Tunable XUV radiation generated by nonresonant frequency tripling in neon

Nonresonant frequency tripling of ultraviolet (UV) radiation (λ_uv = 216–223 nm, generated by sum frequency mixing of the outputs of a frequency doubled pulsed dye laser and of the Nd-YAG pump laser) generates in neon coherent light in the extreme ultraviolet (XUV). The XUV radiation is tunable in spectral regions of negative dispersion λ_xuv = 72.05- 73.58 nm and λ_xuv = 74.3–74.36 nm at the high energy side of the transitions $\text{2p-3s′[}\text{built1}\text{2}\text{, 1]}$ and $\text{2p-3s[}\text{built3}\text{2}\text{, 1]}$. At UV input powers of 0.1-0.3 MW the generated XUV power was typically P_xuv = 0.1-0.4 W (1.5–6 x 10⁸ photons/pulse). Since present UV dye laser systems provide at λ_uv pulse powers of almost 1 MW the XUV output could easily be increased by more than one order of magnitude.     

Characteristics of a Ne-like Ar 46.9-nm soft X-ray laser in capillary discharge at a low Ar pressure

A capillary discharge-pumped Ne-like Ar 46.9-nm soft X-ray laser at a low Ar pressure (28–46 Pa) is proven. To our knowledge, this is the first time an XRD laser output in the condition of the low threshold of a main-current pulse spike (20–21 kA) is demonstrated. The Al₂O₃ ceramic capillary tube is 20 cm in length (Mo electrode is 4 cm in length) and 3 mm in diameter. The maximum laser energy of the precalibrated XRD is 3.5 μJ. The maximum gain coefficient g = 0.46 cm^?1, the maximum gain-length product is 8.28, the beam divergence is 5.4 mrad, and the laser pulse width is 1.65 ns. In addition, the results show that the laser plasma column became difficult to Z-pinch with a increasing Ar pressure, its Z-pinching state of a higher Ar pressure fluctuates more intensely than that of a lower pressure by analyzing the scattering of the delay time between the pre-and main-current pulse.     

Capillary discharge soft X-ray laslng in Ne-like Ar pumped by long current pulses

A capillary discharge soft X-ray laser operating at 46.9 nm on the transition 3p-3s (J = 0-1) of the Ne-like Ar has been realized by exciting the active medium with a long half-cycle duration current pulse of 140 ns. The current is produced by discharging a 10 nF water dielectric capacitor, initially charged to voltages lower than 200 kV by a six stage Marx generator, through a 15-cm long capillary channels. The laser amplification has been obtained by properly adjusting all the other experimental parameters. Utilizing a 3-mm in diameter Al₂O₃ capillary channel initially filled with 0.3 torr of Ar pressure, a laser beam, which has a 4-mrad divergence and a time duration of 1.3 ns, is characterized by a gain of 0.6±0.1 cm^-1. The stability of the plasma compression followed by the laser emission is verified. Received 13 December 2001     

Role of the wall ablation in the operation of a 46.9 nmAr capillary discharge soft x‐ray laser

A capillary discharge pumped soft x‐ray laser operating at 46.9 nm on the 3p–3s transition of the Ne‐like Ar has been realized by pumping the active medium with a relatively slow current pulse (dI/dt ≈ 6 · 10¹¹ A/s). In order to study the role of the ablation in the production of the laser effect, the intensity of the amplified 46.9 nm line has been investigated using the same pumping current pulses in the plastic (polyacetal) and ceramic (Al₂O₃). We showed that the ablation of the capillary walls is unfavorable both for the compression and stability of the plasma and consequently for the soft x‐ray laser production. The amplification and lasing effects are observed only in the ceramic channel. The measurements of the line intensity at 46.9 nm showed the lasing with a gain‐length product of ≈ 9, a laser pulse energy of ≈ 5 μJ, a pulse duration of 1.3 ns and a beam divergence of ≈ 3.5 mrad. In addition, effect of the scaling of the time of lasing with the initial plasma diameter was demonstrated experimentally and compared with a one‐dimensional MHD model.     

水中受激布里渊散射微弱Stokes信号光的高增益放大

采用脉冲宽度为7.2 ns的种子光注入式倍频Nd:YAG脉冲激光器,以CS₂为放大介质,实验并理论研究了水中受激布里渊散射微弱Stokes光的信号增益随延迟时间、放大器池长、抽运光能量的变化规律. 结果表明,当抽运光脉冲相对信号光脉冲延迟进入放大器,且延迟时间为脉冲宽度的一半,抽运光能量略低于介质受激布里渊散射阈值,选择合适的放大器池长可获得最佳的信号增益. 适当选择抽运光能量,亦可实现微弱信号光的线性放大. 实验中采用独立双池放大系统,当水中Stokes信号光的能量为1 pJ时,信 关键词： 布里渊放大器 信号增益 延迟时间 抽运光能量     

Broadband femtosecond OPCPA system driven by the single-shot narrow-band iodine photodissociation laser SOFIA

A two-stage optical parametric amplifier driven by a frequency-tripled beam from the high-energy iodine laser system SOFIA was built. This single-shot Optical Parametric Chirped Pulse-Amplification facility (OPCPA) and the system synchronizing the pump and signal pulses are described in detail. The chirped seed pulse of a Ti:sapphire oscillator running at the central wavelength of 800?nm is amplified in the two-stage (LBO and KDP) optical parametric amplifier over 10⁸ times. The amplified spectral bandwidth of 68?nm corresponds to the pulse duration of 14?fs when a transform-limited pulse is assumed. This implies a compressed pulse of TW power. Systematic gain measurements reveal a good match with the theoretical predictions. Signal and idler beam fluence profiles are presented. The suitability of the iodine photo-dissociation laser as a pump source for the OPCPA technique is thus proved for the first time experimentally. A distinctive feature of the iodine laser is its very narrow gain bandwidth (<0.1?cm^?1) and, therefore, the conventional chirped-pulse amplification technique does not lead to pulse durations at the femtosecond level.     

Tunable XUV radiation generated by nonresonant frequency tripling in argon

Nonresonant frequency tripling of the ultraviolet (UV) radiation of a frequency doubled pulsed dye laser generates in argon coherent radiation in the extreme ultraviolet (XUV). The radiation is tunable in spectral regions of negative dispersion λ_XUV = 97.4–104.75 nm, λ_XUV = 86.8–86.98 nm and λ_XUV = 85.7–86.68 nm located at the high energy side of the transitions 3p-4s′ [$\text{1}\text{2}$,1], 3p-5s′ [$\text{1}\text{2}$,1] and 3p-3d′ [$\text{3}\text{2}$,1], respectively. At UV input pulse powers of 1–2 MW the pr oduced. XUV power was typically 1–8 W (0.2–1.6 × 10¹⁰ photons/pulse).     

Tunable,sub-nanosecond KrF-Raman laser in the ultraviolet

A 0.5 cm^–1 bandwidth injection-locked KrF laser pumps a rare-gas Brillouin cell to produce a reflected pulse with a leading edge risetime of 1 ns, tunable from 248.1 to 248.7 nm. Consistent with Lamb theory of laser amplifiers, subsequent excimer amplification of this pulse produces an intense 500 ps spike on the pulse leading edge. Stimulated Raman scattering then separates the spike from the parent pulse, yielding a tunable short pulse at the first Stokes (S ₁) wavelength. Varying the Raman cell length results in a variable Raman threshold and an adjustable short pulse duration: 250 ps pulses at energies of 3–4 mJ at 268 nm with a 50 cm methane cell and 350 ps, 5 mJ pulses from a 100 cm cell are measured with a streak camera. First pass Raman conversion of the spike toS ₁ followed by second pass backward Raman amplification, where the parent 248 nm pulse serves as the pump beam for the reflectedS ₁ pulse, yields simultaneousS ₁ pulses of 20–25 mJ in the 800 ps range andS ₂ pulses of 550 ps at 5–6 mJ near 290 nm. This laser will avoid collision effects during laser excitation and enable quantitative, single pulse imaging of OH radicals in turbulent combustion because of its high pulse energy.     

Coherent optical pulse evolution in a CO2 amplifier

The coherent reshaping of short duration (2–5 nsec) CO₂ laser pulses in a low-pressure (∽ 5 torr), longitudinal discharge CO₂ amplifier is experimentally studied in the linear regime for a variable number of gain lengths (αL?7). Single pulses grow considerably in duration as well as amplitude in agreement with theoretical considerations. Analysis of the observed pulse evolution is used to obtain the transverse relaxation parameter T₂. Zero-degree pulses {∫^+∞_-∞E ( z, t) dt = 0} are observed to terminate much of the long tail which occurs in single-pulse amplification. Off-resonant amplification of short-duration pulses is shown to lead to dramatic changes in the zero-degree pulse evolution. Numerical calculations relating to pulse amplification in the nonlinear regime for high-pressure CO₂ amplifiers are also presented.     

Two-wave mixing and energy transfer in BaTiO3 application to laser beamsteering

Energy transfer between the two recording beams in a BaTiO₃ photorefractive crystal is analyzed as a function of the following parameters: incident beams ratio, spatial frequency, pump beam intensity. Exponential gain coefficients Γ ≈ 20 cm^-1 are reached for optimized holographic recording conditions. Application of the energy transfer to a new method of laser beam deflection is proposed.     

Output properties of diode-pumped passively Q-switched 1.06 μm Nd:GdVO4 laser using a [100]-cut Cr4+:YAG crystal

A passively Q-switched 1.06???m Nd:GdVO₄ laser with a [100]-cut Cr⁴⁺:YAG saturable absorber was demonstrated. The output characteristics were investigated when the anisotropic transmission of Cr⁴⁺:YAG crystal and the incident pump power level were considered. The experimental results showed that it was feasible to generate laser with narrower pulse width (?? _p ), higher pulse energy and peak power when the polarization direction of laser was parallel to the [001], [010], [ $00\overline{1}$ ], and [ $0\overline{1}0$ ] orientations of the Cr⁴⁺:YAG crystal. The different changes of ?? _p as a function of incident pump power was observed due to the anisotropy of transmission of Cr⁴⁺:YAG and the different gain levels (pump power levels). If the Cr⁴⁺:YAG was fully bleached as a result of high cavity gain or due to the laser polarization direction was parallel to the [001], [010], [ $00\overline{1}$ ], and [ $0\overline{1}0$ ] orientations, ?? _p was constant, otherwise ?? _p decreased when the gain increased.     

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.     

High-peak-power,high-repetition-rate LD end-pumped Nd:YVO<Subscript>4</Subscript> burst mode laser

A compact high-peak-power, high-repetition-rate burst mode laser is achieved by an acousto-optical Q-switched Nd:YVO₄ 1064 nm laser directly pumped at 878.6 nm. Pulse trains with 10–100 pulses are obtained using acousto-optical Q-switch at repetition rates of 10–100 kHz under a pulsed pumping with a 1 ms duration. At the maximum pump energy of 108.5 mJ, the pulse energy of 10 kHz burst mode laser reaches 44 mJ corresponding to a single pulse energy of 4.4 mJ and an optical-to-optical efficiency of 40.5 %.The maximum peak power of ~468.1 kW at 10 kHz is obtained with a pulse width of 9.4 ns. The beam quality factor is measured to be M ² ~1.5 and the pulse jitter is estimated to be less than 1 % in both amplitude and time region.     

SILEX-I: 300-TW Ti:sapphire laser

Based on chirped pulse amplification technology, we have built a Ti:sapphire laser system, called SILEX-I (superintense laser for experiments on extremes), at CAEP, which consists of three stages with 5-, 30-, and 300-TW outputs, respectively. The first and the second stages work at 10 Hz, while the third works at single shot. Pulse durations of 30 fs have been obtained by installing an acousto-optic programmable dispersive filter (AOPDF) to compensate for the spectral gain narrowing in the regen. By taking a number of advanced measures for spatial beam control, such as spatial beam shaping, relay-imaged propagation, precise alignment of compressor gratings, and OAP, near-diffraction limited focal spots (FWHM) have been obtained. Focused intensities are calculated at (1–3) × 10²⁰ W/cm² with an f/2.2 OAP.SILEX-I has shown an excellent stability and reliability in operations for applications since its completion and will soon be able to operate at 500 TW.     

Diode-pumped monolithic Er3+:Yb3+:YAl3(BO3)4 micro-laser at 1.6 μm

Efficient eye-safe 1.6 μm monolithic laser was realized in a c-cut, 0.7-mm-thick Er³⁺:Yb³⁺:YAl₃(BO₃)₄ microchip end-pumped by a quasi-continuous-wave 970 nm diode laser. At incident pump peak power of 20.4 W, a maximum output peak power of 2.6 W with a slope efficiency of 19% was obtained when the waist radius of pump laser beam was 220 μm. The spectra and profiles of output beam of the Er³⁺:Yb³⁺:YAl₃(BO₃)₄ monolithic laser were measured. The influences of the waist radius of pump laser beam on the slope efficiency and threshold of the monolithic laser were also investigated.     

Beam properties of fully optimized,table-top,coherent source at 30 nm

We present results on development and experimental implementation of a 1-kHz, coherent extreme ultraviolet (XUV) radia- tion source based on high-order harmonic generation of the femtosecond, near-infrared laser pulses produced by the titanium-doped sapphire laser system (35 fs, 1.2 mJ, 810 nm) at the Institute of Physics AS CR / PALS Centre. The source comprises a low-density static gas cell filled with a conversion medium, typically argon. The comprehensive optimization of the XUV harmonic source has been performed with respect to major parameters such as gas pressure in the cell, cell length, position of the focus of the driving laser field with respect to the gas cell position, size of the driving near-infrared laser beam, chirp of the femtosecond pulse, and the focal length of the lens deployed in the experimental setup. Harmonic spectra were recorded using an XUV transmission grating spectrometer developed specifically for this purpose. Detailed characterization of the XUV source has been performed including measurement of the XUV beam profile, M² parameter of the beam, absolute energy, and spatial coherence.     

Prepulse dependence ofJ=0−1 lasing at 32.6 nm in neon-like titanium

The dependence of the intensity of the 3p—3s,J = 0–1 lasing line at 32.6 nm in neon-like titanium on the prepulse level has been investigated experimentally. Titanium slabs were irradiated with 1.315 µm/450 ps pulses from the Asterix IV iodine laser using a defined prepulse of 5.2 ns before the main pulse. It is found that for pump energies close to the minimum energy for which lasing is observed, a prepulse level of order 0.5% gives the highest XUV laser intensity, whereas a higher prepulse level, of order 10% and more, is required for optimum XUV lasing far above the threshold. For a 2.7 cm long titanium target lasing was observed down to a pump irradiance of 2.5 TW/cm² (50 J/450 ps) for the 0.5% prepulse.     

Yb3+-doped 200 μm diameter core, gain guided index-antiguided fiber

The lasing in an end-pumped gain guided index-antiguided (GG-IAG) Yb³⁺-doped silicate glass fiber with a 200 μm diameter core is demonstrated. Laser beams with similar beam propagation factors M ² and mode field diameters W ₀ (>160 μm) were observed at the output end of the GG-IAG fibers under different pump powers, which indicated that single mode behavior and excellent beam quality were achieved during propagation. Furthermore, the laser amplifier characteristics in the present Yb³⁺-doped GG-IAG fiber were also evaluated.     

26 mJ Total Output from a Gain-Switched Single-Mode Er3+-Doped Zblan Fiber Laser Operating at 2.8 $$ \mu $$m

We demonstrate a laser-diode-pumped gain-switched Er³⁺-doped ZrF₄-BaF₂-LaF₃-AlF₃-NaF (ZBLAN) fiber laser operating in a single transverse mode at 2.8 \( \mu \)m. The laser pulses produced offer high-pulse energies, with repetition rates ranging from 50 Hz to 10 kHz and a slope efficiency of approximately 14.3% with respect to the launched pump power. The average power at the 50 Hz repetition rate is 1.33 W, giving a maximum total output pulse energy of 26.6 mJ per pump pulse. The fiber laser operates in a single mode, with beam quality factor M² less than 1.2.     

Laser pulse amplification upon Raman backscattering in plasma produced in dielectric capillaries

Femtosecond-pulse amplification upon stimulated Raman scattering is experimentally demonstrated for the case of a counter-propagating femtosecond laser pulse and a frequency-modulated broadband pump pulse with the same carrier frequency in a dielectric capillary filled with gas plasma. A value of ～10³ obtained for the spectral intensity amplification and ～10² for the output energy are the highest ever achieved for these quantities to date. Numerical simulation demonstrates good agreement with the experimental results. Based on the experimental data and the results of theoretical calculations, we propose a hydrodynamic mechanism for plasma-wave breaking as the mechanism playing an important role in the amplification restriction in the scheme considered.