杜永兆
(華僑大學(xué) 工學(xué)院, 福建 泉州 362021)
利用Mach-Zehnder點(diǎn)衍射干涉儀的激光復(fù)振幅實(shí)時(shí)重建方法
杜永兆
(華僑大學(xué) 工學(xué)院, 福建 泉州 362021)
在充分利用Mach-Zehnder自參考干涉系統(tǒng)無需設(shè)置專門參考光優(yōu)點(diǎn)的基礎(chǔ)上,提出一種激光復(fù)振幅實(shí)時(shí)重建方法.在參考臂和測(cè)試臂上分別設(shè)置放大倍率為s且互為倒置的望遠(yuǎn)鏡系統(tǒng),使得經(jīng)針孔濾波、準(zhǔn)直后的參考光被放大s2倍形成波前與振幅(或強(qiáng)度)皆近似于一個(gè)平面的理想?yún)⒖脊?研究表明:利用傅里葉分析法從干涉圖中便可直接重建待測(cè)激光復(fù)振幅分布,具有結(jié)構(gòu)簡(jiǎn)單和測(cè)量快速的優(yōu)點(diǎn).基于該系統(tǒng)分別對(duì)靜態(tài)和動(dòng)態(tài)輸出光場(chǎng)進(jìn)行復(fù)振幅重建實(shí)驗(yàn),并由此求得相應(yīng)的M2因子,從而驗(yàn)證該方法在復(fù)雜光場(chǎng)光束質(zhì)量實(shí)時(shí)檢測(cè)的可行性.
激光光束質(zhì)量; 復(fù)振幅重建; 點(diǎn)衍射干涉儀;M2因子
Abstract: Considering the advantagesof the characteristicssaid as without a special reference wave of Mach-Zehnder point diffraction interference system (MZ-PDI), a real-time complex amplitude reconstruction method is presented. Two telescopes with the magnification s are placed at the reference arm and test arm in MZ-PDI, respectively. The telescope in one arm is inverted with respect to the other one. The reference wave is enlargedS=s2times after filteredby thepinhole filter, consequently, both of the reference wavefrontphaseand reference amplitude (or intensity profile) can be approximately seen as a plane and it services as an ideal reference wave. Thenthe complex amplitude of the test beam can be directly extracted from a single interferogram using the Fourier transform fringe analysis method. The proposed method can provide a simple and fast solutionwhile still maintains high accuracy for reconstructing complex amplitude. The experiments of the static and dynamic output optical wave are demonstrated based on the proposed MZ-PDI system, meanwhile the beam qualityM2factor of the test beam are obtained from the reconstructed complex amplitude. The experimental results showed that feasibility of the proposed system for real-time beam characterization of the complex optical beams.
Keywords: laser beam quality; complex amplitude reconstruction; point diffraction interferometer;M2 factor
為了研究高能激光系統(tǒng)的性能,保證系統(tǒng)的安全運(yùn)行,同時(shí)為光束實(shí)時(shí)控制提供有效反饋數(shù)據(jù),必須對(duì)高能激光輸出光場(chǎng)全場(chǎng)信息即相位和振幅(或強(qiáng)度)分布進(jìn)行快速、準(zhǔn)確檢測(cè).通常的做法是利用波前傳感器,如哈特曼-夏克波前傳感器[1-2]、曲率波前傳感器(CWFS)[3-4]、錐波前傳感器(PWFS)[5-7]或自參考干涉波前傳感器(SRI-WFS)[8-12]對(duì)輸出光場(chǎng)的波前相位進(jìn)行檢測(cè);而強(qiáng)度分布則由光電探測(cè)器(如CCD)測(cè)量得到.但這種相互獨(dú)立的測(cè)量方法不僅增加了測(cè)量系統(tǒng)的復(fù)雜性,也減低了測(cè)量的可靠性與準(zhǔn)確性.激光研究人員在如何實(shí)現(xiàn)單次便可獲得激光輸出光場(chǎng)全場(chǎng)信息上做了大量工作.美國的勞倫斯利弗莫爾國家實(shí)驗(yàn)室Beamlet裝置和英國的AWE實(shí)驗(yàn)室Helen 裝置分別采用徑向剪切干涉儀(RSI)對(duì)輸出光場(chǎng)的全場(chǎng)分布進(jìn)行綜合檢測(cè)[13-14].理論上,利用RSI波前相位和振幅迭代重建算法從單幅干涉圖中便可重建待測(cè)激光的復(fù)振幅[15-18],但因?yàn)椴ㄇ跋辔缓驼穹亟ㄋ惴ㄝ^為復(fù)雜,使得該方法只局限于靜態(tài)或者是瞬態(tài)[18]光場(chǎng)的檢測(cè)應(yīng)用.此外,F(xiàn)uente等[19-21]提出了利用馬赫-曾德衍射光束干涉儀(DBI)通過多次數(shù)值迭代重建待測(cè)激光復(fù)振幅的方法,并進(jìn)行了初步的實(shí)驗(yàn)研究,但同樣的也受到運(yùn)算速度的限制.本文提出一種基于改進(jìn)Mach-Zehnder點(diǎn)衍射干涉儀(MZ-PDI)的激光復(fù)振幅實(shí)時(shí)重建方法,僅需單幅干涉圖便可以直接重建待測(cè)激光復(fù)振幅.
圖1 MZ-PDI光路原理圖Fig.1 Principle diagram of MZ-PDI
1.1MZ-PDI基本原理
為了克服傳統(tǒng)SRI-WFS[8-12,22-23]無法一次性獲得激光復(fù)振幅信息和RSI[15-17],需要多次迭代計(jì)算方可重建激光復(fù)振幅分布的缺點(diǎn),構(gòu)建了如圖1所示的MZ-PDI基本光路結(jié)構(gòu).在系統(tǒng)的兩臂上分別設(shè)置放大倍率一致且互為倒置的望遠(yuǎn)鏡系統(tǒng),使得待測(cè)激光進(jìn)入MZ-PDI系統(tǒng)經(jīng)分光鏡1后分為兩束.其中,一束經(jīng)過由焦距分別為f3,f4的透鏡3,4組成放大倍率為s=f3/f4的倒置望遠(yuǎn)鏡系統(tǒng)后形成包含待測(cè)激光全部信息的縮小光束,作為測(cè)試光;另一光束則依次經(jīng)過透鏡1(焦距為f1)、針孔和透鏡2(焦距為f1)組成的放大倍率為s=f2/f1針孔濾波系統(tǒng)濾波后形成擴(kuò)束光束,作為參考光.參考光和信號(hào)光經(jīng)分光鏡2會(huì)合后并在成像面Pi發(fā)生干涉并形成干涉圖.
定義S=s2>1為MZ-PDI的放大倍數(shù).當(dāng)S足夠大的時(shí),經(jīng)針孔濾波、擴(kuò)束準(zhǔn)直后的波前與振幅(或強(qiáng)度)皆可近似于一個(gè)平面形成理想的參考光.因此,利用傅里葉分析法[24]得到干涉圖的復(fù)振幅調(diào)制函數(shù)[18]即為待測(cè)激光復(fù)振幅分布.
1.2針孔濾波原理分析
如圖1所示,設(shè)A(x0,y0)和W(x0,y0)分別為入射激光E(x0,y0)在物面P0上的振幅與波前分布.那么,經(jīng)分光鏡1的入射激光依次通過透鏡3,4,所形成的測(cè)試光在像平面Pi的復(fù)振幅可表示為
圖2 針孔濾波系統(tǒng)示意圖Fig.2 Schematic of pinhole filtering system
分光鏡1的透射光束則依次通過透鏡1,2和針孔組成的針孔濾波系統(tǒng)后,所形成的擴(kuò)束光束作為參考光.針孔濾波系統(tǒng)可以等效為如圖2所示光路系統(tǒng),其中,針孔放置于透鏡1焦平面Pf處,圖中相應(yīng)地標(biāo)出理論分析過程中需要用到的參數(shù)與坐標(biāo)表達(dá)形式.
根據(jù)傅里葉光學(xué)理論[25],待測(cè)光場(chǎng)E(x0,y0)經(jīng)透鏡1并傳播至其焦平面(xp,yp),相當(dāng)于對(duì)E(x0,y0)作傅里葉變換;焦平面上設(shè)置一個(gè)直徑為dpin的針孔相當(dāng)于一個(gè)理想圓低通濾波器,并對(duì)入射光場(chǎng)進(jìn)行調(diào)制實(shí)現(xiàn)了低通濾波,即針孔后面的光場(chǎng)可表示為
經(jīng)針孔濾波后的光場(chǎng)Ep(xp,yp)經(jīng)過透鏡2后在其成像面Pi上成像,在數(shù)學(xué)上相當(dāng)于對(duì)針孔濾波后的光場(chǎng)Ep(xp,yp)再次作傅里葉變換.忽略復(fù)比例常數(shù)和像的倒置關(guān)系,并考慮針孔濾波系統(tǒng)的放大倍率s=f2/f1,且入射光場(chǎng)聚焦在針孔中心,則在成像面Pi上的參考光場(chǎng)為
式(4)中:?為二維卷積;T(xi,yi)為針孔濾波窗函數(shù)cyl的傅里葉變換,也稱為針孔濾波器的脈沖響應(yīng)函數(shù),其具體表達(dá)式[22-23]為
式(5)中:J1為第一類貝塞爾函數(shù).為了便于分析,式(4)所述的參考光場(chǎng)可以寫為更一般的形式,即
式(6)中:AR(xi/s,yi/s)和WR(xi/s,yi/s)分別為參考光的振幅和波前.
1.3復(fù)振幅重建理論分析
根據(jù)光的干涉原理,測(cè)試光ET(xi,yi)與參考光ER(xi,yi)在像平面Pi重疊區(qū)域發(fā)生干涉,其干涉圖強(qiáng)度分布可以表示為
式(7)中:κ(xi,yi)=sinθ/λ為參考光與測(cè)試光之間夾角θ而引入的線性載頻.結(jié)合MZ-PDI的特點(diǎn),把干涉區(qū)域(sxi,syi)定義為新的定義域(x,y),因此,式(7)可以改寫為
式(8)中:S=s2為MZ-PDI的放大倍數(shù).式(8)中的第3項(xiàng)可以寫成其等價(jià)形式,即
定義c(x,y)為干涉圖的復(fù)振幅調(diào)制函數(shù)[18],其具體形式為
當(dāng)針孔直徑dpin足夠小時(shí),[2J1(βγ)]/βγ趨近于1,即參考波前WR(x/S,y/S)變成原始波前的積分形式,可近似為一個(gè)平面波前;當(dāng)MZ-PDI的放大倍數(shù)S足夠大時(shí),參考振幅AR(x/S,y/S)趨近于一個(gè)高度為AR(0,0)的平面.考慮到實(shí)際應(yīng)用中考慮振幅的相對(duì)值,所得待測(cè)激光的復(fù)振幅可由以下公式確定,即
干涉圖是由線性載頻方法得到的,故式(11)容易由傅里葉變換方法[24]得到.根據(jù)衍射積分理論[25-26],由得到的復(fù)振幅進(jìn)一步求得輸出光場(chǎng)在其傳播方向z上任意位置的光場(chǎng)形式E(x,y,z),進(jìn)而得到待測(cè)激光光束的相關(guān)參數(shù)[27],如束寬、遠(yuǎn)場(chǎng)發(fā)散角及光束質(zhì)量M2因子等[28],實(shí)現(xiàn)對(duì)待測(cè)激光光束質(zhì)量的綜合評(píng)價(jià).
由上述理論分析可知,針孔直徑dpin和MZ-PDI放大倍數(shù)S是決定MZ-PDI系統(tǒng)能否獲得高質(zhì)量參考光的關(guān)鍵,即其直接關(guān)系到 MZ-PDI系統(tǒng)能否精確重建待測(cè)激光復(fù)振幅分布.
假定待測(cè)激光的振幅為1,原始波前相位為峰谷值φPV=2λ的球面波,實(shí)際針孔直徑大小以針孔濾波系統(tǒng)的艾里斑直徑dA為單位.用歸一化光強(qiáng)的峰谷值IPV表示參考光強(qiáng)度誤差,參考波前的均方根值φRMS表示參考波前精度.不同MZ-PDI系統(tǒng)放大倍數(shù)S和針孔直徑dpin對(duì)針孔濾波后參考光強(qiáng)精度的影響,如圖3所示.由圖3可知:隨著dpin增大,濾波后的參考光強(qiáng)的起伏程度越大;但隨著MZ-PDI系統(tǒng)放大倍數(shù)S的增大,參考光強(qiáng)的起伏程度則迅速減小.當(dāng)S=1時(shí),對(duì)應(yīng)的dpin分別為0,5dA,1.0dA,2.0dA和4.0dA時(shí)的IPV分別為0.371 4,0.886 6,0.999 2和0.999 9;而當(dāng)S分別增大到S=3,4,6,8時(shí),對(duì)應(yīng)的dpin濾波后IPV分別減小至0.005 4,0.006 2,0.005 1,0.005 8,幾乎接近與一個(gè)理想的平面,這與后面的實(shí)驗(yàn)結(jié)果相符.
(a) dpin=0.5dA (b) dpin=1.0dA
(c) dpin=2.0dA (d) dpin=4.0dA圖3 系統(tǒng)放大倍數(shù)與針孔直徑對(duì)參考光強(qiáng)精度的影響Fig.3 Simulation results of magnification and pinhole diameter impact on accuracy of reference beam intensity
圖4 系統(tǒng)放大倍數(shù)與針孔直徑對(duì)參考波前精度的影響Fig.4 Simulation results of magnification and pinhole diameter impact on accuracy of reference wavefront phase
不同系統(tǒng)放大倍數(shù)與針孔直徑對(duì)針孔濾波后參考波前精度的影響,如圖4所示.由圖4可知:隨著dpin增大時(shí),參考波前誤差φRMS急劇上升;但是隨著S的增大,參考波前誤差則迅速減小并趨向于一個(gè)定值.當(dāng)dpin=0.5dA且S=1時(shí),φRMS=0.009 1,這與文獻(xiàn)[11,22-23]中的結(jié)果相符.而當(dāng)dpin增大為1.0dA,2.0dA和4.0dA后,適當(dāng)增大S同樣可以獲得高精度的參考波前,即S=2,4,8分別對(duì)應(yīng)于dpin為1.0dA,2.0dA和4.0dA的φRMS分別為0.009 2λ,0.008 7λ和0.010 2λ,幾乎與dpin為0.5dA時(shí)的參考波前一致.
由以上分析可知,當(dāng)增大dpin時(shí),適當(dāng)增大S不但可以獲得高精度的參考光強(qiáng)和參考波前,同時(shí)因?yàn)樵龃罅薲pin也可以提高參考光強(qiáng)度通過率,并且使不同波前像差的光場(chǎng)經(jīng)針孔濾波后的強(qiáng)度通過率的趨于平穩(wěn)[11].即增大dpin可以使得干涉圖的背景光強(qiáng)和對(duì)比度更加的均勻穩(wěn)定,也因此改善了動(dòng)態(tài)光場(chǎng)復(fù)振幅測(cè)量時(shí)條紋對(duì)比度波動(dòng)問題,使得該系統(tǒng)用于動(dòng)態(tài)光場(chǎng)復(fù)振幅實(shí)時(shí)檢測(cè)成為可能.
圖5 復(fù)振幅重建實(shí)驗(yàn)裝置Fig.5 Complex amplitude reconstruction experimental device
為驗(yàn)證所提方案的可行性,對(duì)波長(zhǎng)為532 nm且可通過微調(diào)輸出腔鏡激發(fā)不同階厄米-高斯混合模的二極管泵浦全固態(tài)激光器(DPSSL)的輸出光場(chǎng)進(jìn)行復(fù)振幅重建實(shí)驗(yàn),裝置如圖5所示.圖5中:MZ-PDI系統(tǒng)所采用的分光鏡1,2為5∶5單波長(zhǎng)分光平板;反射鏡1,2為532 nm單波長(zhǎng)介質(zhì)膜高反射鏡;透鏡1,2,3,4分別為焦距f1=f4=100 mm,f2=f3=300 mm的消像差傅里葉透鏡,即MZ-PDI系統(tǒng)的S=9,dpin=25 μm(約為艾里斑直徑dA的2倍).由此計(jì)算相應(yīng)的M2因子,并與標(biāo)準(zhǔn)M2因子測(cè)量?jī)x的測(cè)量結(jié)果相比較.
實(shí)驗(yàn)中,首先微調(diào)DPSSL的輸出腔鏡,使其輸出為一個(gè)類似于厄米-高斯TEM20模的光場(chǎng)作為待測(cè)激光;待測(cè)激光經(jīng)準(zhǔn)直放大后進(jìn)入MZ-PDI復(fù)振幅重建系統(tǒng)并被分光鏡分為兩束.其中,一束經(jīng)反射鏡后進(jìn)入M2因子測(cè)量?jī)x;另一束進(jìn)入MZ-PDI系統(tǒng),參考光和測(cè)試光經(jīng)過成像系統(tǒng)后發(fā)生干涉.為了得到高對(duì)比度的干涉圖,實(shí)驗(yàn)中需要在MZ-PDI兩干涉臂分別放置可調(diào)節(jié)衰減器,以便獲得高對(duì)比度的干涉條紋圖.最后,干涉圖由CCD記錄并經(jīng)數(shù)據(jù)采集系統(tǒng)送至計(jì)算機(jī)PC;然后利用傅里葉變換法重建的待測(cè)激光復(fù)振幅分布,如圖6所示.由圖6可知:計(jì)算得到直接測(cè)量光強(qiáng)和實(shí)驗(yàn)重建光強(qiáng)分布的互相關(guān)系數(shù)C(當(dāng)C=1時(shí)說明完全匹配)為0.972,二者吻合得很好.
(a) 采集到的干涉圖 (b) 直接測(cè)量的強(qiáng)度分布
(c) 重建的強(qiáng)度分布 (d) 重建的相位分布圖6 復(fù)振幅重建實(shí)驗(yàn)結(jié)果Fig.6 Experiment results of complex amplitude reconstruction
圖7 MZ-PDI與標(biāo)準(zhǔn)M2因子測(cè)量?jī)x的結(jié)果比較Fig.7 Measurement results of beam quality M2 factor with the proposed MZ-PDI method and standard M2 factor measurement instrument
圖8 動(dòng)態(tài)光場(chǎng)M2因子隨時(shí)間變化情況Fig.8 Experimental results of dynamic lightfield M2 factor versus change time
為進(jìn)一步驗(yàn)證該方法實(shí)時(shí)檢測(cè)的可行性,對(duì)動(dòng)態(tài)變化光場(chǎng)的復(fù)振幅進(jìn)行在線重建實(shí)驗(yàn).在DPSSL輸出光場(chǎng)MM3中引入空氣抖動(dòng)使輸出光場(chǎng)呈動(dòng)態(tài)變化,用CCD以8 幀·s-1的速度記錄動(dòng)態(tài)干涉圖,利用復(fù)振幅重建方法重建動(dòng)態(tài)變化的復(fù)振幅,對(duì)應(yīng)得到動(dòng)態(tài)光場(chǎng)的M2因子隨時(shí)間變化情況,如圖8所示.由圖8可知:該方法能夠有效地實(shí)時(shí)重建瞬態(tài)光場(chǎng)的復(fù)振幅分布.
結(jié)合針孔濾波理論分析基于MZ-PDI復(fù)振幅重建方法的基本原理,討論針孔直徑dpin和MZ-PDI放大倍數(shù)S對(duì)參考光強(qiáng)和參考波前精度及參考光強(qiáng)度通過率的影響.搭建了MZ-PDI激光復(fù)振幅實(shí)時(shí)重建系統(tǒng),分別對(duì)靜態(tài)和動(dòng)態(tài)光場(chǎng)進(jìn)行復(fù)振幅重建實(shí)驗(yàn),由此求得相應(yīng)光束質(zhì)量M2因子.實(shí)驗(yàn)結(jié)果驗(yàn)證了所提方案在復(fù)雜光場(chǎng)的復(fù)振幅實(shí)時(shí)重建的可行性.
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(責(zé)任編輯: 黃曉楠英文審校: 吳逢鐵)
Real-TimeComplexAmplitudeReconstructionMethodUsingMach-ZehnderPointDiffractionInterferometer
DU Yongzhao
(College of Engineering, Huaqiao University, Quanzhou 362021, China)
10.11830/ISSN.1000-5013.201606119
2016-06-06
杜永兆(1985-),男,講師,博士,主要從事激光光束質(zhì)量評(píng)價(jià)與測(cè)量、光學(xué)成像及光電圖像處理及應(yīng)用的研究.E-mail:yongzhaodu@126.com.
國家自然科學(xué)青年基金資助項(xiàng)目(61605048); 福建省自然科學(xué)基金面上資助項(xiàng)目(2016J01300); 福建省科技創(chuàng)新平臺(tái)資助項(xiàng)目(2013H2002); 華僑大學(xué)高層次人才科研啟動(dòng)項(xiàng)目(600005-Z15Y0073)
TN 401
A
1000-5013(2017)05-0699-07