Photodissociation followed by laser-induced fluorescence at atmospheric pressure and 24 degrees C: a unique scheme for remote detection of explosives

A unique scheme has been applied for sensitive remote detection of 2,4,6-trinitrotoluene (TNT) vapor trace amounts at atmospheric pressure and 24 degrees C. The detection concept is based on a single laser beam inducing a tandem process: photodissociation of TNT vapor followed by highly selective detection of its photofragments vibrationally excited NO, utilizing laser-induced fluorescence with the A2Sigma+(v' = 0) <-- X2Pi(v' = 2) transition. A detection sensitivity of at least 8 parts in 10(9) of TNT vapor with a signal-to-noise ratio of approximately 10 has been experimentally verified for an unfocused approximately 5-mJ laser beam, measured at a distance of approximately 15 cm from the TNT sample.     

Picosecond Diode-Pumped Laser System with 9.3-W Average Power and 2.3-mJ Pulse Energy

We describe a diode-pumped Nd:YAG laser that is suitable for micromachining applications and is capable of generating 2.3-mJ pulses at a 4-kHz pulse repetition frequency. The output pulse duration is 20.5 ps. The system is based on a Nd:YAG regenerative amplifier with a novel four-pass postamplifier. The postamplifier incorporates birefringent depolarization compensation and simultaneously prevents parasitic laser oscillation by use of a nonreciprocal beam path. These output pulse energies are achieved without the use of chirped pulse amplification.     

Kilohertz pulse repetition frequency slab Ti:sapphire lasers with high average power (10 W)

High-average-power broadband 780-nm slab Ti:sapphire lasers, pumped by a kilohertz pulse repetition frequency copper vapor laser (CVL), were demonstrated. These lasers are designed for damage-free power scaling when pumped by CVL's configured for maximum output power (of order 100 W) but with poor beam quality (M(2) ~ 300). A simple Brewster-angled slab laser side pumped by a CVL produced 10-W average power (1.25-mJ pulses at 8 kHz) with 4.2-ns FWHM pulse duration at an absolute efficiency of 15% (68-W pump power). Thermal lensing in the Brewster slab laser resulted in multitransverse mode output, and pump absorption was limited to 72% by the maximum doping level for commercially available Ti:sapphire (0.25%). A slab laser with a multiply folded zigzag path was therefore designed and implemented that produced high-beam-quality (TEM(00)-mode) output when operated with cryogenic cooling and provided a longer absorption path for the pump. Excessive scattering of the Ti:sapphire beam at the crystal surfaces limited the efficiency of operation for the zigzag laser, but fluorescence diagnostic techniques, gain measurement, and modeling suggest that efficient power extraction (>15 W TEM(00), >23% efficiency) from this laser would be possible for crystals with an optical quality surface polish.     

Q-switched 2-micron lasers by use of a Cr2+:ZnSe saturable absorber

Flash-lamp-pumped Ho:YAG (2090-nm) and Tm:YAG (2017-nm) lasers were, for the first time to our knowledge, passively Q switched by use of a Cr(2+):ZnSe saturable absorber. A Q-switched Ho laser with 1.3-mJ pulse energy and ~90-ns pulse duration and a Q-switched Tm laser with ~3.2-mJ pulse energy and 90-ns pulse duration were demonstrated. Compared with the free-running output energies at the Q-switching threshold pump levels, the Q-switching efficiencies were approximately 5% for the Ho:YAG laser and 16% for the Tm:YAG laser.     

24-W average power at 0.537 µm from an externally frequency-doubled Q-switched diode-pumped Nd:YOS laser oscillator

A diode-pumped single Nd:YOS rod laser is operated at 110 Hz with 830-mJ free-lase pulses and 380-mJ, 13-ns (FWHM) Q-switched pulses. External doubling with an 8-mm-long KTP crystal results in 24 W at 0.537 μm after thermalization.     

Experimental investigation of high-power single-pass Raman shifters in the ultraviolet with Nd:YAG and KrF lasers

Single-pass Raman cells pumped by either a quadrupled Nd:YAG (266-nm) laser or a KrF excimer laser are studied. The Raman-active gases comprise H(2), D(2), or CH(4), as well as a mixture of them, with the addition of He, Ne, or Ar. A parametric study, in which the Stokes conversion efficiency and the beam quality (M(2)) were measured, was made. The first Stokes efficiency increases and all the Stokes thresholds decrease with an increase in the lens focal length or the M(2) parameter of the pump beam. The quality of the Stokes beams deteriorates when the active-gas pressure increases but is improved by the addition of an inert gas. Laser-induced breakdown is shown to be a factor that limits the conversion efficiency and the quality of the Stokes beams. With a mixture of D(2), H(2), and Ar, a 10-15-mJ pulse energy is obtained (depending on the pump M(2) parameter) in the first Stokes beam of D(2) (289 nm) and H(2) (299 nm), with a full-angle divergence of 0.5 mrad (at 86% power).     

Saturable absorber Co2+:MgAl2O4 crystal for Q switching of 1.34-microm Nd3+:YAlO 3 and 1.54-microm Er3+:glass Lasers

Saturable-absorber Q switching of a neodymium-doped YAlO(3) laser at 1.34 microm and an erbium-doped glass laser at 1.54 microm with a Co(2+):MgAl(2)O(4) crystal was demonstrated. Q-switched 1.34-microm pulses of 19-mJ energy and 60-ns duration and Q-switched 1.54-microm pulses of 2.7-mJ energy and 75-ns width were obtained. The ground-state absorption cross sections of the Co(2+):MgAl(2)O(4) crystal were estimated to be (2.8 +/- 0.4) x 10(-19) and (3.5 +/- 0.6) x 10(-19) cm(2) at 1.34 and 1.54 microm, respectively. The (4)T(1)((4)F)-->(4)A(2) relaxation time of the Co(2+) ion in the MgAl(2)O(4) crystal was measured to be approximately 350 ns.     

Chromium-doped LiCAF laser passively Q switched with a V3+:YAG crystal

A tunable, lamp-pumped Cr(3+):LiCaAlF(6) laser that operates in the 760-800-nm wavelength range is demonstrated. As a passive Q switch the V(3+):YAG crystal, characterized by wide saturable absorption in the 760-900-nm wavelength range, was applied. The V(3+):YAG parameters were determined by spectroscopic and saturable transmission measurements. The 30-mJ output energy at 2-Hz repetition rate in the free-running regime was obtained in a cavity reinforced by a diaphragm near the fundamental mode. The dispersive prism inserted into the cavity enabled tuning in the 760-800-nm wavelength range. In the passive Q-switching regime we achieved greater than 1-mJ energy with 50-ns pulse duration in a 35-cm-long cavity. A numerical model that takes into account the short recovery time of V(3+):YAG (~5 ns) and excited-state absorption was used to analyze such a laser.     

Development of line-shaped optical system for green laser annealing used in the manufacture of low-temperature poly-Si thin-film transistors

We have developed a new optical system that transforms the circle profile beam generated with near-Gaussian intensity distribution by a pulse green laser (YAG2omega laser; second harmonics of a Q-switched Nd:YAG laser) into a line-profile beam. For homogenization in the longitudinal direction, we employed a waveguide plate-type homogenizer. We successfully reduced interference fringes. In the width direction, the laser beam was focused up to the limited M2 value. This transformed beam has a uniform distribution to within 5% in the longitudinal direction, and it is approximately 100 mm long and 40 microm wide.     

Beam-profile modulation of thulium laser radiation applied with multimode fibers and its effect on the threshold fluence to vaporize water

The threshold fluences at which vaporization is initiated at the tip of a multimode fiber that is submerged in water were investigated when free-running and Q-switched thulium laser pulses (lambda = 2.01 mum) were applied with different pulse energies. We focused on the quantification of temporal and spatial fluence modulations of the beam profile at the tip of a 400-mum fiber. The spatial and the temporal fluence peaks over the average fluence were measured to as high as 1.5 and 4 in the Q-switched mode, respectively, and 2.5 and 40 in the free-running mode, respectively. The fluence peaks significantly influence the vaporization process. An increase in the threshold fluence with increasing pulse energy was found for the Q-switched mode, but there was a decrease for the free-running mode. Pressure transients of the order of 1 kbar and temperatures higher than 200 degrees C were calculated for a 30-mJ Q-switched laser pulse at the onset of vaporization. Collecting all the data allowed us to trace the thermodynamic path of rapid heating and vaporization in a phase diagram of water.     

Room-temperature 1.643-µm Er(3+):Y(3)Sc(2)Ga(3)O(12) (Er:YSGG) laser

Laser operation at 1.643 μm ((4)I(13/2)-(4)I(15/2)) in Er(3+):Y(3)Sc(2)Ga(3)O(12) (Er:YSGG) at 300 K is reported. An Er:glass laser (1.532 μm) was used in an end-pumping arrangement to obtain laser output from a 1-cm-long 0.7% Er(3% Yb, 1% Cr):YSGG crystal in an external cavity. The Er:YSGG laser exhibited an 18-mJ threshold and a 10% slope efficiency.     

Dynamic Correction of Thermal Focusing in Nd:YAG Confocal Unstable Resonators by Use of a Variable Radius Mirror

Excellent beam quality and divergence stability over a wide pump power range was demonstrated in a Q-switched, Nd:YAG, positive branch confocal unstable resonator by using a one degree-of-freedom, adaptive optic. Unlike single-element flexible-membrane mirrors, the variable radius mirror (VRM) consisted of a lens and mirror, whose separation determined the VRM's effective radius of curvature. This simple method enabled low cost and efficient thermal focusing compensation. The VRM was demonstrated by producing a 300-mJ Q-switch or 1-J free-running at a beam quality factor M(2) that varied between 1.2 and 1.8 as the average output power varied between 0 and 33 W.     

Intracavity pumped nanosecond optical parametric oscillator emitting in the eye-safe range

We report on a 1.57-mum wavelength singly resonant optical parametric oscillator (OPO) using a noncritically phase-matched KTP crystal intracavity pumped by an electro-optically Q-switched Nd:YAG laser. A simple model for numerical calculation of the temporal profile of the laser and signal pulses developed inside the intracavity pumped OPO (IPOPO) resonator is described. Working conditions for single-pulse and multipulse parametric oscillation are established. A single signal pulse of 8.1-mJ output energy was obtained when the laser was pumped three times above the pulsed IPOPO threshold. A pulse duration of 5.9 ns was measured at 1.5 times above threshold for the IPOPO.     

Propagation through the ocean's surface at shallow angles with a laser beam

Propagation through the ocean's surface at shallow angles with a laser has been successfully demonstrated experimentally in the ocean. A 6-mJ multipulse laser at a depth of 1.5 m in water with an attenuation coefficient of approximately 3.3 m(-1) was easily detected 600 m away on a sunlit day at an angle of 1 degree to the ocean's surface. The measured efficiency was 3.6 x 10(-2) (J/J)/sr (joules received per receiver steradian per joule transmitted). Statistical ray trace analysis and physical tank modeling produced equivalent efficiencies.     

Energy-Transfer Dye Laser in SolGel Silica

Coumarin 460 transfers radiative energy to disodium fluorescein when solgel silica is doped with the dyes, producing disodium fluorescein lasing in superradiant mode at a 3.3% peak efficiency under low-energy transverse pumping by a 1.5-mJ N(2) laser, with disodium fluorescein being present at a concentration below its lasing threshold value.     

Glass doped with PbS quantum dots for passive Q switching of a 1.54- microm laser

By using a PbS-doped phosphate glass as an intracavity saturable absorber, we achieved passive Q switching of a 1.54-mum Er:Yb:glass laser. We obtained Q-switched pulses with 110-ns duration and 0.2-mJ energy. The saturation intensity at 1.54 mum was estimated to be ~60 kW/cm(2).     

Refractive-index patterning of tellurite glass surfaces by laser spot heating

A dot pattern of a refractive-index change was formed by spot heating with laser-beam irradiation on sodium tellurite glasses. The 15Na(2)O . 85TeO(2) (mol. %) glass doped with 2 mol. % of CoO was irradiated by a green light-beam spot (532 nm) ~800 mum in diameter from the second-harmonic generator of a Q-switched pulsed YAG laser. The map of the refractive index of the glass was determined with an He-Ne laser beam by a scanning ellipsometric technique at a resolution of 100 mum x 50 mum, indicating that the spots possessing a refractive index lower by ~0.05 were formed at the region irradiated by the laser beam.     

Efficient, reliable, long-lifetime, diode-pumped Nd:YAG laser for space-based vegetation topographical altimetry

A highly efficient, diode-pumped, Nd:YAG laser is described. The oscillator utilizes an unstable resonator design with a Gaussian reflectivity output coupler and a side-pumped zigzag slab gain medium. The laser produces 18-mJ, 10-ns pulses at a repetition rate of 242 Hz in a near-TEM00 mode with an optical efficiency of up to 14%. An extended performance test was recently concluded in which the transmitter operated at reduced output for more than 4.8 x 10(9) shots with no optical damage. Design criteria, beam quality, and lifetime data are presented.     

High-energy,all-solid-state,ultraviolet laser power-amplifier module design and its output-energy scaling principle

We demonstrate that a coaxially pumped, large-aperture ultraviolet power-amplifier module with solid-state tunable laser medium Ce(3+):LiCaAlF6 has 98-mJ, 290-nm, and 3-ns output pulses with a sufficient extraction efficiency of 25%. The detailed information of design parameters, including the gain-coefficient dependence on pump condition, is successfully accumulated for further energy scaling for a terawatt-class ultraviolet chirped pulse amplification laser system or a high-pulse-energy laser system.     

Green lasers based on CdSe/ZnSe nanostructures pumped by electron beams with energies below 10 keV

We have studied the output characteristics of pulsed electron-beam-pumped green lasers based on ZnSe-containing quantum-sized structures with thin (20 nm thick) external ZnMgSSe confinement layers. Room-temperature lasing has been observed for electron beam energies above 3.7 keV. At a beam energy of 8–9 keV, the minimum threshold beam current density was 0.4–0.5 A/cm². The maximum laser output pulse energy was 2 W at a pumping electron beam energy of ～5 kW.