Optical trap with spatially varying polarization: application in controlled orientation of birefringent microscopic particle(s)

We report the use of the interference of two orthogonally polarized beams for generation of an optical trap with spatially varying polarization. The spatial variation of polarization in the optical trap has been used for demonstration of simultaneous rotation or orientation of multiple microscopic birefringent particles. Other potential applications of an optical trap with spatially varying polarization are also discussed.This revised version was published online in August 2005 with a corrected cover date.     

Fresnel diffraction model for mode-mismatched thermal lens with top-hat beam excitation

A theoretical model, based on the Fresnel diffraction integral, is developed to describe the thermal lens (TL) signal in a mode-mismatched collinear configuration, which is optimized for a near field detection scheme and excitation by a cw modulated laser beam with a top-hat profile. The TL amplitudes obtained with both top-hat and Gaussian beam excitations are numerically computed and compared, and the dependence of the TL amplitude on the experimental parameters is discussed. Numerical results show that the top-hat beam TL instrument is more sensitive than the Gaussian beam TL instrument, with a potential doubling of the sensitivity. The use of the top-hat beam excitation with TL detection is a significant improvement because a top-hat beam can be easily obtained with a low-cost, wide-spectral emission white-light source. The use of incoherent light sources as the excitation sources would substantially expand the applicability of the TL technique to the general area of chemical analysis.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber

Copropagating fundamental-wavelength and second-harmonic femtosecond pulses of Cr: forsterite laser radiation are used to study cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber. Parametric instability of the second-harmonic probe pulse induced through cross-phase modulation by the fundamental-wavelength pump pulse gives rise to distinct sidebands in the spectrum of the probe field transmitted through the fiber. The wavelength of these sidebands was tuned in our experiments within approximately 100 nm by varying the peak power and the delay time of the pump pulse, suggesting a convenient way of controlled parametric spectral transformation of ultrashort laser pulses.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Double-ring cavity configuration of actively mode-locked multi-wavelength fiber laser with equally tunable wavelength spacing

A novel actively mode-locked multi-wavelength tunable fiber laser with equally increased or decreased wavelength spacing is presented. It is constructed using a double-ring cavity combined with an amplitude modulator and cascaded fiber Bragg gratings (FBGs). The cavity lengths for all FBGs are intrinsically identical due to the double-ring configuration, and simultaneous mode locking of multiple wavelengths is therefore achieved by applying only one mode-locking signal to the modulator. The FBGs are mounted on a organic plate with angles between adjacent gratings such that the forces applied to the FBGs have an equal force increment; thus, stretching or compressing the cascaded FBGs can achieve a wavelength tuning with equally increased or decreased wavelength spacing.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Optical manipulation of third-harmonic generation via either one- or two-photon excitation in diarylethene-polymethylmethacrylate polymer thin films

Optical manipulation of third harmonic (TH) generation in diarylethene-polymethylmethacrylate (DE-PMMA) polymer thin films is obtained by either one- or two-photon excitation. TH intensity generated from a DE-PMMA polymer thin film decreases, when it is pumped by either 325 nm or 442 nm laser irradiation, which changes the molecular structure of DE molecules from an open-form (A form) to a closed-form (B form). TH intensity recovers to its original intensity level when all B form DE molecules return to an A form after being induced by either 532 nm or 1064 nm laser irradiation. The experimental results reveal that the second-order hyperpolarizability () of the A form molecules may be larger than that of the B form molecules. Moreover, TH output efficiency is independent of the angle between the pump and probe polarization directions. This result is attributed to the two-dimensional structure of DE molecules.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Experimental characterisation, optimisation and design of erbium-doped silica fibre lasers

A new characterisation method is described using the new theoretical model for erbium-doped silica fibre lasers (EDSFLs) based on the energy conservation principle. Using this method, we obtained absorption and emission coefficients for the lasing wavelength at lasing operating conditions. After that, an experimental procedure to deduce the spectral profiles of the absorption and emission coefficients is also presented. This procedure allows us to obtain the values of these parameters for the whole fluorescence spectrum through measurements of gain profiles under the lasing operation. Once the absorption and emission coefficients are known, the new model can be applied and a comparison with experimental results for two different laser configurations is shown. The theoretical model is proved to be accurate and in addition some equations are developed to allow the design and optimisation of EDSFLs. This revised version was published online in March 2005. In the previous version, the published online date was missing     

Sapphire-conductive end-cooling of high power cryogenic Yb:YAG lasers

We have demonstrated a high-power laser oscillator with end-cooling using a sapphire-sandwiched Yb:YAG disk at near liquid nitrogen temperature. An output power of 75 W with a near-diffraction-limited beam was obtained from a 0.6-mm thick activemedium. The slope efficiency and themaximum optical–optical efficiency were 80 and 70%, respectively, with respect to absorbed pump power.This revised version was published online in August 2005 with a corrected cover date.     

3D Modeling of amplification processes in CPA laser amplifiers

We present here chirped pulse amplification simulations allowing for the extension of the generally used 1D model. The importance of the beam spatial profiles, diffraction and thermal effects in simulations is clearly shown in order to describe with accuracy the gain saturation. Experimental measurements performed on a 100 TW laser validate this calculation. The effects of dispersion and self-focusing are also studied.This revised version was published online in August 2005 with a corrected cover date.     

Kinetics of helium 3 laser optical pumping

We report on optical pumping experiments in helium-3 using a new broad-band, all fiber laser at 1083 nm. The kinetics of polarization build-up are compared to the numerical results of an appropriate model. Effects of laser power and bandwidth are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Frequency degenerate oscillation in semilinear photorefractive oscillator with reflection gratings

The oscillation intensity of a semilinear photorefractive oscillator with reflection gratings is calculated for different cavity losses, different pump intensity ratios and different crystal orientations. Distinct from an oscillator with transmission gratings, this oscillator always has a soft excitation in the vicinity of the threshold. The theoretical results are in reasonable agreement with that of experimental studies with BaTiO₃:Co as a photorefractive crystal.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Femtosecond thin-disk Yb:KYW regenerative amplifier

We report a compact thin-disk Yb:KYW regenerative amplifier system. Two different concepts are investigated to obtain either subpicosecond pulses with up to 160 μJ or a pulse energy of 20 μJ with a pulse width of about 300 fs. The first concept uses intra-cavity group-velocity dispersion compensation with Gires–Tournois interferometer mirrors to avoid pulse stretching during amplification. The onset of nonlinear effects in this concept inhibits the generation of shorter pulse durations at this energy level. Shorter pulses can be achieved with the second concept, which is based on dispersive pulse stretching during amplification and uses pulse compression after amplification with a grating compressor. Repetition rates up to 45 kHz are demonstrated.This revised version was published online in May 2005. The Article Category was removed.This revised version was published online in August 2005 with a corrected cover date.     

Electron Shell as a Resonator

Main principles of the resonance effect arising in the electron shells in interaction of the nuclei with electromagnetic radiation are analyzed and presented in the historical aspect. Principles of NEET are considered from a more general position, as compared to how this is usually presented. Characteristic features of NEET and its reverse, TEEN, as internal conversion processes are analyzed, and ways are offered of inducing them by laser radiation. The ambivalent role of the Pauli exclusion principles in NEET and TEEN processes is investigated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Time-domain, nuclear-resonant, forward scattering: theclassical approach

This paper deals with the interaction of electromagnetic radiation with matter assuming the matter to have nuclear transitions in resonance with incident electromagnetic radiation. The source of the radiation is taken to be of two types; natural radioactive gamma decay and synchrotron radiation. Numerical examples using ⁵⁷Fe are given for the two types of source radiation. Calculated results are contrasted for the two cases. Electromagnetic radiation produced by recoil-free gamma-ray emission has essentially the natural linewidth. Electromagnetic radiation from a synchrotron, even with the best monochromators available, has a relatively broad-band spectrum, essentially constant for these considerations. Polarization effects are considered. In general, the nuclear-resonant medium changes the polarization of the input radiation on traversing the medium. Calculations are presented to illustrate that synchrotron radiation studies using nuclear-resonant forward scattering have the potential for making high-precision measurements of hyperfine fields and recoilless fractions. An interesting aspect of nuclear-resonant forward scattering, relative to possible gamma-ray laser development, is the so-called “speed-up” effect. This revised version was published online in August 2006 with corrections to the Cover Date.     

All chirped fiber grating based true-time delay for phased-array antenna beam forming

We have demonstrated a continuous phased-array antenna beam former based on a variable photonics true-timedelay unit consisting of four chirped gratings. The system is suitable for the phased-array antenna beam forming at frequencies up to 20 GHz.This revised version was published online in August 2005 with a corrected cover date.     

Optical properties of laser plasma produced by pulses of different duration

The results of experiments on the interaction of laser radiation pulses (at intensities of up to 1015 W cm−2) of different durations (t = 3ps; 300ps; 30ns) with targets in a vacuum chamber are presented. An investigation was made of the optical characteristics of laser plasma in vacuum ultraviolet and X-ray ranges. The intensity of the laser-produced plasma radiation in the range 1–50nm was estimated to be 1010–1012W cm−2. This revised version was published online in November 2006 with corrections to the Cover Date.     

Cascaded frequency doubling and electro-optic coupling in a single optical superlattice

Frequency doubling and electro-optic (EO) coupling are cascaded in a single optical superlattice due to simultaneous modulation of nonlinear and EO coefficients. The coupling equations are deduced and analyzed, and simulations in both periodic and quasi-periodic structures are presented. The results show that both polarization and magnitude of the second harmonic can be manipulated by an external DC electric field, which is useful when simultaneous frequency conversion and signal tuning are desired.This revised version was published online in August 2005 with a corrected cover date.     

Radiation machines for gamma-ray laser research

In the basic research efforts to define the optimum isomers for making a gamma-ray laser, a variety of radiation sources have been used to find the nuclear gateways that make the idea possible. The radiation sources have several roles to play in the gamma-ray laser process, including pumping isomers to excited states with long lifetimes and in subsequently pumping these to short-lived states that quickly decay. This paper reviews the technology of the radiation sources and the desirable characteristics relative to pumping and triggering in gamma-ray laser research. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nuclear Excitation by X-Ray Emission of Femtosecond Laser Plasma

The proposed model of laser plasma emission spectrum formation enables us to determine the absolute value of the laser pulse to plasma emitted radiation conversion factor, profile of the emission spectrum, and frequency distribution of the intensity and energy in the emitted spectra. This is of interest for laser plasma diagnostics and provides a means for direct calculation of the number of excited nuclei in dependent on the parameters of laser pulse. This revised version was published online in August 2006 with corrections to the Cover Date.     

Q-switched Yb 3+ :YVO 4 laser with Raman self-conversion

We report the efficient continuous-wave (CW) and Q-switched laser operation of a diode-pumped Yb:YVO₄ laser. A CW output power of 1 W with a slope efficiency of 59% with respect to absorbed pump power was demonstrated. Passively Q-switched with a Cr⁴⁺:YAG saturable absorber, a Yb:YVO₄ laser with Raman conversion was demonstrated. Q-switched 18.7- J pulses with a pulse duration of 17 ns and a peak power up to 1 kW were obtained at 1018-nm fundamental wavelength and 3.6- J pulses with a pulse duration of 6 ns and a peak power of about 0.6 kW were obtained at 1119.5-nm first-Stokes wavelength.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Supersonic CO2 laser excited by RF discharge in closed cycle

A supersonic gas flow having a Mach number of 2 has been realized in a closed-cycle radio-frequency (RF)-discharge-excited supersonic CO₂ laser system. Stable RF discharge at a high CO₂ gas concentration has become possible using supersonic gas flow and RF discharge generated between dielectric electrodes. As a result, high RF input power density has been obtained. In addition, a high small-signal gain has been obtained in the supersonic section through decreases in gas pressure and gas temperature due to supersonic gas flow.This revised version was published online in August 2005 with a corrected cover date.