1. Introduction

Air-silica microstructure fibers (ASMF), also called photonic crystal or holey fibers, are currently the subject of intense research. Such fibers consist of a pure silica core surrounded by a regular array of air holes [1], that leads to exceptional guiding properties which can not be obtained in conventional step-index fibers. It has been shown that ASMFs can be strictly single-mode over a large wavelength range [2] and the engineerable contribution of the waveguide dispersion can lead to a significant shift of the zero-dispersion wavelength towards the visible spectral region [3].

The gain medium of a fiber laser can be fabricated by introducing a rare-earth ion doping into the core of the microstructure fiber [4]. Even the double-clad concept [5] can be transferred to such fibers [6], with the promising feature of a high numerical aperture of the inner cladding. This is achieved by surrounding the inner cladding with a web of silica bridges which are substantially narrower than the wavelength of the guided radiation. Numerical apertures of up to 0.8 are reported [7]. The benefit of such an air-clad fiber with a high NA is that the diameter of the inner cladding (pump core) can be significantly reduced while retaining brightness acceptance of the pump radiation. Due to the increased ratio of active core area to inner cladding area, the pump light absorption is improved.

Actively doped large-mode-area microstructure fibers also have been realized [6,8]. The high accuracy and flexibility of the control of the refractive index difference between the core and the cladding offers several new design possibilities. Such fibers offer single-mode guidance in core areas that are at least as large as those achievable in conventional step-index fibers. The large-mode-area design, together with the significantly reduced absorption length, reduces nonlinear effects [9], which constitutes in general the performance limitation of high peak power ultrafast fiber laser and amplifier systems.

The combination of these fiber designs with Yb-doped cores results in unique properties for high power lasers. This is due to the small quantum defect of ytterbium, leading to high efficiency and low thermal load. In general, fiber based laser systems are immune against any detrimental thermo-optical problems due to their special geometry. This fact is due to the large ratio of surface to active volume of such a fiber that ensures excellent heat dissipation. The beam quality of the guided mode is determined by the fiber core design and is therefore power-independent. Additionally, the absorption spectrum covers a wavelength range in which powerful diode lasers are commercially available. The low intrinsic loss of an actively doped glass fiber enables long interaction lengths that, together with the simultaneous confinement of pump and laser radiation, leads to a very high single pass gain. These properties make rare-earth-doped fibers in a variety of performance categories superior over other solid-state laser concepts. This becomes obvious by several recent demonstrations of continuous-wave fiber laser systems with output powers well above 100 W with excellent beam quality [10,11].

A further advantage of such an air-clad fiber is that no radiation has direct contact to the coating material, what makes these fibers predestinated for high power operation. On the other hand the main drawback seems to be the thermo-optical management of these fibers due to the interrupted heat dissipation.

In this contribution, we report on the extraction of 80 W of output power from an ytterbium-doped air-clad large-mode-area microstructure fiber, exceeding recently published values by more than one order of magnitude. The 2.3 m long fiber laser emits single transverse mode radiation at around 1070 nm with a slope efficiency of 78%. Even at this power level no thermo-optical distortions are observed.

2. High numerical aperture air-clad ytterbium-doped large-mode-area fiber

The incorporation of rare-earth dopants into the core of an air-silica microstructure fiber is challenging because the dopants and associated codopants itself modify the refractive index of the core. The fiber design has to be chosen carefully that the guiding properties are determined by the hole arrangement and not by the index-step of the dopants [6].

The inner cladding of our large-mode-area photonic crystal fiber, shown in Fig. 1, consists of a hexagonal lattice of air holes with a diameter d of 2 µm and a hole-to-hole spacing Λ of approximately 11.5 µm, therefore d/Λ=0.18. To form the large-mode-area core, three capillaries are replaced by ytterbium-doped rods during the stacking process, resulting in a triangularly shaped core with a diameter of about 28 µm after drawing the fiber. This design has recently been demonstrated to be advantageous for large-mode-area photonic crystal fibers [12]. The doped rods have a diameter of approximately 9 µm and contain 0.6 at% ytterbium-ions. The rods are codoped with aluminum to ensure the solubility of the laser active ions and maintain laser efficiency. The rods are further codoped with fluorine to compensate for the refractive index increase from ytterbium and aluminum and provide a refractive index of the rods that is closely matched to silica. Thus, the refractive index step is as low as ~2·10^-4 at this relatively high ytterbium doping level.

 

Fig. 1. Scanning electron microscope images of the air-clad ytterbium-doped large-mode-area fiber; (b) close-up of core region

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To investigate the influence of the doping on the guiding properties, the mode field and propagation constant are calculated by means of a plane wave expansion, using a freely available software package [13] with and without the index step due to the doped rods. Figure 2 shows the intensity distribution in this large-mode-area core for the two cases. Without doping the core is strictly single-mode at 1070 nm wavelength, and the mode has a nearly triangular shape. Considering the incorporated doped rods the effective index is increased by 5·10^-5 and the mode develops a slight structure. In both cases the fundamental mode occurs in double degenerate pairs of orthogonal linearly polarized modes with the same intensity distribution. Theoretically, the doped fiber guides a second order transverse mode, but very close to the cut-off. Therefore, the fiber is practically single-mode if working at usual bending radii. The mode-field diameter is measured to be as high as 21 µm, corresponding to a mode area of 350 µm². The core numerical aperture is characterized to be 0.05.

 

Fig. 2. Calculated intensity distributions of the air-silica microstructured core without (a) and with doping (b), lines represent 2 dB differences in intensity

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The photonic crystal fiber is produced using stacking of various capillary tubes and doped rods, thereby forming the hexagonally shaped air-cladding, the microstructured inner cladding, and the triangularly shaped core region. The result after the drawing process is already shown in Fig. 1. The silica bridges of the air-cladding are as thin as 390 nm and approximately 50 µm long. The diameter of the inner cladding is 150 µm with a measured numerical aperture of 0.55. The outer cladding diameter of this fiber is about 450 µm, and acrylate is applied as coating material.

The described air-clad microstructure ytterbium-doped large-mode-area fiber has a pump light absorption of 9.6 dB/m at 976 nm. This represents a significant enhancement compared to conventional low-NA large-mode-area fibers with comparable brightness acceptance of the inner cladding, which possess typically a pump light absorption of approximately 2 dB/m.

3. Air-clad photonic crystal fiber laser

A fiber laser in its simplest form is built up using 2.3 m length of the air-clad microstructure fiber. The fiber ends are perpendicularly cleaved and the cavity is formed by a high reflecting dichroic mirror at one end of the fiber and the 4% Fresnel reflections at the other fiber end. The fiber laser is pumped from both sides with fiber coupled (400 µm, NA=0.22) diode lasers operating at 976 nm. The coupling efficiency in the inner cladding is 55%, measured by cut-back method, resulting in a launched pump power of up to 105 W. The length of 2.3 m ensures that the entire launched pump radiation is absorbed.

Figure 3 shows the output characteristics of the high power fiber laser. We were able to reach an output power of up to 80 W with a slope efficiency of 78% with respect to the launched pump power. No degradation of efficiency or any roll over is observed even at this power level. The threshold pump power is about 0.75 W. The high slope efficiency, that is comparable to the highest values reported for conventional ytterbium-doped double-clad fiber lasers, reveals that there are no losses of any kind due to the unconventional shape of the active core and the inner cladding.

 

Fig. 3. Laser characteristics of the high power air-clad microstructure ytterbium-doped large-mode-area fiber

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Figure 4 shows the measured beam profile emitted by this high power fiber laser. In spite of the single transverse operation of this fiber laser, the beam quality is not expected to be diffraction-limited because of the non-circular shape of the fiber core. The beam profile exhibits a triangular shape, however the central part, where most of the power is located, possesses a nearly round and Gaussian like intensity distribution. Thus, the M²-value is characterized to be 1.2±0.1, meaning a nearly diffraction-limited beam quality.

 

Fig. 4. Measured intensity distribution of the emitted beam of the microstructure large-modearea fiber laser

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4. Conclusion

In conclusion, we have demonstrated a high power air-clad photonic crystal fiber laser. To our knowledge, the output power exceeds presently published values by more than one order of magnitude. 80 W of output power are generated from a 2.3 m long ytterbium-doped large-mode-area fiber with a slope efficiency of 78%. Single-transverse mode operation is achieved at a mode-field area of ~350 µm². No thermo-optical problems are observed at this power level. The extracted power per fiber length (~35 W/m) is even higher than that reported for conventional double-clad high power fiber lasers. This leads to the conclusion that air-clad microstructure ytterbium-doped large-mode-area fibers should be scalable to kW-level output powers, where nonlinearity will limit the performance.