A fused fiber combiner is essential for combining multiple laser beams for power scaling. One method to fabricate such a device is to taper a fused fiber bundle, cleave the tapered fiber bundle end and splice it to an output fiber. Therefore, laser beams from multiple fibers are coupled into one fiber.
When combining MM beams, such a device requires brightness conservation, low transmission, and good high power handling capability. Figure 6.1 shows typical performance of a 7:1 MM combiner fabricated using a Vytran GPX-3400 Glass Processing System. This combiner combines 7 MM beams with numerical aperture (NA) of <0.15 from 105μm/125μm fibers and outputs to a 125-μm double clad fiber. This device has average transmission loss of <1%. Total combined output power is 800W. The measured temperature rise at 800W is 22 degrees at the output coating interface.
Examples of a 19:1 MM combiner and (6+1):1 combiner are also shown in Figure 6.2. Large count combiners up to over 61 input ports are possible using large filament with a GPX-3400 system. In addition, filament fusion makes it possible to control the degree of fusion via different fusion temperatures to produce lightly fused or fully fused devices, which result in different beam coupling characteristics.
Further power scaling requires that multiple single mode fiber lasers be combined coherently or incoherently. Incoherent beam combining scales up the output power but results in some beam quality loss. Laser brightness is generally degraded in this case.
Coherent beam combining (CBC) not only scales up the output power but also increases the brightness of the laser. To achieve this, individual lasers must meet an in-phase condition before combining. Figure 7 shows an example of CBC of two fiber lasers in all-fiber and all-passive configuration using a 2x2 fused fiber coupler. In this case, the combined output power from one exit power is 101.5W.
High-performance fused fiber components are essential for high-power fiber lasers. Filament fusion technology has been broadly deployed for several years due to its precise temperature control, uniformity and the high reliability of the resulting splices and components. Some examples of devices that can be produced with this technology include splices, mode-field adapters, end caps and combiners. These components can be produced in high volumes with reliability and consistency, enabling mass production of demanding high-power CW fiber lasers as well as high-peak-power ultrafast lasers.
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