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Abstract
A novel technique is presented for producing micro-hyperboloid lensed fibers for efficient coupling to semiconductor laser chips. A three-step process including a precision mechanical grinding, a spin-on-glass (SOG) coating and an electrostatic pulling process is used to form the hyperboloid lens structure on a flat-end single mold fiber (SMF) with the core diameter of 6.6 μm. Micro-hyperboloid lensed fibers with tunable radii of curvature around 4.18 – 4.83 μm can be formed on the SMF end face. A high average coupling efficiency around 80% and low coupling variation of 0.116 ± 0.044% are obtained for the produced fibers. The developed method is efficient to produce micro-hyperboloid lensed fibers for high-performance light coupling between the SMF and the semiconductor diode lasers.
© 2017 Optical Society of America
1. Introduction
With the rapid growth of the ultra-bandwidth optical communications and network communications, the need for big capacity and long distance data transmission is emerging. Optical fiber communications have been extensively studied due to its advantages with ultra-bandwidth, fast, long range, low loss, and low electricity interference [1]. However, optical fiber communications would also suffer from signal attenuation during transmission and reduce the signal intensity. In general, semiconductor laser diodes with the wavelengths of 980 nm or 1480 nm were used as the pumping laser because of the high gain and low noise characteristics [2]. However, as shown in Fig. 1(A), the typical laser diode has a rectangular light outlet and results in an elliptic light field, which makes it difficult to couple the light from the laser diode into the small and circular core (ϕ~6.6 μm) of the single-mode optical fiber. The difference between the elliptic light field and the rectangular core might cause modal mismatch and significantly increase the coupling loss up to 50% [3]. Therefore, it is essential to achieve the mode match between the laser diode and the optic fiber to increase the coupling efficiency, as shown in Fig. 1(B). There are several approaches to enhance the mode matching between laser diode and optic fiber including design the outlet structure to change the mode field of laser light [4], add a bulky lens between the optical fiber and laser diode for mode matching [5], mount or produce a semispherical or aspherical microlens structure for directly light coupling [6]. Over the developed method, microlens approach exhibited the advantage of easy for system integration and miniaturization. Nevertheless, to mount a precise micro lens structure on the tip of optical fiber relies on time-consuming alignment and fixation procedures. Therefore, producing a lens structure directly on a fiber end is one of the most popular ways for high efficient laser coupling.
Fig. 1 Schematic showing the laser coupling via (A) flat-end fiber and (B) micro-hyperboloid lensed fiber.
Chemical etching method and dispensing are the two major approaches for producing microlensed fibers. The etching method is capable of mass producing highly reproduce lens structures but the light coupling efficiency for this kind of lensed fiber is poor due to rough surface and symmetrical etched pattern [7]. Alternatively, dispensing method exhibited the advantages including simple equipment, low cost and high repeatability feature [8]. The aspherical microlensed structure was directly produced on a multimode fiber using SU-8 photoresist. The tunable radius of curvature significantly enhanced the coupling performance of up to 78% coupling efficiency [9]. However, this approach can only be used in multimode optical fiber with a big core diameter since the radii of curvature were limited to tens microns. Moreover, this method can only produce axisymmetric structures which may not meet the mode match requirement for semiconductor diode laser.
On contrast, non-axisymmetric microlensed optical fiber can match the light field from the laser diode. However, it is challenging to produce the non-axisymmetric structure at the tip of an optic fiber. CO2 laser welding was developed to directly melt the end of the optic fiber to form a specific radius of curvature at the fiber end [10]. However, the reproducibility for this method is difficult to control. Alternatively, compare to laser wielding, mechanical grinding is also an effective method to produce a non-axisymmetric lensed fiber with better repeatability and lower cost. Modavis et al. reported an anamorphic microlensed fiber for coupling diode laser into a single-mode fiber [11]. In addition, a quadrangular-pyramid-shape was machined at the optic fiber end-face for laser coupling [12]. A high coupling efficiency of 83% was achieved due to a better shape match between the elliptical microlenses and the diode laser. However, a five-step precision grinding and then a spark melting processes were required to form the elliptical microlens end-face. An automatic grinding process and series of arc melting were developed to produce the non-axisymmetric microlensed fiber. The produced microlensed fiber exhibited a maximum coupling efficiency of 83% while the offset for the ground tip was controlled within 0.8 μm [13]. Nevertheless, the spark melting process was not easy to control since the arc was generated by a high voltage discharge between two electrodes. Moreover, the discharge machining is risky for the precisely ground fiber tip since arc melting is an irreversible process.
This study develops a novel process to produce hyperboloid microlensed fiber with mechanical grinding and then a spin-on-glass (SOG) coating technique [14]. An electrostatic pulling method is applied to adjust the radius of curvature for the spin-on-glass layer. Therefore, the radius of curvature for the produced microlens structure is determined by the balance of the electrostatic pulling force and the surface tension of the SOG liquid. Therefore, tuning the radius of curvature for the microlens is much consistent. Moreover, the uncured SOG is easy to remove with alcohol if a rework process is required such that the precisely ground optic fiber can be saved. Micro-hyperboloid lensed fibers with the radius of curvature of around 4.5 μm at the minor axis are produced by the developed method. The relationship between the applied electric field and the formed radius of curvature is experimentally investigated. The coupling efficiency and the light coupling stability are measured.
2. Experimental
Single-mode fiber (SM98-PS-U25A, Fujikura, Japan) with the core diameter of 6.6 μm and the cladding diameter of 125 μm was used in the present study. The optical fiber was removed the jacket and flat cut by a fiber cleaver (S325A, Northlab Photonics AB., USA) prior to mechanical grinding. The Single-mode fiber was then dry ground with a rotational polish pad (Diamond Lapping Film 662XW, 3M, USA) to form the hyperboloid structure at the fiber endface. Semiconductor grade SOG (400F, Filmtronics Inc., USA) solution with 0.0% water content, 4% shrinkage, 3 ppm/K thermal expansion coefficient and 1.37 refractive index were used to tune the radius of curvature of the produced hyperboloid lensed fiber. Prior to the fabrication of the microlensed fibers, the optical properties of commercially available SOG was measured. SOG with the thickness 400 nm was spin coated on a quartz coverslip and thermally cured. The optical transmission of the SOG was measured using a visible Spectrophotometer (SP-880, Metertech Inc., USA), and the SOG transmission ratio of around 98.9% over the wavelength from 460 to 1100 nm. The SOG is an ideal solution for this process to fabrication microlensed structure to enhance the coupling efficiency.
Figure 2 presents the simplified fabrication process for producing the proposed microlensed fibers. It is noted that the production process was monitored and inspected with an optical microscope. The ground SMFs with the offset less than 1.0 μm were selected for further SOG coating and electrostatic pulling process. Firstly, the fiber grinder was a three axis controlled variables grinding machine, including servo motor (ϕ) control grinding angle, motorized cylinder (θ) control fiber rotates and motorized linear vertical stage (H) control polish pad height [Fig. 2(A)]. This three-axis system was controlled with home-built computer programs to achieve a fully automated process to grind the SMF end into the minor radius of curvature of 3.3 μm. Details for the precision mechanical grinding procedures can be found in our previous report [15]. After the SMF grinding, a small amount of SOG was applied on the ground SMF tip by moving down the fiber and touching the SOG liquid then leaving upward [Fig. 2(B)]. A uniform electric field with the strength of 4x106 ~10x106 V/m was applied to perform electrostatic pulling for SOG. It is noted that the electrospinning could happen to remove the excess SOG liquid in the initial stage of the pulling process [Fig. 2(C) and 2(D)]. The residual SOG was then balanced with electrostatic pulling force and surface tension of the SOG liquid. This force balance stably formed a specific radius of curvature of SOG layer on the ground SMF tip [Fig. 2(E)]. The SOG coating layer was finally thermally cured to fix the hyperboloid microlens structure with the designed radius of curvature [Fig. 2(F)]. Once the SOG layer formation was out of specification, the SOG coating layer was then wiped with alcohol and steps B-F were repeated. Therefore, this option rework procedure saves lots of time and cost without wasting the precisely ground SMFs.
Figure 3 shows the experimental setup for measuring the light coupling performance for the produced micro-hyperboloid lensed fibers for efficient coupling from laser chips. A 2.1 mm sub-mount and 300 mW power 980 nm laser diode (C2-980-0300-S50, Axcel photonics, USA) was used as the light source for coupling test. The outlet of the laser diode was with an aspect ratio around 1:4 and the divergence angle for laser emission were about 7°and 26°, respectively. The gradually dispersing light emitted from the laser diode resulted in an elliptic light field and a curved wave front. The produced microlensed SMF with 40 cm in length was fixed on a three-axis electronic control stage and monitored with a high magnification CCD camera with the microscope for precision aligning the SMF tip and the laser diode outlet. The maximum light intensity emitted from the other end of the optical fiber was measured with an optical power meter.
3. Results and Discussion
Figure 4 shows the SEM images for a flat end SMF before mechanical grinding [Fig. 4(A)] and the SMF right after mechanical grinding [Fig. 4(B)] and after SOG coating process (Fig. 4(C)). It is clear that the hyperboloid structure was successfully generated on the SMF tip after mechanical grinding. However, there were significant rough features observed on the ground SMF surface. In addition, the sharp edge at the hyperboloid faces could not meet the radius of curvature for high-performance laser chip coupling. Alternatively, the SOG coated fiber showed a smooth surface and the adjusted radius curvature for the short axis such that the SOG microlensed SMFs exhibited better optical performance. The hyperboloid curvature microlensed at the tip could be better matching the mode field phase for laser chip coupling.
Fig. 4 SEM images for the optical fibers (A) right after mechanical grinding and (B) after SOG coating. The insets show the side view picture while measuring the radius of curvature of the formed hyperboloid microlens.
Figure 5(A) presents the relationship between the measured radius of curvature versus the applied electric field for electrostatic pulling SOG. It is noted that the radius of curvature for the mechanical ground fiber without SOG coating was about 3.3 μm. Moreover, there was no electrospinning happened at an applied electric field smaller than 6.7x106 V/m, resulting in SOG accumulation on the SMF tip and formed larger radius of curvature. Electrospinning of SOG liquid happened with an applied electric field higher than 7.0x106 V/m, which reduced the SOG amount and formed a thin SOG layer. Results also showed that the radius of curvature for the produced lenses decreased with the applied electric field. Since only very trace amount of SOG left after the electrospinning, the residual SOG could be cured with a 100 W halogen lame. Although the target radius curvature was set at 4.5 μm for electrostatic pulling the SOG coating layer. The measured radii of curvature for ten individual ground SMFs at short axis were ranging from 4.15 μm to 4.84 μm, and they were measured the light coupling efficiency with a commercial laser diode as the light source. Figure 5(B) presents the measured light coupling efficiency of 10 produced hyperboloid microlensed SMFs and the mechanical ground fiber without SOG coating. A flat-end fiber was used to compare the coupling performance with and without the coupling microlens. Results show that the 10 produced micro hyperboloid lensed fibers were with the coupling efficiently of higher than 75%. The maximum coupling efficiency reached 84% and the high averaged coupling efficiency for 10 fibers was 80%. Alternatively, the coupling efficiency for the ground fiber without SOG coating was lower than 30% due to the shape mismatched to the laser diode. On contrast, the flat-end fiber exhibited a coupling efficiency of around 36% which could meet the requirement for high-efficient light coupling.
Fig. 5 (A) Relationship between the measured radius of curvature versus the applied electric field for SOG pulling, (B) measured coupling efficiency for 10 fibers with different radii of curvature and the fiber without coating.
Figure 6(A) presents the light coupling fluctuation was measured to be less than 0.17% in 5 min for the 5 fibers with coupling efficiency higher than 80%. It is noted that the applied laser power was 250 mW for this test. The average coefficient of variation was as low as 0.116 ± 0.044%, indicating nice light coupling stability for the produced hyperboloid lensed fibers. Figure 6(B) shows the repeating test for the produced lensed fiber. The measured coefficient of variation for the coupling efficiency was 0.326%, indicating that the microlens structure exhibited good repeatability for the repeat coupling high-energy light source. The developed method provides a simple yet high-performance way for producing micro-hyperboroid lensed fiber for semiconductor laser applications.
Fig. 6 (A) Measured light coupling stability for 5 individual hyperboloid microlensed fibers, (B) high-efficiency light source on-off switch test.
5. Summary
This study developed a novel approach for producing hyperboloid microlensed fibers for high-performance light coupling. The hyperboloid microlens structure was produced utilizing a precisely automatic mechanical grinding and SOG coating process. An electrospinning and electrostatic pulling method were used to adjust the radius of curvature of the formed SOG coating layer. Without multiple and risky arc discharges process to adjust the radius of curvature of the ground fibers, the developed SOG coating process could efficiently achieve the target radius of curvature at one electrostatic pulling procedure. The time and cost for producing high quality and precision hyperboloid microlensed fibers could be achieved. Moreover, the measured coupling efficiency for 10 produced lensed fibers was all higher than 75% which was about 100% higher than that obtained with a flat-end fiber. The method developed in the present study will give substantial impacts on producing high performance and low-cost hyperboloid lensed fiber for commercial applications.
Acknowledgment
Financial supports from Ministry of Science and Technology of Taiwan are greatly acknowledged
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