A new instability of counter-propagating waves in BaTiO3?

The self-development of a strong reflection grating in a BaTiO₃ sample illuminated with two mutually incoherent counter-propagating light waves is reported. The two transmitted waves diffracted from this grating become partially mutually coherent. PACS 42.65.Hw; 05.45.-a; 42.65.Pc; 42.65.Sf     

Single-shot dynamics of pulses from a gas-filled hollow fiber

We present measurements of the performance characteristics of few-cycle laser pulses generated by propagation through a gas-filled hollow fiber. The pulses going into the fiber and the compressed pulses after the fiber were simultaneously fully characterized shot-by-shot by using two kHz SPIDER setups and kHz pulse energy measurements. Output-pulse properties were found to be exceptionally stable and pulse characteristics relevant for non-linear applications like high-harmonic generation are discussed. PACS 42.65.Re; 42.65.Ky; 42.65.Sf; 42.65.Jx     

Optimal spectral broadening in hollow-fiber compressor systems

Supercontinuum generation in gas-filled hollow fibers is investigated using numerical simulation of the nonlinear propagation of light pulses in hollow waveguides. The use of the cascading hollow-fiber configuration allows one to significantly enhance the achievable spectral broadening, particularly in the high energy regime. General design criteria for a single- and a double-fiber configuration are presented, which allow the generation of high-energy supercontinua. PACS 42.65.Re; 42.65.-k; 42.65.Jx     

Ultra-fast reconfigurable spatial switching between a quadratic solitary wave and a weak signal

Experimental and numerical investigations of an ultra-fast reconfigurable spatial switch based on the nonlinear interaction between a weak wave (the signal) and a solitary wave (the control) at 1548 nm are reported. The non-collinear interaction in a quadratic nonlinear film waveguide gives birth to a third switched optical beam (the idler). This beam could be steered according to the transverse spatial position of the control beam. PACS 42.65.Ky; 42.65.Re; 42.65.Wi     

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     

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     

Theory and simulation of supercontinuum generation in transparent bulk media

Comprehensive model and computer simulations of the supercontinuum generation in bulk media are presented. Using water and air as representative condensed and gaseous media, the standard model for white-light generation is put to test. It is shown that besides the intensity clamping and plasma-induced frequency blue shift, the linear chromatic dispersion plays an important role. It is the major factor that determines the achievable spectral extent of the supercontinuum. PACS 42.65.-k; 42.65.Hw; 42.65.Ky; 42.65.Sf     

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     

Solvent-dependent optical limiting behavior of lead nanowires stabilized by [60] fullerene derivative

The optical limiting performance of lead nanowires in chloroform, ethanol and dimethylformamide was measured with 532-nm, 8-ns-duration laser pulses. Experiments showed that the optical limiting is strong and solvent dependent. The origins and the solvent effect of the optical limiting were analyzed. It is proposed that the absorption-induced scattering is the main role responsible for the optical limiting behavior and the solvent effect. PACS 42.65.An; 42.65.Hw; 42.65.Jx     

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.     

Optimization of hollow fiber pulse compression using pressure gradients

We present a study on the optimization of multi-mJ spectral broadening in neon-filled hollow fibers using pressure gradients. It is found that the intensity of the laser pulse at the entrance of the hollow fiber should be below or at the level where multi-photon ionization (MPI) starts to occur. If the intensity is higher than the threshold value for MPI, ionization can significantly reduce the throughput efficiency. The extension of the scheme to a terawatt power level is also addressed. PACS 42.65.Re; 42.65.Jx; 42.65.k     

Z-scan measurements of the nonlinear refractive indices of novel Yb-doped laser crystal hosts

We report on the z-scan characterization of the nonlinear refractive indices of three borate crystals (GdCOB, YCOB, and BOYS), which are new promising Yb-doped laser hosts. The results indicate the possibility of substantial Kerr-lensing and self-phase modulation effects in femtosecond oscillators and amplifiers. High values of the nonlinear refractive indices suggest possible KLM operation of such lasers. PACS 42.65.An; 42.65.Hw; 42.55.Rz; 42.65.Re     

Flexible all-optical wavelength conversions of 1.57-ps pulses exploiting cascaded sum- and difference frequency generation (cSFG/DFG) in a PPLN waveguide

Flexible all-optical wavelength conversions of ps-pulses based on cascaded sum- and difference frequency generation (cSFG/DFG) are proposed and experimentally demonstrated using a periodically poled lithium niobate (PPLN) waveguide. The pulsed signal, with a 40 GHz repetition rate and a 1.57 ps pulse width is adopted. Fixed-in variable-out (tunable), variable-in fixed-out and variable-in variable-out wavelength conversions are successfully realized with three proposed tuning schemes. Both wavelength down- and up-conversion are simultaneously observed. PACS 42.65.Ky; 42.65.Wi; 42.65.Re     

High-contrast chemical imaging with gated heterodyne coherent anti-Stokes Raman scattering microscopy

A novel method is presented which substantially improves the signal-to-background ratio for coherent anti-Stokes Raman scattering (CARS) microscopy. It exploits the fixed phase relation between pump, Stokes and CARS fields together with the strong phase coherence in supercontinua generated by femtosecond lasers. Three phase-locked optical parametric amplifiers are used for the realisation of heterodyne signal detection. Proper pulse timing yields a gating mechanism which nearly completely suppresses solvent background signals. PACS 42.65.Dr; 42.65.Hw; 42.65.Yj     

Broadband supercontinuum generation in a single potassium di-hydrogen phosphate (KDP) crystal achieved in tandem with sum frequency generation

We present here an enhanced broadband supercontinuum generation in a potassium di-hydrogen phosphate (KDP) crystal. The enhancement in the bandwidth of the white light is obtained towards the shorter wavelength regime (<400 nm) by employing supercontinuum generation and sum frequency generation in tandem. The tunability in the blue region of the spectrum with angle is demonstrated. The bandwidth of supercontinuum achieved spans from 350 nm to 1300 nm. Further, we show the excellent polarization maintenance of continuum generated in KDP in comparison to that generated in water and BK-7 glass. PACS 42.65.Ky; 42.65.Jx; 42.65.Re; 42.25.Ja     

Optimization of a femtosecond pulse self-compression region along a filament in air

We have identified the pulse self-compression region in a filament produced by 55 fs, 4 mJ, 805 nm radiation propagating in air without geometrical focusing. In our experiment the pulse self-compression region is attained by the propagation distance, where the shortest wavelength in the supercontinuum blue wing reaches a minimum, and the growing conversion efficiency to white light has a large gradient. Numerical tracking of the pulse along the filament shows a single-peak 9 fs pulse with a flat spectral phase at the optimum compression distance. PACS 42.65.Jx; 42.65.Re; 42.65.Ky     

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     

Super-broadband continuum generation with transient self-focusing of a terawatt laser pulse in rare gases

Hyper-continuum that continuously covers from the VUV to the IR spectral regions with a flat spectral shape has been generated in nonlinear propagation in rare gases. The spectral intensity was 1 GW/nm with a conversion efficiency of >50%. The spectral, spatial, and temporal characteristics have been studied. PACS 42.65.Jx; 42.65.Re     

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     

All-fibre wavelength conversion based on four-wave mixing in a ring erbium-doped fibre laser

A new scheme based on four-wave mixing is presented using a ring erbium-doped fibre laser and tuneable fibre Bragg gratings that can effectively perform wavelength conversion in a wide wavelength range. An application of this wavelength-conversion architecture to optical communications is also presented. PACS 42.65.-k; 42.65.Ky