This PDF file contains the front matter associated with SPIE Proceedings Volume 11981, including the Title Page, Copyright information, and Table of Contents.

Er/Yb co-doped YAG nanopowders-derived silica fibers (YNDSFs) are fabricated from UV-curable composites and achieve 1550-nm laser generation with active fiber length of 1 cm. The UV-curable nanocomposites are consisted of 40 wt% home-made Er:YAG/Yb:YAG nanopowders and 60 wt% polymer resins. The Er:YAG/Yb:YAG nanopowders are fabricated by coprecipitation method with diameters in the range of 25 - 165 nm. An Er:YAG/Yb:YAG–polymer hybrid core rod was polymerized using high power UV-lamps, and was drawn to Er/Yb co-doped fiber in silica cladding by melting-in-tube method after debound process to remove organics. The absorption coefficients of the Er/Yb co-doped fiber were measured to be 17 dB cm^-1 and 3.2 dB cm^-1 at 976 nm and 1530 nm, respectively. The background loss was 0.05 dB cm^-1 at 1200 nm. A linear fiber laser cavity was constructed with different length of Er/Yb co-doped fiber and a pair of FBGs (reflectivity: 99 % and 94 %) to quantify the laser performance. The maximum output powers were 7.5 mW and 1.5 mW with active fiber length of 6 cm and 1 cm, respectively. This scheme is expected to be an effective approach for obtaining Er:Yb co-doped fiber with high pump absorption for the generation of 1550-nm single-frequency fiber lasers.

In this paper, we investigate the influence of various nanostructured-core fiber fabrication processes, such as preform elongation or fiber drawing, on the fluorescence lifetime of Yb³⁺ ions. The optical fiber preform was prepared using Modified Chemical Vapor Deposition (MCVD) method combined with Al₂O₃ nanoparticle doping. The optical fiber preform was subjected to various processing treatments involving heat and mechanical stresses, i.e. preform elongation and fiber drawing, and the fluorescence lifetime was measured in all stages of fiber fabrication, i.e. original preform, elongated preform (cane), fiber and overcladded fiber. It was found that the time-resolved photoluminescence properties of Yb³⁺ ions in silica glass are strongly dependent on the processing of the material. The fluorescence lifetime of the 2F5/2 level of Yb³⁺ ions decreased with the heat and mechanical treatment, which was explained by the break-up of Al₂O₃ nanoparticles, diffusion of dopants and changes in the Yb³⁺ phonon environment as well as clustering of the Yb³⁺ ions. The fiber drawing exhibited a stronger effect compared to preform elongation which was ascribed to the high rate of cooling and mechanical stresses during the drawing process. In general, the heat and mechanical processing of Yb-doped optical fiber preforms leads to a deterioration of time-resolved photoluminescence properties.

A polarization-maintaining (PM) solid-core anti-resonant fiber (SC-ARF) based on the anti-resonant waveguide mechanism was proposed in this work. Numerical simulation results indicated that fundamental mode (FM) birefringence could be obtained by filling the core and cladding of designed SC-ARF with As₂S₃ and As₂Se₃ materials respectively. With the cladding tubes thickness of 0.82 μm, our fiber presented the birefringence coefficient (Δn) about 1.56x10^-5 and the confined loss (CL) less than 0.1dB/m. What’s more, the optimized CL values and band-pass transmission properties were observed by increasing the tubes number and adjusting the incident laser wavelength, which are of pivotal importance in the PM laser systems and show great potential to fabricate the filters in the mid-infrared regime (MIR).

Current research focuses on very-large-mode-area fibers (core diameters of 34 μm and above) and all-fiber laser systems to deliver high output power with linear polarization, low noise properties, narrow-band linewidth and high fundamental mode content. All-fiber systems have the advantage of low-maintenance and alignment-free operation. Because power scaling in fiber-based amplifiers is limited by non-linear effects like stimulated Brillouin scattering, very-large-mode-area fibers such as specialty fiber designs like the chirally coupled core (CCC) fiber are investigated. This fiber type offers a signal core of 34 μm or larger while also providing a near single-mode output beam quality. It thereby enables further power scaling in systems that are limited by such nonlinear effects. However, efficient components such as signal-pump-combiners (SPC) with this fiber type need to be developed. The SPC couples the required pump light of multiple high power laser diodes into the gain fiber of the laser system. We report on the development of a SPC with an integrated 34=250 μm feed-through CCCfiber with a pump-to-signal fiber coupling efficiency of 90% and three input pump fibers with a signal-to-pump isolation of 30 dB. The device is tested with an input power of up to 380 W. In addition, different experiments for monolithic implementation of the CCC-fiber type into systems that rely on standard polarization-maintaining (PM) fibers are conducted. We show the polarization maintaining behavior (polarization extinction ratio (PER) < 19 dB over several hours) of the fiber by imprinting externally induced birefringence on the fiber. Experiments with all-fiber setups using the CCC-fiber and a step-index PM-fiber show a PER of < 15 dB with reduced long-term stability.

Fiber laser systems with high average power, narrow linewidth, and nearly diffraction limited beam quality are desirable for applications in areas such as gravitation wave detection or directed energy. In particular, these systems have special fiber design requirements and finding the right balance between large mode areas to mitigate unwanted nonlinear effects such as Stimulated Brillouin Scattering (SBS) while maintaining fundamental mode operation is a challenging task. A promising approach to achieve this goal relies on confining the rare earth dopant to the central region of the central LMA core and therefore providing preferential gain to the fundamental mode while simultaneously minimizing the spatial overlap of the optical wave with the resulting acoustical wave during operation. In this work, we investigate the SBS properties of LMA fibers with confined Ytterbium doping and numerically compare the amplifier performance with homogeneously doped large mode area gain fibers and the corresponding high-power single frequency SBS thresholds.

Using an ytterbium-doped fiber with a 50 μm core and 0.028 NA, a pulse energy of 4.8 mJ was achieved directly from a single-mode Q-switched fiber laser. The repetition rate was 10 kHz and the average power was 48.4 W. The slope efficiencies with regard to the absorbed and launched pump power were ~74% and ~59% respectively. The pulse width decreased with increasing pump power. The 4.8 mJ pulse had a FWHM width of ~300 ns. A shorter pulse of ~200 ns FWHM was also achieved at 2.06 mJ in another configuration. The M² was below 1.3 at all pulse energies. This work demonstrates record pulse energy directly from a single-mode Q-switched fiber laser and the feasibility of operating such laser with high efficiencies.

In-house fabricated Ytterbium-doped rod-type multicore fibers (MCFs) with 4x4 cores were used to generate and amplify ns-class pulses to energies up to 37 mJ. Amplification was compared using tapered and untapered MCFs with large multimode output cores using a Q-switched MCF oscillator as seed source. The tapered fiber improves beam quality (average M2 = 1.56) while maintaining a large waveguide mode area for suppression of nonlinear effects and increased amplification per unit length. These all-glass MCFs can facilitate energy and power scaling in compact high power coherently combined amplifier systems.

In this work a Q-switched mode-locked (QML) noise-like pulse (NLP) emission from a thulium-doped all-fiber laser with passive saturable absorption action through the nonlinear polarization rotation effect is demonstrated. QML-NLP pulses were obtained at 812 kHz repetition rate within a 11 μs QML envelope and pulse energy of 460 nJ. Central modelocked NLP sub-pulses were measured with 8.71 ns of width and 4.17 of peak power. The evolution of QML-NLP is experimentally investigated, and the results demonstrate the feasibility of an unconventional pulse operation regime of NLPs and provide insights into the dynamics of mode-locked fiber lasers.

This article reviews the available research and innovations in high power laser to understand and assess the feasibility and advantage of applying laser in drilling oil and gas wells. Small thermal conductivity in rocks makes them good heat insulator, and based on this fact, current research study and evaluate laser and rock interaction with extensive focus on the principle of rock properties, laser parameters and key variables of heat transfer in different methods of rock removal with laser. This paper demonstrates through examples when advanced mechanical drilling techniques such as MPD/UBDCTD are equipped with appropriate surface and downhole tools to transmit laser to deep bottom hole harsh environment, with no limitation to maintain wellbore stability, hole cleaning, wellbore steering, and real time downhole monitoring, industry would be able to merge or replace mechanical drilling with laser drilling. Drilling hard rocks and developing expensive unconventional resources would be economic with faster and cheaper drilling without compromising safety. Less energy input with higher ROP output and eliminated numerous drilling challenges, makes laser energy an appropriate candidate to begin a revolution to transform the aged mechanical drilling. Laser drilling with low energy consumption is a step towards worldwide energy saving and CO2 emission reduction.

Steering a laser beam is a well-studied method for attaining welds with better quality in reducing pores, spatters, and other defects. However, the steering frequency, the possible shapes, and the total power are limited. Mechanical scanners have speed and power restrictions; diffractive optical elements (DOE) and multi-core lasers have limited shape flexibility. Civan’s Dynamic Beam laser provides a High Power (up to 100 kW), Single Mode, Continuous Wave output with the ability to create arbitrary beam shapes and do beam steering at a speed of 10s of MHz. The Dynamic Beam is derived from using the Coherent Beam Combining (CBC) technology and the Optical Phased Array (OPA) technology. This combination allows to dynamically tailor beam parameters to the required application. The benefits of the Dynamic Beam are significant in many applications such as welding, cutting, drilling, and additive manufacturing. This is due to its ability to have better control of the melt pool and the keyhole, which both affect the quality of products. In cases where the melt pool is not stable, spatter and pores occur and damage the pieces.

Lasers are essential tools for a wide variety of materials processing applications. The speed, quality, and process window are determined in part by the laser beam properties, including size, shape, and divergence. Most laser sources have fixed beam characteristics, resulting in processing and material limitations and nonoptimized performance. nLIGHT has developed a fiber-laser product line that provides rapid tunability of the beam characteristics directly from the delivery fiber using a novel, all-fiber mechanism. The broad range of beam sizes and shapes and real-time programmability allow adjustments on-the-fly and optimization of each process step using a single laser source, enabling development of versatile tools that provide optimum performance for a range of processing needs. We describe the underlying technology, performance, and beam characteristics and show results for the largest industrial laser applications, including metal cutting, welding, and additive manufacturing.

Traditional MIG and TIG welding drawbacks include typically requiring timely initial setup and imparting significant heat while welding resulting in large heat-affected zones with material distortion and deformation resulting in both lower quality results and the need for pre- and post- welding operations that increase overall costs and lower productivity. Further adding to costs is the need to employ experienced welders for welding both complex and simple joints. Automated laser welding resolved these problems but until now, it was not a reasonable solution for metal fabricators due to high costs, required floor space and complexity. IPG Photonics solved these challenges with the introduction of a revolutionary new product that brings manual handheld laser welding and cleaning capabilities to fabricators of any scale for use by both novice and professional welders. The adjustable 1.5 kW fiber laser source with available 2.5 kW peak power delivers the processing power required for welding via an ergonomic handheld gun. Process parameters are stored in the device and optimized for common material-thickness combinations, as well as providing the capability for welding dissimilar materials and those with differing electrical conductivity. Incorporation of beam oscillation (wobbling) both increases the weld seam width up to 5 mm for parts with poor fit-up and creates aesthetic seams as desired. Compact and portable at 0.1 m3 and 53 kg makes this solution both a revolutionary laser innovation and a highly productive industrial tool.

We present a rod-type, Ytterbium-doped, multicore fiber with 4x4 cores. This fiber is employed in a CPA setup for coherent beam combination of femtosecond pulses. High average powers of up to 507 W after combination and compression could be achieved at 10 MHz repetition rate. A high combination efficiency of 85% could be realized together with an excellent beam quality. Additionally, up to 600 μJ pulse energy was measured in a lower repetition rate configuration.

In this report, we will introduce our recent advances in developing the deep-learning-based coherent fiber laser array systems for power scaling and spatial light structuring. Our motivation is to construct a deep-learning network for estimating the thermal and environmental induced phase errors, and further compensate the phase errors by the phase control servo with the assistance of the network outputs. Technical progresses in terms of the network optimization, two-stage control scheme, and optical field information acquisition will be covered. Moreover, the prospects and challenges towards the future implementation of intelligent control for CBC systems will be discussed.

We have developed a scalable, ultrafast laser beam combination scheme, which can combine many beams using two diffractive optics. A feature of this approach is the information contained in the uncombined output beams, which can be used to derive phase error information. We show that a machine-learning algorithm can learn to stabilize beam combination with high efficiency, by finding correlations between uncombined output beam patterns and phase errors.

Heat generation during the operation of lasers and amplifiers negatively affects a wide range of performance and packaging considerations. Accordingly, the thermal management of such systems remains of critical importance. This paper reviews the sources of heat in active fiber lasers and amplifiers, and describes recent successes using all-optical means to reduce heat generation, including anti-Stokes fluorescence cooling, radiation balancing, and dual-wavelength pumping.

Yb:fiber-based pulses are split into 4-pulse bursts, sent into a gas-filled multipass cell for nonlinear pulse compression and recombined again to scale the supported output peak power and pulse energy. The pulse division and recombination are passively stable, being based on birefringent crystals, and enable a final output pulse energy of 3:4mJ at 32 fs pulse duration. Distributing the input energy over a 4-pulse burst allowed to work a factor two above the limitations of single-pulse operation regarding the occurrence of laser induced damage on the multipass cell mirrors. Overall, good efficiency, beam quality and temporal contrast are achieved.

We present a high-power, semi-random Raman fiber laser (SRRFL) where the Stokes wavelengths are determined by discrete fiber Bragg gratings. Utilizing this configuration with Raman fiber, we achieved powers of 264, 181 and 154 Watts at 1480, 1690 and 1824 nm respectively, while with a standard coupler fiber we achieved 366, 265, 247 and 234 Watts at 1174, 1240, 1310 and 1390 nm respectively. The overall peak efficiency was greater than in a standard resonator despite a far simpler configuration and only slightly below the maximum reported for a cascaded Raman amplifier.

Cascaded Raman fiber laser (CRFLs) based on random distributed feedback (RDFB) are proven to be simple, wavelength agile and power scalable technology. However, they are limited in terms of spectral purity where a small amount of output power resides in unwanted Stokes orders. This is due to the inherent intensity noise of pump source used for Raman conversion, which gets transferred and amplified to multiple Stokes orders. Since each Stoke’s signal acts as a pump for the next Stoke’s signal, spectral purity keeps reducing with increasing Stokes order. Further, this increases spectral broadening in these systems due to nonlinear effects of self-phase modulation (SPM) and Cross-Phase Modulation (XPM), thereby, limiting the narrow linewidth operation. Broad spectral width in CRFLs is attributed to 1) inherent broad (~40THz) Raman gain spectrum, and to 2) nonlinear spectral broadening through SPM and XPM. By employing narrow bandwidth spectral filters for feedback and low intensity noise sources for the pump, narrow linewidth operation can be achieved. Here, we demonstrate a low-intensity noise (<-101dBc/Hz from 9kHz to 10GHz) CRFL with <= 99% spectral purity over 6 Stokes orders. A low-intensity noise (<-147dBc/Hz from 9kHz to 10GHz), Narrow linewidth (DBR, 5MHz) 1064nm laser as a pump source. It is line-broadened to ~50GHz bandwidth through phase modulation with a noise source and a sinusoidal signal to suppress stimulated Brillouin scattering (SBS). It is then amplified to 73W using amplifier chain and a passive Raman fiber of length 340m is used for Raman conversion. This work paves a way to achieve narrow linewidth CRFLs with very high conversion efficiency.

We have proposed a MOPA configuration with a stepwise optical frequency pulse train (SOFPT) seed in which the SBS threshold does not decrease even if the duty-cycle of the pulse train is increased up to ~100%. In this paper we present the SBS thresholds of a passive fiber in range of 45ns-500ns pulse duration under the low-duty-cycle condition. Based on the experimental results and theoretical consideration, we estimated the achievable average power of > 8 kW in a MOPA configuration with the SOFPT, in which each pulse has a sub-GHz linewidth.

In this paper, we report a high-power single-frequency all-fiber amplifier at 1064 nm based on cascaded hybrid active fibers. The cascaded gain fiber design, where a piece of active fiber with lower dopant concentration is spliced before the highly doped gain fiber, is proposed to effectively alleviate the thermal loads of high-power amplifier. By virtue of hybrid pump at 976 nm and 915 nm to increase the threshold of transverse mode instability (TMI) effect, an output power of 435 W is achieved from the Yb³⁺-doped fiber with large core diameter of 30 μm. The beam quality is well maintained with a M² of 1.4 even at the maximum laser power.

Saturable absorption effect of unpumped active fiber can be used as a method of achieving single-frequency operation from an all-fiber oscillator. Here, the effect was studied when the signal was highly absorbed by unpumped thulium-doped fiber and the laser threshold was beyond the available pump power. We proved that the oscillator could be manipulated to the “oscillating state” by injecting auxiliary pump light to the saturable absorber fiber. This allowed using unpumped thulium-doped fiber with higher absorption to enhance the frequency selecting capability. A record single-frequency output power of 2.56 W at 1720 nm was obtained, with a slope efficiency of 44% and a linewidth of 3.3 kHz, the power scaling of which was only limited by the available power of the single-mode 1570 nm pump source. This work offers new insights into how high-power, single-frequency fiber oscillators can be produced.

Further power scaling of narrow-linewidth fiber lasers is critical for beam combining. Using allsolid photonic bandgap fibers with large effective mode area and strong higher-order-mode suppression is an interesting approach. Previously we demonstrated ~400 W single-frequency single-mode power at 1064 nm from a 50/400 photonic bandgap fiber amplifier, limited only by TMI. In this work, we demonstrate a TMIlimited single-mode power of 1.37 kW from a monolithic fiber amplifier with a 25/400 photonic bandgap fiber, the highest output power from a photonic bandgap fiber demonstrated to date. The spectral linewidth is broadened to ~8 GHz to suppress stimulated Brillouin scattering.

High power, narrow linewidth all-fiber amplifiers are under intensive investigation in recent years. In this paper, the research status of high power, narrow linewidth all-fiber amplifiers (including those operate at single frequency regime) is briefly summarized. Then the recent progress in our research group is introduced, including more than 500 Watt level single frequency fiber amplifier, more than 4 kW linearly-polarized narrow linewidth all-fiber amplifier and more than 6 kW narrow linewidth all-fiber amplifier. Performance exploring of the operating spectrum property is also discussed.

For the first time, we present the measurement of the stimulated Brillouin gain spectra of all-glass, metal coated, doubleclad, large-mode area fibres. We describe the methodology used to measure the small SBS gains produced by the short lengths of large-mode area fibres. The Brillouin spectra of the fibres were also measured while a segment was heated on a hot plate. Sections of the metal coated fibres were raised to 250 °C, resulting in a ~0.2 GHz shift in the Brillouin gain peak. This is a significantly larger shift in temperature than possible with conventional polymer coated double clad fibres, demonstrating how metal coated fibres can begin to overcome some of the limiting nonlinearities associated with single frequency and narrow linewidth operation.

We report new Yb-doped gain fibers with approximately 20-μm core diameter and 400-μm cladding. These fibers have mode-field diameter greater than 19 μm, and increased higher-order-mode loss compared to conventional 19-μm modefield diameter fibers. The increased higher-order mode loss allows for high transverse-mode instability thresholds in fibers that also have large MFD and high absorption. A 21-μm fiber with 6.5 m operating length, was free from transverse-modal instabilities at 3.6 kW signal power and had 81% optical-optical efficiency. In narrow-linewidth amplifier experiments, the next-generation fiber with 21 μm fiber achieved 2.74 kW output power at 6 GHz linewidth, limited by pump power. This represents almost 2x increase in the ratio of power to signal linewidth compared to existing commercially available 19.5 μm MFD fibers, achieving 510 kW/GHz.

We successfully fabricated a dispersion-shifted 20-um-core LMA fiber by incorporating four resonant side cores. This fiber was designed to operate at 1640-nm eye-safe wavelength and has been used in a pulsed Raman amplifier to overcome modulational instability. A peak power of 100-kW was obtained from the Raman amplifier.

In this study we demonstrate high power master oscillator power amplifier (MOPA) laser system delivering 50 ps pulses and capable to work in wide range of repetition rates. The gain-switched (GS) fiber coupled diode is used as a seed source to provide microwatt power level pulses at 1039.5 nm with FWHM 50 ps duration at a repetition rate in the range of 1 to 20 MHz. The maximum achieved amplified average power at 20 MHz repetition rate was measured to be 513 W corresponding to 481.9 kW of peak power and 25.7 µJ pulse energy. The gain of high-power amplifier reached over 44 dB with around 69% slope efficiency. Almost all optical power (<97%) was enclosed in the pulsed signal. In contrast, with repetition rate decreasing, the part of the optical power contained in the pulsed signal began to decrease, which directly follows from the spectra. The fraction of signal radiation decreased to about 50 % at maximum achieved power of 338.9 W at a repetition rate of 1 MHz. The peak power and the pulse energy in this case may be estimated as 3.2 MW and 169.5 µJ (in the limits of signal spectral bandwidth) respectively and corresponds to over 52 dB of gain with 60 % slope efficiency. The pulse duration changed insignificantly during amplification. Herewith advantages of polarization maintaining tapered double-clad optical fiber (PM-T-DCF) having near single mode input core diameters were allowed to maintain modal content during amplification and preserve near single mode output without significant deterioration

We propose and demonstrate a novel method to achieve pulsed operation of thulium-doped fiber lasers at 1.7 μm. In our design, the thulium-doped fiber laser cavity was embedded within the cavity of an erbium/ytterbium-codoped fiber laser operating at 1.5 μm. The thulium-doped fiber in the erbium/ytterbium-codoped fiber laser cavity served as a saturable absorber, forcing the 1.5 μm laser to operate in a passively Q-switched scheme. The 1.7 μm thulium-doped fiber laser was in-turn pumped by the Q-switched 1.5 μm laser and was gain-switched. This resulted in the pulse duration of 1.7 μm emission was much shorter than that of 1.5 μm emission. 1.02 W average output power at 1720 nm was obtained for 10 W of pump power at 976 nm. The pulse repetition frequency, pulse energy and pulse duration were 134 kHz, 7.6 μJ and 102 ns, respectively.

In this work, we report the fabrication and characterization of a 5-tubes nested hollow-core anti-resonant fiber (Nested HC-ARF), which exhibits outstanding optical performance in terms of a record attenuation value of 0.85 dB/km at 2 μm wavelength and excellent modal. The power handling capability of Nested HC-ARF is also firstly demonstrated in this work. The maximum average power of 60.5 W at 2 μm has been transmitted through the fabricated Nested HC-ARF in robustly single-mode fashion without introducing any damage to the input and output fiber end-faces, which represent the highest average power delivered by HCF in this wavelength, to the best of our knowledge.

We report a new Yb-doped gain fiber with 20 µm core and 400 µm cladding, designed for improving the long-term operation of commercial fiber lasers powered by cost-effective pump diodes, centered near 976 nm wavelength. The fiber was tested in a co-pumped amplifier, reaching 3.8 kW signal power with 89% optical-to-optical efficiency. Negligible photodarkening loss was observed in over 200-hour laser operation at 2 kW. The signal power was limited to 3.8 kW by TMI and can be further increased by optimizing the pumping configuration. The fiber is compatible with available 20/400 commercial fibers and components allowing fast system integration.

In this paper, we demonstrate a broadband anti-reflective (AR) coating over the mid-IR fluoride fiber endface (tip) for high power laser applications. The AR coating consists of multiple-pair Lithium fluoride (LiF) and Al2O3, and was deposited by electron beam physical vapor deposition with an iron assistant source at low temperature (<60°C). In addition, a super thin encapsulation layer (~20nm) of Al2O3 was applied over the AR coating by both traditional PVD and atomic layer deposition technology. The measurements show the coating has a reflectivity of < 1-1.5% in the range of 1.5-5.5μm. The laser induced damage threshold (LIDT) test using high power quantum cascade laser (QCL) shows the damage threshold is greater than 10 MW/cm2 with no sign of any damage on the coating. The durability and environmental tests of the AR coating with PVD coated encapsulation layer show good humidity resistance in open air and 95 RH% at 50ºC for 48 hours, no degradation of film quality and optical performance are observed. Further comparison study of ALD encapsulation layer vs pure PVD layers in more aggressive water vapor testing indicates ALD encapsulation layer has better moisture resistance.

Recent advances in thulium doped fibers have significantly improved the optical conversion efficiencies, indicating their potential utility for a variety of applications. Recently, kilowatt-class fiber amplifiers have been demonstrated with narrow linewidth operation. However, their limitations in scaling to higher powers aren’t well understood. While thermal mode instability (TMI) is a well-known limitation in ytterbium doped fiber amplifiers, no experimental data exists for CW thulium doped fiber amplifiers. As such, it’s difficult to quantify or predict when this limitation is expected to impact TDFA’s. Here, we report on the first measurements of TMI in CW thulium doped fiber amplifiers. We characterize the TMI threshold of both 20μm and 25μm fibers by measuring the beam quality, relative intensity noise (RIN), and polarization extinction ratio (PER). In particular, a strong dependence on the TMI threshold with the fiber length is noted. Counter intuitively, measurements show that short fibers have significantly higher TMI thresholds than longer fiber lengths; indeed doubling the fiber length can reduce the TMI threshold by almost 50%. By optimizing the fiber length, a 20/400μm thulium doped fiber amplifier is scaled to 905W, with 57% optical conversion efficiency and a linewidth 5 GHz.

Several studies have reported mitigation strategies of transverse mode instability. These are usually based either on reducing the strength of the thermally induced refractive index grating (RIG) and/or the modal interference pattern (MIP) along the fiber amplifier or on manipulating the phase shift between the RIG and the MIP. Here we report that a heat load modulation can be achieved by pulsing the seed and/or the pump in bursts, which induces a positive phase shift. In this way, a beam cleaning is achieved, which improves the performance of the amplifier system by allowing for operation with a diffraction-limited beam profile at an average output power significantly higher than the TMI threshold. The heat load modulation technique delivers a stable beam at an average output power higher than 1.5 times the TMI threshold with an instantaneous intra-burst power of around four times higher than the TMI threshold.

In this work, we predict the existence of static modal energy transfer in polarization-maintaining (PM) fibers. This phenomenon, related to the effect of transverse mode instability (TMI), appears because of the coexistence of two modal interference patterns and two thermally induced index gratings (one of each per polarization axis in the fiber) with slightly different periods. This static energy transfer can be exploited to create an early warning system for TMI in PM fibers. Moreover, this technique can become an invaluable tool to gain further insights into the physics of TMI.

Modal decomposition (MD) techniques are a way to characterize the modal amplitudes and phases of eigenmodes in optical fibers. Some applications such as, e.g., diagnosing/analyzing transverse mode instabilities or monitoring a laser beam during operation, require a high-speed modal decomposition. However, the proposed modal decomposition techniques up to now are seldom able to implement a real-time MD due to their complex post-processing (e.g., most of the numerical computing-based MD). In this work a high-speed, real-time modal decomposition of fiber laser beams is presented. It employs a spatial mode multiplexer (MUX), which is a multipass cavity with multiple single-mode fiber (SMF) inputs and a single few-mode fiber (FMF) output, operated in reverse. Thus, if the output of a fiber laser system is imaged on the end facet of the FMF, then the MUX is used as a demultiplexer and, thus, the modal amplitude information can be obtained by reading out the different SMF input ports. We have carried out several experiments with the MUX and it has been proven that this method is accurate and extremely fast. In fact, it is even able to decompose the beam during a Q-switched pulse emission and produce real-time results.

High power amplification of an electro-optic frequency comb to 200W is achieved, featuring sub 500 fs pulses over a widely tunable repetition rate in the 1-18 GHz range. The final amplification stage of the monolithic set-up includes a full-silica large-mode-area microstructured polarization-maintaining ytterbium-doped fiber with a mode field diameter of 24 µm and M² -parameter values below 1.1. Pulse durations as low as 192 fs were recorded from output pickoff compression.

Apart from the classical nonlinear polarization rotation (NPR) mechanism, we incorporate an additional stabilization mechanism of frequency shifting in a Yb-doped all-normal-dispersion fiber laser oscillator. The similariton pulse with a pulse duration of 7.8 ps and a spectrum width of 20.5 nm is generated. By using a grating pair, the pulse duration can be compressed to be 140 fs. By employing time-stretch dispersion Fourier transform (TS-DFT) technique, the mode-locking pulse buildup process is investigated. The use of frequency shifting as a stabilization technique opens a new route towards reliable laser oscillator fabrication for the industrial-grade system.

In single-mode fiber lasers, power scaling is typically limited by effects such as stimulated Raman-scattering and transverse mode instabilities. However, in highly multi-mode fibers, these two effects are usually negligible due to the high number of transverse modes and therefore, higher optical powers can be achieved. To further decrease the complexity, increase the robustness and reliability of multi-mode fiber resonators, we develop highly reflective multi mode Fiber Bragg Gratings (FBGs) in active doped fibers, which is challenging e.g. due to the required homogeneity of the FBG within the fiber. Here, we present a proof of concept of a fiber-integrated multi-mode resonator using a highly reflective multi-mode FBG for spectral stabilization in a fiber with a rectangular core of 20 x 60 µm² and a numerical aperture of 0.1. More than 60 modes can be guided within this fiber, which can result in a homogeneous near-field beam profile. In our experimental verification, we build up a fiber resonator and achieve an optical-optical efficiency of 50 % and stable wavelength operation at 1072 nm. 10 % of the power is emitted as ASE at a wavelength of 1030 nm, which can be removed by using longer fiber lengths to further improve the efficiency. Approximately 12 % of the signal power is propagating through the HR-FBG, which leads to an FBG-reflectivity of 70-80 % based on our simulations. As a next step, we plan to transfer the HR-FBG concept to round-core XLMA-fibers to improve our earlier demonstrated setups and realize full in-fiber multimode resonators.

The state of a soliton is characterized by the eigenvalues of the Zakharov-Shabat problem.1 Generation of high-intensity pulses through inelastic soliton collisions in dispersion oscillating fibre could be carried out. The inelastic collisions leads to the change amplitudes of solitons, group velocities and even of number of solitons. The generation of giant pulse can be controlled by variation of the modulation period and temporal separation between colliding fundamental solitons. The proposed approach allows to generate rogue waves as result of the inelastic collision of optical solitons.

The first higher power 3.5 μm pumping technique. Initial rapid power scaling has stagnated with no higher average power results in recent years. Power levels in excess of 10 W are necessary to unlock industrial and defense applications. Utilizing our rate equation based numerical model we investigate the expected performance of 3.5 μm fiber lasers configurations operating on the 10 W-20 W range. Fiber geometries and pump configurations are investigated. We also compare resonator with the simulated performance of master-oscillator power amplifiers configuration as a potential alternative for power scaling.

A unidirectional ring cavity was developed and characterized, using a Yb-doped double-clad fiber with nonlinear polarization rotation (NPR). Depending on the relative orientations of the quarter and half-wave plates, we observed that the laser operated in different regimes: Continuous-wave(CW), Q-switching(QS), and Modelocking(ML). We observed that the central wavelength was at 1064.5 nm in each of the different laser regimes. Mode-locking a laser with NPR is attractive due to its simplicity, with the locking being induced by the optical Kerr effect along with the fiber while the wave-plates and filters act as an artificial saturable absorber. The total cavity length of the Yb-doped fiber laser was about 10.8 m, including a free space segment, giving us a 18 MHz repetition rate for the mode-locked pulses. In the Q-switched regime, a pulse train with repetition rates from 14 kHz to 35 kHz and pulse widths from 980 ns to 3 μs were observed by varying the pump power. Lastly, we utilized this laser as a pump source to conduct a comparative study of the frequency-doubling characteristics in a single-pass configuration using MgO:PPLN crystal of 10 mm and 20 mm lengths in the three laser regimes. The second harmonic efficiencies obtained at CW, QS, ML for the crystal length of 10 mm were 2.15% , 11.1%, and 13.6% while for 20 mm they were 2.27%, 15.8%, and 18.7% respectively.

A better understanding of stress effects that affect the bending losses in active and passive optical fibers allows us to improve fiber system designs and helps to optimize refractive index profiles in high power, large mode area laser fibers. Bending an optical fiber affects the light in a fiber core by two different phenomena. First, the curved shape of the waveguide changes the light propagation. The second phenomenon is the refractive index change caused by the mechanical stress in a bent optical fiber. The refractive index changes due to bending stresses are estimated by the elasto-optic and stress optic models. The light propagation in a curved waveguide can be modelled applying the electromagnetic wave theory together with the conformally transformed refractive index profiles that include the stress effects. The modelled refractive index profiles that include the bending stress induced index changes are compared to the refractive index profiles that were measured from actual bent optical fibers. We tested if this comparison would allow us to estimate C2 values in stress optic model. Measured bend loss values are compared to the bend loss values simulated with the modelled refractive index profiles.

Large mode area (LMA) fiber types are widely used in many different fiber laser systems. Since the core diameter of an LMA fiber is generally larger than the mode field diameter (MFD), LMA fibers can carry a few higher order modes (HOM). When LMA fibers are spliced together (such as a splice of a passive LMA fiber to an active LMA fiber) if the cores are not well aligned it may result in beam shape deterioration and downstream fiber over-heating. An appropriate core alignment method for fusion splicing of LMA fiber is thus essential for the fiber laser manufacturing process, especially for multi-kW systems. A recent study on LMA fiber core alignment is reported in this paper and has been implemented in new firmware available for FSM-100 series fusion splicers.