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). In general, this study is important to unlock the power and energy scaling potential of the next-generation MCF amplifiers.

Pulses of 32 mJ and 158 fs at 20-kHz repetition rate from a spatiotemporally combined fiber laser system.

A high-energy, high-power ultrafast fiber laser system based on spatiotemporal coherent combination is presented. Bursts of eight subsequent chirped-pulse amplification (CPA)-stretched pulses are amplified simultaneously in 16 parallel ytterbium-doped rod-type amplifiers. After spatial and temporal coherent combination of the total 128 amplified pulse replicas into a single pulse, it is compressed in a partially protective-gas-filled CPA compressor. Finally, nearly Fourier-transform-limited pulses with an energy of 32 mJ and a duration of 158 fs are emitted with a repetition rate of 20 kHz and a close to diffraction-limited beam quality.

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. The frequency-doubled MCF architecture is compatible with high-repetition-rate and high-average-power operation and may provide an efficient alternative to bulk solid-state systems as pump sources for high-energy titanium-doped sapphire lasers.

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. In this work we show different ways to counteract the performance limitations induced by thermal effects in coherently-combined, multicore fiber amplifiers. First, we will show that pumping MCFs in co-propagation will significantly improve the combinable average power since the thermal load at the fiber end is reduced. However, this approach might not be favorable for high energy extraction. Therefore, we will introduce a new MCF design pumped in counter-propagation that leads to a reduction of the thermal load at the fiber end, which will allow for both high combined output power and pulse energy.

Ultrafast Tm-doped fiber laser system delivering 1.65-mJ, sub-100-fs pulses at a 100-kHz repetition rate.

High-energy, ultrafast, short-wavelength infrared laser sources with high average power are important tools for industrial and scientific applications. Through the coherent combination of four ultrafast thulium-doped rod-type fiber amplifiers, we demonstrate a Tm-doped chirped pulse amplification system with a compressed pulse energy of 1.65 mJ and 167 W of average output power at a repetition rate of 101 kHz. The system delivers 85 fs pulses with a peak power of 15 GW. Additionally, the system presents a high long- and short-term stability. To the best of our knowledge, this is the highest average output power short wavelength IR, mJ-class source to date. This result shows the potential of coherent beam combining techniques in the short wavelength infrared spectral region for the power scalability of these systems.

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. This architecture is intrinsically power scalable by increasing the number of cores in the fiber.

Carrier-envelope phase stable few-cycle laser system delivering more than 100 W, 1 mJ, sub-2-cycle pulses.

Two-stage multipass-cell compression of a fiber-chirped-pulse amplifier system to the few-cycle regime is presented. The output delivers a sub-2-cycle (5.8 fs), 107 W average power, 1.07 mJ pulses at 100 kHz centered at 1030 nm with excellent spatial beam quality (M2 = 1.1, Strehl ratio S = 0.98), pointing stability (2.3 µrad), and superior long-term average power stability of 0.1% STD over more than 8 hours. This is combined with a carrier-envelope phase stability of 360 mrad in the frequency range from 10 Hz to 50 kHz, i.e., measured on a single-shot basis. This unique system will serve as an HR1 laser for the Extreme Light Infrastructure Attosecond Light Pulse Source research facility to enable high repetition rate isolated attosecond pulse generation.

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 (M2 ≤ 1.1) and at only 2% loss of combining efficiency. This result indicates that the power-scaling potential of today's beam-splitter-based coherent beam combination is far from being depleted. A potential scaling route to megawatt-level average power is discussed for optimized beam splitter geometry.

1 kW, 10 mJ, 120 fs coherently combined fiber CPA laser system.

An ultrafast fiber chirped-pulse amplification laser system based on a coherent combination of 16 ytterbium-doped rod-type amplifiers is presented. It generates 10 mJ pulse energy at 1 kW average power and 120 fs pulse duration. A partially helium-protected, two-staged chirped-pulse amplification grating compressor is implemented to maintain the close to diffraction-limited beam quality by avoiding nonlinear absorption in air.

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. Taking advantage of the relative insensitivity of the beam combination process to amplitude variations, numerical techniques are used to optimize highly simplified designs with only one, two or three unique coatings. It is demonstrated that with correctly chosen coating reflectivities, the simplified optics are capable of high combination efficiency for several tens of beams. The performance of these optics as beam splitters in multicore fiber amplifier systems is analyzed, and inhomogeneous power distribution of the simplified designs is noted as a potential source of combining loss in such systems. These simplified designs may facilitate further scaling of filled-aperture coherently combined systems in linear array or 2-D array formats.

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. Additionally, we analyze the influence of the core dimensions on the extractable and combinable output power and pulse energy. This includes a detailed study on the thermal effects that influence the propagating transverse modes and, in turn, the amplification efficiency, the combining efficiency, the onset of nonlinear effect, as well as differences in the optical path lengths between the cores. Considering all these effects under rather extreme conditions, the study predicts that average output powers higher than 10 kW from a single 1 m long ytterbium-doped MCF are feasible and femtosecond pulses with energies higher than 400 mJ can be extracted and efficiently recombined in a filled-aperture scheme.

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 (M2≤1.2) and a low relative intensity noise of 0.56% in the frequency range of 1 Hz to 1 MHz. Automated spatiotemporal alignment allows for hands-off operation.

23 mJ high-power fiber CPA system using electro-optically controlled divided-pulse amplification.

The pulse-energy scaling technique electro-optically controlled divided-pulse amplification is implemented in a high-power ultrafast fiber laser system based on coherent beam combination. A fiber-integrated front end and a multipass-cell-based back end allow for a small footprint and a modular implementation. Bursts of eight pulses are amplified parallel in up to 12 ytterbium-doped large-pitch fiber amplifiers. Subsequent spatiotemporal coherent combination of the 96 total amplified pulse replicas to a single pulse results in a pulse energy of 23 mJ at an average power of 674 W, compressible to a pulse duration of 235 fs. To the best of our knowledge, this is the highest pulse energy ever accomplished with a fiber chirped-pulse amplification (CPA) system.

Sequential phase locking scheme for a filled aperture intensity coherent combination of beam arrays.

We present a novel phase locking scheme for the coherent combination of beam arrays in the filled aperture configuration. Employing a phase dithering mechanism for the different beams similar to LOCSET, dithering frequencies for sequential combination steps are reused. By applying an additional phase alternating scheme, this allows for the use of standard synchronized multichannel lock-in electronics for phase locking a large number of channels even when the frequency bandwidth of the employed phase actuators is limited.

3.5 kW coherently combined ultrafast fiber laser.

An ultrafast laser based on the coherent beam combination of four ytterbium-doped step-index fiber amplifiers is presented. The system delivers an average power of 3.5 kW and a pulse duration of 430 fs at an 80 MHz repetition rate. The beam quality is excellent (M2<1.24·1.10), and the relative intensity noise is as low as 1% in the frequency span from 1 Hz to 1 MHz. The system is turn-key operable, as it features an automated spatial and temporal alignment of the interferometric amplification channels.

Electro-optically controlled divided-pulse amplification.

A novel technique for divided-pulse amplification is presented in a proof-of-principle experiment. A pulse burst, cut out of the pulse train of a mode-locked oscillator, is amplified and temporally combined into a single pulse. High combination efficiency and excellent pulse contrast are demonstrated. The system is mostly fiber-coupled, enabling a high interferometric stability. This approach provides access to the amplitude and phase of the individual pulses in the burst to be amplified, potentially allowing the compensation of gain saturation and nonlinear phase mismatches within the burst. Therefore, this technique enables the scaling of the peak power and pulse energy of pulsed laser systems beyond currently prevailing limitations.

High-average-power femtosecond laser at 258 nm.

We present an ultrafast fiber laser system delivering 4.6 W average power at 258 nm based on two-stage fourth-harmonic generation in beta barium borate (BBO). The beam quality is close to being diffraction limited with an M2 value of 1.3×1.6. The pulse duration is 150 fs, which, potentially, is compressible down to 40 fs. A plain BBO and a sapphire-BBO compound are compared with respect to the achievable beam quality in the conversion process. This laser is applicable in scientific and industrial fields. Further scaling to higher average power is discussed.

High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules.

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 1011 photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH3I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.

12 mJ kW-class ultrafast fiber laser system using multidimensional coherent pulse addition.

An ultrafast fiber-chirped-pulse amplification system using a combination of spatial and temporal coherent pulse combination is presented. By distributing the amplification among eight amplifier channels and four pulse replicas, up to 12 mJ pulse energy with 700 W average power and 262 fs pulse duration have been obtained with a system efficiency of 78% and excellent beam quality. To the best of our knowledge, this is the highest energy achieved by an ultrafast fiber-based laser system to date.