Gain fiber structure in a fiber laser
A fiber laser is a type of solid-state laser that uses an optical fiber as a gain medium. The core in the center of the fiber is doped with a rare-earth element, Yb (ytterbium), and has the highest refractive index. A 1.1 μm Laser light and Pump light propagate in this Yb-doped center core. The Pump light also propagates in the inner cladding surrounding the core. The inner cladding is surrounded by the outer cladding; this is called a double-clad fiber because of the two cladding layers.
Laser beam propagation in a double-clad fiber
Figure 2 shows the structure of a double-clad fiber and light intensity profile of beam propagation. The Pump light propagates through the Yb-doped center core and the inner cladding but is confined in the cores by total internal reflection (*Note) at the boundary of outer cladding and outer core. The Laser light propagates in the Yb-doped center core due to total internal reflection at the boundary of inner cladding and center core. As the pump light propagates through the Yb-doped center core, the Yb ions are excited.
Optical circuit configuration in a high-power fiber laser
Figure 3 shows the basic configuration of the optical circuit of a high-power fiber laser.
The optical circuit consists of three major sections: (1) Pump section, (2) Oscillator section, and (3) Beam delivery section.
In the Pump section (1), laser light from pumping laser diodes (LDs) passes through optical fibers into a pump combiner. The pump combiner couples the pump light from the multiple LDs into a single-mode optical fiber.
In the Oscillator section (2), the pump light from the pump combiner propagates through a double-clad fiber (active fiber), as depicted in Fig. 2. The pump light excites the Yb ions and is amplified by the FBGs (Fiber Bragg Gratings). The FBGs act as mirrors with high and low reflectivity. The laser light is emitted from the low-reflectivity FBG.
The Beam delivery section (3) is composed of an optical fiber which passes the laser light from the Oscillator section (2) to a processing head or a beam coupler.
Power enhancement of fiber lasers
Figure 4 shows how power is enhanced by coupling the multiple delivery fibers shown in Fig. 3 with an output beam coupler (combiner).
For example, coupling four 1 kW output beams yields 4 kW, and coupling six 1 kW output beams yields 6 kW.