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Abstract
We experimentally demonstrate that the Mosquito method is capable of fabricating single-mode polymer optical waveguides with multiple cores while maintaining an identical mode-field diameter among them. The Mosquito method we developed is a technique to form circular cores in polymer waveguides using a commercially available microdispenser and multi-axis syringe scanning robot. However, the core shape tends to deteriorate from a circle because of the monomer flow due to the needle scan. We also demonstrate both theoretically and experimentally for the first time in this paper that the needles with a tapered outer form or just titled straight needles enable to form cores with a high circularity even if those cores are dispensed on any heights from the substrate surface.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, with the rapid development of cloud computing services, the number of hyperscale datacenters being built is increasing every year, especially in North America. In such a large-scale datacenter, it is very important to enhance the processing speed of servers and switches as well as to reduce power consumption. One of the key technologies sustaining the growth of datacenter networks is the optical interconnect, which uses optical fibers as signal transmission lines. Today, a large number of single-fiber optical connectors, called local connector (LC), are used to connect fibers to optical transceivers on the front panel. However, those fiber cables are gradually being replaced by fiber ribbons with multi fiber push on (MPO) connectors to increase the bandwidth density on the front panel. In recent years, even the number of fibers to be aligned in an MPO connector has increased to realize much higher bandwidth density [1]. Furthermore, multicore fibers (MCFs), which have multiple cores aligned in one cylindrical cladding, is a promising optical fiber that dramatically enhances the number of optical channels per cross-sectional area. Some optical connectors have been reported that allow several times larger or more channel number per cross-sectional area by replacing the existing single-core fibers (SCFs) with the MCFs [2]. However, there are several important issues in introducing MCFs into datacenter networks. For instance, fan-in/fan-out (FIFO) devices are necessary to connect the MCFs to the transmitters and receivers, which need to convert the channel alignment between a 1D array and a 2D hexagonal array in MCFs. This is because semiconductor lasers are generally aligned inline (one dimension) in optical transceivers. Accordingly, various optical connection methods with FIFO configurations have been proposed [3,4]. In particular, three-dimensional glass or polymer waveguides are expected to realize a compact FIFO device with low connection loss [5]. We have reported the “Mosquito method” to fabricate polymer waveguides with 2D and 3D multiple core alignment using a microdispenser in order to apply the waveguides to on-board optical interconnects [6–8]. Meanwhile, there is a report from other group on directional couplers and mode (De) multiplexers fabricated using the Mosquito method as a three-dimensional device [9]. Here, the unique feature of the Mosquito method is its ability to form circular graded-index cores with variable core diameters from less than 5 µm to tens of micrometers. We already succeeded in fabricating single-mode polymer waveguides with circular cores using the Mosquito method, and demonstrated that the circular cores can easily handle the mode field to coincide with the mode field of conventional single-mode fibers (SMFs). Hence, the single-mode polymer waveguides fabricated using the Mosquito method allow a low connection loss with SMFs very easily. However, in our previous demonstrations, the core cross-sections frequently deteriorated from a circle to an oval or even a heart-like shape. Such core shape degradation can cause high connection loss between the waveguides and SMFs. The reason for the high coupling loss is theoretically calculated by the overlap integral of the fundamental mode profiles in square and circular cores with the same mode-filed diameter (MFD) of 6 µm using the same core and cladding materials, and we obtained the loss of 0.012 dB.
Therefore, in this paper, experimental and theoretical investigations on how to fabricate circular cores by the Mosquito method is presented by introducing a flow analysis simulation, as well as experimental investigations.
2. Mosquito method
A wide variety of fabrication methods have been reported for single-mode polymer waveguides such as photolithography with photomasks [10,11], UV imprinting [12], and laser writing [13,14]. For FIFO structures, the intercore pitch conversion just within one plane (horizontal to the substrate surface) can easily be formed in the above methods using photomasks [15]. In addition, the UV curing through a photomask in the above methods leads to form a rectangular cross-sectional core. Such rectangular core waveguides could cause mode-field mismatch with standard SMFs with circular cores, resulting in high connection loss between them.
On the other hand, we have reported an original fabrication method for multimode and single-mode polymer waveguides named the Mosquito method which uses a microdispenser and a multi-axis syringe scanning robot. The fabrication steps in the Mosquito method are illustrated in Fig. 1.
First, as shown in Fig. 1(a), a cladding monomer is cast on an organic substrate made of polyphenylene sulfide (PPS) doped with glass filler. Since the same PPS polymers are widely employed as the base material of mechanical transfer (MT) ferrules, we investigate the possibility of direct waveguide fabrication in an MT ferrule by applying the PPS based organic substrate. When we test the FIFO function in the fabricated waveguides, an SMF array terminated with another MT or MPO connector is coupled to the waveguide. Here, to connect the FIFO waveguides to SMF arrays, the radial or even rotational offset of the core could be the largest issue in maintaining low connection loss. The thermal expansion coefficients of the waveguide material and substrate should be equal, in order to improve reliability, thermal stability, and connection loss. In Fig. 1(a), the cladding monomer needs to be cast with a uniform thickness on the substrate. Therefore, in this paper, the same microdispenser as the one for the core monomer dispensing is applied to the cladding monomer casting to control the amount of cladding monomer by precisely controlling the monomer temperature and dispensing pressure. After the cladding monomer is cast, the tip of the thin needle on a syringe is inserted into the cladding monomer (as shown in the inset of Fig. 1(b)), and then the core monomer is dispensed into the cladding monomer while the needle scans following the programmed core patterns, as shown in Fig. 1(b). In this paper, the core diameter is controlled by adjusting the dispensing volume of the core monomer. By a precise control of the core diameter and the monomer diffusion, the single-mode and multimode propagation conditions are controlled. Multiple cores are formed by repeating the needle scan. After all the cores are dispensed, both the core and cladding monomers are cured under UV light exposure, as shown in Fig. 1(c).
3. Monomer flow analysis in the Mosquito method
3.1 Monomer dispense through straight needles
We have already demonstrated that the MFD in the single-mode polymer waveguides fabricated using the Mosquito method could be controlled in order to coincide with the MFD of conventional SMFs to be connected [16]. We have also demonstrated that the Mosquito method has allowed to form circular cross-sectional cores, but there has been a tendency for the core cross-sections to deteriorate into elliptic or heart-like shapes [16]. Since a large number of parameters need to be taken into account in the Mosquito method for dispensing the cores, it has been difficult to experimentally investigate the factors that affect the core shape. Therefore, in this paper, we intend to find the appropriate fabrication conditions to form circular cores with the Mosquito method by applying the monomer flow analysis using general-purpose thermal flow analysis software ANSYS Fluent.
Figure 2 illustrates an overall diagram of the flow analysis model that reproduces the Mosquito method. We compose this model by taking two layers of fluid, the cladding monomer and the air, into consideration in addition to the dispensed core monomer. In the monomer flow simulation, three parameters of the two monomers are taken into account: viscosity, surface tension, and density; they are summarized in Table 1. Table 2 shows the parameters of the needle and the conditions in the Mosquito method. It is already confirmed that these polymer materials have a high enough heat resistance to withstand solder reflow process. The dispensing pressure is adjusted so that the diameter of the core monomer dispensed is 10 µm. The cladding width is 7.5 mm (X direction) and the length is 9 cm (Y direction), which are sufficiently wider than the needle size. Therefore, both edge regions (in X and Y-directions) should have little influence on the core shape in the flow analysis. It should be noted that we can accurately reproduce a triangle shape (circled by a dotted line in the inset of Fig. 2) of the cladding monomer on its top creeping up around the needle surface if we involve the parameters of surface tension and the layer of the air above the cladding monomer in the calculation. This cladding monomer creeping up around the needle is quite important for the simulation, which is explained in the later section. Although the needle scans in the Y direction in the actual Mosquito method, in the calculation, the needle is fixed (the needle center is set at Y = 0 mm) and alternatively the cladding monomer flows in the Y-direction.
Table 1. Parameters of the Monomer Used for the Flow Analysis
Table 2. Parameters of the Needle and Fabrication Conditions of the Mosquito Method
The results in Figs. 3 and 4 which were already shown in [16] represent a side view cross-section of the core monomer being dispensed and cross-sectional views perpendicular to the scan direction at three locations shown in Fig. 3, respectively. Here, the positions of the cross-sections in Figs. 4(a) to (c) are on the center of the needle (Y = 0 mm), on the backside edge of the needle (Y = 0.115 mm), and on 9-mm behind the backside edge of the needle (Y = 9.115 mm), respectively. In this flow analysis model, the thickness of the cladding monomer is 0.5 mm and the needle-tip height from the substrate surface (to the center of the core) is set as 0.25 mm.
Fig. 4. Cross-sections perpendicular to the needle-scan direction during the core monomer dispense (a) center of the needle (Y = 0 mm) (b) back-side needle edge (Y = 0.115 mm) (c) 9 mm behind the needle (Y = 9.115 mm) [16].
From Fig. 4(b), it is found that the cross-sectional shape of the core monomer deforms right after it leaves the needle. At the position in Fig. 4(c), the core diameter is as small as 10 µm, decreased significantly from that in Fig. 4(b). The small dip on the upper periphery of the core is found in Fig. 4(b), which remains existed even after it is apart from the needle, as shown in the inset of Fig. 4(c). Furthermore, the vertical to horizontal ratio (VHR) of the core diameter is measured as 0.84, which means the core shape is a horizontally long oval. Here, when VHR is 0.8, which is a representative value with a core height of 0.25 mm and higher, we theoretically estimate that the coupling loss of 0.014 dB is observed.
Since the core shape and output optical field of typical SMFs are circular, there is a concern that fabricated polymer waveguides exhibiting elliptic mode fields could increase the connection loss due to the mode-field mismatch when connecting to the SMFs. Therefore, it is very important to identify the parameters that affect the shape of the core in the Mosquito method and to investigate how to set the parameters to form circular cores. We already empirically found that the horizontally wide elliptical core tended to be formed when the needle height is so low that the core monomer was dispensed very close to the substrate surface. Therefore, we analyze the core shape dependence on the needle-tip height. The flow analysis model is shown in Fig. 5. Two different needle heights are selected: 0.35 mm (Fig. 5(a)) and 0.15 mm (Fig. 5(b)). The other parameters are set to the same as those in Fig. 3. Figures 6 and 7 show cross-sectional views perpendicular to the scan direction at the same three locations as in Fig. 4 at different needle-tip heights.
Fig. 5. Side view cross-section of the core monomer flow in the fluidic simulation (a) needle-tip height of 0.35 mm (b) needle-tip height of 0.15 mm
Fig. 6. Cross-sections perpendicular to the needle-scan direction during the core monomer dispense at a needle-tip height of 0.35 mm (a) center of the needle (b) back-side edge of the needle (c) 9 mm behind the needle.
Fig. 7. Cross-sections perpendicular to the needle-scan direction during the core monomer dispense at a needle-tip height of 0.15 mm (a) center of the needle (b) backside edge of the needle (c) 9 mm behind the needle.
Under both needle-tip height conditions, heart-shape cores are observed in Figs. 6 and 7. However, the VHR of the core diameter is different: At a needle-tip height of 0.35 mm, the VHR is 0.84, which is the same as that at 0.25-mm height. Meanwhile, when the needle-tip height is 0.15 mm, the VHR decreases to 0.76. When the needle-tip height is lower than 0. 25 mm, the VHR decreases to exhibit horizontally longer core diameter, as shown in Figs. 4, 6 and 7.
We already found in [16] that the pressure distribution in the cladding monomer played an important role on the formed core height and cross-sectional shape. Hence, the pressure distribution in the cladding monomer near the backside edge of the needle is analyzed in order to investigate how the core shape deteriorates to a heart-like and horizontally wide oval. Figure 8 shows a zoomed-in view of the pressure distribution in the cladding monomer near the backside edge of the needle at different needle-tip heights. This static pressure represents the gauge pressure, which is relative to the atmospheric pressure. Therefore, if the pressure is lower than the standard atmosphere (101.325 kPa), it is shown as a negative pressure, thus, there is no directionality to this static pressure. Here, the white colored area on the right edge is the cross-section of the needle wall, while the color variation from blue, green, to red indicates the pressure range from negative to positive, as indicated in the color bar. We find a very low pressure region just behind the needle. The calculated pressure distributions on the dashed lines in Fig. 8 are shown in Fig. 9.
Fig. 8. Pressure distribution near the needle backside edge in the X-Y plane on the same height of the needle tip (a) Needle-tip height of 0.15 mm (b) Needle-tip height of 0.25 mm (c) Needle-tip height of 0.35 mm
Fig. 9. Pressure distribution near the needle backside edge on the broken line (X-axis direction) in Fig. 8
It should be noted that one low pressure area widely extended to the X direction is observed on the backside of the needle in Fig. 8(c), which is confirmed by the blue curve in Fig. 9, while the low pressure area splits to two areas with decreasing the needle height, as shown in Figs. 8(a), (b), and the green and red curves in Fig. 9. After the core monomer is dispensed from the needle tip, it flows upward due to the low-pressure area on the backside of the needle. Here, since two split low pressure areas in the X-direction exist as shown in Figs. 8(a), (b), and Fig. 9, the core monomer ascends higher in these two areas compared to the middle of these two regions. Thus, the heart-shape core cross-sections are formed, as shown in Figs. 4 and 7.
Next, we investigate how these pressure distributions in the cladding monomer are formed.
Figure 10 shows the monomer flow velocity map on the same planes as those in Fig. 8. In Fig. 10, the white donut-like circles show the cross-section of the needle tip. Here, we focus on the vertical (Z axis) component of the monomer flow vector. The red area indicates that the flow velocity vector with a positive (+Z directional) component, while the blue area with a negative (−Z directional) component. The velocities of both core and cladding monomers are jointly calculated and the deep color signifies higher flow velocity, as indicated by the color bar in Fig. 10. On the needle front side, since the needle is an obstacle for the monomer to keep flowing in the −Y direction, the monomer has to descend (in blue area) for circumventing the needle. Contrastingly, on the needle backside, the monomer ascends (+Z direction) to flow into the lower pressure regions shown in Fig. 8. Here, because of the split low pressure areas on the needle backside, as shown in Figs. 8 and 9, the two split red regions exist in the map particularly in Fig. 10(a). From Figs. 10(b) and (c), we find the red regions tend to be wider with increasing the needle-tip height, and the two split areas start to merge together.
Fig. 10. Monomer flow velocity distribution around the needle in the X-Y plane (a) Needle-tip height of 0.15 mm (b) Needle-tip height of 0.25 mm (c) Needle-tip height of 0.35 mm
The difference in the flow velocity map among Figs. 10(a) to (c) explains the reason why the heart-like cross-sectional cores are formed as follows: Fig. 11 shows the monomer flow velocity map in the side-view cross-section on the broken line in Fig. 10, corresponding to the needle center (core center). In Fig. 11, the blue, green, and red colors also show the Z-component of the flow velocity vector: blue is negative (descending) and red is positive (ascending). Compared to the map in Fig. 10, the colors on both sides of the needle look opposite. However, the velocity map in Fig. 10 just focuses on the X-Y plane including the needle-tip (ex. broken line in Fig. 11(c)). In this specific plane, the monomer flow direction is opposite compared to other places, which is most clearly observed in Fig. 11(c) with small blue and red (orange) areas near the broken line.
Fig. 11. Monomer flow velocity distribution around the needle in the Y-Z plane, white curves indicate the boundary between the air and cladding monomer (a) Needle-tip height of 0.15 mm (b) Needle-tip height of 0.25 mm (c) Needle-tip height of 0.35 mm
The monomer flow velocity maps on the needle backside in the two different planes in Figs. 10 and 11 are compared in more detail. Near the left edge on the needle tip, the monomer ascends (shown by red color) in Fig. 10. However, in the entire plane on the needle backside shown in Fig. 11, a large blue area exists, which means a large amount of monomer flows downward. Such a downward stream is caused because the cladding monomer, after creeping up around the needle outer surface shown in Fig. 5, falls down after it leaves the needle. Thus, two opposite streams are confluent on the needle backside to form the core.
The downward stream in Fig. 11 largely affects the core cross-section. When the needle tip is the closest to the substrate surface, a larger amount of monomer creeps up on the needle surface, as shown in Fig. 5(b) and thus its downward stream on the needle backside is strong, as shown in Fig. 11(a). So, the core cross-section tends to be a horizontally long oval, as shown in Fig. 7(b). The flow paths of the core monomer dispensed from the needle tip placed on the different heights are calculated and visualized in Fig. 12. The different colors of the line indicate the monomer flow dispensed from the different positions on the circumference of the needle tip.
Fig. 12. Flow path of the dispensed core monomer (a) Needle-tip height of 0.15 mm (b) Needle-tip height of 0.25 mm (c) Needle-tip height of 0.35 mm
It is found from Fig. 12 that the core monomer ascends higher with increasing needle tip height. In addition, the red lines reach the highest point due to the two split low-pressure areas, as shown in Fig. 10. On the other hand, the yellow and green lines indicating the core monomer flow paths near the needle center are not allowed to ascend higher than the red lines, due to the downward monomer stream, as shown in Fig. 11. It is obvious in Fig. 12(a) that the height difference among all the lines is smaller than those in Figs. 12(b) and (c). Here, the pressure distribution in the monomer on the vertical planes (same as those in Fig. 11) is shown in Fig. 13. In Fig. 13, the color indicates just the pressure value as shown in the color bar. It is clearly observed in Fig. 13 that the low-pressure area shown by the blue and green is narrower and does not extended significantly in the Z direction (i.e., it’s relatively flat) with decreasing the needle-tip height. Hence, when the needle-tip height is low, as shown in Fig. 13(a), the core monomer dispensed from the needle tip does not ascend after it leaves the needle. From these results, we confirm the fact that the core tends to be horizontally long oval when the needle-tip height is low. In addition, the downward monomer stream at the needle center on the needle backside forms the dip on the top of cross-section.
Fig. 13. Pressure distribution in the monomer near the needle edge in the Z-Y plane (a) Needle-tip height of 0.15 mm (b) Needle-tip height of 0.25 mm (c) Needle-tip height of 0.35 mm
3.2 Influence of Needle Outer Shape on Core Cross-Section
In the above sections, we confirmed that the direction of monomer flow varied by the pressure distribution in the monomer was the factor that determined the core cross-sectional shape. We also reported in [16] that the needle outer shape affected the monomer pressure distribution and thus the monomer flow velocity. In addition, we have already reported that tilting the needle can improve the core cross-sectional circularity. However, the mechanism of core shape improvement has not been found out, hence the optimum value of the tilted angle was not quantified. In this paper, we apply tapered needles and analyze how the inclined outer wall of a tapered needle contributes to the formation of circular cores. The dimensions of the tapered needle for the calculation are shown in Table 3. These parameters are measured from a tapered needle actually used for the experiment shown in Fig. 14(a). The parameters of the core and cladding monomers summarized in Table 1 are also used.
Fig. 14. Flow analysis of the Mosquito method using a tapered needle. (a) Cross-sectional view of a tapered needle. (b) Side view cross-section of the core monomer flow in the fluidic simulation.
Table 3. Tapered Needle Dimensions
Figure 14(b) shows a side view cross-section of the core monomer being dispensed. In this flow analysis model, the thickness of the cladding monomer is 0.5 mm and the needle-tip height from the substrate surface is 0.25 mm. It should be noted that the cladding monomer creeping up in the Z direction around the needle surface is smaller than that in Fig. 3, by which the monomer flow velocity distribution is affected as follows: The flow velocity map and the pressure distribution in the monomer are shown in Fig. 15.
Fig. 15. Flow velocity and pressure distributions of the monomer in Fig. 14(b) (a) Pressure distribution around needle backside edge in the X-Y plane (b) Monomer flow velocity distribution around the needle in the X-Y plane (c) The pressure distribution near the needle edge in the Z-Y plane
We find from Fig. 15(a) that the low pressure region (blue area) on the needle backside is not completely bifurcated but almost uniformly extends along the needle circumference. In Fig. 15(b), we also find that the monomer flow velocity vector on the needle backside has a + Z directional component even on the broken line that passes through the needle center. Furthermore, from Fig. 15(c), it is confirmed that the low pressure region on the needle backside in the Z-Y plane including the needle center is stretches out in the + Z direction. Due to the tilted outer wall of the tapered needle and the small amount of creeped-up cladding monomer on the needle surface, as shown in Fig. 15(b), the downward monomer flow is not strong compared to that in Fig. 10(b) in the Z-Y plane, including the needle center on the needle backside. Since the dispensed core monomer flows into this low pressure region, the core monomer can vertically diverge following the upward stream. Because of these reasons, it is expected that the tapered needle could eliminate the dip on the top of the core cross-section resulting in forming core with an improved circularity. Simultaneously, the VHR of the core diameter is expected to be close to 1.0. Figure 16 shows cross-sectional views perpendicular to the needle-scan direction during core monomer dispensing when we use the tapered needle, comparison with those in Fig. 4.
Fig. 16. Cross-sections perpendicular to the needle-scan direction during the core monomer dispense (a) center of the needle (b) back-side edge of the needle (c) 9 mm behind the needle.
The VHR of the core diameter is calculated to be 1.04 using the cross-section shown in the inset of Fig. 16(c), from which we confirm the core shape is a circle. So, the heart-shaped cross-sections observed in the core dispensed from straight needles are no longer formed. Next, the relationship between the needle-tip height and the VHR of the core diameter is calculated and summarized in Fig. 17. In the case of the straight needle, it is difficult to form circular cores even if the needle-tip height is adjusted. Contrastingly, when a tapered needle with a 10° taper angle is used, the VHR of the core diameter maintains a value close to 1.0 when the needle-tip height is greater than 0.2 mm, which means that a certain cladding thickness is necessary to fabricate circular core even if a tapered needle is used. Meanwhile, in Fig. 17, the VHR of the core diameter when the needle taper angle is 20° is also indicated. In the case of the 20° taper angle, a VHR close to 1.0 is obtained even when the needle height is less than 0.2 mm. Therefore, if a thinner cladding is needed or if the cores must be formed very close to the substrate surface, the tapered angle should be higher than 10°. Experimental data are marked in Fig. 17 as well. A slight disagreement between the measured and calculated results is observed, and the actual core shape is better than estimated. One of the reasons for the observed core shape improvement is the diffusion of the core and cladding monomers which allows the core-cladding boundary to be blur (due to the graded refractive index profile). The experimental procedures and results are described in more detail in the next section.
In Fig. 18, the dependence of the VHR of the core diameter on the needle-scan velocity is summarized, when the same tapered needle is used. The needle-tip height is set to 0.25 mm. It should be noted that a VHR value very close to 1.0 is maintained, independent of the needle-scan velocity.
On the other hand, it is difficult to adjust the needle tapered angle responding to the needle-tip height during one core dispense. However, this is an important issue to fabricate three-dimensional waveguides in which the core height is varied. Therefore, we expect that tilting the straight needle, which we already reported in [16] works similarly to the tapered needle. Figure 19 shows the calculated core cross-sectional view of the 10° tapered needle compared to those with a straight needle with 10° and 20° tilt at a needle-tip height of 0.25 mm. When the straight needle tilts 10 degrees, the calculated VHR is 0.98, while 20° tilt leads to a VHR of 1.23 showing a vertically long oval. It is confirmed that the same effect as the tapered needle is obtained by tilting the straight needle. Thus, we confirm the possibility to fabricate circular cores at different needle-tip heights by appropriately tilting a straight needle.
Fig. 19. Cross-sections perpendicular to the needle-scan direction during the core monomer dispense (a) 10-degree tapered needle (b) 10-degree tilted straight needle (c) 20-degree tilted straight needle
Finally, the validity of the flow analysis shown above is confirmed by comparing with experimental observations. Figure 20 shows side view cross-sections of the core monomer flow observed using a microscope when a straight needle tilts 0° and 20°. As indicated in the calculated core monomer flow paths in Fig. 3, the core monomer dispensed from the needle-tip flows upward once at the back-side edge of the needle, and then gradually flows downward. In addition, with increasing the tilt angle, the core monomer ascends higher, as theoretically predicted in Fig. 15. Since the flow of the core monomer experimentally observed well agrees with the calculated result, the monomer flow analysis employed in this paper will work to reproduce the other dispensing conditions in the Mosquito method.
Fig. 20. Side view cross-sections of the core monomer flow (see Visualization 1 and Visualization 2) (a) 0-degree tilted straight needle (b) 20-degree tilted straight needle
4. Fabrication of Circular Core Waveguides
4.1 Fabrication of Waveguides Using Tapered Needle
The monomer flow analysis performed in the above sections showed that tapered needles enabled the formation of circular cores when the needle-tip height was adjusted appropriately. In order to compare the experimental and theoretically calculated results, polymer waveguides are fabricated with the tapered needle shown in Fig. 14(a). The fabrication conditions are the same as in Tables 1 and 2 (except for the needle dimension). Figure 21 shows a cross-section of the fabricated polymer waveguide. Figure 22 shows zoomed-in images of some cores. Compared to the cores formed by a straight needle shown in Fig. 4(c), these cross-sectional views show that all the cores have circular shapes with a uniform diameter.
4.2 Evaluation of Optical Properties of Fabricated Polymer Waveguides
The NFPs of all the cores in the waveguide shown in Figs. 21 and 22 are measured at a wavelength of 1.31 µm, and are shown in Fig. 23, from which we confirm that all the cores satisfy the single-mode condition. Here, the propagating mode in the waveguide cores is launched using a 1-m long SMF probe (A typical SMF compliant to ITU-T G. 657. A2, with an MFD of 8.6 ± 0.4 µm at 1.31 µm). From Fig. 23(a), the MFDs measured in the two orthogonal directions, X and Z, show good agreement, and it is close to that of the SMF probe. Then, the VHR of the core diameter is estimated from the NFPs of all the cores. As shown in Fig. 23(b), the measured VHR of the MFD is 1.02 on average, which is almost the same as that of the theoretical value (1.04) shown in Fig. 17. (This value is marked in Fig. 17.) In Fig. 23(a), a slight amount of oscillating MFD variations over the channel number is observed, particularly in the Z-direction. In the step of core monomer dispensing in Fig. 1(b), the needle scans with a zig-zag path, with a + Y directional scan for the odd number channels and a -Y directional scan for the even number ones. The different scan directions could cause the MFD variation particularly in the Z direction.
Fig. 23. NFP measurement at a wavelength of 1.31µm (a) MFD in the X and Z directions (b) MFD ratio of each cores in the waveguide
The insertion loss of the fabricated polymer waveguide between two SMFs is measured using the same SMF for the NFP measurements at 1.31 µm. This insertion loss includes the connection loss with the SMF on the two waveguide facets and the propagation loss in the waveguide (3 cm-long). In the insertion loss measurement, the waveguide and SMF cores are aligned using an automatic stage. Here, a refractive index matching gel is applied between them to reduce the Fresnel loss. Figure 24 shows the results for 12 cores in the fabricated waveguide. Referring to our previous publications [17], the propagation loss inherent to this polymer material is estimated to be 0.30 dB/cm at 1.31 µm. As the waveguide length is 3 cm, the propagation loss in the whole waveguide is estimated to be 0.90 dB. Therefore, the connection loss with the SMF is estimated to be as low as 0.15 dB per connection.
The coupling loss of the two cores can be calculated from the overlap integrals of their NFPs. Here, Fig. 25 shows the calculated coupling loss between the SMF and the fabricated polymer waveguide. Here, the end-face where the light is coupled from the SMF to the waveguide is surface A, while the output end is surface B. The results show that the coupling loss is lower than 0.15 dB in almost all the cores. We find this result is slightly better than the coupling losses experimentally estimated from the insertion and propagation losses. In the estimation of the coupling loss from the measured insertion loss, a propagation loss value of 0.30 dB/cm referred to our previous report is applied. However, if the propagation loss of the waveguide in this paper is slightly higher, such as 0.31 dB/cm, the estimated coupling loss is 0.12 dB, much closer to the calculated coupling loss in Fig. 25. Hence, a slight difference in propagation loss could be one of the causes of the difference.
The coupling loss (0.12 dB) actually observed could be due to the MFD mismatch between the SMF and the polymer waveguide. Therefore, by adjusting the core diameter of the waveguide more carefully, a further reduction in coupling loss is possible.
5. Conclusion
In order to apply to optical connection components such as FIFO devices for MCFs, we designed polymer optical waveguides with circular cores which are fabricated using the Mosquito method to satisfy the single-mode condition, and methods to reduce the fluctuation of the optical characteristics among the multiple cores were investigated both theoretically and experimentally.
In order to fabricate circular cores, we found from the theoretical flow analyses that the core monomer flow in the vicinity of the needle tip just after being dispensed was key: the strong downward stream on the needle backside contributed to the formation of a dip on the cross-section of the core to make a heart cross-sectional shape. In order to address this issue, it was confirmed theoretically and experimentally that using tapered needles allows us to form circular cores because of the reduction of the downward monomer stream on the needle backside.
How to fabricate single-mode waveguides using the Mosquito method while maintaining uniform optical characteristics among the multiple cores as well as a high intercore pitch accuracy, which are required for single-mode core connectors, has been a concern. We found how to address these issues, namely optimization of the fabrication conditions and equipment. We believe that the single-mode waveguides fabricated using the Mosquito method is paving the way to create new waveguide type connection devices.
Funding
Japan Society for the Promotion of Science (JP18H05238).
Disclosures
The authors declare no conflicts of interest.
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