RP Photonics logo
RP Photonics
Encyclopedia
Technical consulting services on lasers, nonlinear optics, fiber optics etc.
Profit from the knowledge and experience of a top expert!
Powerful simulation and design software.
Make computer models in order to get a comprehensive understanding of your devices!
Success comes from understanding – be it in science or in industrial development.
The famous Encyclopedia of Laser Physics and Technology – available online for free!
The ideal place for finding suppliers for many photonics products.
Advertisers: Make sure to have your products displayed here!
… combined with a great Buyer's Guide!
VLib part of the
Virtual
Library

Fiber Lasers

<<<  |  >>>

Definition: lasers with a doped fiber as gain medium, or (sometimes) just lasers where most of the laser resonator is made of fibers

German: Faserlaser

Categories: fiber optics and waveguides, lasers

How to cite the article; suggest additional literature

(British spelling: fibre lasers)

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 laser

Figure 1: A simple erbium-doped femtosecond laser, where the Fresnel reflection from a fiber end is used for output coupling.

DBR fiber laser

Figure 2: Short DBR fiber laser for narrow-linewidth emission.

fiber with lens and end mirror

Figure 3: End reflector with lens and mirror.

fiber loop

Figure 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 setup

Figure 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.

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

Upconversion Fiber Lasers

upconversion in Tm:ZBLAN fiber

Figure 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 lasers, single-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 lasers, DBR 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 laser

Figure 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

figure-of-eight laser

Figure 8: Figure-of-eight laser setup, as explained more in detail in the article on 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.

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

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.

Fiber Laser Modeling

Many technical aspects in fiber lasers are significantly more complicated than in bulk lasers. Reasons for that are manifold:

For these reasons, the operation details of a fiber laser (or fiber laser system) can often not be understood only based on simple analytical calculations. Numerical simulations, carried out with some kind of fiber simulation software, are therefore required for calculating the possible laser performance, analyzing detrimental effects, and optimizing prototype and product designs. Such simulations can address many different technical aspects:

As an example for surprising features even of simple fiber lasers, Figure 9 shows the optical powers and excitation densities along the fiber of an Yb-doped single-mode fiber laser. A fiber Bragg grating with 25% peak reflectivity at 1030 nm on the right side serves as the output coupler, whereas a highly reflecting Bragg grating is used on the left side. The pump light (at 975 nm) is coupled in through that grating. A nearly linear (rather than exponential) decay of pump power on the left side results from strong pump saturation. The fiber is somewhat over-long, resulting in slight signal reabsorption on the right side. That reabsorption maintains a significant ytterbium excitation despite the vanishing pump power, but causes only a negligible reduction in signal output power. Losses via ASE (not shown here) are also negligible.

Yb-doped fiber laser

Figure 9: Optical powers and excitation densities along the fiber of an Yb-doped single-mode fiber laser, core-pumped at 975 nm. Note that the intracavity signal power can be higher than the pump power; only part of that power can be coupled out. The simulation has been done with the software RP Fiber Power.

Figure 10 shows the same for a modified output coupler grating, so that lasing occurs at 1080 nm. The lower emission cross-sections at 1080 nm lead to a higher degree of Yb excitation and thus to weaker pump absorption. This demonstrates that the required fiber length depends not only on the absorption characteristics at the pump wavelength, but also on the details for the signal, such as the signal wavelength and the resonator losses.

Yb-doped fiber laser

Figure 10: Same as in Figure 9, but for a fiber Bragg grating for lasing at 1080 nm.

If the fiber length in the last case would be reduced to 0.7 m, one might expect a moderate reduction in output power due to incomplete pump absorption. However, a simulation (not shown here) tells that lasing would stop completely, and 94% of the pump power would leave the fiber on the right side. The Yb excitation density of about 50% throughout the fiber would not be sufficient to reach the laser threshold. For a reduced pump wavelength of 940 nm, however, lasing would be possible again – despite the reduced pump absorption cross section, because pump saturation effects would be weaker.

Bibliography

[1]E. Snitzer, “Proposed fiber cavities for optical masers”, J. Appl. Phys. 32 (1), 36 (1961)
 [2]E. Snitzer, “Optical maser action in Nd3+ in a Barium crown glass”, Phys. Rev. Lett. 7 (12), 444 (1961)
[3]C. J. Koester and E. Snitzer, “Amplification in a fiber laser”, Appl. Opt. 3 (10), 1182 (1964)
 [4]C. A. Burrus and J. Stone, “Nd3+ doped SiO2 lasers in an end-pumped fiber geometry”, Appl. Phys. Lett. 23 (7), 388 (1973)
[5]J. Stone and C. A. Burrus, “Neodymium-doped fiber lasers: room temperature CW operation with an injection laser pump”, Appl. Opt. 13 (6), 1256 (1974)
[6]R. J.Mears, L. Reekie, S. B. Poole, and D. N. Payne, “Neodymium-doped silica single-mode fibre lasers”, Electron. Lett. 21 (17), 738 (1985)
 [7]L. Reekie et al., “Diode-laser-pumped Nd3+-doped fibre laser operating at 938 nm”, Electron. Lett. 23, 884 (1987)
 [8]W. L. Barnes et al., “Er3+-Yb3+ and Er3+ doped fiber lasers”, J. Lightwave Technol. 7 (10), 1461 (1989)
[9]D. C. Hanna et al., “A 1-watt thulium-doped cw fibre laser operating at 2 μm”, Opt. Commun. 80, 52 (1990)
[10]A. C. Tropper et al., “Thulium-doped silica fiber lasers”, Proc. SPIE 1373, 152 (1991)
[11]R. B. Smart et al., “CW room temperature upconversion lasing at blue, green and red wavelengths in infrared-pumped Pr3+-doped fluoride fibre”, Electron. Lett. 27 (14), 1307 (1991)
[12]H. M. Pask et al., “Ytterbium-doped silica fiber lasers: versatile sources for the 1–1.2 μm region”, IEEE J. Sel. Top. Quantum Electron. 1 (1), 2 (1995)
[13]P. Xie and T. R. Gosnell, “Room-temperature upconversion fiber laser tunable in the red, orange, green, and blue spectral regions”, Opt. Lett. 20 (9), 1014 (1995)
[14]R. Paschotta et al., “230 mW of blue light from a Tm-doped upconversion fibre laser”, IEEE J. Sel. Top. Quantum Electron. 3 (4), 1100 (1997)
[15]Y. Takushima et al., “Polarization-stable and single-frequency fiber lasers”, J. Lightwave Technol. 16 (4), 661 (1998)
[16]V. Dominic et al., “110 W fibre laser”, Electron. Lett. 35, 1158 (1999)
[17]S. D. Jackson et al., “Diode-pumped 1.7 W erbium 3-μm fiber laser”, Opt. Lett. 24 (16), 1133 (1999)
[18]M. Pollnau and S. D. Jackson, “Erbium 3-μm fiber lasers”, IEEE J. Sel. Top. Quantum Electron. 7 (1), 30 (2001)
[19]Y. Jeong et al., “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power”, Opt. Express 12 (25), 6088 (2004)
[20]A. Polynkin et al., “Single-frequency fiber ring laser with 1 W output power at 1.5 μm”, Opt. Express 13 (8), 3179 (2005)
[21]A. Tünnermann et al., “Fiber lasers and amplifiers: an ultrafast performance evolution”, Appl. Opt. 49 (25), F71 (2010)
[22]M. J. F. Digonnet, Rare-Earth-Doped Fiber Lasers and Amplifiers, 2nd edn., CRC Press, Boca Raton, FL (2001)
[23]R. Paschotta, tutorial on "Modeling of Fiber Amplifiers and Lasers"
[24]R. Paschotta, Field Guide to Optical Fiber Technology, SPIE Press, Bellingham, WA (2010)

(Suggest additional literature!)

See also: fibers, rare-earth-doped fibers, double-clad fibers, fiber amplifiers, high-power fiber lasers and amplifiers, distributed feedback lasers, fiber lasers versus bulk lasers, power scaling of lasers, fiber simulation software, Raman lasers, Spotlight article 2011-02-10
and other articles in the categories fiber optics and waveguides, lasers

In the RP Photonics Buyer's Guide, 101 suppliers for fiber lasers are listed.

See also the article on “Fiber-based high-power laser systems”, contributed by external authors.


Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) and software could become very valuable for your business!

If you like this article, share it with your friends and colleagues, e.g. via social media:

arrow