Fan-in/out polymer optical waveguide for a multicore fiber fabricated using the Mosquito method

A fan-in/out polymer optical waveguide is fabricated for connecting multimode multicore (7 cores) fiber with onedimensionally aligned parallel optical components such as a VCSEL/PD array or a multimode fiber ribbon, which is fabricated using the Mosquito method. The Mosquito method we have proposed is a fabrication technique for circular and graded index (GI) cores. One of the unique characteristics of the Mosquito method is a capability of forming threedimensional wirings. In the fan-in/out waveguides, high-density hexagonal alignment of 7 cores at one end is converted to one dimensional alignment with a wider pitch at the other end. For realizing the fan-in/out waveguides, we have issues about low insertion loss, low crosstalk, and the connectability with multicore fibers and optical components. In this paper, we focus in the pitch accuracy of the fan-in/out waveguide. In the Mosquito method, the viscosities of the core and cladding monomers are an important factor of the core figure and the core alignment because the viscosities have a relation to monomer liquid-flow, which could devastate the core alignment. Hence, we investigate the influence of the viscosities of the core and cladding monomers on the interchannel pitch accuracy of the fabricated fan-in/out polymer optical waveguide. With increasing the viscosities of core and cladding monomers, the pitch accuracy is improved, while the appropriate monomer viscosity conditions that can fix all the issues: core circularity and pitch accuracy in both ends still needs to be investigated.


INTRODUCTION
Over the last couple of years, multicore fibers have been expected as a promising solution for over 100-Tbps transmission [1,2]. For realizing multicore fiber based optical links, fusion-splicing and connection techniques with another multicore fiber and fan-in/out devices for single core fibers have been an issue. As for the fan-in/out devices, some solutions such as optical fan-in/out devices using lenses [3], a tapered multicore fiber bundle [4], and a fan-in/out optical waveguide [5,6] are proposed. From the point of view of miniaturization, integration, productivity, and favorable connectivity to on-board optical waveguides, we focus on the fan-in/out optical waveguides composed of organic polymers. In our fan-in/out polymer waveguide design, the cores need to be bent both horizontally and vertically, so three-dimensional core alignment needs to be realized. Such 3-D optical wiring contributes to the further high-density integration of fan-in/out optical waveguides.
We have proposed the Mosquito method that is a fabrication technique for polymer parallel optical waveguides with circular graded-index (GI) cores using a microdispenser [7]. The Mosquito method is capable of forming the cores with three-dimensional alignment in the cladding. By applying the Mosquito method, we fabricate a fan-in/out polymer optical waveguide, in which one end has a high-density hexagonal alignment structure of 25-μm circular GI cores (7 cores), while at the other end, the 7-core alignment is converted to be in line with a pitch as wide as 250 μm [8]. For a high connection efficiency between the fabricated fan-in/out waveguide and a multicore fiber or a fiber ribbon composed of single-core fibers (7 fibers), the low insertion loss, the low crosstalk, and the precise core alignment are important characteristics. Although the waveguide we fabricated in our previous publication [8] showed low insertion loss and low crosstalk, the interchannel pitch accuracy was an issue to be improved. *ltc.sd-ds.hm2945@keio.a5.jp; phone +81 45 566 1593 One of the reasons why the pitch was not accurately controlled is because the repetitive needle scan for dispensing multiple cores caused liquid flow of cladding monomer, which disturbed the core alignments from the originally designed alignment. In order to reduce the monomer liquid-flow effect, the needle should be as thin as possible. So far, we have also investigated a possibility to fabricate single-mode waveguides using the Mosquito method, for which using a thinner needle is effective for dispensing small cores [9,10]. However, the availability of a needle with 80-μm inner diameter and less is a problem. Hence, we focus on the viscosity of the core and cladding monomers, and investigate the pitch accuracy of the fan-in/out polymer optical waveguide fabricated using the Mosquito method. Figure 1 shows the fabrication procedure of the Mosquito method. A thin needle scans while dispensing the core monomer from it. Here, the needle-tip is remains inserted in the cladding monomer while it scans. The needle can scan three dimensionally, which is programmed in the desk-top robot. After forming a waveguide structure in the cladding monomer, both core and cladding monomers are cured under a UV exposure followed by postbaking. In this paper, we use a microdispenser and a desk-top robot for the needle scan made by Musashi Engineering, Inc.

THE MOSQUITO METHOD
In the Mosquito method, we have lots of parameters to adjust. The core diameter depends on the pressure for dispensing the core monomer, the needle inner diameter, the needle-scan velocity, and the viscosity of core monomer. So far, the appropriate combinations of the dispensing pressure, the needle inner diameter, and the needle-scan velocity have been investigated for obtaining desired core diameter. On the other hand, we also found that the viscosities (μ) of the core and cladding monomers mutually affected the cross-sectional core shape. Besides, the viscosity of the cladding monomer also affects the stability of the core position in the liquid-state cladding monomer. The repetitive needle scans causes the monomer liquid flow which could devastate the core alignment if the low viscosity cladding monomer is used. Furthermore, viscous core monomers generally have a higher density than the viscosity cladding monomers, so that the core monomer dispensed in advance could sink down in the cladding monomer due to the gravity. This leads to the position errors of core in the vertical direction.

FAN-IN/OUT POLYMER OPTICAL WAVEGUIDE
In this paper, a multimode multicore fiber with a 26-μm core diameter and a 39-μm interchannel pitch fabricated by OFS [11] is featured because of the ease of fabrication for the fan-in/out waveguides. Figure 2 shows an over-view and a cross-section of the fan-in/out waveguide design. The core alignment at both ends of the fan-in/out waveguide is designed as follows: at the end connected to the multicore fiber, seven cores with 25−μm diameter and a 40−μm pitch are stacked hexagonally with three layers (2-core, 3-core, and 2-core). The three-layer hexagonal alignment is rotated counterclockwise with 15 degrees to obtain the cross-section shown in the inset of Fig. 2. Meanwhile, at the other end, all the cores are aligned in line with a 250−μm pitch. Here, in the first, 2-cm long regions from the both ends, all the cores keep the original alignment, so the seven cores are straight and parallel to the waveguide axis. Then, the 5-cm long region in the middle is used for horizontal pitch conversion, on the other hand, the 1-cm long region at the center, with a red color in Fig. 2, is for vertical pitch conversion. The cores are scanned in order from 1 to 7 indicated in the cross-section, and they are designed not to cross in order to avoid breaking the cores. Figure 3 shows a cross-section of a fabricated fan-in/out waveguide. We applied FX-W712 (μ: 12,000 cps) and FX-W713 (μ: 10,000 cps) for the core and cladding monomers, respectively. Both resins are supplied by ADEKA Co. The fabrication parameters are as follows: The inner needle diameter is 100 μm, the dispensing pressure is 350 kPa, and the needle scan velocity is 24 mm/s. The cross-sections of the fabricated fan-in/out waveguide shown in Fig. 3 are completely different from the designed core alignment shown in Fig. 2. On the other hand, Fig. 4 shows cross-section of a fabricated fan-in/out waveguide after the correction on the needle-scan program. Table 1 and 2 show the measured pitches from core 4 at the hexagonal stacked side and one-dimensional side, respectively. Besides, from the crosssectional photo shown in Fig. 4, the pitch accuracy in the vertical direction at the one-dimensionally aligned side is evaluated. Then, a deviation of ± 12 μm in the pitch is observed. For addressing these pitch error problems, one solution is a real-time feedback using a CCD-camera to the needle-scan program for correcting the position accuracy of the desktop robot. In this report, another solution in which specific monomers are applied for keeping the core alignment without the correction of the needle-scan programs is investigated. In addition, the waveguide shown in design 2 in Fig. 2 is discussed for low insertion loss and crosstalk [12].    Figure 5 shows cross-sections of a fabricated fan-in/out waveguide based on design 2. Here, the waveguide is cut at the middle region into two pieces, and the both sides are separately observed. The cores are not precisely positioned compared to the alignment shown in Fig 3. Then, a lower-viscosity cladding monomer, FX-W715 (μ: 1,000 cps) (ADEKA Co.) is applied, and a fan-in/out waveguide based on design 2 is fabricated again. The dispensing conditions are the same as the case when FX-W713 is applied for the core. The cross-sections are shown in Fig. 6. At the onedimensionally aligned side, the error trends in the core alignment in both horizontal and vertical directions are almost the same as those when a higher-viscosity cladding monomer, FX-W713 (μ: 10,000 cps) is used as shown in Fig. 5. Meanwhile, at the hexagonally stacked side, the cores are no more identified. Figure 7 shows an over-view of the waveguide shown in Fig. 6. The cores aggregate because the lower-viscosity cladding monomer largely flows and would devastate the core alignment with such a high density structure. Hence, with increasing the viscosity of the cladding monomer, the core alignment tends to be maintained as they are dispensed.   Figure 7. Over-view of a fabricated fan-in/out waveguide applying a core monomer (μ: 12,000 cps) and a cladding monomer (μ: 1,000 cps)

PITCH ACCURACY OF FAN-IN/OUT WAVEGUIDE
Next, we investigate the influence of the viscosity of the core monomer on the core alignment. SPR-7782 (μ: 30,000 cps) and SPR-KOU7690 (μ: 92,500 cps), both of which are a modified acrylate monomer, are applied as a higherviscosity core monomer. In this case, SPR-6367 (μ: 9,600 cps), SPR-6373 (μ: 19,000 cps), and SPR-6377 (μ: 32,000 cps), which are also modified acrylate monomer are used as cladding monomers. Here, we use a 100-μm inner diameter needle and the core monomers are dispensed at 650 kPa. In the case of SPR-7782, the needle scans at 16 mm/s, meanwhile in the case of SPR-KOU7690, the needle scans at 5 mm/s. Applying these resins, fan-in/out waveguides are fabricated and the cross-sections at the hexagonally stacked sides are compared in Table 3. Here, the viscosities of the cladding monomers are almost the same: twice and three times higher than the viscosity of FX-W713, meanwhile, the viscosities of SPR-7782 and SPR-KOU7690 are almost three and nine times higher than the viscosity of FX-W712, respectively. Then, the measured pitches and the height errors at the one-dimensionally aligned sides are shown in Table  4.  Almost circular cores are observed when SPR-6367 and SPR-6373 are adopted for the cladding, while ellipse cores are observed in case of the SPR-6377. When the core monomer viscosity is as high as 30,000 cps (SPR-7782), it is noted that the vertical pitch error at the hexagonally stacked side is reduced compared to the other cases in Table 3. Contrastingly, when the core monomer has a viscosity higher than 90,000 cps, SPR-KOU7690, the cross-sections at the hexagonally stacked sides look elongated in the z-direction. Furthermore, in the case when SPR-KOU7690 and SPR-6367 are used for the core and cladding, respectively, the measured pitch in one-dimensionally aligned side is well controlled to 250 μm although the vertical pitch error is large.
From the above discussion, we found that the core and cladding monomers adopted in this experiment were not necessarily appropriate even if those viscosities are high enough for obtaining a complete circular core. For improving the alignment in both horizontal and vertical directions, further detailed investigation is required for the viscosity combinations of core and cladding monomers. In addition, we need to investigate the influence of thickness of cladding monomer on monomer liquid flow.

SUMMARY
The pitch accuracy of the fan-in/out waveguides is investigated. Circular cores, the well aligned hexagonally stack, or the well controlled pitch in horizontal direction at the one-dimensionally aligned side is observed. However, we also need to find out appropriate combinations of core and cladding monomer viscosities in order to fix all the issues: core circularity and interchannel pitch at both ends of the fan-in/out waveguides. In addition, the investigation on the thickness of cladding monomer, which affects the monomer liquid flow, could be a solution.