What is a fiber laser?

Definition: A laser that uses a doped fiber as a gain medium, or a laser whose laser resonator is mostly composed of fiber.

Fiber lasers usually refer to lasers that use fiber as a gain medium, although some lasers that use semiconductor gain media (semiconductor optical amplifiers) and fiber resonators can also be called fiber lasers (or semiconductor optical lasers). In addition, some other types of lasers (for example, fiber-coupled semiconductor diodes) and fiber amplifiers are also called fiber lasers (or fiber laser systems).

In most cases, the gain medium is a rare earth ion-doped fiber, such as erbium (Er3+), ytterbium (Yb3+), thorium (Tm3+), or praseodymium (Pr3+), and one or more fiber-coupled laser diodes are required for pumping. Although the gain medium of fiber lasers is similar to that of solid-state bulk lasers, the waveguide effect and small effective mode area result in lasers with different properties. For example, they usually have high laser gain and high resonator cavity losses. See the entries fiber laser and bulk laser.

Figure 1

Fiber laser resonator

To obtain a laser resonator using an optical fiber, a number of reflectors can be used to form a linear resonator, or to create a fiber ring laser. Different types of reflectors can be used in a linear optical laser resonator:

Figure 2

1. In laboratory setups, ordinary dichroic mirrors can be used at the ends of perpendicularly cleaved fibers, as shown in Figure 1. However, this solution cannot be used in large-scale production and is not durable.

2. The Fresnel reflection at the end of a bare fiber is sufficient to serve as an output coupler for a fiber laser. Figure 2 shows an example.

3. Dielectric coatings can also be deposited directly on the fiber ends, usually by evaporation. Such coatings can achieve high reflectivity over a wide range.

4. In commercial products, fiber Bragg gratings are usually used, which can be prepared directly from doped fibers or by splicing undoped fibers to active fibers. Figure 3 shows a distributed Bragg reflector laser (DBR laser), which contains two fiber gratings. There is also a distributed feedback laser with a grating in the doped fiber and a phase shift in between.

5. If the light emitted from the fiber is collimated by a lens and reflected back by a dichroic mirror, better power handling can be achieved. The light received by the mirror will have a greatly reduced intensity due to the larger beam area. However, slight misalignments can cause significant reflection losses, and additional Fresnel reflections at the fiber end facets can produce filter effects. The latter can be suppressed by using angled cleaved fiber ends, but this introduces wavelength-dependent losses.

6. It is also possible to form an optical loop reflector using a fiber coupler and passive fibers.

Most optical lasers are pumped by one or more fiber-coupled semiconductor lasers. The pump light is coupled directly into the fiber core or at high power into the pump cladding (see double-clad fibers), which will be discussed in detail below.

There are many types of fiber lasers, a few of which are described below.

There are many types of fiber lasers, a few of which are described below.

High-power fiber lasers

Initially, fiber lasers were only able to achieve output powers of a few milliwatts. Today, high-power fiber lasers can achieve output powers of several hundred watts, and sometimes even several kilowatts from single-mode fibers. This is achieved by increasing the aspect ratio and waveguide effects, which avoid thermo-optical effects.

See the entry High-power fiber lasers and amplifiers for more details.

Upconversion fiber lasers

Fiber lasers are particularly suitable for realizing upconversion lasers, which usually operate on relatively infrequent laser transitions and require very high pump intensities. In fiber lasers, high pump intensities can be maintained over long distances, so that the gain efficiency obtained is easily achieved for transitions with very low gain.

In most cases, silica fibers are not suitable for upconversion fiber lasers, because the upconversion mechanism requires a long intermediate state lifetime in the electronic energy level, which is usually very small in silica fibers due to the high phonon energy (see multiphoton transitions). Therefore, some heavy metal fluoride fibers are usually used, such as ZBLAN (a fluorozirconate) with low phonon energy.

The most commonly used upconversion fiber lasers are thorium-doped fibers for blue light, praseodymium-doped lasers (sometimes with ytterbium) for red, orange, green or blue light, and erbium-doped lasers for triode.

Narrow-linewidth fiber lasers

Fiber lasers may operate in only a single longitudinal mode (see single-frequency laser, single-mode operation) with a very narrow linewidth of a few kilohertz or even less than 1 kHz. For long-term stable single-frequency operation, and without additional requirements after considering temperature stability, the laser cavity should be short (e.g., 5 cm), although the longer the cavity, in principle, the lower the phase noise and the narrower the linewidth. The fiber end contains a narrowband fiber Bragg grating (see distributed Bragg reflector laser, DBR fiber laser) to select a cavity mode. The output power typically ranges from a few milliwatts to tens of milliwatts, and single-frequency fiber lasers with output powers up to 1 W are also available.

An extreme form is the distributed feedback laser (DFB laser), where the entire laser cavity is contained within a fiber Bragg grating with a phase shift in between. Here the cavity is relatively short, which sacrifices output power and linewidth, but single-frequency operation is very stable.

Fiber amplifiers can also be used to further amplify to higher powers.

Q-switched fiber lasers

Fiber lasers can generate pulses with lengths ranging from tens to hundreds of nanoseconds, using various active or passive Q switches. Pulse energies of a few millijoules can be achieved with large mode area fibers, and in extreme cases can reach tens of millijoules, limited by the saturation energy (even with large mode area fibers) and the damage threshold (more pronounced for shorter pulses). All fiber devices (except free-space optics) are limited in pulse energy, because they usually cannot implement large mode area fibers and effective Q switching.

Due to the high laser gain, the Q-switching in fiber lasers is very different in nature from that in bulk lasers and is more complex. There are usually multiple spikes in the time domain, and it is also possible to produce Q-switched pulses with a length less than the resonator round-trip time.

Mode-locked fiber lasers use more complex resonators (ultrashort fiber lasers) to produce picosecond or femtosecond pulses. Here, the laser resonator contains an active modulator or some saturated absorbers. Saturated absorbers can be realized by nonlinear polarization rotation effects or by using a nonlinear fiber loop mirror. Nonlinear loop mirrors can be used, for example, in the "figure-of-eight laser" in Figure 8, where the left side contains a main resonator and a nonlinear fiber ring for amplifying, shaping and stabilizing the round-trip ultrashort pulses. Especially in harmonic mode locking, additional devices are required, such as subcavities used as optical filters.