Kexin Chen

Photochromic diarylethene for rewritable holographic data storage

A new diarylethene doped with poly(methyl methacrylate) film is developed and its characteristics of volume holographic recording are investigated. The maximum diffraction efficiency of the 10microm thick film is 1.2%, and the rewritable hologram recording exhibits its high resolution, fatigue resistance, negligible shrinkage, and long lifetime, which are critical to apply this material to high-density rewritable holographic data storage.

Design of freeform mirrors in Czerny-Turner spectrometers to suppress astigmatism

Astigmatism is left uncorrected in traditional Czerny-Turner spectrometers with spherical mirrors, which leads to low throughput of modern instruments applying line-array detectors. By gradually varying the sagittal curvature, freeform mirrors are introduced to suppress astigmatism for a wide wavelength range simultaneously. So as not to reduce spectrum resolution, further calculations of the light path are performed for extra coma compensation. A design example is presented with optimized parameters. The ray-tracing result has revealed a reduction in sagittal spot size from several hundred micrometers to around 10 microm in the wavelength range from 200 nm to 800 nm.

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.

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.

Study of dual-wavelength readout properties in volume holographic storage system

Dual-wavelength method is a research focus in nonvolatile holographic storage. However, both theoretical analysis and experimental demonstration that nonlinear enlargement is inevitable during dual-wavelength readout. By analyzing the speciality of nonlinear enlarge factor, a method of choosing the suitable retrieving angle is proposed, which can prove the uniform enlarge factor and a least efficiency of 44% at a thin thickness such as 10 um to that of Bragg match. In this paper, At last, a novel scheme which achieves simultaneous dual-wavelength readout of two polarization multiplexed holograms is proposed and demonstrated by the experiment.

Bragg match with a diffuser for dual-wavelength method in volume holographic storage system

An effective application of band-limited diffuser is introduced to realize the dual-wavelength method which can reduce the grating decay while retrieving data in the volume holographic data storage. The Diffuser combined with the imaging system can generate enough wavefronts to match the Bragg conditions of all the pixels in the whole image when two wavelengths are used. It is possible to enlarge the size of the retrieved image and get an approximately uniform diffraction efficiency. Experimental setup is presented and the results show that this technique is effective.