This PDF file contains the front matter associated with SPIE Proceedings Volume 12400, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

We report results from a new Yb-doped gain fiber with increased higher-order-mode (HOM) loss, compared to conventional step-index fibers. The fiber had 20 µm mode-field diameter (MFD), high absorption, and high transverse-mode instability (TMI) threshold. TMI-free operation with 5 kW output power was demonstrated from a 9 m length of gain fiber, limited by pump power. The large MFD and high absorption allowed for a 7.5 m long amplifier with greater than 80% o-o efficiency and the Raman peak more than 50 dB below the signal. These results were also enabled by a new, small-size, 7+1:1 pump-signal combiner.

We report the performance of new LMA Yb fibers with increased cladding absorption for pumping in the 915 nm absorption band. A 0.5 dB/m cladding-absorption Yb20/400 fiber showed negligible photodarkening loss in 400-hour laser operation at 3 kW, with 77% optical-to-optical efficiency. Low-SRS and TMI-free operation at 3.5 kW signal power was achieved with a 0.65 dB/m cladding-absorption and 20.2 μm mode-field diameter Yb fiber, tested in a co-pumped amplifier. The Raman peak was 31 dB below the signal peak at the maximum power.

We present the rapid output power modulation of a 200 W continuous-wave all-fiber ytterbium-doped polarization-maintaining fiber amplifier. Pulse durations of 3 ns and MHz modulation frequencies at full power could be achieved. A fiber-coupled electro-optical modulator was used to modulate the polarization of the seed light, transferring linearly to the polarization at the output of the fiber amplifier. Using a thin film polarizer, the perpendicular polarization components at the output were separated, yielding a polarization dependent amplitude modulation. The laser output power then follows the applied voltage signal to the EOM and could be rectangular, saw-tooth or any arbitrary signal.

High-power single-frequency fiber lasers have attracted great attention in the applications of high-resolution spectroscopy, long-distance coherent communication, gravitational wave detection and some other areas, due to the advantage on narrow linewidth, low noise and so on. In this paper, we systematically summarize the recent achievements of high-power singlefrequency fiber laser oscillators and amplifiers as well as performance improvement on noise suppression, linewidth narrowing, and wavelength extension. Besides, the next development of SFFLs has been prospected.

In this work, a single-frequency fiber amplifier with output power of 703 W was demonstrated at 1064 nm in an all-fiber configuration. Cascaded Yb³⁺-doped fiber structure with different dopant concentration and hybrid 915/976 nm pump scheme were employed in power scaling stage to improve the gain saturation for higher transverse mode instability threshold. An overall optical efficiency of 67.5% was achieved at the maximum output power and the M² was measured to be ~1.4. A spectral linewidth of 2 kHz was achieved at the 703-W laser power, which is to the best of our knowledge, the first time that the linewidth of a kHz single-frequency fiber amplifier is characterized at such high laser power.

In this work, to scale laser power while suppressing NL effects, we designed and fabricated a fiber triple-clad Ytterbium doped fiber with large core diameter by using MCVD combined with all-gas-phase doping method. To modify the PD performance of the fiber, P、Al and F were co-doped in the fiber. An excellent laser performance could be demonstrated for such a fiber with an output power of more than 10 kW and a slope efficiency of 82%. Long-term laser stability at 10 kW-level was carried out in a master oscillator power amplifier (MOPA) laser system for 500 hours with power degradation less than 1%.

We recently developed a holmium-doped triple-clad fiber (Ho-3CF) for laser emission beyond 2.1 μm. In a clad-pumped fiber laser oscillator emitting at 2.12 μm, we obtained an optical efficiency of 73% with respect to the absorbed pump power at 1.94 μm, and a maximum signal power of 62 W. We present here the comparison between the laser measurements and a numerical simulation, together with the measurements of the required physical parameters (crosssections, attenuations…). The alumino-silicate core composition of our initial Ho-3CF samples required the introduction of a pedestal to preserve the single spatial-mode guiding. We also present our preliminary results on a new aluminophospho-silicate core composition, in order to suppress the initial pedestal and simplify the fabrication process. Both samples were also analyzed in core-pumped laser configuration.

In this work, we realized broadly tunable mode-locking operation from 1730 nm to 1815 nm in normal dispersion regime employing an acousto-optic tunable fiber (AOTF) in a Tm-doped dispersion-managed fiber laser. The AOTF worked as a multifunctional component in laser cavity suppressing undesired wavelength lasing and introducing a frequency shifting, which improved the stability of laser operation. The hybrid mode-locking incorporated by nonlinear polarization rotation (NPR) effect and frequency shifting effect ensured self-starting stable pulsed operation. The pulse spectral widths ranged from 17 nm to 25 nm. The stretching-free direct amplification in two-cascaded fiber amplifier enabled power scaling up to 310 mW and pulse energy of 19 nJ. Pulse duration was compressed down to 282 fs by a pair of gratings. The seed laser is further optimized. The optimized seed laser enhances output power about 5 times. The laser system was designed for multiphoton imaging of bladder cancer in the third biological window to demonstrate the recently discovered nonlinear effect resulting in improvement of signal contrast at the deeper tissue level.

We report on the first measurement of the temperature dependent silica thulium-doped fiber absorption and emission cross-sections in the spectral range 700–2200 nm for a temperature range −196–300 °C. Using this data, we simulated the effect of temperature-induced shift of cross-sections on high power thulium-doped fiber laser (TDFL) performance when clad-pumped at 790 nm. The simulations show considerable impact on TDFL threshold and efficiency for shorter fiber length. The effect is most apparent for amplifier setup, especially with strong amplified spontaneous emission.

We present the design and performance of a narrow linewidth single frequency 2039 nm distributed feedback (DFB) fiber Bragg grating (FBG) fiber laser source with a novel optical pumping configuration at 1567 nm that significantly increases optical-optical pump conversion efficiency. Our new configuration employs an optical circulator and a reflector at 1567 nm to efficiently recycle pump light that is not absorbed in the first pass through the FBG-DFB fiber laser. We report a comparison of simulations with experimental results for the novel high efficiency single frequency 2039 nm Tm-doped fiber laser source.

Coherent beam combining (CBC) by active phase control is an efficient way to power scale fiber amplifiers but its bandwidth of operation of CBC can be limited. Deep-learning techniques offer some capability for fast retrieval of the laser phases from the shape of the interference pattern generated through combining, in order to increase the speed and bandwidth of operation of CBC. In this paper, we present the development and numerical tests of a Convolutional Neural Network (CNN) used for such fast phase retrieval. After numerically generating tens of thousands of interference patterns corresponding to different phase sets for the combined lasers, we learned the CNN to retrieve the phase set corresponding to a given shape of interference pattern. Unfortunately, due to the central symmetry of the tiled-aperture hexagonal geometry of the array of fiber outputs, there’s not a unique set of phases for the combined lasers that can lead to a given shape of interference pattern. We demonstrate that acquiring the image of the interference pattern in a plane that is not perfectly located in the far-field offers a simple solution to get rid of this non-uniqueness ambiguity. After demonstrating numerically that with this addition, the CNN learning approach operates well resulting in low values for the CBC residual phase error, we explain how it’s possible to transfer this learning that has been done numerically to a real experiment.

We demonstrated 55-fs pulses from spectrally combining two chirped-pulse fiber channels operating at partially-overlapped spectral bands, with a pulse shaper incorporated in each channel. The spectral intensity and phase shaping in two fiber channels are coherently-spectrally synthesized by phase-synchronizing the two channels at the overlapped spectrum. To the best of our knowledge, 55 fs is the shortest pulse duration from a spectrally combined fiber system at one-micron Yb wavelength, and this work is the first demonstration of coherent spectral synthesis of two pulse shapers. This work provides a promising path toward high-energy, tens-of-fs fiber chirped-pulse amplifier systems.

We report on our results of the coherent combination of four Tm-doped rod-type fiber amplifiers. The chirped pulse amplification system emits an average output power of 188 W and a pulse energy of 1.86 mJ at 100.86 kHz repetition rate. The compressed pulses were measured via second order frequency optical gating. The retrieval reveals a compressed pulse duration of the laser system of 84 fs resulting in peak power of 17.7 GW. The amplifier interferometer was stabilized using locking of coherence by single-detector electronic frequency-tagging and piezo driven mirrors in front of the amplifier channels. The long term stability of the source was tracked with a thermal power sensor over a duration of 120 minutes and shows a stability of <0.1 % rms over this measurement period. To the best of our knowledge, this is the highest average power ultrafast mJ-class short-wavelength infrared laser to date. This proves the applicability of coherent combining techniques in Tm-doped fiber laser systems, opening the route towards performance scaling of ultrafast SWIR laser sources to kW-class average power levels with multi-mJ energies. Additionally this renders this technology the ideal candidate for frequency conversion into the soft X-ray, mid infrared and THz spectral region.

Coherent beam combination can be used to overcome limitations associated with the power handling capability of a single fibre laser. However, due to interference effects, the spatial intensity profile of the combined beam is directly affected by the phase of each fibre. Therefore, monitoring and control of the fibre phases is required for practical application. Here, we show that a neural network can extract this phase information from a far-field intensity profile, in a single step, hence unlocking the potential for real-time beam shaping. Further investigation shows that the neural network encoded fundamental rules associated with interference theory.

We report 4kW Single-Mode narrow-linewidth Ytterbium fiber amplifier in all-fiber format and modular package with diffraction-limited beam divergence for the amplification of narrow linewidth seed signals. Measured M² values of output beam throughout full power range are ≤ 1.1. The fiber amplifier has been pumped by laser diodes directly and has 3m output delivery cable terminated with IPG connector. The amplifier module has ≥ 38% electro-optical efficiency. The fiber amplifier operates up to 4kW with depolarized seed signals without onset of SBS and/or MI. We observed the broadening of amplified signal at high powers when depolarizing of the narrow-linewidth seed signal was produced with a classic Lyot scheme on PM Panda fibers. This broadening resulted in dropping of in-band power below 99% at output powers <3.3kW. Negligible broadening at FWHM level of amplified signal have been observed with other type of fiber depolarizer based on single PM Panda fiber scheme. The Yb fiber amplifier with this type of depolarizing of seed signal operates up to 4kW with in-band power ≥ 99% at 60GHz linewidth of the seed source.

We report development of 85µm core Yb-doped and Ge-doped chirally-coupled-core (CCC) fibers, and their integration via fusion-splicing into an all-fiber optical amplifier system. This system, consisting of a CCC fiber amplifier and a 6+1 fusion-spliced signal-pump-combiner with a passive CCC fiber feed-through produces robust single mode output (diffraction-limited) in a counter-pumped configuration with passive-fiber leads as short as ~30cm. The Yb-doped 85µm core CCC fiber amplifiers had produced ~10mJ energy pulses at close to ~100W of average power. This achieved performance and monolithic all-fiber integration are required for compact and robust coherently-combined laser array drivers of laser plasma accelerators.

Within the context of the E-TEST (Einstein Telescope EMR Site & Technology) project, Fraunhofer ILT develops thulium- and holmium-based seed sources and fiber lasers at app. 2 μm wavelength with highest demands on linewidth and stability for usage in a third-generation gravitational wave detector, the Einstein telescope. To fulfill the requirements, we develop a seed laser and a multi-stage fiber amplifier, consisting of holmium-doped fibers. Within this paper, we present our current laser concept and the first results of our dual-stage holmium-doped fiber amplifier stage. We achieve a low linewidth (< 2 MHz) output power of more than 5 W at a wavelength of 2095 nm. By using our in-house developed fiber laser simulation, we show that the efficiency of our amplifier is currently limited by pair induced quenching and the potential for further power scaling.

Continued recent developments in Thulium- (Tm) doped silica fiber design have enabled average power scaling of 2 µm fiber laser system beyond the kW-level. One approach to furthering this development is to maximize the slope efficiency of Tm-doped fiber lasers by selecting highly-doped double-clad fibers (TDF’s) so as to promote the cross-relaxation process. The success of this approach was exemplified by Tumminelli et al. who employed an all-halide vapor-phase fabrication process to produce a single-mode (SM) fiber with a Tm concentration of 8.5 wt% and demonstrated ~70 % slope efficiency. In this work, we report what we believe to be the first high-concentration (8 wt% Tm), double-clad (DC) large-mode area (LMA) Tm-doped fiber (TDF), which was manufactured by the solution-doping MCVD process. Critical performance such as slope efficiency and lasing wavelength are characterized and compared to legacy LMA-TDF-25P/400 fiber.

A single-frequency Tm-doped fiber laser operating at 2050 nm is demonstrated with a ring cavity scheme. A piece of unpumped Tm/Ho-coped fiber saturable absorber incorporating a high reflectivity fiber Bragg grating, which serves as a narrow bandwidth filter, were inserted inside the cavity to select single longitudinal mode and suppress the mode hopping. By optimizing the length of Tm/Ho-codoped fiber, stable single-frequency lasing at 2050 nm was realized. Under 2 W 1570 nm pump power, 215 mW single-frequency output power was obtained, while the slope efficiency with respect to the launched pump power was 22%. This work shows that using Tm/Ho-codoped fiber or Ho-doped fiber as the saturable absorber, which has a higher absorption above 2 μm, could enhance the mode selection capability thus improve the singlefrequency output power.

Ultrafast fiber laser sources emitting fs-pulses around 2 μm have many applications in medicine, metrology and sensing as well as in various frequency-conversion techniques. Thulium-doped fiber amplifiers are a promising platform for power scalable ultrafast amplification in this wavelength region. Usually, these ultrafast, high-power fiber laser systems were pumped at a wavelength around 790 nm and obtain slope efficiencies in the range of 50 % in the 100 W-class. Due to the high quantum defect obtained with this pump technique and the related high heat loads, considerable thermal challenges still must be overcome when scaling the power further. In this contribution we present a concept on highly efficient, high-power thulium-doped fiber amplifiers pumped at 1692 nm. This pump concept is suitable for high-power, high-energy, ultrafast Tm-doped fiber laser systems. In this proof of principle demonstration, we achieve a slope efficiency of 80% in a standard commercially available, thulium-doped photonic crystal fiber (PCF) with ~60 W of average power when pumping at 1692 nm compared to 47 % slope efficiency by pumping at 793 nm. In the simulation we investigated the heat load and core temperature evaluation along the fiber. These findings demonstrate an improvement in the amplification efficiency of large-mode area fiber amplifiers which are suitable for ultrafast operation on Yb-like efficiencies. The reduced heat load paves the way to even higher average powers from ultrafast Tm-doped fiber lasers with the potential to provide multi-mJ energy fs-pulses at kW-level average power from a single amplifier channel.

Transverse mode instability is a key limit to power scaling of high-power fiber lasers. Accurate modeling efforts have, however, been hampered by a lack of experimental data to verify a model. Recently, there have been some good experimental studies, making it possible to validate a model. In this work, we developed a model by integrating a 3D fiber amplifier and stimulated thermal Rayleigh scattering. Since we are only interested in the regime where the fundamental mode dominates, our 3D amplifier divides the core into many cylindrical shells. This limits the model to situations where bend-induced mode distortion of the fundamental mode is negligible, but it is still applicable for most practical scenarios. The benefit of this model is high computational efficiency; it can run in minutes on a PC. This 3D amplifier model considers various pumping configurations and amplified spontaneous emission. It can simulate most experimental conditions. Excellent quantitative fit to experimental data was achieved. Additional studies were also conducted to show that gain saturation is a dominating effect in understanding the observed behaviors of transverse mode instability.

Transverse Mode Instability (TMI) is one of several nonlinear effects that limit power scaling in high power fiber lasers and amplifiers. We demonstrate that TMI can be effectively suppressed by spreading power in multiple modes of the fiber. We show that the TMI threshold scales linearly with the number of modes, upon equal excitation of modes, caused by smearing of thermally induced refractive index grating. The multimode excitation can be focused to a diffraction limited spot, giving high quality beam with increased TMI threshold. We finally show linear scaling of TMI threshold is maintained upon inclusion of gain saturation.

We carry out a stability analysis on the STRS process in two (frequency-detuned) mode fiber amplifiers, in the presence of delayed thermal nonlinearity. We show for the first time that the standard two-wave STRS process is stable against small modal perturbations, and as such it does not describe adequately the TMI process. We further show that inclusion of FWM effects and three-wave interaction, through the addition of an anti-Stokes LP11 wave, is required to describe modal instabilities above a power threshold, and a previously derived TMI power threshold formula is recovered. This work sheds new light on the standard STRS process and adds new insight into its connection with TMI effects in high power fiber amplifiers.

We study modal instabilities in two-mode fiber amplifiers, when the signal is launched either in the lower-order (LP₀₁) or the higher-order (LP₁₁) mode. Launching predominantly into the lowest-order mode (LP₀₁) results in the well-known TMI effects, with a characteristic sharp increase of the TMI gain above a power threshold. Quite surprisingly, launching predominantly into the HOM (LP₁₁) shows no sharp increase of the TMI gain and total absence of a distinctive power threshold. This shows that launching the signal into the HOM results in a much more stable, robust, and resilient performance against perturbations.

A new approach to experimentally characterize the transverse modes of optical fibers is proposed in this submission. It analyzes a large volume of electric field data captured from the fiber under test and obtains the orthogonal modal base set of the fiber using the singular value decomposition. This procedure is similar to the principle of machine learning in the area of artificial intelligence. The results show a good agreement with the simulated transverse modes. Due to its operating principle, this approach can characterize any fiber regardless of its length and size.

We present novel high-speed and mode-resolved polarization measurements with PM and non-PM Ytterbium-doped fiber amplifiers up to their TMI thresholds. The implemented full-Stokes polarimetry technique is based on the simultaneous detection of four parallel channels on a high-speed camera. It enables spatially and mode-resolved polarization analysis with sub-ms temporal resolution, allowing to monitor the polarization of individual modes during TMI. We believe that this high-speed mode-resolved polarization measurement technique is highly interesting for the characterization of PM fiber lasers and could help to explore and analyze new TMI mitigation strategies.

In this work we present experimental results of transverse mode instabilities in dependence of the polarization input angle in a large-mode area polarization maintaining fiber amplifier. The transverse mode instabilities threshold was found at 300 W for an input polarization angle aligned parallel to the slow-axis. We demonstrate that at a constant output power of 300 W the temporal stability can significantly be improved by rotating the input polarization angle with respect the slow-axis, indicating an increased threshold. This allowed for further scaling of the fiber amplifier for linear polarization input angles detuned from the slow-axis of the fiber. For operation in the fast-axis (90° to the slow-axis), the power was scaled to up to 475 W without the onset of transverse mode instabilities. However, a static energy transfer from the fast-axis into the slow-axis was observed at powers above 400 W.

We demonstrate a fast and versatile approach to analyze the modal content of a high power fiber amplifier using a low-loss photonic lantern. By monitoring the first three modes of the photonic lantern on a photodetector we can directly determine the modal content of a laser beam, enabling real time diagnostics of the output and its corresponding beam quality factor, M². We first investigate the beam quality and modal content of the output of a passive LMA double clad fiber commonly used as a delivery fiber in high power fiber laser amplifiers. The output of the fiber is analyzed by both a 6-mode mode-selective photonic lantern and a conventional M² setup utilizing a translation stage and beam profiler. The modal content and beam quality measurements produced in real-time by the photonic lantern are compared to the M² measurements resulting in an RMS error less than 0.098 across M² values between 1.020 to 2.260. We then conduct a follow on experiment using the same photonic lantern to monitor modal instability in a large mode area fiber laser amplifier. In this case, we compare our photonic lantern mode analysis approach versus the commonly used RIN/pinhole method evaluating modal instabilities. Not only does the photonic lantern estimate the modal content and beam quality in real-time but the modal content trends with the RIN metric as the fiber laser amplifier progresses from stable regime below 300W through the chaotic transverse modal instability regime above 400W.

Both transverse mode instability (TMI) and non-linear effects hinder the power scaling in fiber laser amplifiers with a diffraction-limited beam profile. In this context core size scaling is a key design parameter to counteract nonlinear effects and, therefore, it is of outmost importance to understand the impact of this parameter on the TMI threshold. In this work we present the first systematic experimental studies that investigate the impact of core size scaling on TMI characteristics. In this study, the unique characteristics of large pitch photonic crystal fibers is exploited to ensure comparability between fibers with different core sizes. This has been done by manufacturing a set of test fibers (Ytterbium-doped LPFs) with different core sizes which maintain their modal characteristics when operated under the same conditions. Furthermore, our experiments reveal a TMI power threshold of 570 W, the highest ever reported in a rod-type fiber with a very large core diameter.

Large mode area (LMA) fibers form an important part of high-power fiber lasers. There is significant research interest in achieving larger mode field area (MFA) for output power scaling and it is equally desirable to have single-mode operation in these fibers to maintain a good beam quality and suppress transverse mode instabilities. An increase in MFA is typically associated with high sensitivity to bend-induced losses and mode-shrinkage, necessitating several ultra-LMA fiber designs to be supported by thick outer jackets to form a rod-shape fiber. We present a hybrid light guidance mechanism in an all-solid antiresonant fiber, which combines antiresonance guidance with total internal reflection guidance to reduce the confinement loss and bending-induced losses by orders of magnitude. Low-index rods are strategically placed in the cladding to cover the gaps between the antiresonant elements to reduce confinement loss in straight fiber and suppress bending-induced leakage loss by orders of magnitude. We present detailed numerical analysis of a typical hybrid-guidance antiresonant fiber (HGARF) with core diameter 80 μm, optimized for operation in 1 μm wavelength range. The wavelength range of operation in the HGARF is decided solely by the wall thickness of the antiresonant elements and therefore the design principles can be extended to the 2 μm wavelength range.

We present an Ytterbium-doped, multicore fiber with 7×7 cores. The fiber is realized in a rod-type geometry with step index cores and an air-cladding for pump guiding. Using a segmented-mirror beam splitter followed by a double-pass multicore pre-amplifier and a main-amplifier of 1 m length, the stretched femtosecond input pulses emitted from the frontend system are amplified. The cores of the main amplifier have a mode-field diameter of 28 μm. Operating at 10 MHz repetition rate, a high average power in excess of 1 kW with near single-mode operation is achieved.

In this work we present a comprehensive parameter study on core-to-core power coupling in multicore fibers (MCFs). In order to do this, a simulation tool has been developed. We chose MCFs with 3×3 cores in a squared pattern with core sizes ranging between 15 and 50 μm and core-to-core distances of 1.5 to 5 times the core diameter. The central core is seeded by a perfectly matched Gaussian beam and the power evolution in each core along the fiber is calculated up to lengths of 2 m. We will show that coupling effects not only depend on the core distance and the core NA, but also on the core diameter and the wavelength. Our simulations predict that a simplified 3×3 core arrangement can be even used to quantify coupling effects in MCFs with more cores when the core-to-core power coupling is kept low. This comprehensive study is crucial for designing laser-active rod-type MCFs.

Gas-filled anti-resonant hollow-core fiber (ARHCF) constitutes an efficient route towards development of high-energy fiber lasers in the near-infrared (NIR) and mid-infrared (MIR) spectral region. We will present our recent work on developing both vibrational and rotational Raman-active gas-filled fiber lasers spanning from the NIR up to around 4.3 μm wavelength. We will also show how such fiber lasers can be used for high-resolution photoacoustic gas sensing and imaging.

In this work we study the origin of birefringence in multicore fibers. With the help of simulations and experiments we are able to identify a new type of birefringence arising in multicore structures: structural birefringence. Hereby birefringence arises due to the intrinsic stress created by each core in the array and its value and orientation of the main polarization axis is dependent on the position of the cores in the array. We provide a comprehensive analysis of structural birefringence, and discuss ways to solve this problem.

We investigate Tm-doped double-clad fibers drawn from fused silica preforms with inserted Tm:YAG laser crystal rods. Based on the Molten-Core-Method the drawn crystal-derived fibers show typical amorphous properties covering a Tm concentration range from 0.2 to 0.84 mol% Tm2O3. They are studied in terms of their suitability of for multi-Watt level fiber lasers and compared to a Tm-laser fiber conventionally fabricated by Modified Chemical Vapor Deposition and solution doping. For the crystal-derived fibers, we demonstrate up to 4 W output power around 2 μm emission wavelength and a slope efficiency of 47 %, which are to date the highest achieved values for 790 nm pumping.

Numerical analysis of multicore fiber tapers as a mode area scaling technique for use in coherently combined laser systems is demonstrated. Taper designs are modelled with beam propagation method (BPM) numerical simulations to analyze inter-core crosstalk and mode distortion in centimeter-scale taper transitions. By evaluating taper performance for a range of taper lengths on the scale of centimeters, optimized taper designs can be found for a given MCF design. Tapers based on these simulations are fabricated using a CO₂ laser tapering system.

The optimization of large-mode-area fiber design for the amplification of narrow-linewidth content or short pulses, susceptible to nonlinear effects, while reaching average powers exceeding the kW level is not a simple endeavor. The rapidly decreasing TMI-threshold with increasing core size leaves very little room in the 20 to 30 μm core diameter space for improved performance with respect to nonlinear effects while still delivering significant average power. We present results on a 29 μm core, polarization-maintaining LMA fiber, with a 400 μm cladding for high average power scaling. A carefully designed depressed-clad surrounds the core and enhances the bending losses for the Higher-Order Modes (HOM). Even when the fiber is loosely coiled (25 cm diameter), the filtering is very efficient which is advantageous for spreading out the fiber heat load and minimizing the effective area reduction resulting from the coiling-induced mode deformation. The fiber has been tested in a CW MOPA configuration, seeded with a longitudinally single-mode source emitting at 1064 nm, phase-modulated for Stimulated Brillouin Scattering (SBS) mitigation. The resulting slope efficiency has been measured at 88%, the PER was in the 12-15 dB range. The main feature of this fiber is its highly efficient HOM filtering capability, consequently one can maintain single mode-like operation up to the TMI threshold (slightly below 1 kW) without significant beam quality (BQ) degradation. As soon as coupling occurs between the fundamental mode and the first higher-order mode through the thermo-optic long-period grating, the energy is shed away and is coupled out in the fiber cladding.

We present a HHG-based XUV source providing large photon flux across a wide range between 40 eV and 150 eV. It is driven by an industrial-grade TruMicro 2030 20-W average power laser system delivering up to 100 µJ at <400-fs pulse duration. A post-compression unit is part of the device to shorten the pulses to approx. 40 fs at only 10% average power loss. The turnkey source provides photon flux of >10^10 photons/s near 70 eV.

Few-cycle laser systems in the short-wavelength infrared (SWIR) region (from 1.4 μm to 3 μm) with high pulse energy and high average power have become increasingly impactful as driving sources for THz, midinfrared and soft X-ray generation with numerous subsequent applications. Compared with the well-established near-IR region, the longer wavelength enables higher conversion efficiency in the THz and midinfrared generation, and is considered as a favorable tradeoff between pushing the high harmonic generation phase matching cut-off up to the water window region (300 eV – 500 eV) while maintaining reasonable single emitter efficiency. In this contribution, we present our first results of the nonlinear post compression of a high-power ultrafast thulium-doped fiber laser output in a 1.05 m long rod-type hollow-core fiber filled with ~3 bar argon. With around 90 fs input pulse duration (FWHM) and 180 W input average power, the nonlinear compression stage delivers compressed pulses with a duration of 10.2 fs (FWHM), an average output power of 132 W. This corresponds to 1.3 mJ pulse energy at a repetition rate of 101 kHz and 1.64 cycles at a central wavelength of 1.87 μm. With this, we estimate a compressed pulse-peak power of about 80 GW, with an energy content of ~66% in the main feature. It is the highest average power mJ-class few-cycle source in the SWIR region reported to date. Featuring a unique combination of peak- and average- power with less than 2 cycle pulses, this laser source is highly interesting for nonlinear frequency conversion addressing THz, midinfrared and soft X-ray region.

High-harmonics generation (HHG) in solids require high-energy few-cycle laser drivers at near- to mid-infrared wavelengths with excellent beam quality to reach fluences of ~1 TW/cm2. Along this line, soliton sources based on large mode area silica-core singlemode fibers produce ultrashort (70 fs) pulses at remote wavelengths with hundreds of nJ, thus providing a new platform for driving HHG in solids. In this communication, we explore the potential of such soliton-based fiber driver for HHG in thin-films of zinc oxide. The laser delivers 41 nJ 70 fs solitonic pulses at 1764 nm and drives harmonics generation up to H7.

An ytterbium-doped large mode area polarization-maintaining fiber with core/cladding diameters of 35/250 µm was fabricated from modified chemical vapor deposition technique and solution doping process. High cladding absorption and low photodarkening were achieved from aluminophosphosilicate core glass with optimal molar composition. The fiber was tested as a power amplifier using a 1064-nm narrow-linewidth laser oscillator with 34 ps pulse duration and 120 MHz pulse repetition frequency. The slope efficiency was seen to exceed 80% while the average output power was scaled beyond 420 W, before the onset of transverse mode instability. The fabricated fiber also yields near diffraction-limited output, narrow spectral linewidth and high polarization extinction ratio.

We discovered novel phenomenon of periodical spectral peaking in optical fibers. When an ultrashort pulse with sharp spectral dips is coupled into an optical fiber, the spectral dips are transferred into spectral peaks periodically along an optical fiber. If a molecular gas cell is used, sharp spectral peaks with sub-THz spectral interval are generated simultaneously and stably. Intense, multiple spectral peak generation was demonstrated by inserting the molecular gas cell into the fiber laser oscillator. Recently, precise, and freely controllable spectral peak generation was achieved using spatial light modulator. This technique is useful for the highly sensitive spectroscopic applications.

We report a single-frequency pulsed Yb-doped fiber master-oscillator-power-amplifier at 1064nm producing output with pulse energy of 0.601 mJ for a pulse width of 95 ns at a pulse repetition frequency of 5 kHz. The Hybrid active fiber structure consisted with a piece of heavily Yb-doped 50/400 μm phosphosilicate fiber and a piece of 35/250 μm silica fiber with moderate Yb³⁺ doping concentration was employed to mitigate SBS effect. Besides, by pre-shaping the seed pulses, SPM-induced spectral linewidth broadening can be suppressed during power scaling.

Many applications such as nonlinear microscopy and strong field optoelectonics require high-energy (> 100 nJ) ultrashort (< 100 fs) pulses above 1.55 µm out of a singlemode fiber. Here, we report on high-energy amplification in tapered Er-doped fiber fabricated by the powder technique. The system based on direct amplification is free from stretcher and compressor units. We generate 90 fs MW-class pulses at 1600 nm by amplification and management of nonlinear effects in the tapered fiber. Despite the output 100 µm core diameter, the emitted beam is near-diffraction limited.

We have demonstrated for the first time, to the best of our knowledge, the successful direct amplification of a cylindrical-vector beams with axially symmetric polarization and doughnut-shaped intensity profile in picosecond MOPA system based on a double-clad ytterbium-doped spun tapered fiber with a ring-shaped active core. The output radially polarized beam with absolute contrast between bright and dark zones carries 10 ps pulses at 1030 nm with a 14.5 W average power level, 91 kW peak power and 0.97 μJ pulse energy.

We demonstrate a polarization-maintaining passively mode-locked thulium-doped fibre laser that can operate at two different repetition rates (dual-comb) simultaneously. Based on the presented approach, we observe beat notes with a free spectral range of 1.97 kHz and aim to realize a free-running dual-comb source in the 2 μm band.

We report on the development of an Erbium amplifier operating at 1550 nm with an output power of 115 W for 500 hours and power variation of less than 1% when run under an open loop, constant-current configuration. To achieve this level of stability, a Raman pump laser system was configured to optimize the output from a Raman resonator and the output wavelength filtering was staggered to prevent any degradation of downstream components, in particular the amplifier WDM that couples the pump and signal. The power level of this amplifier configuration is adequate for a ground-based transmitter to satellites in geosynchronous orbit at 37,000 km.

A completely spectrally resolved model that incorporates an arbitrary signal spectrum and the SBS gain spectrum, was used to distinguish the resulting Stokes spectrum from the proper interaction between the two. This model was employed in the study of the behavior of the Stokes power spectrum given an input of two narrow signal tones into a fiber. The simulations show several behaviors in the Stokes spectrum that are a function of the input tone separation and reveal the process by which the Stokes spectrum is shifted and broadened before finally splitting as the separating tones couple through the SBS process.

For a study of the European Space Agency ESA, Fraunhofer ILT developed and built single-frequency, linearly polarized, power-stabilized fiber amplifiers as elegant breadboard (EBB) with an output power <3 W for the future space-based gravitational wave detector LISA (Laser Interferometer Space Antenna). The fiber amplifier developed at Fraunhofer ILT in a previous phase has fulfilled most of the optical performance requirements, except the relative intensity noise (RIN) [1]. In this paper, we present our revised and optimized fiber amplifier which now, in addition to the earlier demonstrated parameters, fulfills the LISA RIN requirements. Currently, the engineering model (EM) of the chosen fiber amplifier concept is being implemented by our project partner SpaceTech GmbH. Furthermore, since the Technology Readiness Level (TRL) of the components has to be confirmed for the EM, Fraunhofer ILT conducted 1000hours operational long-term tests of the components installed in a thermal-vacuum chamber and 2-weeks nonoperational tests in a thermal cycling chamber to qualify them for space applications.

A wavelength agile 900 nm 2.5 kW peak power fiber laser is created by four-wave mixing (FWM) in a photonic crystal fiber (PCF), while amplifying a 1300 nm Fourier-domain mode-locked (FDML) laser. The FWM process is pumped by a home-built 1064 nm master oscillator power amplifier (MOPA) laser and seeded by a home-built 1300 nm FDML laser, generating high power pulses at wavelengths, where amplification by active fiber media is difficult. The 900 nm pulses have a spectral linewidth of 70 pm, are tunable over 54 nm and have electronic pulse-to-pulse tuning capability. These pulses can be used for nonlinear imaging like two-photon or coherent anti-Stokes Raman microscopy (CARS) microscopy including spectro-temporal laser imaging by diffracted excitation (SLIDE) and time-encoded (Tico) stimulated Raman microscopy.

We report a compact, robust, and cost-effective ultra-short pulse fiber laser incorporating a NALM-based all-PM modelocked Yb fiber laser oscillator and an Yb-doped fiber amplifier. The Yb-doped fibers are pumped with a low-power laser diode. The laser generates 12 ps pulses at a repetition rate of 20 MHz, center wavelength of 1040 nm, spectral width of 20 nm and average power of 128 mW. We believe that this type of fiber laser is an ideal seed source for further high-power femtosecond fiber laser.

In this work, we present a temporal and spectral study of properties of a pulsed fiber laser in a figure-eight configuration (F8L) through the automatic adjustment of polarization controller plates. The experimental scheme consists of telecom fiber, a double-clad fiber doped with Er/Yb that allows higher power at the laser output, an optical isolator, a saturable absorber, and retarder plates. Automated polarization control system was developed using computer-aided design and additive manufacturing applied to each polarization stage within the fiber laser. The laser operation is automatically adjusted by means of an optical control system, composed of a quarter-wave retarder (QWR) to allow the self-starting operation, while the angles of the QWR, half-wave retarder (HWR), and QWR plates in the polarization controller allows to adjust the temporal duration of the pulses, as well as the spectral width at the laser output. The laser through automated adjustments on retarder plates generates an emission centered at ~1550 nm, with pulse durations between 189.5 ps to 350 ps (spectral widths between 13.4 nm and 53.4 nm), with repetition frequencies of 904 kHz, and it is possible to generate a supercontinuous spectrum of more than 250 nm. The emission obtained corresponds to noise-like pulse operation, which is very useful for the development of applications such as generation of harmonic mode locking pulses, supercontinuum with high flatness, and optical rogue waves, among others. Finally, the proposed study allows showing the advantages of self-adjusting the laser using automated control, with the aim of finding more precisely modes of operation of interest in multivariable systems.

We investigated theoretically and experimentally the feasibility of realizing high-power narrow-linewidth fiber lasers in <1100 nm wavelength range using only ytterbium ions gain. The structure of the laser is much simpler than Yb-Raman fiber lasers. In this laser, amplified spontaneous emission is suppressed by adjusting the amplifier parameters, which are determined by the theoretical calculation results. In addition, the system is built based on FBG-based master oscillator power amplifier configuration. Finally, a 1105 nm fiber laser, which delivers 1.5 kW output power withtheM2 being ~1.4 and a ~70 GHz linewidth, has been realized.

Highly stable, high peak output power pulsed transmitter sources in the 2000 nm band are essential seed lasers for diverse applications such as LIDAR, ground-to-space optical communications, detection of trace gases in the atmosphere, medical applications, and pumping optical parametric oscillators and supercontinuum sources. Previous work utilizing single clad, single mode fibers has demonstrated pulsed mode operation of an optically amplified source at 2051 nm and 2090 nm with pulse widths ranging from 5–500 ns, pulse repetition frequencies (PRFs) of 20–300 kHz, and peak output pulse energies of 10 μJ. In this paper, we report the design and performance of a novel nanosecond MOPA optical transmitter at a signal wavelength of 2070 nm with more than 250 W peak output power and highly stable output pulses. The seed laser is broadened using a phase modulator, to minimize the onset of optical nonlinearities such as SBS and MI and then amplified using a two-stage Ho-doped fiber amplifier (HDFA) employing 8-μm core active fiber. The amplified signal is then transmitted through a tandem arrangement with a 250 MHz acousto-optic modulator (AOM) followed by a high-speed electro-optic amplitude modulator (EOM). This pulses signal is then reamplified by a two-stage HDFA where the second stage employs a 20-μm core active fiber, which reduces the threshold for the onset of nonlinear effects such as modulation instability (MI) and four-wave mixing. We present a comparison of optical simulation results with experimental data for the medium- and large-core Ho-doped fibers in the MOPA transmitter.

Chirped fiber Bragg gratings opened up a way towards investigating dispersion-managed dissipative soliton regime in all-fiber cavities at the wavelength of 1 µm. It has been shown that dispersion management can decrease the chirped pulse duration compared to all-normal-dispersion oscillators. Recent works also prove that operation near-zero net cavity dispersion can reduce the relative intensity noise. Building such systems using only polarization-maintaining optical fibers is of great interest because of their robustness in extreme environmental conditions resulting in various applications outside research laboratories. This work presents an ultrafast Ybdoped fiber laser oscillator made entirely of polarization-maintaining optical fibers and fiberized components. Unlike in typical ring cavities, the ultrashort pulse passes through the rare-earth-doped fiber twice per cavity roundtrip. The system operates in a Raman-free dispersion-managed dissipative soliton regime at the central wavelength of 1031 nm. The negative dispersion is introduced to the cavity via a chirped fiber Bragg grating. At net cavity dispersion of –0.037 ps², the setup delivers stable 3 nJ pulses at a repetition rate of 23.781 MHz. The oscillator, passively mode-locked with a nonlinear optical loop mirror, generates positively chirped 8.2 ps pulses, which can be compressed down to 125 fs with a temporal Strehl ratio of 0.77.

In present work we developed a simulation model which allows to calculate a fraction of amplified spontaneous emission in continuous wave (CW) Er-Yb co-doped fiber amplifier. The simulation shows that high output power at 1550 nm with reasonably small amount of ASE (0.7%) can be reached at very low, mW-level seed signal with single amplification stage. Based on the simulation results, a medium power (3 W) Er-Yb co-doped fiber laser was demonstrated with only one amplification stage.

We report on our recent results of the numeric evaluation of the gain evolution inside a thulium-doped fiber Mamyshev oscillator in the spectral 2 μm region, comparing the core pumping scheme with the double-clad pumping scheme in order to further optimize the output parameters in the experimental setup. By comparing the two pumping schemes, we find spectral gain channeling around 1950 nm for the double-clad pumping scheme owing to a three orders of magnitude lower pump intensity and an order of magnitude higher doping concentration. The found gain distribution is highly effective to suppress amplified spontaneous emission at the maximum emission cross section of thulium ions in silica glass and thus enables operation beyond the water absorption lines. Furthermore based on the gain evolution model, also a novel broadening mechanism inside the gain fiber of Mamyshev oscillators is numerically evaluated. The pulse evolution is determined by the interplay between the anomalous dispersion and self-phase modulation inside the gain fiber and allows to self-compress the pulse, while simultaneously monotonic spectral broadening arises during the amplification. This novel pulse evolution inside a Mamyshev oscillator shows nearly transform limited high peak power pulses with a pulse duration below 100 fs directly at the output without any additional compression stage.

The temporal contrast of multi-pass cells can be increased significantly by implementing the technique of enhanced frequency chirping. By manipulating the pulse form using dispersive self-phase modulation the spectral modulations are reduced resulting in a pulse with decreased side-pulses and more energy in the main feature. Using MPC mirrors with a tailored dispersion and afterwards compensating accumulated higher order dispersions the energy ratio and peak power boost of a MPC can be improved notably.

A polarization-maintaining large-mode-area single-mode solid-core anti-resonant fiber is proposed for the first time in this work based on a double-layer structure. Specifically, the designed fiber obtains a mode field area up to 2135 μm² at 1.064 μm while exhibiting a birefringence of 1.3×10^-5. Single-mode operation can be guaranteed with the confinement losses of all higher order modes exceeding 10 dB/m.

We report on a reliable high power fully monolithic single-frequency Er-Yb fiber amplifier designed for long distance earth to satellite communications. The source is built on three all-fiber master oscillator power amplifier (MOPA) stages emitting over 35 W of output power at 1550 nm. The setup contains home-made highpower endcap and packaging for high power splices (cladding mode stripper (CMS) and input splice). Their development allowed us to become independent from commercially available components which are often incompatible with fibers used in the experimental setups, resulting in additional losses. The amplifier was used for more than 500 h over 35 W, 42 W and 49.5 W. We will discuss some critical parameters especially the mode content evolution using the spectrally and spatially resolved imaging method (S2), the pointing error, four wave mixing (FWM) and long-time stability at high power.

Individual fiber amplifiers with increased average power while maintaining a narrow spectrum and excellent beam quality, enable power scaling of beam combined systems. To accomplish this, fiber amplifiers must contend with two well-known deleterious processes: thermal modal instability (TMI) and stimulated Brillouin scattering (SBS). Changing the fiber host material from silica glass to crystalline Yttrium Aluminum Garnet (YAG) has been reported as a potential means to increase the SBS and TMI power thresholds due to favorable material properties, mainly increased thermal conductivity and decreased electrostrictive constants. In this report, the development of numerical models to examine nonlinear effects in crystalline YAG fiber is described. Fiber simulation code previously developed at the Air Force Research Laboratory (AFRL) for silica are leveraged for crystalline gain media. Results show TMI threshold for a Ytterbium doped YAG (Yb:YAG) fiber 28 times higher than the equivalent silica fiber, and an increase in SBS threshold by over 250 times in YAG compared to silica. The investigations also include thresholds for Holmium doped YAG (Ho:YAG) and Thulium doped (Tm:YAG), which compare well with published experimental data.