What is a fiber laser?

Fiber lasers are defined as lasers that use doped fiber as a gain medium, or lasers whose laser resonator is mostly composed of fiber.

Fiber lasers usually refer to lasers that use fiber as a gain medium. Of course, 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 resonator cavity loss. See the entries fiber laser and bulk laser.

Fiber laser resonator

To obtain a laser resonator using an optical fiber, a linear resonator can be formed using some reflectors or a fiber ring laser can be made. Different types of reflectors can be used in a linear optical laser resonator:

1. In laboratory setups, ordinary dichroic mirrors can be used at the end of a perpendicularly cleaved fiber, as shown in Figure 1. However, this solution cannot be used for large-scale production and is not durable.
2. The Fresnel reflection at the end of a bare fiber is sufficient to act as an output coupler for a fiber laser. Figure 2 shows an example.
3. It is also possible to deposit dielectric coatings directly on the fiber end, usually by evaporation. Such coatings give high reflectivity over a wide range.

2. 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, which has a grating in the doped fiber with a phase shift in the middle.
3. Better power handling can be achieved if the light emerging from the fiber is collimated using a lens and reflected back through a dichroic mirror (see Figure 4). The light received by the mirror has a much smaller intensity due to the larger beam area. However, slight misalignment can cause significant reflection losses, and additional Fresnel reflections from the fiber end face can produce a filter effect. The latter can be suppressed by using an angled cut fiber end, but this introduces wavelength-dependent losses.

6. It is also possible to form an optical loop mirror (Figure 5) using a fiber coupler and passive fiber.

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

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

High-power fiber lasers

Initially, fiber lasers could only achieve a few milliwatts of output power. Today, high-power fiber lasers can achieve hundreds of watts of output power, and sometimes even several kilowatts of power can be obtained from single-mode fibers. At this time, due to the improved aspect ratio and waveguide effect, the thermo-optic effect is avoided.

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

Upconversion fiber lasers

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

In most cases, silica fibers are not suitable for upconversion fiber lasers, because the upconversion mechanism requires a long intermediate 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 (Figure 6), praseodymium-doped lasers (sometimes also doped with ytterbium) for red, orange, green or blue light, and erbium-doped lasers for travel light.

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. To achieve long-term stable single-frequency operation, and without additional requirements for temperature stability, the laser cavity resonator needs to be short (e.g., 5 cm), although longer cavities should in principle result in lower phase noise and narrower linewidth. The fiber ends contain narrowband fiber Bragg gratings (see distributed Bragg reflector laser, DBR fiber laser) that select a cavity mode. Output powers typically range 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), in which the entire laser cavity is contained in the fiber Bragg grating with a phase shift in between. Here the cavity resonator is relatively short, and although this sacrifices output power and linewidth, 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 (see Figure 7). 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 (excluding 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, Q switching in fiber lasers is very different in nature from that in bulk lasers and is more complicated. There are usually multiple spikes in the time domain, and it is also possible to generate Q-switched pulses with lengths less than the resonator round trip time.

Mode-locked fiber lasers

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

For more details on ultrashort fiber lasers, follow us, see Mode-locked fiber lasers.

Raman fiber lasers

Fiber Raman lasers are a special type of fiber laser that exploits the Raman gain in the fiber. The lasers are usually produced with long, sometimes highly nonlinear fibers and pump powers of about 1 W. Using nested optical Bragg grating pairs, a multi-step Raman conversion is performed to obtain an output light that differs from the pump light by several hundred nanometers. Pumping a Raman fiber laser with 1000 nm light gives an output at 1400 nm, which then pumps an erbium-doped fiber amplifier at 1500 nm.

Fiber lasers with semiconductor optical amplifiers

Some lasers contain a semiconductor optical amplifier (SOA) as the gain medium in a resonant cavity consisting of an optical fiber. Although the lasing process does not take place in the fiber, such lasers are sometimes called fiber lasers. The optical power radiated by such lasers is relatively small, on the order of a few milliwatts or less. Compared to rare-earth-doped fibers, such semiconductor gain media have very different properties, in particular, smaller saturation energies and upper state lifetimes. In addition to generating coherent light, such lasers can be used for information processing in fiber-optic communication systems, for example, for wavelength conversion between data channels based on the cross-saturation effect.

Notes on Fiber as Laser Gain Medium

1.Because fibers can be twisted together, the light propagating in the fiber can be well shielded from the environment (e.g., dust), and fiber lasers are small and rugged, provided that the entire laser resonator consists only of fiber components, such as fiber Bragg gratings and fiber couplers (i.e., avoiding free-space optics and alignment requirements).

2.Due to the strong broadening of laser transitions in glass and the large gain bandwidth of the fiber gain medium, a wide range of wavelength tuning and ultrashort pulse generation can be achieved. In addition, the wide spectral region of the fiber laser can be well pumped, so the requirements for the pump wavelength are not high, so there is no need to temperature stabilize the pump diode.

3.Diffraction-limited beam quality can be easily obtained using single-mode fibers, and some multimode fibers with few modes can also be used.

4.Due to the high gain efficiency of doped fibers, fiber lasers can operate at very low pump powers. And high power efficiency can be achieved.

5.In recent years, some possible solutions for obtaining very high output powers have been proposed (up to several kilowatts using double-clad fibers).

6.Also due to the waveguide properties, high pump intensities can be applied to very long fiber lengths, and fiber lasers can be operated at laser transitions that are not easily accessible (e.g., upconversion lasers).

However, fiber lasers also have the following problems:

1.When the pump light enters the single-mode fiber core from free space, strict alignment is required. This problem can be eliminated by using fiber-coupled pump diodes.

2.Most fibers have a very complex polarization evolution with temperature, unless polarization-maintaining fibers or Faraday rotators are used. However, the magnitude of this effect is not comparable to that of nonlinear polarization rotation mode locking.

3.Nonlinear effects often limit the performance of the laser, for example, limiting the output power that can be achieved in single-frequency operation or the pulse quality of a mode-locked laser. For example, Kelly sidebands are very common, but they are almost absent in mode-locked bulk lasers.

4.At high powers, there is a risk of damage even below the damage threshold of the material (see fiber melting).

5.Fibers have limited gain per unit length and pump absorption per unit length, so it is difficult to realize very short resonators for single-frequency lasers or gigahertz mode-locked lasers. However, there has been great progress in the direction of very highly doped fibers, usually using phosphate glasses.

It should be noted that fiber lasers are more difficult to design than bulk lasers. This is due to several reasons, including strong saturation effects at high light intensities, the fact that almost all fiber laser transitions are quasi-three-level, and the complex pulse generation mechanism in mode-locked fiber lasers. As a result, laser development projects are very expensive.

In the article Fiber lasers and bulk lasers, the advantages and disadvantages of fiber and bulk lasers are compared. In the article Laser power scaling, some ideas for high-power fiber devices are included.

Fiber laser model

Many technical details of fiber laser design are more complicated than those of bulk lasers. There are many reasons for this:

1.Fiber lasers usually operate with higher gain and higher resonator losses.

2.The light intensity in fiber lasers is usually greater than the saturation intensity, resulting in strong saturation effects (even for pump light).

3.Most active fibers have a quasi-three-level gain system, and their operating characteristics are more complicated than those of four-level fibers.

4.Fiber laser systems are usually very complex, for example, using a master oscillator power amplifier structure.

For the above reasons, the operating details of fiber lasers cannot usually be obtained by simple analytical calculations. Therefore, numerical simulations using some optical simulation software are required to calculate laser performance, analyze detrimental effects, and optimize models and product designs. Such simulations can obtain the following different technical details:

1.Rate equation models can be used to calculate the energy transfer process for a single laser active ion or multiple ions.

2.Mode solvers, that is, calculators that calculate fiber modes, obtain the incident for further calculations, especially the mode intensity distribution.

3.In some cases, it is necessary to know the numerical simulation results of the beam propagation. For example, in common multimode fibers, cladding pumping is included in double-clad fibers.

4.Calculation of the steady state of lasers and amplifiers requires refined algorithms that can be used to obtain self-consistent solutions for the excitation density and light intensity of laser-active ions throughout the fiber. (Light intensity and excitation density affect each other.)

5.Dynamic models can calculate pulse amplification and Q switching.

6.It is also possible to numerically simulate the propagation of ultrashort pulses in the fiber after considering laser gain, finite gain bandwidth, dispersion, various nonlinearities, etc.

Figure 9 shows the very special properties of a single fiber laser, showing the variation of optical power and excitation density throughout the fiber in an ytterbium-doped single-mode fiber laser. The optical Bragg grating with a peak reflectivity of 25% on the right side functions as an output coupler, while a high-reflectivity Bragg grating is used on the left side. Pump light (975 nm) is coupled in through the grating. The linear decay of the pump power on the left side causes strong pump saturation. The fiber is too long, so there is little signal light reabsorption on the right side. Reabsorption keeps the ytterbium excitation at a high level, even though the pump power is decreasing, but it slightly reduces the signal output power. The losses caused by ASE can also be ignored.

Figure 10 uses the same improved output coupling fiber, and the laser occurs at 1080 nm. The smaller radiation cross section at 1080 nm keeps the ytterbium excitation level high, so the pump absorption is smaller. This shows that the required optical length is not only related to the absorption characteristics of the pump wavelength, but also to the signal light, such as the signal light wavelength and the resonator loss.