Pulse self-compression in normally dispersive bulk media

We experimentally demonstrate that high-power femtosecond pulses can be compressed during the nonlinear propagation in the normally dispersive solid bulk medium. The self-compression behavior was detailedly investigated under a variety of experimental conditions, and the temporal and spectral characteristics of resulted pulses were found to be significantly affected by the input pulse intensity, with higher intensity corresponding to shorter compressed pulses. By passing through a piece of BK7 glass, a self-compression from 50 to 20 fs was achieved, with a compression factor of about 2.5. However, the output pulse was observed to be split into two peaks when the input intensity is high enough to generate supercontinuum and conical emission.     

Multi-stage pulse compression by use of cascaded quadratic nonlinearity

In this paper, the multi-stage compression of picosecond pulses by cascaded quadratic nonlinearity is studied theoretically, and the dependence of pulse compression on phase-mismatch, laser intensity, and crystal characteristics has been discussed in detail. We demonstrate that the multi-stage pulse compression is much more efficient than the single-stage with a same total crystal length. Pulses as short as ∼150 fs can be generated by compressing 30-ps initial pulses in a two-stage configuration under the realistic crystal and laser conditions, and shorter pulses of ∼30 fs may be obtained by three-stage compression. Pulse compression performances with BiBO and BBO crystals are compared and discussed finally.     

Detection of scattered light pulses at femto-Joule level by using a picosecond BBO optical parametric amplifier

We report a technique for detecting weak scattered light pulses based on a 532 nm pumped picosecond β-barium-borate collinear optical parametric amplifier. The measured maximum slope gain factor of the amplifier was found to be around 10⁷, and the energy detection limit was of the order of fJ/pulse for the signal of 730 nm and the idler of 1.5 μm at a pumping intensity of 2.83 GW/cm². The linearity of the gain for this amplifier was found to be excellent for a seeding level of lower than 420 fJ per pulse. The maximum gains and the energy detection limits for the scattered light pulses from various scattering targets were found to be poorer than that from the reflective mirror, owing to the degraded beam quality and the depolarization of the scattered light. A reduction of the maximum gain for the scattered light with the increase of the angle of incidence, which causes pulse broadening and reduces the photon flux of the signal, was investigated. The feasibility of detecting weak scattered light in the infrared by using idler-to-signal frequency up-conversion was also demonstrated, in which the infrared seeder located in the idler branch of the amplifier was injected as the seeding beam and was then parametrically up-converted into the visible signal branch, with an even higher gain.     

Manifestations of sub- and superluminality in filamented femtosecond laser pulse in fused silica

Filamented propagation of femtosecond laser pulses in fused silica is studied using microscopy of Femtosecond Time-resolved Optical Polarigraphy (FTOP). Both super- and subluminal translation of axial intensity maxima in a 100 fs time scale has been directly recorded. The observed spatio-temporal periodicity of the pulse shape in longitudinal and transversal directions is tentatively interpreted as two projections on X and Y axes of the same fringe pattern formed by the interference of the axial part of the beam and spontaneously generated conical beam.     

Cascaded compression of the first and second Stokes pulses during forward transient stimulated Raman amplification

The results of numerical modelling of cascaded compression of the first and second Stokes pulses during regenerative regime of the forward transient stimulated Raman amplification are presented for the case when the walk-off length of the first Stokes pulse due to group velocity mismatch is shorter than the length of the nonlinear medium. The influence of the initial amplitudes of the seed first Stokes pulses, its durations and its time delay with respect to the pump pulse, the Kerr nonlinearity of the medium on the conversion efficiency, duration and propagation factor M² of the first and second Stokes pulse are studied. It is demonstrated that for the pump pulse duration of 1 ps the duration of the compressed second Stokes pulses in a KGW crystal near the beam axis may be approximately 14 times shorter than the pump pulse duration. It is shown that the propagation factor of the compressed pulses increases significantly because of complex spatial-temporal dynamics of compression and the influence of Kerr nonlinearity of Raman medium.     

Effects of electron relaxation on multiple harmonic generation from metal surfaces with femtosecond laser pulses

Calculations are presented for the first four (odd and even) harmonics of an 800 nm laser from a gold surface, with pulse widths ranging from 100 down to 14 fs. For peak laser intensities above 1 GW/cm² the harmonics are enhanced because of a partial depletion of the initial electron states. At 10¹¹ W/cm² of peak laser intensity the calculated conversion efficiency for 2nd-harmonic generation is 3 × 10⁻⁹, while for the 5th-harmonic it is 10⁻¹⁰. The generated harmonic pulses are broadened and delayed relative to the laser pulse because of the finite relaxation times of the excited electronic states. The finite electron relaxation times cause also the broadening of the autocorrelations of the laser pulses obtained from surface harmonic generation by two time-delayed identical pulses. Comparison with recent experimental results shows that the response time of an autocorrelator using nonlinear optical processes in a gold surface is shorter than the electron relaxation times. This seems to indicate that for laser pulses shorter than ∼30 fs, the fast nonresonant channel for multiphoton excitation via continuum-continuum transitions in metals becomes important as the resonant channel becomes slow (relative to the laser pulse) and less efficient.     

High-peak power multi-wavelength picosecond pulses generated from a BaWO4 Raman-seeded optical parametric amplifier

We report the generation of high-peak power multi-wavelength picosecond laser pulses using optical parametric amplification (OPA) in BBO seeded with pulses generated in a 5-mm length BaWO₄ crystal by stimulated Raman scattering of 18-ps laser pulses at 532 nm. The maximum output energy of the amplified first-Stokes component at 559.7 nm was about 1.76 mJ. The corresponding maximum peak power, pulse duration and spectral line width were measured to be 117.3 MW, 15 ps and 18.0 cm⁻¹, respectively. The multi-wavelength picosecond laser pulses were in the visible and near infrared ranges. Using this Raman-seeded OPA technique, the beam quality of the stimulated Raman scattering pulses can be improved.     

SOA-based actively mode-locked fiber ring laser by forward injecting an external pulse train

An actively mode-locked fiber ring laser based on cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) is demonstrated to operate stably with a simple configuration. By forward injecting an easily-generated external pulse train, the mode-locked fiber laser can generate an optical-pulse sequence with pulsewidth about 6 ps and average output power about 7.9 mW. The output pulses show an ultra-low RMS jitter about 70.7 fs measured by a RF spectrum analyzer. The use of the proposed forward-injection configuration can realize the repetition-rate tunability from 1 to 15 GHz for the generated optical-pulse sequences. By employing a wavelength-tunable optical band-pass filter in the laser cavity, the operation wavelength of the designed SOA-based actively mode-locked fiber laser can be tuned continuously in a wide span between 1528 and 1565 nm. The parameters of external-injection optical pulses are studied experimentally to optimize the mode-locked fiber laser.     

Elimination of cracking during UV laser ablation of SrTiO3 single crystals by employing a femtosecond laser

We have performed a comparative study of UV laser ablation of SrTiO₃ with nanosecond- and sub-picosecond sources, respectively. The experiments were performed with lasers at a wavelength of 248 nm and pulse durations of 34 ns and 500 fs. Femtosecond ablation turns out to be more efficient by one order of magnitude and eliminated the known problem of cracking of SrTiO₃ during laser machining with longer pulses. In addition, the cavities ablated with femtosecond pulses display a smoother surface with no indication of melting and well-defined, sharp edges. These effects can be explained by the reduced thermal shock effect on the material by using ultrashort pulses.     

High-flux high harmonic soft X-ray generation up to 10 kHz repetition rate

We report the operation of a Ti:sapphire oscillator-amplifier system with a high, variable repetition rate adjustable between 1 and 15 kHz. The oscillator uses cavity dumping and the multipass amplifier is based on a liquid nitrogen cooled crystal. The system produces pulses with 28 fs duration at 1.1 mJ pulse energy. When pumping the amplifier crystal with 72 W, an average output power of 11 W is obtained at a repetition rate of 10 kHz, resulting in a quantum efficiency of 25%. The output pulses are used to generate high harmonic radiation in argon, neon, and helium, which are detected up to a photon energy of 110 eV, limited by the sensitivity of the toroidal grating employed.     

Nonlinear optical absorption studies of an organo-metallic complex by Z-scan technique

An organo-metallic complex, [(CH₃)₄N][Ni(dmit)₂] (dmit²⁻ = (1,3-dithiole-2-thione-4,5-dithiolate), abbreviated as MeNi, is synthesized. The nonlinear optical absorption properties of MeNi dissolved in acetone have been studied using the open-aperture Z-scan technique with 40 ps pulse width at 1064 nm and 1 ns, 15 ns pulse width at 1053 nm, respectively. Strong saturable absorption has been found when the sample solution is irradiated by 40 ps and 1 ns laser pulses. While irradiated with 15 ns laser pulse, a stronger reverse saturable absorption has been found. The nonlinear optical absorption coefficients are −1.03 × 10⁻¹¹ m/W, −1.85 × 10⁻¹¹ m/W and 3.84 × 10⁻¹⁰ m/W, respectively. The mechanism responsible for the difference between the results is analyzed. All the results suggest that this material may be a promising candidate for the application to laser pulse compression in the near-infrared waveband.     

Generation of fs laser pulses from a ps pulse-pumped optical parametric amplifier with a beat-wave seed signal

We describe a method of ultrashort-pulse and ultrafast-pulse-train generation through optical parametric amplification of a laser beat wave. Numerical simulation shows that 250-fs laser pulses at 1.55 μm are generated from a beat-wave seeded optical parametric amplifier pumped by a 30-ps laser at 1064 nm. The pulse compression is attributable to sideband generation and parametric amplification under group velocity mismatch. Our experimental result confirms efficient generation of comb-like sidebands for the mixing waves from such an optical parametric amplifier.     

14-fs high temporal quality injector for ultra-high intensity laser

We present a chirped pulse amplification (CPA) Ti:Sa laser generating sub-15 fs pulses with expected high temporal quality. Gain-narrowing in the pre-amplifier is balanced by a variable spectral reflectivity mirror and by a fine adaptation of the saturation conditions. A crossed polarized wave generation (XPW) filter is introduced to enhance the contrast, reduce the pulse duration and improve the spectral quality. The pulses are generated at 10 Hz repetition rate, with pulse energy of 110 μJ and very clean Gaussian spectrum. The temporal contrast is evaluated by a measurement before the XPW filter and calculations of the enhancement by the filter. The potential temporal incoherent contrast is 10¹² and the coherent contrast 10¹⁰. The performance of the system makes it suitable as an injector for petawatt lasers operating in the double-CPA scheme.     

Efficient multi-frequency generation of ultrashort light pulses using stimulated Raman scattering and optical parametric amplification

Optical parametric amplification of multi-frequency seed pulses generated in a mixture of compressed hydrogen and methane by stimulated Raman scattering of 1 ps, 1 kHz laser pulses at 395.8 nm has been studied. Efficient generation of spectrally narrow ultrashort pulses with a spatial distribution close to the Gaussian profile of the pump beam was obtained in the visible and near infrared ranges.     

Active phase control and frequency chirp effects on supercontinuum generation in high birefringence photonic crystal fiber

An acoustic-optics programmable dispersive filter (AOPDF) was first employed to actively control the linearly polarized femtosecond pump pulse frequency chirp for supercontinuum (SC) generation in a high birefringence photonic crystal fiber (PCF). By accurately controlling the second order phase distortion and polarization direction of incident pulses, the output SC spectrum can be tuned to various spectral energy distributions and bandwidths. The pump pulse energy and bandwidth are preserved in our experiment. It is found that SC with broader bandwidth can be generated with positive chirped pump pulses except when the chirp value is larger than the optimal value, and the same optimal value exists for the pump pulses polarized along the two principal axes. With optimal positive chirp, more than 78% of the pump energy can be transferred to below 750 nm. Otherwise, negative chirp will weaken the blue-shift broadening and the SC bandwidth.     

Greater signal, increased depth, and less photobleaching in two-photon microscopy with 10 fs pulses

The fundamental advantages to using ultrafast (?100 fs) laser pulses in two-photon microscopy for biomedical imaging are seldom realized due to chromatic dispersion introduced by the required high numerical aperture microscope objective. Dispersion is eliminated here by using the multiphoton intrapulse interference phase scan (MIIPS) method on pulses with a bandwidth greater than 100 nm full width at half maximum. Higher fluorescence intensity, deeper sample penetration, and improved signal-to-noise ratio are demonstrated quantitatively and qualitatively. Due to the higher signal intensity obtained after MIIPS compensation, lower laser power is required, which decreases photobleaching. The observed advantages are not realized if group delay dispersion is compensated for while higher-order dispersion is not. By using a pulse shaper and taking advantage of the broad spectrum of the ultrafast laser, selective excitation of different cell organelles is achieved due to the difference in nonlinear optical susceptibility of different chromophores without requiring an emission filter wheel. Experiments on biological specimens, such as HeLa cells and mouse kidney tissue samples, illustrate the advantages to using sub-10 fs pulses with MIIPS compensation in the field of two-photon microscopy for biomedical imaging.     

Broadband four-wave optical parametric amplification in bulk isotropic media in the ultraviolet

We report on four-wave optical parametric amplification of the ultrashort ultraviolet light pulses in bulk fused silica and CaF₂. Exact phase-matching in these isotropic media is achieved by means of non-collinear interaction with cylindrical beam focusing. Four-wave optical parametric amplifier efficiently operates in the UV spectral range with 1-ps laser pulses, delivering amplified signal energy exceeding 50 μJ using millijoule pump pulses in the visible (527 nm). Results of scanning of the parametric gain profile suggest that broad amplification bandwidth as wide as ∼20 nm (at FWHM) under these experimental settings is achieved, which might support amplification of sub-10-fs ultraviolet pulses with central wavelength around 330 nm. It is also shown experimentally and verified theoretically that the parametric gain profile exposes a distinct inhomogeneity and its bandwidth notably broadens due to effects of self- and cross-phase modulation imposed by the intense pump beam.     

Generation of subpicosecond vacuum ultraviolet pulses at 126 nm by using harmonics of a subpicosecond Ti:Sapphire laser

Subpicosecond vacuum ultraviolet (VUV) pulses at the wavelength of 126 nm have been generated in rare gases as a result of the 7th harmonic radiation of a subpicosecond Ti:Sapphire laser oscillating at 882 nm. The VUV harmonic intensity was optimized in Xe at the pressure of 1.2 Torr. The behavior of the harmonic emission was qualitatively reproduced by the classical nonlinear optics. The increase of the harmonic intensity was limited by multiphoton ionization of Xe.     

Radiation force exerted on nanometer size non-resonant Kerr particle by a tightly focused Gaussian beam

We calculate the radiation force that is exerted by a focused continuous-wave Gaussian beam of wavelength λ on a non-absorbing nonlinear particle of radius a ? 50λ/π. The refractive index of the mechanically-rigid particle is proportional to the incident intensity according to the electro-optic Kerr effect. The force consists of two components representing the contributions of the electromagnetic field gradient and the light scattered by the Kerr particle. The focused intensity distribution is determined using expressions for the six electromagnetic components that are corrected to the fifth order in the numerical aperture (NA) of the focusing objective lens. We found that for particles with a < λ/21.28, the trapping force is dominated by the gradient force and the axial trapping force is symmetric about the geometrical focus. The two contributions are comparable with larger particles and the axial trapping force becomes asymmetric with its zero location displaced away from the focus and towards the beam propagation direction. We study the trapping force behavior versus incident beam power, NA, λ, and relative refractive index between the surrounding liquid and the particle. We also examine the confinement of a Kerr particle that exhibits Brownian motion in a focused beam. Numerical results show that the Kerr effect increases the trapping force strength and significantly improves the confinement of Brownian particles.     

Optimization of optical parametric chirped pulse amplification with different pump wavelengths

Optical parametric chirped pulse amplification with different pump wavelengths was investigated using LBO crystal, at signal central wavelength of 800 nm. According to our theoretical simulation, when pump wavelength is 492.5 nm, there is a maximal gain bandwidth of 190 nm centered at 805 nm in optimal noncollinear angle using LBO. Presently, pump wavelength of 492.5 nm can be obtained from second harmonic generation of a Yb:Sr₅(PO₄)₃F laser. The broad gain bandwidth can completely support ∼6 fs with a spectral centre of seed pulse at 800 nm. The deviation from optimal noncollinear angle can be compensated by accurately tuning crystal angle for phase matching. The gain spectrum with pump wavelength of 492.5 nm is much better than those with pump wavelengths of 400, 526.5 and 532 nm, at signal centre of 800 nm.