A Continuous Core: Side Pump Signal Combiner for kW Fiber Lasers

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High power fiber lasers and amplifiers commonly use pump combiners to couple light from multiple pump sources into the gain fiber. Unlike conventional tapered fiber bundle designs, side pump signal combiners introduce pump light through the fiber cladding without tapering or splicing the signal fiber within the tapering region. This article explores how the technology works, its advantages in polarization maintaining (PM) systems, and its application in kilowatt class designs.

A render model representing a (6+1)x1 side pump combiner. Blue central cylinder with pink tapered cylinders fanning around it on a white background, forming a sleek abstract 3D pattern — (6+1)×1 Side Pump Combiner Configuration

Advantages of a Side Pump Signal Combiner

In a conventional (N+1)×1 end pump combiner, multiple pump fibers surround a central signal fiber. The entire fiber bundle is tapered and spliced to an output fiber. In contrast, a side pump combiner uses individually tapered pump fibers fused to the side of the signal fiber’s cladding, leaving the signal fiber continuous without tapering, cleaving, or splicing within the combiner. This fundamental design difference provides four key advantages:

Signal Transmission
Taper transitions and splice interfaces in an end pump combiner add insertion loss and can degrade beam quality (M²), especially at high power. By preserving a continuous signal core, side pump combiners minimize disruptions to signal propagation. This helps reduce insertion loss and preserve beam quality, with minimal changes to M² in properly designed devices.
Pump Integration and Diode Protection
When reverse isolation is insufficient, backward propagating light may enter the pump fibers and potentially damage the pump diodes, among the most expensive components in the system. The side coupling geometry helps reduce this risk by limiting the amount of backward light entering the pump ports. However, the level of protection depends on the combiner’s optical design and should be verified through reverse isolation measurements.
Scalable Power Architecture
Side pump combiners support flexible pumping architectures, including forward, backward and bidirectional configurations. Multiple side coupling points can also be arranged along a continuous fiber to distribute pump injection. With appropriate system design, this flexibility can help manage the distribution of heat and optical power along the gain fiber.
Manufacturing and Integration
End pump combiners depend on precise fusion, tapering and signal-path splicing, where alignment errors cause loss and rework. By eliminating tapering and splicing within the signal path, side pump combiners reduce the processing required on the signal fiber. However, precise pump fiber tapering, fusion alignment and thermal control remain essential to achieving consistent optical performance.

Side Pump Combiners in PM Fiber Lasers

Polarization maintaining (PM) fibers rely on birefringence to preserve a defined polarization state. Splicing PM fibers requires precise alignment of their polarization axes, while mechanical stress and thermal effects can affect the polarization extinction ratio (PER). By eliminating internal signal fiber splices, side pump combiners reduce one potential source of polarization disturbance.

A (4+1)×1 side pump combiner uses four pump ports to couple pump light into a continuous signal fiber. Versions designed for PM 20/400 fiber are available, making this configuration suitable for applications that require stable signal polarization. Actual polarization performance depends on the fusion geometry, mechanical stress and PER characteristics of the finished device.

The (6+1)×1 Side Pump Combiner for kW Lasers

For non-PM industrial fiber lasers, scaling pump power is a key priority. Tandem pumping offers an effective approach to achieving multi-kilowatt output. Neptec’s (6+1)×1 multimode side pump signal combiner is designed for this application, with six pump fibers fused symmetrically around the signal fiber at 60° intervals. By using 1018 nm fiber laser sources to pump a 1064 nm gain stage, tandem pumping reduces the quantum defect compared with conventional 976 nm diode pumping. This minimizes heat generation in the final gain stage and enables further power scaling. A 2024 SPIE study demonstrated the potential of this architecture, reporting 97.98% pump coupling efficiency at 2.23 kW of pump power and approximately 0.11 dB signal insertion loss using a (6+1)×1 tapered fused side pump and signal combiner.

Neptec’s (6+1)×1 multi-mode side pump signal combiner:

Signal fiber: 30/205/230 µm, NA 0.055/0.20/0.48, 1 m lead
Pump fibers: Six 135/155 µm multimode fibers, NA 0.22, 1 m each
Wavelengths: 1018 nm pump, 1064 nm signal
Pump coupling efficiency: ≥95%, up to 98%
Signal insertion loss: 0.3 dB typ.
Rated pump input power: up to 600 W per port, 3600 W total
Rated signal power handling: up to 1000 W
M²: 1.2 typ. (1 mW light source test, Input M²<1.1)

Diagram of a package L×W×H: 150 × 15 × 10 mm with signal fiber in/out and six red pump input lines.

Conclusion

Side pump signal combiners preserve a continuous signal core while coupling pump light through the side of the fiber cladding. This architecture helps minimize signal loss, maintain beam quality and reduce backward light coupling into pump ports. Neptec's (6+1)×1 configuration extends these advantages to high power tandem pumping applications, with a rated total pump input capacity of up to 3.6 kW.