Joule-class sub-nanosecond pulses produced by end-pumped direct bonded YAG/sapphire modular amplifier.

A Joule-class room-temperature diode-pumped solid-state laser was developed. The energy scaling of the 100 mJ 1064 nm seed pulse was realized by a series of two diode-pumped amplifiers. The gain medium consists in free combinations of Nd:YAG ceramics bonded to sapphire transparent heat sinks, to relax the thermal load induced by the 34 kW pump power. At low repetition rate, parasitic lasing was the main limitation to energy scaling. By choosing a gain module combination producing a step-like gradual doping concentration profile, mitigation of parasitic oscillations was observed, and the system delivered 2.8 J, 800 ps pulses at 2 Hz.

Effects of laser pulse duration on the formation dynamics of laser-induced periodic nanostructures.

Formation dynamics of laser-induced periodic surface structures (LIPSSs) on the SiC substrates were described with varying pulse numbers and pulse duration. As the number of laser pulses increases, two significant transformations become evident in the progression of structural formations. First from surface roughening with nanoparticles to LIPSS with the period that is slightly shorter than the laser wavelength. Second it turns to LIPSS with a period less than half the laser wavelength. It is found that maintaining the crystallinity is the key to changing the structures. In the cases of longer pulse width than sub-nanoseconds, no LIPSS formations are observed or LSFL does not change to HSFL because the irradiated area is poly-crystallized.

Widely tunable near-infrared optical parametric oscillator based on a 5%MgO:PPLN partial cylinder pumped at 1064†nm by a 1-kHz sub-nanosecond microchip laser.

We report the implementation of a singly resonant optical parametric oscillator using a 5%MgO:PPLN partial cylinder pumped by a sub-nanosecond microchip laser emitting 1064 nm at a repetition rate of 1 kHz. It is continuously tunable from 1410 nm up to 4330 nm by rotating the cylinder and a total energy of several microjoules is emitted with a beam quality factor M2 lower than 3.

Laser-induced damage study of bonded material for a high-brightness laser system.

In this work we evaluated the laser-induced damage threshold of the interface between two identical YAG crystals, bonded by an inter-layer assisted surface activated bonding method. The experimental results indicate slight damage threshold degradation for both single- and polycrystalline trivalent rear-earth (RE3+)-ion-doped YAG gain media in the sub-nanosecond pulse regime. Moreover, crystal annealing prior to damage testing could provide additional improvement for the laser damage threshold of the bulk and bonded interface.

>50 MW peak power, high brightness Nd:YAG/Cr4+:YAG microchip laser with unstable resonator.

We demonstrated a flat-convex unstable cavity Nd:YAG/Cr4+:YAG ceramic air-cooled microchip laser (MCL) generating a record 37.6 and 59.2 MW peak power pulses with an energy of 17.0 and 24.1 mJ and a width of 452 and 407 ps at 20 Hz by using a uniform power square and hexagon pump, respectively. For hexagon pump, the near field hexagon donut beam was changed in to a Bessel-like beam in far field, whose beam quality was estimated as 2nd moment M2 of 7.67. The brightness scale of unstable resonator MCL was achieved up to 88.9 TW/(sr·cm2) in contrast with flat-flat cavity MCL. However, the high intense center part of Bessel-like beam increased its brightness effectively more than 8 times, up to 736 TW/(sr·cm2).

High peak-power near-MW laser pulses by third harmonic generation at 355†nm in Ca5(BO3)3F nonlinear single crystals.

In this work, the performance of Ca5(BO3)3F (CBF) single crystals was investigated for the third harmonic generation at 355 nm. A high energy conversion efficiency of 16.9% at 355 nm was reached using a two-conversion-stage setup. First, using a high peak power, passively Q-switched Nd3+:YAG/Cr4+:YAG microlaser based gain aperture in micro-MOPA, the second harmonic at 532 nm was achieved with lithium triborate (LBO) crystal, reaching 1.35 MW peak power. On a second step, laser pulses at 355 nm were generated using a 5 mm-long CBF crystal growth by TSSG method with energy, pulse duration and peak power of 479 µJ, 568 ps and 0.843 MW, respectively. These results are currently the highest reported for CBF material.

Polarity inversion of crystal quartz using a quasi-phase matching stamp.

Stress-induced polarity inversion of crystal quartz using a quasi-phase matching (QPM) stamp is proposed for a QPM frequency conversion quartz device. Fabrication of QPM structure in x-cut quartz plate could be realized using the periodically patterned QPM stamp. Also, second-harmonic 532 nm generation with 16.8 kW peak intensity was demonstrated using a QPM quartz device with QPM period of 124 µm (3rd-order QPM) to confirm its polarity-inverted structure.

>30 MW peak power from distributed face cooling tiny integrated laser.

The Distributed Face Cooling (DFC) chip was fabricated from four pieces of 1 mm-length Nd:YAG plate sandwiched in four pieces of sapphire heat spreaders through advanced surface activated bonding (SAB) at room temperature. A sub-nanosecond (665.7ps) pulsed DFC-chip tiny integrated laser was achieved with output energy of 21.5 mJ and peak power of 32.3 MW with saturable absorber Cr4+:YAG. By finite element analysis, we confirmed the advantages of heat dissipation from DFC-chip compared with conventional bulk-chip. The SAB-DFC-chip based ubiquitous high peak power tiny integrated laser was experimentally within reach for laser-armed robot.

High peak power Nd:YAG/Cr:YAG ceramic microchip laser with unstable resonator.

A doughnut mode microchip laser was demonstrated by introducing a monolithic ceramic Nd:YAG/Cr4+:YAG chip in an unstable resonator to deliver laser pulses with an energy of 13.2 mJ and a pulse width of 476 ps, corresponding to a record peak power of 27.7 MW. The laser beam quality was characterized by M2∼6 at 10 Hz repetition rate. No significant degradation or change of beam pattern, pulse width, and M2 was confirmed during energy scaling in the case of the unstable cavity, promising for further brightness improving. In comparison with a flat-flat cavity, pulse broadening and M2 increase was observed up to ∼1.2 ns and ∼10, respectively, during energy scaling up to 18 mJ due to the beam pattern degradation. The doughnut beam was observed to have an Airy disk at the focal point, which was suitable for laser induced breakdown in air. The measured breakdown threshold of doughnut beam was comparable to a near-Gaussian beam (M2=1.3).

100 Hz operation in 10 PW/sr·cm2 class Nd:YAG Micro-MOPA.

An Nd:YAG Micro-MOPA, based on a microchip master oscillator and power amplifier system with gain aperture beam cleaning, could generate sub-ns 200 mJ pulses with extremely high brightness of 18 PW/sr·cm2 [Opt. Express268609 (2018)]. However, the system repetition rate was limited to 10 Hz, due to thermal problems occurring in the main amplifier rod under high-power operation. In this work, we achieved 100 Hz operation with pulse brightness of 11 PW/sr·cm2 by optical compensation of thermal lensing, which was evaluated through calculations and an experiment.

Spectral phase control of interfering chirped pulses for high-energy narrowband terahertz generation.

Highly-efficient optical generation of narrowband terahertz radiation enables unexplored technologies and sciences from compact electron acceleration to charge manipulation in solids. State-of-the-art conversion efficiencies are currently achieved using difference-frequency generation driven by temporal beating of chirped pulses but remain, however, far lower than desired or predicted. Here we show that high-order spectral phase fundamentally limits the efficiency of narrowband difference-frequency generation using chirped-pulse beating and resolve this limitation by introducing a novel technique based on tuning the relative spectral phase of the pulses. For optical terahertz generation, we demonstrate a 13-fold enhancement in conversion efficiency for 1%-bandwidth, 0.361 THz pulses, yielding a record energy of 0.6 mJ and exceeding previous optically-generated energies by over an order of magnitude. Our results prove the feasibility of millijoule-scale applications like terahertz-based electron accelerators and light sources and solve the long-standing problem of temporal irregularities in the pulse trains generated by interfering chirped pulses.

High brightness energetic pulses delivered by compact microchip-MOPA system.

High brightness compact microchip-seeded MOPA system was realized. Implementing a microchip preamplifier stage acting as gain aperture element lead to excellent output beam quality with M2 = 1.4. At maximum amplification level, 235 mJ (0.4 GW) of output energy (power) was measured. Analysis of the effect of the preamplifier showed that this element increases the available beam intensity by two orders of magnitude without significant increase in system footprint. Final beam brightness was 18 PW/sr.cm2.

Feature issue introduction: Advanced Solid-State Lasers 2017.

The Advanced Solid State Lasers 2017 Conference (ASSL) was held from October 1 to 5, 2017. It was an extraordinary conference at the Nagoya Congress Center in Nagoya, Japan. ASSL 2017 again suggested an impressive platform where miscellaneous perceptions with a variety of approaches to optics, photonics, sensing, laser technology, laser systems, and solid state lasers were presented. This international meeting was highly selective, leading to high level contributions through one plenary conference, 17 invited presentations, 70 regular talks, and 121 posters. The present joint issue of Optics Express and Optical Materials Express features 27 articles written by ASSL 2017 authors and covering the spectrum of solid-state lasers from materials research to sources, and from design innovation to applications.

Randomly polarised beam produced by magnetooptically Q-switched laser.

Diode-pumped solid-state micro lasers are compact (centimetre-scale), highly stable, and efficient. Previously, we reported Q-switched lasers incorporating rare-earth substituted iron garnet (RIG) film. Here, the first demonstration of the magnetooptical (MO) Q-switch in an Nd:YAG laser cavity is performed. We fabricate a quasi-continuous-wave (QCW) diode-pumped Nd:YAG laser cavity, which is shortened to 10 mm in length and which contains an RIG film and a pair of small coils. This cavity yields a 1,064.58-nm-wavelength pulse with 25-ns duration and 1.1-kW peak power at a 1-kHz repetition ratio. Further, the polarisation state is random, due to the isotropic crystal structure of Nd:YAG and the fact that the MO Q-switch incorporating the RIG film does not require the presence of polarisers in the cavity. This is also the first report of an MO Q-switch producing random polarisation.

Process design of microdomains with quantum mechanics for giant pulse lasers.

The power scaling of laser devices can contribute to the future of humanity. Giant microphotonics have been advocated as a solution to this issue. Among various technologies in giant microphotonics, process control of microdomains with quantum mechanical calculations is expected to increase the optical power extracted per unit volume in gain media. Design of extensive variables influencing the Gibbs energy of controlled microdomains in materials can realize desired properties. Here we estimate the angular momentum quantum number of rare-earth ions in microdomains. Using this process control, we generate kilowatt-level laser output from orientation-controlled microdomains in a laser gain medium. We also consider the limitations of current samples, and discuss the prospects of power scaling and applications of our technology. This work overturns at least three common viewpoints in current advanced technologies, including material processing based on magnetohydrodynamics, grain-size control of transparent polycrystals in fine ceramics, and the crystallographic symmetry of laser ceramics in photonics.

Narrowband terahertz generation with chirped-and-delayed laser pulses in periodically poled lithium niobate.

We generate narrowband terahertz (THz) radiation in periodically poled lithium niobate (PPLN) crystals using two chirped-and-delayed driver pulses from a high-energy Ti:sapphire laser. The generated frequency is determined by the phase-matching condition in the PPLN and influences the temporal delay of the two pulses for efficient terahertz generation. We achieve internal conversion efficiencies up to 0.13% as well as a record multicycle THz energy of 40 µJ at 0.544 THz in a cryogenically cooled PPLN.

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.

Sub-nanosecond laser induced air-breakdown with giant-pulse duration tuned Nd:YAG ceramic micro-laser by cavity-length control.

We first demonstrated a continuously and widely giant-pulse duration tunable laser based on a short monolithic Nd:YAG/Cr:YAG ceramic by cavity-length control in 100 Hz operation. The tuning range of pulse duration τ was from 0.5 to 9 ns as keeping peak powers of over 0.5 MW up to 6 MW. The characteristics of the ceramic laser was discussed in detail such as cavity-length dependent beam pattern and divergence, pulse shape, pulse energy due to the transverse and longitudinal modes, and an elliptical polarization status. Laser induced breakdown in laboratory air was investigated as a function of τ in sub-nanosecond region using the developed laser. Air-breakdown threshold intensity Ith was measured using three different focusing conditions. We confirmed that 1) the measured Ith was almost constant at the longer τ than τCI named as limit-pulse-duration of cascade ionization (CI), 2) Ith had ~τ-2 scaling for τ < τCI, 3) the increase of Ith is not connected to a specific intensity level, and 4) τCI was not constant and depended on focusing conditions. These phenomena were discussed with considering temporal-spatial intensity of laser.

Temperature stable operation of YCOB crystal for giant-pulse green microlaser.

In this work the performance of two yttrium calcium oxyborate (YCOB) crystals made by Czochralski and Bridgman growth process was measured. By using high peak power, passively Q-switched Nd3+:YAG/Cr4+:YAG microlaser, high conversion second harmonic generation efficiency were obtained. Laser pulses at 532 nm with 1.14 mJ energy and 223 ps duration were obtained with a 15-mm long YCOB crystal that was grown by Bridgman method. The conversion efficiency was 70.2%, comparable with the conversion efficiency of 72.8% that was achieved with 10-mm long lithium triborate (LBO) nonlinear crystal. Also, for the first time, experimental data on temperature tuning in type I YCOB crystal was measured with linear slope in 200°C temperature range equal to -0.057%/°C and -0.064%/°C for the Czochralski and Bridgman grown crystals, respectively. Such YCOB nonlinear crystal can become a serious option for developing laser sources with high-peak power at high repetition rate that can operate in harsh environment.

Quasi phase-matched quartz for intense-laser pumped wavelength conversion.

Crystal quartz has excellent optical properties as short absorption edge and high laser-damage threshold, which are suitable for intense pulse-laser pumped wavelength conversion by artificial quasi-phase matching structures. We present on initial evaluation of second-harmonic green generation with 14.9 kW peak power using periodic laminar structured quartz, pumped by sub-nanosecond pulse with focused intensity > 100 GW/cm2.