Learn More about Fiber Laser

Fiber lasers are usually meant to be lasers with optical fibers as gain media, although some lasers with a semiconductor gain medium (a semiconductor optical amplifier) and a fiber resonator have also been called fiber lasers (or semiconductor fiber lasers). Also, devices containing some kind of laser (e.g., a fiber-coupled laser diodes) and a fiber amplifier are often called fiber lasers (or fiber laser systems).

In most cases, the gain medium is a fiber doped with rare earth ions such as erbium (Er3+), neodymium (Nd3+), ytterbium (Yb3+), thulium (Tm3+), or praseodymium (Pr3+), and one or several fiber-coupled laser diodes are used for pumping. Therefore, most fiber lasers are diode-pumped lasers. Although the gain media fiber lasers are similar to those of solid-state bulk lasers, the waveguiding effect and the small effective mode area usually lead to substantially different properties of the lasers. For example, they often operate with much higher laser gain and resonator losses. See also the article on fiber lasers versus bulk lasers.

Fiber Laser Resonators

In order to form a laser resonator with fibers, one either needs some kind of reflector (mirror) to form a linear resonator, or one builds a fiber ring laser. Various types of mirrors are used in linear fiber laser resonators:

setup of ultrafast erbium fiber laserFigure 1: A simple erbium-doped femtosecond laser, where the Fresnel reflection from a fiber end is used for output coupling.
  • In simple laboratory setups, ordinary dielectric mirrors can be butted to the perpendicularly cleaved fiber ends, as shown in Figure 5. This approach, however, is not very practical for mass fabrication and not very durable either.

  • The Fresnel reflection from a bare fiber end face is often sufficient for the output coupler of a fiber laser. Figure 1 shows an example.

  • It is also possible to deposit dielectric coatings directly on fiber ends, usually with some evaporation method. Such coatings can be used to realize reflectivities in a wide range.

  • For commercial products, it is common to use fiber Bragg gratings, made either directly in the doped fiber, or in an undoped fiber which is spliced to the active fiber. Figure 2 shows a distributed Bragg reflector laser (DBR laser) with two fiber gratings, but there are also distributed feedback lasers with a single grating in doped fiber, with a phase shift in the middle.

DBR fiber laserFigure 2: Short DBR fiber laser for narrow-linewidth emission.fiber with lens and end mirrorFigure 3: End reflector with lens and mirror.
  • A better power-handling capability is achieved by collimating the light exiting the fiber with a lens and reflecting it back with a dielectric mirror (Figure 3). The intensities on the mirror are then greatly reduced due to the much larger beam area. However, slight misalignment can cause substantial reflection losses, and the additional Fresnel reflection at the fiber end can introduce filter effects and the like. The latter effects can be suppressed by using angle-cleaved fiber ends, which however introduce polarization-dependent losses.

fiber loopFigure 4: Fiber loop mirror.

Most fiber lasers are pumped with one or several fiber-coupled diode lasers. The pump light may be coupled directly into the core, or in high-power into a larger pump cladding (→ double-clad fibers), as discussed below in more detail.

There are many different kinds of fiber lasers, some of which are discussed in the following.

simple fiber laser setupFigure 5: Setup of a simple fiber laser. Pump light is launched from the left-hand side through a dichroic mirror into the core of the doped fiber. The generated laser light is extracted on the right-hand side.

High-power Fiber Lasers

Whereas the first fiber lasers could deliver only a few milliwatts of output power, there are now high-power fiber lasers with output powers of hundreds of watts, sometimes even several kilowatts from a single fiber. This potential arises from a high surface-to-volume ratio (avoiding excessive heating) and the guiding effect, which avoids thermo-optical problems even under conditions of significant heating.

Nowadays, high-power fiber lasers are widely used e.g. in laser material processing. Examples for processes are laser welding and laser cutting e.g. on metals, but also with many other industrial materials. Many applications use a fiber laser machine with continuous-wave operation; limitations concerning pulse generation e.g. with Q switching are substantial, so that bulk lasers reach clearly superior performance in such domains.

See the article on high-power fiber lasers and amplifiers for more details.

Upconversion Fiber Lasers

upconversion in Tm:ZBLAN fiberFigure 6: Level scheme of thulium (Tm3+) ions in ZBLAN fiber, showing how excitation with an 1140-nm laser can lead to blue fluorescence and laser emission.

The fiber laser concept is most suitable for the realization of upconversion lasers, as these often have to operate on relatively “difficult” laser transitions, requiring high pump intensities. In a fiber laser, such high pump intensities can be easily maintained over a long length, so that the gain efficiency achievable often makes it easy to operate even on low-gain transitions.

In most cases, silica glass is not suitable for upconversion fiber lasers, because the upconversion scheme requires relatively long lifetimes of intermediate electronic levels, and such lifetimes are often very small in silica fibers due to the relatively large phonon energy of silica glass (→ multi-phonon transitions). Therefore, one mostly uses certain heavy-metal fluoride fibers such as ZBLAN (a fluorozirconate) with low phonon energies.

The probably most popular upconversion fiber lasers are based on thulium-doped fibers for blue light generation (Figure 6), praseodymium-doped lasers (possibly with ytterbium codoping) for red, orange, green or blue output, and green erbium-doped lasers.

See the article on upconversion lasers for more details.

Narrow-linewidth Fiber Lasers

Fiber lasers can be constructed to operate on a single longitudinal mode (→ single-frequency laserssingle-mode operation) with a very narrow linewidth of a few kilohertz or even below 1 kHz. In order to achieve long-term stable single-frequency operation without excessive requirements concerning temperature stability, one usually has to keep the laser resonator relatively short (e.g. of the order of 5 cm), even though a longer resonator would in principle allow for even lower phase noise and a correspondingly smaller linewidth. The fiber ends have narrow-bandwidth fiber Bragg gratings (→ distributed Bragg reflector lasersDBR fiber lasers), selecting a single resonator mode. Typical output powers are a few milliwatts to some tens of milliwatts, although single-frequency fiber lasers with up to roughly 1 W output power have also been demonstrated.

An extreme form is the distributed-feedback laser (DFB laser), where the whole laser resonator is contained in a fiber Bragg grating with a phase shift in the middle. Here, the resonator is fairly short, which can compromise the output power and linewidth, but single-frequency operation is very stable.

Of course, further amplification to much higher power levels in a fiber amplifier is possible.

Q-switched Fiber Lasers

setup of Q-switched erbium fiber laserFigure 7: Simple Q-switched fiber laser. The setup looks exactly the same as that of a mode-locked laser as shown above (Figure 2), but the SESAM parameters are different.

With various methods of active or passive Q switching, fiber lasers can be used for generating pulses with durations which are typically between tens and hundreds of nanoseconds (see e.g. Fig. 7). The pulse energy achievable with large mode area fibers can be several millijoules, in extreme cases tens of millijoules, and is essentially limited by the saturation energy (even for large mode area fibers) and by the damage threshold (the latter particularly for shorter pulses). All-fiber setups (not containing any free-space optics) are quite limited in terms of the achievable pulse energy, as they can normally not be realized with large mode area fibers and effective Q switches.

Due to the high laser gain, the details of Q switching a fiber laser are often qualitatively different from those of a bulk laser, and more complicated. One often obtains a temporal sub-structure with multiple sharp spikes, and there is a possibility of producing Q-switched pulses with a duration well below the (typically long) resonator round-trip time.

Mode-locked Fiber Lasers

More sophisticated resonator setups are used particularly for mode-locked fiber lasers (ultrafast fiber lasers), generating picosecond or femtosecond pulses. Here, the laser resonator may contain an active modulator or some kind of saturable absorber. An artificial saturable absorber can be constructed using the effect of nonlinear polarization rotation, or a nonlinear fiber loop mirror. A nonlinear loop mirror is used e.g. in a “figure-of-eight laser”, as shown in Figure 8, where there is a main resonator on the left-hand side and a nonlinear fiber loop, which does the amplification, shaping and stabilization of a circulating ultrashort pulse. Particularly for harmonic mode locking, additional means may be used, such as subcavities acting as optical filters.

For more details on ultrafast fiber lasers, see the article on mode-locked fiber lasers.

figure-of-eight laserFigure 8: Figure-of-eight laser setup, as explained more in detail in the article on mode-locked fiber lasers.

Raman Fiber Lasers

A special type of fiber lasers are fiber Raman lasers, relying on Raman gain associated with the fiber nonlinearity. Such lasers usually use relatively long fibers, sometimes of a type with increased nonlinearity, and typical pump powers of the order of 1 W. With several nested pairs of fiber Bragg gratings, the Raman conversion can be done in several steps, bridging hundreds of nanometers between the pump and output wavelength. Raman fiber lasers can e.g. be pumped in the 1-μm region and generate 1.4-μm light as required for pumping 1.5-μm erbium-doped fiber amplifiers.

Fiber Lasers with Semiconductor Optical Amplifiers

There are some lasers which have a semiconductor optical amplifier (SOA) as the gain medium in a resonator made of fibers. Even though the actual laser process does not occur in a fiber, such fibers are sometimes called fiber lasers. They typically emit relatively small optical powers of a few milliwatts or even less. Sometimes they exploit the very different properties of the semiconductor gain medium, as compared with a rare-earth-doped fiber, in particular the much smaller saturation energy and upper-state lifetime. Rather than only generating coherent light, such lasers can be used for information processing in optical fiber communications systems – for example the wavelength conversion of data channels based on cross-saturation effects.

Special Attractions of Fibers as Laser Gain Media

  • As fibers can be coiled and the light propagating in fibers is well shielded from the environment (e.g. concerning dust), fiber lasers can have a compact and rugged setup, provided that the whole laser resonator is built only with fiber components (all-fiber setup) such as fiber Bragg gratings and fiber couplers (i.e., avoiding free-space optics and any requirement for alignment).

  • Fiber gain media have a large gain bandwidth due to strongly broadened laser transitions in glasses, permitting wide wavelength tuning ranges and/or the generation of ultrashort pulses. Also, fiber lasers have broad spectral regions with good pump absorption, making the exact pump wavelength uncritical, so that temperature stabilization of the pump diodes is usually not necessary.

  • Diffraction-limited beam quality is easily obtained when single-mode fibers are used, and sometimes also with slightly multimode fibers.

  • Due to the high gain efficiency of doped fibers, fiber lasers have the potential to operate with very small pump powers. Also, it is possible to obtain very high power efficiencies.

  • In recent years, the potential for very high output powers (several kilowatts with double-clad fibers) has been convincingly demonstrated (see above).

  • Again due to the guidance, which allows high pump intensities to be applied over long lengths, fiber lasers can be operated even on very “difficult” laser transitions (e.g. of upconversion lasers).

On the other hand, fiber lasers can suffer from various problems:

Also note that fiber lasers are in many cases substantially more difficult to design than bulk lasers. This results from very different reasons, including strong saturation effects caused by the high optical intensities, the quasi-three-level behavior of nearly all fiber laser transitions, and the complicated pulse formation mechanisms in mode-locked fiber lasers. As a result, the laser development project can be more costly.

The article on fiber lasers versus bulk lasers compares the strengths and weaknesses of fiber and bulk lasers. See also the article on power scaling of lasers, containing thoughts on high-power fiber devices.

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