A multimode pump combiner is a device used in fiber laser systems. It combines the light from multiple pump laser diodes into a single output fiber. This concentrated light then pumps (energizes) the rare-earth doped fiber to create laser light.

Pumping the Fiber Laser

In a fiber laser, a special fiber is doped with rare-earth ions like ytterbium, erbium or thulium. When these ions absorb pump light of the right wavelength, they become excited to higher energy levels.

The excited ions can then release this stored energy as laser light through a process called stimulated emission. The fiber acts as an optical resonator to amplify and direct this laser light into a coherent beam.

Role of the Multimode Pump Combiner

To achieve enough pump power to efficiently excite the fiber, high-power laser diodes are used as the pump sources. However, the light from a single diode is often not enough.

This is where the multimode pump combiner comes in. It takes the output light from multiple laser diode pumps and combines them into one fiber that leads to the rare-earth doped fiber laser.

By concentrating the pump light into the fiber core, the combiner ensures the fiber laser gets sufficiently pumped to operate optimally.

Working Principle

The pump combiner has multiple input fibers, each coupled to a separate pump diode laser source. These input fibers are bundled together in a fused taper region.

In this tapered region, the fibers are heated, stretched and fused together into a single, larger core output fiber. This output fiber is then spliced to the doped fiber laser.

As the pump light travels through the tapered region, it gradually merges into the larger output core. This merged pump light then enters the doped fiber laser with high brightness and intensity.

Key Advantages

Using a multimode pump combiner offers several advantages over directly coupling pump diodes:

Higher Pump Power

It multiplies the available pump power by combining multiple sources into one output.

Brightness Preservation

The taper geometry preserves the brightness (intensity per unit area) from the input pump sources.

Pump Redundancy

If one pump diode fails, the remaining pumps can still maintain partial lasing.

Flexibility

The number of input ports can be customized based on required pump power levels.

Easier Cooling

Instead of cooling many separate pump diodes, only the combiner output needs elaborate cooling.

Applications

Multimode pump combiners enable high-power industrial fiber lasers for cutting, welding, drilling and additive manufacturing. They are also used in fiber lasers for telecommunications, spectroscopy and medicine.

In essence, the multimode pump combiner is a critical component that delivers the pump power needed to drive high-brightness, efficient rare-earth doped fiber lasers across many application areas.