Introduction to Optical Fiber Couplers

　　Optical fiber couplers are critical passive components in optical communication systems, designed to split or combine optical signals between multiple optical fibers. They operate based on the principle of evanescent field coupling, where light from one fiber is transferred to one or more output fibers through physical proximity or a fused taper structure. These couplers are essential for building optical networks, enabling signal distribution in applications such as telecommunications, cable TV networks, and fiber-optic sensing systems.

　　Optical fiber couplers come in various types, categorized by their structure and function. Fused fiber couplers are created by heating and stretching two or more optical fibers together, forming a tapered region where light can leak between the cores. This process allows precise control over power division, making them suitable for low-loss signal splitting. Planar lightwave circuit (PLC) couplers, on the other hand, are fabricated using integrated optical technology on a silicon or glass substrate, offering high port density and uniformity, which is ideal for high-channel-count networks like wavelength division multiplexing (WDM) systems.

　　Key performance parameters of optical fiber couplers include insertion loss (the power loss during coupling), coupling ratio (the distribution of power between output ports), return loss (measurement of back-reflected light), and polarization dependence. High-quality couplers minimize insertion loss (typically below 0.5 dB for low-port-count devices) and exhibit consistent performance across different wavelengths and polarization states.

　　Applications of optical fiber couplers span from simple signal splitting in local area networks to complex configurations in optical coherence tomography (OCT) for medical imaging. They are also integral in fiber systems, where they direct pump light into the gain medium and extract output laser beams. As optical networks evolve toward higher speeds and greater scalability, the demand for low-loss, high-reliability couplers with advanced features like wavelength selectivity continues to grow, driving innovations in fabrication techniques and material science.

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