A combiner multiplexes light at a specific wavelength, as shown in Fig. 1, and is often used for high power pumping. The combiner is either a polarization beam combiner or pumping combiner, depending on the core diameter. A polarization beam combiner is used for the strong pumping of a rare earth doped singlemode fiber, while an pumping combiner (pump combiner) is used for the strong pumping of a rare earth doped multimode fiber or double clad fiber. As to information on manufacturing processes of the combiner, see the reference.
Fig 1. Sorts of optical fiber coupler (a) branch coupler, (b) WDM coupler, (c) combiner.
Polarization beam combiner
A polarization beam combiner (PBC) is composed of polarization maintaining fibers as schematically shown in Fig. 2. PBC achieves high power by superposing orthogonally polarized waves. PBC is required to be low loss, slight polarization crosstalk, and environment resistance. A slight temperature dependence of the optical property is also required. For a narrowband (within several nanometers range), a low loss combiner is relatively easy to manufacture, while it is difficult for a wideband (of several tenth nanometers range). Therefore, the usable bandwidth is limited.
Fig 2. Structure of polarized wave combiner
An pumping combiner is used in a high power (from several W to kW) CW fiber laser. The generated heat is integrated in CW laser, consequently a laser stability is disturbed. In order to remove the heat, the heat sink is equipped, and a measure of air-cooling or water-cooling is provided. A typical combiner is either Nx1 combiner or (N+1)x1 combiner as shown in Fig. 3.
In an Nx1 combiner, N of pumping fibers are combined in one. The Nx1 combiner is used for producing high power fiber laser oscillator and for combining a lot of rays without a large amount of signal light.
In an (N+1)x1 combiner, N of pumping fibers are combined in one, and a fiber for signal light is integrated. The (N+1)x1 fiber is used for producing a hIgh power fiber laser oscillator and hIgh power fiber amplifier. Commercially available combiners are mainly 2×1, 3×1, (1+1)x1, (6+1)x1, 19×1, and (18+1)x1.
Fig. 3 N x 1 combiner and (N+1) x 1 combiner (N=2)