Two main production methods have been developed over the years — filament fusion and arc fusion.
Graphite filament fusion is based on a resistively heated refractory element — typically an inverted Omega shape for uniform heating around the fiber (Figure 2).
Arc-discharge fusion is an alternative method commonly used for telecom applications. This method generally uses two electrodes to heat the fiber and has limited temperature uniformity around the fiber, especially for large-diameter fibers. Recently, a three electrode configuration has been used to alleviate this issue. However, care must be taken to maintain the same operating condition of the three electrodes to ensure temperature uniformity. In addition, electrodes, typically made of Tungsten, tend to contaminate the fiber during the fusion process. This effect must be mitigated, especially for high-power fiber laser applications.
Filament fusion offers a robust solution for fiber laser applications. This technique has excellent temperature uniformity, good fusion control and consistency. It can be more readily scaled to larger diameter fibers and fiber bundles with different loop diameters. Additionally, filament width can be tailored for different fiber splicing and tapering applications. More importantly, graphite does not oxidize so the filament fusion process is clean. Using this process, high-power components can be easily, reliably, and consistently fabricated.
Splicing Between Fibers
Fusion splicing of dissimilar fibers is important for coupling a fiber laser beam from one fiber to the other. Transmission losses for both laser beam (typically single mode) in the fiber core and multi-mode pump beam in the fiber cladding must be minimized for high power handling capability. Sufficient splice strength must be maintained to ensure long-term reliability of the laser system. Examples of splices between two different 20μm/400μm passive fibers and between active and passive 20μm/400μm fibers are shown in Figures 3.1 and 3.2, respectively. Using filament fusion, splice strength of >200 kpsi can be readily achieved. Splice loss depends on the fiber type; typical splice loss is <0.03 dB for single mode fiber splicing.
Mode Field Adapters
When splicing two different fibers together, the mode-fields of the fibers are generally not the same. In this case, mode-field adapting is necessary to reduce the transmission loss between the fibers.
The thermally expanded core (TEC) method, utilizing sustained heat to diffuse the fiber core and expand the mode-field of the fiber, is a common technique. Figure 4 shows a mode-field adapting example between two fibers with mode-field diameters of 6.5 μm and 12.4 μm, respectively. In this case, the two fibers were spliced together then fire-polished, a method where the splice head is moved back and forth across the splice joint to reduce losses and to enhance the splice strength. Figure 4 shows that by using the fire polish process multiple times (a dot is shown for each pass) the splice loss between these fibers is re duced to <0.03 dB.
A fiber end cap expands the laser beam inside the fiber. It is an important device to reduce power density at the exit surface between glass and air interface, reducing the risk of exit surface damage. To fabricate this device, a MM end-cap fiber with its core significantly larger than the core of the laser fiber is spliced to the laser fiber. Cladding size of the MM fiber can be the same or larger than the laser fiber depending on the output power. The end-cap fiber is then cleaved, typically to the length of around 1mm. To further suppress the reflection from the end-cap exit surface, the end-cap fiber can be either angle cleaved or coated with anti-reflection coating. Figure 5 shows an end cap with a 400μm laser fiber spliced to a 1mm end-cap fiber.