Semilinear coherent oscillator with reflection-type photorefractive gratings

The saturation intensity, oscillation spectrum, and temporal dynamics of oscillation are studied for a semilinear coherent oscillator with two counterpropagating pump waves and a photorefractive crystal with dominating reflection gratings. The instability of the single-frequency oscillation spectrum is revealed, similar to that known for an oscillator with dominating transmission gratings. The experimental manifestation of this transition in the output characteristics of the oscillator is favourably compared with numerical calculations. PACS 42.65.Hw; 05.45.-a; 42.65.Pc; 42.65.Sf     

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     

Measurement and calculation of nonlinear absorption associated with femtosecond filaments in water

We investigate the filamentation dynamics of 200 fs, 527 nm laser pulses in water. By comparing the experimental results with the numerical simulations that use an extended propagation model, the influence of several physical effects, particularly nonlinear losses and free electron plasma generation, is studied. It is shown that a set of relevant numerical values, related to multiphoton absorption, can be extracted with reasonable accuracy. PACS 42.65.Jx; 42.25.Bs     

Time-resolved measurements of high order harmonics confined by polarization gating

We investigate the temporal confinement of high order harmonic pulses generated by a femtosecond (fs) infrared (IR) pulse with a time varying polarization. We use a set of two birefringent quartz plates to modulate the IR polarization. It produces a short temporal gate of linear polarization where harmonics are efficiently generated during a small fraction of the IR pulse. By rotating one of the plates, the gate width can be continuously varied between 70 fs down to 7 fs. The XUV pulse duration is measured by cross-correlation with a probe IR pulse of 12 fs. When the gate width is decreased, a clear temporal confinement of the XUV emission is observed through the cross correlation signal. This experiment is the first direct experimental evidence in the temporal domain that the polarization gating technique can be used to significantly shorten the harmonic pulse duration. PACS 32.80.Wr; 42.65.Ky; 42.65.Re     

Characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We present results of the characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO₄)₂ using a z-scan technique over the 800–1600 nm wavelength range. Based on our experimental and theoretical results we conclude that Yb:KGW crystal is a good candidate for efficient Kerr-lens mode locking. PACS 42.65.An; 42.65.Hw; 42.55.Rz; 42.65.Re     

Optical parametric amplifier pumped by two mutually incoherent laser beams

We report on the experimental proof-of-principle demonstration of the ultrashort pulse single-pass beta-barium borate, BBO optical parametric amplifier pumped by two mutually incoherent laser sources. We show that the amplified signal at 1054 nm gains energy from both pump pulses with wavelengths of 680 and 527 nm, respectively, with overall energy conversion of 36%, and exhibits low wavefront distortions and improved energy stability in the gain saturation regime. PACS 42.65.Yj; 42.65.Re     

Characterization of periodically poled LiTaO3 crystals by means of spontaneous parametric down-conversion

We describe the observation of quasi-phase-matched spontaneous parametric down-conversion in eee-geometry in periodically poled LiTaO₃ crystals. For scattered light, the two-dimensional angular-frequency intensity distribution was studied. Several detuning curves were recorded, corresponding to high orders m=-2, -3, -4, -5 of quasi-phase matching. The measured periods of domain gratings agree with the data obtained by atomic-force microscopy for the etched crystal surfaces. The presence of both odd and even orders indicates that the lengths of positive and negative domains are unequal. To determine the mean duty cycle for regular domain gratings we propose comparing of the intensities of spontaneous parametric down-conversion in different orders of quasi-phase-matching. PACS 42.65.Lm; 42.70.Mp; 81.70.Fy; 78.67.Pt; 42.65.Yj     

UV–Supercontinuum generated by femtosecond pulse filamentation in air: Meter-range experiments versus numerical simulations

We report new experimental and numerical results on supercontinuum generation at ultraviolet/visible wavelengths produced by the propagation of infrared femtosecond laser pulses in air. Spectral broadening is shown to similarly affect single filaments over laboratory distance scales, as well as broad beams over long-range propagation distances. Numerical simulations display evidence of the crucial role of third harmonic generation in the build-up of UV–visible wavelengths, by comparison with current single-envelope models including chromatic dispersion and self-steepening. PACS 52.38.Hb; 42.65.Tg; 42.65.Jx; 42.68.Ay     

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     

Phase diagrams for semilinear photorefractive coherent oscillator

The sequence of optical phase transitions is considered for a photorefractive coherent oscillator with a conventional mirror and a four-wave mixing phase conjugate mirror. Analogous to phase transitions in solids, the phase diagrams are constructed and defined qualitatively as different states of a whole system (light-induced scattering below the threshold of oscillation, single-frequency oscillation, two-frequency oscillation, and multimode mirrorless oscillation). The attempt of experimental mapping of a phase diagram is presented for a coherent oscillator with Co-doped BaTiO₃. PACS 42.65.Hw; 05.45.-a; 42.65.Pc; 42.65.Sf.     

Experimental investigation and numerical simulation of the spatio-temporal dynamics of the light-pulses in nanosecond optical parametric oscillators

This paper reports on the experimental investigation and the numerical simulation of the spatio-temporal dynamics of the pulse formation in nanosecond optical parametric oscillators. The temporal evolution of the spatial intensity distribution is measured with a fast two-dimensional CCD-camera. The measurements are performed for a signal-resonant nanosecond pulsed optical-parametric-oscillator (OPO) which consists of a 12 mm long, critically phase-matched beta-barium-borate (BBO)-crystal in a 40 mm long cavity of two plane mirrors. The OPO was pumped by the third harmonic of a flashlamp-pumped Nd:YAG-Laser at a repetition rate of 10 Hz. At pump energies close to the OPO threshold the emitted OPO-radiation has an almost gaussian intensity distribution throughout the entire pulse. The beam quality factor M² remains below 2.2. At high pump energies the OPO oscillation also starts with an almost gaussian beam-profile. During the build-up and parametric amplification of the pulse the fields experience, however, a spatially inhomogeneous gain, caused by walk-off in the birefringent crystal, pump-depletion, and back-conversion. The spatial intensity distribution thus becomes asymmetric and the M² value increases. The measurements are compared with the results of detailed numerical calculations. The model takes the amplification of the OPO-radiation in the nonlinear crystal, and the properties of the OPO cavity as well as the diffraction of the beams during propagation into account. The spatio-temporal dynamics of the pulse formation predicted by the numerical model are in good agreement with the experimental results. PACS 42.65.Sf; 42.65.Yj     

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     

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     

Chirp-controlled soliton fission in tapered optical fibers

Manipulation of the input pulse chirp during supercontinuum generation in tapered fibers provides precise control of the soliton fission process taking place in the taper waist. Simulations and experiments demonstrate that controlling this pre-chirp by a pulse shaper can be applied to compensate for deleterious effects due to untapered fiber pigtails. Temporal and cross-correlation frequency resolved measurements are utilized to show, that the control of the soliton fission dynamics can be obtained by manipulating the input pulse chirp through the imposition of a quadratic spectral phase. PACS 42.65.Wi; 42.65.Tg; 42.81.Qb; 42.81.DP     

Generation of attosecond pulses with ellipticity-modulated fundamental

We have studied experimentally and theoretically high-order harmonic generation using a laser field with a time-dependent ellipticity. We show that the harmonic emission can be confined into a narrow temporal window, in which the fundamental polarization is quasi-linear. This allows a single attosecond pulse (200 as) with a fundamental field obtained from 10 fs pulse to be generated. PACS 42.65.Ky; 42.65.Re; 32.80.Wr     

Hollow-fiber pulse compressor for KrF lasers

Application of the hollow-fiber pulse compression technique to 100 fs DUV pulses at 248 nm is reported. Sub-20 fs pulses with up to 20 μJ energy were obtained by using krypton in a 65 cm long fiber of 138 μm core radius. The physical process of the spectral broadening is modeled by a nonlinear propagation equation. The experimental data is in good agreement with the results of the model calculations. PACS 42.65.Re; 42.65.Jx; 42.65.-k     

Frequency doubling of multi-terawatt femtosecond pulses

We present an experimental and theoretical study of the influence of the spatial beam quality (fluence and phase distributions) on the second-harmonic generation in KDP crystals pumped by 180-fs pulses at 790 nm. Conversion efficiency and beam focusability are investigated experimentally and theoretically by the numerical analysis of the second harmonic, considering effects due to the cubic nonlinearity, beam diffraction, group-velocity walk-off, and dispersion of the pulses. It was found that the uniform intensity and phase distributions of the fundamental beam are essential to obtain a high focal intensity of the second-harmonic beam. PACS 42.65.Ky; 42.65.Re     

Discrete and dipole-mode gap solitons in higher-order nonlinear photonic lattices

We investigate the formation of fundamental discrete solitons and dipole-mode gap solitons in triangular photonic lattices imprinted in photorefractive nonlinear media. These lattices are strongly affected by the photorefractive anisotropy, resulting in orientation-dependent refractive index structures with reduced symmetry. It is demonstrated that two different orientations of the lattice wave enable the formation of fundamental discrete solitons in the total internal reflection gap. Furthermore, it is shown that one lattice orientation additionally supports dipole-mode solitons in the Bragg reflection gap. The experimental results are corroborated by numerical simulations using the full anisotropic model. PACS 42.65.Tg; 42.65.Wi; 42.70.Qs     

Highly efficient temporal cleaner for femtosecond pulses based on cross-polarized wave generation in a dual crystal scheme

We present an experimental demonstration of a renewed set up, with respect to the one described in [A. Jullien et al, Opt. Lett. 30, 920 (2005)], that enables more reliable and robust performances in the increase of the contrast ratio (CR) of energetic femtosecond pulses. The new approach is based on the use of two successive crystals situated at optimum position that generate cross polarized waves whose individual effect interferes constructively. This arrangement overcomes the limitation of the single crystal schemes for temporal cleaning – the early saturation of the transmission efficiency, previously observed when either using a relatively thick crystal or increasing intensities up to its damage threshold. A theoretical model that predicts the output CR is developed for the first time. It shows that the CR depends on the initial CR and the extinction ratio of the polarizers used. The measured temporal CR 10^-10 is in accordance with theoretical predictions. PACS 42.65.-k; 42.65.Re; 42.65.Jx     

Soliton complexes and flat-top nonlinear modes in optical lattices

We describe a continuous analog of the quasirectangular flat-top nonlinear modes earlier found for discrete nonlinear models. We show that these novel nonlinear modes can be understood as multi-soliton complexes with either in-phase or out-of-phase neighboring solitons trapped by the periodic potential of the lattice. We demonstrate a link between the flat-top states and the truncated nonlinear Bloch waves, and discuss how these nonlinear localized modes can be monitored experimentally in photonics and Bose–Einstein condensates. PACS 42.65.Tg; 42.65.Jx; 03.75.Lm