用于激光诱导等离子体光谱分析的便携式中阶梯光栅光谱仪设计

第34卷 第5期2013年5月发 光 学 报CHINESE JOURNAL OF LUMINESCENCEVo1.34 No.5May，2013Article ID：1000-7032(2013)05-0672-06A Portable Echelle Spectrograph Design forLaser·induced Breakdown SpectroscopyCHEN Shao-jie ，QI Xiang.dong ，Bayanheshig ，TANG Yu-guo ，CAO Hai-xia ，(1.Changchun lnstite of Optics，Fine Mechanics and Physics，Chinese Academy of Sciences，Changchun 130033，China；2.University of Chinese Academy of Sciences，Beijing 100049，China)Corresponding Author，E-mail：haojie. ###gmail.conAbstract：Laser-induced breakdown spectroscopy(LIBS)has atracted increasing interest and is widely used forroutine elemental analysis.In order to enable LIBS to detect complete spectral information within a quite short time，a smal size echelle spectrograph with spectral coverage from 1 80～400 nm was designed.According to the analysis ofthe optical system，the parameters are calculated and the aberrations are corected wel1.In this way，this echelespectrograph can obtain two-dimension spectral graph with high resolution quickly.By testing and calibrating the Hglamp，the actual resolution gets up to 0.036 8 nm at 253.652 nm which can satisfy the analysis requirement of LIBS。

Key words：laser-induced breakdown spectroscopy；LIBS；echelle spectrograph；echele gratingCLC number：TH744.1 Document code：A DOI：10.3788/fgxb20133405.0672用于激光诱导等离子体光谱分析的便携式中阶梯光栅光谱仪设计陈少杰 ，齐向东 ，巴音贺希格 ，唐玉国 ，曹海霞 ，(1.中国科学院 长春光学精密机械与物理研究所，吉林 长春 130033；2.中国科学院大学.北京 100049)摘要：激光诱导等离子体光谱分析技术是-种非接触式实时检测技术，它已成为-种新兴的物质成分与浓度分析手段，并在工业生产等领域有着重要应用。为了使激光诱导等离子体光谱分析技术在极短的时间内同时获得全面的光谱信息，本文设计了-款波段范围为180 400 nm的轻小型中阶梯光栅光谱仪。通过分析其光学性能，确定了系统的结构参数，并对像差进行了分析校正。对汞灯特征光谱进行了测试标定 ，仪器光谱分辨率在 253.652 nm处可达 0.036 8 nm，满足激光诱导光谱分析技术对仪器光学性能的需求。

关 键 词：激光诱导等离子体光谱分析 ；LIBS；中阶梯光栅光谱仪；中阶梯光栅收稿日期 ：2013.01-17；修订日期：2013.03-02基金项目：国家自然科 萎 目(61108032)；国家重大科研装备研制项目(ZDYZ2008-1)；吉林省重大科技攻关项目 (09ZDGG005)资助项目作者简介：陈少杰(1985-)，女，黑龙江宁安人，主要从事光谱仪器研发及光谱信息处理技术的研究。

E-mail：shaojie.csj###gmail.eom第 5期 CHEN Shao-jie.et al：A Portable Echelle Spectrograph Design for Laser-Induced Breakdown Spectroscopy 673IntroductionLaser.induced breakdown spectroscopy(LIBS)，also refered to as laser plasma spectrosco-PY(LIPS)in the technique，is a type of atomicemission spectroscopy which uses a highly energeticlaser pulse as the excitation source[1-3]. LIBS istechnically very similar to many other laser-basedanalytical techniques，such as the vibrational spec-troscopic technique of Raman spectroscopy，and thefluorescence spectroscopic technique of laser-.in.-duced fluorescence(LIF)，and they share much ofthe same hardware.In fact，these devices are nowbeing manufactured in a single instrument，allowingthe atomic，molecular and structural characterisationof a specimen as well as giving a deeper insight intophysical properties[. Because of the advantages ofmicroscale material consumed， remote sensing，depth profile and rapid giving results，LIBS has received much attention.Nowadays，LIBS have beenutilized in many industrial on-line element character。

ization measurements - .In principle，LIBS is ableto analyze any mater regardless of its physical state，such as solid，liquid or gas.When excited to sufi-ciently high temperatures，elements emit lights ofcharacteristic frequencies， so LIBS can determinethem[8-9]. However.what LIBS can detect is limitedby the power of the laser as well as the sensitivity，spectral resolution and wavelength range of the spec-trograph.As the key module of LIBS，the spectro-graph affects the optical perform ance of the entire in-strument signifcantly，including wavelength range，spectral resolution，detection limit and the size ofthe configuration[ 1o]. Besides，recent interest inLIBS has focused on the miniaturization of the COB-ponents and the development of compact，low pow-er，portable systems.In this way，the developmentof miniature spectrograph with high resolution，highsensitivity，and proper spectral coverage is consid-ered necessary。

Echelle spectrographs with internal cross-dis-persion generate two-dimensional spectra on the im--age plane.In this spectroscopic module，the echellegrating dominates the main dispersion. The term echelle denotes a special diffraction grating withrather coarse groove spacing used at high angle of in-cidence with high diffraction orders(usually from 10to 1 50)，so echele grating enables the spectrographpossess of preeminent resolution with miniature con-figuration.While，due to the limit of free spectralrange，many orders are overlap.A cross-dispersingelement，either a grating or a prism，is needed toseparate different orders perpendicular to the echelledispersion.The conventional Czerny-Turner spectro-graphs are very limited either in spectral resolutionor in simultaneously detectable spectral range，which are unable to meet the requirements ofLIBS[ ]. Under this situation. a sma11 s1ze echellespectrograph was designed.Due to the large spectralimage area，it is not easy to gain excellent imagequality and high resolution for the wide spectral COV-erage.The resolution of echelle spectrograph is af-fected by many factors，so it is necessary to analyzethe main parameters of the optical system.This pa-per analyzes and discusses the influence of the pin。

hole diameter， grating parameters，prism parame-ters，CCD pixel size and system aberations.In or-der to reduce the effect of aberation，a cylindricallens is used to decrease the astigmatism which influ。

ences the resolution greatly.According to the analy-sis，a portable echele spectrograph is designed，andthe optical characteristics can satisfy the needs ofI IBS。

2 Spectroscopic Module Design for LIBS2.1 Principle of LIBSThe experimental set-up for LIBS is describedin Fig.1.A Q-switched Nd：YAG laser is utilizedwith high pulse energy at extremely high rate as theexcitation source.The pulse energy is regulated by avariable atenuator，and in addition.it is monitoredvia a beam spliter and a pulse energy monitor.Thelaser beam passing pierced mirror is focused on thesample surface，and the characteristic spectra are in-spired.The plasma emission is collimated with thesame lens，and focused by a second lens on a fiberoptics coupled to an echelle spectrograph with an in-tegrated intensifed CCD camera。

674 发 光 学 报 第 34卷Fig.1 Set-up for LIBS2.2 Fundamentals of Spectrograph DesignThis echelle spectrograph is designed in a kindofdouble-Z” layout.and all optical componentsare arranged one after the other in a zigzag-line ，as showed in Fig.2.Wide spectral coverage，highresolution and smal volume echele spectrograph canbe obtained by this configuration13. The ray froma pinhole is colimated by sphere miror，and thenhits the echelle grating which is mounted in an of-plane mode working at its blaze maximum. A dis-perser，a prism is used to separate the overlappingorders perpendicular to the echele grating disper-sion.The echelle graphic image corected by a cyl-inder lens，is focused by another sphere miror ontothe two-dimensional detector(intensifed or non-in-tensified CCD camera)。

FCollimating miorFig.2 Scheme of echelle spectrometer2.3 Echelle Grating and PropertiesIn the spectroscopic module，the echelle grat-ing dominates the main dispersion，and the prism isa cross disperser orthogonal to the grating[.In thesingle，slit diffraction，most free spectral range of theechele grating are contained in the maximum inten-sity envelop，so the diffractive eficiency can get upto 50% ～ 100% of the peak wavelength，whichmeans the echelle gratings possess high difractiveeficiency within a wide spectral coverage.The ech-elle grating is mounted in an of-plane mode workingat its blaze maximum，which is shown in Fig.3.y-zplane is the main plane of the grating. There is amall angle 6 between the incidence ray and mainplane，i is incidence angle，and is difractive angle。

Ⅳy iFig.3 Work mode of echele spectrometerIn this way，one takes advantage of the ful res-olution power of the grating and any anamorphieeffect of grating diffraction is avoided.In this config-uration，the resolution of the echelle can be de-scribed as follows。

R mKm詈 sin s ， (1)Where，m is the diffraction order，K is the totalnumber of the groves，W is the width of the echellegrating，d is the grating constant， and 0 is theblazed of echele grating. For the specifc wave-length，the resolution depends on the blazed angleand the width of grating.In order to meet the de-mand of the application，the parameters are calculat-ed：d54.49 gr/mm，the blazed angle is 46。，thearea of the echelle grating is 16 mm ×26 mm，andthe wavelength is from 180 nm to 400 nm。

2.4 Cross·Dispersion and PropertiesAs we know，gratings are used to produce a se-ries of repeated spectra through difraction. Tradi-tional spectrographs generally use one dispersive ele-ment，either a grating or prism，which usually worksin 1 st or 2nd order.However，an echelle grating isspecially designed to operate in high difraction or-ders，which leads to serious spatial overlapping be-tween successive orders. Therefore，a second dis。

persion，either a prism or low dispersion grating isneeded to disperse the light in the direction orthogo.-nal to the echelle grating，and it has no effect on thedispersion of the echelle grating. The spectral第 5期 CHEN Sha。-jie， al：A Portable Eche1e Spectrograph Design for Laser-Induced Breakdown Spectmsc。py 675range，crosstalk between orders，and the utilizationration of detector are all needed to be considered toconfirm the parameters of cross-dispersion. Adopting a prism as the cross-disperser，differ。

ent material is chosen according to spectral require-ments. The relationship between dispersion angleand refractive index，incidence angle， and apex angle can be gotten as follows。

csin n arcsin( ))，(2)Where， A is dispersion angle， nA is refractive in。

dex， is incidence angle，咖is apex angle.Dispersion angle varies according to wavelength， so differ。

ent dispersion angles lead different locations of or。

In order minimize the structure and enhance thecross dispersion，the suprasil dispersion prism is a。

1uminum-coated at the back side and acts in the re-flection mode. According to the wavelength rangeand the limit of the detector，the quartz prism ischosen and the apex angle is 20。. the incidence angle is 21。

2.5 Pinhole and DetectorThe pinhole is on the focus plan， so the diame。

ter of the pinhole affects the system resoluti0n seri。

ously. Suppose the diameter of the pinhole is s，each point of the pinhole can considered as asource. In this way，the angle between incidencerays from diferent point sources can be obtained. 手， (3)The grating equation is simplifed as foliows。

，nA d(sin/siH )cos8， (4)This system is in Littrow mode， so the incidence an。

spect to the differential equation. the angular disper-sion of grating is listed as follows。

ddA 2dcosicos6 (5)If△ di，the following relation can be gotten。

：2 dsc。s 。s ， (6)The detector is another mainly factor affecting thespectral resolution.The size of the pixel must matchthe pinhole.In terms of the sampling theorem，thefrequency of the pixel distribution must be twicehigher than the frequency of spectra distribution[。

Suppose the size of the pixel is r，the linear disper。

si。n。f grating is ，s。 the width 0f fhe spectralline for each pixel can be deduced as folows- dA -c0sOcos8 (7) r r， ㈩ Base on the above analysis，△A should be largerthan the two times of△- 。

△A ≥ 2AA，In this case，the relation betweenthe detector can be obtained。

s≥ rpinholeTable 1 Parameters in echelle spectrograph(9)and(10)Wavelength RangeNumber of Grating LinesIncidence Angle of GratingTurning AngleFocus LengthSize of PinholeSize of PixelIncidence Angle of PrismApex Angle of Prism180-4O0 nm54.49 g/mm46。

3 Results and DiscussionIf the echele grating and the cross.dispersermake a combination，this configuration can be sim。

plified as a Czerny.Curner spectrometer. 0n the ba。

sjs of Cary theorem，the aberration can be c0rr.ectedby minimizing the of-axis angle of grating. This of-axis angle depends on the off-axis angles 0f thesphere mirrors and echelle grating. In order t0 meetthe demand of image quality and system structure，the of-axis angles are 5。for the collimating mirr0r. 3。for echelle grating，and -8。for the focusing mir。

ror.The echele grating is mounted in an of-planemode，working at its blaze maximum . which takesadvantage of the ful resolution of the grating and anyanamorphic effect of grating diffraction is avoided. 第 5期 CHEN Shao-jie，et al：A Portable Echelle Spectrograph Design for Laser-Induced Breakdown Spectroscopy 677yong for the mechanical development， and NingChunli for the instrument adjustment.Financial sup-port by the National Engineering Research Center forDifraction Gratings Manufacturing and Applicationis acknowledged。

1]Walid Tawfik Y Mohamed.Improved LIBS limit of detection of Be，Mg，Si，Mn，Fe and Cu in aluminum alloy samples using a p0rtable Eche1e spectrometer with ICCD camera[J].Optics&Laser Technology，2008，40(1)：30-38。

2]Leon J Radziemski.From LASER to LIBS，the path of technology development[J]Spectrochimica Acta P0 B，2002，57(7)：1109-1113。

[3]Ahmed R，Baig M.On the Optimization for enhanced dual-pulse laser-induced breakdown sectroscopy[J].IEEE Transac。

[4]Ta0 M M，Yang P L，Liu W P，et a1.Response characteristics of fiber Bragg gratings irradiated by high energy lasers[J]。

[5]Ye J F，Hong Y J，Wang G Y，et a1.Research progress in micro-laser plasma propulsion[J].Chin.O .(中国光学)，2011，4(4)：319-326(in Chinese)。

6]NoU R，Bette H，Brysch A，et a1.Laser.induced breakdown spectrometry-applications for production control and qualityassuranee in the steel industry [J].Spectromchimica Acta Part B，2001，56：637-649。

7 1 Stank0va A，Gilon N，Dutruch L，et a1.A simple LIBS method forfast quantitative analysis offly ashes[J]. z，2010，89(11)：3468-3474。

[8]Huang Q.The Emission studies on ultraviolet laser ablation of copper in low pressure[J].Chin. Lumin.(发光学报)，2006，27(6)：1021-1025(in Chinese)。

9]Hu J，Chen G，Zen Y.Studies on laser-induced filuorimetry(IX)the luminescence mechanism and analytical applicationof Dv-AA-TOPO-SLS fluorescent svstem[J].Chin.J.Lumin.(发光学报)，1991，12(1)：66-72(in Chinese)。

[10]Bauer H，Leis F，Niemax K.Laser induced breakdown spectrometry with al echele spectrometer and intensified chagecoupled device detectionJ].Spectrochimica Acta Part B，1998，53(12)：1815-1825。

1 1]Haisch C，Panne U，Niessner R.Combination of an intensifed charge coupled device with an echelle spectrograph for a-nalysis of coloidal material by laser-induced plasma spectroscopy[J].Spectrochimica Acta Part B，1998，53：1657-1667。

12]F10rek S，Haisch C，Okruss M，et a1.H.A new，versatile echelle spectrometer relevant to laser induced plasma applica-tions[J].Spectrochimica Acta Part B，2001，56(6)：1027-1034。

[13]Lindblom P.New compact echelle spectrographs with muhichannel time-resolved recording capabilities[J].AnalyticaChimica Acta，1999，380(2/3)：353-361。

[14]Wu N，Tan X，Bayanheshig，Tang Y G，et a1.Simulation and experiments of ion beam etching process for blazed holo-graphic grating[J].Opt.Precision Eng.(光学 精密工程)，2012，20(9)：1904-1912(in Chinese)。

[15]Tang Y G，He M，Cui J C，et a1.Senamont based measuring method for birefringence of infrared crystal[J].Opt.Preci-sion Eng.(光学 精密工程)，2012，20(10)：2176-2183(in Chinese)。

[16]Tang Y，Chen H，Zhang Z，et a1.Multi-Channel Spectrometer based on Linear CCD and its Application[J].HuazhongUniv.ofSci.&Tech.(华中科技大学学报)，2002，30(10)：96-98(in Chinese)。