Amplification Of Ultrashort Pulses Research Articles

Research on the formation of narrow bandwidth long flat-top pulse via coherent addition of ultra-short pulses

Making use of the spectrum gain narrowing effect during the ultra-short pulse amplification in the gain media and the filter effect of the etalon, and adopting the principles of the regenerative amplifier, the multi-pulse coherent addition scheme is proposed and analyzed. In the small signal gain region of the ultra-short pulse amplification in the regenerative amplifier, theoretical analysis and numerical simulation have been made on the multi-pulse coherent addition stage. The simulation suggests that the width of the sub-pulse, the loss in the regenerative amplifier and the mismatch between the oscillator and the regenerative amplifier are the key factors affecting the temporal shape of the composite pulse, although they have little influence on its spectrum. Both the numbers of the constituent sub-pulses and the bandwidth of the etalon have influence on the shape and spectrum of the pulse.

Fifteen Years of Work on Thin-Disk Lasers: Results and Scaling Laws

The principal ideas of the thin-disk laser design will be illustrated and the advantages for operating different laser materials will be explained. The results for continuous-wave (CW) and Q-switched operation as well as for amplification of short (nanosecond) and ultrashort (picosecond, femtosecond) pulses demonstrate the potential of the thin-disk laser design. The scaling laws for this laser design show that the power limit for CW operation is far beyond 40 kW for one single disk and the energy limit is higher than 3 J from one disk in pulsed operation. Also, the applicability of the thin-disk laser concept to optically pumped semiconductor structures will be discussed. When pumping directly into the quantum wells, the energy defect between the pump photon and the laser photon can be smaller than 5%, thus reducing the waste heat generated inside the semiconductor structure. First results demonstrate the potential of this new concept. Finally, a short overview of the industrial realization of the thin-disk laser technology will be given.

Spatial–temporal frequency band of optical parametric amplifier

The spatial–temporal frequency band of optical parametric amplifier for collinear as well as for noncollinear phase-matching is analyzed taking into account angular dispersion of amplified pulsed beam. The simple formulae are derived which make possible determination of the largest frequency bands of parametric amplifiers in noncollinear geometry. It has been demonstrated that a shape of the spatial–temporal frequency band of optical parametric amplifier essentially depends on pump and signal wavelengths, and certain angular dispersion as well as proper beamwidth of signal pulse enable amplification of ultrashort pulses with duration of a few femtoseconds.

All-Fiber Picosecond Laser System at 1.5 $\mu$m Based on Amplification in Short and Heavily Doped Phosphate-Glass Fiber

Amplification of ultrashort pulses in doped fibers is limited by an onset of nonlinear effects in the fiber. At the 1.5-mum wavelength, single-mode fibers typically have anomalous dispersion. The self-phase modulation combined with dispersion leads to instability of multinanojoule pulses in such fibers. Various techniques developed to amplify pulses beyond the nonlinearity limit typically rely on a delicate balance between dispersive and nonlinear effects in different parts of the laser system. We report a simple all-fiber alternative to these complex techniques that utilizes a rapid amplification of pulses in a short and heavily doped phosphate-glass active fiber. In our preliminary experiments, picosecond pulses at 1.5 mum generated by a passively mode-locked fiber oscillator at a repetition rate of 70 MHz are amplified in a 15-cm-long heavily Er-Yb codoped fiber amplifier to the average output power of 1.425 W. The pulse energy and peak power reach 20.4 nJ and 16.6 kW, respectively, while the pulse distortion is minimal in both temporal and spectral domains. Further power up-scaling is possible by using active phosphate fiber with a large mode area, in the amplifier stage

Radiative trapping effect of Yb3+ ions in lead-germanate glasses

Yb is a good candidate as a pump source because there are only two manifolds in the Yb energy level scheme, the F7/2 ground state and the F5/2 excited state, and thus concentration quenching and excited-state absorption should not affect the lasing or the excitation wavelength [1]. Extensive investigations have been of Yb-doped glasses, fibers, and crystals [2–5]. The relatively long fluorescence lifetime, as well as the broad absorption and emission bands, is more prominent for Yb-doped glasses than for crystals, making the glasses attractive for diode-pumped generation of ultra-short pulses, high-power ultra-short pulse amplification and tunable laser sources [6]. In addition, the ability to achieve high concentrations of Yb ions and low loss also triggered the rapid development of various Yb-doped glasses such as phosphate, borate, and silicate glasses. However, up to now, there are few systematic investigations of the optical properties of Yb-doped germanate glasses. In this work, the spectroscopic properties of germanate glasses with different concentrations of Yb ions and different thicknesses were investigated. Glasses with compositions (mol%) 10Na2O–60GeO2– 30PbO–xYb2O3 (x = 0.2, 0.6, 1.0, 1.4, and 2.0) were prepared using regent-grade powders Na2CO3, Pb3O4, and high purity GeO2 (>99.999%). Yb 3+ was introduced as Yb2O3 with 99.9% purity. Batches of 20 g were well mixed in appropriate proportions and melted at 1050 C in a covered alumina crucible in a SiC-resistance electric furnace. The liquids were held for 15 min and the homogenous bubble-free melts were cast on a stainless plate preheated at about 150 C. The samples were well annealed in a muffle furnace to reduce thermal stress. The annealed samples were cut using a low-speed diamond saw and polished with 5 lm diamond paste. Plates of dimensions 20 mm · 10 mm · 2 mm with two polished faces were used for optical measurements. The refractive index of the studied glass was measured using the Brewster angle method. The density of the sample was measured using the Archimedes’ liquidimmersion method on an analytical balance with a precision of 0.0001 g. Absorption spectra of the glass sample were recorded with a Perkin-Elmer-Lambda 900UV/VIS/NIR spectrophotometer in the range 850–1100 nm. Fluorescence measurements were performed with a 940 nm diode laser excitation and detected by a TRIAX550 spectrophotometer controlled by a computer. The infrared fluorescence lifetime was determined by modulating the pump laser diode drive current with a square-pulse duration of 40 ms at a repetition of 10 Hz. All the measurements were performed at room temperature. The important spectroscopic parameters required include the effective emission and absorption cross sections of the F5/2 fi F7/2 transition and the upper laser level lifetimes of Yb. The emission cross section can be determined by either the method of reciprocity or the Fuchtbauer–Ladenburg formula. On the basis of the reciprocity method described by McCumber [7], the emission cross section, remi can be calculated from the measured absorption cross section, rabs, i.e.

Amplification of ultrashort laser pulses upon stimulated Compton scattering in plasma

Stimulated Compton scattering of counterpropagating laser beams in a moving plasma is studied theoretically. It is shown that, by using Compton scattering, picosecond or femtosecond laser pulses can be amplified under certain conditions by two-three orders of magnitude.

Pulse-shaping control of spectral transformations of ultrashort pulses in photonic-crystal fibers

We experimentally demonstrate that the initial pulse width and spectral phase of ultrashort laser pulses may significantly influence the solition dynamics of such pulses in a photonic-crystal fiber. These findings suggest new solutions for the transmission, shaping, stretching, amplification, and spectral transformation of ultrashort pulses in all-fiber laser technologies.

High-power ultrafast fiber laser systems

The recent demonstration of rare-earth-doped fiber lasers with a continuous wave output power well above the kilowatt level with diffraction-limited beam quality has proven that fiber lasers constitute a power-scalable solid-state laser concept. To generate intense pulses from a fiber, several fundamental limitations have to be overcome. Nevertheless, novel experimental strategies and fiber designs offer an enormous potential toward laser systems with high average powers and high pulse energies. This paper reviews the challenges, achievements, and perspectives of ultrashort pulse generation and amplification in fibers.

Ultrafast high power fiber laser systems

Fiber laser systems offer unique properties for the amplification of ultrashort pulses to high powers. Two approaches are discussed, the amplification of linearly chirped parabolic pulses and a fiber based chirped pulse amplification system. Using the first method, we succeeded to generate 17-W average power of linearly chirped parabolic pulses at 75 MHz repetition rate and diffraction-limited beam quality in a large-mode-area ytterbium-doped fiber amplifier. The recompression of these pulses with an efficiency of 60% resulted in 80-fs pulses with a peak power of 1.7 MW. Furthermore, we report on a diode-pumped ytterbium-doped double-clad fiber based chirped pulse amplification system delivering 220-fs pulses, at 1040 nm wavelength, 73 MHz repetition rate and up to 131 W average power, corresponding to a peak power of 8 MW. Key element is a diffraction grating compressor consisting of highly efficient transmission gratings in fused silica allowing the recompression at this high power. To cite this article: J. Limpert et al., C. R. Physique 7 (2006).

Architecture of a blue high contrast multiterawatt ultrashort laser

The strategy used to develop an innovative high contrast multiterawatt femtosecond laser chain based on a hybrid (solid/gas) technology is reported. The laser system includes a Ti:sapphire oscillator generating 50 fs pulses at 950 nm, an optical parametric chirped pulse amplification stage, a second harmonic frequency converter for pulse temporal cleaning, and a final photolytical XeF(C–A) excimer amplifier for direct high peak-power amplification of ultrashort laser pulses in the blue (475 nm) spectral region. Several important issues concerning the design of the laser front-end, the energy extraction and beam phase control in the high peak-power XeF(C–A) amplifier are theoretically addressed. A detailed description of the XeF(C–A) amplifier and careful measurements of its energetic, optical and pumping characteristics are also given, together with the first pilot amplification experiments in the low density XeF(C–A) medium to assert the significance of our approach.

Thin Disk Lasers – Power scalability and beam quality

The results for cw- and q-switched operation as well as for the amplification of short (ns) and ultra-short (ps, fs) pulses demonstrate the potential of the thin disk laser design. The scaling laws for this laser design show that the power limit for cwoperation is far beyond 10 kW for a single disk and the energy limit is higher than 1 J from one disk in pulsed operation. Due to the surface cooling of the disk, the optical distortion of the laser beam is low and therefore operation of the thin disk laser is possible in fundamental mode at extremely high output power.

Tandem chirped quasi-phase-matching grating optical parametric amplifier design for simultaneous group delay and gain control

We present a broadband optical parametric amplifier design using tapered gain and tandem chirped quasi-phase-matching gratings to obtain flat gain and group-delay spectra suitable for applications such as ultrashort-pulse amplification and fiber-optic communication systems. Although a tapered-gain amplifier consisting of a single chirped grating can provide constant gain over a wide frequency range, it cannot be used to control the group delay across the spectrum. We propose controlling both the gain and the group delay profiles using a two-stage amplifier configuration, in which the idler of the first is used as the input signal of the second.

200 kHz electro-optic switch for ultrafast laser systems

A method for obtaining very high electro-optic switching rates is demonstrated and employed for regenerative amplification of ultrashort pulses at high repetition rates. The intracavity polarization switch consists of a transverse-field BBO Pockels cell and a dielectric thin film polarizer. By multiple-shift usage of a single BBO crystal as Pockels cell, it is possible to keep thermal dissipation losses in the Pockels cell driver below 200 W. Using this intracavity polarization switch in an ultrafast thin-disk Yb:KYW regenerative amplifier, pulse energies of about 9 μJ with pulse durations of 280 fs at repetition rates of 200 kHz were generated.

1.4kW high peak power generation from an all semiconductor mode-locked master oscillator power amplifier system based on eXtreme Chirped Pulse Amplification(X-CPA)

The concept of eXtreme Chirped Pulse Amplification (X-CPA) is introduced as a novel method to overcome the energy storage limit of semiconductor optical amplifiers in ultrashort pulse amplification. A colliding pulse mode-locked semiconductor laser is developed as a master oscillator and generates 600fs pulses with 6nm bandwidth at 975nm. Using a highly dispersive chirped fiber Bragg grating (1600ps/nm) as an extreme pulse stretcher and compressor, we demonstrate ~16,000 times extreme chirped pulse amplification and recompression generating optical pulses of 590fs with 1.4kW of peak power. These pulses represent, to our knowledge, the highest peak power generated from an all semiconductor ultrafast laser system.

High-power parametric amplification of 118-fs laser pulses with carrier-envelope phase control

Phase-stable parametric chirped-pulse amplification of ultrashort pulses from a carrier-envelope phase-stabilized mode-locked Ti:sapphire oscillator (11.0 fs) to 0.25 mJ/pulse at 1 kHz is demonstrated. Compression with a grating compressor and a LCD shaper yields near-Fourier-limited 11.8-fs pulses with an energy of 0.12 mJ. The amplifier is pumped by 532-nm pulses from a synchronized mode-locked laser, Nd:YAG amplifier system. This approach is shown to be promising for the next generation of ultrafast amplifiers aimed at producing terawatt-level phase-controlled few-cycle laser pulses.

Dynamics of coherent cooperative effects in extended systems under incoherent pumping

The dynamics of the pulse shape and spectra of superfluorescence, stimulated superradiance, and coherent amplification of ultrashort light pulses is studied for a system of four-level atoms under incoherent cw pumping taking into account homogeneous and inhomogeneous broadening of the luminescence lines. It is found that incoherent pumping at a high level of excitation may significantly change the shape of the emission pulse in cooperative coherent processes. Also, it is found that, depending on the ratio of parameters of the pump, nonradiative transitions, and relaxation, different regimes of superfluorescence and stimulated superradiance may be realized.

Observation of superradiant amplification of ultrashort laser pulses in a plasma.

We demonstrate the amplification of a femtosecond signal pulse in an underdense plasma by a novel mechanism called superradiant amplification. The pulse is amplified by a counterpropagating few picosecond long pump pulse. In the superradiant regime, the ponderomotive forces exceed the electrostatic forces and arrange the plasma electrons to reflect the pump light into the signal pulse. We found a significant amplification in energy and intensity. The time structure of the amplified signal pulse carries intrinsic features of the superradiant regime. Sub-10-fs pulses of petawatt power appear feasible.

Design of a sub-13-fs, multi-gigawatt chirped pulse optical parametric amplification system

We present a design for phase-locked chirped pulse optical parametric amplification of ultra-short pulses based on Ti:sapphire. A realistic description is given by measuring the oscillator pulse (11.6 fs, 4 nJ) with SPIDER and numerically propagating it through the whole chirped pulse amplification system. The interaction is modeled with a full three-dimensional code and compression is ray-trace optimized to yield 12.7-fs, 98-μJ pulses with 1 mJ of pump energy. The design is scalable in energy (e.g. 1 mJ with 10-mJ pump) and is exclusively based on commercially available components .

Application of periodically poled stoichiometric LiTaO3 for efficient optical parametric chirped pulse amplification at 1 kHz

We demonstrate optical parametric chirped pulse amplification (OPCPA) at 1-kHz repetition rate in periodically poled stoichiometric LiTaO3 (PPSLT) with 1 mol % MgO doping. Diode pumping was used both for the fiber laser generating the femtosecond seed pulses and the nanosecond laser/amplifier employed as a pump source. The high gain ( approximately 63 dB) and large bandwidth (20 nm) obtained for single-stage nondegenerate OPCPA operation provide a compact and efficient solution for amplification of ultrashort pulses near 1.57 microm. Stretched femtosecond pulses could be amplified up to 39.5 microJ in a 7-mm long, 2-mm thick sample of PPSLT. The pulse duration of the amplified signal pulses (FWHM) after recompression amounted to 315 fs.

Direct amplification of picosecond pulses in neodymium glass with a power density above 100 GW cm-2

A scheme for amplification of ultrashort laser pulses is studied, which is used in experiments on symmetrisation of ablation pressure with the help of a prepulse upon acceleration of foils by laser radiation of high brightness. The possibility of direct amplification of short pulses before their expansion in order to increase the energy contrast is considered. In experiments performed on the PICO facility, the amplification of a 10-ps pulse with a power density exceeding 100 GW cm-2 is demonstrated with the gain equal to 1.2 and the inversion drop above 30 %.