Scaling the mode instability thresholdwith multicore fibers

Hans-Jürgen Otto,1,* Arno Klenke,1,2 Cesar Jauregui,1 Fabian Stutzki,1

Jens Limpert,1,2 and Andreas Tünnermann1,2,3

1Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena,Albert-Einstein-Str. 15, 07745 Jena, Germany

2Helmholtz-Institute Jena, Fröbelstieg 3, 07743 Jena, Germany3Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Str. 7, 07745 Jena, Germany

*Corresponding author: hans‑juergen.otto@uni‑jena.de

Received January 16, 2014; revised March 24, 2014; accepted March 25, 2014;posted March 25, 2014 (Doc. ID 204770); published April 24, 2014

Mode instabilities (MIs) have quickly become the most limiting effect for the average power scaling of nearlydiffraction-limited beams from state-of-the-art fiber laser systems. In this work it is shown that, by using an advancedmulticore photonic crystal fiber design, the threshold power of MIs can be increased linearly with the numberof cores. An average output power of 536 W, corresponding to 4 times the threshold power of a single core, isdemonstrated. © 2014 Optical Society of AmericaOCIS codes: (140.3510) Lasers, fiber; (060.5295) Photonic crystal fibers; (120.6810) Thermal effects.http://dx.doi.org/10.1364/OL.39.002680

State-of-the-art fiber lasers and amplifiers have alreadyreached the kilowatt level of average output power inboth cw and pulsed operation [1,2]. However, these highpower levels lead to an increased impact of thermaldistortions on the waveguide [3], which ultimately man-ifests itself in the form of mode instabilities (MIs) [4].Above the MI threshold, the former stable and nearlyGaussian-shaped beam profile emitted by an active fiberbecomes temporally unstable and shows an increasedhigher-order mode content [5]. Consequently, MIs limitthe power scalability of nearly diffraction-limited fiberlaser systems. The power level at which MIs occur canbe increased by using advanced fiber designs [6,7],structured doping geometries [8,9], or by an active stabi-lization of the MI-degraded beam [10]. However, even forthese advanced schemes, it is still a challenge to increasethe MI threshold beyond a factor 2 or 3.A promising general approach to overcome the

limitations of single-emitter systems is the coherent com-bination of several emitters [11]. The downside of thisapproach is that it usually involves large, bulky, and com-plex setups. An elegant way to get around this problem isto integrate the different emitters into a single multicorefiber (MCF). This kind of fiber has already been demon-strated in [12] as an approach to reduce the influence ofnonlinearities and to increase the damage threshold inhigh-energy ultrashort pulsed fiber laser systems. Theindividual cores in these fibers are typically coupled toone another in order to produce a supermode or theyhave a small core-to-core distance in order to producea Gaussian-like beam shape in the far field [12]. In thiswork, the promising concept of MCFs is adapted toget around the onset of MI in a fiber laser system. Hereby,in contrast to previous designs, the MCF design demon-strated has a large core-to-core spacing and a tight con-finement of the light inside the signal cores, whichpurposefully inhibits optical coupling between them.This, in principle, should allow for a linear increase ofthe MI threshold with the number of cores, if thermalcoupling between the cores is neglected.

A scanning electron microscope (SEM) image of themulticore photonic crystal fiber (manufactured by NKTPhotonics) is presented in Fig. 1. Each individual coreis formed by seven missing air holes inside a hexagonallattice. Each core has a diameter of 50 μm and the short-est distance between two cores is 56 μm. The structureis surrounded by a common air-clad (dimensions510 μm × 156 μm) to allow for high-power diode pump-ing. The pump absorption of the MCF is ∼22 dB∕m.

The setup used for the experiments is illustrated inFig. 2. The seed system delivers picosecond pulses withan average output power of 8 W at a center wavelength of1033 nm. This seed beam is subsequently split in four spa-tially separated beams. The beam splitter comprises twoparallel mirrors; one of which (M1) is a high-reflectivemirror and the other one (M2) is a semitransparent mir-ror with a reflectivity of 60%. The resulting power distri-bution of the four beams is 2.7∶2.1∶1.7∶0.5 W. Note thatby spatially segmenting the reflectivity of mirror M2 anequal power distribution can be achieved. The distancebetween the beams can be adjusted by tilting the beamsplitter. In this particular setup the beam separation isproportional to the core spacing, which allows couplingin the fiber with a single lens (L1). The MCF is used in acounterpropagating amplification scheme. The pump-light (976 nm) is focused to the size of the smaller air-clad

Fig. 1. SEM image of the MCF (the four Yb3� doped cores arehighlighted in green).

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dimension. Due to mode-mixing effects in the highlymultimode air-clad, it is not necessary to additionallyshape the pump beam.After amplification, the beams are analyzed by a beam

diagnostics setup comprising a power meter, a smallaperture photodiode, and a camera. In this proof-of-principle experiment, no combing element was used inorder to be able to analyze the behavior of each singlecore. The fiber laser system has been characterized ina similar way as that described in [13]. Thus, a photo-diode with a small aperture is employed to measurethe temporal evolution of the intensity distribution emit-ted from a single core. The stability of the beam is thengiven by the standard deviation calculated from the pho-todiode time traces. This measurement is repeated fordifferent power levels and different cores. The resultinggraph is depicted in Fig. 3 and shows the evolution of themeasured standard deviation as a function of the outputpower of each individual core excited one at a time afterseveral measurements (orange color-shaded points). Dueto the onset of MIs the standard deviation increases rap-idly above the threshold. From these experimental datathe threshold powers can be determined to be 165 W(core 1), 153 W (core 2), 149 W (core 3), and 119 W (core4) by using the strict definition of the threshold from [13].The reason for the different thresholds of the individual

cores is mainly due to the inhomogeneous distribution ofthe seed power among the four seed beams. The highestseed power corresponded to core 1 and the lowest one tocore 4. As shown in [14,8] the threshold power dependslinearly on the seed power, thus resulting in the highestand lowest thresholds being those of core 1 and core 4,respectively. The average threshold power of the fourcores is 147 W.

After the single-core excitation, all the cores areseeded at the same time and the threshold measurementis repeated. Figure 4 shows an excerpt from Media 1 thatgives a visual impression of the evolution of all four coreswith increasing power. From the image itself it can beseen that the emitted intensity distributions differ froma Gaussian-like beam shape. The reason for this is a non-uniform index profile within the Yb3� doped core regiondue to the fabrication process of this particular fiber. It isworth noting that current photonic crystal fibers do notpossess this core nonuniformity. Nevertheless, for thecomparison between single-core excitation and multi-core excitation this particular deformation has no influ-ence. However, a MCF without these nonuniformitiesmay show a higher absolute MI threshold power.

From Media 1 it can be seen that the power in core 1 isthe highest of all four cores due to the largest seed powercoupled into this core. Consequently, the thresholdpower of MI is reached in this core first. Thus, the photo-diode has been adjusted to measure the stability of core1. The resulting measured stability evolution is alsoshown in Fig. 3 (green points). The analysis of this tracecasts a combined threshold power of 536 W. This thresh-old power is almost 4 times higher than that of the indi-vidual cores. This result leads to the conclusion that thecores are largely independent (i.e., do not affect eachother) and, therefore, the individual thresholds addlinearly as it would happen in the case of combining spa-tially separated fiber emitters. However, this result is notas trivial as it might seem at first sight and needs moreclarification since the cores can be optically and ther-mally coupled. It can be expected that coupling willcause the superposed threshold to approach the thresh-old of an individual large core. This is supported by thework of Fini et al. [15]. It has been shown that a super-mode formed in a MCF with strong coupling betweenindividual cores and a comparable step-index fiber havethe same modal features.

However, in the case of the particular used MCF theoptical coupling is avoided by the spatial separation ofthe cores and by designing the cores to guide well-confined modes. From Fig. 4 it can be easily confirmedthat any optical coupling is avoided due to the strongguidance of the individual cores. However, the thermalcoupling of the cores is a more complex subject since

Fig. 2. Sketch of the experimental setup. M1–M5 are dielectricmirrors; M2 has 60% reflectivity. D1 is a dichroic dielectricmirror to separate the signal and pump beams; L1 and L2are focusing lenses used to couple the seed signal in the fiberand to collimate the amplified signal, respectively.

Fig. 3. Stability evolution of the laser system when excitingeach individual core of the MCF one at a time (orange color-shaded points) and when all cores are simultaneously excited(green). The corresponding average MI thresholds are 147W forthe individual cores and 536 Wwhen simultaneously exciting allthe cores.

Fig. 4. Excerpt from Media 1 showing the intensity distribu-tions of the four cores slightly below the threshold.

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no thermal shielding has been incorporated in thestructure.In order to evaluate a possible thermal cross talk

between cores, the thermally induced change of therefractive index caused in one core due to the heat loadof a neighboring core is simulated. It should be empha-sized that the scope of the simulation is to study thethermal cross talk between neighboring cores in generaland not for the particular MCF presented in the Letter.Therefore, only two cores are taken into account insteadof simulating the entire structure of the MCF. Figure 5shows a sketch of the simulation scheme. Two cores witha certain distance Δs and the ability to guide the LP01 andLP11 modes are considered. Core 1 is seeded with 10 W(99% in the LP01 and 1% in the LP11). During the ampli-fication of the seed to ∼145 W the resulting temperatureprofile originated in core 1 influences core 2. Addition-ally, two different modal excitation states in core 1 haveto be considered: LP01 � LP11odd and LP01 � LP11even. Inthe case of the odd LP11 orientation the modal beatingwill occur in the vertical direction, while it will be hori-zontal for the even LP11 excitation.In order to get an approximate idea of the influence of

the thermal profile originated in core 1 on core 2, thechange of the transversal refractive index within halfa longitudinal beating period [i.e., refractive indexrefractive index change � n�x; y;position1� −n�x; y; position2� in Fig. 5(b)] in the proximity of the fiberoutput end is analyzed. This allows calculating thecoupling constant κ [16,14] of the quasi-periodic indexchange and, consequently, it gives an estimation of themaximal thermal cross talk between both cores. Figure 6illustrates the thermally induced change of the refractiveindex profile along the transversal cuts of core 1 (bluedashed line in Fig. 5) and core 2 (red and green dashedlines in Fig. 5). The curves are normalized to the maxi-mum value. The change of these index profiles withinhalf a beating period (solid and dashed lines) is usedto estimate the mode coupling strength. It should benoted that for illustration purposes in Fig. 6 the core sep-aration Δs is zero and the longitudinal positions are closeto the output end facet of the fiber where the maximumheat load occurs in case of a counterpumping scheme. Inthis scenario the change of the refractive index is largest.From Fig. 6 it can be seen that there is indeed a change

of the transverse refractive index in core 2 caused by thetemperature profile originated in core 1. When exciting

the even LP11 the refractive index in core 2 goes upand down as a consequence of the shift of the centerof gravity of the temperature profile in core 1. The situa-tion changes when exciting the odd LP11. Here, therefractive index profile becomes also parabolic in core2 and additionally its center of gravity shifts with thelongitudinal beating period. However, the absolutechange of the refractive index in this case is smaller thanin the previous case but since its geometry matches themodal interference pattern better, it can still have impor-tant repercussions.

From the simulations of the thermally induced refrac-tive index evolution the coupling constant κ with respectto the core separation Δs are estimated and shownin Fig. 7.

The value of κ is normalized to the corresponding cou-pling constant in core 1 and is a measure of the strengthof the energy transfer between the guided modes LP01and LP11eo in core 2. It can be seen that the couplingstrength decreases with larger distances Δs betweenthe cores. A separation Δs � 56 μm, comparable to theMCF used in the experiments, leads to a couplingconstant κ ∼ 9%. It has to be expected, that the actualvalue of κ will be higher as the central cores have thermal

Fig. 5. Thermal cross talk between core 1 and core 2 issimulated as a function of their separation Δs. (a) Simulationscheme for different orientations (even/odd) of the LP11 incore 1. (b) The thermally induced change of the refractiveindex in core 1 and core 2 due to the longitudinal modal beatingin core 1 is investigated within half a beating period[refractive index change�n�x;y;position1�−n�x;y;position2�].

Fig. 6. Evolution of the thermally induced index of core 1 and2 generated by modal beating in core 1. The graphs are normal-ized to maximum. Solid and dashed lines represent the changewithin half a beating period.

Fig. 7. Normalized coupling constant κ as a function of thecore separation Δs for the LP11e and LP11o excitation in core 1.

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contributions from two neighboring cores. Nevertheless,the coupling of neighboring cores is still weak enough sothat no significant impact on the MI threshold has beenobserved. Moreover, a further increase in core separationwill reduce the thermal cross talk even further.In conclusion, a MCF concept with decoupled individ-

ual cores is a promising way to circumvent MI and allowfurther average power scaling with nearly diffraction-limited beam quality. The thermal and optical couplingof the individual cores can be efficiently mitigated bychoosing an appropriate core spacing and by designingthe cores to confine the modes well within their areaof optical influence. In addition to the linear increaseof the MI threshold with the number of cores, the multi-core concept provides an effective scaling of the modefield area and, consequently, a reduction of parasitic non-linear effects and an increase in pulse energy. Currently,a compact combining stage of such a MCF system isunder development. It can be expected that any environ-mentally induced phase shift between the channels(i.e., cores) will have a much smaller influence thanin a bulk coherent-combining setup due to the smallchannel-to-channel optical path differences. Moreover,in the MCF all the cores will be affected by external per-turbations in a similar way. Thus, the demands on activephase stabilization will be reduced. A simple way of com-bining the separated beams is the use of the beam splitterin the reverse direction.In the near future this concept will enable the develop-

ment of a compact, kilowatt class, ultrashort pulsed fiberlaser with multi 10 mJ pulse energies.

Financial support from the German Federal Ministryof Education and Research (BMBF), the EuropeanResearch Council under the European Union’s SeventhFramework Programme (FP7/2007-2013)/ERC grantagreement no. [240460] “PECS,” and the Thuringian

Ministry of Education, Science and Culture TMBWK)under contract B514-10061 (Green Photonics).

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