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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 8 — Apr. 11, 2011
  • pp: 7398–7409
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Single-mode ytterbium-doped large-mode-area photonic bandgap rod fiber amplifier

Thomas Tanggaard Alkeskjold, Marko Laurila, Lara Scolari, and Jes Broeng  »View Author Affiliations


Optics Express, Vol. 19, Issue 8, pp. 7398-7409 (2011)
http://dx.doi.org/10.1364/OE.19.007398


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Abstract

Enabling Single-Mode (SM) operation in Large-Mode-Area (LMA) fiber amplifiers and lasers is critical, since a SM output ensures high beam quality and excellent pointing stability. In this paper, we demonstrate and test a new design approach for achieving SM LMA rod fibers by using a photonic bandgap structure. The structure allows resonant coupling of higher-order modes from the core and acts as a spatially Distributed Mode Filter (DMF). With this approach, we demonstrate passive SM performance in an only ~50cm long and straight ytterbium-doped rod fiber. The amplifier has a mode field diameter of ~59µm at 1064nm and exhibits a pump absorption of 27dB/m at 976nm.

© 2011 OSA

1. Introduction

The rapid development and deployment of high-peak power and high pulse energy fiber amplifier systems have been fuelled by the development of large-mode-area (LMA) fiber amplifiers, having larger and larger effective mode area. The continuous demand for larger effective area is driven by the need to mitigate nonlinear effects such as Four-Wave Mixing (FWM), Self-Phase Modulation (SPM), and Stimulated Raman Scattering (SRS), which can seriously distort pulse amplification due to spectral and/or temporal broadening. Larger effective area is also needed in order to increase the damage threshold at the fiber facets, which ultimately sets the limit of the maximum possible extractable pulse energy. In most cases, pure silica endcaps can be fused to the fiber facet, thereby enabling higher pulse energies. Equally important is the need for having high beam quality and excellent pointing stability. This is required for successfully applying fiber amplifiers in for example semiconductor and solar cell scribing applications or for stable and efficient frequency conversion of NIR pulses. As the core diameter of LMA fibers is increased to beyond approx. 15µm, single-mode fibers, based on a conventional step-index design, becomes difficult to manufacture with sufficient yield due to the required index precision obtainable even with state-of-the-art rare-earth-doped core manufacturing processes (+/−1e-4). For the fabrication of larger SM cores other strategies must be applied. For example, manufacturing of low-NA SM LMA Photonic Crystal Fibers (PCFs), using an air/silica microstructured cladding, has typically higher yield than the step-index approach and have successfully been applied in amplifier systems [1

1. C. D. Brooks and F. Di Teodoro, “Multi-megawatt peak-power, single-transverse-mode operation of a 100 µm core diameter, Yb-doped rod-like photonic crystal fiber amplifier,” Appl. Phys. Lett. 89(11), 111119 (2006). [CrossRef]

,2

2. J. Limpert, O. Schmidt, J. Rothhardt, F. Röser, T. Schreiber, A. Tünnermann, S. Ermeneux, P. Yvernault, and F. Salin, “Extended single-mode photonic crystal fiber lasers,” Opt. Express 14(7), 2715–2720 (2006). [CrossRef] [PubMed]

]. Another approach is to use Multi-Mode (MM) step-index fiber cores, where the Higher-Order-Modes (HOMs) are either suppressed by utilizing differential bend loss [3

3. 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]

], chirally coupled cores [4

4. C. Liu, G. Chang, N. Litchinister, D. Guertin, N. Jacobson, K. Tankala, and A. Galvanauskas, “Chirally coupled core fibers at 1550-nm and 1064-nm for effectively single-mode core size scaling,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper CTuBB3.

] or differential mode loss in so-called leaky-channel fibers [5

5. L. Dong, H. A. McKay, L. Fu, M. Ohta, A. Marcinkevicius, S. Suzuki, and M. E. Fermann, “Ytterbium-doped all glass leakage channel fibers with highly fluorine-doped silica pump cladding,” Opt. Express 17(11), 8962–8969 (2009). [CrossRef] [PubMed]

]. SM operation in highly MM fibers can also be obtained by matching the launched beam to the FM and carefully exciting the Fundamental Mode (FM) only [6

6. M. E. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt. Lett. 23(1), 52–54 (1998). [CrossRef]

]. Finally, it is possible to excite and amplify only one specific HOM, which typically has large effective area, by using long-period gratings as mode converters at the input and output to achieve FM operation and good beam quality [7

7. J. W. Nicholson, J. M. Fini, A. M. DeSantolo, E. Monberg, F. DiMarcello, J. Fleming, C. Headley, D. J. DiGiovanni, S. Ghalmi, and S. Ramachandran, “A higher-order-mode erbium-doped-fiber amplifier,” Opt. Express 18(17), 17651–17657 (2010). [CrossRef] [PubMed]

].

In this paper, we demonstrate and test a new design approach, which utilizes high-index ring-shaped DMFs. A low-NA SM ytterbium-doped fiber rod amplifier with 59µm mode field diameter is realized.

2. Fiber design

A cladding structure should be able to be adjusted such that the NA of the core ensures SM operation without introducing high confinement loss to the FM. Taking typical core batch to batch variations into account (+/−1e-4), this means that the upper cladding state (often the Fundamental Space-filling Mode (FSM)) should be able to cross all four LP11 lines of Fig. 1, by adjusting the hole size only. The mode-spacing between the FM and HOM is in this case ~5e-5, and the accuracy of the effective cladding index should, therefore, be manufactured to within approx. +/−1e-5, which is difficult but feasible.

Figure 2
Fig. 2 Modal LP11 indices, as in Fig. 1, and FSM for the hexagonal cladding (FSMhex) and for a honeycomb-type cladding structure having pure silica elements (FSMhoney0) and + 3e-4 updoped elements(FSMhoney3).
shows the FSM as function of d/Λ of two cladding structures: 1) a hexagonal cladding structure (Fig. 1, inset) and 2) a honeycomb-type cladding structure (Fig. 2, inset).

A more manufacturing ‘friendly’ cladding structure should, therefore, support cladding states having an upper effective mode index that is equal to silica for small holes (d/Λ~0.1), such that a HOM is not guided for a slightly up-doped core ( + 1e-4). For a larger hole size (d/Λ~0.3), the effective cladding mode index should be somewhat lower than silica to ensure that the FM is guided in a slightly down-doped core (−2e-4) and that any HOM is not guided. The highest index cladding state should, therefore, cross all four LP11 lines of Fig. 1 in the interval 0.1<d/Λ<0.3.

One way of achieving this, is to use multiple spatially localized elements in the cladding, which exhibit waveguiding properties that are strongly affected by a change in the size of an air hole. Figure 3
Fig. 3 Schematic and microscope image of a DMF element having a central air hole surrounded by a high-index germanium ring with index n2 and an outer silica shell with index n3.
shows an example of such a structure, which is a ring-shaped high-index germanium element. The element contains a center element with index n1 (this case an air hole with n1 = 1), a high-index ring (this case germanium doped silica) with index n2 and an outer ring (this case pure silica) with index n3. As the air hole is inflated or deflated, the thickness of the ring changes and thereby affects the effective index of the (super) modes that this structure supports.

This type of element can be arranged for example in a hexagonal lattice as shown in Fig. 4a
Fig. 4 Schematic of rod fiber design with DMF elements arranged in a hexagonal lattice (a) and in a honeycomb-type lattice (b).
, in a honeycomb-type lattice as shown in Fig. 4b, or in a different pattern than shown here. In these cases, the individually elements are optically coupled elements and they form cladding states that ensures that HOMs are not guided in the core. This is achieved by adjusting the air holes of the cladding structure such that the core NA becomes sufficiently small to only support a single-mode. The elements thereby form a Distributed Mode Filter (DMF).

An important manufacturing parameter for the DMF structure is how sensitive the effective cladding index is to a change in hole size. This can be expressed as a dn/dD [1/] value (where n represents the effective index and D the hole diameter), which should be negative and not excessive large but not excessive small either. If dn/dD is too large, it will become difficult to adjust the hole size with sufficient precision and if too small, large hole size variation is needed and it will be difficult to maintain the specified dimensions of the final rod fiber.

Figure 5
Fig. 5 Modal LP11 indices as in Fig. 1a and the FSM of the DMF elements arranged in a hexagonal (FSMDMF-HEX) and in a honeycomb-type lattice (FSMDMF-HONEY).
shows the FSM of the DMF element either arranged in a hexagonal lattice (Fig. 4a) and in a honeycomb-type lattice (Fig. 4b), having dn/dD values of −3.8e-3 and −2.4e-3, respectively. It can be observed that both structures supports cladding modes that crosses all four LP11 lines and that the honeycomb-type lattice has the lowest slope and is, therefore, more feasible for achieving SM performance.

3. DMF rod fibers

The transmission notch could furthermore be utilized for suppression of Amplified Spontaneous Emission (ASE), for example for 1030nm ASE suppression in a 1064nm low rep-rate amplifier or gain-shaping for operating at longer wavelengths.

We also manufactured a 100µm core rod fiber by up-scaling the 85µm core by 117%. The SM region scales almost linearly with the thickness of the germanium rings, and we therefore expected the 100µm core rod to be SM at approx. 1240nm. The rod fiber was measured to be SM at 1224nm wavelength i.e. at slightly lower wavelength than expected. The near field at 1224nm is shown on Fig. 11
Fig. 11 Near field image of a passive 100µm core DMF rod at 1224nm wavelength. The mode field diameter was measured to 72.5µm at 1224nm.
. The rod exhibited a mode field diameter of ~72µm, which was measured using an InGaAs camera (Spiricon XEVA XC-130) and a super-continuum source (SuperK Extreme, NKT Photonics) combined with a dual acousto-optic filter for NIR and visible wavelength selection (SpectraK Dual, NKT Photonics), which generated an 8nm wide signal centered at 1224nm.

We designed and manufactured an ytterbium-doped version of the DMF rod fiber described earlier. We used silica indexed-matched ytterbium as the core material. The rod fiber contained a MM pump air-clad with 267µm diameter, had 1.7mm outer diameter and a length of 120cm. We measured the pump absorption to be ~27dB/m (nominal) at 976nm using the cutback technique. A small fraction of the pump is coupled directly to the DMF elements and does not get absorbed, but is guided along the length of the rod. The total area of the DMF elements is about 2-3% of the total area of the pump cladding, but the NA of the DMF elements is only 0.085. If the rod is pumped with a standard 200µm 0.22NA MM pump, the pump light coupled to the DMF elements will only be about 0.7%. In case of a 267µm 0.6NA pump beam, it will only be 0.05% of the total pump.

4. Conclusion

We have successfully tested and demonstrated a new photonic bandgap design for achieving SM performance in an ytterbium-doped LMA rod fiber. We have manufactured both passive and active rods based on a distributed mode filter design and both types showed SM performance. The ytterbium-doped rod exhibited SM performance in lengths longer than ~50cm. The rod fibers had a mode field diameter of 59µm at 1064nm wavelength. The ytterbium-doped rod was manufactured with a MM pump cladding and the pump absorption was measured to ~27dB/m at 976nm, thereby enabling effective device lengths of about 60-70cm.

Acknowledgment

The project is supported with funding from the European Union (EU) FP7 project LIFT (CP- IP 228587-1- LIFT).

References and links

1.

C. D. Brooks and F. Di Teodoro, “Multi-megawatt peak-power, single-transverse-mode operation of a 100 µm core diameter, Yb-doped rod-like photonic crystal fiber amplifier,” Appl. Phys. Lett. 89(11), 111119 (2006). [CrossRef]

2.

J. Limpert, O. Schmidt, J. Rothhardt, F. Röser, T. Schreiber, A. Tünnermann, S. Ermeneux, P. Yvernault, and F. Salin, “Extended single-mode photonic crystal fiber lasers,” Opt. Express 14(7), 2715–2720 (2006). [CrossRef] [PubMed]

3.

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]

4.

C. Liu, G. Chang, N. Litchinister, D. Guertin, N. Jacobson, K. Tankala, and A. Galvanauskas, “Chirally coupled core fibers at 1550-nm and 1064-nm for effectively single-mode core size scaling,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper CTuBB3.

5.

L. Dong, H. A. McKay, L. Fu, M. Ohta, A. Marcinkevicius, S. Suzuki, and M. E. Fermann, “Ytterbium-doped all glass leakage channel fibers with highly fluorine-doped silica pump cladding,” Opt. Express 17(11), 8962–8969 (2009). [CrossRef] [PubMed]

6.

M. E. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt. Lett. 23(1), 52–54 (1998). [CrossRef]

7.

J. W. Nicholson, J. M. Fini, A. M. DeSantolo, E. Monberg, F. DiMarcello, J. Fleming, C. Headley, D. J. DiGiovanni, S. Ghalmi, and S. Ramachandran, “A higher-order-mode erbium-doped-fiber amplifier,” Opt. Express 18(17), 17651–17657 (2010). [CrossRef] [PubMed]

8.

F. Jansen, F. Stutzki, H.-J. Otto, M. Baumgartl, C. Jauregui, J. Limpert, and A. Tünnermann, “The influence of index-depressions in core-pumped Yb-doped large pitch fibers,” Opt. Express 18(26), 26834–26842 (2010). [CrossRef]

9.

N. Mortensen and J. Folkenberg, “Near-field to far-field transition of photonic crystal fibers: symmetries and interference phenomena,” Opt. Express 10(11), 475–481 (2002). [PubMed]

OCIS Codes
(060.2310) Fiber optics and optical communications : Fiber optics
(060.4005) Fiber optics and optical communications : Microstructured fibers
(060.3510) Fiber optics and optical communications : Lasers, fiber

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 1, 2011
Revised Manuscript: March 21, 2011
Manuscript Accepted: March 27, 2011
Published: April 1, 2011

Citation
Thomas Tanggaard Alkeskjold, Marko Laurila, Lara Scolari, and Jes Broeng, "Single-mode ytterbium-doped large-mode-area photonic bandgap rod fiber amplifier," Opt. Express 19, 7398-7409 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-8-7398


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References

  1. C. D. Brooks and F. Di Teodoro, “Multi-megawatt peak-power, single-transverse-mode operation of a 100 µm core diameter, Yb-doped rod-like photonic crystal fiber amplifier,” Appl. Phys. Lett. 89(11), 111119 (2006). [CrossRef]
  2. J. Limpert, O. Schmidt, J. Rothhardt, F. Röser, T. Schreiber, A. Tünnermann, S. Ermeneux, P. Yvernault, and F. Salin, “Extended single-mode photonic crystal fiber lasers,” Opt. Express 14(7), 2715–2720 (2006). [CrossRef] [PubMed]
  3. 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]
  4. C. Liu, G. Chang, N. Litchinister, D. Guertin, N. Jacobson, K. Tankala, and A. Galvanauskas, “Chirally coupled core fibers at 1550-nm and 1064-nm for effectively single-mode core size scaling,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper CTuBB3.
  5. L. Dong, H. A. McKay, L. Fu, M. Ohta, A. Marcinkevicius, S. Suzuki, and M. E. Fermann, “Ytterbium-doped all glass leakage channel fibers with highly fluorine-doped silica pump cladding,” Opt. Express 17(11), 8962–8969 (2009). [CrossRef] [PubMed]
  6. M. E. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt. Lett. 23(1), 52–54 (1998). [CrossRef]
  7. J. W. Nicholson, J. M. Fini, A. M. DeSantolo, E. Monberg, F. DiMarcello, J. Fleming, C. Headley, D. J. DiGiovanni, S. Ghalmi, and S. Ramachandran, “A higher-order-mode erbium-doped-fiber amplifier,” Opt. Express 18(17), 17651–17657 (2010). [CrossRef] [PubMed]
  8. F. Jansen, F. Stutzki, H.-J. Otto, M. Baumgartl, C. Jauregui, J. Limpert, and A. Tünnermann, “The influence of index-depressions in core-pumped Yb-doped large pitch fibers,” Opt. Express 18(26), 26834–26842 (2010). [CrossRef]
  9. N. Mortensen and J. Folkenberg, “Near-field to far-field transition of photonic crystal fibers: symmetries and interference phenomena,” Opt. Express 10(11), 475–481 (2002). [PubMed]

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