Huaidong Yang

Resolution enhancement by combination of subpixel and deconvolution in miniature spectrometers

The resolution of a miniature spectrometer with a multichannel detector is limited by its throughput. A subpixel deconvolution method is proposed to enhance resolution without physically reducing the throughput. The method introduces subpixel reconstruction to overcome undersampling during deconvolution processing. The experimental result has shown a 36.6% reduction in FWHM of spectral lines, indicating the effectiveness of the method.

Generalized method for calculating astigmatism of the unit-magnification multipass system

A generalized method to accurately calculate astigmatism of the unit-magnification multipass system (UMS) is proposed. A practical coaxial optical transmission model is developed for the UMS. Astigmatism analysis is then made convenient by a 4 by 4 general transfer matrix. Astigmatism correction is significantly promoted, and hence further improvement in imaging quality can be expected. Good agreement between numerical simulations and Zemax ray tracing results verifies the effectiveness of this method. The resulted RMS spot size of this method is only 25% to 64% of other previous methods based on the golden section search for minimum astigmatism in real design cases. This method is helpful for the optical design of the UMS.

Approximate analytic astigmatism of unit-magnification multipass system.

We develop a way to estimate the approximate analytic astigmatism with a high accuracy for any unit-magnification multipass system (UMS). The coaxial optical transmission model for UMS is simplified based on the systems features. Furthermore, astigmatism is derived as a distinct form of vector addition and, thus, feasible analytic astigmatism can be obtained. The effectiveness of our method is verified by simulations for a Bernstein-Herzberg White cell. In our cases, the relative error of optimization for astigmatism correction by our method is smaller than 5 per thousand, which is only one-tenth of that by Kohns method. Our method significantly improves the efficiency for astigmatism correction, and further benefits the optical design of a UMS.

Astigmatism analysis by matrix methods in White Cells

White Cell is a typical multiple-path absorption cell for absorption spectroscopy. For multi-path enhanced absorption application, we should make sure that the input light of the multiple-path cell can be output as much as possible to enhance absorption signal. Thus the astigmatism of the White cell should be analyzed for optimization as a key factor. In White cell, the astigmatism is difficult to be calculated as the meridian and sagittal planes of the beam would rotate with a different angle while at each reflection. In this article, the White cell is presented by defined 4×4 optical matrices as a nonsymmetrical system. The astigmatic difference would then be obtained from the eigenvalues of the partitioned matrices. The numerical computation and ray-tracing simulation results presented have proved the efficiency of this method. The astigmatism can be improved by properly choosing the distance h between the two rows of image spots on the field mirror.

Improvement of miniature grating spectrometers

The trade-off between resolution and signal to noise ratio is a shackle to develop high performance miniature grating spectrometers. Concentrating on breaking this shackle, freeform optics and super-resolution restoration method for miniature grating spectrometers are proposed in this paper. Substituting a varying sagittal surface for a toroidal one, not only aberrations along dispersive direction can be reduced, but also aberrations perpendicular to dispersive direction can be reduced in a broad spectral range. This means both resolution and throughput would be multiplied. To reduce the remnant imperfection of system, subpixel-deconvolution process may be supplemented. By subpixel reconstruction, under-sampling due to finite pixel size of array detector would be got rid of. By deconvolution, blurring duo to slit and other system imperfection would be eliminated. Consequently, resolution and throughput would be further increased.

An on-line monitoring technique for trace gases in atmosphere based on differential optical absorption spectroscopy

Differential optical absorption spectroscopy (DOAS) has become a widely used method to measure trace gases in the atmosphere. The concentrations of trace gases can be retrieved by fitting differential absorption spectra with standard differential absorption cross-section using the linear least-square method. The basic principle of DOAS is introduced. The construction of DOAS on-line monitoring system is designed and the retrieval method of trace gases concentration based on the principle of least-squares is discussed. The properties of DOAS system are tested by experiments. The advantages of DOAS system used in atmosphere quality monitoring are shown.

Optimization of sampled imaging system with baseband response squeeze model

When evaluating or designing a sampled imager, a comprehensive analysis is necessary and a trade-off among optics, photoelectric detector and display technique is inevitable. A new method for sampled imaging system evaluation and optimization is developed in this paper. By extension of MTF in sampled imaging system, inseparable parameters of a detector are taken into account and relations among optics, detector and display are revealed. To measure the artifacts of sampling, the Baseband Response Squeeze model, which will impose a penalty for undersampling, is clarified. Taken the squeezed baseband response and its cutoff frequency for favorable criterion, the method is competent not only for evaluating but also for optimizing sampled imaging system oriented either to single task or to multi-task. The method is applied to optimize a typical sampled imaging system. a sensitivity analysis of various detector parameters is performed and the resulted guidelines are given.

A wavelength calibration process for micro-spectrometers with multichannel detectors

An advanced wavelength calibration process with higher wavelength accuracy is developed based on the conventional calibration method of micro-spectrometers with multichannel detectors. The deficiencies of the conventional method in acquiring sufficient well-spaced and adequately accurate wavelength-pixel data for calibration are analyzed. And three steps are added to the conventional method before the final pixel-wavelength fit is carried out. First, segmented data collection is carried out to ensure sufficient well-spaced lines for calibration. Second, sub-pixel analysis is executed to increase sampling rate. Third, peak fit is implemented to acquire more accurate central wavelength positions. The simulated experiment was based on a compact spectrometer with a crossed Czerny-Turner optical design. Mercury and Argon line spectra are used as wavelength standards. A linear image sensor with 1024 pixels each 25μm in width is used as the detector. In the new calibration process the whole spectral region was divided into two segments with different integral time of the detector; the sampling rate was increased by 2 times by sub-pixel analysis; and log-normal function is applied in the peak fit. The results show that by applying the new method, the wavelength accuracy improves from above 1.0nm to around 0.6nm.