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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 18 — Aug. 27, 2012
  • pp: 20238–20254
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Semi-guiding high-aspect-ratio core (SHARC) fiber amplifiers with ultra-large core area
for single-mode kW operation
in a compact coilable package

John R. Marciante, Vladimir V. Shkunov, and David A. Rockwell  »View Author Affiliations


Optics Express, Vol. 20, Issue 18, pp. 20238-20254 (2012)
http://dx.doi.org/10.1364/OE.20.020238


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Abstract

A new class of optical fiber, the SHARC fiber, is analyzed in a high-power fiber amplifier geometry using the gain-filtering properties of confined-gain dopants. The high-aspect-ratio (~30:1) rectangular core allows mode-area scaling well beyond 10,000 μm2, which is critical to high-pulse-energy or narrow-linewidth high-power fiber amplifiers. While SHARC fibers offer modally dependent edge loss at the wide “semi-guiding” edge of the waveguide, the inclusion of gain filtering adds further modal discrimination arising from the variation of the spatial overlap of the gain with the various modes. Both methods are geometric in form, such that the combination provides nearly unlimited scalability in mode area. Simulations show that for kW-class fiber amplifiers, only the fundamental mode experiences net gain (15 dB), resulting in outstanding beam quality. Further, misalignment of the seed beam due to offset, magnification, and tilt are shown to result in a small (few percent) efficiency penalty while maintaining kW-level output with 99% of the power in the fundamental mode for all cases.

© 2012 OSA

1. Introduction

High-power and high-energy fiber lasers and amplifiers have greatly benefitted from large-mode-area (LMA) fibers [1

1. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]

]. Since nonlinear effects typically depend on intensity, scaling to larger mode areas means that higher cw and peak powers can be carried in the fiber with the same intensities as in singe-mode fibers. Since LMA fibers are nominally multimode in nature, some sort of mode filtering is required in order to deliver an output beam having the desired beam quality that is provided exclusively by the fundamental (lowest order) mode.

Numerous LMA fiber architectures have been developed to provide the requisite mode filtering, such as low-NA fibers with bend loss [2

2. J. P. Koplow, D. A. V. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett. 25(7), 442–444 (2000). [CrossRef] [PubMed]

], chirally-coupled core fiber [3

3. C. Liu, G. Chang, N. Litchinitser, A. Galvanauskas, D. Guertin, N. Jabobson, and K. Tankala, “Effectively Single-Mode Chirally-Coupled Core Fiber,” ASSP 2007, paper ME2.

], higher-order-mode (HOM) fiber [4

4. S. Ramachandran, J. W. Nicholson, S. Ghalmi, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello, “Light propagation with ultralarge modal areas in optical fibers,” Opt. Lett. 31(12), 1797–1799 (2006). [CrossRef] [PubMed]

], photonic crystal rod-type fibers [5

5. J. Limpert, N. Deguil-Robin, I. Manek-Hönninger, F. Salin, F. Röser, A. Liem, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen, “High-power rod-type photonic crystal fiber laser,” Opt. Express 13(4), 1055–1058 (2005). [CrossRef] [PubMed]

], leakage-channel fibers [6

6. L. Dong, X. Peng, and J. Li, “Leakage channel optical fibers with large effective area,” J. Opt. Soc. Am. B 24(8), 1689–1697 (2007). [CrossRef]

], and gain filtering via confined-gain dopants [7

7. J. R. Marciante, “Gain filtering for single-spatial-mode operation of large-mode-area fiber amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15(1), 30–36 (2009). [CrossRef]

]. Although each fiber architecture has demonstrated scaling beyond the conventional single-mode-fiber limit, each also has drawbacks that prohibit indefinite scaling, even if only due to practical packaging considerations.

In this paper, we explore the benefits of the SHARC fiber in high-power fiber amplifiers. The specific architecture adopted for this case is the inclusion of gain filtering through the spatial distribution of gain dopants. Specifically, we show that gain-filtered SHARC fiber can provide kW-level amplification with loose tolerances for seed beam injection and >99% fundamental-mode content at the amplifier output under all conditions.

This paper is organized as follows. In Section 2, this new class of active semi-guiding high-aspect-ratio-core (SHARC) fibers is reviewed along with the gain-filtering benefits of tailored dopant profiles. Detailed analytic modeling results, which elucidate the principles and fundamental operation of the active fiber and serve as a guide in the fiber design, are described in Section 3. In Section 4, intense numerical simulations confirm the analytical predictions and further demonstrate the strength of the gain-filtered SHARC concept, including strict evaluation of alignment tolerances. A discussion of practical implications of this new class of active fibers is presented in Section 5, with concluding remarks following in Section 6.

2. Semi-guiding high-aspect-ratio fiber and gain filtering

The semi-guiding high-aspect-ratio core (SHARC) fiber concept schematically shown in Fig. 1
Fig. 1 Schematic of semi-guiding high-aspect-ratio core (SHARC) fiber in coiled configuration (not to scale). Expansion shows the schematic cross section of a SHARC fiber and the relative orientation of the fiber core with respect to the coil. The central slow-axis region of the core is doped with ytterbium to provide spatially confined gain when pumped. Similar to the definition used for semiconductor lasers, the fast and slow axes of the SHARC fiber are indicated in the figure.
is a radical departure from conventional fiber designs. It embodies a high-aspect-ratio rectangular core that enables scaling to a very large area, up to 30,000 μm2 or more, while retaining a thin, mechanically flexible narrow dimension. Despite the large core area, a SHARC fiber can still be coiled in the fast-axis direction, depicted in Fig. 1. This enables SHARC fibers to form a compact package comparable to that of state-of-the-art LMA fibers (which have significantly smaller mode area ~400 μm2). Despite this common packaging strategy, coiled SHARC fibers do not suffer from the significant performance challenges of conventional LMA fibers as the core area is increased. In particular, as the core size of conventional LMA fibers increases, the mode area becomes severely reduced due to bend-induced mode deformation [9

9. J. M. Fini, “Bend-resistant design of conventional and microstructure fibers with very large mode area,” Opt. Express 14(1), 69–81 (2006). [CrossRef] [PubMed]

, 10

10. R. C. G. Smith, A. M. Sarangan, Z. Jiang, and J. R. Marciante, “Direct measurement of bend-induced mode deformation in large-mode-area fibers,” Opt. Express 20(4), 4436–4443 (2012). [CrossRef] [PubMed]

]. In contrast, the SHARC fiber increases the core area by expanding only the slow-axis dimension, while maintaining a constant thickness in the fast-axis direction in which the coiling occurs. Although mode deformation exists, in principle, in the fast-axis dimension of the SHARC fiber, the small fast-axis waveguide dimension (~15 µm) prohibits any significant mode deformation similar to what is observed in small-core (~20 µm) LMA fibers.

As denoted by the key descriptor “semi-guiding,” this fiber design specifies conventional index-based guiding via total internal reflection (TIR) in only one transverse dimension (the “fast axis,” as labeled in Fig. 1). Specifically, and as Fig. 1 indicates, index-based TIR guiding only occurs along the two large surfaces of the rectangular core where they are in contact with the fast-axis cladding layers. The slow-axis core edges are designed not to support TIR; in fact, the index step at the core-edge boundary is kept very small (Δn ~100 ppm or less), such that all modes suffer propagation losses as they radiate out beyond the slow-axis core edges. Although the fast-axis direction can be designed to be single-mode, slow-axis mode control is achieved in the passive fiber by exploiting the natural process of “loss filtering” [8

8. D. A. Rockwell, V. V. Shkunov, and J. R. Marciante, “Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package,” Opt. Express 19(15), 14746–14762 (2011). [CrossRef] [PubMed]

]. This process arises from the fact that all slow-axis modes suffer radiation loss into the cladding through the “open” core edges, with the lowest-order mode having significantly less loss than any other mode. Using this approach, the mode-dependent loss can effectively discriminate in favor of the lowest-order mode, analogous to the desirable mode filtering in other LMA fibers.

Perhaps most importantly, the SHARC-fiber core-area scaling necessary to access higher power levels is achieved by increasing only the slow-axis direction, and this proceeds at a practically fixed core-to-cladding ratio. This fact allows power to be scaled with a constant effective pump absorption coefficient and therefore a constant total fiber length. The SHARC fiber architecture also scales output power at a constant pump-etendue per output watt, thereby ensuring the possibility of generating higher output power levels without having to invent new pump-diode packages with increasingly higher brightness. As a quantitative example, carrying 3-kW of single-frequency optical power will require core dimensions of 20 μm × 1.5 mm, for a total core area of 30,000 μm2, which is equivalent to a circular core having a diameter of ~200 μm. In this example, stimulated Brillouin scattering (SBS) suppression occurs by virtue of the large core area and low intensity, which lead to an SBS threshold power in excess of 3 kW even for a kHz-range laser bandwidth. Hence, in order to deliver multi-kW-level optical powers, SHARC fibers do not require additional SBS suppression techniques such as multi-GHz signal modulation [11

11. G. D. Goodno, S. J. McNaught, J. E. Rothenberg, T. S. McComb, P. A. Thielen, M. G. Wickham, and M. E. Weber, “Active phase and polarization locking of a 1.4 kW fiber amplifier,” Opt. Lett. 35(10), 1542–1544 (2010). [CrossRef] [PubMed]

, 12

12. C. M. Zeringue, I. Dajani, and G. T. Moore, “Suppression of stimulated Brillouin scattering in optical fibers through phase-modulation: a time dependent model,” Proc. SPIE 7914, 791409 (2011). [CrossRef]

], with its associated system complexity, or acoustic waveguide management [13

13. P. D. Dragic, C.-H. Liu, G. C. Papen, and A. Galvanauskas, “Optical fiber with an acoustic guiding layer for stimulated Brillouin scattering suppression,” CLEO 2006, paper CThZ3.

15

15. L. Dong, “Limits of stimulated Brillouin scattering suppression in optical fibers with transverse acoustic waveguide designs,” J. Lightwave Technol. 28, 3156–3161 (2010).

].

The maximum benefit of gain filtering is obtained by designing the transverse ytterbium dopant profile to optimize the overlap of the gain with the fundamental mode while minimizing the gain-overlap of all other modes, performing a global optimization at all levels of saturation. It has been recently shown that gain filtering in fiber amplifiers can lead to better beam quality than the injected seed beam [7

7. J. R. Marciante, “Gain filtering for single-spatial-mode operation of large-mode-area fiber amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15(1), 30–36 (2009). [CrossRef]

, 17

17. T. Eidam, S. Hädrich, F. Jansen, F. Stutzki, J. Rothhardt, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, “Preferential gain photonic-crystal fiber for mode stabilization at high average powers,” Opt. Express 19(9), 8656–8661 (2011). [CrossRef] [PubMed]

].

One anticipated drawback to gain filtering in round fibers is the aforementioned mode deformation, where the mode becomes compressed towards the outside edge of the bend for large core diameters. Although the reduced mode size is detrimental to most LMA fiber applications, for very large cores (~100 µm) the displacement of the mode towards the edge of the waveguide can reduce the effectiveness of gain filtering by altering the overlap of the deformed modes with the centralized gain region [7

7. J. R. Marciante, “Gain filtering for single-spatial-mode operation of large-mode-area fiber amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15(1), 30–36 (2009). [CrossRef]

].

The SHARC fiber offers a unique advantage that can exploit gain filtering without this packaging limitation. The SHARC fiber is coiled in the fast-axis direction, but the gain filtering is applied in the slow-axis direction, as depicted in Fig. 1. Therefore, no slow-axis mode offset will be incurred, and the mode overlap with the gain will remain unchanged regardless of core area or coiling diameter. Consequently, the integration of gain filtering into the SHARC fiber yields the ideal architecture for significant core-area scaling of high-power fiber amplifiers to 30,000 µm2 and beyond.

3. SHARC fiber amplifier analytic calculations

As was discussed in detail in [8

8. D. A. Rockwell, V. V. Shkunov, and J. R. Marciante, “Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package,” Opt. Express 19(15), 14746–14762 (2011). [CrossRef] [PubMed]

], the SHARC fiber geometry lends itself nicely to separation of variables such that the fast- and slow-axis physics can be handled nearly independently of each other. This makes direct analytical modeling possible, from which the primary physics can be obtained. The validity of this assumption and its benefits were confirmed with rigorous three-dimensional beam propagation modeling (BPM) simulations [8

8. D. A. Rockwell, V. V. Shkunov, and J. R. Marciante, “Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package,” Opt. Express 19(15), 14746–14762 (2011). [CrossRef] [PubMed]

].

The first analysis pertains to the impact of gain filtering on the SHARC modes. Simplistically, the net gain experienced by a particular mode is mathematically given by the spatial overlap of the mode with the available gain. In practical terms, the available gain is the nominal gain as saturated by all of the modes in the fiber. For the case of interest, the fundamental mode will carry most of the power in the fiber. As such, the differential gain gk experienced by a given mode k as it propagates along the z-direction is given by the equation
gk(z)=[gss(x)1+I0(x,z)/Isat]|Φk(x)|2dx
(1)
where gss is small-signal gain profile, Isat is the saturation intensity, Φk is the profile of the kth mode (power-normalized to unity), and I0(x,z) is the intensity of the fundamental mode. Note that this term carries both power and spatial dependence such that the gain will be locally saturated in the transverse dimension.

Using Eq. (1), the differential gain of each mode can be calculated as a function of saturation level. Figure 3
Fig. 3 Relative differential gain of various slow-axis SHARC fiber modes as a function of gain width for various saturation levels (noted in figure) for flat-top (left) and Gaussian (right) gain profiles. The dashed line represents the normalized gain of the fundamental mode. The colored curves represent the higher-order modes as labeled in the top-left figure.
shows the results of these calculations at various saturation levels for flat-top and Gaussian gain profiles of varying distribution width. In each case, the gains are normalized to that of the fundamental mode, which is therefore represented by a black dashed line at unity. The other (higher-order) modes are labeled in the upper left figure.

For the cases of no saturation (I0/Isat = 0, bottom figures), the plots simply represent the overlap of the modes with the gain dopant profile. As would be expected in this case, the fundamental mode experiences the highest gain when the waveguide is doped to match the mode profile. However, it has been previously shown that saturation can drastically change this picture, since the gain becomes locally saturated where the mode has the highest intensity (and therefore extracts the highest gain locally) [7

7. J. R. Marciante, “Gain filtering for single-spatial-mode operation of large-mode-area fiber amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15(1), 30–36 (2009). [CrossRef]

, 18

18. T. Bhutta, J. I. Mackenzie, D. P. Shepherd, and R. J. Beach, “Spatial dopant profiles for transverse-mode selection in multimode waveguides,” J. Opt. Soc. Am. B 19(7), 1539–1543 (2002). [CrossRef]

]. Similar to the case for conventional (round) optical fibers [7

7. J. R. Marciante, “Gain filtering for single-spatial-mode operation of large-mode-area fiber amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15(1), 30–36 (2009). [CrossRef]

], the optimum operation condition for mode discrimination changes with local gain saturation. All saturated (I/Isat > 0) plots in Fig. 3 show that the fundamental mode has the smallest gain of all calculated modes when the normalized gain width is unity. However, similar to the case of round fibers, by confining the gain to the central portion of the waveguide where the fundamental mode has the highest intensity, the saturation happens more uniformly, without leaving residual gain near the edge of the waveguide for higher-order modes to exploit. This is clearly observable in all plots of Fig. 3. For the flat-top case (left-hand side), the optimal gain width is ~45% of the waveguide width, allowing the fundamental mode 1.4x higher differential gain than any other mode in the fiber.

In the quasi-three-level kinetics model, the small signal gain factor gss depends on the doping density and the signal wavelength λs, but is also nominally dependent on the pump saturation level Ipump/Ipsat. However, this dependence approaches a constant value for typical conditions of strong pump saturation (the case relevant to high-power Yb:fiber lasers) and can therefore be ignored, allowing gss to be constant along the length of the fiber.

To first order, bi-directional pumping leads to nearly uniform pump distribution along the length of the fiber. Consequently, Eq. (3) can be taken as constant along the fiber by assuming proper values for the pump intensity and its saturation intensity. In this approximation, Eq. (2) can be solved for the fundamental mode assuming an amplifier with a fixed gain, and gss as the constant fitting parameter. Typical fiber amplifier performance (gain = 30, efficiency = 80%) and a 1-kW nominal output power lead to the starting values listed in Table 1

Table 1. Parameters used to Derive Small Signal Gain Parameter

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. The geometrical cross-sections are taken from the core and cladding of the fabricated SHARC fiber described in [8

8. D. A. Rockwell, V. V. Shkunov, and J. R. Marciante, “Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package,” Opt. Express 19(15), 14746–14762 (2011). [CrossRef] [PubMed]

].

4. SHARC fiber amplifier numerical simulations

In order to confirm the behavior of the SHARC fiber amplifier predicted by the analytic modeling, full numerical simulations of the active SHARC fiber were performed using the Beam Propagation Method (BPM) [22

22. H. Rao, M. J. Steel, R. Scarmozzino, and R. M. Osgood Jr., “Complex propagators for evanescent waves in bidirectional beam propagation method,” J. Lightwave Technol. 18(8), 1155–1160 (2000). [CrossRef]

]. The primary advantage of this method is its ability to model spatially dependent structures, such as the refractive index and gain profiles, with an arbitrary launch field. The output of the model describes the complete spatial profile of the optical field without having a priori knowledge of the modes of the fiber. In this method, the paraxial wave equation is used to model the signal beam along the fiber axis, z, as
E(x,y,z)z=i2k0ncoT2E+ik0[n2(x,y)nco22nco]E+12[gss(x,y)1+|E|2/Isat]E
(5)
where k0 = 2π/λ and nco is the refractive index of the waveguide core. On the right hand side of Eq. (5), the first term represents diffraction, where T2 is the transverse Laplacian, and the second term accounts for the cross-sectional refractive index profile, n(x,y). The last term incorporates the spatial dependence of the gain doping profile through gss(x,y) and accounts for spatially localized saturable gain in three dimensions.

To illustrate this logic of this approach, Fig. 6
Fig. 6 Waveguide and ytterbium doping cross-sections (top) relative to the transverse profiles of the first five fiber modes. The dashed lines denote the slow-axis edges of the waveguide and ytterbium doping regions. The dimensions of the frames are 56 μm (vertical) × 600 µm (horizontal) and are not shown to relative scale.
shows the calculated 2D modes of the SHARC waveguide in relative scale to the waveguide and ytterbium-doping structures. The fundamental mode (k = 0) is nearly entirely contained in the gain region, while the overlap of the second mode with the gain region is less than 35%. It is precisely this concept that leads to exceptional mode filtering by using confined gain regions. However, the geometrical overlap of the mode with the ytterbium-doped region is not sufficient to fully describe the problem since local transverse saturation of the gain changes each mode’s ability to extract gain, as discussed in Section 3.

Although Fig. 5 demonstrates the power of gain filtering in the SHARC fiber, its practical aspects result from alignment of the seed beam into the SHARC fiber. One of the most difficult tasks facing the use of LMA fibers in general is launching the seed beam into the fundamental mode with high efficiency. This is particularly difficult in practical applications where the physical environments are not as stable as in typical laboratory settings. Therefore, understanding the alignment tolerances of the gain-filtered SHARC fiber is of critical importance for practical applications.

Figure 8(a)
Fig. 8 (a) Higher-order mode content of the amplifier output (red) and reduction in amplifier efficiency (blue) as a function of the offset of the injected seed beam in the slow-axis direction, also cast as fundamental-mode launch efficiency (top horizontal axis). (b) Slow-axis intensity profile along the propagation axis for a fundamental-mode-matching Gaussian seed beam that is injected at the left side of the figure with a 120-µm offset in the slow-axis dimension.
shows the results of simulations when a Gaussian beam matching the fundamental mode is injected into the fiber with an offset in the slow-axis direction. The first observation is that although the higher-order-mode (HOM) content increases with increasing misalignment, the fraction of output power in the fundamental mode remains above 99% even for a 120-µm offset, more than 25% of the waveguide width. This is particularly striking considering the poor launch efficiency (~40% into higher-order modes). The second observation is that there is a price to misaligning the input beam: reduced amplifier efficiency. The misaligned seed beam excites many modes in the SHARC fiber, with larger offsets translating to lower powers being launched into the fundamental mode. Since the higher-order modes experience net loss (as per Fig. 7), the saturation level in the amplifier is dictated by the power in the fundamental mode, translating to reduced amplifier saturation and less power in the output. However, for reasonable alignment tolerances (50 µm = 11% of the waveguide width) the penalty to the amplifier efficiency is less than 2%.

As a demonstration of the filtering phenomenon, Fig. 8(b) shows the slow-axis field amplitude propagating through the amplifier when seeded with a 120-µm offset. The offset of the launched seed beam can be observed at the left side of the figure. Similar to the case shown in Fig. 5 for the flat-top launch, the strong mode filtering inherent in this configuration leads to amplifier output in the fundamental mode.

Figure 9(a)
Fig. 9 (a) Higher-order mode content of the amplifier output (red) and reduction in amplifier efficiency (blue) as a function of magnification of the injected seed beam in the slow-axis direction, also cast as fundamental-mode launch efficiency (top horizontal axis). (b) Slow-axis intensity profile along the propagation axis for a Gaussian seed beam that is injected at the left side of the figure and is de-magnified in the slow-axis dimension to 0.65x the size that would match the fundamental mode.
shows the results of similar simulations when a Gaussian beam with improper slow-axis beam width is injected on-axis into the fiber. Again, the combination of gain filtering and SHARC fiber edge loss leads to very high mode discrimination at the amplifier output, greater than 99.4% in this case. Due to the symmetric launch conditions, a large fraction of power is always launched into the fundamental mode, as indicated by the upper horizontal axis in Fig. 9(a). Correspondingly, the amplifier efficiency reduction is much smaller, less than 4% over the entire simulated range. Note that this range ( ± 35% mode mismatch) implies rather poor alignment conditions.

Figure 9(b) shows the shows the slow-axis field amplitude propagating through the amplifier when the Gaussian seed beam is de-magnified in the slow-axis dimension to 0.65 times the width that would best match the fundamental mode. The smaller beam is readily observable at the left-hand side of the simulation domain. Although this beam is coupled into many SHARC fiber modes, their observed beating rapidly diminishes as the higher-order modes are filtered out during propagation through the amplifier.

Figure 10(b) shows the slow-axis field amplitude propagating through the amplifier when the Gaussian seed beam is tilted to 1.5 times the diffraction limit. The off-axis propagation direction of the beam is clearly evident early in the propagation (left-hand side of plot), but eventually is corrected as only the fundamental mode experiences significant gain during propagation through the amplifier.

Figures 8-10 demonstrate that the gain-filtered SHARC fiber amplifier is extremely tolerant to misalignment in the injection of the seed laser beam. In fact, this architecture shows a “beam clean-up” type of behavior: regardless of the injection HOM content, even as large as 40%, the output HOM content it always less than 1%.

It should be noted that the simple model used in Section 3 to derive the small-signal gain parameter was for an amplifier with a gain of 30 without including the effects of edge loss or confined gain. With perfect launch conditions, the results of the BPM simulations including edge loss and confined gain results in a ~4% higher net gain, implying higher amplifier efficiency. Although the edge loss in SHARC fibers will contribute to reducing the amplifier efficiency, it is a distributed loss. The saturable nature of the gain enables a reduced signal to extract higher gain, resulting in a smaller efficiency reduction than one might expect. Moreover, the confined gain used here for gain filtering also leads to higher efficiency since the gain is confined to the high-intensity portion of the mode, which can extract the gain very efficiently [16

16. J. R. Marciante, R. G. Roides, V. V. Shkunov, and D. A. Rockwell, “Near-diffraction-limited operation of step-index large-mode-area fiber lasers via gain filtering,” Opt. Lett. 35(11), 1828–1830 (2010). [CrossRef] [PubMed]

, 27

27. J. Oh, C. Headley, M. J. Andrejco, A. D. Yablon, and D. J. DiGiovanni, “Increased pulsed amplifier efficiency by manipulating the fiber dopant distribution,” CLEO 2006, paper CTuQ3.

]. The net result is that SHARC amplifier efficiency is slightly higher than what one might expect from a conventional LMA fiber amplifier.

5. Discussion and practical implications of the SHARC fiber amplifier

As implied in Fig. 1, the SHARC fiber has numerous thermal advantages over conventional round fiber for use in high-power amplifiers. First, since the core is much larger than conventional LMA fibers, the pump light is absorbed over a much larger area resulting in lower heat-source density. Second, the slab-like geometry allows for easier heat extraction than for cylindrical fibers, both in terms of heat-transfer surface area of the core into the cladding as well as the flat surface of the cladding wrapped around a cooled mandrill (shown in Fig. 1). This argument is similar to the advantages of slab vs. rod geometries in high-power solid-state laser systems. Finally, the large core area of the SHARC fiber significantly increases the SBS threshold, allowing the use of longer fibers and enabling lower heat load per unit length.

It is important to realize that, unlike large-core photonic crystal fibers, a SHARC fiber amplifier enables very large core areas while maintaining the all-glass monolithic architecture that is one of the principal attractions of existing fiber lasers. As an example, it is reasonable to expect that the front-end of a SHARC fiber amplifier chain will have one or more LMA fiber preamplifiers. In this case, one would like to have an all-glass coupler that can be spliced between the final LMA fiber preamplifier and the final-stage SHARC fiber power amplifier to (a) shape the circular mode of the LMA preamplifier into a high-aspect-ratio (~30:1 or greater) ellipse, and (b) collimate the launched signal as it enters the SHARC fiber amplifier. These two functions can be performed simultaneously, without any free-space optics, using the signal coupler schematically shown in Fig. 11
Fig. 11 Schematic diagram indicating a signal coupler spliced between an LMA fiber preamplifier and a SHARC fiber power amplifier without any free-space optics. The coupler is a passive SHARC-like fiber waveguide that propagates the signal from the LMA to the SHARC unchanged in the fast-axis dimension (perpendicular to the plane of the figure). In the slow-axis dimension, the coupler functions as a planar quarter-pitch GRIN lens, which expands the signal beam to match the SHARC slow-axis mode and collimates the signal as it enters the SHARC fiber. The red curves indicate the wavefront of the beam (propagating from left to right) while the dashed gray line indicates the classical GRIN ray trajectory path.
. This coupler comprises a short length of passive SHARC-like fiber that exploits index-based guiding to maintain a constant fast-axis dimension all the way from the LMA fiber output to the SHARC fiber amplifier input. In the slow-axis dimension, the coupler functions as a one-dimensional quarter-pitch planar GRIN lens by employing a slow-axis index gradient. Such a scheme allows the signal beam to expand and match the optimum input size for the SHARC amplifier input, providing a nominally collimated beam in the slow-axis dimension. Simulations show that coupler lengths in the range 10-20 mm will suffice for this type of application, depending on the precise dimensions of the LMA and SHARC amplifier fibers.

In considering this pumping approach, it is useful to appreciate how large the acceptance etendue is for pumping a SHARC fiber. Consider a 20 μm x 1 mm core contained within a 200 μm x 1.2 mm pump cladding, and an outer cladding providing an NA of 0.45 for the outer boundaries of the pump cladding. This geometry presents a full-angle beam-parameter product of 1120 mm-mrad in the wide dimension, which is roughly equivalent to a linear array of > 10 pump fibers having a 200-μm core diameter and 0.22 NA. A SHARC fiber amplifier therefore easily accommodates efficient launch of a large number of state-of-the-art fiber-coupled pump diode packages.

6. Conclusions

Acknowledgment

The authors wish to thank Robert Byren, Roberta Gotfried, and John Zolper for the research grant that enabled the collaborative relationship between the University of Rochester and Raytheon.

References and links

1.

D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]

2.

J. P. Koplow, D. A. V. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett. 25(7), 442–444 (2000). [CrossRef] [PubMed]

3.

C. Liu, G. Chang, N. Litchinitser, A. Galvanauskas, D. Guertin, N. Jabobson, and K. Tankala, “Effectively Single-Mode Chirally-Coupled Core Fiber,” ASSP 2007, paper ME2.

4.

S. Ramachandran, J. W. Nicholson, S. Ghalmi, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello, “Light propagation with ultralarge modal areas in optical fibers,” Opt. Lett. 31(12), 1797–1799 (2006). [CrossRef] [PubMed]

5.

J. Limpert, N. Deguil-Robin, I. Manek-Hönninger, F. Salin, F. Röser, A. Liem, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen, “High-power rod-type photonic crystal fiber laser,” Opt. Express 13(4), 1055–1058 (2005). [CrossRef] [PubMed]

6.

L. Dong, X. Peng, and J. Li, “Leakage channel optical fibers with large effective area,” J. Opt. Soc. Am. B 24(8), 1689–1697 (2007). [CrossRef]

7.

J. R. Marciante, “Gain filtering for single-spatial-mode operation of large-mode-area fiber amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15(1), 30–36 (2009). [CrossRef]

8.

D. A. Rockwell, V. V. Shkunov, and J. R. Marciante, “Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package,” Opt. Express 19(15), 14746–14762 (2011). [CrossRef] [PubMed]

9.

J. M. Fini, “Bend-resistant design of conventional and microstructure fibers with very large mode area,” Opt. Express 14(1), 69–81 (2006). [CrossRef] [PubMed]

10.

R. C. G. Smith, A. M. Sarangan, Z. Jiang, and J. R. Marciante, “Direct measurement of bend-induced mode deformation in large-mode-area fibers,” Opt. Express 20(4), 4436–4443 (2012). [CrossRef] [PubMed]

11.

G. D. Goodno, S. J. McNaught, J. E. Rothenberg, T. S. McComb, P. A. Thielen, M. G. Wickham, and M. E. Weber, “Active phase and polarization locking of a 1.4 kW fiber amplifier,” Opt. Lett. 35(10), 1542–1544 (2010). [CrossRef] [PubMed]

12.

C. M. Zeringue, I. Dajani, and G. T. Moore, “Suppression of stimulated Brillouin scattering in optical fibers through phase-modulation: a time dependent model,” Proc. SPIE 7914, 791409 (2011). [CrossRef]

13.

P. D. Dragic, C.-H. Liu, G. C. Papen, and A. Galvanauskas, “Optical fiber with an acoustic guiding layer for stimulated Brillouin scattering suppression,” CLEO 2006, paper CThZ3.

14.

D. Walton, S. Gray, J. Wang, M.-J. Li, X. Chen, A. B. Ruffin, J. Demeritt, and L. Zenteno, “High power, narrow linewidth fiber lasers,” Proc. SPIE 6102, 610205 (2006). [CrossRef]

15.

L. Dong, “Limits of stimulated Brillouin scattering suppression in optical fibers with transverse acoustic waveguide designs,” J. Lightwave Technol. 28, 3156–3161 (2010).

16.

J. R. Marciante, R. G. Roides, V. V. Shkunov, and D. A. Rockwell, “Near-diffraction-limited operation of step-index large-mode-area fiber lasers via gain filtering,” Opt. Lett. 35(11), 1828–1830 (2010). [CrossRef] [PubMed]

17.

T. Eidam, S. Hädrich, F. Jansen, F. Stutzki, J. Rothhardt, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, “Preferential gain photonic-crystal fiber for mode stabilization at high average powers,” Opt. Express 19(9), 8656–8661 (2011). [CrossRef] [PubMed]

18.

T. Bhutta, J. I. Mackenzie, D. P. Shepherd, and R. J. Beach, “Spatial dopant profiles for transverse-mode selection in multimode waveguides,” J. Opt. Soc. Am. B 19(7), 1539–1543 (2002). [CrossRef]

19.

H. W. Bruesselbach, D. S. Sumida, R. A. Reeder, and R. W. Byren, “Low-heat high-power scaling using InGaAs diode-pumped Yb:YAG lasers,” IEEE J. Sel. Top. Quantum Electron. 3(1), 105–116 (1997). [CrossRef]

20.

J. R. Marciante and J. D. Zuegel, “High-gain, polarization-preserving, Yb-doped fiber amplifier for low-duty-cycle pulse amplification,” Appl. Opt. 45(26), 6798–6804 (2006). [CrossRef] [PubMed]

21.

OFS Laboratories has fabricated a SHARC fiber with refractive-index uniformity of 200 ppm across the core width (Private communications with D. J. Trevor, OFS Laboratories, 2012).

22.

H. Rao, M. J. Steel, R. Scarmozzino, and R. M. Osgood Jr., “Complex propagators for evanescent waves in bidirectional beam propagation method,” J. Lightwave Technol. 18(8), 1155–1160 (2000). [CrossRef]

23.

Y. Chung and N. Dagli, “An assessment of finite difference beam propagation method,” IEEE J. Quantum Electron. 26(8), 1335–1339 (1990). [CrossRef]

24.

J. Yamauchi, T. Ando, and H. Nakano, “Beam propagation analysis of optical fibres by alternating direction implicit method,” Electron. Lett. 27(18), 1663–1666 (1991). [CrossRef]

25.

G. R. Hadley, “Transparent boundary condition for the beam propagation method,” IEEE J. Quantum Electron. 28(1), 363–370 (1992). [CrossRef]

26.

M. Born and E. Wolf, Principles of Optics, 6th ed. (Permagon Press, 1991), pp 127–128.

27.

J. Oh, C. Headley, M. J. Andrejco, A. D. Yablon, and D. J. DiGiovanni, “Increased pulsed amplifier efficiency by manipulating the fiber dopant distribution,” CLEO 2006, paper CTuQ3.

OCIS Codes
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(060.2400) Fiber optics and optical communications : Fiber properties
(060.2430) Fiber optics and optical communications : Fibers, single-mode
(140.3280) Lasers and laser optics : Laser amplifiers
(060.3510) Fiber optics and optical communications : Lasers, fiber

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 27, 2012
Revised Manuscript: June 15, 2012
Manuscript Accepted: July 24, 2012
Published: August 20, 2012

Citation
John R. Marciante, Vladimir V. Shkunov, and David A. Rockwell, "Semi-guiding high-aspect-ratio core (SHARC) fiber amplifiers with ultra-large core area
for single-mode kW operation
in a compact coilable package," Opt. Express 20, 20238-20254 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-18-20238


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References

  1. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B27(11), B63–B92 (2010). [CrossRef]
  2. J. P. Koplow, D. A. V. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett.25(7), 442–444 (2000). [CrossRef] [PubMed]
  3. C. Liu, G. Chang, N. Litchinitser, A. Galvanauskas, D. Guertin, N. Jabobson, and K. Tankala, “Effectively Single-Mode Chirally-Coupled Core Fiber,” ASSP 2007, paper ME2.
  4. S. Ramachandran, J. W. Nicholson, S. Ghalmi, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello, “Light propagation with ultralarge modal areas in optical fibers,” Opt. Lett.31(12), 1797–1799 (2006). [CrossRef] [PubMed]
  5. J. Limpert, N. Deguil-Robin, I. Manek-Hönninger, F. Salin, F. Röser, A. Liem, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen, “High-power rod-type photonic crystal fiber laser,” Opt. Express13(4), 1055–1058 (2005). [CrossRef] [PubMed]
  6. L. Dong, X. Peng, and J. Li, “Leakage channel optical fibers with large effective area,” J. Opt. Soc. Am. B24(8), 1689–1697 (2007). [CrossRef]
  7. J. R. Marciante, “Gain filtering for single-spatial-mode operation of large-mode-area fiber amplifiers,” IEEE J. Sel. Top. Quantum Electron.15(1), 30–36 (2009). [CrossRef]
  8. D. A. Rockwell, V. V. Shkunov, and J. R. Marciante, “Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package,” Opt. Express19(15), 14746–14762 (2011). [CrossRef] [PubMed]
  9. J. M. Fini, “Bend-resistant design of conventional and microstructure fibers with very large mode area,” Opt. Express14(1), 69–81 (2006). [CrossRef] [PubMed]
  10. R. C. G. Smith, A. M. Sarangan, Z. Jiang, and J. R. Marciante, “Direct measurement of bend-induced mode deformation in large-mode-area fibers,” Opt. Express20(4), 4436–4443 (2012). [CrossRef] [PubMed]
  11. G. D. Goodno, S. J. McNaught, J. E. Rothenberg, T. S. McComb, P. A. Thielen, M. G. Wickham, and M. E. Weber, “Active phase and polarization locking of a 1.4 kW fiber amplifier,” Opt. Lett.35(10), 1542–1544 (2010). [CrossRef] [PubMed]
  12. C. M. Zeringue, I. Dajani, and G. T. Moore, “Suppression of stimulated Brillouin scattering in optical fibers through phase-modulation: a time dependent model,” Proc. SPIE7914, 791409 (2011). [CrossRef]
  13. P. D. Dragic, C.-H. Liu, G. C. Papen, and A. Galvanauskas, “Optical fiber with an acoustic guiding layer for stimulated Brillouin scattering suppression,” CLEO 2006, paper CThZ3.
  14. D. Walton, S. Gray, J. Wang, M.-J. Li, X. Chen, A. B. Ruffin, J. Demeritt, and L. Zenteno, “High power, narrow linewidth fiber lasers,” Proc. SPIE6102, 610205 (2006). [CrossRef]
  15. L. Dong, “Limits of stimulated Brillouin scattering suppression in optical fibers with transverse acoustic waveguide designs,” J. Lightwave Technol.28, 3156–3161 (2010).
  16. J. R. Marciante, R. G. Roides, V. V. Shkunov, and D. A. Rockwell, “Near-diffraction-limited operation of step-index large-mode-area fiber lasers via gain filtering,” Opt. Lett.35(11), 1828–1830 (2010). [CrossRef] [PubMed]
  17. T. Eidam, S. Hädrich, F. Jansen, F. Stutzki, J. Rothhardt, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, “Preferential gain photonic-crystal fiber for mode stabilization at high average powers,” Opt. Express19(9), 8656–8661 (2011). [CrossRef] [PubMed]
  18. T. Bhutta, J. I. Mackenzie, D. P. Shepherd, and R. J. Beach, “Spatial dopant profiles for transverse-mode selection in multimode waveguides,” J. Opt. Soc. Am. B19(7), 1539–1543 (2002). [CrossRef]
  19. H. W. Bruesselbach, D. S. Sumida, R. A. Reeder, and R. W. Byren, “Low-heat high-power scaling using InGaAs diode-pumped Yb:YAG lasers,” IEEE J. Sel. Top. Quantum Electron.3(1), 105–116 (1997). [CrossRef]
  20. J. R. Marciante and J. D. Zuegel, “High-gain, polarization-preserving, Yb-doped fiber amplifier for low-duty-cycle pulse amplification,” Appl. Opt.45(26), 6798–6804 (2006). [CrossRef] [PubMed]
  21. OFS Laboratories has fabricated a SHARC fiber with refractive-index uniformity of 200 ppm across the core width (Private communications with D. J. Trevor, OFS Laboratories, 2012).
  22. H. Rao, M. J. Steel, R. Scarmozzino, and R. M. Osgood., “Complex propagators for evanescent waves in bidirectional beam propagation method,” J. Lightwave Technol.18(8), 1155–1160 (2000). [CrossRef]
  23. Y. Chung and N. Dagli, “An assessment of finite difference beam propagation method,” IEEE J. Quantum Electron.26(8), 1335–1339 (1990). [CrossRef]
  24. J. Yamauchi, T. Ando, and H. Nakano, “Beam propagation analysis of optical fibres by alternating direction implicit method,” Electron. Lett.27(18), 1663–1666 (1991). [CrossRef]
  25. G. R. Hadley, “Transparent boundary condition for the beam propagation method,” IEEE J. Quantum Electron.28(1), 363–370 (1992). [CrossRef]
  26. M. Born and E. Wolf, Principles of Optics, 6th ed. (Permagon Press, 1991), pp 127–128.
  27. J. Oh, C. Headley, M. J. Andrejco, A. D. Yablon, and D. J. DiGiovanni, “Increased pulsed amplifier efficiency by manipulating the fiber dopant distribution,” CLEO 2006, paper CTuQ3.

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