Input Laser Pulse Research Articles

Laser pulse-stretching with multiple optical ring cavities.

We describe a simple and passive nanosecond-long laser-pulse stretcher using multiple optical ring cavities. We present a model of the pulse-stretching process for an arbitrary number of optical ring cavities. This new model explicitly includes the effects of cavity delay time, beam-splitter reflectivity, total number of optical cavities, and describes the effects of spatial profile sensitivity. Using the model, we optimize the design of a pulse stretcher for use in a spontaneous Raman-scattering excitation system that avoids laser-induced plasma spark problems. From the optimized design, we then experimentally demonstrate and verify the model with a three-cavity pulse-stretcher system that converts a 1000-mJ, 8.4-ns-long input laser pulse into an approximately 75-ns-long (FWHM) output laser pulse with a peak power reduction of 0.10x and an 83% efficiency.

Optimal control of transient sum-frequency mixing processes

An optimal control technique is applied to design a pair of input laser pulses that control the temporal shape of an optical pulse generated in a process of transient sum-frequency mixing. Specific calculations performed for a model crystal in the case of fifth-harmonic generation are discussed as an example. The efficiency of the generation process is enhanced by the use of temporally tailored laser pulses.

Control of electron current by double-barrier structures using pulsed laser fields

Tunneling current through a double-barrier structure is considered in a strong pulsed ac electric field. By absorbing and emitting photons, an electron is able to penetrate through the barriers. An analytical expression for the current is found in the resonance approximation for electric fields with slowly varying amplitudes. The time-dependent evolution of the current exhibits positive and negative amplitudes, depending on the intensity of the field. With an increase of temperature, the current increases substantially. At particular field intensities, a sequence of input laser pulses can be mapped to no pulses, a single pulse, or two pulses in the output electric current. Thus, the decoding of information is essentially impossible.

Optical processing on a femtosecond time scale

An all optical full logic unit, operating at femtosecond switching times, is demonstrated, based on transient index gratings from three independent, but mutually coherent, subpicosecond input laser pulses. The output signal is made up by the third harmonic of the input beams, resulting in a unique contrast ratio.

Dynamics of photon-induced degradation and fluorescence in riboflavin microparticles

An unexpected blue-fluorescence band (around 420 nm) from both micrometer-sized dried particles and aqueous droplets of riboflavin [7,8-dimethyl-10-(D-1′-ribityl)-isoalloxazine] is observed when the microparticles are irradiated with a pulsed UV (355- or 351-nm) laser. The intensity of the band increases quadratically with input laser energy density (fluence) and is attributable to a one-photon-excited fluorescence of lumichrome (7,8-dimethyl-alloxazine) that is produced by photo-degradation of riboflavin. The well-known greenish-yellow fluorescence band (at 560 nm for dried particles and 535 nm for aqueous droplets) from riboflavin increases sublinearly with UV-laser fluence. With a laser input fluence above 5 J/cm2 the riboflavin fluorescence decays earlier and the lumichrome fluorescence reaches a maximum later than the peak of the input laser pulse. The temporal dynamics of the 420- and 535-nm fluorescence peaks are consistent with a rate-equation simulation of photon-induced conversion of riboflavin to lumichrome and the subsequent fluorescence of lumichrome.

LASER HEATING OF SILICON: A KINETIC THEORY APPROACH

The present study considers laser heating of silicon. A three-dimensional form of electron kinetic theory approach is considered when deriving the governing heat transfer equation. The numerical method employing a finite difference scheme is used to discretize the resulting equations. The step input laser pulse with Gaussian spatial distribution is introduced. The present predictions are compared with the results of the Fourier heating model. It is found that the temperature rise in the surface vicinity depends on the electron lattice site atom collisions. As heating progresses, the internal energy gain dominates the conduction losses in this region. The electron kinetic theory predictions and the Fourier theory results become similar as the laser pulse length increases.

Phase measurements in nonlinear optics

Measurement of the phase, and not only the intensity, of the nonlinear optical response of a system is important in many applications which are reviewed here. We demonstrate and compare several techniques for direct measurement of the phase of the nonlinear susceptibilities through nonlinear interferometry. Different ways of imparting a controllable phase shift are discussed. Unless the coherence length is impractically large, the preferred method is to use a variable-pressure gas cell to control the phase difference between the input laser pulse and the nonlinear optical signal.

Intensity-dependent polarization twisting within the temporal profiles of optical pulses in a nonbirefringent single-mode optical fiber

An intensity-dependent polarization-twisting phenomenon has been observed within the temporal profiles of linearly polarized picosecond laser pulses propagating through a nonbirefringent single-mode optical fiber. The substructures of output pulse temporal profiles for the two perpendicular linearly polarized components from linearly polarized input laser pulses have been measured. These strong intensity-dependent temporal structures have been attributed to the twisting action of the linear polarization state as a function of pulse local time.

Nonlinear optical effects in Raman calibrations of a Thomson scattering system

We present an investigation of the occurrence of stimulated Raman scattering and other nonlinear optical effects during Raman calibrations of Thomson scattering diagnostic systems for magnetic fusion plasmas. When these effects take place, the intensity of the Raman lines is unpredictable, and the calibrations are impossible. In this research Raman scattering from H(2) and D(2) at filling pressures up to 1 atm has been experimentally investigated using the Thomson scattering system of the ETA-BETA II reversed field pinch device. Stimulated Raman Stokes light has been observed at filling pressures above 230 and 500 mbars for H(2) and D(2), respectively, for input laser pulses of 8 J and 30 ns (FWHM) duration. Evidence has been found that the stimulated Raman light does not originate from the observed scattering volume but is detected as light diffused into the vacuum chamber. To explain these results, the Raman gain and the intensity of the stimulated Raman light are calculated, taking into account the multimode structure of the laser beam. We find that significant power conversion from the input laser beam to the Stokes wave takes place near the output window of the vacuum chamber. Part of this radiation is diffused back into the machine, and this part is detected as superimposed on the spontaneous Raman signal. Finally we discuss the Raman calibrations in RFX, a larger plasma device in which the Raman medium will be N(2) at a temperature up to 350°C, and show that a filling pressure of 100 mbar gives a sufficient calibration signal, avoiding any nonlinear effect.

Power extraction study of an e-beam-pumped atmospheric pressure, Kr-rich KrF laser amplifier

Single-pass (50-cm) amplification characteristics of a short-pulse [65 ns full width half maximum (FWHM)] e-beam-pumped KrF laser amplifier with an atmospheric pressure Kr-rich and Kr/F2 mixture were investigated theoretically for wide excitation rate from 0.1 to 2.0 MW/cm3. We have constructed a one-dimensional propagation amplifier model which considers the gain depletion and the saturated absorption spatially and temporally in the amplifying medium. With this numerical model, good agreement was obtained between the calculated result and the experiment. As a result of this theoretical study, extraction efficiencies of over 40% and power efficiencies of over 10% are obtainable at the highest output powers for all atmospheric mixtures containing 10%–99% Kr gas for excitation rates ranging from 0.5 to 2.0 MW/cm3 using a 30-ns (FWHM) input laser pulse, indicating that high efficiency power extraction from an atmospheric pressure KrF laser amplifier is achievable.

Time dependence of multiorder stimulated Raman scattering from single droplets

An optical multichannel detection technique was used to measure simultaneously the time profiles of the input laser pulse and the elastic scattering, as well as the time profiles of the spectrally resolved multiorder stimulated Raman scattering (SRS), from single droplets. The time delay between the multiorder SRS and the input laser pulse is consistent with the generalized four-wave mixing process for first-order stimulated Raman growth, starting from spontaneous noise or the parametric signal. The presence of an internal plasma associated with laser-induced breakdown within a droplet quenches the SRS and increases the elastic scattering.

STUDY ON TRANSIENT OPTICAL BISTABILITY OF QUASI-RESONANT-ABSORPTIVE DYE MEDIUM

Using intense pulsed laser beam of 5320 ? to excite the BDN dye solution inside the F-P interferometer, we have found the transient and intrinsic optical bistability effect. The various possible mechanisms to induce the refractive-index change of the sample medium and their relative contributions have been estimated; finally it is confirmed that in the duration of input laser pulse the refractive-index variation increases with time approximately in the form of semi-cubic parabola, and the main mechanism to cause this variation is the opto-thermal effect of the sample medium.

A self-setting attenuator for laser pulse energy stabilization

An active optical filter has been developed which when applied to single shot pulsed lasers dramatically improves the pulse to pulse energy reproducibility, essentially eliminating shot to shot fluctuations and day to day drift. The device acts as an intelligent (i.e. self setting) electro optic attenuator, which samples the input laser pulse and applies an appropriate transmission correction to that same laser pulse to give a quasi-constant output energy. The basic insertion loss can be as low as 50%. We present both the design and experimental demonstration of the device.