High-power ultrafast fiber laser systems - IEEE Xplore

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

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High-Power Ultrafast Fiber Laser Systems Jens Limpert, Fabian Röser, Thomas Schreiber, and Andreas Tünnermann, Associate Member, IEEE (Invited Paper)

Abstract—The recent demonstration of rare-earth-doped fiber lasers with a continuous wave output power well above the kilowatt level with diffraction-limited beam quality has proven that fiber lasers constitute a power-scalable solid-state laser concept. To generate intense pulses from a fiber, several fundamental limitations have to be overcome. Nevertheless, novel experimental strategies and fiber designs offer an enormous potential toward laser systems with high average powers and high pulse energies. This paper reviews the challenges, achievements, and perspectives of ultrashort pulse generation and amplification in fibers. Index Terms—Chirped pulse amplification, fiber laser and amplifiers, microstructured fibers, nonlinear fiber optics, ultrafast optics.

I. INTRODUCTION NUMBER of important practical as well as fundamental research applications of ultrafast lasers appeared over the last decades [1], a trend initiated by the step from old dye-laser technology toward solid-state lasers. These high-power ultrafast solid-state lasers use small rods as the amplifier media— for instance, Titanium-doped sapphire as the most widespread one [2]—and have the potential to generate significantly higher pulse energies, higher powers, and shorter pulse durations, in combination with greater reliability than dye-lasers. However, these systems are difficult to scale in average power and suffer from low efficiencies because direct diode pumping is not possible. Furthermore, the complexity of short pulse high energy Ti:sapphire lasers still constrain the employment of ultrafast laser 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 forward in terms of efficiency. In order to overcome thermooptical effects, which limit the power scaling capability of these systems, several novel gain media designs, such as thin disk or slab, have been introduced [3], [4]. However, due to the low single pass gain of these amplifier materials, very complex systems; e.g., regenerative amplification schemes, are required to obtain a reasonable output. Therefore, robustness, compactness, and long-term stability are restricted in short pulse bulk solid-state laser systems. Alternatively, forming the gain medium to be long and thin not only leads to outstanding thermooptical properties, but also

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Manuscript received November 17, 2005. J. Limpert, F. Röser, 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. Tünnermann is with the Institute of Applied Physics, Friedrich Schiller University Jena, D-07745 Jena, Germany, and also with the Fraunhofer Institute for 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 have the reputation of being immune to any thermooptical problems due to their special geometry. Their excellent heat dissipation is due to the large ratio of surface-to-active volume of such fiber. The beam quality of the guided mode is determined by the fiber core 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 not limited to the Rayleigh length, as is the case in longitudinally pumped bulk lasers. The gain of the laser medium is determined by the product of pump light intensity and interaction length with the laser radiation in the gain medium. Therefore, the decisive product can be orders of magnitude higher in fibers than in other bulk solid-state lasers. This results in very efficient operation of fiber laser systems exhibiting very high gain and low pump threshold values. Additionally, complete integration of the laser process in a waveguide allows for inherent compactness and long-term stability of fiber lasers. In particular, Ytterbium-doped glass fibers, which have a quantum defect of less than 10%, can provide optical-to-optical efficiencies well above 80% and, therefore, low thermal load. These fiber laser systems are especially interesting for highpower ultrashort pulse generation and amplification because of several unique properties [5]: Firstly, a broad emission spectrum allows for short pulse amplification. In ytterbium-doped glass fibers, the amplification bandwidth of approximately 40 nm supports, in principle, pulses of durations as short as ∼30 fs. Furthermore, the absorption spectrum covers a wavelength range in which powerful diode lasers are commercially available. An additional point to note is that the long fluorescence lifetime (∼1 ms) results in a high-energy storage capability. Exited-state absorption of pump or signal radiation, or concentration quenching by ion-ion energy transfer processes, does not occur with ytterbium because only two energy level manifolds are relevant for all optical wavelengths. High power fiber lasers usually use the double-clad fiber concept, invented in 1988 by Snitzer [6]. Such a double-clad fiber is characterized by a second waveguide, which is highly multimode, surrounding the active core. Into this second waveguide, also called inner cladding or pump core, low brightness high power diode laser radiation can be launched. This pump light is gradually absorbed over the entire fiber length and is converted into high brightness high power laser radiation. Thus, doubleclad rare-earth doped fibers can provide a highly efficient brightness improvement by pump-to-laser radiation conversion by the laser process itself. The aforementioned properties make rare-earth-doped fibers superior to other solid-state laser concepts in a variety of performance categories. This has become obvious following several

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recent demonstrations of continuous wave fiber laser systems exhibiting more than 1 kW of average power while maintaining an excellent beam quality [7]–[10]. II. BASIC LIMITATIONS OF SHORT PULSE AMPLIFICATION IN OPTICAL FIBERS The confinement of the laser radiation and the long interaction length makes fibers attractive for solid-state laser systems; however, nonlinear effects impose fundamental limitations on these systems, which may lead to severe pulse distortions and even to damage of the fiber. The lowest-order nonlinear effect in optical fibers originates from the third-order susceptibility χ(3) , and is responsible for an intensity-dependent refractive index in the form n = n0 + n2 I. Consequently, an optical field propagating through a fiber experiences a self-induced phase shift, a phenomenon referred to as self-phase modulation (SPM). A second important class of nonlinear effects results from stimulated inelastic scattering, whereby the radiation transfers a part of its energy to the glass host in form of excited vibrational modes. A large frequency shift (∼13 THz) is observed due to the excitation of optical phonons, a process called stimulated Raman scattering (SRS), whereas the excitation of acoustical phonons leads to a much smaller shift (∼17 GHz) called stimulated Brillouin scattering (SBS). Both manifest themselves as a significant power loss mechanism in fiber-based laser systems. In the context of ultrashort pulse amplification in fibers, where the spectra are significantly broader than the Brillouin gain bandwidth, the effect of SBS can be neglected [11]. In general, the nonlinearity coefficients in glass fibers are intrinsically small. Both the nonlinear index coefficient n2 and the gain coefficients of SRS and SBS are at least two orders of magnitude smaller than in other common nonlinear media [12]. Nevertheless, due to the large product of intensity and interaction length inside the fiber core, nonlinear effects can be observed at very low power levels and basically limit the performance of pulsed rare-earth-doped fiber systems. Pulse propagation in fibers is described by the nonlinear Schrödinger equation (NLSE), shown in (1), which describes the temporal and longitudinal dependency of the slowly varying pulse envelope A(z, T ) along the fiber in the retarded time frame T ∞ in −1 ∂ n g ∂A − βn (1) A − A = iγ|A|2 A n ∂z n! ∂T 2 n =2 where g represents the gain or loss of the fiber, βn are the Taylor expansion coefficients of the dispersion parameter β2 (ω), and the pulse amplitude A(z, t) is assumed to be normalized such that |A|2 represents the optical power. The nonlinearity coefficient γ is defined by n2 ω0 γ= (2) cAeff where Aeff is the effective mode field area, n2 is the nonlinear refractive index coefficient, c equals the speed of light, and ω0 is the 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 recompressed pulses assuming a Gaussian pulse shape during amplification in comparison with transform-limited pulse (B = 0).

self-phase modulation as the lowest-order nonlinear effect. The phase imposed by SPM can lead to a nonlinear chirp, which cannot be compensated by standard dispersive elements. An important parameter is the accumulated nonlinear phase of the pulse traveling through the fiber, the so-called B-integral, defined by (3) with I(z) as the pulse intensity varying over the fiber length L [13] L 2π · n2 · I(z) · dz. (3) B= λ 0 A B-integral of smaller than ∼1 is considered as a linear propagation, pulse quality degradation is expected above this value. To illustrate the consequence of SPM, a chirped pulse amplification (CPA) system is simulated by solving the NLSE using the split-step Fourier method and assuming a Gaussian pulse shape during amplification. Fig. 1 illustrates the recompressed pulse intensity profiles after accumulating a nonlinear phase shift in comparison to the transform-limited pulse. As revealed, SPM leads to a pulse broadening and the evolution of sidelobes containing a considerable amount of pulse energy. Hence, even moderate self-phase modulation can significantly distort a recompressed pulse after amplification, with the result of reduced peak power and degraded pulse contrast. In contrast to SPM, SRS is a process which is initiated at a certain threshold value. Above this threshold, energy is rapidly transferred to a frequency-downshifted Stokes wave. This process can be described quantum-mechanically as an inelastic scattering process, where an incident photon is annihilated to create a photon at a downshifted frequency, the Stokes wave, and a 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 momentum is available. The threshold for the onset of SRS in a fiber amplifier is, to a good approximation, given by (4), where gR is the peak Raman gain and Leff is the effective interaction length [11] SRS Pthreshold ≈

16 · Aeff . gR · Leff

(4)

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

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Fig. 4.

Fig. 2.

Coreless fiber end cap to avoid facet damage.

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

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

Fig. 3.

Damaged fiber facet.

As an example, the threshold behavior of SRS and energy loss of the propagating optical pulse in a single-mode shortpulse fiber amplifier is shown in Fig. 2. The measured emitted spectrum reveals strong changes with increasing peak power. At lower pulse energies, only the signal at 1060 nm is observable. At peak powers higher than 2.3 kW, a signal at about 1120 nm increases rapidly, indicating that the threshold of stimulated Raman scattering is passed. At a peak power of 3.4 kW, even the frequency up-shifted anti-Stokes wave at about 1010 nm becomes detectable. At 4.6 kW, the pulse breaks up and most of the intense central part of the spectrum is transferred into the Stokes wave. Beside these basic nonlinear effects, fiber damage has to be considered as a scaling limitation of high-energy fiber laser systems. The surface damage fluence threshold of fused silica, which is significantly lower than the bulk damage fluence threshold, 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, the damage threshold is in general significantly lower than the extractable energy. Fig. 3 shows an example of a damaged fiber facet.

This problem can be solved by special treatment of the fiber end. One solution is to splice a coreless end cap on the output side of the fiber amplifier, as shown in Fig. 4. The expansion of the beam reduces the fluence and avoids fiber facet damage. Due to the aforementioned nonlinear distortions, the generation of high peak powers in pulsed fiber laser systems is very challenging. Novel fiber designs and experimental strategies have to be applied to scale up the output parameters. There are basically two approaches to amplify ultrashort laser pulses in fibers: the parabolic pulse amplification, which actually uses nonlinearity, and the chirped pulse amplification, which relies on avoidance of nonlinearity. Before reviewing these two basic amplification concepts, fiber designs with intrinsically reduced nonlinearity; i.e., an increased mode-field diameter and a reduced fiber length, will be discussed. III. LOW-NONLINEARITY FIBER CONCEPTS As discussed in Section III and shown in (3) and (4), the nonlinear effects are proportional to the fiber length and the intensity in the fiber core, and therefore are inversely proportional to the mode-field area of the guided radiation in the fiber. Thus, an enlargement of the mode-field diameter and a reduction of fiber length would help to avoid disturbing nonlinear effects. Using special techniques and fiber designs the mode-field-area of single-transverse mode fiber devices could be significantly increased in the past years. To investigate the modal properties, the normalized frequency V is introduced in (6). It is related

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to the effective refractive indices of the core nC and cladding nClad of the fiber with a core radius aeff . One can show that a fiber becomes single-mode for V < 2.405 [15] 2π 2π aeff · NA = aeff n2C − n2Clad . V = (6) λ λ Thus, one approach to increase the core diameter is to decrease the numerical aperture (NA) at a certain value of V . However, in conventional step-index fibers, a reduction in the numerical aperture below a certain limit is not tolerable in terms of propagation losses and precision of core fabrication method. More sophisticated fiber designs are based on modified index profiles, which increase the single-mode area by using an outer ring structure. Such large-mode area fibers with up to few 10-µm core diameter and diffraction-limited output could be demonstrated [16]. Preferential gain to the fundamental mode is created by an optimally overlapping rare-earth dopant distribution [17]. This concept can be extended to gain and loss managed multimode fibers to discriminate higher order modes [18]. Stable fundamental mode propagation over more than 20 m at 1.5-µm wavelength is obtained in a conventional step-index core double-clad fiber with a core diameter of 45 µm with a numerical aperture of 0.13 [19]. In [19], it is stated that the mode-coupling between transversal modes is affected by the cladding diameter and the fiber fabrication process. Thus, for maintaining the fundamental mode, it is preferable to use large cladding diameters relative to the core diameters and the high-quality modified chemical-vapor deposition (MCVD) process. Further discrimination of higher order modes is achieved by a careful optimization of the seed launching conditions and incorporated tapered sections [20] inside the fiber laser or amplifier. Using these techniques, the single-mode operation of a 50-µm core (MFD ∼30 µm) fiber amplifier at 1.06 µm was reported [21]. In recent years, the most commonly employed lownonlinearity fiber concept is the low-numerical aperture largemode-area (LMA) fiber. Such an LMA fiber has V-parameters in the range of 5–10, and can therefore guide several higher order transverse modes. However, bending losses can be applied to achieve stable fundamental mode operation of a fiber laser or amplifier [22]. Fig. 5 shows the calculated bending losses for the fundamental and three higher-order modes as a function of bending radius in a 30-µm LMA fiber (NA = 0.06) [23]. This calculation reveals a significant discrimination of higher order transverse modes. As an example, at a bending radius of 50 mm, the induced bending loss for the LP01 mode is 0.01 dB/m and for the first higher order mode LP11 is 52 dB/m. This difference of approximately five orders of magnitude enforces fundamental mode operation. Robust and environmentally stable fundamental mode operation in even larger cores is only possible in a truly single-mode fiber. Therefore, a strictly single-mode fiber with large mode areas would be a significant achievement. Microstructuring the fiber by including air-holes adds several attractive properties to conventional 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 of air-filled holes characterized by the hole diameter d and the

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

pitch Λ [Fig. 6(a)]. To describe the modal properties, (6) can also 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 effective cladding index is strongly dependent on λ/Λ. Also, in the limit of λ/Λ → 0, the effective cladding index reaches the core index. This property becomes more interesting by plotting the single mode boundary V = 2.405 of the normalized wavelength over the relative hole diameter, as done in Fig. 6(b). For d/Λ > 0.4, the fiber turns from multimode operation into single mode operation if the relative wavelength is large enough. For d/Λ < 0.4 the fiber becomes single mode for all wavelengths λ/Λ due to the fact that the wavelength dependence of nClad keeps the NA low enough to stay single mode. This regime is called endlessly single mode. It was observed in the first photonic crystal fibers and was investigated theoretically soon afterward [25], [26]. Such an endlessly single mode operation is not known from step index fibers, and leads to a concept of scaling the core diameter of a photonic crystal fiber. If the modal properties do not depend on the normalized wavelength λ/Λ, the mode diameter of a PCF, which is proportional to Λ, can theoretically be scaled to infinity at a given wavelength. Of course, the scaling of the core size is limited by increasing propagation losses [27]. If the V -parameter of the photonic crystal fiber is smaller than one, confinement of the mode is too weak and leakage occurs through a PCFs finite cladding. On the other hand, if the value λ/Λ becomes too small (1 kW output of a continuous-wave fiber laser with nearly diffraction-limited beam quality endorses this statement. Furthermore, the spectral width of the oscillator is completely supported by the amplifier system and stayed unchanged at about 8 nm at all power levels; therefore, even sub-100 fs pulses are obtainable just by employing an oscillator which delivers shorter pulses.

B. High-Energy Fiber CPA System The generation of high-energy pulses from a fiber based laser system is certainly the most challenging task. Even significant stretching of pulses cannot avoid the formation of enormous peak power levels in the fiber amplification stages. As discussed in Section II, fiber facet damage can be prevented by proper fiber preparation. Thus, nonlinear pulse distortion is the most challenging scaling limitation. The experimental setup of a high-energy fiber CPA system is quite similar to that presented in the previous section. Pulses from a femtosecond oscillator are stretched, amplified by the high single-pass gain in a certain number of fiber amplifiers, and finally recompressed with a pair of gratings or other suitable dispersive elements. The main difference is the larger necessary stretching factor and the employment of an acoustooptical modulator to reduce the pulse repetition rate. Consequently, the pulse energy is increased if the average power is kept high. Pursuing this approach, we realized a fiber CPA system producing 100 µJ at a 200-kHz repetition rate corresponding to an average power of 20 W. The pulse duration of the recompressed pulses is 800 fs. The system employs identical fibers as used in the high average power fiber CPA system; i.e., a 40 µm core single-transverse-mode ytterbium-doped photonic crystal fiber. The pulse duration during amplification is 700 ps. It is noteworthy that the average power is only limited by the use of gold-coated gratings in the compressor; thus, an upscaling is possible by employment of dielectric gratings, as previously described. To show the pulse energy scalability, Fig. 18 summarizes all of the important restricting effects in a fiber CPA system using a fiber with 50 µm mode-field diameter in a final amplification stage and a stretched pulse duration of 1 ns. As can be seen, the surface damage limits the pulse energy to few 100 µJ if no mode expansion is applied. By preparation of the fiber with a coreless endcap, the damage threshold can be increased to well above 10 mJ. A fundamental limitation is self-focusing; i.e., the spatial Kerr effect. The critical peak power for self-focusing in fused silica is about 3.8 MW [45]. The assumed pulse duration

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

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 before any other limitations. However, the mJ-level can be reached by using fibers with extremely low nonlinearity. If a modefield diameter of 50 µm is assumed, a fiber length as short as 0.5 m is necessary to avoid a B-integral larger than 1. Such a fiber is exactly the design referred to as a rod-type photonic crystal fiber in Section III. As previously discussed, pure linear amplification is not mandatory. By choosing a suitable pulse shape; e.g., parabolic pulses, an accumulated nonlinear phase of 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 a fiber-based CPA system delivering 100 W average power at 100 kHz repetition rate, corresponding to 1 mJ pulse energy and subpicosecond pulse duration, in the near future. Such a laser system would be an ideal source for a number of interesting applications, such as high-speed high precision micromachining of various solid materials. VI. CONCLUSION We reviewed the main advantages of ultrafast fiber laser systems compared to bulk solid-state lasers. Their heat dissipation capability, very high single-pass gain, broad gain bandwidth, compactness, robustness, and simplicity of operation make fiber lasers attractive for a host of applications. In particular, rareearth-doped photonic crystal fibers offer several unique properties, which allow an upward scaling of performance compared to conventional fiber lasers. Their extended optical parameter range and the transfer of additional functionality to the fiber by microstructuring indicate that such systems have an enormous potential to scale the performance of next-generation laser systems. To prove this statement, we demonstrated high power and high energy photonic crystal fiber laser and amplifier systems using the advantages of an air-clad PCF with very high pump core NA, and an extended mode field area of an intrinsically single-mode core. Furthermore, we summarized the limitations of ultrafast fiber laser systems. Besides the innovative fiber designs, experimental strategies were discussed to push these restrictions as far as possible. In conclusion, in the near future, such short pulse fiber laser systems will deliver very high average powers, and therefore high repetition rates, in combination with high pulse energies, so that applications demanding high processing speed such as high precision micromachining will be in reach.

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Jens Limpert was born in Jena, Germany, on December 10, 1975. He received the Diploma degree and the Ph.D. degree in general physics from the Friedrich Schiller University Jena, Jena, Germany in 1999 and 2003, respectively. After a one-year Postdoctoral position at the University of Bordeaux, France, he is currently leading the Laser Development Group at the Institute of Applied Physics, Friedrich Schiller University Jena. He has published over 50 conference and journal papers in the field of fiber lasers. His research interests include high-power fiber lasers in the pulsed and continuous-wave regime, in the near-infrared and visible spectral range. Dr. Limpert is member of the German Physical Society and the Optical Society of America.

Fabian Röser was born in Saalfeld, Germany, on September 17, 1977. He received the Diploma degree in general physics from the Friedrich Schiller University Jena, Jena, Germany, in 2003. Currently, he is working toward the Ph.D. degree in the Laser Development Group at the Institute of Applied Physics, Friedrich Schiller University Jena. His research works include high-power fiber lasers and amplifiers, focusing on ultrashort pulse generation and amplification. Mr. Röser is a member of the German Physical Society and the Optical Society of America.

Thomas Schreiber was born in Gera, Germany, on September 24, 1976. He received the Diploma degree in general physics from the Friedrich Schiller University Jena, Jena, Germany, in 2001. Currently, he is working toward the Ph.D. at the Laser Development Group, Institute of Applied Physics, Friedrich Schiller University Jena. He worked with fluorescence lifetime imaging as a biology application of laser physics with Prof. P. French at Imperial College, London, U.K., in 2003. His research work includes fiber lasers and amplifiers, especially applications of photonic crystal fibers in the field of ultrafast optics, and modeling ultrashort pulse propagation. Mr. Schreiber is a member of the German Physical Society and the Optical Society of America.

¨ Andreas Tunnermann (M’95–A’96) was born in Ahnsen, Germany on June 10, 1963. He received the Diploma and Ph.D. degrees in physics from the University of Hannover, Hannover, Germany, in 1988 and 1992, respectively. In 1997, he received the habilitation for his work on ultrastable light sources for interferometric gravitational wave detectors. He was the Head of the Department of Development at the Laser Zentrum Hannover, Hannover, Germany, from 1992 to 1997. In 1994, he became the National Scientific Coordinator for the LASER 2000 program. In 1998, he joined the Friedrich Schiller University Jena, Jena, Germany, as a Professor. Since 2003, he has been the Director of the Institute of Applied Physics and also the Director of Fraunhofer Institute for Applied Optics and Precision Engineering, Jena, Germany. His main research interests include scientific and technical aspects associated with the tailoring of light, and the design and manufacturing of novel micro- and nanooptical photonic devices using high-end microlithography. Prof. Tünnermann is a member of the German Physical Society, the Optical Society of America, and the Institute of Electrical and Electronics Engineers. He received the Roentgen Award in 1997, the WLT Award in 1998, the Otto Schott Award in 2003, the Leibinger Innovation Award in 2004, and the Leibniz Award in 2005.