Lightrun’s Science Popularization: Fiber Combiner - Structure and Principle.

Precise Fused Taper Process Achieves High - efficiency Energy Coupling

As a core optical coupling device relying on the Fused Biconical Taper (FBT) technology, the multimode fiber combiner plays a crucial "optical energy integration hub" role in high - power fiber laser systems. Its core function is to couple the discrete beams output from multiple input fibers (usually multimode pump fibers) into a single output fiber (multimode or double - clad fiber) with low loss and high stability, providing efficient energy supply for subsequent laser amplification and output.

(I) The Core Structure of the Fiber Combiner

It consists of three parts: the input fiber array, the fused taper region (referred to as the taper region for short), and the output fiber. The technical core lies in achieving low - loss coupling of multiple optical signals through the precise fused taper process.

1.Input Fiber Array: According to the combing requirements, it is composed of N (or N+1) specific types of fibers (such as single-mode, multimode, double-clad fibers, etc.) arranged in a preset manner (such as close-packed arrangement or surrounding the central fiber), responsible for receiving multiple input laser beams.

2.Fused Taper Region: High-precision fused taper equipment is used to perform high - temperature melting treatment on the input fiber array, enabling the fiber cladding and core to form a continuously tapered structure in the molten state, achieving energy fusion and mode matching of multiple optical signals.

3.Output Fiber: Usually with a single - core structure (multimode fiber or double - clad fiber), it is precisely spliced to the end of the taper region, outputting the fused laser as a single beam. This structural design not only ensures high-power output but also facilitates connection with subsequent laser transmission links (such as transmission fibers and laser heads), reducing the difficulty of system integration.

(II) The Working Principle of the Fiber Combiner

Its working principle is essentially based on the adiabatic mode conversion theory. This theory requires that during the optical field transmission process, the mode evolution rate caused by the waveguide structure change (such as the gradual change of the fiber diameter) is much lower than the oscillation frequency of the optical field itself, ensuring that the optical field energy is only transferred orderly between modes without additional radiation loss.

This implementation process can be vividly analogized to "fiber braiding". First, through a precise process, multiple pre-processed input fibers and a single output fiber are arranged into a bundle in a specific array. Then, the temperature, tension, and time are precisely controlled in a high-temperature melting furnace to make the fibers melt and eutectic and be tapered. Subsequently, the core diameter of the input fibers in the taper region becomes smaller, and the optical mode is transferred to the output fiber. The "adiabatic property" is used to reduce losses and achieve efficient optical integration. Finally, all input beams are stably transmitted in the same mode in the output fiber, with a coupling efficiency of over 95% and an insertion loss of less than 0.5 dB, guaranteeing the performance of high-power laser systems.

The multimode fiber combiner is an indispensable key passive optical device in the field of optical communication. Its core structure consists of an input fiber array, a taper region, and an output fiber. Based on the precise fused taper manufacturing process and combined with the adiabatic mode conversion theory, this device can achieve efficient and low-loss coupling of multiple optical signals. It also has advantages such as low cost, high cost-performance ratio, and wavelength selectability. It not only meets the requirements of scenarios such as optical communication, fiber optic sensors, and optical amplifiers but also serves as an "optical energy integration hub" to support the stable operation of high-power laser systems, strongly promoting the engineering application and industrial development of related technologies.