定量相位成像技术在超快激光加工检测中的应用
Laser technology has become increasingly widespread in various research fields in recent years. Compared with continuous laser processing, femtosecond laser processing can improve or even eliminate the thermal effects caused by laser reactions, while being highly designable and controllable because of the wide range of materials that can be processed. Currently, the atomic force microscopy is widely used for the inspections of the morphology of femtosecond laser etching processes. This method can achieve nanoscale precision measurements of the sample morphology; however, the inspection process is slow and expensive and can only detect the physical dimensions of surface etching, which is a significant constraint when studying the morphology of transparent materials after femtosecond laser internal processing. A bright-field microscope can only qualitatively measure the edges of the process without information on the refractive index. In contrast, quantitative phase imaging (QPI) is an imaging method that can measure the phase information of transparent samples by allowing light beams to pass through the processed area while quantitatively detecting the optical properties around the processed area. Due to its non-contact nature, high sensitivity, and wide field of view, QPI has been used extensively in industrial inspection and biomedicine. However, to the best of our knowledge, its application in femtosecond-laser processing has not yet been reported. Therefore, this study proposes performing QPI measurements on femtosecond laser-processed glass samples. The results demonstrate the potential of this method in detecting the sizes and refractive indices of machined cavities inside glass cubes, as well as verifying the effects of different glass dopants with different femtosecond laser pulse energies.
In this experiment, a femtosecond laser was focused on a glass sample, creating linear cavities inside the glass with the aid of high pulse energy. Initially, the processed sample inside the calcium-sodium glass was characterized using a bright-field microscope and QPI system to determine the size of the machined cavity. During this process, the changes in the modified region around the cavity can be quantitatively measured using a QPI system. To analyze the three-dimensional physical characteristics of the laser processing area from a side view, a four-sided polished K9 glass cube was employed. Finally, to further investigate the effects of cavity processing on undoped glass materials, the same process was performed on fused silica and analyzed quantitatively using the QPI system.
Femtosecond lasers with different pulse energies were used to process cavities inside doped (calcium-sodium glass and K9 glass) and undoped (fused silica) glass cubes, and the cavity structures were characterized in three dimensions using QPI. After femtosecond laser processing, the doped glass exhibits a symmetrical area of tubes and bands in the top-view direction. In this region, the phase undergoes a semicircular change, with the phase falling in the center and rising at the edges of the cavity (Fig. 3). In the side-viewing direction, there is an extension, and the phase first increases and then decreases. By analyzing the processing area inside the glass from various angles, we restore the morphological changes in the modified area around the processing location inside the calcium-sodium and K9 glasses and describe them in three dimensions (Fig. 5). For undoped glass, the phase decreases in the processed area in the top-view direction and increases on both sides. However, there is no semicircular modified area or abrupt phase change at the edge of the processed cavity. In the side-view direction, the phase drops and rises rapidly in the machined area, whereas the average phase is slightly higher than that in the unmachined area (Fig. 6).
QPI is an important technique for analyzing optical-microscopic characteristics and has the potential to be a valuable tool in ultrafast laser processing. Unlike atomic force microscopy, QPI can probe the interior of transparent materials and recover their internal morphology using quantitative phase information. Through the three-dimensional analysis of the machined areas inside the glass, it is possible to restore and depict the morphological changes around the modified areas of calcium-sodium and K9 glasses. The results indicate a significant difference in the range of the modified areas produced by different doped glass materials, when processed at the same energy. When the undoped fused silica is subjected to femtosecond laser processing, a “pearl chain” structure appears and the semicircular modification of the refractive index around the processed position is not readily apparent. This phenomenon is related to a change in the refractive index of the glass itself caused by the doped materials. In conclusion, QPI holds promise for playing an important role in the field of laser processing inspection.
1 引言
近年来,激光技术在各领域的应用愈发广泛,其中以飞秒激光为代表的超短脉冲激光技术为研究实验反应和变化过程中的超快现象提供了一种新思路[1-2]。飞秒激光加工过程中会发射飞秒量级的超短脉冲,当脉冲通过聚焦物镜聚焦到透明介质材料表面或内部时,由于飞秒激光极高的瞬时功率,这些超短脉冲在激光焦点位置处的能量密度极高,从而形成极强的激光场,并与材料发生非线性作用[3],在激光焦点处形成大范围的等离子体结构,当该区域的密度较大时,透明介质材料内部就可以形成永久性的折射率调制[4-5]。相较于连续激光加工,飞秒激光加工的过程主要集中于激光焦点附近,其与材料的作用时间特别短,可以很好地改善甚至消除加工过程中由激光反应带来的热效应,实现“冷加工”[6-7]。同时,飞秒激光加工的适用性很广,加工过程是真三维加工,具有很高的可设计性和可控性[8]。
透明介质材料内部的折射率调制主要取决于入射激光的光强,当入射激光的光强比较低时,能量可以均匀地作用于透明介质材料,此时激光作用区域内的材料会发生局部反应,作用区域未出现明显损伤,但光学性质会发生变化(折射率及密度会增大)。材料改性后的折射率变化表现为各向同性的变化,这种折射率变化类型被称为Ⅰ型折射率调制,可用于在玻璃内部直写波导结构。当入射激光脉冲能量较大时,在焦点附近可以极快地产生等离子体结构,此区域内的物质及能量会以一种复合型冲击波的形式从激光焦点作用位置向四周扩张,最终使焦点处形成空洞,折射率也随之降低[9],而处于加工位置附近的介质材料由于受到来自焦点位置的冲击波的作用,密度会增加,从而导致折射率增大,这种折射率调制被称作Ⅱ型折射率调制。本文测试分析的玻璃内部空腔结构就是在高强度Ⅱ型损伤机制下写入的。
玻璃材料的折射率可近似看作是各组氧化物折射率的总和,与材料的极化率、密度成正比。当石英玻璃无掺杂时,其主要成分为二氧化硅。作为石英的非晶态,熔融石英具有极佳的光谱透过性和化学稳定性,被广泛应用于晶体振荡器、磨料、铸造材料、陶瓷和水泥等。向玻璃中掺杂氧化钙、氧化钠等改性氧化物,可以降低玻璃材料的软化点,同时可以增大材料的离子导电性及热膨胀特性。与钙钠玻璃相比,硼酸盐玻璃的热膨胀系数小[10],性质相对稳定。硼酸盐玻璃主要用于光学玻璃、精密实验用具玻璃等的生产制造。使用飞秒激光直写多组分玻璃时,其折射率变化可以是正的、负的或不均匀的,表现出对玻璃成分的强烈依赖性[11]。
在现阶段,研究人员大都采用原子力显微镜对飞秒激光刻蚀加工样品进行形貌检测。该方法虽然可以对样品形貌进行纳米级的精准测量,但检测时间较长且成本较高,而且只能检测表面刻蚀的物理尺寸,对于透明介质材料内部加工形貌的检测有着很大的局限性。使用明场显微镜观测,只能看到加工处的边缘,不能看到内部的折射率变化。随着光学三维测量技术的日益成熟[12-13],其在众多测量技术中脱颖而出。其中,定量相位成像技术是一种能够获得透明介质材料相位信息的成像方法。基于光学干涉原理,该方法使激光穿过加工区域,然后通过相位成像的方法来表征加工区域的属性[14-15]。该技术可以对加工区域周围光学性质发生改变的区域进行检测,并进行定量分析。
定量相位成像是一种基于双光束干涉测量的光学显微成像技术。传统的明场显微镜对高透过的散射介质或样品内部微小的缺陷结构进行探测时,采用的是直接测量透射光强度的探测方式,无法得到良好的探测结果;而定量相位成像技术通过探测光与给定参考光的干涉将探测光的相位变化转化为强度变化,获得样品的折射率信息和厚度信息[16],从而实现对微小结构的精确测量。美国中佛罗里达大学的Dogariu研究小组[17-18]在2019年报道了基于米氏散射原理和定量相位成像的技术,对光束通过亚波长粒子后的相位延迟进行了理论分析及实验验证。西北工业大学的邸江磊研究团队[19]在2020年使用定量相位成像技术获取了近场区域介质样品的强度和相位图像,进而实现了对介质样品相关物理参量的测量和表征。南京理工大学的左超研究团队[20]在2022年报道了基于环形照明傅里叶显微术的自适应光学定量相位成像技术,该技术只需要6张原始图像就可以重建图像,进行活细胞的实时成像;他们采用该方法实现了HeLa细胞的长期和宽视场成像。凭借高效、精准、高灵敏度、非接触,以及能够对目标物体结构进行纳米级精度测量的优势,定量相位显微成像技术已被广泛应用于血红细胞检测[21]、癌细胞诊断[22]、神经细胞观测[23]等领域的研究,并在工业检测[24]及生物医学[25]等领域大放异彩。
目前在超快激光加工领域尚未有与定量相位成像相关的研究。鉴于此,笔者对飞秒激光加工的玻璃样品进行了定量相位成像测量,结果显示:该方法在探测透明介质材料内部所加工空腔的尺寸、加工区域周围折射率改变区域的形貌以及验证不同掺杂物质对玻璃折射率的影响等方面有着良好的效果。
2 实验分析
2.1 飞秒激光在有掺杂玻璃内部的加工
飞秒激光在钙钠玻璃内部加工空腔的过程如
图 1. 飞秒激光在钙钠玻璃内部加工空腔结构示意图以及空腔结构。(a)使用飞秒激光器在钙钠玻璃内部加工示意图;(b)明场显微镜下观测到的4.1 μJ下加工的空腔结构
Fig. 1. Diagram of femtosecond laser processing cavity inside calcium-sodium glass and the cavity structure. (a) Schematic of processing cavity inside calcium-sodium glass using femtosecond laser; (b) cavity structure machined at 4.1 μJ observed under bright-field microscope
首先对有掺杂的钙钠玻璃载玻片进行超声处理,将其表面的碎屑和杂质去除,然后用乙醇将其擦净并将其固定在三维位移平台上,采用不同的脉冲能量进行加工。每加工完一条空腔后,将载物台调至初始位置,同时横向平移1 cm(以确保相邻的两组加工区域互不影响),并改变脉冲能量来加工第二条空腔。实验中分别使用5组不同的脉冲能量(1.5~4.7 μJ,能量间隔为0.8 μJ),依次加工5条空腔结构。钙钠玻璃内部的加工区域会出现线状空腔,并且空腔的两侧会出现光学性质改性的致密区域。
基于马赫-曾德尔干涉仪的透射型定量相位成像实验装置如
式中:a和φ分别代表样品的振幅和相位。叠加光场通过45°的四分之一波片后,参考光和探测光转换为具有相反自旋角动量的圆偏振态,此时叠加光场可以表示为
图 2. 马赫-曾德尔定量相位成像实验装置图
Fig. 2. Experimental setup of Mach-Zehnder quantitative phase imaging
由于本系统采用聚焦光束对样品进行照明,即引入了附加的球面相位,因此在样品后方使用物镜OL和透镜F4组合对聚焦光束的球面波进行补偿,将物平面上的球面波准直成平面波后再进行成像。这种方法可抵消球面波引起的相位差,只保留样品的相位信息。为了获得准确的样品相位信息,先在不放置样品(此时光波通过样品位置时仍按照平面波传播)的情况下对系统进行校正,但受系统误差的影响,实验获得的相位始终带有一定的曲率(即球面相位)。因此,将其视作背景相位,在后续实验中进行相位补偿,进而得到准确的样品相位。
理论上,在三个不同的偏振方向上进行测量即可获得精确的相位及强度。为了进一步减小噪声和测量误差的影响,通过控制检偏器的旋转角
式中:I(·)表示光强。
在实验中,通过改变半波片的偏转角度可以控制偏振分光镜的分光比,从而达到最佳的干涉成像效果及探测灵敏度。此外,值得一提的是,使用偏振相机代替检偏器和CCD,可以实现单帧定量相位成像[26]。像素分辨率(即相机中单像素点对应的成像大小)可通过对标尺进行成像获得。实验中,10 μm标尺对应的像素点数为43个,即每个像素点对应的尺寸为0.233 μm,但该尺寸小于阿贝衍射极限[0.61λ/(nsin α)=0.61λ/NA=965 nm,n表示物镜与样品之间填充介质的折射率],故本实验中的横向分辨率仍然受到阿贝衍射极限的制约,最小分辨率为965 nm。
图 3. 飞秒激光加工的钙钠玻璃内部的定量相位成像分析。(a)4.1 μJ单脉冲能量加工空腔的定量相位图像;(b)4.1 μJ脉冲能量加工空腔横截面处的相位平均曲线;(c)半圆形折射率改性区域直径与空腔宽度随脉冲能量的变化
Fig. 3. Quantitative phase imaging analysis of the interior of calcium-sodium glass processed by femtosecond laser. (a) Quantitative phase image of the processed cavity with the single pulse energy of 4.1 μJ; (b) phase average curve of the cross-section of the processed cavity with the single pulse energy of 4.1 μJ; (c) diameter of the semicircle refractive-index modified region and the cavity width change with the pulse energy
受限于实验所用钙钠玻璃样品的尺寸限制,实验中只能从俯视加工面的角度进行二维分析,无法从三维角度分析加工区域的相位变化。为了进一步对加工结果进行全面分析,采用四面抛光的玻璃立方体进行实验,使观测面变为俯视面和侧视面,通过增加侧视角度的相位测量和分析来探索加工区域的物理模型及特征。
图 4. 飞秒激光加工K9玻璃立方体示意图及结果分析。(a)K9玻璃立方体加工示意图;(b)四面抛光的K9玻璃立方体的实物图;(c)不同脉冲能量下加工的空腔在显微镜下的俯视图;(d)不同脉冲能量下加工的空腔在显微镜下的侧视图
Fig. 4. Schematic of femtosecond laser processing K9 glass cube and the results analysis. (a) Schematic of femtosecond laser processing K9 glass cube; (b) a photo of the four-sided polished K9 glass cube; (c) top-view of the cavity processed at different pulse energies captured by a microscope; (d) side-view of the cavity processed at different pulses of energies captured by a microscope
使用定量相位成像系统对4.9 μJ单脉冲能量下加工的空腔进行分析,从
图 5. 飞秒激光器在K9玻璃立方体中加工时的物理模型
Fig. 5. Physical model of femtosecond laser processing in K9 glass cube
本文首次通过定量相位成像方法验证了有掺杂玻璃内部的超快激光加工区域的性质变化和形貌特征,通过对比所加工区域的横向和纵向定量相位成像,可以直观定量地还原出空腔加工区域的物理模型。
2.2 飞秒激光在熔融石英内部的加工
为了探究无掺杂玻璃材料内部空腔的加工效果,进一步对熔融石英进行了相同的加工,并使用定量相位成像系统进行了定量分析。
图 6. 飞秒激光在熔融石英内部加工空腔的成像结果。(a)~(c)飞秒激光加工后,石英立方体内部俯视方向的相位图、相位曲线和明场显微镜图;(d)~(f)飞秒激光加工后,石英立方体内部侧视方向的相位图、相位曲线和明场显微镜图
Fig. 6. Imaging of cavity processed by femtosecond laser in fused quartz. (a)-(c) Phase diagram, phase curve, and bright-field microscopy image of quartz cube interior in overlooking direction after femtosecond laser machining; (d)-(f) phase diagram, phase curve, and bright-field microscopy image of quartz cube interior in side-view direction after femtosecond laser machining
3 结论
利用飞秒激光分别对有掺杂玻璃(钙钠玻璃和K9玻璃)和无掺杂玻璃(熔融石英玻璃)内部进行了空腔的加工,并通过定量相位成像技术对所加工的空腔结构进行多维度表征。结果显示:有掺杂玻璃在加工的俯视方向会出现管状和带状的对称区域,在此区域内相位呈现一种半圆形的变化趋势,相位在空腔中心下降,在边缘骤升;加工区域在侧视方向出现延伸,且相位先升高后减小。在无掺杂玻璃中,俯视方向加工区域的相位下降,两侧相位升高,但两侧并无半圆形改性区域,加工的空腔边缘处也没有骤升的相位变化;加工区域在侧视方向上也有延伸,但相位会在加工区域下降后迅速升高,延伸区域的相位略高于未加工区域。
定量相位成像技术是一种重要的光学微观表征手段,在超快激光加工效果的检测中具有很重要的作用。相较于原子力显微镜,定量相位成像技术可以探测到玻璃材料的内部,还原其内部及改性区域的形貌,对目标样品进行精确表征。通过对玻璃内部加工区域的多角度分析,本文还原了钙钠玻璃和K9玻璃内部加工位置及其周围改性区域的形貌变化,并用三维视图进行了描述。结果表明:不同掺杂的玻璃材料在相同能量下加工时产生的改性区域范围有着很显著的区别;在无掺杂的熔融石英中,随着飞秒激光的加工,出现了“珍珠链”结构,加工位置周围的透明改性区域不明显。导致这一现象的原因与玻璃中掺杂的物质改变了介质材料本身的折射率有关。因此,定量相位成像技术有望在激光加工检测领域发挥重要作用。
[1] Perevoznik D, Tajalli A, Zuber D, et al. Writing 3D waveguides with femtosecond pulses in polymers[J]. Journal of Lightwave Technology, 2021, 39(13): 4390-4394.
[2] Liao Y, Ni J L, Qiao L L, et al. High-fidelity visualization of formation of volume nanogratings in porous glass by femtosecond laser irradiation[J]. Optica, 2015, 2(4): 329-334.
[3] 吴雪峰, 梅三林. 飞秒激光加工机理及仿真研究进展[J]. 激光与光电子学进展, 2021, 58(19): 1900005.
[4] Sun M Y, Eppelt U, Schulz W, et al. Role of thermal ionization in internal modification of bulk borosilicate glass with picosecond laser pulses at high repetition rates[J]. Optical Materials Express, 2013, 3(10): 1716-1726.
[5] Sakakura M, Lei Y H, Wang L, et al. Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass[J]. Light: Science & Applications, 2020, 9(1): 1-10.
[6] Sugioka K, Cheng Y. Ultrafast lasers: reliable tools for advanced materials processing[J]. Light: Science & Applications, 2014, 3(4): e149.
[7] 姜玺阳, 王飞飞, 周伟, 等. 飞秒激光与材料相互作用中的超快动力学[J]. 中国激光, 2022, 49(22): 2200001.
[8] Wang X D, Yu H B, Li P W, et al. Femtosecond laser-based processing methods and their applications in optical device manufacturing: a review[J]. Optics & Laser Technology, 2021, 135: 106687.
[9] 李佳群, 闫剑锋, 李欣, 等. 透明介质材料的超快激光微纳加工研究进展[J]. 中国激光, 2021, 48(2): 0202019.
[10] 汤李缨, 高栋良, 向光. BaO、Li2O对钠钙硅玻璃热膨胀系数和化学稳定性的影响[J]. 材料导报, 2013, 27(S1): 195-197.
Tang L Y, Gao D L, Xiang G. Effect of BaO and Li2O content on the thermal expansion coefficient and chemical stability of Na2O-CaO-SiO2 glass[J]. Materials Review, 2013, 27(S1): 195-197.
[11] Bhardwaj V R, Simova E, Corkum P B, et al. Femtosecond laser-induced refractive index modification in multicomponent glasses[J]. Journal of Applied Physics, 2005, 97(8): 083102.
[12] 付莉娜, 杨静雯, 李雁玲, 等. 二值条纹离焦投影技术综述[J]. 激光与光电子学进展, 2022, 59(14): 1415011.
[13] 郭文博, 张启灿, 吴周杰. 基于相移条纹分析的实时三维成像技术发展综述[J]. 激光与光电子学进展, 2021, 58(8): 0800001.
[14] 季颖, 龚凌冉, 傅爽, 等. 基于卷积神经网络的相位体自动识别方法研究[J]. 激光与光电子学进展, 2022, 59(6): 0617026.
[15] 季颖, 韦鑫宇, 张明明, 等. 相位边缘检测下形态特征快速提取的实验采样策略[J]. 激光与光电子学进展, 2022, 59(6): 0617030.
[16] 杨泽文, 张璐, 吕宁, 等. 生物折射率三维无标记定量成像研究进展[J]. 中国激光, 2022, 49(5): 0507201.
[17] Shen Z A, Dogariu A. Meaning of phase in subwavelength elastic scattering[J]. Optica, 2019, 6(4): 455-459.
[18] Shen Z A, Cui S W, Dogariu A. Polarization-encoded field measurement in subwavelength scattering[J]. Optics Letters, 2019, 44(14): 3446-3449.
[19] 戴思清, 豆嘉真, 张继巍, 等. 基于数字全息术的近场成像与应用[J]. 光学学报, 2020, 40(1): 0111008.
[21] Wang Y, Zhu L D, Zhou H X, et al. Quantitative phase imaging using spectral domain phase microscopy without phase wrapping ambiguity[J]. Optics Letters, 2019, 44(1): 151-154.
[22] Li Y F, Fanous M J, Kilian K A, et al. Quantitative phase imaging reveals matrix stiffness-dependent growth and migration of cancer cells[J]. Scientific Reports, 2019, 9(1): 1-8.
[23] Hu C F, Popescu G. Quantitative phase imaging (QPI) in neuroscience[J]. IEEE Journal of Selected Topics in Quantum Electronics, 2019, 25(1): 6801309.
[24] 张润南, 蔡泽伟, 孙佳嵩, 等. 光场相干测量及其在计算成像中的应用[J]. 激光与光电子学进展, 2021, 58(18): 1811003.
[25] 满天龙, 万玉红, 菅孟静, 等. 面向生物样品三维成像的光干涉显微技术研究进展[J]. 中国激光, 2022, 49(15): 1507202.
[26] Cui S W, Gao S, Li C H, et al. Quantitative phase imaging based on polarization encoding[J]. Optics Express, 2022, 30(24): 43622-43632.
[27] 孙劭伟, 齐乃杰, 孔艳, 等. 熔石英玻璃激光损伤的三维应力场研究[J]. 中国激光, 2021, 48(1): 0101001.
[28] HnatovskyC, SimovaE, RajeevP P, et al. Applications of femtosecond laser-induced self-assembled nanocracks in fused silica glass[C]// Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides 2007, September 2-6, 2007, Québec City, Canada. Washington, D.C.: Optica Publishing Group, 2007: BTuD1.
[29] 周强, 王俊波, 邱荣, 等. 纳秒激光诱导熔石英玻璃损伤的超快诊断[J]. 中国激光, 2014, 41(3): 0303003.
[30] Ohmura E. Analyses of self-focusing phenomenon and temperature rise in fused silica by ultrashort pulse laser irradiation[J]. Procedia CIRP, 2013, 5: 7-12.
Article Outline
李昌恒, 崔省伟, 姚晓天. 定量相位成像技术在超快激光加工检测中的应用[J]. 中国激光, 2023, 50(12): 1202403. Changheng Li, Shengwei Cui, X. Steve Yao. Application of Quantitative Phase Imaging in Ultrafast Laser Processing Inspection[J]. Chinese Journal of Lasers, 2023, 50(12): 1202403.