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High Power Fiber Lasers: A Review
Article in IEEE Journal of Selected Topics in Quantum Electronics · September 2014

DOI: 10.1109/JSTQE.2014.2321279
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IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER/OCTOBER 2014 0904123
High Power Fiber Lasers: A Review

Michalis N. Zervas and Christophe A. Codemard
(Invited Paper)
Abstract—In this paper, we summarize the fundamental proper- perior overall performance. Due to large surface-to-volume ra-
ties and review the latest developments in high power fiber lasers. tio, fiber lasers provide better thermal management and total
The review is focused primarily on the most common fiber laser elimination of thermal lensing, which plagues solid-state crys-
configurations and the associated cladding pumping issues. Spe-
cial attention is placed on pump combination techniques and the
tal counterparts. The well controlled spatial distribution of the
parameters that affect the brightness enhancement observed in signal, provided by the continuous guidance, results in supe-
single-mode and multimode high power fiber lasers. The review rior beam quality and stability, while small quantum defect,
includes the major limitations imposed by fiber nonlinearities and as well as, low cavity and transmission losses result in record
other parasitic effects, such as optical damage, transverse modal in- wall-plug efficiencies. Also, fiber lasers show turn-key opera-
stabilities and photodarkening. Finally, the paper summarizes the tion and small foot print. Their unique properties, in particular
power evolution in continuous-wave and pulsed ytterbium-doped
fiber lasers and their impact on industrial applications.
the output power stability and unparalleled beam quality at high
output powers, have increased their market penetration and have
Index Terms—Beam quality, brightness, cladding-pumping, enabled a number of new applications [7].
fiber amplifiers, fiber lasers, high power, holmium-doped, indus- The amorphous nature of glass host in the fiber core produces
trial lasers, material processing, modal instabilities, optical dam-
age, optical fibers, optical fiber nonlinearities, optical pulses, pho- inhomogeneously broadened active-ion emission and absorp-
todarkening, pump combiners, thulium-doped, transverse mode tion spectra, which are wider than they would be in crystals [8],
instabilities, ytterbium-doped. [9]. This enables fiber lasers to be widely tuned and work ef-
ficiently from continuous-wave (CW) operation to ultra-short
I. INTRODUCTION optical pulses. They show high gain, which enables master-
oscillator power amplifier (MOPA) and cascaded amplifier con-
IBER lasers [1], [2] were proposed and studied as a promis-
F ing laser configuration soon after the discovery and first
laser demonstration by Maiman [3]. Ever since lasers have
figurations and makes them suitable for average power scaling.
However, the small saturation energy, associated with the rel-
atively small—compared to solid-state rod counterparts—fiber
played central role in the fast developing field of Photonics, core diameter compromises the energy storage capabilities and
which in turn has revolutionized existing, as well as, enabled high energy operation.
entire new scientific and industrial sectors [4]. Over the last decade, the performance advances in fiber
Lasers exploit the quantum effect of stimulated emission to lasers have been spectacular making fiber lasers a success-
generate light and share a number of common features, such as ful, fast increasing commercial business currently worth over
an active medium to provide gain, an optical cavity to enhance $800 M/year, with compound annual growth rate of about
and control the optical field and a pumping source to provide 13%—the highest among the different laser technologies [7].
the energy [5]. However, the details of these features play an im- Key for the power scaling of high power fiber (YDF) lasers are
portant role in differentiating the laser performance, the power the developments in major technologies, such as high-quality
scaling capabilities, stability, footprint and cost. Different gas- passive and active fibers, high-power passive fiber components,
filled tubes, crystal rods and discs have been traditionally used as including, beam combiners, fiber Bragg gratings (FBG), isola-
active media, incorporated in various bulk-optic cavities. Fiber tors, cladding mode strippers and end caps, and bright diode
lasers are the latest entry in the solid-state laser technology laser pump modules.
arena [6], fast increasing their penetration in all sectors of in- The expansion of the high-power fiber laser field has been
dustrial, medical and directed energy application space [7]. already captured in a number of excellent reviews and book
There are a number of features that differentiate fiber lasers monographs available in the literature [6], [10]–[14]. The fast
from the other existing laser technologies and give them su- observed pace though warrants frequent reviews of the latest de-
velopments. This paper summarizes the fundamental properties
and reviews the latest developments in high power fiber lasers,
Manuscript received January 20, 2014; revised March 29, 2014; accepted which so far have been the most commercially successful. The
April 23, 2014. The work was partially supported by the EPSRC Center for review is focused primarily on the most commonly used fiber
Innovative Manufacturing in Photonics, University of Southampton. laser configurations and the issues related to cladding pumping,
M. N. Zervas is with the Optoelectronics Research Center, University of
Southampton, Southampton, SO17 1BJ, U.K., and also with SPI Lasers,
the preferred technique for power scaling. Special attention is
Southampton, SO30 2QU, U.K. (e-mail: mnz@orc.soton.ac.uk). given on pump combination techniques and the parameters that
C. A. Codemard is with SPI Lasers, Southampton, SO30 2QU, U.K. (e-mail: affect the brightness enhancement observed in high power fiber
christophe.codemard@spilasers.com). lasers. The review also includes the major limitations imposed
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org. by fiber nonlinearities and other parasitic effects, such as opti-
Digital Object Identifier 10.1109/JSTQE.2014.2321279 cal damage and photodarkening and gives a brief account of
1077-260X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications standards/publications/rights/index.html for more information.
0904123 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER/OCTOBER 2014
Fig. 2. Schematic of cladding pumping principle of operation.
tion with intra-cavity pump launching, which alleviates some of

these issues.
High power hybrid fiber laser configurations require careful
bulk-optic mirror alignment and special fiber-end facet prepa-
ration [17], [18] to avoid unwanted backreflections, as well as,
avoid surface damage. Such configurations are primarily more
suitable for high power lab demonstrations [19]–[21] or low
average power laser systems. However, the overall robustness
of the laser can be improved if mirrors are butt-coupled or di-
rectly deposited onto the fiber facet [22]. Such approaches are
Fig. 1. Cladding-pumped fiber laser configurations (a) hybrid end-pumped
more suitable for multimode (MM) operation. All fiber config-
(b) all-fiber end-pumped and (c) all fiber intra-pumped. urations, on the other hand, are preferable if all the benefits of
the fiber technology are to be harnessed, and such systems are
suitable for service-free, reliable industrial systems [23]–[30].
transverse mode instabilities (TMI). The paper summarizes
FBGs are usually written in single-mode (SM) fibers, which
the power evolution in continuous-wave and pulsed ytterbium-
in addition to wavelength can also determine the output beam
doped fiber lasers and their impact on material processing
modality.
and other industrial applications. It concludes with the future
prospects in the field of high power fiber lasers.
B. Cladding Pumping
II. FUNDAMENTALS In his seminal paper [1] E. Snitzer states that “the major dis-
advantage (of the fiber laser) is that of getting the pump power
A. Fiber Laser Cavity Configurations
into the fiber. However, it should be possible to overcome this
The gain in fiber lasers is provided by fibers of various types difficulty with proper design of the fiber and the illuminating
with cores doped with active rare-earth ions, such as ytterbium, optics.” Subsequently in 1988 E. Snitzer proposes an elegant so-
erbium, thulium, or holmium. Typically the cavity is formed lution to this problem in the form of cladding pumping [31], [32]
either by bulk mirrors placed on either fiber end, or FBGs [15] which has proven to be the most powerful enabling technique
written directly into the fiber core [16]. The pumping is achieved for power scaling fiber lasers.
by combining laser diodes (single emitters, bars or stacks) and In cladding pumping schemes (see Fig. 2), instead of launch-
launching either in the core or cladding of the fiber. ing into the highly restrictive—in terms of size and numerical
1) Most Common Fiber Laser Configurations: A number of aperture (NA)—active core, high-power low-brightness pump
different configurations have been used for fiber laser demon- light is launched into the much larger, in size and numerical
strations, depending on the active fiber and availability in pump- aperture, cladding. As the pump light rays propagate down the
ing technology. highly multimoded fiber cladding they cross and get absorbed
The most commonly used fiber laser configurations are shown gradually by the active core. However, the generated light is effi-
in Fig. 1(a)–(c). Fig. 1(a) shows a hybrid end-pumped arrange- ciently trapped inside the much smaller size and lower NA core,
ment with the active fiber placed inside an optical cavity formed and as a result the cladding-pumped amplifier or laser output
by two bulk mirrors, a high reflector (HR) with R > 99% and a is much brighter and intense. In this respect, cladding-pumped
lower reflectivity output coupler. The pump is launched through fiber lasers are extremely efficient brightness converters (see
the fiber ends with appropriately placed dichroic mirrors (DM) Appendix A for definitions).
that transmit the signal and reflect the pump wavelengths or vice From the definition of brightness (see Appendix A), the max-
versa. imum pump power that can be launched into a circular fiber
Fig. 1(b) shows an all-fiber end-pumped configuration, where cladding is given by
the bulk-optic mirrors are replaced by intra-core FBGs and the 2 1
combined pumps are launched through the FBGs. This config- Ppin = Bp πrcl πN A2cl = Bp λ2p Np . (1)
uration puts extra stress on the FBGs as they are subjected to 2
strong pump and signal powers and special care should be taken The launched power is proportional to the brightness of the
to protect them. Finally, Fig. 1(c) shows an all-fiber configura- pump source (Bp ), the square of the cladding radius (rcl ),
ZERVAS AND CODEMARD: HIGH POWER FIBER LASERS: A REVIEW 0904123
Fig. 4. Cladding-pumped fiber cross-sections.
these improved designs “scatter” skew rays towards the active

core [35]–[42].
Fig. 3. Modal space for multimode step-index fibers. Region I (II) shows the
Fig. 4 shows a number of commonly used cladding-pumped
group of modes overlapping (non-overlapping) with the doped core. fiber cross-sections. The pump NA is defined by the choice of the
outer cladding material. Typically, these fibers use fluorinated
polymer outer cladding giving NAs of ∼0.46. In this case, at
and numerical aperture (NAcl ). Alternatively, the maximum high power operation special cooling arrangements are required
launched pump power isproportional to the number of supported to avoid excess heating of the polymer. Cooling requirements are
pump modes Np = Vcl2 2, where Vcl is the cladding V-number. relaxed considerably if low index fluorosilicate glass is used as
Therefore, power scaling in fiber lasers relies on the develop- the inner cladding. Unfortunately, although such glass–glass in-
ment of high brightness pump modules (see Section B.3)), and terface has superior power and thermal handling capabilities, the
fibers with high cladding NA and large cladding area. However, obtained NAs with current technologies are rather low (∼0.22–
the launched pump power cannot be increased indefinitely due 0.26), reducing the amount of pump power that can be launched.
to limitations imposed by the practically achievable NA, the In this case, large diameter coiling is required to avoid excessive
onset of nonlinear effects and optical damage (see Section III). pump bend-induced loss. Also, compatibility with standard high
The overall cladding-pumped fiber laser and amplifier length power TFB fused combiners would also be compromised.
depends on the pump absorption coefficient (αC P ), which scales To overcome these problems, novel jacketed-air clad (JAC)
as αC P = ηS αco (Aco /Acl ), where αco is the small-signal fiber designs [Fig. 4(d)] have been developed, which rely on a
2
pump absorption when launched into the core, and Aco = πrco row of cylindrically arranged air holes to provide effectively a
is the doped core area. ηS is a coefficient, which defines the ef- glass/air interface with N As > 0.8 [43], [44]. Such JAC fibers
fectiveness of modal absorption of the various mode scrambling can also be used to reduce the cladding diameters considerably,
techniques. ηS = 1 implies that all cladding modes are excited increase the pumping rate and enable efficient 3-level operation,
equally, overlap equally with the doped core and are absorbed e.g., at 980 nm [45].
uniformly. This can only be achieved by a cladding shape that The choice of the cladding perturbation should be consider
fully scrambles the propagating modes [33]. In the case of a judiciously in order to maximize mode scrambling, while avoid-
straight circular fiber with centered core, this assumption ap- ing excess scattering loss [46]. It was found, for example, that
plies to all LP 0n modes, which correspond to meridional rays, for the same material compositions, the effective NA of rect-
and only to a small fraction of the LPm n (m 0) modes, cor- angular fibers is smaller than that of circular fibers and the
responding to skew rays. The majority of the LPm n (m = 0) pump light propagation loss in rectangular inner-cladding fibers
modes miss the doped core entirely. For a straight fiber with [Fig. 4(c)] is larger [47]. Boron-doped stress elements can be
circular cladding the fraction of modes, which overlap with the incorporated into the inner cladding to maintain polarization. In
doped core and are effectively absorbed, is approximated by the case of cladding pumped hi-bi fibers, the low refractive index
ηS ≈ (πrco )/(2rcl ) [34]. of the borosilicate stress-applying elements ensures that pump
Fig. 3 shows the power overlap with the doped core in the light will not be trapped in these elements, and their presence
cladding modal space, in the case of a fiber with rco = 3 μm, along with the applied stresses scrambles helical modes/skew
rcl = 65 μm, and NAcl = 0.46. It is shown that the majority rays within the inner cladding [48], [49]. From the various
of the overlapping modes (region I) have ∼10% of their power inner-cladding shapes, shown in Fig. 4, the ones with multiple
within the doped core, independently of their order. The steep truncations, e.g. Fig. 4(f) and (g), are shown to be more efficient
transition between regions I and II is defined by the higher order in mode scrambling [38].
modes whose internal caustic coincides with the active core In most practical cases, breaking the cladding cross-section
perimeter [34]. rotational symmetry alone is not enough to absorb effectively
In order to improve the overall pump absorption, a number the launched pump power over the entire fiber length and over
of fiber designs have been proposed which break the cladding’s all wavelengths. In order to increase the pump absorption close
rotational symmetry and increase the fraction of cladding modes to its limiting value, given by the core over cladding area ratio,
overlap with the active core. Equivalently, we can consider that the modes should be continuously mixed over the entire length,
Fig. 6 Brightness enhancement factor for different cladding radii and NAs
and single-mode signal output (λp = 945 nm and η o −o = 75%).
Fig. 5. Effect of longitudinal mode mixing and wavelength on pump absorp-
tion, in Yb 3 + -doped alumina-silicate fiber lasers [34].
which is achieved by properly perturbing the fiber along its

length [50], [51], using periodic/quasi-periodic fiber bending
[52], [53] or fiber tapering [54].
Fig. 5 plots the increase in pump absorption of a cladding-
pumped fiber as a function of length for different wavelengths,
with and without mode mixing. In this case the mode mixing
is achieved by periodic fiber bending. It is shown that in the
case without mode mixing, the pump absorption at both wave-
lengths is small and saturates quickly with fiber length. In this
+
case, pumping at 976 nm, the Yb3 absorption peak, offers no
significant advantage over 940 nm. This results in an effective
absorption spectrum deformation and can be observed irrespec-
tive of the pumping scheme (discussed in Section II) [34]. Mode
mixing, increases the pump absorption considerably and restores
to large extent the absorption (in dB) linearity with length. How-
ever, at the 976 nm absorption peak the absorption still saturates Fig. 7. Main cladding-pumping schemes for (a)–(c) end-pumping, and (d)–(f)
with length, demonstrating that wavelengths with higher absorp- side-pumping.
tion require stronger mode mixing. Nonlinear pump absorption
affects the signal evolution and overall efficiency in fiber laser
signal core mode numbers. In the case of single mode core,
cavities [55]. More importantly, though, for fixed overall pump
Nco = 2.
absorption it results in highly non-uniform heat generation along
Fig. 6 plots the brightness enhancement factor in single-mode,
the fiber length, with most of the heat generated over short length
cladding-pumped fiber lasers and amplifiers as a function of the
at the launching side. Cladding pumping has also been imple-
cladding radius for different cladding NAs and λp = 945 nm
mented for power scaling in planar waveguide lasers [56]–[58].
and optical-to-optical efficiency of 75%. For commonly used
1) Brightness Enhancement in Cladding-Pumped Lasers: It
fibers with cladding radius between 65 and 250 μm and NA
has already been mentioned that cladding pumping combined
larger than 0.4 the expected brightness enhancement factor lies
with high output beam quality, in addition to power scaling,
between 103 and 104 .
provides extraordinary brightness enhancement. The brightness
2) Cladding-Pumping Schemes: Over the years, a number
enhancement factor is given by
of different cladding pumping schemes have been proposed in
2 V 2 pursue for higher fiber laser output power. These schemes can
Bsout λp cl 2
∗ Ncl
≈ ηoo be broadly classified into two main categories of end-pumping
Vco2
ηB = in = ηoo (2)
Bp λs Nco and side-pumping. Fig. 7(a)–(c) shows schematically the main
2
end-pumping techniques used to-date, while Fig. 7(d)–(f) shows
where Bsout and Bpin are the brightness of the output signal and the most prevalent side-pumping techniques.

input pump (see Appendix A), respectively. ηoo = Psout Ppin is Fig. 7(a) shows free-space geometric combination of pump
the optical-to-optical power conversion efficiency, where Psout modules [59], while Fig. 7(b) shows a scheme based on TFBs
and Ppin are the output signal and input pump powers, respec- [60], [61]. The number of combined free space or fiber-coupled
tively. Ncl (Nco ) is the total number of cladding (core) modes pump modules depends on the cladding diameter and NA and is
(including degenerate spatial orientations and orthogonal po- restricted by the étendue conservation principle (see Appendix
larizations). The maximum brightness enhancement factor is A). Fig. 7(c) shows an end-pumping scheme where the pump
simply proportional to the ratio of the cladding pump mode to modules are wavelength multiplexed, using a series of bulk-optic
[Fig. 7(f)], has the additional advantage of distributing the pump

power more uniformly along the fiber length, resulting in better
heat management [71].
3) Pump Combination Schemes for Cladding Pumping:
High-brightness, high-power pumping modules are key com-
ponents for the development of robust, high-power fiber lasers.
Pump brightness is a commodity that should always be spared
and used wisely. Once it has been compromised, it cannot be re-
covered by passive means. Single broad-area emitters launched
into MM fibers (typically 105/125 μm, and 0.22 NA) reduce to
∼1/100 of their initial brightness. After fast axis collimation,
this is primarily due to the large mismatch between the rect-
angular shape of the diode emitting aperture and the receiving
circular fiber. This mismatch is even more pronounced in the
case of diode bars.
Fig. 8. Pump combination modules for cladding pumping.
In order to fully utilize the brightness of pump fibers, a num-
ber of different techniques have been developed to re-organize
wavelength-division-multiplexing (WDM) couplers [20]. In ge- and aggregate the outputs of high power single-diode emitters,
ometric combination schemes [see Fig. 7(a) and (b)], the bright- as well as, minimize the in-between “dead” space of diode bars
ness of the combined pump module is usually lower than the or stacks and turn them into high brightness modules suitable
brightness of the contributing pump modules. It is equal in the for cladding-pumping high power fiber lasers. The pump com-
lossless case. In contrast, in wavelength-multiplexed schemes bination in most of the cases is achieved in two stages. It in-
[see Fig. 7(c)], since multiple aligned beams are superimposed, volves a relatively low count combined single-emitter diodes, or
the brightness of the pumping module is actually increased. It diode mini-bars (stage #1), feeding into a tapered multi-fiber
should be mentioned that, in this case, the spectral brightness bundle (stage #2). These can then be used for either end- or
(see Appendix A) of the module is reduced. This though is not side-coupling into the active fiber cladding, using one of the
a major handicap given that most dopants in silica fibers show schemes presented in Fig. 7.
large absorption bandwidths. The brightness of a combined multiple-pump module can
In the aforementionedend-pumping schemes the signal beam in some cases exceed the brightness of the individual pump
is usually intertwined with the pump-combiner optics. In high elements [72]. This can be achieved if mutual coherence is es-
power laser systems, the overlap of strong pump and signal tablished across the pump lasers and the output of the entire
beams increases the risk of bulk optics and fiber end-face fail- source behaves as a single spatial supermode. The resulting
ures. In the case of the fiber tapered bundle, special care should brightness can in this case be equal to the sum of the individual
be taken to minimize signal losses in the taper region. laser brightness. Another way to increase the brightness of the
Fig. 7(d) shows a side-pumping scheme based on total internal combined module is to use pump lasers with different eigen-
reflection taking place in a V-groove milled in the cladding [62]. properties, such as wavelength or polarization. Passive optical
Such an invasive approach, although employed in low pow- elements such as diffraction gratings or polarizing beam split-
ers [63], it is very difficult to be scaled to the currently achieved ters, respectively, can be used to multiplex several beams [72].
kilowatt levels. Fig. 7(e) shows a side-pumping scheme con- In the case of wavelength combination, the spectral brightness
sisting of an angle-polished [64], [65] or tapered [66] pump is reduced and these modules can only be used with active ions
fiber attached to or fused into the cladding of the signal fiber. with broad absorption spectra, such as Yb3+ around 940 nm.
The angle-polished fiber approach pump scheme is found not High brightness laser pump sub-modules capable of cou-
only to launch light into the cladding but also to leak out, which pling over 100 W of optical power into a 105 μm, 0.15 NA
leads to efficiency loss and compromises the laser integrity [67]. fiber at 976 nm have been demonstrated with N A < 0.13 and
Fig. 7(f) shows a side-pumping scheme based on a multi-fiber an electrical to optical efficiency >40% [73]. The pump sub-
assembly in optical contact surrounded and held together by a module brightness is ∼0.21 W/(μm2 sr). Such sub-modules
common low-index polymer cladding, applied the usual way have been spliced to a 7:1 fused fiber combiner, providing
during the drawing (trade name GTWave) [68], [69]. Remov- 500 W coupled into a 220 μm, 0.22 NA fibers. The resulting
ing part of the polymer over-cladding frees the individual fiber combined pump module brightness reduces to ∼0.086 W/(μm2
members, which then can be accessed independently. Such fiber sr). Commercially available pump modules using TFBs and ge-
assembly provides multiple ports for pump power to be injected ometrically combined single, large-area emitters can provide
into the cladding and be absorbed by the core of the optically 140 W in 106.5 μm, 0.22 NA fibers. The resulting brightness is
coupled signal fiber. A variant of this scheme uses multiple bare 0.1 W/(μm2 sr) [74]. Also, wavelength-beam-combined pump
pump and signal fibers held together by an external heat-shrunk modules, using diode stacks, have been demonstrated, which
tube [70]. In the side-pumping schemes, the signal path is kept provide 200 W at 91 Xnm in 200 μm/0.22 NA fibers [75].
separate from the pump injection paths. This simplifies the op- The resulting brightness is 0.04 W/(μm2 sr), less than half to
tical design and enhances the overall laser robustness. Finally, what has been achieved with geometrically combined single
the side-pumping scheme, based on evanescently-coupled fibers emitters. Seven such modules were then combined with a 7:1
tapered fused fiber bundle to provide ∼1.5 kW pump power in

400 μm diameter fiber.
Most of the measured pump insertion loss in combined mod-
ules is due to the brightness loss across the tapered fiber com-
biner. Therefore, designs closer to the brightness limit are con-
siderably more sensitive to variations of the input power distri-
bution as a function of NA [76] By proper design and optical
loss minimization in both directions, TFBs with kW-level power
handling capabilities are possible [75], [77], [78]. A number
of different pump modules, using different combination tech-
niques, have been developed. The choice of diode type (e.g.,
single emitter or bar size) and optimum arrangement (i.e., num-
ber of emitters and input NA) is finally influenced by the result-
Fig. 9. (a) Typical energy level diagram of Yb 3 + ions in silica, (b) typical
ing wall-plug efficiency, life-time, manufacturability and final emission and absorption cross-sections in aluminosilicate (thicker lines) and
cost. These considerations favour large area, single emitters and phosphosilicate (thinner lines) fibers (the arrow shows the peak emission and
small-size diode bars. absorption for phosphosilicate fibers).
In addition to MM TFBs, used extensively to combine MM
pumps, there have been demonstrations of SM-to-MM TFBs. Up
to three hundred 0.12 NA, 9 μm-core and 125 μm-clad single
mode fibers can be combined into a single output multi-mode
fiber with 0.15 NA, 105 μm-core and 125 μm-clad diameter [79].
The SM-to-MM TFBs can be used to combine the outputs of
high power SM fiber lasers into a scaled-up MM output beams
for industrial applications. It can also be used to combine short
wavelength, SM fiber lasers for cladding in-band (or tandem)
pumping [80] of other fiber lasers. This is the prevalent pumping
scheme in SM diffraction-limited fiber lasers with >3 kW output
power [30], [81], [82].
Finally, both MM TFBs and SM-to-MM TFBs have been
used together to demonstrate multi-kW fiber laser outputs [83] Fig. 10. (a) Typical energy level diagram of Tm 3 + ions (only lower levels
First, single-emitter diodes are combined by 91:1 MM TFBs are shown), and (b) typical emission and absorption cross-sections in alumi-
to produce 900 W pump modules, which are used to cladding- nosilicate fibers [96].
pump SM fiber lasers. Seven such SM fiber lasers were then
combined with SM-to-MM combiners to produce >4 kW MM
output beam. The limit in power scaling of pump and/or signal
rial [94]. Although phosphosilicate glasses reduce considerably
fused TFBs will be ultimately set by the NA and power handling
the emission and absorption cross-sections, they allow for much
capabilities of coating materials.
larger dopant concentrations, without significant clustering ef-
fects [95], and reduce or even eliminate photo-darkening effects
(see Section III-D). The simplicity of the Yb3+ energy-level
C. Active Ion Spectroscopy and Pump Wavelength Selection structure also eliminates other efficiency reduction effects, such
A number of rare-earth dopants have been incorporated suc- as excited-state absorption, multiphonon non-radiative decay
cessfully into optical fibers, using modified chemical vapor and concentration quenching.
deposition (MCVD) process [84], to form lasers. They in- Yb3+ shows a broadband absorption spectrum, extending
clude Nd3+ [85], Er3+ [86], Er3+ /Yb3+ [87], Yb3+ [88], from ∼850 to ∼1080 nm, enabling multi-pump or multi-
Tm3+ [89], and Ho3+ [90]. From the extended range of dopants wavelength pumping schemes, which in turn facilitate power
used in fiber lasers, we consider here only the Yb3+ and Tm3+ scaling. The broadband absorption spectrum also enables the
ions, which so far have shown excellent power scaling with use of unstabilized and low cost pumps, simplifying the design
output powers exceeding 1 kW (see Fig. 22). and reducing the overall cost and long-term stability of high
Yb3+ comprises a simple two-level system and provides effi- power fiber lasers. Interestingly, the small but finite absorption
cient lasing around the 1 μm window. Fig. 9(a) shows a typical in the 1010–1020 nm band enables in-band (or tandem) pump-
energy diagram of Yb3+ ions in silica, with indicative sub-level ing with high brightness fiber lasers, which is key for the power
Stark splitting. The exact sub-level splitting depends on the glass scaling to multi-kW levels [26], [30]. In addition, the broadband
composition and Yb3+ concentration [91], [92]. Stark splitting emission spectrum enables wide wavelength coverage and tun-
enables three- or four-level-system operation, depending on the ability, from 980 nm to about 1100 nm and short pulse (down
choice of pump and lasing wavelengths. Fig. 9(b) plots typi- to few 10 s of fs) amplification.
cal emission and absorption cross-sections in aluminosilicate The Tm3+ ion, on the other hand, shows a much more
and phosphosilicate fibers [93]. The emission and absorption complex energy-level structure. Fig. 10(a) shows the lower en-
spectra details and level lifetimes depend on the host mate- ergy levels of a Tm3+ ion. The diagram indicates the main
Fig. 11. Typical efficiency budget for Yb 3 + -doped fiber lasers.
ground-state absorption transitions and the most important,

in the context of this review, lasing transition around 2 μm.
Fig. 10(b) shows typical Tm3+ emission and absorption cross-
sections in aluminosilicate fibers [96]. The most technologically
important absorption bands are the one around 1600 nm, which
enables in-band pumping with high power Er3+ -doped fiber
lasers, and the 790 nm band, which can make efficient use of
available powerful diode pumps through the cross-relaxation
process [97]. Cross-relaxation creates two excited Tm3+ ions
in the upper laser level for every absorbed pump photon and can
potentially result in 100% optical-to-optical efficiency. Cross-
relaxation in Tm3+ -doped fiber lasers pumped at 790 nm has
resulted in record 74% efficiency and provides the root for effi-
cient power scaling [97]. Fig. 12. (a) Beam quality for different modes as a function of V number
of step-index fiber, (b) M 2 of highest-order mode (dashed line), average M 2
(assuming mode equipartition) (and measured M 2 for different V-number fiber
D. Fiber Laser Efficiency Budget lasers (experimental data taken from Ref. [103]).
Emission and absorption cross-sections define also to a large

extent the efficiency of a laser system. Along with the fiber
E. Beam Quality
parameters, they define the signal saturation energy and power
extraction efficiency. The choice of pumping wavelength defines Laser beam quality can be defined in a number of different
the fundamental heat dissipation limit through the quantum de- ways. The M 2 definition, based on the second moment of the
fect (pump/signal wavelength ratio), as well as, the absorption beam intensity profile, is the most commonly used method [98],
and total fiber length. Fig. 11 shows a typical power efficiency [99]. From a practical point of view, though, the “quality” of
budget for Yb3+ doped fiber lasers. In addition to the fun- a laser beam depends on the specific application for which the
damental loss due to the quantum defect, there are inevitable beam is intended for. The M 2 parameter denotes also how many
loss contributions from excess pump and signal losses and non- times faster the beam diverges compared to a diffraction-limited
optimized cavity. These losses can be minimized by proper Gaussian beam with the same waist diameter. Therefore, it can
choice of core and cladding materials and proper cavity design be defined as the ratio of the beam-parameter product (BPPB )
(choice of optimum reflector wavelength and strength). of the beam in question divided by the BPPG of a diffraction-
Losses due to the quantum defect depend on the choice of limited Gaussian beam, namely [99]:
pumping and lasing wavelengths. In the case of in-band (or
BPPB ωB θ B π
tandem) pumping, the quantum defect can be very small (∼1%) M2 = = = ωB θ B (3)
and the optical-to-optical output efficiency can be increased BPPG ωG θ G λ
considerably. However, if the conversion efficiency of the (fiber where ωB and θB are the mode-field radius and far-field diver-
or disc) pump laser is taken into account, the overall efficiency gence of the beam. For Gaussian beams M 2 = 1, while for any
drops again to the levels shown above. Nevertheless, in-band other practical beam M 2 > 1. The beam quality of a fiber laser
pumping is a powerful approach for effective heat management, output can be tailored by proper fiber design.
and power scaling SM fiber lasers beyond 3 kW relies almost Fig. 12(a) shows the M 2 variation, calculated by the sec-
exclusively on it. ond moment of the corresponding mode intensity profile, as a
function of the normalized frequency (V number) of a step- peak intensity, and pointing stability can occur by varying the
index fiber, for different modes. The M 2 of the fundamental relative phase of the constituent fiber modes [106], [111].
mode (FM) LP01 is shown to depart considerably from 1 for
V < 1.5. This is due to the increased evanescent extension into
the cladding and departure from the Gaussian profile. It fol- F. Fiber Types
lows closely the LP01 mode-to-core radius ratio variation with High power fiber lasers require the development of active and
V number, as approximated by Marcuse’s formula [100]. The passive LMA fibers, in order to reduce peak intensities and di-
same behavior is observed for all modes as they approach cut- minish nonlinear effects. In most occasions these fibers end up
off and their field extends deep into the cladding. The modes being multimoded, and they are operated in the SM or low-mode
LPm 1 , corresponding to skew rays or whispering gallery modes (LM) regime by introducing sufficient HOM differential losses,
with high orbital momentum, show considerably lower M 2 , without increasing significantly the FM loss. This becomes in-
which grows at a much slower rate compared to other LPm n creasingly challenging as the fiber dimensions increase and the
modes of the family characterized by the same composite mode mode effective index differences decrease. Additionally, in the
number p = 2n + m [101]. Within each p family, the LP0n presence of unavoidable small external perturbations, such as
modes, which correspond to meridional rays or modes with no fiber drawing or packaging-induced microbending, small modal
orbital momentum, show the highest and fastest growing M 2 . effective index differences enhance modal cross-coupling be-
This resembles the M 2 behavior of elegant Laguerre–Gaussian tween FM and HOMs. This results in efficiency and beam qual-
(LG) modes, compared with the standard LG modes supported ity deterioration.
by cavities with cylindrical symmetry [102]. In the case of 1) Active Fibers: Modality in MM fibers is generally con-
real fibers with complex refractive-index profiles (RIPs) the trolled by properly matching the FM at the input [112], launch-
mode intensity distributions differ from the corresponding mode ing through a mode-field adapter (MFA) [113], [114] and/or by
distributions of the step-index fiber and, therefore, show differ- properly bending of the fiber [115]. However, in the last case,
ent values of M 2 . care should be taken to avoid excessive bending, which exceeds
The M 2 of an incoherent MM beam is given by the weighted the safe bend-induced stresses level. This level depends on the
average of the M 2 of the participating modes [104]. In this case, fiber outer diameter. Depending on the core radius and NA in ac-
the centroid of the beam is stationary [105], [106]. Using simple tive fibers, bending does not always reduce modality or improves
arguments applicable to MM fibers [107], it can be shown that output beam quality. This is due to induced mode coupling,
the M 2 of the group of modes characterizedby the composite modal deformation and modal gain competition [116], [117].
mode number p is given by Mp2 = πrco θpout λ = πp/4, where Furthermore, in high-power lasers and amplifiers short LMA
θpout is the half-angle divergence of the mode in free space. active fiber lengths and small cladding diameters are needed
2
The weighted-average 2 < M > of a MM beam can then be in order to maximize pump absorption and minimize length-
approximated by M ≈ π (2pm ax + 1)/12 = π/12 + V /3, dependent nonlinear effects. However, small cladding diameter
where pm ax = 2V /π and both polarizations are counted [108]. and low-NA LMA fibers have the disadvantage of being ex-
The weighted-average < M 2 > is obtained when all supported tremely sensitive to external perturbations, which has adverse
modes are equally excited. For a given V number, the M 2 of the effects on efficiency and optical beam quality [112], [118].
highest-order mode group (p = pm ax ) can be approximated by Another issue associated with low-NA, LMA active fibers is
Mm2 ax = V /2. related to the uniformity of the refractive index across the core
Fig. 12(b) plots the weighted average < M 2 > and the Mm2 ax area. Making low NA, highly doped Yb3+ fibers requires high
of a step index fiber, as a function of the V number. It also phosphorous concentration to increase dopant solubility. This,
shows the measured M 2 of the output of kW-level fiber lasers in addition to increasing the core refractive index, results usually
as a function of the V-number of the active fiber [103]. In prac- in refractive index central dips due to uncontrolled phosphorous
tice, the number of modes and their relative power depends on evaporation in MCVD fabricated fibers. Such refractive index
the gain saturation level, their overlap with the active region feature can deteriorate the beam quality and power stability of
and effective modal reflectivities [109]. For relatively low V LMA fiber lasers.
numbers (V < 8) the lower-order modes dominate and the M 2 An alternative method has been demonstrated, which places
remains small. For higher V-number fibers, though, the higher- much less stringent requirements on the MCVD process. Solid
order modes (HOMs) attain substantial power and the output rod has been fabricated, with small index step and quasi-uniform
beam quality approaches the equipartition < M 2 > value. From doping to form the core region of a LMA photonic crystal fiber
the relations derived above and Eqn. (A-2), it can be deduced (PCF) laser, by repeated “stacking and drawing” [119] of Yb3+ -
that the brightness of a MM beam is inversely proportional to doped and undoped silica. The ensemble therefore can form an
the core V number squared. A similar relation (∝ 1/D2 ) has effective-index medium with V < 1.3. The composite doped rod
been predicted for MM VCSEL devices in the limit of large has been then stacked along with silica capillaries to form a PCF
D [110]. in the usual manner [120]. Solid active fibers with similar novel
A drastically different situation arises in the case of coherent core structures composed of small doped cores to give a LMA
MM beams in large-mode area (LMA) fibers. Even with large structure with a low effective core NA and without a central dip
HOM content (e.g., 30% LP11 ) the resulting M 2 can be decep- have also been demonstrated [121], [122]. In another approach,
tively low (M 2 < 1.1), depending on the relative phase between based on PCF technology, by adjusting the hole size and spacing
LP01 and HOMs. However, significant changes in beam shape, of the air-holes around the LMA doped core, effectively single
mode operation has been achieved for core diameters up to

100 μm [123].
LMA fibers have been demonstrated using solid-core PCF
technology utilizing the “modal sieve” effect [124]. LMA oper-
ation has been extended even further with very large-mode-area
(VLMA) fiber designs using large-pitch PCFs (LPFs). These
fibers use claddings with hole-to-hole spacing of ∼10–30 times
the operating wavelength. The aforementioned designs offer
different degrees of HOM leakage or delocalization into the
cladding [125], [126]. Compared to LMA PCFs, LPFs relax con-
siderably the fabrication tolerances and have resulted in record
core diameters of 135 μm and mode field diameter (MFD) of
∼130 μm in passive operation. In order to avoid bend-induced
MFD collapse, such rod-type fibers have to be kept straight dur-
ing operation. However, under high power operation thermally-
induced waveguide changes have been observed in Yb3+ -doped Fig. 13. Nonlinearity enhancement factor as a function of amplifier gain, for
LPF, resulting in substantial fundamental MFD reduction and L 0 = 15 m and NA = 0.1.
increased modality [127].
An alternative LMA fiber design is based on the chirally-
coupled core (CCC) concept, which provides resonant filtering fibers (LCFs) have been demonstrated using a small number of
of HOMs and enables effective SM index-guiding. Single-mode air holes inserted in the silica cladding, providing differential
CCC fibers have been produced with core sizes exceeding stan- loss for the HOMs. LCFs with core diameters in the range of
dard 50 μm [128]. Being resonant thought their length, CCC 170 μm have been demonstrated, with effective areas in excess
LMA fibers require tight fabrication tolerances. of 10 000 μm2 [134]. In another approach, modal filtering is
Finally, LMA MM active fibers can be operated effectively in achieved by inserting small cores around the main MM core,
the SM regime by tailoring the dopant distribution inside the core which are resonant to the HOMs. The HOMs are then leaking
to provide gain and favor predominantly the FM [129]. Fiber out of the main core through evanescent coupling [135].
lasers based on gain-guided, index antiguiding mechanism are Lately, state-of-the-art hollow microstructured fibers have
another interesting approach to achieving LMA operation and been used for high peak power delivery. In such fibers optical
power scaling [130]. These waveguides, though, are fundamen- nonlinearities are contained effectively, since the vast majority
tally leaky and additional measures should be taken to manage of the power (>99%) is guided in air. Hypocycloid core Kagome
the leaking power during high power operation. lattice microstructured fibers have been used for delivering high
2) Passive Fibers: LMA passive fibers play an important beam quality, high peak power 500 fs, 1 mJ pulses over 10 m
role in the construction of high performance, high power fiber length, without beam quality degradation. The energy threshold
lasers, particularly in delivering efficiently the generated power damage was found to be >10 mJ, with output power density af-
into the work-piece. In some applications, delivery fibers up ter focusing approaching TW/cm2 . This opens up the possibility
to 10–20 m long are required. Optical power LMA delivery of using such fibers for practical high peak power delivery [136].
fibers should be designed properly to avoid excessive spectral
broadening and onset of temporal instabilities due to non-linear
interactions, such as stimulated Raman scattering (SRS), stim- III. NONLINEAR AND OTHER PARASITIC EFFECTS
ulated Brillouin scattering (SBS), self-phase modulation (SPM) Most of the fiber nonlinearities are χ(3) based and are in-
and four-wave mixing, as well as, beam quality degradation due tensity and fiber length dependent. Hence they become more
to modal scrambling. severe in pulsed, high peak power and CW kW-level oper-
Standard step-index (SI) fibers with core diameters of ation. Despite the very small nonlinear coefficient in silica
∼30 μm and effective areas ∼360 μm2 can be SM at a 5 cm (n2 = 3.2 × 10−16 cm2 /W)), due to the high intensities and
bending radius with NA of ∼0.06, which is considered to be the lengths involved, the nonlinearity enhancement factor (NEF),
lowest limit that can be achieved repeatedly and it is manufac- which compares the strength of the nonlinear interaction in
turable with current fiber fabrication techniques [131]. When fibers with that of a focused beam in bulk glass (see Appendix
kept straight, SI fibers can maintain single mode operation for B) can take quite large values.
diameters below ∼15 μm [12]. Fig. 13 plots the NEF, given by Eqn. (B.3), as a function
A number of different fiber designs have been proposed and of amplifier gain, for different V numbers. For λ = 1μ m, a
experimentally demonstrated, which extend the FM area well fiber length L0 = 15 m and NA = 0.1, the obtained NEF is
beyond the SI capabilities. These fibers are fundamentally mul- ∼105 . This actually makes optical fibers one of the highest
timoded but utilize different techniques to filter out HOMs. An nonlinearity media. It is shown that the NEF decreases with
early example is the W-type fiber, which uses a refractive-index gain. This is because the power distribution varies significantly
dip around the core to drive the second-order mode beyond cut- along the length for higher gains. Also, NEF decreases with V
off into leakage [132] or LMA segmented-cladding fibers, which number, because in this case the FM effective radius increases.
leak out HOMs [133]. Recently more advanced leakage-channel For V > 10, the NEF reduces by about one order of magnitude.
From Eqn. (B.3) to (B.5), it can be shown that the NEF is

inversely proportional to wavelength, and therefore, moving to
λ = 2μ m is expected to half the nonlinearity strength.
Nonlinear effects are one of the most limiting factors in
scaling-up the power in fiber lasers. In general, they transfer
energy in unwanted spectral regions and can potentially desta-
bilize laser operation. They can be reduced significantly by
special fiber designs and/or appropriate spectral filtering.
A. Stimulated Brillouin and Stimulated Raman Scattering

SBS and SRS are related to inelastic nonlinear processes and
involve power interactions with acoustic and optical phonons,
respectively [137].
1) Stimulated Raman Scattering: Under controlled condi-
tions and special fiber designs, both effects can be either min-
imized or used effectively to enhance wavelength coverage of
available fiber laser output spectrum. In high peak power pulsed
lasers, SRS can be minimized by employing short lengths of
phosphosilicate fibers [138]. Fibers with wavelength-selective
transmission can suppress the Stokes wave of Raman scattering
and result in length-independent nonlinearity threshold, which
could be particularly advantageous for high power lasers and
fiber beam delivery in material processing applications [139],
Fig. 14. Output power overshoot (relaxation oscillation) of a pump modulated
[140]. Residual backreflections at splices or fiber ends can de- fiber laser. (a) Total forward power (signal + SRS), Signal and SRS traces
crease considerably the SRS threshold [20]. resolved, (b) total forward power (signal + SRS), forward signal and backward
SRS is in general a non-catastrophic effect, resulting in only SRS traces.
in power transfer to longer wavelengths. Such spectral broad-
ening can complicate the design of focusing optics and result in dex contrast of 0.09 and acoustic index slope of 0.01/μm [148].
effective focal shifts that can compromise processing capabili- It is shown that SBS can be effectively suppressed by broadening
ties. In some cases, the presence of strong forward and backward the signal linewidth to a value above 0.07 nm [149].
SRS can destabilize fiber laser cavities. SBS in an Yb3+ -doped double-clad pulsed fiber amplifier
Fig. 14(a) shows the output power overshoot (relaxation os- with multi-ns-duration can break-up the original pulse and pro-
cillation) of a pump-modulated fiber laser. By inserting appro- duce high peak-power sub-pulses. Fig. 15(a) shows three differ-
priate spectral filters, the total forward propagating beam can be ent cases of backward-propagating 1st-Stokes SBS measured
analyzed into its signal content (centered around 1070 nm de- at the input of a ns pulsed fiber amplifier. SBS is stochastic in
termined by FBGs) and the forward propagating SRS (centered nature and the backward SBS pulses are usually characterized
around 1130 nm). The SRS threshold is shown to be ∼250 W. A by a sharp (∼10 ns) spike followed by a longer tail. Fig. 15(b)
small ripple starts appearing after the first overshoot. Fig. 14(b) shows the corresponding amplified forward pulses with 200 ns
shows the output overshoot at higher pump power. In addition duration and output peak power of ∼15 kW. It is shown that
to forward signal and SRS, it plots the backward SRS, measured strong backward SBS excitation results in forward pulse distor-
on the HR side of the cavity. It is shown that the first overshoot is tion [150], as power is transferred into acoustic waves, as well
terminated abruptly by a sharp, high peak power backward prop- as, generation of forward propagating 2nd Stokes, appearing
agating SRS. This is then followed by a secondary relaxation as superimposed sharp ∼10 ns spikes. Generation of forward
oscillation, modulated at the cavity roundtrip. This behavior is propagating 2nd Stokes is usually followed by optical damage
repeatable and predictable, occurring each time there is strong and catastrophic fiber failure (see Section IIC).
enough backward SRS. In MM fibers, in addition to normal backward SBS, SBS
2) Stimulated Brillouin Scattering: Techniques to suppress in a forward direction (FSBS) has been observed, transferring
SBS, while maintaining FM operation, include increasing the power between LP01 and LP11 forward propagating modes.
mode area with appropriate NA reduction [141], using fibers FSBS is possible because although the overlap between flexural
with tailored acoustic speed profiles [142], increasing the ef- fiber modes and the light is small, the phonon lifetime is much
fective linewidth via phase modulation [143]–[145], laser gain longer than in conventional SBS. Unlike in normal SBS, FSBS
competition [146], and using highly doped fibers to absorb the does not depend significantly on the laser linewidth, and may
pump light in a short length. Self-heating and strong temperature also be the first example of a nonlinear effect, which for a
gradients due to pump absorption can contribute to substantial given power is actually enhanced by increasing the optical mode
SBS threshold increase [147]. 11.2 dB suppression of SBS in an area [151]. FSBS can take place in both active and passive fibers
Yb3+ -doped, Al/Ge co-doped LMA gain fiber is demonstrated and can transfer power from the FM into HOMs and potentially
with a ramp-like acoustic index profile exhibiting an acoustic in- destabilize the output beam.
Fig. 16. SBS induced damage.
mitigated just by scaling the mode size. For 1060 nm operation,

SF in silica fibers occurs at a power of ∼4–5 MW [160] and,
therefore, it can only be relevant to ultrashort pulse propaga-
tion [161]. Surprisingly, transmission of powers of ∼20 MW,
much higher than the widely accepted limit, have been reported
using highly MM fibers [162]. However, fiber core design can
affect the SF threshold. Numerical results suggest that optical
fibers with a strong central dip at the center of the refractive index
profile can guide stable fundamental modes at more than 10x the
bulk silica critical power for SF [163]. SF threshold increases
to ∼6–8 MW with the use of circularly-polarized light [164],
[166]. SF threshold has a wavelength squared dependence and,
therefore, operating at 2 μm results in quadrupling the afore-
mentioned thresholds. The onset of SF can potentially result in
Fig. 15. (a) Backward-propagating 1st-Stokes SBS measured at the input of spatial beam collapse and optical damage.
a ns pulsed fiber amplifier, (b) corresponding amplified forward pulses with
forward propagating second Stokes.
C. Optical Damage
Optical damage for ns and sub-ns pulses is a catastrophic ef-
B. SPM and FWM fect associated usually with electron avalanche effects [164],
[165]. Damage initiates if the electron density exceeds 2 ×
A direct consequence of the Kerr effect is the nonlinear intra-
108 μm−3 , beyond which the plasma frequency approaches the
pulse phase shift, which results in SPM and equivalent fre-
optical frequency and the propagating light is strongly absorbed.
quency shifts. The SPM-induced spectral broadening depends
The deposited energy is then sufficient to melt or fracture the sil-
on the pulse shape and it is more pronounced for pulses with
ica glass. For pulses longer than ∼50 ps the bulk optical damage
steep leading and trailing edges. SPM is a limiting factor in
irradiance is found to be constant at ∼ 4.75 kW/μm2 , which
short-pulse energy scaling when using phase locking or coher-
makes the threshold fluence linearly-dependent on the pulse du-
ent combination of multiple lasers.
ration. For shorter pulses, the electron avalanche effects evolve
Four-wave mixing (FWM) in fibers is an elastic χ(3) nonlin-
slower than the pulse envelope, so that the threshold fluence
ear process involving two pump photons, which annihilate to
increases and departs from the above linearity. Preliminary re-
create one Stokes and one anti-Stokes photon with frequencies
sults indicate that Yb3+ doping does not affect appreciably the
defined by the energy conservation principle. FWM is a coherent
aforementioned optical damage thresholds [164]. By proper pol-
process and its efficiency depends critically on the exact phase
ishing of end-faces, surface damage is measured to be equal to
matching between the waves involved. In high power fiber lasers
the bulk value [165].
and amplifiers the signal beam serves as the FWM pump and
Optical damage of a different type is associated with the onset
increases exponentially along the length. This results in sub-
of strong SBS in pulsed fiber lasers or self Q-switched highly
stantial FWM generation despite the phase mismatching [152],
inverted fiber amplifiers. This damage is caused by internal
[153]. FWM generation is also enhanced in birefringent or MM
stresses induced by the acoustic waves generated by the SBS
fibers, since phase matching is greatly facilitated by the fact
material/light interaction. Fig. 16 shows SBS induced damage
that the Stokes and anti-Stokes beams can propagate in different
in the core and cladding a pulsed fiber amplifier. The fiber has
fiber modes with the appropriate group velocities [17], [154].
turned into white powder over a ∼2–3 cm length, while the
Instead of being always parasitic, SRS and FWM can co-
coating remains intact. This damage mechanism is different to
operate and be beneficial in specially designed fibers for certain
the optical fuse effect [167], and appears to be similar to the fast
applications, such as efficient supercontinuum generation. High
optical discharge mechanism observed in fibers [168].
peak powers enhance the spectral broadening via FWM and
Raman shifting and result in record bandwidth and spectral
D. Photodarkening
density supercontinuum sources [155]–[157].
The ultimate limit in high power can be set by self-focusing High power fiber lasers based on Yb3+ -doped silicate glasses
(SF) in fibers [158], [159]. SF is the only nonlinearity that de- are known to suffer from light-induced optical losses, known as
pends on power rather than intensity and, therefore, cannot be photodarkening (PD). The optical losses are believed to be due
Fig. 17. Power evolution with time in high-power Yb 3 + -doped cladding-

pumped fiber lasers. [multimode (MM), single-mode, diode pumped (SM - Fig. 18. Brightness enhancement in high-power Yb-doped cladding-pumped
DP), single-mode, tandem pumped (SM - TP), and coherent beam combination fiber lasers. [single-mode (SM), multimode (MM) - direct diode (DD) is shown
(CBC)]. also for comparison].
to formation of color centers in the glass matrix, which in- SM outputs. It has been predicted that if the fiber’s MFD could be
crease the background loss and reduce the output power. The increased arbitrarily, about 36 kW of diffraction-limited power
exact color-center formation mechanisms are still under debate. could be obtained from single fiber lasers or amplifiers. This
There have been a number of studies showing that the PD rate power limit is imposed primarily by thermal and SRS effects,
and saturated level are dependent on the Yb3+ inversion [169], and does not take into account modal instability effects, which
[170]. The order of the inversion dependence, however, varies can reduce it considerably [180]. As already mentioned, scal-
between 3.5 [169] and 7 [170]. It has also been observed that ing the SM fiber laser power above 3 kW requires in-band (or
photodarkening in LMA fibers is non-uniformly distributed over tandem) pumping to reduce the thermal load on the final power
the fiber cross-section [171]. The PD-induced loss is also non- amplifier, which has resulted in SM output of 20 kW [181]. It
uniformly distributed along the length of the active fiber [172], has been predicted that in-band pumping can extend the SM
following closely a dependence to the calculated Yb3+ inversion operation to about 70 kW [182]. Scaling the power to such high
to the power of 2. It is shown that increasing (decreasing) the SM levels involves very large diameter fibers and stable SM
operating temperature results in decrease (increase) of the laser operation will prove quite challenging. It is conceivable that
output power, reaching the new equilibria over time scales of robust fiber lasers can reach ∼25 kW quasi-diffraction-limited
∼200 h [172]. However, the occurrence of PD is entirely depen- SM output.
dent on the materials composing the fiber core [173]. The Yb3+ Fig. 17 also includes the case of MM outputs obtained with
concentration and co-dopants such as aluminum [174], [175], geometric incoherent combination, currently reaching the 100
phosphorous [176] or cerium [177] can reduce significantly or kW level [181]. Such power levels promise even higher fiber
even eliminate PD. Photodarkening has also been observed in laser penetration into the industrial and directed energy appli-
Tm3+ doped fibers [178], [179]. cation space. Fig. 17 finally includes the power evolution of
single beam, near diffraction-limited output obtained with co-
IV. HIGH POWER FIBER LASERS herent combination of SM fiber lasers [183]–[187]. Currently,
coherently-combined fiber lasers have demonstrated multi-kW
In the last decade, fiber technology has grown quite diverse
quasi-Gaussian outputs [186], [187]. Coherent combination dis-
and mature and can provide an excellent platform for fabricating
tributes effectively the optical gain and thermal load among
robust, high performance laser systems. The core and cladding
several contributory fiber strands and can break the vicious
structures can be tailored appropriately to control the beam
circle of increased-power/increased-heat-generation/increased-
modality, optical nonlinearities and scale-up the power.
nonlinearities and potentially provide high quality output beams
with power increased well above the current single fiber strand
A. Single Fiber, Single-Mode Continuous-Wave (CW) Output
limits.
In addition to fiber technology advancements, power evolu- In addition to raw power scaling, one of the most impor-
tion in fiber lasers with near diffraction-limited output has fol- tant characteristics of diode-pumped fiber lasers is the achieved
lowed and depended critically on the maturity of the pumping brightness enhancement. The generated laser output beams are
technologies, and their progress from low brightness diodes to characterized by much higher brightness than that of the pump
combined high-brightness diode modules, and lately to in-band sources. Fig. 18 compares the brightness of SM and MM fiber
tandem pumping [30]. lasers and the corresponding diode pump modules [188] as a
Fig. 17 shows the power evolution of single mode (SM) function of power. It is shown that the experimentally observed
near diffraction-limited Yb3+ -doped fiber lasers, when entirely SM output brightness increase in the range of 10+3 –10+4 is in
diode pumped (SM-DP) and with the final amplifier in-band very close agreement with the theoretical predictions shown in
tandem pumped (SM-TP). Introduction of high brightness diode Fig. 6. The SM fiber brightness increases monotonically with
pump modules after year 2000 has resulted in a fast increase of the output power, although it is showing signs of saturation.
Power scaling in direct diode (DD) output, on the other hand,

relies on various geometrical beam shaping techniques and the
resulting brightness decreases with power. Due to the geomet-
rical incoherent combination, MM fiber laser outputs show a
brightness increase ∼1–2 orders of magnitude smaller, almost
independent of the power level.
B. Pulsed Fiber Laser Parameter Space

Although fiber lasers are ideal for average power scaling, it is
generally perceived that they suffer in terms of energy storage
and peak power handling. However, even in this front, progress
in fiber technology has enabled substantial improvements in
pulsed fiber laser performance and has resulted in increased
penetration into the industrial and scientific application space.
Q-switched fiber lasers have been used extensively in the
field of low-cost laser marking [7]. They combine configura-
tion simplicity and substantial pulse energies [189]–[194]. As Fig. 19. Nanosecond pulsed fiber laser performance parameter space.
a practical approximate rule of thumb, the extractable energy
from a fiber laser or amplifier is limited to about ten times the
saturation energy [190] and can be controlled by fiber design.
The pulse duration in Q-switched fiber lasers is directly propor- 4.5 MW and near-Gaussian, single-transverse-mode profile of
tional to the round trip time of the laser cavity. It also depends M 2 ∼ 1.3, using a 100 μm-core rod-like PCF used as the final
on the inversion level and it reduces with increasing inversion amplifier [200].
levels and, therefore, increasing small-signal gain. Q-switched Fig. 19 shows that high performance nanosecond pulsed fiber
fiber laser has been demonstrated producing pulse durations lasers with SM and LM output beams have shown output pow-
well below 10 ns by using a short length Yb-doped rod-type ers below 250 W. In order to circumvent this limitation spectral
photonic crystal fiber as gain medium. Pulse energies up to 0.5 beam combination has been used to achieve 1.1 kW, 5 ns pulsed
mJ and average powers in excess of 30 W have been obtained laser [201]. Spectral and coherent beam combination (CBC)
in single-transverse mode beam quality, at repetition rates up will be required to extend the SM/LM nanosecond fiber laser
to 100 kHz [192]. At low repetition rates, below the ytterbium performance to the high average/high energy parameter space
inverse fluorescence lifetime, the generated ASE can reduce the region (highlighted in Fig. 19). Such beam combination tech-
inversion and limit the pulse energies. Optimized pump modula- niques will be necessary to avoid modal instability issues, likely
tion can minimize the effects of intra-pulse ASE and maximize to be encountered in this regime (see discussion below).
the extractable energy [193], [194]. Ultrashort pulse lasers have opened up new scientific and in-
MOPAs based on a diode-seeded nanosecond fiber system dustrial application areas, such as time-resolved material and
offer adaptive pulse shape control [195], [196] that can cover chemical studies, nonlinear microscopy, metrology and preci-
an extremely large range of pulse duration (from ps to CW) and sion material micro-machining. Compared to solid-state crys-
repetition rates (from pulse-on-demand to MHz). They combine tal counterparts, fiber lasers are characterized by broad emis-
the fast dynamics and turn-on characteristics of semiconductor sion spectra and offer themselves for ultrafast tens-of-fs op-
lasers and the high gain, high average power capabilities of fiber eration [14], [202]–[205]. Chirp and nonlinear propagation in
amplifiers, resulting in high performance pulsed laser systems fibers can also be combined to achieve even shorter pulses [206].
with energies and beam qualities suitable for a number of diverse Lately, significant power scaling in superfast pulse fiber lasers
applications, such as marking and material micro-processing has been achieved based on the powerful chirped-pulse amplifi-
[197], [198]. cation (CPA) technique [207] and special fibers, such as the ones
Fig. 19 summarizes the progress in peak power, pulse en- using rod-type PCF fiber technology [13], [204], [205]. Com-
ergy and average power achieved by nanosecond pulsed fiber pared to diode-pumped solid-state crystal and thin-disc lasers,
lasers within the last decade. The main results are catego- fiber ultrashort pulse lasers offer superior thermo-mechanical
rized in terms of beam quality: single-mode (SM–M 2 < 1.5), behavior and more robust operation. They offer the potential for
low-moded (LM–M 2 ∼ 3) and multimoded (MM–M 2 ∼ 6–8). highly integrated and passively cooled ultrashort pulse sources.
Most of the record results presented in Fig. 19 have been ob- Fig. 20 summarizes the progress in pulse energy and av-
tained using large core rod-type fibers and SM to LM output erage power achieved ultrafast (ps and fs) pulsed fiber lasers
beams. Q-switched 60 ns pulses with 26 mJ pulse energy and within the last decade. High average powers, in excess of 800 W,
near diffraction-limited beam quality (M 2 < 1.3) with average and moderate pulse energies (Ep ≤ 10 μJ) have been demon-
output power of 130 W and peak power of 500 kW is achieved, strated [204]. This was achieved by using 27 μm) MFD pixilated
using a large-pitch fiber with a core diameter of 135 μm [199]. core [120] and 500 μm outer diameter JAC fibers. At the other
A micro-chip seed, Yb-doped fiber MOPA produced 1-ns-long, extreme, appreciably high energies (>2 mJ) and moderate av-
4.3 mJ pulses, with average power of 42 W, peak power of erage powers (>10 W) with record (∼4 GW) peak power have
Fig. 21. Transverse mode instability threshold versus nominal core diameter.
(PCF-LPF#1&#2: [211]- [214], DMF#1-#3: [215], PCF-B1: [216], PCF-
B2: [217], PCF-SAT: [218], SI-MC: [204], (SI-NS): [210], SI & SI-GT: [219],
PM25: [220]).
Fig. 20. Ultrafast pulsed fiber laser parameter space.

fibers, with and without gain tailoring (SI and SI-GT: [219]),
as well as, polarization-maintaining step-index fibers (PM25:
been achieved [208], using large pitch PCF fibers with 105 μm [220]). PM25 is monolithic, fully-spliced amplifier, in contrast
MFD. with all the other configurations, which are free-standing and
Scaling up the average power requires special fibers with in- end-pumped.
creasingly large diameters. However, exceeding 100 W in the Fig. 21 shows that the TMI threshold generally decreases
high pulse energy regime (Ep ≤ 10 μJ) [209] or 1 kW in the with the nominal core diameter, irrespective of the fiber type
lower energy regime (Ep ≤ 10 μJ) [204], delineated by the used. TMI threshold, however, has been observed to be affected
dashed line, the power fiber amplifier operation is severely lim- by a number of other amplifier characteristics. In the case of
ited by TMIs [204], [211], resulting in significant output power DMF [215], it is shown that HOM excess leakage results in
variations and beam pointing drift. The limit is established ex- approximately double the TMI threshold. Also, in the case of
perimentally and it is related to the fiber effective core diameters. SI-GT [219], gain tailoring results in substantial (∼ x3) TMI
This currently appears to be a hard limit and intensive research threshold increase. However, gain tailoring is not as effective
is underway to understand the root cause and develop robust in the case of PCF-GT [214]. In the case of PCF-SAT [218],
solutions to the modal instability effects [210]–[229]. the TMI threshold has been shown to increase substantially by
efficient fiber cooling. Finally, in the case of PM25 polarization-
maintaining fiber it is observed that increasing the input power
C. Transverse Mode Instabilities and pumping the amplifier off the absorption peak results in
In high power LMA fiber amplifiers, TMI manifests itself as almost double the TMI threshold. Interestingly, in this case,
sharp and rapid output beam profile deterioration above an out- a similar non-PM fiber shows no TMI instability up to 1 kW
put power threshold. For near single fundamental-mode input level [220]. It should be also remarked that an all-fiber, spliced-
excitation, the output beam shows large content of and strong up and coiled amplifier with core diameter of 20 μm showed
competition with HOMs (dominantly LP11 mode), after a cer- no signs of TMI for >2 kW output power [230]. To the best of
tain threshold has been reached. A number of research groups our knowledge, so far there has been no TMI observed in fiber
have recently reported on TMI effects in PCF and LPF high amplifiers with core diameter smaller than 20 μm. Finally, It
power Yb3+ -doped fiber amplifiers [211]–[218]. Lately, TMI should be mentioned that due to lack of experimental details,
has been observed also in standard solid-core LMA fiber ampli- it is not clear if all the observed TMI effects follow the same
fiers [219], [220]. evolution patterns [213], or are all due to the same root cause.
Fig. 21 plots the TMI threshold power as a function of the Different theoretical models have been proposed for the root
active fiber nominal diameter. The highest TMI thresholds to cause of the observed TMI effects [221]–[229]. All models con-
date has been obtained with actively cooled, step index fiber of sider thermo-optically induced refractive-index gratings as the
22 μm [210] and ∼30 μm core diameter (SI-MC) [204]. The main mechanism of forward mode coupling. The instability has
fibers had micro-structured doped core to achieve low NA and been attributed to either stimulated thermal Rayleigh scatter-
was bent to preferentially attenuate HOMs. Fig. 21 includes also ing (STRS) [221], [223]–[227] or other thermal mode coupling
results obtained with short length, straight rod-type PCFs and effects [222], [228], [229]. So far, the proposed models have
LPFs (PCF-LPF#1 & #2: [211]–[214], DMF#1-#3: [215]), as had different degrees of success in predicting mode coupling
well as, longer bendable PCFs (PCF-B1: [216], PCF-B2: [217], and instabilities. However, to the best of our knowledge, there
PCF-SAT: [218]). It also includes more conventional step-index has been limited success to quantitatively predict instability
TABLE I
COMPARISON OF DIFFERENT HIGH POWER LASER TECHNOLOGIES.
Fig. 22. Fiber laser wavelength coverage and maximum output power achieved
to date.
evolution and dynamics. This is not surprising, given the lack of

experimental details and other uncertainties regarding published
data. Specially designed experiments are required to prove or
disprove proposed theories.
VI. INDUSTRIAL FIBER LASERS AND APPLICATIONS
Industrial high power fiber lasers are almost exclusively based
V. OTHER FIBER LASERS on all-fiber monolithic configurations, exploiting the excellent
Fiber lasers can cover an extended range of wavelengths by power scaling capabilities of fiber amplifiers in MOPA con-
simply doping the core with different active dopants. Fig. 16 figurations. In such monolithic configurations, a high power
shows schematically the wavelength ranges offered by dopants all-fiber laser seed is followed usually by one matched, low-
such as Nd3+ , [231], [232] Er3+ /Yb3+ [233], [234] and Tm3+ gain and well saturated fiber amplifier [23]–[30]. Additionally,
[235]. Power scaling in Nd3+ and Er3 +/Yb3 + doped fiber lasers hybrid high power fiber laser systems have been demonstrated,
has been severely hampered by the relative large quantum de- using free-space optics for pump coupling and laser cavities
fect and excessive thermal management requirements. Tm3+ - with mirrors butt-coupled or directly deposited onto the fiber
doped fiber lasers operate around the eye-safe 2 μm region and facets [22].
have been scaled up to kW level and appear to be promising All-fiber MOPA configurations offer the possibility of using
for new directed energy and industrial applications [236]. A multiple pump power injection points and therefore distribute
monolithic, robustly single-mode, resonantly cladding-pumped evenly the pump absorption and thermal load, providing service-
Ho3+ -doped fiber laser producing more than 400 W of out- free, reliable industrial laser systems.
put power in the 2.05–2.15 μmwavelength range [237]. Using Table I shows a comparison of the wall-plug efficiency
nonlinear processes such as SRS, high power lasers emitting in (WPE), expected lifetime, maintenance requirements and fiber
the spectral regions shown in Fig. 22 can be used as pumps and, delivery capabilities for the main industrial high power laser
combined with properly designed and optimised fibers, can offer technologies. It is shown fiber lasers outperform all other tech-
substantial power in almost any spectral region in the 1–2.5 μm nologies combining record >30% WPE, 100k hours lifetime
span. (defined by the single-emitter (SE) diode pumps) and mainte-
Fiber lasers have also been used as seeds to produce high per- nance free operation.
formance supercontinuum sources [238]. CW supercontinuum Fig. 23 compares the typical beam quality, quantified by the
generation extending to the visible spectral region has been beam-parameter product (BPP = ω0 θ0 ) variation with output
demonstrated by pumping photonic crystal fibers at 1.07 μm power for the most common laser applications in material pro-
with a 400 W single mode CW Ytterbium fiber laser. The con- cessing and manufacturing to date. It also superimposes the
tinuum spans over 1300 nm with average powers up to 50 W and BPP and power requirements for the main materialprocessing
spectral power densities over 50 mW/nm [157]. High-energy laser applications. The contours show parameters averaged over
pulsed supercontinuum spanning the 450–1750 nm region with different operational conditions and materials. These applica-
energy spectral density in excess of 1 nJ/nm in the visible, suit- tions are based primarily on thermal processes, such as heating,
able for STED microscopy, has also been generated [156]. melting and vaporization. In addition to replacing traditional
Fiber distributed-feedback (DFB) lasers [239]–[241] can pro- mechanical or chemical techniques, lasers have also enabled a
vide high purity, single frequency, single polarization [242], low number of novel processes. Laser cutting, for example, allows
phase noise seeds ideal for advanced high power MOPA config- repeatable high-precision patterns at high speeds that cannot be
urations, sensors [243] and other applications [244]. It should produced via conventional methods. Also use of lasers enables
be mentioned that in the highest power laser MOPA system, welding of dissimilar materials, like steel and aluminum, which
demonstrated to date in the world at the National Ignition Facil- is of growing interest to shipbuilding and car industries and
ity, the only laser used is actually a fiber DBF laser [245]. known to be impossible in conventional welding.
come more challenging to achieve stable focal position of the

focused beam at the work-piece, because of the time dependent
thermal lensing and optical distortions in the transmitting optics
inside the laser processing head [188], [246]. It is rather ironic
that although eliminating the thermal lensing from the laser cav-
ity is one of the major contributors to scaling-up the diffraction-
limited fiber laser output, this deleterious effect “sneaks” back
into the processing head optics. To gain the full potential and
make efficient use of high power fiber lasers with diffraction-
limited beam quality new processing heads have been devel-
oped [188], [247], [248].
VII. SUMMARY—FUTURE PROSPECTS

Fig. 23. BPP and average power requirements for laser applications & BPP Fiber lasers have come of age and are currently fast increasing
versus average output power for main industrial lasers [4].
their share in the industrial applications market. They uniquely
combine high average powers, unparalleled beam quality, small
In automotive industry, welding car bodies, transmission and footprint, and record efficiencies. They gradually replace con-
engine components, air bags, exhaust systems, etc., are now ventional laser technologies offering substantially lower cost
made robotically using laser systems. In healthcare, lasers are of ownership, higher processing speeds in existing applications
used for welding deep brain stimulator implants, pacemakers while enabling new ones.
and prosthetics. In electronics industry, for drilling and cutting In this paper, we have summarized the fundamental properties
of printed circuit boards. In photovoltaics for scribing, drilling and reviewed the latest developments in high power ytterbium-
and cutting of Si-wafer, ablation of conduction or dielectric doped fiber lasers. The review has been focused primarily on the
layers of thin film solar and crystalline Si solar cells. More main fiber laser configurations, used in industrial applications,
recently in additive manufacturing, 3-D rapid prototyping and have considered issues related to cladding pumping. Special at-
manufacturing by selective sintering, melting and 3-D cladding tention has been placed on pump combination techniques and
directly from CAD files is enabled by lasers. Rapid prototyping the parameters that affect the brightness enhancement observed
has evolved from polymer components to tool-free rapid manu- in high power fiber lasers. The review also included the major
facturing of high quality metallic parts using materials, such as limitations imposed by fiber nonlinearities and other parasitic
titanium, aluminum and cobalt chrome powders. effects, such as optical damage, TMI and photodarkening. The
So far most of the industrial applications are based on lasers paper finally summarized the power evolution in continuous-
operating predominantly in CW or relatively long pulse mode. wave and pulsed ytterbium-doped fiber lasers and highlighted
Recently, advances in laser technology have resulted in industry- their impact on material processing and other industrial appli-
worthy ultrashort laser systems capable of efficient material cations.
processing. Femtosecond pulses can extend the laser processing Following the spectacular progress in their performance so
capabilities into materials inaccessible by traditional lasers. For far, future innovations in materials and fiber designs are ex-
example, transparent materials can be processed efficiently by pected to continue pushing the performance boundaries with
focusing fs pulses tightly to induce nonlinear absorption through new radical fiber laser solutions.
a combination of athermal effects, such as multiphoton absorp- So far, the tremendous success of fiber lasers has been al-
tion, tunneling ionization, and avalanche ionization. As a result, most entirely based on Yb3+ -doped fibers operating around
the induced structural changes are confined into tiny volumes 1 μm. However, moving forward extending the output beam
with nm precision and are ideal for 3-D micromachining. This wavelength into the mid-infrared (mid-IR) will be beneficial
area of highly nonlinear matter/light interaction is still in its for a large number of existing or enable novel new applica-
infancy and is expected to take fiber laser material processing tions [235], [249]. For industrial material processing applica-
to a new level, with a number of novel applications expected in tions, such as plastic welding, or glass processing, fiber laser
the near future. sources in the mid-IR with substantial power (>100 W) will
One of the main attributes of high power fiber lasers is their be required. Processing glass will require robust laser sources
superior beam quality at high powers. For the same collimated in the 3–5 μm range. At present, there are no industrial-grade,
beam size, higher beam quality radiation result in smaller spot fiber-based lasers in this spectral region. Emission in this spec-
size at the work-piece. Alternatively, for the same spot size at the tral region relies entirely on ZBLAN or possibly chalcogenide
work-piece, higher beam quality requires smaller and therefore glasses [235], the power handling capabilities of which have not
lighter focusing beam optics. This results in lighter processing been proven yet.
heads and higher processing speeds. Finally, for the same beam Further power scaling with diffraction limited outputs, well
size and focusing optics, as well as, same spot size, beams beyond the current single-fiber performance, can be achieved
with higher beam quality can be focused further away enabling by spectral (SBC) and/or CBC of high power SM fiber lasers
remote material processing [188]. [182], [185]. SBC and CBC distribute spatially the thermal load
However, with the development of high-power, high- and intensity and can effectively mitigate thermal and nonlinear
brightness near-diffraction-limited fiber laser sources it has be- effects, which set the hard limits to the power generated and
transmitted in currently used fibers. In addition to power scaling, For rectangular cross-sections [110], A = dx dy and
CBC and SBC increase the output beam spatial brightness. SBC
though is limited by the amplification bandwidth of the active θx θy

Ω = 4 sin−1 sin sin ≈ θx θy (A.3)
medium and results in spectral brightness deterioration, which 2 2
might complicate the processing head design. Four 2 kW fiber
lasers have been spectrally combined to provide 8.2 kW of where dx and dy are the lengths and θx , θy are the divergence
output power [250]. The beam quality was retained to M 2 < 1.5 full angles in the two orthogonal directions. Again, brightness
up to 2.5 kW and degraded to M 2 ∼ 3.5 at full power due to is given in terms of beam quality parameters as
the onset of TMI.
P P π 2 P
CBC has been already demonstrated by tiling eight SM fiber
B= = = .
lasers side-by-side in the near field (tiled aperture) to provide dx dy θ x θ y 16 (BPPx BPPy ) 4 Mx2 My2 λ2
a record 4 kW of diffraction-limited beam (M 2 = 1.25) [186]. (A.4)
Using a single diffractive optical elements (filled aperture) five There are two important theorems that govern brightness in
SM fiber lasers have been combined coherently into one beam a passive optical system. First, assuming that both object and
with M 2 = 1.1, exceeding that of the contributory lasers [187]. image spaces have the same index of refraction, the brightness
Power scaling in CBC is achieved by increasing the number of theorem states that the brightness of beam produced by an imag-
tiled contributory fiber lasers. So far, a maximum of 64 fiber ing system cannot be greater than the original source brightness.
lasers have been combined successfully combined coherently The brightness is preserved only when the system is lossless.
[251], making CBC an extremely powerful technique for future The underlying principle is conservation of energy, or conser-
fiber laser power scaling. SBC and CBC can also be combined vation of number of rays. The second theorem states that the
for multidimensional power scaling [252]. brightness of a collection of mutually incoherent beams cannot
In addition to average power scaling, research is now con- be higher than the brightness of the brightest beam.
centrated into novel techniques for scaling the pulse en- These two theorems imply that there is an upper limit to the
ergy and peak power in ultrafast laser systems, using re- brightness achieved by a combined pump module, which defines
cently demonstrated promising techniques such as divided- to large extend the efficiency of the various cladding pumping
pulse amplification [253]–[255] and/or the stack-and-dump schemes.
concept [256]. Currently, these are areas of intensive re-
The above theorems are sometimes expressed in terms of
search and new exciting results are expected in the near
étendue, which is defined as
future.
Finally, fiber lasers offer themselves for massive “paral-
E = n2 A Ω (A.5)
lelism” and can go beyond the classic MOPA configurations
into schemes such as the coherent amplifier network (ICAN).
where n is the surrounding medium refractive index. Etendue
Such radical concepts can potentially produce pulses with en-
describes the light gathering power or acceptance of an optical
ergies of >10 J at repetition rates of several kHz, as required
system.
for the next-generation particle accelerators [257]. Given the
Finally, the spectral brightness or spectral brilliance is es-
size, power requirements and expected cost, practical imple-
sentially the brightness per unit optical bandwidth, expressed in
mentation of such future coherent amplified network concepts
can only be realized by the fiber laser technology, which can W/(μm2 sr Hz).
provide 10 s of kW of diffraction-limited beams, with record
wall-plug efficiencies (>30%) robustly in small foot-print and APPENDIX B
low cost. NONLINEARITY ENHANCEMENT FACTOR
The NEF is a figure of merit that compares the strength of
APPENDIX A the nonlinear interaction in fibers with that of a focused beam in
BEAM BRIGHTNESS bulk silica. For a Gaussian beam of power P0 and focused waist
radius ω0 , the (intensity) x (effective length) product is given
The brightness (or radiance) of a beam (B) is defined as the
by [258]:
beam power (P ) per unit area (A) and unit solid angle (Ω),
namely [99], [259]: P0
ILbulk = (B.1)
λ
P
B= . (A.1)
AΩ where the effective length is equal to Rayleigh length. A quasi-
Gaussian beam with waist radius ω0 , propagating in a fiber
For circular cross-sections A = πr2 and Ω = πN A2 , where amplifier of length L0 and gain G = Pout /Pin = exp(γL0 ),
r is the radius and NA the numerical aperture. The brightness where Pout = P0 , is characterized by (intensity) x (effective
can also be given in terms of beam quality as length) product

P P L0
G−1
B= = . (A.2) ILam p = Iin eγ z dz = I0 (B.2)
π 2 BPP2 (M 2 λ)2 0 γG

where I0 = P0 πω02 = GIin and γ is the gain coefficient. [18] S. Bohme, S. Fabian, T. Schreiber, R. Eberhardt, and A. Tunnermann,
“End cap splicing of photonic crystal fibers with outstanding quality for
The NEF is then defined as high power applications,” Proc. SPIE, vol. 8244, 824406, 2012.
ILam p λ G−1 [19] Y. Jeong, J. K. Sahu, D. N. Payne, and J. Nilsson, “Ytterbium-doped
NEF = = L0 . (B.3) large-core fiber laser with 1.36 kW continuous-wave output power,”
ILbulk πω02 G ln G Opt. Exp., vol. 12, pp. 6088–6092, 2004.
[20] C.-H. Liu, B. Ehlers, F. Doerfel, S. Heinemann, A. Carter, K. Tankala,
In the quasi-Gaussian approximation, the fundamental fiber J. Farroni, and A. Galvanauskas, “810 W continuous-wave and single-
mode is given by [100]: transverse-mode fibre laser using 20 μ mcore Yb-doped double-clad
fibre,” Electron. Lett., vol. 40, pp. 1471–1472, 2004.
[21] J. Nilsson, W. A. Clarkson, R. Selvas, J. K. Sahu, P. W. Turner,
1.619 2.879
ω0 ≈ r0 0.65 + 3/2 + (B.4) S. U. Alam, and A. B. Grudinin, “High-power wavelength tunable
V V6 cladding-pumped rare-earth-doped silica fiber lasers,” Opt. Fiber Tech-
nol., vol. 10, pp. 5–30, 2004.
and [22] J. D. Minelly, L. Spinelli, R. Tumminelli, S. Govorkov, D. Anthon,
E. Pooler, R. Pathak, D. Roh, D. Grasso, D. Schleuning, B. Perilloux,
V and P. Zambon, “All-glass kW fibre laser end-pumped by MCCP-cooled
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Opt. Eur., 2011, p. 1.
[23] V. Gapontsev, D. Gapontsev, N. Platonov, O. Shkurikhin, V. Fomin,
ACKNOWLEDGMENT A. Mashkin, M. Abramov, and S. Ferin, “2 kW CW Ytterbium fiber
laser with record diffraction-limited brightness,” in Proc. Conf. Lasers
The authors would like to thank helpful discussions and fruit- Electro-Opt. Eur., 2005, p. 508.
ful collaboration over the years with colleagues and coworkers [24] S. Norman, M. N. Zervas, A. Appleyard, P. Skull, D. Walker, P. Turner,
and I. Crowe, “Power scaling of high power fiber lasers for microma-
in SPI (Dr. S. R. Norman, Dr. M. P. Varnham, Dr. M. K. Durkin, chining and materials processing applications,” Proc. SPIE, vol. 6102,
Dr. F. Ghiringhelli, and Dr. L. Walker) and the ORC (Prof. D. N. p. 61021P, 2006.
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and Prof. D. J. Richardson). [26] D. Gapontsev, “6 kW CW single mode ytterbium fiber laser in all-fiber
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Science, Alexandria, VA, USA, Tech. Rep. CRL-ID-155446, 2003. gree from École Nationale Supérieure Des Sciences
[246] F. Abt, A. Hess, and F. Dausinger, “Temporal behaviour of the focal shift Appliquées et de Technologie, Lannion, France, in
of beam forming optics for high power single mode lasers,” presented optoelectronics in 1999 and the Ph.D. degree from
at the 27th Int. Congr. Appl. Lasers Electro-Opt., CA, USA, 2008, pp. the Optoelectronics Research Centre, Southampton
561–568, Paper #1302. University, U.K., in 2002 that he completed in 2007.
[247] H. Zimer, R. Niedrig, and B. Wedel, “Beam delivery systems and pro- He worked for Point Source Ltd. and Southampton
cessing heads for 1 μm high brightness laser cutting systems,” Proc. Photonics Ltd. as a Development Engineer in 1999
SPIE, vol. 8239, 82390V, 2012. and 2000, respectively. After a year as a Senior Devel-
[248] C. Thiel, R. Weber, J. Johannsen, and T. Graf, “Stabilization of a laser opment Engineer at SPI Laser working on high power
welding process against focal shift effects using beam manipulation,” laser from January 2006 to April 2007, he joined the
Phys. Procedia, vol. 41, pp. 209–215, 2013. ORC as a Research Fellow in the High Power Fiber Laser group until January
[249] P. Zhou, X. Wang, Y. Ma, H. Lu, and Z. Liu, “Review of recent progress 2010. Subsequently, he joined Fianium Ltd. as an Engineering Manager and
on mid-infrared fiber lasers,” Laser Phys., vol. 22, pp. 1744–1751, 2012. then as the Head of Laser Development. He then joined SPI Laser Ltd., where
[250] C. Wirth, O. Schmidt, I. Tsybin, T. Schreiber, R. Eberhardt, J. Limpert, he currently leads the Advanced Laser Laboratory. His main research interests
A Tünnermann, K. Ludewigt, M. Gowin, E. ten Have, and M. Jung, include high power rare-earth doped fiber lasers and amplifiers, pulsed fiber
“High average power spectral beam combining of four fiber amplifiers lasers, nonlinear effects and numerical simulations. He has published more than
to 8.2 kW,” Opt. Lett., vol. 36, no. 16, pp. 3118–3120, 2011. 100 papers in scientific journals and conference proceedings.
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High Power Fiber Lasers: A Review

Article in IEEE Journal of Selected Topics in Quantum Electronics · September 2014

Michalis N Zervas Christophe Codemard

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High Power Fiber Lasers: A Review

Fig. 2. Schematic of cladding pumping principle of operation.

tion with intra-cavity pump launching, which alleviates some of

Fig. 4. Cladding-pumped fiber cross-sections.

these improved designs “scatter” skew rays towards the active

which is achieved by properly perturbing the fiber along its

[Fig. 7(f)], has the additional advantage of distributing the pump

tapered fused fiber bundle to provide ∼1.5 kW pump power in

Fig. 11. Typical efficiency budget for Yb 3 + -doped fiber lasers.

ground-state absorption transitions and the most important,

Emission and absorption cross-sections define also to a large

mode operation has been achieved for core diameters up to

From Eqn. (B.3) to (B.5), it can be shown that the NEF is

A. Stimulated Brillouin and Stimulated Raman Scattering

Fig. 16. SBS induced damage.

mitigated just by scaling the mode size. For 1060 nm operation,

Fig. 17. Power evolution with time in high-power Yb 3 + -doped cladding-

Power scaling in direct diode (DD) output, on the other hand,

B. Pulsed Fiber Laser Parameter Space

Fig. 20. Ultrafast pulsed fiber laser parameter space.

evolution and dynamics. This is not surprising, given the lack of

come more challenging to achieve stable focal position of the

VII. SUMMARY—FUTURE PROSPECTS

though is limited by the amplification bandwidth of the active θx θy

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