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Cleaving of polymer optical fiber by 193-nm excimer laser
Optical Fiber Technology 54 (2020) 102069
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Cleaving of polymer optical fiber by 193-nm excimer laser a
Xianfeng Luo , Libing You a b c
b,⁎
a,c
, Le Luo , Xiaodong Fang
T
b
Institute of Intelligent Manufacturing, Hefei University of Technology, Hefei 230009, China Anhui Provincial Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China School of Electronic Science and Applied Physics, Hefei University of Technology, Hefei 230009, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cleaving Polymer optical fiber PS PMMA Laser cutting
A smooth end-face is difficult to obtain when cleaving a polymer optical fiber (POF) due to the complex mechanical properties of polymers. In this article, a cleaving method is reported, which use laser ablation of a 125μm solid multi-mode POF with a 193-nm ultraviolet laser. To achieve the best POF end-face quality, we designed a new beam shaping system that makes a 180 × 22-μm beam size. As a result, we found that an energy density between 550 and 600 mJ/cm2 and a repetition rate of 3 Hz were the optimal parameters cleaving the 125-μm solid multi-mode polymer optical fiber.
1. Introduction
close to (or slightly below) that of the fibers. Lower blade speeds, ranging between 0.7 and 0.5 mm/s were generally effective. Atakaramians et al. exploited a SD saw to cleave PMMA and TOPAS® cyclic olefin copolymer highly porous fibers developed for guiding terahertz radiation in 2009 [13]. But the results showed that the end-face of PMMA porous fibers was chipped along the direction of cleave and the end-face of TOPAS porous fibers was smeared due to the grinding action of the blade. Atakaramians et al also used an FIB with an ion beam current of 21 nA to cleave 400 μm PMMA POFs and achieved even better end-face [13]. Nevertheless, the cleaving time was nearly 17.5 h and FIB milling is very expensive. Ghirghi et al. presented a new method for preparing POF end-faces in 2014 [14] that included using liquid nitrogen or liquid nitrogen vapor to cool the fibers and breaking them using a special curved form. Canning et al. first developed a method of cleaving air-polymer structure fibers using 193-nm excimer laser in 2002 [16] and found that the repetition rate should be below 2 Hz to avoid thermal damage by laser irradiation. However, the cleaving time scales with repetition, and it was found that the intensity of 1.6 J/cm2 and 4 Hz gave a quality end-face. Atakaramians et al. also used a 193-nm excimer laser with an intensity of 312 or 425 mJ/cm2 to cleave the POFs, and two auxiliary rotary apparatuses at 3.6˚/s to reduce the damage by thermal buildup during the cleaving progress [13]. In this article, we first used a 193-nm excimer laser to cleave the multi-mode solid POFs with a PS core. The interaction between the 193nm excimer laser and polymers is known as ablation [17]. This course of action produces a photon decomposition reaction by projecting a laser beam on a POF. Increasing the pulse intensity to the threshold value, the majority of the photon absorption will cause etching and the
The first polymer optical fiber (POF) was developed by DuPont in 1968 [1]. There are many kinds of materials with which to fabricate POFs, but the most common materials are polymethyl methacrylate (PMMA), polystyrene (PS), and polycarbonate (PC) [2]. POFs have the same structure as quartz optical fibers with a core, cladding, and sometimes a jacket. However, Young’s modulus of POFs is almost 28 times lower than that of quartz [3]. Owing to this characteristic, POFs have many advantages, such as higher sensitivity, excellent flexibility, lower stiffness and higher thermo-optic coefficients. In addition, POFs are increasingly considered a potential alternative to quartz optical fibers in many fields. In view of these advantages, POFs were used in many fields, including short distance telecommunication [4], the automotive industry [5], biosensing, and strain sensing [6,7]. However the attenuation in POFs is higher than in conventional quartz optical fibers [8]. Therefore, a smooth end-face of POF is of critical significance to avoid excessive insertion loss. However, the materials of low stiffness limit the cleaving of POFs. The end-face of POF is jagged and mutilated scene by using a normal diamond blade. Several methods for preparing POF end-face have been reported, such as polishing the connector endface [9], hot-blade cleaving [10–12], focused ion beam (FIB) milling [9,13], using a semiconductor dicing (SD) saw [13], termination with use of liquid nitrogen and laser cutting [14]. Law et al. first used a hot blade to cleave POFs in 2006 [11,15], and found the effect of temperature and cutting-blade speed in the process. In addition, the best results were achieved when the POFs temperature was controlled between 70 ℃ and 80 ℃, and the blade temperature was
⁎
Corresponding author. E-mail address: [email protected] (L. You).
https://doi.org/10.1016/j.yofte.2019.102069 Received 11 March 2019; Received in revised form 30 October 2019; Accepted 30 October 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 54 (2020) 102069
X. Luo, et al.
Fig. 1. (a) The linear cutting beam shaping system, (b) A liner cutting beam and (c) The fiber.
rest will be transformed into heat [18]. After transformation, the etching residue was vaporized by the previously heat. Owing to the shortwave characteristics of the excimer laser, the high lateral resolution, and the larger energy of photons, the 193-nm laser beam is deemed to be an appropriate “knife”. In addition, the remaining material shows some thermal damage and produces sharp steps at the edge of the irradiated area. In experiments, a new optical system was designed to produce a smooth POF end-face and no additional processing was carried out on the fibers, such as heating/cooling the fibers or changing their mechanical structure.
180 × 22 μm. Fig. 1(b) shows a microscope image of the linear cutting beam that works on photographic paper. A charge-coupled-device (CCD) camera (NAVITAR) allows in situ monitoring of the entire process. Fig. 1(c) shows a CCD image of the fiber with a very flat incision when it was cut off. A three dimensional (3D) motorized platform (Zolix Instruments Co., Ltd) was used to precisely adjust the position of fibers. All end-face images were acquired by a microscope (OLYMPUS BX51M). The sides of fibers were glued to an aluminum block with a hole. At the same time, the fibers were suspended above the hole to avoid damage from the splashing of aluminum.
2. Instruments
3. Experiments
An argon-fluoride (ArF) excimer laser (Lambda Physik,Germany) was used in these experiments (repetition rate, 20 Hz; pulse energy, 200 mJ; pulse width, 25 ns; beam size, 24 × 10 mm2). The POF material is essentially PS and PMMA cladding with a diameter of 125 μm, and was purchased from Paradigm Optics. Fig. 1(a) shows the new linear cutting beam-shaping system. It consists of a light source, a collimating and beam expanding unit, a mask, an objective lens, and three reflectors. The output energy of the excimer laser is typically characterized by a nearly Gaussian distribution along the short axis and a flat-top distribution with Gaussian wings along the long axis [19]. Therefore, the output beam was reshaped by the collimating beam expanding unit and then the center area of the beam was intercepted by the mask to achieve a rectangular distribution along the long axis. The unit also can decrease the angle of divergence of the beam. An LMU-3 × -193 focusing objective lens (THORLABS, 3X, 193 nm, NA = 0.08) with a focal length of 49 mm was used to create a fine line beam to the fibers in the image plane area. In the image plane, the linear cutting beam has a more homogeneous energy distribution with steep and well-defined edges, and its size is approximately
All experiments were conducted at room temperature under atmospheric conditions. The length of the fibers was controlled at 10 mm and the distance of two bonding positions was controlled below 1 mm. The two bonding positions of fibers should be closed to avoid a slight shaking of fibers in the cleaving process. Before the best laser conditions for cleaving POFs were found, we investigated several factors that will affect the results. 3.1. Previous experiments From past experiments, it was found that variable beam widths have different cleaving effects. Fig. 2(a) shows the image that the fiber was cut off by a 180 × 40 μm beam. A large area of damage on the fiber upper surface with the width of beam is 40 μm. However, the situation did not arise when by using a 22 μm beam width of the beam in Fig. 1(c). Therefore, the beam width was maintained at nearly 22 μm to reduce this damage in experiments. Another important factor impacting on the quality of the end-face is the uniformity of the laser energy distribution. When the energy Fig. 2. Images of bad cleaves.
2
Optical Fiber Technology 54 (2020) 102069
X. Luo, et al.
to 326 s. From the images, the quality of end-faces is smoother than in Fig. 4(a) and (b). However, it can be clearly observed that the outer ring of Fig. 4(c) and (d) locates slightly lower than the center of the endface. There is also some slight thermal damage and a few distinct black dots. In addition, the images show that the color of cladding is deeper than that of the core. The reason for this phenomenon is that the materials comprising the core and cladding interact with the laser in different ways. The optical images of Fig. 4(f) and (g) are of the end-face that was cleaved at ~550–650 mJ/cm2, and the cleaving time is approximately 300 s. Those two groups have the best end-face quality, exhibit no black dots, and incurred minimal thermal damage. Compared to the other optical images of end-faces, the center of the end-faces is not seriously higher than the outer rings, and there are few residues on the end-faces. Unfortunately, an end crack in an end-face is shown in Fig. 4(g). The optical images in Fig. 4(h) and (i) are of the end-face cleaved at ~690 mJ/cm2, and the cleaving time is approximately 280 s. It can be obviously seen that the cladding of fibers is much thinner than before cleaving. Compared to other images, it was found that the extent of this situation is directly related to pulse intensity. Furthermore, a large area of cross-section appeared to damage the fiber’s core and a serious end crack formed in the end-face in Fig. 4(i). Interestingly, the cladding and core have different depths of color, and the cladding is slightly deeper, a possible solution for which could be the dissimilarity of the photochemical reaction between PS and PMMA with the laser. On one hand, when the pulse intensity is less than 280 mJ/cm2, the photon density is not sufficient to cause the complete decomposition of the PS/PMMA in the depth of light penetration; On the other hand, the part of the transformation thermal energy is not sufficient to cause the complete vaporization of the etched material. It gives rise to abundant black residues attached to the end-faces. Cleaving for excessive time leads to the deformation of fibers and the appearance of stepped end-faces. From the analysis of Fig. 4(a) and (b), it was found that pulse intensities below 300 mJ/cm2 cannot produce good end-faces of POFs. The images in Fig. 4(c)–(e) show a complete photochemical reaction between the laser and PS/PMMA. However, the amount of energy transformed to thermal energy is also not sufficient to vaporize the remaining materials, resulting in some black spots on the end-faces. Accordingly, a pulse intensity below 530 mJ/cm2 is also not suitable for cleaving. Above 650 mJ/cm2, the main reason that the brown cladding and upper cladding are thinner than the other side is the thermal build-up caused by excessive thermal excess during the cleaving process. Most of the end-faces actually exhibit such problems. Fig. 4(f) and 4(g) show good-quality end-faces, and we found that a suitable pulse intensity was one of the key factors in the experiments. The resulting images indicate that a pulse intensity of 550–650 mJ/cm2 is best to cleave PS/PMMA fibers.
distribution in the long axis is not uniform, it was found that the process of inconsistent cleaving speed leads to an imbalance stress on both sides, resulting in cleavage of an end-face that resembles a wave, as shown in Fig. 2(b). It is also important to keep the cleaving angle of fibers and laser beam at approximately ~90°. Fig. 2(c) shows the damage to nearly half the area of the end-face that caused a rough surface by oblique incidence. Moreover, when the cleaving process creates a larger axial stress in the fibers, it will consequently produce an end crack in the end face, which is another situation that reduces the tension of fibers and which should demand more attention during the cleaving process. Although this parameter is currently not measured, the smaller the axial stress, the lower the possibility of elastic deformation during the cleaving process.
3.2. Optimum energy density range All cleaving processes were carried out with an output energy of the 193-nm laser between 22 and 120 mJ, taking into account the result obtained by Canning et al., namely that the repetition rate should be below 2 Hz to avoid damaging the fiber end-face. In subsequent studies, a repetition rate of 2 Hz was used to cleave the fibers. The laser pulse intensity was divided into nine groups to cleave fibers. Table 1 lists details of the output energy of the 193-nm ultraviolet (UV) laser, the energy of the linear cutting beam, and the pulse intensity. In this study, we obtained fibers with an energy density of 0.18–0.75 J/cm2, an energy density gradient of 0.05–0.08 J/cm2, and a laser spot area of ~3.9 × 10–5 cm2. Fig. 3 shows the time required for the pulse intensity when the fibers were fully cleaved. A decreasing linear relationship between the pulse intensity and time of cleaving fibers was found. With increasing pulse intensity, fiber cleaving time fundamentally presents a linear gradient, but when the pulse intensity is higher than 600 mJ/cm2, the decreasing trend became gradually slower and the time leveled off in the last three groups. In this study, the longest time for cleaving fibers was approximately 441 s and the shortest time 280 s when the repetition rate was 2 Hz. When the energy density was increased to 600 mJ/ cm2, the cleaving time was not significantly different and always close to 280 s. Fig. 4 shows the microscope images of fiber end-faces using nine pulse intensity groups. The optical images in Fig. 4(a) and (b) are the end-faces that were cleaved at ~ 185 and 280 mJ/cm2, respectively. The cleaving time of two groups was 441 and 421 s, respectively. Multiple rough faults, like a ladder, appeared in the surface, but the reason is not the same as in Fig. 2(b). In addition, it was found that some black residues that were accumulated by ablation were attached to the fibers’ cladding. The majority of the regions of end-faces were not smooth in those two groups. Moreover, Fig. 4(a) shows many raised streaks along the cleaving direction. The optical images in Fig 0.4(c)–(e) are the end-faces that were cleaved at ~350–530 mJ/cm2, and their cleaving times range from 380
3.3. Optimum repetition rate The repetition rate of the UV laser was also analyzed. During this process, the pulse intensity was controlled at ~600 mJ/cm2, and then the repetition rate was increased from 1 to 10 Hz successively to determine its influence on the quality of cleaving of end-faces. First, three different repetition rates 1, 5, and 10 Hz were selected to better discover the variation of cleaving effect by repetition rates. From this step, the time of cleaving fibers was one of obvious variation. The time to cleave fibers was 630, 130, and 67 s at 1, 5, and 10 Hz, respectively. All end-faces produced at various repetition rates are shown in Fig. 5. Fig. 3 shows a plot of time against different repetition rates. Fig. 5(a) shows the unsmooth end-face cleaved at 1 Hz. It was found that a longer fibercleaving time easily causes deformation of the fiber, which results in a deviation from the original cleaving direction. Therefore, a repetition rate of 1 Hz is not a suitable choice to cleave PS/PMMA fibers. Fig. 5(b) shows the fiber cleaved at 5 Hz. Although the fiber’s core is slick, the cladding was first exposed to a UV laser and exhibited severe burning
Table 1 Energy density of cleaving fibers at different output energies. Number
Output energy of laser (mJ)
Energy of liner cutting beam (μJ)
Pulse intensity of liner cutting beam (J/cm2)
1 2 3 4 5 6 7 8 9
120 108 96 84 72 56 46 34 22
29.1 27.2 25.3 23 20.7 17.6 14 10.9 7.2
0.744 0.696 0.647 0.588 0.529 0.450 0.358 0.279 0.184
3
Optical Fiber Technology 54 (2020) 102069
X. Luo, et al.
Fig. 3. Time required for a pulse intensity or a repetition before a full cleave is achieved.
the repetition rate does not damage the end-face. Finally, we compromised on a repetition rate of 3 Hz.
on account of local thermal accumulation. Higher repetition rates gave rise to the thermal buildup, which leads to excessive thermal accumulation and causes chipped cladding. Increasing the repetition rate to 10 Hz, although the cleaving time is only 67 s, half of the cladding was seriously damaged, as shown in Fig. 5(c). It is found from the images that the balance between cleaving time and end-face quality should be maintained. Therefore, the repetition rate should be controlled below 5 Hz. In addition, the fibers were cleaved at 2, 3, and 4 Hz, with the results shown in Fig. 5(d)–(f), respectively. Fig. 5(f) shows that the endface cladding was damaged by the UV laser. Fig. 5(d) and (e) shows that the end-faces of fibers are smooth and a small degree of damage was incurred by the cladding. From these optical images, it was found that the quality of cleaving fibers could be improved by setting the repetition rate at 2 or 3 Hz as much as possible. Considering the cutting time, the optimal repetition rate is 3 Hz. Although one might conclude that the effect of repetition rate on end-face smoothness of fiber is less than the energy density, high repetition rate can reduce the time required for cleaving fibers as long as
4. Conclusion In this article, we studied cleaving of a 125-μm PS/PMMA multimode polymer optical fiber with a 193-nm excimer laser. This work depended on the linear cutting beam system, which improved the uniformity of the pulse intensity and the quality of shaping. The optimal energy density range of cleaving fibers was determined to be 550–650 mJ/cm2 with an optimal repetition rate of 3 Hz. This study confirmed that 193-nm excimer laser cleaving can produce excellent end-faces in POFs. UV laser cleaving does not change the state of POF materials, so it is a good candidate for POFs cutting. Of course, for different types of POFs, the laser parameters are different. Moreover, the end-face cracks and upper surface damage of laser cleaving are worthy of attention. A nitrogen-purged environment could possibly positively impact this problem. Next, we plan to add a
Fig. 4. The microscope images of end-face which cleaved by different pulse intensity. 4
Optical Fiber Technology 54 (2020) 102069
X. Luo, et al.
Fig. 5. Microscope images of end-face cleaved at (a) 1 Hz, (b) 5 Hz, (c) 10 Hz, (d) 2 Hz, (e) 3 Hz, (f) 4 Hz.
nitrogen-purged environment and measure the insertion loss of fibers cleaved by 193-nm laser.
[9]
Acknowledgements [10]
This work was supported by the National Natural Science Foundation of China (No. 41627803) and the Major Research and Development program of Anhui Province of China (No. 1804a0802219).
[11]
[12]
References [13]
[1] W. Daum, J. Krauser, P.E. Zamzow, et al., Development of the polymer optical fiber (POF), POF — Polym. Opt. Fibers Data Commun. (2002), https://doi.org/10.1007/ 978-3-662-04861-0_3. [2] E. Pone, C. Dubois, N. Gu, et al., Drawing of the hollow all-polymer Bragg fibers, Optics Express. 14 (13) (2006) 5838–5852, https://doi.org/10.1364/oe.14.005838. [3] M.C.J. Large, L. Poladian, G.W. Barton, et al., Microstructured Polymer Optical Fibres, Springer, USA, 2008, pp. 165–179, , https://doi.org/10.1007/978-0-38768617-2_5. [4] Y. Shao, R. Cao, Y.K. Huang, et al., 112-Gb/s transmission over 100m of gradedindex plastic optical fiber for optical data center applications, Optical Fiber Communication Conference and Exposition, 2012, https://doi.org/10.1364/ofc. 2012.ow3j.5. [5] S. Teshima, H. Munekuni, S. Katsuta, Plastic optical fiber for automotive networks, Automot. Eng. (1991) 99, https://doi.org/10.4271/910896. [6] S. Liehr, P. Lenke, M. Wendt, et al., Polymer optical fiber sensors for distributed strain measurement and application in structural health monitoring, IEEE Sensors J. 9 (11) (2009) 1330–1338, https://doi.org/10.1109/jsen.2009.2018352. [7] C.A.F. Marques, G.D. Peng, D.J. Webb, Highly sensitive liquid level monitoring system utilizing polymer fiber Bragg gratings, Optics Express. 23 (5) (2015) 6058–6072, https://doi.org/10.1364/oe.23.006058. [8] Y. Koike, K. Koike, Progress in low-loss and high-bandwidth plastic optical fibers, J.
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Polym. Sci. Part B Polym. Phys. 49 (1) (2010) 2–17, https://doi.org/10.1002/polb. 22170. O. Abdi, K.C. Wong, T. Hassan, et al., Cleaving of solid single mode polymer optical fiber for strain sensor applications, Optics Commun. 282 (5) (2009) 856–861, https://doi.org/10.1016/j.optcom.2008.11.046. R. Oliveira, L. Bilro, R. Nogueira, Smooth end face termination of microstructured, graded-index, and step-index polymer optical fibers, Applied Optics. 54 (18) (2015) 5629, https://doi.org/10.1364/ao.54.005629. S.H. Law, J.D. Harvey, R.J. Kruhlak, et al., Cleaving of microstructured polymer optical fibres, Optics Commun. 258 (2) (2006) 193–202, https://doi.org/10.1016/j. optcom.2005.08.011. A. Stefani, K. Nielsen, H.K. Rasmussen, et al., Cleaving of TOPAS and PMMA microstructured polymer optical fibers: Core-shift and statistical quality optimization, Optics Commun. 285 (7) (2012) 1825–1833, https://doi.org/10.1016/j.optcom. 2011.12.033. S. Atakaramians, K. Cook, H. Ebendorff-Heidepriem, et al., Cleaving of extremely porous polymer fibers, IEEE Photon. J. 1 (6) (2010) 286–292, https://doi.org/10. 1109/jphot.2009.2038796. M.V.P. Ghirghi, V.P. Minkovich, A.G. Villegas, Polymer Optical Fiber Termination With Use of Liquid Nitrogen, IEEE Photon. Technol. Lett. 26 (5) (2014) 516–519, https://doi.org/10.1109/lpt.2013.2295885. S.H. Law, M.A.V. Eijkelenborg, G.W. Barton, et al., Cleaved end-face quality of microstructured polymer optical fibres, Opt. Commun. 265 (2) (2006) 513–520, https://doi.org/10.1016/j.optcom.2006.04.059. J. Canning, E. Buckley, N. Groothoff, et al., UV laser cleaving of air–polymer structured fibre, Opt. Commun. 202 (1–3) (2002) 139–143, https://doi.org/10. 1016/S0030-4018(01)01727-8. L.I. Yang-Long, W.P. Wang, Laser interaction with polymers and its applications, Electro-Optic Technol. Appl. 25 (2) (2010) 8–13, https://doi.org/10.3969/j.issn. 1673-1255.2010.02.003. L. Zhang, Q.H. Lou, Y.R. Wei, et al., Comparison of etching characteristics of polymers by 193 nm and 308 nm excimer laser radiation, Chinese J. Lasers 1 (2002) 25–28, https://doi.org/10.1002/mop.10502. T. Kajava, M. Kaivola, J. Turunen, et al., Excimer laser beam shaping using diffractive optics. Conference on Lasers and Electro-Optics Europe, 2000, Conference Digest, 2002, https://doi.org/10.1109/CLEOE.2000.910356.
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Cleaving of polymer optical fiber by 193-nm excimer laser a
Xianfeng Luo , Libing You a b c
b,⁎
a,c
, Le Luo , Xiaodong Fang
T
b
Institute of Intelligent Manufacturing, Hefei University of Technology, Hefei 230009, China Anhui Provincial Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China School of Electronic Science and Applied Physics, Hefei University of Technology, Hefei 230009, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cleaving Polymer optical fiber PS PMMA Laser cutting
A smooth end-face is difficult to obtain when cleaving a polymer optical fiber (POF) due to the complex mechanical properties of polymers. In this article, a cleaving method is reported, which use laser ablation of a 125μm solid multi-mode POF with a 193-nm ultraviolet laser. To achieve the best POF end-face quality, we designed a new beam shaping system that makes a 180 × 22-μm beam size. As a result, we found that an energy density between 550 and 600 mJ/cm2 and a repetition rate of 3 Hz were the optimal parameters cleaving the 125-μm solid multi-mode polymer optical fiber.
1. Introduction
close to (or slightly below) that of the fibers. Lower blade speeds, ranging between 0.7 and 0.5 mm/s were generally effective. Atakaramians et al. exploited a SD saw to cleave PMMA and TOPAS® cyclic olefin copolymer highly porous fibers developed for guiding terahertz radiation in 2009 [13]. But the results showed that the end-face of PMMA porous fibers was chipped along the direction of cleave and the end-face of TOPAS porous fibers was smeared due to the grinding action of the blade. Atakaramians et al also used an FIB with an ion beam current of 21 nA to cleave 400 μm PMMA POFs and achieved even better end-face [13]. Nevertheless, the cleaving time was nearly 17.5 h and FIB milling is very expensive. Ghirghi et al. presented a new method for preparing POF end-faces in 2014 [14] that included using liquid nitrogen or liquid nitrogen vapor to cool the fibers and breaking them using a special curved form. Canning et al. first developed a method of cleaving air-polymer structure fibers using 193-nm excimer laser in 2002 [16] and found that the repetition rate should be below 2 Hz to avoid thermal damage by laser irradiation. However, the cleaving time scales with repetition, and it was found that the intensity of 1.6 J/cm2 and 4 Hz gave a quality end-face. Atakaramians et al. also used a 193-nm excimer laser with an intensity of 312 or 425 mJ/cm2 to cleave the POFs, and two auxiliary rotary apparatuses at 3.6˚/s to reduce the damage by thermal buildup during the cleaving progress [13]. In this article, we first used a 193-nm excimer laser to cleave the multi-mode solid POFs with a PS core. The interaction between the 193nm excimer laser and polymers is known as ablation [17]. This course of action produces a photon decomposition reaction by projecting a laser beam on a POF. Increasing the pulse intensity to the threshold value, the majority of the photon absorption will cause etching and the
The first polymer optical fiber (POF) was developed by DuPont in 1968 [1]. There are many kinds of materials with which to fabricate POFs, but the most common materials are polymethyl methacrylate (PMMA), polystyrene (PS), and polycarbonate (PC) [2]. POFs have the same structure as quartz optical fibers with a core, cladding, and sometimes a jacket. However, Young’s modulus of POFs is almost 28 times lower than that of quartz [3]. Owing to this characteristic, POFs have many advantages, such as higher sensitivity, excellent flexibility, lower stiffness and higher thermo-optic coefficients. In addition, POFs are increasingly considered a potential alternative to quartz optical fibers in many fields. In view of these advantages, POFs were used in many fields, including short distance telecommunication [4], the automotive industry [5], biosensing, and strain sensing [6,7]. However the attenuation in POFs is higher than in conventional quartz optical fibers [8]. Therefore, a smooth end-face of POF is of critical significance to avoid excessive insertion loss. However, the materials of low stiffness limit the cleaving of POFs. The end-face of POF is jagged and mutilated scene by using a normal diamond blade. Several methods for preparing POF end-face have been reported, such as polishing the connector endface [9], hot-blade cleaving [10–12], focused ion beam (FIB) milling [9,13], using a semiconductor dicing (SD) saw [13], termination with use of liquid nitrogen and laser cutting [14]. Law et al. first used a hot blade to cleave POFs in 2006 [11,15], and found the effect of temperature and cutting-blade speed in the process. In addition, the best results were achieved when the POFs temperature was controlled between 70 ℃ and 80 ℃, and the blade temperature was
⁎
Corresponding author. E-mail address: [email protected] (L. You).
https://doi.org/10.1016/j.yofte.2019.102069 Received 11 March 2019; Received in revised form 30 October 2019; Accepted 30 October 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 54 (2020) 102069
X. Luo, et al.
Fig. 1. (a) The linear cutting beam shaping system, (b) A liner cutting beam and (c) The fiber.
rest will be transformed into heat [18]. After transformation, the etching residue was vaporized by the previously heat. Owing to the shortwave characteristics of the excimer laser, the high lateral resolution, and the larger energy of photons, the 193-nm laser beam is deemed to be an appropriate “knife”. In addition, the remaining material shows some thermal damage and produces sharp steps at the edge of the irradiated area. In experiments, a new optical system was designed to produce a smooth POF end-face and no additional processing was carried out on the fibers, such as heating/cooling the fibers or changing their mechanical structure.
180 × 22 μm. Fig. 1(b) shows a microscope image of the linear cutting beam that works on photographic paper. A charge-coupled-device (CCD) camera (NAVITAR) allows in situ monitoring of the entire process. Fig. 1(c) shows a CCD image of the fiber with a very flat incision when it was cut off. A three dimensional (3D) motorized platform (Zolix Instruments Co., Ltd) was used to precisely adjust the position of fibers. All end-face images were acquired by a microscope (OLYMPUS BX51M). The sides of fibers were glued to an aluminum block with a hole. At the same time, the fibers were suspended above the hole to avoid damage from the splashing of aluminum.
2. Instruments
3. Experiments
An argon-fluoride (ArF) excimer laser (Lambda Physik,Germany) was used in these experiments (repetition rate, 20 Hz; pulse energy, 200 mJ; pulse width, 25 ns; beam size, 24 × 10 mm2). The POF material is essentially PS and PMMA cladding with a diameter of 125 μm, and was purchased from Paradigm Optics. Fig. 1(a) shows the new linear cutting beam-shaping system. It consists of a light source, a collimating and beam expanding unit, a mask, an objective lens, and three reflectors. The output energy of the excimer laser is typically characterized by a nearly Gaussian distribution along the short axis and a flat-top distribution with Gaussian wings along the long axis [19]. Therefore, the output beam was reshaped by the collimating beam expanding unit and then the center area of the beam was intercepted by the mask to achieve a rectangular distribution along the long axis. The unit also can decrease the angle of divergence of the beam. An LMU-3 × -193 focusing objective lens (THORLABS, 3X, 193 nm, NA = 0.08) with a focal length of 49 mm was used to create a fine line beam to the fibers in the image plane area. In the image plane, the linear cutting beam has a more homogeneous energy distribution with steep and well-defined edges, and its size is approximately
All experiments were conducted at room temperature under atmospheric conditions. The length of the fibers was controlled at 10 mm and the distance of two bonding positions was controlled below 1 mm. The two bonding positions of fibers should be closed to avoid a slight shaking of fibers in the cleaving process. Before the best laser conditions for cleaving POFs were found, we investigated several factors that will affect the results. 3.1. Previous experiments From past experiments, it was found that variable beam widths have different cleaving effects. Fig. 2(a) shows the image that the fiber was cut off by a 180 × 40 μm beam. A large area of damage on the fiber upper surface with the width of beam is 40 μm. However, the situation did not arise when by using a 22 μm beam width of the beam in Fig. 1(c). Therefore, the beam width was maintained at nearly 22 μm to reduce this damage in experiments. Another important factor impacting on the quality of the end-face is the uniformity of the laser energy distribution. When the energy Fig. 2. Images of bad cleaves.
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to 326 s. From the images, the quality of end-faces is smoother than in Fig. 4(a) and (b). However, it can be clearly observed that the outer ring of Fig. 4(c) and (d) locates slightly lower than the center of the endface. There is also some slight thermal damage and a few distinct black dots. In addition, the images show that the color of cladding is deeper than that of the core. The reason for this phenomenon is that the materials comprising the core and cladding interact with the laser in different ways. The optical images of Fig. 4(f) and (g) are of the end-face that was cleaved at ~550–650 mJ/cm2, and the cleaving time is approximately 300 s. Those two groups have the best end-face quality, exhibit no black dots, and incurred minimal thermal damage. Compared to the other optical images of end-faces, the center of the end-faces is not seriously higher than the outer rings, and there are few residues on the end-faces. Unfortunately, an end crack in an end-face is shown in Fig. 4(g). The optical images in Fig. 4(h) and (i) are of the end-face cleaved at ~690 mJ/cm2, and the cleaving time is approximately 280 s. It can be obviously seen that the cladding of fibers is much thinner than before cleaving. Compared to other images, it was found that the extent of this situation is directly related to pulse intensity. Furthermore, a large area of cross-section appeared to damage the fiber’s core and a serious end crack formed in the end-face in Fig. 4(i). Interestingly, the cladding and core have different depths of color, and the cladding is slightly deeper, a possible solution for which could be the dissimilarity of the photochemical reaction between PS and PMMA with the laser. On one hand, when the pulse intensity is less than 280 mJ/cm2, the photon density is not sufficient to cause the complete decomposition of the PS/PMMA in the depth of light penetration; On the other hand, the part of the transformation thermal energy is not sufficient to cause the complete vaporization of the etched material. It gives rise to abundant black residues attached to the end-faces. Cleaving for excessive time leads to the deformation of fibers and the appearance of stepped end-faces. From the analysis of Fig. 4(a) and (b), it was found that pulse intensities below 300 mJ/cm2 cannot produce good end-faces of POFs. The images in Fig. 4(c)–(e) show a complete photochemical reaction between the laser and PS/PMMA. However, the amount of energy transformed to thermal energy is also not sufficient to vaporize the remaining materials, resulting in some black spots on the end-faces. Accordingly, a pulse intensity below 530 mJ/cm2 is also not suitable for cleaving. Above 650 mJ/cm2, the main reason that the brown cladding and upper cladding are thinner than the other side is the thermal build-up caused by excessive thermal excess during the cleaving process. Most of the end-faces actually exhibit such problems. Fig. 4(f) and 4(g) show good-quality end-faces, and we found that a suitable pulse intensity was one of the key factors in the experiments. The resulting images indicate that a pulse intensity of 550–650 mJ/cm2 is best to cleave PS/PMMA fibers.
distribution in the long axis is not uniform, it was found that the process of inconsistent cleaving speed leads to an imbalance stress on both sides, resulting in cleavage of an end-face that resembles a wave, as shown in Fig. 2(b). It is also important to keep the cleaving angle of fibers and laser beam at approximately ~90°. Fig. 2(c) shows the damage to nearly half the area of the end-face that caused a rough surface by oblique incidence. Moreover, when the cleaving process creates a larger axial stress in the fibers, it will consequently produce an end crack in the end face, which is another situation that reduces the tension of fibers and which should demand more attention during the cleaving process. Although this parameter is currently not measured, the smaller the axial stress, the lower the possibility of elastic deformation during the cleaving process.
3.2. Optimum energy density range All cleaving processes were carried out with an output energy of the 193-nm laser between 22 and 120 mJ, taking into account the result obtained by Canning et al., namely that the repetition rate should be below 2 Hz to avoid damaging the fiber end-face. In subsequent studies, a repetition rate of 2 Hz was used to cleave the fibers. The laser pulse intensity was divided into nine groups to cleave fibers. Table 1 lists details of the output energy of the 193-nm ultraviolet (UV) laser, the energy of the linear cutting beam, and the pulse intensity. In this study, we obtained fibers with an energy density of 0.18–0.75 J/cm2, an energy density gradient of 0.05–0.08 J/cm2, and a laser spot area of ~3.9 × 10–5 cm2. Fig. 3 shows the time required for the pulse intensity when the fibers were fully cleaved. A decreasing linear relationship between the pulse intensity and time of cleaving fibers was found. With increasing pulse intensity, fiber cleaving time fundamentally presents a linear gradient, but when the pulse intensity is higher than 600 mJ/cm2, the decreasing trend became gradually slower and the time leveled off in the last three groups. In this study, the longest time for cleaving fibers was approximately 441 s and the shortest time 280 s when the repetition rate was 2 Hz. When the energy density was increased to 600 mJ/ cm2, the cleaving time was not significantly different and always close to 280 s. Fig. 4 shows the microscope images of fiber end-faces using nine pulse intensity groups. The optical images in Fig. 4(a) and (b) are the end-faces that were cleaved at ~ 185 and 280 mJ/cm2, respectively. The cleaving time of two groups was 441 and 421 s, respectively. Multiple rough faults, like a ladder, appeared in the surface, but the reason is not the same as in Fig. 2(b). In addition, it was found that some black residues that were accumulated by ablation were attached to the fibers’ cladding. The majority of the regions of end-faces were not smooth in those two groups. Moreover, Fig. 4(a) shows many raised streaks along the cleaving direction. The optical images in Fig 0.4(c)–(e) are the end-faces that were cleaved at ~350–530 mJ/cm2, and their cleaving times range from 380
3.3. Optimum repetition rate The repetition rate of the UV laser was also analyzed. During this process, the pulse intensity was controlled at ~600 mJ/cm2, and then the repetition rate was increased from 1 to 10 Hz successively to determine its influence on the quality of cleaving of end-faces. First, three different repetition rates 1, 5, and 10 Hz were selected to better discover the variation of cleaving effect by repetition rates. From this step, the time of cleaving fibers was one of obvious variation. The time to cleave fibers was 630, 130, and 67 s at 1, 5, and 10 Hz, respectively. All end-faces produced at various repetition rates are shown in Fig. 5. Fig. 3 shows a plot of time against different repetition rates. Fig. 5(a) shows the unsmooth end-face cleaved at 1 Hz. It was found that a longer fibercleaving time easily causes deformation of the fiber, which results in a deviation from the original cleaving direction. Therefore, a repetition rate of 1 Hz is not a suitable choice to cleave PS/PMMA fibers. Fig. 5(b) shows the fiber cleaved at 5 Hz. Although the fiber’s core is slick, the cladding was first exposed to a UV laser and exhibited severe burning
Table 1 Energy density of cleaving fibers at different output energies. Number
Output energy of laser (mJ)
Energy of liner cutting beam (μJ)
Pulse intensity of liner cutting beam (J/cm2)
1 2 3 4 5 6 7 8 9
120 108 96 84 72 56 46 34 22
29.1 27.2 25.3 23 20.7 17.6 14 10.9 7.2
0.744 0.696 0.647 0.588 0.529 0.450 0.358 0.279 0.184
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Fig. 3. Time required for a pulse intensity or a repetition before a full cleave is achieved.
the repetition rate does not damage the end-face. Finally, we compromised on a repetition rate of 3 Hz.
on account of local thermal accumulation. Higher repetition rates gave rise to the thermal buildup, which leads to excessive thermal accumulation and causes chipped cladding. Increasing the repetition rate to 10 Hz, although the cleaving time is only 67 s, half of the cladding was seriously damaged, as shown in Fig. 5(c). It is found from the images that the balance between cleaving time and end-face quality should be maintained. Therefore, the repetition rate should be controlled below 5 Hz. In addition, the fibers were cleaved at 2, 3, and 4 Hz, with the results shown in Fig. 5(d)–(f), respectively. Fig. 5(f) shows that the endface cladding was damaged by the UV laser. Fig. 5(d) and (e) shows that the end-faces of fibers are smooth and a small degree of damage was incurred by the cladding. From these optical images, it was found that the quality of cleaving fibers could be improved by setting the repetition rate at 2 or 3 Hz as much as possible. Considering the cutting time, the optimal repetition rate is 3 Hz. Although one might conclude that the effect of repetition rate on end-face smoothness of fiber is less than the energy density, high repetition rate can reduce the time required for cleaving fibers as long as
4. Conclusion In this article, we studied cleaving of a 125-μm PS/PMMA multimode polymer optical fiber with a 193-nm excimer laser. This work depended on the linear cutting beam system, which improved the uniformity of the pulse intensity and the quality of shaping. The optimal energy density range of cleaving fibers was determined to be 550–650 mJ/cm2 with an optimal repetition rate of 3 Hz. This study confirmed that 193-nm excimer laser cleaving can produce excellent end-faces in POFs. UV laser cleaving does not change the state of POF materials, so it is a good candidate for POFs cutting. Of course, for different types of POFs, the laser parameters are different. Moreover, the end-face cracks and upper surface damage of laser cleaving are worthy of attention. A nitrogen-purged environment could possibly positively impact this problem. Next, we plan to add a
Fig. 4. The microscope images of end-face which cleaved by different pulse intensity. 4
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Fig. 5. Microscope images of end-face cleaved at (a) 1 Hz, (b) 5 Hz, (c) 10 Hz, (d) 2 Hz, (e) 3 Hz, (f) 4 Hz.
nitrogen-purged environment and measure the insertion loss of fibers cleaved by 193-nm laser.
[9]
Acknowledgements [10]
This work was supported by the National Natural Science Foundation of China (No. 41627803) and the Major Research and Development program of Anhui Province of China (No. 1804a0802219).
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