Multipass Amplifier Research Articles

Performance of a prototype for a large-aperture multipass Nd:glass laser for inertial confinement fusion

The Beamlet is a single-beam prototype of future multibeam megajoule-class Nd:glass laser drivers for inertial confinement fusion. It uses a multipass main amplifier, adaptive optics, and efficient, high-fluence frequency conversion to the third harmonic. The Beamlet amplifier contains Brewster-angle glass slabs with a clear aperture of 39 cm x 39 cm and a full-aperture plasma-electrode Pockels cell switch. It has been successfully tested over a range of pulse lengths from 1-10 ns up to energies at 1.053 mum of 5.8 kJ at 1 ns and 17.3 kJ at 10 ns. A 39-actuator deformable mirror corrects the beam quality to a Strehl ratio of as much as 0.4. The 1.053-mum output has been converted to the third harmonic at efficiencies as high as 80% and fluences as high as 8.7 J/cm(2) for 3-ns pulses.

High-average-power kilohertz-repetition-rate sub-100-fs Ti:sapphire amplifier system

We report on a femtosecond Ti:sapphire laser amplifier system that generates pulse energies >5 mJ at a 1-kHz repetition rate. The system consists of regenerative and multipass amplifiers and uses the technique of chirped-pulse amplification. When the system was seeded with 70-fs pulses at 800 nm from a self-mode-locked Ti:sapphire oscillator, amplified pulses of 94-fs duration at a repetition rate of 1 kHz and an average output power of 5.4 W were produced. The amplified pulse-repetition rate is variable from 250 Hz to 3 kHz. Pulse energies of >7.5 mJ were obtained at 500 and 250 Hz.

Increase in the output energy of multipass neodymium glass amplifiers

It was found that overlapping of the pulses on the time scale and a spatial inhomogeneity of the amplified radiation reduced, like stimulated Brillouin scattering, the output energy of 10-7 s pulses amplified in a multipass neodymium glass amplifier. Methods for eliminating the influence of these factors were proposed and implemented.

THEORETICAL AND EXPERIMENTAL STUDIES ON THE CHIRPED PULSE MULTIPASS AMPLIFICATION IN DOUBLE SIDE PUMPED Ti:SAPPHIRE

A novel and more realistic calculation model and method for multipass laser pulse amplification has been put forward based upon the double side pumping scheme. We present here the systematic calculation and analysis on the amplification characteristics of the double side pumped confocal cavity Ti:sapphire amplifier and the chirped pulse multipass amplification through a new amplification configuration. Our experimental results show good agreement with the calculations. With such amplifier,the pass number is noticably reduced and some electro optical elements in regenerative amplifiers have been omitted.

Effective stimulated emission cross-sections determined by transient fluorescence amplification in a Fabry–Pe´rot resonator

The amplification of fluorescence light of an active medium in a Fabry-Pe´rot resonator, transversely pumped with picosecond pulses, is exploited to determine the effective stimulated emission cross-section spectrum, σem,eff(λ). A theory is presented for the ratio of amplified fluorescence output to spontaneous emission signal (resonator mirrors removed). The detection sensitivity is enhanced by the multi-pass amplification. Its dependence on the output mirror reflectivity and residual ground-state absorption in the emission region is analysed. As an example the effective stimulated emission cross-section spectrum of coumarin 2 in methanol is measured in the wavelength region 435nm to 610nm.

Gain-narrowing and gain-shifting of ultra-short pulses in Ti: sapphire amplifiers

We have developed a simple model for chirped-pulse titanium: sapphire amplification that includes gain narrowing and gain shifting effects. We analyse the spectral limitation in ultra-short pulse amplification and compare two different kinds of pre-amplifiers: multi-pass and regenerative amplifiers. We compare the model to multi-pass Ti: sapphire amplifier measurements using different input bandwidths.

Laser drilling of advanced materials: Effects of peak power, pulse format, and wavelength

The quality of laser-drilled holes is significantly influenced by the laser peak power, pulse format, and wavelength. Three advanced materials are used to demonstrate the importance of choosing the correct laser parameters for a specific material. The materials are: (1) intermetallic single crystal nickel aluminide (NiAl) alloy; (2) N5, a single crystal nickel-based superalloy; and (3) a silicon carbide (SiC) ceramic matrix composite (CMC). The laser peak power is varied in terms of the pulse duration and format. For low peak powers, long pulse format (pulse lengths on the order of ms), conventional and also shaped-pulse (burst) commercial rod Nd:YAG laser systems were used. For high peak powers, a short pulse format was adopted for the laser used, namely a cw-pumped Q-switched or modelocked/Q-switched Nd:YAG oscillator followed by a cw-pumped multipass amplifier using a face-pumped, total internal reflection Nd:YAG slab as the gain medium. Finally, a KrF excimer laser operating at a 248 nm wavelength was used. The excimer wavelength was chosen to be close to the Nd:YAG fourth harmonic.

The inverse problem of a laser amplifier

Starting from the recurrence relations of a multipass amplifier, an iterative computing method has been presented to solve the inverse problem of a laser amplifier, i.e., to find the initial fluence and pulse shape for a given output pulse and amplifier parameters. Numerical calculations have been performed to illustrate the results.

Multi-pass amplification of sub-50 fs pulses up to the 4 TW level

We describe the design and operation of a 2-stage multi-pass Ti:sapphire amplifier laser system, which is optimised for a broad transmission bandwidth and incorporates a novel reflective spherical mirror pulse-stretcher. Output energies of 180 mJ after compression, and amplified bandwidths up to 30 nm are observed.

Multipass optical parametric amplifier

A compact, low-threshold, multipass optical parametric amplifier has been developed for the conversion of short-pulse (360-ps) 1064-nm Nd:YAG laser radiation into eye-safe 1572-nm radiation for laser ranging and radar applications. The amplifier had a threshold pump power of as low as 45 microJ, and at three to four times this threshold pump power the amplifier converted 30% of the input 1064-nm radiation into 1572-nm output radiation.

Frequency sweeping of radiation in a system with a monolithic laser and a pulsed slab amplifier

Theoretical and experimental investigations were made of frequency sweeping in a system formed by a highly stable cw master oscillator and a multipass pulsed slab amplifier. It is shown that in a four-pass small-signal amplification system the chirp of the amplified radiation does not exceed 7.7 kHz when the energy of the pump pulses is 36 J and the duration of these pulses is 300 μs.

Concept and numerical investigation of an ultrafast high-contrast high-power KrF laser system

The concept of an ‘all-excimer’ ultrafast high-power UV laser system implementing the KrF excimer medium both in the oscillator and in the amplifiers is presented. The system includes a short-pulse (10-11–10-10s) KrF oscillator employing fast pulse period Q-switching or truncated Brillouin scattering and a multipass KrF amplifier with a multilayer saturable absorber applied as a highly effective pulse compressor. By means of computer simulations, it is shown that the system, incorporating currently available KrF discharge modules and acridine dye as the saturable absorber, is capable of generating high-contrast pico-or subpicosecond pulses of near terawatt power level.

Competition between spontaneous scattering and stimulated scattering in an injection-seeded Raman amplifier.

The amplification of a visible laser-diode beam in a gain-guided, multipass Raman amplifier is studied as a function of laser-diode and pump-laser powers. At a relatively high laser-diode power (178 nW) the amplified output grows as a simple exponential from very low gains to amplifier saturation. At a relatively low laser-diode power (2.3 nW) the behavior is more complicated, since the seeded growth and the growth from spontaneous scattering are comparable in power. The experiments, which also include an experiment with no injected laser-diode light, are modeled by a nonorthogonal-mode theory that accounts for the three-dimensional nature of the amplification process. The theory and data are found to be in close agreement in all experimental regimes studied, except at low gains with low input seed power. \textcopyright{} 1996 The American Physical Society.

A tunable femtosecond pulses amplifier

We have designed a multipass amplifier combined with a folded prisms optical compressor that generates 80 fs light pulses tunable between 580 and 630 nm when using sulforhodamine B in water. The external energy source is a 20 Hz repetition rate, doubled Nd: YAG laser, delivering 8 ns pulses. A numerical model has been developed that describes the dynamics of gain saturation and amplified spontaneous emission growth in a multipass pulses amplifier. This model predicts a gain Q-switched behavior of the multipass amplifier for high pumping levels.

SELF-PHASE MODULATION IN CHIRPED-PULSE AMPLIFICATION

Based on semiclassical approach, the transport equations of chirped-pulse amplification are presented. Time-and spectral-domain evolution of chirped pulse and effects of self-phase modulation in multipass amplification are numerically analyzed. Theoretical results show that the new frequency chirp induced by self-phase modulation changes the instantaneous frequency of chirped pulse and affects the quality of amplified pulse. The study of chirped-pulse amplification in the gain saturation regime shows that the linear frequency chirp of input pulse tends to suppress the effects of self-phase modulation.

Characterization of KNbO_3 crystal for frequency doubling of a tunable pulsed near-infrared laser

Using KNbO(3) crystals for second-harmonic generation in the blue spectral range, we have obtained conversion efficiencies as high as 53.7%. To our knowledge this value is the highest reported for a pulsed laser. A gainswitched nanosecond Ti:sapphire oscillator, amplified by a Cr:LiSrAlF(6) multipass amplifier, was used as the fundamental light source. The maximum energy obtained at the harmonic wavelength of 430 nm was 38 mJ for a bandwidth of 230 MHz.

Performance of large-aperture optical switches for high-energy inertial-confinement fusion lasers

We describe the design and performance of large-aperture (>30 cm × 30 cm) optical switches that have demonstrated, for the first time to our knowledge, active switching of a high-energy (>5 kJ) optical pulse in an inertial-confinement fusion laser. These optical switches, which consist of a plasma-electrode Pockels cell (PEPC) and a passive polarizer, permit the design of efficient, multipass laser amplifiers. In a PEPC, plasma discharges on the faces of a thin (1-cm) electro-optic crystal (KDP or KD*P) act as highly conductive and transparent electrodes. These plasma electrodes facilitate rapid (<100 ns) and uniform charging of the crystal to the half-wave voltage and discharging back to 0 V. We discuss the operating principles, design, optical performance, and technical issues of a 32 cm × 32 cm prototype PEPC with both KDP and KD*P crystals, and a 37 cm × 37 cm PEPC with a KDP crystal for the Beamlet laser. This PEPC recently switched a 6-kJ, 3-ns pulse in a four-pass cavity.

Nd:YAG amplifier with a semiconductor phase-conjugate mirror

We investigate an all-solid-state Nd:YAG multipass amplifier with a phase-conjugate mirror operating in the nanosecond pulsed regime. The phase conjugator employed a dynamic phase hologram written within a semiconductor (Si, CdTe:V, GaAs, GaAs:Cr) by free carrier generation. The amplification factor of such architecture is calculated to be as high as 100 with a four-pass amplifier. Net amplifications of 3.5 are obtained experimentally with a two-pass flash-lamp-pumped Nd:YAG amplifier with a semiconductor phase-conjugate mirror. The ability of this device to compensate for severe phase distortions is demonstrated.

Amplification of 26-fs, 2-TW pulses near the gain-narrowing limit in Ti:sapphire

We report the generation of 26-fs-duration pulses, with an energy of 60 mJ, from a simple multipass Ti:sapphire amplifier system. The peak power of our amplified pulses is 2 TW, and the repetition rate is 10 Hz. Our amplifier design consists of two highly doped multipass amplifiers and is simple and compact. We use an all-reflective, low-groove-density grating stretcher and compressor, combined with a relatively short material path length in the amplifier. This design allows us to minimize higher-order dispersion. The result is a laser system that generates multiterrawatt transform-limited pulses, with good beam quality and low amplified-spontaneous-emission levels, at a duration near the theoretical limit imposed by gain narrowing in Ti:sapphire.

Numerical three-dimensional modelling of a cw CO2 amplifier with transverse pumping of the active medium

A three-dimensional mathematical model of a cw CO2 amplifier with transverse flow is constructed. The model takes into account the refraction of radiation and the saturation of the active-medium gain. Numerical methods with higher precision are proposed for approximate integration of a parabolic equation describing the propagation of radiation and for determination of two-dimensional intensity distributions in the far-field zone. The cases of amplification of beams of various forms are considered for certain multipass amplifier configurations with electric-discharge excitation. The dependences of the angular divergence of the output radiation on the length of the active medium are obtained.