We report on a novel ytterbium-doped fiber design that combines the advantages of rod and fiber gain media. The fiber design has outer dimensions of a rod laser, meaning a diameter in the range of a few millimeters and a length of just a few tens of centimeters, and includes two important waveguide structures, one for pump radiation and one for laser radiation. We obtained 120-W output power in single-mode beam quality from a 48-cm-long fiber cane that corresponds to an extracted power of 250 W/m. The fiber has significantly reduced nonlinearity, which therefore allows for scalability in the performance of a high-peak-power fiber laser and amplifier system.
©2005 Optical Society of America
Over the past few years rare-earth-doped fiber lasers and amplifiers have proved to be a power-scalable solid-state laser concept. Continuous-wave output powers in the kilowatt range with excellent beam quality [1–3] have been demonstrated. These results are possible because of several inherent properties of active fibers. Their main performance advantages arise from the confinement of pump and laser radiation in waveguide structures. The refractive-index profile of the doped core determines the mode quality of the laser output, which is therefore power independent. Furthermore, the large ratio of surface to active volume ensures excellent heat dissipation, which makes a fiber laser basically immune to thermo-optic problems. The guidance of pump radiation leads to a large product of pump intensity and interaction length, which is the reason for the high single-pass gain of rare-earth-doped fiber. However, the tight confinement of laser radiation, mode-field diameters of conventional single-mode fiber being ~10 µm, together with the considerably long fiber length, typically longer than 1 m and even as much as 100 m, creates nonlinear effects such as stimulated Raman scattering (SRS) and self-phase modulation (SPM) . These nonlinear effects are in general the main performance limitation of fiber lasers and amplifiers. The nonlinearity of a fiber scales with the fiber length and is inversely proportional to the mode-field area. Therefore, the employment of short large-mode-area fibers allows for significant power scaling.
We report on a novel large-mode-area photonic crystal fiber design with the outer dimensions of a rod laser and the performance of a fiber laser. We obtained 120 W of single-transverse-mode laser output power from a just 48-cm-long fiber sample with a slope efficiency of 74%. This corresponds to an extracted power of 250 W/m, which to our knowledge is the highest value ever reported for fiber lasers.
2. Photonic Crystal Rod-Type Ytterbium-Doped Fiber
The basic idea of this fiber design is to have outer dimensions of a rod laser, meaning a diameter in the range of a few millimeters and a length of just a few tens of centimeters, but including two important waveguide structures, one for pump radiation and one for laser radiation. Finally, such a fiber has an extremely reduced nonlinearity and therefore allows for significant power and energy scaling.
One can achieve the reduction of fiber length by reducing the ratio of pump core area to active core area, which increases the pump light absorption of double-clad fiber . The inner cladding of the discussed fiber is surrounded by an air-cladding region , as shown in Fig. 1. The inner cladding has a hexagonal shape with a flat-to-flat diameter of ~117 µm and a corner-to-corner diameter of ~141µm. The numerical aperture (NA) is as high as 0.6. The ytterbium-doped core has a diameter of ~35 µm, leading to pump light absorption of this structure of ~30 dB/m at 976 nm. Because of the high NA of the inner cladding, which is realized by microstructuring, the pump core is even compatible with 400-µm, 0.22-NA standard pump delivery fibers. Consequently, the fiber is designed for several hundred watts of output power by use of state-of-the-art diode laser technology. It is important to note that, because of the unchanged product of pump intensity and fiber length compared with other double-clad fiber designs, the high single-pass gain of the fiber is maintained even with an extremely short fiber length. Therefore, this rod-type fiber also features a low lasing threshold and high efficiency as is the case for conventional fiber dimensions.
It has been shown that microstructuring an optical fiber can lead to several new properties of the fiber. The basic characteristics, such as dispersion and nonlinearity, can be tailored with high precision [7,8]. Furthermore large, true, single-transverse mode waveguides  can be realized. Especially in the case of a perfectly straight fiber for which bending losses are negligible, the high precision of microstructuring provides the possibility to create low NA large-mode-area cores. The active core of the presented fiber has a diameter of ~35 µm. The hole diameter-to-pitch ratio is approximately 0.33. This core supports just one transverse mode, therefore the emitted beam quality is close to diffraction limited. However, single-mode operation of an ytterbium-doped photonic crystal fiber has been recently demonstrated with mode field diameters even as high as 45 µm .
Usually, the extraction of high-power levels from short fiber lengths is limited by thermo-optic problems. A detailed analysis of the thermo-optic behavior of high-power fiber lasers (including photonic crystal fibers)  has revealed that power scaling is restricted by damage to the polymer coating, which occurs at fiber surface temperatures between 100 and 200°C. These temperatures are easily reached if power levels in the 100 W/m range are extracted. In a conventional double-clad fiber the coating has an optical function. The coating has to have a lower refractive index than fused silica and therefore forms the waveguide for pump radiation. But in a microstructured air-clad fiber it just serves to protect the fiber from mechanical damage and chemical attack. The most straightforward way to avoid the coating damage is to remove the coating. This can be done if the fiber itself has enough mechanical stability, i.e., if the fiber is thick enough. The fiber shown in Fig. 1 has an outer cladding diameter as large as 1.70 mm and possesses no coating. In addition, the larger outer diameter improves the heat dissipation capabilities of this fiber  and also reduces the propagation loss of weakly guided radiation that is due to the increased rigidity.
3. Experiment and Results
A fiber laser in its simplest form is built by use of just a 48-cm length of the discussed rod-type fiber. The laser is pumped from just one side, and the fiber ends are perpendicularly polished. The cavity is formed by a high reflecting mirror on one side and ~4% Fresnel reflections on the other side. Figure 2 shows a photograph of the experimental setup.
At a launched pump power of 165 W at 976 nm we achieved 120 W of laser output power with a slope efficiency of 74%, as shown in Fig. 3. This value corresponds to an extracted power of 250 W/m. No roll over was observed up to this power level, which illustrates further the scaling potential of this fiber design. The fiber laser is pumped from just one side, i.e., simple power scaling could also be achieved by use of the second fiber end to launch the pump light. The emission spectrum of the short-length fiber laser is shown in Fig. 4; the free-running laser operates around 1035 nm. This short wavelength is related to the high excited population density achieved in this fiber. The performance (lasing threshold, slope efficiency) of the laser is comparable with conventional fiber lasers.
In a second experiment, 200-ps pulses at a repetition rate of 100 kHz were amplified in the short-length rod-type fiber. Therefore, the fiber had to be angle polished. We measured a saturated single-pass gain of greater than 25 dB, which illustrates that this fiber can definitely be used as a power amplifier for short and ultrashort high-peak-power pulses.
In conclusion, we have demonstrated the high power extraction out of a short-length rod-type fiber laser. The laser is not externally cooled; nevertheless, we reached a power level of 250 W/m, which is just pump power limited. To our knowledge this the highest value ever reported for fiber lasers. The design has an extremely reduced nonlinearity and therefore offers a significant scaling potential in several operating regimes.
We acknowledge the support of the Conseil Régional d’Aquitaine and the European Fund for Regional Economic Development.
References and links
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