Optical fibers are glass-based cylindrical structures drawn from a SiO₂ matrix material. Their light-guiding principle relies on total internal reflection: when light strikes the boundary between a denser medium with a higher refractive index and a less dense medium with a lower refractive index at an angle greater than the critical angle, total internal reflection occurs. In this case, all the incident light is reflected back into the denser medium, while no light passes into the less dense, outer medium. Typically, a bare optical fiber consists of three main components: a central core made of high-refractive-index glass, an intermediate cladding layer of low-refractive-index silica glass, and an outermost protective coating made of reinforced resin. Optical fibers can be classified into two primary types based on the mode of light propagation: single-mode fibers and multi-mode fibers. Single-mode fibers have a smaller core diameter, allowing only one mode of light to propagate through them, which results in minimal intermodal dispersion. In contrast, multi-mode fibers feature a larger core diameter, enabling multiple modes of light to travel simultaneously—but this also leads to greater intermodal dispersion. Additionally, optical fibers can be categorized according to their refractive index profile into two types: step-index (SI) fibers, where the refractive index drops abruptly at the core-cladding interface, and graded-index (GI) fibers, where the refractive index gradually decreases from the center of the core toward the cladding.
Take the rare-earth-doped fiber laser as an example: the fiber core doped with rare-earth ions serves as the gain medium, and the doped fiber is fixed between two mirrors to form a resonant cavity. Pump light enters the fiber through M1, while the laser output is extracted from M2.
 

When pump light travels through the optical fiber, rare-earth ions in the fiber absorb the pump light, exciting their electrons to higher excited energy levels and thus achieving population inversion. The inverted particles then emit radiation as they transition back from the excited state to the ground state, producing the output laser beam. Figure 1’s mirror resonator is primarily used to illustrate the principle of the fiber laser. In practice, fiber lasers can employ a variety of all-fiber resonator configurations.
 

Figure 2 shows a fiber-loop reflector constructed using a 2×2 fiber coupler, as well as an all-fiber laser built around this type of reflector. (a) illustrates the configuration where the two output ports of the fiber coupler are connected to form a loop, while (b) depicts an equivalent optical system assembled from discrete optical components. (c) presents a setup in which two fiber-loop reflectors are cascaded with a segment of rare-earth-doped optical fiber, creating an all-fiber laser. As an example, an Nd³⁺-doped quartz fiber laser is used, with an AlGaAs (aluminum-gallium arsenide) semiconductor laser operating at 806 nm serving as the pump source. The laser emission wavelength of the fiber laser is 1064 nm, and the pump threshold is approximately 470 μW.
 

A fiber ring laser can be constructed using a 2×2 fiber coupler. As shown in Figure 3(a), one end (port 2) of the fiber coupler is connected to a segment of rare-earth-doped fiber, and the doped fiber is then looped back to connect to the coupler’s output port (port 4), forming a closed loop. Pump light is injected through coupler port 1, entering the fiber loop to excite the rare-earth ions embedded within it. The resulting laser light circulates within the fiber loop and is finally extracted via coupler port 3. This configuration represents a traveling-wave-type laser. Notably, the lower the coupling ratio of the fiber coupler, the greater the energy stored inside the fiber loop—and consequently, the lower the laser’s threshold power. A typical Nd³⁺-doped fiber ring laser, with a coupling ratio ≤10%, is pumped using the 595 nm output from a dye laser, generating a 1078 nm laser beam with a threshold as low as just a few milliwatts. The equivalent discrete optical components and their corresponding optical path arrangement for this fiber ring laser cavity are illustrated in Figure 3(b).
By leveraging the broad fluorescence spectral bandwidth of rare-earth ions in optical fibers, wavelength-selective optical elements such as gratings can be incorporated into the aforementioned laser cavities to create tunable fiber lasers. For instance, a typical Er³⁺-doped fiber laser can be tuned over 14 nm around 1536 nm and 11 nm around 1550 nm. Moreover, with a specially designed fiber laser cavity, single-longitudinal-mode operation can be achieved, enabling laser linewidths as narrow as tens of megahertz—potentially even down to the 10 kHz range. When an acousto-optic modulator is integrated into the fiber laser cavity, the device can switch between Q-switched or mode-locked operation. In a Q-switched Er³⁺-doped quartz fiber laser, the pulse width was measured at 32 ns, with a repetition rate of 800 Hz, delivering a peak power as high as 120 W. Meanwhile, mode-locking experiments yielded ultrashort pulses with a duration of just 2.8 picoseconds and a repetition rate of 810 MHz, making this laser system highly promising for applications as a soliton-based laser source.
Rare-earth-doped quartz fiber lasers leverage the mature technology of conventional quartz fiber fabrication, ensuring low loss and precise parameter control. By carefully selecting the doping elements, the fiber can operate in single-mode at both the pump and lasing wavelengths, enabling high pumping efficiency. Additionally, the fiber boasts an exceptionally high surface-to-volume ratio, which facilitates excellent heat dissipation. As a result, these fiber lasers typically require only low-power pumping to achieve continuous-wave operation. Moreover, fiber lasers are highly compatible with standard optical fibers used in various fiber-based systems, allowing for efficient splicing. Their flexible, compact design further enhances their versatility, making them invaluable not only in fiber-optic communications and sensing applications but also in fields such as medical treatments, precision measurement, and even instrumentation manufacturing.
Laser systems using undoped optical fibers as the gain medium can be developed based on fiber amplifiers. Under the action of pump light, these fibers easily achieve high power density, leading to "population inversion" of the laser energy levels in the laser-active material. By carefully incorporating a positive feedback loop (which forms a resonant cavity), laser oscillations and output can be sustained.