A pump combiner is a fiber optic coupler that is designed to send pump light into a fiber amplifier or fiber laser. In a pump combiner, there is a fiber connected directly to a specific active fiber on one side, and sometimes, between these two fibers, there is an additional passive fiber to install an FBG (Fiber Bragg Grating) if FBG array cannot be written into the active fiber. On the other side, a combiner has several input MM optical fibers connected to fiber-coupled pump laser diodes.

A pump combiner with n pump inputs can be denoted as Nx1 pump combiner or N:1 pump combiner. Such a device may be placed in a metallic housing because it may need to be put in thermal contact to avoid excessive heating caused by dissipated power.

For instance, a pump combiner with 6 pump inputs can be denoted as 6×1 pump combiner or 6:1 pump combiner. In Nx1 pump combiner, there is no additional signal input.

In principle, there should be no problem to not use all pump input ports, with the only exception that there will be loss of pump brightness.

The Fabrication Process of a Pump Combiner

In a pump combiner, several input fibers are arranged around another pump fiber (around a signal fiber if it is a pump-signal combiner). The whole bundle of input fibers is tapered down such that its dimensions perfectly fit to those of the active fiber.

During construction process of a tapered Nx1 pump combiner, all fibers are fused together to form a stable rigid device. The pump fibers may be mounted at a slight angle to the axis of the active fiber in a way their bending can be reduced and the pump light travels toward the active fiber core.

The beam divergence of the pump light delivered to the active fiber must remain within the numerical aperture of that fiber so that the pump light remains guided in the cladding. It also should be noted down that tapering of the pump fibers not only diminishes the beam areas but also increases divergence of the guided light. Hence, a pump combiner is designed taking these two factors into account.

Most double-clad fibers come with pump cladding with standard diameter of 125μm. Hence, many pump combiners are made compatible with that dimension. However, there are also active fibers with smaller pump cladding, which helps with obtaining higher pump absorption. As a result, one can use shorter pieces of active fiber.

The diameters of fibers can further be increased to 200μm or 250μm for even higher power levels and reduced needs on the radiance of pump radiation. However, increasing the fibers’ diameters reduces the effective pump absorption and make fibers more sensitive to bending.

Even if the pump radiation is inserted fully into the pump cladding only, it will travel or propagate in a way that its intensity profile needs a certain overlap with the doped fiber core, where pump light can be absorbed.

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