If you connect a fiber with a thick core and a fiber with a thin core, will the light get stuck?
Light does not “stick” or stagnate at splices like physical matter, but serious light energy loss (radiation loss) and mode conversion can occur due to Mode Field Mismatch.
I. Physical Mechanism and Loss Analysis
Light propagates in optical fibers as specific electromagnetic transverse modes. When two fibers with different core diameters or Mode Field Diameters (MFD) are spliced, the coupling efficiency depends on the overlap integral of the electromagnetic fields at the cross-sections of the two fibers.
1. Larger Core Fiber \rightarrow Smaller Core Fiber (High Loss Direction)
- Physical Phenomenon: For example, connecting a multimode fiber with a core diameter of 50\ \mu\text{m} to a single-mode fiber with a core diameter of 9\ \mu\text{m}.
- Propagation Process: The energy distribution cross-section conducted in the larger core is much larger than the receiving area of the smaller core. When the light beam reaches the interface, only the light in the central overlapping region that satisfies the numerical aperture (NA) and guided mode conditions of the smaller core can be coupled into the smaller core to continue propagating; the rest of the light energy cannot enter the core and directly leaks into the cladding of the smaller core fiber, becoming cladding radiation modes and dissipating over a short distance.
- Result: Extremely high insertion loss (typically above 10\ \text{dB} \sim 20\ \text{dB}).
2. Smaller Core Fiber \rightarrow Larger Core Fiber (Low Loss Direction)
- Physical Phenomenon: Connecting a single-mode fiber with a core diameter of 9\ \mu\text{m} to a multimode fiber with a core diameter of 50\ \mu\text{m}.
- Propagation Process: The output fundamental mode Gaussian beam from the smaller core has a smaller spot size than the cross-section of the larger core. Most of the energy can fall within the larger core and excite the fundamental mode and low-order modes within the larger core.
- Result: Small forward coupling loss (typically around 0.2\ \text{dB} \sim 1\ \text{dB}), but the output light changes from single-mode to multimode propagation, introducing intermodal dispersion.
II. Mode Field Mismatch Formula
For two fibers that are both operating in single-mode but have different mode field radii (w_1 and w_2 respectively, approximating as Gaussian beams), the theoretical power transfer coefficient \eta for an ideal coaxial splice with no gap is:
\eta = \left( \frac{2 w_1 w_2}{w_1^2 + w_2^2} \right)^2
The mode field mismatch loss \alpha_{\text{MFD}} (in dB) caused by this is:
\alpha_{\text{MFD}} = -10 \log_{10}(\eta) = -20 \log_{10}\left( \frac{2 w_1 w_2}{w_1^2 + w_2^2} \right)
The larger the difference in mode field diameter between the two fibers, the lower the transfer coefficient \eta and the more significant the loss.
III. Engineering Reference
In optical communication and optical sensing systems, unified standard optical fibers are typically used to avoid mode field mismatch. For example:
- Standard communication and sensing systems commonly use single-mode fibers with a core diameter of 9\ \mu\text{m} and a cladding diameter of 125\ \mu\text{m}, such as OFSCN® G.652D Optical Fiber;
- When it is necessary to connect fibers with different core/mode field specifications, engineering practices employ tapered transitions, Mode Field Adapters (MFAs), or specific fusion splicing heating expansion techniques (TEC) to achieve smooth mode field transitions and reduce mismatch loss.
