This PDF file contains the front matter associated with SPIE Proceedings Volume 11665, including the Title Page, Copyright information, and Table of Contents.

Welcome and Introduction to SPIE Photonics West LASE conference 11665: Fiber Lasers XVIII: Technology and Systems

We report 3kW test data of Single-Mode (SM) Ytterbium-doped fiber amplifier with diffraction-limited divergence for narrow linewidth seed sources in compact modular package with all-fiber format. Measured M² values of output beam in full power range are < 1.1. The fiber amplifier has been pumped by direct laser diodes and has 3m output delivery cable terminated with IPG connector. The fiber amplifier in modular package has ≥ 40% wall-plug efficiency and 15nm spectral bandwidth in 1055-1070nm wavelength range. The amplifier operates up to 3kW at linewidths of 60GHz and 30GHz with polarized and depolarized seed sources respectively. The amplifier data and non-linear SBS, SRS and MI effects are discussed for different input linewidth and test conditions.

In this study, we design and fabricate a novel type of active fiber——double-tapered double-clad fiber (DT-DCF). Based on this self-developed DT-DCF, we have constructed an all-fiberized fiber amplifier that is operating under a continuous-wave (CW) regime at 1080 nm wavelength. The maximum output power of the system reaches 4 kW, which, to the best of our knowledge, is the highest output power of tapered fiber-based laser systems. The amplifier exhibits near-single-mode beam quality (M²=1.33) at the highest output power with a slope efficiency of 83%. Our result successfully verifies the potential of power scalability of DT-DCF, and the performance of our system can be further enhanced by fiber design optimization.

Stimulated Brillouin scattering (SBS) is the lowest threshold nonlinear effect that limits power scaling in narrow linewidth continuous wave fiber lasers. While several SBS mitigation techniques exist, optical linewidth broadening through external phase modulation has become the predominant method for SBS suppression. Arbitrary waveform generators (AWG) and pseudo-random binary sequence (PRBS) modulation schemes provide enhanced control of lineshape to effectively mitigate SBS but are expensive and difficult to implement. White noise (WNS) phase modulation is simple and easy to implement, but the resulting line shape is non-ideal and has a slow roll-off. Thus, the SBS enhancement obtained experimentally with WNS broadening is reasonably lower than the theoretical value. We attribute this to the increased SBS seeding due to the overlap between the WNS broadened signal and the Brillouin gain spectrum. A modulation scheme that is implemented easily and provides adequate line shape control would be of great advantage from a practical and engineering point of view. In this work, we propose a simple, yet powerful modulation technique to synthesize a line shape to have fast roll-off and improved flatness by incorporating dual sine and noise modulation. An in-house built kW-class, polarization maintaining, multi-stage Ytterbium-doped fiber amplifier is used to quantify the SBS enhancement of the proposed modulation scheme. We experimentally compare the results to that of pure noise broadened modulation at similar RMS linewidths and demonstrate over 2.3x enhancement in SBS limited output power at ~7.3GHz and <1kW SBS unlimited output power at ~10.4GHz in a fully polarization maintained system.

We are reporting a monolithic all-fiber highly multimode laser reaching up to 4.03 kW stable output power. The phosphosilicate glass matrix used as a gain medium is highly multimode optical fiber. This allows to enhance dramatically the Stimulated Raman Scattering rejection without observation of a Transverse Mode Instability neither a photodarkening phenomenon. The achievement of a high reflectivity fiber Bragg grating written on highly multimode passive fiber made the multimode laser cavity feasible.

A short-pulse Yb-doped fiber laser based on a master oscillator and power amplifier scheme is reported to yield an average power exceeding 500 W and pulse energy over 1 mJ. The final amplifier stage features a polarization-maintaining, large mode area tapered fiber with core/cladding diameters of 35/250 μm and 56/400 μm at each end of the flared section. The latter yields excellent optical conversion efficiency, near diffraction-limited output, narrow spectral linewidth and high polarization extinction ratio. The threshold for the onset of stimulated Raman scattering was further investigated using a pulsed seeder with ps-ns digitally programmable waveforms. Besides, no indication for transverse mode instability could be observed below the stimulated Raman scattering threshold, as beam quality M² was measured < 1.3 and no fluctuations were further detected from photodiode time-traces of near-field laser beam samples.

A 1.1 kW CW fiber amplifier emitting at 1.95µm and phase modulated at 5 GHz, with 51% optical conversion efficiency and near diffraction limited beam quality (M2<1.1) is demonstrated. The fiber amplifier consists of 9m of active 20/400um thulium doped fiber, and 2m of passive delivery fiber. Several limiting nonlinearities critical to maintaining coherence were analyzed, including stimulated Brillouin Scattering and modulation instability. To our knowledge, this is the first kw-class thulium doped fiber amplifier operating with narrow linewidth.

The principle of a Mamyshev oscillator depends on alternating spectral filtering between sections of spectral broadening by self-phase modulation. In the 2 µm wavelength range, this concept faces the difficulty that standard fibers are anomalous dispersive which limits the possible pulse energy to the pJ-regime without proper dispersion management. We applied ultra-high numerical aperture fibers with normal dispersion in order to achieve up-chirped pulses in an anomalous dispersive Thulium-doped gain fiber. With that design, we achieved mode-locked pulses with energies of 6.4 nJ and a compressed autocorrelation duration of 195 fs at a repetition rate of 16 MHz.

We demonstrated an ultra-low noise polarization-maintaining (PM) single frequency fiber laser (SFFL) at 2 μm. By suppressing the pump relative intensity noise (RIN) using a feedback loop control, the RIN and frequency noise of the SFFL are simultaneously reduced, and the reduction is about 3 - 15 dB and 3 - 8.4 dB, respectively. After two stage Tm³⁺-doped PM fiber amplifier, the output power reached about 5 W. Meanwhile, the frequency noise almost has no increases, which is still below 100 Hz/√Hz after 13 Hz. And the frequency-tunable range is approximately 2 GHz with frequency response of 46 MHz/V.

We have demonstrated an efficient 1720-nm all-fiber laser with ring-cavity configuration based on commercial Tmdoped silica fiber and 1570-nm in-band pump source. The rate equation model was built up to analyze the laser performance of Tm-doped fiber, which exhibits strong absorption in 1.7-μm region. The results show that efficient laser operation can be achieved through the optimization of output coupling and the length of Tm-doped fiber. By using homemade couplers, we experimentally achieved 2.36-W laser output power under 6-W launched pump power. The slope efficiency with respect to the absorbed pump power and optical efficiency were 50.2% and 39.3%, respectively. Due to the employment of ring resonator, a narrow laser linewidth of ~4 GHz at maximum output power was observed.

We report the design and performance of Holmium-doped fiber amplifiers (HDFAs) with novel alternative in-band pump wavelengths in the 1720—2000 nm spectral region. We demonstrate through simulations that pump wavelengths of 1840—1860 nm can yield significantly improved output power (3—6 dB), gain (8—10 dB) , and optical-optical conversion efficiency compared to the previous technical and industry standard pump wavelength of 1940 nm. Experimental results fully confirm our simulations.

Stimulated Brillouin scattering (SBS) is typically the lowest order nonlinearity encountered in ytterbium doped fiber amplifiers (YDFA), and the simplest means of suppressing it is though linewidth broadening from phase modulation. However, to maintain compatibility with beam combining techniques critical to scaling to high output powers, narrow linewidths are needed, and bandwidth efficient means of suppressing SBS are key to scaling to high powers. Here, the scalability of novel phase modulation techniques in combination with laser gain competition are explored. Ultimately, LGC is shown to improve the TMI threshold by 15%, and reduce the linewidth by a factor of 2.1. A 1.8 kW fiber amplifier with 7 Ghz linewidth is demonstrated.

The stimulated Brillouin scattering (SBS) is the main power limiting factor in the high-power narrow-line fiber laser circuits. A good way to increase the SBS power-threshold is to reduce its coherent gain, which is directly connected with the width of the Brillouin gain spectrum (BGS). The Brillouin gain peak and the phonons life-time are inversely proportional to the full-width-at-half-maximum of the BGS. The fine structure of the BGS and its ranges are sufficiently affected by parameters of the acoustic waveguide of the fiber. We propose a novel approach for increasing the BGS width and lowering its maximum (related directly to the SBS threshold) for a given optical refractive index profile. The aim of the approach is to maximize both the number and spectral spread of guided acoustic modes, as well as equate the acousto-optic interaction coefficients (acousto-optical overlap integrals) for the maximum possible number of these modes. This is due to the fact that an increase in the number of acoustic modes effectively contributing to the BGS, while preserving the distribution of the optical mode intensity, causes the scattered Stokes optical power to be redistributed accordingly between the corresponding number of Brillouin spectral lines, providing proportional damping of the Brillouin gain. Such an acoustically multimode SBS suppression can be achieved by tailoring a proper radial acoustic refractive index profile which can be fabricated by co-doping of silica with phosphorous oxide and fluorine.

Fiber lasers of high-order spatial modes are advantageous in a number of applications. We experimentally demonstrate a transverse mode-selective few-mode Brillouin fiber laser using a pair of mode-selective photonic lanterns as spatial mode filters. The three LP lasing modes were generated based on both intra- and inter-modal stimulated Brillouin scattering. Their slope efficiencies, optical spectra, mode profiles, and linewidths were characterized for both cases. Based on the ring-cavity configuration, distributed temperature and strain sensing with 1-m spatial resolution is also demonstrated. The Brillouin dynamic grating was generated in the LP01 mode and high-order modes were used as probes. The feasibility of simultaneous temperature and strain sensing has also been investigated.

We observed a large optical bistability in a single-frequency thulium fiber laser with ring cavity configuration. A piece of unpumped Tm-doped fiber served as nonlinear saturable absorber (SA), which also acted as a narrow-bandpass filter by forming self-induced gratings with counter-propagate lasers and enabled single-frequency laser operation at 1720 nm. Due to the large absorption cross section of thulium ions at 1720 nm, the unpumped Tm-doped fiber has large variable losses, hence resulting in strong optical bistability. With 0.75-m SA fiber, a 4.8-W wide bistable region was achieved. The evolution of bistable region with different lengths of SA fibers was investigated. The bistable region became narrower with decreasing SA fiber length, and totally disappeared at a SA fiber length of 0.15 m. To the best of our knowledge, this is the first observation of optical bistability in thulium fiber lasers.

We show how neural networks can be used to model complex and predict nonlinear propagation dynamics in optical fibres for a widerange of input conditions and fibre systems, including pulse compression, ultra-broadband supercontinuum generation, and multimode fiber systems. Our results open up novel perspectives to model and optimize complex nonlinear dynamics and systems.

We analyzed the nonlinear dynamics of pulsed beam self-cleaning in nonlinear tapered Ytterbium doped and Erbium-Ytterbium codoped graded-index multimode optical fibers, with quasi-uniform doping distribution in the core cross-section. By increasing the net gain when operating in active configuration we observed that the output spatial intensity distribution changed from a speckled into a high-quality and bell-shaped beam. By launching pulses in the normal dispersion regime of the taper, from the wider into the smaller core diameter, we generated a supercontinuum emission between 520 nm and 2600 nm. When the laser pulses were launched into the small core side of the tapered fiber or in the Erbium-Ytterbium fiber, self-cleaning was obtained without any self-phase modulation-induced spectral broadening or frequency conversion.

We have demonstrated a new record of 302W single-mode power from an Er/Yb co-doped fiber master oscillator power amplifier (MOPA) with a record optical efficiency of 56%. This single-mode power is a new record for any lasers in this wavelength range. The previous record of single-mode power from an Er/Yb fiber laser pumped by a 9xx nm diode was 207 W at 1560 nm. The new optical efficiency of 56%, very close to the quantum-limited efficiency of 58.6%, is also a new record for Er/Yb fiber lasers. This new result is made possible mainly due to new fiber development from Nufern and off-resonant pumping of the Er/Yb fiber, which demonstrates further power-scaling potentials of Er/Yb fiber lasers pumped by widely available 9xx nm diodes. We also show that further power scaling is no longer limited by Yb³⁺ parasitic lasing near 1.06μm, but by fiber fuse in the Er/Yb fiber. The Yb³⁺ amplified spontaneous emission (ASE) was found to be negligible in all the cases we tested. Nonetheless, our numerical investigation shows that off-resonance pumping at 915 nm or 940 nm only plays a small role in the above-mentioned negligible ASE. We believe that the major cause may be the high Er³⁺ doping level in the Nufern Er/Yb co-doped fiber. Our results provide significant new insights and will stimulate further power scaling of Er/Yb fiber lasers and amplifiers.

Conventional models of Er/Yb co-doped fibers assume all ytterbium ions are equally involved in the energy transfer with erbium ions, governed by a singular transfer rate. This would predict output power clamping once ytterbium parasitic lasing starts, contrary to the observations that the output continued to grow albeit at a slower rate. One study explained this using elevated temperature at high powers. Our study, however, shows that elevated temperature and mode-dependent effects only play insignificant roles. A new model is developed based on the existence of isolated ytterbium ions, which can explain all the observed experimental behaviors.

Single-frequency Er³⁺:Yb³⁺ co-doped fiber amplifiers (EYDFAs) are promising candidates for laser sources in the next-generation of gravitational wave detectors. The high power scalability of EYDFAs can decrease the quantum shot noise while the wavelength around 1.5 µm is the most favorable for cryogenic cooling of the optics to reduce the thermal noise. In this work, we present the recent progress on a fully monolithic, 2-stage single-frequency EYDFA that utilizes polarization-maintaining fibers. We present a comprehensive study on different pre-amplifier concepts with a seed input power of 8 mW and sub-MHz linewidth. We discuss the limitations, i.e. ASE, SBS or technical issues, and demonstrate that cladding-pumping with 940 nm provides the highest gain without the onset of ASE and a maximum output power of 1.07W. Furthermore, we demonstrate SBS-free operation of the pre-amplifier by relative intensity noise (RIN) measurements. The pre-amplifier was on an engineering-ready level, i.e. possesses temperature control, monitoring and housing. The pre-amplifier was long-term tested and characterized with regards to its noise properties. The high-power amplifier utilized an Er³⁺:Yb³⁺ codoped and polarization maintaining LMA fiber. The high-power amplifier was also pumped at 940 nm in counter-propagation direction. An additional cladding light stripper was introduced at the output to eliminate residual ASE light from the cladding. The high-power amplifier provided an output power of 110W in a Gaussian-like mode, had an ASE extinction ratio of > 50 dB and only marginal Yb³⁺ ASE power levels. We show that the amplifier operated SBS-free and discuss the polarization, i.e. PER, and long-term performance, i.e. cooling requirements.

We report the results from Raman fiber lasers with CW output powers of 224 Watts at 1.48 μm and 168 Watts at 1.7 μm, both of which are to the best of our knowledge the highest output powers from cascaded Raman resonators at these wavelengths. The Raman fiber lasers use unique Raman filter fibers to prevent threshold at the next Stokes wavelength, allowing for high spectral purity in the desired Stokes. In addition, the 1.7 μm output is pulsed to achieve 1.4 mJ in an 8.5 µs FWHM pulse.

Cascaded Raman Fiber Lasers (CRFL) bridges the gap between conventional fiber lasers emission bands by providing output in the intermediate wavelengths by cascaded Raman stokes conversion. Recently, random distributed feedback (RDFB), which provides broadband feedback, has shown to provide a great amount of wavelength tunability. Currently, coarse tuning in these lasers is achieved by controlling the Raman Stokes order through the power and feedback tuning, while fine-tuning within a Stokes order is achieved using a wavelength-tunable pump laser. Feedback tuning is achieved through simple filters such as short-pass filters to terminate the Raman cascade or a cascade of filters to enhance filtering complexity. In either approach, to achieve wavelength fine-tuning over several Raman Stokes orders, the system complexity is substantially enhanced. Here, we overcome this limitation by using a Fourier spectral shaper, a technique for achieving arbitrary spectral control to modify the feedback. A photolithographically fabricated 2-D mask in the shaper whose spatial co-ordinates can be altered together with the presence of multiple patterns on the mask enables a wide variety of filtering functions with high spectral resolution. In this work, we demonstrate a proof of concept cascaded Raman laser system pumped with a fixed wavelength laser at 1064nm, which can achieve tunable laser output around the first, second and third Stokes components of the 1064 nm pump at 1117 nm, 1175 nm, and 1240 nm. Multiwatt class output powers are demonstrated with a high degree of wavelength conversion of < 95 % in all cases.

Modal interference can lead to intensity modulations in optical fibers, which can produce refractive index gratings under the influence of quantum defect heating in a fiber laser. These gratings are perfectly phased-matched for mode couplings, which can lead to transverse mode instabilities at high average powers in fiber lasers. A detailed understanding of this process is critical for further power scaling of fiber lasers. We have directly observed and characterized this quantum-defect-assisted mode coupling for the first time using polarization modes in a PM fiber amplifier, providing solid experimental evidence for this key mechanism for transverse mode instability in fiber lasers.

We present an overview of theoretical analysis and experimental observation of the spatio-temporal transverse mode instability in polarization-maintaining high-gain fiber amplifiers with a high concentration of Yb³⁺ ions and core diameters of 8-10 μm. The nonlinear power transformation of the LP₀₁ fundamental mode into the high-order modes is explained by a traveling refractive index grating accompanying a population grating induced by the mode interference field. The origin of the refractive index gratings is discussed. The threshold power for the transverse mode instability is found to depend on the fiber core diameter, numerical aperture, dopant concentration and the signal bandwidth. The threshold power is observed to decrease dramatically in the presence of a backward reflection of a signal from the output fiber facet.

In this work we study the conditions for the onset of modal instabilities in two-mode fiber amplifiers. We show that transverse-mode instability (TMI) results as a consequence of cross-phase modulation and four-wave mixing processes. The TMI threshold condition corresponds to a self-phase and cross-phase modulation induced nonlinear phase matching between LP01 and LP11 modes. The derived TMI power threshold shows a functional dependence on fiber and amplification parameters similar to previously derived formula applicable to single-mode fiber amplifiers.

A new passive mitigation strategy for the effect of transverse mode instability is presented in this work. This technique requires the use of a polarization-maintaining fiber in which light is coupled with a polarization state oriented around 45° with respect to the main polarization axes of the fiber. Since the modal beat length in each of the main polarization axes of the fiber is slightly different, the aforementioned coupling condition leads to the modal interference pattern being periodically washed out. Such a situation at the end leads to a weakening of the thermally-induced refractive index grating and to an increase of the threshold for the effect of transverse mode instability that can amount to ~50%.

Transverse mode instability (TMI) induces detrimental mode coupling in fiber-laser systems at high average powers and still represents the main limitation for the further power scaling of diffraction-limited systems. In this contribution, we describe a new approach to mitigate TMI in fiber amplifiers by dynamically modifying the inversion profile in the fiber. When periodically changing the excitation of the active fiber (e.g. with the help of an acousto-optic deflector), the intensity distribution along the fiber is also dynamically modified. If this is done with a frequency of a few hundreds of kHz (i.e. so that the inversion cannot completely adapt to the new intensity pattern), the inversion grating will be washed out and a homogeneous inversion profile can develop. Consequently, the resulting heat distribution will also be homogenized and the formation of a thermally-induced refractive index grating, which is responsible for the TMI-induced mode coupling, can be largely suppressed. Hence, the presented mitigation approach tackles TMI at an early stage by acting upon the root cause of the detrimental modal energy transfer. At the conference, simulations will be presented which will illustrate the working principle of the new mitigation approach and show its potential to increase the TMI threshold of high-power fiber amplifiers by washing out the inversion profile.

We report on the observation and experimental characterization of backward power fluctuations with the temporal characteristics of transverse mode instability (TMI). A quasi-monolithic, counter-pumped amplifier system in 20/400 μm geometry was developed to investigate forward and backward propagating core- and cladding power as well as their temporal evolution. By experimentally observing the backward propagating core power on a photodiode, we can correlate the temporal traces to those in forward direction. The degree of correlation is found to be highly increased above the TMI threshold. Simultaneous investigations on the modal content in forward and backward direction were enabled by a free-space optical coupling between the first and second amplification stage and performed utilizing a high-speed camera (HSC). In the case of TMI mode content fluctuations are found to occur only in forward direction. Additionally, the evaluations reveal a varying core power content in both directions. The forward core power fluctuations are shown to be induced by the partial coupling of higher-order mode (HOM) content to the cladding. Meanwhile the backward core power fluctuations appear to be a consequence of the ones in forward direction induced by backreflections. Our measurements demonstrate the detection of TMI at various amplifier positions and could be helpful for scientific as well as industrial applications.

In this work we investigate transverse mode instability (TMI) in the presence of pump intensity noise and a controlled perturbation of the input coupling for a rod-type fiber amplifier using spatially and temporally resolved imaging (ST). We show that inherent pump intensity noise from the power supply can define significant peaks in the resulting TMI spectrum. ST measurements in the transition region of TMI also indicates that the simple picture of TMI being seeded by the combination of a static initial fraction of LP11 and pump or signal intensity noise is not valid for our measurements. Furthermore, we present seeding of TMI by perturbing the input coupling dynamically which allows measurements of the TMI gain as a function of frequency and signal power.

A new fast dynamic model for TMI investigation is presented and used to study the evolution of the internal powers, inversion and thermal gratings and the impact of relative phase shifts on the amplifier dynamics, showing the prevalence and importance of nonlinearly-induced beat-length variations. The optical field is represented in a modal basis, with overlaps with thermal field components precomputed. This allows optical propagation to be achieved with an efficient Runge-Kutta scheme, with mode coupling represented by a coupling term. This substantially reduces the computational load of the simulation. New features of TMI dynamics are revealed.

Results of steady periodic finite element beam propagation (SP-FE-BPM) simulations of transverse mode instability (TMI) in representative coiled step index and large pitch photonic crystal fiber amplifiers with pump noise and modulation are presented and discussed. Modulating the pump power of fiber amplifiers near the TMI threshold complicates the conceptual picture due to the introduction of the additional dynamical time scale of the pump modulation. The time dependent mode field throughout the length of the amplifier calculated with the SP-FE-BPM was used to synthesize spatially and temporally resolved imaging measurements including spatial distribution of the amplitude and phase of different frequency components as a means to characterize the effects of pump modulation on TMI dynamics. Pump modulation was found to affect the threshold and dynamics of TMI in two amplifier configurations and a range of modulation cases. The step index fiber (SIF) amplifier was found on average to be approximately 20% more susceptible to pump noise than the large pitch fiber (LPF) amplifier. Furthermore a TMI dynamic not associated with stimulated thermal Rayleigh scattering that has been experimentally observed was observed in both SIF and LPF SP-FE-BPM simulations.

The analysis of TMI has advanced over the last decade, with added observation parameters depending on the complexity of the experimental system. Increasing levels of information have been extracted, from camera images in the beginning over modal decomposition, time trace and frequency analysis, on towards bi directional measurements at multiple system positions and separation of spectral components. We will give an overview of the evolution of TMI analysis for different model systems and discuss the applicability and the additional insight that can be gained from advanced observation methods.

A new approach to characterize the effect of transverse mode instability (TMI) in high-power fiber-laser systems is presented. A position-sensitive detector is employed to detect the trajectories of the center of gravity of the fluctuating beam (in a short time-window) at different average power levels. With an increasing average output power, the area covered by the trajectories increases which is used as a measure for the stability of the system. The new concept allows for a simple, fast, and detailed characterization of TMI, which accurately determines its threshold even in complex operating regimes. This new technique can easily distinguish between spatial and power fluctuations of the beam. Furthermore, the trajectories contain information about the movement of the center of gravity of the beam, which can be used to gain additional insight about the dynamics of TMI. The technique robustness was tested by characterizing the TMI behavior in a complex operating regime. e.g., using a pump-power modulation.

High power femtosecond fiber laser systems typically rely on self-similar amplification, large scale chirped pulse amplification (CPA), or higher-order soliton pulse compression. In any of these system architectures the control or minimization of fiber nonlinearities is paramount. To date, the scalability of Er fiber lasers in particular has been limited due to their intrinsic anomalous dispersion (limiting self-similar amplification) and their small gain bandwidth (limiting CPA) whereas higher-order soliton compression generally limits pulse quality. Here, we address these limitations by enabling highly nonlinear pulse propagation in Er fiber amplifiers with minimal pulse distortions based on adaptive control of the input pulse. Though the system is subject to high levels of self-phase modulation, the output pulse quality remains high, moreover, the output bandwidth is greatly increased, easily surpassing the bandwidth limitation of classical CPA systems. Adaptive control is enabled via a compact adaptive chirped fiber Bragg grating (FBG) pulse shaper/stretcher, paired with a matched, static FBG compressor. We generate 110 nJ pulses, with a FWHM of 62 fs in a single mode Er fiber amplification system at a repetition rate of 99.8 MHz, corresponding to an average power of 11.0 W. This corresponds to a maximum peak power of 1.4 MW, which should yield focused intensities above 1 TW/cm2, providing the high intensity and repetition rate needed to combine strong-field effects such as high harmonic generation in solids with the precision of frequency combs. At a lower 25 MHz repetition rate, we reach 340 nJ, 63 fs.

We report demonstration of a new spectrally-controllable device, based on a sequence of linear polarizers and birefringent plates, which allows to accurately and adjustably tailor its spectral filtering properties for achieving complete gain-narrowing compensation over ~30nm of signal bandwidth in an Yb-doped fiber system with the total gain reaching 150dB. The experimental demonstration was performed in a regenerative Yb-fiber amplifier system with controllable number of passes, allowing to characterize both signal spectral-narrowing, and as well as spectral compensation at varying levels of achieved total gain. This result opens a pathway towards 100fs duration multi-mJ pulses from fiber CPSA systems.

In this work, we present an all-polarization-maintaining, all-large-mode-area fiber laser oscillator passively mode-locked using a nonlinear optical loop mirror. The ytterbium-doped system, working in a Raman-free regime, operates at a central wavelength of 1.03 μ;m. The oscillator emits 12 nJ pulses at a repetition rate of 7.56 MHz. Positively chirped pulses from an all-normal-dispersion cavity can be externally compressed to the duration of 250 fs.

Recently, Mamyshev oscillators (MO) have attained a lot of attention, due to their generation of mode-locked pulses with outstanding output parameters in terms of output energy, spectral bandwidth and pulse duration. We present a MO with output pulse energies in the range of 0.5µJ, an optical spectrum ranging from 1010nm to 1060nm and an externally compressed autocorrelation duration of less than 100fs. This MO completely consists of commercially available standard step-index fibers. In order to handle the high pulse energies, we apply a few-mode gain fiber with a core-diameter of 20µm in the second arm of the oscillator.

We report on a simple and robust all-fiber femtosecond MOPA system based on an optimized active tapered double-clad fiber (T-DCF). The main feature of this active medium is that the thin part, acting as a pre-amplifier, allows to propagate mostly the fundamental mode LP₀₁, while the signal experiences an extremely high gain in the thick part of the tapered fiber, where most of the pump power is absorbed. These T-DCF properties make them particularly well suited for designing high energy CPA based femtosecond amplifiers, with a high non-linear threshold (SPM, SRS) and a diffraction limited output beam. After the compressor, we obtained pulses amplified up to 40 μJ energy, 412 fs duration and 97 MW peak power at 1036 nm. To the best of our knowledge, it is the highest peak power reported in such T-DCF amplifier in the femtosecond regime.

Fiber based Mamyshev regenerators provide potentially a low cost, pulse-on-demand source with pulse durations down to the hundred femtosecond scale. Based on nonlinear broadening of gain switched diode laser pulses in fiber and consequent pulse shaping methods, these sources could provide an alternative for mode-locked systems. Here we study numerically the properties of such sources in terms of input pulse duration and power as well as various fibers. We show that an optimum operating region can be found for each input parameter combination, that is limited mainly by the onset of spontaneous Raman scattering and optical wave breaking.

In Fourier domain mode locked (FDML) lasers, extremely precise and stable matching of the filter tuning period and light circulation time in the cavity is essential for ultra-low noise operation. During the operation of FDML lasers, the ultra-low noise mode can be lost due to temperature drifts of the already temperature stabilized cavity resulting in increased intensity noise. Until now, the filter frequency is continuously regulated to match the changing light circulation time. However, this causes the filter frequency to constantly change by a few mHz and leads to synchronization issues in cases where a fixed filter frequency is desired. We present an actively cavity length controlled FDML laser and a robust and high precision feedback loop algorithm for maintaining ultra-low noise operation. Instead of adapting the filter frequency, the cavity length is adjusted by a motorized free space beam path to match the fixed filter frequency. The closed-loop system achieves a stability of ~0.18 mHz at a sweep repetition rate of ~418 kHz which corresponds to a ratio of 4×10^-10. We investigate the coherence properties during the active cavity length adjustments and observe no noise increase compared to fixed cavity length. The cavity length control is fully functional and for the first time, offers the possibility to operate an FDML laser in sweet spot mode at a fixed frequency or phase locked to an external clock. This opens new possibilities for system integration of FDML lasers.

Fourier domain mode locked (FDML) laser are fast swept light sources. Measuring the linewidth and coherence length of such light sources is not straightforward, but very important for a physical understanding of FDML lasers and their performance in optical coherence tomography (OCT). In order to characterize the dynamic (“instantaneous”) linewidth, we performed beat signal measurements between a stationary narrowband continuous wave laser and an FDML laser and detected the signals with a 63 GHz real time oscilloscope. The evaluation of the beat signals of consecutive FDML wavelength sweeps yields information about the phase evolution within one sweep and over several sweeps. These measurements suggest the existence of a distinct comb like mode structure of the FDML laser and help to determine the locking strength of individual modes (comb lines).

A narrow-linewidth single-frequency Yb-doped nanosecond pulsed fiber laser with 27-kW peak power was demonstrated in this work. By employing the novel triangle-shape pulse with a 1-ns rise time and 13-ns fall time, the spectral linewidth of 88 MHz was achieved at the maximum output power. The signal to noise ratio was about 33 dB and no obvious residual pump power within the output laser. To the best of our knowledge, it is the first time that the performance of triangle-pulse fiber amplifier was demonstrated.

We investigate femtosecond pulse generation from a CW Laser diode by optical gating with a Mach-Zehnder electro-optic modulator. 45 ps pulses are amplified to ten’s of watt peak power and propagated through a standard polarization maintaining fiber to reach enough spectral broadening by self-phase-modulation. Pulses are then compressed down to about 500 femtoseconds with a grating compressor. We analyse the measured spectral broadening with respect to pulse repetition rate and average power and do some comparison with numerical simulations. This approach paves the way to versatile ultrashort pulse Lasers that could be easily synchronized.

We present the results of compact and cost-efficient high-power and energy laser system based on a picosecond gain-switched DFB laser diode operating at a wavelength of 1064 nm with spectral linewidth less than 0.1 nm and pulse duration of 50 ps and tapered double clad fiber (T-DCF) amplifier. The unique properties of T-DCF as efficient amplification of low power seed signal and suppressed threshold of nonlinearities allow achievement of both high peak power of 170 kW and pulse energy of 9 μJ at pulse repetition rate of 10 MHz while maintaining spectrum linewidth as narrow as 0.1 nm. High average output power of 150 W was obtained with slightly broader laser linewidth. We also demonstrate second harmonic generation with over 30 W at 532 nm wavelength with conversion efficiency of 38%. These results make MOPA system based on gain-switched DFB laser diode and T-DCF amplifier an attractive source for material processing and sensing application including time resolved Raman spectroscopy.

We describe an approach capable of generating structured light beams from a compact laser source based on the coherent combination of multiple tailored Gaussian beams emitted from a multicore fiber (MCF) amplifier. We report a proof-of-concept structured light generation experiment, using a cladding-pumped 7-core MCF amplifier as an integrated parallel amplifier array and a spatial light modulator to actively control the amplitude, polarization and phase of the signal light input to each fiber core. We demonstrate the generation of various structured light beams including high-order linearly polarized spatial fiber modes, cylindrical vector beams and helical phase front optical vortex beams.

We present a large-bandwidth high-power, high-energy fiber laser system based on coherent beam combination of 16 ytterbium-doped rod-type amplifiers. The CPA pulse stretching as well as extensive spectral shaping to counteract gain narrowing are implemented in a mostly fiber-integrated front end. A two-staged and partially helium-filled CPA grating compressor allows to compress the amplified pulses while maintaining a nearly diffraction-limited beam quality. Two laser operation regimes were investigated. The first one aimed for the shortest possible pulse duration, whereby 106 fs were reached at an average power of 910 W and a pulse energy of 910 μJ. In a second experiment, the primary aim of increasing the average power to 1 kW and the pulse energy to 10 mJ was successfully reached while the secondary objective, again being a minimum pulse duration, was optimized to 120 fs, posing a record value for fiber-CPA systems at this performance level.

Tiled-aperture Coherent Beam Combination architecture opens the way to digital laser operating in high peak and average power regimes.

Coherent beam combining (CBC) by active phase control is an efficient way to power scale fiber amplifiers. Most often, CBC operates from measuring the phase differences between the lasers at their outputs, hence resulting in efficient combination of the laser beams in the very near-field. We developed a laser testbed coherently combining seven 1.5-µm fiber lasers through active phase control, using frequency-tagging to assess the phase fluctuations to be compensated for. The testbed can operate in a target-in-the-loop (TIL) configuration, with a detection sub-system designed to analyse the optical signal back-scattered by a remote target, in order to achieve coherent combining on the target rather than at the output of the lasers. In this paper, we present the testbed and its components, as well as the results obtained in direct coherent combining, operated at the output of the lasers, during the preliminary tests of the setup. Then, we present the results of the outdoor experimental campaign where the testbed is operated in a TIL-CBC configuration. Measurement of TIL-CBC efficiency when distance to the target is progressively increased from 15 meters to 1 km is detailed. As the experimental campaign took place in hot weather, with a close to the ground horizontal path of propagation for the laser beams, very strong turbulence conditions were encountered. However, efficient atmospheric turbulence compensation was demonstrated, confirming that TIL-CBC can be achieved, even under such detrimental turbulence conditions.

Gravitational wave detectors require single-frequency laser sources with challenging requirements regarding beam quality and noise properties. We developed a reliable single-frequency fiber amplifier architecture based on standard step-index large-mode-area fibers and coherently combined two high power beams to enable further power scaling. A combined power of 391 W could be achieved with a combining efficiency of approx. 92 %. The TEM00-mode content of the combined beam was analyzed and a higher-order-mode content of 6.8 % was measured. This yields 365 W linearly polarized output power in the TEM00-mode that is usable for the application.

PISTIL (PISton and TILt) interferometry is a segmented wavefront metrology technique that can fulfill the role of being an independent phase analyzer for tiled laser arrays used in coherent beam combining (CBC). It presents a plug-and-play characteristics enabling others research or industrial applications such as metrology of segmented mirrors, MOEMS or measurement standards. It can operate onto complex optical benches. Alongside the PISTIL concept, we developed methods for phase extraction and meta-analysis, with best accuracy to rightfully address an end user needs in term of segmented wavefront diagnosis. We demonstrate those functionalities onto the HIBISCUS optical testbed equipped with a segmented mirror, specifically designed test data analysis pipelines and improve the control-command based on PISTIL wavefront analysis. In the current configuration, it can emulate CBC near field piston and tilt variations.

Specialty fibers such as chirally-coupled-core fibers (3C®-fiber) show a high potential for further power scaling of single-frequency fiber amplifiers. Especially, the application of gravitational wave detectors requires a high optical output power at low noise characteristics. The output power of fiber-based single-frequency amplifiers is typically limited by nonlinear effects (e.g. stimulated Brillouin scattering). In general, to reduce the impact of nonlinearities, the mode area of the fiber core is enlarged and 3C®-fibers have been specifically designed to enable single-mode operation with a large mode area core. This fiber type consists of a step-index fiber structure, whose signal core is additionally chirally surrounded by one or more satellite cores. Because of the phase matching and the helical geometry, the higher order modes are pulled out of the signal core, which allows a high-purity modal content in the core. The development of compact all-fiber lasers in conjunction with specialty fibers combines the advantages of both techniques. For the first time, we demonstrate a spliceless all-fiber amplifier, where all optical components are directly integrated in a single Yb³⁺-doped 3C®-fiber. Such a spliceless laser design allows a compact and robust architecture using specialty fibers, while maintaining excellent beam properties. At an output power of 336 W, a fundamental mode content of 90.4% was demonstrated. This work emphasizes the suitability of 3C®-fibers in high-power laser and amplifier systems and the potential as laser sources for the next generation of gravitational wave detectors.

We present in this work, the development of a nanosecond pulsed Master-Oscillator Power-Amplifier (MOPA) laser system near 905 nm based on the 3-level transition of Neodymium using a novel low NA polarization-maintaining Nd-doped silica fiber with a 30µm core and 130µm cladding. The MOPA delivered up to 24 W of average power (0.6 mJ energy per pulse) with good beam quality (M²~1.4). Cascaded LBO and BBO crystals are used respectively for second-harmonic generation and fourth-harmonic generation, giving respectively average output powers of 4.9W at 452nm (conversion efficiency of 20%) and 550mW at 226nm (conversion efficiency of 10%).

Multicore Photonic Crystal Fiber (MC-PCF) have multiple cores close to each other, which allows coherent beam combining (CBC). MC-PCF can scale the output power by increasing the number of cores by CBC if the in-phase mode is selected. We demonstrate simultaneous realization of phase-locked and mode-locked laser by using Yb-doped 7-core MC-PCF by a semiconductor saturable absorber placed in the near-filed inside a resonator. High energy 333 nJ pulses were obtained directly from a mode-locked fiber laser oscillator at a 42.4 MHz repetition rate with an average power of 14.1 W at 24 W excitation. We observed the direct output pulse duration of 52 ps by assuming a sech2 profile. However, it may be noise-like pulses because of no variation when we performed pulse compression. Single-pulse operation was achieved by increasing the bandwidth of intracavity filter. At this time, 129 nJ, 42.4 MHz pulses were generated with a 5.5 W average power and the direct output pulse duration of 42 ps.

We report on the observation of a new phenomenon, occurring in a fiber ring laser. This phenomenon describes the transition from an initially bidirectional emission of a reciprocal fiber ring laser to a unidirectional emission at a certain pump power threshold. In addition, the final direction is not predefined but appears to be randomly chosen every time the threshold is exceeded. Therefore, we term this new phenomenon as direction instability. In addition we provide a first discussion of how the pump power threshold and the final direction can be influenced by the length and the loss of the cavity.

In this work we optimize the design of coherently-combined multicore fiber amplifiers. It has been shown that increasing the number of cores in such fibers helps to increase the combinable output power. However, in counter-pumped multicore fibers, thermal effects will finally lead to strong non-uniform mode-shrinking in each core. This, in turn, will result in a significant reduction of the combining efficiency. In this study we will examine the power and energy scaling potential for different pumping schemes and different fiber designs. To this purpose, a simulation tool is used that solves the laser rate equations taking into account the resulting temperature gradient and the transverse mode distortions caused by it. In the simulation co- and counter pumped multicore fibers with a square core arrangement and a core number ranging from 2x2 up to 10x10 will be considered. Moreover, we investigate the influence of the active core size in terms of thermal effects as well as the extractable output power and energy. Particular attention is paid to the mitigation of non-uniform mode-shrinking at the fiber end-facet. By comparing the co- and counter-pumped cases, we will show that a combinable output power of 26 kW (co-pump) instead of 14 kW (counter-pump) with a 10x10 MCF and 30 μm cores should be achievable.

A silica leakage channel microstructured optical fiber (LCMOF) is firstly drawn from a 3D printed preform. The LCMOF realized the optical signal transmission of supercontinuum spectrum from 600 nm to 1650 nm and the measured propagation loss is 15 dB/m at 632 nm and 19.5 dB/m at 1064 nm. The bending loss is 5 dB/m with 15 cm radius and 7.3 dB/m with 5 cm radius. The refractive index fluctuation is less than 7×10^-4 in the fiber core region. The structure of LCMOF can be adjusted effectively by altering the pre-designed preform and optimizing drawing temperature, which result in the less defects and lower loss.

In this work, we present a simplified method for manufacturing microstructured optical fibers, which was applied for an Yb-doped pentagonal large pitch fiber (LPF). Instead of using the classical stack & draw technique, which can become time-consuming and delicate, we used a one-step structuring of the cladding via deep-hole drilling in glass. The Ybdoped core was also realized in one-step by gas phase doping via MCVD. Additional nanostructuring steps of the core material were redundant due to the high radial homogeneity of the refractive index achieved, matched with high precision to the refractive index of the glass cladding. Numerical simulations were used to verify the method and analyze the scaling of the fiber towards larger mode field diameters, which is crucial for further scaling the power in fiber lasers.

We developed ytterbium-doped double-clad large mode area (MFD = 30 μm) spun tapered fibers with low internal birefringence and perfect beam quality (M² < 1.2). Picosecond MOPA system (95ps/100 MHz, 1064 nm) based on proposed active tapered fiber with output average power of 64 W (gain 32 dB) is demonstrated.

In this paper, we demonstrate the spectrally selective fundamental core mode suppression in single-mode fiber by mode anti-crossing technique. A unique feature of the method proposed in the current work is that the fundamental core mode can have rectangular-like excess loss spectrum with bandwidth controllable by fiber bending. This property made the proposed fiber design to be promising for suppression of lasing at unwanted wavelengths in different fiber lasers and amplifiers.

High-power fiber lasers have experienced a dramatic development over the last decade. Further increasing the output power needs an upscaling of the fiber mode area, while maintaining a single-mode output. Here, we propose an all-solid anti-resonant fiber (ARF) structure, which ensures single mode operation in broadband by resonantly coupling high order modes (HOMs) into the cladding. A series of fibers with core sizes ranging from 40 to 100 μm are proposed exhibiting maximum mode area exceeding 5000 μm². Numerical simulations show this resonant coupling scheme provides a HOMs suppression ratio more than 20 dB, while keeping the fundamental mode loss lower than 1 dB/m. The proposed structure also exhibits high tolerance for core index depression.

Power demands from high beam quality fiber-based lasers have reached levels where nominally weak optical nonlinearities now limit continued scalability. Amongst the parasitic nonlinearities, transverse mode instability (TMI) is especially problematic because it is the dominant scaling limitation. To manage optical nonlinearities, the fiber laser community has singularly focused on large mode area (LMA) designs to spread the optical power out over a larger cross-sectional area and reduce the effective intensity to increase nonlinear thresholds. Such LMA designs are necessarily multimode and, so, TMI, while not necessarily predictable, was not surprising in hindsight. This paper will focus on an alternative and complementary approach; one where nonlinearities are managed materially through understanding and judicious design of the glass compositions from which the fiber is comprised. Indeed, optical nonlinearities are fundamentally light-matter interactions and so attacking them through the ‘matter’ component is the purest approach. A further benefit of a materials approach to mitigating nonlinearities is that multiple nonlinearities, e.g., SBS and TMI, can be simultaneously reduced while permitting a much simpler fiber design, which aids in manufacturability and cost. In other words, discussed here, is a materials approach to larger core, simple step-index fibers bypassing TMI. This paper highlights several material approaches to specifically mitigating TMI including < 1% quantum defect fiber laser compositions, power-scaling in intrinsically low thermo-optic core fibers, and novel fullypassive “thermally self-single moding” LMA fibers that intrinsically become single moded as the fiber lases and reaches its operating power and temperature.

This presentation recorded for SPIE Photonics West LASE, Digital Forum, 2021.

This presentation was recorded for SPIE Photonics West LASE, 2021.

In this work, the linewidth performance of triangle-shaped pulsed fiber laser (TPL) with 6.5-ns pulse width was investigated numerically and experimentally. The spectral linewidth of TPL changed with different rising times and fixed pulse width. The minimum spectral linewidth could be obtained when the rising time or falling time is equal to 0. Besides, the self-phase modulation (SPM) could be suppressed when the rising time of TPL is less than 3.25 ns, and the tunable linewidth from 77.73 MHz to 86.69 MHz was obtained at 2.1-kW peak power of pulse. It was found that experimental results are consistent with the theoretical analysis.

We report the design, optical architecture, and performance of a multi-watt tunable polarization-maintaining Tm-doped fiber laser that can be tuned from 1890—2050 nm. The compact OEM laser exhibits peak fiber coupled output powers of > 3.5 W CW and a linewidth of < 0.05 nm. Data as a function of output wavelength are presented for the output spectrum, output power, OSNR, and long term power stability.

Beam-combinable, high-power, narrow-linewidth Yb-doped fiber amplifiers are presently being evaluated as high energy laser weapons where the rapid turn-on of the amplifiers is critical. These amplifiers are optically pumped at the narrow 976nm, high-absorption peak of ytterbium. The fiber amplifier turns on when the emission spectra of the diode pump modules significantly overlap the 976nm Yb absorption peak. The thermal wavelength tuning behavior of two types of laser diode sources was analyzed to predict the cold-start turn-on-time of a fiber amplifier. Turn-on-times of ~4ms and ~4s were predicted for a fiber amplifier employing a laser diode bar directly attached to a micro-channel cooler and a single emitter package cooled by a cold plate, respectively.

We report an Yb-doped fiber laser oscillator that is generating femtosecond pulses with tunable spectral properties. Central wavelength and the bandwidth of the oscillator is adjusted via a manual iris aperture used as a spatial filter in the laser cavity. The tunability range of the central wavelength of the optical pulses generated from the oscillator is in the range of 1022 nm to 1038 nm and the bandwidth tunability range is 8 nm to 32 nm. The obtained pulses are all suitable for further amplification via fiber-based amplifiers and mostly in the mW average power range. The adjustability is satisfied by changing the aperture width and the system is open for improvement by adjusting the position of the filter too. We believe that this tunable oscillator is promising to be used as a seed for high power femtosecond pulse amplification schemes that necessitate certain central wavelength and bandwidth properties at the input of the amplifier and may enable more efficient and shorter pulse duration systems.

We report superiorities of a NALM-based all-PM fiber laser incorporating dual-bandpass filter instead of ordinal single narrow bandpass filter. The fiber laser starts mode-locking quickly with relatively low pump powers. Shorter wavelength bandpass window component of dual-BPF is used for inducing Q-switched mode-locking and enlarging amplitude noise. Longer wavelength bandpass window component is used for keeping CW mode-locking. Once the laser starts modelocking at longer wavelength window, spectrum components in shorter wavelength window are negligible. The laser produces various of ultra-fast pulses by adjusting longer wavelength window property.

Current progress in infrared LIDAR, atmospheric sensing, and DWDM transmission system experiments highlights the need for large bandwidth, high dynamic range polarization-maintaining (PM) optical amplifiers in the 1900 nm—2100 nm band [1—6]. Amplifiers that can operate efficiently near the high wavelength end of this band at 2090—2100 nm are particularly attractive for many emerging applications. In this paper we present the first simulated and experimental results for a newly developed miniature packaged Ho-doped fiber amplifier that is optimized for operation at 2090—2100 nm and employs high performance single clad PM Hodoped fiber (iXblue IXF-HDF-PM-8-125). Our goal in building a packaged PM Holmium-doped fiber amplifier (HDFA) at 2100 nm is to provide a miniaturized device with output powers of > 200 mW CW, high small signal gain, low noise figure, and large OSNR that can be used in many applications as a versatile wideband preamplifier or power booster amplifier. Our novel miniature HDFA package, shown in the photograph of Figure 1, has dimensions of 97 × 78 × 15 mm³, incorporates full pump control electronics, and communicates via an RS232 interface. The device is fully isolated against external and internal reflections and employs FC/APC connectors for the input and output ports.