Long-wave-infrared pulse production at 11 µm via difference-frequency generation driven by femtosecond mid-infrared all-fluoride fiber laser.

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.

Mid-IR pulse amplification to ∼millijoule energies in a single transverse mode using large core Er:ZBLAN fibers operating at 2.8µm.

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.

Demonstration of 0.67-mJ and 10-ns high-energy pulses at 2.72 µm from large core Er:ZBLAN fiber amplifiers.

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 ∼0.7mJ 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 KTiOAsO4-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.

All-in-fiber method of generating orbital angular momentum with helically symmetric fibers.

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.

All-in-fiber method of generating orbital angular momentum with helically symmetric fibers: publisher's note.

This publisher's note amends a figure caption in Appl. Opt.57, 8182 (2018)APOPAI0003-693510.1364/AO.57.008182.

High-energy pulse stacking via regenerative pulse-burst amplification.

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.

Focus issue introduction: Advanced Solid-State Lasers (ASSL) 2016.

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.

Coherent pulse stacking amplification using low-finesse Gires-Tournois interferometers.

We demonstrate a new technique of coherent pulse stacking (CPS) amplification to overcome limits on achievable pulse energies from optical amplifiers. CPS uses reflecting resonators without active cavity-dumpers to transform a sequence of phase- and amplitude-modulated optical pulses into a single output pulse. Experimental validation with a single reflecting resonator demonstrates a near-theoretical stacked peak-power enhancement factor of ~2.5 with 92% and 97.4% efficiency for amplified nanosecond and femtosecond pulses. We also show theoretically that large numbers of equal-amplitude pulses can be stacked using sequences of multiple reflecting resonators, thus providing a new path for generating very high-energy pulses from ultrashort pulse fiber amplifier systems.

Single-mode chirally-coupled-core fibers with larger than 50 µm diameter cores.

In this paper, we report an advance in increasing core size of effective single-mode chirally-coupled-core (CCC) Ge-doped and Yb-doped double-clad fibers into 55 µm to 60 µm range, and experimentally demonstrate their robust single-mode performance. Theoretical and numerical description of CCC fibers structures with multiple side cores and polygon-shaped central core is consistent with experimental results. Detailed experimental characterization of 55 µm-core CCC fibers based on spatially and spectrally resolved broadband measurements (S(2) technique) shows that modal performance of these large core fibers well exceeds that of standard 20 µm core step-index large mode area fibers.

Femtosecond pulse spectral synthesis in coherently-spectrally combined multi-channel fiber chirped pulse amplifiers.

We demonstrate coherent spectral beam combining and femtosecond pulse spectral synthesis using three parallel fiber chirped pulse amplifiers, each amplifying different ultrashort-pulse spectra. This proof-of-concept experiment opens a path to simultaneously overcome individual-amplifier energy and power limitations, as well as limitations on amplified pulse spectra due to the gain narrowing in a single fiber amplifier.

High-energy similariton fiber laser using chirally coupled core fiber.

We present a high-energy amplifier similariton laser based on a chirally coupled core (3C) fiber. Chirped pulse energies up to 61 nJ at 3.3 W average power are obtained with effectively single-mode output. The pulses can be compressed with a simple grating compressor to durations below 90 fs. We demonstrate for the first time a fused pump-signal combiner to confirm the integration potential of 3C fiber.

Coherent femtosecond pulse combining of multiple parallel chirped pulse fiber amplifiers.

We report on femtosecond pulse combining with up to four parallel chirped-pulse fiber amplifier channels. Active phase locking is implemented using the LOCSET (Locking of Optical Coherence by Single-detector Electronic-frequency Tagging) single detector feedback technique, resulting in 96.4%, 94.0%, and 93.9% relative combining efficiency with two, three, and four channels respectively. Theoretical and experimental analysis of combining efficiency dependence on amplitude and phase noise shows convergence to a fixed value with increasing number of channels, indicating that multi-channel pulse combining with LOCSET feedback should be scalable to very large numbers of channels.

Propagation-length independent SRS threshold in chirally-coupled-core fibers.

Both analytical study and numerical simulations show that the propagation-length independent Stimulated Raman Scattering (SRS) threshold can be achieved by Stokes wave suppression in optical fibers. We propose a specific design based on Chirally-Coupled-Core (CCC) fibers with spectrally-tailored wavelength-selective transmission to suppress the Stokes wave of Raman scattering. Fibers with length-independent nonlinearity threshold could be particularly advantageous for high power lasers and fiber beam delivery for material processing applications.

Energy scaling of mode-locked fiber lasers with chirally-coupled core fiber.

We report a mode-locked dissipative soliton laser based on large-mode-area chirally-coupled-core Yb-doped fiber. This demonstrates scaling of a fiber oscillator to large mode area in a format that directly holds the lowest-order mode and that is also compatible with standard fiber integration. With an all-normal-dispersion cavity design, chirped pulse energies above 40 nJ are obtained with dechirped durations below 200 fs. Using a shorter fiber, dechirped durations close to 100 fs are achieved at pump-limited energies. The achievement of correct energy scaling is evidence of single-transverse-mode operation, which is confirmed by beam-quality and spectral-interference measurements.

Angular-momentum coupled optical waves in chirally-coupled-core fibers.

A new type of interaction between optical waves occurs in chirally-coupled-core (CCC) fibers. Instead of linear-translational symmetry of conventional cylindrical fibers, CCC fibers are helical-translation symmetric, and, consequently, interaction between CCC fiber modes involves both spin and orbital angular momentum of the waves. Experimentally this has been verified by observing a multitude of new phase-matching resonances in the transmitted super-continuum spectrum, and theoretically explained through modal theory developed in helical reference frame. This enables new degrees of freedom in controlling fiber modal properties.

Chirally-coupled-core Yb-fiber laser delivering 80-fs pulses with diffraction-limited beam quality warranted by a high-dispersion mirror based compressor.

We demonstrate a high-energy femtosecond laser system that incorporates two rapidly advancing technologies: chirally-coupled-core large-mode-area Yb-fiber to ensure fundamental-mode operation and high-dispersion mirrors to enable loss-free pulse compression while preserving the diffraction-limited beam quality. Mode-locking is initiated by a saturable absorber mirror and further pulse shortening is achieved by nonlinear polarization evolution. Centered at 1045 nm with 39-MHz repetition rate, the laser emits 25-nJ, positively chirped pulses with 970-mW average power. 6 bounces from double-chirped-mirrors compress these pulses down to 80 fs, close to their transform-limited duration. The loss-free compression gives rise to a diffraction-limited optical beam (M2 = 1.05).

Dynamical, bidirectional model for coherent beam combining in passive fiber laser arrays.

We generalize the recently proposed model for coherent beam combining in passive fiber laser arrays [Opt. Express 17, 19509 (2009)] to include the transient gain dynamics and the complication of counterpropagating waves, two important features characterizing actual experimental conditions. The extended model reveals that beam combining is not affected by the population relaxation process or the presence of backward propagating waves, which only serve to co-saturate the gain. The presence of nonresonant nonlinearity is found to reduce the coherent combining efficiency at high power levels. We show that the array lases at the frequencies with minimum overall losses when multiple loss mechanisms are present.

Array size scalability of passively coherently phased fiber laser arrays.

We explore, by means of experiments and simulation, the power combining efficiency and power fluctuation of coherently phased 2, 4, 6, 8, 10, 12, 14, 16-channel fiber-laser arrays using fused 50:50 single-mode couplers. The measured evolution of power combining efficiency with array size agrees with simulations based on a new propagation model. For our particular system the power fluctuations due to small wavelength-scale length variations are seen to scale with array size as N(3). Beat spectra support the notion that a lack of coherently-combined supermodes in arrays of increasing size leads to a decrease in combined-power efficiency.

Model for passive coherent beam combining in fiber laser arrays.

We present a new model for studying the beam combining mechanism, spectral and temporal dynamics, the role of nonlinearity, and the power scaling issue of discretely coupled fiber laser arrays. The model accounts for the multiple longitudinal modes of individual fiber lasers and shows directly the formation of the composite-cavity modes. Detailed output power spectra and their evolution with increasing array size and pump power are also explored for the first time. In addition, it is, to our knowledge, the only model that closely resembles the real experimental conditions in which no deliberate control of the fiber lengths (mismatch) is required while highly efficient coherent beam combining is still attained.

Femtosecond Yb-fiber chirped-pulse-amplification system based on chirped-volume Bragg gratings.

A 100 W amplified (75 W compressed) femtosecond (650 fs) Yb-fiber chirped-pulse-amplification system is demonstrated using broadband chirped-volume Bragg gratings (CVBGs) for the stretcher and compressor. With a 75% compression efficiency, the CVBG-based compressor exhibits an excellent average power handling capability and indicates the potential for further power scaling with this compact and robust technology.