Very short far-infrared pulses from optically pumped CH3OH/D, CH3F, and HCOOH lasers using an EQ-switched CO2laser as a pump source

Very short far-infrared (FIR) pulses were generated from optically pumped CH3F, CH3OH/D, and HCOOH lasers using aQ- switched, current pulsed (200 mA, 100 μs), low pressure (20 torr) CO2laser as a pump source. Values of 20 ns for the rise time and 50 ns for the decay time of the FIR pulses have been observed. The dependence of the FIR pulse shape parameters, i.e., rise time, decay time, and pulse buildup time, on the width of the pump pulse and the pressure of the molecular gas have been investigated experimentally. Due to the regular pulse shape, high repetition rate (350 Hz), high peak power (≳1 W), and broad spectral range (lambda = 100-500 mum), the pulses are very useful for purposes of solid-state and molecular time resolved spectroscopy.     

Application of a phase-conjugate Brillouin mirror to generation ofhigh-quality variable-duration KrF pulses

Phase conjugation by Brillouin scattering in SF₆ gas has been implemented on a two-pass KrF amplifier to provide near-diffraction-limited output pulses having a very low background level of amplified spontaneous emission. The pulse duration is variable between 1 and 10 ns by simply attenuating the input pulse. These pulses have also been used for the generation of much shorter duration pulses by truncated Brillouin scattering at the surface of cyclohexane     

A highly efficient two-stage Er3+-doped optical fiberamplifier employing an optical gate to effectively reduce ASE

Highly efficient amplification of ultrashort optical pulses is demonstrated with a two-stage Er³⁺-doped optical fiber amplifier that includes an optical gate to efficiently reduce amplified spontaneous emission (ASE) generated from the first Er³⁺-doped fiber. A gain of 49 dB, an amplified peak power 0f 105 W, and 1.05 nJ pulse energy are achieved for 2-Mb/s, 10-ps pulses at a total pumping power of 90 mW from 1.48-μm LDs     

Many-pass resonant laser amplifier

A very high gain amplifier has been developed by arranging for many amplifying passes to be made through one system. Input pulses of millijoule energy have been amplified to the damage limit of the materials (∼10 J/cm²) by this method. One such system can replace several when amplification of low-energy pulses is required.     

Improvements in aluminum-silicon Schottky barriers due to processing with a pulsed ruby laser

Improvements in Schottky barrier characteristics (increases in barrier heights and reductions in n-values or ideality factors) have been obtained for aluminum-n-type silicon contacts upon irradiation of single (1 msec) pulses from a ruby laser (λ = 0.694 µm). Increases in φbof up to 0.2 volts and decreases in n by a factor of ≃ 2 have been observed for optical pulse energies in the range of 25-40 J cm^-2.     

Generation and kilohertz rate amplification of synchronizedfemtosecond and picosecond laser pulses

Synchronized femtosecond and tunable picosecond laser pulses have been generated using a linear cavity configuration in both lasers. The long-term stability of the synchronization, as well as the frequency of the synchronously pumped femtosecond dye laser, is assured by active stabilization of the cavity length. Additionally, the picosecond and femtosecond pulses have been amplified in dual amplifier chains to pulse energies in the microjoule range at a 1 kHz repetition rate. The total peak intensity available for two-photon, nonlinear excitations is on the order of 10⁹ times greater than that available from unamplified beams     

Nonlinear photoemission from organic cathodes excited by a pulsed near-ultraviolet laser

High current density electron beams with fast modulation capability can be generated using laser-driven photoemission. Organic film cathodes having a real intermediate energy level can work in a poor vacuum environment and with near ultraviolet (∼ 350 nm) illumination where temporal formats from CW to picosecond pulse are available. Measured nonlinear photoemission characteristics of several organic films are presented and compared to the response of a rate equation model. Pulse distortions for 5 ns and 5 ps pulses also have been computed. Current densitities above 30 A/cm²for 5 ns, 337 nm pulses have been measured: an increase up to 3 kA/cm³for 5 ps pulses is predicted by the model.     

Picosecond CO2 laser-pulse generation and amplification

The generation of 30-ps 10-μm laser pulses from an optical-free-induction-decay (OFID) 10-μm CO₂ laser system and the first realization of a UV-preionized high-pressure CO₂-laser-pulse amplifier with Ernst-electrode profiles for these ultrashort CO₂ laser pulses are described. For a given transverse electric discharge width, the Ernst profile makes it possible to position the UV-radiation sources nearer to the discharge than with the Rogowski or Chang profile. A more homogeneous preionization of the discharge area is possible. Thus, the amplifier works up to 15 bar with a laser-gas mixture of 5:5:90 (CO₂:N₂:He). The authors amplified 30-ps OFID 10-μm CO₂-laser pulses up to ten times by passing through the amplifier twice. Thus, 10-μm OFID pulse energies of 1 mJ were achieved. With the amplifier used as an oscillator, a maximum output energy of 600 mJ was obtained in longitudinal-multimode operation for 50-ns pulses     

Pulsed laser damage thresholds in vitro for intraocular lenses and membranes

Energy and power density damage thresholds were determined in air, for plastic IOL's and membranes at the focal point of several solid-state laser systems: 1) 694 nm,Q-switched single pulse (30 ns), multimode, 2) 1064 nm,Q-switched single pulse (20 ns), TEM00, 3) 1060 nm, mode-locked single pulse, 15 ps, TEM11, 4) 530 nm, mode-locked single pulse, 15 ps, TEM11, and 5) 1064 nm, mode-locked pulse train (9-11 pulses, 30 ps), TEM00. Pulse energies bracketing damage thresholds as well as focal diameter and pulse duration for each system were determined. Energy density thresholds are lower, and power density thresholds higher, for shorter duration pulses-e.g., 23 J/cm²(1.15 GW/cm²) versus 6 J/cm²(400 GW/cm²) at the same wavelength as in systems 2) and 3) (p = 0.005). Damage thresholds for glass IOL's are 37 J/cm²(1.9 GW/cm²) and 37 J/cm²(1235 GW/cm²) as in systems 2) and 5). Damage threshold values for plastic membranes (Saran Wrap^®) exposed to nanosecond and picosecond pulse trains of Nd:YAG at 1064 nm are about half that of plastic IOL's. When laser pulses with a cone angle of 14° from systems 2) and 5) are focused on plastic membrane next to the IOL, damage thresholds are 30 J/cm²(1.5 GW/cm²) for 20 nsQ-switched pulses and 20 J/cm²(670 GW/cm²) for trains of 30 ps mode-locked pulses. Damage thresholds of IOL's immersed in 0.9 percent saline are approximately the same as those obtained in air.     

An EBS (electron bombarded semiconductor) high-current pulse amplifier

An EBS (electron bombarded semiconductor) pulse amplifier which generates high-current fast-risetime variable-width pulses into low impedance loads is described. Current pulses of 100 A into a 1-Ω load have been obtained with a risetime of 2.2 ns. A di/dt of 40 000 A/µs and a dV/dt of 71 000 V/µs have been obtained. Pulse lengths to 1 µs at 0.1-percent duty have been achieved. The risetime and peak current capabilities are presently limited by internal circuit parasitics. Without parasistics, the theoretical peak output capabilities for this EBS are 340 A with a di/dt of 6 × 10⁵A/µs.     

High power optically pumped far infrared laser systems

Single mode, 60-80 ns pulse width, 50-70 kW peak power laser oscillators operating on the 384.6 μm line of D2O have been developed. The characteristic linewidths of these oscillators are less than 25 MHz full width at half maximum which favorably compares with the intrinsic width of 6-8 MHz associated with the pulse length of about 60 ns. A 12.7 mJ, 195 kW, 384.6μm D2O laser oscillator-amplifier combination has been constructed and tested. Although single longitudinal mode operation is attained from this oscillator-amplifier system, amplified spontaneous emission (superradiance) from the amplifier adds low power level wide-bandwidth background radiation. Studies of far infrared lasing action in CH3F and CH3I are also described.     

Characteristics of organic dye lasers as tunable frequency sources for nanosecond absorption spectroscopy

The properties of 1, 1' diethyl-γ-cyano-2,2'-di-carbocyanine-tetrafluoroborate (DTCDCT)and 1, 1'-diethyl-γ-nitro- 4, 4'-di-carbocyanine-tetrafluoroborate (DTNDCT) lasers have been investigated. High-efficiency (25 percent) spectral narrowing of the normal 150-Å-wide, 2-MW output of DTCDCT (around λ 7600 Å) to <0.01 Å, corresponding to a single longitudinal mode, is achieved with a novel longitudinally pumped (by giant pulse ruby laser) dye laser cavity. Beam divergence is <0.5 mrad, and spectral tunability is obtained by rotating the echelle grating and Fabry-Perot etalon cavity elements. DTNDCT transversely pumped by a mode-locked ruby laser (>150 MW peak power) produces a train of 100 percent modulated mode-locked dye laser pulses whose risetimes (<0.5 ns) and pulse widths (0.6 ns) are detector limited. The production of broad continua for nanosecond absorption spectroscopy is also described, together with measurements on the synchronization of the pumping and dye laser pulses. Finally, the direct spectrographic detection of near-resonant optical-frequency Stark effects in potassium is briefly described. Employing the DTCDCT continuum as absorption source, a red shift of75 times 10^{-3}cm^-1of the λ 7699-Å resonance line was detected for a ruby laser perturbing field of power density ∼ 30 MW/cm^-2.     

High peak power and high repetition rate characteristics in acurrent-pulsed Q-switched CO2 laser with a mechanical shutter

An electro-mechanical Q-switched (EMQ) CO₂ laser is Q-switched by a mechanical beam chopper in combination with a pulsed discharge current. Such a system can produce pulses with high peak powers (>10 kW) and high repetition rates (>1 kpps). In order to analyze the output characteristics, the peak power and the duration of the output pulses have been measured experimentally in detail over a wide range of Q-switching times up to 250 ns. For a low-pressure (<4 kPa) CO₂ gas system, the standard rate equations adequately explain the experimental results by introducing a new switching function for the form of the cavity loss for the mechanical chopper. In an EMQ-laser with a high initial inversion density (4.5·10¹⁵ /cm³ at 150 mA peak current), multiple peak pulses or pulse distortion have been observed. This is due to the plasma screening effect induced by the burning of the metal shutter blades placed inside the cavity. It is found that tungsten metal shutter blades can be used up to a power density of 259 MW/cm² for a focused beam without this effect occurring, The solutions of the rate equations show that optimum coupling can prevent the plasma screening effect even for a Q-switching time longer than the pulse buildup time. The EMQ-laser configured for optimum coupling has produced a peak output power of 30 kW for the 9P20 transition branch in the CO₂ spectrum without any pulse distortion. This value has been obtained even though the discharge length was only 1.3 meters     

Gigabit optical pulse generation with FM mode-locked LiNdP4O12lasers

FM mode locking of an LiNdP4O12(LNP) laser with an intracavity modulator is reported. Mode-locked optical pulses with a pulse width of 49 ps at a repetition rate of 960 MHz, corresponding toc/2L(c:light velocity,L:cavity length), have been obtained. A pulse repetition rate as high as 2.88 GHz has been achieved with a multiple mode-locking technique.     

Pulse propagation stability in absorbing and amplifying media

Simple stability conditions are obtained for pulse propagation in a two-level system with linear scattering and combined homogeneous and inhomogeneous broadening. Properties of steady-state π andpisqrt{2}area pulses are reviewed. Computer simulation of an approach to equilibrium pulse propagation is presented in a few cases for initially perturbed pulses.     

High-power dye laser pulses by cascaded controlled transient oscillation

The output of a picosecond cascade-pumped controlled transient oscillation (CTO) dye laser has been amplified to the millijoule level. Single-mode operation and frequency selection with a spectral width of 0.2 nm were achieved by using an angle-tuned narrow-band interference filter. A plasma shutter was employed as a pulse clean-up device for these high-power laser pulses. This device had a transmission of 86 percent and provided background-free operation for the laser system. Sychronizable output pulses with an energy of 2 mJ and a duration of 20 ps at 10 pps were routinely produced. The gain of the amplifier chain was about7.5 times 10^{4}.     

Picosecond laser diode pulse amplification up to 12 W by laserdiode pumped erbium-doped fiber

Intense picosecond optical pulse generation from a gain-switched laser diode (LD) was demonstrated using a 1.48-μm LD-pumped Er³⁺ -doped fiber laser amplifier. Saturation characteristics of the amplifier output power were also measured as a function of input repetition frequency. An amplified peak power of 12 W and 105-pJ pulse energy were obtained for 9-ps pulses at a 33-GHz repetition frequency. This is the highest peak power yet demonstrated in pulse generation employing all-laser diodes as active devices     

Properties of high-energy electrically pulsed CO2lasers

The output properties of an electrically pulsed CO2laser have been investigated as a function of partial gas pressure and discharge voltage. The output pulse became sharper with increasing CO2pressure. The output pulse energy increased with increasing helium pressure up to a pressure where the output became independent of further increases. Output pulses of 24 joules at 3.6 MW and 3.6 percent efficiency were obtained at 1/10 atmosphere in a 3.5-meter-long 5-cm-bore laser tube.     

Ultra-shallow high-concentration boron profiles for CMOS processing

The fabrication of ultra-shallow high-concentration boron profiles in silicon has been carried out utilizing a XeCl excimer laser. The Gas Immersion Laser Doping (GILD) process relies on a dopant species, in this case diborane (B2H6), to be adsorbed on the clean silicon surface and subsequently driven in during a melt/regrowth process initiated upon exposure to the short laser pulse. Secondary Ion Mass Spectrometry and spreading resistance profiles show peak boron concentrations from 5 × 10¹⁹cm^-3to 5 × 10²⁰cm^-3depending on the number of laser pulses, with junction depths from 0.08 to 0.16 µm depending on the laser energy. Electrical characteristics show essentially ideal diode behavior following a 10-s 950°C anneal.     

High-efficiency pulse compression with intracavity Raman amplifiers

High-efficiency pulse compression using intracavity Raman amplifiers has been computed. The energy of a pump laser stored in a lossless cavity is extracted at the Stokes frequency by means of Raman amplification of an input Stokes pulse. Calculations are made for both long and short duration input Stokes pulses for different lossless cavities. As an example, we use a hydrogen-argon mixture as the Raman medium and 1.5 J/cm²energy fluence stored in the lossless cavity at the ruby frequency. By comparing amplified Stokes pulses to a 30 ns pulse duration conventional ruby laser delivering the same energy fluence, pulse shortening factors larger than 20 are computed with quantum conversion efficiencies higher than 80 percent. These values compare favorably to backward Raman amplification. Moreover, this technique is proved to be able to provide a pulse compression rate larger than 14, even for a broad-band laser, which is impossible with backward Raman amplification. This technique could be used with any laser, even with absorbing laser media (excimer lasers) provided pump energy is stored in the lossless cavity by shifting of the laser frequency with any nonlinear process.