High-gain Laser Research Articles

Rhodamine 6G impregnated porous silica: A photoluminescence study

The optical properties of rhodamine 6G dye confined in porous silica are reported. Photoluminescence properties of embedded chromophores in mesoporous hosts can be affected by the surrounding matrices: shifts in emission spectra and variations of photoluminescence quantum yield are found as compared to dye solutions. Host–guest interactions are studied here by varying both SiO 2 xerogels porosity and the dye concentration. Comparing samples obtained by impregnating matrices with 5.4 and 18.2 nm pores with solutions having concentrations in the rhodamine 6G high laser gain, matrices with 5.4 nm pores impregnated with a dye concentration of 5 × 10 − 4 M are found to be the most stable and efficient in the examined range.

Model for high-gain fiber laser arrays

Recent experiments have shown that a small number of fiber lasers can spontaneously form coherent states when suitably coupled. The observed synchrony persisted for a long time without any active control. In this paper, we develop a dynamical model for fiber laser arrays that is valid in the high gain regime. In the limiting case of a single laser analysis and simulations are presented that agree with physical expectations. Using simulations to examine array behavior we report results that are in qualitative agreement with laboratory observations.

Oxide crystals for electro-optic Q-switching of lasers

A new generation of oxide crystals is emerging for electro-optic Q-switching or control of high-power pulsed lasers. Unlike the acousto-optic Q-switches in which the total turn-off time is limited by the duration of sound wave propagation (110–220 ns/mm) across the beam diameter, the electro-optic devices provide a short (<10 ns) response needed for minimum losses. Extinction ratios of better than 100 : 1 for electro-optic crystals ensure their reliable hold-off. By contrast, acousto-optic devices are characterized by single-pass dynamic losses of approximately 40%, which hinders their use in high-gain lasers. The basic principles of electro-optic Pockels cells are discussed. The performance characteristics of Q-switching for traditional electro-optic materials [deuterated potassium dihydrogen phosphate (DKDP), lithium niobate (LNB)] and other new electro-optic crystals, such as barium metaborate (BBO) and crystals belonging to the langasite (LGS) and potassium titanyl phosphate (KTP) families, are reviewed comparatively. Particular emphasis is placed on KTP-type electro-optic crystals, primarily on rubidium titanyl phosphate RbTiOPO4 (RTP), which stand out in their ability to provide Q-switching at extremely high frequencies or repetition rates up to 200 kHz. The use of both X- and Y-oriented double crystals as Q-switches in order to combine large electro-optic coefficients and low quarter-wave hold-off voltages with excellent thermal stability of the device is considered.

Skin depth plasma front interaction mechanism with prepulse suppression to avoid relativistic self-focusing for high-gain laser fusion

Measurements of the ion emission from targets irradiated with neodymium glass and iodine lasers were analyzed and a very significant anomaly observed. The fastest ions with high charge number Z, which usually are of megaelectron volt energy following the relativistic self-focusing and nonlinear-force acceleration theory, were reduced to less than 50 times lower energies when 1.2 ps laser pulses of about 1 J were incident. We clarify this discrepancy by the model of skin depth plasma front interaction in contrast to the relativistic self-focusing with filament generation. This was indicated also from the unique fact that the ion number was independent of the laser intensity. The skin layer theory prescribes prepulse control and lower (near relativistic threshold) laser intensities for nonlinear-force-driven plasma blocks for high-gain ignition similar to light ion beam fusion.

Superexcited state dynamics probed with an extreme-ultraviolet free electron laser.

We present the first experimental results obtained with a high-gain harmonic generation extreme ultraviolet free electron laser. The experiment probes decay dynamics of superexcited states of methyl fluoride via ion pair imaging spectroscopy. Velocity mapped ion images of the fluoride ion, obtained with excitation via intense, coherent, subpicosecond pulses of 86-89 nm radiation, reveal low translational energy, implying very high internal excitation in the methyl cation cofragment. Angular distributions show changing anisotropy as the excitation energy is tuned through this region. The dynamics underlying the dissociation are discussed with the aid of theoretical calculations.

Characterisation of solid-state dyes and their use as tunable laser amplifiers

This paper reports on the spectroscopic and photo physical characteristics of solid-state dyes and their use as tunable high-gain laser amplifiers. A comparison of the absorption and fluorescence spectra is made for the dyes pyrromethene 567, pyrromethene 597, pyrromethene 650, rhodamine 6G in the solid hosts, polymethyl methacrylate, polycom and ormosil, and compared to the case of the dyes in the solvent methanol. Results show a reduced Stokes’ shift and an increased fluorescence lifetime for dyes in the solid hosts compared to the solvent. The use of pyrromethene 650 in polymethyl methacrylate as a tunable laser amplifier was investigated. The dye amplifier was shown to exhibit photo-induced birefringence, and the fluorescence polarisation and intensity dependent anisotropy effects of gain and refractive index was characterised. The molecular reorientation time for pyrromethene 650 in the solid host was calculated to be 6.8 ns. The solid-state dye exhibited a single pass gain of 500 at 616 nm. The saturation behaviour was investigated and values of emission cross-section determined. Operation of the solid-state dye in a laser oscillator showed effects of strong shift in the lasing wavelength as a function of the cavity Q due to the small Stokes’ shift of the dye in the solid host.

Scheme for attophysics experiments at a X-ray SASE FEL

We propose a concept for production of high power coherent attosecond pulses in X-ray range. An approach is based on generation of eighth harmonic of radiation in a multistage HGHG FEL (high-gain high-harmonic free electron laser) configuration starting from shot noise. Single-spike phenomena occurs when electron bunch is passed through the sequence of four relatively short undulators. The first stage is a conventional “long” wavelength (0.8nm) SASE FEL which operates in the high-gain linear regime. The 0.1nm wavelength range is reached by successive multiplication (0.8nm→0.4nm→0.2nm→0.1nm) in a stage sequence. Our study shows that the statistical properties of the high-harmonic radiation from the SASE FEL, operating in linear regime, can be used for selection of radiation pulses with a single spike in time domain. The duration of the spikes is in the range of 400–600attoseconds. Selection of single-spike high-harmonic pulses is achieved by using a special trigger in data acquisition system. The potential of X-ray SASE FEL at TESLA at DESY for generating attosecond pulses is demonstrated. The use of a 10GW-level attosecond X-ray pulses at X-ray SASE FEL facility will enable us to track process inside atoms for the first time.

Demonstration of a transient-gain nickel-like xenon-ion x-ray laser

We demonstrate a high-gain nickel-like xenon-ion x-ray laser, using a picosecond-laser-irradiated gas-puff target. The elongated x-ray laser plasma column was produced by irradiation of the gas-puff target with line-focused double picosecond laser pulses with a total energy of 18 J in a traveling-wave excitation scheme. Strong lasing at 9.98 nm was observed, and a high gain coefficient of 17.4 cm(-1) was measured on the transient collisionally excited 4d-4p , J=0-1 transition for nickel-like xenon ions with target lengths as great as 0.45 cm. A weak nickel-like lasing line at a shorter wavelength of 9.64 nm was also observed, with a gain coefficient of 5.9 cm(-1) .

Effects of thin high-Z layers on the hydrodynamics of laser-accelerated plastic targets

Experimental results and simulations that study the effects of thin metallic layers with high atomic number (high-Z) on the hydrodynamics of laser accelerated plastic targets are presented. These experiments employ a laser pulse with a low-intensity foot that rises into a high-intensity main pulse. This pulse shape simulates the generic shape needed for high-gain fusion implosions. Imprint of laser nonuniformity during start up of the low intensity foot is a well-known seed for hydrodynamic instability. Large reductions are observed in hydrodynamic instability seeded by laser imprint when certain minimum thickness gold or palladium layers are applied to the laser-illuminated surface of the targets. The experiment indicates that the reduction in imprint is at least as large as that obtained by a 6 times improvement in the laser uniformity. Simulations supported by experiments are presented showing that during the low intensity foot the laser light can be nearly completely absorbed by the high-Z layer. X rays originating from the high-Z layer heat the underlying lower-Z plastic target material and cause large buffering plasma to form between the layer and the accelerated target. This long-scale plasma apparently isolates the target from laser nonuniformity and accounts for the observed large reduction in laser imprint. With onset of the higher intensity main pulse, the high-Z layer expands and the laser light is transmitted. This technique will be useful in reducing laser imprint in pellet implosions and thereby allow the design of more robust targets for high-gain laser fusion.

Zone melting growth of LiSrAlF6:Cr crystals for diode laser pumping

Abstract In this work we demonstrate the growth and the laser operation of high-quality Cr:LiSAF crystals by zone melting method. CrF3 dehydration and oxidation were carefully studied to optimize Cr incorporation in the LiSAF host. The obtained crystals have presented low losses and high laser gain, characteristics that are very suitable for diode-pumped laser operation. We studied the thermal behavior of these crystals during laser action and compared their CW performance with that of a commercial highly doped Czochralski-grown crystal.

Laser action in temperature-controlled scattering media

We used a novel system based on a lower critical solution temperature (LCST) mixture containing a high gain laser dye to study laser actions in random scattering media. The LCST material comprised of hydroxypropyl cellulose (HPC) in water can be reversibly transformed from a transparent state to a highly scattering colloid as the temperature is increased above 41 °C. The resulting temperature-dependent scattering length and photon diffusivity in the presence of an amplifying optically pumped Kiton Red 620 laser dye can tune the linewidth of output spectrum from 30 to 5 nm. Besides its suitability for random laser studies, this system may find its applications in remote temperature sensing and displays.

The puzzle of natural lasers

Hundreds of strong astrophysical masers have been detected since the first, serendipitous detection of such a maser in 1965. Surprisingly, no high-gain natural lasers in the optical domain and only one in the infrared domain have been detected during these decades. What does inhibit natural lasing? An answer is proposed in this presentation.

Efficient high-gain laser amplification from a low-gain amplifier by use of self-imaging multipass geometry

We characterize a self-imaging multipass amplifier scheme that provides both high extraction efficiency and overall gain. A diode-pumped slab amplifier with a single-pass small-signal gain of 2.5 is used in a 16-pass mode to amplify an input pulse from 50 muJ to 50 mJ, extracting approximately 22% of the stored energy. A stimulated Brillouin-scattering phase-conjugate mirror provides isolation from amplified spontaneous emission, prevents gain depletion, and also ensures good beam quality. The system can be operated from 10 Hz to in excess of 450 Hz, with modest changes in the beam quality and energy. The scheme has the potential to be scaled to higher-energy and higher-power systems.

Image Mode Projection using TN and PDLC Displays

A laser video projection method that is based on the image mode laser has been developed. The image mode laser is a laser with a spatial light modulator placed inside an optically pumped high-gain laser cavity to provide area-selective Q-modulation. The transverse mode of this laser creates a desired image that can be projected onto a viewing screen. This method of creating a lasing projected image has the potential advantages of higher contrast and brightness at the faceplate over the traditional method of spatial light modulation of a uniform optical source. The elimination of scanning systems results in a simpler design without mechanical parts; also, this system can potentially be adapted to full color with a single laser providing optical pumping for image mode lasers of all three primary colors.

High-gain Compton free electron laser driven by pre-bunched electrons

A theoretical analysis of high-gain Compton free electron laser dynamics when the radio frequency (rf) field evolves self-consistently with single resonant particles representing a pre-bunched electron beam is presented and the single particle phase space is studied. Using a set of universally scaled equations the optimal parameters of excitation for a free electron laser (FEL) in the single particle approximation are found. Using a new approach the relations between the initial rf field amplitude, the output field amplitude and the particle’s initial detuning from resonance are also found. A set of equations describing the self-consistent evolution of the rf field with e-bunch macro-parameters, such as bunch width and bunch mean phase, is derived from the universally scaled equations under the condition of the uniform initial distribution of electrons in the bunch. The analysis of optical field generation and amplification by compact bunches of electrons is provided. The saturation regime and the physical reason for the saturation in the high-gain Compton FEL driven by pre-bunched electrons are studied and discussed.

Full characterization of a high-gain saturated x-ray laser at 13.9 nm

We report the characterization of a saturated collisional Ni-like Ag soft-x-ray laser at 13.9 nm. A main pumping pulse of 140 J in 100 ps $(I\ensuremath{\approx}8\ifmmode\times\else\texttimes\fi{}{10}^{13} \mathrm{W}{\mathrm{cm}}^{\ensuremath{-}2}),$ the preceded 3 ns before by a prepulse of variable intensity was used to create the amplifying medium. The target length was varied up to 2 cm for the single target and 4 cm for the double target. For the double target the quasi-traveling-wave excitation (QTWE) technique has been used. Time-integrated as well as time-resolved measurement of the $4d\ensuremath{-}4p$ $J=0\ensuremath{-}1$ line have been performed for different pumping configurations and target geometry. Due to the very high pumping intensity and the resulting plasma overheating, an exceptionally high-gain coefficient of $19 {\mathrm{cm}}^{\ensuremath{-}1}$ has been measured leading to the saturation regime for plasma length longer than 8 mm. Time-resolved and time-integrated $3d\ensuremath{-}4f$ spectra of the plasma are also displayed and ionization balance is discussed. Using double-target geometry a narrow divergence is observed due to efficient coupling factor as well as a temporal increase of the x-ray laser pulse duration in the direction of QTWE. The far-field pattern of the x-ray laser beam has been characterized. Two vertically separated bright spots were observed for the double target, while a single spot was observed for the single target. The energy of the double-target x-ray laser beam has been measured to be $\ensuremath{\sim}300 \ensuremath{\mu}\mathrm{J}$ which closely corresponds to an output power of 5 MW.

Power self-regulation in double-pass high-gain laser amplifiers

Double-pass laser amplifiers can provide automatic passive regulation of the power in an optical signal. This regulation can significantly reduce the amplitude noise on a laser beam that is intended as a continuous wave light source. Analytic expressions are derived to describe the optical-noise-reduction region of double-pass amplifier operation and the dependence of the self-regulation properties on gain, saturation, and mirror reflectivity.

Double-pass high-gain laser amplifiers

Double-pass laser amplifiers have advantages of compactness and efficiency in the amplification of optical signals, and such amplifiers have been employed in a wide range of optical systems. Work in this area is reviewed briefly, and analytical solutions are obtained for the intensity of the electromagnetic waves in double-pass homogeneously-broadened high-gain laser amplifiers. Expressions are derived relating the output power to the input, including the effects of arbitrary mirror reflectivity and frequency detuning from line center. In the limits of weak saturation and of high reflectivity the results are consistent with earlier studies.

Lasing Pixels: A New Application for Polymer Dispersed Liquid Crystals (PDLCs)

A lasing pixel device that implements a spatially patterned variable loss element placed inside an optically pumped high-gain laser cavity is presented. A polymer dispersed liquid crystal (PDLC) modulator was used as the loss element. A projection device has been developed with high output, high on-screen contrast (<1000:1), and narrow spectral linewidth (∼3 nm). These positive characteristics make such a device attractive for a non-scanned laser projection system. A model of PDLC lasing pixel output versus element loss will be presented and compared to electro-optic measurements. PDLC materials from EM Industries have been optimized for this application.

Beam matter interaction physics for fast ignitors

The fast ignitor scheme for achieving inertial fusion energy precompresses the DT fuel to more than 1000 times the solid state density with nanosecond laser pulses. It then employs a picosecond 10–100-PW laser pulse to deposit with very high efficiency the energy necessary to achieve the ignition temperature of approx. 4 keV. In this way the necessary driver energy for inertial fusion energy reactors is reduced from the megajoule level to the 100-kJ level. In this article, we present results relevant to the development of the fast ignitor scheme that have been achieved in recent years by Australian teams in collaboration with international teams. The topics that are specifically addressed are: (1) forces and relativistic mechanisms of laser interaction with the electrons in the intense picosecond beams; (2) electron and ion emission from the focused beam into the precompressed plasma, including double layer effects and collective stopping power; and (3) the energy of the picosecond beam, which when nearly uniformly deposited into the precompressed DT-fuel can achieve the conditions for high-gain volume ignition. Positive results are derived for the volume ignition scheme from considerations of recent high neutron gain laser fusion experiments.