The basic principle of single-mode fiber lasers

The basic principle of single-mode fiber lasers

The generation of laser requires meeting three basic conditions: population inversion, an appropriate resonant cavity, and reaching the laser threshold (the gain of light in the resonant cavity must be greater than the loss). The working mechanism of single-mode fiber lasers is precisely based on these fundamental physical principles and achieves performance optimization through the special structure of fiber waveguides.

Stimulated radiation and population inversion are the physical basis for the generation of lasers. When the light energy emitted by the pump source (usually a semiconductor laser diode) is injected into the gain fiber doped with rare earth ions (such as Ytterbium Yb³⁺, erbium Er³⁺), the rare earth ions absorb energy and transition from the ground state to the excited state. When the number of ions in the excited state exceeds that in the ground state, a population inversion state is formed. At this point, the incident photon will trigger the stimulated radiation of the excited-state ion, generating new photons of the same frequency, phase and direction as the incident photon, thereby achieving optical amplification.

The core feature of single-mode fiber lasers lies in their extremely fine core diameter (typically 8-14μm). According to wave optics theory, such a fine core can only allow one electromagnetic field mode (i.e., fundamental mode LP₀₁ or HE₁₁ mode) to be stably transmitted, that is, the single mode. This eliminates the intermodal dispersion problem existing in multimode fibers, that is, the pulse broadening phenomenon caused by the propagation of different modes at different speeds. From the perspective of transmission characteristics, the path difference of light propagating along the axial direction in single-mode optical fibers is extremely small, which makes the output beam have perfect spatial coherence and Gaussian energy distribution, and the beam quality factor M² can approach 1 (M²=1 for an ideal Gaussian beam).

Fiber lasers are outstanding representatives of the third-generation laser technology, which use rare earth element-doped glass fibers as the gain medium. Over the past decade, single-mode fiber lasers have occupied an increasingly important share in the global laser market, thanks to their unique performance advantages. Compared with multimode fiber lasers or traditional solid-state laser, single-mode fiber lasers can generate an ideal Gaussian beam with a beam quality close to 1, which means that the beam can almost reach the theoretical minimum divergence Angle and minimum focused spot. This feature makes it irreplaceable in the fields of processing and measurement that require high precision and low thermal impact.