- Email: [email protected]
Temperature characteristics of fusion splicing Hollow Core Photonic Crystal Fiber by sinusoidal modulation CO2 laser
Optics & Laser Technology 49 (2013) 64–67
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Temperature characteristics of fusion splicing Hollow Core Photonic Crystal Fiber by sinusoidal modulation CO2 laser Guangwei Fu a,b, Kuixing Li a, Xinghu Fu a,b, Weihong Bi a,b,n a b
Department of Optoelectronic Engineering, Yanshan University, Qinhuangdao 066004, PR China The Key Laboratory for Special Fiber and Fiber Sensor of Hebei Province, Yanshan University, Qinhuangdao 066004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2012 Received in revised form 6 December 2012 Accepted 11 December 2012 Available online 12 January 2013
During the fusion splicing Hollow Core Photonic Crystal Fiber (HC-PCF), the air-holes collapse easily due to the improper fusion duration time and optical power. To analyze the temperature characteristics of fusion splicing HC-PCF, a heating method by sinusoidal modulation CO2 laser has been proposed. In the sinusoidal modulation, the variation relationships among laser power, temperature difference and angular frequency are analyzed. The results show that the theoretical simulation is basically in accordance with the experimental data. Therefore, a low-loss fusion splicing can be achieved by modulating the CO2 laser frequency to avoid the air-holes collapse of HC-PCF. Further, the errors are also given. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Hollow core photonics crystal fiber Sinusoidal modulation CO2 laser
1. Introduction Photonic Crystal Fiber (PCF) has many excellent characteristics [1–3], including endlessly single-mode operation, extraordinary chromatic dispersion, high nonlinearity, high birefringence, and so on. So PCF can be used in optical communication system and has attracted extensive attentions from scientists all over the world. HC-PCF with air-holes [4] arranged as triangular structure was designed by Cregan in 1999. With the light trapped in airholes, this fiber has little material dispersion and material loss [5,6]. However, the temperature characteristic is critical between two HC-PCFs or different HC-PCF and single mode fibers (SMF) in fusion splicing. The air-holes collapse can be caused by the improper fusion duration time and optical power. So a method is required for analyzing the temperature characteristics of fusion splicing HC-PCF. To resolve this problem, various solutions have been proposed for researching the temperature characteristics of fusion splicing HC-PCF. For example, Kristensen et al. [7] reported on highly reproducible low-loss fusion splicing of polarization-maintaining single-mode fiber (PM-SMF) and HC-PCF, and the results can be used in fiber lasers. Thapa et al. [8] analyzed the fusion splicing loss of arc splicing HC-PCF and Single Mode Fiber (SMF). The low splicing loss can be obtained by filling one end of the HC-PCF spliced to SMF with acetylene gas and performing saturation
n Corresponding author at: Department of Optoelectronic Engineering, Yanshan University, Qinhuangdao 066004, PR China. Tel.: þ 86 335 8051386. E-mail address: [email protected] (W. Bi).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.12.010
spectroscopy. Carvalho et al. [9] used a laser splicing system for splicing between the SMF and HC-PCF. The low splicing losses can be obtained with high reproducibility. Chong et al. [10,11] and Grellier et al. [12] used CO2 laser splicing system for splicing between the large-mode-area PCF and SMF. The low splicing loss can be obtained by adjusting the laser power precisely. However, the temperature characteristics of fusion splicing HC-PCF have not been discussed in detail. In this paper, a heating method by frequency modulation CO2 laser has been proposed. In the sinusoidal modulation, the variation relationships among the laser power, temperature difference and angular frequency are analyzed. The temperature characteristics of fusion splicing HC-PCF have been demonstrated. Thereby, a low-loss fusion splicing can be obtained by greatly reducing air-holes collapse.
2. Basic principle The CO2 laser works in TEM00 mode with Gaussian beam distribution [13]. So the relative position between the laser spot and fiber is shown in Fig. 1. The radiant flux density of CO2 laser after modulation is " !# 2y2 2z2 þ 2 Iðy,zÞ ¼ I0 exp f ðt Þ ð1Þ w2y wz where, I0 is the light intensity of CO2 laser. wy and wz are 600 mm, and are the laser beam widths in y and z direction, respectively.
G. Fu et al. / Optics & Laser Technology 49 (2013) 64–67
65
Table 1 Material properties. Material
SiO2
Air
Density r (kg m 3) Specific heat capacity cp (J kg 1 K 1) Thermal conductivity k (W m 1 K 1)
2200 1345 2.68
0.93 1010 0.032
Fig. 1. The relative position between the laser spot and fiber.
f(t) is the modulating signal; it is a periodic function of time, such as sinusoidal signal, cosine signal, square wave signal, sawtooth wave, and so on. The total laser power is ZZ P total ¼ Iðy,zÞdydz ð2Þ A
For integrating the laser spot area, Eq. (2) can be described as Z þ Rz Z þ Ry 2y2 2z2 dy dz ð3Þ exp exp P total ¼ I0 Wy Wz Ry Rz where, Ry and Rz are the laser spot radii at fiber splicing plane. So the integral in Eq. (3) is 2 8R pffiffiffi þ Ry Wy p 2y > < Ry exp W y dy ¼ pffiffi2 2 pffiffiffi ð4Þ R þR Wp z ffiffi p > : Rz z exp 2z W z dz ¼ 2 Substituting Eq. (4) into Eq. (3), we can obtain I0 ¼
2Ptotal
pW y W z
ð5Þ
So the radiant flux density of CO2 laser after modulation can be described as " !# 2Ptotal 2y2 2z2 f ðt Þ ð6Þ Iðy,zÞ ¼ exp þ pwy wz w2y w2z By changing the frequency of modulation signal, heat conduction and temperature distribution inside the fiber will be affected. So temperature characteristics of fusion splicing HC-PCF can be analyzed. The mechanism of the effect of the sinusoidal modulation CO2 laser on the splicing performance is as follows: the laser beam is irradiated onto the surface of the HC-PCF, and optical energy is converted into thermal energy. By adjusting the frequency of modulation signal, the fiber surface can absorb more energy from the incident laser and then reach the splicing temperature. Moreover, the air-holes around the fiber core cannot reach the splicing temperature due to the insufficient absorbed energy. Thereby, this method can maintain the optical characteristics of the spliced fiber by reducing the collapse of the air-holes and achieve low splicing loss.
3. Simulation and analysis In simulation, we used HC-PCF with a length of 3 mm and a heating time of 200 ms. The air-holes core radius is 4.2 mm, the air-holes diameter is 2.46 mm, air-space size is 3.28 mm, and the SO2 melting point is 1700 75 K. Other parameters are as shown in Table 1. The sinusoidal signal is used to modulate the laser. By sinusoidal modulation, Eq. (6) becomes " !# 2Ptotal 2y2 2z2 Iðy,zÞ ¼ exp þ ð7Þ sinðot Þ pwy wz w2y w2z where, sin(ot) is the modulating signal, t is the variation range of heating time, o is the sinusoidal modulating angular frequency
and the variation range of o is 0.5–5p rad/s. So the temperature distribution along the HC-PCF length can be obtained by finite element method as shown in Fig. 2. In Fig. 2, we can see that the temperature is 1705 K (the melting point of the fiber) at the end of fusion splicing HC-PCF and decreases along the fiber length. When the variation range of the fiber length is 0–0.85 mm, the temperature distribution will decrease rapidly. After this, the variation trend of temperature distribution becomes smooth and almost invariable. It reaches 298 K at 3 mm length. Moreover, when the angular frequency o is 0.5 rad/s, the temperature distribution at the cross-section of the fiber is also obtained as shown in Fig. 3. In Fig. 3, we can see that the temperature distribution is different at the cross-section of HC-PCF. When the variation range of radial distance is 0–10 mm, the temperature distribution is less than 1692 K. With the radial distance increasing, the temperature distribution increases. Here, the air-holes core radius of 4.2 mm is used for analyzing, so the temperature around the HC-PCF core is below the melting point (1705 K), while the temperature outside the fiber core can reach the fusion temperature by sinusoidal modulation. Furthermore, the temperature distribution in airholes has a relatively large gradient due to the periodically arranged air-holes. In contrast, temperature distribution in SiO2 materials is a smooth curve, which is caused by the hindered heat conduction by air and the heat conductivity of air is far less than that of SiO2. Thereby, the relationships among laser power, temperature difference and angular frequency can be obtained by sinusoidal modulation as shown in Fig. 4. In Fig. 4, for the most part, the required laser power decreases with increasing angular frequency. Especially, when o is changing from 0.5 to 11.5 rad/s and then 11.5 to 5p rad/s, the laser power firstly decreases and reaches a minimum value, for which the laser power is 1.58 W at the angular frequency of 11.5 rad/s. Then, it increases with angular frequency. Moreover, the temperature difference along the fiber length decreases with increasing modulating angular frequency. The maximum value of temperature difference is 14.386 K at the angular frequency of 0.5 rad/s.
4. Experimental analysis We then performed a straightforward experiment to prove the theory we proposed on temperature characteristics of fusion splicing HC-PCF as shown in Fig. 5. In Fig. 5, the setup is composed of a CO2 laser, a laser modulator, transmitting energy fibers, focusing lens, and so on. The laser modulator is used to modulate the CO2 laser and different frequency of modulation signal can be obtained. The CO2 laser has a maximum output power of 40 W. The transmitting energy fiber is GeO2 dielectric hollow waveguide. The ZnSe lens with focal length 150 mm is used to couple laser into optical coupler. By focusing the laser signal, the HC-PCF can be fused. The laser spot diameter is controlled within the range from 500 to 800 mm. In the experiment, the sinusoidal modulation signal sin(ot) with different angular frequency o is used for heating HC-PCF ten times. Optris IR Plus2000 infrared thermometer is used for measuring temperature variations of heating zone
66
G. Fu et al. / Optics & Laser Technology 49 (2013) 64–67
Fig. 2. The temperature distribution along the HC-PCF length.
Fig. 3. The temperature distribution at the cross-section of fiber and along the radial distance.
Fig. 4. Relationships among the laser power, temperature difference and angular frequency. Fig. 6. Theoretical data and experimental results.
Fig. 5. Experimental setup.
and it can measure temperature range from 873 to 2273 K, with the accuracy of 70.5% and repeatability of 70.25%. A simple automatic control system for the thermometer is developed so that the measurement error can be reduced. Thereby, the experimental results and theoretical data are compared as shown in Fig. 6. In Fig. 6, the experimental results are basically consistent with the theoretical data. In modulation, experimental and theoretical temperature difference decreases with increasing angular frequency; the greater the angular frequency, the smaller the temperature difference. When o is changing from 0.5 to 5 rad/s, the temperature difference is more than 13 K, this system has
large temperature difference and achieves the purpose of modulation. In addition, the experimental temperature difference is larger than the theoretical, which declared that modulation is more effective in fusion splicing than simulation. When the modulating frequency is small, the air hole collapse is small, or can even be avoided. Moreover, the relationship between the splicing loss and the angular frequency in experiment is shown in Fig. 7. In Fig. 7, we can see that the splicing loss is very low when angular frequency is changing from 0.5 to 4 rad/s. Along with the angular frequency increasing, the splicing loss increases due to air-holes collapse. The minimum and maximum of splicing loss are 0.02 dB and 3.35 dB, respectively. However, there are some errors between experimental results and theoretical data which are as follows: (1) The stepper motor control of heating region may be not precise enough, so the HC-PCF heating zone can be affected.
G. Fu et al. / Optics & Laser Technology 49 (2013) 64–67
67
greatly reduced or even be avoided, resulting in a low-loss fusion splicing HC-PCF.
Acknowledgments The Project is supported by the National Natural Science Foundation of China (Nos. 61077067 and 60927009), the China Postdoctoral Science Foundation (No. 2012M510767), the Hebei Provincial Natural Science Foundation (Nos. F2012203148 and F2011203116), the Qinhuangdao Technology Development Program (No. 2012021A043), the Doctoral Funds of Yanshan University of China (Nos. B643 and B501). Fig. 7. The relationship between the splicing loss and the angular frequency.
Moreover, the heating time for HC-PCF is short, so the realtime measurement accuracy of thermometer may be inadequate. (2) In simulation, the mesh size in finite element method may be not detailed enough. The simulation time will increase as the detailed meshing increases. For example, the unit numbers of 189658 and grids numbers of 85634 are used in Fig. 2, and the unit numbers of 59662 and grids numbers of 29863 are used in Fig. 4. (3) Sinusoidal signal is created by the function generator and may have a little difference with that in simulation. So there may be a time delay leading to inaccurate heating time control.
5. Conclusion A method to control air-holes collapse in the heating process of HC-PCF has been proposed. A simple experiment was performed to verify the temperature characteristics of HC-PCF. Theoretical data and experimental results show that it is feasible to change the temperature distribution in fiber by frequency modulation CO2 laser. Thereby, the air-holes collapse can be
References [1] Knight JC, Birks TA, Russell J, Atkin DM. All-silica single-mode optical fiber with photonic crystal cladding. Optics Letters 1996;21(19):1547–9. [2] Russell J. Photonic crystal fibers. Journal of Lightwave Technology 2006;24(12):4729–49. [3] Russell J. Photonic crystal fibers. Science 2003;299(5605):358–62. [4] Cregan RF, Mangan BJ, Knight JC, Birks TA, Russell J, Roberts PJ, et al. Single-mode photonic band gap guidance of light in air. Science 1999;285(5433):1537–9. [5] Roberts PJ, Couny F, Sabert H, Mangan BJ, Williams DP, Farr L, et al. Ultimate low loss of hollow-core photonic crystal fibres. Optics Express 2005;13(1):236–44. [6] Mishra SS, Singh Vinod Kumar. Study of non-linear properties of hollow core photonic crystal fiber. Optik 2011;122(8):687–90. [7] Kristensen Jesper Toft, Houmann Andreas, Liu Xiaomin, Turchinovich Dmitry. Low-loss polarization-maintaining fusion splicing of single-mode fibers and hollow-core photonic crystal fibers, relevant for monolithic fiber laser pulse compression. Optics Express 2008;16(13):9986–95. [8] Thapa R, Corwin KL, Washburn BR. Arc fusion splicing of hollow-core photonicbandgap fibers for gas-filled fiber cells. Optics Express 2006;14(21):9576–83. [9] Carvalho JP, Magalha~ es F, Fraza~ o O, Santos JL, Arau´jo FM, Ferreira LA. Splicing and coupling losses in hollow-core photonic crystal glass fibers. Solid State Phenomena 2010;161:43–9. [10] Chong JH, Rao M. Development of a system for laser splicing photonic crystal fiber. Optics Express 2003;11(12):1365–70. [11] Chong JH, Rao MK, Zhu Y, Shum P. An effective splicing method on photonic crystal fiber using CO2 laser. IEEE Photonics Technology Letters 2003;15(7):942–4. [12] Grellier AJC, Zayer NK, Pannell CN. Heat transfer modelling in CO2 laser processing of optical fibres. Optics Communications 1998;152:324–8. [13] Kurosaki Yasuo. Radiative heat transfer in plastic welding process. Journal of Quantitative Spectroscopy & Radiative Transfer 2005;93:25–41.
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Temperature characteristics of fusion splicing Hollow Core Photonic Crystal Fiber by sinusoidal modulation CO2 laser Guangwei Fu a,b, Kuixing Li a, Xinghu Fu a,b, Weihong Bi a,b,n a b
Department of Optoelectronic Engineering, Yanshan University, Qinhuangdao 066004, PR China The Key Laboratory for Special Fiber and Fiber Sensor of Hebei Province, Yanshan University, Qinhuangdao 066004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2012 Received in revised form 6 December 2012 Accepted 11 December 2012 Available online 12 January 2013
During the fusion splicing Hollow Core Photonic Crystal Fiber (HC-PCF), the air-holes collapse easily due to the improper fusion duration time and optical power. To analyze the temperature characteristics of fusion splicing HC-PCF, a heating method by sinusoidal modulation CO2 laser has been proposed. In the sinusoidal modulation, the variation relationships among laser power, temperature difference and angular frequency are analyzed. The results show that the theoretical simulation is basically in accordance with the experimental data. Therefore, a low-loss fusion splicing can be achieved by modulating the CO2 laser frequency to avoid the air-holes collapse of HC-PCF. Further, the errors are also given. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Hollow core photonics crystal fiber Sinusoidal modulation CO2 laser
1. Introduction Photonic Crystal Fiber (PCF) has many excellent characteristics [1–3], including endlessly single-mode operation, extraordinary chromatic dispersion, high nonlinearity, high birefringence, and so on. So PCF can be used in optical communication system and has attracted extensive attentions from scientists all over the world. HC-PCF with air-holes [4] arranged as triangular structure was designed by Cregan in 1999. With the light trapped in airholes, this fiber has little material dispersion and material loss [5,6]. However, the temperature characteristic is critical between two HC-PCFs or different HC-PCF and single mode fibers (SMF) in fusion splicing. The air-holes collapse can be caused by the improper fusion duration time and optical power. So a method is required for analyzing the temperature characteristics of fusion splicing HC-PCF. To resolve this problem, various solutions have been proposed for researching the temperature characteristics of fusion splicing HC-PCF. For example, Kristensen et al. [7] reported on highly reproducible low-loss fusion splicing of polarization-maintaining single-mode fiber (PM-SMF) and HC-PCF, and the results can be used in fiber lasers. Thapa et al. [8] analyzed the fusion splicing loss of arc splicing HC-PCF and Single Mode Fiber (SMF). The low splicing loss can be obtained by filling one end of the HC-PCF spliced to SMF with acetylene gas and performing saturation
n Corresponding author at: Department of Optoelectronic Engineering, Yanshan University, Qinhuangdao 066004, PR China. Tel.: þ 86 335 8051386. E-mail address: [email protected] (W. Bi).
0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.12.010
spectroscopy. Carvalho et al. [9] used a laser splicing system for splicing between the SMF and HC-PCF. The low splicing losses can be obtained with high reproducibility. Chong et al. [10,11] and Grellier et al. [12] used CO2 laser splicing system for splicing between the large-mode-area PCF and SMF. The low splicing loss can be obtained by adjusting the laser power precisely. However, the temperature characteristics of fusion splicing HC-PCF have not been discussed in detail. In this paper, a heating method by frequency modulation CO2 laser has been proposed. In the sinusoidal modulation, the variation relationships among the laser power, temperature difference and angular frequency are analyzed. The temperature characteristics of fusion splicing HC-PCF have been demonstrated. Thereby, a low-loss fusion splicing can be obtained by greatly reducing air-holes collapse.
2. Basic principle The CO2 laser works in TEM00 mode with Gaussian beam distribution [13]. So the relative position between the laser spot and fiber is shown in Fig. 1. The radiant flux density of CO2 laser after modulation is " !# 2y2 2z2 þ 2 Iðy,zÞ ¼ I0 exp f ðt Þ ð1Þ w2y wz where, I0 is the light intensity of CO2 laser. wy and wz are 600 mm, and are the laser beam widths in y and z direction, respectively.
G. Fu et al. / Optics & Laser Technology 49 (2013) 64–67
65
Table 1 Material properties. Material
SiO2
Air
Density r (kg m 3) Specific heat capacity cp (J kg 1 K 1) Thermal conductivity k (W m 1 K 1)
2200 1345 2.68
0.93 1010 0.032
Fig. 1. The relative position between the laser spot and fiber.
f(t) is the modulating signal; it is a periodic function of time, such as sinusoidal signal, cosine signal, square wave signal, sawtooth wave, and so on. The total laser power is ZZ P total ¼ Iðy,zÞdydz ð2Þ A
For integrating the laser spot area, Eq. (2) can be described as Z þ Rz Z þ Ry 2y2 2z2 dy dz ð3Þ exp exp P total ¼ I0 Wy Wz Ry Rz where, Ry and Rz are the laser spot radii at fiber splicing plane. So the integral in Eq. (3) is 2 8R pffiffiffi þ Ry Wy p 2y > < Ry exp W y dy ¼ pffiffi2 2 pffiffiffi ð4Þ R þR Wp z ffiffi p > : Rz z exp 2z W z dz ¼ 2 Substituting Eq. (4) into Eq. (3), we can obtain I0 ¼
2Ptotal
pW y W z
ð5Þ
So the radiant flux density of CO2 laser after modulation can be described as " !# 2Ptotal 2y2 2z2 f ðt Þ ð6Þ Iðy,zÞ ¼ exp þ pwy wz w2y w2z By changing the frequency of modulation signal, heat conduction and temperature distribution inside the fiber will be affected. So temperature characteristics of fusion splicing HC-PCF can be analyzed. The mechanism of the effect of the sinusoidal modulation CO2 laser on the splicing performance is as follows: the laser beam is irradiated onto the surface of the HC-PCF, and optical energy is converted into thermal energy. By adjusting the frequency of modulation signal, the fiber surface can absorb more energy from the incident laser and then reach the splicing temperature. Moreover, the air-holes around the fiber core cannot reach the splicing temperature due to the insufficient absorbed energy. Thereby, this method can maintain the optical characteristics of the spliced fiber by reducing the collapse of the air-holes and achieve low splicing loss.
3. Simulation and analysis In simulation, we used HC-PCF with a length of 3 mm and a heating time of 200 ms. The air-holes core radius is 4.2 mm, the air-holes diameter is 2.46 mm, air-space size is 3.28 mm, and the SO2 melting point is 1700 75 K. Other parameters are as shown in Table 1. The sinusoidal signal is used to modulate the laser. By sinusoidal modulation, Eq. (6) becomes " !# 2Ptotal 2y2 2z2 Iðy,zÞ ¼ exp þ ð7Þ sinðot Þ pwy wz w2y w2z where, sin(ot) is the modulating signal, t is the variation range of heating time, o is the sinusoidal modulating angular frequency
and the variation range of o is 0.5–5p rad/s. So the temperature distribution along the HC-PCF length can be obtained by finite element method as shown in Fig. 2. In Fig. 2, we can see that the temperature is 1705 K (the melting point of the fiber) at the end of fusion splicing HC-PCF and decreases along the fiber length. When the variation range of the fiber length is 0–0.85 mm, the temperature distribution will decrease rapidly. After this, the variation trend of temperature distribution becomes smooth and almost invariable. It reaches 298 K at 3 mm length. Moreover, when the angular frequency o is 0.5 rad/s, the temperature distribution at the cross-section of the fiber is also obtained as shown in Fig. 3. In Fig. 3, we can see that the temperature distribution is different at the cross-section of HC-PCF. When the variation range of radial distance is 0–10 mm, the temperature distribution is less than 1692 K. With the radial distance increasing, the temperature distribution increases. Here, the air-holes core radius of 4.2 mm is used for analyzing, so the temperature around the HC-PCF core is below the melting point (1705 K), while the temperature outside the fiber core can reach the fusion temperature by sinusoidal modulation. Furthermore, the temperature distribution in airholes has a relatively large gradient due to the periodically arranged air-holes. In contrast, temperature distribution in SiO2 materials is a smooth curve, which is caused by the hindered heat conduction by air and the heat conductivity of air is far less than that of SiO2. Thereby, the relationships among laser power, temperature difference and angular frequency can be obtained by sinusoidal modulation as shown in Fig. 4. In Fig. 4, for the most part, the required laser power decreases with increasing angular frequency. Especially, when o is changing from 0.5 to 11.5 rad/s and then 11.5 to 5p rad/s, the laser power firstly decreases and reaches a minimum value, for which the laser power is 1.58 W at the angular frequency of 11.5 rad/s. Then, it increases with angular frequency. Moreover, the temperature difference along the fiber length decreases with increasing modulating angular frequency. The maximum value of temperature difference is 14.386 K at the angular frequency of 0.5 rad/s.
4. Experimental analysis We then performed a straightforward experiment to prove the theory we proposed on temperature characteristics of fusion splicing HC-PCF as shown in Fig. 5. In Fig. 5, the setup is composed of a CO2 laser, a laser modulator, transmitting energy fibers, focusing lens, and so on. The laser modulator is used to modulate the CO2 laser and different frequency of modulation signal can be obtained. The CO2 laser has a maximum output power of 40 W. The transmitting energy fiber is GeO2 dielectric hollow waveguide. The ZnSe lens with focal length 150 mm is used to couple laser into optical coupler. By focusing the laser signal, the HC-PCF can be fused. The laser spot diameter is controlled within the range from 500 to 800 mm. In the experiment, the sinusoidal modulation signal sin(ot) with different angular frequency o is used for heating HC-PCF ten times. Optris IR Plus2000 infrared thermometer is used for measuring temperature variations of heating zone
66
G. Fu et al. / Optics & Laser Technology 49 (2013) 64–67
Fig. 2. The temperature distribution along the HC-PCF length.
Fig. 3. The temperature distribution at the cross-section of fiber and along the radial distance.
Fig. 4. Relationships among the laser power, temperature difference and angular frequency. Fig. 6. Theoretical data and experimental results.
Fig. 5. Experimental setup.
and it can measure temperature range from 873 to 2273 K, with the accuracy of 70.5% and repeatability of 70.25%. A simple automatic control system for the thermometer is developed so that the measurement error can be reduced. Thereby, the experimental results and theoretical data are compared as shown in Fig. 6. In Fig. 6, the experimental results are basically consistent with the theoretical data. In modulation, experimental and theoretical temperature difference decreases with increasing angular frequency; the greater the angular frequency, the smaller the temperature difference. When o is changing from 0.5 to 5 rad/s, the temperature difference is more than 13 K, this system has
large temperature difference and achieves the purpose of modulation. In addition, the experimental temperature difference is larger than the theoretical, which declared that modulation is more effective in fusion splicing than simulation. When the modulating frequency is small, the air hole collapse is small, or can even be avoided. Moreover, the relationship between the splicing loss and the angular frequency in experiment is shown in Fig. 7. In Fig. 7, we can see that the splicing loss is very low when angular frequency is changing from 0.5 to 4 rad/s. Along with the angular frequency increasing, the splicing loss increases due to air-holes collapse. The minimum and maximum of splicing loss are 0.02 dB and 3.35 dB, respectively. However, there are some errors between experimental results and theoretical data which are as follows: (1) The stepper motor control of heating region may be not precise enough, so the HC-PCF heating zone can be affected.
G. Fu et al. / Optics & Laser Technology 49 (2013) 64–67
67
greatly reduced or even be avoided, resulting in a low-loss fusion splicing HC-PCF.
Acknowledgments The Project is supported by the National Natural Science Foundation of China (Nos. 61077067 and 60927009), the China Postdoctoral Science Foundation (No. 2012M510767), the Hebei Provincial Natural Science Foundation (Nos. F2012203148 and F2011203116), the Qinhuangdao Technology Development Program (No. 2012021A043), the Doctoral Funds of Yanshan University of China (Nos. B643 and B501). Fig. 7. The relationship between the splicing loss and the angular frequency.
Moreover, the heating time for HC-PCF is short, so the realtime measurement accuracy of thermometer may be inadequate. (2) In simulation, the mesh size in finite element method may be not detailed enough. The simulation time will increase as the detailed meshing increases. For example, the unit numbers of 189658 and grids numbers of 85634 are used in Fig. 2, and the unit numbers of 59662 and grids numbers of 29863 are used in Fig. 4. (3) Sinusoidal signal is created by the function generator and may have a little difference with that in simulation. So there may be a time delay leading to inaccurate heating time control.
5. Conclusion A method to control air-holes collapse in the heating process of HC-PCF has been proposed. A simple experiment was performed to verify the temperature characteristics of HC-PCF. Theoretical data and experimental results show that it is feasible to change the temperature distribution in fiber by frequency modulation CO2 laser. Thereby, the air-holes collapse can be
References [1] Knight JC, Birks TA, Russell J, Atkin DM. All-silica single-mode optical fiber with photonic crystal cladding. Optics Letters 1996;21(19):1547–9. [2] Russell J. Photonic crystal fibers. Journal of Lightwave Technology 2006;24(12):4729–49. [3] Russell J. Photonic crystal fibers. Science 2003;299(5605):358–62. [4] Cregan RF, Mangan BJ, Knight JC, Birks TA, Russell J, Roberts PJ, et al. Single-mode photonic band gap guidance of light in air. Science 1999;285(5433):1537–9. [5] Roberts PJ, Couny F, Sabert H, Mangan BJ, Williams DP, Farr L, et al. Ultimate low loss of hollow-core photonic crystal fibres. Optics Express 2005;13(1):236–44. [6] Mishra SS, Singh Vinod Kumar. Study of non-linear properties of hollow core photonic crystal fiber. Optik 2011;122(8):687–90. [7] Kristensen Jesper Toft, Houmann Andreas, Liu Xiaomin, Turchinovich Dmitry. Low-loss polarization-maintaining fusion splicing of single-mode fibers and hollow-core photonic crystal fibers, relevant for monolithic fiber laser pulse compression. Optics Express 2008;16(13):9986–95. [8] Thapa R, Corwin KL, Washburn BR. Arc fusion splicing of hollow-core photonicbandgap fibers for gas-filled fiber cells. Optics Express 2006;14(21):9576–83. [9] Carvalho JP, Magalha~ es F, Fraza~ o O, Santos JL, Arau´jo FM, Ferreira LA. Splicing and coupling losses in hollow-core photonic crystal glass fibers. Solid State Phenomena 2010;161:43–9. [10] Chong JH, Rao M. Development of a system for laser splicing photonic crystal fiber. Optics Express 2003;11(12):1365–70. [11] Chong JH, Rao MK, Zhu Y, Shum P. An effective splicing method on photonic crystal fiber using CO2 laser. IEEE Photonics Technology Letters 2003;15(7):942–4. [12] Grellier AJC, Zayer NK, Pannell CN. Heat transfer modelling in CO2 laser processing of optical fibres. Optics Communications 1998;152:324–8. [13] Kurosaki Yasuo. Radiative heat transfer in plastic welding process. Journal of Quantitative Spectroscopy & Radiative Transfer 2005;93:25–41.