Dopant Diffusion
Localized thermal treatment of optical fibers causes core dopants to diffuse radially outward, altering the guide characteristics. The thermally expanded core technique increases the mode field diameter at the fiber end face without changing the outer cladding diameter. It allows for easier alignment and reduced coupling loss between single-mode fibers or optoelectronic devices.
Coupling Efficiency
Alignment tolerances are significantly relaxed when the mode field diameter is enlarged. A thermally expanded core splice reduces sensitivity to lateral offset during alignment, which is required for automated assembly lines. This expanded mode field reduces the power density at the fiber connection point, preventing optical damage in high-power laser applications.
The expanded core must be tapered gradually to avoid converting the guided core mode into lossy cladding modes.
Fabrication Parameter
Process parameters include the heating temperature, duration, and the localized hot-zone width. Creating a thermally expanded core requires temperatures above the glass transition point of silica, typically using a micro-burner, CO2 laser, or electric arc. Precise control over the heat source profile prevents asymmetric diffusion, which would degrade the beam quality.
Loss Evaluation
Measurement of the insertion loss of the processed fiber confirms the quality of the transition. A poorly optimized thermally expanded core taper introduces loss due to radiation of the guided optical power. System engineers verify this parameter by measuring directional loss before and after the thermal process to ensure the taper remains adiabatic.
