Optical couplers are made by bringing fiber cores into close proximity to enable evanescent wave coupling, typically using fused biconical taper (FBT) or planar lightwave circuit (PLC) techniques.

Operating Principle

An optical coupler is a passive device that splits or combines light between fibers without external power. Its operation relies on evanescent coupling, where a small portion of the light field extends beyond the fiber core into the cladding. When two fiber cores are brought within a few micrometers, the evanescent waves overlap, allowing light to transfer from one fiber to another. The splitting ratio—the percentage of light directed to each output—is controlled by the interaction length and core separation, while insertion loss quantifies the total light loss during coupling .

Fused Biconical Taper (FBT) Method

The traditional FBT method involves:

1. Stripping and cleaning fibers to expose the cores.
2. Twisting or aligning fibers in close contact.
3. Heating the fibers until the glass softens.
4. Stretching the fibers to taper the cladding, bringing the cores into sufficient proximity for evanescent coupling.
5. Monitoring the splitting ratio in real time and stopping the process when the desired ratio (e.g., 50/50, 70/30) is achieved. This method allows flexibility for custom ratios and is suitable for low-port-count couplers. However, the process is sensitive to small variations in elongation, requiring careful measurement and adjustment to achieve the intended performance .

Planar Lightwave Circuit (PLC) Method

PLC couplers are fabricated using photolithographic etching on a silicon substrate:

1. Waveguides are etched to form precise Y-branch structures.
2. Fibers are coupled to the input and output ports.
3. The device is thermally stabilized to ensure uniform performance across all channels. PLC technology is preferred for high-port-count splitters (e.g., 1×32) due to its high uniformity, thermal stability, and scalability. Unlike FBT, PLC devices are highly reproducible and suitable for mass production .

Performance Considerations

Key metrics in coupler fabrication include:

• Splitting ratio: Determines how light is distributed among outputs.
• Insertion loss: Total light loss, including coupling and excess loss.
• Directivity: Fraction of input light lost internally.
• Polarization-dependent loss (PDL): Variation in transmission due to polarization states. These parameters are measured during and after fabrication to ensure the coupler meets design specifications .

Summary

The process of making an optical coupler combines precise physical alignment, controlled heating, and stretching (FBT) or microfabrication techniques (PLC) to achieve efficient light transfer between fibers. The choice of method depends on the number of ports, desired splitting ratio, and production scale, with FBT suited for small-scale, customizable couplers and PLC ideal for high-volume, multiport applications .