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A novel method is presented for fabricating lensed optical fibers for enhancing the coupling efficiency between high-power IR laser diodes and gradient-index fibers (GIF). SU-8 photoresist is attached to the fiber tip by means of surface tension forces and a cone-shaped micro-lens structure is then formed using an electrostatic pulling method. It is shown that micro-lenses with various radii of curvature can be easily formed by tuning the intensity of the electric field used in the pulling process. Experimental results show that for a laser diode chip with a central wavelength of 1310 nm, a coupling efficiency of 78% can be obtained using a lensed optical fiber with a radius of curvature of 48 μm. By contrast, the coupling efficiency of a traditional flat-end fiber is just 40%. Overall, the fabrication method proposed in this study provides a rapid and low-cost solution for the mass production of high-quality lensed optical fibers.
©2011 Optical Society of America
1. Introduction
Optical fibers have revolutionized the telecommunications, high-end audio, and remote sensing technology fields [1]. Fiber-optic systems typically use a high-power laser diode as the optical source. Since the light from the laser disperses after leaving the laser cavity, the incident angle of the light beams coupled into the optical fiber depends on the angle at which they were emitted from the source. Thus, the coupling efficiency depends on a number of parameters including laser light spot, core of the fiber, distance between the laser and fiber, alignment issue and many others. In practice, some of the coupled rays fall outside of the acceptance cone of the fiber and are therefore not propagated along its length, resulting in a low coupling efficiency. This coupling loss between the laser diode and the optical fiber has a critical effect on the system performance and is therefore a crucial issue in the telecommunications field.
Various approaches have been proposed for improving the coupling efficiency [2] between the light source and an optical fiber. Amongst these techniques, the use of directional optical fiber couplers is one of the most common [3]. However, commercial fiber couplers are typically bulky, and are therefore not easily integrated with laser diode chips. Furthermore, the use of discrete optical elements within such couplers complicates the device assembly and increases the overall price of the system. Accordingly, some researchers have proposed mounting a micro-lens at the end of the optical fiber in order to transform the mode size of the light emitted from the laser diode such that it more closely matches the core size and mode of the fiber [4, 5]. However, the assembly process is delicate since the micro-lens must be aligned with an extremely high degree of precision.
Lensed optical fibers, i.e., optical fibers whose end faces are formed in a lens shape, provide a far cheaper and simpler means of increasing the coupling efficiency between the fiber and laser diodes or semiconductor optical amplifiers. The first lensed optical fiber was demonstrated by Cohen and Schneider as far back as 1974 [6]. A small amount of negative-tone photoresist (PR) was applied to the end of the optical fiber and was then exposed using UV light. Small amount of photoresist was cross-linked and formed hemispherical lens structures in front of the core position. The PR lens increased the light coupling efficiency by around 26% compared to that achieved of flat-end fiber. Inspired by their success, many other researchers also proposed lensed optical fibers fabricated using a variety of techniques, including laser machining [7], mechanical grinding [8], chemical etching [9, 10], and melting/stretching [11]. Nearly 100% coupling was theoretically achieved [7]. Nevertheless, fabricating lensed fibers using laser or mechanical grinding techniques relies on delicate alignment and machining processes such that the production rate is relatively slow. Furthermore, ensuring the reproducibility of batch-produced grinded optical fibers is highly challenging. To resolve these difficulties, various researchers have fabricated lensed optical fibers using high-refractive index UV-curable polymers. For example, Sakata et al. [12] presented a lensed plastic optical fiber with a concave end face filled with high-index resin. Kim et al. [13] proposed a lensed optical fiber incorporating a refractive acrylate polymer micro-lens formed by surface tension forces. In both cases, the lensed fibers had a spherical profile. However, as reported in the previous report, spherical-shaped lensed fibers result in only a modest improvement in the light coupling efficiency [14, 15]. Therefore, to meet the demands of high-performance light-coupling applications, non-spherical lensed fibers are required.
Accordingly, the present study proposes a simple and novel method for fabricating non-spherical lensed optical fibers using SU-8 PR. SU-8 PR is attached to the fiber tip by means of surface tension forces and a cone-shaped micro-lens structure is then formed using an electrostatic pulling method performed at a temperature of 60°C. The curvature of the lens structure can be easily controlled by tuning the intensity of the electric field used in the pulling process. The relationship between the electric field strength and the radius of curvature of the lens is quantified. Moreover, the performance of the various lensed fibers is evaluated by measuring the light coupling efficiency between the fiber and a IR laser diode.
2. Fabrication
Prior to fabricating the lensed fibers, the optical transmission behavior of commercially available SU-8 PR (SU-8-50, MicroChem Corp., USA) was measured over the telecommunications range. SU-8 PR was coated on a quartz slide with a thickness of 50 μm, followed by the soft-bake process at 95°C for 20 min, and then fully cross-linked via flux exposure. The optical transmittance of the exposed substrate was measured using a near infrared spectrometer (DTS-1700, Polychromix Inc., USA). The results show that the SU-8 substrate has a transmission ratio of around 98% over the wavelength from 1250 to 1550 nm, confirming that SU-8 PR is an ideal material for micro-lens fabrication in optical communication applications.
Figure 1 presents a schematic overview of the fabrication process used to realize the lensed optical fibers. The lensed fibers were fabricated using commercial polymer-based gradient-index optic fibers (GIF, LGR01A012L, Asahi, Japan) with an outer diameter of 500 μm and a core diameter of 120 μm. The GIFs were first polished mechanically using water-rinsed #3000 silicon carbide sandpaper (Fig. 1(A)) then cleaned in an ultrasonic bath containing absolute alcohol. An SU-8 PR layer with a thickness of 50 μm was spin-coated on a bare glass substrate and then soft-baked on a hotplate at a temperature of 90°C in order to remove the solvent content from the PR (Fig. 1(B)). The glass transition temperature (Tg) of unexposed SU-8 PR is 55°C [16]. Following the soft-baking process, the substrate was heated to a temperature of 60°C and the polished GIFs were then dipped into the molten PR layer (Fig. 1(C)). Due to surface tension effects, a constant volume of SU-8 PR self-attached to the end face of each GIF in the form of a semi-spherical droplet. Based on the dimensions of the optical fiber, the volume of attached SU-8 PR was calculated to be around 17.7 nL. Moreover, the variation of the droplet volume was measured within 3%. Non-spherical cone-shaped lenses with different radii of curvature were then formed using an electrostatic pulling method with electric field strengths ranging from1.6x106 to 2.38x106 V/m (Fig. 1(D)). To maintain the fluidity of the PR, the pulling process was performed at a temperature of 60°C; with the heat source provided by a 150 W halogen lamp. Finally, the pulled lens structures were cured for 3 min using a 20 W hand-held UV-lamp (Fig. 1(E)).
Fig. 1 Schematic overview of lensed optical fiber fabrication process. (A) End polishing of cut GIFs. (B) Spin-coating and soft-baking of SU-8 layer. (C) SU-8 application. (D) SU-8 pulling. (E) SU-8 curing.
In fabricating the lensed optical fibers proposed in this study, the aspherical lens structures are formed via the Taylor cone effect [17]. SU-8 is based on an epoxy group with a triangular structure comprising two carbon atoms and a single oxygen atom. Due to the difference in the electro-negativities of the carbon and oxygen atoms, the epoxy group contains a polarity. Therefore, when the SU-8 is exposed to a high-intensity electric field, the polarity of the epoxy group orients toward the electric dipole. Consequently, an electrical charge generates and gathers on the droplet, and the resulting electrostatic force between the droplet and the electrode deforms the droplet shape. In other words, the cone tip diameter is inversely proportional to the electric field strength. That is, a higher electrostatic field strength results in a smaller lens diameter. Figure 2 presents SEM images showing the SU-8 droplet before and after the electrostatic pulling process, respectively. (Note that the pulling process was performed using an electric field strength of 2x106 V/m.) The images confirm the effectiveness of the proposed electrostatic pulling method in forming non-spherical lens structures. In addition, it is observed that the pulled micro-lens has an excellent surface quality and is therefore suitable for optical applications.
Fig. 2 SEM images of SU-8 lensed fibers before (A) and after (B) electrostatic pulling process. (Note that the pulling process was performed using an electric field strength of 2x106 V/m.)
For the GIFs used in the present study (outer diameter: 500 μm), the minimum radius of curvature was found to be 62 μm, and was obtained using an electric field strength of 2.25 x106 V/m. At higher values of the electric field strength (>2.25 x106 V/m), an electro-spinning effect was observed and failed the experiment [18]. Therefore, the SU-8 application and electrostatic pulling processes shown in Figs. 1(B) to 1(E) were repeated using a produced lensed fiber in order to further reduce the radius of curvature of the micro-lens. Figure 3 shows that variation of the radius of curvature of the pulled lens structure with the electric field strength after first and second pulling process. The radii of curvature are measured by means of analysis the contour of capture lens image. As expected, the radius of curvature decreases with an increasing electric field strength. As discussed above, the minimum radius of curvature following the first pulling process is around 62 μm. However, as shown in Fig. 4 , the radius of curvature can be reduced to around 40 μm by performing a second pulling process using an electric field strength of 2.38x106 V/m.
Fig. 3 Variation of radius of curvature of pulled micro-lens structure with electric field strength after first and second pulling process.
4. Experimental Results
The light focusing ability of the lensed fibers was evaluated using a green laser (530 nm, 100 mW, Unice Optic Ltd., Taiwan) and a fluorescence solution composed of 10−4 M Rhodamine B. In performing the experiments, the laser beam was coupled into the lensed fiber from the flat end and emitted from the lensed end into the fluorescence solution. The far-field intensity distribution of the emitted light was examined using a microscope (E400, Nikon, Japan) equipped with a CCD module (DXC-190, Sony, Japan). Figures 4(A) and 4(B) show the far-field intensity distributions of the light emitted from a flat-end GIF and a lensed GIF (R = 90 μm) fabricated using the proposed method, respectively. It is clear that the flat-end fiber results in a divergent light profile, whereas the proposed lensed fiber produces a convergent light profile. In other words, the fabricated micro-lens provides an excellent focusing performance.
Figure 5 shows the normalized intensity distributions of the light emitted from an aspherical lensed GIF, a spherical lensed GIF, and a flat-end GIF, respectively. Note that the corresponding far-field images are presented as inserts (a), (b) and (c), respectively. The results show that the spherical and non-spherical lenses both produce a symmetric focusing effect. As a result, both lenses can be expected to improve the light coupling efficiency. However, comparing the two lenses, it is seen that the aspherical lens yields a narrower (i.e., more focused) distribution of the emitted light. Thus, the experimental results indicate that the aspherical lens will achieve a higher light coupling efficiency.
Fig. 5 Light emitting properties of (a) aspherical lensed GIF (R = 90 μm), (b) spherical lensed GIF (R = 226 μm), and (c) flat-end GIF.
The light coupling efficiency of the fabricated lensed fibers was evaluated using a laser chip with a central wavelength of λ= 1310 nm and a power of 5 W. The laser chip was bonded to a copper sub-mount and was maintained at a constant temperature of 25°C using a TE cooler. The laser chip was powered by a laser driver (Model 325, Newport, USA) with a current of 500 mA. The output power of the light emitted from the other end of the lensed fiber was measured using a commercial power meter (2832-C, Newport, USA). Figure 6(A) shows the variation of the coupling efficiency with the fiber tip–to–laser diode distance for the aspherical lensed GIF (R = 48 μm), spherical lensed GIF (R = 226 μm), and flat-end GIF, respectively. The results confirm that the aspherical lensed fiber provides the best light coupling performance of the three fibers. From inspection, the light coupling efficiency is around 78% when the fiber tip is positioned at a distance of 90 μm from the laser diode. By contrast, the maximum light coupling efficiency of the spherical lensed fiber is around 42% when positioned at a distance of 60 μm from the diode. Significantly, the maximum light coupling efficiency of the aspherical lensed fiber is around twice that of the flat-end GIF fiber (i.e., 40% at a coupling distance of 0 μm). Figure 6(B) shows the relationship between the optimal coupling efficiency and the radius of curvature of the aspherical microlens. The results confirm that the maximum light coupling efficiency (78%) is obtained using a microlens with a radius of curvature of 48 μm. Significantly, this coupling efficiency is much higher than that achieved by optical fibers equipped with lenses of a larger radius. The developed method provides a simple, low-cost and batch fabrication way for producing lens fibers with high coupling efficiency.
Fig. 6 (A) Coupling efficiency versus distance between fiber tip and laser diode. (B) Optimized coupling efficiency versus radius of curvature of micro-lens.
5. Conclusions
This paper has presented a simple method for fabricating SU-8-based lensed GIFs for high-power laser coupling applications. In the proposed approach, the SU-8 PR is attached to the end of the GIF via surface tension effects and is then stretched to form an aspherical lens by means of an electrostatic pulling process. It has been shown that the tip radius of the aspherical lens can be easily tuned by adjusting the intensity of the electric field strength used to pull the droplet. The experimental results have shown that a coupling efficiency of 78% can be obtained using an aspherical lens with a radius of curvature of 48 μm. Overall, the results presented in this study confirm that the proposed fabrication method provides a straightforward, high-throughput and low-cost solution for the mass production of lensed optical fibers for laser coupling applications.
Acknowledgments
The financial support from National Science Council of Taiwan is greatly acknowledged.
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