In recent years, optical interconnect technology has emerged as the preferred choice for high-speed communication channels, particularly for long-distance transmission. Its widespread adoption in telecommunication networks is fueled by the growing demand for increased bandwidth. Optical interconnect offers significant advantages in terms of broadband bandwidth compared to traditional copper-based connections that rely solely on electrical signals. By utilizing optical fibers or waveguides, it becomes possible to seamlessly connect various optical devices, enabling efficient and reliable data transmission. However, achieving reliable connections with low coupling loss between different optical devices using optical fibers remains a challenge. Mode-field mismatch is one key reason which causes coupling loss. Size and geometric difference are two critical factors for the mismatch. With the application of high refractive index contrast materials and development of nanofabrication technologies, optical devices tend to be integrated, which is an effective way to increase the packing density of optical chips in optical interconnect systems. Due to etching process and design constraints, optical waveguides usually feature asymmetric structures, which substantially differ from the geometry of optical fibers. Therefore, integration poses a severe problem when it comes to connecting optical devices/waveguides with optical fibers. When they are butt-coupled, high coupling loss generates due to the large mode-field mismatch caused by their size and geometric difference. Conventional optical interconnect techniques (such as fusion splicing and laser welding) cannot effectively address the mode-field mismatch coupling issues since they connect similar mode-field size devices with fibers. To overcome these challenges, grating couplers and spot size converters have been proposed as potential solutions. However, the grating couplers approach is wavelength dependent, and it exhibits low coupling efficiency due to its surface-coupling scheme. The spot size converters approach requires specially designed tapers, which may increase the complexity of overall designs and fabrication cost.

In this thesis, we propose dye-doped epoxy-based self-written waveguide (SWW) technique to tackle the coupling issues in optical interconnects. The mechanism of this approach is to form a light-induced optical waveguide in photocurable epoxy, serving as a connection between optical devices. The optical waveguide, once cured, will not change its structure even when the light source is removed, which acts as an effective optical link between cores of optical devices. Light can effectively transmit through this optical link from devices to devices. The approach can apply in connecting a single-mode fiber (SMF) to any waveguide surface in principle, even with a large mode-field mismatch, significantly alleviating the tight alignment requirements typically needed for end-fire coupling into integrated waveguides. Based on our material systems (appropriate dye and epoxy) and induced laser (green light, at a wavelength of 532 nm), we have successfully fabricated SWWs between optical devices. We demonstrate how self-written waveguides solve the practical coupling issues in optical interconnects on different platforms and study the parameters to further improve the performance of SWW. Additionally, we summarize the advantages and impacts of the SWW approach in optical interconnects.

In our first work, we proposed the SWW approach to tackle the mode-field mismatch coupling issues between optical devices. We quantitatively investigated the effectiveness of using dye-doped epoxy-based SWW for coupling loss reduction and relaxation of the lateral alignment tolerance between a SMF and a 5 × 2 µm rectangular channel waveguide-under-test (WUT) with a large mode-field mismatch due to their size and geometric difference. Based on our findings, we found that the more significant the mode-field mismatch between two optical devices, the greater coupling loss can be reduced by SWW. However, it is important to note that a larger mode-field mismatch condition will result in a higher initial insertion loss. To evaluate the effectiveness of the SWW approach in mitigating this issue, we defined a figure of merit and conducted a study on the coupling loss between two types of single-mode fibers and the waveguide-under-test (WUT) using SWW connections. Our study revealed a significant reduction in coupling loss. Specifically, between the SMF130V fiber & SM1500 fiber and the WUT, the coupling loss decreased by 8.34 dB from -11 dB and by 3.22 dB from -4.27 dB, respectively. Furthermore, the SWW approach demonstrated a remarkable lateral alignment tolerance, with the coupling loss between the SM1500 fiber and the WUT remaining below 0.2 dB across a lateral offset range of ± 2 µm.

Lithium niobate (LN) has been widely applied in optical communication networks due to its high-speed electro-optical (EO) modulation ability. With the increasing demand for integration, thin-film lithium niobate-on-insulator (LNOI) has been adopted in many nanophotonic devices. Due to the high refractive index contrast, it is difficult to achieve low-loss optical interconnects between fibers and thin-film LN chips. In our second work, we applied SWW approach to reduce the substantial coupling loss between optical fibers and integrated LN chips. The SWW was realized by irradiating green light into dye-doped epoxy from single side and double side under a photo-polymerization process. We investigated two critical parameters (green light power and dye concentration) that dictate the SWW formation process and offered insights into their relationships. After the formation of the SWW under double-side irradiation condition, the measured fiber-to-chip coupling loss was reduced from an initial value of -14.27 dB by 8.66 dB. This approach can reduce significant coupling loss between optical fibers and integrated LN chips without a complex design and nanofabrication process. Therefore, it offers a cost-effective and flexible fiber-to-LN chip optical interconnect.

With the increasing requirements of bandwidth in optical networks, multi-core fiber (MCF) has been widely adopted to realize the space division multiplexing (SDM) to improve the data transmission capacity. It is of great significance to maintain good optical interconnects between multi-core fibers. In our third work, we applied SWW approach to multi-core fiber connections. We have achieved significant progress in establishing optical interconnects between two four-core single-mode fibers. We investigated and illustrated characterizations of the SWW approach and compared with commercial polarization maintaining fiber patch cords. This approach offers notable advantages, including ease of fabrication, low optical coupling loss (-0.5 dB in average) and inter-core crosstalk (-30.4 dB in average), and high lateral alignment tolerance. Besides, the optical transmission characteristics over C and L bands are good. The approach holds the potential to provide a cost-effective and flexible solution for optical interconnects in multi-core fiber systems, with the prospect of making a significant impact in optical networks.

In conclusion, the SWW approach proposed in this thesis offers a practical and effective solution, supported by case studies, to address mode-field mismatch and multi-core fiber coupling issues in optical interconnects. The approach demonstrates its effectiveness in reducing coupling loss and enhancing reliability. It provides several significant advantages, including ease of fabrication, low optical coupling loss, relaxed lateral alignment tolerance, and versatility across different platforms. With these inherent advantages, the SWW approach opens up new avenues and paves the way for practical applications in the field of optical interconnects.