Ultrafast semiconductor laser-diode-seeded Cr:LiSAF regenerative amplifier system

An ultrafast, hybrid mode-locked semiconductor laser-diode system has been used to seed a flash-lamp-pumped Cr:LiSAF regenerative amplifier system, producing subpicosecond pulses with millijoule output pulse energy. This system has the potential to eliminate argon-ion-pumped-based, ultrafast laser systems.     

Pulse compression with a high-energy Nd:YAG regenerative amplifier system

We describe a method for the generation of readily synchronizable, near-transform-limited, 1064-nm, 6-mJ pulses with <20-ps duration at a repetition rate of 20 Hz. The method employs chirped pulse amplification of spectrally broadened and temporally stretched pulses from a cw mode-locked Nd:YAG laser in a commercial Nd:YAG regenerative amplifier followed by pulse compression with a grating pair. Linear amplification subsequent to regenerative amplification is not required with this method, although higher energies would be easily obtained.     

Characterization of a high-brilliance soft x-ray laser at 13.9 nm by use of an oscillator-amplifier configuration

We have improved a highly coherent x-ray laser at 13.9 nm using an oscillator-amplifier configuration. To improve a high-brilliance x-ray laser, we adopted traveling wave pumping for the amplifier target and rotated the amplifier target 3-4 mrad in the counterclockwise direction. Thereby, a seed x-ray laser can be amplified by medium plasma of the amplifier target with a high gain coefficient. The amplified x-ray laser has the output energy of approximately 1.3 microJ, corresponding to a large photon flux of 6.5 x 10(10) photons/pulse and a high peak brilliance of 5 x 10(26) photons/(s x mm(2) x mrad(2) x 0.01% bandwidth).     

Optimization of the power and control of the shape of amplified trains of laser pulses

We have performed an optimization study on a train of laser pulses in a Nd:YLF master-oscillator power amplifier chain. Instead of using a flat train of input pulses and proper timing of the input pulses with respect to the pump pulse to keep the output pulse flat, we used a pulse-shaping technique. Then the maximum gain could be used, resulting in a 70% increase in output pulse energy. We constructed a special feed-forward loop to control the temporal shape of the train. We compare the results with a computational model based on the rate equations in the Nd:YLF amplifiers.     

Picosecond Diode-Pumped Laser System with 9.3-W Average Power and 2.3-mJ Pulse Energy

We describe a diode-pumped Nd:YAG laser that is suitable for micromachining applications and is capable of generating 2.3-mJ pulses at a 4-kHz pulse repetition frequency. The output pulse duration is 20.5 ps. The system is based on a Nd:YAG regenerative amplifier with a novel four-pass postamplifier. The postamplifier incorporates birefringent depolarization compensation and simultaneously prevents parasitic laser oscillation by use of a nonreciprocal beam path. These output pulse energies are achieved without the use of chirped pulse amplification.     

Generation of a train of ultrashort pulses from a compact birefringent crystal array

A linear array of n calcite crystals is shown to allow the generation of a high contrast (>10:1) train of 2(n) high energy (>100 microJ) pulses from a single ultrafast laser pulse. Advantage is taken of the pulse-splitting properties of a single birefringent crystal, where an incident laser pulse can be split into two pulses with orthogonal polarizations and equal intensity, separated temporally in proportion to the thickness of the crystal traversed and the difference in refractive indices of the two optic axes. In the work presented here an array of seven calcite crystals of sequentially doubled thickness is used to produce a train of 128 pulses, each of femtosecond duration. Readily versatile properties such as the number of pulses in the train and variable mark-space ratio are realized from such a setup.     

Kilohertz femtosecond UV-pumped visible beta-barium borate and lithium triborate optical parametric generator and amplifier

We demonstrate optical parametric generation and amplification of femtosecond pulses in the entire visible range using type-I phase-matched beta-barium borate and lithium triborate crystals pumped by the frequency-doubled output of a Ti:sapphire regenerative amplifier at 395 nm. The output is tunable from 470 to 770 nm with a pulse width of ~170 fs at a repetition rate of 1 kHz and a maximum output energy of ~1.1 muJ/pulse. The visible optical parametric amplifier output was then frequency doubled and sum frequency mixed with the fundamental output of Ti:sapphire at 790 nm to produce UV pulses with a conversion efficiency of greater than 25%. The second harmonic generated UV pulses are tunable from 240 to 380 nm with a maximum pulse energy of ~260 nJ/pulse.     

Experimental and theoretical investigation of a multipass, plane mirror, femtosecond dye laser amplifier

Femtosecond pulses of a passive mode-locked Rhodamine-6G dye laser are amplified in a double-stage, three-pass, plane mirror, Sulforhodamine-101 amplifier system. Saturable filters (Schott glass RG645 and Malachite Green) are used to suppress amplified spontaneous emission. Input pulses of 110-fs duration are broadened to 240 fs in the amplifier system and recompressed to 75 fs in a prism-pair compressor. Using a 20-Hz Q-switched Nd:YAG pump laser of 50-mJ second-harmonic output energy, we obtained amplified and recompressed pulses of 180-μJ energy at 625 nm starting with 40-pJ input pulses. The small-signal amplification dynamics is studied numerically. Relevant gain dye and saturable filter parameters are derived. The influence of amplified spontaneous emission is analyzed.     

Optical parametric chirped-pulse amplifier as an alternative to Ti:sapphire regenerative amplifiers

We demonstrated a high-pulse energy, femtosecond-pulse source based on optical parametric chirped-pulse amplification. We successfully amplified 1-microm broadband oscillator pulses to 31 mJ and recompressed them to 310-fs pulse duration, at a 10-Hz repetition rate. The gain in our system is 6 x 10(7), achieved by the single passing of only 40 mm of gain material pumped by a commercial Q-switched Nd:YAG laser. This relatively simple system replaces a more complex Ti:sapphire regenerative-amplifier-based chirped-pulse amplification system. Numerous features in design and performance of optical parametric chirped-pulse amplifiers make them a preferred alternative to regenerative amplifiers based on Ti:sapphire in the front end of high-peak-power lasers.     

Reliable Stimulated Brillouin Scattering Compression of Nd:YAG Laser Pulses with Liquid Fluorocarbon for Long-Time Operation at 10 Hz

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 eta(SBS) = 94% and the overall device efficiency a value of eta(dev) = 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.     

Two-photon-pumped frequency-upconverted blue lasing in coumarin dye solution

Two-photon-pumped (TPP) frequency-upconverted blue lasing of Coumarin 500 dye solution has been experimentally investigated. The shortest lasing wavelength was measured to be ~479 nm from a Coumarin 500 solution in chloroform pumped with ~800-nm laser pulses of ~5-ns duration. The spectral, temporal, and spatial structures as well as the output-input characteristics of TPP cavity lasing were measured with a 1-cm-long Coumarin 500 solution-filled quartz cuvette. The cavity lasing spectral structure and the numbers of longitudinal modes were easily controlled simply by attachment of an optical plate to the output window of the dye-solution cuvette. The net conversion efficiency from the absorbed 800-nm pump pulse energy to the blue-upconverted cavity lasing energy was ~4.8%.     

Chromium-doped LiCAF laser passively Q switched with a V3+:YAG crystal

A tunable, lamp-pumped Cr(3+):LiCaAlF(6) laser that operates in the 760-800-nm wavelength range is demonstrated. As a passive Q switch the V(3+):YAG crystal, characterized by wide saturable absorption in the 760-900-nm wavelength range, was applied. The V(3+):YAG parameters were determined by spectroscopic and saturable transmission measurements. The 30-mJ output energy at 2-Hz repetition rate in the free-running regime was obtained in a cavity reinforced by a diaphragm near the fundamental mode. The dispersive prism inserted into the cavity enabled tuning in the 760-800-nm wavelength range. In the passive Q-switching regime we achieved greater than 1-mJ energy with 50-ns pulse duration in a 35-cm-long cavity. A numerical model that takes into account the short recovery time of V(3+):YAG (~5 ns) and excited-state absorption was used to analyze such a laser.     

Development of a chirped pulse amplification laser with zigzag slab Nd:glass amplifiers dedicated to x-ray laser research

A chirped pulse amplification laser with zigzag slab Nd:glass amplifiers dedicated to x-ray laser research is described. The laser provides a 1.6 ps duration pulse with approximately 7 J energy at a repetition rate of 0.1 Hz. In the power amplifier system, laser light is amplified in a two-step manner: The first step is image-relayed multipass amplification up to approximately 1 J with a 10 mm x 10 mm beam. The second step is double-pass amplification up to >10 J with a 10 mm x 90 mm beam. By using this laser system, the saturated amplification of the Ni-like Ag laser at a wavelength of 13.9 nm has been successfully demonstrated.     

Femtosecond Traveling-Wave Optical Parametric Amplification in MgO:LiNbO(3)

A mid-infrared femtosecond optical parametric amplifier tunable in the chemically important spectral region between 3.1 and 3.9 mum (at >10-muJ idler pulse energy) has been constructed on the basis of MgO:LiNbO(3) with 7% doping. With femtosecond pumping near 800 nm (Ti:sapphire regenerative amplifier) and narrow-band (long-pulse) seeding, this simple single-stage device provides maximum conversion efficiencies of 40% and exhibits extremely low seed threshold (<10-mW peak seed power for >1-muJ idler output). The generated idler pulses are almost transform limited with <200-fs pulse duration. The pulse-to-pulse fluctuations reproduce the stability of the pump source at 1-kHz repetition rate.     

Acoustic detection of controlled laser-induced microbubble creation in gelatin

A high-frequency (85 MHz) acoustic technique is used to identify system parameters for controlled laser-induced microbubble creation inside tissue-mimicking, gelatin phantoms. Microbubbles are generated at the focus of an ultrafast 793-nm laser source and simultaneously monitored through ultrasonic pulse-echo recordings. Displayed in wavefield form, these recordings illustrate microbubble creation, and integrated backscatter plots provide specifics about microbubble characteristics and dissolution behavior. By varying laser parameters, including pulse fluence (or pulse energy flux, J/cm2), total number of pulses delivered, and the period between pulses, the size, lifetime, and dissolution dynamics of laser-induced microbubbles may be independently controlled. Pulse fluence is the main size-controlling parameter, whereas both increases in pulse fluence and pulse number can lengthen microbubble lifetime from tens to hundreds of milliseconds. In short, a microbubble of particular lifetime does not necessarily have to be of a particular size. Microbubble behavior, furthermore, is independent of pulse periods below a fluence-dependent threshold value, but it exhibits stochastic behavior if pulse repetition is too slow. These results demonstrate that laser pulse fluence, number, and period may be varied to deposit energy in a specific temporal manner, creating and stabilizing microbubbles with particular characteristics and, therefore, potential uses in sensitive acoustic detection and manipulation schemes.     

Q-switched 2-micron lasers by use of a Cr2+:ZnSe saturable absorber

Flash-lamp-pumped Ho:YAG (2090-nm) and Tm:YAG (2017-nm) lasers were, for the first time to our knowledge, passively Q switched by use of a Cr(2+):ZnSe saturable absorber. A Q-switched Ho laser with 1.3-mJ pulse energy and ~90-ns pulse duration and a Q-switched Tm laser with ~3.2-mJ pulse energy and 90-ns pulse duration were demonstrated. Compared with the free-running output energies at the Q-switching threshold pump levels, the Q-switching efficiencies were approximately 5% for the Ho:YAG laser and 16% for the Tm:YAG laser.     

A generation of 2 μm Q-switched thulium-doped fibre laser based on anatase titanium(IV) oxide film saturable absorber

We demonstrate a Q-switched thulium-doped fibre laser operating at approximately 1935 nm wavelength using anatase titanium(IV) oxide (TiO₂) embedded in polyvinyl alcohol as the passive newly saturable absorber (SA). The film has absorption loss of 3.5 dB and modulation depth of 33%. It is sandwiched between two fibre ferrules in a ring laser cavity to produce self-started pulse train with a repetition rate that is tuned from 30.12 to 36.96 kHz as the 1552-nm pump power is increased from 289 to 485 mW. At maximum pump power, the laser produced a Q-switching pulse train with pulse duration, output power, pulse energy and peak power of 1.91 μs, 11 mW, 0.3 μJ and 146 mW, respectively. These results show that the TiO₂ is a new potential SA material for pulsed laser applications.     

Single-frequency pulsed laser source with hybrid MOPA configuration

A unique approach with a hybrid master oscillator power amplifier configuration to obtain single-frequency, high-energy laser pulses at 1064 nm is presented. The setup consists of a single-frequency seed laser, a multistage fiber amplifier, and a four-pass crystal rod amplifier. Pulse energy of 10 mJ is obtained at the repetition rate of 100 Hz. The pulse width is about 110 ns with a transform-limited linewidth of 3.2 MHz. The M(2) factor of the output beam is about 1.5. To our knowledge, this is the first report of using a hybrid amplifier to obtain 10 mJ pulses with long pulse width and transform-limited linewidth.     

Theoretical analysis of a noncollinear phase-matched optical parametric amplifier seeded by an optical parametric generation

We have theoretically analyzed the characteristics of an optical parametric amplifier system seeded by an optical parametric generation. We investigated the influences of the energy, pulse duration, material dispersion, and the third-order nonlinear effect in beta-barium borate. The group-velocity mismatch (GVM) becomes the most important factor for the amplification of bandwidths. Even though tilting the wave front of the pump can decrease the GVM, it seems impossible to generate pulses smaller than 10-fs with 400-nm pumping. However, 10-fs pulses can be achieved with a 30-fs pump duration with pumping at 800 nm.     

High-power dye laser using steady-state amplification with chirped pulses

Steady-state chirped-pulse amplification was applied to a five-stage dye amplifier system to extract available energy over the full gain duration and at the same time suppress the amplified spontaneous emission (ASE). An output energy of 41 mJ was generated with 1.4-ns chirped pulses having an ASE of 3% and a pumping efficiency of 8.8% for the final amplifier. After five-stage amplification these pulses were compressed to 320 fs FWHM.