## 4 Optical Measurement Process
### Loss Analysis of the Waveguides Fabricated Using the Mosquito Method  
In the measured results of the insertion loss of polymer waveguide shown in Figure 12, the uniformity of the loss over the whole parallel cores is a concern. The results in Figure 12c show the loss variation from 2 to 4 dB in a 5-cm long waveguide at 850 nm. In the Mosquito method, the parallel cores are dispensed one by one in order, and the core dispensed first has longer interim time than the last one after being dispensed to start UV cure. This interim time difference among the channel could lead to the difference of the index profile, core diameter, and NA, which could cause the variation of the insertion loss. Hence, the interim time dependence of the insertion loss is analyzed in more detail. Figure 13 shows the interim time dependence of the insertion loss. In Figure 13, two different measurement conditions are employed by Keio University in accordance with the IEC measurement  
Figure 11: Total insertion loss per cm including coupling loss. Figure 12: Insertion loss measurements at 850 nm and 1310 nm on glass and polymer waveguide samples, (**a**) Fraunhofer IZM glass waveguide sample set 1, (**b**) Fraunhofer IZM glass waveguide sample set 7 and (**c**) Keio University polymer Mosquito waveguides, (**d**) Sumitomo Bakelite polymer waveguide (850 nm measurements only). requirements [11]: OM3 GI-MMF is used for both launch and capturing fibers (50 GI-50 GI) and a standard single-mode fiber is used for the launch fiber (SMF-50 GI). Meanwhile, Figure 14 shows the interim time dependence of the output Near Field Pattern (NFP) from the waveguide. From Figure 14, the insertion loss monotonically decreases with respect to the interim time from 0 to 270 s under both conditions. Then, once the minimum insertion loss is observed at 300- to 430-s interim time, a gradual increase is observed with increasing the interim time. From Figure 14, it is found that the core-cladding boundary in the cross-section becomes blurred with increasing interim time, which means the refractive index gradation is gradually formed. By comparing the NFP image to the scale bar of 50 \(\upmu\)m in Figure 14, it is also found that originally the output optical field of the NFP is slightly larger than the designed core diameter (50 \(\upmu\)m). Since the core has a profile close to an SI when the interim time is short, the NFP is widely spread to the whole core in the first two cores, even if the core is launched with a small beam spot via an SMF. Therefore, the insertion losses are as high as 1.8 to 3 dB when the interim time is shorter than 105 s, due to the core size and optical field mismatching between the cores of waveguide and GI-MMF (50 GI, capturing fiber). Meanwhile, after an interim time of longer than 205 s, the size of output NFPs are almost the same and smaller than 50 \(\upmu\)m under 50 GI and SMF launch conditions, respectively, although the actual core diameter observed from the cross section is much larger than 50 \(\upmu\)m due to the monomer diffusion. The small spot of the output NFP is a well-known feature of the GI core, which leads to a high efficiency coupling to the GI-MMF capturing fiber. Hence, the low insertion loss is observed after an interim time longer than 205 s, as shown in Figure 14. Here, the insertion loss slightly increases when the interim time is long enough (longer than 500 s). The excess diffusion of the core and cladding monomers contributes to the decrease of the NA of the core, resulting in lowering the light confinement effect. From the NFPs of the last two cores in Figure 14, a gradual spot size increment is visually observed. Figure 14: Interim time dependence of the cross-section, Near Field Pattern (NFP) and insertion loss of the polymer waveguide. Figure 13: Interim time dependence of the insertion loss of the 5-cm long polymer waveguide.