High-power Fiber Lasers and Amplifiers

Whereas the first fiber lasers could deliver only a few milliwatts of output power, there have subsequently been rapid developments which have lead to high-power fiber lasers and particularly amplifiers with output powers of tens or hundreds of watts, sometimes even several kilowatts from a single fiber. This potential arises from a very high surface-to-volume ratio (avoiding excessive heating) and the guiding (waveguide) effect, which avoids thermo-optical problems even under conditions of significant heating.

There is no generally agreed minimum power level for the attribute “high power”. It may mean more than 100 W of average power, but some lasers are labeled as high-power levels even for 10 W only.

Fiber laser technology now competes strongly with other high-power laser technologies based on solid-state bulk lasers, such as thin-disk lasers.

Double-clad Fibers and Beam Quality

High-power fiber lasers and fiber amplifiers are nearly always realized with rare-earth-doped double-clad fibers, which are pumped with fiber-coupled high-power diode bars or other kinds of laser diodes. The pump light is launched into an inner cladding rather than into the (much smaller) fiber core, in which the laser light is generated. The laser light can have very good beam quality – even diffraction-limited beam quality if the fiber has a single-mode core. Therefore, the brightness of the fiber laser output can be orders of magnitude higher than that of the pump light, even though the output power is of course somewhat smaller. Double-clad fiber lasers can thus effectively be used as brightness converters. Typical optical-to-optical power efficiencies are above 50%, sometimes even above 80%; the overall electrical-to-optical power conversion efficiency of a diode-pumped fiber laser can be of the order of 50% for an Yb-based fiber laser pumped with efficient high-power diode lasers.

For the highest powers, the core area needs to be fairly large (→ large mode area fibers) because the optical intensities would otherwise become too high, and often also because a double-clad fiber with a large ratio of cladding to core area has a weak pump absorption. For core areas up to the order of a few thousand μm², it is feasible to have a single-mode core. Larger mode areas with still fairly good output beam quality are possible with a slightly multimode core, where most of the light propagates in the fundamental mode. (The excitation of higher-order fiber modes can be suppressed to some extent e.g. by coiling the fiber, except if one obtains strong mode coupling at high power levels [15].) For even larger mode areas, the beam quality can no longer be nearly diffraction-limited, but it can still be fairly good compared with, e.g., rod lasers operating at similar power levels.

The required fiber length is often several tens of meters, dictated by the need to absorb the injected pump light with sufficiently high efficiency. Although the fiber length can 'in principle be reduced e.g. by using a fiber core with a higher doping concentration, a large fiber core or a smaller pump cladding (for a smaller cladding/core ratio), there are various limits to that, e.g. arising from limits to the doping concentration, the beam quality of the pump source, beam quality requirements on the signal or thermal considerations.

Launching the Pump Light

There are several options for launching pump light at very high power levels. The simplest is to launch directly into the pump cladding at one or both fiber ends (see Figure 2). This technique does not require special fiber components; however, it needs the propagation of the high-power pump radiation through air (with free-space optics) and particularly through an air–glass interface, which is then very sensitive to dust and misalignment. In many cases, it is therefore preferable to use one of several techniques where one employs fiber-coupled pump diodes and keeps the pump light in fibers from there on (see Figure 3). One option is to launch the pump light into passive (undoped) fibers which are wound around the active fiber so that the light is gradually transferred into the active fiber (GTWave fiber). Other techniques are based on special pump combiner devices (fiber-optic pump combiners), where several pump fibers and a single active signal fiber are fused together. Yet other approaches are based on side-pumped fiber coils (fiber disk lasers) [3], or on grooves in the pump cladding through which pump light can be injected. The latter technique allows for multi-point pump injection and thus for a better distribution of the heat load.

A comparison of all the different pump launching techniques is complex, since it involves many aspects: not only the transfer efficiency, but also the loss of brightness, the ease of manufacturing, the flexibility of handling, possible back-reflections, leakage of light from the fiber core back to the pump source, the option to maintain the polarization, etc.

Case Studies

Case Study: Designing a Double-clad Fiber Amplifier

We develop a double-clad fiber amplifier with high gain, where we have to care about limiting losses by ASE.

Lasers and Amplifiers

The conceptually simplest option for generating high-power laser light is to build a fiber laser directly with some kind of mirrors on both ends. However, high-power fiber devices are very often built as laser-amplifier combinations, i.e., with a MOFA (master oscillator fiber amplifier) architecture. This concept has several advantages. It is easier to control the emission properties of a lower-power seed laser in terms of linewidth, laser noise, wavelength tunability, pulse generation, etc. Also, the fiber of an amplifier has to stand only a power about equal to the output power, whereas in a laser the intracavity power is higher (even though fiber lasers as high-gain devices allow for strong output coupling). Furthermore, it can be advantageous to use a modular design approach, where amplifier stages can be combined as required. In many cases, particularly when a low-power seed laser is used, one even uses several amplifier stages, typically with increasing mode areas and pump powers along the chain.

Nanosecond Pulses

Many high-power lasers for material processing are Q-switched bulk lasers, generating intense nanosecond pulses. The direct application of that technique to fiber lasers is very much limited in terms of achievable peak power, basically due to the strong fiber nonlinearities. However, the high gain of fiber devices allows the realization of very flexible MOPA devices with a gain-switched laser diode (→ picosecond diode lasers) as seed laser. This approach, unlike Q switching of a laser, allows one, e.g., to modify the pulse duration independently of the pulse repetition rate. Still, the possible pulse energies are far lower than possible with bulk lasers. For many applications, however, the possible performance is sufficient.

Ultrashort Pulses

The large gain bandwidth of fiber amplifiers allows for the amplification of ultrashort pulses. That can be exploited both in mode-locked fiber lasers and in ultrashort pulse fiber amplifiers.

Particularly in this pulse duration regime, substantial challenges arise from the strong fiber nonlinearity because high-energy femtosecond pulses have huge peak powers. In addition to the risk of fiber damage, strong nonlinear distortions can result from the fiber nonlinearity, and the high level of chromatic dispersion (including higher-order dispersion) can also be problematic. Particularly if pulse quality matters, these matters need to be carefully considered.

One possible approach is to make fiber-based chirped-pulse amplification systems, where the pulse duration within the amplifier is strongly increased, so that the effect of nonlinearities is accordingly decreased. A less powerful alternative to this approach is the amplification of parabolic pulses, where up-chirped pulses experiencing the gain and nonlinearity of the amplifier fiber evolve in a self-similar fashion, and their close to linear chirp makes it possible to obtain a high pulse quality with dispersive compression.

An interesting direction is the development of all-fiber ultrashort pulse sources, possibly allowing for a final free-space pulse compressor (e.g. a transmission grating), but eliminating the need to launch pulses from free space into a fiber.

Limiting Factors

There are various possible limiting factors for the performance of high-power fiber amplifiers and lasers, which are briefly discussed in the following. Which of those factors limits the performance of a particular device, depends very much on the operation regime.

Available Pump Power

Some devices are limited by the amount of pump power which can be launched into the fiber. Note that even when using double-clad fibers, the beam quality of the pump source needs to be sufficiently high. Effectively, that means that only pump sources with sufficiently high radiance (brightness) can be used.

Fortunately, the continuous development of diode bars and broad area laser diodes results in devices with higher and higher radiance, so that the corresponding performance limits are extended more and more.

Heat Generation

A common problem for high-power lasers is the heat generation in the gain medium. Fiber amplifiers and lasers, however, are affected by this only at very high powers, basically because (a) the often quite high power conversion efficiency (resulting in a relatively low heat load), (b) the distribution of the heat load over a relatively long length (e.g. several meters) and (c) the waveguide function of the fiber core.

The heat load in a fiber may become a problem particularly in cases where the fiber length has to be minimized, e.g. in order to mitigate nonlinear effects. This means using fibers with relatively high doping concentration, allowing to turn over the required power within a shorter length. The temperature rise in the fiber core can then be quite substantial. Besides, the resulting thermal lensing may significantly modify the mode properties, e.g. reducing the effective mode area and increasing the number of guided modes.

The dissipated power per unit length often reached values of the order of 100 W/m, which causes significant heating for air-cooled fibers. Water cooling may be used for significant further increases in power, although it cannot remove the thermal lensing problem.

If the fiber is not strictly single-mode (which is often the case in high-power devices, particularly the last amplifier stage), the heating can also trigger another problem, called thermally induced transverse mode instability (TMI), which is based on stimulated thermal Rayleigh scattering (STRS). That phenomenon usually occurs above a relatively well defined pump power level; a typical characteristic is that one then suddenly obtains a substantially reduced output beam quality, which is associated with a substantial transfer of power into higher-order guided modes. The distribution of optical power shows an oscillatory or chaotic behavior. The tendency for that mode instability grows for fibers with particularly large mode area.

The physical mechanism underlying the thermally induced mode instability is relatively complicated. It involves the generation of a long-period grating in the fiber [14] which couples light from the fundamental mode to higher-order modes. At the same time, the grating is generated by modal interference, which is leading to a spatially varying intensity pattern and (via Kramers–Kronig) to a refractive index pattern. An important aspect is a slight difference in optical frequency between the involved modes, which leads to a easily detected beat note and to a movement of the mentioned refractive index grating [15].

Nonlinear Limitations

Various high-power fiber devices are essentially limited in performance by consequences of fiber nonlinearities of different types:

• Particularly single-frequency devices suffer from stimulated Brillouin scattering (SBS). The problem is that the intense signal light in an amplifier, for example, leads to a high Brillouin gain for counterpropagating light with a slightly lower optical frequency. Once that nonlinear gain reaches approximately 90 dB, quantum noise is amplified to a substantial power level. The resulting counter propagating wave then extracts power from the signal, which in turn reduces the nonlinear gain; that interaction usually leads to chaotic power fluctuations. There are various mitigation techniques. For example, it is common to apply a first phase modulation to the input signal, which can be removed after the amplifier, and reduces the Brillouin gain by increasing the signal linewidth beyond the Brillouin gain bandwidth. One may also optimize the fiber design, particularly concerning the effective mode area and used length, and exploit a temperature gradient along the fiber.
• Similarly, a nonlinear gain arises from stimulated Raman scattering (SRS), where however various parameters are very different from those of Brillouin scattering: the gain coefficient is much lower, while the nonlinear gain bandwidth is far higher, and the nonlinear gain also occurs for co-propagating light. SRS is often the dominant limiting factor for the amplification of ultrashort pulses, where SBS is not relevant due to the broader signal bandwidth and the rather limited spatial overlap region for counterpropagating short pulses.
• A rather hard limit for the possible peak power in a fiber is set by nonlinear self-focusing. Above a certain critical power, that effect leads to a catastrophic collapse of the beam profile, which is usually followed by instant destruction of the fiber material. Interestingly, the critical power is not increased by using a fiber with larger effective mode area, even though the optical intensity for the undisturbed mode is reduced by that. For silica fibers, having a relatively low nonlinear index, the critical power is relatively high (several megawatts), but still a serious limitation in some cases with ultrashort pulse amplification. Even when applying chirped-pulse amplification, the pulse energy is limited to the order of 10 mJ because there are practical limits to the usable chirped pulse duration.
• There can also be the problem of laser-induced damage at fiber ends. That problem can be substantially mitigated by using core-less end caps, in which the beam can expand to a substantially larger area until it reaches the more sensitive glass–air interface.

Note that the optimization of high-power file devices often involves a trade-off between different kinds of limitations; best results are usually obtained by finding an optimal compromise.

Prospects for Further Improvements

Even though the progress in the development of high-power fiber devices has been tremendous in recent years, various kinds of limitations are now encountered, which are expected to slow this progress:

• In most cases (particularly for pulsed operation), fiber devices are not limited by available pump power, or at least the pump power could be increased further with already available laser diodes.
• The optical intensities in high-power fiber devices have been enormously increased. Now they are often close to the damage threshold of the material. Therefore, increased mode areas (→ large mode area fibers) are used. However, it seems that the limits of this approach have also been approximately reached, at least if high output beam quality is required.
• There still seems to be some potential for mitigating the thermally induced mode instability, but probably not a possibility to eliminate that limitation altogether.
• Fiber nonlinearities are usually the limiting factor for devices involving short or ultrashort pulses. For various devices (particularly for mode-locked fiber lasers), improved mitigation methods have led to substantial performance improvements; it is unclear whether substantial further advances along that route are possible.

Due to the explained limitations, high-power fiber devices can – contrary to common opinions – usually not be considered as power-scalable in a reasonable sense, at least not beyond the already achieved performance level. Previous advances have also largely not been achieved with simple power scaling, but with a combination of other methods, such as using improved pump diodes and fiber designs, and optimizing parameters to ideally deal with existing trade-offs. See the article on power scaling of lasers for more details.

Once the limit of the power per fiber has been reached, beam combining is a further option, albeit at the cost of substantially reducing the system complexity.

Although the mentioned optimization techniques have allowed for often astonishing performance of fiber-based high-power laser sources, these achievements are usually based on setups which involve a lot of free-space optics and hence lose many of the specific advantages of all-fiber systems. However, it may be possible to develop more all-fiber systems which still offer good enough performance for many applications. For an example, see Ref. [28].

More to Learn

Case studies:

• Case Study: Designing a Double-clad Fiber Amplifier
• Case Study: Ytterbium-doped 975-nm Fiber Lasers

Encyclopedia articles:

• fiber lasers
• fibers
• rare-earth-doped fibers
• double-clad fibers
• fiber-optic pump combiners

Blog articles:

• The Photonics Spotlight 2010-01-22