Dye-dispersed polymer based self-written waveguide for alignment-free optical interconnect

Abstract

Passive optical network (PON) in optical communications has gained much attention in the past few years in field of telecommunications. Huge bandwidth demand and the increase in transmission speed of telecommunication traffic have also influenced the fast maturity of fiber-to-the-home (FTTH) technology. To implement a full PON system, multiple passive optical components are necessary. Various types of planar lightwave circuit (PLC) passive devices are needed in FTTH systems, and a suitable optical interconnection method to connect them with conventional optical fibers is required. Three conventional optical interconnection techniques that have been identified are fusion splicing, laser welding, and adhesive bonding, of which each has its own advantages and disadvantages. In relation, this study mainly aims to improve the existing adhesive bonding packaging method for photonic packaging between optical devices. The light-induced self-written waveguide (SWW) method is proposed to provide an optical linking property to interconnect two cores of optical devices. This method has been found to manifest inherently the benefits of adhesive bonding technology, while compensating for the drawbacks of coefficient of thermal expansion (CTE) mismatch. The formation mechanism of SWW is achieved via photo-polymerization, an increase in the refractive index (RI) by irradiating the laser with suitable wavelength in dye well-doped adhesive to form a core layer. Such is useful in reducing optical loss of the optical path for compact photonic component. To achieve lowest coupling loss and high stability interconnection, the formation process and reliability should be carefully investigated while great effort should be given in identifying influencing parameters and for reliability testing. Optical coupling is characterized by loss measurement at different conditions: (i) with an air gap of 100 μm between two single-mode fibers (SMFs), (ii) after adding dye-doped epoxy, and (iii) after forming the SWW. First, the study aims to investigate influencing factors, dye-doped concentration, and lasing power intensity, which could cause variation in optical performance. The experiment is then tested using the optimized value. Second, the SWW formed by single-side irradiation technique is characterized. Consequently, loss tolerances due to the lateral displacement after SWW are formed at perpendicular axis for the different tested conditions. In view of further improvement in coupling efficiency and higher mechanical strength, double-side irradiation is applied and tested. To test the availability of the alignment relaxation property of this interconnection method, the experiment with different lateral misalignment in perpendicular axis prior SWW formation is tested and compared for both single- and double-side irradiation. Finally, the SWW is tested to interconnect between the fiber arrays (FAs) using a specially designed fixture. The reliability test of the SWW against temperature changes has been proven to be less temperature-sensitive. The influencing factors that affect the performance of the formed SWW are optimized to achieve lowest coupling loss. Mechanical strength is reinforced and coupling efficiency is improved by comparing double-side irradiation with single-side irradiation. The former can provide better tolerance on alignment relaxation, and more lateral displacement before SWW formation is allowed, when compared with single-side irradiation. The reliability test of the SWW between two FAs is also tested and with only a small change throughout temperature testing. The result provides confidence that SWW formed by double-side irradiation can provide low coupling loss and higher stability with less susceptibility due to mechanical stress and changes in temperature. The findings are believed to be very useful for PLC packaging, which is widely used in PON for optical communication.