In-band pumped, Q-switched thulium-doped fiber laser system delivering 140 W and 7 mJ pulse energy.

We report on a highly efficient, in-band pumped, Q-switched, Tm-doped, rod-type master oscillator power amplifier (MOPA) system delivering up to 140 W average output power and 7 mJ pulse energy with a slope efficiency of 77% at 20 kHz repetition rate. The amplifier is pumped with Raman-shifted fiber lasers centered at 1692 nm. This in-band pump scheme for Tm-doped fiber lasers can significantly mitigate their quantum defect-related heat load limitations.

Analysis of optical core-to-core coupling: challenges and opportunities in multicore fiber amplifiers.

In this work we study in detail core-to-core coupling effects in multicore fibers (MCFs) using a simulation tool based on supermodal interference. We pay particular attention to the impact of core area scaling, which plays an important role in prospective amplifier systems. We consider geometrical and optical properties of the MCF structure, including the ability for dense packaging of the cores but also the influence on the core guidance (V-parameter).

Frequency-doubled Q-switched 4 × 4 multicore fiber laser system.

Frequency doubling of a Q-switched Yb-doped rod-type 4 × 4 multicore fiber (MCF) laser system is reported. A second harmonic generation (SHG) efficiency of up to 52% was achieved with type I non-critically phase-matched lithium triborate (LBO), with a total SHG pulse energy of up to 17 mJ obtained at 1 kHz repetition rate. The dense parallel arrangement of amplifying cores into a shared pump cladding enables a significant increase in the energy capacity of active fibers.

Mitigation of thermally-induced performance limitations in coherently-combined multicore fiber amplifiers.

Multicore fiber (MCF) amplifiers have gained increasing interest over the past years and shown their huge potential in first experiments. However, high thermal loads can be expected when operating such an amplifier at its limit. Especially in short MCF amplifiers that are pumped in counter-propagation, this leads to non-uniform mode-shrinking in the cores and, consequently, to a degradation of the system performance.

High-energy Q-switched 16-core tapered rod-type fiber laser system.

High-energy Q-switched master oscillator power amplifier systems based on rod-type 4 × 4 multicore fibers are demonstrated, achieving energy up to 49 mJ in ns-class pulses. A tapered fiber geometry is tested that maintains low mode order in large multimode output cores, improving beam quality in comparison to a similar fiber with no taper. The tapered fiber design can be scaled both in the number of amplifying cores and in the dimensions of the cores themselves, providing a potential route toward joule-class fiber lasers systems.

500 W rod-type 4 × 4 multicore ultrafast fiber laser.

We present a coherently combined femtosecond fiber chirped-pulse-amplification system based on a rod-type, ytterbium-doped, multicore fiber with 4 × 4 cores. A high average power of up to 500 W (after combination and compression) could be achieved at 10 MHz repetition rate with excellent beam quality. Additionally, < 500 fs pulses with up to 600 µJ of pulse energy were also realized with this setup.

Scaling potential of beam-splitter-based coherent beam combination.

The impact of nonlinear refraction and residual absorption on the achievable peak- and average power in beam-splitter-based coherent beam combination is analyzed theoretically. While the peak power remains limited only by the aperture size, a fundamental average power limit is given by the thermo-optical and thermo-mechanical properties of the beam splitter material and its coatings. Based on our analysis, 100 kW average power can be obtained with state-of-the-art optics at maintained high beam quality (M ≤ 1.

Impact of thermo-optical effects in coherently combined multicore fiber amplifiers.

In this work we analyze the power scaling potential of amplifying multicore fibers (MCFs) used in coherently combined systems. In particular, in this study we exemplarily consider rod-type MCFs with 2 × 2 up to 10 × 10 ytterbium-doped cores arranged in a squared pattern. We will show that, even though increasing the number of active cores will lead to higher output powers, particular attention has to be paid to arising thermal effects, which potentially degrade the performance of these systems.

Simplified design of optical elements for filled-aperture coherent beam combination.

A simplification strategy for segmented mirror splitters (SMS) used as beam combiners is presented. These devices are useful for compact beam division and the combination of linear and 2-D arrays. However, the standard design requires unique thin-film coating sections for each input beam; thus, potential for scaling to high beam-counts is limited due to manufacturing complexity.

10.4 kW coherently combined ultrafast fiber laser.

An ultrafast laser delivering 10.4 kW average output power based on a coherent combination of 12 step-index fiber amplifiers is presented. The system emits close-to-transform-limited 254 fs pulses at an 80 MHz repetition rate, and has a high beam quality (≤1.

Photonic lantern kW-class fiber amplifier.

Pump-limited kW-class operation in a multimode fiber amplifier using adaptive mode control and a photonic lantern front end was achieved. An array of three single-mode fiber inputs was used to adaptively inject the appropriate superposition of input modes in a three-mode gain fiber to achieve the desired mode at the output. Mode fluctuations at high power were compensated by adjusting the relative phase, amplitude, and polarization of the single-mode fiber inputs.

Efficient cryogenic near-infrared Tm:YLF laser.

Operation of a cw thulium laser emitting at 816 nm has been demonstrated in bulk Tm:YLF with 46% slope efficiency. Prior cw demonstrations of this transition have been limited to ZBLAN fiber hosts and prior lasing in bulk crystalline host material has been limited to quasi-cw operation due to population trapping. Trapping at the F level was mitigated by co-lasing at 1876 nm.