In this paper, a visible-near infrared imaging spectrometer optical system is designed, which is used to detect seawater elements and provides a favorable monitoring means for the protection of marine resources. According to the application background of ocean color detection instruments, the index parameters of the instrument are determined, and a high resolution imaging spectrum system is designed. The front telescope system adopts an off-axis three mirror anastigmat without intermediate real image, with the focal length of 590.75mm and the entrance diameter of 118.15mm;The system comprises an innovative spectrometer design with 0.6 magnification. The de-magnifying optical design allows the telescope to operate at F#/5, while the spectrometers are built in a more compact arrangement at F#/3.The main mirror and three mirrors use Zernike Fringe Sag surface to compensate for the aberration introduced by breaking the classical Offner concentric and isometric structure. The whole optical system comprises a single telescope feeding two functionally identical spectrometers. The spectral range is 0.4-0.9μm, field of view angle is 19 °, spectral resolution is 5nm and instantaneous field of view angle is 0.048mrad, the full field of view and full band MTF is more than 0.8, which is close to the diffraction limit, the RMS radius is less than 4 μm, and the smile and keystone of the spectrometer are less than 10% pixels.
In view of the problems that the existing subwavelength transmission grating structure parameters require high processing technologies and are difficult to prepare, this paper explores and investigates the design simulation of encapsulated grism grating based on the finite element method (FEM). The variation of diffraction efficiency and polarization sensitivity of encapsulated grism is explored for different groove structures, trench depth and duty cycle, according to the requirements of high diffraction efficiency and low polarization sensitivity. The grating surface etched onto a fused silica substrate is formed by binary structure of grooves and trenches filled by a high refractive index multi-layer coating, working at Littrow configuration in the SWIR-1 (1590-1625nm) and SWIR-2 (1635-1670nm). The simulation results show that the average diffraction efficiency exceeds 85% and the polarization sensitivity is less than 5% with a wide tolerance range when the depth-period ratio is about 2 and the duty cycle is around 0.6. The diffraction efficiency and polarization sensitivity meet the design requirements, substantially improving the efficiency of transmission grating design and processing. This enables compact optical design to achieve high signal-to-noise ratio and low stray light to meet the critical radiation measurement accuracy requirements.
The immersion grating is one of the key components of the imaging spectrometer due to its high spectral resolution. The silicon immersion grating is conducive to the miniaturization and weight reduction of related satellite-borne instruments to reduce the launch cost of satellite payloads. This paper introduces the application of silicon immersion grating at home and abroad, its working principle and manufacturing methods. The symmetrical trapezoidal groove silicon immersion gratings with groove density of 200 lp/mm and 400 lp/mm were designed. The grating diffraction efficiency and polarization were analyzed by finite element analysis. The photoresist mask was fabricated by UV exposure and holographic exposure, and then the uniform arrays of V-shaped grating on a 2-inch Si substrate were achieved by reactive ion etching and wet etching. The measured results of groove parameters met the design requirements.
To obtain the fine characteristics of the atmospheric carbon dioxide absorption spectrum, the imaging spectrometer must have high spectral resolution, but the volume of the instrument will increase simultaneously which is not conducive to remote sensing detection on satellite platforms. To solve this problem, we used a silicon immersion grating as dispersion element that diffraction occurs inside the silicon grating medium, the dispersion and spectral resolution are increased by 3.4 times respect to the conventional grating, and the volume of the spectrometer can be reduced to 1/40 at most. Smile is introduced because of the dispersion difference of light between the non-principal section and the principal section, which affects the spectral fidelity, data retrieval and detection accuracy. We established a model to analyze the smile by the silicon immersion grating and used the distortion of off-axis lens to correct. To miniaturize the instrument, optical system adopted the Littrow structure that the dimension is 410 mm×130 mm×110 mm. The optical system works in the short-wave infrared carbon dioxide weak absorption band 1.594-1.624 μm with a spectral resolution of 0.08 nm, entrance slit length is 16 mm and F number is 2. The optimized MTF is greater than 0.87 at the Nyquist frequency of the detector, the RMS radius of the spot diagram is within the Airy disk, and the maximum smile is less than 0.1 m, which is less than 1% of a single detector pixel. The optical system has excellent imaging performance, meeting the requirements of atmospheric carbon dioxide detection.
Offner spectrometer is widely applied in hyperspectral imaging spectrometers, in which the design, fabrication, and testing of convex gratings are crucial to ensure the spectral and imaging performance of the whole system. A conical diffraction Offner spectrometer is proposed with the spectral range of 400 to 900 nm, spectral resolution of 5 nm, slit length of 1 mm, and spectral dispersive width of 10 mm. A finite-element analysis is adopted to optimize the groove parameters of the conical diffraction blazed convex grating that is used in the aforesaid spectrometer. Holographic scan ion beam etching method is employed to fabricate such convex grating. Experimental facilities for testing the diffraction efficiency are built in the lab, showing that the efficiency is higher than 50% in the whole waveband and the peak efficiency is over 75%, which is consistent with the design result. The result indicates that it is feasible to design and optimize the grating groove using the finite-element analysis method.
The increase of atmospheric concentration of anthropogenic greenhouse gases(GHGs), primarily carbon dioxide(CO2) and methane(CH4), is concerned as a main cause of the global climate change. From the previous experiences in GHG detecting, satellite imaging spectral remote sensing provides the unique potentials in accuracy, precision, coverage, temporal sampling and spectral resolution, having been developing as an effective and efficient means for monitoring GHGs’ accumulation and emission in the atmosphere. This paper reports a promising optical design of very high spectral resolution imaging spectrometer on LEO satellite with a swath of over 100 km and a spatial resolution of less than 3 km. Its specification satisfies with the requirement of high column concentration retrieval precision of 1ppm for CO2 and 9ppb for CH4 within four absorption bands (755-765nm, 1595-1625nm, 2040-2080nm and 2275-2325nm). Above all, up to 23000 spectral resolving power hints us the superiorities of immersed grating in increasing resolution but decreasing volume. A holographic flat plane grating is directly etched on a wedge prism, operating in reflective near-Littrow condition, having optimized diffraction efficiency of over 85%. Additional prisms are introduced to correct the smile distortion of the slit image produced by the grism. This method is crucial for the fidelity of the instrument spectral response function (ISRF) and data processing. Moreover, to desensitize the instrument to the polarization state of the income radiation, four polarization scramblers are adopted after the shared fore-optics, specially designed for each bands. Thanks to the scramblers, the predicted polarization sensitivity is lower than 1% at worst.
The grating imaging spectrometer has the characteristics of good linearity, wide dispersion range and is widely used in the field of remote sensing. Distortions (including smile and keystone) are one of the important parameters of the grating imaging spectrometer, which directly affects the quality of the image and spectral information obtained by the imaging spectrometer. In order to get the requirements of two kinds of distortions in the design process of the grating imaging spectrometer, the effect of the smile and keystone on the target detection is simulated and analyzed respectively. Based on the spectral response function with the Gaussian, the change of the spectral signal acquired by the grating imaging spectrometer with the amount of the different smile is calculated by combining with the spectral data of the atmospheric in the visible and near-infrared (0.4~1μm). The results show that the amount of smile should be no more than 1nm, 0.6nm and 0.2nm respectively when the spectral resolutions of the imaging spectrometer are 20nm, 10nm and 5nm. With the assumption that the spatial response function is the rectangle function, the effect of the different keystone on spectral signal acquisition of the imaging spectrometer is simulated by using the hyperspectral data. The results indicate that the offset of the keystone should be controlled within 0.04d (d is the pixel width).
In this paper, the generation mechanism of stray light is analyzed for a visible and near infrared imaging spectrometer with a spectral range of 400nm to 900nm. The optical mechanical model of the instrument was established and its stray light level was simulated. Based on the notch method, A stray light measuring device is built. The veiling glare index of the imaging spectrometer is measured to be 0.84%. The uncertainty of measurement is assessed by GUM method, and the influence of uncertainty components on the measurement results is analyzed. When the confidence probability of the measuring device is 95.45%, the measurement uncertainty of veiling glare index is 0.15%. Finally, a comparison and analysis are made between the simulated values of the veiling glare index and the actual measured values. This work provides technical support for the development of high resolution imaging spectrometer.
Astigmatism and distortion aberrations of conventional Offner-type imaging spectrometer with an in-plane diffraction grating will increase dramatically as its spectral dispersion width so that such spectroscopic mounting is usually suitable for such situation that both slit length and spectrum width are medium and that the spectrum width is less than the slit length. To short slit and high dispersion, novel conical diffraction Offner mounting is more appropriate. Based on the operation principle of this kind mounting, a set of optimized designs, which the focal ratio is 4,the spectral region from 400nm to 900nm, the slit length from 0.5mm to 1mm, and the dispersion width from 9.8mm to 28mm are obtained under the same optical size. To evaluate the imaging quality of the designed and to get the relation between slit length and dispersion width, the merit function and spectral response function are considered. The results show that conical diffraction Offner imaging spectrometers can image well while the spectrum width is less than the slit length, but no more than its 20 times.
As one kind of light source simulation devices, spectrally tunable light sources are able to generate specific spectral shape and radiant intensity outputs according to different application requirements, which have urgent demands in many fields of the national economy and the national defense industry. Compared with the LED-type spectrally tunable light source, the one based on a DMD-convex grating Offner configuration has advantages of high spectral resolution, strong digital controllability, high spectrum synthesis accuracy, etc. As a key link of the above type light source to achieve target spectrum outputs, spectrum synthesis algorithm based on spectrum matching is therefore very important. An improved spectrum synthesis algorithm based on linear least square initialization and Levenberg-Marquardt iterative optimization is proposed in this paper on the basis of in-depth study of the spectrum matching principle. The effectiveness of the proposed method is verified by a series of simulations and experimental works.
Owing to the ability of generating designated spectrums as special requirements, spectrum-controllable light source has attracted huge interesting in several fields, e.g. medical science, industrial detection, defense-related testing. In principle, optical performance of a spectrum-controllable light source can be predicted by some transfer functions of the corresponding system, e.g. modulation transfer function (MTF). Unfortunately, the aforementioned research work is still lacking at present although it is meaningful for the optical design and evaluation of this new kind of light sources. Hence, a MTF model for a modified version of our previously-proposed spectrum-controllable light source system based on a Digital Micromirror Device (DMD) and an Offner dispersion configuration with a convex grating is deduced as an example. Related preliminary analyses have been present in this paper as well.
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