High-power fiber lasers have been increasingly used for material processing, metal cutting, and welding applications due to high wall-plug efficiency, compactness, and superb long term reliability[ 1, 2]. Fused fiber components enable reliable, efficient, high-power fiber laser systems operating in continuous wave (high power) or pulsed regimes such as ultrafast (high peak power).
Common fused fiber components include the fiber mode-field adapter (MFA), fused fiber combiner (FFC), and fiber end cap (FEC) in addition to fusion splicing of dissimilar fibers[3, 4]. These devices allow power scaling of high-power fiber lasers. Key requirements for the components are low transmission loss, good long-term reliability, and high power handling capability. Fabrication of these parts requires both specific optical design and advanced fiber fusion process solutions.
A bi-directionally pumped single stage fiber laser is shown in Figure 1. The laser cavity consists of active gain fiber (e.g. Yb-doped double clad fiber), a high-reflection fiber Bragg grating (HR FBG) with reflectivity of typically >99%, and an output coupler (OC) FBG with reflectivity of typically around 5-10%. Low loss mode-field adapting splicing between gain fiber and HR/OC passive fiber is important for efficient laser operation and good output beam quality. The laser is pumped by combined multi-mode (MM) laser diodes (LD) from both directions to enable high-power laser output. Multiple LDs are combined with a fused fiber combiner to deliver highenergy power for pumping the laser cavity in an all-fiber configuration.
A fiber end cap, which allows expansion of the laser beam in the glass before exiting to the free space, mitigates detrimental end-face damage at high power by reducing the power density of the exit beam. Performance of these components is critical for maintaining efficient, reliable, long-term operation of the laser system.
To enable high output power of the fiber laser, efficient coupling of MM pump beams into the fiber and low loss transmission of the laser beam between different fibers are both critical. This imposes challenges for fabricating the necessary fused fiber components.
The fused-component optical design must be sound to ensure proper coupling of MM beams and laser beams between fibers. Brightness must be conserved for MM beam propagation and the fiber mode-field between dissimilar fibers needs to be properly matched. Fiber taper, geometric or diffusion taper, must be adiabatic.
A fusion process with a high degree of fusion temperature control, a high temperature uniformity and an accurate heating time control are necessary to ensure high-quality fiber fusion without introducing abnormality to the fibers. A good process results in low transmission loss and high splice strength. Macro and micro bending loss and excitation of high order beams must be minimized for single mode fiber laser operation.
This poses stringent requirements for the heat source used in the fusion process, including:
- uniform heating around the fiber due to its circular shape,
- wide temperature range to accommodate different fiber sizes,
- consistent heating to allow a precise fusion process repeatedly, and
- “clean” fusion process to ensure minimum temperature rise around the processed fiber area as the MM pump beam “sees” the surface of the fiber cladding. Minute surface contaminants could absorb laser energy and generate heat on the fiber surface.
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
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
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.
Fused Combiners for Power Scaling
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
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.
- V. P. Gapontsev, “Penetration of fiber lasers into industrial market,” Proc. SPIE 6873, Fiber Lasers V: Technology, Systems, and Applications, edited by J. Broeng, et. al., (2008)
- D. Kliner et. al. “4-kW fiber laser for metal cutting and welding”, Proc. of SPIE 7914, Fiber Lasers VIII: Technology, Systems, and Applications, edited by Jay W. Dawson (2011)
- A. D. Yablon, Optical Fiber Fusion Splicing, Springer, (2005)
- B. S. Wang and E. Mies, “Review of Fabrication Techniques for Fused Fiber Components for Fiber Lasers,” Proc. SPIE 7195, Fiber Lasers VI: Technology, Systems, and Applications, edited by D. V. Gapontsev et. al., (2009)
- B. S. Wang and Anthony Sanchez, “Allfiber passive coherent combining of high power lasers,” Opt. Eng., 50(11), 111606 (2011)