This PDF file contains the front matter associated with SPIE Proceedings Volume 12865, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

A first theoretical study of Transverse Mode Instability (TMI) in oscillators based on a Stimulated Thermal Rayleigh Scattering (STRS) model is conducted. Higher Order Mode (HOM) lasing is found to happen at high powers. Further Fundamental Mode (FM) growth is limited once HOM lasing takes place, with further increase of pump power mostly going to HOM growth, a fundamentally different phenomenon from that in fiber amplifiers. TMI thresholds defined as when the HOM lasing condition is met is studied. The results are consistent with the measured TMI thresholds and their dependence on pumping configurations and pump wavelengths.

We experimentally investigate the impact of the modal content of the seed beam on the Transverse Mode Instability (TMI) threshold in high-power fiber amplifiers. Theoretical models have predicted that the TMI threshold should decrease with Higher-Order Mode (HOM) content in the seed. However no systematic experiment has been done so far to probe such a statement. In this work we have built a system comprising a spatial mode multiplexer that allows manipulating the power content of the Fundamental Mode (FM) and the HOM in the seed beam. Such beams, with the same power but different modal contents, were coupled into a multimode active rod-type fiber and the evolution of the TMI threshold was studied.

We investigate Transverse Mode Instability (TMI) in an in-house large-mode-area Polarization Maintaining (PM) fiber amplifier. The TMI threshold was systematically measured at different linear polarization input angles with respect to the slow axis of the fiber. At a polarization input angle of 50°, the TMI threshold increased by more than 100% with respect to the threshold of the slow axis and 60% with respect to one of the fast axis. Furthermore, the temporal characteristic of TMI were studied in detail at different polarization input angles but fixed power of 290W, which was above the TMI threshold of the slow and fast axis. This analysis revealed the three different temporal regimes associated to TMI: chaotic fluctuations in the slow-axis, stable at 50°, and periodic fluctuations in the fast axis. These new results provide with valuable insights into the effect of TMI, especially concerning PM fibers, as well as with a relatively simple way of mitigating TMI in these fibers.

Multicore Fibers (MCF), the integrated alternative to spatially separated amplification in fibers, are a promising laser architecture thanks to their capability to deliver high pulse energy and average power in a compact format. The introduction of 7x7 MCF laser systems represents a significant advancement of this technology, bringing us closer to realizing multi-kilowatt and J-class fiber laser amplifiers. In this context, Ytterbium-doped MCFs have already demonstrated power scalability proportional to the number of amplifying cores. Using 4x4 MCFs already showcases high pulse energy and high average power and, if coherently combined, offers nearly diffraction-limited beam quality. This work complements the Coherent Beam Combination (CBC) testbed by Incoherent Beam Combination (IBC). IBC emerges as a straightforward and robust solution, providing the opportunity to achieve performance capabilities equivalent to Multimode Fibers (MMFs) while demonstrating a better beam quality. These IBC systems are appealing for various applications, including pumping solid-state lasers and incoherent frequency conversion towards shorter wavelengths, e.g., to the Extreme Ultraviolet (EUV) or soft-Xray through laser-produced plasma sources.

In this work, we discuss a novel architecture for an all-fiber linearly polarized counter-pumped linear amplifier utilizing an 85 μm core diameter rod fiber. Signal light is launched directly into the core region of the rod fiber with high coupling efficiency via a monolithic Mode-Field Adapter (MFA) splice. The free-space coupling optics and alignment for counter-pumping the amplifier are contained and fixed in a small, ruggedized packaging. Both the monolithic MFA splice and fixed free-space optics lock the signal and pump coupling efficiencies, allowing the device to be handled as if it were entirely monolithic. Over 18 dB of gain was achieved during power testing as a single-stage linear amplifier. Methodologies for advancing this architecture into a multistage linear amplifier to achieve higher peak and average powers are discussed. Simulations and models are used to define the signal power, pump power, gain fiber geometries, and gain stage lengths required to achieve 1 mJ pulse energies in 1 ns pulse durations, as well as predict the resulting B-integral, Amplified Spontaneous Emission (ASE) levels, and unabsorbed pump power. The feasibility of realizing such an amplifier architecture is then discussed as a conclusion.

We report pulse energy of 1.6 mJ at 1 kHz repetition rate from a single-frequency fiber amplifier with high energy gain of 19 dB at 1560 nm, using a 2.4 m long, Very-Large Mode Area (VLMA) Polarization-Maintaining (PM) erbium-doped fiber with 50 μm core diameter and absorption of 50 dB/m, core-pumped with a CW, 1480 nm Raman fiber laser. The in-pulse power fraction was 85%. The FWHM of the output pulses was 1.48 μs at 1 kHz repetition rate. At 150 Hz repetition rate, of interest in coherent wind LIDAR systems, we report 1 mJ, with energy gain of 17 dB, in-pulse power fraction of 72%, and FWHM of 1.24 μs. Stimulated Brillouin scattering limited the maximum energy reported here. The output of the fiber amplifier was diffraction limited, with mean beam quality factor, M², of 1.04. The polarization extinction ratio exceeded 20 dB.

The Laser Interferometer Space Antenna (LISA) mission is a collaborative consortium led by the European Space Agency (ESA) in cooperation with the National Aeronautics and Space Administration (NASA). LISA’s goal is to capture gravitational wave events, such as spacetime distortions caused by massive orbiting bodies. Multiple low noise, single frequency Master Oscillator Power Amplifier (MOPA) lasers with a polarization-maintaining Yb-fiber amplifier will serve as the light sources for the LISA observatory. The NASA Goddard Space Flight Center (GSFC) is currently developing the laser transmitters for this project. The LISA lasers will carry Radio Frequency (RF) sidebands for exchanging reference RF clock information between spacecrafts. Excess phase noise, especially carrier-sideband differential phase noise added to the modulation sideband by the phase modulator and the fiber amplifier, becomes a concern. In the gravitational wave frequency ranges, leading sources of this RF “differential” phase noise can be both intrinsic and extrinsic. Examples of intrinsic sources include thermal load on the active fiber, thermo-mechanical induced birefringence, and nonlinearities in the various passive and active fibers. Extrinsic sources can include external temperature fluctuations and infrasonic mechanical strain or vibrations. These noise spectra usually include both white and pink (1/f) thermal noise contributions. This work investigates the physical origins and impact of these sources of RF phase noise, and their characteristic spectra and management will be discussed.

We present an efficient way to remove unwanted Amplified Stimulated Emission (ASE) in high-power fiber lasers and amplifiers using intracavity Chirped Tilted Fiber Bragg Grating (CTFBG) filters. The grating is written with tilted fringes so that the unwanted ASE is reflected into the fiber cladding where it is no longer amplified. Depending on the desired emission wavelength and active fiber, one or several filters are spliced within the active fiber to suppress ASE before it reaches a detrimental power. Numerical simulations clearly show that adding the filters allows amplification in configurations that would just be impossible due to the onset of ASE. The filter bandwidth and extinction, and the maximum allowed active fiber length between each filter are also computed depending on the core/cladding diameter ratio of the active fiber used and the targeted emission wavelength. As an example, a fiber laser at 1018 nm is assembled in a 20/400μm core/cladding diameter ytterbium fiber that is cladding pumped at 976 nm. Two CTFBGs with 20 dB attenuation from 1025 nm to 1070 nm are spliced within the 6-meter-long ytterbium fiber. 432 W of laser emission at 1018 nm is efficiently achieved at 77% slope efficiency with respect to the absorbed pump power. The extinction between the 1018 nm signal and the ASE is greater than 50 dB. Removing the ASE filters from the cavity clearly leads to only self-pulsation of the ASE between 1030 nm and 1050 nm, no generation of 1018 nm light was possible. The measured thermal slope of the filters shows scalability above the kW level. Demonstration at 1908 nm with a 25/400 core/cladding diameter thulium doped fiber is also done. Tests were done to inscribe the CTFBG directly in an ytterbium fiber for simpler implementation and avoid additional splicing.

Advancements in laser material processing, particularly in EV battery production, require innovative solutions. Dynamic beam control of fiber lasers has emerged as a promising technology to meet these needs. This contribution examines the challenges in EV battery production and the solutions provided by these lasers. Through dynamic beam control, fiber guided lasers enable precise and efficient welding of copper and aluminum components. Adjustable beam parameters such as intensity, shape, and size enhance precision, productivity, and material utilization. These capabilities of dynamic beam-controlled lasers drive innovation in manufacturing processes for the EV industry and beyond.

We introduce a new approach for the generation of high-power multi-spot fiber lasers, employing square core fiber segments to enhance the efficiency of material processing. Utilizing an all-fiber strategy with Single Mode Fiber– Square Core Fiber (SMF-SCF) structures, our approach exemplifies notable progress in beam shaping technologies. Through laser drilling experiments on metals, the effectiveness of this technique is demonstrated, promising diverse applications in advanced manufacturing and industrial processes.

We experimentally demonstrate single frequency pulsed amplification at 1550 nm using an erbium and ytterbium co-doped multimode fiber. At a repetition rate of 10 kHz, 3.5 kW of peak power with 15 dB gain were generated with a pulse duration of 200 ns. By adjusting the seed wavefront using a spatial light modulator, the output can be shaped to a focused spot while maintaining a gain exceeding 10 dB. Further power scaling is anticipated, and our latest results will be presented.

We report the performance of an LMA Yb-doped fiber, designed for increasing the transverse mode instability threshold and minimizing nonlinear effects in multi-kilowatt class fiber lasers, by reducing the thermo-optic coefficient of the fiber core, compared with that of standard aluminophosphosilicate Yb-doped fibers. A TMI-free 5.2 kW single-mode output power from a Yb 20/400 fiber with a 17.5 μm mode-field diameter was achieved in a broad bandwidth, co-pumped amplifier with 78% optical-to-optical efficiency, while a 4 kW signal output was attained in a 26 GHz linewidth amplifier. Negligible photodarkening loss was observed during 150 hour laser operation at 2 kW.

We report a novel approach to linewidth broadening of the seed laser spectrum for the suppression of Stimulated Brillouin Scattering (SBS) in high power fiber lasers. The seed laser linewidth is broadened through frequency modulation induced by current modulation of a Distributed-Feedback (DFB) laser, while the amplitude modulation is suppressed through a similar, out-of-phase current modulation of a Semiconductor Optical Amplifier (SOA). This approach produces an ideal, uniformly distributed seed laser spectrum for efficient suppression of SBS. SBS suppression is demonstrated on a 1kW fiber amplifier with 95% of output power within a 5.5GHz linewidth (5.7GHz FWHM).

This contribution presents temperature-dependent and site-selective spectroscopy measurements of Yb, Al, F co-doped silica. White light absorption and fluorescence measurements using a multimode 915 nm diode for excitation were both made over the range 77 K to 420 K. Low temperature measurements allow determination of the Stark levels. The high temperature measurements allow quantification of how the laser cross-sections vary with temperature over intervals applicable to high power laser operation. Between room temperature and 420 K, the cross-sections for some spectral regions change by more than 10%, whereas other regions are essentially unchanged over the same temperature range.

A manufacturing chain that revolutionizes the production of fibers with highly precise refractive index matching is presented. By integrating iterative refractive index adjustments using reference fibers, a remarkable improvement in homogeneity, reaching 1E-5, was achieved. Conventional methods, such as the MCVD process, are limited to approximately 1E-4. This enhanced precision was demonstrated through meticulous measurements of core materials and validated in the final fiber. Importantly, the presented manufacturing method considers that the refractive index of fused silica is not a fixed property of the material. It depends on the thermal treatment history and the exact conditions during fiber drawing. This breakthrough manufacturing process ensures the reproducible manufacturing of fibers with fundamental mode guiding properties and mode field diameters exceeding 100 μm. These specialized fibers play a pivotal role in scaling the power of fiber lasers for ultrashort pulse applications.

An empirical TMI threshold formula is derived based on a recently developed model and used to analyze the power-scaling performance of ytterbium-doped silica glass and YAG (Y₃Al₅O₁₂) and Lutetia (Lu₂O₃) single-crystalline fiber amplifiers. Overall, the single-crystalline fiber lasers are found to scale potentially to higher average powers mostly due to their higher thermal conductivities compared to silica glass. This work serves as a useful extension to earlier works and shines significant new light on optimal fiber and amplifier designs for maximum average output power with TMI considered.

We study a phenomenon related to TMI in PM fiber amplifiers: polarization instability. Hereby the polarization of the Fundamental Mode (FM) and of the Higher Order Mode (HOM) in a PM fiber fluctuate during TMI, but the HOM shows significantly stronger polarization changes than the FM. In this work, we will explain the reasons for this behavior and also show that these instabilities, or at least the increase of the sensitivity of the polarization of the HOM to external perturbations, occur even below the TMI threshold.

We achieved the first demonstration of a wavelength-tunable mode-locked fiber laser in the L-band using all-Polarization-Maintaining (all-PM) Nonlinear Polarization Rotation (NPR). The all-PM configured laser features excellent repeatability and reliability. By increasing the pump power from 82.5 mW to 135 mW, a center wavelength-tuning from 1576.2 nm to 1592.2 nm is obtained. This non-mechanical tuning mechanism opens new possibilities for L-band wavelength-tunable lasers and their applications.

Integrated quantum clocks exemplify ultracold-atom-based quantum sensors that rely on lasers as a crucial component. Precise control over the quantum states of ions and atoms used in such devices necessitates lasers with narrow linewidths, high spectral stability, and minimal phase noise. To transfer the absolute spectral characteristics to the cooling and trapping lasers, frequency combs come into play. A reduction of the intrinsic linewidths of frequency combs below a few kHz without need of locking to an optical stabilization cavity would simplify quantum clock experiments significantly. Frequency combs based on mode-locked Er:fiber oscillators are state-of-the-art systems exhibiting several advantages over solid-state lasers like compactness, alignment-free operation and robustness against environmental influences. By employing supercontinuum generation, amplification stages and nonlinear conversion processes, the wavelength range of fiber frequency combs can be extended from 420 to more than 2000 nm. Fiber frequency combs typically have comb lines with an intrinsic optical linewidth in the range of several tens of kilohertz. The broadening of the linewidth is attributed to factors such as pump-induced noise, sensitivity on environmental effects as well as on quantum noise effects. In our recent work we have demonstrated frequency combs exhibiting exceptionally low phase noise resulting in comb linewidths as low as 700 Hz. In this work we employ this technique aiming intrinsically narrow linewidths at wavelengths used in an integrated quantum clock experiment based on Strontium atoms (813 nm, 689 nm).

Our research team has achieved a significant milestone by generating pulses with sub-20-attosecond (as) timing jitter from a 200-MHz all-Polarization-Maintaining (PM) erbium-doped (Er:) Nonlinear Amplifying Loop Mirror (NALM) fiber laser. Accurate measurement of these temporal fluctuations was conducted using the Balanced Optical cross-Correlation (BOC) technique. Through comprehensive investigation, we identified the critical parameters responsible for timing jitter, including dispersion and pump power, and validated their impact. The fine-tuning of the contributing factors allowed us to demonstrate an exceptionally low integrated timing jitter of only 15.59 attoseconds, integrated from 10 kHz to 10 MHz. This accomplishment stands as the lowest value ever documented for any free-running mode-locked fiber lasers that are erbium-doped.

There are limited fiber-based single-mode laser sources with wavelength emissions in the visible and near infrared range. Nonlinear conversion through four-wave mixing in photonic crystal fibers allows for the generation of new wavelengths far from a pump wavelength. Utilizing an all-fiber spliced configuration, we convert 1064 nm light into a W-level signal in the 750 nm – 820 nm spectral region. Out of our custom photonic crystal fiber, we demonstrate 11.3 watts in the signal band, with M² ⪅ 1.15, at an optical-optical conversion efficiency of 37.2%.

Cascaded Raman Fiber Lasers (CRFL) are a versatile source for achieving power in Near-Infrared (NIR) wave-lengths, spanning from 1 to 1.9 μm. This is due to transfer of power from the pump to successive Stokes using cascaded Raman conversion. Frequency doubling the output of such widely tunable CRFLs can generate tunable visible lasers from green to red and beyond. But conventional CRFLs have broad linewidths due to the broad Raman gain, which limits the power and efficiency of nonlinear conversion to visible wavelengths. Only a fraction of NIR power is converted to visible, so linewidth control of the generated Stokes is desired. To address this issue, a custom feedback mechanism is used that combines broadband feedback at all wavelengths with filtered feedback at a desired wavelength. This mechanism allows for linewidth control of CRFLs up to the sixth order of Stokes shift and can potentially be extended further till approximately 2 μm, limited by loss and Zero Dispersion Wavelength (ZDWL) in Raman fiber. The result is significantly reduced linewidths from approximately 4-5 nm to ⪅1 nm along with multi-watt class output, as well as fine tuning of wavelength within the Stokes band using a fixed wavelength pump. As proof of concept, over 100 mW of visible power at 557 nm is generated through frequency doubling of the first Stokes along with wavelength tunability.

We demonstrate a low relative intensity noise (RIN) (⪅-125dBc/Hz from 1 MHz to 10MHz), Continuous Wave (CW) Supercontinuum (SC) generation in standard telecom fiber by using a low intensity noise 1064nm Ytterbium (Yb) doped fiber amplifier as pump source. The generated SC has ⪆8dB of RIN improvement (over a bandwidth of 300nm from 1400nm to 1700nm) when compared to the RIN of SC generated by a conventional Fiber Bragg Grating (FBG) cavity based Yb fiber pump laser. Integrated Root Mean Square (RMS) RIN of 0.16 % (from 1MHz to 10MHz) was better than the low noise CW supercontinua reported so far.

We present a novel photonic Crystal Fiber (PCF), designed for degenerate four-wave mixing (FWM), with a Yb-doped core to amplify the FWM pump light via stimulated emission. Using a 1030 nm Q-switched microchip laser as the FWM pump, the generation of anti-Stokes light at 691 nm was enhanced by using a 976 nm CW laser diode to create a population inversion in the Yb-doped core of the PCF, which amplifies the 1030 nm pulses. For a 1030 nm incident average power of 15 mW (4 kW peak power), the 691 nm anti-Stokes power generated increased from 0 to 1.15 mW when the incident 976 nm power was increased from 0 to 287 mW. FWM was not observed for this 1030 nm input power level in a similar length of a conventional PCF with the same phase-matching properties. Hence, we demonstrate that amplification of the FWM pump pulse through stimulated emission boosts the generated anti-Stokes power, providing a promising route to increasing the pump to anti-Stokes conversion efficiency beyond what is possible with non-rare-earth-doped FWM PCFs.

We report the highest average Stokes power using a single frequency methane filled Hollow Core Fiber (HCF) Raman amplifier to the best of our knowledge. HCFs guide light within their core and have high thresholds for detrimental nonlinearities. Gas filled HCF amplifiers use the long interaction lengths of the HCF to lase in hard-to-access wavelength bands with narrow linewidths. Results were obtained using a methane or deuterium filled HCF to convert 1.06 μm, nanosecond-scale, 0.52 mJ pulses to ≈1.55 μm. Optical-to-optical efficiency decreased at high pressures which analysis indicates is due to secondary Raman shifts. Measurements and simulations relating to the power scaling effort of the methane or deuterium filled HCF Raman amplifiers are presented.

Thanks to a high atmospheric transmission, 2 μm fiber lasers offer unique benefits for both civil and military applications, such as LIDAR, laser weapons or optical countermeasures. All-fibered sources are of particular interest since they allow compact, robust and alignment-free systems. Furthermore, they are well-suitable for power upscaling thanks to a good thermal dissipation. We present in this contribution the recent results achieved on 2 μm fiber lasers and fibered components allowing all-fibered architectures. In particular, the power upscaling up to 500 W-class and the efficiency of Tm³⁺ -doped and Tm³⁺, Ho³⁺ -codoped fiber lasers are discussed.

Thulium-doped fiber amplifiers have been limited to average output powers of around 1 kW for over a decade. To achieve multi-kW powers around 2 μm wavelength, we propose using dual-grating Spectral Beam Combining (SBC). Three customized kW-class Tm-doped fiber amplifiers operating in the 2030-2050 nm range were developed. The amplifiers consist of three stages and are pumped with non-stabilized, fiber-coupled diode lasers at 790 - 795 nm. Singlemode, TMI-free output powers exceeding 800 W, with narrow linewidths of FWHM ⪅115 pm, were achieved and subsequently combined using highly efficient in-house fabricated reflection gratings. With an overall combining efficiency of 90 % and a thermal slope of the combining grating measured as 6.8 K/kW, scalability to kW-level powers is enabled. The combined output power achieved a record-breaking 1.91 kW with good beam quality (M² ⪅2) and potential for further optimization. Finally, the potential power scalability of this non-coherent combining approach to power levels exceeding 20 kW is discussed.

High power laser sources in the 2 μm wavelength region and emitting ns-pulses have many applications in material processing, biology as well as for laser plasma sources targeting the extreme ultraviolet region. Thulium (Tm)-doped fibers represent a very attractive platform for Q-switched systems emitting ns pulses in the 2 μm wavelength range. Usually, high-power, Tm-doped fiber amplifiers are pumped at 790 nm. By exploiting Cross-Relaxations (CR) the slope efficiency can be significantly increased beyond the Stokes limit. However, in those fiber systems targeting mJ pulse energy, the slope efficiency is barely above 35 %. Due to the large quantum defect of approximately 60 % such systems face considerable thermal challenges. In this contribution an in-band pumped Q-switched oscillator is demonstrated. This source is used to seed an in-band pumped, Tm-doped, rod-type amplifier which delivers up to 6.1 mJ pulse energy and up to 128 W average power with 77% slope efficiency.

We present the design and experimental and simulated results for a 2050 nm band fiber amplifier with high optical-optical slope efficiency and low ion pairing, using a novel high performance single clad Ho-doped fiber from the Naval Research Laboratory (NRL). We report a measured optical-optical slope efficiency of 57% using 1 mW input signal power and 1860 nm pumping which we believe is the highest slope efficiency measured to date for a single clad copumped HDFA. This efficiency is linked to a low ion pairing coefficient of 4% in the doped fiber derived from our data.

We developed a high power and high efficiency 1.55 μm fiber laser for eye-safe autonomous driving lidars. A drive current directly modulated DFB laser provides a seed pulse selectable from 2 ns to 10 ns. A high gain (47 dB), low ASE noise (-23 dB), and cladding-pumped multi-pass fiber amplifier is designed and used as the power amplifier. The laser can deliver laser pulses from 80 kHz to 10 MHz with peak powers greater than 5 kW, and pulse energy more than 10 uJ with a single output mode of M² = 1.05. The laser works in -40 °C to approximately 105 °C environment temperature range with total consumed electrical power ⪅10 W.

We present on the nonlinear downconversion of a post-compressed Tm-doped fiber laser system via intrapulse difference frequency generation. In a preliminary experiment, with sampled 15 fs driving pulses and a 3 mm Gallium Selenide crystal, we achieved an octave spanning spectrum (5-16 μm) with a conversion efficiency of 3.6%. Utilizing the full 100 W driving laser power, we believe that this approach can pave the way towards W-class emission in the molecular fingerprint region enabling unprecedented sensitivity and precision in future spectroscopic applications.

In this work, we demonstrated that a single pump source in the 1.57-1.72 μm band can enable mid-infrared 2.8 μm lasing in Er-doped fluoride fiber via ground state absorption and excited-state absorption. The efficiency evolution of 2.8-μm fiber lasers with respect to pump wavelength was experimentally investigated. The high slope efficiency of ⪆50% and low laser threshold of ⪅0.1 W have been achieved with optimized fiber doping concentration and pump wavelength. This pioneer work paves the way for the further power scaling of mid-infrared 3 μm Er fiber lasers pumped by near-infrared fiber lasers.

We demonstrate a pulsed Er-doped ZBLAN fiber laser at 3750 nm based on the gain-switching scheme. A diffraction grating was introduced as a wavelength selection component to enable stable lasing in this long-wavelength region, which has deviated from the emission peak of ⁴F_9/2→ ⁴I_9/2 transition in Er³⁺. Different from the convention gain-switching behavior where the Pulse Repetition Frequency (PRF) of output laser is same as the that of the pump, three switchable gain-switched temporal states with 1/n (n=4,3,2) pump PRF rates are observed in experiment. The pulse evolution, including average output power, PRF, pulse duration, and peak power, in response to the 1950 nm pump power are investigated in detail. Over 200 mW average output power at 3750 nm was obtained under 12 W of 1950 nm pump power. The pulse repetition frequency, pulse energy and pulse duration were 35.4 kHz, 6.7 μJ and 1.2 μs, respectively. This work represents the longest pulsed emission wavelength generated from rare-earth doped fiber lasers.

We present the first polarization-maintaining, Ytterbium-doped, rod-type, multicore fiber with 5x7 cores. This fiber is drawn from a drilled 7x7 preform in which the outermost columns were filled with Boron-doped stress-rods, resulting in a stress-field strong enough to induce linear birefringence in all the cores simultaneously. Experimentally we have observed the preservation of linear polarization in the cores, albeit to a different degree depending on the core positions.

We investigate cladding pump light absorption in double-clad multicore amplifier fibers in dependence of the pump Numerical Aperture (NA). Results indicate that for the investigated case the assumptions for Beers law break down and need to be revised to include an NA dependent absorption. In this work we present a method for NA resolved absorption measurements and focus on the absorption behavior of a double clad multicore fibers, where experimental results show that the cladding absorption is changing drastically with the NA of the pump light. Additionally pump modes are investigated numerically to assist these findings. These results have not only implications for amplifier performance but impact characterization of cladding pump light absorption for such fibers as well. Lastly it is also expected that such behavior can be mitigated or exploited by fiber design.

In this work we will cover the basic considerations of incoherent combination of fiber laser systems and, in particular of multicore fibers, since these are ideal sources for this type of combination. Our analysis will have a special emphasis on what separates incoherent combination from its coherent counterparts. Additionally, we will discuss the scalability of the beam quality with the number and arrangement of the cores in a MCF, and we will compare the incoherently combined emission from a multicore fiber with that of a multimode fiber.

Nanostructured or “pixelated” core fibers have attracted great attention thanks to possible design of optical fibers with almost arbitrary refractive index profile, including gradient index nanostructured core, large mode area fibers for high power applications or fiberized free-form optical components. A short review of applications of (nano)structured core active fibers in fiber lasers will be given followed by detailed study of the effect of heat treatment and fiber drawing on the luminescence properties important for fiber laser performance; and application of the erbium- and ytterbium structured-core active fibers in fiber lasers that operate simultaneously at 1 and 1.55 micrometer wavelengths.

The output power of single-frequency fiber amplifiers is usually limited by nonlinear effects such as stimulated Brillouin scattering (SBS). To obtain higher power thresholds for the onset of unwanted nonlinear effects, the mode area of the fibers in use needs to be increased. Specialty fibers can provide larger mode areas and thus push the current power limits of single-frequency fiber amplifiers while maintaining single-mode beam quality as required by next generation gravitational wave detectors. Fibers with a large core diameter, depressed cladding around the core and a confined doping (DCCD-fiber) inside the core are by now commercially available and address the need for large mode area fibers while maintaining single-mode operation. The depressed cladding leads to a smaller effective refractive index difference for Higher Order Modes (HOM) in comparison to the fundamental mode which results in a significant increase of bending losses for the HOM. The confined doping results in a selective gain increase for the fundamental mode. Here, we present a forward pumped single-frequency amplifier based on an Yb³⁺-doped DCCD fiber. With this fiber, an output power of 400W was achieved with a slope efficiency of 75%, and a PER of 15 dB. The amplifier showed no signs of SBS or parasitic lasing of the amplified spontaneous emission. This work will evaluate the potential of the used DCCD fiber in the context of next generation gravitational wave detector lasers.

A polarization-maintaining ytterbium-doped tapered double-cladding fiber was made from an aluminophosphosilicate glass preform, with core/cladding diameters of 17/125 μm and 56/400 µm at the small and large ends of the flared section, respectively. Amplifier gain exceeding 50 dB and average output power beyond 200 W were simultaneously achieved after the 1032-nm seed laser, while amplified spontaneous emission was measured ⪅ 1% of total output power. This fiber also yields an optical efficiency close to 90%, near diffraction-limited output with M² ⪅ 1.2 and polarization extinction ratio ⪆ 18 dB. The newly developed fiber holds the potential to combine two successive amplifier stages in a single device, with foreseen benefits for ultrafast fiber amplifiers and laser harmonics generation.

In this work, we optimize singlemode-to-multimode doped fiber splices by varying the arc duration and monitoring HOM excitation, using the standard S² technique. By measuring the diffused Refractive Index Profile (RIP) distributions around the splice point, contrary to current belief, we show that the optimum arc duration does not corresponds to RIP that match the MFDs on either side of the splice point. Furthermore, we show that the MM doped fiber diffused RIP is longitudinally non-adiabatic and results in additional power transfer from the excited FM (LP₀₁) to HOMs (LP₀₂). As a result, we find that the optimum arc duration corresponds to a substantial MFD mismatch which results in an initial excitation of the HOM (LP₀₂) with the appropriate magnitude and relative phase to nullify the corresponding HOM power generated in the non-adiabatic region.

We demonstrate record high energy of 2 mJ, with four nanosecond pulses a peak power of ⪆420 kW and average power of 660 W, in a fiber amplifier using a novel 26 μm mode-field diameter Yb-doped gain fiber. The TMI threshold for this fiber was measured to be 1kW. This is achieved at a diffraction limited beam quality of M²=1.14.

An actively Q-switched diode-pumped Tm³⁺-doped fiber laser (TDFL) operating at 2050 nm is reported based on a flexible Photonic Crystal Fiber (PCF) with a core diamter of 50 μm. Using a fiber length of 3 m, the TDFL delivers gaussian shaped pulses with a maximum pulse energy of 1.5 mJ, corresponding to a peak power of 16 kW and a pulse width of 88 ns. The measured output spectrum shows a single peak at 2050 nm with a 3-dB-linewidth of 100 pm and 10-dB-linewidth of 270 pm. For a longer fiber length of 7 m, the effective gain is redshifted by reabsorbtion, increasing the achievable pulse energy up to 1.9 mJ. The average output power of the pulsed TDFL can be scaled to more than 100 W with a slope efficiency of 46 %. In all configurations the TDFL delivers nearly diffraction limited beam quality (M² ⪅1.3).

Lasers and amplifiers at 2.1 μm window are of great interest for applications that require high atmospheric transmission. To date, fiber lasers and amplifiers operating at this wavelength are based on single-mode Holmium Doped Fibers (HDF) so that a high-quality output beam can be obtained. However, as can be referred from the case of ytterbium doped fiber power amplifier, limiting nonlinear and thermal effects such as Stimulated Brillouin Scattering (SBS) and Transverse Mode Instability (TMI) will become obstacles in scaling single-mode holmium doped fiber amplifiers into the multi-kW power regime. The use of multimode HDF can help to mitigate the SBS and TMI effects, facilitating future power scaling of HDF amplifiers (HDFA). Here we propose and experimentally demonstrate a multimode HDF amplifier where the typical speckle pattern output is shaped into a quality focus by wavefront-shaping the amplifier’s input seed.

We are developing a 10W-level tunable fibre laser source with a simple design. The source serves dual purposes: firstly, it enables the exploration of how altering pump wavelengths affects the efficiency of 3.5 micron, dual-wavelength pumped mid-infrared fibre laser systems. Secondly, it facilitates the investigation of the exact absorption coefficients of optical materials relevant to the LIGO gravitational wave observatory. This technology holds promise for advancing knowledge and enhancing the sensitivity and precision of gravitational wave detection, our latest results will be presented.

Thulium-doped double-clad fiber (Nufern, LMA-TDF-25/250, 3 m long) based laser was tested under resonant excitation. As a source of pumping radiation at 1.7 μm a fiber-coupled laser diode was used (fiber diameter 400 μm, maximum power 25 W). Fiber laser resonator consisted of dichroic (high reflectivity at 2 μm, high transparency at 1.7 μm) free-space plane mirror, placed between lenses used to collimate and to focus the pumping radiation, and Fresnell reflection of active fiber end which serves as an output coupler. Under CW pumping this Tm-laser generated laser radiation at wavelength 1972 nm with power up to 4.5W for absorbed pumping power 11W. The laser efficiency in respect to absorbed pumping power was 63 % in this simple configuration. The goal of our work is to attract attention to the possibility of pumping thulium fiber lasers resonantly using laser diodes that are already readily available today. Thanks to resonance pumping, compared to cross-relaxation pumping at 0.8 μm, it can be achieved higher efficiency and lower heat generation, which can help to reach high mean powers from thulium fiber lasers. An important fact is that both the generated laser radiation and pumping radiation belong to the eye-safe spectral region, which reduces the risk of vision damage.

In this work, the full emission spectrum of Tm-doped fiber (1700 nm – 2000 nm) is employed for tunable noise-like pulse operation. By using an Acousto-Optic Tunable Filter (AOTF), the operation wavelength can be electrically tuned within the 300 nm wavelength range. Based on an all-anomalous dispersion cavity and hybrid mode-locking technique, noise-like pulse operations at different wavelengths are obtained with a 10-dB spectral width ranging from 22 nm to 37 nm and output average power from 25.9 mW to 104 mW. The cavity repetition is 35 MHz. Additionally, when the laser operates in the water absorption region, we observe giant pulses with intensity several times higher than average pulse intensity are generated in the output pulse train.

We demonstrate ASE pumping of rare-earth-doped fiber amplifiers, fiber lasers, and broadband ASE sources. Pumping with an ASE source yields the advantages of optical-optical efficiencies comparable to conventional pumps, generation of ultra-broad-band ASE sources, and reduced low frequency noise transferred from the pump to the signal.

Thulium-Doped Fiber Lasers (TDFL) emitting at 2 μm wavelength are used in various applications such as imaging, telecommunication and optical countermeasures. Many of these applications require highly integrated and passively cooled lasers with low SWaP (size, weight, and power) architecture that can work in harsh environment at different temperatures. We investigated the temperature dependence of a multi-watt TDFL with a low SWaP architecture for temperatures ranging from 253 K till 573 K. Cladding-pumping with 793 nm diode lasers is used for high-power TDFLs to take advantage of the cross-relaxation effect to double the quantum efficiency. However, since the 3 H4 absorption band is relatively narrow with a 16 nm FWHM compared to the diode wavelength shift of 0.3 nm/K, these diode lasers have to be wavelength or temperature stabilized using volume Bragg gratings or Peltier elements. Both approaches either limit the applicable temperature range1 or decrease the overall efficiency. In contrast in-band core-pumping directly into the 3 F4 level offers a broad absorption band ranging from 1550 nm till 1720 nm and is therefore preferred for low SWaP TDFLs. We investigated therefore a low SWaP TDFL that is core-pumped by an in-house built erbium:ytterbium-codoped fiber laser (EYDFL) with pump wavelength of 1567 nm.

We demonstrate that the PM 56/400 Yb tapered fiber achieves a good combination of large mode field diameter and beam quality for use in a pulsed, monolithic fiber amplifier. We evaluate this fiber over a wide test regime to determine its viability when compared to other amplifier fibers. Our focus is achieving as much energy, peak power, and average power as possible while remaining conservative to protect the fiber long-term. The fiber is tested using our Counter-Pumped Tapered Endcap (CPTEC) design that retains the benefits of counter-pumping without the drawbacks of free-space coupling.

We demonstrate a few-picosecond pulse laser of 8-GHz repetition rate by electro-optic modulation from continuous signal. The generated pulse has a 1-nm of spectrum bandwidth and a 67-ps of pulse duration since the pulse is modulated. To compress the long pulse to few-picosecond level, a triple-pass dispersion compensator with a high-groove grating of 1800-l/mm is applied. After compression, we achieve 3-ps pulse duration, which is closed to the transform-limited pulse duration.

A single-frequency Er³⁺ -doped ring-cavity silica fiber laser operating at 1610.06 nm based on cascaded single-mode-multimode-single-mode devices and sub-ring cavities was demonstrated. The laser presents maximum single-frequency output power of 31.6 mW with slope efficiency of 9% and laser linewidth of 2.8 kHz.

In this work, the architecture of a machine learning model which is strongly constrained by the physical boundary conditions of the observed optical fibers is presented. The procedure of extraction of the physical relevant information from the trained model is shown. This work aims to estimate the eigenmodes of an optical fiber with the main focus on fibers with few guided modes. We will give an overview of the transit scheme of the expected electromagnetic field properties in a system with low eigenstates by introducing conform ML-Model architecture and corresponding merit (loss) functions for the learning procedure. The introduced ML-Model is proposed as a glass box, where the internal states are equivalent or equivalent up to an isomorphism to the corresponding physical model. The model itself will be presented as a trainable transfer function of a few modes or, in general multimode, optical fiber. The proposed training procedure is equivalent to the approximation of a transfer function of a certain physical system with a limited number of eigenstates, which are finally extractable from the trained model. The presented model contains physically motivated dot-product layers in the complex plane. The gradient-descent-based learning procedure is performed with the Wirtinger derivative technique. The presented estimation technique can be applied in a wide range of tasks, where the eigenstates are unknown or the system is only partially temporal invariant e.g., for analyzing the effect of Transverse Mode Instabilities (TMI) in optical fibers.

In this publication, we present a linearly polarized MOFPA (Master Oscillator Fiber Power Amplifier) setup for generating and amplifying short pulses in the ps- to ns-regime at wavelengths around 2 μm to ⪆20 W of average power. The Master Oscillator consists of a directly gain-switched seed diode laser at a center wavelength of approx. 1950 nm which is capable to generate pulses with durations down to 50-70 ps by separating the gain spike and suppressing the trailing edge by using specially designed driver electronics. The subsequent fiber amplifier is formed by three cascaded amplifier stages based on polarization-maintaining Thulium-doped active fibers pumped by multimode pump laser diodes around 800 nm. The realized setup achieves a total gain of ⪆60 dB, which leads to a spectrally filtered output power of ⪆20 W with slightly broadened pulse durations ⪅350 ps due to nonlinear effects, for example when an input pulse duration of 222 ps is used. Since our fully monolithic fiber amplifier system consists of single-mode and LMA fibers, the output beam quality is nearly diffraction limited. Due to the separately variable pulse durations and pulse repetition rates, our here presented approach offers technical and economic advantages over commercially available laser systems with comparable pulse durations, which are largely based on mode-locked oscillators.

We here present our holmium-doped fiber amplifier developed for research on topics on a future third-generation gravitational wave detector for the ETpathfinder research facility. For lasers to be used in gravitational wave detectors, there are not only highest demands on typical laser parameters, such as linewidth, spectral purity, and polarization, but especially on the Relative Intensity Noise (RIN) properties. Our developed laser system consists of a compactly packaged holmium-doped fiber amplifier, which is pumped by a thulium-doped fiber amplifier. By setting up both fiber amplifiers in the same, thermally stabilized and compactly engineered laser housing, we aim to achieve highest output power stabilities. With our current setup, we achieve an output power of app. 450 mW with linear polarization and a low linewidth of app. 2 MHz at a wavelength of 2095 nm. To analyze the RIN, we use our in-house developed measurement setup. We present the achieved RIN results in the frequency range from 10^-3 to 10⁴ Hz, and for example at a frequency of 100 Hz, we achieve a RIN of app. 10^-6 1/Hz^0.5, which shows the suitability of our concept to achieve highest power stabilities.

Concepts of Large Mode Area (LMA) fibers tailored for passive mitigation of Stimulated Brillouin Scattering (SBS) are discussed. The presented designs are based on the management of the overlap between the optical fundamental mode and the acoustic modes by lowering the contrast of acoustic velocity in the region surrounding the fiber core and considering the coiling of the fiber on a support. Analogously to the electromagnetic case, the circular curvature of the optical fiber changes the intensity distribution of the acoustic modes due to the propagation path lengthening with increasing distance from the winding axis. Numerical calculations were performed to determine the acoustic velocity profiles leading to an increased SBS threshold in the optical fiber by assessing the contribution of both guided and leaky acoustic modes.

For a study of the European Space Agency ESA, Fraunhofer ILT developed and built single-frequency, linearly polarized, power-stabilized highly stable 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). While the fiber amplifier developed at Fraunhofer ILT has fulfilled most of the optical performance requirements, e.g. the relative intensity noise (RIN), we further optimized our setup to investigate the power scaling to expand the number of potential applications. In this paper, we present the power scaling of our optimized breadboard setup of our single-stage Ytterbium-doped fiber amplifier. Our fiber amplifier is seeded by a commercial Non-Planar Ring Oscillator (NPRO) with a linewidth ⪅10 kHz at a wavelength of 1064 nm. The fiber amplifier is based on cladding-pumped 10 μm-core Ytterbium-doped SM-PM fiber and provides an SBS-free amplification to an output power of more than 15 W while maintaining narrow-linewidth and polarization. Additionally, we investigate the relative intensity noise to analyze the temporal stability of the output power and achieve high power stabilities.

We demonstrate the dramatic progress in Yb-doped spun tapered double-clad fiber amplifiers delivering up to 550 W of average power with single mode spatial profile and 50 ps pulses at 20 MHz repetition rate. The special geometrical architecture of the fiber enables the direct amplification of short pulses from tens of mW to hundreds of watt levels in a single amplification stage, leading towards the realization of a compact and highly efficient picosecond fiber-based laser system with excellent output spatial and temporal characteristics.

Femtosecond-pulse inscription of Fiber Bragg Gratings (FBGs) in a Multicore Fiber (MCF) offers new opportunities of controlling the spatio-spectral properties of the generated beam in all-fiber scheme. With coupled cores, interference of partially reflected beams from individual FBGs in different cores becomes important. We present our recent results on the effect of narrowing/collapse of the laser spectrum generated in a cavity based on FBG array fs-inscribed in coupled cores of active (Yb-doped) MCF, which is shown to arise due to the supermodes formation and hybridisation. Output beam concentration in one core is observed at Raman tasing in passive MCF with FBG arrays and is potentially possible in Yb-doped MCFs. Applications and benefits of such all-fiber lasers will be discussed.

This paper presents for the first time to the best of our knowledge the spatiotemporal dynamics of soliton propagation through rare-earth doped fiber gain media under diverse gain conditions. Utilizing numerical simulations based on the Ginzburg-Landau equation, we show significant differences in dynamics under saturated and unsaturated gain conditions. Specifically, the spatiotemporal dynamics reveal recurring soliton creation and collapse, early filamentous soliton formation, and collision and repulsion interactions between them. These findings significantly contribute to our understanding of the fundamental behavior and applications of soliton-based systems in optical communications, fiber lasers, and nonlinear optics in general.