1. Introduction

The confinement of the laser radiation is the most important property making a fiber laser superior to other solid-state laser concepts. The beam quality of a fiber laser is typically not affected by thermal lensing or other thermo-optical problems which prevent the power scaling of single-transverse-mode emission of conventional solid-state lasers. This has been proven by the output power evolution of single-mode fiber laser over the recent years. Continuous-wave emission well above the kW-level with excellent beam quality is achieved [1–3].

However, the tight confinement of the optical field over a considerably long interaction length makes a fiber based laser system susceptible to nonlinear effects [4] which indeed constitute the main performance limitation of fiber lasers and amplifiers. This statement is already valid in the continuous-wave regime but becomes most challenging in the short-pulse regime. In general, all restricting nonlinear effects scale with the fiber length and are inversely proportional to the mode-field-area of the fiber core. Consequently, a reduction of nonlinearity and, therefore, a reveal of scaling potential are achieved by increasing the fiber core diameter and shortening the fiber length. The increase of core diameter is usually accompanied by a transition to multimode guidance above a certain value. The resulting degradation of beam quality is unwanted in most of experiments and applications. A multimode core causes a loss of the main fiber laser talent: the outstanding and power independent beam quality.

Over the recent years every endeavor has been made to develop fiber designs with reduced nonlinearity. A reduction of core numerical aperture (NA) from standard values of larger than 0.1 to the smallest precisely adjustable by MCVD fiber preform fabrication technology of about ~0.06 allowed for an increase of single-mode core diameters to ~15μm in the 1 μm wavelength region. Such fibers are termed low-NA large-mode-area fibers. The core becomes multi-mode by a further enlargement of the core dimensions. However, especially at a low numerical aperture the significant difference of bending loss between the fundamental mode and the higher-order transverse modes can be used to force such a multimode fiber to single-mode operation [5]. This technique works well up to core diameters in the range of 30 μm, robust single-mode behavior in even larger cores is nearly impossible.

It has been shown that micro-structuring a fiber can add several novel possibilities to control the guiding properties of the waveguide [6,7]. Consequently, the potential of photonic crystal structures inside active fibers has been discussed already few years ago [8,9]. Under certain circumstances such a photonic crystal fiber, consisting of a regular arrangement of air holes around a missing hole, can be single-mode for all wavelengths. This property is termed endlessly single-mode. In order to have this characteristic the ratio of air hole diameter d to hole-to-hole distance Λ has to be smaller than ~0.4 in a one missing hole core [10]. Hence, the single mode behavior can be simply achieved by proper design of the holey cladding. The inverse interpretation of the endlessly single-mode characteristic is that the core diameter can be scaled to infinity while staying single-mode if the criteria d/Λ < 0.4 is fulfilled.

In principle, the guiding mechanism in a step-index fiber and microstructured fiber is basically the same, total internal reflection. Consequently, the possible core enlargement of a photonic crystal fiber is accompanied by increased bending sensitivity and increased propagation losses. However, highly efficient ytterbium-doped microstructured fibers possessing an intrinsically single-transverse-mode core with diameters up to 40 μm have been reported [11]. Recently, a novel fiber design has been discussed which allows further up-scaling in single-mode core dimensions by significantly reducing the propagation losses. This fiber design features the outer dimensions of a rod, few millimeters in diameter and just few ten centimeters in length, combined with two waveguide structures, one for the pump and one for the laser radiation. Hence, such a fiber is referred to as rod-type photonic crystal fiber [12]. Basically, the large outer diameter makes the fiber that stiff and, thus, inherently straight allowing for the propagation of weakly guided modes in these extended single-mode cores.

In this contribution, we report on a single-transverse-mode emission of a 60 μm core ytterbium-doped fiber with an absorption length of just ~0.5 m. To our knowledge this fiber possesses the lowest nonlinearity ever reported for single-mode rare-earth-doped double-clad fibers. In a first continuous-wave experiment we were able to extract 320 W of output power from a 58 cm long rod-type fiber corresponding to a record power per unit length of 550 W/m. Furthermore, robust single-mode propagation in a passive 100 μm core is discussed making us confident that further single-mode core size scaling is possible.

2. The 60 μm core photonic crystal rod-type ytterbium-doped fiber

A microscope image of the cross section of the 60 μm ytterbium-doped core air-cladding photonic crystal fiber is shown in Fig. 1. The cladding consists of a triangular hole structure possessing a d/Λ = 0.19. The core is formed by 19-missing holes surrounded by 4 rings of airholes. The core is microstructured as well and comprises a balanced composition of ytterbium and fluorine doped glass to match the refractive index as close as possible to the silica index. In order to obtain producible hole sizes the resulting refractive index of the doped region is slightly below the silica index. By precise adjustment of effective cladding and core index to eachother the numerical aperture and therefore the number of guided modes can be more precisely defined as in conventional step-index fibers fabricated by MCVD-technology.

The mode-field diameter of the fundamental mode is ~50 μm corresponding to a mode-field area of ~2000 μm². Figure 2 shows the measured and calculated near-field intensity profile of the single-mode emission of this fiber. The calculated mode profile is obtained by using a finite-difference algorithm to solve the scalar Helmholtz equation.

The round inner cladding has a diameter of 175 μm. The air-cladding region is formed by silica bridges of ~400 nm width and ~10 μm length leading to a numerical aperture of the inner cladding of ~0.6 at 975 nm wavelength. This fiber design has a pump light absorption of 30 dB/m resulting in a very short absorption length of ~0.5 m. The fiber diameter is as large as 1.5 mm, thus, the fiber itself has a sufficient rigidity and mechanical stability that no coating material is necessary. Furthermore, propagation losses for the fundamental mode can be neglected making the single-transverse-mode guidance in these extended dimensions possible.

 

Fig. 1. (a). Microscope-image of the extended-mode-area rod-type photonic crystal fiber and (b) SEM-picture of the microstructured region.

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Fig. 2. Measured (a) and calculated (b) near-field intensity profile of the ytterbium doped 60 μm core emission.

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A comparison of nonlinearity between a double-clad ytterbium-doped step-index single-mode fiber, a low-numerical large-mode-area fiber and the described single-transverse mode rod-type photonic crystal fiber shows the achievement of this novel fiber design (Tab. 1). The nonlinearity is basically given by the effective mode area and the effective length of the fiber (taking into account the gain of the doped fiber). A gain as high as 25 dB has been demonstrated in rod-type ytterbium-doped fibers [12]. At high gain factors the ratio of effective fiber length to fiber length stays constant to a large extent with differing gain [4]. Therefore, the following comparison of nonlinearity, which is normalized to the value of the standard single mode step-index fiber in the 1 μm wavelength region, considers the real fiber length. This comparison reveals a reduction of nonlinearity by a factor of about 2000 in the rod-type PCF. As stated above, whenever power or energy scaling is limited by nonlinearity such a fiber offers a significant potential.

Table 1. Comparison of nonlinearity of different fiber designs in the 1 μm wavelength region

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3. Continuous-wave fiber laser experiment

To demonstrate the average power handling capability of this fiber design we built up a simple continuous-wave fiber laser. The fiber is pumped from both ends by fiber-coupled diode lasers emitting at 976 nm. The resonator is formed by one high reflecting mirror and Fresnel reflections on the other end. The rod fiber is perpendicularly polished after collapsing the air-holes. It has to be mentioned that the rod is water-cooled. Actually, the water cooling is not of essential need, because the fiber design could handle the thermal load by its own [13]. But any misalignment of the cavity would lead to not extracted population inversion, hence more thermal load, consequently a de-population of the lower laser level and therefore a reduction of pump light absorption. This avalanche process could eventually destroy the conversely emitting diodes.

Figure 3 shows the output characteristics of the high power short-length fiber laser. At a launched pump power of 425 W we achieved 320 W of laser output power with a slope efficiency of 78%. This value corresponds to an extracted power per unit length of 550 W/m. To our knowledge this represents the highest extracted power per fiber length ever reported for fiber lasers. No roll over is observed up to this power level, which illustrates the further scaling potential of this fiber design. Four rings of air holes are sufficient to ensure low propagation losses of the fundamental mode in this short-length fiber design. The high efficiency, which is comparable to the most efficient ytterbium-doped fiber lasers, is in contrast to large-core multimode fibers forced to operate single-mode, e.g. by applying bending losses, where the efficiency penalty increases which increasing core diameter.

 

Fig. 3. Output characteristics of the high power short-length rod-type fiber laser.

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4. Single-mode propagation in a passive 100 μm core fiber

To show the potential of the approach a fiber with up-scaled dimensions is investigated. The fiber has a similar structure as the above discussed 60 μm core (Fig. 1). Just the core diameter is increased to 100 μm and the d/Λ of the considered fiber is ~0.2. The fiber possesses also a round inner cladding with a diameter of 300 μm (NA ~0.6). In fact, the core guides also few higher order transverse modes, however, optimized launching conditions and mode-matching made [14] it possible to excite the fundamental mode only, which propagates stable along the straight fiber rod. Hence, scattering to higher order modes can be neglected in this fiber. The mode-field diameter of the fundamental mode is ~75 μm corresponding to a mode-field area of ~4500 μm². Figure 2 shows the measured near-field intensity profile of the single-mode emission of this fiber.

Finally, tapering of one fiber end [15] or the incorporation of losses for higher order transverse modes [16, 17] could be done to force the fiber to intrinsic single-transverse-mode emission.

 

Fig. 4. Measured near field intensity profile of the passive 100 μ m core emission.

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5. Conclusion

In conclusion, we have reported on single-transverse-mode emission of active and passive photonic crystal fibers with core diameters of 60 μm and 100 μm, respectively. These fiber designs based on the rod-type fiber concept possess record low nonlinearity combined with high efficiency and, therefore, an enormous peak power scaling potential for fiber based laser systems. Furthermore, we have demonstrated the high power extraction out of a short-length rod-type fiber laser. We reached a power level of 550 W/m, which is just pump power limited. To our knowledge this is the highest value ever experimentally demonstrated in fiber lasers.