Pump combiner (tapered fiber bundle)

A pump combiner joins the outputs of several fiber-coupled diode lasers into the inner cladding of a double-clad fiber, so that a fiber laser or amplifier can be pumped with more power than any single diode provides. The common all-fiber form is the tapered fiber bundle: $N$ multimode pump fibers are packed together, fused, drawn down into a taper and cleaved, and the taper waist is spliced to the double-clad fiber. In the $(N+1)\times 1$ version a signal fiber runs through the center of the bundle, so the core signal passes through while the pump enters the cladding; $(6+1)\times 1$ and $(18+1)\times 1$ layouts follow from hexagonal close packing. Pump ports are typically 105–200 µm core fibers with NA 0.15–0.22, and well-made combiners transmit more than 90% of the pump.

Brightness limit

A passive combiner cannot increase radiance, so the total étendue of the inputs must fit within the étendue the output fiber accepts. Since étendue scales as $(d\cdot\text{NA})^2$, the condition is

$$N\,(d_\text{in}\,\text{NA}_\text{in})^2 \le (d_\text{out}\,\text{NA}_\text{out})^2,$$

where $d$ is the guiding diameter (pump-fiber core, output inner cladding). For 105 µm, NA 0.22 pump fibers and a 400 µm, NA 0.46 inner cladding, the right side allows at most 63 inputs; in practice packing gaps and the cladding of each pump fiber reduce the usable number well below this.

Taper ratio and NA growth

Tapering conserves étendue within each fiber: as the diameter is reduced by the taper ratio $R = D_\text{out}/D_\text{in}$, the ray angles grow, and the NA of the light at the waist becomes

$$\text{NA}_\text{waist} = \frac{\text{NA}_\text{in}}{R}.$$

The bundle must be tapered enough to fit inside the output cladding, but not so much that $\text{NA}_\text{waist}$ exceeds the output fiber's numerical aperture. As a worked example, seven fibers of 200 µm core and 220 µm cladding at NA 0.22 form a hexagonal bundle 660 µm across. Tapering it to 400 µm gives $R = 0.606$ and a waist NA of 0.363, below the 0.46 of a low-index-polymer double-clad fiber. The inputs fill 7 × (200 × 0.22)² / (400 × 0.46)², or about 40%, of the output étendue, leaving margin for the higher-angle light that real diodes and bends produce.

Signal feed-through

In an $(N+1)\times 1$ combiner the central signal fiber is tapered along with the bundle, which shrinks its core and enlarges the mode, so the signal fiber's taper ratio is limited, or a fiber with a larger core is chosen so that after tapering its mode matches the output core. Signal loss and mode quality through the combiner are specified separately from pump transmission and matter most for large-mode-area amplifiers, where higher-order modes can be excited at the splice. Side-pumping combiners, in which pump fibers are fused alongside an untapered signal fiber, avoid this at the cost of lower pump coupling per port.

Measurement

Pump transmission is measured port by port with a power meter at the output, after stripping any light that remains in the outer polymer. A thermal camera on the splice and taper region reveals light that escapes because it exceeded the acceptance NA. Backward pump transmission, from the output to each pump port, indicates how much returning light or signal could reach and damage the diodes.

Pitfalls

Light launched above the design NA, from over-bent pump fibers or diodes with wide far fields, leaks out at the taper or into the recoat of the splice and heats it; this is the usual cause of combiner failure at high power. Contamination or a poor cleave at the waist splice also creates a local hot spot. Signal light leaking from the core into the cladding in the backward direction travels up the pump fibers to the diodes, so a cladding light stripper and diode protection filters are often added.

Common questions

How many pump diodes can one combiner take?

The étendue condition sets the upper bound: the sum of $(d\cdot\text{NA})^2$ over the inputs cannot exceed that of the output cladding. Packing efficiency, taper NA growth and the need for margin keep practical port counts below this bound, commonly 6, 18 or similar hexagonal numbers.

Why does the NA increase in a taper?

A guided ray bouncing in a tapering core meets the wall at a steeper angle after each reflection. To a good approximation the product of diameter and NA is conserved along an adiabatic taper, so reducing the diameter raises the NA in proportion.

References: D. J. Richardson, J. Nilsson and W. A. Clarkson, "High power fiber lasers: current status and future perspectives," J. Opt. Soc. Am. B 27, B63 (2010); B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019).