Understanding Fiber Combiners

Optical fiber combiner is a kind of optical fiber connection device, which can couple the optical energy output from the transmitting optical fiber to the receiving optical fiber to the maximum through the precision fusion splicing technology of optical fibers, and minimize the impact on the system due to its intervention in the optical path.

The fiber combiner is an important component in the fiber laser system. Its quality not only directly determines the power of the fiber laser and the quality of the beam, but also an important guarantee for the safe and stable operation of the laser.

Classification of Fiber Combiners

According to the functional classification, fiber optic combiners can be divided into two categories: power combiners and pump combiners.

(1) The pump beam combiner is mainly to combine multiple pump lights into one optical fiber for output, which is mainly used to improve the pump power

Rate. (2) The power beam combiner is to combine multiple single-mode lasers into one optical fiber for output, which is used to increase the output power of the laser.

pump combiner

power combiner

According to the classification of the composition, the fiber combiner can be divided into two categories: N×1 fiber combiner without signal fiber and (N+1)×1 fiber combiner including signal fiber. Unlike the N×1 fiber combiner, the central fiber of the (N+1)×1 fiber combiner is the signal fiber. During the manufacturing process, N optical fibers must be closely and symmetrically arranged around the signal optical fiber, and the middle signal optical fiber is used for the input of signal light.

The N×1 beam combiner has both a power beam combiner and a pump beam combiner, and its function depends on the type of the N-channel input fibers. If the N-channel fibers are all single-mode fibers or large-mode-field fibers, then It can be directly connected to N lasers to increase the output power of the laser, which is a power beam combiner; if the N fibers are all multimode fibers, it can be connected to N pump sources to increase the pump power of the laser , which is the pump combiner.

▲ N×1 fiber combiner

(N+1)×1 beam combiners are all pump combiners, mainly used in optical fiber amplification systems. The single-mode optical fiber in the middle of the beam combiner is a signal optical fiber, which is used for the transmission of signal light, and the N multimode optical fibers surrounding it are pump optical fibers, which are used for the transmission of pump light. Such combiners are commonly used in MOPA configurations.

▲ (N+1)×1 Fiber Combiner

Side-pumped beam combiners and end-pumped beam combiners

The center of the side-pumped beam combiner is a signal fiber, the core is a single-mode or quasi-single-mode waveguide for transmitting laser light, and the peripheral six optical fibers are pumping fibers for transmitting pump light. The seven optical fibers are neatly arranged, melted and thinned, and fused with the output double-clad optical fiber.

▲ End-pumped beam combiner Fiber combiner

The difference between the side-pump combiner and the end-pump combiner is that the pump fiber of the side-pump combiner is thinned and then attached to the cladding of the signal fiber, but the signal fiber is not melted and thinned. Therefore, the signal transmission of the side-pump combiner is better than that of the end-pump combiner in principle.

▲ Side-pumped fiber combiner

The making of the beam combiner

The basic structure of a power combiner mainly includes three parts: input fiber, fused tapered fiber bundle and output fiber.

▲ Basic structure of power beam combiner

First of all, in order to make the optical fiber bundle fused and tapered to be well fused with the output optical fiber, the cross section of the optical fiber bundle must be circular, and the pump optical fiber must be closely arranged in a certain geometric way, usually the optical fiber is in the shape of a regular hexagon closely arranged. In the production process, the input optical fiber bundles are firstly bundled, and then the bundled input optical fiber bundles are melted and tapered to form a fused tapered fiber bundle, and then the tapered fiber bundle is cut by the tapered waist part and fused together with the output fiber . Finally, a suitable packaging and heat dissipation structure is designed to ensure that the beam combiner can work stably for a long time. Metal copper or aluminum with high thermal conductivity is usually used as the housing for packaging and heat dissipation, and if necessary, a water-cooling structure is designed on the metal packaging.

Fiber lasers use fusion splicing to realize the connection of fiber optic devices. In order for the laser to achieve higher power indicators, high-quality fiber fusion is very important. In the process of splicing optical fibers, losses are inevitable, and these losses will continuously accumulate light and heat during the operation of the laser, which may lead to degradation of beam quality or damage to optical components.