Dynamics of the discharge-pumped vacuum ultraviolet Kr/sub 2//sup */ laser

A numerical simulation code has been developed to investigate the dynamics of the discharge-pumped vacuum ultraviolet Kr/sub 2//sup */ laser. Dynamics of spectral narrowing of the laser emission down to 0.2-nm full-width at half-maximum (FWHM) was well reproduced. Small signal gain behavior as a function of the applied voltage was predicted to have a gain coefficient of 0.08 cm/sup -1/ at the applied voltage of 32 kV, which was more than twice as large as the experimentally observed maximum gain value (0.035 cm/sup -1/). The code predicted that the laser output energy could become more than 1 mJ at the applied voltage at the Kr gas pressure of 10 atm.     

Characteristics of a saturated 18.9-nm tabletop laser operating at 5-Hz repetition rate

We report the characteristics of a saturated high-repetition rate Ni-like Mo laser at 18.9 nm. This table-top soft X-ray laser was pumped at a 5-Hz repetition rate by 8-ps 1-J optical laser pulses impinging at grazing incidence into a precreated Mo plasma. The variation of the laser output intensity as a function of the grazing incidence angle of the main pump beam is reported. The maximum laser output intensity was observed for an angle of 20/spl deg/, at which we measured a small signal gain of 65 cm/sup -1/ and a gain-length product g/spl times/l>15. Spatial coherence measurements resulting from a Young's double-slit interference experiment show the equivalent incoherent source diameter is about 11 /spl mu/m. The peak spectral brightness is estimated to be of the order of 1/spl times/10/sup 24/ photons s/sup -1/ mm/sup -2/ mrad/sup -2/ within 0.01% spectral bandwidth. This type of practical, small scale, high-repetition soft X-ray laser is of interest for many applications.     

Comparison of laser-produced plasma target materials for pumpingthe 109-nm Xe2+ Auger laser

We compared cadmium, copper, gold, and zinc targets to stainless steel for laser-produced plasma pumping the Xe²⁺ 109 nm laser. A unique target geometry allowed us to measure the laser output using two materials under identical conditions. Small signal gain coefficients and maximum output powers are presented for each material relative to stainless steel; we observed a ~20% improvement in the gain coefficient and a ~112% improvement in total energy using a cadmium target. This increase probably results from an improved overlap of the laser-produced plasma emission spectrum with the Xe inner shell 4d photoionization cross section, although published data to support this hypothesis are scarce and inconsistent     

A 2.65-kW Yb:YAG single-rod laser

We report a continuous-wave average output power of 2.65 kW from a single Yb:YAG laser rod pumped with 9000 W from 940 nm InGaAs laser diodes. To the best of our knowledge, this is the highest average output power ever reported from a single Yb:YAG gain element. The optical-to-optical efficiency (i.e., output power to raw laser diode optical power) was 28%. We also obtained 860 W with an M/sup 2/ of 2.1 when pumping with 6000 W, obtaining 14% optical-to-optical efficiency.     

High-power continuous-wave intracavity frequency-doubled Nd:GdVO/sub 4/-LBO laser under diode pumping into the emitting level

The continuous-wave high power laser emission of Nd:GdVO/sub 4/ at the fundamental wavelength of 1.06 /spl mu/m and its 531-nm second harmonic obtained by intracavity frequency doubling with an LBO nonlinear crystal is investigated under pumping by diode laser at 808 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 5/2/ transition) and 879 nm (on the /sup 4/I/sub 9/2//spl rarr//sup 4/F/sub 3/2/ transition). It is shown that, in spite of a lower absorption at 879 nm, the infrared emission is comparable under these two wavelengths of pump. The green emission performances were, however, improved by the 879 nm pump: 5.1 W at 531 nm with M/sup 2/=1.46 and 0.31 overall optical-to-optical efficiency was obtained from a 3-mm-thick 1-at.% Nd:GdVO/sub 4/ laser medium and a 10-mm-long LBO nonlinear crystal in a Z-type cavity for 16.5 W pump power. In similar conditions, the maximum green power for the 808 nm pump was 4.4 W, with 0.26 overall optical-to-optical efficiency and M/sup 2/=3.40 beam quality; at this pump wavelength the green emission shows evident saturation for pump power in excess of 9.9 W. This behavior is connected with the enhanced heat generation under 809-nm pumping, as evidenced by the increased thermal lensing of the fundamental emission. A careful alignment of the laser enables emission almost free of chaotic intensity fluctuations.     

High-power scaling (>100 W) of a diode-pumped TEM/sub 00/ Nd:GdVO/sub 4/ laser system

We present results of an investigation to scale a diode-pumped A Nd:GdVO/sub 4/ laser system to high powers (>100 W) using a bounce amplifier configuration. A Nd:GdVO/sub 4/ laser oscillator with a bounce amplifier geometry, pumped by a single 40-W diode bar, gave 24 W of multimode output power (60% optical efficiency), and 20 W of TEM/sub 00/ output with M/sup 2/<1.05. Power scaling of the oscillator system with pumping by multiplexing two (nominally) 40-W diode bars gave 50.1 W of multimode output at 83-W diode pumping, 40 W of predominantly TEM/sub 00/ output from 81-W of diode pumping, and 34 W TEM/sub 00/ output (M/sup 2//sub x/=1.05,M/sup 2//sub y/=1.1) with an external slit spatial filter. Higher power scaling is achieved by using a master-oscillator power-amplifier (MOPA) configuration with the double-diode-pumped oscillator and a bounce amplifier pumped by a 3-bar diode stack. A multimode MOPA output of 100 W is achieved by single pass amplification with 145-W amplifier diode pumping and 104-W TEM/sub 00/ mode using a double bounce amplifier configuration with 180-W amplifier diode pumping.     

Petawatt laser system and experiments

We have developed a hybrid Ti:sapphire-Nd:glass laser system that produces more than 1.5 PW of peak power. The system has produced up to 680 J of energy on target at 1054 nm in a compressed 440/spl plusmn/20-fs pulse by use of 94-cm diffraction gratings. A focused irradiance of up to 6/spl times/10/sup 20/ W/cm/sup 2/ was achieved using an on-axis parabolic mirror and adaptive optic wavefront control. Experiments with the petawatt laser system focused the beam on solid targets and produced a strongly relativistic interaction. Energy content, spectra, and angular pattern of the photon, electron, and ion radiations were diagnosed in a number of ways, including several nuclear activation techniques. Approximately 40-50% of the laser energy was converted to broadly beamed hot electrons, with an associated bremsstrahlung beam. High luminosity ion beams were observed normal to the rear surface of various targets with energies up to /spl ges/55 MeV, representing /spl sim/7% of the laser energy. These and other results are presented.     

Plasma formation in water by picosecond and nanosecond Nd:YAG laserpulses. II. Transmission, scattering, and reflection

For pt.I see ibid., vol.2, no.4, p.847-60 (1996) We investigated the transmission, scattering, and reflection of plasmas produced in water by Nd:YAG laser pulses of 6-ns and 30-ps duration. The transmission measurements comprise a large energy range at wavelengths of 1064 and 532 nm and various focusing angles between 1.7° and 22°. This parameter range covers the parameters used for intraocular microsurgery, but also allows one to assess the influence of self-focusing on plasma shielding, which is only relevant at small focusing angles. We found that most of the laser light is either absorbed or transmitted; scattering and reflection amount to only a few percent of the incident laser energy. The transmission is considerably higher for picosecond pulses than for nanosecond pulses, regardless of the focusing angle. The plasma transmission increases with decreasing focusing angle. Self-focusing, which occurs at focusing angles below 2°, leads to a further increase of transmission. The experimental results were compared with the predictions of the moving breakdown distributed shielding model. Only partial agreement could be achieved, because the model assumes a spatially and temporally constant absorption coefficient within the plasma which is not realistic. The model can, however, be used to determine the average absorption coefficient. Fits of calculated transmission curves to the experimental data at &thetas;=22° yielded 900 cm^-1⩽α⩽1800 cm^-1 nanosecond plasmas and 360 cm^-1⩽α⩽570 cm^-1 picosecond plasmas. The efficacy of plasma-mediated intraocular laser surgery is higher with 6-ns pulses than with 30-ps pulses, because with the nanosecond pulses nearly 50% of the laser pulse energy is absorbed already at threshold, whereas it is only 8% with the picosecond pulses     

Transient inversion soft X-ray lasers

The collisionally excited transient inversion scheme is shown to produce exceptionally high gain coefficients and gain-length products. Data are presented for the Ne-like titanium and germanium and Ni-like silver X-ray lasers (XRL's) pumped using a combination of nanosecond and picosecond duration laser pulses. This method leads to a dramatic reduction of the required pump energy and makes down-sizing of XRL's possible, an important prerequisite if they are to become commonly used tools in the long-term     

Transient collisional excitation X-ray lasers with 1-ps tabletopdrivers

Recent results are presented for 1-ps driven X-ray laser amplification in Ne-like and Ni-like transient collisional excitation research at the Lawrence Livermore National Laboratory. Plasma formation, ionization and collisional excitation are optimized using two laser pulses of 600- and 1-ps duration at tabletop energies of typically 5 J or less in each beam. Gain of 35 cm^-1 and higher has been measured on the 147 Å 4d→4pJ=0→1 transition of Ni-like Pd and is a direct consequence of the nonstationary population in, version produced by the high intensity picosecond pulse. We report the characterization of the X-ray laser properties including the transient gain lifetime and beam divergence for different Ne-like and Ni-like X-ray lasers     

Thermal effects in Cr/sup 2+/:ZnSe thin disk lasers

We report the modeling and experimental characterization of thermal lensing in Cr/sup 2+/:ZnSe face cooled laser disks using the phase shift interferometry technique. The thermal lens powers of the 1 mm and 0.5 mm thick disks were strong (23 and 7 diopters at 8 W pumping). The thermal lens power scaled with disk thickness and pump power, and temperatures were reached in the disks such that nonradiative relaxation was significant. Laser output greater than 4 W average power was achieved using face cooled thin disks of Cr/sup 2+/:ZnSe.     

Demonstration of a fully spatial coherent X-ray laser at 13.9 nm

We have recently reported the successful development of a fully coherent X-ray laser (XRL) at 13.9 nm by an oscillator-amplifier configuration with two targets. In the experiment, a seed XRL beam from the first target is injected into a plasma amplifier at the second target. The observed XRL beam has full spatial coherence and 0.2 mrad of nearly diffraction-limited divergence. In order to improve the output fluence, the amplification properties of the XRL beam have been investigated using various plasma lengths of the second amplifier target. The output energy has been improved by a factor of ten, increasing the length of the gain region to 10 mm, resulting in about 0.2 /spl mu/J of output energy.     

Resonantly pumped eyesafe erbium lasers

The viability of high-power and high-energy, direct eyesafe emission from bulk erbium lasers has recently been demonstrated. In this paper, we present a review of eyesafe erbium lasers that are resonantly pumped by both fiber and diode lasers. High brightness pumping with a 1.53-/spl mu/m erbium fiber laser has yielded 60 W of continuous wave (CW) output, 10 W of repetitively Q-switched output, and as much as 16 mJ of pulse energy. Diode laser pumping has yielded 38 W of quasi-CW output and >40 mJ of Q-switched output.     

Saturation in neon- and nickel-like collisional soft-X-ray lasersat low pump energy

We report on the experimental demonstration of saturated X-ray laser output from collisionally pumped Ne-like Fe at 25.5 nm as well as Ni-like Ag and Pd at 14.0 nm and 14.7 nm, respectively, using a 100-ps drive pulse irradiation. A double-prepulse scheme and a 3-m radius-of-curvature target resulted in a gain-length product of gL=16.5 in the case of Fe, With a single prepulse and flat slab targets, gain-length products of 15.3 and 15.8 were obtained for Ag and Pd, respectively. Saturation was also confirmed by the observed reduction in beam divergence with increasing target length. The required drive energy used was only 30 J in a 100-ps pulse, corresponding to an irradiance of 12 TW/cm². A key role in the achievement of these results was played by the reduction in the roughness of the target surface in the case of the Pd X-ray laser     

Generation of attosecond X-ray laser pulses

We propose to generate as-X-ray laser pulses by beating of two or more X-ray laser lines with a frequency separation in the range of 10/sup 15/ Hz. We focus on nickel-like X-ray lasers, some of which have a few almost equidistant laser gain lines with an appropriate difference frequency. It is shown that in the case of three or more lines, these can be phase-locked by means of a Langmuir wave generated in the gain medium at a suitable electron density.     

High-power/high-brightness diode-pumped 1.9-/spl mu/m thulium and resonantly pumped 2.1-/spl mu/m holmium lasers

We report high power (>36 W) with beam propagation factor M/sup 2//spl sim/2 in a diode end-pumped Tm:LiYF/sub 4/ (Tm:YLF) laser generating output near the 1.91-/spl mu/m region. Using the 1.91-/spl mu/m emission and high brightness achieved with the Tm:YLF laser we resonantly end-pump the Holmium /sup 5/I/sub 7/ manifold in Ho:YAG and demonstrate /spl sim/19 W of continuous-wave (CW) output. The diode-to-Holmium optical to-optical conversion efficiency achieved is /spl sim/18%. Using a CW pumped and repetitively Q-switched configuration, the Tm:YLF pumped Ho:YAG laser achieves >16 W of output power with an M/sup 2//spl sim/1.48 at 15 kHz. A Q-switched frequency range of 9 to >50 kHz is also achieved.     

Mode-locking in broad-area semiconductor lasers enhanced by picosecond-pulse injection

We present combined experimental and theoretical investigations of the picosecond emission dynamics of broad-area semiconductor lasers (BALs). We enhance the weak longitudinal self-mode-locking that is inherent to BALs by injecting a single optical 50-ps pulse, which triggers the output of a distinct regular train of 13-ps pulses. Modeling based on multimode Maxwell-Bloch equations illustrates how the dynamic interaction of the injected pulse with the internal laser field efficiently couples the longitudinal modes and synchronizes the output across the laser stripe. Thus, our results reveal insight into the complex interplay between lateral and longitudinal dynamics in BALs, at the same time indicating their potential for short optical pulse generation.     

Atmospheric pressure of nitrogen plasmas in a ferroelectric packed bed barrier discharge reactor. Part I. Modeling

Numerical modelling of ferroelectric packed bed nonthermal plasma reactor has been conducted to predict plasma parameters in a pure nitrogen environment. Simplified time averaged one-dimensional physical model based on Poisson's equation for electric field and transport equation for electrons was developed. The mean electron energy was obtained by a swarm relationship from calculated electric field profiles and plasma neutral conditions. For chemical model, N/sup +/, N/sub 2//sup +/, N/sub 3//sup +/, N/sub 4//sup +/, N/sup */, N/sub 2//sup */ and electron were considered where N/sup */ and N/sub 2//sup */ are the total excited atoms and molecules, respectively. The results show that all the plasma parameters increase with increasing applied AC voltage and pellet dielectric constant. The numerical results also show that the dominant ion is N/sub 4//sup +/ and the metastable molecule density is much higher than radical and the electron densities at atmospheric gas pressure.     

High-average power EUV light source for the next-generation lithography by laser-produced plasma

Laser-produced plasma is expected to fulfill the strict requirement as an extreme ultraviolet (EUV) light source for the next-generation lithography with 115-W average power at the intermediate focus, in terms of stability, minimum contamination, and cost of ownership. A liquid xenon micro jet is employed in our experimental facility to confirm the scalability up to the 115-W clean output power. The present experimental device is composed of a 1-kW 10-kHz 6-ns Nd:YAG laser with a xenon jet of up to 50-/spl mu/m diameter of 35 m/s speed inside a vacuum chamber, combined with a xenon recirculation system. The observed EUV power is 4 W at 13.5 nm (2% bandwidth, 2/spl pi/sr) from the plasma source with 0.5% stability (1 /spl sigma/, 50-pulse moving average). Debris mitigation and contamination control is now studied for fast ions by time-of-flight measurements, and confinement and exhaust by a magnetic field is shown to be effective. Xe/sup +/ to Xe/sup 13+/ ions were observed with Xe/sup 2+/ being the main charged state. Experimental study is presented on these subjects and further parametric study on the laser wavelength and pulsewidth is reported, discussing the scalability toward the realization of a 115-W system.