Inner cladding microstructuration based on ... - OSA Publishing

Inner cladding microstructuration based on symmetry reduction for improvement of singlemode robustness in VLMA fiber Romain Dauliat,1,∗ Dmitry Gaponov,1 Aur´elien Benoit,1,2 Franc¸ois Salin,2 Kay Schuster,3 Rapha¨el Jamier,1 and Philippe Roy1 1

Xlim Research Institute, UMR CNRS / Universit´e de Limoges n°7252, 123 avenue Albert Thomas, 87060 Limoges, France 2 Eolite Lasers, 11 avenue de la Canteranne, 33600 Pessac, France 3 Institute of Photonic Technology, Albert Einstein Straße 9, 07745 Jena, Germany ∗

[email protected]

Abstract: Very large mode area, active optical fibers with a low high order mode content in the actively doped core region were designed by removing the inner cladding symmetry. The relevance of the numerical approach is demonstrated here by the investigation of a standard air-silica Large Pitch Fiber, used as a reference. A detailed study of all-solid structures is also performed. Finally, we propose new kinds of geometry for 50 μ m core, all-solid microstructured fibers enabling a robust singlemode laser emission from 400 nm to 2200 nm. © 2013 Optical Society of America OCIS codes: (060.2280) Fiber design and fabrication; (060.2430) Fibers, single-mode; (060.4005) Microstructured fibers; (140.3510) Lasers, fiber.

References and links 1. D. Gapontsev, “6kW CW single mode ytterbium fiber laser in all-fiber format,” Proc. Solid State and Diode Laser Technology Review (2008). 2. F. Stutzki, F. Jansen, A. Liem, C. Jauregui, J. Limpert, and A. T¨unnermann, “26mJ, 130W Q-switched fiber laser system with near-diffraction-limited beam quality,” Opt. Lett. 37(6), 1073-1075 (2012). 3. P. F. Moulton, T. Ehrenreich, R. Leveille, I. Majid, K. Tankala, and G. Rines, “1-kW, all-glass Tm : fiber laser,” Proc. of SPIE 7580, paper 7580112 (2010). 4. S. Jetschke, S. Unger, A. Schwuchow, M. Leich, and J. Kirchhof, “Efficient Yb laser fibers with low photodarkening by optimization of the core composition,” Opt. Express 16(20), 15540-15545 (2008). 5. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H.-J. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. T¨unnermann, “Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers,” Opt. Express 19(14), 13218-13224 (2011). 6. K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Lægsgaard, “Theoretical analysis of mode instability in highpower fiber amplifiers,” Opt. Express 21(2), 3997-4008 (2013). 7. F. Stutzki, F. Jansen, C. Jauregui, J. Limpert, and A. T¨unnermann, “Non-hexagonal large-pitch fibers for enhanced mode discrimination,” Opt. Express 19(13), 12081-12086 (2011). 8. M. M. Jørgensen, S. R. Petersen, M. Laurila, J. Lægsgaard, and T. T. Alkeskjold, “Optimizing single mode robustness of the distributed modal filtering rod fiber amplifier,” Opt. Express 20(7), 7263-7273 (2012). 9. F. Jansen, F. Stutzki, H. Otto, C. Jauregui, J. Limpert, and A. T¨unnermann, “High-power thermally guiding index-antiguiding-core fibers,” Opt. Lett. 38(4), 510-512 (2013). 10. J. Limpert, F. Stutzki, F. Jansen, H. J. Otto, T. Eidam, C. Jauregui, and A. T¨unnermann, “Yb-doped large-pitch fibres: effective single-mode operation based on higher-order mode delocalisation,” Light: Science & Applications 1, e8, (2012). 11. F. Jansen, F. Stutzki, C. Jauregui, J. Limpert, and A. T¨unnermann, “Avoided crossings in photonic crystal fibers,” Opt. Express 19(14), 13578-13589 (2011).

#191556 - $15.00 USD Received 31 May 2013; revised 20 Jul 2013; accepted 22 Jul 2013; published 1 Aug 2013 (C) 2013 OSA 12 August 2013 | Vol. 21, No. 16 | DOI:10.1364/OE.21.018927 | OPTICS EXPRESS 18927

12. R. Dauliat, D. A. Gaponov, A. Benoit, F. Salin, K. Schuster, S. Jetschke, S. Grimm, and P. Roy, “Ytterbium doped all solid large pitch fiber made from powder sintering and vitrification,” International Conference on Fibre Optics and Photonics - OSA Technical Digest, paper TPo.7 (2012). 13. J. M. Fini, “Improved symmetry analysis of many-moded microstructure optical fibers,” J. Opt. Soc. Am. B 21(8), 1431–1436 (2004). 14. P. McIsaac, “Symmetry-induced modal characteristics of uniform waveguides,” IEEE Trans. Microwave Theory Tech. 23(5), 421-433 (1975). 15. R. Guobin, W. Zhi, L. Shuqin, and J. Shuisheng, “Mode classification and degeneracy in photonic crystal fibers,” Opt. Express 11(11), 1310-1321 (2003). 16. M. Steel, “Reflection symmetry and mode transversality in microstructured fibers,” Opt. Express 12(8), 1497-509 (2004). 17. P. M. Agruzov, K. V Dukelskii, and V. S. Shevandin, “Three types of microstructured large core fibers : development and investigation,” Conference on Lasers and Electro-Optics , paper CE.P.28 (2009). 18. V. V Demidov, K. V Dukelskii, and V. S. Shevandin, “Novel bend-resistant design of single-mode microstructured fibers,” Conference on Lasers and Electro-Optics , paper CE.P35 (2011). 19. M.-Y. Chen, Y. Li, J. Zhou, and Y.-K. Zhang, “Design of asymmetric large-mode area optical fiber with low bending loss,” J. of Lightwave Technol. 31(3), 476-481 (2013). 20. J. Limpert, “Large-pitch fibers: pushing very large mode areas to highest powers,” International Conference on Fibre Optics and Photonics - OSA Technical Digest, paper T2A.1 (2012).

1.

Introduction

Over the last decade, rare-earth doped optical fibers have shown outstanding potential in increasing the average optical power (in CW operation) and energy (in the pulsed regime) delivered by fiber laser systems at 1 μ m [1, 2] and more recently at 2 μ m [3]. These performances were driven by the development of Very Large Mode Area Photonic Crystal Fibers (VLMAPCFs) that exhibit a Mode Field Area (MFA) larger than 1600 μ m2 . This has resulted in the onset of non linearities, being pushing away by a decrease in the optical power density in the fiber core and/or a reduction in the fiber length. The presence of a pump cladding and the intrinsic nature of PCFs (composed of a solid core surrounded by a classic hexagonal array of air holes) do not lead to operation that is strictly singlemode. Nevertheless, these structures provide efficient modal discrimination by favoring the High-Order Modes (HOM) leakage through the inner cladding, thus delocalizing them out of the gain region. The fundamental mode (FM), which exhibits the largest overlap with the gain region, can be preferentially amplified. Large Pitch PCFs, referred as Large Pitch Fibers (LPFs) due to their cladding hole-to-hole spacing Λ larger than 10 times the wavelength, have initiated this technological breakthrough by enabling singlemode emission from fibers that exceed 100 μ m in core diameter [2]. Unfortunately, the power scaling in such VLMA-LPFs faced new key hurdles that going beyond the sole nonlinear effects and photodarkening [4]. Indeed, fast modal instabilities were demonstrated when the pump/signal powers overcome a certain threshold above which the quality of the emitted beam is suddenly degraded. This phenomenon is due to a transversal (across the fiber section) and longitudinal (along the fiber length) temperature gradient, which modifies the refractive index profile of the structure via a thermo-optical effect, and induces a thermal long period grating into the fiber core. This grating leads to a temporally varying mode coupling between the fundamental mode and at least one undesired HOM. These disturbances influence the guiding properties of the fiber and no longer allow efficient singlemode operation [5, 6]. In this context, new fiber designs have to be proposed to circumvent these limitations, i.e. by enhancing the modal discrimination in very large core LPFs. This condition is essential to achieve the power scaling expected with these fibers. Commonly, the proposed air/silica LPFs are characterized by a cladding microstructuration preserving the symmetrical hexagonal lattice of standard PCFs. However, alternative non-hexagonal designs have recently been explored. A pentagonal lattice, in particular, was pointed out for the improvement of mode discrimination allowing the extension of the MFD to 125 μ m [7].

#191556 - $15.00 USD Received 31 May 2013; revised 20 Jul 2013; accepted 22 Jul 2013; published 1 Aug 2013 (C) 2013 OSA 12 August 2013 | Vol. 21, No. 16 | DOI:10.1364/OE.21.018927 | OPTICS EXPRESS 18928

In this paper, we propose going further by developing new kinds of LPF structures based on a symmetry reduction of the cladding lattice, allowing the improvement of the singlemode robustness, and providing a better modal discrimination than that obtained using well-known photonic crystal lattices. Moreover, the proposed fiber structures are all-solid, used to obtain efficient heat diffusion, and subsequently achieve an efficient thermal cooling. Furthermore, such fibers facilitate fiber preparation (polishing, cleaving and splicing). Unlike air/silica LPFs, the solutions presented here exhibit a core refractive index which does not have to match that of silica. This suppresses the restriction on the rare-earth concentration and makes it possible to achieve higher levels of gain. An efficient mitigation for photo-darkening can also be envisaged with the help of index raising dopants such as phosphorus, without having to compensate the final refractive index. The background material should accordingly match that the index the gain region. This paper is organized as follows. First, we will introduce our investigation method and apply it to the well-known air/silica LPF, the fiber core of which is created by omitting only one cell of the periodical lattice. How this fiber theoretically performs will be a reference for our work. Then, we will present several fiber designs, on which the periodical lattice has been modified in order to enhance modal discrimination, giving birth to the ”Vortex” and ”Hexagonal symmetry free” fibers. Here, we will demonstrate that a reduction in the microstructured cladding symmetry is beneficial to the improvement of modal discrimination, and thus to the beam quality. 2.

Definition of our simulation procedure - Application to an air/silica LPFs chosen as reference

One criterion that is essential to the development of high power fiber lasers/amplifiers is the robustness of the singlemode emission. However, VLMA fibers undoubtedly feature a fewmodes content and some efforts have to be taken in order to devise fibers that exhibit a selective amplification of the FM, principally by HOMs efficiently leaking out the gain area. In order to qualify the quality of the beam emitted by an inherently slightly multimode fiber, we decided to compare the overlap integrals of the different modes with the gain region. For this purpose, all fiber designs discussed in this paper were simulated using software based on a full-vector finite-element method that is commercially available. Then, the overlap factors of the guided modes, designated OF and expressed in percent, were computed using the following formula: 

|E|2 dS

Ad

OF = 

|E|2 dS

(1)

Ap

Here, Ad and A p represent the areas of gain region and the pump cladding respectively, E is the electric field distribution of the guided mode with |E|2 as its intensity, and dS is referred as the cross section of integration. Due to the double clad, that is to say both core and cladding modes may be propagated without confinement losses and exhibit a significant mode overlap with the gain region. For this reason, we decided to examine at least the first 300 guided modes (identified by their refractive effective indices) throughout this study to ensure that the most competitive HOM is taken adequately into account. In order to achieve a robust singlemode emission, we are looking for the highest overlap of the FM with the gain region (maximizing its amplification) and conversely the lowest for all others (limiting the gain competition with the FM). In this way, the modal discrimination (and with it the singlemode behavior) of the designed structures is defined as the difference between the overlap factor of the fundamental

#191556 - $15.00 USD Received 31 May 2013; revised 20 Jul 2013; accepted 22 Jul 2013; published 1 Aug 2013 (C) 2013 OSA 12 August 2013 | Vol. 21, No. 16 | DOI:10.1364/OE.21.018927 | OPTICS EXPRESS 18929

mode (designated OFFM ) and the most confined HOM (OFHOM ): ΔOF = OFFM − OFHOM

(2)

It is commonly assumed that a ΔOF value larger than 30% ensures the preferential amplification and emission of the sole fundamental mode [8]. It is straightforward logic to establish that when the modal discrimination is larger, the beam quality better, as HOMs contribute less and less to the guided radiation. As a reference to our modus operandi, we initially considered the classic air/silica LPF described in reference [2]. The 45μ m actively doped core is composed of 7 hexagons [9] (see Fig. 1(a)), and the pitch Λ is equal to 30 μ m. The normalized hole diameter d/Λ is variable.

(a)

(b)

Fig. 1. : (a) 2D refractive index repartition of the air/silica LPF described in [2]. The gain region is in red, the pure silica in blue and the air holes in yellow. (b) Overlap factor of the fundamental mode (solid lines) and modal discrimination (dashed lines) computed for the air/silica LPF. Calculations have been done for three air-clad diameters: 170 μ m (black), 180 μ m (red), and 190 μ m (blue) and various ratio d/Λ. Insets: computed intensity distributions corresponding to the fundamental mode coupling (top), and the most disturbing HOM (bottom).

Figure 1(b) represents the evolution of the fundamental mode confinement (solid lines) and the modal discrimination (dashed lines) for a ratio d/Λ that varies from 0.1 to 0.45, as well as for different air clad diameters: 170 μ m (black), 180 μ m (red) and 190 μ m (blue). Solid curves confirm that the fundamental mode is more confined into the gain region for larger air holes (narrower leakage channels). The dashed lines illustrate that the modal discrimination is at its maximum when the d/Λ value is close to 0.325 for an air-clad of 170 μ m. Indeed, above this limit, the LP31 -like mode becomes competitive and reduces the modal discrimination. Moreover, one can note that for a d/Λ smaller than 0.325, the most restrictive HOM is the LP11 -like mode. The largest discrimination obtained for this air/silica LPF is close to 45%, which is congruent to the results reported in [10], validating our simulation procedure. Finally, on the curves relating to the air-clad diameter of 190 μ m, a mode coupling between the LP01 and a cladding HOM is observed, as in reference [11]. 3.

Design of all-solid microstructured fibers

In this section, we discuss an approach aiming to improve the HOMs discrimination in microstructured leaky fibers. Designs investigated hereinafter are based on an array of hexagonal #191556 - $15.00 USD Received 31 May 2013; revised 20 Jul 2013; accepted 22 Jul 2013; published 1 Aug 2013 (C) 2013 OSA 12 August 2013 | Vol. 21, No. 16 | DOI:10.1364/OE.21.018927 | OPTICS EXPRESS 18930

cells as depicted in Table 1 and Table 2. In leaky fibers, the refractive index of the core has to match that of the background material in order to provide efficient leakage channels. In this way, for current air/silica fibers, the refractive index of the gain medium match that of the pure silica. Here we chose to relieve the restriction on the core refractive index (RI) and to increase the RI of the background material (silica RI is increased using an index-raising dopant). This made it possible to introduce a strong concentration of rare-earth ions (RE) into the fiber core, thus reducing the required fiber length and pushing away the non linear effects. Moreover, it offers a potential for high linear gain and finally, an appropriate composition of the core material can be envisaged to efficiently compete with photodarkening. New fiber designs presented throughout this paper are composed of actively (red hexagons in Table 1 and Table 2) and passively (clear blue hexagons) doped rods, both of which have a positive refractive index contrast of Δn = 6·10−3 compared to that of pure silica (dark blue hexagons). This value of refractive index contrast has been arbitrarily fixed and can be adjusted without changing our approach. However, it can be pointed out that it is realistic considering the RI of current highly RE-doped silica materials. The inner cladding geometry enabling the mode discrimination is obtained by inserting pure silica rods into the passively-doped background material. According to the state-of-the-art air/silica PCF/LPF operating in a singlemode regime, the ratio d/Λ is fixed to 0.33. The 50 μ m core, involving a pitch of 30 μ m, is fully doped and composed of 19 actively doped rods. It should be pointed out that to mitigate the request on the volume of fabricated material, all structures we propose are limited to 7 layers of hexagons (two of them belonging to the core) [12]. Thus, the influence of a homogeneous enlargement of the fiber core from 50 to 70 μ m will be studied and, by the way, an increase of the double clad diameter from 150 to 210 μ m is induced. It is also important to underline that the pure silica clad (outer dark blue region, see Table 1 and Table 2) is not the outer cladding, but rather a part of the pump core. Thus, suck kind of pedestal acts on the mode’s confinement in the fiber core. An air-clad or fluorine-doped layer has to be added to propagate the pump radiation, resulting in a triple clad design. Guided modes of the different structures are computed at an emitted wavelength of 1.03 μ m. The flagship all-solid designs we studied are depicted in Table 1 and Table 2 with their corresponding overlap factor graphs. These graphic representations allow the clear identification of the most competitive HOM (its intensity distribution is depicted in inset). Moreover, they provide information about the fundamental mode overlap factor and the evolution of the modal discrimination versus the core size, as calculations have been done for three core diameters: 50, 60 and 70 μ m). Later, fiber designs will be referred to their numbering in the tables. 3.1.

HOM leakage behavior

Structure 1 (see Table 1) is made of a single layer of low-index rods. The conventional six fold symmetry is retained and the HOMs leakage behavior examined. In order to study the cladding region where the most disruptive HOMs are delocalized, the cladding of structure 1 is divided into six identical sections, according to the minimum sector of its pattern (C6ν symmetry) [13]. Each of the cells in a section is associated with the other five cells at the same position in the other sections, forming a set of subdomains. Then, the analysis of the mode’s intensity localized in each group of cladding subdomains make it possible to strive toward an optimum cladding microstructuration for HOMs discrimination (as depicted in Table 1). Structure 1 exhibits a strong mode coupling of the fundamental core mode with the cladding mode (see inset on Table 1), reducing its confinement in the core to 50%. Nevertheless, HOMs leaky behavior can be observed (OFHOMs