessment based on Zernike polynomials, and an instrument experiment is implemented to validate the effect of astigmatism correction. The results demonstrated that astigmatism of the echelle spectrometer was corrected to a large extent, and high spectral resolution better than 0.1 nm was achieved.

Simultaneous nano-tracking of multiple motor proteins via spectral discrimination of quantum dots

Simultaneous nanometric tracking of multiple motor proteins was achieved by combining multicolor fluorescent labeling of target proteins and imaging spectroscopy, revealing dynamic behaviors of multiple motor proteins at the sub-diffraction-limit scale. Using quantum dot probes of distinct colors, we experimentally verified the localization precision to be a few nanometers at temporal resolution of 30 ms or faster. One-dimensional processive movement of two heads of a single myosin molecule and multiple myosin molecules was successfully traced. Furthermore, the system was modified for two-dimensional measurement and applied to tracking of multiple myosin molecules. Our approach is useful for investigating cooperative movement of proteins in supramolecular nanomachinery.

Miniature anastigmatic spectrometer design with a concave toroidal mirror

An advanced optical design for a low-cost and astigmatism-corrected spectrometer with a high resolution is presented. The theory and method of astigmatism correction are determined with the use of a concave toroidal mirror. The performances of a modified spectrometer and a traditional spectrometer are compared, and the analysis is verified. Experimentally, the limiting resolution of our spectrometer is 0.1 nm full width at half-maximum, as measured for 579.1 nm.

Toroidal grating astigmatism of high-harmonics characterized by EUV Hartmann sensor

The beam transport of single high-order harmonics in a monochromator arrangement is studied. A toroidal grating combines spectral filtering and focusing in order to produce a small individual spot for each harmonic. Here, the effect of small deviations from perfect alignment is investigated. Experimentally, a Hartmann sensor monitors the EUV wavefront while the grating is subjected to an online alignment procedure. The obtained results are confirmed by a simple theoretical description employing optical matrix methods.

Tandem gratings spectrometer for spectroscopy broadband anastigmatic imaging

A tandem gratings spectrometer with high imaging quality is designed. By applying the geometric analysis, the spectral broadband anastigmatic imaging conditions have been obtained. It offers an advanced design with low aberrations for the whole spectral range of the small-scale spectrometer both in the off-axis and coaxial telescope applications. A UV design exhibiting excellent optical performance is presented. The specifications of design have also been investigated.

Modified Schwarzschild imaging spectrometer with a low F-number and a long slit

A modified Schwarzschild imaging spectrometer utilizing three nonconcentric aspheric mirrors and a plane grating is designed that can handle low F-number, long slit, and broad spectral range. Based on the geometrical aberration theory and Rowland circle condition, the astigmatism-correcting method of the Schwarzschild imaging spectrometer is analyzed. The design procedure of initial parameters is programmed using Matlab software. As an example, a modified Schwarzschild imaging spectrometer operating in 400-1000 nm waveband with F-number of 2.5 and slit length of 13 mm is designed, and good imaging quality is obtained.

Micrometer axial resolution OCT for corneal imaging

An optical coherence tomography (OCT) for high axial resolution corneal imaging is presented. The system uses 375 nm bandwidth (625 to 1000 nm) from a broadband supercontinuum light source. The system was developed in free space to minimize image quality degradation due to dispersion. A custom-designed spectrometer based on a Czerny Turner configuration was implemented to achieve an imaging depth of 1 mm. Experimentally measured axial resolution was 1.1 μm in corneal tissue and had a good agreement with the theoretically calculated resolution from the envelope of the spectral interference fringes. In vivo imaging was carried out and thin corneal layers such as the tear film and the Bowman's layer were quantified in normal, keratoconus, and contact lens wearing eyes, indicating the system's suitability for several ophthalmic applications.

Schwarzschild spectrometer

This is a proposal and description of a new spectrometer based on the Schwarzschild optical system. The proposed design contains two Schwarzschild optical systems. Light diverging from the spectrometer entrance slit is collimated by the first one; the collimated light beam hits a planar diffraction grating and the light dispersed from the grating is focused by the second system, which is concentric with the first. A very simple procedure obtains designs that are anastigmatic for the center of the slit and for a particular wavelength. A specific example shows the performance of this type of spectrometer.

Astigmatism-corrected Czerny-Turner imaging spectrometer for broadband spectral simultaneity

A low-cost, broadband, astigmatism-corrected Czerny-Turner arrangement with a fixed plane grating is proposed. A wedge cylindrical lens is used to correct astigmatism over a broadband spectral range. The principle and method of astigmatism correction are described in detail. We compare the performance of this modified Czerny-Turner arrangement with that of the traditional Czerny-Turner arrangement by using a practical Czerny-Turner imaging spectrometer example.

Broadband astigmatism-corrected Czerny-Turner spectrometer

We report an optical design for a low-cost optics, broadband, astigmatism-corrected practical spectrometer. An off-the-shelf cylindrical lens is used to remove astigmatism over the full bandwidth. Results show that better than 0.1 nm spectral resolution and more than 50% throughput were achieved over a bandwidth of 400 nm centered at 800 nm.

Broadband astigmatism-free Czerny-Turner imaging spectrometer using spherical mirrors

We describe the elimination of the astigmatism of a Czerny-Turner imaging spectrometer, built using spherical optics and a plane grating, over a broad spectral region. Starting with the principle of divergent illumination of the grating, which removes astigmatism at one chosen wavelength, we obtain design equations for the distance from the grating to the focusing mirror and the detector angle that remove the astigmatism to first order in wavelength. Experimentally, we demonstrate near diffraction-limited performance from 740 to 860 nm and over a 5 mm transverse spatial extent, while ray-tracing calculations show that barring finite-aperture and detector size limitations, this range extends from 640 to 900 nm and over 10 mm transversely. Our technique requires no additional optics and uses standard off-the-shelf components.