Temporal pulse quality of a Yb:YAG burst-mode laser post-compressed in a multi-pass cell.

Nonlinear pulse post-compression represents an efficient method for ultrashort, high-quality laser pulse production. The temporal pulse quality is, however, limited by amplitude and phase modulations intrinsic to post-compression. We here characterize in frequency and time domain with high dynamic range individual post-compressed pulses within laser bursts comprising 100-kHz-rate pulse trains. We spectrally broaden 730 fs, 3.2 mJ pulses from a Yb:YAG laser in a gas-filled multi-pass cell and post-compress them to 56 fs. The pulses exhibit a nearly constant energy content of 78% in the main peak over the burst plateau, which is close to the theoretical limit. Our results demonstrate attractive pulse characteristics, making multi-pass post-compressed lasers very applicable for pump-probe spectroscopy at, e.g., free-electron lasers or as efficient drivers for secondary frequency conversion stages.

All-normal-dispersion nonlinear polarization rotation mode-locked Tm:ZBLAN fiber laser.

We demonstrated an all-normal-dispersion nonlinear polarization rotation mode-locked Tm:ZBLAN fiber laser in the 2 µm wavelength band. All fibers in the experiment were ZBLAN fibers with normal dispersion in the wavelength band. An average power of 63 mW with a characteristic cat-ear shaped optical spectrum of an ANDi laser was obtained. The center wavelength and the spectral bandwidth were 1880 nm and ~80 nm, respectively. The repetition rate was 70.6 MHz and the corresponding pulse energy was 0.9 nJ. The pulse duration directly from the oscillator was 860 fs and it was compressed to 107 fs.

Deep reinforcement learning for coherent beam combining applications.

Coherent beam combining is a method to scale the peak and average power levels of laser systems beyond the limit of a single emitter system. This is achieved by stabilizing the relative optical phase of multiple lasers and combining them. We investigated the use of reinforcement learning (RL) and neural networks (NN) in this domain. Starting from a randomly initialized neural network, the system converged to a phase stabilization policy, which was comparable to a software implemented proportional-integral-derivative (PID) controller. Furthermore, we demonstrate the capability of neural networks to predict relative phase noise, which is one potential advantage of this method.

Delay line coherent pulse stacking.

Ultrafast fiber amplifiers are usually limited in terms of pulse energy and peak power due to fiber damage and nonlinearities. On the other hand, fiber amplifiers are very energy efficient. To take advantage of this efficiency, but still scale the pulse energy, coherent combining and pulse stacking have become the method of choice. Here we show how to directly and symmetrically stack up the pulses from an oscillator chain, using delay lines and phase preshaping of a 48 MHz, 1.5 µm master oscillator. This method avoids the need for high-power low-loss dumpers as in passive stack and dump cavities and, compared to divided pulse amplification (DPA), it eliminates the need for splitting pulses and pulse picking and, thus, optimally utilizes the available seed power. Furthermore, it avoids the asymmetrical gain of the pulse bursts, which typically limits the efficiency in DPA.

Self-focusing in multicore fibers.

Self-focusing is the ultimate power limit of single mode fiber amplifiers. As fiber technology is approaching this limit, ways to mitigate self-focusing are becoming more and more important. Here we show a theoretical analysis of this limitation in coupled multicore fibers. Significant scaling of the self-focusing limit is possible even for coupled multicore fibers if the out-of-phase mode is chosen. On the other hand the in-phase mode can - depending on the coupling strength - be prone to instabilities.

TEM00 mode content measurements on a passive leakage channel fiber.

Mode content measurements with a scanning ring cavity were performed in order to determine the TEM00 mode content of the output beam profile of a resonantly enhanced leakage channel fiber. The measurements were performed at 1.0 and 1.5 µm. In addition, the influence of different bending diameters as well as launching conditions has been investigated. Furthermore, a numerical simulation was used to determine the maximum theoretical TEM00 overlap, if only the fundamental fiber mode is guided. The simulation was also used to analyze how the TEM00 overlap for the case of any additional higher order fiber mode can be determined consistently.

Beam quality degradation of a single-frequency Yb-doped photonic crystal fiber amplifier with low mode instability threshold power.

A current limit in power scaling of Yb-doped fiber amplifiers is the sudden onset of mode instabilities. We investigated this effect on a single-frequency Yb-doped photonic crystal fiber amplifier with a low mode instability threshold power. By measuring the overlap of the fiber output beam with the fundamental mode of an external cavity to be about 95%, we could exclude significant modal power transfer below a sharp power threshold. Furthermore, we directly measured the frequency resolved intensity noise spectra. No fluctuations in the overall output power were observed, but for the modal content different oscillation regimes were identified.

Frequency resolved analysis of thermally induced refractive index changes in fiber amplifiers.

Temperature induced refractive index changes are an important aspect in today's fiber amplifiers with high average power. For many processes, their time dependence is critical. Here, we analyze the impact of radial heat diffusion on the optical phase. We modulated the pump power in a 10 W amplifier and measured the frequency response of the optical phase. We compared the result with the calculated frequency response of the temperature in the fiber core, which shows the same characteristics. Additionally, we analyzed the influence of fiber parameters on the temperature dynamics.

Gain dynamics and refractive index changes in fiber amplifiers: a frequency domain approach.

Gain dynamics and refractive index changes in fiber amplifiers are important in many areas. For example, the knowledge of the frequency responses for seed and pump power modulation are required to actively stabilize low noise fiber amplifiers. Slow and fast light via coherent population oscillations rely on the change of group index to delay or advance pulses, and refractive index changes in fiber amplifiers are a possible explanation for mode fluctuations in high power fiber amplifiers. Here, we analyze the frequency dependent influence of seed and pump power modulation on the fiber amplifier output power and the refractive index. We explain the observed power and refractive index modulation with an analytic model originally developed for telecom amplifiers and discuss a further simplification of the model.

All-fiber coherent beam combining with phase stabilization via differential pump power control.

Coherent beam combining enables power scaling beyond the limits of single amplifiers. Therefore, improving the performance and simplicity of coherent combination techniques is of great interest for many high power applications. Here, we show all-fiber coherent beam combining of two ytterbium doped amplifiers with and without a dedicated phase actuator and a total output power up to 25 W. Instead of a dedicated phase actuator, we directly controlled the two ytterbium amplifiers to also stabilize their relative phase. We compared the performance of this method with phase stabilization using two piezo driven fiber stretchers. In both cases, power noise was dominated by the single amplifier.

Beam quality and noise properties of coherently combined ytterbium doped single frequency fiber amplifiers.

Collinear coherent combination of multiple single frequency fiber amplifiers is a promising approach to realize the high power laser sources required for 3rd generation gravitational wave detectors (GWD), as long as the stringent requirements on the beam quality and noise properties can be met. Here, we report the beam quality and noise properties of two coherently combined 10 W single frequency amplifiers with respect to the requirements of GWD. The combining efficiency was larger than 95% with 97% of the combined beam in the fundamental spatial mode. There was no significant noise increase compared to the fluctuations of the single amplifier.

Linearly polarized single-mode Nd:YAG oscillators using [100]- and [110]-cut crystals.

The output power and efficiency of linearly polarized high power Nd:YAG lasers is limited by depolarization and bifocusing. Both effects degrade the beam quality and decrease the output power. In a single pass configuration, [100]- and [110]-cut crystals can be used to reduce the depolarization. Here, we compare [100]-, [110]- and [111]-cut crystals in an oscillator configuration. As expected it was possible to reduce the depolarization loss by using [100]-cut crystals in our configuration, while the depolarization loss was higher for [110]-cut crystals. The thermal lens establishing in these crystals is not circular, which can degrade beam quality in high power operation.

All-fiber phase actuator based on an erbium-doped fiber amplifier for coherent beam combining at 1064 nm.

Active phase control in fiber amplifiers is of considerable interest for low-noise single-frequency amplifiers and for coherent beam combining. We demonstrate phase control at 1064 nm by use of an erbium-doped fiber. We investigated the phase shift by guiding the beam through an erbium-doped fiber amplifier in a Mach-Zehnder configuration and applied the results to stabilize the relative phase of two ytterbium-doped fiber amplifiers. To the best of our knowledge, this is the first demonstration of an all-fiber coherent beam combining at 1064 nm employing an erbium-doped fiber as a phase actuator.

Intrinsic reduction of the depolarization in Nd:YAG crystals.

The output power of linearly polarized Nd:YAG lasers is typically limited by thermally induced birefringence, which causes depolarization. However, this effect can be reduced either by use of some kind of depolarization compensation or by use of crystals which are cut in [110]- and [100]-direction, instead of the common [111]-direction. Investigations of the intrinsic reduction of the depolarization by use of these crystals are presented. To our knowledge, this is the first probe beam-experiment describing a comparison between [100]-, [110]- and [111]-cut Nd:YAG crystals in a pump power regime between 100 and 200 W. It is demonstrated that the depolarization can be reduced by a factor of 6 in [100]-cut crystals. The simulations reveal that a reduction of depolarization by use of a [110]-cut crystal in comparison with a [100]-cut crystal only becomes possible at pump powers in the kW region. Analysis also shows that the bifocusing for [100]-cut is slightly smaller and more asymmetrical than for [111]-cut.