聚合物光波导端面磁流变抛光工艺

聚合物光波导端面磁流变抛光工艺彭文强;吴宇列;刘勇;冯宗杰【摘要】The end-face surface quality has great influence on the optical waveguide device's coupling performance,so the end-face polishing must be conducted before coupling.However,the end-face polishing of the polymer optical waveguide mainly relies on the traditional disk polishing,whose complex process and low polishing efficiency have become the bottleneck that restricts the wide application of polymer optical waveguide.A unique method for polishing end-faces of polymer optical waveguides via magnetorheological finishing (MRF) technique was put forward to achieve high optical quality surfaces based on the excellent machining properties of polymer.Experiments of polishing 3 nun × 3 mm polymer optical waveguide end-face were conducted on specially designed end-polishing equipment based on traditional disk polishing fluid and MRF technique polishing fluid with CeO2 polishing abrasives with the diameters of 5 μm,O.5 μm and 1 μm,separately.Experimental results show that a 2-minute MRF with raster scan tool path for one time on the end-face has better polishing effect compared with traditional method of 3-hour rough and finishing polishing ing white-light interferometer we can observe that the rootmean-square (RMS) of surface roughness of the end-face of polymer optical waveguide is 2.6 nm polished by MRF while 128.7 nm by traditional polishing method.Optical coupling tests results show that the insert loss of the polymer opticalwaveguide has reduced from 32.7 dB to 17.8 dB after polished by MRF technique.With fast polishing speed and high quality polished end surfaces,MRF technique will become a promising method for the end-face polishing of polymer optical waveguides.%光波导端面的表面质量会严重影响光波导器件的先耦合封装性能,耦合封装前必须对波导器件端面进行抛光处理.目前聚合物光波导端面主要依靠传统研磨盘进行抛光处理,该工艺工序复杂、抛光效率低已成为制约聚合物波导器件应用的瓶颈.基于聚合物光波导材料优良的加工特性,通过对比实验提出了聚合物光波导的磁流变端面抛光工艺.采用5μm、0.5μm和1 μm粒径的氧化铈抛光粉分别配制研磨盘抛光液及磁流变抛光液对3mm×3mm聚合物光波导端面进行抛光实验,发现磁流变加工对聚合物光波导端面进行一次2min 光栅扫描抛光就具有比传统研磨盘约3h精、粗抛光较好的端面质量.经过白光干涉仪测量,磁流变抛光后光波导端面表面粗糙度的均方根值达到了2.6 nm,传统端面抛光端面粗糙度均方根值为128.7 nm.通过自动对准耦合平台测试,结果显示通过磁流变端面抛光的光波导的插入损耗由抛光前的32.7 dB降低到了17.8 dB.磁流变抛光方法可以对聚合物光波导端面进行快速、高性能的抛光,在光波导应用领域具有非常广阔的应用前景.【期刊名称】《纳米技术与精密工程》【年(卷),期】2011(009)004【总页数】6页(P364-369)【关键词】磁流变抛光(MRF);聚合物光波导;端面抛光;表面粗糙度;插入损耗【作者】彭文强;吴宇列;刘勇;冯宗杰【作者单位】国防科学技术大学机电工程与自动化学院,长沙410073;国防科学技术大学机电工程与自动化学院,长沙410073;国防科学技术大学机电工程与自动化学院,长沙410073;国防科学技术大学机电工程与自动化学院,长沙410073【正文语种】中文【中图分类】TN252Polymer optical materials offer a number of interesting features for fabrication of optical waveguides compared with the traditional inorganic optical materials. Organic polymers have attracted a lot of attentions due to their excellent optical properties, low cost-effectiveness, simple fabrication and low structural flexibility. They offer such advantages as low temperature process of fabrication, easy control of mechanical properties and mass production possibility, high refractive index, good adhesion to substrate, low dielectric constants, and high optical transparency in the infrared wavelength region as well as high glass transition and high thermal decomposition temperatures[1]. Due to those advantages, polymer optical waveguides have been widely applied in optical interconnection and integrated optics areas [2-3]. Optical polymer waveguides are generally fabricated using reaction icon etching, lithograph technique, laser ablation, molding, direct laser writing, electron beam writing and photo-bleaching. Among these methods, molding with micro mould made by diamond-cutting technique on an ultra-precision machine is a promising method for its high accuracy and fastmanufacturing speed[4]. However, there are still many problems to be solved for this method. The end-face polishing is one of those problems. There exists lots of cracks and pits on the end-face of the waveguide before it is polished, which will cause severe light scattering and absorption. The optimal insert loss of the single mode optical fiber was 1.45 dB when the end-face roughness was 0.1 μm[5]. Lee et al [6] in Korea had measured the scattering loss with 2.78 dB of a single-mode rectangle polymer optical waveguide by separating the end-face scattering loss from the insert loss. The end-face roughness has great influence on the performance of the waveguide when signal light is coupled into it. Therefore, that is why the end-face needs to be grounded and polished to achieve minimum roughness after cutting step. At present the traditional polishing methods mainly rely on special disk polishing machine (SDPM). The waveguide end-face was rough and finishing polished repeatedly by the disk of SDPM driving different sizes sandpapers or the polishing fluid compounded with different sizes polishing powder[7-8]. Traditional polishing methods involve many processing steps and can lead to a long fabrication time and a low yield.Magnetorheological finishing (MRF) technique has excellent performance on polishing polymer materials such as high material-removal rates and smooth polished surface. It has been proved that the smooth surface can be achieved by MRF if appropriate polishing powder by the Center for Optics Manufacturing at California in America [9] is used. MRF technique has been applied successfully to the finishing for general large opticalsurfaces with high accuracy. However, to the best of our knowledge, this technique has not yet been applied to the polishing for end-faces of polymer optical waveguides. This paper will employ MRF technique to achieve fast and high quality polishing for end-faces of polymer optical waveguides.1 Polymer optical waveguideThe polymer optical waveguides used in this experiment are multi-mode embedding trapezoid optical waveguide with the cross section dimensions of top-side 65 μm, bottom-side 38 μm and height 50 μm as shown in Fig.1. The cladding and core polymer materials used in the trapezoid optical waveguide are PMMA with refractive index 1.48@1 550 nm and ultraviolet ************************************************************ propagation property of the polymer waveguide was simulated by the Rsoft Company’s professio nal optical waveguide design software named BeamProp.Fig.1 Polymer trapezoid waveguideThe polymer optical waveguides were made by injection molding technique. The fabrication of high quality optical microstructure based on ultra-precision machining technology was proved a method with high precision and quality and short fabrication time [10-11].The waveguide mould fabrication using ultra-precision machining technique was so far only reported in Korea Photonics Technology Institute [4]. The technique for polymer optical waveguide fabrication with ultra-precision machining is a newly developed method and is still not mature. The polymer opticalwaveguide aluminum mould was fabricated by the specially designed ultra-precision fly-cutting processing system with a diamond tool in the waveguide fabrication experiment. The waveguide structure on the mould is shown in Fig.2.Fig.2 Microscope waveguide image on the mouldThe end-faces of the polymer optical waveguide should be glabrous in order to reduce the insert loss of the waveguide when coupled into the signal light on the auto-aligning platform. In order to get the glabrous end-faces, the cracks and pits on its surface must be removed while the structure of the cladding and core material should not be destroyed.2 Experiment2.1 MethodologyIn order to get smooth end-faces polishing method the polymer optical waveguides with cross section of 3 mm×3 mm were polished by the SDPM and MRF separately. The microstructures of end-faces were observed by the digital light microscope before and after being polished. In order to analyze the end-face quality, we used white-light interferometer to measure the polished surface. At last the waveguide insert loss was measured on the auto-aligning platform to examine the end-face polished effect.The polishing abrasives in magnetorheological(MR) fluid were CeO2 with the average abrasive diameter of 1 μm, while the polishing abrasives of polishing fluid in rough and finishing polishing with SDPM were CeO2 with the average abrasive diameters of 5 μm and 0.1 μm. The MRF machineused in the experiment was developed by our laboratory and SDPM machine developed by Central South University.2.2 Polishing experimentsFirstly to get smooth waveguide end-face, rough and finishing polish processes were conducted on the traditional SDPM using different polishing fluids with the average polishing abrasive diameters of 5 μm and 0.1 μm. The diameter of grinding disk is 500 mm with polishing speed 30 r/min. The waveguide end-faces were polished by the polishing fluid repeatedly and the total polish time was about 3 h. The waveguide end-faces polished by SDPM is shown in Fig.3.Fig.3 Polymer optical waveguide polished by SDPMExperiment of MRF end-face polishing was conducted in order to obtain a better end-face polishing method. The end-faces of the polymer optical waveguide polished by MRF mainly relied on MR ribbon to transport the fine CeO2 powder particles to process the surface. The dimension of the MR ribbon on the polishing wheel was 10 mm ×1 mm (width × height). In order to get the smooth surface, the end-faces of the waveguide were polished by MRF under the process parameters with polishing wheel speed of 180 r/min, feed rate of 20 mm/min and ribbon bite of 0.3 mm. The MR polishing wheel polished the end-face with raster scan tool path for one time and the total polishing time was about 2 min. The waveguide end-faces polished by MRF is shown in Fig.4.Fig.4 Polymer optical waveguide polished by MRF3 Results and discussionThe end-faces of polymer optical waveguide samples were cut by a diamond cutter. The microstructure of the end-faces were observed in lens with the magnification of 500 by the digital light microscope before and after being polished. The roughness caused by mechanical cut at the waveguide end-faces is obvious as shown in Fig.5(a), from which we can see the obvious tool marks. The waveguide end-faces are more glabrous after being polished by SDPM, but the end-faces in core material are still very coarse as shown in Fig.5(b). The end-faces polished by MRF look very smooth in both cladding and core polymer materials. The tool marks and pits on the surface are obviously removed. The boundary line between the cladding and core polymers is not affected. The end-faces polished by MRF are shown in Fig.5(c). This means that the MRF technique is very suitable for the polymer optical waveguide polishing process.Fig.5 End-faces of polymer optical waveguide before and after being polishedTo quantify surface roughness, the waveguide end-face was measured by white-light interferometer. Fig.6 shows the measurement results of the end-faces polished by the two different methods. The root-mean-square (RMS) of surface roughness of the end-face polished by MRF is 2.6 nm, while 128.7 nm polished by SDPM. The measurement results mean that the MRF has a better effect on polishing polymer optical waveguides.Fig.6 Surface roughness measured by white-light interferometerThe SPDM process mainly relies on grinding disk to transport the powder particles to polish the workpiece surface. As powder particles are not in thesame dimension, not all particles take part into polishing in SPDM process. While, depending on denaturation of MR fluid, MRF polishes the workpiece surface by the shearing force of denaturation. MRF makes use of flexible polishing ribbon formed by MR fluid in high gradient magnetic field to polish the workpiece surface. MR fluid is magnetized into chain structures with the action of magnetic field and then the MR fluid becomes many tiny grinding head group of thin grinding film with certain thickness and gas porosity. The MRF polishing model is shown in Fig.7.Fig.7 Principle of MRF polishing modelThe new style grinding film can capture and confine the polishing abrasives effectively compared with the grinding in the SPDM process, so the big and small polishing abrasives can all take part into polishing the workpiece surface which leads to high material-removal rates. The MRF can get high machining precision for MR fluid, which confines the polishing abrasive flexibly with the intensity change of magnetic field and the polishing wheel does not exist in abrasion and truing. So the MRF can get a better effect in waveguide end-faces polishing with the same polishing power particle but large average diameters and short polishing time compared with the SPDM.4 Optical testTo examine the polishing effect of polymer optical waveguide end-faces, the insert loss of polished strip waveguides was measured by an optical auto-aligning platform shown in Fig.8. The device is based on the auto-aligning platform equipped with computer controlled nano-movers and anoptical power meter. Laser sourced light with light-wave 1 500 nm is coupled into the waveguides with a multi-mode fiber (50/125 μm) and the output intensity of the waveguides is monitored with an identical fiber of optical power meter. With this device, the insert loss of the strip waveguide is 32.7 dB@1 550 nm before being polished, 20.5 dB@1 550 nm after polished by SDPM, and 17.8 dB@1 550 nm after polished by MRF. The polished polymer optical waveguide has greatly reduced insert loss with MRF technique. It is proved that the MRF has good effect on polymer end-face polishing.Fig.8 Auto-aligning platform used to test polymer optical waveguide5 ConclusionsThe main purpose of this paper is to develop a reliable and effective end-face polishing method for the polymer optical waveguides. As MR ribbon can capture and confine the polishing abrasives effectively, polymer strip waveguide polished by MRF has the smoother surface with the RMS of roughness of 2.7 nm and higher material-removal rate with polishing time of about 2 min compared with the traditional SDPM polishing method. The polished effect is further proved by the waveguide optical loss measurement on auto-aligning platform. It is found that the waveguide insert loss reduces from 32.7 dB to 20.5 dB polished by SDPM and to 17.8 dB polished by MRF. The average diameter of the polishing abrasives used in MRF can be larger than that used in SDPM to reach the same polishing effect. With fast polishing speed and high quality polished surface, MRF technique will become a promising method for the end-face polishing ofpolymer optical waveguide.References：【相关文献】[1] Ma H, Jen A K Y, Dalton L R. Polymer-based optical waveguides: Materials, processing, and devices[J]. Adv Mater, 2002, 14 (19): 1339-1365.[2] Cho I K, Lee W J, Rho B S, et al. Polymer waveguide with integrated reflector mirrors for an inter-chip link system[J]. Optics Communications, 2008, 281(19): 4906-4909. [3] Yang J Y, Zhao F, An D C, et al. Multifunctional optical waveguide chips[C]// Conference on WDM and Photonic Switching Devices for Network Applications III . San Jose, CA, USA, 2002, 4653:105-110.[4] Lee W J, Lim J W, Hwang S H, et al. Imprint master fabricated by ultra precision machining for optical waveguide[C]// OECC/ACOFT 2008. Sydney, Australia, 2008, 1/2: 556-557.[5] Liu Defu, Duan Ji’an. Mechanism research on lapping of optical fiber end-face[J]. Optics and Precision Engineering, 2004, 12(6): 570-575(in Chinese).[6] Lee H M, Oh M C, Park H, et al. End-face scattering loss in integrated-optical waveguides[J]. Applied Optics, 1997, 36(34): 9021-9024.[7] Ab-Rahman M S, Ab-Aziz F, Arif N A A M, et al. The study of the good polishing method for polymer SU-8 waveguide[J]. Optica Applicata, 2009, 39 (3):459-464.[8] Qian Shubing, Hu Guohua, Yun Binfeng, et al. The research of machining for integrative polymer waveguide facet[J]. Optoelectronic Technology, 2007, 27(4):228-230(in Chinese).[9] DeGroote J E, Romanofsky H J, Kozhinova I A, et al. Polishing PMMA and other optical polymers with magnetorheological finishing[C]// Conference on Optical Manufacturing and Testing V. San Diego , CA, USA, 2003, 5180: 123-134.[10] Zhao Q L, Guo B, Yang H, et al. A mechanistic cutting force model for diamond fly-cutting of microstructured surface[C]// Proceedings of SPIE. Chengdu, China, 2009, 7282: 728204-1-6.[11] Jiang W D. Diamond turning microstructure optical components[C]// Proceedings of SPIE. Chengdu, China, 2009, 7284: 728404-1-6.。