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  • Editor: Alan E. Willner
  • Vol. 37, Iss. 21 — Nov. 1, 2012
  • pp: 4546–4548
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High-power very large mode-area thulium-doped fiber laser

Florian Jansen, Fabian Stutzki, Cesar Jauregui, Jens Limpert, and Andreas Tünnermann  »View Author Affiliations


Optics Letters, Vol. 37, Issue 21, pp. 4546-4548 (2012)
http://dx.doi.org/10.1364/OL.37.004546


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Abstract

Large-pitch photonic-crystal fibers have demonstrated their unique capability of combining very large mode areas, high output powers and robust single-mode operation at a wavelength of 1 μm. In this Letter, we present the experimental realization of thulium-doped very large mode-area fibers based on the large-pitch fibers with record mode-field diameters exceeding 60 μm and delivering more than 52 W of output power.

© 2012 Optical Society of America

The output power of both CW and pulsed fiber laser systems has undergone an unparalleled growth during the past decade, but this impressive rise in power has been focused mainly on 1 μm laser sources due to the excellent properties of ytterbium [1

1. D. J. Richardson, J. Nilsson, and W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010). [CrossRef]

]. This progress was fueled by the development of large mode-area fibers that were capable of mitigating nonlinear effects while maintaining nearly diffraction-limited output beams even under high-power operation [2

2. J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann, Light Sci. Appl. 1, e8 (2012). [CrossRef]

].

The wavelength region around 2 μm lately has been receiving an increasing amount of attention due to the existence of attractive medical, industrial, communication, and defense applications and, additionally, due to the eye-safe nature of the light scattered at this wavelength. Many of these applications would also significantly profit from high pulse energies. Several dopants for active fibers in the 2 μm region have been demonstrated, but thulium and holmium have become the dopants of choice for emission around 2 μm [3

3. S. D. Jackson, Nat. Photonics 6, 423 (2012). [CrossRef]

].

Step-index fibers with moderate mode-field areas have already delivered more than 1 kW of continuous output power in the 2 μm region [4

4. T. Ehrenreich, R. Leveille, I. Majid, and K. Tankala, Proc. SPIE 7580, xxxvii (2010).

]. Recently, first experimental demonstrations of Tm-doped photonic-crystal fibers (PCFs) proved the feasibility of index-guiding PCF [5

5. N. Modsching, P. Kadwani, R. A. Sims, L. Leick, J. Broeng, L. Shah, and M. Richardson, Opt. Lett. 36, 3873 (2011). [CrossRef]

], delivering several watts of output power with a mode-field diameter (MFD) of 36 μm and nearly diffraction-limited beam quality. Further mode-area scaling, as required for high-energy pulsed systems, implies the adaption of fiber designs able to combine high-power handling capability with robust single-mode operation and very large MFDs exceeding 50 μm. Large-pitch PCFs (LPFs) combine these requirements, and they have achieved the highest average powers and the highest pulse energies for Yb-doped fibers. The function principle of LPFs is based on the delocalization of higher-order modes, yielding a preferential excitation and amplification of the fundamental mode [2

2. J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann, Light Sci. Appl. 1, e8 (2012). [CrossRef]

].

In this Letter, we experimentally show that LPFs are also ideally suited for 2 μm operation. Furthermore, we discuss the advantages and challenges faced when scaling the mode-field area of fiber amplifiers into the 2 μm region.

On one hand, the transition to longer wavelengths reduces nonlinearities [3

3. S. D. Jackson, Nat. Photonics 6, 423 (2012). [CrossRef]

]. On the other hand, in principle, it additionally facilitates larger mode-field areas. A step-index fiber with a fixed numerical aperture maintains its V parameter, and therefore its guiding properties, when scaling the wavelength and the core size proportionally. As the maximum core diameter for strictly single-mode operation of step-index fibers is currently limited by technological processes to 15μm in the 1 μm region, it should be possible to obtain strictly single-mode step-index fibers with MFDs of around 30 μm in the 2 μm region. The same holds true for PCFs as the guiding properties are maintained for a proportional scaling of the structure dimensions with wavelength.

Recent experiments at 1 μm with Yb-doped large mode-area fibers have demonstrated that thermal effects have to be considered for fibers operating at some hundred watts of average power and with MFDs exceeding 50 μm, as the resulting thermally induced index gradient significantly alters the waveguide properties. This thermally induced index profile will lead to a decrease of the mode-field area [7

7. F. Jansen, F. Stutzki, H. Otto, T. Eidam, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Express 20, 3997 (2012). [CrossRef]

]. Furthermore, it can be very problematic for resonant fiber designs because it shifts the band gaps [8

8. M. Laurila, M. M. Jørgensen, K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Lægsgaard, Opt. Express 20, 5742 (2012). [CrossRef]

].

Tm-doped fibers are typically pumped at 793 nm and operated around 2 μm. This results in a very large quantum defect compared to Yb-doped systems. Even though this quantum defect can be alleviated by exploiting cross-relaxation processes [9

9. S. D. Jackson, Opt. Commun. 230, 197 (2004).

], finding the required experimental conditions for them to take place is not always easy or possible. Therefore, in general, Tm-doped fibers are plagued by much stronger thermal effects than Yb-doped fibers. Moreover, it has to be noted that, in contrast to standard Yb-doped fibers pumped around 976 nm (i.e., the zero phonon line), the energy level scheme of Tm results in heat generation even without amplification when pumped at 793 nm. Thus, a significant thermal index gradient has to be expected simply by pumping a Tm-doped fiber. Therefore, a robust design, such as the LPF, is mandatory for high-power very large mode-area Tm-doped fibers.

In our experiments, we employed a 1.2 m long Tm-doped LPF in CW-oscillator configuration. Figure 1 shows a structural scheme of the LPF. The Tm-doped LPF with a hole-to-hole distance Λ of 45 μm has a core diameter of 81 μm (of which 63 μm was doped), a relative hole size d/Λ of 0.2, and an air-clad diameter of 260 μm. It was doped with approximately 2.5 wt. % Tm and codoped with Al. The active fiber possesses an estimated pump absorption of about 9dB/m at 793 nm.

Fig. 1. Large-pitch fiber structure.

The LPF is a straight rod-type fiber to ensure the largest possible MFD. Typically, such short straight fibers maintain the incoming polarization, which makes any stress-applying elements redundant. Furthermore, the absence of a polymer coating, which is intrinsic of rod-type fibers, is advantageous for high-power operation in a Tm-doped fiber laser due to the high temperatures reached at the fiber surface. In this experimental demonstration, the LPF has been placed in aluminum V grooves that were water-cooled to 14°C.

The experimental setup is depicted in Fig. 2. The oscillator was pumped with a 400 μm fiber-coupled 793 nm high-power diode laser able to deliver up to 300 W. The oscillator was created by a high-reflectivity mirror (HR) (butt-coupled, >99% reflectivity for 1900–2100 nm) and the Fresnel reflection of the perpendicularly cleaved opposite end facet. Two fused silica aspheric lenses with focal lengths of 40 mm (antireflection coated from 600–1050 nm, A2) and 25 mm (uncoated, A1), were used as the pump coupling optics. Between these two lenses, there was a dichroic mirror (HR 1900–2100 nm, high transmissivity around 800 nm) to allow the extraction of the signal beam.

Fig. 2. Experimental setup of the oscillator: HR, high-reflectivity mirror; A1 and A2, aspheric lenses; DM, dichroic mirror; PM, power meter.

The signal output beam was filtered with an additional dichroic mirror, and it was sampled with a fused silica wedge, which enabled the synchronous measurement of output power, optical spectrum, and beam profile. The measured output power was corrected with respect to the Fresnel reflections at the surfaces of the silica wedge.

This free-running laser exhibited multiple peaks below 1900 nm (Fig. 3), which is where the maximum of the emission cross section of Tm is located. This rather short operating wavelength can be attributed to the short fiber length and to the large output-coupling ratio [10

10. S. D. Jackson and T. A. King, Opt. Lett. 23, 1462 (1998). [CrossRef]

].

Fig. 3. Emitted spectrum of the free-running LPF oscillator.

Figure 4 illustrates the slope of the output power versus the launched pump power (as measured after the coupling optics). A maximum output power of 52 W was achieved, limited by the available pump power. The high lasing threshold of 115 W launched pump power and the deviations from a linear slope (at 10W output power) can be explained by the aforementioned index depression. A strongly depressed core can deform the fundamental mode up to a point where it has reduced overlap with the doped region (at the so-called avoided-crossing points). This will result in decreased gain and in a deviation from a linear slope. In an oscillator configuration, this index-mismatch can increase the laser threshold, as no localized solution can be found in the core at low output powers.

Fig. 4. Output characteristics of the Tm-doped LPF oscillator with 81 μm core.

Pumping the fiber imprints a parabolic thermal index gradient, and, therefore, the strength of the index mismatch inside the core is reduced. With rising pump power, this thermally induced gradient becomes stronger, which pulls the fundamental mode back into the core. This phenomenon was studied in detail in Yb-doped fibers [7

7. F. Jansen, F. Stutzki, H. Otto, T. Eidam, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Express 20, 3997 (2012). [CrossRef]

], where mode deformations and dips in the amplifier slope are discussed. Judging from the facts that Tm-doped fibers are less sensitive to index depression and that an increased heat load is deposited in the fiber, the high lasing threshold can be interpreted as the result of a very large index depression. In fact, some preliminary investigations point toward an index depression larger than 5·104. Hence, a smaller index depression (1·104 should be technologically feasible) will significantly decrease the lasing threshold, and, possibly, it will even increase the slope efficiency.

Furthermore, it is worth mentioning that the large thermal load manifested itself in a bulging of the end facet of the fiber, which gave rise to a slowly varying defocusing of the near-field image of the fiber end facet. Forced-air cooling significantly reduced the slow beam deformations, which leads to the conclusion that improved cooling techniques can substantially enhance the stability of the system.

Fig. 5. Near-field image of the fiber end facet at highest output power.

In conclusion, we have demonstrated, to the best of our knowledge, the highest output power of very large mode-area fibers in the 2 μm region. Despite a strong index-mismatch between the Tm-doped core and the silica matrix, the LPF design is capable of delivering a single-mode output beam. With an improved index matching, significantly larger MFDs should be feasible, ultimately enabling pulsed fiber laser systems with several millijoule pulse energies and high-power ultrashort pulses in the 2 μm regime.

The research leading to these results has received funding from the German Federal Ministry of Education and Research (BMBF), the European Research Council (ERC) under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. (240460) “PECS,” and the Thuringian Ministry for Economy, Labour, and Technology (TMWAT) (project no. 2011 FGR 0103) with a European Social Fund (ESF) grant. Additionally, F. J. acknowledges financial support by the Abbe-School of Photonics Jena.

References

1.

D. J. Richardson, J. Nilsson, and W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010). [CrossRef]

2.

J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann, Light Sci. Appl. 1, e8 (2012). [CrossRef]

3.

S. D. Jackson, Nat. Photonics 6, 423 (2012). [CrossRef]

4.

T. Ehrenreich, R. Leveille, I. Majid, and K. Tankala, Proc. SPIE 7580, xxxvii (2010).

5.

N. Modsching, P. Kadwani, R. A. Sims, L. Leick, J. Broeng, L. Shah, and M. Richardson, Opt. Lett. 36, 3873 (2011). [CrossRef]

6.

F. Jansen, F. Stutzki, H.-J. Otto, M. Baumgartl, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Express 18, 26834 (2010). [CrossRef]

7.

F. Jansen, F. Stutzki, H. Otto, T. Eidam, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Express 20, 3997 (2012). [CrossRef]

8.

M. Laurila, M. M. Jørgensen, K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Lægsgaard, Opt. Express 20, 5742 (2012). [CrossRef]

9.

S. D. Jackson, Opt. Commun. 230, 197 (2004).

10.

S. D. Jackson and T. A. King, Opt. Lett. 23, 1462 (1998). [CrossRef]

OCIS Codes
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(140.3510) Lasers and laser optics : Lasers, fiber
(140.6810) Lasers and laser optics : Thermal effects
(060.5295) Fiber optics and optical communications : Photonic crystal fibers
(060.3510) Fiber optics and optical communications : Lasers, fiber

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: September 17, 2012
Revised Manuscript: October 1, 2012
Manuscript Accepted: October 3, 2012
Published: October 30, 2012

Citation
Florian Jansen, Fabian Stutzki, Cesar Jauregui, Jens Limpert, and Andreas Tünnermann, "High-power very large mode-area thulium-doped fiber laser," Opt. Lett. 37, 4546-4548 (2012)
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-37-21-4546


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