Study on thermal effect and optical attenuation characteristics of SiO2-GeO2 single-mode fiber for silicon photonic modulator

Abstract

Optical fibers are critical components of silicon photonic modulators. To investigate the effects of copper wire Joule heating and electro-optical energy conversion loss on optical fibers, communication single-mode fiber (C-SMF) and waveguide single-mode fiber (W-SMF) were selected for the experiments in this study. The core diameter of the C-SMF was 9  μm, whereas that of the W-SMF was 8.2 μm; both fibers had an overall diameter of 125  μm. Both fiber cores were composed of 96 wt% SiO₂ and 4 wt% GeO₂, while their claddings are made of 100 wt% SiO₂. In this study, a transmission electron microscope (TEM) was used to perform elemental analysis of the cores, and the mechanical properties and crystallinity of the fibers were compared. Various heating effects were simulated through heat treatment, thermal cycling, and point-heating tests to investigate the changes in the properties of the fibers. In addition, a self-designed polymer-based photonic integrated circuit (PIC) chip was used to analyze the core morphology and observe the light profile.

The results indicate that C-SMF exhibits lower tensile strength but higher elongation, whereas the center of W-SMF contains SiO structures that absorb optical signals and reduce transmission efficiency under high-temperature conditions. Owing to the rapid cooling after the thermal treatment, the fibers could not form stable structures, leading to reductions in both the crystallinity and tensile properties. The crystallinity decreased owing to the impact of thermal stress on the structure after 100 cycles of the thermal cycling test (TCT). After TCT 700 cycles, the slow diffusion of Ge and the localized Ge-rich area promoted grain growth in the W-SMF. Regarding optical attenuation, the W-SMF showed improved performance after thermal treatment owing to defect reduction through the annealing effect. However, prolonged high-temperature exposure leads to significant Ge diffusion, increased defect density, and refractive index fluctuations, ultimately leading to optical attenuation degradation. In addition to the annealing effect observed after three-point heating, the refractive index difference between the core and cladding increased, resulting in a more stable optical signal. The PIC light profile confirmed that the optical signal was transmitted not only through the core but also through the cladding, and the signal was transmitted in the fundamental mode. This study confirms the high-temperature applicability of optical fibers in silicon photonic modulators and the compatibility of optical fibers and PICs.