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The Improvement on the Performance of DMD Hadamard Transform Near-Infrared Spectrometer by Double Filter Strategy and a New Hadamard Mask

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The Improvement on the Performance of DMD Hadamard Transform Near-Infrared Spectrometer by Double Filter Strategy and a New Hadamard Mask micromachines Article The Improvement on the Performance of DMD Hadamard Transform Near-Infrared Spectrometer by Double Filter Strategy and a New Hadamard Mask Zifeng Lu 1,2, Jinghang Zhang 1, Hua Liu 1,2,*, Jialin Xu 3 and Jinhuan Li 1,2 1 Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China; [email protected] (Z.L.); [email protected] (J.Z.); [email protected] (J.L.) 2 Demonstration Center for Experimental Physics Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China 3 Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China; [email protected] * Correspondence: [email protected]; Tel.: +86-180-0443-0180 Received: 7 December 2018; Accepted: 15 February 2019; Published: 23 February 2019 Abstract: In the Hadamard transform (HT) near-infrared (NIR) spectrometer, there are defects that can create a nonuniform distribution of spectral energy, significantly influencing the absorbance of the whole spectrum, generating stray light, and making the signal-to-noise ratio (SNR) of the spectrum inconsistent. To address this issue and improve the performance of the digital micromirror device (DMD) Hadamard transform near-infrared spectrometer, a split waveband scan mode is proposed to mitigate the impact of the stray light, and a new Hadamard mask of variable-width stripes is put forward to improve the SNR of the spectrometer. The results of the simulations and experiments indicate that by the new scan mode and Hadamard mask, the influence of stray light is restrained and reduced. In addition, the SNR of the spectrometer also is increased. Keywords: spectrometer; infrared; digital micromirror device (DMD); signal-to-noise ratio (SNR); stray light 1. Introduction In the 1970s, the Hadamard transform (HT) was proposed and developed into a relatively mature theory [1]. With the emergence of the mechanical encoding mask, the HT was applied to the near-infrared (NIR) spectrometer. The encoding mask is a key device in spectrometers. However, adopting the mechanical mask, the spectrometer exhibits a complex structure, low resolution, and short life. Compared with the traditional instrument, it possesses no advantage. The development of HT spectrometers is restricted by the encoding mask. Later, the digital micromirror device (DMD) was developed and applied to the HT spectrometer as an encoding mask. Because the HT spectrometer based on the DMD has several advantages such as a higher signal-to-noise ratio (SNR), wider spectral range, and low cost [2,3], DMD-based HT spectrometers have attracted significant research attention. At present, the performance of HT spectrometers has been greatly improved, but they still have defects, such as the grating diffraction of the spectrometer, the two-dimensional grating diffraction of the DMD, and the poor spectral efficiency of the light source; these defects can make the spectral energy distribution uneven. Thus, the influence of stray light on the absorbance of the whole spectrum is varied; the lower the spectral energy is, the greater the influence by stray light is. The low-energy spectral band exhibits a low SNR, nonlinearity, whereas the high-energy spectral Micromachines 2019, 10, 149; doi:10.3390/mi10020149 www.mdpi.com/journal/micromachines MicromachinesMicromachines 20182019,, 910, x, 149FOR PEER REVIEW 2 2of of 13 13 low-energy spectral band exhibits a low SNR, nonlinearity, whereas the high-energy spectral band performsband performs well in wellthose in aspects. those aspects.To improve To improvethe energy the of energythe entire of thespectrum, entire Wang spectrum, and colleagues Wang and proposedcolleagues a proposedspectrum-folded a spectrum-folded structure of a structure HT spectr ofometer a HT and spectrometer a special illumination and a special optical illumination device, butoptical the device,structure but of the the structure spectrometer of the was spectrometer complex [4,5]. was complexZhang et [ 4al.,5 ].proposed Zhang et a al. new proposed algorithm a new to realizealgorithm energy to realize compensation energy compensation of the spectrum of and the spectrumanalyzed andthe effect analyzed of the the HT effect on the of thenoise HT without on the consideringnoise without the considering noise distribution the noise [6]. distribution Quan et [6al.]. Quananalyzed et al. the analyzed spectral the distortion spectral distortion in the HT in spectrometerthe HT spectrometer and presented and presented a correction a correction approach approach [7]. However, [7]. However, for forthe the analysis analysis of of stray stray light, light, especiallyespecially thatthat with with a higha high correlation correlation of energy of distribution,energy distribution, their processing their effectprocessing was unsatisfactory. effect was unsatisfactory.Xu et al. analyzed Xu et the al. influence analyzed of the the influence HT on the of noisethe HT before on the and noise after before coding. and They after also coding. proposed They alsoa new proposed encoding a new mask encoding to correct mask the anomalyto correct inthe the anomaly spectra in caused the spectra by optical caused defects by optical [8]. With defects the [8].variation With the in variation the height inof the the heig stripes,ht of the their stripes, new their encoding new encodi maskng exhibited mask exhibited a low utilization a low utilization rate of ratethe DMD.of the DMD. ToTo improve the performance of the DMD HT NIR spectrometer, a new method of the split waveband scanning is proposed in this paper to mitigate the impact of the stray light. It It can can not not only only reducereduce the influenceinfluence ofof stray stray light light on on the the low-energy low-energy spectral spectral bands, bands, but but also also improve improve the linearitythe linearity and andaccuracy accuracy of absorbance of absorbance in the in low-energy the low-energy spectral spectral bands. bands. On the On other the other hand, hand, a new a Hadamardnew Hadamard mask maskof variable of variable width-stripe width-stripe matching matching with each with scanning each scanning area is area presented is presented to improve to improve SNR.Based SNR. onBased the onnew the scanning new scanning method method and coding and coding mask, the mask, simulation the simulation and experimental and experimental results indicate results thatindicate the straythat thelight stray is suppressed light is suppressed and the spectral and the energy spectral distribution energy distribution is more uniform. is more Theuniform. SNR ofThe the SNR spectrum of the spectrumis also improved, is also especiallyimproved, in especially the low-energy in the spectral low-energy band thespectral SNR isband increased the SNR significantly. is increased It is significantly.demonstrated It thatis demonstrated by the proposed that approach,by the proposed the minimum approach, SNR the in minimum the low-energy SNR in spectralthe low-energy band is spectralimproved band by ais factor improved of 7.434 by a greater factor thanof 7.434 that greater of the traditionalthan that of HT the method. traditional HT method. 2. Theory of Hadamard Transform (HT) Spectrometer with Digital Micromirror Device (DMD) 2. Theory of Hadamard Transform (HT) Spectrometer with Digital Micromirror Device (DMD) AA schematic of the spectrometer designed by us is illustrated in Figure 11a.a. TheThe incidentincident lightlight emittedemitted from thethe samplesample poolpool is is dispersed dispersed by by the the grating, grating, and and the the dispersion dispersion spectrum spectrum imaged imaged on theon theDMD DMD plane plane by theby the imaging imaging lens lens is encoded is encoded and and reflected. reflected. Then, Then, the the reflected reflected light light is focused is focused onto onto the thedetector detector by the by convergingthe converging lens. Finally,lens. Finally, the detector the detector signal issignal decoded is decoded and processed and processed by a computer. by a computer.Because DMD Because is programmable, DMD is programmable, multiple scan multiple modes scan are availablemodes are to available the spectrometer to the spectrometer such as the suchcolumn as the scan column mode, scan the Hadamard mode, the scanHadamard mode, andscan other mode, multiplexed and other scanmultiplexed mode. To scan our mode. spectrometer, To our spectrometer,the major scan the mode major is Hadamardscan mode scan. is Hadamard The coding scan. matrix The of coding the Hadamard matrix spectrometerof the Hadamard is an spectrometerS-matrix determined is an S-matrix by quadratic determined residue by quadrati methodc andresidue can method be used and to describecan be used the to patterns describe to the be patternsdisplayed to onbe thedisplayed DMD [ 9on]. Bythe thisDMD approach, [9]. By this the approach, spectrum canthe bespectrum modified. can be modified. FigureFigure 1. 1. (a()a )Optical Optical system system of of Hadamard Hadamard transform transform (HT) (HT) spectrometer. spectrometer. (b) (Opticalb) Optical structure structure of HT of spectrometer.HT spectrometer. Based on our theory, the HT NIR spectrometer with a DMD can be realized. The photograph of the spectrometer is shown in Figure1b. The parameters of the spectrometer are listed in Table1. Before the spectrometer is used, the calibration must be performed
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