Multiple-beam interference patterns in optical fiber generated with ultrafast pulses and a phase mask

We compare the cladding patterns present in grating structures fabricated with an ultrafast laser and a phase mask with a cw beam interference model. We find that the observed patterns agree well with the model results for picosecond pulses; however, for femtosecond pulses, we show that the full bandwidth and the pulsed nature of the sources must be considered because the pattern can be affected by group-velocity walk-off. An interesting consequence of order walk-off is the possibility of pure two-beam interference generation with a phase mask in the femtosecond pulse regime.     

Ultrashort-pulse dual-wavelength source for digital holographic two-wavelength contouring

Digital holographic shape measurements using femtosecond laser pulses are reported. For contouring of very fast moving objects, the simultaneous generation of at least two spectrally separated ultrashort pulses is required. To deliver this particular spectral signature at high pulse energies, a chirped-pulse Ti:sapphire laser amplifier was modified to emit two spectrally separated pulses with energies above 1 μJ each. The wavelength separation of these pulses was adjustable within the 50 nm gain bandwidth, cutting out two distinct wavelength peaks by a variable double-slit assembly in a prism sequence. A Michelson-type interferometer was employed to perform the two-wavelength contouring. The phases of the holograms and the phase differences are calculated numerically, which allow us to deduce the contour lines of the topology of the object. The suitability of the light source for digital holography is demonstrated with contouring of stationary objects and the potential for high-speed applications is indicated. PACS 42.40.-i; 42.60.By     

Measurement of harmonic phase differences by interference of attosecond light pulses

By using a self-referencing technique, we have experimentally measured the influence of the carrier-envelope phase of femtosecond light pulses on the phase of the electric field of the radiation produced by high-order harmonic generation. We show that, in particular experimental conditions, the temporal evolution of the electric field of the attosecond pulses, is directly controlled by the carrier-envelope phase of the driving pulses.     

Generation and detection of shear acoustic waves in metal submicrometric films with ultrashort laser pulses

We present experimental and calculational results demonstrating the thermoelastic generation of shear acoustic waves using femtosecond laser pulses in submicrometric isotropic aluminum films. We show that the generation of the shear waves is correlated to the reduction of the width of the optoacoustic source on the surface. The presence of shear waves is related to acoustic diffraction and acoustic mode conversion at the thin film interfaces.     

Controlled rotational Raman echo recurrences and modulation of high-intensity ultrashort laser pulses by molecular rotations in the gas phase

We show that sequences of femtosecond laser pulses can control the rotational Raman response of a gas mixture, giving rise to a tunable manifold of echo recurrences in the retarded nonlinear-optical response of the gas. Tailored phase masks for high-intensity ultrashort laser pulses are experimentally demonstrated using molecular rotations in the gas phase driven by optimally time-ordered pulse sequences.     

Femtosecond pulse shaping by dynamic holograms in photorefractive multiple quantum wells

Femtosecond pulses can be shaped in the time domain by diffraction from dynamic holograms in a photorefractive multiple quantum well placed inside a Fourier pulse shaper. We present several examples of shaped pulses obtained by controlling the amplitude or the phase of the hologram writing beams, which modifies the complex spectrum of the femtosecond output.     

Investigation of parameters of terahertz pulses generated in single-domain LiNbO<Subscript>3</Subscript> crystal by step-wise phase mask

A new scheme for the efficient generation of broadband terahertz radiation via optical rectification of femtosecond laser pulses in the single-domain lithium niobate crystal equipped with the step-wise phase mask (SPM) is investigated. It is shown that using the SPM one can provide the phase matching for all the spectral components of a terahertz pulse by providing the effective conversion of laser radiation in the terahertz region. The angular distribution of spectral components, as well as the temporal shape of terahertz pulses in the wave zone is studied. These results can be applied in the time-domain spectroscopy, the imaging of hidden objects, and etc.     

Nonlinearity management in optics: experiment, theory, and simulation

We conduct an experimental investigation of nonlinearity management in optics using femtosecond pulses and layered Kerr media consisting of glass and air. By examining the propagation properties over several diffraction lengths, we show that wave collapse can be prevented. We corroborate these experimental results with numerical simulations of the (2+1)-dimensional focusing cubic nonlinear Schr?dinger equation with piecewise constant coefficients and a theoretical analysis of this setting using a moment method.     

Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located. PACS 42.65.Re; 42.72.Bj; 32.80.Qk     

Dispersion compensation for a femtosecond self-pumped phase conjugator

Dispersion compensation for a photorefractive self-pumped phase conjugator with femtosecond pulses is analyzed. The self-pumped phase conjugator consists of a pair of dynamic gratings coupled by total internal reflections at the crystal surfaces (cat conjugator). The negative angular dispersions of refraction at the air-crystal interface and the gratings inside the crystal compensate for the positive dispersion of the finite crystal path. The experimental results show that with partial dispersion compensation the width of the self-pumped phase conjugation at 450 nm of femtosecond pulses is narrower than that of the transmitted pulses.     

Self-assembled volume vortex grating induced by femtosecond laser pulses in glass

We presented a microfabrication process for optical volume vortex grating inside glass by femtosecond laser pulses. The self-trapped filament of femtosecond laser pulses can induce hundreds μm-long region refractive-index changes in glass. We realized the restructured optical vortex beams using a collimated He–Ne laser beam. The maximum first-order diffraction efficiency was about 19.6%. The volume vortex grating structure fabricated in glass is polarization dependent.     

X-ray analysis of mechanical and thermal effects induced by femtosecond laser treatment of aluminum single crystals

Surface marking of aluminum single crystal is performed with femtosecond laser pulses. X-ray analysis allows to measure thermal and mechanical effects induced by the femtosecond laser pulses. These effects are estimated by comparing the pole figures (crystallinity) and the broadening of the diffraction peaks (mechanical contribution) before and after the laser irradiation. The results show that the femtosecond laser treatment ensures a re-crystallization of the structure and the presence of mechanical residual stresses. The analysis of the pole figures provides the sign of a re-crystallization on smaller volumes compared to initial ones. After the laser irradiation, the crystallization is perfectly oriented like the (1 1 0) orientation of the massive sample. Moreover, following the laser treatment, we show that the crystallographic structure is purer than the initial one. We also prove that the laser effect is persistent on a typical scale of 10 μm beyond the surface.     

Investigation of ultrashort pulse parameters under SRS temporal compression

Based on numerical simulations, we analyze the compression of femtosecond pulses by forward stimulated Raman scattering (SRS). We have revealed the main patterns and determined the optimal conditions of this process to achieve the highest conversion efficiency. The influence of diffraction effects on the generation and compression of Gaussian beams is investigated.     

不同波长飞秒脉冲的相位测量

飞秒脉冲技术的不断发展能够较方便地产生不同波段和结构特性的飞秒脉冲. 对已有的改进型零附加相位光谱相位相干电场重构系统进行优化整合,使之胜任不同特性飞秒脉冲的测量. 利用该系统测量了两台飞秒激光系统输出不同波长的脉冲及其通过厚度为80 mm的BK7玻璃块而得到的啁啾脉冲. 实验结果表明,该系统的适用范围较宽. 关键词： 光谱相位相干直接电场重构法 飞秒脉冲测量 波长     

Characteristic of femtosecond laser-pulsed digital holography

Digital holography can be applied to ultrafast detection when a femtosecond laser pulse is used. In this paper, the interference process of two femtosecond laser pulses is studied and the recording process of the femtosecond laser pulsed digital hologram is simulated. Holograms at different recording angles are generated by integrating the instantaneous interference field. By analyzing the distribution of the reconstructed phase error, the characteristics of femtosecond laser pulsed digital holography are discussed.     

Spatiotemporal control of ultrashort optical pulses by refractive-diffractive-dispersive structured optical elements

Structured optical elements that control the spatial and temporal characteristics of femtosecond light pulses are analyzed and synthesized. We show that unique spatiotemporal effects can be attained based on the diffraction, refraction, and dispersive effects that appear in the femtosecond regime. We argue that the design requirements for ultrafast optics are beyond the achromatization considerations that are usually applied to incoherent illumination because of the need to consider coherent effects. Despite fundamental limitations in the space-time control of ultrashort pulses, we show the potential of this technique to improve simultaneously the spatial and the temporal resolution of a lens and to generate ultrafast pulse sequences.     

Experimental investigation of the closest parallel pulses in holographic femtosecond laser processing

In holographic femtosecond laser processing, diffractive parallel pulses are distorted by phase discontinuities and mutual interference between the neighborhoods in the reconstructed image of a Fourier computer-generated hologram when the interval is smaller than the beam diameter. We investigated holographic fabrication on a glass surface using parallel pulses with different intervals. We found the closest parallel pulses with sufficient separation to avoid mutual interference in holographic femtosecond laser processing. The minimum interval was 2.8 times larger than the diffracted beam diameter. The experimental results were also supported by a computer simulation. Our findings will be very useful in the design of holographic laser processing systems.     

Nonlinear Talbot self-healing in periodically poled LiNbO_3 crystal [Invited]

The nonlinear Talbot effect is a near-field nonlinear diffraction phenomenon in which the self-imaging of periodic objects is formed by the second harmonics of the incident laser beam. We demonstrate the first, to the best of our knowledge, example of nonlinear Talbot self-healing, i.e., the capability of creating defect-free images from faulty nonlinear optical structures. In particular, we employ the tightly focused femtosecond infrared optical pulses to fabricate Li Nb O_3 nonlinear photonic crystals and show that the defects in the form of the missing points of two-dimensional square and hexagonal periodic structures are restored in the second harmonic images at the first nonlinear Talbot plane. The observed nonlinear Talbot self-healing opens up new possibilities for defect-tolerant optical lithography and printing.     

Measurement of subpicosecond optical waveforms using a resonator-based phase modulator

We propose a method for the measurement of periodic optical waveforms based on the use of an electrooptic phase modulator placed in an optical Fabry-Perot or ring resonator. Significant broadening of the modulation spectrum extends the recently developed method of periodic modulation for pulse characterization into femtosecond scales. We numerically demonstrate the characterization of a 300-fs optical pulse. We also present a technique based on the temporal fractional Talbot effect for restoration of the pulse phase profile. After fast linear processing, subpicosecond pulses will be observed on the screen of a real-time oscilloscope. This complete characterization of optical pulses is entirely linear and therefore highly sensitive and simple in implementation.     

Generation of multiple femtosecond pulses by step gratings

A simple method for generating multiple isolated femtosecond pulses by step gratings is presented, and a criterion for judging whether or not an incident pulse is just split into multiple independent pulses is obtained. The dependences of multiple pulses on the structural parameters of the step gratings are explored. The results show that an incident pulse can split into multiple isolated similar diffraction pulses for enough step height, and the pulse repetition interval increases with the increasing of the step height.