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Performance analysis of CO2 laser polished angled ribbon fiber
Optical Fiber Technology 33 (2017) 77–82
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Regular Articles
Performance analysis of CO2 laser polished angled ribbon fiber Ik-Bu Sohn a,⇑, Hun-Kook Choi a,e, Young-Chul Noh a, Man-Seop Lee b, Jin-Kyoung Oh c, Seong-min Kim c, Md. Shamim Ahsan d,⇑ a
Advanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology (GIST), 123 Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea Phoco Co. Ltd., KAIST Munji Campus, 103-6 Munji-dong, Yuseong-gu, Daejeon 34051, Republic of Korea c P-CUBE Co. Ltd., Gwangju Technopark, 958-3 Daechon-dong, Buk-gu, Gwangju 61008, Republic of Korea d Elecctronics and Communication Engineering Discipline, Khulna University, Khulna 9208, Bangladesh e Department of Photonic Engineering, Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 61452, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 28 August 2016 Accepted 6 November 2016
Keywords: CO2 laser polishing Angled ribbon fiber Insertion loss Return loss
a b s t r a c t This paper demonstrates CO2 laser assisted simultaneous polishing of angled ribbon fibers consisting eight set of optical fibers. The ribbon fibers were rotated vertically at an angle of 12° and polished by repetitive irradiation of CO2 laser beam at the end faces of the fibers. Compared to mechanically polished sharp edged angled fibers, CO2 laser polishing forms curve edged angled fibers. Increase in the curvature of the end faces of the ribbon fibers causes the increase of the fibers’ strength, which in turn represents great robustness against fiber connections with other devices. The CO2 laser polished angled fibers have great smoothness throughout the polished area. The smoothness of the fiber end faces have been controlled by varying the number of laser irradiation. After CO2 laser polishing, the average value of the fiber angle of the ribbon fibers is 8.28°. The laser polished ribbon fibers show low insertion and return losses when connecting with commercial optical communication devices. The proposed technique of polishing the angled ribbon fibers is highly replicable and reliable and thus suitable for commercial applications. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction A significant interest in optical communication was stimulated in the early 1960s with the discovery of the functional lasers. Optical communication using dielectric waveguides or optical fibers fabricated from glass to maintain the quality of the optical signal were proposed almost simultaneously by Kao & Hockhham [1] and Werts [2]. The advent of optical fibers enable us long distance guiding of optical signals using the concept of total internal reflection. In parallel with conventional core-clad based fiber, microstructured fibers such as photonic crystal fibers [3] and photonic band-gap fibers [4] have attracted many researchers. Since their discovery, optical fibers have shown promises in a wide range of application areas including optical communication [5–7], optical imaging [8], fiber lasers [9], optical sensors [10,11], and biomedical engineering [12,13]. During last several decades, lasers have been considered as a versatile tool for micro/nano-scale patterning of a large variety of materials including optical fibers. Due to the capability of high precision and debris free micro-machining of both transparent and non-transparent materials, pulsed lasers
⇑ Corresponding authors. E-mail addresses: [email protected] (I.-B. Sohn), [email protected] (M.S. Ahsan). http://dx.doi.org/10.1016/j.yofte.2016.11.009 1068-5200/Ó 2016 Elsevier Inc. All rights reserved.
have been considered as the most promising lasers [14–16]. Although the laser assisted micro-machining industry is primarily dominated by pulsed laser systems, the melting & polishing capabilities of the CO2 lasers have attracted many researchers as well as industry. CO2 lasers have shown their potential in micropatterning of transparent materials. Among various applications of CO2 lasers, some of the most significant applications include the formation of micro-lenses [17–20], diffraction gratings [21,22], microfluidic system [23], printed circuit board [24], and polishing transparent materials [25–27]. Optical fibers are simply a passive wave guide those require additional active/passive elements or devices to design a complete operating device or system. Ribbon fibers, consisting a group of optical fibers, are suitable to carry a large number of fibers together. These kinds of fibers have been used extensively in optical communication or sensors to interface optical fibers with various devices or other optical fibers. The interfaces among optical fibers and various optical/electronic devices introduce loss in the system. One of the critical issues arises when the reflection from the fiber-device interface causes any damage in the optical source or other active/passive elements. Researchers find ways to resolve this issue by polishing the end faces of the fibers. Mechanical polishing system is the most widely used technique in polishing opti-
78
I.-B. Sohn et al. / Optical Fiber Technology 33 (2017) 77–82
cal fibers. Mechanical polishing involves long processing time and complicated fabrication steps. In addition, the sharp end faces sometimes cause mechanical damage of fibers while connecting with other optical/electronic devices. As a result, the demand for a simple processing technique comes to the forefront of the research field that can not only reduce the processing time but also guarantee high quality angled fibers. CO2 lasers can be considered to overcome the shortcomings of mechanical polishing techniques for polishing optical fibers. Thus, the material processing ability and the strength of the CO2 laser sources deserve more investigation. In this paper, we demonstrate a novel technique of polishing ribbon fibers by CO2 laser assisted polishing technique. The sharp end faces of a 1 8 ribbon fiber have been polished several times to achieve curve edged optical fibers. For simultaneous polishing of all the fibers in a ribbon fiber, the ribbon fiber was rotated at an angle of 12° and CO2 laser beam was irradiated to polish the end faces of the fibers. After polishing, the fiber angle in all the eight fibers was almost constant. The curved end faces, induced by CO2 laser polishing, represent great robustness against fiber connections with other devices/fibers. The insertion and return losses of the CO2 laser polished angled fibers are comparable (in some cases better) to the losses induced in mechanically polished fibers. Because of the low roughness of the CO2 laser polished optical fibers, we expect significant reduction of light reflection from the end faces of the optical fibers. More importantly, CO2 laser based polishing system has reduced the polishing time from 5 to 10 min (required in mechanical polishing systems) to 10 s. The consistent results guarantee the reliability and stability of the proposed CO2 laser based polishing technology for polishing ribbon fibers as well as other optical fibers.
2. Experimental details To polish the end faces of the ribbon fibers, we utilized a pulsed CO2 laser (Coherent C-55L) operating at the central wavelength (k) of 10.6 lm and pulse repetition rate of 5 kHz. The pulse width of the CO2 laser beam was 120 ± 40 ls. The schematic diagram of the experimental setup is illustrated in Fig. 1(a). The CO2 laser beam having laser fluence of 142 J/cm2 was focused on top of the fibers’ tips by means of a galvanometer scanner (magnification: 80; focal length: 170 mm) and scanned at a scanning speed of 500 mm/s and a scanning step of 20 lm. The spot size of the CO2 laser beam at the focal point was 30 lm. Since the wavelength of the CO2 laser beam is invisible, it is hard to locate the exact position of the beam. To ensure proper polishing of all the optical fibers of the ribbon fiber, the CO2 laser beam was irradiated over an area of 3 mm 3 mm. Fig. 1(b) shows the schematic diagram of CO2 laser polishing process. As mentioned before, the ribbon fiber consists of 8 set of fibers, where the gap between the fibers was 125 lm. The core and cladding diameters of the optical fibers of the ribbon fiber were 8 lm and 125 lm. As a result, the ribbon fiber has an overall width of 1.875 mm excluding the buffer layer and jacket. The ribbon fiber was placed inside a mechanically rotatable holder that was placed on top of a 3-axis translation stage having resolution of 100 nm in the x, y, and z directions. For polishing, the fiber holder was rotated vertically at an angle of 12°, as shown in Fig. 1(c). The CO2 laser beam was irradiated 3 times (forward & backward) on the end faces of the optical fibers. To investigate the reliability & stability of the proposed polishing system, we repeated the same experiment in 25 different ribbon fibers. After CO2 laser polishing, the end faces of the polished fibers were examined by means of an optical microscope. The fiber angle of the optical fibers was measured using the software associated with the optical microscope
having accuracy in the range of ±0.1°. Furthermore, we measured the insertion and return losses of the optical fibers polished by CO2 laser polishing and mechanical polishing techniques using loss measurement and bonding equipment. The loss measurement meters were connected to the single optical fibers those were coupled & bonded with the angled, i.e. polished ribbon fibers.
3. Results and discussion As mentioned before, we curved the end faces of the sharply angled ribbon fibers by polishing the fibers by irradiating a CO2 laser beam having laser fluence of 142 J/cm2 at a scanning speed of 500 mm/s and a scanning step of 20 lm. The optical fibers in a ribbon fiber are arranged in a common plane. To form angled ribbon fibers, the ribbon fiber was perfectly aligned and rotated vertically by an angle of 12°, as shown in Fig. 1(c). Because of the invisible wavelength of the CO2 laser beam, precise focusing of the laser beam on the fiber surface is difficult. Simultaneous polishing of the fibers of a 1 8 ribbon fiber using a single CO2 laser beam was challenging as well. A slight mismatch in location of the fibers from the common plane may results in polishing at the wrong place or absence of polishing. We overcame this difficulty by irradiating the CO2 laser beam over a large area. CO2 laser beam was focused and irradiated 3 times (forward & backward) over an area of 3 mm 3 mm to polish the 1 8 ribbon fibers having overall width of 1.875 mm without jacket & buffer zone. Consequently, angled ribbon fibers having curved end faces were evolved, as depicted in Fig. 2(a). The fiber angles measured for the 1st and 8th fibers of 25 different 1 8 ribbon fibers after laser polishing are plotted in Fig. 2(b). From the experimental results it is evident that, the fiber angles of the ribbon fibers stay around 8.28°. The highest value of fiber angle for the 1st fiber of the 25 ribbon fibers was 8.33°, whereas, the highest value of fiber angle for the 8th fiber was 8.31°. In contrast, the lowest values of fiber angle for the 1st and 8th fiber of the 25 ribbon fibers were 8.03° and 7.97°. The variation in the fiber angles might have caused due to dislocation of optical fibers from the common plane. When any fiber or a group of fibers present in a ribbon fiber are slightly moved from their original plane, the CO2 laser beam may focus above or inside the fiber tip. As a result, the laser energy at the focal point of the laser beam impinged on a fiber tip may vary, which reason might be responsible for the inconsistency in fiber angle. If all the fibers are placed in the same plane, we can avoid such variations in fiber angle. In order to investigate the smoothness of the polished fiber ends, we captured the top view images of the fiber tips by means of an optical microscope. Fig. 3(a) represents the optical microscope images of the end faces of all the fibers present in an 8chanel ribbon fiber after CO2 laser polishing. The optical microscope images exemplify the smoothness of the fiber tips. Fig. 3(b) depicts the arrangements of optical fibers in a ribbon fiber. Compared to the sharp edge of the mechanically polished angled fibers, CO2 laser polished angled fibers show curved edges. Fig. 3(c) explains the difference between mechanical polishing technique and CO2 laser assisted polishing technique. In order to compare the surface roughness of the ribbon fibers with the roughness of various mechanically polished fibers, we polished some optical fibers using mechanical polishing technique. The optical microscope images of the mechanically polished fibers are presented in Fig. 4. Compared to the roughness of the mechanical polished fibers (shown in Fig. 4), the CO2 laser polished ribbon fibers (shown in Fig. 3(a)) show better surface quality. We also examined various losses such as insertion loss and return loss while incident light passed through the CO2 laser polished and mechanically polished ribbon fibers. The diagrams of
I.-B. Sohn et al. / Optical Fiber Technology 33 (2017) 77–82
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Fig. 1. (a) Schematic diagram of the experimental setup; (b) CO2 laser polishing process; (c) CO2 laser polishing of ribbon fibers.
Fig. 2. Fiber angle for different fibers of a ribbon fiber after 3 times (forward & backward) CO2 laser polishing at a rotation angle of 12°. (a) Optical microscope images of the 3rd, 5th, and 7th fibers of a 1 8 ribbon fiber; (b) plot of fiber angles after CO2 laser polishing for 25 ribbon fibers.
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Fig. 3. (a) Optical microscope images of the end surfaces of all the fibers of a 1 8 ribbon fiber after CO2 laser polishing; (b) arrangement of fibers in a ribbon fiber; (c) comparison of mechanical polishing and CO2 laser polishing.
Fig. 4. Optical microscope images of the end faces of various optical fibers after mechanical polishing.
the loss measurement and bonding equipment are illustrated in Fig. 5. The first step of loss measurement of ribbon fibers is to couple each fiber of a ribbon fiber with a single optical fiber, the schematic diagram of which is depicted in Fig. 5(a). Each angled fiber of the ribbon fiber and another single optical fiber was placed inside the splitter at the bottom of a cover glass for coupling. The coupling process is illustrated in Fig. 5(c). Fig. 5(b) shows the images of a coupled 1 8 ribbon fiber with 8 single optical fibers. The second step for loss measurement is the bonding of the coupled fibers. During our experiments, we utilized epoxy bonding for the coupled devices; the bonding process is shown in Fig. 5(e). The final step is to connect the single fibers, coupled with the ribbon fiber, with the loss measurement equipment shown in Fig. 5(d). The loss measurement device measured the values of the insertion and return
losses of the polished fibers present in the 1 8 ribbon fiber and displayed the values, as represented in Fig. 5(f). This process was applied to measure the insertion and return losses of both the CO2 laser polished and mechanically polished ribbon fibers. Table 1 summarizes the insertion and return losses of the fibers present in 8-channel ribbon fibers polished by mechanical and CO2 laser based techniques. From the experimental results of Table 1, we find that the average losses in the CO2 laser polished angled fibers are almost same as the average losses involved in the mechanically polished angled ribbon fibers. These results indicate the potential of the CO2 laser polished ribbon fibers in optical communication and other optical sensing devices. From our experimental results, it is evident that the CO2 lasers can be utilized to polish the end surfaces of optical fibers to achieve curve edged ribbon fibers. The fiber angle of the
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Fig. 5. Loss measurement, coupling, and bonding technology of the ribbon fibers. (a) Schematic diagram of the coupling process of angled ribbon fibers with single optical fibers; (b) photograph of the coupled fibers; (c) photograph of the loss measurement and bonding equipment; (d) coupling process of the ribbon fiber with single optical fibers; (e) insertion loss and return loss measurements of the angled ribbon fibers; (f) bonding process of the coupled ribbon fiber attached with single optical fibers.
Table 1 Comparison of energy losses in the angled ribbon fibers polished using mechanical and CO2 laser polishing technique. Mechanical Technique (Unit: dB)
CO2 Laser Technique (Unit: dB)
Fiber Number
Return Loss
Insertion Loss
Fiber Number
Return Loss
Insertion Loss
1 2 3 4 5 6 7 8 Average
53.6 52.6 52.2 50.7 54.9 50.5 48.0 51.7 51.8
9.87 9.89 9.93 9.98 9.93 9.92 10.06 9.93 9.92
1 2 3 4 5 6 7 8 Average
54.2 60.7 42.5 58.7 47.3 52.6 55.9 54.5 53.3
10.07 10.07 9.68 9.87 9.90 9.95 10.00 9.89 9.93
optical fibers is controllable, which is directly influenced by the rotation angle and the number of polishing. Besides, CO2 laser processing can polish the fiber samples precisely. The CO2 laser polishing technology is consistent and reliable that can produce similar results in a large number of samples (Fig. 2(b)) indicating the applicability of the proposed CO2 laser assisted polishing technique in
commercial or industrial applications. The proposed CO2 laser polishing technique is advantageous over the conventional mechanical polishing technique due to several reasons. Firstly, CO2 laser polishing can produce smooth surfaces with lower roughness compared to mechanical polishing, which is obvious from the optical microscope images of Figs. 3(a) and 4. Secondly, the curved surface
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of the optical fibers, formed by means of CO2 laser polishing, has reduced the chance of mechanical damage as oppose to mechanical polishing induced sharp edged angled fibers while connecting the ribbon fibers with splitters or other connecting devices. Finally, the processing time has reduced significantly when CO2 laser beam is used to produce angled fibers. For a 1 8 ribbon fiber, we need only 10 s to polish the end faces of the fibers and develop angled ribbon fibers. For the same ribbon fiber, we require at least 10 min to polish using conventional mechanical approach, where the fiber angle and surface roughness parameters are inconsistent. Consequently, CO2 laser beam assisted polishing technique provides high throughput. On the contrary, the difference in losses (insertion loss and return loss) between the CO2 laser polished and mechanically polished ribbon fibers are negligible. These results confirm the applicability of the CO2 laser polished angled ribbon fibers in various applications including optical communication and sensing. 4. Conclusion In summary, by utilizing CO2 laser polishing technique we fabricated curve edged angled ribbon fibers. Before polishing, the optical fibers were rotated at an angle of 12° using the rotatable holder. 25 samples of 8-channel ribbon fibers were polished using CO2 laser beam where the average end face angle of the ribbon fibers was 8.28°. The surface of the CO2 laser polished fibers show great consistency in smoothness. Experimental results confirmed the reliability, stability, and preciseness of the proposed CO2 laser based polishing technique. Compared to mechanically polished sharp edged angled ribbon fibers, the CO2 laser polished curve edged angled fibers have lower probability of mechanical damage while connecting with various optical devices because of their curved structure. The insertion and return losses of the CO2 laser polished angled ribbon fibers were comparable with the losses observed in the mechanically polished angled ribbon fibers. We strongly believe that, the proposed CO2 laser polished angled fibers will have dynamic applications in optical communication and sensing. Acknowledgments This research work was supported by the GIST Research Institute (GRI), Republic of Korea in 2016. This work was also supported by Bio-industry Technology Development Program, Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry, and Fisheries, Republic of Korea. References [1] K.C. Kao, G.A. Hockham, Dielectric fiber surface waveguides for optical frequencies, Proc. IEE 113 (1966) 1151–1158. [2] A. Werts, Propagation de la lumière cohérente dans les fibers optiques, L’Onde Eleztrique 46 (1966) 967–980.
[3] K. Morishita, Y. Miyake, Fabrication and resonance wavelengths of long-period gratings written in a pure-silica photonic crystal fiber by the glass structure change, IEEE J. Lightwave Technol. 22 (2004) 625–630. [4] Y. Li, C.Y. Wang, M. Hu, B. Liu, X. Sun, L. Chai, Photonic bandgap fibers based on a composite honeycomb lattice, IEEE Photon. Technol. Lett. 18 (2006) 262–264. [5] M.S. Ahsan, M.S. Lee, S.H. Shah Newaz, Migration to the next generation optical access networks using hybrid WDM/TDM-PON, J. Netw. 6 (2011) 18–25. [6] M.J. O’Mahony, C. Politi, D. Klonidis, R. Nejabati, D. Simeonidou, Future optical networks, IEEE J. Lightwave Technol. 24 (2006) 4684–4696. [7] L. Cognolato, Chemical vapour deposition for optical fibre technology, J. Phys. IV 05 (1995), C5-975–C5-987. [8] H.Y. Choi, S.Y. Ryu, J. Na, B.H. Lee, I.B. Sohn, Y.C. Noh, J.M. Lee, Single-body lensed photonic crystal fibers as side-viewing probes for optical imaging systems, Opt. Lett. 33 (2008) 34–36. [9] M. Hofer, M.H. Ober, F. Haberl, M.E. Fermann, Characterization of ultrashort pulse formation in passively mode-locked fiber lasers, IEEE J. Quantum Electron. 28 (1992) 720–728. [10] J.K. Lee, Investigation of sound pressure detection of fiber optic sensor in transformer oil according to TLS and CW laser source, J. Acoust. Soc. Korea 30 (2011) 33–41. [11] K.H. Lee, B.J. Ahn, D.H. Kim, Fiber optic displacement sensor system for structural health monitoring, J. Korea Soc. Nondestr. Test. 31 (2011) 374–381. [12] I.B. Sohn, Y.S. Kim, Y.C. Noh, I.W. Lee, J.K. Kim, H. Lee, Femtosecond laser and arc discharge induced microstructuring on optical fiber tip for the multidirectional firing, Opt. Express 18 (2010) 19755–19760. [13] J.K. Kim, J.K. Kim, K.H. Oh, I.B. Sohn, W. Shin, H.Y. Choi, B. Lee, Fabrication of micro Fresnel zone plate lens on a mode-expanded hybrid optical fiber using a femtosecond laser ablation system, IEEE Photon. Technol. Lett. 21 (2009) 21– 23. [14] I.B. Sohn, Y.S. Kim, Y.C. Noh, J.C. Ryu, J.T. Kim, Microstructuring of optical fiber using a femtosecond laser, J. Opt. Soc. Korea 13 (2009) 33–36. [15] M.S. Ahsan, M.S. Lee, Colorizing mechanism of brass surface by femtosecond laser induced microstructures, OPTIK 124 (2013) 3631–3635. [16] I.B. Sohn, M.S. Ahsan, Y.C. Noh, H.K. Choi, J.T. Kim, M.J. Ko, Fabrication of Fresnel zone plate lens in fused silica glass using femtosecond laser lithography technology, Opt. Eng. 53 (2014) 055107. [17] H.K. Choi, M.S. Ahsan, D. Yoo, I.B. Sohn, Y.C. Noh, J.T. Kim, D. Jung, J.H. Kim, H. M. Kang, Formation of cylindrical micro-lens array on fused silica glass surface using CO2 laser assisted reshaping technique, Opt. Laser Technol. 75 (2015) 63–70. [18] M. Wakaki, Y. Komachi, G. Kanai, Microlenses and microlens arrays formed on a glass plate by use of a CO2 laser, Appl. Opt. 37 (1998) 627–631. [19] H.K. Choi, D. Yoo, I.B. Sohn, Y.C. Noh, J.H. Sung, S.K. Lee, T.M. Jeong, M.S. Ahsan, J.T. Kim, CO2 laser assisted fabrication of micro-lensed single-mode optical fiber, J. Opt. Soc. Korea 19 (2015) 327–333. [20] S. Calixto, M.R. Aguilar, F.J.S. Martin, L.C. Escobar, Rod and spherical silica microlenses fabricated by CO2 laser polishing, Appl. Opt. 44 (2005) 4547– 4556. [21] H.H. Choi, I.B. Sohn, Y.C. Noh, Y.T. Lee, J.T. Kim, M.S. Ahsan, Femtosecond-laserassisted fabrication of double-layer diffraction gratings inside fused silica glass, J. Korean Phys. Soc. 66 (2015) 55–60. [22] H.K. Choi, D. Jung, I.B. Sohn, Y.C. Noh, Y.T. Lee, J.T. Kim, M.S. Ahsan, Diffraction efficiency enhancement of femtosecond laser-engraved diffraction gratings due to CO2 laser polishing, J. Korean Phys. Soc. 65 (2014) 1559–1565. [23] H. Klank, J.P. Kutter, O. Geschke, CO2-laser micromachining and back-end processing for rapid production of PMMA-based microfluidic systems, Lab Chip 2 (2002) 242–246. [24] C.J. Moorhouse, F. Villarreal, J.J. Wendland, H.J. Baker, D.R. Hall, D.P. Hand, CO2 laser processing of alumina (Al2O3) printed circuit board substrates, IEEE Trans. Electron. Packag. Manuf. 28 (2005) 249–258. [25] D. Jung, I.B. Sohn, Y.C. Noh, C.H. Kim, H. Lee, Optical fiber polishing with CO2 laser technology, Korean Soc. Precis. Eng. 3 (2011) 99–100. [26] M. Udera, H. Arun, A. Alacakir, Laser polishing of fiber end surface, Opt. Eng. 40 (2001) 2026–2030. [27] H.K. Choi, D. Jung, I.B. Sohn, Y.C. Noh, Y.T. Lee, M.S. Lee, J.K. Oh, H.J. Lee, Fabrication and characterization of angled ribbon fibers using a CO2 laser, Proc. COIN 2014 (2014), TP-05.
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Regular Articles
Performance analysis of CO2 laser polished angled ribbon fiber Ik-Bu Sohn a,⇑, Hun-Kook Choi a,e, Young-Chul Noh a, Man-Seop Lee b, Jin-Kyoung Oh c, Seong-min Kim c, Md. Shamim Ahsan d,⇑ a
Advanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology (GIST), 123 Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea Phoco Co. Ltd., KAIST Munji Campus, 103-6 Munji-dong, Yuseong-gu, Daejeon 34051, Republic of Korea c P-CUBE Co. Ltd., Gwangju Technopark, 958-3 Daechon-dong, Buk-gu, Gwangju 61008, Republic of Korea d Elecctronics and Communication Engineering Discipline, Khulna University, Khulna 9208, Bangladesh e Department of Photonic Engineering, Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 61452, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 28 August 2016 Accepted 6 November 2016
Keywords: CO2 laser polishing Angled ribbon fiber Insertion loss Return loss
a b s t r a c t This paper demonstrates CO2 laser assisted simultaneous polishing of angled ribbon fibers consisting eight set of optical fibers. The ribbon fibers were rotated vertically at an angle of 12° and polished by repetitive irradiation of CO2 laser beam at the end faces of the fibers. Compared to mechanically polished sharp edged angled fibers, CO2 laser polishing forms curve edged angled fibers. Increase in the curvature of the end faces of the ribbon fibers causes the increase of the fibers’ strength, which in turn represents great robustness against fiber connections with other devices. The CO2 laser polished angled fibers have great smoothness throughout the polished area. The smoothness of the fiber end faces have been controlled by varying the number of laser irradiation. After CO2 laser polishing, the average value of the fiber angle of the ribbon fibers is 8.28°. The laser polished ribbon fibers show low insertion and return losses when connecting with commercial optical communication devices. The proposed technique of polishing the angled ribbon fibers is highly replicable and reliable and thus suitable for commercial applications. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction A significant interest in optical communication was stimulated in the early 1960s with the discovery of the functional lasers. Optical communication using dielectric waveguides or optical fibers fabricated from glass to maintain the quality of the optical signal were proposed almost simultaneously by Kao & Hockhham [1] and Werts [2]. The advent of optical fibers enable us long distance guiding of optical signals using the concept of total internal reflection. In parallel with conventional core-clad based fiber, microstructured fibers such as photonic crystal fibers [3] and photonic band-gap fibers [4] have attracted many researchers. Since their discovery, optical fibers have shown promises in a wide range of application areas including optical communication [5–7], optical imaging [8], fiber lasers [9], optical sensors [10,11], and biomedical engineering [12,13]. During last several decades, lasers have been considered as a versatile tool for micro/nano-scale patterning of a large variety of materials including optical fibers. Due to the capability of high precision and debris free micro-machining of both transparent and non-transparent materials, pulsed lasers
⇑ Corresponding authors. E-mail addresses: [email protected] (I.-B. Sohn), [email protected] (M.S. Ahsan). http://dx.doi.org/10.1016/j.yofte.2016.11.009 1068-5200/Ó 2016 Elsevier Inc. All rights reserved.
have been considered as the most promising lasers [14–16]. Although the laser assisted micro-machining industry is primarily dominated by pulsed laser systems, the melting & polishing capabilities of the CO2 lasers have attracted many researchers as well as industry. CO2 lasers have shown their potential in micropatterning of transparent materials. Among various applications of CO2 lasers, some of the most significant applications include the formation of micro-lenses [17–20], diffraction gratings [21,22], microfluidic system [23], printed circuit board [24], and polishing transparent materials [25–27]. Optical fibers are simply a passive wave guide those require additional active/passive elements or devices to design a complete operating device or system. Ribbon fibers, consisting a group of optical fibers, are suitable to carry a large number of fibers together. These kinds of fibers have been used extensively in optical communication or sensors to interface optical fibers with various devices or other optical fibers. The interfaces among optical fibers and various optical/electronic devices introduce loss in the system. One of the critical issues arises when the reflection from the fiber-device interface causes any damage in the optical source or other active/passive elements. Researchers find ways to resolve this issue by polishing the end faces of the fibers. Mechanical polishing system is the most widely used technique in polishing opti-
78
I.-B. Sohn et al. / Optical Fiber Technology 33 (2017) 77–82
cal fibers. Mechanical polishing involves long processing time and complicated fabrication steps. In addition, the sharp end faces sometimes cause mechanical damage of fibers while connecting with other optical/electronic devices. As a result, the demand for a simple processing technique comes to the forefront of the research field that can not only reduce the processing time but also guarantee high quality angled fibers. CO2 lasers can be considered to overcome the shortcomings of mechanical polishing techniques for polishing optical fibers. Thus, the material processing ability and the strength of the CO2 laser sources deserve more investigation. In this paper, we demonstrate a novel technique of polishing ribbon fibers by CO2 laser assisted polishing technique. The sharp end faces of a 1 8 ribbon fiber have been polished several times to achieve curve edged optical fibers. For simultaneous polishing of all the fibers in a ribbon fiber, the ribbon fiber was rotated at an angle of 12° and CO2 laser beam was irradiated to polish the end faces of the fibers. After polishing, the fiber angle in all the eight fibers was almost constant. The curved end faces, induced by CO2 laser polishing, represent great robustness against fiber connections with other devices/fibers. The insertion and return losses of the CO2 laser polished angled fibers are comparable (in some cases better) to the losses induced in mechanically polished fibers. Because of the low roughness of the CO2 laser polished optical fibers, we expect significant reduction of light reflection from the end faces of the optical fibers. More importantly, CO2 laser based polishing system has reduced the polishing time from 5 to 10 min (required in mechanical polishing systems) to 10 s. The consistent results guarantee the reliability and stability of the proposed CO2 laser based polishing technology for polishing ribbon fibers as well as other optical fibers.
2. Experimental details To polish the end faces of the ribbon fibers, we utilized a pulsed CO2 laser (Coherent C-55L) operating at the central wavelength (k) of 10.6 lm and pulse repetition rate of 5 kHz. The pulse width of the CO2 laser beam was 120 ± 40 ls. The schematic diagram of the experimental setup is illustrated in Fig. 1(a). The CO2 laser beam having laser fluence of 142 J/cm2 was focused on top of the fibers’ tips by means of a galvanometer scanner (magnification: 80; focal length: 170 mm) and scanned at a scanning speed of 500 mm/s and a scanning step of 20 lm. The spot size of the CO2 laser beam at the focal point was 30 lm. Since the wavelength of the CO2 laser beam is invisible, it is hard to locate the exact position of the beam. To ensure proper polishing of all the optical fibers of the ribbon fiber, the CO2 laser beam was irradiated over an area of 3 mm 3 mm. Fig. 1(b) shows the schematic diagram of CO2 laser polishing process. As mentioned before, the ribbon fiber consists of 8 set of fibers, where the gap between the fibers was 125 lm. The core and cladding diameters of the optical fibers of the ribbon fiber were 8 lm and 125 lm. As a result, the ribbon fiber has an overall width of 1.875 mm excluding the buffer layer and jacket. The ribbon fiber was placed inside a mechanically rotatable holder that was placed on top of a 3-axis translation stage having resolution of 100 nm in the x, y, and z directions. For polishing, the fiber holder was rotated vertically at an angle of 12°, as shown in Fig. 1(c). The CO2 laser beam was irradiated 3 times (forward & backward) on the end faces of the optical fibers. To investigate the reliability & stability of the proposed polishing system, we repeated the same experiment in 25 different ribbon fibers. After CO2 laser polishing, the end faces of the polished fibers were examined by means of an optical microscope. The fiber angle of the optical fibers was measured using the software associated with the optical microscope
having accuracy in the range of ±0.1°. Furthermore, we measured the insertion and return losses of the optical fibers polished by CO2 laser polishing and mechanical polishing techniques using loss measurement and bonding equipment. The loss measurement meters were connected to the single optical fibers those were coupled & bonded with the angled, i.e. polished ribbon fibers.
3. Results and discussion As mentioned before, we curved the end faces of the sharply angled ribbon fibers by polishing the fibers by irradiating a CO2 laser beam having laser fluence of 142 J/cm2 at a scanning speed of 500 mm/s and a scanning step of 20 lm. The optical fibers in a ribbon fiber are arranged in a common plane. To form angled ribbon fibers, the ribbon fiber was perfectly aligned and rotated vertically by an angle of 12°, as shown in Fig. 1(c). Because of the invisible wavelength of the CO2 laser beam, precise focusing of the laser beam on the fiber surface is difficult. Simultaneous polishing of the fibers of a 1 8 ribbon fiber using a single CO2 laser beam was challenging as well. A slight mismatch in location of the fibers from the common plane may results in polishing at the wrong place or absence of polishing. We overcame this difficulty by irradiating the CO2 laser beam over a large area. CO2 laser beam was focused and irradiated 3 times (forward & backward) over an area of 3 mm 3 mm to polish the 1 8 ribbon fibers having overall width of 1.875 mm without jacket & buffer zone. Consequently, angled ribbon fibers having curved end faces were evolved, as depicted in Fig. 2(a). The fiber angles measured for the 1st and 8th fibers of 25 different 1 8 ribbon fibers after laser polishing are plotted in Fig. 2(b). From the experimental results it is evident that, the fiber angles of the ribbon fibers stay around 8.28°. The highest value of fiber angle for the 1st fiber of the 25 ribbon fibers was 8.33°, whereas, the highest value of fiber angle for the 8th fiber was 8.31°. In contrast, the lowest values of fiber angle for the 1st and 8th fiber of the 25 ribbon fibers were 8.03° and 7.97°. The variation in the fiber angles might have caused due to dislocation of optical fibers from the common plane. When any fiber or a group of fibers present in a ribbon fiber are slightly moved from their original plane, the CO2 laser beam may focus above or inside the fiber tip. As a result, the laser energy at the focal point of the laser beam impinged on a fiber tip may vary, which reason might be responsible for the inconsistency in fiber angle. If all the fibers are placed in the same plane, we can avoid such variations in fiber angle. In order to investigate the smoothness of the polished fiber ends, we captured the top view images of the fiber tips by means of an optical microscope. Fig. 3(a) represents the optical microscope images of the end faces of all the fibers present in an 8chanel ribbon fiber after CO2 laser polishing. The optical microscope images exemplify the smoothness of the fiber tips. Fig. 3(b) depicts the arrangements of optical fibers in a ribbon fiber. Compared to the sharp edge of the mechanically polished angled fibers, CO2 laser polished angled fibers show curved edges. Fig. 3(c) explains the difference between mechanical polishing technique and CO2 laser assisted polishing technique. In order to compare the surface roughness of the ribbon fibers with the roughness of various mechanically polished fibers, we polished some optical fibers using mechanical polishing technique. The optical microscope images of the mechanically polished fibers are presented in Fig. 4. Compared to the roughness of the mechanical polished fibers (shown in Fig. 4), the CO2 laser polished ribbon fibers (shown in Fig. 3(a)) show better surface quality. We also examined various losses such as insertion loss and return loss while incident light passed through the CO2 laser polished and mechanically polished ribbon fibers. The diagrams of
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Fig. 1. (a) Schematic diagram of the experimental setup; (b) CO2 laser polishing process; (c) CO2 laser polishing of ribbon fibers.
Fig. 2. Fiber angle for different fibers of a ribbon fiber after 3 times (forward & backward) CO2 laser polishing at a rotation angle of 12°. (a) Optical microscope images of the 3rd, 5th, and 7th fibers of a 1 8 ribbon fiber; (b) plot of fiber angles after CO2 laser polishing for 25 ribbon fibers.
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Fig. 3. (a) Optical microscope images of the end surfaces of all the fibers of a 1 8 ribbon fiber after CO2 laser polishing; (b) arrangement of fibers in a ribbon fiber; (c) comparison of mechanical polishing and CO2 laser polishing.
Fig. 4. Optical microscope images of the end faces of various optical fibers after mechanical polishing.
the loss measurement and bonding equipment are illustrated in Fig. 5. The first step of loss measurement of ribbon fibers is to couple each fiber of a ribbon fiber with a single optical fiber, the schematic diagram of which is depicted in Fig. 5(a). Each angled fiber of the ribbon fiber and another single optical fiber was placed inside the splitter at the bottom of a cover glass for coupling. The coupling process is illustrated in Fig. 5(c). Fig. 5(b) shows the images of a coupled 1 8 ribbon fiber with 8 single optical fibers. The second step for loss measurement is the bonding of the coupled fibers. During our experiments, we utilized epoxy bonding for the coupled devices; the bonding process is shown in Fig. 5(e). The final step is to connect the single fibers, coupled with the ribbon fiber, with the loss measurement equipment shown in Fig. 5(d). The loss measurement device measured the values of the insertion and return
losses of the polished fibers present in the 1 8 ribbon fiber and displayed the values, as represented in Fig. 5(f). This process was applied to measure the insertion and return losses of both the CO2 laser polished and mechanically polished ribbon fibers. Table 1 summarizes the insertion and return losses of the fibers present in 8-channel ribbon fibers polished by mechanical and CO2 laser based techniques. From the experimental results of Table 1, we find that the average losses in the CO2 laser polished angled fibers are almost same as the average losses involved in the mechanically polished angled ribbon fibers. These results indicate the potential of the CO2 laser polished ribbon fibers in optical communication and other optical sensing devices. From our experimental results, it is evident that the CO2 lasers can be utilized to polish the end surfaces of optical fibers to achieve curve edged ribbon fibers. The fiber angle of the
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Fig. 5. Loss measurement, coupling, and bonding technology of the ribbon fibers. (a) Schematic diagram of the coupling process of angled ribbon fibers with single optical fibers; (b) photograph of the coupled fibers; (c) photograph of the loss measurement and bonding equipment; (d) coupling process of the ribbon fiber with single optical fibers; (e) insertion loss and return loss measurements of the angled ribbon fibers; (f) bonding process of the coupled ribbon fiber attached with single optical fibers.
Table 1 Comparison of energy losses in the angled ribbon fibers polished using mechanical and CO2 laser polishing technique. Mechanical Technique (Unit: dB)
CO2 Laser Technique (Unit: dB)
Fiber Number
Return Loss
Insertion Loss
Fiber Number
Return Loss
Insertion Loss
1 2 3 4 5 6 7 8 Average
53.6 52.6 52.2 50.7 54.9 50.5 48.0 51.7 51.8
9.87 9.89 9.93 9.98 9.93 9.92 10.06 9.93 9.92
1 2 3 4 5 6 7 8 Average
54.2 60.7 42.5 58.7 47.3 52.6 55.9 54.5 53.3
10.07 10.07 9.68 9.87 9.90 9.95 10.00 9.89 9.93
optical fibers is controllable, which is directly influenced by the rotation angle and the number of polishing. Besides, CO2 laser processing can polish the fiber samples precisely. The CO2 laser polishing technology is consistent and reliable that can produce similar results in a large number of samples (Fig. 2(b)) indicating the applicability of the proposed CO2 laser assisted polishing technique in
commercial or industrial applications. The proposed CO2 laser polishing technique is advantageous over the conventional mechanical polishing technique due to several reasons. Firstly, CO2 laser polishing can produce smooth surfaces with lower roughness compared to mechanical polishing, which is obvious from the optical microscope images of Figs. 3(a) and 4. Secondly, the curved surface
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of the optical fibers, formed by means of CO2 laser polishing, has reduced the chance of mechanical damage as oppose to mechanical polishing induced sharp edged angled fibers while connecting the ribbon fibers with splitters or other connecting devices. Finally, the processing time has reduced significantly when CO2 laser beam is used to produce angled fibers. For a 1 8 ribbon fiber, we need only 10 s to polish the end faces of the fibers and develop angled ribbon fibers. For the same ribbon fiber, we require at least 10 min to polish using conventional mechanical approach, where the fiber angle and surface roughness parameters are inconsistent. Consequently, CO2 laser beam assisted polishing technique provides high throughput. On the contrary, the difference in losses (insertion loss and return loss) between the CO2 laser polished and mechanically polished ribbon fibers are negligible. These results confirm the applicability of the CO2 laser polished angled ribbon fibers in various applications including optical communication and sensing. 4. Conclusion In summary, by utilizing CO2 laser polishing technique we fabricated curve edged angled ribbon fibers. Before polishing, the optical fibers were rotated at an angle of 12° using the rotatable holder. 25 samples of 8-channel ribbon fibers were polished using CO2 laser beam where the average end face angle of the ribbon fibers was 8.28°. The surface of the CO2 laser polished fibers show great consistency in smoothness. Experimental results confirmed the reliability, stability, and preciseness of the proposed CO2 laser based polishing technique. Compared to mechanically polished sharp edged angled ribbon fibers, the CO2 laser polished curve edged angled fibers have lower probability of mechanical damage while connecting with various optical devices because of their curved structure. The insertion and return losses of the CO2 laser polished angled ribbon fibers were comparable with the losses observed in the mechanically polished angled ribbon fibers. We strongly believe that, the proposed CO2 laser polished angled fibers will have dynamic applications in optical communication and sensing. Acknowledgments This research work was supported by the GIST Research Institute (GRI), Republic of Korea in 2016. This work was also supported by Bio-industry Technology Development Program, Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry, and Fisheries, Republic of Korea. References [1] K.C. Kao, G.A. Hockham, Dielectric fiber surface waveguides for optical frequencies, Proc. IEE 113 (1966) 1151–1158. [2] A. Werts, Propagation de la lumière cohérente dans les fibers optiques, L’Onde Eleztrique 46 (1966) 967–980.
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