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Improved coating method for uniform polymer layer in infrared hollow fiber
ARTICLE IN PRESS
Optics & Laser Technology 39 (2007) 1528–1531 www.elsevier.com/locate/optlastec
Improved coating method for uniform polymer layer in infrared hollow fiber Katsumasa Iwaia, Yi-Wei Shib,, Mitsunobu Miyagia, Yuji Matsuurac a
Sendai National College of Technology, 4-16-1 Ayashi-chuo, Aoba-ku, Sendai 989-3128, Japan School of Information Science and Engineering, Fudan University, Handan road 220, Shanghai 200433, China c Graduate School of Engineering, Tohuku University, 05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
b
Received 3 April 2006; received in revised form 25 December 2006; accepted 1 January 2007 Available online 15 February 2007
Abstract An improved coating method was proposed in order to form a uniform polymer layer in the fabrication of cyclic olefin polymer-coated silver (COP/Ag) hollow fiber. A COP solution was flowed in a closed loop, in which the silver-coated tube was used as a part of the loop. Owing to the constant flowing speed of the COP solution and the airtight flowing environment, a COP layer was uniformly formed. The hollow fibers attain high performance and deliver multi-wavelength laser light from the infrared to the visible simultaneously. The method was successfully applied to the fabrication of practical hollow fibers with 2 m length for the near and mid-infrared lasers. r 2007 Elsevier Ltd. All rights reserved. Keywords: Hollow fiber; Polymer film; Coating
1. Introduction Hollow fibers [1–4] with an inner metal layer and a dielectric layer have attained low loss properties in several key wavelength bands in the visible [5] and the infrared regions [6,7] for medical laser light transmission. Silver is normally used as the metal layer because of its high reflection rate. The cyclic olefin polymer (COP) is one of the best dielectric layer materials due to its low absorption in the wavelength regions. In the fabrication of COP coated silver (COP/Ag) hollow fiber, a liquid-phase coating method is normally used for the COP layer. The uniformity of the COP layer is one of the most important factors to obtain high-performance hollow fibers [8]. Improvements have been proposed and experimentally discussed on the method, such as the optimum combination [9] of the flowing speed and the concentration of COP solution, the non-cure method [10] for the hollow fiber with high strength. In this paper, we improved the coating method in order to obtain a uniform COP layer. COP solution was flowed Corresponding author.
E-mail address: [email protected] (Y.-W. Shi). 0030-3992/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2007.01.001
in a closed loop, so as to attain a constant flowing speed. Also the airtight flowing environment provides a better ambient for COP film formation. According to the experimental results, a more uniform COP layer was coated by using the improved method. The method was successfully applied to the fabrication of hollow fibers with 2 m length for the near and mid-infrared lasers, such as Nd:YAG, Er:YAG and CO2 lasers. More stable fabrication and better reproduction were also observed by using the method, especially in the fabrication for hollow fibers with thin inner diameter of 320 or 200 mm. 2. Improved coating method for COP layer The COP was selected among several polymer candidates as the dielectric layer for hollow fiber due to its low absorption in infrared regions and the possibility to form a uniform layer upon the silver layer. A COP layer is normally coated by using the liquid-phase coating method. Fig. 1 shows the conventional flowing method (a) and the closed-loop coating method (b) newly developed in this paper. A COP solution diluted by cyclohexane is first flowed into the silica glass tube with a short length. In the new method, the silica glass tube is then connected with the
ARTICLE IN PRESS K. Iwai et al. / Optics & Laser Technology 39 (2007) 1528–1531
1529
0.6
COP 12 wt %
Polymer Thickness (µm)
0.5
0.4
10 wt %
0.3 8 wt% 0.2
Fig. 1. Liquid-phase coating method for COP layer in hollow optical fiber: (a) conventional flowing method; (b) closed loop flowing method.
0.1 0
20
40 60 Length (cm)
80
100
Fig. 2. COP thickness along a COP/Ag hollow fiber (700 mm 1 m) fabricated by the conventional method, where the flowing speed of COP solution is around 4 cm/min. The concentration of COP solution is depicted in the figure.
0.8
Polymer Thickness (µm)
long-closed silica tube as shown in Fig. 1(b). The peristaltic pump draws the COP solution from downside in both methods. In the improved method, the COP solution flows more stably because it is pressed on both sides of the solution. The loop is fixed perpendicularly. A liquidphase COP film remains on the inner surface due to the effect of surface tension of liquid COP solution. Then, the tube is cured while nitrogen gas flows through the hollow core. Cyclohexane solvent in the liquid-phase film is evaporated and a solid COP layer forms upon the silver layer. In the conventional method, we do not have the left part of the closed loop as shown in Fig. 1(a). The COP solution is open to the air when it was flowing down along the Ag-coated tube. The flowing speed can be unstable and the concentration of COP solution tends to change easily. We evaluated the thickness distribution of COP layer along the hollow fibers that was fabricated by using the conventional method. The thickness of COP layer along the fiber is shown in Fig. 2, where the flowing speed was around 4 cm/min for three kinds of COP solutions, whose concentration were 8, 10, and 12 wt%, respectively. As we have known, thicker COP solution leads to a thicker COP layer. We note that the COP layer becomes less uniform when a thicker layer was formed. For laser light in mid-infrared region, such as CO2 laser radiating at 10.6 mm, a relatively thicker COP film of about 1 mm is required. The obvious fluctuations of COP film thickness are seen especially at both ends of the fiber. This is because that the COP solution tends to move abnormally at both ends due to the junction of tubes with different inner diameters. Furthermore, with the consumption of the COP solution during being flowed in the silvercoated tube, the concentration and the viscosity become higher. Therefore, the film thickness becomes thicker at the output end of the hollow fiber. Based on our experiments so far done, it is difficult to improve the film uniformity of COP layer by using the conventional method. This is the reason why the new method of coating should be developed. We compare the uniformity of COP layer made by the conventional and newly developed methods, as shown in
Closed-loop method
0.6 Conventional
0.4 0
50 Length (cm)
100
Fig. 3. COP thickness along a COP/Ag hollow fiber (700 mm 1 m) fabricated by the conventional and closed-loop method, where the concentration of COP solution was 11 wt% and the flowing speed was 8 cm/min.
Fig. 3. The film thickness tends to be unstable when it becomes thicker than 0.6 mm in the conventional method. On the other hand, the new method produces much more stable thickness distribution along the fiber. The uniform COP layer owes to not only the stable flowing speed of the COP solution but also the airtight flowing environment. The airtight environment depresses the evaporation of cyclohexane in the COP solution, which maintains a constant COP concentration during flowing through the whole silver tube. We noticed that there were still relatively large fluctuations at the input and output ends. Therefore, one has to cut off the both ends for the practical hollow fibers.
ARTICLE IN PRESS K. Iwai et al. / Optics & Laser Technology 39 (2007) 1528–1531
1530
3. Characteristics of fibers made by the improved method 25 Conventional
Attenuation (dB)
COP/Ag hollow fibers for low-loss delivery of Er:YAG (2.94 mm), and CO2 (10.6 mm) laser light were fabricated using the newly developed closed-loop coating method. Fig. 4 shows the spectral loss properties of the fibers. At the object wavelengths, the COP/Ag hollow fiber obtains low losses. In Fig. 4(a), the Er:YAG hollow fiber has a low-loss dip in the visible region. Green laser light can be delivered as a pilot beam with low loss. In Fig. 4(b), we show the spectral loss of a fiber designed for CO2 laser light with a COP film thickness of 0.9 mm [11]. The fiber has a uniform COP layer. The loss is low over the wide wavelength regions from the visible to the mid-infrared regions. Sharp interference peaks induce several low-loss dips. The loss dips coincide with the Er:YAG laser light and a red pilot beam wavelength without affecting the loss properties at
20
15 Closed loop 10 0.5
1.0 Wavelength (µm)
1.5
Fig. 5. Loss spectra of COP/Ag hollow fibers (700 mm 2 m) in the visible and near-infrared regions for CO2 laser light fabricated by the conventional and closed-loop methods.
20
Attenuation (dB)
Er:YAG
10
Pilot 0 0.5
1
2
3
5
Wavelength (µm)
Attenuation (dB)
20
10
Pilot 0 0.5
CO2
Er:YAG 1
2
3
5
7
the CO2 laser light wavelength. The fiber can deliver the three lasers with low losses simultaneously. It is important for an infrared fiber to deliver a pilot beam in medical and industrial applications. The pilot beam gives an aiming point for the invisible infrared laser light. Simultaneous delivery of two or more infrared laser light provides a useful tool in many laser applications, because different lasers have their own therapeutic effect. We can deliver the sources to the tissue site simultaneously, separately, or alternatively. In Fig. 5, we show a spectral loss in the visible and NIR regions for 2 m long hollow fibers. The two fibers were designed for CO2 laser transmission. The newly developed coating method produces many sharp interference peaks and attains low loss because of a more uniform COP layer. Power durability is one of the most important characteristics of the hollow fiber in various applications. We have made a durability experiments on the COP coated silver hollow fibers cooperating with J. MORITA MFG. CORP. [12]. Fibers with 2 m length and 0.7 mm inner diameter were used for Er:YAG laser light delivery in the experiment. A 950 h delivery experiment was made with the output energy of 350 mJ and the pulse repetition rate of 25 Hz. More than 85 million shots were delivered and no deterioration was observed. Furthermore, more than 600 pieces of COP coated hollow fibers have been used in medical laser systems in recent 2 years. No damage report was received up to now. 4. Conclusions
10
Wavelength (µm) Fig. 4. Loss spectra of COP/Ag hollow fibers (700 mm 1 m) for Er:YAG (a) and CO2 laser light (b) fabricated by a closed-loop method, where the concentration of COP were 8 and 12 wt% and the flowing speed was the same 8 cm/min.
A closed-loop flowing method is newly proposed to obtain a uniform COP layer in the fabrication of infrared hollow fibers that can transmit the pilot beams for invisible infrared laser light. In the method, a constant flowing speed and stable concentration for COP solution is attained. The
ARTICLE IN PRESS K. Iwai et al. / Optics & Laser Technology 39 (2007) 1528–1531
method is shown to provide a useful method to lower the loss in the whole visible and the infrared regions. The method is also proved to be efficient in the fabrication for 2 m long fibers. Acknowledgments This research was supported by the Ministry of Education, Science, Sports and Culture of Japan through Grant-in-Aids for Scientific Research (C), and (A), 2004–2006. References [1] Miyagi M, Hongo A, Aizawa Y, Kawakami S. Fabrication of germanium coated nickel hollow waveguide for infrared transmission. Appl Phys Lett 1983;43:430–2. [2] Levy MB, Laakman KD. Flexible waveguides for CO2 laser surgery. Proc Soc Photo-Opt Instrum Eng 1986;605:57–8. [3] Hongo A, Morosawa K, Masumoto K, Shiota T, Hashimoto T. Transmission of kilowatt-class CO2 laser light through dielectriccoated metallic hollow waveguides for material processing. Appl Opt 1992;31:5114–20.
1531
[4] Morrow CE, Gu G. FIBERLASEs a monolithic hollow waveguide. Proc Soc Photo-Opt Instrum Eng 1994;2131:18–27. [5] Matsuura Y, Takada G, Yamamoto T, Shi YW, Miyagi M. Hollow fibers for delivery of harmonic pulses of Q-switched Nd:YAG lasers. Appl Opt 2002;41:442–5. [6] Abe Y, Matsuura Y, Shi YW, Uyama H, Miyagi M. Polymer-coated hollow fiber for CO2 laser delivery. Opt Lett 1998;23:89–90. [7] Shi YW, Wang Y, Abe Y, Matsuura Y, Miyagi M, Sato S, et al. Cyclic olefin polymer-coated silver hollow glass waveguides for the infrared. Appl Opt 1998;37:7758–62. [8] Shi YW, Ito K, Matsuura Y, Miyagi M. Multiwavelength laser light transmission of hollow optical fiber from the visible to the midinfrared. Opt Lett 2005;30:2867–9. [9] Shi YW, Matsuura Y, Miyagi M. Pilot beams for polymer-coated silver hollow glass fibers. Proc Soc Photo-Opt Instrum Eng 2002;4253:50–7. [10] Shi YW, Pan ZY, Matsuura Y, Miyagi M. New and simple method for fabricating polymer-coated silver hollow fibers with large mechanical strength. Opt Laser Technol 2000;32:273–5. [11] Miyagi M, Kawakami S. Design theory of dielectric-coated circular metallic waveguides for infrared transmission. J Lightwave Technol 1984;LT-2:116–26. [12] http://www.jmorita-mfg.co.jp/.
Optics & Laser Technology 39 (2007) 1528–1531 www.elsevier.com/locate/optlastec
Improved coating method for uniform polymer layer in infrared hollow fiber Katsumasa Iwaia, Yi-Wei Shib,, Mitsunobu Miyagia, Yuji Matsuurac a
Sendai National College of Technology, 4-16-1 Ayashi-chuo, Aoba-ku, Sendai 989-3128, Japan School of Information Science and Engineering, Fudan University, Handan road 220, Shanghai 200433, China c Graduate School of Engineering, Tohuku University, 05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
b
Received 3 April 2006; received in revised form 25 December 2006; accepted 1 January 2007 Available online 15 February 2007
Abstract An improved coating method was proposed in order to form a uniform polymer layer in the fabrication of cyclic olefin polymer-coated silver (COP/Ag) hollow fiber. A COP solution was flowed in a closed loop, in which the silver-coated tube was used as a part of the loop. Owing to the constant flowing speed of the COP solution and the airtight flowing environment, a COP layer was uniformly formed. The hollow fibers attain high performance and deliver multi-wavelength laser light from the infrared to the visible simultaneously. The method was successfully applied to the fabrication of practical hollow fibers with 2 m length for the near and mid-infrared lasers. r 2007 Elsevier Ltd. All rights reserved. Keywords: Hollow fiber; Polymer film; Coating
1. Introduction Hollow fibers [1–4] with an inner metal layer and a dielectric layer have attained low loss properties in several key wavelength bands in the visible [5] and the infrared regions [6,7] for medical laser light transmission. Silver is normally used as the metal layer because of its high reflection rate. The cyclic olefin polymer (COP) is one of the best dielectric layer materials due to its low absorption in the wavelength regions. In the fabrication of COP coated silver (COP/Ag) hollow fiber, a liquid-phase coating method is normally used for the COP layer. The uniformity of the COP layer is one of the most important factors to obtain high-performance hollow fibers [8]. Improvements have been proposed and experimentally discussed on the method, such as the optimum combination [9] of the flowing speed and the concentration of COP solution, the non-cure method [10] for the hollow fiber with high strength. In this paper, we improved the coating method in order to obtain a uniform COP layer. COP solution was flowed Corresponding author.
E-mail address: [email protected] (Y.-W. Shi). 0030-3992/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2007.01.001
in a closed loop, so as to attain a constant flowing speed. Also the airtight flowing environment provides a better ambient for COP film formation. According to the experimental results, a more uniform COP layer was coated by using the improved method. The method was successfully applied to the fabrication of hollow fibers with 2 m length for the near and mid-infrared lasers, such as Nd:YAG, Er:YAG and CO2 lasers. More stable fabrication and better reproduction were also observed by using the method, especially in the fabrication for hollow fibers with thin inner diameter of 320 or 200 mm. 2. Improved coating method for COP layer The COP was selected among several polymer candidates as the dielectric layer for hollow fiber due to its low absorption in infrared regions and the possibility to form a uniform layer upon the silver layer. A COP layer is normally coated by using the liquid-phase coating method. Fig. 1 shows the conventional flowing method (a) and the closed-loop coating method (b) newly developed in this paper. A COP solution diluted by cyclohexane is first flowed into the silica glass tube with a short length. In the new method, the silica glass tube is then connected with the
ARTICLE IN PRESS K. Iwai et al. / Optics & Laser Technology 39 (2007) 1528–1531
1529
0.6
COP 12 wt %
Polymer Thickness (µm)
0.5
0.4
10 wt %
0.3 8 wt% 0.2
Fig. 1. Liquid-phase coating method for COP layer in hollow optical fiber: (a) conventional flowing method; (b) closed loop flowing method.
0.1 0
20
40 60 Length (cm)
80
100
Fig. 2. COP thickness along a COP/Ag hollow fiber (700 mm 1 m) fabricated by the conventional method, where the flowing speed of COP solution is around 4 cm/min. The concentration of COP solution is depicted in the figure.
0.8
Polymer Thickness (µm)
long-closed silica tube as shown in Fig. 1(b). The peristaltic pump draws the COP solution from downside in both methods. In the improved method, the COP solution flows more stably because it is pressed on both sides of the solution. The loop is fixed perpendicularly. A liquidphase COP film remains on the inner surface due to the effect of surface tension of liquid COP solution. Then, the tube is cured while nitrogen gas flows through the hollow core. Cyclohexane solvent in the liquid-phase film is evaporated and a solid COP layer forms upon the silver layer. In the conventional method, we do not have the left part of the closed loop as shown in Fig. 1(a). The COP solution is open to the air when it was flowing down along the Ag-coated tube. The flowing speed can be unstable and the concentration of COP solution tends to change easily. We evaluated the thickness distribution of COP layer along the hollow fibers that was fabricated by using the conventional method. The thickness of COP layer along the fiber is shown in Fig. 2, where the flowing speed was around 4 cm/min for three kinds of COP solutions, whose concentration were 8, 10, and 12 wt%, respectively. As we have known, thicker COP solution leads to a thicker COP layer. We note that the COP layer becomes less uniform when a thicker layer was formed. For laser light in mid-infrared region, such as CO2 laser radiating at 10.6 mm, a relatively thicker COP film of about 1 mm is required. The obvious fluctuations of COP film thickness are seen especially at both ends of the fiber. This is because that the COP solution tends to move abnormally at both ends due to the junction of tubes with different inner diameters. Furthermore, with the consumption of the COP solution during being flowed in the silvercoated tube, the concentration and the viscosity become higher. Therefore, the film thickness becomes thicker at the output end of the hollow fiber. Based on our experiments so far done, it is difficult to improve the film uniformity of COP layer by using the conventional method. This is the reason why the new method of coating should be developed. We compare the uniformity of COP layer made by the conventional and newly developed methods, as shown in
Closed-loop method
0.6 Conventional
0.4 0
50 Length (cm)
100
Fig. 3. COP thickness along a COP/Ag hollow fiber (700 mm 1 m) fabricated by the conventional and closed-loop method, where the concentration of COP solution was 11 wt% and the flowing speed was 8 cm/min.
Fig. 3. The film thickness tends to be unstable when it becomes thicker than 0.6 mm in the conventional method. On the other hand, the new method produces much more stable thickness distribution along the fiber. The uniform COP layer owes to not only the stable flowing speed of the COP solution but also the airtight flowing environment. The airtight environment depresses the evaporation of cyclohexane in the COP solution, which maintains a constant COP concentration during flowing through the whole silver tube. We noticed that there were still relatively large fluctuations at the input and output ends. Therefore, one has to cut off the both ends for the practical hollow fibers.
ARTICLE IN PRESS K. Iwai et al. / Optics & Laser Technology 39 (2007) 1528–1531
1530
3. Characteristics of fibers made by the improved method 25 Conventional
Attenuation (dB)
COP/Ag hollow fibers for low-loss delivery of Er:YAG (2.94 mm), and CO2 (10.6 mm) laser light were fabricated using the newly developed closed-loop coating method. Fig. 4 shows the spectral loss properties of the fibers. At the object wavelengths, the COP/Ag hollow fiber obtains low losses. In Fig. 4(a), the Er:YAG hollow fiber has a low-loss dip in the visible region. Green laser light can be delivered as a pilot beam with low loss. In Fig. 4(b), we show the spectral loss of a fiber designed for CO2 laser light with a COP film thickness of 0.9 mm [11]. The fiber has a uniform COP layer. The loss is low over the wide wavelength regions from the visible to the mid-infrared regions. Sharp interference peaks induce several low-loss dips. The loss dips coincide with the Er:YAG laser light and a red pilot beam wavelength without affecting the loss properties at
20
15 Closed loop 10 0.5
1.0 Wavelength (µm)
1.5
Fig. 5. Loss spectra of COP/Ag hollow fibers (700 mm 2 m) in the visible and near-infrared regions for CO2 laser light fabricated by the conventional and closed-loop methods.
20
Attenuation (dB)
Er:YAG
10
Pilot 0 0.5
1
2
3
5
Wavelength (µm)
Attenuation (dB)
20
10
Pilot 0 0.5
CO2
Er:YAG 1
2
3
5
7
the CO2 laser light wavelength. The fiber can deliver the three lasers with low losses simultaneously. It is important for an infrared fiber to deliver a pilot beam in medical and industrial applications. The pilot beam gives an aiming point for the invisible infrared laser light. Simultaneous delivery of two or more infrared laser light provides a useful tool in many laser applications, because different lasers have their own therapeutic effect. We can deliver the sources to the tissue site simultaneously, separately, or alternatively. In Fig. 5, we show a spectral loss in the visible and NIR regions for 2 m long hollow fibers. The two fibers were designed for CO2 laser transmission. The newly developed coating method produces many sharp interference peaks and attains low loss because of a more uniform COP layer. Power durability is one of the most important characteristics of the hollow fiber in various applications. We have made a durability experiments on the COP coated silver hollow fibers cooperating with J. MORITA MFG. CORP. [12]. Fibers with 2 m length and 0.7 mm inner diameter were used for Er:YAG laser light delivery in the experiment. A 950 h delivery experiment was made with the output energy of 350 mJ and the pulse repetition rate of 25 Hz. More than 85 million shots were delivered and no deterioration was observed. Furthermore, more than 600 pieces of COP coated hollow fibers have been used in medical laser systems in recent 2 years. No damage report was received up to now. 4. Conclusions
10
Wavelength (µm) Fig. 4. Loss spectra of COP/Ag hollow fibers (700 mm 1 m) for Er:YAG (a) and CO2 laser light (b) fabricated by a closed-loop method, where the concentration of COP were 8 and 12 wt% and the flowing speed was the same 8 cm/min.
A closed-loop flowing method is newly proposed to obtain a uniform COP layer in the fabrication of infrared hollow fibers that can transmit the pilot beams for invisible infrared laser light. In the method, a constant flowing speed and stable concentration for COP solution is attained. The
ARTICLE IN PRESS K. Iwai et al. / Optics & Laser Technology 39 (2007) 1528–1531
method is shown to provide a useful method to lower the loss in the whole visible and the infrared regions. The method is also proved to be efficient in the fabrication for 2 m long fibers. Acknowledgments This research was supported by the Ministry of Education, Science, Sports and Culture of Japan through Grant-in-Aids for Scientific Research (C), and (A), 2004–2006. References [1] Miyagi M, Hongo A, Aizawa Y, Kawakami S. Fabrication of germanium coated nickel hollow waveguide for infrared transmission. Appl Phys Lett 1983;43:430–2. [2] Levy MB, Laakman KD. Flexible waveguides for CO2 laser surgery. Proc Soc Photo-Opt Instrum Eng 1986;605:57–8. [3] Hongo A, Morosawa K, Masumoto K, Shiota T, Hashimoto T. Transmission of kilowatt-class CO2 laser light through dielectriccoated metallic hollow waveguides for material processing. Appl Opt 1992;31:5114–20.
1531
[4] Morrow CE, Gu G. FIBERLASEs a monolithic hollow waveguide. Proc Soc Photo-Opt Instrum Eng 1994;2131:18–27. [5] Matsuura Y, Takada G, Yamamoto T, Shi YW, Miyagi M. Hollow fibers for delivery of harmonic pulses of Q-switched Nd:YAG lasers. Appl Opt 2002;41:442–5. [6] Abe Y, Matsuura Y, Shi YW, Uyama H, Miyagi M. Polymer-coated hollow fiber for CO2 laser delivery. Opt Lett 1998;23:89–90. [7] Shi YW, Wang Y, Abe Y, Matsuura Y, Miyagi M, Sato S, et al. Cyclic olefin polymer-coated silver hollow glass waveguides for the infrared. Appl Opt 1998;37:7758–62. [8] Shi YW, Ito K, Matsuura Y, Miyagi M. Multiwavelength laser light transmission of hollow optical fiber from the visible to the midinfrared. Opt Lett 2005;30:2867–9. [9] Shi YW, Matsuura Y, Miyagi M. Pilot beams for polymer-coated silver hollow glass fibers. Proc Soc Photo-Opt Instrum Eng 2002;4253:50–7. [10] Shi YW, Pan ZY, Matsuura Y, Miyagi M. New and simple method for fabricating polymer-coated silver hollow fibers with large mechanical strength. Opt Laser Technol 2000;32:273–5. [11] Miyagi M, Kawakami S. Design theory of dielectric-coated circular metallic waveguides for infrared transmission. J Lightwave Technol 1984;LT-2:116–26. [12] http://www.jmorita-mfg.co.jp/.