Flexible light-induced self-written optical waveguide with 50 μm core size

We demonstrate a NIR light-induced self-written (LISW) optical waveguide between graded-index (GI) glass optical fibers with a 50 μm core size (50GIFs) using gel material. We describe the optical properties of the LISW optical solder in terms of its flexibility, adhesiveness, and loss. The results demonstrate that the two 50GIFs were self-coupled through the LISW optical waveguide, and the connection maintained adhesiveness against displacement. A low loss and relaxation of alignment tolerance were demonstrated for the optical interconnection between two 50GIFs using flexible LISW optical soldering. This technology is applicable to future autonomous driving systems using high-speed optical data transmission.


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or future autonomous driving, vehicles should be equipped with multiple sensors that acquire information about the surroundings.There is therefore a need for high-speed in-vehicle network systems that can transmit in real time large amounts of data acquired from multiple sensors, such as cameras, light detection and ranging, and radar. 1)][4] Normally, to prevent damage to the POF end face owing to vibrations, in-vehicle connectors have air gaps of about 0.1-0.2mm at the connection point. 5)However, such gaps cause problems with respect to maintaining lowloss optical coupling between the POFs, especially for highspeed networks with transmission speeds over 1 Gbit s −1 . 6,7)o solve this problem, we are developing a light-induced self-written (LISW) optical waveguide technique 8,9) using gel materials. 10,11)When photocurable resins are irradiated with light from optical fibers, polymerization begins at the center of the optical fiber core end because of the concentrated light intensity.The refractive index of the irradiated region was increased by polymerization, and a difference in the refractive index was generated between the exposed core center region and the surrounding region owing to the self-focusing effect.The LISW optical waveguide was grown through repeated photopolymerization and the self-focusing effect.The advantage of using an LISW optical waveguide is that no misalignment occurs between the optical fiber and waveguide because the waveguide is produced automatically from the fiber tip.Moreover, bidirectional light irradiation enables automatic optical coupling between two optical fibers, which is called an "optical solder". 9)Further, various extended technologies based on LISW optical waveguides have been demonstrated, [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] and there is a wider range of applications of LISW optical waveguides.
However, recently reported conventional LISW optical waveguide materials are predominantly crosslinked hard polymer resins.Although such hard materials are useful for applications under standstill conditions, such as data centers, they are not applicable to in-vehicle optical networks exposed to vibration conditions.Mohammed et al. reported flexible LISW optical solder using single-mode fibers. 26)In the report the use of adhesion promoter is inevitable to maintain mechanical and optical properties.In our previous work, an LISW optical solder was demonstrated between two SI-POFs (980 μm core size) using gel materials, and the LISW optical solder was robust to mechanical displacement without using adhesive. 9,10)][29][30] Previous papers on flexible LISW optical solder 9,10,26) are not suitable for application to automotive high-speed data link.Therefore, this study designed a flexible LISW optical interconnection between GI glass optical fibers with core and cladding sizes of 50 μm and 125 μm (named 50GIF), respectively.Here, we describe the optical properties of the LISW optical solder in terms of its flexibility, adhesiveness, and loss measurement.
Figure 1 shows the stress-strain curve for the "soft" resin which is photo-cured at around 850 nm.As a comparison, the curve for conventional NIR-light-cured "hard" resin for LISW optical waveguide 20) is also measured and shown in the graph.From Fig. 1 the resin used in this study has smaller Young's modulus than the conventional hard one, which reveals that the present resin is flexible enough to perform the mechanical experiment.
Upon increasing the amount of butyl acrylate, the photocured mixture became flexible.A detailed optimization of the mixing ratio of the four components will be reported elsewhere considering factors such as flexibility, refractive index controllability, and propagation loss.
Figure 2 shows the sample preparation for fabricating (a) an LISW optical waveguide and (b) an LISW optical solder.First, the prepared material was dropped onto a glass slide, and the tip of the optical fiber was soaked in the material (a-1) and (b-1).It was moved upward so that a small amount of material adhered to the tip of the optical fiber (a-2) and (b-2).Because the prepared material has a high viscosity, it can adhere to the end face of the optical fiber.In (b-3), another optical fiber was prepared under the same conditions as in (b-1) and (b-2) and placed face-to-face.The gap between the optical fibers was set to approximately 100 μm because air gaps of the order of 100 μm exist in automotive optical connectors practically. 5)fter curing by laser light with a wavelength of 853 nm (a-3) and (b-3), the uncured portion around the LISW optical waveguide was removed with acetone (a-4) and (b-4).A laser source with a wavelength of 853 nm was used in the experiment because this wavelength lies within the in-vehicle optical data transmission wavelength range.
Figure 3 shows the experimental setup for the bidirectional laser irradiation.A laser source with a wavelength of 853 nm was used.The light source was connected to an isolator, circulator, and splitter using 50GIFs.After the input light was split into two, 50GIFs were connected to each output port of the splitter, and the end surfaces of the 50GIFs were mounted such that they faced each other.Each optical fiber was fixed to a positioning stage, and fine adjustments, such as axis misalignment on the micrometer scale, could be performed.
Figure 4 shows a photograph of the LISW optical waveguide fabricated using one-sided laser irradiation.The output light power from the 50GIF tip was set to 1 μW, 5 μW, and 10 μW, and the irradiation time was set to 10 s.Table I lists the sizes of the LISW waveguides fabricated under various laser conditions.With a laser output power of 1 μW, the waveguide could not be grown (10 s), or a diameter less than that of 50GIF core was grown (30 s).However, with a laser output power that exceeded 5 μW, the LISW optical waveguide grew and reached a size similar to that of the 50GIF core, and the waveguide size did not change.Further irradiation time increased the insertion loss because of the thicker waveguide.From these results, we found that the threshold condition corresponded to an irradiation power of 1 μW and an irradiation time of 30 s, which corresponded to 30 μJ.Moreover, the conditions of 5 μW and 10 s, which correspond to an irradiation energy of 50 μJ, were appropriate according to results considering the low fabrication energy.
A mechanical experiment was conducted to demonstrate the high flexibility and adhesiveness of LISW optical waveguides.Figure 5(a) shows a microscopic photograph of the LISW optical waveguide after optical interconnection by bidirectional light irradiation and the subsequent removal of the cladding.Two 50GIFs were coupled by a LISW optical waveguide with a length of 120 μm. Figure 5(b) shows a photograph of the LISW optical waveguide axis stretched horizontally from state (a).The optical fiber was stretched by 60 μm in the horizontal direction for an overall length of 180 μm.The LISW optical waveguide was connected without peeling it from the optical fiber end faces.In addition, the

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd original shape was confirmed to have been restored when returning from (b) to (a). Figure 5(c) shows a photograph of the LISW optical waveguide stretched in the direction perpendicular to the axis from state (a).The optical fiber was stretched by about 50 μm in the direction vertical to the axis.In this case, the LISW optical waveguide was also connected without peeling it from the end surface of the optical fiber.In addition, the original shape was restored when returning from (c) to (a).These results indicate that LISW   optical waveguides using gel materials have high adhesive strength and restoring force against mechanical displacement, and they are suitable for flexible optical data transmission under vibration conditions.
Loss measurements were performed to determine the effect of LISW optical soldering on insertion loss changes at a wavelength of 853 nm. Figure 6 shows microscopic photographs of the two optical coupling conditions for loss evaluation: (a) air gap and (b) LISW optical soldering without washing the uncured cladding.When the air gap has a distance of 100 μm, the measured loss was 3.0 dB.By performing optical soldering using an LISW optical waveguide, the loss became 1.2 dB.The measured insertion loss includes not only the loss of coupling between fibers, but also the propagation loss in 50GIF (10 −3 dB m −1 at 850 nm 29) ) and input coupling loss of the fibers.This measurement experiment considers not only the axial misalignment of the two fibers but also the loss caused by the angular misalignment; reproducing the initial condition in Fig. 6(a), the loss due to the angular misalignment of 0.3 to 0.5 dB is included in the measurement error.Based on results obtained, the two interconnecting optical fibers with the LISW optical waveguide led to a significant reduction in the coupling loss.
Figure 7 shows insertion loss measurement results obtained when the axial misalignment is given in the vertical direction.In this experiment, LISW optical coupling was performed with no axial misalignment between the two fibers, followed by axial misalignment displacement.An evaluation of loss due to misalignment in the axis-perpendicular direction was performed for air gap and LISW optical waveguide interconnections.The blue circles represent losses under the air-gap condition [Fig.6(a)], and the red triangles represent losses in the LISW optical coupling [Fig.6 (b)].It was confirmed that compared with the air gap, the LISW optical coupling suppresses not only the absolute loss value but also the tolerance by axial misalignment, even if the degree of misalignment increases.The LISW optical waveguide was bent owing to optical axis misalignment, resulting in a slight increase in loss.However, this increase was suppressed by the flexible LISW optical self-coupling.From these results, the flexible LISW optical waveguide is considered effective for high-speed optical data transmission because the mechanical motion has less influence.
In summary, this paper presents the fabrication of a flexible NIR-LISW optical waveguide using a gel material from a GI glass fiber with a 50 μm core.A low insertion loss and relaxation of alignment tolerance were demonstrated for the optical interconnection between two 50GIFs using flexible LISW optical soldering.The fabricated self-assembled LISW optical waveguide has high adhesive strength and flexibility despite its small contact area, and exhibits potential for future movable applications such as automotive applications, which require high-speed data transmission.
Although flexibility and adhesiveness were maintained for the LISW optical solder without cladding, the insertion loss increased by approximately 1 dB because of the numerical aperture mismatch between the 50GIF and LISW optical waveguides at the connections as well as the higher scattering loss between the core and cladding (air).In future, the fabrication of flexible claddings is important not only for low-loss coupling but also for the long-term stability of optical coupling against mechanical vibrations.Considering the flexibility of the LISW optical solder, this technique is considered applicable to other movable applications such as soft robotics. 31)

Fig. 5 .
Fig. 5. Photograph of optical interconnection by flexible LISW optical waveguide; cladding is washed away.(a) No alignment deviation.(b) Extension in the horizontal direction of the axis.(c) Extension in the vertical direction of the axis.The arrows indicate the displacement directions.

Fig. 7 . 4 ©
Fig. 7. Insertion loss as a function of axial misalignment for two optical coupling conditions; (a) air gap.(b) connection using LISW optical waveguide before removing claddings.