We present highly efficient three-channel dual-grating spectral beam combining with a combined signal output power of 5.5 kW at an excellent beam quality of M<sup>2</sup> = 1.5. Three 2-kW all-fiber narrow-linewidth continuous-wave Ytterbium-doped fiber amplifiers at 1050 nm, 1070 nm and 1090 nm were combined using in-house fabricated polarizationindependent dielectric reflection gratings. The total combining efficiency was 94% at full power level, which is close to the expected value referred to the incorporated grating’s efficiency in a dual-grating setup.
We present and compare the performance of bidirectionally pumped Yb-doped monolithic amplifier and oscillator setups in 20/400 μm geometry tested up to signal powers of 3.5 kW and 5 kW without the occurrence of transverse mode instabilities and maintaining a single mode beam quality of M<sup>2</sup> ~ 1.3. The scaling was primarily limited by the nonlinear effect of Stimulated Raman Scattering. This contribution contains detailed analysis of the temporal and spectral behavior of both configurations. The results show the excellent feasibility of monolithic oscillators and FBG for high power operation, even outperforming the amplifier pendant in terms of output power.
We present our current results on the fabrication of arbitrary shaped fiber tapers on our tapering rig using a CO2-laser as heat source. Single mode excitation of multimode fibers as well as changing the fiber geometry in an LPG-like fashion is presented. It is shown that this setup allows for reproducible fabrication of single-mode excitation tapers to extract the fundamental mode (M<sup>2 </sup>< 1.1) from a 30 μm core having an NA of 0.09.
State-of-the-art ultrafast fiber lasers currently are limited in peak power by excessive nonlinearity and in average power by modal instabilities. Coherent beam combination in space and time is a successful strategy to continue power scaling by circumventing these limitations. Following this approach, we demonstrate an ultrafast fiber-laser system featuring spatial beam combination of 8 amplifier channels and temporal combination of a burst comprising 4 pulses. Active phase stabilization of this 10-armed interferometer is achieved using LOCSET and Hänsch-Couillaud techniques. The system delivers 1 kW average power at 1 mJ pulse energy, being limited by pump power, and delivers 12 mJ pulse energy at 700 W average power, being limited by optically induced damage. The system efficiency is 91% and 78%, respectively, which is due to inequalities of nonlinearity between the amplifier channels and to inequality of power and nonlinearity between the pulses within the burst. In all cases, the pulse duration is ~260 fs and the M<sup>2</sup>-value is better than 1.2. Further power scaling is possible using more amplifier channels and longer pulse bursts.
We report on two low-loss 7:1 single-mode to multi-mode fiber coupler-designs generating more than 5 kW of output
power. These all-glass fiber-optical devices have the objective to keep the brightness at its theoretical maximum and the
heat load at its technical minimum. To the best of our knowledge, regarding all-fiber geometrical combined power
generation, beam quality and heat stability, the presented results had never been reported before.
In this contribution we demonstrate a single mode continuous wave laser amplifier with 146 W of power at a wavelength of 1009 nm. On one hand this experiments constitutes an extension of the wavelength range of high power fiber lasers, furthermore, emission wavelength well below 1030 nm find use for efficient high-brightness tandem pumping of high power fiber amplifiers. The wavelength and bandwidth of the seed oscillator are defined by a pair of fiber Bragg gratings. This seed is amplified in a two-stage Ytterbium-doped rod-type amplifier to 146 W with a high slope efficiency of 64 %, an excellent beam quality and an ASE-suppression as high as 63 dB.
Fiber lasers have reached kW levels of output power. To achieve this level it is necessary to use reliable high-power components that sustain these power levels. Double-clad fibers (DCFs) are often used in high-power fiber lasers. Cladding-light strippers (CLSs) are used to remove unwanted light from the inner cladding of the DCF. This unwanted light consists of residual pump light or signal light that leaked into the cladding, thus requiring that the CLS removes both high-NA (>0.4) and low-NA (<0.1) light. Often high-index polymers are used to remove the unwanted light from DCFs<sup>1,2,3</sup>. Because the CLS has to be able to withstand several 100W and most polymers are not capable of exceeding temperatures more than 200°C, we investigated a CLS without polymers, based on an etching process. We present results from a CLS that was tested up to 500W of stripped power. We determined the angle dependency of the stripping efficiency by launching both high- and low-NA light into the fiber and evaluating the NA attenuation. Furthermore, we measured the dependency of the stripping efficiency on the length of the etched area and the etching time. With optimized parameters an attenuation of more than 20 dB when launching high-NA light and 6 dB with low-NA light was achieved. The CLS did not show any degradation in terms of attenuation or thermal behavior in a six-hour stability test.
We report on a novel concept to scale the performance of ultra-fast lasers by means of coherent combination. Pulses
from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently
combined after amplification. The splitting and combination process is based on the polarization combining technique
using polarization cubes. A Hansch-Couillaud detector measures the polarization state of the combined beam. The error
signal (deviation from linear polarization) is used to stabilize the optical path lengths in the different channels with a
piezo mounted mirror. In a proof-of-principle experiment the combination of two femtosecond fiber-based amplifiers in
a CPA systems is presented. A combining efficiency as high as 97% has been achieved. Additional measurements were
carried out to investigate the stability of the system. The concept offers a unique scaling potential and can be applied to
all ultrafast amplification schemes independent of the architecture of the gain medium.
We demonstrate a femtosecond fiber laser system delivering >5-μJ, sub-400-fs pulses at a pulse repetition rate of
200 kHz. At constant average power the pulse repetition rate of this Watt-level femtosecond laser can be adjusted up to
several MHz. The laser is monolithically integrated from the oscillator to the booster amplifier stage. The system was
applied for structuring metallic as well as transparent media as e.g. biological tissues in ophthalmology.
We report on experimental generation of wave-breaking-free pulses from an environmentally stable Yb-doped all-fiber
laser. The compact linear cavity is constructed with saturable absorber mirror directly glued to the fibers end-facet as
nonlinear mode-locking mechanism and chirped fiber Bragg grating (CFBG) for dispersion management, thus, without
any free-space optics. Further, the laser was intrinsically environmentally stable, as only polarization maintaining (PM)
fibers were used. In the wave-breaking-free regime, the fiber laser directly generates positively-chirped picosecond
pulses at a repetition rate of 20.30 MHz. These pulses can be compressed to 218 fs in a HC-PBG providing a
femtosecond all-fiber laser system. Adapting the intra cavity dispersion we have also generated chirped pulses with a
parabolic spectral profile in the stretched pulse regime. We confirm numerically the wave-breaking-free pulse and
stretched pulse evolution and discuss advantages and disadvantages of both regimes in terms of pulse quality.