Laser printed fiber microlens for fiber-diode coupling by direct laser writing Hongmei Zou,1 He Huang,1 Shijie Chen,1 Qing Li,1 Jian Fu,1 Feng Lin,2 and X. Wu1,* 1
State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China 2
School of Computer Engineering, Nanyang Technological University, 637553, Singapore *Corresponding author: [email protected] Received 2 September 2014; revised 24 October 2014; accepted 15 November 2014; posted 17 November 2014 (Doc. ID 221829); published 17 December 2014
A printable microlens on fiber end for coupling between a laser diode to a single-mode fiber (SMF) is demonstrated. The fiber microlens fabricated by the novel technique based on laser direct writing using a quasi-Bessel beam has gained high coupling efficiency, long working distance, and sufficient alignment tolerance. A coupling efficiency of 53.5% was measured for SMF coupling at a working distance of 16 μm. The tolerances for a 1-dB loss increment for translational displacements and angular deviations between the fiber microlens and the laser diode were 2.5 and 1.2 μm, and 2.0 and 5.0 degrees, respectively. The printable fabrication of a microlens on a fiber endface by laser direct writing allows for a batch process to reproduce a multiple microlens with a high consistency in a simple and fast fabrication cycle, with no need of individual fiber loading and unloading. © 2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.4005) Microstructured fibers. http://dx.doi.org/10.1364/AO.53.008444
1. Introduction
The elliptical mode field of a laser diode provides a low direct coupling efficiency between a laser diode and a single-mode fiber (SMF). To improve mode match and coupling efficiency, a common approach is to fabricate a microlens on a fiber tip for the SMF and LD to couple directly. Up until now, there were several methods of microlens fabrication, such as heating and drawing fiber [1], hydrofluoride (HF) chemical etching [2,3], micromachining with a CO2 laser [4,5], and grinding followed by polishing [6–10]. Among these methods, the most commonly used at present are the mechanical grinding/polishing and chemical etching techniques, with a high coupling efficiency of around 70% [7,8]. Several complex structures have been fabricated based on grind and polish techniques to achieve even higher coupling efficiency and alignment tolerances, including 1559-128X/14/368444-05$15.00/0 © 2014 Optical Society of America 8444
APPLIED OPTICS / Vol. 53, No. 36 / 20 December 2014
quadrangular-pyramid-shaped fiber endfaces [6], conical-wedge-shaped fiber endfaces [7], asymmetric elliptic-cone-shaped microlenses [8], double-variablecurvature microlenses [9], and hyperboloid microlenses [10]. Although some structures can obtain a coupling efficiency above 80%, the process of grinding demands a precise control over the endface orientation and offset, and some microlenses require a high intensity arc fusion process to polish and reshape [7–10]. A chemical etching technique is typically performed by etching the fibers in a HF solution, then fusing the fiber tip with a splicing machine to form a microlens. The coupling efficiency of this method was reported to be as high as 80%–90% [2]. However, the fabrication process involves the use of HF, a hypertoxic chemical containing latent risks of accidental inhalation or contact with the skin, which can lead to incurable burns. A common requirement of all the aforementioned fabrication techniques is the utilization of individual
fiber loading and unloading not suitable for batch fabrication. In an ever-increasing market demand for laser-fiber coupling, a technique capable of mass production of fiber lens on fiber endface will be of great interest. In search of such a fabrication process, much effort has been devoted to feature both the ability to be continually printable on a substrate and a cost-effective creation of highly consistent optical-quality 3D surfaces [11,12]. In this study, we demonstrate a novel fabrication process for a fiber microlens by laser direct writing. We utilize a focused Bessel beam to form a microlens on the fiber endface. A single laser pulse with a cone-shaped beam has been used to expose UV optical epoxy, producing a fiber microlens with optical surface finishing and size uniformity. Through controlling the writing parameters such as beam shape, exposure time, and development process, we were able to generate a microlens at various form parameters to optimize fiber coupling. 2. Fabrication
The setup for laser direct writing fabrication of fiber microlenses is illustrated in Fig. 1 (Upper), which includes the following devices: an individual custom made fiber-axicon on a single mode fiber tip with an apex angle ranging from 75° to 125° and a surface roughness of about 0.02 to 0.04 μm to generate Bessel beams, a high precision 3D piezoelectric nanostage with a movement resolution of 20 nm (Thorlabs Nano
Fig. 1. (Upper) Schematic of setup for fiber microlenses fabrication; (lower) (a) coating of UV curable epoxy to a fiber end; (b) laser exposure with a quasi-Bessel beam; (c) removal of residual epoxy using an air knife; (d) post-exposure UV curing.
Max, Max311D), an angle adjuster used for both horizontal and vertical angle control of the fiber end, and a light emitting diode (LED) to provide the illumination needed for in situ CMOS-monitoring of the fabrication sequence. The microscope camera measures the distance between the tip of the fiber axicon and the fiber end, which is coated with UV curable epoxy (DSM, G.652 optical fiber secondary coating). The entire process of direct writing is automated by the custom in-house programming environment. Figure 1 (lower) (a)–(d) displays the sequence of fabrication [11]. UV curable optical epoxy was coated onto a fiber end by dip-coating. The viscosity of the epoxy was kept at ∼4800 mPa · s at 50 s−1 to maintain a uniform coating thickness by controlling the working temperature at 20°C. The thickness of the coated film was 50 μm, with a curved edge on the circumference due to surface tension. Alignment between the fiber and the fiber-axicon was achieved by adjusting the three-dimensional translation stage and the angle adjuster. A quasi-Bessel beam was then used to expose the UV curable epoxy. The exposed viscous liquid prepolymer undergoes a crosslinking reaction upon UV exposure, and solidifies immediately. The unexposed prepolymer was removed both mechanically and chemically. A post-exposure treatment was then conducted to the exposed polymer to develop direct-writing patterns. Instead of using traditional wet developing methods, which were found to be difficult in maintaining the fine feature of preliminarily solidified epoxy, we developed an air knife-based technique. Compressed air (0.2 Mpsi) was pressured out from air knife openings onto the plant fiber end with variable angles ranging from 20° to 40° with respect to the fiber endface, driving the uncured epoxy off the fiber end. The air knife was found to be capable of blowing off approximately 90% of the unexposed epoxy, with the remaining unexposed UV epoxy cleaned off by spraying a small amount of ethanol onto the fiber end to enhance the air knife developing process. After this pattern developing process, the sample was placed into a UV irradiance chamber (a 20 W mercury lamp) for 4 min, where the epoxy structure was thoroughly cured to reach a stable refractive index of 1.475. This process also improves surface finishing by re-flowing the uncured epoxy over the cured structure, forming a smooth optical surface with the surface tension of UV epoxy. This setup is capable of generating various microlenses depending on the laser beam profile and size of the UV illumination on the epoxy. With the fiberaxicon used in the experiment, a single mode laser beam traveling in the fiber was converted from a Gaussian profile to a Bessel-like beam (λ 408 nm, minimum FWHM 1.5 μm, maximum output power 20 mW). By using fiber-axicons with different apex angles, microlenses and lens arrays with various shapes can be fabricated [11]. Figure 2 shows a microscope image of a fabricated microlens on the SMF 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS
8445
Fig. 2. Microscopic image of a fabricated microlens on a singlemode fiber endface.
endface. The shape of the fiber microlens has an optimized coupling efficiency among the different shapes that were fabricated and measured. Other measured shapes with larger or smaller radius of inscribed circle (defined in the next section) showed lower coupling efficiencies. 3. Measurements and Results
A 650 nm laser diode with typical far-field divergence angles of 6.5°horizontal × 35°vertical, and an aspect ratio of 5 was used to measure the coupling efficiency between the fabricated fiber microlens and laser. A single mode fiber with an operating wavelength of 980–1600 nm (Nufern1060-XP, mode field diameter of 5.9 0.5 μm at 980 nm and 9.5 0.5 μm at 1550 nm) was used in both experiment and modeling. Microlenses with different shapes were tested to evaluate the correlation between shape and the coupling efficiency. The radius of curvature of the lens tip is described by using an inscribed circle (as shown in Fig. 3). Figure 3 shows the relationship between the coupling efficiency and the radius of the inscribed circle. It was found experimentally that for an inscribed circle radius ranging from 6 to 20 μm, the lens height for maximized coupling is inversely proportional to
Fig. 3. Measured coupling efficiency as a function of the inscribed circle radius. 8446
APPLIED OPTICS / Vol. 53, No. 36 / 20 December 2014
the inscribed circle radius. For an optimized inscribed radius of 8.1 μm, the lens height was found to be 50 μm with a tolerance of 5∕ − 0 μm and a maximum coupling efficiency of 53.5% was obtained. When the radius was below 7.5 μm, the coupling efficiency was observed to decrease rapidly. For example, the coupling efficiency was at 27.7% when the radius reached 7.1 μm. The fiber microlens with an inscribed circle radius of 8.1 μm was obtained by using a fiber axicon with an apex angle of 120°, and the distance between the fiber-axicon and the UV curable epoxy was 2 μm; the laser power was set to be 50 μW and exposed for 1 s. Utilizing the microlens with optimized coupling efficiency, we further investigated the position/angle tolerance of the coupling efficiency. The experimental measurement was compared with simulated result using the ZEMAX software program where both the microlens and laser diode were modeled by curve fitting to lens and emission specification of the laser diode. Figures 4(a)–4(c) display both measured and calculated coupling efficiency as a function of the working distance, horizontal, and vertical displacement. Defining the separation between the LD and the fiber microlens tip as the working distance, Fig. 4(a) shows the dependence of coupling efficiency on working distance. Coupling efficiency peaks when the working distance was at 15.8 μm, in comparison to a simulated optimal working distance at 15.2 μm. The coupling efficiency falls off fast when the working distance was either below 14 μm or above 18 μm. The lateral misalignment between the LD and the fiber results in a decrease of the coupling efficiency. Figure 4(b) shows the coupling efficiency as a function of horizontal displacement. The measured result was in a good agreement with that of simulation. A horizontal displacement of 2 μm yields a 10% reduction in coupling efficiency. Figure 4(c) shows that the coupling efficiency dropped by 12% for a vertical displacement of 1 μm. But the simulated vertical misalignment is more sensitive; a 1 μm of vertical displacement causes a drop in coupling efficiency by 17%. The discrepancy between the measured and calculated results is likely to be due to two reasons: (a) shape of modeled lens deviates from the actual form and (b) asymmetric distribution differs from the specified distribution of the laser diode emission which the model was based on. Table 1 lists the measured 1-dB and 3-dB coupling tolerances of both lateral displacements ΔX; ΔY and angular deviations, θX , θY , between the fiber microlens and the laser diode, along with the reported results of microlens fabricated by other methods. The horizontal misalignment tolerance is close to that in the vertical direction, in contrast to the property of a microlens fabricated by the grinded/polished technique, mainly due to the symmetrical structure of the fabricated microlens. It also shows that the angular misalignment of the vertical direction is more tolerable than the horizontal direction because of the elliptical mode field distribution diode.
Table 1.
Measured 1/3-dB Misalignment Tolerance for the Fiber Microlens
Misalignment Melt/drawn taper [1] Grinded/polished [7] Laser direct writing Laser direct writing
Loss (dB)
ΔX (μm)
ΔY (μm)
ΔθX (deg.)
ΔθY (deg.)
1 3 1 3
3.0 2.34 2.5 3.5
1.3 0.6 1.2 1.7
2.0 31.7 2.0 5.2
2.0 27.4 5.0 8.8
fiber microlens, allowing a batch print process at a low-cost, high-consistency, and high-fabrication rate. The laser direct writing with a quasi-Bessel beam can be used to create a 3D optically polymerized microlens with good repeatability and high optical performance. The fabricated fiber microlens has gained reasonably good coupling efficiency, long working distance, and wide tolerance. In conclusion, a novel one-step laser direct writing process for fabricating a fiber microlens for coupling 650-nm laser diodes to SMFs was demonstrated. A coupling efficiency of 53.5% was measured for the single mode fiber at an optimum working distance of 16 μm. The tolerances for a 1-dB loss increment for X, Y translational displacements and θX , θY angular deviations between the fiber microlens and the laser diode were 2.5, 1.2 μm, and 2.0, 5.0 degrees, respectively. A good agreement was achieved between the measured coupling efficiency and the simulated result based on the beam propagation method using ZEMAX software. This work was supported by the National Natural Science Foundation of China (NSFC) under grant 61178049. References
Fig. 4. Coupling efficiency of fabricated fiber microlens and simulated result by ZEMAX as a function of (a) working distance, (b) horizontal displacement, and (c) vertical displacement.
4. Discussion and Conclusion
In comparison to other fabrication techniques of lensed fiber used in the coupling of the laser diode, including heating and drawing, chemical etching, and grinding/polishing, the laser direct writing technique offers a printable fabrication methodology for a
1. H. Kuwahara, M. Sasaki, and N. Tokoyo, “Efficient coupling from semiconductor lasers into single-mode fibers with tapered hemispherical ends,” Appl. Opt. 19, 2578–2583 (1980). 2. H. M. Yang, S. Y. Huang, C. W. Lee, T.-S. Lay, and W.-H. Cheng, “High-coupling tapered hyperbolic fiber microlens and taper asymmetry effect,” J. Lightwave Technol. 22, 1395–1401 (2004). 3. W. H. Cheng, C. S. Wang, and C. J. Hwang, “Highly efficient power coupling between GaAIAs lasers and taperedhemispherical-end multimode fibers,” Appl. Opt 21, 3409– 3410 (1982). 4. H. M. Presby and C. A. Edwards, “Efficient coupling of polarization-maintaining fiber to laser diodes,” IEEE Photon. Technol. Lett. 4, 897–899 (1992). 5. N. M. Presby and C. R. Giles, “Asymmetric fiber microlenses for efficient coupling to elliptical laser beams,” IEEE Photon. Technol. Lett. 5, 184–186 (1993). 6. S. M. Yeh, Y. K. Lu, S. Y. Huang, H.-H. Lin, C.-H. Hsieh, and W.-H. Cheng, “A novel scheme of lensed fiber employing a quadrangular-pyramid-shaped fiber endface for coupling between high-power laser diodes and single-mode fibers,” J. Lightwave Technol. 22, 1374–1379 (2004). 7. S. M. Yeh, S. Y. Huang, and W. H. Cheng, “A new scheme of conical-wedge-shaped fiber endface for coupling between high-power laser diodes and single-mode fibers,” J. Lightwave Technol. 23, 1781–1786 (2005). 8. Y. K. Lu, Y. C. Tsai, Y. D. Liu, S.-M. Yeh, C.-C. Lin, and W.-H. Cheng, “Asymmetric elliptic-cone-shaped microlens for 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS
8447
efficient coupling to high-power laser diodes,” Opt. Express 15, 1434–1442 (2007). 9. Y. D. Liu, Y. C. Tsai, Y. K. Lu, L.-J. Wang, M.-C. Hsieh, S.-M. Yeh, and W.-H. Cheng, “New scheme of double-variablecurvature microlens for efficient coupling high-power lasers to single-mode fibers,” J. Lightwave Technol. 29, 898–904 (2011). 10. Y.-C. Huang, W.-H. Hsieh, Y.-C. Hsu, M.-T. Sheen, Y.-S. Lin, J.-L. Chen, Y.-C. Tsai, and W.-H. Cheng, “New scheme of
8448
APPLIED OPTICS / Vol. 53, No. 36 / 20 December 2014
hyperboloid microlens for high-average and high-yield coupling high-power lasers to single-mode fibers,” J. Lightwave Technol. 31, 1681–1686 (2013). 11. H. Huang, S. J. Chen, H. M. Zou, Q. Li, J. Fu, F. Lin, and X. Wu, “Fabrication of micro-axicons using direct-laser writing,” Opt. Express 22, 11035–11042 (2014). 12. Y. L. Zhang, Q. D. Chen, H. Xia, and H. B. Sun, “Designable 3D nanofabrication by femtosecond laser direct writing,” Nano Today 5(5), 435–448 (2010).
State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China 2
School of Computer Engineering, Nanyang Technological University, 637553, Singapore *Corresponding author: [email protected] Received 2 September 2014; revised 24 October 2014; accepted 15 November 2014; posted 17 November 2014 (Doc. ID 221829); published 17 December 2014
A printable microlens on fiber end for coupling between a laser diode to a single-mode fiber (SMF) is demonstrated. The fiber microlens fabricated by the novel technique based on laser direct writing using a quasi-Bessel beam has gained high coupling efficiency, long working distance, and sufficient alignment tolerance. A coupling efficiency of 53.5% was measured for SMF coupling at a working distance of 16 μm. The tolerances for a 1-dB loss increment for translational displacements and angular deviations between the fiber microlens and the laser diode were 2.5 and 1.2 μm, and 2.0 and 5.0 degrees, respectively. The printable fabrication of a microlens on a fiber endface by laser direct writing allows for a batch process to reproduce a multiple microlens with a high consistency in a simple and fast fabrication cycle, with no need of individual fiber loading and unloading. © 2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.4005) Microstructured fibers. http://dx.doi.org/10.1364/AO.53.008444
1. Introduction
The elliptical mode field of a laser diode provides a low direct coupling efficiency between a laser diode and a single-mode fiber (SMF). To improve mode match and coupling efficiency, a common approach is to fabricate a microlens on a fiber tip for the SMF and LD to couple directly. Up until now, there were several methods of microlens fabrication, such as heating and drawing fiber [1], hydrofluoride (HF) chemical etching [2,3], micromachining with a CO2 laser [4,5], and grinding followed by polishing [6–10]. Among these methods, the most commonly used at present are the mechanical grinding/polishing and chemical etching techniques, with a high coupling efficiency of around 70% [7,8]. Several complex structures have been fabricated based on grind and polish techniques to achieve even higher coupling efficiency and alignment tolerances, including 1559-128X/14/368444-05$15.00/0 © 2014 Optical Society of America 8444
APPLIED OPTICS / Vol. 53, No. 36 / 20 December 2014
quadrangular-pyramid-shaped fiber endfaces [6], conical-wedge-shaped fiber endfaces [7], asymmetric elliptic-cone-shaped microlenses [8], double-variablecurvature microlenses [9], and hyperboloid microlenses [10]. Although some structures can obtain a coupling efficiency above 80%, the process of grinding demands a precise control over the endface orientation and offset, and some microlenses require a high intensity arc fusion process to polish and reshape [7–10]. A chemical etching technique is typically performed by etching the fibers in a HF solution, then fusing the fiber tip with a splicing machine to form a microlens. The coupling efficiency of this method was reported to be as high as 80%–90% [2]. However, the fabrication process involves the use of HF, a hypertoxic chemical containing latent risks of accidental inhalation or contact with the skin, which can lead to incurable burns. A common requirement of all the aforementioned fabrication techniques is the utilization of individual
fiber loading and unloading not suitable for batch fabrication. In an ever-increasing market demand for laser-fiber coupling, a technique capable of mass production of fiber lens on fiber endface will be of great interest. In search of such a fabrication process, much effort has been devoted to feature both the ability to be continually printable on a substrate and a cost-effective creation of highly consistent optical-quality 3D surfaces [11,12]. In this study, we demonstrate a novel fabrication process for a fiber microlens by laser direct writing. We utilize a focused Bessel beam to form a microlens on the fiber endface. A single laser pulse with a cone-shaped beam has been used to expose UV optical epoxy, producing a fiber microlens with optical surface finishing and size uniformity. Through controlling the writing parameters such as beam shape, exposure time, and development process, we were able to generate a microlens at various form parameters to optimize fiber coupling. 2. Fabrication
The setup for laser direct writing fabrication of fiber microlenses is illustrated in Fig. 1 (Upper), which includes the following devices: an individual custom made fiber-axicon on a single mode fiber tip with an apex angle ranging from 75° to 125° and a surface roughness of about 0.02 to 0.04 μm to generate Bessel beams, a high precision 3D piezoelectric nanostage with a movement resolution of 20 nm (Thorlabs Nano
Fig. 1. (Upper) Schematic of setup for fiber microlenses fabrication; (lower) (a) coating of UV curable epoxy to a fiber end; (b) laser exposure with a quasi-Bessel beam; (c) removal of residual epoxy using an air knife; (d) post-exposure UV curing.
Max, Max311D), an angle adjuster used for both horizontal and vertical angle control of the fiber end, and a light emitting diode (LED) to provide the illumination needed for in situ CMOS-monitoring of the fabrication sequence. The microscope camera measures the distance between the tip of the fiber axicon and the fiber end, which is coated with UV curable epoxy (DSM, G.652 optical fiber secondary coating). The entire process of direct writing is automated by the custom in-house programming environment. Figure 1 (lower) (a)–(d) displays the sequence of fabrication [11]. UV curable optical epoxy was coated onto a fiber end by dip-coating. The viscosity of the epoxy was kept at ∼4800 mPa · s at 50 s−1 to maintain a uniform coating thickness by controlling the working temperature at 20°C. The thickness of the coated film was 50 μm, with a curved edge on the circumference due to surface tension. Alignment between the fiber and the fiber-axicon was achieved by adjusting the three-dimensional translation stage and the angle adjuster. A quasi-Bessel beam was then used to expose the UV curable epoxy. The exposed viscous liquid prepolymer undergoes a crosslinking reaction upon UV exposure, and solidifies immediately. The unexposed prepolymer was removed both mechanically and chemically. A post-exposure treatment was then conducted to the exposed polymer to develop direct-writing patterns. Instead of using traditional wet developing methods, which were found to be difficult in maintaining the fine feature of preliminarily solidified epoxy, we developed an air knife-based technique. Compressed air (0.2 Mpsi) was pressured out from air knife openings onto the plant fiber end with variable angles ranging from 20° to 40° with respect to the fiber endface, driving the uncured epoxy off the fiber end. The air knife was found to be capable of blowing off approximately 90% of the unexposed epoxy, with the remaining unexposed UV epoxy cleaned off by spraying a small amount of ethanol onto the fiber end to enhance the air knife developing process. After this pattern developing process, the sample was placed into a UV irradiance chamber (a 20 W mercury lamp) for 4 min, where the epoxy structure was thoroughly cured to reach a stable refractive index of 1.475. This process also improves surface finishing by re-flowing the uncured epoxy over the cured structure, forming a smooth optical surface with the surface tension of UV epoxy. This setup is capable of generating various microlenses depending on the laser beam profile and size of the UV illumination on the epoxy. With the fiberaxicon used in the experiment, a single mode laser beam traveling in the fiber was converted from a Gaussian profile to a Bessel-like beam (λ 408 nm, minimum FWHM 1.5 μm, maximum output power 20 mW). By using fiber-axicons with different apex angles, microlenses and lens arrays with various shapes can be fabricated [11]. Figure 2 shows a microscope image of a fabricated microlens on the SMF 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS
8445
Fig. 2. Microscopic image of a fabricated microlens on a singlemode fiber endface.
endface. The shape of the fiber microlens has an optimized coupling efficiency among the different shapes that were fabricated and measured. Other measured shapes with larger or smaller radius of inscribed circle (defined in the next section) showed lower coupling efficiencies. 3. Measurements and Results
A 650 nm laser diode with typical far-field divergence angles of 6.5°horizontal × 35°vertical, and an aspect ratio of 5 was used to measure the coupling efficiency between the fabricated fiber microlens and laser. A single mode fiber with an operating wavelength of 980–1600 nm (Nufern1060-XP, mode field diameter of 5.9 0.5 μm at 980 nm and 9.5 0.5 μm at 1550 nm) was used in both experiment and modeling. Microlenses with different shapes were tested to evaluate the correlation between shape and the coupling efficiency. The radius of curvature of the lens tip is described by using an inscribed circle (as shown in Fig. 3). Figure 3 shows the relationship between the coupling efficiency and the radius of the inscribed circle. It was found experimentally that for an inscribed circle radius ranging from 6 to 20 μm, the lens height for maximized coupling is inversely proportional to
Fig. 3. Measured coupling efficiency as a function of the inscribed circle radius. 8446
APPLIED OPTICS / Vol. 53, No. 36 / 20 December 2014
the inscribed circle radius. For an optimized inscribed radius of 8.1 μm, the lens height was found to be 50 μm with a tolerance of 5∕ − 0 μm and a maximum coupling efficiency of 53.5% was obtained. When the radius was below 7.5 μm, the coupling efficiency was observed to decrease rapidly. For example, the coupling efficiency was at 27.7% when the radius reached 7.1 μm. The fiber microlens with an inscribed circle radius of 8.1 μm was obtained by using a fiber axicon with an apex angle of 120°, and the distance between the fiber-axicon and the UV curable epoxy was 2 μm; the laser power was set to be 50 μW and exposed for 1 s. Utilizing the microlens with optimized coupling efficiency, we further investigated the position/angle tolerance of the coupling efficiency. The experimental measurement was compared with simulated result using the ZEMAX software program where both the microlens and laser diode were modeled by curve fitting to lens and emission specification of the laser diode. Figures 4(a)–4(c) display both measured and calculated coupling efficiency as a function of the working distance, horizontal, and vertical displacement. Defining the separation between the LD and the fiber microlens tip as the working distance, Fig. 4(a) shows the dependence of coupling efficiency on working distance. Coupling efficiency peaks when the working distance was at 15.8 μm, in comparison to a simulated optimal working distance at 15.2 μm. The coupling efficiency falls off fast when the working distance was either below 14 μm or above 18 μm. The lateral misalignment between the LD and the fiber results in a decrease of the coupling efficiency. Figure 4(b) shows the coupling efficiency as a function of horizontal displacement. The measured result was in a good agreement with that of simulation. A horizontal displacement of 2 μm yields a 10% reduction in coupling efficiency. Figure 4(c) shows that the coupling efficiency dropped by 12% for a vertical displacement of 1 μm. But the simulated vertical misalignment is more sensitive; a 1 μm of vertical displacement causes a drop in coupling efficiency by 17%. The discrepancy between the measured and calculated results is likely to be due to two reasons: (a) shape of modeled lens deviates from the actual form and (b) asymmetric distribution differs from the specified distribution of the laser diode emission which the model was based on. Table 1 lists the measured 1-dB and 3-dB coupling tolerances of both lateral displacements ΔX; ΔY and angular deviations, θX , θY , between the fiber microlens and the laser diode, along with the reported results of microlens fabricated by other methods. The horizontal misalignment tolerance is close to that in the vertical direction, in contrast to the property of a microlens fabricated by the grinded/polished technique, mainly due to the symmetrical structure of the fabricated microlens. It also shows that the angular misalignment of the vertical direction is more tolerable than the horizontal direction because of the elliptical mode field distribution diode.
Table 1.
Measured 1/3-dB Misalignment Tolerance for the Fiber Microlens
Misalignment Melt/drawn taper [1] Grinded/polished [7] Laser direct writing Laser direct writing
Loss (dB)
ΔX (μm)
ΔY (μm)
ΔθX (deg.)
ΔθY (deg.)
1 3 1 3
3.0 2.34 2.5 3.5
1.3 0.6 1.2 1.7
2.0 31.7 2.0 5.2
2.0 27.4 5.0 8.8
fiber microlens, allowing a batch print process at a low-cost, high-consistency, and high-fabrication rate. The laser direct writing with a quasi-Bessel beam can be used to create a 3D optically polymerized microlens with good repeatability and high optical performance. The fabricated fiber microlens has gained reasonably good coupling efficiency, long working distance, and wide tolerance. In conclusion, a novel one-step laser direct writing process for fabricating a fiber microlens for coupling 650-nm laser diodes to SMFs was demonstrated. A coupling efficiency of 53.5% was measured for the single mode fiber at an optimum working distance of 16 μm. The tolerances for a 1-dB loss increment for X, Y translational displacements and θX , θY angular deviations between the fiber microlens and the laser diode were 2.5, 1.2 μm, and 2.0, 5.0 degrees, respectively. A good agreement was achieved between the measured coupling efficiency and the simulated result based on the beam propagation method using ZEMAX software. This work was supported by the National Natural Science Foundation of China (NSFC) under grant 61178049. References
Fig. 4. Coupling efficiency of fabricated fiber microlens and simulated result by ZEMAX as a function of (a) working distance, (b) horizontal displacement, and (c) vertical displacement.
4. Discussion and Conclusion
In comparison to other fabrication techniques of lensed fiber used in the coupling of the laser diode, including heating and drawing, chemical etching, and grinding/polishing, the laser direct writing technique offers a printable fabrication methodology for a
1. H. Kuwahara, M. Sasaki, and N. Tokoyo, “Efficient coupling from semiconductor lasers into single-mode fibers with tapered hemispherical ends,” Appl. Opt. 19, 2578–2583 (1980). 2. H. M. Yang, S. Y. Huang, C. W. Lee, T.-S. Lay, and W.-H. Cheng, “High-coupling tapered hyperbolic fiber microlens and taper asymmetry effect,” J. Lightwave Technol. 22, 1395–1401 (2004). 3. W. H. Cheng, C. S. Wang, and C. J. Hwang, “Highly efficient power coupling between GaAIAs lasers and taperedhemispherical-end multimode fibers,” Appl. Opt 21, 3409– 3410 (1982). 4. H. M. Presby and C. A. Edwards, “Efficient coupling of polarization-maintaining fiber to laser diodes,” IEEE Photon. Technol. Lett. 4, 897–899 (1992). 5. N. M. Presby and C. R. Giles, “Asymmetric fiber microlenses for efficient coupling to elliptical laser beams,” IEEE Photon. Technol. Lett. 5, 184–186 (1993). 6. S. M. Yeh, Y. K. Lu, S. Y. Huang, H.-H. Lin, C.-H. Hsieh, and W.-H. Cheng, “A novel scheme of lensed fiber employing a quadrangular-pyramid-shaped fiber endface for coupling between high-power laser diodes and single-mode fibers,” J. Lightwave Technol. 22, 1374–1379 (2004). 7. S. M. Yeh, S. Y. Huang, and W. H. Cheng, “A new scheme of conical-wedge-shaped fiber endface for coupling between high-power laser diodes and single-mode fibers,” J. Lightwave Technol. 23, 1781–1786 (2005). 8. Y. K. Lu, Y. C. Tsai, Y. D. Liu, S.-M. Yeh, C.-C. Lin, and W.-H. Cheng, “Asymmetric elliptic-cone-shaped microlens for 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS
8447
efficient coupling to high-power laser diodes,” Opt. Express 15, 1434–1442 (2007). 9. Y. D. Liu, Y. C. Tsai, Y. K. Lu, L.-J. Wang, M.-C. Hsieh, S.-M. Yeh, and W.-H. Cheng, “New scheme of double-variablecurvature microlens for efficient coupling high-power lasers to single-mode fibers,” J. Lightwave Technol. 29, 898–904 (2011). 10. Y.-C. Huang, W.-H. Hsieh, Y.-C. Hsu, M.-T. Sheen, Y.-S. Lin, J.-L. Chen, Y.-C. Tsai, and W.-H. Cheng, “New scheme of
8448
APPLIED OPTICS / Vol. 53, No. 36 / 20 December 2014
hyperboloid microlens for high-average and high-yield coupling high-power lasers to single-mode fibers,” J. Lightwave Technol. 31, 1681–1686 (2013). 11. H. Huang, S. J. Chen, H. M. Zou, Q. Li, J. Fu, F. Lin, and X. Wu, “Fabrication of micro-axicons using direct-laser writing,” Opt. Express 22, 11035–11042 (2014). 12. Y. L. Zhang, Q. D. Chen, H. Xia, and H. B. Sun, “Designable 3D nanofabrication by femtosecond laser direct writing,” Nano Today 5(5), 435–448 (2010).
