Cw Laser Beam Research Articles

Surface nanostructuring of borosilicate glass by femtosecond nJ energy pulses

We report on a feature, that of hillock-shaped damage, formed on a glass surface by femtosecond pulses of tp=180 fs [full width at half maximum (FWHM) value] duration produced by a recording beam focus with energy of 5 nJ/pulse at 800 nm wavelength (the corresponding irradiance of about 7.6 TW/cm2 was evaluated for a 0.68 μm FWHM spot size). Single hillocks of 40–50 nm height were recorded reproducibly in single-pulse irradiation. Surface nanopatterning over a large, curved area (over 200 μm2) can be achieved by implementing a confocal surface curvature tracking method that utilizes the reflection of a supplementary cw-laser beam. The ablation pattern achieved by this method is consistent with that of a Coulomb explosion.

Fabrication of microlenses in Ag-doped glasses by a focused continuous wave laser beam

A technique for the fabrication of small (2–10 μm in diameter) microlenses on the surface of glass with embedded silver nanoclusters in a subsurface diffusion layer is demonstrated. The dependence of the microlens size on the exposure time and the laser power of a focused continuous wave (cw) laser beam is discussed. It is shown that the optical transmission of the lenses increases with increasing laser power used for the exposure. The temperature distribution in the glass around the focal spot is calculated taking into account the temperature dependence of heat conductivity, shape of the beam, and decrease of absorption coefficient with depth through the diffusion layer containing Ag clusters. The measured microlens sizes are in good agreement with the calculations.

Measurement of a small vertical emittance with a laser wire beam profile monitor

We describe in this paper a measurement of vertical emittance in the Accelerator Test Facility (ATF) damping ring at KEK with a laser wire beam profile monitor. This monitor is based on the Compton scattering process of electrons with a laser light target which is produced by injecting a cw laser beam into a Fabry-Perot optical cavity. We installed the monitor at a straight section of the damping ring and measured the vertical emittance with three different ring conditions. In all cases, the ATF ring was operated at 1.28 GeV in a single bunch mode. When the ring was tuned for ultralow emittance, the vertical emittance of ${\ensuremath{\epsilon}}_{y}=(1.18\ifmmode\pm\else\textpm\fi{}0.08)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\mathrm{mrad}$ was achieved. This shows that the ATF damping ring has realized its target value also vertically.

Performance Studies of a Laser Wire Beam Profile Monitor

We describe a new type of beam profile monitor developed for measurements of an electron beam as small as 10 µm in size. This monitor is based on the Compton scattering of electrons with a laser light target. A thin and intense laser beam (laser wire) is produced by injecting a CW laser beam into a Fabry–Perot optical cavity. We were able to obtain a stable laser wire having a beam waist of 14.52±0.55 µm and a power gain of 220±20. This monitor was installed in the ATF damping ring at KEK, and was used to measure its vertical emittance. We report in this paper the development and performance studies of this monitor in detail.

Improvement in friction by cw Nd:YAG laser surface treatment on cast iron cylinder bore

The reduction of the oil-film thickness in the piston-assembly of automobile engines has lead to it now being necessary to add a cylinder–bore surface treatment, so that oil reserves can be created on it. However in doing this, the initial plateaued texture must be kept. With a Nd:YAG laser source treatments oil reserves holes are formed, which respect the function of the initial bore surface texture. The material, cast iron, is heterogeneous: perlitic matrix and carbon lamellae. The laser treatment will make use of this heterogeneity: as under a cw laser beam, using graphite, with its good insulating material properties, carbon lamellae, which are near the surface, are revealed. Also a high density of lamellae in a hollow against the surface are obtained. In order to improve these new surfaces, friction tests were conducted on a friction bench using samples from engine components as test pieces. In order to know bore roughness evolution, 3D surface topography measurements were made before and after as well as friction tests on surfaces without laser treatment and with laser treatment. The results indicate that these holes can improve oil lubrication time.

Building blocks for a two-frequency laser lidar-radar: a preliminary study.

A new principle of lidar-radar is theoretically and experimentally investigated. The proposed architecture is based on the use of an rf modulation of the emitted light beam and a direct detection of the backscattered intensity. Use of a radar-processing chain allows one to obtain range and Doppler measurements with the advantages of lidar spatial resolution. We calculate the maximum range of this device, taking into account different possible improvements. In particular, we show that use of a pulsed two-frequency laser and a spatially multimode optical preamplification of the backscattered light leads to calculated ranges larger than 20 km, including the possibility of both range and Doppler measurements. The building blocks of this lidar-radar are tested experimentally: The radar processing of an rf-modulated backscattered cw laser beam is demonstrated at 532 nm, illustrating the Doppler and identification capabilities of the system. In addition, signal-to-noise ratio improvement by optical pre-amplification is demonstrated at 1.06 microm. Finally, a two-frequency passively Q-switched Nd:YAG laser is developed. This laser then permits two-frequency pulses with tunable pulse duration (from 18 to 240 ns) and beat frequency (from 0 to 2.65 GHz) to be obtained.

Atomic resonance behavior in laser-induced Rb atom desorption—the effect of ambient gas atoms and molecules

Abstract We report an observation that in a typical optical pumping cell containing Rb metal and 150 Torr N 2 gas, a cw laser beam of a few tens of mW and beam size 5.5 mm 2 can desorb Rb films on cell surfaces when the laser is tuned to the Rb D 1 line. The frequency dependence of the Rb desorption rate displays atomic resonance behavior. The desorption is suggested to be mediated by the ambient gas atoms (Rb) and molecules (N 2 ). The phenomenon was used to provide evidence for the existence of thin Rb films on seemingly clear cell surfaces.

Entangled two-photon wave packet in a dispersive medium.

We report an experimental study of group-velocity dispersion effect on an entangled two-photon wave packet, generated via spontaneous parametric down-conversion and propagating through a dispersive medium. Even in the case of using cw laser beam for pump, the biphoton wave packet and the second-order correlation function spread significantly. The study and understanding of this phenomenon is of great importance for quantum information applications, such as quantum communication and distant clock synchronization.

헤테로다인 방법을 이용한 펄스 증폭된 레이저빔의 주파수 Chirping연구

좁은 선폭을 갖는 연속 레이저 빔이 제이조화된 Nd:YAG레이저빔과 Bethune셀을 사용하여 좁은 선폭을 갖는 펄스 레이저빔이 생성되었다. 증폭된 펄스빔의 주파수 chirping정도는 헤테로다인 방법을 이용하여 측정되었으며, 헤테로다인 맥놀이신호로부터 순간적인 위상 변화량을 추출함으로서 분석되었다. 10 ns 펄스에 의해 MHz 이하의 선폭을 가지는 연속 레이저빔으로부터 증폭된 주파수 chirping은 80MHz정도로 매우 좁은 선폭의 증폭된 펄스 레이저빔이 얻어졌다. Amplified pulsed laser beam with narrow linewidth was generated from CW laser beam with narrow linewidth by using frequency doubled Nd:YAG laser beam and Bethune cell. The degree of the frequency chirping of the amplified pulse laser was measured by using the heterodyne method and obtained by calculating instantaneous phase change from heterodyne beating signals. The frequency chirping of amplified pulsed laser beam from CW laser beam with sub-MHz linewidth by 10 ns pulse was 80 MHz so that pulsed laser beam with very narrow linewidth was obtained.

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.

Dynamic Holographic Endoscopy: New Perspectives in Minimally Invasive Diagnostics

An endoscopic holographic camera system based on a dynamic holographic technique, i.e. Electronic-Speckle-Pattern Interferometry (ESPI), is presented which can be applied to in-vitro and in-vivo minimally invasive medical diagnostics as well as to examinations of technical objects. Integration of optical fibers for the guidance of a cw-laser beam and an endoscopic imaging system yield a compact ESPI system which opens up new possibilities for highly sensitive interferometric intra-cavity inspection under hand-held conditions with dynamic on-line (video-frame-rate) information presentation of interferograms. A CCD-camera in combination with a fast frame-grabber system allows dynamic image subtractions up to a frequency of 25 Hz with high fringe contrast. Results from investigations of biological specimen in-vitro and in-vivo in the fields of gastroenterology and coloscopy are obtained, demonstrating the capability for structural differentiation by analysis of tissue elasticity even underneath the visible surface as well as for compensation of the loss of tactile sense impression in endoscopic surgery by dynamic optical information (“endoscopic taction”).

Studies in light field of the mesomorphic state of some biological membrane models

The paper presents studies under light beams of some systems which model the biological membrane (BM), namely fatty acids (FAs) and mixtures of fatty acids with cholesterol (Ch), and surfactant systems. Under a weak external electric field, the systems behave as weak dielectrics, with a hysteretic evolution of the current versus the applied voltage, and a negative resistance, which suggest a liquid crystal state. The microscopic aspect of both the systems in water showed a self-assembly of the molecules as a double layer, similar with the membranes structure. The hydrophobic part presents a lyotropic liquid crystal texture that changes in terms of the concentration. Some of the samples exhibit also a thermotropic mesomorphism, that is much influenced by adding cholesterol. Maintained under integral and polarized light, the textures and their colors change in a few seconds. In a low power cw laser beam, the light spreading has been evidenced and some non-linear optical effects were observed. The results seem to lead to the necessity to study both the lyotropic and thermotropic mesomorphism together and in interaction with different external stimuli, among them the light being an important tool for biological investigations.

Self-focusing on bounded domains

The critical nonlinear Schrödinger equation (NLS) on bounded domains models the propagation of cw laser beams in hollow-core fibers. Unlike the NLS on unbounded domains which models propagation in bulk media, the ground-state waveguide solutions are stable and the condition of critical power for singularity formation is generically sharp.

Thermal strain distributions and change of refractive indices in 4mm crystals illuminated with Gaussian cw laser beams

The strain variations and change of refractive indices both induced by thermal effect are calculated in parallelepipedic crystals owing to the 4mm point group. The laser beam illuminating the crystal is responsible of the heating and is supposed to be a TE 00 mode. The stress are calculated on the faces of the crystal and in the median planes. The expressions for the changes of refractive indices are also given.

Phase-coherent light scattering spectroscopy. II. Depolarized dynamic light scattering

In our preceding paper, we demonstrate the basic principle of the phase-coherent light scattering (COLS) method, focusing on polarized light scattering. In this paper we describe the principle of depolarized COLS. We explain how to excite coherent orientational motion of molecules in liquids and transverse sound waves in amorphous solids by using the electric-field grating, which is generated by crossing two cw laser beams in the media. We also explain how to observe complex spectra of depolarized light scattering from these anisotropic modes. The advantage of COLS over the conventional method is also discussed.

Measurement of an electron beam size with a laser wire beam profile monitor

We describe the first measurement of an electron beam size in the accelerator test facility damping ring at KEK with a laser wire beam profile monitor. This monitor is based upon the Compton scattering process of electrons with a laser light target, which is produced by injecting a cw laser beam into a Fabry-P\'erot optical cavity. We have observed clear signals of the Compton scattered photons and confirmed that the observed energy spectrum as well as the count rate agree with the expected ones. From the measurement, we have deduced the vertical beam size ${\ensuremath{\sigma}}_{b}$ to be $9.8\ifmmode\pm\else\textpm\fi{}1.1\ifmmode\pm\else\textpm\fi{}0.4\ensuremath{\mu}\mathrm{m}$, where the first (second) error represents statistical (systematic) uncertainty. Various improvements are in progress to enhance the signal-to-noise ratio, which is essential for the detailed study of the beam dynamics.

Two-dimensional sideband Raman cooling and Zeeman-state preparation in an optical lattice

A method of sideband Raman cooling to the vibrational ground state of the $m=0$ Zeeman sublevel in a far-detuned two-dimensional optical lattice is proposed. In our scheme, the Raman coupling between vibrational manifolds of the adjacent Zeeman sublevels is shifted to the red sideband due to the ac Stark effect induced by a weak pump field. Thus cooling and optical pumping to $m=0$ are achieved by purely optical means with coplanar cw laser beams. The optical lattice and cooling parameters are estimated in the framework of simple theoretical models. An application of the transverse sideband cooling method to frequency standards is discussed. Coherent population trapping for the sideband Raman transitions between the degenerate vibrational levels is predicted.

Enhanced photoinduced birefringence in polymer microcavities

We investigate photoinduced birefringence in functionalized polymer microcavities illuminated by a visible polarized cw laser beam. The cavity studied in this experiment consists of an assembly of two Bragg mirrors embedding a polymer film with tethered azobenzene dyes. Linearly polarized light induces orientationally selective excitation of the chromophore guests, resulting in optical anisotropy of the film. The mechanism of the phenomenon is based on random-to-axial statistical reorientation of the azochromophores following repetitive trans-cis-trans photoisomerization cycles. The resulting birefringence is inferred from analysis of the spectral shifts of the cavity Fabry–Pérot longitudinal modes as a result of illumination. A large value of the birefringence of the order of 10−2 is noted after currently unoptimized durations of the order of 30′ in the 1.3–1.5 μm range with possible applications in optical storage at WDM telecom-compatible wavelengths.

High resolution cw laser beam preamplified in a multipass cavity pumped by a CuHBr (HyBrID) laser

The amplification of a cw high resolution (1 MHz) single-mode dye laser in a single-stage, multipass preamplifier based on a free flowing dye jet as amplifying medium is reported. The device is pumped by a homemade CuHBr laser (HyBrID laser) (18 kHz repetition rate, 30 ns pulse duration at half-maximum and 80 ns at its base) and yields pulses of duration of 10 ns (at half-maximum). In the studied pump energy domain, the gain is a linear function of pump energy and the maximum value obtained so far with pump energy of 400 μJ is 25 μJ per pulse for an input beam power of 30 mW. The corresponding gain is of the order of 10 4. We assume that a higher gain can be reached using better quality elements. The output beam is a high quality TM 00 mode. The pulse shapes of the pump beam and the output beam are compared and a phenomenological model (“switched photon lifetime”) is introduced that reproduces our observations with a good agreement.

Measurement of the thermal contribution to the nonlinear refractive index of air at 1064 nm

The thermal contribution to the nonlinear refractive index of air at 1.064mum was measured with a high-finesse Fabry-Perot cavity and a 500-mW cw laser beam. At room temperature and pressure, the nonlinear refractive-index coefficient of air was found to be n(2)((th))=(-1.9+/-0.2)x10 (-14) cm(2)/W for a beam waist radius of 0.23 mm and was found to be independent of the relative humidity. The thermal nonlinearities of N(2) , O(2) , and CO(2) were also measured, and it was found that the dominant contribution to air is its O(2) content.