We have demonstrated a monolithic cladding-pumped ytterbium-doped single all-fiber laser oscillator generating 1 kW of CW signal power at 1080 nm with 71% slope efficiency and near diffraction-limited beam quality. Fiber components were highly integrated on “spliceless” passive fibers to promote laser efficiency and alleviate non-linear effects. The laser was pumped through a 7:1 pump combiner with seven 200-W 91x nm fiber-pigtailed wavelength-beam-combined diode-stack modules. The signal power of such a single all-fiber laser oscillator showed no evidence of roll-over, and the highest output was limited only by available pump power.
©2012 Optical Society of America
Ytterbium-doped large-core fiber lasers have shown tremendous progress in the last decade in terms of power scaling, reaching kilowatt and even multi-kilowatt of near diffraction-limited signal power [1–7]. Relying on their high efficiency, high brightness and superior reliability, kilowatt-class fiber lasers rapidly increase their market share in defense and industrial applications. The highest reported signal power of single-mode fiber laser is 10 kW , which is uniquely achieved with pumping by fiber lasers. However, all the existing fiber laser designs are either non-all-fiber approaches [1, 2], pump power of which is coupled in free space with bulk optics, or use concatenated multi-stage amplifiers [3–7]. It is desirable to have even compact and monolithic solutions for kilowatt fiber laser and amplifiers. This work presents a 1 kW single-mode CW all-fiber laser oscillator and explores the limits of current state-of-the-art fiber components. Because it requires no free-space optics and no hard-to-obtain isolators used in between multi stages of amplifiers, such an all-fiber single oscillator architecture is suitable not only for power scaling demonstration of high power fiber components but also for potential commercialization of monolithic fiber laser enabled with high-brightness diode pumps.
Advance of high power ytterbium-doped fiber (YDF) lasers is fueled by three major technologies: high-quality active fibers; high-power passive fiber components, including pump combiners, fiber Bragg gratings (FBG), isolators, cladding mode strippers and end caps; and bright diode laser pumps. Ahead of the other two factors, well-developed YDF technologies are currently not the limitation for power scaling of fiber lasers, such that the research effort on passive fiber components and diode laser pumps has been intense. Here we report the results of a 1 kW single-mode CW all-fiber laser oscillator that was built for the demonstration of passive fiber components capable of handling kilowatt-level pump and signal power. The 1 kW single-mode (M2 < 1.3) fiber laser output from a 20 μm / 0.06 NA core has a center wavelength around 1080 nm and slope efficiency (SE) of 71%. Pump power was launched from the high-reflection (HR) FBG side through a 7:1 tapered fiber bundle (TFB). To the best of our knowledge, this is the first reported kilowatt-class all-fiber oscillator in such a compact architecture with integration of passive fiber components. We also developed seven 200-W 91x-nm wavelength-beam-combined (WBC) diode-stack pump modules fiber-pigtailed with 200 μm / 0.22 NA fibers in this work. The typical power conversion efficiency (PCE) of these pumps is 30%. Both designs and results for pumps and the fiber laser are discussed in the following sections. Conclusions are drawn in the last section.
2. Setup and performance of WBC pumps
The fiber laser was pumped with seven wavelength-beam-combined diode laser stack modules [8–10]. The pump layout in the slow-axis plane and beam footprints modeled with ZEMAX are shown in Fig. 1 . The external cavity consists of a 6-bar micro-channel cooled diode laser stack, a transform lens of 150-mm effective focal length (EFL), a 2000-line/mm diffraction grating and an output coupler with 15% reflectivity. The combined output beam is focused into a 200-μm core / 0.22 NA fiber with a multi-element objective lens of 37 mm EFL. Each bar in the 6-bar diode laser stack is 1 cm long and contains 19 emitters with 2-mm cavity length and 20% fill factor. Laser output from each bar is collimated with a fast-axis collimation (FAC) lens and a slow-axis collimation (SAC) lens array. The reflective gold-coated diffraction grating used for beam combining is commercially available and has a first order diffraction efficiency above 90%. The copper substrate of the grating is mounted on a water-cooled heat sink for dissipating excess heat that causes grating distortion. Emitters in a bar operate at different center wavelengths due to their displacement along the slow axis, respected to the axis of the transform lens. Each emitter receives the feedback from the output coupler in a different first-order diffraction angle. The cylindrical lens transforms the angles into the displaced locations of emitters. The three beam footprints at locations of transform lens, output coupler and fiber entrance illustrate the brightness enhancement through wavelength-beam-combining.
As shown in Fig. 1, we kept the WBC pump layout as a “V” configuration rather than a “Z” configuration used in , by which means we saved a folding mirror and minimized the cavity length to achieve a relative small form factor, although it was a little more complicated for alignment. The pump cavity length played a critical role in selection of cavity mode, in other words, suppression of higher order modes. Large divergence of broad-area laser diode is one of the issues limiting WBC diode laser arrays and stacks. Because the large divergent beam in the slow axis can be captured not only by the SAC lens in front of the emitter but also by neighboring lenses, the cross-coupled beam forms a multi-lobed output. The side lobes, which are higher order cavity modes, can contain up to 50% of the total power, extremely harmful for fiber coupling due to their larger launching angles than the fiber NA. We optimized the pump cavity length to the minimum while maintaining a fundamental spatial mode at the output. The other issue limiting WBC laser arrays and stacks is bar “smile”, which reduces WBC feedback into individual emitters. The “smile” of our laser bars is typically smaller than 3 μm.
Typical performance of the WBC diode pumps is shown in Fig. 2 . L-I curves for WBC diode laser output and fiber output as well as the total PCE of the fiber-coupled pump module are depicted in Fig. 2(a). We obtained more than 200 W of output power at 70 A through a 200 μm core / 0.22 NA pigtail fiber. The micro-lensed six-bar stacks have a PCE of about 60%. The optical-to-optical efficiency of the WBC laser cavity is around 55%, and the fiber coupling efficiency is 90~92%. Since both sides of the fiber pigtail are uncoated, the fiber coupling loss is mainly caused by Fresnel reflection loss. The total PCE of the pump module is about 30%. A typical pump spectrum is depicted in Fig. 2(b), showing a spectral width of 16 nm as the result of WBC. Each peak in the spectrum has a width about 0.3 nm (FWHM), originating from six emitters (one from each bar) at the same position on the slow-axis. The non-uniformity of peak intensities is mainly due to the stack-up effect of six bar- “smiles”. The gratings that we tested showed efficiency drop when CW power density was above 500 W/cm2. Such efficiency degradation can be attributed to grating distortion under excess heat. We set the operating current at 70 A corresponding to more than 200 W of output power and below 450 W/cm2 of power density on the grating. The total pump power available with the seven WBC pump modules is nearly 1.5 kW. The center of the spectrum can be slightly tuned by adjusting the grating angle for maximizing the output power. The center wavelength for each of the seven pump modules is between 915 nm and 920 nm.
3. Experiments and results of the all-fiber laser oscillator
The all-fiber laser oscillator architecture is sketched in Fig. 3 . Seven fiber-pigtailed WBC diode laser pump modules were combined with a 7:1 TFB and launched through the HR-FBG (integrated in the TFB package) end of the laser cavity. The TFB combined seven 200/220-μm silica fibers with 0.22 NA cores that were fused and tapered down to match the 20/400-μm output fiber. This bundle was then spliced to the 20/400-um passive relay fiber on which an HR FBG was inscribed. Scattering from the fiber splices generates heat and reduces laser efficiency, such that every splice is a potential reliability issue at the kilowatt power level. This is especially true in a resonator configuration, where part of the signal light will see multiple round trips. To promote the efficiency of the fiber laser, we reduced the number of fiber splices and the length of relay fiber to a minimum. The TFB and HR FBG were integrated in one high-power package, which eliminated the need for a spliced relay fiber between the two components. The yielded insertion loss of this integrated combiner was below 0.2 dB. All other passive fiber components, including cladding mode stripper, tap monitor, OC FBG and AR-coated end cap, were fabricated on one piece of relay fiber and also sealed in high-power packages for effective heat sinking. The only two intracavity splices are located between active and passive fibers, indicated with red crosses in Fig. 3. The gain fiber is an ytterbium-doped large-mode-area double-clad fiber with a 20 μm/0.06 NA core and a 400 μm/0.46 NA inner clad. The total cavity length of the fiber laser was minimized with the highly integrated design, helping to alleviate non-linear effects.
Compared with a design compositing both master-oscillator and power amplifier, this monolithic all-fiber laser oscillator does not require a high-power isolator. Moreover, the highly integrated design of passive components promotes laser efficiency and reduces non-linear effects by eliminating intracavity splices and unnecessary cavity length. The intracavity cladding mode striper reduces the ASE and residual pump in the cladding, maintaining the reliability for kilowatt operation. We optimized the active-to-passive fiber splices with assistance of mode field adaptors for LP01 operation of the cavity and coiled the gain fiber with a minimum diameter of 10 mm, resulting in a single-mode lasing condition. A tap monitor inserted inside the laser cavity provides an additional means for monitoring and characterizing the behavior of the fiber laser. The two single-mode fiber taps with 40 dB tap ratio are connected to an optical spectrum analyzer (OSA) and a photo detector, in order to monitor spectrum and signal noise level with forward and backward propagating laser beams coupled from the relay fiber.
The fiber laser was tested up to 1 kW of output signal power with no evidence of roll-over, shown in Fig. 4(a) . The highest output power was restricted only by available pump power. Pump absorption of the 20/400 μm YDF was characterized using the WBC diode pumps, so the total residual pump after propagation in the gain fiber could be estimated as about 50 W at the highest total launched pump power of 1485 W. The SE of fiber laser power with respect to injected pump power was 71% without taking the residual pump into account, and the beam quality factor was around 1.3. A spectrum of the fiber laser at 1-kW of signal power measured with scans of 0.02-nm resolution is overlaid on the wavelength dependent reflectivity of the OC FBG, shown in Fig. 4(b). The center wavelengths matched at 1080.3 nm, but the laser spectrum with a 2-nm bandwidth (FWHM) was broadened from the 0.15-nm (FWHM) OC FBG due to nonlinear effects, such as self-phase modulation.
Nonlinear effects resulting from stimulated inelastic scattering processes, such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), are usual limitations for the power scaling of rare-earth-doped fiber systems. The performance of single-frequency fiber lasers is typically limited by SBS. The SBS threshold increases substancially with emission bandwidth, such that SRS usually dominates when the spectral bandwidth exceeds ~0.5 GHz . A good approximation for the SRS threshold power is given by Eq. (1) as 1.8 kW, which is well above the 1-kW value.
We described the monolithic architecture and the performance of a 1-kW single-mode CW all-fiber laser oscillator pumped with WBC diode laser pumps. The fiber laser showed no evidence of roll-over in laser output power even at the highest launched pump power (~1.5 kW). The output power was limited only by available pump power in this work, and we demonstrated the 1-kW signal power handling capability for the fiber laser components. Further power scaling of such an all-fiber laser and exploration of component damage threshold can be achieved with improving pump brightness and adding counter directional pumping. A “spliceless” cavity, with FBGs inscribed on gain fiber directly, will be the ultimate goal for efficiency promotion.
This work is sponsored by the High Energy Joint Technology Office under contract number FA9451-08-D-0201/002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States government.
References and links
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