i. This PDF file contains the front matter associated with SPIE Proceedings Volume 10512, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Pump-limited kW-class operation in a multimode fiber amplifier using adaptive mode control was achieved. A photonic lantern front end was used to inject an arbitrary superposition of modes on the input to a kW-class fiber amplifier to achieve a nearly diffraction-limited output. We report on the adaptive spatial mode control architecture which allows for compensating transverse-mode disturbances at high power. We also describe the advantages of adaptive spatial mode control for optical phased array systems. In particular, we show that the additional degrees of freedom allow for broader steering and improved atmospheric turbulence compensation relative to piston-only optical phased arrays.

With their advantages like good beam quality, easy thermal management, high robustness and compact size, fiber lasers are one of the most promising solid state laser concepts for high power scaling with excellent beam quality. One issue of further power scaling is the reduction of nonlinear effects, especially Raman scattering, which consequently led to increased mode field areas. However, for large mode area fibers, new challenges, namely transversal mode instabilities (TMI) have to be taken into account. Beside our investigations in the power scaling of ytterbium doped fiber amplifiers up to 4.4kW output power, we present our investigations of the TMI threshold in dependence on bend diameter and absorption length of a well-known, commercial fiber. Within this scope, we used a 13m piece of the fiber and gradually reduced the bend diameter from 60cm slightly below 14cm within a pump wavelength of 976nm. Furthermore, we increased the fiber length to 30 m, presuming the bend diameter of 14 cm and all experimental conditions. However, in a next step, we detuned the pump wavelength up to 980 nm in order to increase the pump absorption length As a result, we achieved 2.9kW of single mode output at a bend diameter of 14cm. The 4.4kW result was obtained with a separately manufactured low-NA fiber, allowing for a slope efficiency of 90% with regards to the absorbed pump light and an extremely temporal stability.

Thermally-induced refractive index gratings (RIG) in high-power fiber laser systems lead to transverse mode instabilities (TMI) above a certain average power threshold. The effect of TMI is currently the main limitation for the further average power scaling of fiber lasers and amplifiers with nearly diffraction-limited beam quality. In this work we experimentally investigate, for the first time, the growth of the RIG strength by introducing a phase-shift between the RIG and the modal interference pattern in a fiber amplifier. The experiments reveal that the RIG is strong enough to couple energy between different transverse modes even at powers significantly below the TMI threshold, provided that the introduced phase-shift is high enough. This indicates that, as the strength of the RIG further increases with increasing average output power, the RIG becomes more and more sensitive to even small noise-induced phase-shifts, which ultimately trigger TMI. Furthermore, it is shown that a beam cleaning also occurs when a positive phase-shift is introduced, even above the TMI threshold. This finding will pave the way for the development of a new class of mitigation strategies for TMI, which key feature is the control of the introduced phase-shift.

We introduced a simple formula providing the mode-field diameter shrinkage, due to heat load in fiber amplifiers, and used it to compare the traditional thermal-lensing power limit (P^TL) to a newly developed transverse-mode instability (TMI) power limit (P_TMI), giving a fixed ratio of P_TMI/P_TL≈0.6, in very good agreement with experiment. Using a failure-in-time analysis we also introduced a new power limiting factor due to mechanical reliability of bent fibers. For diode (tandem) pumping power limits of 28kW (52kW) are predicted. Setting a practical limit of maximum core diameter to ~35μm, the limits reduce to 15kW (25kW).

Over the last decade, significant work has been carried out in order to increase the energy/peak power provided by fiber lasers. Indeed, new microstructured fibers with large (or very large) mode area cores (LMA) such as Distributed Mode Filtering (DMF) fibers and Large-Pitch Fibers (LPF) have been developed to address this concern. These technologies have allowed diffraction-limited emission with core diameters higher than 80 μm, and have state-of-the-art performances in terms of pulse energy or peak power while keeping an excellent spatial beam quality. Although these fibers were designed to reach high power levels while maintaining a single transverse mode propagation, power scaling becomes quickly limited by the onset of transverse modal instabilities (TMI). This effect suddenly arises when a certain average power threshold is exceeded, drastically degrading the emitted beam quality. In this work, we investigate the influence of the core dimensions and the refractive index mismatch between the active core and the background cladding material, on the TMI power threshold in rod-type Fully-Aperiodic-LPF. This fiber structure was specifically designed to enhance the higher-order modes (HOMs) delocalization out of the gain region and thus push further the onset of modal instabilities. Using a 400W pump diode at 976 nm, the power scaling, as well as the spatial beam quality and its temporal behavior were investigated in laser configuration, which theoretically provides a lower TMI power threshold than the amplifier one due to the lack of selective excitation of the fundamental mode.

We propose, to the best of our knowledge, the first mitigation strategy for TMI based on controlling the phase shift between the thermally-induced index grating and the modal intensity pattern. In particular, in this work we present a study of transverse mode instabilities in burst operation in a high-power fiber laser system. It is shown that, with a careful choice of the parameters, this operation regime can potentially lead to the mitigation of TMI by forcing an energy transfer from the higher-order-modes into the fundamental mode during the burst.

An ultrafast ytterbium-doped fiber laser system based on coherent beam combination of 16 amplifier channels is presented. The system delivers 1.83 kW average power at 2.3 mJ pulse energy and 240 fs pulse duration. The combining efficiency of 82% and the beam M²-value of 1.8 currently is limited by thermal lensing in some optical components, which were identified and are to be replaced.

10mJ energy extraction from a single Yb-doped 85µm core CCC fiber has been achieved using coherent pulse stacking amplification (CPSA) technique. This has been achieved by amplifying a burst of 81 stretched pulses with modulated amplitudes and phases, in a Yb-doped fiber CPA system where it is amplified to 10mJ with low nonlinearity, and coherently stacked into a single pulse with 4+4 cascading GTI cavities. The burst is generated by sending femtosecond pulses from a 1GHz repetition rate mode-locked fiber oscillator into a pair of amplitude and phase electro-optic modulators, where the burst is carved out and pre-shaped to compensate for strong saturation effect in fiber CPA system and to provide correct relative phases for coherent pulse stacking. After each pulse is stretched to approximately 1-ns, the burst is amplified through several cascading fiber amplifiers and down-counted to 1kHz repetition rate, and it extracts >90% stored energy from the last Yb-doped 85µm core CCC fiber. This multi-mJ burst of 81 pulses is then coherently stacked into a single pulse in 4+4 multiplexed GTI cavities consisting of 4 sets of 1ns-roundtrip cavities followed by 4 sets of 9-ns roundtrip cavities. After stacking, the stretched pulse is compressed to <540fs using diffraction-grating pulse compressor. CPSA enables generation of multi-mJ femtosecond pulses with one fiber amplifier channel.

We develop a novel, femtosecond beam combination technique, which can coherently combine large numbers of ultrashort pulse beams using a diffractive optic pair. Existing methods of ultrashort pulse beam combining increase the number of combining optics with the number of beams. Diffractive combiners add many beams on one optic, but exhibit loss for femtosecond pulses due to pulse front tilt. We solve this problem by adding a second diffractive optic to cancel pulse front tilt. By selecting parameters, uncorrected temporal and spatial dispersions from the two DOEs can be made negligible for >30fs pulse widths. We numerically model a proof-of-principle case of 1-D, 4-beam combination, showing that four 120fs beams can be combined with 92% efficiency. This has been demonstrated experimentally with the preservation of 120fs pulse duration and a relative combining efficiency of >85%. A 120fs Yb fiber oscillator output is amplified in a YDFA, split into four phase-controlled channels, and collimated to produce a beam array. This is sent to the DOE pair, forming a combined beam which is compressed and sent to FROG diagnostic. The measured output pulse duration is identical to the oscillator pulse duration. Combining efficiency theory for a 2-D array of ultrashort pulse beams is developed, showing that hundreds of beams can be combined with >90% efficiency. We calculate that for a 224-beam case with practical optical parameters, and temporal dispersion causes 1% extra loss, while spatial dispersion causes 2.5% extra loss, in addition to possible DOE imperfections and beam aberrations.

The XCAN project aims at the coherent combination of 61 fiber amplifiers in the femtosecond regime. An important intermediate step towards this goal is the implementation of a seven fiber test setup, which allows to address key scientific and technical challenges which might occur in the scaled version of 61 fibers. This work includes the design and characterization of a support unit able to hold 61 fibers with the high precision required for an efficient coherent combination in tiled aperture configuration. This configuration, in combination with an interferometric phase measurement and active phase control, is particularly well suited for the coherent combination of a very large number of beams. Our first preliminary results with seven fibers include a combination efficiency of 30 % and a residual phase error between two fibers as low as λ/40 rms. Experiments conducted with three fibers in order to evaluate technical improvements revealed an increase of efficiency to 54 %. The combined beam was temporally compressed to 225 fs, which is Fourier transform limited with respect to the measured spectrum.

Stimulated Raman scattering gives back reflection sensitivity to a high power fiber laser. Therefor SRS suppression is necessary in order to realize stable laser processing by a high power fiber laser. A 5-kW single-mode ytterbium doped fiber laser with a 20-m long delivery fiber has been realized. The fiber laser is an all-fiber single-stage Fabry-Perot system in a co-pumping configuration. The optical to optical efficiency was 80% at the output power of 5.0 kW. And the M-squared figure of 1.3 was obtained. The Stokes light by SRS is suppressed to 45 dB below the laser output by using fibers with the effective mode area of 600 μm² . While SRS was well suppressed, four wave mixing was observed with the frequency shift of ~6 THz. Four wave mixing between the fundamental mode and the secondary modes is believed to take place. Four wave mixing is believed not to give back reflection sensitivity to the fiber laser. The 5-kW single-mode fiber laser was applied to laser processing. Bead-on-plate tests were carried out with a galvanometer scanner. The laser ran without stopping nor damaging the laser system even during processing highly reflective material. This implies that our SRS suppressed single-mode fiber laser can be used practically in most of processing systems.

We report a 1kW, 14 μm-MFD, Yb fiber laser operating at 1117nm, without any parasitic lasing at shorter wavelengths, and negligible intra-cavity Raman gain at longer wavelengths (>50dB extinction at 1175nm). To the best of our knowledge, this is the first experimental demonstration of 1kW output at 1117nm from an Yb fiber laser. In contrast to the 14 μm-MFD of the gain fiber in this report, our previous records at 1117nm of 400W for an Yb oscillator and 450W for an Yb MOPA, were limited primarily by a 6 μm-MFD gain fiber.

We report test data for narrow linewidth fiber amplifiers in a compact modular package and in all-fiber format with power up to 2.5kW and 2.0kW in non-PM and PM fiber configuration respectively for various scientific and advanced applications. Both fiber amplifiers have been pumped directly by laser diodes and have 3m output delivery cable. The fiber amplifiers have more than 35% wall-plug efficiency. Measured M² values are ≤ 1.1. The amplifier data are discussed for different input linewidth and test conditions.

We report on the performance of a standard Yb-doped DC-LMA fiber and compare it to a similar core-size chirally-coupled core (3C®) fiber in a co-pumped fiber amplifier configuration. We used Yb-doped 20/400/0.064 DC LMA fiber for the power amplifier and achieved ~2.4 kW of signal power at 2.79 kW of absorbed pump power. However, we observed an onset of TMI at ~2.2 kW. The spectral bandwidth of this amplifier was 20 GHz and there was no sign of SBS at 2.4 kW of output power. We then used an Yb-doped 21.9/400/0.059 DC 3C fiber with a coiling diameter of ~30 cm to test the efficacy of HOM suppression in this fiber with respect to improving TMI threshold. We achieved 2.6 kW of output power (pump combiner limited) without TMI. Further power-scaling experiments are underway and we will report on the latest findings. However, it is clear from these results that 3C fiber has a better HOM suppression capability compared to 10-cm diameter coiled DC-LMA fiber. Even a 30-cm coiled 3C fiber shows no sign of TMI at 2.6 kW; while, a slightly smaller diameter and tightly coiled 10-cm diameter LMA fiber amplifier shows signs of TMI ~ 2.2 kW. We also measured Brillouin shift, gain bandwidth and gain coefficient and they were found to be ~15.3 GHz, ~83 MHz and 0.47 to 0.7 ×10^-11 m/W respectively compared to reported values of 16.1 GHz, ~64 MHz and 5 ×10^-11 m/W. This significantly lower Brillouin gain and slightly larger gain bandwidth leads to eight times higher SBS threshold for amplifiers using nLIGHT fiber with near single-frequency seed compared to literature values. This is a distinct advantage which will enable optimization of both the LMA and 3C fiber geometry to achieve higher TMI threshold in the future.

We present our recent advances on power scaling of a high-power single-pass pumped CVD-diamond Raman oscillator at 1.2 μm. The single pass scheme reduced feedback to the high gain fiber amplifier, which pumps the oscillator. The Yb-doped multi-stage fiber amplifier itself enables up to 1 kW output power at a narrow linewidth of 0.16 nm. We operate this laser in quasi-cw mode at 10% duty cycle and on-time (pulse) duration of 10 ms. With a maximum conversion efficiency of 39%, a maximum steady-state output power of 380 W and diffraction limited beam quality was achieved.

Thermally induced transverse mode instability (TMI) has been recognized as one of the major limits to average power scaling of single-mode fiber laser. Mitigating the thermal load in single-mode high-power fiber lasers by operating lasing closer to the pump wavelength is one of the effort directions. Here, we demonstrate 220w single–mode output power at 1018nm from an ytterbium-doped all-solid photonic bandgap fiber (ASPBF) pumped at 976nm. The quantum defect is only 4.1%, helping to mitigate the thermal load. The ASPBF fiber has the multiple-cladding-resonant design, leading to better higher-order modes (HOM) suppression in its ~50µm core. The large core/cladding ratio also benefits the 1018nm lasing, providing the higher cladding pump absorption so shorter fiber length is needed with better ASE suppression at longer wavelength. In addition, the use of a phosphosilicate host in this fiber also enhances ytterbium gain at 1018nm, leading to a reduction in the required inversion, further increasing efficiency. In the laser test, one end of fiber is spliced to a high-reflective fiber-Bragg-grating at 1018nm and the other end is right-angle cleaved. ~62% and ~77% lasing efficiency has been achieved around maximum power with respective to the launched and absorbed pump power. The M2 was measured at 130W as 1.06 and 1.17 with respective to the x and y axis.

Single-frequency fiber amplifiers in MOPA configuration operating at 1064 nm (Yb³⁺) and around 1550 nm (Er³⁺ or Er³⁺:Yb³⁺) are promising candidates to fulfill the challenging requirements of laser sources of the next generation of interferometric gravitational wave detectors (GWDs). Most probably, the next generation of GWDs is going to operate not only at 1064 nm but also at 1550 nm to cover a broader range of frequencies in which gravitational waves are detectable. We developed an engineering fiber amplifier prototype at 1064 nm emitting 215 W of linearly-polarized light in the TEM₀₀ mode. The system consists of three modules: the seed source, the pre-amplifier, and the main amplifier. The modular design ensures reliable long-term operation, decreases system complexity and simplifies repairing and maintenance procedures. It also allows for the future integration of upgraded fiber amplifier systems without excessive downtimes. We also developed and characterized a fiber amplifier prototype at around 1550 nm that emits 100 W of linearly-polarized light in the TEM₀₀ mode. This prototype uses an Er³⁺:Yb³⁺ codoped fiber that is pumped off-resonant at 940 nm. The off-resonant pumping scheme improves the Yb³⁺-to-Er³⁺ energy transfer and prevents excessive generation of Yb³⁺-ASE.

Ultra-narrow-linewidth lasers are of great interest in many applications, such as precise spectroscopy, optical communications, and sensors. Stimulated Brillouin scattering (SBS), as one of the main nonlinear effects in fibers, is capable of generating narrow-linewidth light emission. We establish a compact Brillouin/erbium fiber laser (BEFL) utilizing 4-m erbium-doped fiber as both the Brillouin gain and linear media. A 360-kHz-linewidth laser diode is injected into the cavity as the Brillouin pump (BP) light and generates Brillouin Stokes lasing light. Both of the phase noise of the BP and BEFL output are measured by a high-accuracy unbalanced Michelson interferometer. It is demonstrated that 53- dB phase noise reduction is achieved after the BP is transferred into Brillouin Stokes emission. The linewidth of the BEFL is indicated at Hz-range by both calculation and experiment.

In the last years, great efforts have been made to power scale single-mode fiber amplifiers at 1.5μm. In this context, the advantages of Er:Yb co-doped fibers as active media have been widely accepted even though their limitation due to Yb-band ASE are well known. To overcome this limitation the novel technique of off-resonant pumping (e.g. at 940nm) has demonstrated to be effective in cladding-pumped amplifiers. However, the absorption cross-section of Er:Yb fibers at these wavelengths requires a long piece of fiber to achieve high pump power absorption. This is usually an issue in single-frequency systems, where high power levels in a large interaction length reduce the threshold of non-linearities. Nonetheless, the off-resonant technique has not been yet investigated in core-pumped systems because of the low power of single-mode pump sources at 940nm. In addition, the wavelengths longer than 980nm as main pump wavelength are neither sufficiently investigated. In this work we study the viability of the off-resonant pumping technique in purely single-mode systems core-pumped at 1018nm. A 6μm core Er:Yb fiber is core-pumped with a previously generated 1018nm signal and seeded with a single-frequency signal at 1.5μm. The output signal was scaled up to >3.2W before the onset of a significant amount of ASE in the 1μm band occurred. The optical efficiency was 30%. This work represents a proof of principle of a fiber amplifier core-pumped off-resonantly for single-frequency and purely single-mode applications.

We report on a very highly doped and very highly efficient cladding pumped single mode Tm3+ silica fiber laser operating at 1.985 microns. The preform was fabricated using a proprietary all halide vapor phase modification of the MCVD process. The preform core has a maximum Tm2O3 doping concentration of greater than 8.5 wt%. A laser based on the resulting double clad fiber was pumped at 793 nm and demonstrated a record slope efficiency of 74.5 % wrt absorbed pump power.

In this paper we present a high power, polarized 2 μm Thulium-doped fiber laser with high beam quality. Such laser systems are ideally suited for the processing of plastic materials which are highly transparent in the visible and 1 μm wavelength range and for the pumping of laser sources for the mid-IR wavelength region. For most applications polarized lasers are beneficial, as they can be easily protected from back reflections and combined with other laser sources or power scaled by polarization combining. The Tm-doped fiber laser is pumped in an all-fiber configuration by using a fiber coupled pump diode emitting around 790 nm. This pumping scheme allows the exploitation of the crossrelaxation process to populate the upper laser level.

A compact and robust laser configuration was achieved by using an all-fiber configuration with single mode fibers and fiber Bragg gratings (FBG). Different FBG pairs with wavelength around 2 μm were tested. To achieve stable polarized output power the fibers with the FBG were 90° twisted at the splices. Stable linearly polarized output power up to 38 W with an extinction ratio of up to 50:1 was observed. With respect to the diode output power an optical-to-optical efficiency of 51 % was reached with a correspondent slope efficiency of 52 %. The emission linewidth at maximum power was measured to be < 0.3 nm which is well suitable for Ho-laser pumping. First tests of the precise processing of highly transparent plastic materials demonstrate the potentials of these laser systems.

Delivering high peak powers from fiber lasers is limited by the accumulation of nonlinear effects due to the high optical intensities and the long interaction lengths of fibers. Peak power scaling at 2 μm is limited by modulation instability (MI), which is not found for 1 μm sources. This work investigates the performance of a spectrally broadband, nanosecond pulsed thulium-doped fiber laser. The average power and pulse energy performance of the single-mode amplifier delivers >20 W and ~280 μJ. A variable spectral filter is incorporated to study the onset of MI and subsequent spectral broadening as a function of seed linewidth. It is observed that MI-induced spectral broadening is enhanced for larger linewidths. However, when the seed linewidth is increased beyond >10 nm, this trend is reversed. A fiber amplifier model including MI (treated as degenerate four-wave mixing) simulates a parametric gain bandwidth of ~900 GHz for this amplifier configuration, which is equivalent to ~11.5 nm at the 1960 nm center wavelength. Therefore, the decrease in spectral broadening for seed linewidths <10 nm is due to a reduced overlap with the MI gain bandwidth. The capability to scale 2 μm fiber lasers to high powers is strongly dependent on the spectral quality of the seed. Any power initially located within the MI gain bandwidth will degrade performance, and must be considered for power scaling.

A simple engineering design is important for achieving high Thulium-doped amplifier (TDFA) performance such as good power conversion, low noise figure (NF), scalable output power, high gain, and stable operation over a large dynamic range. In this paper we report the design, performance, and simulation of two stage high-power 1952 nm hybrid single and double clad TDFAs. The first stage of our hybrid amplifier is a single clad design, and the second stage is a double clad design. We demonstrate TDFAs with an output power greater than 20 W with single-frequency narrow linewidth (i.e. MHz) input signals at both 1952 and 2004 nm. An optical 10 dB bandwidth of 80 nm is derived from the ASE spectrum. The power stage is constructed with 10 μm core active fibers showing a maximum optical slope efficiency greater than 50 %. The experimental results lead to a 1 dB agreement with our simulation tool developed for single clad and double clad TDFAs. Overall this hybrid amplifier offers versatile features with the potential of much higher output power.

We explore the potential of a new mid-infrared laser transition in praseodymium-doped fluoride fiber for emission around 3.4 μm, which can be conveniently pumped by 0.975 μm diodes via ytterbium sensitizer co-doping. Optimal cavity designs are determined through spectroscopic measurements and numerical modeling, suggesting that practical diode-pumped watt-level mid-infrared fiber sources beyond 3 μm could be achieved.

Short pulse operation of fiber lasers operating at wavelengths up 3 micron have been reported in recent years. At longer wavelengths, fiber lasers have only been demonstrated with a continuous operation mode. Short pulse operation in the mid-IR is necessary for utilizing such lasers in laser radars and for medical applications. Our previous numerical work suggested that Q-switching is possible on the 3.5 μm transition in erbium-doped ZBLAN in a similar manner to work demonstrated on the 2.8 μm transition in erbium. In this work we report on initial experimental results of a Q-switched, dualwavelength pumped fiber laser operating on the 3.5 μm transition in erbium-doped ZBLAN glass fibers. Using a hybrid fiber and open resonator configuration utilizing an acousto-optic modulator we demonstrated stable single pulse Q-switching while operating at repetition rates of 20 kHz and up to 120 kHz. The laser achieved a peak power of 8 W with pulse energy of 7 μJ while operating at 25 kHz. Long pulse widths on the order of 1 μs were obtained. The low peak power and long pulses are likely the result of both low gain of the transition and additional losses in the resonator which are currently being investigated. Our latest results will be presented.

Previously reported progress in 3 micron dysprosium doped ZBLAN fiber lasers achieved record conversion efficiency but was limited in tuneability due to the inband nature of the pumping scheme. Near infrared pumping has also been demonstrated but was limited in conversion efficiency due both to pump excited state absorption and large quantum defect. We address these limitations by employing a Raman fiber laser operating at 1700 nm as a pump source. Reduced quantum defect shows promise for efficiency gains while maintaining near infrared pumping and the increased gain bandwidth shows promise for pulsed operation.

In this contribution we demonstrate the nonlinear pulse compression of an ultrafast thulium-doped fiber laser down to 14 fs FWHM duration (sub-3 optical cycles) at a record average power of 43 W and 34.5 μJ pulse energy. To the best of our knowledge, we present the highest average power few-cycle laser source at 2 μm wavelength. This performance level in combination with GW-class peak power makes our laser source extremely interesting for driving high-harmonic generation or for generating mid-infrared frequency combs via intra-pulse frequency down-conversion at an unprecedented average power.

The experiments were enabled by an ultrafast thulium-doped fiber laser delivering 110 fs pulses at high repetition rates, and an argon gas-filled antiresonant hollow-core fiber (ARHCF) with excellent transmission and weak anomalous dispersion, leading to the self-compression of the pulses. We have shown that ARHCFs are well-suited for nonlinear pulse compression around 2 μm wavelength and that this concept features excellent power handling capabilities. Based on this result, we discuss the next steps for energy and average power scaling including upscaling the fiber dimensions in order to fully exploit the capabilities of our laser system, which can deliver several GW of peak power. This way, a 100 W-class laser source with mJ-level few-cycle pulses at 2 μm wavelength is feasible in the near future.

We present a novel approach for temporal contrast enhancement of energetic laser pulses by filtered SPM broadened spectra. A measured temporal contrast enhancement by at least 7 orders of magnitude in a simple setup has been achieved. This technique is applicable to a wide range of laser parameters and poses a highly efficient alternative to existing contrast-enhancement methods.

Photonic crystal fiber (PCF) technology has radically impacted the scientific and industrial ultrafast laser market. Reducing platform dimensions are important to decrease cost and footprint while maintaining high optical efficiency. We present our recent work on short 85 μm core ROD-type fiber amplifiers that maintain single-mode performance and excellent beam quality. Robust long-term performance at 100 W average power and 250 kW peak power in 20 ps pulses at 1030 nm wavelength is presented, exceeding 500 h with stable performance in terms of both polarization and power. In addition, we present our recent results on hollow-core ultrafast fiber delivery maintaining high beam quality and polarization purity.

Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber is reported. Single-stage amplifier gain larger than 43 dB is achieved, with energy of seed pulses (35 ps, 200 kHz) boosted up to 50 μJ at the amplifier output. Spectral broadening induced by self-phase modulation is shown to take place advantageously along the larger end of the counter-pumped active tapered fiber, where the mode area scales beyond 1000 μm². Pulse durations as short as 1 ps and peak powers exceeding 16 MW are demonstrated thereafter using a chirped volume Bragg grating as a dispersive compressor. Efficient suppression of higher-order modes in the large mode area tapered fiber yields diffraction-limited output (M² < 1.2) for optimal pulse compression.

Mode-locked oscillators are fantastic scientific instruments. Today, femtosecond pulses are now in demand for applications outside physics and chemistry laboratories, most notably for precision materials processing and nonlinear optical microscopy in biomedical laboratories and clinics. These applications require short pulses with high peak power and excellent beam quality. However, they also require devices that are cheap, reliable, and – crucially -- flexible. For example, synchronization of pulses with scanning optics is crucial for high efficiency machining and imaging alike. Lasers in materials processing are often required to exhibit pulse-on-demand operation in order to deposit energy only in specific locations. In biology, such behavior would enable region-of-interest monitoring, allowing e.g. ultrahigh resolution monitoring of a neuronal circuit’s dynamics. This kind of behavior is not compatible with the regular pulse train of a mode-locked oscillator. To this end, we seed an ultrafast fiber amplifier with a gain-switched diode. This a robust, integrated device that can be electronically triggered at virtually any repetition rate, or on-demand, yielding partially-coherent, ~10 ps duration pulses. Remarkably, a single Mamyshev regenerator (nonlinear spectral broadening in fiber followed by offset spectral filtering) allows isolation of a coherent component of the diode pulse. After subsequent nonlinear pulse shaping, amplification to the µJ-regime, and linear compression, we obtain transform-limited ~140-fs pulses with 13 MW peak power. As will be discussed, such a source cannot replace all mode-locked oscillators (e.g., for frequency comb applications), but is extremely attractive for the key mainstream applications of ultrashort pulses.

A 1064 nm, 1 mJ pulsed fiber MOPA module, housed in 16”x14”x2.5” package for application in a lunar and planetary in-situ surface dating instrument is demonstrated. The module is based on a three-stage MOPA with a 60 μm core tapered fiber terminal amplifier. The master oscillator and first two preamplifier stages, which generate 20 μJ pulses, are all contained on a 13”x11”x1” board. Several improvements to the electronic signal control were instrumental to the laser development, including bipolar drive of the phase modulator for SBS suppression, shaping of the seed pulse to compensate pulse steepening, and pulsed operation of the power amplifier pump to reduce spontaneous emission at low pulse repetition frequency. The packaged laser runs at a repetition rate of 10 kHz and generates 10 ns pulses at 1 mJ with a 40 GHz linewidth, an M² ~ 1.2 beam quality, and an 18 dB polarization extinction ratio. The modular design enables seven independent lasers to be stacked in a 20”x18”x16.25” enclosure, supporting a path towards a fiber laser based LARIMS for advanced materials characterization and chronological dating in harsh and remote environments.

Photonic crystal fiber (PCF) technology for ultrafast fiber amplifiers traditionally uses air holes as key elements for large mode area (LMA) fiber designs. These air holes are crucial for the performance of high-end LMA PCFs, but makes splicing and interfacing more complex. To reduce this complexity in mid-range amplifiers, we present single-mode polarization-maintaining Yb-doped LMA PCFs without air holes for easier splicing into monolithic all-fiber amplifier designs.

A 30 μm core all-solid spliceable PCF is presented, and amplification of 1064 nm light above 50 W with an optical to optical efficiency of 80 % is demonstrated.

Furthermore, to demonstrate the excellent reliability of PCF based monolithic amplifiers, we demonstrate ultra-longterm performance data of > 35 khrs on a 14 μm core step-index type PCF amplifier with low long-term power degradation slope of < 1.5 % / 10,000 h.

We present the application of the dispersion scan (d-scan) technique in order to fully characterize intensity and phase of ultrafast pulses compressed by a grating-prism (grism) compressor. Ultrafast pulses are generated in an Yb-doped fiber oscillator, mode-locked by non-linear polarization evolution (NPE) and operating at a central wavelength of 1030 nm with a repetition rate of 50 MHz. These output pulses are stretched in a 150 m long fiber to roughly 50 ps and then amplified in an Yb-doped fiber to an average power of 150 mW. A grating-prism combination is used to compensate the second and third order dispersion that was applied to the pulse by the fiber material. For the optimal compression of the pulse, information on its spectral and temporal phase is required. Therefore, the dispersion scan is applied. In this technique, second harmonic spectra of the pulse are generated and recorded for different amounts of dispersion compensation applied to the pulse by the grism compressor. Depending on the chirp of the pulse, the maximum of the generated second harmonic spectra shifts, leading to a characteristic trace. The amount of dispersion is changed by varying the separation of the prisms within the compressor, while the relation of second and third order dispersion is kept almost constant. The separation is changed by a stepper motor in order to enable a fully automated recording of the spectra. The spectral and temporal course and phase of the pulse are then retrieved from the d-scan trace by a genetic computer algorithm.

We report an all-fiber, linearly polarized, 1.1-W, 1064-nm fiber laser based on a two-stage Ytterbium-doped fiber amplifier seeded by a gain-switched diode laser with tunable pulse width from 21 to 200 ps at repetition rates of 0.5–1.5 GHz. Timing jitter of our 1064-nm fiber laser was measured to be 0.60 ps over 10 Hz–40 MHz when the gain-switched diode laser was operated at a repetition rate of 0.5, 1, and 1.5 GHz. The fiber laser offers an excellent long term power stability of ± 0.3% and wavelength stability of ± 0.01 nm over 8 hours

We report on novel approach to switch the optical beam profile in a fibre laser using a short piece of special passive fibre. The switching mechanism relies on selective wavelength resonance between the core and a high index surrounding ring in the fibre. Beam switching at 530 kHz from a Gaussian beam shape to a flat-top beam with a ring with corresponding M² from 1.4 to 3.5 is demonstrated in a modified industrial pulsed fibre laser.

An all-fiber pulsed erbium laser with pulse width of 2.4 ns working in a MOPA configuration has been created. Cladding pumped double clad erbium doped large mode area fiber was used in the final stage amplifier. Peculiarity of the current work is utilization of custom-made multimode diode wavelength stabilized at 981±0.5 nm – wavelength of maximum absorption by Er ions. It allowed us to shorten Er-doped fiber down to 1.7 m and keep a reasonably high pump-to signal conversion efficiency of 8.4%. The record output peak power for all-fiber amplifiers of 84 kW was achieved within 1555.9±0.15 nm spectral range.

In surface processing applications the correlation of laser power to processing speed demands a further enhancement of the performance of short-pulsed laser sources with respect to the investment costs. The frequently applied concept of master oscillator power amplifier relies on a complex structure, parts of which are highly sensitive to back reflected amplified radiation. Aiming for a simpler, robust source using only a single ytterbium doped XLMA fiber in a q-switched resonator appears as promising design approach eliminating the need for subsequent amplification. This concept requires a high power-tolerant resonator which is provided by the multikilowatt laser platform of Laserline including directly water-cooled active fiber thermal management.

Laserline GmbH and Fraunhofer Institute for Laser Technology joined their forces¹ to upgrade standard high power laser sources for short-pulsed operation exceeding 1 kW of average power. Therefor a compact, modular qswitch has been developed.

In this paper the implementation of a polarization independent q-switch into an off-the-shelf multi-kilowatt diodepumped continuous wave fiber source is shown. In this early step of implementation we demonstrated more than 1000 W of average power at pulse lengths below 50 ns FWHM and 7.5 mJ pulse energy. The M² corresponds to 9.5. Reliability of the system is demonstrated based on measurements including temperature and stability records. We investigated the variation possibilities concerning pulse parameters and shape as well as upcoming challenges in power up-scaling.

In this paper we present a simple approach to achieving nanosecond pulses from a directly q-switched high-power resonator based on extra-large mode area (XLMA) fibers with a beam quality factor M² < 15. An average output power of > 500 W has been demonstrated for repetition frequencies between 50-100 kHz. The resonator consists of a single fiber q-switched with soldered Pockels-cells which exhibit a very high contrast ratio leading to output pulses down to about 10 ns and peak powers up to > 250 kW at 1064 nm wavelength.

By using this design instead of a fiber MOPA setup, a cost-effective and less complex system could be implemented.

A high-energy, single-frequency fiber laser with long pulse duration of 100 ns has been experimentally investigated in an all-fiber architecture. Only 34-cm long heavily Yb-doped phosphate fiber was employed in power scaling stage to efficiently suppress the Stimulated Brillouin effect (SBS). In the experiment, 0.47 mJ single pulse energy was achieved in power scaling stage at the pump power of 16 W. The pre-shaped pulse was gradually broadened from 103 to 140 ns during the amplification without shape distortion.

In this work we propose a simple, all-fiber dispersion managed Ho-doped laser mode-locked with graphene saturable absorber (SA). Dispersion compensating fiber was used to balance the anomalous dispersion of the active and standard single mode fibers, which allows to overcome the limitations set by soliton area theorem. The laser was pumped by a custom-made 1940 nm thulium-doped fiber laser delivering up to 3 W of power. Graphene/PMMA composite sandwiched between fiber connectors serves as a fully fiberized SA. The generated Gaussian pulses centered at 2059 nm reached 53.6 nm of bandwidth. Compressing the pulses by guiding it through SMF-28 fiber yields pulse durations of 190 fs. We also compare the graphene-based laser with a setup mode-locked with nonlinear polarization rotation - an artificial SA.

To the best of our knowledge, this is a first demonstration of a nanomaterial-based stretched pulse fiber laser operating beyond 2 μm. The all-fiber cavity is a more robust alternative to setups that include bulk optics, and requires significantly less alignment compared to lasers mode-locked with real SAs

Ultrafast fiber lasers could provide a compact, robust and cheaper alternative to solid-state sources, such as Ti:sapphire oscillators. If fiber sources can provide equal performance at a fraction of the cost, then many applications of ultrafast pulses and frequency combs will be poised for explosive adaption beyond research laboratories. For the past decade, a major impediment to this has been the reliance on nonlinear polarization rotation (NPR) to mode-lock the lasers. Without NPR, performance is usually insufficient. With NPR, the laser is environmentally unstable. In the past year, remarkable developments have been made based on the use of the Mamyshev regenerator, which simply combines the power-dependent spectral broadening of self-phase modulation with offset spectral filtering to achieve power-dependent transmission. A gain fiber is used to compensate for filtering losses. By arranging two Mamyshev regenerators with opposing spectral filter offsets in a loop, we created a Mamyshev oscillator. Polarization maintaining fibers are used for environmental stability. The power-dependent transmission means that continuous wave lasing is suppressed, and only pulsed operation is possible. This simple but significant change means that incredibly large nonlinear phase shifts can be tolerated. We leverage this large nonlinearity to create spectral bandwidth in a parabolic pulse. We find that the Mamyshev oscillator provides an order-of-magnitude better performance – 1 MW from standard single-mode fiber - than any previous design. Simulations suggest that still greater performance is achievable, and experiments in our laboratory show promise to 10 MW and beyond. Finally, self-starting Mamyshev oscillator results will be discussed.

Additive pulsed-mode mode-locked fiber lasers are known for their wide range of operating states, which can be achieved by tuning the artificial saturable absorber with built-in polarization controllers. We have equipped our laser with three motorized polarization controllers and online monitoring. We recorded three dimensional high-resolution maps of the oscillation states by stepping trough all possible polarization controller settings. In addition, we demonstrate the single and multi-objective optimization by a genetic algorithm of the fiber laser towards a desired operating state. To the best of our knowledge this is the first demonstration of multi-objective optimization of a fiber laser. Doing so, we can selectively operate the cavity in pulsed and continuous-wave mode and tune the wavelength of the laser emission by more than 55 nm. The high wavelength tunability is possible because the polarization controllers and the inline-polarizer act as a tunable birefringent filter.

Higher-order mode (HOM) fibers have been engineered to allow propagation of linearly polarized symmetric modes LP_0,N in a robust way. Compared with the fundamental mode LP(0,1), HOMs exhibits an effective area that can be larger by over two order magnitude, and thus propagating light in these modes could greatly suppress the effect of nonlinear effects. HOM fibers could also be doped with rare earth ions in order to amplify light propagating in these modes, which offers the enormous potential for generating high-intensity pulses. Excitation of HOM gain fiber using cladding pumping with multimode pump source is attractive for ytterbium based amplifiers, because of the availability of low-cost multimode pump diodes in the 975nm wavelength range. One problem associated with cladding pumping which leads to excitation of the large doped core (over 100 μm diameter) is that it could result in a large amount of amplifiedspontaneous- emission (ASE) noise, particularly when the input signal is weak. Optimization of amplifier design is critical in order to suppress ASE and achieve high gain and pump-to-signal conversion efficiency. We conducted numerical modeling of a cladding pumped HOM-amplifier, which revealed that this problem could be mitigated by using a relatively long gain-fiber that allowed reabsorption of the forward propagating ASE resulting in a further amplification of the signal. We demonstrate efficient amplification of a LP_0,10 mode with an effective area 3140μm² in an Yb-doped HOM amplifier cladding pumped at 975nm. We have successfully obtained a 20.2dB gain for 0.95 W 1064 nm input seed signal to more than 105W.

We present investigations on the seed source dependence of stimulated Raman scattering (SRS) created in a high power fiber amplifier. It is shown that fiber oscillators are much worse in terms of SRS than other seed sources. The longitudinal mode composition was found to be of less importance. We reinforce the experimental observations by a numerical investigation, which shows that temporal power variations on the ps-scale and their propagation along the fiber are crucial for the SRS creation in high-power fiber systems, extending the well-known but simplified SRS threshold description.

This work investigates the feasibility of passive cooling in high-power Yb amplifiers. Experimentally, an all-glass airclad step-index (ACSI) amplifier is diode-pumped with 400W and provides 200W power levels. With only natural convection to extract heat, core temperatures are estimated near 130°C with no degradation of performance relative to cooled architectures. Further, advanced analysis techniques allow for core temperature determination using thermal interferometry without the need for complicated stabilization or calibration.

Fiber amplifiers with a robust monolithic seed coupling and very high peak power in a near diffraction-limited beam are increasingly demanded by many industrial applications in laser materials processing. A tapered all-solid rod-type fiber amplifier scheme is proposed. The principle of this approach is the use of a local adiabatic taper to provide a monolithic signal path and selectively excite the fundamental mode in highly multimode fiber. A large mode area fiber is used to scale up the peak power and suppress the nonlinear effects. The powder-sintering technology (REPUSIL) was employed to achieve rod-type fibers with excellent refractive index homogeneity. In this work, we present a double-clad Yb3+-doped rod-type REPUSIL fiber with a core/clad diameter of 45µm/190µm and a core/clad numerical aperture of 0.09/0.19.This developed fiber has a relatively low Al3+-concentration to reduce diffusion during tapering and an optimized outer cladding material to reduce the taper process temperature. Finally it is demonstrated that the diffusion phenomenon is successfully eliminated and the near-diffraction limited beam quality during the amplification process is maintained. First experiments with improved Yb3+-doped rod-type amplifiers delivered 2ns pulses with peak powers of 210kW for the non-tapered rod and 140kW for the tapered rod (limited by facet damage). For the tapered fiber, the beam quality was between 1.3 and 1.7, significantly improved compared to the beam quality of the non-tapered fiber (M2 = 3.3 ~ 4.5). Future work will concentrate on adopting endcaps to protect the fiber facets from damage while scaling up the peak power.

The average output power of fiber lasers have been scaled deep into the kW regime within the recent years. However a further scaling is limited due to nonlinear effects like stimulated Raman scattering (SRS). Using the special characteristics of femtosecond laser pulse written transmission fiber gratings, it is possible to realize a notch filter that mitigates efficiently this negative effect by coupling the Raman wavelength from the core into the cladding of the fiber. To the best of our knowledge, we realized for the first time highly efficient gratings in large mode area (LMA) fibers with cladding diameters up to 400 μm. The resonances show strong attenuation at design wavelength and simultaneously low out of band losses. A high power fiber amplifier with an implemented passive fiber grating is shown and its performance is carefully investigated.

In this paper, we demonstrate an extremely low-concentrated solar-pumped laser (SPL) that uses a transversely excited fiber laser geometry. To eliminate the need for precise solar tracking with an aggressive cooling system and to considerably increase the number of laser applications, low-concentration factors in SPLs are highly desired. We investigate the intrinsic low-loss property of SiO₂ optical fibers; this property can be used to compensate for the extremely low gain coefficient of the weakly-pumped active medium by sunlight. As part of the experimental setup, a 40-m long Nd³⁺-doped SiO2 fiber coil was packed in a ring-shaped chamber filled with a sensitizer solution; this solution functioned as a down-shifter. The dichroic top window of the chamber transmitted a wide range of sunlight and reflected the down-shifted photons, confining them to the highly-reflective chamber until they were absorbed by the Nd³⁺ ions in the active fiber. We demonstrated a lasing threshold that is 10 times the concentration of natural sunlight and two orders of magnitude smaller than that of conventional SPLs.

We present a novel approach for linear polarized operation of a fiber laser not requiring any intra-cavity components, but relying on an external self-seeding effect. A small fraction of back-reflected polarized light created in an external cavity consisting of fused coupler, polarizer and fiber Bragg grating is sufficient to seed and lock the laser to linear polarized operation. This approach enables the construction of polarized monolithic fiber lasers, minimizes intra-cavity losses and drastically reduces parasitic amplified spontaneous emission. We experimentally demonstrate strong polarization locking in an Yb-doped fiber laser with extinction ratio of 500:1 at 1154 nm.

A multiplication method of the Sagnac effect scale factor in ring laser gyroscopes is presented based on the Vernier effect of a dual-coupler passive ring resonator coupled to the active ring. The multiplication occurs when the two rings have comparable lengths or integer multiples and their scale factors have opposite signs. In this case, and when the rings have similar areas, the scale factor is multiplied by ratio of their length to their length difference. The scale factor of the presented configuration is derived analytically and the lock-in effect is analyzed. The principle is demonstrated using optical fiber rings and semiconductor optical amplifier as gain medium. A scale factor multiplication by about 175 is experimentally measured, demonstrating larger than two orders of magnitude enhancement in the Sagnac effect scale factor for the first time in literature, up to the authors’ knowledge.

We present the carrier envelope offset (CEO) frequency stabilization of a fiber laser using two novel methods. The first one is based on cross gain modulation (XGM) implemented with a low-power auxiliary light sharing the laser gain medium. The second method is based on opto-optical-modulation (OOM) of a semiconductor absorber mirror in the laser cavity, used for the first time in a fiber laser. We achieved CEO stabilization with feedback bandwidths of up to 600 kHz using these two easy-to-integrate methods. A tight CEO lock was achieved with a residual integrated phase noise of less than 400 mrad.

The effect of gas pressure, fiber length, and optical pump power on an acetylene mid-infrared hollow-core optical fiber gas laser (HOFGLAS) is experimentally determined in order to scale the laser to higher powers. The absorbed optical power and threshold power are measured for different pressures providing an optimum pressure for a given fiber length. We observe a linear dependence of both absorbed pump energy and lasing threshold for the acetylene HOFGLAS, while maintaining a good mode quality with an M-squared of 1.15. The threshold and mode behavior are encouraging for scaling to higher pressures and pump powers.

We present 10W single-mode fiber laser based on Nd⁺³ fiber operating at 1428nm. All-solid fused silica microstructured waveguide fiber design is employed to suppress amplification at 1μm. The Nd⁺³ fiber is pumped by commercial multi-mode 880nm diode.

Scaling the power-level of fiber sources has many practical advantages, while also enabling fundamental studies on the light-matter interaction in amorphous guiding media. In order to scale the power-level of fiber-sources without encountering nonlinear impairments, a strategy is to increase the effective-area of the guided optical-mode. Increasing the effective-area of the fundamental mode in a fiber, however, presents the challenges of increased susceptibility to mode-distortion and effective-area-reduction under the influence of bends. Therefore, higher-order-mode (HOM) fibers, which guide light in large effective-area (A_eff) Bessel-like modes, are a good candidate for scaling the power-level of robust fiber-sources. Many applications of high-power fiber-sources also demand a deterministic control on the polarization-state of light. Furthermore, a polarization-maintaining (PM)-type HOM fiber can afford the added possibility of coherent-beam combination and polarization multiplexing of high-power fiber-lasers. Previously, we reported polarization-maintaining operation in a 1.3 m length of PM-HOM fiber that was held straight. The PM-HOM fiber guided Bessel-like modes with A_eff ranging from 1200-2800 μm². In this work, we report, for the first time, that the polarization-extinction-ratio (PER) of the HOM exceeds 10 dB in an 8 m long fiber that is coiled down to a diameter of 40 cm. This opens a path towards compact and polarization-controlled high-power fiber-systems.

Monolithic 976 nm laser design based on a newly developed saddle-shaped Yb-doped fiber has been proposed. The fiber has central single-mode part with core diameter of about 12 μm and ultra-thin square-shaped clad with side of about 42x42 μm. At the both ends of the saddle-shaped fiber the core and the clad sizes were adiabatically increased up to 20/(70x70) μm and the fiber could be spliced with standard (80..125 μm clad) passive fibers using commercially available equipment. Single-mode laser at 976 nm based on the developed fiber has been fabricated and photodarkening-free operation with output power of 10.6 W, which is the record high for all-fiber laser schemes, has been demonstrated.

The power scaling of fiber lasers and amplifiers has triggered an extensive development of large-mode area fibers among which the most promising are the distributed mode filtering fibers and the large-pitch fibers. These structures enable for an effective higher-order modes delocalization and subsequently a singlemode emission. An interesting alternative consists in using the fully-aperiodic large-pitch fibers, into which the standard air-silica photonic crystal cladding is replaced by an aperiodic pattern made of solid low-index inclusions cladding. However, in such a structure, the core and the background cladding material surrounding it must have rigorously the same refractive index. Current synthesis processes and measurement techniques offer respectively a maximum resolution of 5×10^-4 and 1×10^-4 while the indexmatching must be as precise as 1×10^-5 . Lately a gain material with a refractive index 1.5×10^-4 higher than that of the background cladding material was fabricated, thus re-confining the first higher-order modes in the core. A numerical study is carried out on the benefit of bending such fully-aperiodic fiber to counteract this phenomenon. Optimized bending axis and radius have been determined. Experiments are done in a laser cavity operating at 1030 nm using an 88cm-long 51μm core diameter ytterbium-doped fiber. Results demonstrate an improvement of the M² from 1.7 when the fiber is kept straight to 1.2 when it is bent with a 100 to 60 cm bend radius. These primary results are promising for future power scaling.

We demonstrates all-fiber master oscillator – power amplifier delivered 70W output power at 1033.33nm with 8 kHz FWHM linewidth without any problems with SBS. The anisotropic ytterbium doped tapered double clad amplifier with 50 μm MFD and polarization extinction ratio about 30 dB is developed as a burst stage. The output radiation demonstrated perfect beam quality (M²=1.03/1.08).

In this work, we demonstrate an architecture to perform Raman-based power combining and simultaneous wavelength conversion of two independently controlled high-power Ytterbium doped fiber lasers operating at different wavelengths into a single laser line at the 1.5-micron band. Specifically, we have been able to achieve an in-band output power of ∼99W with a conversion of ∼64% of the quantum limited efficiency. This power combining is illustrated for different cases of the input wavelengths of the Ytterbium fiber laser. In each case, we have been able to demonstrate a power combining of >87 W in the final 1.5-micron band, with more than 85% of the fraction of the power residing in the final desired band.

We report on the first all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode laser diodes. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of fiber core, results in high-efficiency conversion of a multimode (M²~26) pump at 915 nm into a high-quality output beam at 954 nm. Fibers with core diameter of 62.5, 85 and 100 um are compared. With core enlargement, the output power and slope efficiency increase sufficiently (from 47% to 84%) at the expense of slight beam-quality parameter reduction (M²=1.3-3).

We have demonstrated a ~34 W continuous wave supercontinuum using the standard telecom fiber (SMF 28e). The supercontinuum spans over a bandwidth of ~1000 nm (>1 octave) from 880nm to 1900 nm with a substantial power spectral density of >1mW/nm from 880-1350 nm and ~50-100mW/nm in 1350-1900 nm. The distributed feedback Raman laser architecture was used for pumping the supercontinuum which ensured high efficiency Raman conversions and helped in achieving a very high efficiency of ~44% for supercontinuum generation. Using this architecture, Yb laser operating at any wavelength can be used for generating the supercontinuum and this was demonstrated by using two different Yb lasers operating at 1117nm and 1085 nm to pump the supercontinuum.

Raman fiber lasers have seen increased interest recently, due to their ability to access difficult wavelength ranges without the use of specially doped materials and to avoid some of the obstacles of very high power rare-earth doped fiber lasers, including modal instability and photodarkening. Though most modern works in Raman fiber lasers are based on fiber laser or direct diode pumping, solid state lasers have been developed with extremely high average powers and are readily available commercially.

This work explores a very short fiber length high average power multi-mode Raman laser system. The custom 200um graded index fiber is pumped by 30ns pulses with average powers up to 750W and pulse energies up to 7.5mJ at 1030nm, by a solid state commercial laser system. Pump-only and seeded configurations are examined. In the seeded case, higher order mode activation is demonstrated by detuning the single mode seed to preferentially feed energy to the less confined modes.

5 orders of Stokes are demonstrated, ranging from 1078nm to 1350 nm. Beam enhancement is observed by qualitative measurement of minimum beam waist, and average powers up to 70W are achieved at an energy of 1.4mJ.

We have shown that the relative phase between the fundamental fiber mode and the transverse perturbation affects significantly the local transverse modal instability (TMI) gain. The gain variation is more pronounced as the core diameter increases. This finding can be used in conjunction with other proposed approaches to develop efficient strategies for mitigating TMI in high power fiber amplifiers and lasers. It also provides some physical insight on the physical origin of the observed large differences in the TMI threshold dependence on core diameter for narrow and broad linewidth operation.

Large-mode-area Yb-doped fiber amplifiers operating at high powers are of interest in many areas of manufacturing, processing, and defense as they bypass the catastrophic power density limits of single-mode fiber devices. As power is increased, thermal mode instability limits the beam quality of these devices due to thermally induced, long-period gratings that couple light out of the fundamental mode. In this work, three fiber designs for delaying the onset threshold of thermal mode instability are investigated using a spatio-temporally resolved model based on that developed by Shadi Naderi, modified for additional physics of temperature dependent modes and our design criteria. The model includes amplifier gain, temperature dependent mode shapes and effective indexes, cross- and self-phase modulation terms, and spatially-dependent coupling between fiber modes based on thermals determined by deposition and diffusion. The three designs include a trefoil multi-core design, a clad linear index-graded design, and a confined gain design. The three independent designs result in over 27%, 50%, and 100% improvement in threshold, respectively. All designs were approached based on the objective to reduce the overlaps between the fundamental mode and the higher-order modes, as well as reduce the higher-order mode overlaps with the gain dopant and thus the area of greatest thermal deposition. All three fibers are designed to have a fundamental mode of similar spatial extent to that of the fundamental mode a of a 50 micron core optical fiber with 0.06NA.

Two approaches are presented for detailed analysis of transverse mode instability in fiber amplifiers based on a scalar finite element beam propagation method (BPM). The first employs two beams: one propagating at a fundamental frequency and one de-tuned to the middle of the stimulated thermal Rayleigh scattering (STRS) gain peak. This method was found to suffer from a computational artifact causing it to converge in some cases to an unphysical solution. The second was based on the steady periodic method. This required more computational resources but was found to be reliable and not susceptible to the artifact mentioned above. This method was used to simulate step-index fiber amplifiers, large pitch photonic crystal fiber amplifiers, and a hybrid large pitch photonic bandgap fiber amplifier with reduced symmetry. Results for reference step index fiber amplifiers were found to be consistent with those obtained by other methods. The simulated instability threshold values all fell between 200 and 310 Watts showing relatively little variation among designs. Some areas for improvement in the method are discussed.

Publisher’s Note: This paper, originally published on 2/26/18 was replaced with a corrected/revised version on 5/25/18. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

The thermal grating (TG) and inversion grating (IG) TMI gain dependence on the light beating intensity spectrum is investigated. TMI gain is restricted to intensity bandwidths comparable to the thermal gain bandwidth of ∼20 kHz. Seed laser phase noise generates intensity spectra determined by the laser linewidth and the relative group delay time of the gain fiber. These spectral bandwidths exceed the thermal gain bandwidth by orders of magnitude in both the coherent and incoherent regimes, making them unlikely sources of TMI. It is suggested that phase noise generated in the gain fiber due to external perturbations may be the source of the TMI.

We have developed a Watt-level single-frequency tunable fiber laser in the 915-937 nm spectral window. The laser is based on a neodymium-doped fiber master oscillator power amplifier architecture, with two amplification stages using a 20 mW extended cavity diode laser as seed. The system output power is higher than 2 W from 921 to 933 nm, with a stability better than 1.4% and a low relative intensity noise.

To answer a growing demand in development of high power pulsed and continuous wave sources at 2 micron spectral range we have participated in several projects, which resulted in a delivery of versatile monolithic sources providing picosecond, nanosecond and CW laser signal. As an example of pulsed sources we developed all-fiber monolithic devices based on a directly modulated laser diode and gain-switched laser diode to generate nanosecond and picosecond pulses, respectively, which are amplified in the same fiber amplifier chain up to 50 µJ with 96 ps and more than 1 mJ with pulses longer than 35 ns.

We developed the dual line fiber laser module based on FBG combination. The proposed configuration has several advantages such as compact, simple, and inexpensive. The laser was composed pump LD (40W), two HR FBGs for 1053 nm and 1058 nm, Yb-doped fiber, two OC FBGs for 1053 nm and 1058 nm, and delivery fiber. All single mode fibers were polarization maintained with approximately 6 micron core. All FBGs were mounted on holders with TECs and their temperatures were controlled independently. The center wavelengths of HR and OC FBGs were temperature dependent and their shifts are approximately 7 nm/degree-C for all integrated FBG. By adjusting the temperature, it is possible to realize the resonant condition for only 1053 nm or only for 1058 nm. Based on this configuration, we demonstrated dual line CW fiber laser module. This module was compact with the size of 200 mm X 150 mm X 23 mm. By adjusting the FBG temperatures, we obtained the output power of more than 10 W at 1053 nm and 1058 nm with linear polarization.

Ultra-wide emission in bismuth doped optical fiber has been extremely studied for the development of the laser and amplifier working at near infrared band. In our homemade bismuth/erbium co-doped optical fiber, bismuth active center associated with silica (BAC-Si) has been found that when pumping at its resonant wavelength at 830 nm the NIR emission could be partially bleached. In addition, a self-recovery process has been observed at room temperature. However, the exact mechanism is still unclear. In this work, we have investigated the photo-bleaching effect on the BAC-Si via the pump power, pump wavelength and temperature dependence. Based on analyzing the result using stretched exponential function, it shows that the bleaching effect on BAC-Si has a strong link with the excitation process of Bi ion in BAC-Si. A potential energy curve model is used to illustrate the BAC-Si photo-bleaching process.

Yb-doped Photonic Crystal Fibers (PCFs) have triggered a significant power scaling into fiber-based lasers. However thermally-induced effects, like mode instability, can compromise the output beam quality. PCF design with improved Higher Order Mode (HOM) delocalization and effective thermal resilience can contain the problem. In particular, Fully- Aperiodic Large-Pitch Fibers (FA-LPFs) have shown interesting properties in terms of resilience to thermal effects. In this paper the performances of a Yb-doped FA-LPF amplifier are experimentally and numerically investigated. Modal properties and gain competition between Fundamental Mode (FM) and first HOM have been calculated, in presence of thermal effects. The main doped fiber characteristics have been derived by comparison between experimental and numerical results.

We report the design, experimental performance, and simulation of a single stage, co- and counter-pumped Tmdoped fiber amplifier (TDFA) in the 2 μm signal wavelength band with an optimized 1567 nm shared pump source. We investigate the dependence of output power, gain, and efficiency on pump coupling ratio and signal wavelength. Small signal gains of >50 dB, an output power of 2 W, and small signal noise figures of <3.5 dB are demonstrated. Simulations of TDFA performance agree well with the experimental data. We also discuss performance tradeoffs with respect to amplifier topology for this simple and efficient TDFA.

High power single frequency lasers are very attractive for a wide range of applications such as nonlinear conversion, gravitational wave sensing or atom trapping. Power scaling in single frequency regime is a challenging domain of research. In fact, nonlinear effect as stimulated Brillouin scattering (SBS) is the primary power limitation in single frequency amplifiers. To mitigate SBS, different well-known techniques has been improved. These techniques allow generation of several hundred of watts [1]. Large mode area (LMA) fibers, transverse acoustically tailored fibers [2], coherent beam combining and also tapered fiber [3] seem to be serious candidates to continue the power scaling. We have demonstrated the generation of stable 200W output power with nearly diffraction limited output, and narrow linewidth (Δν<30kHz) by using a tapered Yb-doped fiber which allow an adiabatic transition from a small purely single mode input to a large core output.

We report on the generation of dual-wavelength noise-like pulse (NLP) from a passively mode-locked all-fiber laser based on multimode interference (MMI) effect. The theory to evaluate and design transmission spectrum of MMI filter is analyzed. A homemade MMI filter was employed in an Er-doped fiber ring laser with NPE configuration and dual-wavelength NLP at 1530 and 1600 nm was obtained with 3-dB bandwidth of 18.1 and 41.9 nm, respectively. The output had a signal-to-noise ratio higher than 35 dB and can achieve self-started operation.

We obtain a switchable and tunable dual-wavelength single-frequency Er-doped ring fiber laser. In order to realize single-longitudinal output, two saturable-absorber-based tracking narrow-band filters are formed in 3- meter-long unpumped Er-doped fiber to narrow the linewidth via using the PM-FBG as a reflection filter. The maximum output power is 2.11 mW centered at 1550.16 nm and 1550.54 nm when the fiber laser operates in dual-wavelength mode. The corresponding linewidths of those two wavelengths are measured to be 769 Hz and 673 Hz, respectively. When the temperature around the PM-FBG is changed from 15 °C to 55 °C, the dual-wavelength single-frequency fiber laser can be tuned from 1550.12 nm to 1550.52 nm and from 1550.49 nm to 1550.82 nm, respectively.

A novel narrow line-width Single longitudinal mode (SLM) dual wavelength random fiber laser of 20 nm separation between wavelengths of 1530 and 1550 nm is presented. The laser is based on Rayleigh backscattering in a standard single mode fiber of 2 Km length as distributed mirrors, and a semiconductor optical amplifier (SOA) as the optical amplification medium. Two optical bandpass filters are used for the two wavelengths selectivity, and two Faraday Rotator mirrors are used to stabilize the two lasing wavelengths against fiber random birefringence. The optical signal to noise ratio (OSNR) was measured to be 38 dB. The line-width of the laser was measured to be 13.3 and 14 KHz at 1530 and 1550 nm respectively, at SOA pump current of 370 mA.

We experimentally demonstrate the use of a high-speed, solid-state optical switch as an efficient Mid-IR Q-switch. The switch could be modulated upto 2.2 MHz and its rising and falling time were measured to be 160 ns and 140 ns, respectively. Using the optical switch, stable actively Q-switched pulses were readily generated from a thulium doped-fiber laser cavity. The minimum pulse width of the pulses was measured to be 160 ns at 100 kHz with the average output power of 0.5 mW.

We have presented fiber laser configuration with an injection locked DFB (IL-DFB) laser for application in phase sensitive optical time domain reflectometry (φ-OTDR). A low-loss fiber optical ring resonator (FORR) is used as a high finesse filter for the self-injection locking of the DFB laser.

We have used IL-DFB laser configuration for detection and localization of the vibrations in phase sensitive OTDR system. By applying moving differential processing algorithm for the Rayleigh backscattering traces we demonstrate accurate localization of the vibrations with a frequency ~50 Hz at a distance about 10 km with the same signal to noise ratio that is achievable with an ultra-narrow linewidth OEwaves laser (OE4020-155000-PA-00).

Based on the 1550 nm single-frequency Erbium-Ytterbium co-doped fiber amplifier, the output linewidth with different seed power and gain fiber temperature was experimental investigated. The results demonstrated that, to obtain the same output power, the increment of the seed power was benefit to improve the output signal to noise ratio (SNR), and to reduce the linewidth broadening. The increment of gain fiber temperature can improve slope efficiency and enhance the ASE intensity but broaden linewidth. Meanwhile, the ASE was considered as one of the reason of linewidth variation.

We present result of achieving a random fiber laser at a working wavelength of 1178nm while pumping at 1018nm. The laser power is realized by 200m long cavity which includes three high reflectivity fiber Bragg gratings. This simple and efficient random fiber laser could provide a novel approach to realize low-threshold and high-efficiency 1178nm long wavelength laser. We theoretically analyzed the laser power in random fiber lasers at different pump power by changing three high reflectivity fiber Bragg gratings. We also calculated the forward and backward power of 1st-order stokes, 2nd-order stokes, 3rd-order stokes. With the theoretical analysis, we optimize the cavity’s reflectivity to get higher laser power output. The forward random laser exhibits larger gain, the backward random laser has lower gain. By controlling the value of angle-cleaved end fiber’s reflectivity to 3×10^-7, when the high reflectivity increases from 0.01 to 0.99, the laser power increases, using this proposed configuration, the 1178nm random laser can be generated easily and stably.

In this work we present an ultrafast laser system distinguished by its industry-ready reliability and its outstanding flexibility that allows for real-time process-inherent parameter. The robust system design and linear amplifier architecture make the all-fiber series TruMicro 2000 ideally suited for passive coupling to hollow-core delivery fibers. In addition to details on the laser system itself, various application examples are shown, including welding of different glasses and ablation of silicon carbide and silicon.

We present an axially tapered large mode area W-fiber for high peak power femtosecond pulse propagation at 1550-nm wavelength. Our numerical simulations show pulse compression with output pulse of 36 kW peak power and 81 fs duration in down-tapered case and nearly undistorted propagation of 20 kW, 150 fs pulse in up-tapered case. Such direction dependent propagation of the pulse in the fiber should find applications in high power laser systems.

Ever extending applications of fiber lasers require energy efficient, high-power, small footprint and reliable fiber lasers and laser wavelength versatility. To meet these demands, next generation of active fibers for high-power fiber lasers is coming out that will eventually offer tailored spectroscopic properties, high robustness and reduced cooling requirements and improved efficiency through tailored pump absorption. We report on numerical modelling of the efficiency of the pump absorption in double clad active fibers with hexagonal shape of the inner cladding cross section and rare-earth-doped core. We analyze both the effect of different radii of the spool on which the fiber is coiled and different fiber twisting rates. Two different launching conditions were investigated: the Gaussian input pump beam and a speckle pattern that mimics the output of the pump laser diode pigtail. We have found that by asymmetric position of the rare-earth-doped core we can significantly improve the pump absorption.

We report an in-fiber structure based on the use of a multimode fiber segment and a double cladding fiber segment, and its application as spectral filter in an erbium-doped fiber laser for selection and tuning of the laser line wavelength. The output transmission of the proposed device exhibit spectrum modulation of the input signal with free spectral range of ~21 nm and maximum visibility enhanced to more than ~20 dB. The output spectrum of the in-fiber filter is wavelength displaced by bending application which allows a wavelength tuning of the generated laser line in a range of ~12 nm. The use of the proposed in-fiber structure is demonstrated as a reliable, simple, and low-cost wavelength filter for tunable fiber lasers design and optical instrumentation applications.

We demonstrate an all-fiberized, linear-polarized, narrow spectral linewidth laser system with kilowatts-level output power at 1030 nm in master oscillator-power amplifier (MOPA) configuration. The laser system consists of a linear-polarized, narrow linewidth (~28 GHz) fiber laser oscillator and two stages of linear-polarized fiber amplifiers. A 925 W linear-polarized fiber laser with a polarization extinction ratio (PER) of 15.2 dB and a spectral width of ~60 GHz at the central wavelength of 1030.1 nm is achieved. Owing to the setting of the appropriate parameters for the laser, no indication of Stimulate Brillouin Scattering (SBS) is observed in the system. Moreover, thanks to the excellent quantum efficiency of the laser and the thightly coiling of the active fiber in the main amplifier, the mode instability (MI) is successfully avoided. As a result, the near diffraction-limited beam quality (M²<1.3) is achieved.

We report a linear cavity all-fiber passive Q-switched thulium-doped fiber laser operating at the 2 μm wavelength range. The laser configuration is based on a thulium-doped fiber used as a gain medium and an unpumped segment of holmium-doped fiber which acts as a fiber saturable absorber. The cavity is formed by a fiber optical loop mirror and the flat end facet of the holmium-doped fiber. The fiber segments as saturable absorber is a 1-m long single mode doubleclad holmium-doped fiber. Q-switched pulses are obtained at the wavelength of 2024.5 nm with a pulse width of 1.1 μs. The pulse repetition rate increases as a linear function of the applied pump power. The maximum pulse repetition rate of 100 kHz was obtained with a pump power of 2.4 W.

We report a soliton mode locked femtosecond oscillation with all-polarization maintaining erbuim doped fiber laser based on Carbon Nanowalls saturable absorber (CNWs SA). To improve the stability and the capability of the oscillator, the all-polarization maintaining(all-PM) fiber is generally used since PM fiber is tolerant of stretches and bends. The saturable absorber is an optical device that placed in a laser cavity to suppress continuous wave operation to promote cooperation between many modes to sustain ultrashort pulse operation. We apply CNWs for the material of SAs in our oscillator. CNWs are one of the nanocarbon materials, which are a high-aspect-ratio structure in the cross-section, where, although their width and height range in a few micrometers, the thickness is as small as ten nanometers or so. A sheet of CNWs is made up of nano-size graphite grain aggregates. Then CNWs structure is expected to have a high absorption to the incident light and large modulation depth due to a small number of carbon layers as well as CNT and Graphene. With this all-PM fiber laser oscillator based on CNWs SA, the soliton mode-locked laser oscillated with 66.3MHz repetition frequency and its spectrum width is 5.6nm in FWHM. Average output power is 8.1mW with 122.5mW laser diode pump power. In addition, the laser amplification system with erbium-doped fiber is constructed and amplifies the femtosecond pulse laser into 268.2mW and 3000mW pumping power.

We have proposed a germanium doped core fiber design for large mode area single-mode applications. The designed fiber effective index, dispersion and bend loss of the fundamental mode and next higher order modes have been calculated using the numerical method. The fiber exhibits a high differential loss between the fundamental mode and higher order modes. Therefore, the designed fiber structure effectively suppresses the higher order modes and retains only the first mode (or fundamental mode) in the core region. Our simulation results demonstrate that, a low loss of 0.1dB/m is achieved for fundamental mode at 1060nm wavelength with 10cm bend radius, along with it also exhibits a high loss of 4.8 dB/m to first higher order mode. The fiber shows a large mode area of 831.4 μm² at 1060nm wavelength. The proposed paper further explores the fiber properties such as dispersion and fabrication tolerances. Our design shows a dispersion of 39 ps/km-nm at 1060nm, and also the structure shows a less dispersion variation over a wavelength band of 400nm. The fiber reduces the fabrication difficulties as compared to the other designed fibers. We fabricated the present fiber using the renowned vapor axial deposition technique. In this method, we can achieve the large diameter preforms and also the method decreases the tolerances when dealing with glasses.

We report an experimental investigation on the onset of SBS-induced modulation instability in a CW Yb-doped fiber amplifier. Threshold for such backscattered signals is found to be well below SBS threshold.

We report the dynamics of multi-pulse in a ring cavity passively mode-locked fiber laser with a strict control of the polarization state. We study the relation between the polarization state of the pulses propagating in the cavity and the regimes of generation. We have found that small ellipticities, the laser generates one bunch of pulses in the cavity, while at higher ellipticities the laser generates multiple bunches. At constant ellipticity we rotated the polarization azimuth and observed a regime transition from the generation of a bunch of solitons to that of noise-like pulses (NLP).