Fiber lasers are composed of three basic elements: pump source, gain medium, and resonant cavity. The pump source is generally a semiconductor laser diode (LD), the gain medium is a rare-earth doped fiber (usually ytterbium-doped fiber, YDF), and the resonant cavity is generally composed of a grating and a gain fiber. 

The pump light generated by multiple laser diodes enters the resonant cavity through the forward and backward beam combiners, and the gain fiber forms a particle number reversal and generates radiation after absorbing the pump light, and the resulting radiation is amplified by the excitation and forms a stable laser output through the backward beam combiners.

Basic structure of resonant cavity : Fiber laser resonant cavity mainly has three parts: high-reflection grating, ytterbium-doped fiber, and low-reflection fiber grating.

The basic structure of ytterbium-doped optical fiber is shown in the figure, from the inside to the outside of the core, the inner cladding, the outer cladding and the coating layer. The core is doped with the rare earth element "ytterbium". Pump light is injected into the inner cladding layer repeatedly through the core, absorbed by the ytterbium ion, which is converted into a 1070 nm fiber laser.

The grating in the resonant cavity structure not only plays the role of positive feedback but also the role of mode selection. Photons are reflected back and forth between the two gratings and repeatedly pass through the gain medium, each time they pass through, new photons are excited, and eventually, the beam energy becomes larger and larger. Only light of a specific wavelength and direction can be output from the low-reflection grating.