Continuum generation of the third-harmonic pulse generated by an intense femtosecond IR laser pulse in air

The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV. PACS 42.65.Jx; 42.65.Ky; 52.35.Mw     

Filamentation nonlinear optics

A filamenting femtosecond laser pulse self-stabilizes the intensity fluctuation inside the filament core due to intensity clamping and generates an excellent spatial beam quality inside the core due to self-spatial filtering. The high quality of the core can be sampled by nonlinear processes. A few experimental examples are shown: self-phase modulation, four-wave mixing, third-harmonic generation and waveguide writing in glass. PACS 42.65.Jx; 52.35.Mw     

Ultrabroadband continuum generated in air (down to 230 nm) using ultrashort and intense laser pulses

We present results on supercontinuum generation extended up to 230 nm in air during the propagation of a powerful femtosecond laser pulse. The broad supercontinuum generated in air is contributed by self-phase modulation and self-steepening of the fundamental laser pulse, the third-harmonic pulse and their interaction. In particular, the strong interaction between the fundamental and the third-harmonic pulses leads to broad and efficient continuum generation of the third-harmonic pulse itself. The spectrum of the third-harmonic generated in air extends over several tens of nm and overlaps with the shorter wavelength extent of the fundamental continuum. PACS 42.65.Ky; 42.65.Jx; 52.35.Mw     

Third-harmonic generation and self-channeling in air using high-power femtosecond laser pulses

It is shown, both theoretically and experimentally, that during laser pulse filamentation in air an intense ultrashort third-harmonic pulse is generated forming a two-colored filament. The third-harmonic pulse maintains both its peak intensity and energy over distances much longer than the characteristic coherence length. We argue that this is due to a nonlinear phase-locking mechanism between the two pulses in the filament and is independent of the initial material wave-vector mismatch. A rich spatiotemporal propagation dynamics of the third-harmonic pulse is predicted. Potential applications of this phenomenon to other parametric processes are discussed.     

Spatial optimization of filaments

We demonstrate the spatial homogenization of intense laser pulses by adaptive minimization of spatial chirp of the spectrally broadened output pulses of a filament. A liquid-crystal-based two-dimensional spatial light modulator is used to control the spatial phase of the driver pulse. An evolutionary algorithm finds the optimal spatial laser phase distribution that introduces minimal distortions during filamentation and enhances the beam quality of the output pulse. A homogeneous intensity distribution favours efficient temporal compression close to the bandwidth limit without the need for spatial filtering after the filament. PACS 42.65.-k; 42.65.Re; 41.85.Ct     

Self-transformation of a powerful femtosecond laser pulse into a white-light laser pulse in bulk optical media (or supercontinuum generation)

We present experimental and theoretical results on white-light generation in the filamentation of a high-power femtosecond laser pulse in water and atmospheric air. We have shown that the high spatio-temporal localization of the light field in the filament, which enables the supercontinuum generation, is sustained due to the dynamic transformation of the light field on the whole transverse scale of the beam, including its edges. We found that the sources of the supercontinuum blue wing are in the rings, surrounding the filament, as well as at the back of the pulse, where shock-wave formation enhanced by self-steepening takes place. We report on the first observation and demonstration of the interference of the supercontinuum spectral components arising in the course of multiple filamentation in a terawatt laser pulse. We demonstrate that the conversion efficiency of an initially narrow laser pulse spectrum into the supercontinuum depends on the length of the filament with high intensity gradients and can be increased by introducing an initial chirp. PACS 42.65.Jx; 42.65.Re; 42.25.Bs; 42.50.Hz     

Observation of filamentation-induced continuous self-frequency down shift in air

We observed a new filamentation nonlinear process: continuous self-frequency down shift inside the filament zone during the propagation of a femtosecond laser pulse in air. The frequency shift depends strongly on the length of the self-guided column (filament). PACS 42.65.Dr; 42.65.Jx; 52.35.Mw     

Delayed pulse dynamics in single mode class B lasers

Single slow–fast intensity pulses are generated by quickly increasing the pump from a below to an above threshold value during a finite time interval. Under particular conditions, a pulse appears after the pump perturbation has ended and with a significant delay. We demonstrate that this delayed pulse is not a classical turn-on pulse and that it verifies unusual properties. Experimental and numerical observations of the amplitude and the delay of these pulses are compared quantitatively and indicate that they emerge from zero at a bifurcation point. PACS 42.60.Mi; 42.65.Sf     

Role of the carrier-envelope phase in laser filamentation

We numerically study the influence of the initial carrier-envelope phase (CEP) on the filamentation of ultrashort laser pulses in noble gas. Emphasis is put on the CEP-induced changes of pulses that reach their clamping intensity during near-cycle self-compression. In other propagation regimes, the CEP does not significantly alter the pulse evolution. Our results indicate that third-harmonic generation, compared to plasma generation, is dominant in driving these changes. Finally, the stability of the filament CEP against shot-to-shot fluctuations is examined.     

Enhanced harmonic conversion efficiency in the self-guided propagation of femtosecond ultraviolet laser pulses in argon

Harmonic generation during the self-guided propagation of femtosecond ultraviolet (UV) laser pulses (248-nm, 450-fs) in argon is investigated. The third (82.7-nm) and fifth (49.6-nm) harmonics are generated in the UV filament. The energy-conversion efficiencies for the harmonics are found to be at least two orders of magnitude higher than those reported in the literature for similar gas pressures. The enhancement is attributed to the quasi-phase matching of the harmonics due to the self-guiding of the driving pulse. PACS 42.25.Bs; 42.65.Jx; 42.65.Re; 42.65.Ky     

Third-harmonic generation in relative-phase-controlled two-color laser field

We experimentally demonstrate third-harmonic generation (THG) by intense femtosecond laser pulse at a central wavelength of ??800?nm superposed by its second harmonic in air. The third-harmonic signal shows a periodic modulation with a period of ??0.67?fs when the delay between fundamental and second-harmonic wave is continuously changed. The periodic modulation of THG can be attributed to the interference of third-harmonic signals generated from a direct THG channel (3??=??+??+??) and a four-wave mixing (FWM) channel (3??=2??+2?????). With high pump intensity, the fitting of the measured TH spectrum as a function of delay implies that the pump pulse undergoes a strong pulse splitting and self-phase modulation at the focus.     

Long-range spectrally and spatially resolved radiation from filaments in air

The distance-resolved spectral intensity distribution of the backscattered light from long filaments generated in air using ultra-short and intense laser pulses is presented. A clean fluorescence spectrum from N₂ molecules and ions, which is produced by the high peak intensity inside the plasma filament of the fundamental pulse, was clearly resolved from the backscattered supercontinuum. The supercontinuum generated by both the fundamental and the third-harmonic pulses developed progressively and became fully developed only at the end of the filamentation.     

Experimental investigation of the spectro-temporal dynamics of the light pulses of Q-switched Nd:YAG lasers and nanosecond optical parametric oscillators

We report an experimental investigation of the spectro-temporal dynamics of the pulse formation in Q-switched Nd:YAG lasers and in nanosecond optical parametric oscillators (OPOs). The temporal evolution of the spectral intensity distribution of the light pulses was measured with a 1-m Czerny–Turner spectrometer in combination with a fast streak camera. This detection system allows the analysis of temporal changes in the spectrum of single nanosecond pulses. The measurements were performed for a flashlamp-pumped, Q-switched Nd:YAG laser and for an unseeded as well as for a seeded singly-resonant nanosecond OPO. The laser output spectrum varies strongly from pulse to pulse and even within a single pulse due to mode beating. In an unseeded OPO, individual spectral modes start to oscillate statistically from the parametric noise for pump powers close to the OPO threshold. With increasing pump power a strong modulation in the spectral formation of the pulse is observed, resulting from a strong interaction of parametric conversion and back conversion of signal and idler radiation into pump radiation. By means of injection seeding, the starting condition was controlled for a single mode. Due to the seed radiation, the seeded mode starts sooner than the unseeded modes. These are suppressed completely in the case of sufficient seed power and moderate pump power. The observations are in good agreement with results of corresponding numerical simulations. PACS 42.65.Sf, 42.65.Yj     

A subnanosecond pulsed laser-source for mid-infrared LIDAR

A new subnanosecond pulsed laser-source for mid-infrared (MIR) LIDARbased on difference-frequency generation (DFG) in periodically poled LiNbO₃ (PPLN) is developed. An output pulse energy of up to 36 pJ is achieved by amplification of the pump and signal laser. A pulse duration of Δτ=538 ps has been measured using silver thiogallate as a non-linear device for an upconversion cross correlation experiment. The mid-infrared pulse duration corresponds to a spatial resolution of Δs≈8 cm in the LIDAR system. PACS 07.07.Df; 42.60.Da; 42.65.Ky     

Two-photon-absorption of frequency converter crystals at 248 nm

The two-photon-absorption coefficient of KDP, BBO, LTB, and CLBO crystals has been determined from the measurement of the intensity dependent transmission through long samples. The intensity of the sub-picosecond KrF excimer laser pulses on the samples was varied from 0.2–80 GW/cm². The linear absorption of the samples was determined by using a low intensity, long pulse KrF laser. The first-principle simulations to the experimental data show a TPA value of 0.48 cm/GW for KDP, 0.5 cm/GW (o-ray) and 0.34 cm/GW (e-ray) in BBO, 0.22 cm/GW in LTB and 0.53 cm/GW in CLBO. PACS 78.20.Ci; 42.65.Ky; 42.65.Yj     

Computer simulation of laser pulse filament generation in rain

Initiation of femtosecond laser pulse filamentation in rain conditions is investigated numerically. It is shown that coherent scattering on rain water particles generates filament initiation sites. Results of computer simulation are consistent with full-scale experimental data. PACS 42.65.Jx; 42.68.Jg; 42.60.Jf     

Generation of flat-top waveform in the time domain based on stimulated Brillouin scattering

A method of generating a flat-top waveform in the time domain based on stimulated Brillouin scattering (SBS) is proposed. The transmitted pulses are simulated with pump wavelength at 1064 nm and 532 nm, respectively, and validated in the experiment performed with the Nd:YAG seed-injected laser. The experimental results agree well with the theoretical simulation. With 532 nm pump wavelength, the top of the transmitted pulse is almost a platform, while there is a peak in the front and a platform in the back with 1064 nm pump wavelength. The mechanism behind the generation of flat-top waveform with 532 nm pump wavelength is analyzed in details. PACS 42.65-k; 42.65.Es; 42.65.Hw     

InGaAsP quantum-wells saturable absorber for diode-pumped passively Q-switched 1.3-μm lasers

We demonstrate the first use of InGaAsP quantum wells as a saturable absorber in the Q-switching of a diode-pumped Nd-doped 1.3 μm laser. The barrier layers of the InGaAsP quantum-well device are designed to be a strong absorber for the suppression of the transition channel at 1.06 μm. With an incident pump power of 1.8 W, an average output power of 160 mW with a Q-switched pulse width of 19 ns at a pulse repetition rate of 38 kHz was obtained. PACS 42.60.Gd; 42.55.Xi; 42.65.Re     

Effect of pulse slippage on density transition-based resonant third-harmonic generation of short-pulse laser in plasma

The resonant third-harmonic generation of a self-focusing laser in plasma with a density transition was investigated. Because of self-focusing of the fundamental laser pulse, a transverse intensity gradient was created, which generated a plasma wave at the fundamental wave frequency. Phase matching was satisfied by using a Wiggler magnetic field, which provided additional angular momentum to the third-harmonic photon to make the process resonant. An enhancement was observed in the resonant third-harmonic generation of an intense short-pulse laser in plasma embedded with a magnetic Wiggler with a density transition. A plasma density ramp played an important role in the self-focusing, enhancing the third-harmonic generation in plasma. We also examined the effect of the Wiggler magnetic field on the pulse slippage of the third-harmonic pulse in plasma. The pulse slippage was due to the group-velocity mismatch between the fundamental and third-harmonic pulses.     

Polarization-sensitive non-3ω third-harmonic generation by femtosecond Cr: Forsterite laser pulses in birefringent microchannel waveguides of photonic-crystal fibers

Femtosecond Cr: forsterite laser pulses coupled into small-diameter birefringent channel waveguides off the central core of a photonic-crystal fiber are shown to generate multiple narrowband spectral peaks within the 380–460 nm wavelength region through multimode-phase-matched third-harmonic generation. Some of these peaks are shifted by tens of terahertz from the tripled frequency of the pump field, dictated by standard energy conservation for third-harmonic generation in monochromatic fields. The spectral contents of the third-harmonic signal generated in such a regime are controlled by changing polarization and the intensity of the input pump field.