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We demonstrate single transverse mode and high energy nanosecond pulse amplification at ∼2.8-µm using large core Er:ZBLAN fibers. The highest energies achieved are 0.75mJ from a 50 µm core, and 420µJ from a 30 µm core fibers respectively, seeded with 95 ns long pulses generated by a ring-cavity Q-switched Er:ZBLAN fiber laser. Nearly diffraction-limited beams with M2 = 1.2-1.3 were obtained using a single-mode excitation technique of multi-mode core fibers. Achieved pulse energies exceed by approximately an order of magnitude the previously reported highest pulse energies in a single transverse mode from a fiber laser or amplifier at these mid-IR wavelengths.

We present an ultrafast long-wave infrared (LWIR) source driven by a mid-infrared fluoride fiber laser. It is based on a mode-locked Er:ZBLAN fiber oscillator and a nonlinear amplifier operating at 48 MHz. The amplified soliton pulses at ∼2.9 µm are shifted to ∼4 µm via the soliton self-frequency shifting process in an InF3 fiber. LWIR pulses with an average power of 1.25-mW centered at 11 µm with a spectral bandwidth of ∼1.3 µm are produced through difference-frequency generation (DFG) of the amplified soliton and its frequency-shifted replica in a ZnGeP2 crystal. Soliton-effect fluoride fiber sources operating in the mid-infrared for driving DFG conversion to LWIR enable higher pulse energies than with near-infrared sources, while maintaining relative simplicity and compactness, relevant for spectroscopy and other applications in LWIR.

We demonstrate high energy pulses in a single transverse mode from 50µm and 30µm core Er:ZBLAN fibers in mid-IR at ~2.8µm. 100ns pulses with up 450µJ have been achieved at kHz repetition rates.

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.

Ultrashort mid-IR pulse train with 95 fs pulse width, 37.4 nJ pulse energy and 1.8 W average power was generated at ~2.85 µm from a simple Er:ZrF4 fiber nonlinear amplifier seeded by a mode-locked oscillator.

We show equivalence of coherent pulse stacking system to a deep recurrent neural network, and experimentally demonstrate real-time learning on stacking cavities and input pulses, necessary for high fidelity coherent temporal combining with ∼102 pulses.

We demonstrate coherent beam combining of pulses from 3 chirally-coupled core optical fibers into a single 22mJ pulse, achieving >7mJ per channel combining and allowing for future temporal combination and compression at high energy.

We apply momentum stochastic parallel gradient descent (MSPGD) and policy gradient algorithms to optimize coherent pulse stacking (CPS), and demonstrate their increased effectiveness compared to traditionally used stochastic parallel gradient descent (SPGD) algorithm.

We explored generation of high-energy nanosecond short pulses in the mid-IR wavelength range using 30–70-µm-core Er:ZBLAN fiber amplifiers. The highest energies achieved were ∼ 0.7 m J at 2.72 µm in 11.5-ns-long pulses, with the corresponding peak power of 60.3 kW, obtained with a 70-µm-diameter core fiber amplifier pumped at 976 nm and seeded by a K T i O A s O 4 -based optical parametric oscillator/optical parametric amplifier system. To the best of our knowledge, these pulse energies are the highest achieved to date from mid-IR fiber lasers at longer than 2-µm wavelengths with nanosecond pulses. The achieved highest pulse energies were limited by the surface damage of unprotected fiber output facets.

In this paper, we report a high power and compact mid-IR ultrafast laser system consisting of an Er:ZBLAN fiber-based mode-locked oscillator and a nonlinear amplifier. The mode-locked pulses are amplified and simultaneously nonlinearly compressed to sub-100 fs, without using any external pulse compressor or dispersion-managing stretcher. The output pulses have ∼85 fs pulse durations at ∼2.85 µm, with an average power of up to 2.4 W, and pulse energies of >40 nJ. This constitutes the highest average power sub-100fs duration pulses generated from a mid-IR fiber laser system to date.

© 1965-2012 IEEE. Coherent pulse stacking (CPS) is a new time-domain coherent addition technique that stacks several optical pulses into a single output pulse, enabling high pulse energy from fiber lasers. We develop a robust, scalable, and distributed digital control system with firmware and software integration for algorithms, to support the CPS application. We model CPS as a digital filter in the Z domain and implement a pulse-pattern-based cavity phase detection algorithm on an field-programmable gate array (FPGA). A two-stage (2+1 cavities) 15-pulse stacking system achieves an 11.0 peak-power enhancement factor. Each optical cavity is fed back at 1.5kHz, and stabilized at an individually-prescribed round-trip phase with 0.7deg and 2.1deg rms phase errors for Stages 1 and 2, respectively. Optical cavity phase control with nanometer accuracy ensures 1.2% intensity stability of the stacked pulse over 12 h. The FPGA-based feedback control system can be scaled to large numbers of optical cavities.

Small-signal parametric gain at 10.6 pm in GaSe crystal pumped at 2.75 pm is experimentally demonstrated. Simulation predicts that nanosecond optical parametric chirped-pulse amplification in GaSe supports sub-three-cycle pulses.

We experimentally demonstrate a novel broadband dispersion compensator designed for ~50fs-150fs pulses that uses controlled optical aberrations to compensate third and fourth order dispersion accumulated in ~50m long fiber path of a fiber CPA system.

High power laser science and engineering 7, e54 (2019). doi:10.1017/hpl.2019.36, Published by Cambridge Univ. Press, Cambridge

We demonstrate 667µJ and 11.5ns pulses at 2.72μm from an LMA Er:ZBLAN fiber amplifier with 70µm core. This represents the highest energy and peak power ever obtained in mid-IR with a fiber laser source.

An all-in-fiber method of generating orbital angular momentum (OAM) is proposed. A simple device composed with a section of helically symmetric fiber and another section of regular fiber is designed to convert input light to optical vortices. Finite element method calculation of first- and second-order OAM generation based on the coordinates transformation technique is taken to show that the eigenmodes of the helically symmetric fiber structures carry orbital and spin angular momentum. Simulation using the self-developed beam propagation method algorithm is also performed to verify the orbital angular momentum generation and evaluate the performance of the OAM generator.

© 2018 Optical Society of America. To stabilize the combined pulse energy for coherent temporal pulse stacking using interferometer cavities, we have developed a direct cavity phase measurement method based on analysis of the response to modulated probe pulses. An experiment has demonstrated optical phase control within 50 mrad for four cavities, resulting in a combination of 25 pulses with 1.5% root mean square stability over 30 h.

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

In this paper, we study fiber laser arrays coupled in a spatially filtered ring oscillator geometry that produces a tiled-array output. In particular, we examine the passive phasing dynamics, the efficiency of coherent combination, and the dependence of the system’s behavior on nonlinearities in the fibers. For an array containing a small number of elements, we find that the fibers achieve a co-phased state within two round trips after a perturbation. Steady-state results agree with the previous work. We also find that the Kerr nonlinearity decreases the combining efficiency as determined from the on-axis intensity in the far-field output.

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.

Mode-locked oscillator pulse phase is stabilized and locked to an external cavity in a novel way by using a phase-sensitive peak power response of the Gires-Tournois interferometer, which enables significant increase in phase-measurement sensitivity.

We have temporally combined 25 equal-amplitude pulses using four cavities, stabilizing cavity round-trip phase by measuring the response to a probe pulse train. Energy enhancement of 18.4 is maintained within 1% RMS.

Here we present a coherent pulse stacking approach for upscaling the energy of a solid-state femtosecond chirped pulse amplifier. We demonstrate pulse splitting into four replicas, amplification in a burst-mode regenerative Yb:CaF2 amplifier, designed to overcome intracavity optical damage by colliding pulse replicas, and coherent combining into a single millijoule level pulse. The thresholds of pulse-burst-induced damage of optical elements are experimentally investigated. The scheme allows achieving an enhancement factor of 2.62 using a single-stage stacker cavity and, potentially, much higher enhancement factors using cascaded stacking.

Pulse burst generation has important applications in material processing [1], seeding of free electron lasers [2] and coherent pulse stacking [3]. In particular, coherent pulse stacking technique allows further scaling of fiber and solid-state amplifiers by splitting the seed pulse into multiple replicas and stacking them into a single dominant pulse after the amplification. For this task, a pulse burst consisting of 4–90 pulses, with fully controllable amplitudes and phases is required. In this contribution we demonstrate pulse burst generation method based on the Vernier effect. The pulse burst with controllable amplitudes and phases is formed using a femtosecond oscillator and regenerative amplifier cavity that have slightly different round trip times. This operation mode can be used for the purposes of coherent pulse stacking, rapid material miroprocessing and rapid scan spectroscopy. Unlike in the case of pulse burst formation from a GHz repetition rate oscillator [4], the pulse burst formation using the Vernier effect allows very high repetition rate (much higher than the driving oscillator frequency) pulse burst formation with controllable amplitudes and phases.

The editors introduce the focus issue on "Advanced Solid-State Lasers (ASSL) 2016", which is based on the topics presented at a conference of the same name held in Boston, USA, from October 30 to November 3, 2016. This focus issue, jointly prepared by Optics Express and Optical Materials Express, includes 20 contributed papers ( 14 for Optics Express and 6 for Optical Materials Express) selected from the voluntary submissions from attendees who presented at the conference and have extended their work into complete research articles. We hope this focus issue provides a useful link to the variety of topical discussions held at the conference and will contribute to the further expansion of the associated research areas. (C) 2017 Optical Society of America

We report multi-mJ energy (>5mJ) extraction from femtosecond-pulse Yb-doped fiber CPA using coherent pulse stacking amplification (CPSA) technique. This high energy extraction has been enabled by amplifying 10’s of nanosecond long pulse sequence, and by using 85-µm core Yb-doped CCC fiber based power amplification stage. The CPSA system consists of 1-GHz repetition rate mode-locked fiber oscillator, followed by a pair of fast phase and amplitude electro-optic modulators, a diffraction-grating based pulse stretcher, a fiber amplifier chain, a GTI-cavity based pulse stacker, and a diffraction grating pulse compressor. Electro-optic modulators are used to carve out from the 1-GHz mode-locked pulse train an amplitude and phase modulated pulse burst, which after stretching and amplification, becomes equal-amplitude pulse burst consisting of 27 stretched pulses, each approximately 1-ns long. Initial pulse-burst shaping accounts for the strong amplifier saturation effects, so that it is compensated at the power amplifier output. This 27-pulse burst is then coherently stacked into a single pulse using a multiplexed sequence of 5 GTI cavities. The compact-footprint 4+1 multiplexed pulse stacker consists of 4 cavities having rountrip of 1 ns, and one Herriott-cell folded cavity - with 9ns roundtrip. After stacking, stretched pulses are compressed down to the bandwidth-limited ~300 fs duration using a standard diffraction-grating pulse compressor.

In this paper we discuss recent progress in exploring short-term and long-term stability of high power Yb-doped fiber lasers and amplifiers. Long-term stability is associated with photodarkening effects that can significantly reduce operational lifetime of a high-power laser system. Short-term stability is associated with so-called transverse modal instabilities that degrade output beam quality at average powers above a certain threshold. In this paper we review ongoing studies that provide experimental characterization, and explore physical causes and mitigation strategies of the different physical phenomena involved. Those studies are critical for achieving stable operation of high power fiber lasers.

Multi-mJ 81ns effectively-long burst of chirped pulses is amplified through fiber amplification system based on 85μm Yb-doped Chirally-Coupled-Core fiber and coherently stacked into a single pulse. 5.4mJ energy extraction at 1kHz repetition rate is demonstrated.

A new technique of time-domain pulse combining — coherent pulse stacking amplification — is enabling nonlinearity-free energy extraction at the stored energy limit from rare-earth doped fiber based ultrashort pulse amplification systems.

81ns effectively-long burst of chirped pulses is amplified to 10mJ with low nonlinearity in a Yb-doped 85µm core CCC-fiber based system, and coherently stacked with a multi-GTI arrangement, and compressed into a single

A novel phase-locking method, which locks cavity phase based on the pulse patterns detected from each cavity, has stabilized the output pulse intensity in a four-cavity Coherent Pulse Stacking experiment.

This year’s Working Group 8 included two broad themes: laser technology development and research for accelerator applications, and reports on ongoing programs at various beam and laser facilities. One particular focus in the first category was on emerging high average power and pulse energy laser technologies for the next generation of laser plasma accelerator drivers. Other well-represented theme was associated with mid-IR lasers for acceleration applications.

In order to develop a design for a laser-plasma accelerator (LPA) driver, we demonstrate key technologies that enable fiber lasers to produce high energy, ultrafast pulses. These technologies must be scalable, and operate in the presence of thermal drift, acoustic noise, and other perturbations typical of an operating system. We show that coherent pulse stacking (CPS), which requires optical interferometers, can be made robust by image-relaying, multipass optical cavities, and by optical phase control schemes that sense pulse train amplitudes from each cavity. A four-stage pulse stacking system using image-relaying cavities is controlled for 14 hours using a pulse-pattern sensing algorithm. For coherent addition of simultaneous ultrafast pulses, we introduce a new scheme using diffractive optics, and show experimentally that four pulses can be added while a preserving pulse width of 128 fs.

We discuss nonlinearity management versus energy scalability and compressibility in a three-stage monolithic 100-kHz repetition rate Yb-fiber amplifier designed as a driver source for the generation and tunable parametric amplification of a carrier-envelope phase stable white-light supercontinuum.

Photonic-crystal fibers (PCFs) with a specifically designed dispersion profile and nonlinearity are shown to enable an accurate broadband compensation of the stretcher-compressor dispersion in fiber laser sources of high-peak-power ultrashort light pulses. We demonstrate that the nonlinear phase shift in such systems can partially compensate for the fourth-order dispersion, allowing the stretcher—compression group delay to be compensated up to the fourth-order dispersion terms by using a sequence of only two fibers—a standard optical fiber and a PCF.

One critical issue for the realization of Inertial Fusion Energy (IFE) power plants is the driver efficiency. High driver efficiency will greatly relax the driver energy requested to produce a fusion gain, resulting in more compact and less costly facilities. Among lasers, systems based on guided wave such as diode pumped Yb:glass fiber-amplifiers with a demonstrated overall efficiency close to 70% as opposed to few percents for systems based on free propagation, offer some intriguing opportunities. Guided optics provides the enormous advantage to directly benefit from the telecommunication industry where components are made cheap, rugged, well tested, environmentally stable, with lifetimes measured in tens of years and compatible with massive manufacturing. In this paper, we are studying the possibility to design a laser driver solely based on guided wave optics. We call this concept FAN for Fiber Amplification Network. It represents a profound departure from already proposed laser drivers all based on free propagation optics. The system will use a large number of identical fibers to combines long (ns) and short (ps) pulses that are needed for the fast ignition scheme. Technical details are discussed relative to fiber type, pump, phasing, pulse shaping and timing as well as fiber distribution around the chamber. The proposed fiber driver provides maximum and independent control on the wavefront, pulse duration, pulse shape, timing, making possible reaching the highest gain. The massive manufacturing will be amenable to a cheaper facility with an easy upkeep.

Coherent pulse stacking with a 9ns effectively-long burst of equal amplitude chirped pulses into a single pulse using a compact cascade of four Gires-Tournois interferometers is experimentally demonstrated with a fiber chirped pulse amplification system.

Coherent stacking of 27 equal-amplitude pulses is achieved in 5-GTI sequence, with 4 cavities having rountrip of 1 ns, and one cavity - 9ns. Compression of effectively ~27ns long stretched-pulses down to 330fs is demonstrated.

A turn-key, robust, all-digital, and software-based control system based on stochastic parallel gradient descent algorithm is demonstrated to efficiently stabilize N-cascaded optical cavities of coherent pulse stacking systems.

Supercontinuum (SC) with a continuous spectrum from ~0.8-3 mum is generated in a standard single-mode fiber followed by high-nonlinearity fiber. The SC is pumped by 2-ns laser diode (LD) pulses amplified in a multistage fiber amplifier, and the two octave spanning continuum is achieved by optimizing a two-stage process that separates pulse breakup and soliton formation from spectral broadening. We also demonstrate scalability of the average power in the continuum from 27 mW to 5.3 W by increasing the pulse repetition rate from 5 kHz to 1 MHz, while maintaining comparable peak power. We attribute the generated SC spectrum to the ensemble average of multiple solitons and the superposition of their corresponding spectra. The hypothesis is confirmed through simulation results obtained by solving the generalized nonlinear Schrodinger equation (NLSE). Similar SC spectra can also be obtained by using both femtosecond and nanosecond pump pulses. Furthermore, by tailoring the input pulse shape, we propose and simulate the generation of the entire SC spectrum in one single soliton under quasi-continuous-wave (CW) pulse pumping scheme.

High peak power fiber lasers are important for a variety of applications ranging from material processing and remote sensing to laser-plasma produced extreme ultraviolet lithography (EUV) generation. These applications require high peak powers in the megawatt range, 5-MW peak power with subnanosecond pulses, the highest peak power achieved so far from a fiber laser. Use of a variety of core sizes between 65 and 200 mum has been explored and it has been shown that for core sizes as large as 80 mum, a good output beam quality of M2 ~ 1.3 can be maintained. This corresponds to the largest ever demonstrated mode area (2750 mum2) of a conventional LMA fiber with nearly diffraction-limited output. We also demonstrate average-power scaling of megawatt peak power pulses of up to ~90 W.

© 2015, EDP Sciences and Springer. This paper describes novel time-domain multiplexing techniques that use various resonant cavity configurations for increasing pulse energy extraction per each parallel amplification channel of a coherently combined array. Two different techniques are presented: a so-called N2 coherent array combining technique, applicable to a periodic pulse train, and a coherent pulse stacking amplification (CPSA) technique, applicable to a pulse burst. The first technique is a coherent combining technique, which achieves simultaneous beam combining and time-domain pulse multiplexing/down-counting using traveling-wave Fabry-Perot type resonators. The second technique is purely a time-domain pulse multiplexing technique, used with either a single amplifier or an amplifier array, which uses traveling-wave Gires-Tourmois type resonators. The importance of these techniques is that they can enable stacking of very large number of pulses, thus increasing effective amplified-pulse duration potentially by 102 to 103 times, and reducing fiber array size by the corresponding factor. This could lead to very compact coherently combined arrays even for generating very high pulse energies in the range of 1 to 100 J.

Study of nonlinear, intensity-dependent polarization evolution in 55μm core polygonal-CCC fibers reveals that both nonlinear polarization switching as well as robust and intensity-independent polarization maintenance can be achieved depending on input signal polarization.

We show stacking of multiple equal-amplitude pulses into a single pulse using properly configured sequences of Gires-Tournois interferometers, which in conjunction of chirped pulse amplification in fibers can enable large increase in extractable pulse energies.

Coherent pulse stacking utilizing cascaded Gires-Tournois interferometers to coherently stack multiple equal amplitude pulses in the time domain is experimentally demonstrated with a fiber chirped pulse amplification system.

A new generation of compact and robust ultrashort pulse lasers is currently emerging based on rare-earth doped fiber gain media. This paper reviews the development of high-power fiber technology, which recently has led to millijoule energies and /spl sim/10 W average powers from various femtosecond fiber systems. These results indicate that fiber technology has a significant potential to replace conventional solid-state lasers and promises important advantages both for practical use and for achieving high powers and energies. Chirped pulse amplification and different mode-size scaling techniques compose the foundation of this ultrashort-pulse fiber technology. Mode-size scaling can be achieved either by using multimode core fibers, which can produce a diffraction-limited beam at the fiber amplifier output, or by "mode-cleaning" of multimode core fiber output through saturated optical parametric amplification.

Ultrafast pulse sources based on multi-mode optical fibers are discussed. High-power passively mode-locked fiber lasers based on multi-mode rare-earth-doped optical fibers greatly exceed the power limitations of single-mode oscillators. Ultrafast multi-mode fiber amplifiers operating in conjunction with multi-mode oscillators provide even higher power levels, where nonlinear propagation effects enable pulse compression to below 100 fs. Multi-mode fiber oscillators can be combined with single-mode Raman-shifting fibers to produce widely wavelength-tunable sources of femtosecond pulses. Further amplification in Yb fibers allows for the generation of sub-100-fs pulses with W-level average powers.