Generation of femtosecond double pulse by adjusting the refractive index modulation of volume holographic grating

Based on the modified Kogelnik’s coupled-wave theory, expressions of diffraction field distribution of volume holographic grating (VHG) are derived when it is illuminated by an ultrashort pulse. It is found that the diffracted pulse is involved into two pulses, and the two-pulse interval is found to be proportional to the refractive index modulation of the VHG. Moreover, the emergence of double pulses is periodic. An overmodulation effect of refractive index modulation of transmitted VHG is used to explain this particular diffraction behavior of the VHG. Results of our study provide a feasible method to generate interval adjustable double pulse with a simple system.     

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.     

Generation of femtosecond pulses by two-photon pumping supercontinuum-seeded collinear traveling wave amplification in a dye solution

Visible femtosecond (fs) laser pulses have been obtained in a dye solution with a very simple traveling wave collinear configuration. A femtosecond Ti:sapphire laser (790 nm) pumps the dye solution by two-photon absorption and simultaneously generates supercontinuum, which seeds a light-amplification mechanism. Cross-correlation frequency-resolved optical gating measurements reveal a chirped structure in the dye pulse. The shortest pulse duration achieved is 170 fs and the overall energy efficiency of the process is typically 25%. Received: 22 March 2001 / Revised version: 4 July 2001 / Published online: 19 September 2001     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

Filter characteristics of a chirped volume holographic grating

We compare and analyze the filter properties of transmission-type volume holographic gratings, especially the dispersion characteristics for uniform and chirped gratings. It is theoretically and experimentally shown that the dispersion characteristics can be controlled by introducing one-dimensional chirping to the volume grating in a photorefractive crystal. The filter response including output power and dispersion comes from a combined effect of the spatial spectra of the grating structure, input beam, and output-coupling fiber mode. Filter responses can be designed by controlling these parameters for optical communication applications.     

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.     

Generation of polarization-shaped ultraviolet femtosecond pulses

We experimentally demonstrate the generation and characterization of polarization-shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 400 nm. Near-infrared laser pulses are first polarization shaped and then frequency doubled in an interferometrically stable setup that employs two perpendicularly oriented nonlinear crystals. A new pulse shaper design involving volume phase holographic gratings reduces losses and hence leads to an increase in pulse energy.     

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     

Compensation of second-order dispersion in femtosecond pulses after filamentation using volume holographic transmission gratings recorded in dichromated gelatin

We have designed and developed a pulse compressor with volume transmission holographic gratings to be implemented in post-compression experiments based on filamentation in gases. Pulse compression down to 13?fs has been demonstrated. The gratings have been recorded in commercial PFG-04 dichromated gelatin emulsions with a recording wavelength of 532?nm, attaining sufficient index modulation to achieve high efficiency when they are illuminated by an 800-nm laser.     

800 nm飞秒激光长周期光纤光栅的特性

利用800 nm飞秒激光脉冲作为光源,在标准通信单模光纤上直接刻写周期分别为100,200,300和400 m的长周期光纤光栅(LPFG),得到波长范围为1 280～1 680 nm的透射谱,分析研究了在不同刻写条件下LPFG透射谱的共振波长、透射深度和插入损耗等参数的变化。通过对比分析发现透射衰减与刻写长度、条数以及平台高度等有着一定的对应关系。优化实验参数制作出共振波长分别约为1 407,1 311,1 669,1 551 nm,透射深度分别约为24.0,22.3,27.8,23.4 dB,插入损耗分别约为2.5,1.7,3.2,2.0 dB的LPFG。     

An external cavity diode laser using a volume holographic grating

This study presents an external cavity diode laser (ECDL) system, utilizing a volume holographic grating (VHG) and a microfabricated silicon flexure as the VHG holder. The laser design is aimed for easy assembly, controllability, and better stability of the laser cavity. The laser frequency was stabilized to a D₂ transition of rubidium at 780.247 nm, with a mode-hop-free tuning range of 16 GHz and 9.6 GHz with and without feed-forward on the diode injection current. The measured linewidth was 850 kHz in 500 s, qualified for laser cooling experiments.     

Generation of a directed nanolocalized photoelectron beam by means of femtosecond laser pulses

A directed photoelectron beam is obtained when photoelectrons from a sharp metal tip irradiated by femtosecond laser pulses are passed through a quartz nanocapillary. Such a nanolocalized photoelectron train makes it possible to conduct experiments with simultaneous femtosecond time and nanometer spatial resolutions.     

Generation of synchronized femtosecond and picosecond pulses in a dual-wavelength femtosecond Ti:sapphire laser

We obtain synchronized 45-fs and 0.848-ps pulses by achieving cross-mode locking in a double-cavity dual-wavelength femtosecond Ti:sapphire laser. Autocorrelation and cross correlation show that the femtosecond and picosecond pulses are well synchronized, with a timing jitter of 41 fs. Cross-phase modulation dominates the processes of cross-mode locking and synchronization.     

Generation of regular femtosecond pulses in AlGaInP semiconductor laser

It is shown experimentally that stable ~ 200 fs regular pulses and optical frequency comb spectrum containing ~ 70 discrete frequencies can be achieved using semiconductor injection laser working in the conditions of superfluorescence under dc pumping current, which is lower than the threshold of laser action.     

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.     

Generation and measurement of 200 femtosecond optical pulses

Pulses as short as 0.2 ps have been obtained from a passively mode locked dye laser. The technique is simpler than those previously used, but one must pay the price of a loss of tunability. The pulse durations were determined from their second order correlation functions measured with a highly stable interferometer which is described.     

Anisotropic diffraction of stratified volume holographic grating in photorefractive media

We have studied the anisotropic diffraction properties of the stratified volume holographic gratings recorded in photorefractive media using the anisotropic coupled wave theory. It is shown that the diffraction efficiency of such system exhibit the uniform periodic Bragg selectivity properties. In addition the dependence of the stratified volume holographic optical elements (SVHOEs) diffraction properties on the buffer-layer thickness, grating-layer thickness, number of modulation layers, and total thickness of system are discussed in detail.