Showing papers on "Pulse duration published in 1998"

Pulse wave diagnosing device

Abstract: When a pulse wave sensor unit (130) senses a pulse wave MH, a wavelet conversion unit (10) applies wavelet conversion to this pulse wave MH and generates pulse wave analysis data MKD. The MKD includes data corresponding to a time region defined by dividing one heartbeat into eight parts and data corresponding to a frequency region defined by dividing the frequency range of 0 to 4 Hz into eight parts. A frequency correction unit (11) applies frequency correction to the MKD and generates pulse wave correction data MKD'. A pulse wave image data generation unit (12) compares the MKD' in each frequency region and time region and generates pulse wave image data ZD representing a pulse wave image. A display unit (13) displays the pulse wave image of the pulse wave MH on the basis of the ZD.

Dispersion pre-compensation of 15 femtosecond optical pulses for high-numerical-aperture objectives

Abstract: The excitation efficiency in two-photon absorption (TPA) microscopy depends strongly — owing to the square dependence of the TPA fluorescence on the excitation intensity — on the temporal width of the excitation pulse. Because of their inherently large frequency bandwidth, ultrashort optical pulses tend to broaden substantially because of dispersion from propagation through the dispersive elements in the microscope. In this paper, the dispersion characteristics of a wide range of microscope objectives are investigated. It is shown that the induced dispersion can be pre-compensated in all cases for pulses as short as 15 fs. Because of the excellent agreement between the results from theoretical modelling and the experimental data, predictions of the possibility of dispersion control for microscope objectives in general, as well as for even shorter pulses, can be inferred. Since for TPA imaging the background due to single photon absorption processes and scattering is independent of the pulse width, proper dispersion pre-compensation — which minimizes the pulse duration at the focal point and hence maximizes the excitation efficiency — provides optimal image contrast in TPA microscopy.

Influence of pulse duration on mechanical effects after laser-induced breakdown in water

Abstract: The influence of the pulse duration on the mechanical effects following laser-induced breakdown in water was studied at pulse durations between 100 fs and 100 ns. Breakdown was generated by focusing laser pulses into a cuvette containing distilled water. The pulse energy corresponded to 6-times breakdown threshold energy. Plasma formation and shock wave emission were studied photographically. The plasma photographs show a strong influence of self-focusing on the plasma geometry for femtosecond pulses. Streak photographic recording of the shock propagation in the immediate vicinity of the breakdown region allowed the measurement of the near-field shock pressure. At the plasma rim, shock pressures between 3 and 9 GPa were observed for most pulse durations. The shock pressure rapidly decays proportionally to r−(2⋯3) with increasing distance r from the optical axis. At a 6 mm distance of the shock pressure has dropped to (8.5±0.6) MPa for 76 ns and to <0.1 MPa for femtosecond pulses. The radius of the cavitation bubble is reduced from 2.5 mm (76 ns pulses) to less than 50 μm for femtosecond pulses. Mechanical effects such as shock wave emission and cavitation bubble expansion are greatly reduced for shorter laser pulses, because the energy required to produce breakdown decreases with decreasing pulse duration, and because a larger fraction of energy is required to overcome the heat of vaporization with femtosecond pulses.

Method of, and apparatus for, surgery of the cornea

Abstract: A laser-based method and apparatus for corneal surgery. The present invention is intended to be applied primarily to ablate organic materials, and human cornea in particular. The invention uses a laser source which has the characteristics of providing a shallow ablation depth (0.2 microns or less per laser pulse), and a low ablation energy density threshold (less than or equal to about 10 mJ/cm 2 ), to achieve optically smooth ablated corneal surfaces. The preferred laser includes a laser emitting approximately 100-50,000 laser pulses per second, with a wave-length of about 198-300 nm and a pulse duration of about 1-5,000 picoseconds. Each laser pulse is directed by a highly controllable laser scanning system. Described is a method of distributing laser pulses and the energy deposited on a target surface such that surface roughness is controlled within a specific range. Included is a laser beam intensity monitor and a beam intensity adjustment means, such that constant energy level is maintained throughout an operation. Eye movement during an operation is corrected for by a corresponding compensation in the location of the surgical beam. Beam operation is terminated if the laser parameters or the eye positioning is outside of a predetermined tolerable range. The surgical system can be used to perform surgical procedures including removal of corneal scar, making incisions, cornea transplants, and to correct myopia, hyperopia, astigmatism, and other corneal surface profile defects.

Relativistic X-band BWO with 3-GW output power

Abstract: Results from a study of a relativistic X-band backward-wave oscillator (RBWO) with 3-GW output microwave power are presented. The RBWO was driven by the high-current electron accelerator SINUS-7. The dependence of radiated microwave pulse duration on microwave power was obtained. Pulse shortening occurring at the higher power levels is probably attributable to explosive electron emission from the slow wave structure (SWS). An increase in the cross section of the electrodynamic structure seems to be a way to increase the microwave pulse duration. Experimental results from a moderately oversized X-band RBWO using a resonance reflector are described. This tube can be operated with a low external magnetic field.

100-TW sub-20-fs Ti:sapphire laser system operating at a 10-Hz repetition rate.

Abstract: We developed a compact three-stage Ti:sapphire amplifier laser system that produced peak power in excess of 100 TW for a pulse duration of less than 19 fs and an average power of 19 W at a 10-Hz repetition rate. A final 40-mm-diameter Ti:sapphire amplifier is pumped by a Nd:YAG master-oscillator–power-amplifier system that produces ?7-J output of 532-nm radiation. The spatial beam quality is approximately 2 times diffraction limited for the full amplified compressed output pulse. With f/3 optics, this system should therefore be capable of producing a focused intensity of ?3×1020 W/cm2.

Ultraviolet laser ablation of polymers: spot size, pulse duration and plume attenuation effects explained.

Abstract: A versatile model for ultraviolet (UV) laser ablation of polymers is presented, which is very successfully applied to the calculation of a variety of different properties of this process, including the influence of plume attenuation dynamics. The polymer is described as a system of chromophores with two possible electronic states. The model is based on the combination of photothermal decomposition and photodissociative bond breaking in the electronically excited state. Laser induced chemical modifications are incorporated via different absorption coefficients for the initial and for the modified polymer after absorption of UV light. Dynamic attenuation of the expanding ablation plume and heat conduction are taken into account. The results of the theoretical calculations are compared with the results of three different series of experiments performed with polyimide (PI) and polymethylmethacrylate at the excimer laser wavelength 248 nm and with PI also at 308 nm: (1) Measurement of the ablation rate as a function of fluence for four different pulse durations between 20 and 250 ns; (2) Measurements of the ablation rate as a function of fluence for five different laser irradiation spot radii between 10 and 150 μm, and (3) Time resolved measurement of the dynamic plume attenuation at the ablating laser wavelength as a function of fluence for four different pulse durations between 20 and 250 ns. The model leads to a prediction of etch rates, ablation thresholds, plume attenuation, and surface temperatures during the ablation process, which is in good agreement with the experimental results. The observed increase of the ablation rate with increasing pulse length and with decreasing laser spot size can be explained by the model as a consequence of laser induced modified absorption in combination with the dynamic shielding of the expanding plume.

Pulse energy control for excimer laser

Abstract: A process for controlling pulse energy in burst of pulses produced by an excimer laser. The energy of each pulse in each burst is measured. The rate of change of pulse energy with charging voltage is determined. A pulse energy error is determined for a previous pulse of the present burst. An integrated dose error is also determined for all previous pulses in the current burst. A charging voltage for the next pulse is determined using the pulse energy error, the integrated dose error, the rate of change of energy with charging voltage and a reference voltage. In a preferred embodiment, the rate of change of energy with voltage is determined by dithering the voltage during two pulses of each burst, once lower and once higher. The reference voltage is a voltage calculated using prior voltage and energy data. In this embodiment the method of determining the reference voltage during a first portion of the pulse is different from the method used during a latter portion of the burst. During the first set of pulses, for each pulse, (40 in this embodiment) a specified voltage calculated using voltage and energy data from a corresponding pulse in a previous burst is utilized as a prediction of the voltage needed to produce a pulse energy converging on a target pulse energy. For pulses (41) and thereafter the reference voltage for each pulse is the specified voltage for the previous pulse.

Cylindrical microlasers and light emitting devices from conducting polymers

Abstract: Substantially improved, photopumped polymer lasers are demonstrated using microrings and microdisks of various diameters D ranging from 5 to 200 μm. Various cavity-dependent laser modes were observed, which for D<10 μm were dominated by a single longitudinal mode with linewidth of less than 1 A. These microlasers were also characterized by Q of order 5000, low threshold excitation energy of order 100 pJ/pulse for pulse duration ranging from 100 ps to sub-μs, and an abrupt increase in the emission directionality and polarization degree. Light emitting diodes with cylindrical geometry, fully compatible with these microlasers are also demonstrated.

Femtosecond infrared pulses tunable from 9 to 18 mum at an 88-MHz repetition rate.

Abstract: Femtosecond mid-infrared laser pulses that are continuously tunable in the wavelength range from 9 to 18 µm are demonstrated. These nearly bandwidth-limited pulses are generated by phase-matched difference-frequency mixing within the broad spectrum of 20-fs pulses from a mode-locked Ti:sapphire laser in GaSe. A direct determination of the pulse duration at 11.5 µm gives a value of 140??fs. The average mid-infrared power of 1 µW is ?100 times greater than that for infrared generation by non-phase-matched optical rectification.

Transient Cavitation and Acoustic Emission Produced by Different Laser Lithotripters

Abstract: Transient cavitation and shockwave generation produced by pulsed-dye and holmium:YAG laser lithotripters were studied using high-speed photography and acoustic emission measurements. In addition, stone phantoms were used to compare the fragmentation efficiency of various laser and electrohydraulic lithotripters. The pulsed-dye laser, with a wavelength (504 nm) strongly absorbed by most stone materials but not by water, and a short pulse duration of ∼1 μsec, induces plasma formation on the surface of the target calculi. Subsequently, the rapid expansion of the plasma forms a cavitation bubble, which expands spherically to a maximum size and then collapses violently, leading to strong shockwave generation and microjet impingement, which comprises the primary mechanism for stone fragmentation with short-pulse lasers. In contrast, the holmium laser, with a wavelength (2100 nm) most strongly absorbed by water as well as by all stone materials and a long pulse duration of 250 to 350 μsec, produces an e...

Ultrashort pulse laser ablation of silicon: an MD simulation study

Abstract: Molecular Dynamics (MD) simulation has been employed in order to study laser ablation of silicon surfaces. The impact of laser pulses with lengths ranging from 5 ps down to 10 fs on a target of approximately 100A x 100A x 50A was investigated. The ablation shows a strong dependence on pulse length, on pulse energy and on the number of laser shots. With decreasing pulse length the amount of removed particles increases, and with decreasing pulse energy the holes become narrower. Especially in multishot ablations, holes with a diameter of just a fraction of the focus could be observed. This can be attributed mainly to ablation of atoms from lower areas and their redeposition close to the surface, leading to amorphous areas around the ablation hole. For pulses of picosecond duration, and even for femtosecond pulses, the main material removal occurs on a timescale of a few ps. Interestingly, the simulations show two thresholds: the onset of damage at the surface. which depends on the pulse energy but only insignificantly on the pulse length; and the onset of the removal of particles, which shows a strong dependence on the pulse length of the laser.

Analysis of terahertz pulse measurement with a chirped probe beam

Abstract: In this letter, terahertz (THz) pulse measurements with a chirped probe pulse are analyzed. The method of stationary phase is used to explore the relation between the temporal waveform of an input THz pulse and spectral distribution of an output probe pulse on a detector array. The dependence of the temporal resolution on the chirp rate, the spectrum of the chirped probe beam, and the spectral resolution of the spectrometer is discussed. We confirm that the temporal resolution of the chirped pulse technique is equal to the square root of the product of the original unchirped probe pulse duration and the chirped pulse duration.

Method of producing microstructural medical implants

Abstract: In a method of producing microstructural medical implants by laser material processing, it is provided to make use of a tunable laser beam of a pulse length in the order of magnitude of femtoseconds, and of a laser irradin that is variable in its frequency of pulse repetition, laser capacity and/or velocity of displacement for gentle material treatment without melting to take place.

Effects of self-steepening and self-frequency shifting on short-pulse splitting in dispersive nonlinear media

Abstract: We characterize the propagation of short optical pulses through dispersive media with a cubic self-focusing nonlinear polarization, which incorporates self-steepening and self-frequency shifting. These effects can significantly affect pulse propagation dynamics, both in the normal but especially in the anomalous dispersion regimes. The nature of the dynamics is significantly different in the two regimes.

Significant pulse-lengthening in a multigigawatt magnetically insulated transmission line oscillator

Abstract: The Air Force Research Laboratory/Phillips Laboratory magnetically insulated transmission line oscillator (MILO) is a gigawatt-class, L-band, high-power microwave tube driven by a 500-kV, 60-kA electron beam. A previous version of this tube generated 1.5 GW pulses, but with significant RF pulse shortening, The paper reports on improvements to the tube that have allowed us to increase the output power by 25% and to increase the RF pulse duration by a factor of two and a half.

Delta-sigma pulse width modulator

Abstract: A delta sigma pulse width modulator control circuit (103) uses a delta sigma modulator (201) as a first stage to create a sequence of pulses representing an input control signal. A pulse width modulator (205) weighs each pulse, accumulates the sequence of weighted pulses and defines a pulse width modulated output signal (603) from the accumulated pulses. The pulse width modulated signal (603) is given a randomly generated offset (605) to the time of pulse value transition and an adjacent pulse value matching technique is used to reduce harmonically related noise generation.

Preferential Vaporization during Laser Ablation Inductively Coupled Plasma Atomic Emission Spectroscopy

Abstract: The Zn-to-Cu ratio in brass was measured by laser ablation inductively coupled plasma atomic emission spectroscopy. The influence of laser beam properties (pulse width, wavelength, and power density) on fractional laser ablation was investigated. The behavior of the Zn/Cu ratio vs. laser power density shows that there are different mechanisms influencing ps and ns laser ablation. With the use of a 30 ns pulse duration from an excimer laser, thermal vaporization appears to be the dominant process in the low-power density region. The Zn/Cu ratio approaches stoichiometry at higher power density, but the ablated mass still remains Zn rich. With a 35 ps pulse Nd:YAG laser, a nonthermal mechanism appears to govern the laser ablation process. When a 3 ns Nd:YAG laser is used, both thermal and nonthermal processes exist. For both 3 ns and 30 ps Nd:YAG lasers, stoichiometric ablation can be achieved at higher power densities.

Ultrasound imaging method and apparatus for generating pulse width modulated waveforms with reduced harmonic response

Abstract: An improvement to the method for harmonic imaging including the steps of transmitting ultrasonic energy at a fundamental frequency, and receiving reflected ultrasonic energy at a harmonic of the fundamental frequency is provided. The transmitting step includes the step of transmitting a waveform comprising at least a sequence of at least a first and second pulse characterized by first and second pulse durations, respectively, where the second pulse duration is different than the first duration. This arrangement can reduce harmonic energy in the waveform. The system (10) includes a transmit beam former (12) that supplies high voltage transmit waveform in a plurality of channels via a TX/RX switch (14) to a transducer array (16).

Adaptive control of pulse phase in a chirped-pulse amplifier.

Abstract: Using experimental feedback, we demonstrate that a chirped-pulse amplifier can adaptively learn to compensate for the higher-order phase dispersion that is inherent in the amplification process. A genetic algorithm-based search routine is used to repetitively update the pulse phase in a programmable pulse stretcher during a plasma breakdown experiment to maximize the magnitude of spectral blueshift. Reductions in pulse duration from 37 to 30 fs and substantially better wing structure are typically obtained as a result of the optimization.

Second-harmonic generation from regeneratively amplified femtosecond laser pulses in BBO and LBO crystals

Abstract: The spectral and temporal characteristics and optical-conversion efficiency of ∼150-fs laser pulses at 400 nm generated by second-harmonic generation (SHG) of a regeneratively amplified mode-locked Ti:sapphire laser were investigated both theoretically and experimentally. The theoretical investigation was done by taking into account cubic nonlinearity, pulse walk-off, group-velocity dispersion, Kerr nonlinearity, quadratic broadening, frequency chirping of the fundamental pulse, and higher-order nonlinear mixing such as backconversion and optical parametric processing. The experimental studies of the effects of crystal length and pumping intensity on the pulse duration, the spectrum, and the optical-conversion efficiency of the SHG were carried out in BBO and LBO crystals of various thicknesses and compared with the theory. It was found that in a non-transform-limited pulse, the most significant contribution to the temporal and spectral distortion of the ∼150-fs SHG pulses is mainly due to the chirping of the fundamental beam and self-phase modulation at high pumping intensity and long crystal length. The optimum crystal length and pumping intensity for obtaining a high optical-conversion efficiency and a pure spectrum in SHG are also calculated and experimentally investigated. It was found that a transform-limited fundamental pulse is essential to obtain a high conversion efficiency and to preserve the temporal profile of the second-harmonic pulse. It is also found that for a non-transform-limited ∼150-fs pulse, a 0.5–0.6-mm BBO crystal and a modest pumping intensity of ∼40 GW/cm2 are the most suitable for SHG.

Reliable stimulated brillouin scattering compression of nd:yag laser pulses with liquid fluorocarbon for long-time operation at 10 hz

Abstract: Stokes pulses of high energy and high-average power were obtained by stimulated Brillouin scattering (SBS) compression of long Nd:YAG laser pulses. The SBS medium used in the single-cell compressor was liquid ultrafiltered Fluorinert FC-75 fluorocarbon. An output pulse duration of 0.9 ns and a peak-power enhancement by 1 order of magnitude were observed for 10-ns, 0.57-J input pulses at a 10-Hz repetition rate. The compressor internal SBS efficiency reached a value of ηSBS = 94% and the overall device efficiency a value of ηdev = 87%; both values are the highest reported so far to the best of our knowledge. The simple single-cell SBS geometry provided excellent energy and pointing stability of the Stokes pulse. Its temporal shape turned out to be somewhat less stable. The SBS process also partially improved the laser beam quality. The Stokes pulses proved to be capable of generating radiation in the extreme-ultraviolet and soft-x-ray regions over a period of two months without any significant output deterioration.

Solar blind chemically vapor deposited diamond detectors for vacuum ultraviolet pulsed light-source characterization

Abstract: A major difficulty in characterizing vacuum ultraviolet (VUV) radiation produced by harmonic generation or four-wave sum frequency mixing arises in differentiating between the desired VUV signal and the remaining fundamental pump laser beam. To overcome this problem, visible and near UV blind VUV detectors, made from natural and synthetic diamond, have been developed. Such detectors have been used to characterize coherent VUV pulses (λ=125 nm, pulse duration at full width half maximum (FWHM) τFWHM∼7 ns) generated by resonance-enhanced four-wave sum mixing in mercury vapor. They allow full characterization of the intensity profile of the VUV pulses, without any significant parasitic signal from simultaneous stray light irradiation at λ=313 nm. Detectors were fabricated exhibiting response times of less than 70 ps at FWHM, corresponding to the lowest response time obtainable with a 7 GHz bandwidth single-shot oscilloscope.

Subfemtosecond pulse generation by molecular modulation

Abstract: This Letter suggests and analyzes a technique for producing subfemtosecond pulses of radiation. We will show that two laser beams whose frequency difference is slightly offset from a molecular transition will, for an appropriate choice of gas pressure and cell length, generate a spectrum of Raman sidebands whose Fourier transform is a periodic train of subfemtosecond pulses. The essence of the technique is the concurrent generation of a frequency modulated (FM) waveform and the use of group velocity dispersion to temporally compress this waveform. By numerically modeling this process in molecular deuterium sD2d, we calculate a generated train of pulses with a pulse spacing of 11.1 fs and a pulse length of 0.21 fs. The coherence rab (Fig. 1) of the driven molecular transition is central to this work. This coherence is established by driving the molecular transition slightly off resonance with driving lasers of sufficient intensity that the product of their Rabi frequencies exceeds the product of the detuning from the molecular electronic states and the detuning from the Raman transition. With the linewidth of the applied laser pulses small as compared to the Raman detuning, the magnitude of the molecular coherence approaches 0.5, and its sign is determined by the sign of the Raman detuning. For the conditions of the previous paragraph, the generation and phase-slip lengths are of the same order, and Raman sideband generation proceeds collinearly and very differently than in the conventional low coherence regime. In essence, the molecular motion now modulates the electronic refractive index much in the same way that a low frequency electric field modulates the refractive index of a polar crystal. Incident optical frequencies become frequency modulated with peak (sinusoidal) frequency deviations and spectral bandwidths that substantially exceed the width of the visible spectrum. This allows the extension of compression techniques, such as the use of group velocity dispersion as described here, to a new regime of short pulse compression. In pertinent prior work, (1) the authors have noted how an electromagnetically induced transparency (EIT)-like excitation may be used to generate a broad comb of coherent sidebands, but the Bessel-function nature of the spectrum and the possibility of pulse compression were not

HVAC fan-powered terminal unit having preset fan CFM

Abstract: An arrangement is disclosed for controlling fan motors (18) in fan-powered terminal units (10) which permit the air flow of the terminal units to be factory preset and reset using a voltmeter. The fan motor is responsive to the pulse width of pulses of a pulse width modulated signal to provide an air flow which is proportional to the pulse width. By adjusting the pulse width, the air flow may be set as desired. A pulse width modulated signal having pulses with a set amplitude and frequency is generated. The dc voltage of the pulse width modulated signal is measured using a voltmeter. As the pulses have a set amplitude and frequency, the pulse width of the pulses may be adjusted by controlling the dc voltage of the pulse width modulated signal. The fan motor is programmed to deliver the set air flow over a range of static pressures.

All-solid-state cw mode-locked picosecond KTiOAsO4 (KTA) optical parametric oscillator

Abstract: (KTA) optical parametric oscillator (OPO) synchronously pumped by a cw diode-pumped mode-locked Nd:YVO4 oscillator–amplifier system. The laser system (pumped by 84 W of cw 808-nm diode radiation) generates 7-ps-long pulses at 1.064 μm with a repetition rate of 83.4 MHz and an average power of 29 W. The OPO, synchronously pumped by the 1.064-μm laser pulses, consists of a 15-mm-long KTA crystal (cut for type II noncritical phase-matching) in a folded signal resonant linear resonator. The average powers of the 1.54-μm signal radiation and the 3.47-μm idler radiation are 14.6 W and 6.4 W, respectively. The total OPO output of 21 W corresponds to an internal efficiency of 75%. The experimental investigations include measurements of the OPO output power (and its dependence on the pump power, the transmission of the output coupler, and the resonator length) and of the pulse properties (such as pulse duration and spectral width). The measured results are in good agreement with the predictions of a numerical analysis based on a split-step Fourier method.