Controllable parabolic lensed liquid-core optical fiber by using electrostatic force Chun Yin Tang,1 Xuming Zhang,1 Yang Chai,1 Long Hui,1 Lili Tao,1 and Yuen H. Tsang1,* 1
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hong Kong, China * [email protected]
Abstract: For typical optical fiber system, an external lens accessory set is required to adjust the optical path of output light, which however is limited by the fixed parameter of the lens accessory setup. Considering spherical aberration in the imaging process and its small focusable spot size, a complicated lens combination is required to compensate the aberration. This paper has demonstrated a unique method to fabricate liquid-core lensed fibers by filling water and NOA61 respectively into hollow Teflon AF fibers and silicate fiber, the radius of curvature of the liquid lens can be controlled by adjusting the applied voltage on the core liquid and even parabolic shape lens can be produced with enough applied voltage. The experiment has successfully demonstrated a variation of focal length from 0.628mm to 0.111mm responding to the change of applied voltage from 0V to 3.2KV (L = 2mm) for the Teflon AF fiber, as well as a variation of focal length from 0.274mm to 0.08mm responding to the change of applied voltage from 0V to 3KV (L = 2mm) for the silicate fiber. Further simulation shows that the focused spot size can be reduced to 2µm by adjusting the refractive index and fiber geometry. Solid state parabolic lensed fiber can be produced after NOA61 is solidified by the UV curing. ©2014 Optical Society of America OCIS codes: (230.3990) Micro-optical devices; (350.4600) Optical engineering; (230.7370) Waveguides; (220.0220) Optical design and fabrication.
References and links Y. H. Tsang, T. A. King, T. Thomas, C. Udell, and M. C. Pierce, “Efficient high power Yb3+ silica fiber laser cladding pumped at 1064nm,” Opt. Commun. 215(4–6), 381–387 (2003). 2. Y. H. Tsang, T. A. King, D. K. Ko, and J. Lee, “Output dynamics and stabilisation of a multi-mode double-clad Yb-doped silica fiber laser,” Opt. Commun. 259(1), 236–241 (2006). 3. Y. H. Tsang, A. F. El-Sherif, and T. A. King, “Broadband amplified spontaneous emission fiber source near 2μm using resonant in-band pumping,” J. Mod. Opt. 52, 109–118 (2005). 4. B. Zhou, L. Tao, Y. H. Tsang, W. Jin, and E. Y.-B. Pun, “Superbroadband near-IR photoluminescence from Pr3+-doped fluorotellurite glasses,” Opt. Express 20(4), 3803–3813 (2012). 5. J. John, T. S. M. Maclean, H. Ghafouri-Shiraz, and J. Niblett, “Matching of single-mode fiber to laser diode by microlense at 1.5um wavelength,” IEEE Proc. Optoelectron. 3, 178–184 (1994). 6. H. Liu, “The approximate ABCD matrix for a parabolic lens of revolution and its application in calculating the coupling efficiency,” Optik 119(14), 666–670 (2008). 7. Y. H. Tsang, D. J. Coleman, and T. A. King, “High power 1.9μm Tm3+-silica fiber laser pumped at 1.09 μm by a Yb3+-silica fiber laser,” Opt. Commun. 231(1-6), 357–364 (2004). 8. K. Y. Hung, et al., “Electrostatic force modulated microaspherical lens for optical pickup head,” J. Micromech Syst. 17(2), 370–380 (2008). 9. Y. Murakami, J.- Yamada, J.- Sakai, and T. Kimura, “Microlens tipped on a single-mode fiber end for InGaAsP laser coupling improvement,” Electron. Lett. 16(9), 321–322 (1980). 10. Y. T. Tsang, J. B. Huang, and W. J. Su, “Fabricating lensed fiber using a novel polishing method,” J. Manuf. Sci. Eng. 131(4), 041016 (2009). 11. A. Bozolan, C. J. S. de Matos, C. M. B. Cordeiro, E. M. Dos Santos, and J. Travers, “Supercontinuum generation in a water-core photonic crystal fiber,” Opt. Express 16(13), 9671–9676 (2008). 1.
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20948
12. R. Altkorn, I. Koev, R. P. Van Duyne, and M. Litorja, “Low-loss liquid-core optical fiber for low-refractiveindex liquids: fabrication, characterization, and application in Raman spectroscopy,” Appl. Opt. 36(34), 8992– 8998 (1997). 13. C.-Y. Tang, G. Bai, K. L. Jim, X. Zhang, K. H. Fung, Y. Chai, Y. H. Tsang, J. Yao, and D. Xu, “Lensed watercore Teflon-amorphous fluoroplastics optical fiber,” J. Lightwave Technol. 32(8), 1538–1542 (2014). 14. G. Bai, Y. H. Tsang, K. L. Jim, and X. M. Zhang, “UV-curable liquid-core fiber lenses with controllable focal length,” Opt. Express 21(5), 5505–5510 (2013). 15. G. F. Zheng, W. W. Li, X. Wang, H. Wang, D. H. Sun and L. W. Lin, “Experiment and simulation of coiled nanofiber deposition behavior from near-field electrospinning,” NEMs, 284-288 (2010). 16. R. Coelho and J. Debeaus, “Properties of the tip-plane configuration,” J. Phys. D Appl. Phys. 4(9), 1266–1280 (1971).
1. Introduction Fiber offers favorable geometry that possesses large surface area to volume ratio, long interaction length and small cross sectional area and therefore, it has been employed for various photonic devices e.g. fiber lasers [1, 2], broadband light source or amplifiers etc [3, 4]. Micro-lens fabricated directly on the end faces of the optical fiber can be used for efficient coupling of laser light from diode laser to the optical fibers, as well as coupling laser light from the fiber devices to another optical fiber. Such technology is essential for many applications including pumping the fiber amplifier of the optical communication system, diode laser fiber pig tails, fiber sensing and fiber laser systems [5–7]. The micro-parabolic lens on the fiber tip is a favorable choice compared with the lenses of hemispherical shape and other conventional coupling systems due to its higher coupling efficiency [6] for these fiber optical devices. Focusing lens for imaging is another potential application of the parabolic micro-lenses. Typically spherical lens is used most frequently in the imaging lens system, however, the spherical lens cannot bring on-axial and off-axial ray into the same point because of its spherical aberration, resulting in blurry image [8] or large focused spot size. However, parabolic lenses can further minimize the focusable spot size and also improve the image quality. Therefore, it is very important to carry out the research on focusing the output signal light out of the water core fiber into other micro-optoelectronic device or detector for further analysis, especially for high numerical aperture fibers with a high divergence angle. Typically, the complicated bulky focusing system is used as the focusing optics to focus signal light into the microelectronic devices. However, it involves expensive components, complicated alignment and assembly procedures, time consuming and complex mechanical polish or molten process [9, 10], and the great difficult fabrication of micro-sized parabolic lenses. Liquid core fiber has found increasing applications in supercontinuum generation [11] and Raman spectroscopy [12]. For biomedical application, water is a non-toxic basic solvent for lots of biochemical substances, such as blood or amniotic fluid, making it possible and practical to introduce biomedical species into water core fiber for further analysis. Therefore, water core optical fiber with controllable coupling power [13] was selected for this study. Additionally Norland Optical Adhesive NOA 61 was also used as one of the core liquids for this research because of its solidifiability under UV illumination [14]. In this paper, a unique method to fabricate liquid-core lensed fibers by filling water and NOA61, respectively, into hollow Teflon AF fibers and silicate fiber were demonstrated. The radius of curvature of the liquid lens can be controlled by adjusting the applied voltage on the core the fiber so as to produce parabolic shaped lens with enough applied voltage, followed by solidifying the NOA61 liquid photopolymer lensed fiber core under UV illumination to form a solid lensed fiber. 2. Experimental setup and results The variations of shapes and focal length of these liquid lenses with respect to different applied voltages have been quantified. Two types of liquid core fiber are used for these
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20949
experiments: Type 1: Teflon AF fiber cladding with DI water as the liquid core, the refractive indices of the fiber core and the cladding are 1.33 and 1.29, respectively, giving an NA value of 0.324. The diameters of core and cladding are 183.4 µm and 867.6 µm, respectively. Type 2: Silicate glass fiber cladding with NOA61 as the liquid core, the refractive index of core and cladding are 1.527 and 1.525, respectively, giving an NA value of 0.0781. The core and cladding diameters are 90 µm and 180 µm, respectively. Figure 1 shows the experimental setup in which a strong electric field is formed between the core liquid lenses and the conducting surface due to the applied voltage. The conducting surface is fabricated by spin coating a layer of RS Silver loaded electronically conductive paint on a 5 x 10 mm stainless steel plate soldered with an electrical wire. The conducting surface also acts as a 45 degree titled view screen for the spot size measurement. Initially, a pressure is applied to the fiber core using a flow rate programmable fusion pump with 180N thrust to push the liquid slightly out of the fiber end. This initial push is essential as the available voltage is not high enough to move out the core liquid. Besides a copper wire with 100µm diameter is inserted into the fiber core through the liquid inlet to form the electrical contact to the core liquid. Afterwards, with the gradual increase of applied voltage, the major axis of the elliptical light spot size projected on the screen and the geometry of the induced liquid lens can be visualized by the optical microscope. The simulation of the projected spot size on the screen is achieved by using the TracePro software.
Fig. 1. Schematic diagram of the experimental set-up to produce variable focal length of the lensed liquid core fiber under the strong electrical field created at the fiber tip.
Fig. 2. (a) & (b) The schematic view and photo show the variation of radii of the water lenses under the increased electrostatic force, the aperture is limited by the Teflon AF fiber core diameter. (c) & (d) The schematic view and photo show that more liquid comes out from the core, the lens aperture is limited by the cladding diameter. (e) The phase profile of the radius of curvature of Teflon AF fiber liquid lens with respect to different applied voltages (Correspond to Fig. 2(b) and the distance between fiber tip and conducting plane L = 2mm. The phase profile is produced by fitting the equation of parabola curve to the experimental data, the R2 of the fitting is in between 0.933 to 0.992.)
The aperture of the electric field induced lens is affected by the fiber geometry and the attraction force between the liquid and the cladding material. For example, Figs. 2(a) and 2(b)
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20950
show that the hydrophobic nature of Teflon AF and water, lens aperture is limited by the fiber core. Nevertheless, the aperture can go as larger as the core diameter when high voltage is applied. Figures 2(c) and 2(d) show the water lenses with large apertures but different shapes. In the case of Type I fiber, the aperture of the water lens can be limited either by core diameter or the cladding diameter, depending on the applied voltage. Once the voltage is applied to the liquid core, positive charges are accumulated on the surface of the liquid lens, resulting in the formation of E-field between the liquid lens and the conductor plane. According to the Tip-plane model [15,16], the E-field can be represented by E( X ) =
L ⋅V 1 ⋅ X (2 L − X ) + ( L − X )r In[2( L / r )1/ 2 ]
(1)
where V is the applied voltage, L is the distance between the fiber tip and the conductor plane, r represents the radius of the fiber core or the cladding depending on the lens limited by either the core diameter or cladding diameter. X is the distance from the fiber tip to the calculated position. With the formation of the E-field at the apex of the liquid lens, electroosmosis flow occurs and carries the core liquid toward the apex of the lens [8]. Nevertheless, a pressure field is induced to balance the osmosis flow. The result net flow v is, v = −(
h2 ε Eϕ ) − ( )∇p η 3η
(2)
where ε is the dielectric constant, η is the viscosity of the core liquid, φ is the surface potential, and h is the maximum liquid lens thickness. In the equilibrium, the net flow v is equal to zero and the applied voltage V(x,y) is proportional to the induced pressure p(x,y) by
−εφV ( x, y ) = 1/ 3(h 2 p ( x, y ))
(3)
The induce pressure field is inversely proportional to radius of curvature of the lens R by p = 2 γ /R, where γ is the liquid surface tension and further on, 1/ R = 3 / 2(εφ / γ ⋅ h 2 )V ( x, y)
(4)
These equations indicates a decrease in the radius of curvature of the liquid lens with the increase of the applied voltage.
Fig. 3. (a) Experimental results show the relationship between the applied voltage and the radius of curvature of the lens of the two types of fibers (distance between fiber tip and conducting plane L = 2mm). (b) The corresponding variation of the focal length of the liquid lenses with respect to the applied voltage.
The viscosity of liquid plays an important role in the sensitivity of the changes of the radius of curvature of the lens with respect to the applied voltage. Figure 3(a) shows the change of radius of curvature of the lens with the applied voltage. The two types of fibers
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20951
share the same trend that higher voltage leads to smaller radius of curvature of the lens. However, the silicate fiber with NAO61 has a smaller slope (i.e., lower sensitivity). This can be explained by Eq. (2) that the higher viscosity of NOA61 lens leads to smaller net flow of liquid toward center of lens under the same voltage. These experiments have also measured the sensitive change in radius of curvature of the lens to applied voltage higher than 2.6 kV when turning the lens into parabolic shape. For the two types of fibers, the parabolic lens started to transform into Talyor cone at the voltage of 3.2kV. The produced Talyor cone is shown in the inset of Fig. 3(b). From Fig. 3(b), it can be concluded that a variation of focal length from 0.628mm to 0.111mm responding to the change of applied voltage from 0V to 3.2KV (L = 2mm) for the Type 1: Teflon AF fiber, as well as a variation of focal length from 0.274mm to 0.08mm responding to the change of applied voltage from 0V to 3KV (L = 2mm) for the Type 2: silicate fiber.
Fig. 4. (a) The change of the projected laser spot size diameter (the major axis of the elliptical spot on the screen) with respect to various focal lengths of the NOA61 liquid lens. Blue square: calculated size; Red circle with error bar: measured size. (L = 0.8 mm). (b) The simulated spot size diameter at the focus against different value of refractive index of core liquid of Teflon AF cladding fiber (Refractive index of Teflon is 1.29). Hollow core diameter = 8 μm, cladding diameter = 125 μm, the radius of curvature of the parabolic lens apex is 3.11 μm and its thickness is 5.16 μm.
According Eq. (1), reducing the distance between the fiber end and conducting surface can further enhance the produced E-field with respect to the applied voltage and therefore the tunable range of the radius of curvature of the liquid lens can be increased with respect to available applied voltage. The tunable range of liquid lens is also related to the surface tension and conductivity of the liquid and therefore some additive that may changes these parameters can further enlarge the tunable range as well. Type 2 fiber has smaller projected spot size produced on screen compared with Type 1 fiber due to its much smaller NA value and dimension. Therefore, silicate fiber is chosen to demonstrate the projected spot size variation with respect to different focal length through adjusting the applied voltages and the results are shown in Fig. 4(a). The projected spot size is reduced by increasing focal length as the screen is placed beyond the focus length of the liquid lens (see Fig. 1 for details). The Type 2: silicate-NOA61 fiber was solidified by the UV curing, with the maintenance of the applied voltage at 2.9kV. The radius of curvature of the apex of the produced parabolic NOA61 lens is 55.5 μm and the thickness of the lens is 94 μm, given the theoretical focal length of 0.13mm. The solidified lens surface remains smooth. The theoretical major axis of the elliptical projected spot on screen with L = 0.236mm is 100.9 μm and the measured value is 104.3 μm, the difference is 3.36%. The error may partly due to the 1.5% of the linear shrinkage of the NOA61 after solidification. Further spot size reduction of lensed water-core optical fiber
Due to high NA value and large core diameter of Type 1 fiber, no focused spot from Type 1 fiber is achieved experimentally. However, it is possible to further reduce the produced focused spot size down to around 2μm by using standard single mode optical fiber geometry with core and cladding diameters of 8 μm and 125 μm and refractive index close to the Teflon #213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20952
as computer simulation shown in Fig. 4(b) assuming parabolic lens formed with the highest thickness of 5.16μm Under the same conditions for Type 2 fiber, the smallest simulated spot size calculated is close to 1.2μm at focus after UV solidification. 3. Conclusions
In this study, it has demonstrated a unique method to fabricate liquid-core lensed fibers with controllable lens shape (semi-spherical / parabolic) and radius of curvature of the lens by adjusting the applied voltage. The experiment has successfully demonstrated a variation of focal length from 0.628mm to 0.111mm responding to the change of applied voltage from 0V to 3.2KV (L = 2mm) for the Teflon AF fiber (water core dia. 183.4 µm, Teflon cladding dia. 867.6 µm, NA = 0.324), as well as a variation of focal length from 0.274mm to 0.08mm responding to the change of applied voltage from 0V to 3KV (L = 2mm) for the silicate fiber (NOA61 core dia. 90 µm, Silicate cladding dia. 180 µm, NA = 0.0781). Further simulation shows that the focused spot size can be reduced to 2µm by adjusting the refractive index and fiber geometry. Solid state parabolic lensed fiber was produced after NOA61 is solidified by the UV curing. The demonstrated method is practical and can potentially be used to produce voltage controlled parabolic lensed liquid or solid fibers. Acknowledgments
This work is financially supported by the Research Grants Council (RGC) of Hong Kong (GRF 526511/PolyU B-Q26E, ECS 533412/PolyUF-PP07) and PolyU internal research grants G-YL06, G-YM44.)
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20953
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hong Kong, China * [email protected]
Abstract: For typical optical fiber system, an external lens accessory set is required to adjust the optical path of output light, which however is limited by the fixed parameter of the lens accessory setup. Considering spherical aberration in the imaging process and its small focusable spot size, a complicated lens combination is required to compensate the aberration. This paper has demonstrated a unique method to fabricate liquid-core lensed fibers by filling water and NOA61 respectively into hollow Teflon AF fibers and silicate fiber, the radius of curvature of the liquid lens can be controlled by adjusting the applied voltage on the core liquid and even parabolic shape lens can be produced with enough applied voltage. The experiment has successfully demonstrated a variation of focal length from 0.628mm to 0.111mm responding to the change of applied voltage from 0V to 3.2KV (L = 2mm) for the Teflon AF fiber, as well as a variation of focal length from 0.274mm to 0.08mm responding to the change of applied voltage from 0V to 3KV (L = 2mm) for the silicate fiber. Further simulation shows that the focused spot size can be reduced to 2µm by adjusting the refractive index and fiber geometry. Solid state parabolic lensed fiber can be produced after NOA61 is solidified by the UV curing. ©2014 Optical Society of America OCIS codes: (230.3990) Micro-optical devices; (350.4600) Optical engineering; (230.7370) Waveguides; (220.0220) Optical design and fabrication.
References and links Y. H. Tsang, T. A. King, T. Thomas, C. Udell, and M. C. Pierce, “Efficient high power Yb3+ silica fiber laser cladding pumped at 1064nm,” Opt. Commun. 215(4–6), 381–387 (2003). 2. Y. H. Tsang, T. A. King, D. K. Ko, and J. Lee, “Output dynamics and stabilisation of a multi-mode double-clad Yb-doped silica fiber laser,” Opt. Commun. 259(1), 236–241 (2006). 3. Y. H. Tsang, A. F. El-Sherif, and T. A. King, “Broadband amplified spontaneous emission fiber source near 2μm using resonant in-band pumping,” J. Mod. Opt. 52, 109–118 (2005). 4. B. Zhou, L. Tao, Y. H. Tsang, W. Jin, and E. Y.-B. Pun, “Superbroadband near-IR photoluminescence from Pr3+-doped fluorotellurite glasses,” Opt. Express 20(4), 3803–3813 (2012). 5. J. John, T. S. M. Maclean, H. Ghafouri-Shiraz, and J. Niblett, “Matching of single-mode fiber to laser diode by microlense at 1.5um wavelength,” IEEE Proc. Optoelectron. 3, 178–184 (1994). 6. H. Liu, “The approximate ABCD matrix for a parabolic lens of revolution and its application in calculating the coupling efficiency,” Optik 119(14), 666–670 (2008). 7. Y. H. Tsang, D. J. Coleman, and T. A. King, “High power 1.9μm Tm3+-silica fiber laser pumped at 1.09 μm by a Yb3+-silica fiber laser,” Opt. Commun. 231(1-6), 357–364 (2004). 8. K. Y. Hung, et al., “Electrostatic force modulated microaspherical lens for optical pickup head,” J. Micromech Syst. 17(2), 370–380 (2008). 9. Y. Murakami, J.- Yamada, J.- Sakai, and T. Kimura, “Microlens tipped on a single-mode fiber end for InGaAsP laser coupling improvement,” Electron. Lett. 16(9), 321–322 (1980). 10. Y. T. Tsang, J. B. Huang, and W. J. Su, “Fabricating lensed fiber using a novel polishing method,” J. Manuf. Sci. Eng. 131(4), 041016 (2009). 11. A. Bozolan, C. J. S. de Matos, C. M. B. Cordeiro, E. M. Dos Santos, and J. Travers, “Supercontinuum generation in a water-core photonic crystal fiber,” Opt. Express 16(13), 9671–9676 (2008). 1.
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20948
12. R. Altkorn, I. Koev, R. P. Van Duyne, and M. Litorja, “Low-loss liquid-core optical fiber for low-refractiveindex liquids: fabrication, characterization, and application in Raman spectroscopy,” Appl. Opt. 36(34), 8992– 8998 (1997). 13. C.-Y. Tang, G. Bai, K. L. Jim, X. Zhang, K. H. Fung, Y. Chai, Y. H. Tsang, J. Yao, and D. Xu, “Lensed watercore Teflon-amorphous fluoroplastics optical fiber,” J. Lightwave Technol. 32(8), 1538–1542 (2014). 14. G. Bai, Y. H. Tsang, K. L. Jim, and X. M. Zhang, “UV-curable liquid-core fiber lenses with controllable focal length,” Opt. Express 21(5), 5505–5510 (2013). 15. G. F. Zheng, W. W. Li, X. Wang, H. Wang, D. H. Sun and L. W. Lin, “Experiment and simulation of coiled nanofiber deposition behavior from near-field electrospinning,” NEMs, 284-288 (2010). 16. R. Coelho and J. Debeaus, “Properties of the tip-plane configuration,” J. Phys. D Appl. Phys. 4(9), 1266–1280 (1971).
1. Introduction Fiber offers favorable geometry that possesses large surface area to volume ratio, long interaction length and small cross sectional area and therefore, it has been employed for various photonic devices e.g. fiber lasers [1, 2], broadband light source or amplifiers etc [3, 4]. Micro-lens fabricated directly on the end faces of the optical fiber can be used for efficient coupling of laser light from diode laser to the optical fibers, as well as coupling laser light from the fiber devices to another optical fiber. Such technology is essential for many applications including pumping the fiber amplifier of the optical communication system, diode laser fiber pig tails, fiber sensing and fiber laser systems [5–7]. The micro-parabolic lens on the fiber tip is a favorable choice compared with the lenses of hemispherical shape and other conventional coupling systems due to its higher coupling efficiency [6] for these fiber optical devices. Focusing lens for imaging is another potential application of the parabolic micro-lenses. Typically spherical lens is used most frequently in the imaging lens system, however, the spherical lens cannot bring on-axial and off-axial ray into the same point because of its spherical aberration, resulting in blurry image [8] or large focused spot size. However, parabolic lenses can further minimize the focusable spot size and also improve the image quality. Therefore, it is very important to carry out the research on focusing the output signal light out of the water core fiber into other micro-optoelectronic device or detector for further analysis, especially for high numerical aperture fibers with a high divergence angle. Typically, the complicated bulky focusing system is used as the focusing optics to focus signal light into the microelectronic devices. However, it involves expensive components, complicated alignment and assembly procedures, time consuming and complex mechanical polish or molten process [9, 10], and the great difficult fabrication of micro-sized parabolic lenses. Liquid core fiber has found increasing applications in supercontinuum generation [11] and Raman spectroscopy [12]. For biomedical application, water is a non-toxic basic solvent for lots of biochemical substances, such as blood or amniotic fluid, making it possible and practical to introduce biomedical species into water core fiber for further analysis. Therefore, water core optical fiber with controllable coupling power [13] was selected for this study. Additionally Norland Optical Adhesive NOA 61 was also used as one of the core liquids for this research because of its solidifiability under UV illumination [14]. In this paper, a unique method to fabricate liquid-core lensed fibers by filling water and NOA61, respectively, into hollow Teflon AF fibers and silicate fiber were demonstrated. The radius of curvature of the liquid lens can be controlled by adjusting the applied voltage on the core the fiber so as to produce parabolic shaped lens with enough applied voltage, followed by solidifying the NOA61 liquid photopolymer lensed fiber core under UV illumination to form a solid lensed fiber. 2. Experimental setup and results The variations of shapes and focal length of these liquid lenses with respect to different applied voltages have been quantified. Two types of liquid core fiber are used for these
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20949
experiments: Type 1: Teflon AF fiber cladding with DI water as the liquid core, the refractive indices of the fiber core and the cladding are 1.33 and 1.29, respectively, giving an NA value of 0.324. The diameters of core and cladding are 183.4 µm and 867.6 µm, respectively. Type 2: Silicate glass fiber cladding with NOA61 as the liquid core, the refractive index of core and cladding are 1.527 and 1.525, respectively, giving an NA value of 0.0781. The core and cladding diameters are 90 µm and 180 µm, respectively. Figure 1 shows the experimental setup in which a strong electric field is formed between the core liquid lenses and the conducting surface due to the applied voltage. The conducting surface is fabricated by spin coating a layer of RS Silver loaded electronically conductive paint on a 5 x 10 mm stainless steel plate soldered with an electrical wire. The conducting surface also acts as a 45 degree titled view screen for the spot size measurement. Initially, a pressure is applied to the fiber core using a flow rate programmable fusion pump with 180N thrust to push the liquid slightly out of the fiber end. This initial push is essential as the available voltage is not high enough to move out the core liquid. Besides a copper wire with 100µm diameter is inserted into the fiber core through the liquid inlet to form the electrical contact to the core liquid. Afterwards, with the gradual increase of applied voltage, the major axis of the elliptical light spot size projected on the screen and the geometry of the induced liquid lens can be visualized by the optical microscope. The simulation of the projected spot size on the screen is achieved by using the TracePro software.
Fig. 1. Schematic diagram of the experimental set-up to produce variable focal length of the lensed liquid core fiber under the strong electrical field created at the fiber tip.
Fig. 2. (a) & (b) The schematic view and photo show the variation of radii of the water lenses under the increased electrostatic force, the aperture is limited by the Teflon AF fiber core diameter. (c) & (d) The schematic view and photo show that more liquid comes out from the core, the lens aperture is limited by the cladding diameter. (e) The phase profile of the radius of curvature of Teflon AF fiber liquid lens with respect to different applied voltages (Correspond to Fig. 2(b) and the distance between fiber tip and conducting plane L = 2mm. The phase profile is produced by fitting the equation of parabola curve to the experimental data, the R2 of the fitting is in between 0.933 to 0.992.)
The aperture of the electric field induced lens is affected by the fiber geometry and the attraction force between the liquid and the cladding material. For example, Figs. 2(a) and 2(b)
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20950
show that the hydrophobic nature of Teflon AF and water, lens aperture is limited by the fiber core. Nevertheless, the aperture can go as larger as the core diameter when high voltage is applied. Figures 2(c) and 2(d) show the water lenses with large apertures but different shapes. In the case of Type I fiber, the aperture of the water lens can be limited either by core diameter or the cladding diameter, depending on the applied voltage. Once the voltage is applied to the liquid core, positive charges are accumulated on the surface of the liquid lens, resulting in the formation of E-field between the liquid lens and the conductor plane. According to the Tip-plane model [15,16], the E-field can be represented by E( X ) =
L ⋅V 1 ⋅ X (2 L − X ) + ( L − X )r In[2( L / r )1/ 2 ]
(1)
where V is the applied voltage, L is the distance between the fiber tip and the conductor plane, r represents the radius of the fiber core or the cladding depending on the lens limited by either the core diameter or cladding diameter. X is the distance from the fiber tip to the calculated position. With the formation of the E-field at the apex of the liquid lens, electroosmosis flow occurs and carries the core liquid toward the apex of the lens [8]. Nevertheless, a pressure field is induced to balance the osmosis flow. The result net flow v is, v = −(
h2 ε Eϕ ) − ( )∇p η 3η
(2)
where ε is the dielectric constant, η is the viscosity of the core liquid, φ is the surface potential, and h is the maximum liquid lens thickness. In the equilibrium, the net flow v is equal to zero and the applied voltage V(x,y) is proportional to the induced pressure p(x,y) by
−εφV ( x, y ) = 1/ 3(h 2 p ( x, y ))
(3)
The induce pressure field is inversely proportional to radius of curvature of the lens R by p = 2 γ /R, where γ is the liquid surface tension and further on, 1/ R = 3 / 2(εφ / γ ⋅ h 2 )V ( x, y)
(4)
These equations indicates a decrease in the radius of curvature of the liquid lens with the increase of the applied voltage.
Fig. 3. (a) Experimental results show the relationship between the applied voltage and the radius of curvature of the lens of the two types of fibers (distance between fiber tip and conducting plane L = 2mm). (b) The corresponding variation of the focal length of the liquid lenses with respect to the applied voltage.
The viscosity of liquid plays an important role in the sensitivity of the changes of the radius of curvature of the lens with respect to the applied voltage. Figure 3(a) shows the change of radius of curvature of the lens with the applied voltage. The two types of fibers
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20951
share the same trend that higher voltage leads to smaller radius of curvature of the lens. However, the silicate fiber with NAO61 has a smaller slope (i.e., lower sensitivity). This can be explained by Eq. (2) that the higher viscosity of NOA61 lens leads to smaller net flow of liquid toward center of lens under the same voltage. These experiments have also measured the sensitive change in radius of curvature of the lens to applied voltage higher than 2.6 kV when turning the lens into parabolic shape. For the two types of fibers, the parabolic lens started to transform into Talyor cone at the voltage of 3.2kV. The produced Talyor cone is shown in the inset of Fig. 3(b). From Fig. 3(b), it can be concluded that a variation of focal length from 0.628mm to 0.111mm responding to the change of applied voltage from 0V to 3.2KV (L = 2mm) for the Type 1: Teflon AF fiber, as well as a variation of focal length from 0.274mm to 0.08mm responding to the change of applied voltage from 0V to 3KV (L = 2mm) for the Type 2: silicate fiber.
Fig. 4. (a) The change of the projected laser spot size diameter (the major axis of the elliptical spot on the screen) with respect to various focal lengths of the NOA61 liquid lens. Blue square: calculated size; Red circle with error bar: measured size. (L = 0.8 mm). (b) The simulated spot size diameter at the focus against different value of refractive index of core liquid of Teflon AF cladding fiber (Refractive index of Teflon is 1.29). Hollow core diameter = 8 μm, cladding diameter = 125 μm, the radius of curvature of the parabolic lens apex is 3.11 μm and its thickness is 5.16 μm.
According Eq. (1), reducing the distance between the fiber end and conducting surface can further enhance the produced E-field with respect to the applied voltage and therefore the tunable range of the radius of curvature of the liquid lens can be increased with respect to available applied voltage. The tunable range of liquid lens is also related to the surface tension and conductivity of the liquid and therefore some additive that may changes these parameters can further enlarge the tunable range as well. Type 2 fiber has smaller projected spot size produced on screen compared with Type 1 fiber due to its much smaller NA value and dimension. Therefore, silicate fiber is chosen to demonstrate the projected spot size variation with respect to different focal length through adjusting the applied voltages and the results are shown in Fig. 4(a). The projected spot size is reduced by increasing focal length as the screen is placed beyond the focus length of the liquid lens (see Fig. 1 for details). The Type 2: silicate-NOA61 fiber was solidified by the UV curing, with the maintenance of the applied voltage at 2.9kV. The radius of curvature of the apex of the produced parabolic NOA61 lens is 55.5 μm and the thickness of the lens is 94 μm, given the theoretical focal length of 0.13mm. The solidified lens surface remains smooth. The theoretical major axis of the elliptical projected spot on screen with L = 0.236mm is 100.9 μm and the measured value is 104.3 μm, the difference is 3.36%. The error may partly due to the 1.5% of the linear shrinkage of the NOA61 after solidification. Further spot size reduction of lensed water-core optical fiber
Due to high NA value and large core diameter of Type 1 fiber, no focused spot from Type 1 fiber is achieved experimentally. However, it is possible to further reduce the produced focused spot size down to around 2μm by using standard single mode optical fiber geometry with core and cladding diameters of 8 μm and 125 μm and refractive index close to the Teflon #213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20952
as computer simulation shown in Fig. 4(b) assuming parabolic lens formed with the highest thickness of 5.16μm Under the same conditions for Type 2 fiber, the smallest simulated spot size calculated is close to 1.2μm at focus after UV solidification. 3. Conclusions
In this study, it has demonstrated a unique method to fabricate liquid-core lensed fibers with controllable lens shape (semi-spherical / parabolic) and radius of curvature of the lens by adjusting the applied voltage. The experiment has successfully demonstrated a variation of focal length from 0.628mm to 0.111mm responding to the change of applied voltage from 0V to 3.2KV (L = 2mm) for the Teflon AF fiber (water core dia. 183.4 µm, Teflon cladding dia. 867.6 µm, NA = 0.324), as well as a variation of focal length from 0.274mm to 0.08mm responding to the change of applied voltage from 0V to 3KV (L = 2mm) for the silicate fiber (NOA61 core dia. 90 µm, Silicate cladding dia. 180 µm, NA = 0.0781). Further simulation shows that the focused spot size can be reduced to 2µm by adjusting the refractive index and fiber geometry. Solid state parabolic lensed fiber was produced after NOA61 is solidified by the UV curing. The demonstrated method is practical and can potentially be used to produce voltage controlled parabolic lensed liquid or solid fibers. Acknowledgments
This work is financially supported by the Research Grants Council (RGC) of Hong Kong (GRF 526511/PolyU B-Q26E, ECS 533412/PolyUF-PP07) and PolyU internal research grants G-YL06, G-YM44.)
#213899 - $15.00 USD Received 11 Jun 2014; revised 18 Jul 2014; accepted 18 Jul 2014; published 21 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. 17 | DOI:10.1364/OE.22.020948 | OPTICS EXPRESS 20953
