Laser Architecture Research Articles

Monolithic thin-disk laser and amplifier concept

Thin-disk lasers are indispensable in photonics research as well as in a multitude of industrial applications. They represent a unique class of laser and amplifier architecture that provides kW output power with excellent properties concerning beam quality, long-term stability, thermal management, and power scalability. For many applications, a reduced complexity of the laser and its size would be highly beneficial. The necessary multipass transitions in thin-disk lasers and amplifiers typically require sophisticated multi-mirror arrangements. Here, we present a monolithic version of the pump concept for thin-disk lasers and amplifiers, where the thin disk is replaced by a thin, wedged gain medium acting as a wedged optical trap. The wedge is coated in a peculiar manner in order to allow for efficient in- and out-coupling of the pump and laser radiation from the wedge. This concept transfers the complexity of the multi-mirror optics into the thin disk itself in a monolithic fashion. With this concept, we achieved 890 W of CW output power, 59% slope efficiency, optical-to-optical efficiency of 50%, and a gain factor greater than 10 for small signals. This demonstrates that this new concept is capable of reaching the kW power regime with minimum complexity and size.

Advances in burst-mode laser diagnostics for reacting and nonreacting flows

High-energy burst-mode lasers have enabled the development of kHz–MHz rate, multi-dimensional diagnostics for characterizing highly dynamic and transient phenomena in reacting and non-reacting flows. In addition to their high power, which enables frequency conversion and tunability over a wide range of the electromagnetic spectrum, the master oscillator power amplifier (MOPA) architecture enables ample flexibility in the shape of the pulse train through selection of the oscillator temporal and spectral characteristics. This article reviews both the development and application of burst-mode lasers, whose design has evolved significantly over a half century to include pulse widths from 250 femtoseconds to 50 microseconds, burst durations up to 0.1 s for sequences of up to 100,000 pulses, energies up to 2 J per pulse, and frequency conversion from the deep ultraviolet to the near-infrared, and repetition rates up to 5 MHz. Such performance characteristics have laid the foundation for the advancement of a wide array of ultra-high-speed laser-based diagnostics including (i) linear techniques such as laser-induced fluorescence, particle image velocimetry, Mie scattering, Rayleigh scattering, Raman scattering, and laser-induced incandescence; (ii) nonlinear techniques such as coherent anti-Stokes Raman scattering, multiphoton fluorescence, and molecular tagging velocimetry; (iii) and multi-dimensional imaging from planar (2D) slices to volumetric (3D) tomography. The development of high-power and multileg burst-mode lasers has also enabled simultaneous multi-parameter measurements (temperature, species, and velocity) and extension to 4D imaging (3D in space + time) using many of the aforementioned techniques. More specialized configurations include multi-pulsing (from doublets to quadruplets), pulse-to-pulse wavelength tuning, and multi-pulsewidth approaches combining femtosecond and picosecond amplification. The pace of development and transition to robust operation continues to expand through advances in opto-electronics, predictive modeling of laser performance, and more compact/portable laser architectures. Future prospects include growing utilization of burst-mode laser-based diagnostics in hypersonic flows, detonations, energetic reactions, plasmas, high-pressure combustion, turbulent flames, and other extreme aerodynamic and thermochemical environments.

All-fiber Mamyshev oscillator enabled by chirped fiber Bragg gratings.

We present a linear cavity self-polarizing Mamyshev oscillator at 1550 nm made fully of polarization-maintaining fibers. The cavity filters are chirped fiber Bragg gratings with a Gaussian reflectivity profile that allow for greater reflectivity, larger bandwidth, and dispersion control. This mode-locked fiber laser architecture shows an unprecedented simplicity while delivering 21.3-nJ pulses compressed to 108 fs with a competitive 22.3% pump conversion efficiency. Mode locking is initiated with an external saturable absorber mirror. Numerical simulations show how nonlinearity can be managed with highly dispersive filters inside a Mamyshev oscillator.

Flexible all-PM NALM Yb:fiber laser design for frequency comb applications: operation regimes and their noise properties

We present a flexible all-polarization-maintaining (PM) mode-locked ytterbium (Yb):fiber laser based on a nonlinear amplifying loop mirror (NALM). In addition to providing detailed design considerations, we discuss the different operation regimes accessible by this versatile laser architecture and experimentally analyze five representative mode-locking states. These five states were obtained in a 78-MHz configuration at different intracavity group delay dispersion (GDD) values ranging from anomalous (-0.035 ps2) to normal (+0.015 ps2). We put a particular focus on the characterization of the intensity noise as well as the free-running linewidth of the carrier-envelope-offset (CEO) frequency as a function of the different operation regimes. We observe that operation points far from the spontaneous emission peak of Yb (∼1030 nm) and close to zero intracavity dispersion can be found, where the influence of pump noise is strongly suppressed. For such an operation point, we show that a CEO linewidth of less than 10-kHz at 1s integration can be obtained without any active stabilization.

Stepped frequency comb generation based on electro-optic phase-code mode-locking for moderate-speed circular-ranging OCT.

Circular-ranging (CR) optical coherence tomography (OCT) uses frequency comb sources to improve long-range imaging. While the initial development of CR-OCT focused on extremely high-speed imaging (i.e., operation at A-line rates of several to tens of MHz), there are many applications and imaging strategies for which more moderate speeds are preferred. However, we lack suitable frequency comb sources to enable moderate speed CR-OCT imaging. Here, we describe a novel phase-code mode-locking (PCML) laser architecture that can be operated from the kilohertz to megahertz range, while also offering novel features such as dynamic re-configurability and simplified linear-in-time frequency stepping. We demonstrate a prototype CR-OCT system with a PCML laser and present imaging results at A-line rates from 176 kHz to 3.52 MHz with coherence-length limited imaging depths as high as 170 mm.

Random laser in dye-doped electrospun nanofibers: Study of laser mode dynamics via temporal mapping of emission spectra using Pearson's correlation

The development of optically active flexible nanostructures has been pursued in the last years owing to their potential application in photonic devices. Herein, we report a random laser architecture based on a flexible Rhodamine B-doped PMMA nanofibers produced via electrospinning technique with 1% and 5% (w/w) of dye. The lower concentrated sample provided only incoherent emission due to the low absorption mean free path. The most concentrated one provided both coherent and incoherent random laser emission depending on the position of the sample the excitation took place. Pearson's correlation coefficient maps showed a clear correspondence with the already established mode locking transition in random laser. A temporal mapping of successive emission spectra was performed, which provided qualitative information about mode competition dynamic when applied to coherent emission spectra and a highly sensitive way of tracking line widening and red/blueshift in incoherent random laser emission spectra. The developed system can be further employed to design novel miniaturized chemical and temperature sensors.

Compact Ho:YLF-pumped ZnGeP2-based optical parametric amplifiers tunable in the molecular fingerprint regime.

We report on a compact mid-infrared laser architecture, comprising a chain of $ {\rm ZnGeP}_2 $ZnGeP2-based optical parametric amplifiers (OPAs), which afford a higher energy yield ($ \mathbin{\lower.3ex\hbox{$\buildrel \lt \over{\smash{\scriptstyle\sim}\vphantom{_x}}$}} 60\;\unicode{x00B5} {\rm J} $∼x<60µJ at 1 kHz) compared to most conventional OPA gain media transparent in the 2-8-µm wavelength range. Specifically, our OPA scheme allows ready tunability in the molecular fingerprint regime and is tailored for strong-field excitation and coherent control of both stretch and bend (or torsional) vibrational modes in molecules. The OPAs are pumped and directly seeded (via supercontinuum generation) by a 2-µm, 3-ps Ho:YLF regenerative amplifier. The compressibility of the OPA output is demonstrated by a representative measurement of the near-Gaussian temporal profile of a dispersion-compensated 105-fs idler pulse at a central wavelength of 5.1 µm, corresponding to ${\sim}6 $∼6 optical cycles. Detailed numerical simulations closely corroborate the experimental measurements, providing a benchmark and a platform to further explore the parameter space for future design, optimization, and implementation of high-energy, ultrafast, mid-infrared laser schemes.

Burst-mode laser architecture for the generation of high-peak-power MHz-rate femtosecond pulses

A novel master-oscillator power-amplifier burst-mode laser system is reported for generating femtosecond (fs) pulse trains with high peak power and a broad spectral bandwidth of >10 nm without the use of nonlinear compression techniques. A mode-locked, 1064.6 nm fundamental-wavelength broadband master oscillator, a fiber amplifier/pulse stretcher, and four Nd:glass power amplifiers are used to generate a sequence of high-repetition-rate, transform-limited 234 fs pulses over a 1 ms burst duration at a 0.1 Hz burst repetition rate. Peak powers are 1.24 GW at 100 kHz and 500 MW at 1 MHz with M2∼1.5. Burst-mode fs laser design features, performance characteristics relevant to MHz-rate flow diagnostics, and prospects for scaling peak powers further by an order of magnitude are discussed.

UV random laser emission from flexible ZnO-Ag-enriched electrospun cellulose acetate fiber matrix

We report an alternative random laser (RL) architecture based on a flexible and ZnO-enriched cellulose acetate (CA) fiber matrix prepared by electrospinning. The electrospun fibers, mechanically reinforced by polyethylene oxide and impregnated with zinc oxide powder, were applied as an adsorbent surface to incorporate plasmonic centers (silver nanoprisms). The resulting structures – prepared in the absence (CA-ZnO) and in the presence of silver nanoparticles (CA-ZnO-Ag) - were developed to support light excitation, guiding and scattering prototypes of a RL. Both materials were excited by a pulsed (5 Hz, 5 ns) source at 355 nm and their fluorescence emission monitored at 387 nm. The results suggest that the addition of silver nanoprisms to the ZnO- enriched fiber matrix allows large improvement of the RL performance due to the plasmon resonance of the silver nanoprisms, with ~80% reduction in threshold energy. Besides the intensity and spectral analysis, the RL characterization included its spectral and intensity angular dependences. Bending the flexible RL did not affect the spectral characteristics of the device. No degradation was observed in the random laser emission for more than 10,000 shots of the pump laser.

Optically controlling the emission chirality of microlasers

Orbital angular momentum (OAM) carried by helical light beams is an unbounded degree of freedom of photons that offers a promising playground in modern photonics. So far, integrated sources of coherent light carrying OAM are based on resonators whose design imposes a single, non-tailorable chirality of the wavefront (i.e. clockwise or counter clockwise vortices). Here, we propose and demonstrate the realization of an integrated microlaser where the chirality of the wavefront can be optically controlled. Importantly, the scheme that we use, based on an effective spin-orbit coupling of photons in a semiconductor microcavity, can be extended to different laser architectures, thus paving the way to the realization of a new generation of OAM microlasers with tunable chirality.

Numerical modelling and optimization of actively Q-switched waveguide lasers based on liquid crystal transducers.

We have recently experimentally demonstrated that a novel liquid crystal-based photonic transducer for sensing systems could be utilized as an active Q-switch in a miniaturised and integrated waveguide laser system. In this paper, we now present a comprehensive numerical modelling study of this novel laser architecture by deriving a set of equations that accurately describe the temporal optical response of the liquid crystal cell as a function of applied voltage and by combining this theoretical model with laser-rate equations. We validate the accuracy of this model by comparing the results with previously obtained data and find them in excellent agreement. This enables us to predict that under realistic conditions and moderate pump power levels of 500 mW, the laser system should be capable of generating peak power levels in excess of 1.1 kW with pulse widths of about 20 ns, corresponding to pulse energies > 20 μJ. We believe that such a low-cost and ultra-compact laser source could find applications ranging from trace gas sensing and LIDAR to material processing.

Surface Emitting, Tunable, Mid-Infrared Laser with High Output Power and Stable Output Beam

A reflective outcoupler is demonstrated which can allow for stable surface emission from a quantum cascade laser and has potential for cost-effective wafer-scale manufacturing. This outcoupler is integrated with an amplified, electrically tunable laser architecture to demonstrate high power surface emission at a wavelength near 4.9 μm. Single mode peak power up to 6.7 W is demonstrated with >6 W available over a 90 cm−1 (215 nm) spectral range. A high quality output beam is realized with a simple, single-layer, anti-reflective coating. The beam shape and profile are shown to be independent of wavelength.

Temporal solitons in free-space femtosecond enhancement cavities

Temporal dissipative solitons in nonlinear optical resonators are self-compressed, self-stabilizing and indefinitely circulating wave packets. Owing to these properties, they have been harnessed for the generation of ultrashort pulses and frequency combs in active and passive laser architectures, including mode-locked lasers1–4, passive fibre resonators5 and microresonators6–11. Here, we demonstrate the formation of temporal dissipative solitons in a free-space enhancement cavity with a Kerr nonlinearity and a spectrally tailored finesse. By locking a 100-MHz-repetition-rate train of 350-fs, 1,035-nm pulses to this cavity-soliton state, we generate a 37-fs sech²-shaped pulse with a peak-power enhancement of 3,200, which exhibits low-frequency intensity-noise suppression. The power scalability unique to free-space cavities, the unprecedented combination of peak-power enhancement and temporal compression, and the cavity-soliton-specific noise filtering attest to the vast potential of this platform of optical solitons for applications including spatiotemporal filtering and compression of ultrashort pulses and cavity-enhanced nonlinear frequency conversion. Temporal dissipative soliton formation in a free-space femtosecond enhancement cavity with a thin Kerr medium is reported. Locking a 350-fs, 1,035-nm pulse train with a repetition rate of 100 MHz to this cavity-soliton state generates a 37-fs sech2-shaped pulse with a peak-power enhancement of 3,200.

Single-laser, polarization-controlled optical sampling system.

Optical sampling systems traditionally require either one mode-locked laser with an external delay line or two mode-locked lasers with a controllable repetition rate difference. In this paper we present a novel polarization-multiplexed laser architecture combining the benefits of both approaches. The laser emits two mode-locked pulse trains sharing only one gain section without any external delay line. The colliding pulses in the laser have orthogonal polarization as well as opposite propagation directions to reduce coupling effects. With this, the two pulse trains can be freely phase controlled to conduct pump-probe measurements. To further analyze the timing stability of the system, we conducted a two-photon-absorption experiment, leading to a timing accuracy of 30 fs. Based on the novel laser architecture, we call this new approach single-laser polarization-controlled optical sampling, or SLAPCOPS.

Laguerre-Gaussian and beamlet array as second generation laser heater profiles

The microbunching instability is known to be detrimental to x-ray free electron laser performance. At the Linear Coherent Light Source, the microbunching instability is suppressed with a laser heater, which increases the uncorrelated energy spread of e-beam in the injector. While the current system has been shown to improve x-ray brightness, other laser architectures could further enhance performance. In this study, we model the interaction between a laser and e-beam with arbitrary transverse profiles and examine the effect of various laser designs on the energy distribution of the electrons after the injector and laser heater, as well as their ability to suppress microbunching instability. This simulation incorporates random transverse jitter in order to reproduce physically representative operation of the Linac Coherent Light Source. We compare Gaussian and Laguerre-Gaussian modes, and explore composite beams in the form of an array of Gaussian beamlets. We conclude that the Gaussian laser profile is highly susceptible to e-beam ellipticity and random transverse jitter. The Laguerre-Gaussian profile, a mathematically ideal solution to suppressing microbunching, is less susceptible to these effects and can provide effective suppression even with a distorted e-beam, though performance can be improved by increasing stabilization. The array of beamlets presents a solution that produces consistent and smooth energy distributions with significantly less variance in heating than the Laguerre-Gaussian profile.

Transverse mode switchable all-fiber Brillouin laser.

A transverse mode switchable all-fiber Brillouin laser architecture is proposed. This laser architecture consists mainly of an optical switch, a set of cascaded mode selection couplers (MSCs), and a few-mode fiber ring cavity. By switching the output signal of the optical switch, specific transverse modes generated by MSCs can be coupled into the ring cavity to excite the corresponding Brillouin scattering light. Based on the Brillouin nonlinear effect, the desired mode resonant amplification is guaranteed, and a transverse mode switchable laser beam is obtained. As a proof of principle, we have implemented an all-fiber Brillouin laser with switchable output of fundamental transverse (LP01) mode and second-order transverse (LP11) mode at the wavelength of 1550nm. The slope efficiency and linewidth for LP01 and LP11 modes are 25.1% and 4.9kHz, 20.9% and 4.96kHz, respectively. Additionally, the orbital angular momentum laser beam with l=+1, -1, or 0 switchable output is also demonstrated from our all-fiber Brillouin laser system. Owing to the compact all-fiber architecture, this transverse mode switchable Brillouin fiber laser is reliable during long-term operation and thus promising for many practical applications.

Cumulative plasma effects in cavity-enhanced high-order harmonic generation in gases

Modern ultrafast laser architectures enable high-order harmonic generation (HHG) in gases at (multi-) MHz repetition rates, where each atom interacts with multiple pulses before leaving the HHG volume. This raises the question of cumulative plasma effects on the nonlinear conversion. Utilizing a femtosecond enhancement cavity with HHG in argon and on-axis geometric extreme-ultraviolet (XUV) output coupling, we experimentally compare the single-pulse case with a double-pulse HHG regime in which each gas atom is hit by two pulses while traversing the interaction volume. By varying the pulse repetition rate (18.4 and 36.8 MHz) in an 18.4-MHz roundtrip-frequency cavity with a finesse of 187, and leaving all other pulse parameters identical (35-fs, 0.6-μJ input pulses), we observe a dramatic decrease in the overall conversion efficiency (output-coupled power divided by the input power) in the double-pulse regime. The plateau harmonics (25–50 eV) exhibit very similar flux despite the twofold difference in repetition rate and average power. We attribute this to a spatially inhomogeneous plasma distribution that reduces the HHG volume, decreasing the generated XUV flux and/or affecting the spatial XUV beam profile, which reduces the efficiency of output coupling through the pierced mirror. These findings demonstrate the importance of cumulative plasma effects for power scaling of high-repetition-rate HHG in general and for applications in XUV frequency comb spectroscopy and in attosecond metrology in particular.

Wavelength stability in a hybrid photonic crystal laser through controlled nonlinear absorptive heating in the reflector

The need for miniaturized, fully integrated semiconductor lasers has stimulated significant research efforts into realizing unconventional configurations that can meet the performance requirements of a large spectrum of applications, ranging from communication systems to sensing. We demonstrate a hybrid, silicon photonics-compatible photonic crystal (PhC) laser architecture that can be used to implement cost-effective, high-capacity light sources, with high side-mode suppression ratio and milliwatt output output powers. The emitted wavelength is set and controlled by a silicon PhC cavity-based reflective filter with the gain provided by a III–V-based reflective semiconductor optical amplifier (RSOA). The high power density in the laser cavity results in a significant enhancement of the nonlinear absorption in silicon in the high Q-factor PhC resonator. The heat generated in this manner creates a tuning effect in the wavelength-selective element, which can be used to offset external temperature fluctuations without the use of active cooling. Our approach is fully compatible with existing fabrication and integration technologies, providing a practical route to integrated lasing in wavelength-sensitive schemes.

Introduction to the Special Issue on Fiber Lasers

The sixty-two papers in this special issue focus on fiber lasers which are increasingly becoming the light source of choice for a large range of scientific and industrial applications thanks to their inherent technological advantages. The invited papers authored by worldrenowned research groups and promising scientists from all over the world present extended reviews on recent progress and significant perspectives on future research directions. The emergence of distinctive operation regimes and new materials for fiber laser systems such as random fiber lasers, and novel saturable absorbers based on two-dimensional layered materials is covered in this issue. Furthermore, this special issue covers a range of advances on theory and simulation, new design and fabrication, novel laser architectures and pumping methods, advanced fiber laser characterization methodologies, ultra-high power fiber lasers, mid-infrared fiber lasers and ultrafast fiber lasers. This issue also illustrates the versatility of the fiber laser platform for a broad range of newand emerging applications. For example, fiber lasers are being exploited in biophotonics, state-ofthe- art material processing and, supercontinuum and frequency comb generation, as well as being considered for the nextgeneration gravitational wave detection and key distribution.

Nanowire-Intensified Metal-Enhanced Fluorescence in Hybrid Polymer-Plasmonic Electrospun Filaments.

Hybrid polymer-plasmonic nanostructures might combine high enhancement of localized fields from metal nanoparticles with light confinement and long-range transport in subwavelength dielectric structures. Here, the complex behavior of fluorophores coupling to Au nanoparticles within polymer nanowires, which features localized metal-enhanced fluorescence (MEF) with unique characteristics compared to conventional structures, is reported. The intensification effect when the particle is placed in the organic filaments is remarkably higher with respect to thin films of comparable thickness, thus highlighting a specific, nanowire-related enhancement of MEF effects. A dependence on the confinement volume in the dielectric nanowire is also indicated, with MEF significantly increasing upon reduction of the wire diameter. These findings are rationalized by finite element simulations, predicting a position-dependent enhancement of the quantum yield of fluorophores embedded in the fibers. Calculation of the ensemble-averaged fluorescence enhancement unveils the possibility of strongly enhancing the overall emission intensity for structures with size twice the diameter of the embedded metal particles. These new, hybrid fluorescent systems with localized enhanced emission, and the general nanowire-enhanced MEF effects associated to them, are highly relevant for developing nanoscale light-emitting devices with high efficiency and intercoupled through nanofiber networks, highly sensitive optical sensors, and novel laser architectures.