Management of the high-order mode content in large(40 μm) core photonic bandgap Bragg fiber laser

D. A. Gaponov,1 S. Février,1,* M. Devautour,1 P. Roy,1 M. E. Likhachev,2 S. S. Aleshkina,2

M. Y. Salganskii,3 M. V. Yashkov,3 and A. N. Guryanov3

1Xlim, UMR 6172 CNRS, University of Limoges 123 Avenue A. Thomas, 87060 Limoges, France2Fiber Optics Research Center of Russian Academy of Sciences, 38 Vavilov Street, Moscow, 119333, Russia

3Institute of Chemistry of High Purity Substances, 49 Tropinin Street, Nizhny Novgorod, 603950, Russia*Corresponding author: sebastien.fevrier@xlim.fr

Received April 19, 2010; revised June 5, 2010; accepted June 6, 2010;posted June 11, 2010 (Doc. ID 127236); published June 25, 2010

Very large-mode-area Yb3þ-doped single-mode photonic bandgap (PBG) Bragg fiber oscillators are considered. Thetransverse hole-burning effect is numerically modeled, which helps properly design the PBG cladding and the Yb3þ-doped region for the high-ordermode content to be carefully controlled. A ratio of the Yb3þ-doped region diameter tothe overall core diameter of 40% allows for single-mode emission, even for small spool diameters of 15 cm. Such afiber was manufactured and subsequently used as the core element of a cw oscillator. Very good beam quality para-meterM2 ¼ 1:12 and slope efficiency of 80%were measured. Insensitivity to bending, exemplified by the absence oftemporal drift of the beam, was demonstrated for curvature diameter as small as 15 cm. © 2010 Optical Society ofAmericaOCIS codes: 060.2280, 060.2270.

In today’s oscillators or amplifiers, large-mode-area(LMA) fibers able to withstand very high light intensities,either in continuous or pulsed regimes, are indispensa-ble. Recently, the highest performances in the femtose-cond regime have been obtained using 80 μm corediameter rod-type photonic crystal fibers (PCFs) fromwhich 100 μJ, 370 MW, 270 fs pulses can be extracted[1]. This outstanding result was obtained with a rigid2-mm-thick rod fiber. Very recently, 830 W averagepower 640 fs pulses were extracted from a three-stagechirped-pulse amplifier system incorporating 27 μmmode field diameter fibers [2]. According to Eidam etal., using such fibers was dictated by the requirementof single-mode operation, which is not met at very highaverage powers in in large core diameter PCFs [2]. Thisdetrimental behavior of LMA fibers has been attributed tothe transverse hole-burning effect.In standing-wave laser cavities, spatial (longitudinal)

hole burning is responsible for shaping the population in-version. The spatial distribution of the population inver-sion determines the gain of the different cavity modesand, therefore, which modes will oscillate. In LMA fiberamplifiers, Jiang and Marciante [3] showed that, even atmoderate powers, the fundamental mode can dramati-cally deplete the inversion population near the fiber axis.This effect is referred to as transverse hole burning.Hence, the high-order transverse modes (HOM), takingbenefit of their good overlap with peripheral ions, canbe amplified. Therefore, in accurate modeling tools,the transverse variations of light intensities and popula-tion densities are taken into account [3,4].The photonic bandgap (PBG) fiber was recently pro-

posed as an alternative architecture for LMA fibers [5].Shortly afterward, it was demonstrated to be a conveni-ent candidate for cw [6] and femtosecond [7] oscillationregimes, as well as for the femtosecond amplification re-gime [8]. The fiber is composed of a low-index core sur-rounded by a periodic cladding composed of alternatinghigh-n and low-n layers, which act as a cylindrical Bragg

mirror. Thanks to its cylindrical symmetry, the fiber pre-form can be fabricated by the widespread modified che-mical vapor deposition technique, which allows theextension of the doped area to be accurately controlled.

In this Letter, a very accurate modeling of modal com-petition in LMA PBG Bragg fiber cw oscillators givesguidelines to properly design single-mode oscillators.The performances of a 40 μm diameter core Yb3þ-dopedfiber are then experimentally assessed.

In the model developed [4], the rate equations [9] aresolved, taking into account the transverse dependence ofthe light intensity Ikðx; y; zÞ, where the subscript k refersto any combination of wavelength (pump or signal),mode number, and parity (e.g., x-polarized LP01, y-polarized LP11, and so on). The population densitiesand, therefore, the population inversion are also spacedependent. This allows for the possible multiple oscillat-ing modes to be clearly identified. The pump and thesignal wavelengths are fixed at λP ¼ 0:977 μm and λS ¼1:04 μm, respectively. The design is very similar to thatproposed in [10], that is, a 40 μm diameter down-dopedcore surrounded by three pairs of layers with index con-trast Δn ¼ 15 × 10−3 relative to the silica background.Down-doping of the core, relative to the low n layers,has two major advantages. First, the HOMs are madeleakier. Second, other things being equal, and particu-larly the wavelength, the period of the optimized clad-ding, enabling the PBG guidance, is made smaller,which implies a thinner overall cladding. As a conse-quence, this thin and bendable fiber is more likely tobe single mode. The eight lowest-order guided modes(LP01 and LP02; odd and even LP11, LP21, LP12) are thensearched for by the finite-element method. For each k,the rate equations are integrated over the fiber lengthand the Ikðx; y; zÞ are evaluated over the Yb3þ -dopedarea, which is supposed to be a disk of diameter Dd.The Yb3þ ion concentration is equal to N0 ¼ 3:35×1025 m−3, corresponding to 10,000 parts per million(ppm) by weight. In Fig. 1(a), the modal lasing efficiency

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above threshold ηk is computed versus the ratio Dd=D.When the whole fiber core is doped with Yb3þ ions,the population inversion is depleted near the fiber axiswhere the fundamental mode peaks. Yb3þ ions locatedat the core edge are still inverted and opportunist HOMswith rather good overlap with these peripheral ions canoscillate. On the contrary, when the ratio Dd=D is de-creased to 70%, the fundamental mode depletes thewhole transverse population inversion, leaving the HOMsunamplified. They are, therefore, prohibited from oscil-lating. The HOM content can be managed by properly tai-loring the doped diameter. The bent fiber should also beoperated single mode. The modal lasing efficiency wasthen computed versus the curvature radius Rb andplotted in Fig. 1(b) for Dd=D ¼ 40% and 70%. It is worth

noting that too large a ratio Dd=D hinders the fiber frombeing operated single mode, even with moderate Rb ¼15 cm. On the other hand, decreasing Dd=D to 40% en-sures a single-mode behavior as well as high slope effi-ciency for curvature radii larger than 5 cm. Figure 1(c)shows the modal intensity distributions for Rb ¼ 10and 4:5 cm when Dd=D ¼ 40%. The boundary of theYb-doped area is shown with a dashed white circle.For Rb ¼ 10 cm, the overlap of the LP11 mode with thedoped area is much smaller than that of the LP01, therebyimplying a single-mode emission. For Rb ¼ 4:5 cm, theLP01 field intensity localizes at the core edge and over-laps less with the inverted ions, whereas one intensitylobe of the LP11 mode enters the doped area due tothe bend-induced field shift. As a consequence, the LP11mode starts to be amplified, as shown in Fig. 1(b). More-over, the LP01 lasing efficiency decreases.

A fiber preform was fabricated in several stages. First aYb3þ-doped preform was fabricated. Highly Yb3þ (about10,000 ppm by weight) core doping was obtained by uti-lization of metal–organics substances as precursors. Thecore was doped with Al2O3 (to increase solubility ofYb3þ) and codoped with F (to provide the negative indexdifference). The diameter of the Yb3þ-doped section was3:5 mm, and the preform outer diameter was 8:5 mm (ra-tio Dd=D approximately equal to 40%). The preform re-fractive index profile (RIP) was measured and showedindex variations along the radius that were attributedto the evaporation of core dopants during the preformcollapse. In particular, a large index dip was measuredat the fiber axis. Numerical optimization of the claddingwas carried out, taking into account the actual core RIP.After the cladding preform has been fabricated, the corepreform was inserted in the cladding preform. The over-all preform was consolidated, given an octagonal shape,and drawn down into a double-clad low-index polymer-coated active fiber with D ¼ 40 μm. The fiber RIP wasmeasured and plotted in Fig. 2. The outer cladding dia-meter is equal to 125 μm. The cladding small signal ab-sorption is then approximately equal to 6 dB=m at thepump wavelength (977 nm). Taking into account the ac-tual RIP, the mode field diameter was estimated fromcomputations to be 26 μm.

A 2-m-long fiber sample was then used as the core ele-ment of a continuous wave oscillator. The cavity wasclosed by a 100%mirror at λS on one end, and the 4% Fres-nel reflection was provided by a 90° cleave on the distalend. The signal output power, at the wavelength of1:04 μm, was measured versus the launched pump power

Fig. 1. (Color online) Modal lasing efficiency above thresholdηk computed versus (a) the ratio Dd=D and (b) the curvatureradius Rb. Inset, refractive index profile of the PBG Bragg fiber.In (b), the labels refer to Dd=D. (c) LP01 and LP11 intensity dis-tributions for Dd=D ¼ 40% and two curvature radii.

Fig. 2. Refractive index profile of the manufactured fiber.

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for several bend configurations. The slope efficiency,plotted as a function of the inverse curvature radius inFig. 3, was measured to be approximately 80%, versusthe launched pump power. Also shown in the inset ofFig. 3 are the near-field intensity distributions of thebeam emitted for various bend radii. When Rb ≥ 7:5 cm,the near-field intensity distribution is Gaussian-like. Thebeam quality parameter was measured to beM2 ¼ 1:12 inboth orthogonal directions. The M2 parameter can be,however, very good in slightly multimode fibers. As a sec-ond check, we have observed the temporal drift of thebeam, which is detrimental in any beam pointing applica-tions. For Rb ¼ 7:5 cm, the beam does not drift in time.These two measurements confirm the successful man-agement of the high-order mode content in LMA PBGBragg fibers. For a tight bend (Rb ¼ 5 cm), the fieldintensity is noticeably shifted toward the exterior ofthe curvature, as expected from computations. ForRb ¼ 4 cm, HOMs become excited, as shown in the bot-tom inset of Fig. 3. In such a case, a nonnegligible tem-poral drift of the beam was observed. This “threshold”value is in excellent agreement with the numerical pre-dictions of Fig. 1(b).In conclusion, we have designed and fabricated a

single-mode PBG Bragg fiber featuring a 40 μm Yb-dopedcore. A very high slope efficiency, of approximately 80%(versus the launched pump power), as well as a verygood beam quality parameter of 1.12 in both orthogonaldirections were measured in the cw oscillation regime.To ensure that the fiber is indeed single mode, we have

checked that no temporal drift can be observed for a cur-vature radius as small as 7:5 cm. In another experimentto be described elsewhere, an average pump power ashigh as 150 W was launched into an air-clad version ofthe fiber without excitation of high-order modes, con-firming the fact that the management of high-ordermodes operates at higher power.

The authors acknowledge the financial support ofthe Agence Nationale de la Recherche through grantANR06BLAN-0091-01. We thank M. Hanna from theLaboratoire Charles Fabry de l’Institut d’Optique, France,and Y. Zaouter from Amplitude Systèmes, France, for theM2 measurement. This work was partially supported bythe Russian Fund for Basic Research, grant 10-08-01226-a.

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Fig. 3. (Color online) Slope efficiency measured versus thelaunched pump power for various bend radii. Inset, correspond-ing near-field intensity profiles.

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