Non-contact acoustic tests based on nanosecond laser ablation: Generation of a pulse sound source with a small amplitude

A method to generate a pulse sound source for acoustic tests based on nanosecond laser ablation with a plasma plume is discussed. Irradiating a solid surface with a laser beam expands a high-temperature plasma plume composed of free electrons, ionized atoms, etc. at a high velocity throughout ambient air. The shockwave generated by the plasma plume becomes the pulse sound source. A laser ablation sound source has two features. Because laser ablation is induced when the laser fluence reaches 10¹²–10¹⁴ W/m², which is less than that for laser-induced breakdown (10¹⁵ W/m²), laser ablation can generate a lower sound pressure, and the sound source has a hemispherical radiation pattern on the surface where laser ablation is generated. Additionally, another feature is that laser-induced breakdown sound sources can fluctuate, whereas laser ablation sound sources do not because laser ablation is produced at a laser beam–irradiation point. We validate this laser ablation method for acoustic tests by comparing the measured and theoretical resonant frequencies of an impedance tube.     

Laser-induced Alignment and Coulomb Explosion of CO2

实验研究了CO₂分子在飞秒强激光脉冲作用下的动力学过程,包括分子取向,隧穿电离和库仑爆炸,激光强度从1×10¹³W/cm²变化到6×10¹⁴W/cm². 当激光强度小于分子的电离阈值时,CO₂分子的非绝热转动激发形成一个相干转动波包,波包演化导致分子沿激光电场方向取向. 激光脉冲结束后,分子取向可以周期性地再现,利用另一束激光可以对取向结构进一步进行修饰. 当激光强度大于分子     

Picosecond fluorescence studies by intracavity gain spectroscopy in a modelocked dye laser

This article presents measurements which combine modelocking technique with intracavity spectroscopy. To test this approach, a sample (10^–5 m ethanolic solution of 1,4-dihydroxyanthranquinone) was inserted in a modelocked Ar⁺ ion laser and probed by intracavity pulses of a synchronously pumped dye laser. The probing of the sample results in an amplification of the dye laser output. Maximum output was measured if the pulses of the dye laser temporally overlapped with those of the Ar⁺ ion laser inside the sample. Under this condition, the spectral laser intensity was shaped by the spectrum of stimulated fluorescence which originated from a molecular vibronic state populated by pump laser excitation.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Boron isotope enrichment in nanosecond pulsed laser-ablation plume

Boron isotopic enrichment is observed in the laser ablation of B₄C target using nanosecond (ns) wide 532 nm laser beam of a Nd-YAG laser. B¹⁰/B¹¹ ratio of 0.9 against the natural abundance of 0.25 is obtained at a laser power density of 8×10⁸ W/cm² (fluence of 6.4 J/cm²). The enrichment as a function of laser power density is demonstrated using a quadrupole mass spectrometer. Apart from higher enrichment factor, only singly charged ions are found in the laser plume from the B₄C target, in contrast to the multiply charged ions from the BN target reported in a recent report using femtosecond (fs) laser pulses. This study indicates the possibility of using less expensive, widely used ns lasers, which can also yield a higher throughput per pulse than a fs laser for isotope enrichment. Received: 28 September 2001 / Accepted: 4 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +91-4114/480-065, E-mail: mj@igcar.ernet.in     

Performance assessment of a new laser system for efficient spin exchange optical pumping in a spin maser measurement of <Superscript>129</Superscript>Xe EDM

We demonstrate spin-exchange optical pumping of ¹²⁹Xe atoms with our newly made laser system. The new laser system was prepared to provide higher laser power required for the stable operation of spin maser oscillations in the ¹²⁹Xe EDM experiment. We studied the optimum cell temperature and pumping laser power to improve the degree of ¹²⁹Xe spin polarization. The best performance was achieved at the cell temperature of 100 ^°C with the presently available laser power of 1 W. The results show that a more intense laser is required for further improvement of the spin polarization at higher cell temperatures in our experiment.     

A 20 W Cr, Tm, Ho: YAG laser

A 2.1 μm Cr³⁺, Tm³⁺, Ho³⁺ : YAG laser is operated with an output power of 20 W at 12 Hz. The laser output is successfully transmitted through a silica fibre. The laser system was designed based on the technology of an alexandrite laser oscillator.     

Temporal evolution of the xenon laser pulse

An experimental investigation was made of the xenon excimer laser. Several laser cavities have been employed with excitation by electron beams of cross section (15×2) cm² and (55×4) cm² and current density 100–150 A cm^-2 injected transverse to the cavity axis. A numerical model of the xenon laser is compared with experimental results. The laser pulse width was found to be dependent on laser intensity and cavity mirrors. Early termination of the laser pulse was observed consistent with changing reflectivity of the cavity mirrors. An uncoated MgF₂ retroreflector produced a laser output of significantly longer pulse width. Quasi-cw laser action was observed.     

Surface modulation of silicon surface by excimer laser at laser fluence below ablation threshold

Controlled single step fabrication of silicon conical surface modulations on [311] silicon surface is reported utilizing KrF excimer laser [λ=248 nm] at laser fluence below ablation threshold laser fluence. When laser fluence was increased gradually from 0 to 0.2 J/cm² for fixed 200 numbers of shots; first nanopores are observed to form at 0.1 J/cm², then very shallow nanocones evolve as a function of laser fluence. At 0.2 J/cm², nanoparticles are observed to form. Up to 0.15 J/cm² the very shallow nanocone volume is smaller but increases at a fast rate with laser fluence thereafter. It is observed that the net material volume before and after the laser irradiation remains the same, a sign of the melting and resolidification without any ablation.     

Effects of laser energy density on impulse coupling coefficient of laser ablation of water for propulsion

Time-resolved force sensing and intensified charge-coupled device (ICCD) imaging techniques were applied to the study of the effects of laser energy density on impulse coupling coefficient of laser ablation of water for propulsion. A Transversely Excited at Atmospheric pressure (TEA) CO₂ laser operated at 10.6 μm, 30 J pulse energy was used to ablate water contained in a quadrate quartz container. Net imparted impulse and coupling coefficients were derived from the force sensor data and relevant results were presented for various laser energy densities. ICCD imaging was used in conjunction with the dynamic force techniques to examine the dependencies on laser energy density. Results showed that the impulse coupling coefficient could reach a maximum value when laser energy density was about 10⁵ J/m², and it would increase before laser energy got to this point and would decrease after this point, and ICCD imaging supplied important phenomenon to explain this variation, which were water ablation before laser energy density got to 10⁵ J/m² and laser-induced air-breakdown with water as an induction when laser energy density was higher than 10⁵ J/m².     

Investigations of Yb-doped optical fiber using selective laser excitation

The method of tunable site-selective laser excitation was used for a study of the spectral and fluorescence characteristics of Yb³⁺ optical centers in laser optical fiber. Decay curves and positions of fluorescence spectra maxima at different wavelengths of selective laser excitation and positions of excitation spectra maxima at different fluorescence selective registration provide new information on lifetimes and Stark energy distribution among inhomogeneously broadened Yb³⁺ lines. The obtained Stark splitting energy distribution diagram demonstrates the tuneability of Yb³⁺ laser oscillation wavelength under tunable excitation of a pump laser.     

Lepton pair production in high-frequency laser fields

The production of electron-positron and muon-antimuon pairs in high-frequency laser fields via few-photon absorption is considered. It is assumed that an intense X-ray laser beam collides either with a relativistic ion beam or with a second, equally intense laser beam. We study the generation of free e ⁺ e ^? pairs, free μ⁺μ^? pairs, and bound-free e ⁺ e ^? pairs where in the latter case the electron is born in a low-lying atomic orbit of the projectile nucleus. Effects resulting from the finite nuclear size, the laser’s polarization state, and its magnetic field component are examined, which are testable experimentally by virtue of upcoming X-ray free-electron laser (XFEL) devices.     

The above-threshold detachment of negative ions in half-cycle laser field

We study partial detachment rate and photodetachment asymmetry of F⁻ detached by half-cycle linearly polarized laser field using numerical simulation. Similar to photodetachment for negative ions in few-cycle laser fields, we find that partial detachment rates of a couple opposite directions in the above-threshold detachment of F⁻ are not equal, the detachment is asymmetric. Furthermore, the photodetachment asymmetry degree increases with carrier-envelop phase (CEP) as the peak laser intensity becoming stronger or the pulse width becoming shorter. The maximal asymmetry degree is stronger with higher laser intensity. We confirm the effect of the CEP, laser intensity and pulse width on the above-threshold detachment of F⁻ in half-cycle laser fields. It provides a possible way to determine the CEP of half-cycle laser fields by measuring detached photoelectrons.     

LD-pumped passively Q-switched Nd:YVO4/YVO4 laser with an Nd3+:Cr4+:YAG saturable absorber

By using a piece of codoped Nd³⁺:Cr⁴⁺:YAG crystal as a saturable absorber, a laser-diode pumped passively Q-switched Nd:YVO₄/YVO₄ laser has been realized. The maximum laser output power of 2.452 W has been obtained at the incident pump power of 8.9 W for an 8.8% transmission of the output coupler at 1064 nm, corresponding to a slope efficiency of 30%. The other output laser characteristics of the laser have also been investigated. The laser with a Nd³⁺:Cr⁴⁺:YAG saturable absorber has a lower threshold pump power and a higher slope efficiency compared to that with a similar small-signal transmission of a Cr⁴⁺:YAG saturable absorber.     

Femtosecond laser-induced stimulation of a single neuron in a neuronal network

We demonstrated the stimulation of neurons at a single-cell level in cultured neuronal network by a focused femtosecond laser. When the femtosecond laser was focused on a neuron loaded with a fluorescent calcium indicator, the fluorescence intensity immediately increased at the laser spot, suggesting that intracellular Ca²⁺ increases in the neuronal cell due to the femtosecond laser irradiation. The probability of Ca²⁺ elevation at the laser spot depended on the average laser power, irradiation time, and position of the focal point along the optical axis, indicating that the femtosecond laser activates neurons because of multiphoton absorption. Moreover, after laser irradiation of a single neuron cultured on multielectrode arrays, the evoked electrical activity of the neurons was demonstrated by electrophysiological systems, which concluded that the focused femtosecond laser could achieve stimulating a single neuron in a neuronal network with high spatial and temporal resolution.     

A 750-mW,continuous-wave,solid-state laser source at 313 nm for cooling and manipulating trapped <Superscript>9</Superscript>Be<Superscript>+</Superscript> ions

We present a solid-state laser system that generates 750 mW of continuous-wave, single-frequency output at 313 nm. Sum-frequency generation with fiber lasers at 1550 and 1051 nm produces up to 2 W at 626 nm. This visible light is then converted to ultraviolet by cavity-enhanced second-harmonic generation. The laser output can be tuned over a 495-GHz range, which includes the ⁹Be⁺ laser cooling and repumping transitions. This is the first report of a narrow-linewidth laser system with sufficient power to perform fault-tolerant quantum-gate operations with trapped ⁹Be⁺ ions by use of stimulated Raman transitions.     

Near-diffraction-limited segmented broad area diode laser based on off-axis spectral beam combining

The beam quality of a 500-μm-wide broad area diode laser with five active segments has been improved beyond the beam quality of the individual segments. The principle of this new laser system is based on off-axis feedback in combination with spectral beam combining. By using a double-feedback scheme we are able to improve the beam quality of the laser by a factor of 23 from M²=55 for the free-running diode laser to M²=2.4 for the laser with feedback at a drive current of 2.2 A. The improved M² value is a factor of 3.4 below M²=8.2 for a single free-running segment. This is the first time that the beam quality of a segmented broad area diode laser has been improved beyond the beam quality of the individual segments. PACS 42.55.Px; 42.60.Da; 42.60.Jf     

Tunable single-mode pulsed Ti3+:Sapphire laser injection-seeded by a continuous-wave Cr3+:LiSrAlF6 laser

Spectrally pure high-power tunable single-mode operation of a pulsed Ti³⁺:sapphire laser by a tunable injection-seeding is reported. The injection laser was a cw diode laser pumped, spectrally narrowed tunable Cr³⁺:LiSrAlF₆ (Cr³⁺:LiSAF) laser with a grating in the auxiliary cavity. Single-mode tunable operation of a pulsed Ti³⁺: sapphire ring oscillator was obtained at different wavelengths in the range between 818 nm and 848 nm with a typical linewidth of 0.006 cm^-1. To extend the applicability of this operation to a differential absorption lidar system, the single-mode Ti³⁺:sapphire oscillator output was amplified and a high energy output of 38 mJ was obtained with the same linewidth.     

Pulse properties of a synchronously mode-locked,cavity dumped,picosecond dye laser system

A synchronously mode-locked, cavity-dumped picosecond dye laser is described. The structure and intensity of the picosecond pulses measured under different conditions are reported. It was found that the structure of the pulses from the synchronously pumped dye laser depends critically on the length of the Ar⁺ laser pulses. At the shortest Ar⁺ laser pulses of about 70 ps the dye pulses are as short as 1.1 ps. With Ar⁺ laser pulses of 200 ps the dye laser pulses contains a broad satellite pulse which contains a large fraction of the total intensity. When a cavity dumper is added to the system one gets dye laser pulses 15–20 ps long with a substructure, which indicates incomplete mode-locking. Well mode-locked 1.5–2.0 ps pulses were obtained in the red part of the dye laser action spectrum, i.e. 620–650 nm for R6G, 595–608 nm for R 110 and 657–662 nm for RB, respectively. Addition of mode-locking dyes also improved the pulse quality at some wavelengths.     

Neutron production in a picosecond laser plasma at a radiation intensity of 3×1017W/cm2

Neutron production as a result of the reaction ²H(d, n)³He in a picosecond laser plasma is reported. A considerable neutron yield of 5×10⁴ per pulse is obtained for the first time in a picosecond laser plasma on the surface of a solid deuterated target at laser radiation intensity of 3×10¹⁷ W/cm².     

Wide-band interferometric tuning of a multiatmosphere CO2 laser

We describe the operating characteristics of a compact, frequency tunable, electron-beam-controlled, 15 atmosphere CO₂ laser. Two intracavity Fabry-Perot etalons have been used to obtain laser line narrowing and continuous tuning within the 10.4 μm and 9.4μm bands of the CO₂ gain spectrum. The laser output consists of a 100 nsec pulse (fwhm) with energy up to 100 mJ. The overall laser tuning range is 70 cm^-1 and we have measured a laser linewidth of 0.03 cm^-1.