Laser Axis Research Articles

Fabrication of Laser Beam Axis Compensator by Photonic Crystal Technique for Optical Pickup Units

A laser beam axis compensator was fabricated for an optical pickup unit with blue and red lasers. The device consisted of multiwavelength wave plates and a double-refractive-index material. The wave plates were realized by a photonic crystal technique. By measuring the beam profiles of the blue and red lasers, the device was confirmed to function as a laser axis compensator. Under 110 µm laser beam axis shift setting conditions, readout signal patterns from an optical disc were measured.

Control of the pulse width in a diode-pumped passively Q-switched Nd:GdVO4 laser with GaAs saturable absorber

Different techniques to control the pulse width of a diode-pumped passively Q-switched Nd:GdVO 4 laser with GaAs saturable absorber have been studied. It is shown that varying the positions of the saturable absorber in the laser axis and the pump beam waist in the gain medium provides an efficient means to control the pulse width. A rate equation model is introduced to theoretically analyze the results obtained in the experiment, in which the Gaussian spatial distribution of the intracavity photon density, the longitudinal variation of the photon density and the pump beam spatial distribution are taken into account. The numerical calculations of the rate equations are consistent with the experimental results.

On the stability of the output characteristics of a grazing incidence grating dye laser transversely pumped by a copper vapor laser

The output characteristics of a high repetition rate pulsed dye laser has both short-term fluctuations and long-term drift. In high power high repetition rate lasers flow induced variations dominate over those due to other factors. In this paper it is shown by dye laser measurements that bandwidth fluctuations can be traced to the effective changes of the resonator dispersion due to fluctuations in the penetration depth of the pump beam in the dye medium. Short-term wavelength fluctuations can be traced to instantaneous deflection of the dye laser axis by the refractive index changes due to absorption of the pump beam. The fluctuations in both the bandwidth and the wavelength decreases with increasing flow rates within a laminar region. A copper vapor laser operating at 5.6 kHz repetition rate pumped the Rhodamine 6G dye laser used. The wavelength fluctuation of ±0.0035, 0.0030, 0.0004 nm and the bandwidth fluctuation of ±710, 132, 45 MHz over approx. 60 minutes were observed at 1.2, 3.7, 5.5 lpm flow rates respectively.

Relativistic reversal of the ponderomotive force in a standing laser wave

Effect of relativistic reversal of the ponderomotive force (PF), reported earlier for a collinear configuration of electron and laser standing wave [A. E. Kaplan and A. L. Pokrovsky, Phys. Rev. Lett., 95, 053601 (2005)], is studied here theoretically for various types of polarizations of the laser beam. We demonstrated that the collinear configuration, in which the laser wave is linearly polarized with electric field $\stackrel{P\vec}{E}$ parallel to the initial electron momentum ${\stackrel{P\vec}{p}}_{0}$, is the optimal configuration for the relativistic reversal. In that case, the transverse PF reverses its direction when the incident momentum is ${p}_{0}=mc$. The reversal effect vanishes in the cases of circular and linear with $\stackrel{P\vec}{E}\ensuremath{\perp}{\stackrel{P\vec}{p}}_{0}$ polarizations. We have discovered, however, that the counter-rotating circularly polarized standing waves develop attraction and repulsion areas along the axis of laser, in the laser field whose intensity is homogeneous in that axis, i.e., has no field gradient.

Laser Feedback Interferometers Based on Orthogonally Polarized He-Ne Lasers

A novel unattached laser feedback interferometer by combining the laser feedback effect with the frequency splitting technology is presented and demonstrated. Light source is the birefringence dual frequency He Ne laser which generates o-light and e-light with orthogonally polarized states. When the feedback mirror is moved along the laser axis, the intensity of both o-light and e-light is changed periodically. If a threshold intensity for the output light is appropriately given by the signal processing circuits, the remaining intensity curves in each period shows four different polarization states: e-light, e-light and o-light, o-light, and no-light. Each change of polarization state corresponds to λ/8 displacement of the feedback mirror. We determine the movement direction by the difference of the order of the polarization state appearance. The resolution of the system is 0.079µm. In the experiment, the reflectivity of feedback has no influence on waveform. The laser feedback interferometer is likely to become an instrument which has no real contact with a measured object.

Control of the pulse width in a diode-pumped passively Q-switched Nd:GdVO_4/KTP green laser with a Cr^4+:YAG saturable absorber

Different techniques to control the pulse width of a diode-pumped passively Q-switched Nd:GdVO4/KTP green laser have been theoretically and experimentally studied. It is shown that, when we vary the positidns of the saturable absorber in the laser axis and the pump beam waist in the gain medium, we obtain an efficient means to control the pulse width. The experiments performed on a diode-pumped Nd:GdVO4/KTP green laser passively Q-switched by a Cr4+:YAG saturable absorber are consistent with the theoretical calculations obtained from the rate equation model, in which the intracavity photon density that is assumed to be the Gaussian spatial distribution, the longitudinal variation of the intracavity photon density, and the pump beam spatial distribution are also considered.

Tunneling of a relativistic laser pulse through an overdense plasma slab

An ultra-fast super-radiant laser, with Gaussian radial intensity profile, when impinges on an overdense plasma slab, induces large oscillatory velocity on electrons raising their mass and lowering the plasma frequency. The ponderomotive force driven electron cavitation further lowers the plasma frequency. These effects are pronounced for the central portion of the beam and weaken as one moves away from the laser axis. Consequently, central portion of the beam tunnels through the plasma slab, while the marginal rays are blocked. The output beam thus has a smaller spot size. It also possesses a curvature in the wavefront that may help diagnostics.

Electron acceleration in combined intense laser fields and self-consistent quasistatic fields in plasma

The acceleration of plasma electron in intense laser-plasma interaction is investigated analytically and numerically, where the conjunct effect of laser fields and self-consistent spontaneous fields (including quasistatic electric field Esl, azimuthal quasistatic magnetic field Bsθ and the axial one Bsz) is completely considered for the first time. An analytical relativistic electron fluid model using test-particle method has been developed to give an explicit analysis about the effects of each quasistatic fields. The ponderomotive accelerating and scattering effects on electrons are partly offset by Esl, furthermore, Bsθ pinches and Bsz collimates electrons along the laser axis. The dependences of energy gain and scattering angle of electron on its initial radial position, plasma density, and laser intensity are, respectively, studied. The qualities of the relativistic electron beam (REB), such as energy spread, beam divergence, and emitting (scattering) angle, generated by both circularly polarized (CP) and linearly polarized (LP) lasers are studied. Results show CP laser is of clear advantage comparing to LP laser for it can generate a better REB in collimation and stabilization.

High-energy proton generation and suppression of transverse proton divergence by localized electrons in a laser-foil interaction

A suppression of a transverse divergence of high-energy protons generated by an interaction of a laser with a thin slab foil is investigated in this paper by 2.5-dimensional particle-in-cell simulations. When an intense (approximately 10(24) W/m(2)) short-pulse (a few ten femtoseconds) laser illuminates a thin foil target of a hydrogen, foil electrons are accelerated and compressed longitudinally by a laser light pressure and fast electron bunches are produced in the thin foil target. The fast electron bunches pass through the foil target, and a strong magnetic field is produced near the opposite side of the foil target. Because the strong magnetic field confines the electrons, a localization of the electrons is observed at the opposite side of a laser illumination surface. The local electron bunch produces not only a longitudinal electric field, but also a transverse electric field, which is directed toward the laser axis. Protons are accelerated and extracted from the foil, and the proton bunch divergence is successfully suppressed by the transverse electric field.

Control of the pulse width in a diode-pumped passively Q-switched Nd:YVO4/KTP green laser with GaAs saturable absorber

By varying the positions of the saturable absorber in the laser axis and the pump beam waist in the gain medium, respectively, we have theoretically and experimentally studied the control of the pulse width in a diode-pumped passively Q-switched Nd:YVO 4 /KTP green laser with GaAs saturable absorber. A rate equation model is introduced, in which the intracavity photon density is assumed to be Gaussian spatial distribution, the longitudinal variation of the intracavity photon density and the pump beam spatial distribution are also considered. The experimental results are consistent with the numerical calculations of the rate equations.

Tracking-FCS: Fluorescence correlation spectroscopy of individual particles

We exploit recent advances in single-particle tracking to perform fluorescence correlation spectroscopy on individual fluorescent particles, in contrast to traditional methods that build up statistics over a sequence of many measurements. By rapidly scanning the focus of an excitation laser in a circular pattern, demodulating the measured fluorescence, and feeding these results back to a piezoelectric translation stage, we track the Brownian motion of fluorescent polymer microspheres in aqueous solution in the plane transverse to the laser axis. We discuss the estimation of particle diffusion statistics from closed-loop position measurements, and we present a generalized theory of fluorescence correlation spectroscopy for the case that the motion of a single fluorescent particle is actively tracked by a time-dependent laser intensity. We model the motion of a tracked particle using Ornstein-Uhlenbeck statistics, using a general theory that contains a umber of existing results as specific cases. We find good agreement between our theory and experimental results, and discuss possible future applications of these techniques to passive, single-shot, single-molecule fluorescence measurements with many orders of magnitude in time resolution.

Design of a transversely pumped, high repetition rate, narrow bandwidth dye laser with high wavelength stability

The paper presents the design and performance of a transversely pumped, narrow bandwidth, high wavelength stability tunable dye laser that neither uses low expansion coefficient materials for construction nor incorporates any active control on the wavelength or the dye solution and environmental temperature as generally used in such lasers. The scheme essentially involves designing the mechanical assembly in such a way that, when bolted together it forms a massive monoblock, enclosing all the optical components and the dye laser axis within itself. This ensures the environmental temperature changes can only affect the output characteristics over long time scale. Short term (pulse to pulse) fluctuations in wavelengths and bandwidths, generally associated with the dye flow instabilities, were minimized by using a specially designed a dye cell made of a near 360°-curved rectangular duct, in which the turbulent flow is transformed itself into laminar flow as it reaches the dye laser axis. The laser was operated with Rhodamine 6G-ethanol-ethylene glycol solution, pumped by a copper vapor laser operating at 5.6 kHz. The dye laser output, consisting of three axial modes, separated by about 990 MHz, was stable over the observation period of about 90 min. Maximum long term (>1 h) fluctuation in Δν/ν was about ±3.6×10−6. The bandwidth of the individual mode varied between 245 MHz to 315 MHz.

Monochromatic imaging and angular distribution measurements of extreme ultraviolet light from laser-produced Sn and SnO2 plasmas

Properties of extreme ultraviolet (EUV) emission from laser-produced Sn and SnO2 plasmas were investigated. EUV emission images were taken with a monochromatic imager for 13.5nm with 4% bandwidth. It was found that the EUV emitting plasma is not formed symmetrically with respect to the target surface normal but extends toward laser incident axis. This result is consistent with the angular distribution of EUV emission peaked toward the direction nearly perpendicular to the laser axis. The asymmetric plasma can be attributed to the interaction of a long laser pulse with expanding plasma along the path of laser incidence.

The band of ND3 studied by laser-induced amplified spontaneous emission and transient gain spectroscopy

Abstract Two nonlinear laser spectroscopic methods have been employed for interrogating the C ′ 1 A ′ 1 → A 1 A 2 ′′ transition of ND 3 . The first method utilized laser-induced amplified spontaneous emission (LIASE) as the signal radiation. When single rovibronic levels in the C ′ 1 A ′ 1 ( 2 n , n=1–7 ) states of the ND 3 molecules were populated through two-photon excitation from the X 1 A 1 state by pulsed UV laser light, yellow LIASE was generated along the input laser axis. The q Q branches with J = K dominated both the C ′ ← X 2 0 0 excitation and C ′ → A 2 1 1 LIASE spectra. LIASE was almost perfectly polarized parallel to the polarization plane of the incident UV radiation. The second method, transient gain spectroscopy (TGS), in which the stimulated emission induced by the injection of weak DUMP radiation is detected, was also applied to the C ′ → A 2 1 1 band system. The amplification of DUMP laser intensities at the exit of the ND 3 sample cell was successfully observed when the DUMP frequencies were resonant with the specific rovibronic lines in the C ′ → A band. The maximum gain was as large as 75. TGS spectra exhibited narrower line widths than the natural line widths estimated from the lifetimes of lower levels.

High-energy electrons produced in subpicosecond laser-plasma interactions from subrelativistic laser intensities to relativistic intensities.

The characteristics of the forward hot electrons produced by subpicosecond laser-plasma interactions are studied for different laser polarizations at laser intensities from subrelativistic to relativistic. The peak of the hot electron beam produced by p-polarized laser beam shifts to the laser propagation direction from the target normal direction as the laser intensity reaches the relativistic. For s-polarized laser pulse, hot electrons are mainly directed to the laser axis direction. The temperature and the maximum energy of hot electrons are much higher than that expected by the empirical scaling law. The energy spectra of the hot electrons evolve to be a single-temperature structure at relativistic laser intensities from the two-temperature structure at subrelativistic intensities. For relativistic laser intensities, the forward hot electrons are less dependent on the laser polarization under the laser conditions. The existing of a preplasma formed by the laser amplified spontaneous emission pedestal plays an important role in the interaction. One-dimensional particle-in-cell simulations reproduce the most characteristics observed in the experiment.

Application of laser ignition on laminar flame front investigation

The first stages of laser-induced spark ignition were investigated as a function of time. Experiments were conducted using a premixed laminar CH4/air burner. Laser-induced breakdown was achieved by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. An anti-reflection coated lens with a focal length of 100 mm was used. The results obtained from an intensified high-speed and PIV CCD camera and a Cassegrain optics system coupled to an ICCD spectrometer provided information about the formation of laser-induced plasma and its transition to a flame kernel and a self-sustaining flame. The localization of the kernel and its time development were reproducible. Two types of flame fronts develop: one that expands against the flow direction, and one that moves with the flow. The initial flame expansion along the laser axis is asymmetric because of the shape of the plasma, different ionization levels inside the plasma, and the shock-wave expansion. Development of the fast flame occurs behind the shock wave induced by the plasma. This is important when laser ignition is used as a flame holder. An ICCD spectrometer coupled to an optical fiber permitted chemiluminescence visualization. The spectrum obtained during the plasma and flame kernel formation defined different stages in flame formation. The results obtained with these two optical techniques were synchronized to obtain the temporal resolution of the flame kernel evolution. Laser-induced ignition of a very lean mixture can be controlled to provide local heat release and extinction in a flame.

Tuning a grating-controlled CO2 laser by means of a scanning slit.

The use of diffraction gratings as tuning devices in laser resonators is well known and has been demonstrated abundantly for CO2 lasers. As the grating rotates, the lines of the infrared spectrum of the CO2 molecule reach threshold and oscillate one by one. Here it is shown that it is possible to obtain the same spectrum by moving an aperture inside the cavity perpendicularly to the laser axis while keeping the grating stationary. The conditions for obtaining a well-resolved spectrum are formulated for both the rotating grating and the moving aperture case.

Spatial distribution of high-energy electron emission from water plasmas produced by femtosecond laser pulses.

High energy electrons emitted by water plasmas produced by a single or a multiple laser pulse are investigated. The multipulse mode greatly enhances the generation and the temperature of hot electrons. Directional emission of high energy electrons over 25 keV is observed in two symmetric directions with respect to the laser axis and at 46 degrees from the directions of the laser electric field. Two-dimensional particle-in-cell simulations reproduce well the experimental results and indicate that the acceleration mechanism of the high energy electrons is due mainly to the resonance absorption at the edge of the spherical droplets formed by the leading pulse.

Piezoacoustic modulation of gain and distributed feedback for quantum cascade lasers with widely tunable emission wavelength

Due to the piezoelectric properties of III–V materials, an acoustic wave propagating along the optical axis of a unipolar laser produces a periodic modulation of the carrier density and the optical gain, sufficient for providing distributed feedback (DFB) with a mode suppression ratio exceeding 30 dB. In contrast to bipolar lasers, the piezoelectric modulation of unipolar carrier density is not accompanied by a degradation of the average gain. Inasmuch as the acoustic frequency can be easily changed, the wavelength of the main DFB mode can be tuned in a wide range. This property should be very attractive for spectroscopic applications of the quantum cascade laser.

Displacement sensors based on feedback effect of orthogonally polarized lights of frequency-split HeNe lasers

The novel principle of laser feedback displacement sensors is presented and demonstrated. A birefringent dual-frequency laser is used as the light source, which outputs two polarization lights, o light and e light. The two polarization lights are divided by a Wollaston prism and detected separately by two photoelectric detectors. When the feedback mirror is moved along the laser axis, the intensities of both the o light and the e light change periodically. And an increase of o light intensity always accompanies a decrease of e light intensity. If a threshold intensity for the output light is appropriately given by the signal processing circuits, the remaining intensity curves in each period shows four different polar- ization states: e light, e light and o light, o light, and no light. Each change of polarization state corresponds to a l/8 displacement of the feedback mirror. We judge the movement direction by the difference of the order of the polarization states that appear. The resolution of the system is 0.079 mm. The possibility of employing the fold feedback cavity is discussed, and its resolution is believed to reach 0.0395 mm. © 2003