high-power ultrafast fiber laser systems

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 12, NO. 2, MARCH/APRIL 2006 233

High-Power Ultrafast Fiber Laser SystemsJens Limpert, Fabian Roser, Thomas Schreiber, and Andreas Tunnermann, Associate Member, IEEE

(Invited Paper)

Abstract—The recent demonstration of rare-earth-doped fiberlasers with a continuous wave output power well above the kilowattlevel with diffraction-limited beam quality has proven that fiberlasers constitute a power-scalable solid-state laser concept. To gen-erate intense pulses from a fiber, several fundamental limitationshave to be overcome. Nevertheless, novel experimental strategiesand fiber designs offer an enormous potential toward laser systemswith high average powers and high pulse energies. This paper re-views the challenges, achievements, and perspectives of ultrashortpulse generation and amplification in fibers.

Index Terms—Chirped pulse amplification, fiber laser and am-plifiers, microstructured fibers, nonlinear fiber optics, ultrafastoptics.

I. INTRODUCTION

ANUMBER of important practical as well as fundamentalresearch applications of ultrafast lasers appeared over the

last decades [1], a trend initiated by the step from old dye-lasertechnology toward solid-state lasers. These high-power ultra-fast solid-state lasers use small rods as the amplifier media—for instance, Titanium-doped sapphire as the most widespreadone [2]—and have the potential to generate significantly higherpulse energies, higher powers, and shorter pulse durations, incombination with greater reliability than dye-lasers. However,these systems are difficult to scale in average power and sufferfrom low efficiencies because direct diode pumping is not pos-sible. Furthermore, the complexity of short pulse high energyTi:sapphire lasers still constrain the employment of ultrafastlaser technology in industrial environments.

Recently, the development of diode pumped solid-state lasers,such as Yb:YAG or Cr:LiSAF, has constituted a big step for-ward in terms of efficiency. In order to overcome thermoopticaleffects, which limit the power scaling capability of these sys-tems, several novel gain media designs, such as thin disk or slab,have been introduced [3], [4]. However, due to the low singlepass gain of these amplifier materials, very complex systems;e.g., regenerative amplification schemes, are required to obtaina reasonable output. Therefore, robustness, compactness, andlong-term stability are restricted in short pulse bulk solid-statelaser systems.

Alternatively, forming the gain medium to be long and thinnot only leads to outstanding thermooptical properties, but also

Manuscript received November 17, 2005.J. Limpert, F. Roser, and T. Schreiber are with the Institute of Applied

Physics, Friedrich Schiller University Jena, D-07745 Jena, Germany (e-mail:[email protected]; [email protected]; [email protected]).

A. Tunnermann is with the Institute of Applied Physics, Friedrich SchillerUniversity Jena, D-07745 Jena, Germany, and also with the Fraunhofer Institutefor Applied Optics and Precision Engineering, D-07745 Jena, Germany (e-mail:[email protected]).

Digital Object Identifier 10.1109/JSTQE.2006.872729

to a very high single pass gain. Fiber-based laser systems havethe reputation of being immune to any thermooptical problemsdue to their special geometry. Their excellent heat dissipation isdue to the large ratio of surface-to-active volume of such fiber.The beam quality of the guided mode is determined by the fibercore design, and is therefore power-independent.

Due to the confinement of both the laser and pump radiation,the intensity is maintained over the entire fiber length and is notlimited to the Rayleigh length, as is the case in longitudinallypumped bulk lasers. The gain of the laser medium is determinedby the product of pump light intensity and interaction length withthe laser radiation in the gain medium. Therefore, the decisiveproduct can be orders of magnitude higher in fibers than in otherbulk solid-state lasers. This results in very efficient operation offiber laser systems exhibiting very high gain and low pumpthreshold values. Additionally, complete integration of the laserprocess in a waveguide allows for inherent compactness andlong-term stability of fiber lasers.

In particular, Ytterbium-doped glass fibers, which have aquantum defect of less than 10%, can provide optical-to-opticalefficiencies well above 80% and, therefore, low thermal load.These fiber laser systems are especially interesting for high-power ultrashort pulse generation and amplification because ofseveral unique properties [5]: Firstly, a broad emission spectrumallows for short pulse amplification. In ytterbium-doped glassfibers, the amplification bandwidth of approximately 40 nm sup-ports, in principle, pulses of durations as short as ∼30 fs. Fur-thermore, the absorption spectrum covers a wavelength rangein which powerful diode lasers are commercially available. Anadditional point to note is that the long fluorescence lifetime(∼1 ms) results in a high-energy storage capability. Exited-stateabsorption of pump or signal radiation, or concentration quench-ing by ion-ion energy transfer processes, does not occur withytterbium because only two energy level manifolds are relevantfor all optical wavelengths.

High power fiber lasers usually use the double-clad fiber con-cept, invented in 1988 by Snitzer [6]. Such a double-clad fiberis characterized by a second waveguide, which is highly multi-mode, surrounding the active core. Into this second waveguide,also called inner cladding or pump core, low brightness highpower diode laser radiation can be launched. This pump light isgradually absorbed over the entire fiber length and is convertedinto high brightness high power laser radiation. Thus, double-clad rare-earth doped fibers can provide a highly efficient bright-ness improvement by pump-to-laser radiation conversion by thelaser process itself.

The aforementioned properties make rare-earth-doped fiberssuperior to other solid-state laser concepts in a variety of perfor-mance categories. This has become obvious following several

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234 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 12, NO. 2, MARCH/APRIL 2006

recent demonstrations of continuous wave fiber laser systemsexhibiting more than 1 kW of average power while maintainingan excellent beam quality [7]–[10].

II. BASIC LIMITATIONS OF SHORT PULSE AMPLIFICATION

IN OPTICAL FIBERS

The confinement of the laser radiation and the long interac-tion length makes fibers attractive for solid-state laser systems;however, nonlinear effects impose fundamental limitations onthese systems, which may lead to severe pulse distortions andeven to damage of the fiber.

The lowest-order nonlinear effect in optical fibers originatesfrom the third-order susceptibility χ(3), and is responsible for anintensity-dependent refractive index in the form n = n0 + n2I.Consequently, an optical field propagating through a fiber ex-periences a self-induced phase shift, a phenomenon referredto as self-phase modulation (SPM). A second important classof nonlinear effects results from stimulated inelastic scattering,whereby the radiation transfers a part of its energy to the glasshost in form of excited vibrational modes. A large frequencyshift (∼13 THz) is observed due to the excitation of opticalphonons, a process called stimulated Raman scattering (SRS),whereas the excitation of acoustical phonons leads to a muchsmaller shift (∼17 GHz) called stimulated Brillouin scattering(SBS). Both manifest themselves as a significant power lossmechanism in fiber-based laser systems. In the context of ultra-short pulse amplification in fibers, where the spectra are signif-icantly broader than the Brillouin gain bandwidth, the effect ofSBS can be neglected [11].

In general, the nonlinearity coefficients in glass fibers are in-trinsically small. Both the nonlinear index coefficient n2 andthe gain coefficients of SRS and SBS are at least two or-ders of magnitude smaller than in other common nonlinearmedia [12]. Nevertheless, due to the large product of inten-sity and interaction length inside the fiber core, nonlinear ef-fects can be observed at very low power levels and basicallylimit the performance of pulsed rare-earth-doped fiber sys-tems. Pulse propagation in fibers is described by the nonlinearSchrodinger equation (NLSE), shown in (1), which describesthe temporal and longitudinal dependency of the slowly vary-ing pulse envelope A(z, T ) along the fiber in the retarded timeframe T

∂A

∂z−

( ∞∑n=2

βnin−1

n!∂n

∂Tn

)A − g

2A = iγ|A|2A (1)

where g represents the gain or loss of the fiber, βn are theTaylor expansion coefficients of the dispersion parameter β2(ω),and the pulse amplitude A(z, t) is assumed to be normalizedsuch that |A|2 represents the optical power. The nonlinearitycoefficient γ is defined by

γ =n2ω0

cAeff(2)

where Aeff is the effective mode field area, n2 is the nonlinearrefractive index coefficient, c equals the speed of light, and ω0 isthe center frequency of the optical field. With these parameters,(1) includes the linear effects of dispersion and gain/loss and

Fig. 1. Impact of SPM in CPA system on pulse quality of recompressedpulses assuming a Gaussian pulse shape during amplification in comparisonwith transform-limited pulse (B = 0).

self-phase modulation as the lowest-order nonlinear effect. Thephase imposed by SPM can lead to a nonlinear chirp, whichcannot be compensated by standard dispersive elements. Animportant parameter is the accumulated nonlinear phase of thepulse traveling through the fiber, the so-called B-integral, de-fined by (3) with I(z) as the pulse intensity varying over thefiber length L [13]

B =2π

λ·∫ L

0

n2 · I(z) · dz. (3)

A B-integral of smaller than ∼1 is considered as a linear prop-agation, pulse quality degradation is expected above this value.To illustrate the consequence of SPM, a chirped pulse amplifi-cation (CPA) system is simulated by solving the NLSE usingthe split-step Fourier method and assuming a Gaussian pulseshape during amplification. Fig. 1 illustrates the recompressedpulse intensity profiles after accumulating a nonlinear phaseshift in comparison to the transform-limited pulse. As revealed,SPM leads to a pulse broadening and the evolution of sidelobescontaining a considerable amount of pulse energy. Hence, evenmoderate self-phase modulation can significantly distort a re-compressed pulse after amplification, with the result of reducedpeak power and degraded pulse contrast.

In contrast to SPM, SRS is a process which is initiated at acertain threshold value. Above this threshold, energy is rapidlytransferred to a frequency-downshifted Stokes wave. This pro-cess can be described quantum-mechanically as an inelasticscattering process, where an incident photon is annihilated tocreate a photon at a downshifted frequency, the Stokes wave, anda phonon with the right energy and momentum. Additionally,a frequency up-shifted photon, the so-called anti-Stokes wave,can be created if a phonon with the right energy and momentumis available. The threshold for the onset of SRS in a fiber ampli-fier is, to a good approximation, given by (4), where gR is thepeak Raman gain and Leff is the effective interaction length [11]

P SRSthreshold ≈ 16 · Aeff

gR · Leff. (4)

LIMPERT et al.: HIGH-POWER ULTRAFAST FIBER LASER SYSTEMS 235

Fig. 2. Emission spectrum of t fiber amplifier as a function of pulse energy.

Fig. 3. Damaged fiber facet.

As an example, the threshold behavior of SRS and energyloss of the propagating optical pulse in a single-mode short-pulse fiber amplifier is shown in Fig. 2. The measured emittedspectrum reveals strong changes with increasing peak power. Atlower pulse energies, only the signal at 1060 nm is observable.At peak powers higher than 2.3 kW, a signal at about 1120 nmincreases rapidly, indicating that the threshold of stimulatedRaman scattering is passed. At a peak power of 3.4 kW, eventhe frequency up-shifted anti-Stokes wave at about 1010 nmbecomes detectable. At 4.6 kW, the pulse breaks up and mostof the intense central part of the spectrum is transferred into theStokes wave.

Beside these basic nonlinear effects, fiber damage has to beconsidered as a scaling limitation of high-energy fiber lasersystems. The surface damage fluence threshold of fused sil-ica, which is significantly lower than the bulk damage fluencethreshold, at a wavelength of ∼1 µm, is given by [14]

Fdamage = 22 · (∆τ)0.4 J/cm2 (5)

where ∆τ is the pulse duration in nanoseconds. Therefore, thedamage threshold is in general significantly lower than the ex-tractable energy. Fig. 3 shows an example of a damaged fiberfacet.

Fig. 4. Coreless fiber end cap to avoid facet damage.

Fig. 5. Calculated bending losses in 30-µm LMA fiber (NA = 0.06) for firstfour transverse modes subject to bending radius.

This problem can be solved by special treatment of the fiberend. One solution is to splice a coreless end cap on the outputside of the fiber amplifier, as shown in Fig. 4. The expansion ofthe beam reduces the fluence and avoids fiber facet damage.

Due to the aforementioned nonlinear distortions, the genera-tion of high peak powers in pulsed fiber laser systems is verychallenging. Novel fiber designs and experimental strategieshave to be applied to scale up the output parameters. There arebasically two approaches to amplify ultrashort laser pulses infibers: the parabolic pulse amplification, which actually usesnonlinearity, and the chirped pulse amplification, which relieson avoidance of nonlinearity. Before reviewing these two basicamplification concepts, fiber designs with intrinsically reducednonlinearity; i.e., an increased mode-field diameter and a re-duced fiber length, will be discussed.

III. LOW-NONLINEARITY FIBER CONCEPTS

As discussed in Section III and shown in (3) and (4), thenonlinear effects are proportional to the fiber length and the in-tensity in the fiber core, and therefore are inversely proportionalto the mode-field area of the guided radiation in the fiber. Thus,an enlargement of the mode-field diameter and a reduction offiber length would help to avoid disturbing nonlinear effects.Using special techniques and fiber designs the mode-field-areaof single-transverse mode fiber devices could be significantlyincreased in the past years. To investigate the modal properties,the normalized frequency V is introduced in (6). It is related


to the effective refractive indices of the core nC and claddingnClad of the fiber with a core radius aeff . One can show that afiber becomes single-mode for V < 2.405 [15]

V =2π

λaeff · NA =

2π

λaeff

√n2

C − n2Clad. (6)

Thus, one approach to increase the core diameter is to decreasethe numerical aperture (NA) at a certain value of V . However, inconventional step-index fibers, a reduction in the numerical aper-ture below a certain limit is not tolerable in terms of propagationlosses and precision of core fabrication method. More sophisti-cated fiber designs are based on modified index profiles, whichincrease the single-mode area by using an outer ring structure.Such large-mode area fibers with up to few 10-µm core diameterand diffraction-limited output could be demonstrated [16]. Pref-erential gain to the fundamental mode is created by an optimallyoverlapping rare-earth dopant distribution [17]. This concept canbe extended to gain and loss managed multimode fibers to dis-criminate higher order modes [18]. Stable fundamental modepropagation over more than 20 m at 1.5-µm wavelength is ob-tained in a conventional step-index core double-clad fiber with acore diameter of 45 µm with a numerical aperture of 0.13 [19].In [19], it is stated that the mode-coupling between transversalmodes is affected by the cladding diameter and the fiber fabri-cation process. Thus, for maintaining the fundamental mode, itis preferable to use large cladding diameters relative to the corediameters and the high-quality modified chemical-vapor depo-sition (MCVD) process. Further discrimination of higher ordermodes is achieved by a careful optimization of the seed launch-ing conditions and incorporated tapered sections [20] inside thefiber laser or amplifier. Using these techniques, the single-modeoperation of a 50-µm core (MFD ∼30 µm) fiber amplifier at1.06 µm was reported [21].

In recent years, the most commonly employed low-nonlinearity fiber concept is the low-numerical aperture large-mode-area (LMA) fiber. Such an LMA fiber has V-parametersin the range of 5–10, and can therefore guide several higher or-der transverse modes. However, bending losses can be appliedto achieve stable fundamental mode operation of a fiber laseror amplifier [22]. Fig. 5 shows the calculated bending lossesfor the fundamental and three higher-order modes as a functionof bending radius in a 30-µm LMA fiber (NA = 0.06) [23].This calculation reveals a significant discrimination of higherorder transverse modes. As an example, at a bending radius of50 mm, the induced bending loss for the LP01 mode is 0.01dB/m and for the first higher order mode LP11 is 52 dB/m. Thisdifference of approximately five orders of magnitude enforcesfundamental mode operation.

Robust and environmentally stable fundamental mode opera-tion in even larger cores is only possible in a truly single-modefiber. Therefore, a strictly single-mode fiber with large modeareas would be a significant achievement. Microstructuring thefiber by including air-holes adds several attractive properties toconventional fibers. These so called photonic crystal fibers (or“holey fibers”) are currently a subject of intense research [24].Solid core photonic crystal fibers consist of a regular array ofair-filled holes characterized by the hole diameter d and the

Fig. 6. (a) Structural parameters, and (b) modal characteristics of a one hole-missing photonic crystal fiber.

pitch Λ [Fig. 6(a)]. To describe the modal properties, (6) canalso be used for such one-hole-missing PCFs where the param-eter aeff corresponds to Λ/

√3. The main difference between a

standard step index fiber and a PCF is that for PCF, the effectivecladding index is strongly dependent on λ/Λ. Also, in the limitof λ/Λ → 0, the effective cladding index reaches the core index.This property becomes more interesting by plotting the singlemode boundary V = 2.405 of the normalized wavelength overthe relative hole diameter, as done in Fig. 6(b). For d/Λ > 0.4,the fiber turns from multimode operation into single mode oper-ation if the relative wavelength is large enough. For d/Λ < 0.4the fiber becomes single mode for all wavelengths λ/Λ due tothe fact that the wavelength dependence of nClad keeps the NAlow enough to stay single mode. This regime is called endlesslysingle mode. It was observed in the first photonic crystal fibersand was investigated theoretically soon afterward [25], [26].

Such an endlessly single mode operation is not known fromstep index fibers, and leads to a concept of scaling the core di-ameter of a photonic crystal fiber. If the modal properties donot depend on the normalized wavelength λ/Λ, the mode diam-eter of a PCF, which is proportional to Λ, can theoretically bescaled to infinity at a given wavelength. Of course, the scalingof the core size is limited by increasing propagation losses [27].If the V -parameter of the photonic crystal fiber is smaller thanone, confinement of the mode is too weak and leakage occursthrough a PCFs finite cladding. On the other hand, if the valueλ/Λ becomes too small (<0.1), scattering losses due to longitu-dinal nonuniformities increase; e.g., losses due to microbending,macrobending, and dielectric imperfections play an important


role. If further scaling of the mode-field diameter is necessary,the core can be extended to consist of more than one missinghole; i.e., three missing holes [28]. Even seven missing holePCFs are demonstrated with mode field diameters of ∼50 µmand fundamental mode guidance [29].

The gain medium of a fiber laser can be fabricated by replac-ing the pure silica core by a rare-earth-doped rod. In general,the core is codoped with fluorine to compensate for the refrac-tive index increase due to the rare-earth ion. This ensures thatthe refractive index of the rod closely matches that of the silicacladding. The refractive index step can be reduced to ∼10−5,even at relatively high ytterbium doping levels, so that the guid-ing properties are determined by the photonic crystal structuresurrounding the core, and not by any index step due to thedopants.

A further advantage of microstructuring a fiber is the possi-bility of forming an air-cladding region to create double-cladfibers [30]. Double-clad PCFs can be achieved by surround-ing the inner cladding with a web of silica bridges, which aresubstantially narrower than the wavelength of the guided radi-ation. The result is a very high numerical aperture of the innercladding up to 0.8. This allows for reducing the diameter ofthe inner cladding, while maintaining sufficient brightness ac-ceptance for efficient pumping. The advantage of shrinking theinner cladding is that the overlap ratio of the core to the innercladding increases, leading to shorter absorption lengths, andtherefore to reduced nonlinearity [31], [32].

Recently, a fiber design has been developed which exhibitsextremely reduced nonlinearity. This type is called rod-typephotonic crystal fiber [33]. The basic idea of this fiber design isto have outer dimensions of a rod laser, meaning a diameter inthe range of a few millimeters and a length of just a few tens ofcentimeters, but to include two important waveguide structures,one for pump radiation and one for laser radiation. The pumplight is confined by an air cladding and the laser light is guidedin an array of air holes. Finally, such a fiber has an extremelyreduced nonlinearity, and therefore allows for significant powerand energy scaling. The cross section of this fiber is shown inFig. 7. The inner cladding has a diameter of ∼180 µm. Thenumerical aperture is as high as 0.6. The ytterbium-doped corehas a diameter of 60 µm; to our knowledge, this represents thelargest single-mode core ever reported for doped fibers. Thisfiber possesses a pump light absorption as high as ∼30 dB/m at976 nm. Usually, the extraction of high-power levels from shortfiber lengths is limited by thermooptical problems. A detailedanalysis of the thermooptical behavior of high-power fiber lasers(including photonic crystal fibers) has revealed that power scal-ing is restricted by damage to the polymer coating, which occursat fiber surface temperatures between 100 ◦C and 200 ◦C [34].These temperatures are easily reached if power levels in the 100-W/m range are extracted. In a conventional double-clad fiber,the coating has an optical function. The coating has to have alower refractive index than fused silica, and therefore forms thewaveguide for pump radiation. In contrast, in a microstructuredair-clad fiber, it just serves to protect the fiber from mechanicaldamage and chemical attack. The most straightforward way toavoid damage of the coating is to remove it. This can be done if

Fig. 7. (a) Microscope image of rod-type photonic crystal fiber. (b) Close-upview of inner cladding and core regions.

the fiber itself has enough mechanical stability; i.e., if the fiberis thick enough. The fiber shown in Fig. 7 has an outer claddingdiameter as large as 2 mm and possesses no coating. In addition,the larger outer diameter improves the heat dissipation capabil-ities of this fiber [34], and also reduces the propagation loss ofweakly guided radiation because of the increased rigidity.

A comparison of nonlinearity between a double-cladytterbium-doped step-index single-mode fiber, a low-numericallarge-mode-area fiber, and the previously described single-transverse mode rod-type photonic crystal fiber, shows theachievement of this novel fiber design (Table I). The nonlinear-ity, basically given by the effective mode area and the absorptionlength of the fiber, is normalized to the value of the standard sin-gle mode step-index fiber in the 1 µm wavelength region. Thiscomparison reveals a reduction of nonlinearity by a factor ofabout 2000 in the rod-type PCF. As stated previously, wheneverpower or energy scaling is limited by nonlinearity, such a fiberoffers a significant potential.

IV. SHORT PULSE AMPLIFICATION IN THE PRESENCE

OF NONLINEARITY

If short optical pulses are amplified in a fiber, nonlinearitycannot be neglected as long as there are no special techniques


TABLE ICOMPARISON OF NONLINEARITY IN DIFFERENT FIBER DESIGNS

Fig. 8. Evolution of Gaussian input pulse toward parabolic output pulse in 10-m normal-dispersion fiber amplifier. Intensity in 2-m increments in: (a) time domain(linear scale) and (b) spectral domain (logarithmic scale).

applied to avoid it. Dispersion, gain, and nonlinearity af-fect the pulse evolution during the amplification in a fiber.This section concentrates on the interplay of these effectsand how they can be used to generate high power ultrashortpulses.

As discussed in Section II, SPM leads to a phase shift acrossthe pulse that is directly proportional to the pulse intensity itself.Thus, as the instantaneous frequency is the negative first deriva-tive of the phase, new frequencies are generated across the pulse,where a red shift occurs on the leading edge and a blue shift onthe trail of the pulse. In addition to the temporal frequencychirp, a second consequence of SPM is, of course, a spectralbroadening due to this generation of new frequencies. The in-terplay of SPM and dispersion can lead to dramatic temporaland spectral changes with the result that the generated frequencychirp cannot be compensated to obtain transform-limited pulsesby standard dispersive elements such as a pair of gratings orprisms [35], which require a linear chirp. Thus, the evolutionof a pulse, which maintains a linear chirp during amplification,would be favorable. The pulse shape with a perfect linear chirpis a parabolic shape. If this shape is maintained, the limits ofthe accumulated phase, the B -integral, can even be overcome.The next two sections discuss the experimental realization ofthis idea. Firstly, parabolic pulses are generated during directamplification of femtosecond pulses by a balanced interplay ofdispersion, gain, and nonlinearity. Secondly, the amplificationof parabolic pulses generated directly by a fiber oscillator isdemonstrated.

A. The Generation of Parabolic Pulses in a Fiber Amplifier

The numerical investigation of the NLSE with gain and sec-ond order dispersion β2 (1) reveals that the interplay of normaldispersion, nonlinearity (self-phase modulation), and gain pro-duces a linearly chirped pulse with a parabolic shape, whichresists optical wave breaking [36], [37]. The linear chirp can beefficiently removed, allowing for high quality pulse compres-sion. For an amplifier with constant distributed gain, an exactasymptotic solution has been found corresponding to a parabolicpulse that propagates self-similarly [38]. The asymptotic pulsecharacteristics are not influenced by the shape or width of theinput pulse. Only the initial pulse energy determines the finalpulse amplitude and width.

As an example, the spectral and temporal evolution of aGaussian input pulse at 1060 nm center wavelength with re-alistic parameters of a 10 m long ytterbium-doped fiber ampli-fier is plotted in Fig. 8. The initial pulses have a duration of∆TFWHM = 500 fs and energy Ei = 400 pJ. The fiber ampli-fier provides a gain of 30 dB (g = 0.7 m−1), β2 = 0.02 ps2

m−1, and γ = 5.0 · 10−4 W−1 m−1. The final parabolic pulseshave a pulse duration (FWHM) of 6.7 ps and a spectral widthof 49.3 nm, corresponding to a time-bandwidth product of 84.6.It can be seen that the temporal shape is parabolic (Fig. 8(a)),which is also indicated by steep edges in the logarithmic spec-trum (Fig. 8(b)).

The setup to experimentally demonstrate this evolution intoa linearly chirped parabolic pulse is shown in Fig. 9. The fiber


Fig. 9. Experimental setup of parabolic pulse fiber amplifier.

amplifier system consists of a passively mode-locked, diode-pumped solid-state laser system, a diode-pumped ytterbium-doped fiber amplifier, and a diffraction-grating compressorbased on transmission gratings. As a femtosecond seed source,a Nd:glass laser system is applied, which is based on a semi-conductor saturable absorber mirror. The laser is running at a75 MHz repetition rate, producing pulses as short as 180 fs at∼1060 nm center wavelength and an average power of 100 mW.The high power amplifier is constructed using 9 m length oflow-NA large-mode-area fiber with a 30-µm diameter, 0.06-NAstep-index ytterbium-doped core, and a 400-µm D-shaped innercladding with NA = 0.38. Bending losses are used to extractfundamental mode beam quality from this fiber.

The combined action of gain, nonlinearity, and normal disper-sion ensures the generation of linearly chirped parabolic pulseswhich experience large temporal and spectral broadening. Thespectral broadening in the parabolic amplifier is illustrated inFig. 10, showing the emitted spectrum at different output pow-ers in a logarithmic scale. The conversion of the spectrum of theinitial sech2 pulse to the spectrum of a linearly chirped parabolicpulse can be identified and compared to the simulation in Fig. 8.At an average output power of 17 W, the spectral width hasincreased to 40.5 nm.

Transmission gratings in fused silica are employed to removethe chirp of the high power parabolic pulses [39]. Fig. 11 showsthe measured autocorrelation trace of the recompressed pulsesat an output power of 17 W of the fiber amplifier. We deter-mined the FWHM pulse duration τp = 80 fs corresponding toa time-bandwidth product of 0.86. The low-intensity wings inthe autocorrelation trace have their origin in uncompensatedhigher-order phase contributions of the fiber amplifier gratingcompressor setup. A compressor efficiency of 60 % is reached,resulting in an average power of 10.2 W after compression, cor-responding to a pulse energy of 140 nJ and a pulse peak powerof 1.7 MW.

B. Amplification of Parabolic Pulses in a Fiber Amplifier

As previously described, special pulse shapes, herein calledparabolic pulses, can create themselves during amplification innormal dispersive amplifier fibers and overcome limitations dueto nonlinear effects. Of course, these pulse shapes can also beexternally generated and injected into the amplifier. The mostelegant way of doing this is to create parabolic pulses directly

Fig. 10. Experimentally obtained output spectra of fiber amplifier.

Fig. 11. Intensity autocorrelation trace of recompressed 80-fs pulses (dashedcurve: fit).

in a short pulse fiber oscillator. Indeed, the similariton regimehas been realized in fiber oscillators [40]. Such a laser emitslinearly chirped parabolic pulses which, due to their pulse shape,constitute an ideal seed source for high power fiber amplifiers.

For this purpose, we developed a mode-locked all-polarization maintaining fiber oscillator comprising a saturable


Fig. 12. Experimental setup of nonlinear regime parabolic pulse amplifier.

Fig. 13. SEM image of Yb-doped air-clad photonic crystal fiber with sixindex-matched stress applying parts.

absorber mirror (SAM) [41]. The SAM makes the oscillatorself-starting, and the exclusive use of polarization maintain-ing fibers provides environmental stability. Linearly chirpedparabolic pulses with a spectral bandwidth of ∼ 10 nm around1035 nm, about 0.2 nJ pulse energy, and a pulse repetition rate of17 MHz are emitted. These pulses can be externally compressedto a pulse duration of 260 fs. The setup of the oscillator and thesubsequent amplification stages are shown in Fig. 12.

The output of the oscillator is preamplified in a ytterbium-doped single-clad PM fiber. The average power is boosted to50 mW and, due to the additional fiber length, the pulsesare stretched to ∼10 ps. The power amplifier consists of aytterbium-doped single-polarization single-mode double-cladphotonic crystal fiber [42]. This special fiber comprises index-matched stress-applying elements (SAP) as part of the photoniccladding. A cross section of this fiber is shown in Fig. 13.

Fig. 14. Measured autocorrelation trace of the recompressed pulses out of thenonlinear regime amplifier. In comparison, simulation of recompressed ampli-fied pulse is shown in the inset using chirped parabolic pulse (black) and chirpedsech2 pulse (gray).

The proximity of the SAP to the core ensures high differentialstress, and thus high birefringence. Additionally, due to the indexmatching of the SAP array to the air-hole cladding, the light isconfined by both parts of the photonic cladding: the air holesand the index matched regular arrays of SAPs. Furthermore,the amount of birefringence is enough to split two polarizationstates of the weakly guided fundamental mode in that way thatthe effective index of one polarization is below the cladding in-dex. This results in a single-polarization large-mode-area fiberin a certain wavelength range, which is determined by the de-sign and, in our case, lies within the amplification bandwidthof Ytterbium. The mode field area of ∼700 µm2 is larger thanthat of any step-index single-mode fiber. The pump cladding isformed by an air-cladding outside the inner photonic cladding,and has a diameter of 170 µm and an NA of 0.6. This structurehas a pump light absorption of ∼14 dB/m.

A fiber coupled diode laser emitting at 976 nm pumps themain amplifier, which is built using 1.2 m of the Yb-doped


Fig. 15. Schematic setup of high average power fiber CPA system. OI: optical isolator.

single-polarization fiber. Up to 29 W average power with aslope efficiency of 72% is obtained at a launched average seedpower of ∼30 mW. The polarization extinction ratio is as highas 24 dB (1:250) even at the highest power level. Again, a pairof transmission gratings is applied to recompress the pulses.The grating separation is just ∼2 cm, and due to the perfectlinear polarization of the amplified output the throughput ef-ficiency is 73%, resulting in 21 W of recompressed averagepower. The measured autocorrelation is shown in Fig. 14. Theautocorrelation width is 320 fs, corresponding to a 240-fs pulseduration. The recompressed pulse quality is almost unaffectedby the imposed nonlinear phase, even if the B-integral is in therange of ∼6. The slight reduction in pulse duration compared tothe oscillator is made possible by the bandwidth enhancementin the amplification stages. At the highest power levels, wingsin the autocorrelation trace are developing; however, they arecontaining only a minor part of energy. Consequently, the pulsepeak power is as high as 5 MW. In comparison, a simulationof the system is done by solving the NLSE with constant gain.The resulting autocorrelation after recompression is shown inthe inset of Fig. 14 for a chirped parabolic pulse and a compara-ble chirped sech2 pulse coming out of the laser. As shown, thequality of the output pulses is significantly better when usinga parabolic pulse shape. The autocorrelation of the amplifiedsech2 pulse has a considerable wing structure containing a sub-stantial part of the pulse energy. To our knowledge, this is thefirst experimental confirmation that parabolic pulses can lead toa significant improve of pulse quality in fiber amplifier systems.Further power and energy scaling will be possible in the futureby stretching the parabolic pulses to nanosecond duration, asdescribed in the following experiments.

V. SHORT PULSE AMPLIFICATION IN THE ABSENCE

OF NONLINEARITY

To significantly scale up the average power or pulse energyof fiber laser systems, the well-known technique of chirpedpulse amplification (CPA) [43] must be employed. Stretchingthe pulses in the time domain reduces the pulse peak powerand, consequently, nonlinear pulse distortion can be avoided toa certain extent. In the following sections, the achievements and

Fig. 16. Output characteristics of ytterbium-doped fiber power amplifier.

scaling potential of fiber based chirped pulse amplification isdiscussed.

A. High Average Power Fiber CPA System

In a fiber-based CPA system, sufficient pulse stretching andthe enlargement of the mode-field diameter of the fiber to reducethe peak power, and therefore nonlinear effects such as thepreviously discussed effects of SRS and SPM, are the key pointsto scale the output parameters. The setup of the state-of-the-art high average power femtosecond fiber laser system [44] isshown in Fig. 15. It consists of a passively mode-locked solid-state laser system, a grating stretcher, a two-stage ytterbium-doped single-mode photonic crystal amplifier, and a fused-silicatransmission grating compressor.

The femtosecond seed source is a passively mode-lockedYb:KGW laser system producing 150 fs pulses at a 73 MHzrepetition rate, and a 1040 nm center wavelength. These pulsesare stretched to 120 ps using a conventional gold-coated 1200lines/mm diffraction grating. The preamplifier and the poweramplifier are constructed with air-clad photonic crystal fiberspossessing an active core diameter of 40 µm (NA = 0.03) andan inner cladding diameter of 170 µm (NA = 0.62). The usedfiber length is just 1.2 m. In the first stage, the signal is amplifiedto 5 W, which is the seed of the power amplifier. The output


Fig. 17. Measured autocorrelation trace of high average power femtosecondpulses.

characteristic of the main amplifier is shown in Fig. 16. Up to175 W of average power is obtained with a slope efficiency of∼75% with respect to the launched pump power. The emittedbeam quality is nearly diffraction limited (M2 < 1.2).

The stretched and amplified pulses are recompressed usingthe transmission gratings in fused silica, and are almost matchedto the grating stretcher by having a period of 1250 lines/mm.Due to the higher average power damage threshold compared toconventional gold-coated diffraction gratings, these gratings areagain one of the key elements of the high average power fiberCPA system.

Fig. 17 shows the measured autocorrelation trace at the high-est output power of the fiber amplifier. The best compression isachieved at a grating separation of 0.90 m. This optimal gratingdistance stayed unchanged with increasing power. This meansthat the influence of nonlinearity can be neglected, which wasalso confirmed by numerical simulations. The autocorrelationwidth is determined to be 340 fs, corresponding to 220-fs pulseduration (FWHM), assuming a sech2 pulse shape. The pulseduration is constant over the whole power range, no limita-tions or pulse quality degradation are observed. The differencein pulse duration between the oscillator and the recompressedpulses is due to slightly mismatched gratings in the stretcher andcompressor. The measured compressor throughput efficiency is∼75%, resulting in 131 W average power of femtosecond pulses.The peak power is as high as 8.2 MW. To our knowledge, thisis the highest average power ever reported for ultrashort pulsesolid-state laser systems.

The achieved results were only limited by the available pumppower and not by nonlinear pulse distortions; therefore, furtherpower scaling is possible even with the current system. Therecent demonstration of >1 kW output of a continuous-wavefiber laser with nearly diffraction-limited beam quality endorsesthis statement. Furthermore, the spectral width of the oscillatoris completely supported by the amplifier system and stayedunchanged at about 8 nm at all power levels; therefore, evensub-100 fs pulses are obtainable just by employing an oscillatorwhich delivers shorter pulses.

Fig. 18. Summary of all limiting effects in fiber CPA system, assuming MFDof final amplifier of 50 µm, and stretched pulse duration of 1 ns.

B. High-Energy Fiber CPA System

The generation of high-energy pulses from a fiber based lasersystem is certainly the most challenging task. Even significantstretching of pulses cannot avoid the formation of enormouspeak power levels in the fiber amplification stages. As discussedin Section II, fiber facet damage can be prevented by properfiber preparation. Thus, nonlinear pulse distortion is the mostchallenging scaling limitation.

The experimental setup of a high-energy fiber CPA systemis quite similar to that presented in the previous section. Pulsesfrom a femtosecond oscillator are stretched, amplified by thehigh single-pass gain in a certain number of fiber amplifiers,and finally recompressed with a pair of gratings or other suitabledispersive elements. The main difference is the larger necessarystretching factor and the employment of an acoustooptical mod-ulator to reduce the pulse repetition rate. Consequently, the pulseenergy is increased if the average power is kept high.

Pursuing this approach, we realized a fiber CPA system pro-ducing 100 µJ at a 200-kHz repetition rate corresponding toan average power of 20 W. The pulse duration of the recom-pressed pulses is 800 fs. The system employs identical fibers asused in the high average power fiber CPA system; i.e., a 40 µmcore single-transverse-mode ytterbium-doped photonic crystalfiber. The pulse duration during amplification is 700 ps. It isnoteworthy that the average power is only limited by the useof gold-coated gratings in the compressor; thus, an upscalingis possible by employment of dielectric gratings, as previouslydescribed.

To show the pulse energy scalability, Fig. 18 summarizes allof the important restricting effects in a fiber CPA system usinga fiber with 50 µm mode-field diameter in a final amplificationstage and a stretched pulse duration of 1 ns. As can be seen,the surface damage limits the pulse energy to few 100 µJ if nomode expansion is applied. By preparation of the fiber with acoreless endcap, the damage threshold can be increased to wellabove 10 mJ. A fundamental limitation is self-focusing; i.e., thespatial Kerr effect. The critical peak power for self-focusing infused silica is about 3.8 MW [45]. The assumed pulse duration


of 1 ns translates this to 3.8 mJ of energy. As shown in Fig. 18,the nonlinear effects of SPM and SRS are occurring beforeany other limitations. However, the mJ-level can be reachedby using fibers with extremely low nonlinearity. If a mode-field diameter of 50 µm is assumed, a fiber length as short as0.5 m is necessary to avoid a B-integral larger than 1. Sucha fiber is exactly the design referred to as a rod-type photoniccrystal fiber in Section III. As previously discussed, pure linearamplification is not mandatory. By choosing a suitable pulseshape; e.g., parabolic pulses, an accumulated nonlinear phaseof several π can be handled without pulse quality degradation.Thus, even multi-mJ femtosecond pulses are feasible.

As a result, we are convinced that we can demonstrate afiber-based CPA system delivering 100 W average power at 100kHz repetition rate, corresponding to 1 mJ pulse energy andsubpicosecond pulse duration, in the near future. Such a lasersystem would be an ideal source for a number of interestingapplications, such as high-speed high precision micromachiningof various solid materials.

VI. CONCLUSION

We reviewed the main advantages of ultrafast fiber laser sys-tems compared to bulk solid-state lasers. Their heat dissipationcapability, very high single-pass gain, broad gain bandwidth,compactness, robustness, and simplicity of operation make fiberlasers attractive for a host of applications. In particular, rare-earth-doped photonic crystal fibers offer several unique proper-ties, which allow an upward scaling of performance comparedto conventional fiber lasers. Their extended optical parameterrange and the transfer of additional functionality to the fiber bymicrostructuring indicate that such systems have an enormouspotential to scale the performance of next-generation laser sys-tems. To prove this statement, we demonstrated high power andhigh energy photonic crystal fiber laser and amplifier systemsusing the advantages of an air-clad PCF with very high pumpcore NA, and an extended mode field area of an intrinsicallysingle-mode core.

Furthermore, we summarized the limitations of ultrafast fiberlaser systems. Besides the innovative fiber designs, experimentalstrategies were discussed to push these restrictions as far aspossible. In conclusion, in the near future, such short pulsefiber laser systems will deliver very high average powers, andtherefore high repetition rates, in combination with high pulseenergies, so that applications demanding high processing speedsuch as high precision micromachining will be in reach.

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Jens Limpert was born in Jena, Germany, on De-cember 10, 1975. He received the Diploma degreeand the Ph.D. degree in general physics from theFriedrich Schiller University Jena, Jena, Germany in1999 and 2003, respectively.

After a one-year Postdoctoral position at the Uni-versity of Bordeaux, France, he is currently leadingthe Laser Development Group at the Institute of Ap-plied Physics, Friedrich Schiller University Jena. Hehas published over 50 conference and journal papersin the field of fiber lasers. His research interests in-

clude high-power fiber lasers in the pulsed and continuous-wave regime, in thenear-infrared and visible spectral range.

Dr. Limpert is member of the German Physical Society and the Optical So-ciety of America.

Fabian Roser was born in Saalfeld, Germany, onSeptember 17, 1977. He received the Diploma degreein general physics from the Friedrich Schiller Uni-versity Jena, Jena, Germany, in 2003. Currently, heis working toward the Ph.D. degree in the Laser De-velopment Group at the Institute of Applied Physics,Friedrich Schiller University Jena.

His research works include high-power fiber lasersand amplifiers, focusing on ultrashort pulse genera-tion and amplification.

Mr. Roser is a member of the German PhysicalSociety and the Optical Society of America.

Thomas Schreiber was born in Gera, Germany, onSeptember 24, 1976. He received the Diploma de-gree in general physics from the Friedrich SchillerUniversity Jena, Jena, Germany, in 2001. Currently,he is working toward the Ph.D. at the Laser Develop-ment Group, Institute of Applied Physics, FriedrichSchiller University Jena.

He worked with fluorescence lifetime imaging asa biology application of laser physics with Prof. P.French at Imperial College, London, U.K., in 2003.His research work includes fiber lasers and ampli-

fiers, especially applications of photonic crystal fibers in the field of ultrafastoptics, and modeling ultrashort pulse propagation.

Mr. Schreiber is a member of the German Physical Society and the OpticalSociety of America.

Andreas Tunnermann (M’95–A’96) was born inAhnsen, Germany on June 10, 1963. He receivedthe Diploma and Ph.D. degrees in physics from theUniversity of Hannover, Hannover, Germany, in 1988and 1992, respectively.

In 1997, he received the habilitation for his workon ultrastable light sources for interferometric grav-itational wave detectors. He was the Head of theDepartment of Development at the Laser ZentrumHannover, Hannover, Germany, from 1992 to 1997.In 1994, he became the National Scientific Coordi-

nator for the LASER 2000 program. In 1998, he joined the Friedrich SchillerUniversity Jena, Jena, Germany, as a Professor. Since 2003, he has been theDirector of the Institute of Applied Physics and also the Director of FraunhoferInstitute for Applied Optics and Precision Engineering, Jena, Germany. Hismain research interests include scientific and technical aspects associated withthe tailoring of light, and the design and manufacturing of novel micro- andnanooptical photonic devices using high-end microlithography.

Prof. Tunnermann is a member of the German Physical Society, the OpticalSociety of America, and the Institute of Electrical and Electronics Engineers.He received the Roentgen Award in 1997, the WLT Award in 1998, the OttoSchott Award in 2003, the Leibinger Innovation Award in 2004, and the LeibnizAward in 2005.

high-power ultrafast fiber laser systems

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