This PDF file contains the front matter associated with SPIE Proceedings Volume 11260, including the title page, copyright information, table of contents, and author and conference committee lists.

The kW-level single-mode Yb fiber lasers at wavelengths in 1000 -1030nm spectral range with diffraction-limited beam and with direct diode pumping are remarkable high brightness and low quantum defect sources for tandem pumping of multi-kilowatt fiber and crystal laser systems. In this paper we present SM Yb fiber lasers in all-fiber format with powers 0.75kW, 0.90kW, 1.33kW and 1.40kW at 1007nm, 1010nm, 1018nm and 1030nm respectively with M² values of output beams < 1.1. These are the highest powers of single mode CW Yb fiber lasers at wavelengths near 1 micron to the best of our knowledge. The fiber oscillators with direct diode pumping are highly efficient and have optical slope efficiencies from 64% at 1007nm to 78% at 1018nm relative to total laser diode pump power. The fiber lasers have 3-5m output delivery cable terminated with a LC-8 connector in dependence on power level and MFD of output fiber. The pump and non-linear process (SRS and MI) limitations are discussed.

We recently demonstrated a seed modulation format that is effective in suppressing SBS in kW-class fiber amplifiers. By applying synchronous amplitude and frequency modulation, a broad low power seed spectrum can be spectrally narrowed via self-phase modulation as it is amplified, thus reducing the SBS gain and enabling narrower linewidths. Since the approach leverages the fiber Kerr nonlinearity, SBS suppression becomes more effective as fiber powers and lengths increase. We will present updated results extending this approach toward higher modulation depths, higher powers, and narrower linewidths.

We investigated the limitations in output power generated by a high power narrow-linewidth Raman fiber amplifier. The pump was produced by a kW-level all-fiber Yb-doped amplifier emitting at 1060 nm, whose seed linewidth could be changed. The Raman seed was a narrow-linewidth signal at 1110 nm co-propagating with the laser at 1060 nm. The main Raman conversion occurred in the passive fiber at the amplifier output. We identified cross-phase modulation (XPM) as a main reason for broadening of the Raman light by using different pump sources, which is a first limitation. An improved setup was limited at approximately 600 W of Stokes output power by a threshold-like onset of a transverse mode instability. Since the instability was not observed without a Stokes seed and the temperatures of the active fiber with and without Stokes seed are equal, this constitutes the first direct observation of transverse mode instabilities (TMI) induced by SRS in a passive fiber.

We report on the results of a narrow linewidth fibre laser with <1kW laser output. Pseudo-random bit sequence (PRBS) phase modulation at 6.9GHz was employed to suppress Stimulated Brillouin Scattering (SBS). The laser demonstrated wavelength tunable operation from 1036nm to 1071nm, with spectral purity exceeding 40dB over most of the range. A delivery fibre length of 3m was enabled by exploiting the short gain lengths in a customized 250μm cladding fibre design. Near diffraction limited beam quality of M²=1.19 was measured with no indication of transverse modal instability (TMI).

New large mode area Ytterbium-doped fiber (LMA-YDF) designs for 400µm clad laser oscillators are investigated. Designs are optimized to simultaneously overcome limitations of Stimulated Raman Scattering (SRS) and Thermal Mode Instability (TMI) within the limitations set by the industrial laser market. The optimization enable industrial fiber lasers operating at 1µm wavelengths to robustly provide > 2kW with single mode beam quality. 915 nm pumping and 976 nm pumping require different designs due to different thermal loads. Wavelength specific options are given for both that robustly increase power from the current state of the art 1-1.5kW to up to 2.3kW.

Here we present the latest experimental results of a high-power CEP-stable FCPA system. The 16-channel FCPA runs at 0.3% RMS power stability (>9hours) delivering more than 1kW and 10mJ after the compressor at a pulse duration of 280fs. To generate 6fs pulses, stretched hollow-core fibers are being employed. We present a significant up-scaling of this technique towards an output of 5mJ, 100kHz and 6fs.

We present a coherently-combined ultrafast fiber laser system consisting of twelve amplifier channels delivering 10.4 kW average power at 80 MHz repetition rate with a pulse duration of 240 fs FWHM and an almost diffraction-limited beam quality of M² ≤ 1.2. The system incorporates an automated self-adjustment of the beam combination with 3 degrees of freedom per channel. The system today is, to the best of our knowledge, the world’s most average-powerful femtosecond laser. Thermographic analysis indicates that power scaling to 100 kW-class average power is feasible.

We developed a simulation of a beam tiled aperture combining system with n emitters. Using the Fourier propagation method, we calculate the far-field pattern depending on (n-1) relative phase parameters. We then simulate the phase noise as a time series using Gaussian random walk phase noise, with the possibility to actuate the phase parameters. For the stabilization scheme, we exclusively use a far filed intensity pattern. This image we feed through a simple two-layer neural network trained by a deep deterministic gradient reinforcement learning algorithm. This allows for a large amount of flexibility in terms of optimization metrics, which easily allows for beam combination in the far filed and beam shaping. The control policy is automatically derived from reward functions which can be phase patterns, target beam shapes or maximum power through a pinhole without the need for further optical analysis. After training only the image is used for stabilization.

In this work we present theoretical investigations of the power scaling potential of multicore fibers. In principle it is widely accepted that increasing the number of active cores helps to overcome current challenges such as transversal mode instabilities and non-linear effects. However, in order to do a proper analysis of the average power scaling potential of multicore fibers it is required to pay particular attention to thermal effects arising in such fibers. Therefore, a simulation tool has been developed that is capable of solving the laser rate equations, taking into account the resulting temperature gradient and the distortions in the mode profiles that it causes. In the study several different multicore fibers possessing a rectangular core position layout of 2×2 to 7×7 of active cores have been analyzed. Moreover, we have investigated the influence of the active core size in terms of thermal effects as well as the extractable output power and energy. This includes a study in the maximum achievable coherent combination efficiency of the multicore channels (that is strongly influenced by the distorted mode profile at the fiber end facet), the impact on nonlinear effects, the optical path differences between the cores and the amplification efficiency which are all triggered by thermal effects. Finally the scaling potential as well as the challenges of such fibers will be discussed.

A simplification of segmented-mirror splitters for coherent beam combination based on numerical optimization of coating designs is presented. The simplified designs may facilitate the production of such elements for coherent beam combination while maintaining high combination efficiency. The achievable efficiency and error tolerance, and additional performance characteristics are analyzed in the context of coherently combined multicore fiber laser systems.

Output power levels from 1907 nm Tm fiber laser sources are restricted by low lasing efficiency and challenging thermal management. Here we develop a novel fiber core structure where the Tm dopant material is confined to a ring within the larger undoped glass core. This evolution of dopant profiling has tripled the fiber device lengths and produced 131 W of 1907 nm emission with 64.5% absorbed slope efficiency. This, to the best of our knowledge, is the most efficient 1907 nm Tm fiber laser that has been demonstrated with the capability to perform beyond 100 W output power.

A route to high power continuous-wave generation around 1.7 µm has been explored using an Er-only doped LMA fiber laser for high brightness core pumping of a thulium-doped fiber laser (TDFL). The maximum output power of 47 W with a slope efficiency of 80% was achieved at 1726 nm which, to the author’s best knowledge, is the highest recorded power in this wavelength region from a TDFL. The motivation for an Er-only pump source is scalability; these results show that this configuration has significant potential for further power scaling towards hundred-watt class systems at 1.7 µm and beyond.

Thulium-doped fibers pumped with 79xnm high power diodes enable a cross-relaxation process to achieve two excited ions for one pump photon, reaching a quantum efficiency up to 200% in the two-micron band. For high power operation at longer wavelengths <2100nm, where the Thulium-doped fiber laser (TDFL) efficiency drops considerable, holmiumdoped fiber lasers (HDFLs) are usually the preferred choice. However, as the Ho3+ ion has no absorption bands where high power diodes are available, TDFLs are traditionally used as a pump source. Therefore, the overall optical to optical conversion efficiency is dependent on the performance of TDFL. This approach also brings additional challenges to the fiber fabrication process as conventional low-index polymers that serve as a pump cladding in double clad fibers cannot be used due to the high absorption of polymer at two microns band and thus, there is a need for an all-glass fiber structure. In this work, we report on the fabrication and laser characterization of a Tm:Ho co-doped fiber in an aluminosilicate host fabricated by using a hybrid gas phase-solution doping technique combined with the MCVD process, where aluminum was introduced into the silica matrix through vapor phase deposition and the rare-earths by solution doping process. The proposed fabrication technique allows more uniform dopant distribution within the fiber core region that helps to achieve a good laser performance. We have demonstrated a free running laser, operating at 2105nm with an output power of <37W and a laser efficiency of ~56% with respect to the absorbed pump power when cladding pumped by a 793nm laser diode. As evident from the experimental results that the donor-acceptor energy transfer from Tm³⁺ to Ho³⁺ ions is working in our Tm:Ho co-doped fiber.

Numerous molecules important for environmental and life sciences feature strong absorption bands in the molecular fingerprint region from 3 μm – 20 μm. While mature drivers at 1 μm wavelength are the workhorse for the generation of radiation up to 5 μm (utilizing down-conversion in nonlinear crystals) they struggle to directly produce radiation beyond this limit, due to impeding nonlinear absorption in non-oxide crystals. Since only non-oxide crystals provide transmission in the whole molecular fingerprint region, a shift to longer driving wavelengths is necessary for a power scalable direct conversion of radiation into the wavelength region beyond 5 μm. In this contribution, we present a high-power single-stage optical parametric amplifier driven by a state of the art 2 μm wavelength, thulium-doped fiber chirped pulse amplifier. In this experiment, the laser system provided 23 W at 417 kHz repetition rate with 270 fs pulse duration to the parametric amplifier. The seed signal is produced by supercontinuum generation in 3 mm of sapphire and pre-chirped with 3 mm of germanium. Combining this signal with the pump radiation and focusing it into a 2 mm thick GaSe crystal with a pump intensity of 160 GW/cm² lead to an average idler power of 700 mW with a spectrum spanning from 9 μm – 12 μm. To the best of our knowledge, this is the highest average power reported from a parametric amplifier directly driven by a 2 μm ultrafast laser in the wavelength region beyond 5 μm. Employing common multi-stage designs, this approach might in the future enable multi-watt high-power parametric amplification in the long wavelength mid infrared.

Coaxial fiber lasers, consisting of a Ho-doped core surrounded by a Tm-doped ring, are studied via experiments and numerical simulations. Previous simulations indicated that coaxial fiber lasers have the potential to reach power conversion efficiencies of up to 54%, but experiments have yielded much lower efficiencies. To understand this difference, a wavelength dimension is added to the standard model and simulations are compared to experiments. An output coupler is designed to optimize the efficiency of coaxial fiber systems, and a path towards realizing the predicted 54% efficiency is presented.

Applications such as material processing, spectroscopy, particle acceleration, high-harmonic and mid-IR generation can greatly benefit from high repetition rate, high power, ultrafast laser sources emitting around 2 μm wavelength. In this contribution we present a single-channel Tm-doped fiber chirped-pulse amplifier delivering 108 W of average output power at 417 kHz repetition rate with 250 fs pulse duration and 0.73 GW of pulse peak power. To the best of our knowledge, this is the first demonstration of an ultrafast Tm-doped fiber laser with more than 100 W of average power and GW-level peak power.

We report on the first coherent beam combining of 60 fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beams recombination in the far field, this technique appears well suited for the combination of a large number of fiber amplifiers. The 60 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal microlens array. The measured residual errors within the fiber array yields standard deviations of 4.2 μm for the fiber pitch and 3.1 mrad for the beam-to-beam pointing, allowing a combining efficiency of 50 %. The phasing of 60 fiber amplifiers demonstrates both pulse synchronization and phase stabilization with a residual phase error as low as λ/100 RMS.

Multi-photon microscopy (MPM) has become an indispensable tool for observing biological structures and functions in vivo, benefitting from its deep penetration depth and high spatial resolution. Femtosecond pulses featuring a broad wavelength tuning range are highly desired by MPM. We demonstrate a 1-MHz ultrafast fiber-optic source that produces ~100-fs pulses tunable from 940 nm to 1250 nm with 100-nJ level pulse energy. For example, we achieved 120-fs pulses with 105-nJ energy centered at 1150 nm. This broadly tunable, energetic fs source constitutes an ideal source for deep-tissue multi-photon imaging.

We report on a fiber laser setup optimized to generate and propagate high energy solitons with megawatt peak power around 1700 nm. Picosecond pulses from a chirped-pulse amplifier system at 1560 nm trigger the formation of sub-100 fs solitons with approximately 70 nJ energy in an all-solid photonic bandgap Bragg fiber with ultra-large mode area. Upon propagation in the same fiber the soliton self-frequency shift effect allows for tuning the central wavelength up to 1680 nm in a 1.5 m long piece of fiber. This work paves the way to miniaturized endomicroscopes in the biologically relevant window around 1700 nm and, thanks to the 20-cm critical bend radius of the delivery fiber, opens the way to deep in vivo imaging of freely moving animals.

The key advantages of ultrafast fiber lasers compared to their bulk competitors are their small footprints, low costs and nearly diffraction limited beam quality. The pulse energy of fiber based ultrafast oscillators is still inferior comparted to ultrafast bulk oscillators. In the last decade substantial effort has been devoted to overcome this hurdle, including the development of novel saturable absorber schemes. In this work, we discuss the pulse energy scaling potential of all normal dispersion (ANDI) fiber lasers utilizing Ytterbium doped multicore fiber as saturable absorber.

We present the characteristics of a high-energy ultrafast Yb-fiber laser system, based on a Mamyshev oscillator and a subsequently arranged fiber amplifier stage. The Mamyshev oscillator emits pulses at a repetition rate of 11 MHz and pulse energies of 31 nJ. These pulses are spectrally filtered and amplified in a Yb-doped fiber up to 1 μJ pulse energy which could be temporally dechirped to less than 50 fs autocorrelation duration. We discuss the scaling as well as limiting options related to pulse energies and duration.

We experimentally demonstrate the first bidirectional all-normal dispersion (ANDi) fiber laser. Single-pulse bidirectional mode-locking was established at 36 MHz and stably maintained for hours. Both directions showed broad spectra of more than 20 nm (10 dB) while the shapes were slightly different. The repetition rate in both directions can be tuned from 100 Hz to 150 Hz by changing the pump power. More significantly, the pulse energies in both directions were larger than 2 nJ and more than 20 nJ can be potentially achieved. Therefore, our laser represents a promising solution for compact dual-comb nonlinear spectroscopy.

An all-fiber supercontinuum light source based on a polarization-maintaining, self-starting femtosecond oscillator with repetition rate in the MHz range is presented. The supercontinuum stretches from 950 – 2100 nm and exhibits a high level of stability – both in terms of amplitude and repetition rate. The supercontinuum source displays an rms intensity noise of 0.05 % when integrated from 10 Hz to 1 MHz. The light source is contained in box with dimensions 25×25×5 cm³ with a single mode output fiber. This together with the high level of stability makes this light source well suited for applications in, e.g., spectroscopy and OCT.

We present the first spatiotemporally mode-locked fiber laser with self-similar pulse evolution, to the best of our knowledge. Our multimode fiber laser produces amplifier similaritons with near-Gaussian beam quality (M2<1.4) at the output. Ytterbium based laser generates 2.3 ps pulses at 1030 nm with 2.4 nJ energy. The output pulses are externally compressed to 192 fs with a grating compressor. Intracavity large spectral breathing (>6) and less chirped pulses than the cavity induced total dispersion are the verifications of the spatiotemporal self-similar pulse propagation.

We report an all-polarization maintaining fiber laser operating at central wavelength of 1020-1064 nm with pulse duration of 2-8 ps and the repetition rate of 6.12 MHz. The laser is mode-locked using a nonlinear amplifying loop mirror, and the wavelength can be tuned continuously by installing an electrically programmable optical filter into the cavity. Our ultrafast fiber laser will be attractive for various applications such as bio-imaging, laser micro-processing, optical measurements, medical treatment, and supercontinuum generation.

Intense, ultrafast laser sources with an emission wavelength beyond the well-established near-IR are important tools for exploiting the wavelength scaling laws of strong-field, light-matter interactions. In particular, such laser systems enable high photon energy cut-off HHG up to, and even beyond, the water window thus enabling a plethora of subsequent experiments. Ultrafast thulium-doped fiber laser systems (providing a broad amplification bandwidth in the 2 μm wavelength region) represent a promising, average-power scalable laser concept in this regard. These lasers already deliver ~100 fs pulses with multi-GW peak power at hundreds of kHz repetition rate. In this work, we show that combining ultrafast thulium-doped fiber CPA systems with hollow-core fiber based nonlinear pulse compression is a promising approach to realize high photon energy cut-off HHG drivers. Herein, we show that thulium-doped, fiber-laser-driven HHG in argon can access the highly interesting spectral region around 90 eV. Additionally, we show the first water window high-order harmonic generation experiment driven by a high repetition rate, thulium-doped fiber laser system. In this proof of principle demonstration, a photon energy cut-off of approximately 400 eV has been achieved, together with a photon flux <10⁵ ph/s/eV at 300 eV. These results emphasize the great potential of exploiting the HHG wavelength scaling laws with 2 μm fiber laser technology. Improvements of the HHG efficiency, the overall HHG yield and further laser performance enhancements will be the subject of our future work.

A 1720-nm thulium-doped all-fiber laser based on a ring-cavity configuration is demonstrated. The long-wavelength lasing near the 1.9-μm thulium emission peak was suppressed using a wavelength division multiplexer and single-mode– multimode–single-mode (SMS) fiber device, which together served as a short-pass filter instead of the grating devices usually used in 1.7-μm thulium fiber lasers. A stable hundred-milliwatt-level 1720-nm laser output with a narrow spectral linewidth on the order of gigahertz was obtained after optimizing the output coupling, the active fiber length and the SMS device.

Stimulated Brillouin scattering (SBS) is a well established method to narrow the laser linewidth to kilohertz level, which however suffers high threshold due to the low SBS gain at the region of 2 μm. The hybrid Brillouin/thulium fiber laser (BTFL) is such an approach which could suppress the laser linewidth with low threshold and high efficiency. In this paper, an ultra-narrow linewidth hybrid Brillouin/thulium fiber laser (BTFL) was demonstrated. Through experimentally optimizing the output coupling, pump scheme, Brillouin pump power and cavity length of the laser, 344-mW laser output with a narrow linewidth of 0.93 kHz was obtained, in which the linewidth of Stokes light was suppressed more than 43 times compared with the 40 kHz linewidth of the Brillouin pump. Besides, the influences of output coupling and pump scheme on the power and linewidth behavior of a single-frequency BTFL were also experimentally investigated, and there exists a performance balance among linewidth narrowing, output power and SBS threshold. The BTFL output power was further boosted to 5.5 W by a one-stage cladding-pumped fiber amplifier, and the corresponding spectral linewidth was broadened to 1.93 kHz. The output coupling exerted a significant influence on the BTFL performance.

Recent developments in LIDAR, atmospheric sensing, and WDM transmission system experiments highlight the need for large bandwidth, high dynamic range polarization-maintaining (PM) optical amplifiers in the 1.9—2.1 μm band. While some results for all-double-clad PM amplifier designs have been presented, single clad PM amplifiers are particularly attractive in these applications because of their potential for high gain and low noise figures approaching the quantum limit. There is also a strong and growing need for need for compact and rugged amplifiers to boost modest (1 mW CW) semiconductor laser source output powers in a space/satellite environment. In this paper, we report first experimental results for a newly developed single clad PM Thulium-doped fiber with the parameters shown in Table 1 below and an optical signal bandwidth of < 120 nm, and then present performance of a miniature packaged optical amplifier using this new fiber.

In fiber lasers, operating in normal dispersion regime benefits high peak power operation thanks to no pulse breakup as in anomalous dispersion. However, the spectral range of Thulium (Tm) emission lies in anomalous dispersion regime for conventional optical fibers. Hence, a customized W-type step-index Normal Dispersion Thulium Fiber (NDTF) is designed to have strong waveguide dispersion at the Tm emission band. The dispersion of NDTF is -28.97 ps/nm.km at 1.9 μm wavelength. An all-fiber seed source based on a ring oscillator was built with the NDTF as the active fiber and produce mode-locked soliton pulses near 2 μm. Subsequently, the pulses are amplified through the NDTF in an all-fiber amplifier stage. The NDTF amplifier produced pulses of ~593 nJ pulse energy in a ~4.4 ps FWHM pulse width. The amplified pulse is then compressed to ~1.91 ps giving a peak power of ~310 kW in an all-fiber compressor consisting of SMF28 fiber. This represents a potential to generate high peak powers in ultrashort pulses at 2 μm wavelength in all-fiber configuration.

We proposed and demonstrated an all-fiber broadband tunable filter, with “band-stop” characteristics using bending of a resonant coupling fiber. Unlike conventional step-index fiber bend-induced “low-pass” filtering, with this novel approach we are able to retain the “band-stop” filter characteristics and tune the long wavelength edge of the rejected spectral band whilst the short wavelength edge is fixed by fiber design. The fiber filter can be readily spliced to and integrated with high-power single mode output ytterbium- or thulium-doped fiber lasers to provide efficient filtering of four-wave-mixing and/or stimulated Raman scattering, as well as parasitic 1um lasing in erbium/ytterbium co-doped lasers.

In this work we present a multicore fiber design that exploits the Talbot effect to carry out the beam splitting and recombination inside of the fiber. This allows reducing the complexity of coherent combining systems since it makes the splitting and combining subsystems together with the active stabilization redundant. In other words, such a multicore fiber behaves for the user as a single core fiber, since the energy is coupled just in a single core and it is extracted from the same core. This work describes the operating principle of this novel fiber design and analyzes its performance in high power operation using a simulation model based on the supermode theory. This includes a study on the impact on non-linear effects, on the amplification efficiency, on the thermal resilience of this design and on the performance dependence on the pump direction. Moreover, some design guidelines will be provided to tailor the characteristics of the fiber. Finally, it will be discussed how these fibers can be used to increase the TMI threshold of fiber laser systems.

Cladded linear index graded (CLING) fibers are designed to reduce the spatial overlap between the fundamental mode and higher-order modes in order to mitigate thermal mode instability. Numerical simulations were used to study the effects of bending on loss and mode deformation in CLING fibers. It was found that CLING fibers experience mode deformation at a >10x tighter bend radius than conventional large-mode-area (LMA) fiber. The CLING fiber therefore retains all the advantages of an LMA fiber but at small bend radii, enabling much larger mode areas than allowable in conventional LMA fibers (currently 25-µm core).

Gravitational wave detectors require single-frequency laser sources with challenging properties regarding beam quality, polarization, and noise properties. We developed a single-frequency fiber amplifier engineering prototype based on standard step-index polarization maintaining fibers and characterized the 200 W output beam with the complete set of measurements necessary to evaluate the system's performance with respect to the application requirements. The output beam has a TEM₀₀-mode content of 94:8% at 200W and a polarization extinction ratio of 19 dB. In the crucial frequency range from 1 Hz to 100 kHz the frequency noise, relative power noise, and relative pointing noise measurements demonstrated low noise properties. In addition, the pointing noise below 100 Hz is the lowest reported for single-frequency amplifiers with 200W output power. SBS-free operation was demonstrated by monitoring the relative power noise in the MHz frequency range. The system was operated above 200W for 695 h and evaluated again after 650 h of operation. No signs of photodarkening were found.

Fourier domain mode locking (FDML) is a recently developed technique for lasers to generate ultra-rapid wavelength sweeps, equivalent to a train of extremely chirped pulses. FDML lasers are the light sources of choice for fastest megahertz optical coherence tomography (MHz-OCT). Measuring the coherence properties of FDML lasers is of particular importance for the image quality in OCT but it is also crucial to develop a better understanding of this unconventional mode locking mechanism. Usually, experiments to analyze the phase stability of FDML lasers use interferometers to generate interference of a single laser by delaying a part of the output to generate a beat signal. Here, for the first time, we present real time beat signal measurements between two independent FDML lasers over the entire sweep range of ~5 THz width for more than 80 roundtrips (~200 μs), evaluate their phase stability and explain the consequence for our understanding of the FDML mechanism. Beat signal measurements allow direct access to the phase difference between the FDML lasers and therefore the difference in timing of the circulating sweeps as well as their instantaneous frequency.

We developed a novel technique based on a short range supercontinuum Lidar for robust nonintrusive combustion diagnostics using just one inspection window. The approach enables simultaneous remote measurement of combustion gas parameters like temperature and concentration which plays a key role in the performance of combustion power plants. We demonstrate preliminary industrial scale measurement of water vapor temperature and concentration in a full scale boiler. Looking forward, we emphasize that the technique possesses a great potential for simultaneous 3D profiling of temperature and concentration, which can be achieved by varying the direction of the probe beam in a non-parallel plane.

Results of numerical simulations of transverse mode instability (TMI) in Ytterbium-doped coiled large mode area step index fiber amplifiers are presented and discussed. The TMI thresholds for counter-pumped amplifiers were found to be significantly greater than for co-pumped amplifiers. A greater dependence of the TMI threshold on amplifier length was observed in the counter-pumped case than in the co-pumped case. Furthermore the onset of TMI was found to be localized in a specific critical position along the amplifier in the co-pumped case. In this configuration, amplifiers could be pushed slightly beyond the TMI threshold and still produce stable output due to bend loss filtering of the higher order modes. The TMI thresholds were found to have exhibit a non-monotonic behavior with respect to coiling diameter and only weakly dependent on core launch misalignment. Power stripped from the core into the cladding was in some cases found to interfere with the signal within the core leading to small power fluctuations that appear unrelated to the TMI process. In some co-pumped cases the beam was distorted at the amplifier output only during a small window within the overall simulated time period. This indicates that all of the different components of the broadened frequency spectrum are coherent. At this time, the physical explanation for this effect is unknown.

In this work a novel accurate method of measuring beam fluctuations is presented and applied to analyze transverse mode instabilities (TMI). The new measurement, ST-measurement, uses Fourier analysis on data from a high-speed camera to achieve raw spatial information about beam fluctuations. TMI in a 65 μm mode- field-diameter aeroGain-ROD-PM85 fiber is investigated using both the ST- and standard photo detector measurement. A comparison of the two measurements shows the quantitative and qualitative superiority of the new ST-measurement due to the spatial information. Numerical simulations are carried out to support the interpretation of the data.

In this work we experimentally and theoretically investigate the impact of seed intensity-noise on the threshold of transverse mode instability (TMI) in Yb-doped, high-power fiber laser systems and compare it to the impact of pump intensity-noise. Former studies have shown that pump intensity-noise significantly decreases the TMI threshold due to the introduction of a phase shift between the modal interference pattern and the thermallyinduced refractive index grating in the fiber. However, it can be expected that fluctuations of the seed power will also induce such phase shifts due to a change of the extracted energy and the heat load in the fiber. Thus, it is important to investigate which one, i.e. the seed- or the pump intensity-noise, has a severer impact on the TMI threshold. Our experiments have shown that the TMI threshold of a fiber amplifier was decreased by increasing the seednoise amplitude. However, contrary to conventional belief, the impact of seed intensity-noise was much weaker than the one of pump intensity-noise. The measurements are in good agreement with our simulations and can be well explained with previous studies about the noise transfer function. The reason for the weaker impact of seed intensity-noise on the TMI threshold is the attenuation of its frequency components below 20 kHz in saturated fiber amplifiers, which includes the frequencies relevant for TMI. Thus, the main trigger for TMI in saturated fiber amplifiers can be considered to be pump intensity-noise. A suppression of this noise below 20 kHz represents a promising way to increase the TMI threshold of fiber laser systems.

Supported by both experimental and simulated results, this contribution demonstrates the heat load distribution in a co-pumped, ytterbium (Yb)-doped fiber amplifier seeded with two different wavelengths can be significantly changed depending on the seed power ratio. Longitudinal temperature measurements in a Yb-doped 10.5 m 20/400 μm fiber confirm a significant shift of the heat load maximum by 3.5 m towards the fiber output when decreasing the seed power ratio from P_1030nm/P_1080nm = 1.7 to 20. In single-tone operation with a seed power of P_1080nm = 3.5 W, the amplifier is limited by the onset of transverse mode instabilities at a power-level of 1950 W. However, dual-tone seeding with a seed power ratio up to P_1030nm/P_1080nm = 10 reduces the TMI-threshold dramatically down to 1050 W. Additionally we show, that the modal instability threshold is very susceptible to 1030 nm seed noise in the frequency regime up to 10 kHz.

In this work we present a novel way to mitigate the effect of transverse mode instability in high-power fiber amplifiers. In this technique a travelling wave is induced in the modal interference pattern by seeding the amplifier with two modes that have slightly different frequencies. The interference pattern thus formed will travel up- or downstream the fiber (depending on the sign of the frequency difference between the modes) with a certain speed (that depends on the absolute value of the frequency difference). If the travelling speed is chosen properly, the thermally-induced index grating will follow the travelling modal interference pattern creating a constant phase shift between these two elements. Such a constant controllable phase shift allows for a stable energy transfer from the higher-order modes to the fundamental mode or viceversa. Thus, this technique can be adjusted in such a way that, at the output of the fiber almost all the energy is concentrated in the fundamental mode, regardless of the excitation conditions. Moreover, this technique represents one of the first examples of the new family of mitigation strategies acting upon the phase shift between the modal interference pattern and the refractive index grating. Additionally, it even exploits the effect of transverse mode instability for gaining control over the beam profile at the output of the amplifier. Therefore, by adjusting the frequency difference between the seed modes, it is possible to force that the beam at the output acquires the shape of the fundamental mode or that of a higher order mode.

The output power of fiber-based single-frequency amplifiers, e.g. for gravitational wave detectors, is typically limited by nonlinear effects (e.g. stimulated Brillouin scattering). In general, to reduce the impact of nonlinearities, the mode area of the fiber core is enlarged. Chirally-coupled-core (3C®) fibers have been specifically designed to enable single-mode operation with a large mode area core. 3C®-fibers consist of a step-index fiber structure, whose signal core is additionally chirally surrounded by one ore more satellite cores. Because of the phase matching and the helical geometry, the higher order modes are pulled out of the signal core, which enables a high-purity modal content in the core. We present a robust and monolithic fiber amplifier based on an ytterbium-doped 3C®-fiber in combination with commercially available standard fibers. For the realization of such a monolithic system, a mode field adapter (MFA) was designed and installed between a standard polarization-maintaining fiber and an active 3C®-fiber for the first time. The MFA was tested regarding the guided modal content by means of a S²-system. Overall, the fiber amplifier achieves a polarization extinction ratio of 17.6 dB and an optical output power of 100.1W in a linearly polarized TEM00-mode. To our knowledge, the fundamental mode content of 98.9% is the highest TEM₀₀-mode content of fiber amplifiers reported at this power level. This work emphasizes the high potential of fiber amplifiers based on 3C®-fibers as laser sources for the next generation of gravitational wave detectors and demonstrates that compact and robust amplifiers can be realized using 3C®-fibers.

We developed a highly efficient double-clad Yb-doped polarization-maintaining fiber to be implemented for small-signal amplification near 0.976 μm. The fiber was designed to have a relatively small mode field diameter compatible with standard step-index single-mode optical fibers. Another feature of the fiber was a small threshold for 0.976 μm signal amplification, which was achieved by a creation of a thin inner cladding (80 μm diameter). The unique design of the fiber allowed us to construct successfully an all-fiber picoseconds mode-locked laser at 0.98 μm for the first time to the best of our knowledge.

The 4F consortium has obtained new results on microstructured optical fibers for the development of Ytterbium active fibers with a large core but also of hollow fibers for the transport of the laser beam. The active fiber core composition aims to ensure good performance in aging and photodarkening. The fiber specifications are a Mode Field Diameter greater than 30 μm, a Polarization Extinction Ratio more than 15 dB, a cladding absorption at 976 nm more than 10 dB/m, bending over a relatively small diameter (about 20cm). The fiber will allow a laser operation exceeding 150 W.

We designed and analyzed a novel optically pumped gas THz fiber laser (OPTFL) based on hollow-core anti-resonant fiber (HC-ARF). The OPTFL filled with methanol vapor and pumped by a continuous-wave tunable CO2 laser. Combining the rate equations and the THz wave transmission theory in HC-ARF, factors that impacting THz output are analyzed. By investigating the inner structure of HC-ARF, a HC-ARF with an intrinsic single-mode transmission and a low confinement loss lower than 0.2 dB/m in OPTFL is proposed for efficiently transmitting THz waves. Theoretical analysis indicate that by appropriately increasing the cavity length, the output power of OPTFL is expected to reach the magnitude of 100 milliwatts with optimal operating conditions. The results provide an available method for highperformance THz laser source.

For space missions, there is a need for fiber lasers of minimum power consumption involving stabilized frequency combs. We exploit the extreme sensitivity of the polarization state of circularly polarized light sent through polarization maintaining (PM) fibers to power and temperature variations. Low power nonlinear transmission is demonstrated by terminating a PM fiber by an appropriately oriented polarizer. The strong correlation between the power sensitivity of the polarization state and the temperature dependence of the birefringence of the PM fiber can be exploited for stabilization the optical length in fiber lasers and interferometers.

We report a high pulse, all-fiber supercontinuum source generated by pumping 2 ns long pulses at 100kHz from an ytterbium-doped fiber amplifier (YDFA) into a 15 m tapered photonics crystal fiber (PCF). The YDFA operates at 1066 nm, and the zero-dispersion wavelength of the PCF is at 1040 nm. The PCF is a 15 µm core at the input, tapered down to 5 µm at the output. The tapering is done such that the core-pitch ratio is maintained along the length of the fiber used. The resulting supercontinuum spans from 450 nm to 2400 nm, with a total pulse energy of more than 7uJ. The supercontinuum covers the visible spectrum making it useful for applications that require substantial pulse energies such as photoacoustic tomography.

In this work, we show that the enhanced backscatter originating from the recently discovered ROGUE (Random Optical Grating by Ultraviolet or ultrafast laser Exposure) increases the performance of random distributed feedback lasers. The inscription of ROGUEs inside regular optical fibers leads to lower power thresholds, and more compact lasers requiring shorter lengths to achieve lasing. These ROGUEs can also be used in half-open cavities, to further decrease the lasing threshold.

In this paper, we report the first (to the best of our knowledge) GaN laser diode, emitting at 445 nm, pumped dysprosium (Dy) doped ZBLAN fiber laser for yellow emission using a simple setup. In our yellow laser experiment, we have used a commercially available Dy-doped ZBLAN fiber, which is originally designed for mid-infrared lasers demonstration and not optimized for visible laser design, as a laser active medium. For yellow (∼576 nm) lasing, we have exploited the ⁴F_9/2 to ⁶H_13/2 laser transition of a Dy ion, which is a quasi four level system. The performance of the yellow laser system is investigated by using two different Dy-doped fiber lengths (0.6 m and 5.95 m). The measured lasing thresholds are 7 mW and 28 mW for 0.6 m and 5.95 m of Dy-doped fiber, respectively. However, the maximum laser slope efficiency with respect to absorbed pump power is only 2.3% for 0.6 m of Dy-doped fiber. The laser slope efficiency decreases to 0.9% and the threshold increases to 28 mW for 5.95 m of Dy-doped fiber, which are result of fiber background loss at the signal wavelength. In addition, we have observed the pump excited state absorption at 445 nm pumping wavelength and estimated the pump ESA cross-section via numerical simulation.

High repetition rate with broadband spectrum optical pulses generated directly from an Yb – doped fiber oscillator were realized. The output spectrum has a 10 – dB spectral width of 106 nm over 1100nm. The average power of the pulses is higher than 100 mW and the repetition rate was 169.43 MHz. To the best of our knowledge, this is the first time the mode – locked Yb – doped fiber laser with the optical spectrum broader than 100 nm, pumped by low pump power has been reported.

Nowadays, the request for femtosecond lasers operating between 1.7 μm and 2 μm is continuously growing for many applications. Mode-locked Holmium- or Thulium-doped fiber lasers based on Saturable Absorber Mirror (SAM) are typically the first approach to generate pulses in this spectral range but this technique suffers from a lack of tunability. Indeed, the operating wavelength is fixed by the SAM and the gain fiber. Another way to reach the 2 μm-spectral range consists to exploit the nonlinear phenomena appearing in optical fibers and in particular the Soliton-Self Frequency Shift (SSFS) effect from an Erbium-fiber laser. Several systems based on this phenomenon allowed the generation of ultrashort pulses at different wavelengths and in different type of fibers (step-index, PCF, …). In this paper, we report on the design of a compact and robust all-Polarization-Maintaining (PM) fiber system entirely based on commercial PM components. This system allows to generate a single femtosecond pulse continuously tunable from 1700 nm to 2050 nm. We also demonstrate that the sub-150 fs pulses are transform-limited over all the spectral range and thanks to an optimized rate conversion close to 50 %, the pulse energy and the peak power can reach the nJclass and the kW-class respectively, which represents a gain of a factor 2 compared to the previous works.

We prove the possibility of generating an amplified spontaneous emission (ASE) source using all fiber laser configuration that uses 1.2-meter single mode (SM) Dysprosium ions (Dy³⁺) doped ZBLAN fiber with core diameter and concentration of 12.5μm and 2000ppm respectively as the gain medium. The ZBLAN fiber is directly fusion spliced to the output of 5-watt ytterbium (Yb) doped fiber laser with the center wavelength of 1094nm. The measured spectrum spans about 600nm starting around 2.75μm to 3.35μm and provides a total power of about 83mW. We demonstrate that our approach is more cost-effective and efficient than other systems previously reported and it opens new windows for wideband tunable fiber laser sources at 3μm spectral region.

We report on the use of OH diffusion barrier coatings to suppress fiber tip degradation in high-power 3 μm fiber lasers. To this extent, silicon nitride thin-films are sputtered on the endface of a 20 W erbium-doped fluoride fiber laser at 2.83 μm. By monitoring the temperature evolution of the output fiber tip over a 24 hour time period, we validate the effectiveness of silicon nitride thin films in mitigating fiber tip degradation. These results, along with previous demonstrations, underline the potential of 3 μm fiber lasers to reach 100 W of output power in the near future.

We present preliminary results relating to laser emission at 3:16 µm from a Dy fiber laser that is diode pumped at 800 nm. To allow strong diode pump absorption and to capture improved quantum efficiencies resulting from cross relaxation, the Dy:ZBLAN fiber was co-doped with Tm ions in a 10 to 1 concentration ratio to Dy. A resonant energy transfer from Tm to Dy provides an inversion on the ⁶H_13/2 to ⁶H_15/2 transition. Maximum output power of 5.5 mW at a slope efficiency of 1.3 % was produced from a highly non-optimal arrangement. System performance is bench marked against well established resonant pumping of the Dy upper state. Measurement of fluorescence lifetimes of both dopants allows for qualitative assessment of the energy transfer efficiency. A potentially detrimental energy transfer mechanism is identified and discussed.

We demonstrate a high repetition rate (3 MHz) Mid-IR supercontinuum (SC) source spanning whose spectrum spanning 1000-4200 nm using a cascade of different nonlinear fibers. Multi-tone absorption spectroscopy measurements are subsequently carried out using this source and a scanning spectrometer probing various concentrations and a combination of different analytes. We further explore a novel algorithm for rearranging the absorption in the IR-region and the NIR region for three-dimensional modeling. We show this method of analyzing the data is robust, that is being able to predict newly added samples of slightly different nature without having to the recalibrate the model.

Mid-infrared (mid-IR) fiber lasers that are based on dysprosium (Dy) as the active laser ion provide emission in the wavelength range between 2.6–3.4 μm and can thus bridge the spectral gap between holmium (Ho) and erbium (Er) based mid-IR lasers. Another distinct feature is the wide choice of pump wavelengths (1.1 μm, 1.3 μm, 1.7 μm, and 2.8 μm) that can be used. To date, pump wavelengths shorter than 1.1 μm have not been reported and all demonstrated pump wavelengths apart from in-band pumping suffer from pump excited state absorption (ESA). In this paper, we report new excitation wavelengths, 0.8 μm and 0.9 μm, for Dy-doped mid-IR fiber lasers. We have measured 18.5% and 23.7% slope efficiency (relative to launched pump power) for 0.8 μm and 0.9 μm pumping wavelengths, respectively. By comparing the residual pump power of experimental and numerical simulation data of a 0.5 m Dy-doped fiber, we have found that these new excitation wavelengths are free from pump ESA. Moreover, the high power laser diodes are commercially available at these new excitation wavelengths; therefore, the realization of a diode-pumped Dy-doped mid-infrared fiber laser might become feasible in the near future.

We demonstrate 104 Watts of in-band output power from a cascaded Raman fiber laser operating around 1.7 μm with a spectral purity of over 90% operating in both continuous wave and pulsed regimes. Through the use of a filter fiber with its cutoff wavelength designed between the 6th and 7th Stokes orders, output above 1.8 μm is suppressed below threshold values. In the pulsed regime the laser produces output pulses ranging from 11.5 mJ pulses with 100 μs pulse width to 10 J pulses with 100 ms pulse width.

We report the first fully monolithic, polarized, fiber amplifier based on resonantly pumped Holmium doped optical fibers at wavelength longer than 2050 nm. With commercially available fibers, it was possible to achieve Watt level output powers at 2088 nm.

We report gamma radiation influence on an active Er3+ doped fiber amplifier. Hydrogenation of active fibers under special condition allowed for a radiation hardness increase by an order of magnitude. Stability and longevity of hydrogenation effects are investigated.

A short-pulse Yb-doped fiber laser is reported to yield more than 50 W of UV light after single-pass second- and thirdharmonic generation using LBO crystals. The NIR laser generates ns-mJ pulses with average power exceeding 200 W after multipart fiber amplifier stages. The polarization-maintaining LMA tapered fiber used as the last amplifier stage features near diffraction-limited output, narrow linewidth and high polarization extinction ratio. Conversion efficiency from NIR to UV is found near 30%, with output pulse energy as high as 260 μJ for an oscillator pulse repetition frequency of 200 kHz. Generated UV light is seen close to diffraction limit, with M² factor measured < 1.3 both along and perpendicular to walk-off axis.

We present a single-mode narrow band linear-polarized picosecond green fiber source delivered up to 146.4 kW of peak power. The laser architecture is composed of frequency-doubled all-fiber MOPA system operating at 1064 nm. The commercially available gain-switched semiconductor laser diode was used as a seed source delivered 77 ps pulses with the repetition rate between 100 kHz - 80 MHz. Two stages of pre-amplifiers based on the single-mode Yb-doped fibers were designed to amplify microwatt pulsed signal up to milliwatt level. A high-power amplification cascade comprised a double-clad polarization-maintaining tapered Yb³⁺-doped fiber as a gain medium. The frequency doubling was realized in a single-pass scheme with LBO crystal. The MOPA design with the active tapered fiber enabled to amplify effectively a narrowband picosecond IR radiation with relatively small spectral broadening. We obtained stable laser radiation with 77 ps pulses at repetition rate of 1 MHz, 290 pm spectral bandwidth with a central wavelength of 532 nm, the average power of 12 W corresponding to 12 μJ of pulse energy and 146.4 kW of peak power. The overall efficiency of secondharmonic generation reached 37 % in a single pass scheme. The obtained results showed advantages of the MOPA system based on a tapered amplifier in comparison with already published picosecond green laser systems exploited standard amplifiers based on cylindrical fixed-core fibers. The single-mode green laser with high peak power and narrow line are in high demand for a wide range of Raman spectroscopy applications.

High power diffraction-limited 1064 nm fiber lasers operating in the nanosecond regime can be used for long-range LIDAR and micromachining applications. Peak power is limited by non-linearities, there is therefore an interest to develop fibers exhibiting a very large mode field effective area. New fibers are being developed in the frame of the 4F consortium ("French laser Fibers for Factories of the Future") to fulfill this need. We report on results obtained with a new 39 μm core diameter polarization maintaining ytterbium doped fiber that has been manufactured using the powder sintering technology. It features a large cladding absorption close to 20 dB/m at 976 nm (small signal) and a mode field diameter close to 32 μm. We built a pulsed MOPA. The preamplifier generates 2.5 ns pulses at 1064 nm with 8.5 W average power at 1 MHz pulse repetition frequency. The power amplifier is based on the 39 μm core fiber with 215/230μm hexagonal cladding counterpumped at 976 nm. It features 72 % slope efficiency delivering 72.2 W average power at a pulse repetition rate of 1 MHz. An end-cap was spliced to the fiber output to increase the damage threshold. At 100 kHz a peak power of 351 kW was measured for an average power of 59.9 W. The efficiency is then 70 %. We also studied the influence of the bending radius on the slope efficiency. We do not observe any slope efficiency reduction down to 25 cm bending diameter. It decreased to 68 % for the 20 cm bending diameter. The laser shows a quasisinglemode output beam with a good quality factor M² of 1.2.

In this paper, we demonstrate possibility of simultaneous achievement of high peak and high average power in picosecond pulses using a monolithic amplifier based on a long Yb-doped tapered fiber. Due to a very high pump absorption (~ 25 dB/m at 976 nm) in the realized 2.4 m long tapered fiber most of the pump is absorbed near the thick tapered fiber end and a very small fraction of pump power reaches thin fiber end. As a result, signal passes through the thin part of the tapered fiber without an amplification and exhibits fast growth only near the output tapered fiber end, where a mode field diameter is large (35 μm at 1064 nm for 46 μm output core diameter), so that pulses can be amplified to a high peak power. Moreover, only a negligible fraction of pump radiation leaks at the conic part of the tapered fiber, because its most part was absorbed in the thick tapered fiber part. Thus a safe operation without polymer burning at a leakage point is possible up to a very high pump power. The developed tapered fiber was used in a final amplification stage of the all-fiber pulsed laser system, which allowed us to amplify 8.3 ps pulses with repetition rate of 18.4 MHz and central wavelength of 1064 nm to 150 W of average power and 0.92 MW peak power. The average power level was limited only by available pump power (230 W): no signs of transverse mode instability effects were observed.

The principles of coherent beam combining for multiple fiber laser amplifiers is presented and discussed, with a focus on the optical phased array (OPA) beam combining technique. Several unique properties of OPA beam combining such as real time control over beam scanning, beam shape, focal plane position and intensity modulation with a MHz bandwidth open up several important new degrees of freedom for materials processing applications, enabling higher throughput, more efficient operation and advanced processing techniques. A 16kW dynamic laser beam at 1064nm based on coherent combination of 32 parallel ytterbium doped fiber amplifiers is presented, together with some example beam profiles.

Single-mode fiber lasers with excellent beam quality and several-kilowatts output power are expected to realize both extraordinary processing speed and high aspect ratio in the material processing field. In this paper, we report an 8-kW output single-stage Yb-doped fiber laser with a BPP of 0.50 mm-mrad. The laser has a delivery fiber with a length of 3 m. To realize an 8-kW single-mode output without excess a stimulated Raman scattering threshold, fibers with a considerably large effective core area are employed. An ytterbium doped fiber is directly pumped by newly developed high-power laser diode modules with a total available pump power over 10 kW through a bi-directional pumping scheme. The laser has a high slope efficiency of 81%. The power of the SRS light around 1120 nm was 22 dB smaller than the fundamental laser power at 1070 nm. To the best of our knowledge, it is the smallest BPP with more than 8 kW output power pumped by an end-pumping regime. We believe the laser will contributes laser processing using a galvano scanner for high speed and high aspect ratio welding.

In this work we demonstrate, a high brightness, high transmission Pump Signal Combiner (PSC) operating at 2.1 kW for 10 minutes mounted on a metal plate without running cooling water (Passively Cooled). The 6+1 to 1 PSC tested in this work has six 247 μm cladding diameter 0.22NA pump pigtails, a 10/125um signal pigtail and a 20/400um double clad output fiber. The combiners achieve a high pump transmission efficiency of 98%. During the passively cooled highpower test conducted at a power of 2.1 kW the maximum temperature recorded on the surface of the package was at a temperature of 38.5°C for 10min of power aging.

VAD technology for laser fibers are offering the extremely large rare-earth doped core, which makes it possible to improve the productivity and reproducibility of laser fibers. At the previous report, Yb:Ce co-doped fibers have shown relatively large background loss and the issues of surface crystallization. In this report, the additional process development for the reduction of the loss and the efficiency improvement has been successful. Based in that, Yb:Ce codoped aluminosilicate PM fiber has been fabricated and it used for power amplification of narrow linewidth source. The slope efficiency was 72.8% w.r.t the absorbed pump power and the output power has been reached to 124W at the initial experiment. The further power scaling will be performed and SBS characterization will be reviewed.

Within the EKOLAS consortium, which is part of the BMBF-funded EffiLAS (Efficient high-performance laser beam sources) research initiative, we are developing fiber Bragg gratings (FBG) directly written into multimode fibers. Fiber lasers are an established beam source for high-power materials processing due to their high efficiency and high average output power at high beam quality. By using FBG as fiber-integrated output mirrors, which is state-of-the-art in singlemode fiber lasers, we aim to reduce the complexity and increase robustness and reliability of multimode fiber resonators. Therefore, we are investigating the use of FBG as outcoupling mirrors in multimode high-power multimode fiber lasers. As a first step, we directly write an FBG into an active extra-large mode area (XLMA) fiber with <100 μm core and use the FBG as low reflective outcoupling mirror for the fiber resonator with simultaneous frequency stabilization. The setup delivers an output power of more than 800 W at 1077 nm. The output power of the system was limited by the pump laser setup and not by the FBG or its temperature. The FBG is passively cooled and the measured temperature of the fiber at the grating is below 130 °C at 800 W output power. As the next step, we set up an active XLMA-fiber (core <100 μm) with an FBG as outcoupling mirror into a laser resonator with water cooling of the resonator fiber and optimized pump coupling. This setup delivers an output power of more than 8 kW at 1077 nm without failure of the FBG.

Nonlinear effects and transverse mode instabilities (TMI) limit power scaling of single-mode fiber lasers. To overcome these limitations not only the fiber design but also laser relevant properties of the actively doped material itself need to be optimized. By being able to fabricate Yb-doped fibers for high power applications in-house, we have direct access to laser relevant material parameters.We fabricated fibers using three different co-doping systems, namely Yb:Al:P, Yb:Al:F, and Yb:Al:F:Ce. Afterwards we characterized and compared their laser relevant properties. All three co-doping systems showed nearly identical background losses and absorption cross-sections. In contrast, we found that the PD losses and the factor between PD losses @633nm and the laser wavelength range (1μm) to be significantly different. The retrieved characterization results were implemented into our simulations tool in order to improve the reliability of predictions. Finally, we characterized the fibers in kW-amplifier setups according to their power scaling limits, especially the TMI threshold. This cycle of fiber fabrication, characterization, and simulation enabled us to identify the impact of individual fiber parameters on the TMI threshold. We demonstrated that the impact of PD loss leads to a reductions of the TMI threshold for Yb:Al:F co-doping system of 13% to 23% (depending on the Yb-concentration). The PD loss for the two other systems was proved to be significantly lower and was found to have no impact on the TMI threshold. We experimentally proved that your in-house Yb:Al:P and Yb:Al:F:Ce fibers performed like PD-free fibers.

It is known that eigenvalues of Zakharov-Shabat problem can be used to encode a signal in soliton communication lines. We propose to use dispersion oscillation fiber as a new versalite tool to control the eigenvalues. A fiber with sine-wave variation of the core diameter can be used to manage both real and imaginary parts of the eigenvalues. Change of real part of the eigenvalues results in a splitting of an optical breather into two distinct pulses propagating with the different group velocities. Change of imaginary parts of the eigenvalues allows to realize a reverse process of merge of two solitons into high-intensity pulse. The splitting of optical breather and merge of solitons can be obtained even under the strong effect of stimulated Raman scattering. We believe tools and techniques based on use of dispersion oscillating fiber will grant unprecedented control over soliton eigenstates.

We report several techniques affecting line-width and long-term stability of single longitudinal mode (SLM) random fiber laser (RFL) based on Rayleigh backscattering in a standard single mode fiber as distributed mirrors, and a semiconductor optical amplifier (SOA) as the gain medium. Three parameters were investigated; single mode fiber (SMF) length, the optical filter bandwidth, and the type of the cavity reflecting mirror. Line-width of the laser was measured at three values of SMF length; 100 m, 1 km, and 2 km. Impact of the optical filter bandwidth on the laser line-width was studied. Finally, long-term stability was characterized for two types of reflecting mirrors; a silver coated mirror, and a Faraday rotating mirror (FRM).

We report a passive mode-locked fiber laser (PMLFL) in a novel configuration to generate a single soliton with ultra-low repetition rate. The configuration includes a Faraday mirror after the first half of the cavity length to counteract the nonlinear polarization rotation effects. The total cavity length is 428 m including a 400-m SMF-28 fiber which was twisted to cancel the linear birefringence. The strict polarization control establishes a relation between the regimes of generation and the polarization state of the pulses propagating in the cavity. By properly adjusting the initial polarization state, we observed three different emission regimes, the single soliton regime (SR), conventional noise-like pulses (NLP) and noise-like square-waveform pulse (NLSWP). In the SR, we obtain a 2.9 ps pulse duration centered at 1558.7 nm with a 467.2 kHz repetition rate.

We demonstrate an all-fiber picosecond laser with a repetition rate of 1.2 GHz. The seed source is a passively modelocked Yb-doped fiber laser. The pulse width of the seed laser is 4.2 ps with a repetition rate of 19.2 MHz, and the seed exhibits a spectral bandwidth of 3.6 nm at the center wavelength of 1064.1 nm. Due to the length of the cavity, it is difficult to increase the repetition rate of the fiber laser. To solve the above problem, we provide a high-repetition-rate pulsed laser modulator comprising: five 2 × 2 type, splitting ratio 50:50 couplers, two 1 × 2 type, splitting ratio 50:50 couplers, to increase the repetition rate of the seed laser to 1.2 GHz. Meanwhile, the novel combination of the devices can realize a low-loss mutual coupling of laser pulses.

High peak power mode locked fiber lasers are effective tools for many applications like optical metrology, biomedical imaging, micromachining and so on. All-fiber architecture and high pulse energy mode locked fiber lasers are very attractive to these applications due to their compactness and robustness. In recent years, an attractive mode-locking scheme based on Mamyshev regenerator was demonstrated to realize high pulse energy mode locked pulse output leading to breakthroughs in the mode locked fiber laser performance. We experimentally demonstrate an all-fiber linear Mamyshev regenerator operating at 1550 nm. Self-phase modulation and off-set spectral filtering provide high peak power pulse pick-up effect in the laser cavity and impel the laser to operate in the mode locking regime. With properly setting of the parameters, the all-fiber Mamyshev regenerator can achieve self-starting of the mode locking easily. No external pulse seeds or auxiliary starting arms are needed for the self-starting of the mode locking, which makes the laser very convenient to operate. Pulses with maximum energy of ~18 nJ and pulse width of 230 fs were achieved. The pulse width almost keeps unchanged with increasing in the pump power and the output power increases almost linearly with the pump power. The spectra from the two outputs with different pump powers were experimentally investigated as well. This high pulse energy Mamyshev regenerator can be used as a high quality and cost-effective laser source for many applications.

A compact high-power femtosecond (~640 fs) GHz intra-burst repetition rate all-in-fiber CPA system operating at 3.26 GHz intra-burst and 200 kHz burst repetition rate regime with two configurations of large mode area (LMA and PCF) cladding-pumped Yb-doped fiber power amplifiers were presented in this work. Significantly high average power levels of 6 W (LMA fiber power amplifier) and <20 W (PCF power amplifier) were achieved which corresponded to a maximum energy of 30 μJ and <100 μJ per burst respectively. Two burst shaping layouts were introduced in this experimental setup obtaining desired burst shape using one or two acousto-optic modulators, pulse repetition rate multiplier based on a cascaded 2×2 fiber coupler sequence with a splitting ratio of 50/50 and controlled using arbitrary waveform generator. High power all-in-fiber CPA system with LMA fiber power amplifier operating at GHz burst regime was compared to the system operating at MHz pulse repetition rate regime which allowed to achieve 1.5 μJ energy pulses of good pulse quality at the output of the laser system at a repetition rate of 4 MHz.