Space discharge and gas lasers

The main processes in TE lasers are analyzed. Significant attention is given to models for the formation of a homogeneous plasma column and space charge. The reasons for channel growth at the stage of increasing voltage are demonstrated. A method to increase the energy input to the gas using a cathode with high emission properties is proposed. It is demonstrated that, in TEA CO₂ lasers, the limiting deposited and radiated energy densities may amount to 1.35 and 0.085 J/cm³, respectively. Experimental results show that the duration of radiation can be increased using a variation in the composition and pressure of the active medium, the sectionalization of the active medium and the sequential excitation of the sections, and the excitation of the medium with a pulse train at a small (about 10⁷ s) pulse duration and an interpulse interval of about 10⁻⁵ s. Original Text ? Astro, Ltd., 2006.     

Thyratron-based Pockels cell driver for single pulse switch-out in mode-locked lasers

A circuit is described which utilizes a hydrogen filled thyratron driven by a high current MOSFET as a high voltage (up to 12.5 kV), low jitter (0.2 ns) driver for a Pockels cell. Its use as a single pulse switch-out unit for a passively mode-locked Nd:YAG laser is demonstrated.     

A Q-switched Nd:YAlO3 laser emitting 1080 and 1342 nm

In this paper, the study of a switchable electro-optic Q-switched Nd:YAlO₃ laser emitting 1080 and 1342 nm wavelength was demonstrated. The two-wavelength lasers were eradiated by a Nd:YAlO₃ crystal pumped by a xenon lamp. Two KD*P Pockels cells were adopted as Q-switches. The output energy of 277.2 mJ at 1080 nm wavelength with a pulse width of 20 ns and that of 190.67 mJ at 1342 nm wavelength with a pulse width of 40 ns was obtained, respectively, in our experiment. By switching on or off the voltages applied to the Pockels cells, the operation of the two-wavelength lasers can be selected in free running mode or Q-switching mode.     

Active mode-locking of lasers using GaAs and GaP picosecond switches

Active mode-locking of coumarin dye lasers has been achieved using either a GaAs or GaP picosecond high voltage switch to drive an intra cavity Pockels cell in a Q modulation technique. Pulses of 30–50 ps were generated with a peak power of ≈500 kW, while pulses as short as 20 ps were observed. The method can be generally applied to various laser systems over wide spectral ranges.     

Terahertz pulse driven Josephson junctions

The voltage response of a Josephson junction to a pulsed terahertz current is evaluated in the limit of a negligible junction capacitance (overdamped limit). The time-dependent superconductor phase difference across the junction is calculated in the framework of the standard resistive shunted junction model by using a perturbative method. The pulsed current bias affects the time average value of the voltage across the junction and current steps are induced in the current–voltage characteristics for voltage values depending on the pulse repetition rate. The current step height is proportional to the square of the pulse time width (τ) to the period (T) ratio. A fast response detector for pulsed Terahertz radiation is proposed, with an expected responsivity of the order of 0.1 V/W and an equivalent noise power of about 3 × 10^?10 W/Hz^1/2.     

Electrooptic prepulse suppression for fusion laser systems

We report on an effective electrooptic prepulse suppression technique with particular applicability to fusion laser systems. Three optically timed Pockels cells in series have a contrast of 10⁹. They are driven by a laser activated Si high voltage switch of 5 ps jitter and < 100 ps rise time. The resulting overall rise time of the three Pockels cells is 600 ps over the first 6 decades and 1.5 to 2 ns over 9 decades.     

20 GW紧凑Marx型重复频率脉冲驱动源研制进展

介绍了一种基于电触发技术的重复频率脉冲驱动源,其突破了双电容结构脉冲成形、紧凑型结构高压产生、大电流条件下重复频率稳定运行等关键技术,采用电路结构最简单的Marx电压叠加技术,解决了Marx在重复频率运行中的技术难点。脉冲功率驱动源设计输出功率20 GW、脉冲宽度180 ns、重复频率1~50 Hz,输出功率和重复频率在一定范围内可调。研制的脉冲功率驱动源体积仅2.5 m3,重量低至2.2 t,脉冲形成单元储能密度高达23 kJm-3,驱动源单次工作状态下输出功率约20 GW;在重复频率30 Hz工作状态下,输出功率16 GW、连续运行时间10 s、系统抖动约6 ns,系统运行稳定可靠。     

Simple electro-optic technique to generate temporally flat-top laser pulses

A simple technique is presented to generate temporally flat-top shaped laser pulses using electro-optic modulator (Pockels cell). It involves splitting of input laser pulse into two halves of equal intensity and then stacking together with appropriate optical delay to get a temporally flat-top laser pulse. It also allows generation of other pulse shapes by varying the relative intensity, delay, and phase between two halves of the input laser pulse. Temporally flat-top laser pulses of duration ~ 9 ns have been generated using ~ 7 ns duration incident laser pulses from a flash lamp pumped Q-switched Nd:glass laser oscillator. The rise and fall-time of the shaped pulse is limited by speed of electro-optic switch (Pockels cell), which is ~ 2 ns in the present case.     

Far-infrared pump-probe measurement of the lifetime of the 2p?1 shallow donor level in n-GaAs

The low temperature lifetime of electrons excited in the 2p^–1 donor level of n-GaAs has been studied in a far-infrared pump-probe experiment. The measurement has been carried out using a pulsed far-infrared molecular gas laser working at a wavelength of 292µm, with the sample in a magnetic field of 5.1 T, resonant with the 1s_o–2p^–1 transition. Two FIR pulses are sliced from one FIR-laser pulse by means of optical switching techniques using two Q-switched Nd:YAG lasers. The first pulse is used to saturate the transition, while the second pulse probes the return of the population in the excited state towards thermal equilibrium as a function of the time delay after the excitation pulse. The value of 350±50 ns found for the lifetime falls in line with CW saturation results on materials with other doping concentrations.     

Injection-seeded Tm:YAG laser at room temperature

The solid-state, tunable, narrowband, high pulse energy and high reliability lasers are attractive source for LIDAR system. In this paper, we demonstrated a diode pumped injection-seeded 2 μm Tm:YAG laser. By inserting two F-P etalons into the laser cavity, linear-polarized single-frequency seed-laser was achieved at a wavelength of 2013 nm, with a maximum output power of 60 mW. Long-term and short-term frequency stability for the seed-laser were 1.27 × 10^− 7 and 97 Hz/μs, respectively. High power Q-switched laser was operated using a bowtie cavity, the bidirectional output of which was favorable for the injection-seeded. After injecting the seed-laser to the power-laser, single-frequency, nearly transform-limited pulsed 2 μm laser was obtained. As much as 2.0 mJ output energy was achieved at an operating repetition rate of 15 Hz, with a pulse width of 356.2 ns.     

Photoconductive semiconductor switches

Optically activated GaAs switches operated in their high-gain mode are being used or tested for pulsed power applications as diverse as low-impedance, high-current firing sets in munitions; high impedance, low-current Pockels cell or Q-switch drivers for lasers; high-voltage drivers for laser diode arrays; high-voltage, high-current, compact accelerators; and pulsers for ground penetrating radar. This paper will describe the properties of high-gain photoconductive semiconductor switches (PCSS), and how they are used in a variety of pulsed power applications. For firing sets, we have switched up to 7 kA in a very compact package. For driving Q switches, the load is the small (30 pF) capacitance of the Q switch which is charged to 6 kV. We have demonstrated that we can modulate a laser beam with a subnanosecond rise time. Using PCSS, we have demonstrated gain switching a series-connected laser diode array, obtaining an optical output with a peak power of 50 kW and a pulse duration of 100 ps. For accelerators, we are using PCSS to switch a 260 kV, 60 kA Blumlein. A pulser suitable for use in ground-penetrating radar has been demonstrated at 100 kV, 1.3 kA. This paper will describe the specific project requirements and switch parameters in all of these applications, and emphasize the switch research and development that is being pursued to address the important issues     

高重复频率脉冲功率技术及其应用:(5)脉冲叠加的意义

通过对多个脉冲功率发生器的输出进行同步叠加可以获得功率合成。脉冲叠加还可以使脉冲功率源的设计和制作走向模块化。从直线变压器驱动源（LTD） 的观点出发,综合论述了脉冲叠加在重复频率脉冲功率技术中的意义。特别针对固态LTD,通过实验结果归纳了它的技术特征,并在此基础上提出了智能化脉冲功率的概念。     

Generation of arbitrary waveforms with electro-optic pulse-shapers for high energy—multimode lasers

Precisely controlled pulse-shaping operation with large-aperture high-energy laser beams may imply numerous limitations when some minimal flexibility together with high optical performance are required. This paper presents an original electro-optical concept for the production of very high dynamic range arbitrary-waveforms with high temporal resolution, using laser–diode coupled photo-conductive bars coupled onto large coplanar micro-strip lines. This concept simply involves a short-impulse high-voltage generator at the input and a standard Pockels cell at the output. A number of prototypes have been tested on large-aperture fast Pockels cells for the validation of both electrical and optical performance. Such pulse-shapers may find specific applications in the field of optical smoothing related to fusion lasers and other areas, i.e. whenever one needs to provide computer-controlled, low- or high-voltage, pulses with any pre-defined arbitrary waveform. Apart from the coupling application to a Pockels cell, this concept is fully scalable for higher electrical energies and higher voltages.     

Modulateur electrooptique pour l'extraction d'une impulsion lumineuse d'un train

The performance of a new light modulator are presented. It uses the Pockels effect in order to select a single pulse from a train. The repetition rate of commutation is greater than 100 Hz, the contrast ratio exceeds 10⁴ and the lifetime is about 5 x 10⁷ shots. The main advantage of this modulator is an easy working-up.     

Limitations of an optically pumped rubidium laser imposed by atom recycle rate

A rubidium laser pumped on the 5²S_1/2–5²P_3/2 D₂ transition by a pulsed dye laser at pump intensities exceeding 3.5 MW/cm² (>1000 times threshold) has been demonstrated. Output energies as high as 12 μJ/pulse are limited by the rate for collision relaxation of the pumped ²P_3/2 state to the upper laser ²P_1/2 state. More than 250 photons are available for every rubidium atom in the pumped volume during each pulse. For modest alkali atom and ethane spin–orbit relaxer concentrations, the gain medium can only process about 50 photons/atom during the 2–8 ns pump pulse. At 110°C and 550 Torr of ethane, the system is bottlenecked in the ²P_3/2 state and all of the incident photons cannot be absorbed. The output energy is linearly dependent on pump pulse duration for a given pump energy. The highly saturated pump limit of the recently developed three-level model for diode pumped alkali lasers (DPALs) is developed. The system efficiency based on absorbed photons approaches 36% even for these extreme pump conditions.     

Diode-pumped actively Q-switched Nd:GGG laser operating at 938 nm

The stimulated emission cross-section of Nd:GGG crystal in 938 nm transition was measured by the amplifier approach. It is 2.3×10^?20 cm². A quasi-continuous-wave diode pumped, actively Q-switched Nd:GGG laser operating at 938 nm was demonstrated. Pumped by laser diodes with 900 W peak power and 300 μs pulse duration, it generated 168 mJ energy in long pulse mode. The slope efficiency was 36%. Q-switched by a KD?P Pockels cell, 41 mJ output pulse energy was obtained. The pulse duration and peak power were 120 ns and 340 kW, respectively. The optical to optical efficiency was 7%.     

High-energy single pulse and multi-spike operation with a passive polymer Q-switch

Stable highly efficient polymer passive switch based on a polyurethane composite and organic dye BDN were developed for the neodymium lasers. The maximum energy of 0.98 J was obtained for a single pulse at passive Q-switching of these lasers. Repetition of single pulses at 50 Hz was possible. Repetitively pulsed operation with a peak output power up to 2 MW and an energy of train of nanosecond pulses 14.1 J were achieved in an yttrium aluminate laser for the first time. Variation of the initial switch transmittance made it possible to vary the pulse (spike) duration in the range 28–90 ns. The maximum pulse repetition rate in a train was 350 kHz when the Q-switching efficiency was 98%. The damage threshold of the investigated polymer matrix was achieved 14 and 18 J/cm² for single pulse duration (15 and 35 ns correspondingly) and 52 J/cm² in multi-spike generation regime (duration 80 ns).     

Computer controlled Fabry-Perot wavemeter

A combination of three Fabry-Perot etalons with a small polychromator is used to measure wavelengths and line profiles of pulsed and cw-lasers. A small computer calculates from the interference fringe patterns monitored by silicon-diode arrays the wavenumber of laser lines within 0.002 cm^-1. In case of pulsed lasers a single pulse with a minimum energy of 5 × 10^-6 J is sufficient for wavelength determination.     

Picosecond pulsed laser ablation and micromachining of 4H-SiC wafers

Ultra-short pulsed laser ablation and micromachining of n-type, 4H-SiC wafer was performed using a 1552 nm wavelength, 2 ps pulse, 5 μJ pulse energy erbium-doped fiber laser with an objective of rapid etching of diaphragms for pressure sensors. Ablation rate, studied as a function of energy fluence, reached a maximum of 20 nm per pulse at 10 mJ/cm², which is much higher than that achievable by the femtosecond laser for the equivalent energy fluence. Ablation threshold was determined as 2 mJ/cm². Scanning electron microscope images supported the Coulomb explosion (CE) mechanism by revealing very fine particulates, smooth surfaces and absence of thermal effects including melt layer formation. It is hypothesized that defect-activated absorption and multiphoton absorption mechanisms gave rise to a charge density in the surface layers required for CE and enabled material expulsion in the form of nanoparticles. Trenches and holes micromachined by the picosecond laser exhibited clean and smooth edges and non-thermal ablation mode for pulse repetition rates less than 250 kHz. However carbonaceous material and recast layer were noted in the machined region when the pulse repetition rate was increased 500 kHz that could be attributed to the interaction between air plasma and micro/nanoparticles. A comparison with femtosecond pulsed lasers shows the promise that picosecond lasers are more efficient and cost effective tools for creating sensor diaphragms and via holes in 4H-SiC.     

Laser fluence, repetition rate and pulse duration effects on paint ablation

The efficiency (mm³/(J pulse)) of laser ablation of paint was investigated with nanosecond pulsed Nd:YAG lasers (λ = 532 nm) as a function of the following laser beam parameters: pulse repetition rate (1-10,000 Hz), laser fluence (0.1-5 J/cm²) and pulse duration (5 ns and 100 ns). In our study, the best ablation efficiency (η ≅ 0.3 mm³/J) was obtained with the highest repetition rate (10 kHz) at the fluence F = 1.5 J/cm². This ablation efficiency can be associated with heat accumulation at high repetition rate, which leads to the ablation threshold decrease. Despite the low thermal diffusivity and the low optical absorption of the paint (thermal confinement regime), the ablation threshold fluence was found to depend on the pulse duration. At high laser fluence, the ablation efficiency was lower for 5 ns pulse duration than for the one of 100 ns. This difference in efficiency is probably due to a high absorption of the laser beam by the ejected matter or the plasma at high laser intensity. Accumulation of particles at high repetition rate laser ablation and surface shielding was studied by high speed imaging.