Quasi-continuous Laser Research Articles

Generation of higher-order harmonics upon the addition of high-frequency XUV radiation to IR radiation: Generalization of the three-step model

The irradiation of atoms by a strong, quasicontinuous IR laser field of frequency $\ensuremath{\omega}$ results in the emission of odd harmonics of $\ensuremath{\omega}$ (``IR harmonics''), by high-harmonic generation. It has been recently shown [A. Fleischer and N. Moiseyev, Phys. Rev. A 77, 010102(R) (2008)] that the addition of a weak XUV field of harmonic frequency $q\ensuremath{\omega}$ to the IR driver field leads to the appearance of a new set of higher-order harmonics (``XUV harmonics'') $q\ifmmode\pm\else\textpm\fi{}2K$ ($q,K$ integers) which were absent in the spectra in the presence of the IR field alone. Here we generalize these results by studying the generation of XUV harmonics upon the addition of an arbitrary high-frequency XUV field $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}\ensuremath{\omega}$, with frequency not necessarily a harmonic of $\ensuremath{\omega}$, and amplitude ${\ensuremath{\epsilon}}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}}^{\mathit{in}}$, which might be large. We have found that as the intensity of the XUV field increases, higher sets of XUV harmonics $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}\ifmmode\pm\else\textpm\fi{}2K,2\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}\ifmmode\pm\else\textpm\fi{}(2K\ensuremath{-}1),3\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}\ifmmode\pm\else\textpm\fi{}2K,\dots{}$. gradually appear, where each XUV harmonics set ${n}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}\ifmmode\pm\else\textpm\fi{}[2K\ensuremath{-}1+\mathrm{mod}({n}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}},2)]$ $({n}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}}=1,2,3,\dots{})$ is ${(\frac{{\ensuremath{\epsilon}}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}}^{\mathit{in}}}{{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}}^{2}{\ensuremath{\omega}}^{2}})}^{2{n}_{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}}}$ times weaker than the set of IR harmonics. The mechanism responsible for the appearance of the XUV harmonics is analyzed analytically using a generalization of the semiclassical recollision (three-step) model of high-harmonic generation (HHG). It is shown that the emitted HHG radiation field can be written as a series of terms, with the zeroth-order term representing the three-step recollision mechanism in its most familiar context [P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993)] and giving rise to IR harmonics in the spectra. The higher-order terms, which are responsible for the appearance of the new sets of XUV harmonics in the spectra, are shown to originate from the polarization (ac-Stark oscillations) which is induced by the XUV field on the returning electronic wave packet in the recollision process. The XUV harmonics are formed by the same electron trajectories which form the IR harmonics and have the same emission times as the IR harmonics. Due to the small quiver amplitude of the ac-Stark oscillation, they are, however, much weaker than the IR harmonics. Nevertheless, this mechanism allows an extension of the cutoff in the harmonic generation spectra without the necessity of increasing the IR field intensity, as is verified numerically by a quantum-mechanical simulation of a Xe atom irradiated by a combination of IR and XUV classical fields.

532 nm laser based on V-type doubly resonant intra-cavity frequency-doubling

LD side-pumped dual interconnected V-type quasi-continuous wave green laser has been demonstrated. The two Nd:YAG modules were placed in a plane-concave V-type resonator and a plane-concave straight cavity formed two stable operation beam of the 1064-nm fundamental frequency laser. Through acousto-optic Q-switched and frequency doubling crystal, two double-frequency laser beams arrived at the folded flat mirror, which were unidirectional output by the folded flat mirror at the end. As the pumped current was 50 A, the 532 nm green laser maximum average output power of 206 W at a repetition of 22.4 kHz was achieved with a pulse width of 201 ns and the largest single pulse energy of 9.2 mJ, corresponding to a peak power of 45.8 kW and a double frequency efficiency of 60.2%.

Formation of silicon nanostructures when a target is ablated by a quasi-continuous laser pulse

When a target is ablated in an atmosphere of an inert buffer gas by low-intensity laser radiation (no greater than 105W∕cm2), nanoparticles self-assemble into low-dimensional fractal structures. The dependences of the efficiency of the self-assembly process on the composition of the silicon–silica target and the pressure of the buffer gas manifest a distinct correlation with percolation in the plasma of the laser flare. A structure with a fractal dimension of df>2 is formed most efficiently close to the three-dimensional percolation threshold pc≈0.3, where pc is the ratio of the number density of silicon atoms to the number of all the atoms in the laser flare. Close to the two-dimensional percolation threshold (pc≈0.5), fractal structures are observed with df<2, assembled on a plane. The main structural elements of the observed objects are one-dimensional structures—chains of tens of nanoparticles, with a characteristic size of about 80nm. Results of a study of fractal nanostructures by scanning electron microscope are presented, an x-ray structural analysis is carried out, and the photoluminescence is studied.

Laser dazzling of infrared focal plane array cameras

Modern weapons systems often use infrared imaging for acquisition and tracking of targets. In the acquisition phase, the operator uses the system to detect the target and aim the weapon. In the tracking phase, the images from the camera are used to guide the weapon to the target. One possible method of protection against such imaging systems is laser dazzling. The intended effect is to deny the imager a clear view of the target by saturating the detector and consequently preventing the weapon from making a hit. The problem of locating the threat and pointing the laser at it is closely connected to the countermeasure effectiveness. This has been investigated particularly for specific applications such as directed infrared countermeasure (DIRCM) systems for aircraft self-protection. Another application is sniper detection, which uses the ‘cat’s-eye’ or retro-reflection effect, which occurs when a searching laser beam is directed at the visor of the rifle. We currently aim to use the retro-reflection effect in a wider range of applications for locating optics.1 The aim of our dazzling work is to quantify the effects of laser countermeasures on mid-wave infrared (MWIR) imaging to predict the decrease in the detection and pointing capabilities of weapons systems. Dazzling with a continuous or quasicontinuous pulsed laser causes a reversible saturation effect on the detector array. Most of the current MWIR lasers provide output power levels that do not exceed saturation levels for practical applications. Permanent damage effects require much higher irradiance levels at the camera front optics. This might be within reach at shorter ranges with these lasers, but is not addressed in our current research. In our experiments we used a periodically poled lithium niobate optical parametric oscillator (PPLN OPO) laser pumped by an Nd:YVO4 (yttrium orthovanadate) laser to generate a 3.9μm laser wavelength.2 Figure 1 presents images of a standard 256×320 indium antimonide (InSb) camera irradiated by Figure 1. Impact of increasing laser irradiance on the InSb camera image.

A novel design and operation scheme for a 224 nm segmented hollow-cathode silver ion laser

Operational data of a novel segmented hollow-cathode 224 nm silver ion laser are reported. The laser represents a next step in a series of segmented hollow-cathode laser designs: starting from simple laboratory test versions heading towards a practical sealed-off laser tube. 150 mW quasi-continuous laser power is reached during the 100 μs long discharge current pulses. The maximal obtained average power is 1.8 mW. Promising lifetime tests provide necessary information for further design.

Semiclassical limits to the linewidth of an atom laser

We investigate the linewidth of a quasi-continuous atom laser within a semiclassical framework. In the high flux regime, the lasing mode can exhibit a number of undesirable features such as density fluctuations. We show that the output therefore has a complicated structure that can be somewhat simplified using Raman outcoupling methods and energy-momentum selection rules. In the weak outcoupling limit, we find that the linewidth of an atom laser is instantaneously Fourier limited, but, due to the energy `chirp' associated with the draining of a condensate, the long-term linewidth of an atom laser is equivalent to the chemical potential of the condensate source. We show that correctly sweeping the outcoupling frequency can recover the Fourier-limited linewidth.

Analysis of Laser Surface Hardened Layers of Automobile Engine Cylinder Liner

Gray cast iron that is used for automobile engine cylinder liners was laser surface hardened using Nd: YAG quasi-continuous and CO2 continuous wave laser, respectively. The macromorphology and microstructure of the laser surface hardened layers were investigated using an optical microscope. Geometric dimensions including depth and width and microhardness distribution of the hardened layers were also examined in order to evaluate the quality of the hardened layers.

Precision high-aperture calorimetric sensors for high-energy laser beams

Aspects are discussed of raising the accuracy of a wide-aperture calorimetric converter for high levels of pulsed and quasicontinuous laser radiation.

Multi-frequency quasi-continuous wave solid-state Raman laser for the ultraviolet, visible, and near infrared

An efficient 1kHz barium nitrate Raman laser excited with the second harmonic radiation of a lamp pumped Nd:YAG laser has been developed and its characteristics studied. The threshold of the Raman laser generation corresponds to only 0.4kW of peak pumping power. The laser system generates 15 wavelength components in the 280–800nm spectral range with average powers from 5mW to 1.2W. The maximum Raman conversion efficiency reaches 40%. The minimum spectral width of the generated radiation is equal to 0.15cm−1. The laser system made can be used for Raman spectroscopy studies, engineering, medicine, and for other applications.

Guided Quasicontinuous Atom Laser

We report the first realization of a guided quasicontinuous atom laser by rf outcoupling a Bose-Einstein condensate from a hybrid optomagnetic trap into a horizontal atomic waveguide. This configuration allows us to cancel the acceleration due to gravity and keep the de Broglie wavelength constant at 0.5 microm during 0.1 s of propagation. We also show that our configuration, equivalent to pigtailing an optical fiber to a (photon) semiconductor laser, ensures an intrinsically good transverse mode matching.

Space qualification and environmental testing of quasicontinuous wave laser diode arrays

NASA’s mission requirements for spaceborne laser diode arrays lead to a set of tests peculiar to space flight. The goal of these tests is to determine if vibration, radiation, or vacuum will impair the operation or lifetime of nominally 100W quasicontinuous wave 808nm laser diode arrays. To simulate the stresses expected during a mission, terrestrial tests involve mechanical vibration, simulating the acceleration of launch, exposure to the equivalent doses of ionizing radiation, and operation in a vacuum. Three sets of devices were tested: one set with random vibration up to 20 g root-mean-square (grms) applied along three axes, a second set of devices was irradiated with γ radiation (1.17 and 1.33MeV) at 744rad(Si)∕min up to 200krad(Si), and the third set was exposed to a flux of 5×1011 or 1012p∕cm2 of 200MeV protons up to 60krad total dose. Only the proton irradiated devices showed any effect attributable to the test: a slight rise in lasing threshold, which recovered over time with self-annealing. A selection of the devices were then operated in vacuum for a further 1400h accumulating 500×106 pulses without any signs of degradation. Similar results were obtained in a parallel test, conducted in air, accumulating over 109 pulses on a selection of the remaining devices.

Performance of high-power laser diode arrays for spaceborne lasers

The adequacy of commercial quasi-continuous high-power laser diode arrays (HPLDAs) as pump sources for spaceborne lasers has been assessed by endurance tests up to 3 x 10(9) shots under various stress conditions, vacuum operation up to 0.36 x 10(9) shots, and proton radiation tests. Observations of the evolution of the electro-optic parameters and of the near-field patterns of the HPLDAs during endurance tests have revealed that some diode bars could reach the required lifetime of a multibillion shots, suggesting how to build long lifetime HPLDAs by proper selection of the diode bars. The robustness of the HPLDAs against the proton environment experienced in a typical low Earth orbit has been checked. Finally, high-power laser diode arrays have been operated under vacuum, showing a behavior similar to that of HPLDAs operating in atmospheric conditions.

Dynamics of radiation scattering by particles of condensed phase in quasicontinuous laser irradiation of metals

Radiation scattering by particles of condensed phase in an ablation plasma plume has been experimentally studied during quasicontinuous laser irradiation (λ = 1.06 µm, q = 0.1–9 MW/cm2, τ ∼ 1.5 msec) of duraluminum D16T, aluminum A99, and bismuth. The particle size distribution and the nature of their dispersal during irradiation was studied in scattered light (λ = 0.69 µm) from individual particles that could be visually observed on photographs. It was found that under the pressure developed in the plume, large particles ejected from the irradiated zone can move backward and return to the target (D16T). The plume (Bi) becomes brighter due to ablation of particles in the path of the laser beam. The directional scattering coefficients for scattering from the local zone on the axis of the plume, measured during the laser pulse, were used to study the relationship between the dynamics of entry of condensed phase into the plume, shielding of the target by the particles, and brightening of the plume under the action of the incident laser radiation.

Generation of 7.6-W blue laser by frequency-tripling of a Nd:YAG laser in LBO crystals

High-power, quasi-continuous wave blue laser output is obtained by intracavity frequency-tripling of a 1.319 microm Nd:YAG laser with two LiB(3)O(5) crystals. The conversion efficiency and the stability of the blue laser output power at 439.7 nm were both improved through the suppression of 1.338 microm operation by means of a thin YAG etalon. 7.6 W average power at 439.7 nm was demonstrated at 5 kHz with pulse width of 170+/-10 ns (FWHM). The beam quality factor M(2) is 12+/-2 in both horizontal and vertical directions. The long-term stability of the blue light is better than 1% at an output of 6 W for 200 operation hours.

PHOTOSELECTIVE VAPORIZATION OF THE PROSTATE: A VOLUME REDUCTION ANALYSIS IN PATIENTS WITH LOWER URINARY TRACT SYMPTOMS SECONDARY TO BENIGN PROSTATIC HYPERPLASIA AND CARCINOMA OF THE PROSTATE

In this study preoperative and postoperative transrectal ultrasound prostate volume was evaluated in patients undergoing photoselective vaporization of prostate using an 80 W potassium-titanyl-phosphate (KTP) laser (Greenlight PV Laser System, Laserscope, San Jose, California) for obstructive uropathy secondary to benign prostatic hyperplasia or carcinoma of the prostate. A total of 18 patients with lower urinary tract symptoms secondary to benign prostatic hyperplasia (8) and carcinoma of the prostate (10) were treated with an 80 W quasicontinuous KTP laser. Preoperative and immediate postoperative treatment prostate volume measurements were recorded by transrectal ultrasound. The end point of treatment was complete vaporization of the obstructive adenoma to the level of the capsular fibers and the creation of an adequate transurethral resection-like prostatic cavity. KTP/532 laser energy was delivered by a side firing glass fiber through a 27Fr continuous flow resectoscope. Photoselective vaporization of the prostate using sterile water irrigation was performed with all patients under spinal anesthesia. Mean lasting time +/- SEM was 33.5 +/- 12 minutes (range 11 to 53). Mean preoperative prostate volume +/- SEM was 53.2 +/- 24.7 ml (range 23.6 to 110), while mean postoperative prostate volume was decreased to 26.2 +/- 14.8 ml (range 8 to 58) during a mean followup of 2.8 +/- 2.3 months (range 1 to 10), resulting in a 51% mean decrease in prostate volume, as measured by transrectal ultrasound. There was no significant intraoperative bleeding and no change in serum sodium postoperatively. One patient sustained a small capsular perforation with persistent venous bleeding, which could not be controlled with KTP laser. Because of poor vision, the procedure was completed with electroresection. Complications included mild dysuria in 2 patients (11%) and mild hematuria longer than 2 weeks in duration in 4 (22%). Photoselective prostate vaporization can effectively vaporize obstructive benign and malignant prostatic tissue, leading to a significant decrease in the total volume of the treated prostate (p = 0.000).

Highly efficient high-power quasi-continuous diode laser bars for pumping solid-state lasers based on Yb-containing active media

Highly efficient high-power quasi-continuous diode laser bars (DLBs) emitting in the region of λ=0.95 μm have been developed for pumping solid-state lasers based on Yb-containing active media. The parameters of DLBs and the results of long-term tests are presented. The proposed DLBs are implemented in a solid-state laser based on an Yb-Er glass operating in the 1.5 mm range.

Erratum: Quasicontinuous Atom Laser in the Presence of Gravity [Phys. Rev. Lett. 86, 4729 (2001)

In a recent Letter [1], we analyzed theoretically the extraction of a quasicontinuous atom laser out of a trapped Bose-Einstein condensate. The explicit expression for the outcoupling rate given in this Letter was underestimated by a factor of 2, and the correct expression should read (using the notation of [1]).

LD side-pumped passively Q-switched Yb:YAG slab laser

A quasi-continuous wave laser diode side-pumped passively Q-switched Yb:YAG slab laser with Cr 4+:YAG saturable absorber has been demonstrated in order to understand the pulse properties of Yb:YAG crystal. To our knowledge the maximum 69% extraction efficiency is achieved by the system. 44 μJ pulse energy and 1.64 KW peak power with near diffraction-limited beam quality are presented at 25 Hz repetition rate. The build-up time of the Q-giant in the passively Q-switched laser is shown.

Experimental observation of a new modulational instability spectral window induced by fourth-order dispersion in a normally dispersive single-mode optical fiber

We experimentally investigate the influence of the fourth-order dispersion (FOD) on the onset of scalar spontaneous modulational instability (MI) in a single-mode optical fiber. This study was performed in the normal dispersion regime of a dispersion-shifted fiber pumped by a quasi-continuous laser diode operating near the zero-dispersion wavelength. We experimentally demonstrate the evidence of a new MI spectral window due to the FOD effect in the normal dispersion regime. The effect of random fluctuations of the zero-dispersion wavelength along the fiber is considered for effective gain prediction. The measured modulation frequencies, which range from 5 up to 9 THz for an input power of 7 W, are in very good agreement with those obtained from the linear stability analysis. The FOD induced-MI can be used for broadband wavelength conversion. In addition it can be of a great interest for the generation of ultra-high repetition rate train of femtosecond pulses.

Diffraction Signals of Aligned Molecules in the Gas Phase: Tetrazine in Intense Laser Fields

We performed a theoretical study of the electron diffraction patterns that arise from aligning a molecule, s-tetrazine (C2N4H2), in a high intensity laser field using the Friedrich−Herschbach approach. The molecule is modeled using geometries and zero-point vibrations from a coupled-cluster level ab initio calculation. The alignment is achieved by a circularly polarized, quasicontinuous high-intensity laser pulse that intersects the molecular sample at selected geometries. The molecular ensemble is taken to be at a rotational temperature of 5 K. We find that even at 1 TW/cm2 there is a very noticeable effect on the diffraction pattern stemming from the alignment. Moreover, using different laser geometries, it is possible to observe different diffraction patterns corresponding to specific projections of the molecular geometry onto the Fourier plane of the detector, making the laser a kind of “optical goniometer.” Predictably, higher alignment laser intensities lead to narrower orientational distributions, which in turn provide for diffraction patterns with larger modulation depths. We find, however, that for small diffraction angles (s ≤ 15 Å-1) the effect appears to saturate at an alignment laser intensity of about 4 TW/cm2. This suggests that there is a window of opportunity where alignment is very beneficial for extracting information from gas-phase diffraction patterns and where detrimental multiphoton processes do not yet compromise the interpretation of the molecular structure.