253 J at 0.2†Hz, LD pumped cryogenic helium gas cooled Yb:YAG ceramics laser.

A 253 J with 26 ns at 0.2 Hz laser performance was demonstrated using a LD pumped cryogenically cooled Yb:YAG ceramics laser amplifier. A high energy storage of 344 J was achieved with a stored energy density of 0.58 J/cm3 using a 1 kJ output multidirectional-pumping system. High energy-extraction efficiency of 56.5% was achieved with high energy fluence of 4.63 J /cm2. To the best of our knowledge, this is the highest output energy obtained with a repetitive nanosecond pulse by LD pumped solid-state laser. This paper presented a design of 1 kJ amplifier based on experimentally proven numerical data.

10 J operation of a conductive-cooled Yb:YAG active-mirror amplifier and prospects for 100 Hz operation.

We report, to the best of our knowledge, the highest power conductive-cooled active-mirror amplifier (CcAMA) using Yb:YAG with a pulse energy of 10 J. By using four liquid-nitrogen circulating cooled laser heads, we achieved a repetition rate, pulse energy, and average power of 33.3 Hz, 9.3 J, and 310 W, respectively. The problem of wavefront distortion, which is difficult to solve with a large-aperture active-mirror laser, is suppressed by using reinforcing materials with the same thermal expansion coefficient. We have confirmed that the wavefront distortion is small (0.15λ P-V per head) at 100 Hz operation, which paves the way for 100 Hz operation with the CcAMA concept.

Raman lidar for remote sensing of oil in water.

We describe a portable Raman lidar system that can remotely detect oil leakages in water. The system has been developed based on a frequency-doubled, Q-switched Nd:YAG laser, operated at 532 nm with a receiver telescope equipped with some filters and photomultipliers. Stand-off detection of oil is achieved in a 6-m-long water tank, which allowed us to considerably increase the survey capability of subsea infrastructures, including both the range observation and target identification.

40 kHz, 20 ns acousto-optically Q-switched 4 µm Fe:ZnSe laser pumped by a fluoride fiber laser.

An actively Q-switched mid-infrared Fe:ZnSe laser pumped by a continuous wave fluoride fiber laser has been developed. Stable operation with a pulse duration of 20 ns and a repetition rate of 40 kHz at 4 µm was achieved. The maximum peak power was 1.1 kW. The high-repetition rate, high-peak power nanosecond pulsed laser, which has been created for the first time, to the best of our knowledge, in an actively Q-switched Fe:ZnSe laser, should prove a suitable light source for laser processing and molecular sensing.

Petapascal Pressure Driven by Fast Isochoric Heating with a Multipicosecond Intense Laser Pulse.

Fast isochoric laser heating is a scheme to heat matter with a relativistic intensity (>10^{18} W/cm^{2}) laser pulse for producing an ultrahigh-energy-density (UHED) state. We have demonstrated an efficient fast isochoric heating of a compressed dense plasma core with a multipicosecond kilojoule-class petawatt laser and an assistance of externally applied kilotesla magnetic fields for guiding fast electrons to the dense plasma. A UHED state of 2.2 PPa is achieved experimentally with 4.6 kJ of total laser energy that is one order of magnitude lower than the energy used in the conventional implosion scheme. A two-dimensional particle-in-cell simulation confirmed that diffusive heating from a laser-plasma interaction zone to the dense plasma plays an essential role to the efficient creation of the UHED state.

Complex spatiotemporal coupling distortion pre-compensation with double-compressors for an ultra-intense femtosecond laser.

In an ultra-intense femtosecond chirped-pulse amplification laser, the imperfect diffraction wave-fronts of the second and the third gratings of the compressor, where spatio-spectral coupling exists, could introduce a complex spatiotemporal coupling distortion (STCD) and degrade the pulsed beam in both near- and far-fields. Here, we propose a method of double-compressors for pre-compensation. By inserting a scaled down compressor (small compressor) with a deformable retro-reflection mirror into the beam-line, the frequency-dependent wave-front distortion, i.e., the complex STCD, could be removed. We simulate the results in two different ultra-intense femtosecond lasers with 80 and 400 nm bandwidths for comparison, and near ideal focused peak intensities could be obtained in both cases. Meanwhile, the influences of several miss-matching effects, which might appear in engineering, are also analyzed and discussed for applications.

High beam quality and high peak power Yb:YAG/Cr:YAG microchip laser.

The prospect for developing a passively Q-switched Yb:YAG/Cr:YAG monolithic microchip laser that operates at cryogenic temperature is theoretically analyzed. It is concluded that such a system has the potential to deliver laser pulses with improved energy and increased peak power in comparison with composite Yb:YAG/Cr:YAG or Nd:YAG/Cr:YAG devices that are operated at room temperature. Consequently, a cryogenically cooled Yb:YAG/Cr:YAG system is built and the emission performances are investigated. Laser pulses with 3.2 mJ energy, 6.1 MW peak power and high beam quality of M2 = 1.8 are achieved. By increasing the pump beam diameter, laser pulses with higher energy 32 mJ are obtained at 25 MW peak power with M2 = 5.4. To our knowledge, these are the best results obtained from passively Q-switched composite Yb:YAG/Cr:YAG monolithic microchip lasers.

Arbitrarily distorted 2-dimensional pulse-front measurement and reliability analysis.

A method of 2-dimensional (2-D) space-scanned (in the x-y plane) spatiotemporal double-slit interference is used to reconstruct the 2-D pulse-front (in the x-y-t domain) of a femtosecond pulsed beam. While comparing with recent other methods, the method possesses two advantages: no reference pulse/beam is required anymore, and an arbitrarily distorted pulse-front, not just pulse-front tilt and pulse-front curvature, could be detected. Meanwhile, the influence of different factors of unknown pulsed beams and optical elements on the measurement reliability is also analyzed for engineering applications.

Stable ultra-broadband gain spectrum with wide-angle non-collinear optical parametric amplification.

Comparing with the non-collinear optical parametric amplification (NOPA), the gain bandwidth could be significantly enhanced by the wide-angle NOPA (WNOPA), i.e., with a divergent signal (WNOPA-S) or pump (WNOPA-P). In a uniaxial crystal, the spectral symmetry/asymmetry of WNOPA is introduced. In WNOPA-S, the ultra-broadband gain spectrum can be obtained in two phase-matching directions at both sides of the pump, however, the output is heavily angularly dispersed. In WNOPA-P, although the gain bandwidth enhancement is only achieved in one phase-matching direction, i.e., on the opposite side of the crystal axis, it is free of angular dispersion. The stabilities of the gain spectrum in NOPA and in WNOPA-P are experimentally compared and theoretically analyzed. Compared with NOPA, WNOPA-P supports an even broader and more stable gain spectrum, and compared with WNOPA-S, WNOPA-P is angular-dispersion-free. The conversation efficiency of WNOPA-P is the same as NOPA. We suppose WNOPA-P is ideally suitable for the amplification of stable ultra-broadband few-cycle pulse lasers.

Optimization of laser emission at 2.8 µm by Er:Lu2O3 ceramics.

We have demonstrated the continuous-wave operation of a highly efficient 2.8 µm Er-doped Lu2O3 ceramic laser at room temperature. An Er:Lu2O3 ceramic with a doping concentration of 11 at.% afforded a slope efficiency of 29% and an output power of 2.3 W with pumping at 10 W. To our knowledge, these are the highest slope efficiency and output power obtained to date for an Er:Lu2O3 ceramic laser at 2.8 µm. In addition, we prepared ceramics with various doping concentrations and determined their emission cross sections by fluorescence lifetime measurements and emission spectroscopy.

Single-shot real-time detection technique for pulse-front tilt and curvature of femtosecond pulsed beams with multiple-slit spatiotemporal interferometry.

Spatiotemporal coupling (STC) of femtosecond pulsed beams could significantly reduce the focal-spot intensity of ultra-intense lasers. We theoretically present a very simple method for single-shot real-time detecting pulse-front tilt, curvature, or tilt and curvature (PFT, PFC or PFT&PFC) by using multiple-slit spatiotemporal interferometry (MSTI). An unknown input pulsed beam is spatially cut by a high-density multiple-slit and changed into a series of spatially separated sub-pulses. By only measuring the spatial distribution of the interference pattern in the far-field, PFT, PFC, or PFT&PFC can be detected. Comparing with recent methods, no reference pulses or beams, no temporal or spatial scanning, and no temporal or spectral measurement is required. The single-shot and spatial-only measurement will greatly simplify the real-time detection of PFT and PFC.

12 mJ Yb:YAG/Cr:YAG microchip laser.

We have developed a quasi-continuous wave diode end-pumped cryogenically cooled Yb:YAG/Cr:YAG passively Q-switched microchip laser. A maximum energy of 12.1 mJ with 3.7 MW of peak power was obtained. To the best of our knowledge, this is the highest energy and peak power obtained by an Yb:YAG/Cr:YAG microchip laser so far.

PCSEL pumped coupling optics free Yb:YAG/Cr:YAG microchip laser.

The passively Q-switched operation of a cryogenically cooled Yb:YAG/Cr:YAG microchip laser was demonstrated with end pumping by a photonic crystal surface emitting laser (PCSEL). This laser generated 70 µJ/1.7 ns/3.2 kHz pulses with near diffraction limited beam quality (M2=1.1) at 1029.4 nm. There were no coupling optics between the microchip laser crystal and PCSEL, which made the system simple and compact.

Generation of 300 nm bandwidth 0.5 mJ pulses near 1 µm in a single stage gas filled hollow core fiber.

A simple and compact spectral-broadening system is presented that is based on a single-stage statically pressurized Ar filled hollow core fiber. By optimizing the inner diameter of the hollow core fiber, a bandwidth of 300 nm is obtained. This is the broadest bandwidth known to date with millijoule level energy near the 1-µm wavelength by a single stage gas filled hollow core fiber.

Efficient continuous wave and quasi-continuous wave operation of a 2.8 µm Er:Lu2O3 ceramic laser.

We have demonstrated a highly efficient 2.8 µm Er-doped Lu2O3 ceramic laser and investigated the lasing dynamics by time-resolved spectroscopy. During room-temperature continuous wave operation, a slope efficiency of 22% was achieved with a high-quality transparent ceramic. To our knowledge, this is the highest slope efficiency obtained by an Er:Lu2O3 ceramic laser. In addition, an output peak power of 1.2 W was obtained during quasi-continuous wave operation. Time-resolved spectroscopy showed that the emission wavelengths exhibited a red shift from 2715 to 2845 nm, which indicated that continuous wave operation may be possible at 2740 and 2845 nm.

Demonstration of a photonic crystal surface-emitting laser pumped Yb:YAG laser.

We developed a cryogenically cooled Yb:YAG continuous wave oscillator directly pumped with a photonic crystal surface-emitting laser (PCSEL). A high slope efficiency of 65.7% was obtained at an output power of 208 mW. The beam quality was close to the diffraction limit, with M2<1.2 in both directions. To the best of our knowledge, this is the first PCSEL pumped solid state laser to be developed.

1-J operation of monolithic composite ceramics with Yb:YAG thin layers: multi-TRAM at 10-Hz repetition rate and prospects for 100-Hz operation.

Experimental amplification of 10-ns pulses to energy of 1 J at repetition rate of 10-100 Hz in cryogenic multipass total-reflection active-mirror (TRAM) amplifier is reported for the first time. By using a monolithic multi-TRAM, which is a YAG ceramic composite with three thin Yb:YAG active layers, efficient energy extraction was achieved without parasitic lasing. A detailed measurement of output characteristics of the laser amplifier is presented; results are discussed and compared with numerical calculations.

Amplification characteristics of a cryogenic Yb³âº:YAG total-reflection active-mirror laser.

We have studied the amplification characteristics of a cryogenically cooled Yb³âº:YAG total-reflection active-mirror (TRAM) ceramic laser including wavefront distortion, birefringence loss, small signal gain (SSG), and temperature rise for developing high-performance master oscillator power amplifier (MOPA) systems. A 0.6 mm thick Yb³âº:YAG ceramic sample was used, and maximum pump intensity ~10 kW/cm² was reached. The transmitted wavefront was measured by using a Shack-Hartmann wavefront sensor, and we evaluated the thermal lens focal length and Strehl ratio for different pump conditions. We have also observed a butterfly-like leakage profile of thermally induced birefringence loss at the maximum pump intensity. From SSG measurements, we obtained moderate laser gain of G=3 for one bounce with a near aberration-free wavefront. Gain calculations, which included also temperature dependence of the emission cross section and reabsorption of Yb³âº:YAG, were in good agreement with the experiments. These experimental results will be useful as benchmark data for numerical simulations of temperature distribution in TRAM and for designing multikilowatt-class high-performance MOPA systems.

Temperature dependence of thermo-optic effects of single-crystal and ceramic TGG.

The temperature dependence of the thermo-optic effects in single crystal and ceramic TGG were evaluated by using the Fizou interferometer method. The temperature dependence of the refractive index and thermal expansion are significantly improved at low temperature for both ceramics and single crystals. Our estimation using a figure of merit indicated that a TGG ceramics cooled to liquid nitrogen temperature can reduce thermal wave-front distortion by a factor of up to 4.7 with respect to that at 300 K, and can reduce thermal birefringence effects by up to a factor of 12 with respect to those at 300 K.

ASE and parasitic lasing in thin disk laser with anti-ASE cap.

The amplified spontaneous emission (ASE) and parasitic lasing (PL) effects in thin disk laser with an anti-ASE cap have been investigated in detail by measuring both time-resolved radiated intensity at longer axis of elliptical pump profile (dominant ASE direction) and small signal gain (SSG) in laser amplifier. A cryogenically-cooled total-reflection active-mirror laser consisting of 9.8 at.% doped, 0.6-mm thick Yb:YAG and un-doped YAG trapezoidal ceramics cap was used as a sample. The phased transitions from spontaneous emission (SE) to ASE and from ASE to PL have been unambiguously observed. For several pump beam diameters, the ASE gain parameter g(0)l(ASE) at ASE threshold was about 3, and the SSG coefficient was down to about 65% until PL started. To the best of our knowledge, this is the first quantitative characterization of the ASE/PL effects in the thin disk laser with an anti-ASE cap.