Spatial Chirp Research Articles

Generation of background-free pulses at 1064nm accurately synchronized with femtosecond laser pulses at 794nm

This paper reports on generation of background-free pulses at 1064nm accurately synchronized with femtosecond pulses at 794 nm, by the use of a continuous-wave He-Ne laser seeded non-collinear optical parametric amplification (NOPA) pumped by frequency-doubled Ti:sapphire femtosecond laser at 396nm. Experimental results showed that the 1064 nm light pulses can be used as the pump seed for optical parametric chirped pulse amplification to realize an all-optical synchronization between pump and signal beams. Background-free pulses at 1064 nm could also be generated by using a NOPA with continuous-wave seed from the intracavity 632.8 nm light of an appropriate He-Ne cavity. The spectral and spatial chirp properties of the idler pulses were quite similar in both cases according to our measurements. The generated idler pulses in the intracavity seed case have single pulse energy 10 times that of the external cavity case.

Describing first-order spatio-temporal distortions in ultrashort pulses using normalized parameters

We develop a first-order description of spatio-temporal distortions in ultrashort pulses using normalized parameters that allow for a direct assessment of their severity, and we give intuitive pictures of pulses with different amounts of the various distortions. Also, we provide an experimental example of the use of these parameters in the case of spatial chirp monitored in real-time during the alignment of an amplified laser system.

Direct noninterference cylindrical vector beam generation applied in the femtosecond regime

This letter describes the generation of femtosecond cylindrical vector beams by passing the optical pulse through a microfabricated spiral phase plate and an azimuth-type polarization analyzer. The resulting beam resembles a doughnut and has a polarization that is axially symmetric. Portions of the wave front were sampled via the GRENOUILLE frequency resolved optical grating device; it was found that the original pulse of 200fs was stretched slightly by ∼13fs. Compared to an interferometric beam combination technique, this experiment geometry is simple, minimizes material dispersion, and has negligible spatial chirp.

Comparison of the filamentation and the hollow-core fiber characteristics for pulse compression into the few-cycle regime

The gas-filled hollow-core fiber compression and the optical filamentation technique are compared experimentally in a parameter regime suitable for intense few-cycle pulse generation. In particular, pointing stability, spectral properties, and spatial chirp are investigated. It is found that in the case of filamentation, the critical parameter for pointing stability is gas pressure inside the generation cell whereas for the hollow-core fiber it is alignment that plays this role. The hollow-core fiber technique yields spectra that are better suited for chirped-mirror pulse compression whereas filamentation offers higher throughput and prospects for easy-to-implement self-compression. We present spectral phase interferometry for direct electric-field reconstruction (SPIDER) measurements that directly show the transition in the spectral phase of the output continua into the self-compression regime as the gas pressure is increased.

Generation of femtosecond optical vortices using a single refractive optical element

It is shown experimentally for small spectral bandwidth that spiral phase plates (SPPs) can generate optical vortices in femtosecond beams without the need for additional dispersion compensation elements. An autocorrelation measurement performed on the obtained optical vortex (OV) beam showed that the SPP did not significantly change the pulse duration. Hence the single element SPP, already an established beam shaping technique in continuous wave and high power regimes, is presented as an efficient and practical means to obtain OVs that are free from spatial chirp in femtosecond beams.

ON A STUDY OF DIFFRACTION AND DISPERSION MANAGED SOLITON IN A CYLINDRICAL MEDIA

Analytical and numerical techniques are developed for the analysis of the solitary pulse propagation in a cylindrical media, where both the dispersion and diffraction management are known to exist. In the first part we treat the situation when there is no diffraction management by a simple analytical approach and show that it is possible to control both spatial and temporal width over one period of the dispersion map. An important out put of our analytical treatment is that we can predict the value of length of the second link and the amount of group velocity dispersion there if the initial conditions are given. Unfortunately since the spatial chirp can not be controlled, the treatment can not be repeated a second time. So for a long distance propagation a different treatment in needed. We then show that for long period of transmission, it is necessary to introduce diffraction management term, the best form turns out to be periodic function of the distanced travelled. The detailed variation of spatial and temporal width, chirp, and amplitude are explicitly given.

Study of propagation characteristics and control of one-dimensional optical lattice soliton

Propagation of optical lattice soliton in nonlinear Kerr medium with harmonic modulation of refractive index is investigated analytically and numerically. The equations governing evolution of the soliton parameters and the conditions for soliton formation and stable propagation are obtained. It is shown that the beam is finally trapped in a guide-like channel and propagates stably when the launching angle is smaller than a critical value. The critical angle increases as the depth and period of modulation of refractive index increase, and increases as the beam width decreases. Furthermore, linear spatial chirp upsets the balance between diffraction and nonlinearity, thus affects the formation and stable propagation of the soliton, although it doesn't affect the central position of the beam. In order to maintain the soliton propagation one can offset the effect of the chirp by using proper beam power.

Extremely simple single-prism ultrashort- pulse compressor

We have designed and demonstrated a very simple and compact ultrashort-pulse compressor using a single prism and a corner-cube. Our design is significantly easier to align and tune compared with previous designs. Angle-tuning the prism wavelength-tunes, and translating the corner cube varies the group-delay dispersion over a wide range. When tuned, the device automatically maintains zero angular dispersion, zero pulse-front tilt, zero spatial chirp, and unity magnification. The device can easily be built so that its output beam remains collinear with the input beam, and when the input beam or pulse compressor moves, the input and output beams remain collinear.

Compression, splitting and switching of bright and dark solitons in nonlinear directional coupler

Abstract A detailed numerical simulation of the switching, compression and splitting characteristics of various solitary pulses (bright, grey and dark) are carried out by a direct solution of the associated coupled NLS equation. Important physical parameters of the out going pulse such as, intensity distribution, root mean square spatial and temporal width and chirp are calculated. Both the cases of symmetric and asymmetric couplers are considered. The important phenomenon of periodic power transfer from one channel to the other unfolds. The compression varies with the type of pulse launched in the initial channel. It is observed that the chirping of the initial pulse has an optimum value and it vary quite noticeably with the character of the pulse and couplers, symmetric and asymmetric.

Far-field spectral characterization of conical emission and filamentation in Kerr media

By use of an imaging spectrometer we map the far-field ($\theta-\lambda$) spectra of 200 fs optical pulses that have undergone beam collapse and filamentation in a Kerr medium. By studying the evolution of the spectra with increasing input power and using a model based on stationary linear asymptotic wave modes, we are able to trace a consistent model of optical beam collapse high-lighting the interplay between conical emission, multiple pulse splitting and other effects such as spatial chirp.

Experimental and theoretical study of hyperfine WDM demultiplexer performance using the virtually imaged phased-array (VIPA)

We have developed a Fresnel diffraction analysis that provides an analytic expression for the passband response of virtually imaged phased-array (VIPA) demultiplexers. Our analysis shows that although the passband can be inherently symmetric, a strong asymmetry can develop when the output plane is detuned longitudinally. The symmetric passband has the minimum -3 dB transmission bandwidth. We also identify a spatial chirp effect that arises when the passband becomes asymmetric. Our theoretical predictions are confirmed via experiment. The experimental results include a demonstration of a hyperfine wavelength demultiplexing response with 10 pm (1.25 GHz) -3 dB transmission bandwidth.

Creation of optical vortices in femtosecond pulses

We experimentally created a femtosecond optical vortex using a pair of computer-synthesized holographic gratings arranged in a 2f - 2f optical setup. We present measurements showing that the resulting donut mode is free of spatial chirp, and support this finding with an analysis of the optical wave propagation through our system based on the Kirchhoff-Fresnel diffraction integral. An interferogram confirms that our ultrashort vortex has topological charge 1, and a conservative experimental estimation of its duration is 280 fs. We used 25-fs radiation pulses (bandwidth approximately 40 nm) produced by a Ti:sapphire laser oscillator.

Self-referenced measurement of the complete electric field of ultrashort pulses

A self-referenced technique based on digital holography and frequency-resolved optical gating is proposed in order to characterize the complete complex electric field E (x, y, z, t) of a train of ultrashort laser pulses. We apply this technique to pulses generated by a mode-locked Ti:Sapphire oscillator and demonstrate that our device reveals and measures common linear spatio-temporal couplings such as spatial chirp and pulse-front tilt.

Spatial chirp in ultrafast optics

Spatial chirp is a common spatio-temporal effect, but its parameterization is currently unsatisfactorily vague. In this paper, we propose and compare two definitions of spatial chirp, which we call “spatial dispersion” and “frequency gradient.” The appropriate definition to use depends on the application. For Gaussian beams and pulses, the relationship between the two definitions is found to be analogous to that between the definitions of temporal chirp in the time and frequency domains.

Femtosecond X-ray pulses from a spatially chirped electron bunch in a SASE FEL

We propose a simple method to produce short X-ray pulses using a spatially chirped electron bunch in a SASE FEL. The spatial chirp is generated using an RF deflector which produces a transverse offset (in y and/or y′) correlated with the longitudinal bunch position. Since the FEL gain is very sensitive to an initial offset in the transverse phase space at the entrance of the undulator, only a small portion of the electron bunch with relatively small transverse offset will interact significantly with the radiation, resulting in an X-ray pulse length much shorter than the electron bunch length. The X-ray pulse is also naturally phase locked to the RF deflector and so allows high precision timing synchronization. We discuss the generation and transport of such a spatially chirped electron beam and show that tens of femotsecond long pulse can be generated for the linac coherent light source.

Spatial chirp and angular dispersion of the nonlinear crystal for a femtosecond OPO

In this paper spatial chirp and angular dispersion are calculated theoretically and analyzed for a four-mirror, standing-wave cavity femtosecond OPO. The analytical expression of spatial chirp in the nonlinear crystal is deduced. The analytical expressions of second- and third-order dispersion including angular dispersion of the nonlinear crystal are presented, for the noncollinear type-I phase-matched Fs BBO OPO using angle tuning. Moreover, variations of GDD and TOD with cavity parameters are calculated exactly. The analysis method and the obtained result also are efficacious for a same type Fs OPO.

Spatial chirp and angular dispersion of a laser crystal for a four-mirror cavity Kerr-lens mode-locked laser.

We describe oscillating loops in a laser cavity and optical paths in a laser crystal of different wavelength rays for a four-mirror cavity Kerr-lens mode-locked laser. The relation between different wavelength ray paths and laser resonator parameters is deduced. The analytical expressions of second- and third-order dispersion including angular dispersion of the crystal are presented. The variations of group-delay dispersion (GDD) and third-order dispersion (TOD) with cavity parameters are calculated exactly. The calculation shows that GDD and TOD increase rapidly when the spacing between two folding mirrors approaches the boundary of a cavity stability zone. The rapid dispersion increase influences the mode-locked pulse width and the mode-locked stability.

Pulse-front tilt caused by spatial and temporal chirp

Pulse-front tilt in an ultrashort laser pulse is generally considered to be a direct consequence of, and equivalent to, angular dispersion. We show, however, that, while this is true for certain types of pulse fields, simultaneous temporal chirp and spatial chirp also yield pulse-front tilt, even in the absence of angular dispersion. We verify this effect experimentally using GRENOUILLE.

Influence of off-waist incidence on evolution of the Gaussian beam in the nonlocal nonlinear media

In this paper, we discuss the evolution of the offwaist incident Gaussian beam in the nonlocal nonlinear media by use of the (1+1)dimensional strongly nonlocal model. An exact analytical solution for the evolution is obtained. We find that no matter how high the power of the incident beam is, the beam width will oscillate periodically and no spatial soliton will exist, which is substantially different from the onwaist incident case. The influence of the input power and incident position on beam evolution is also discussed. The higher the input power, the narrower the beam width is. The incident position determines the initial evolution trend. The definition of a “spatial chirp” is given, to our knowledge, for the first time. The physical origin of the above phenomenon is that the different incident position corresponds to the different spatial chirp. The concept of the spatial chirp provides a new physical qualitative insight into the evolution of optical beams. The comparisons of beam evolutions in focusing media, in defocusing media and in linear homogeneous media are also given.

Spatial chirp and angular dispersion of the nonlinear crystal for a femtosecond optical parametric oscillation

In this paper the spatial chirp and the angular dispersion for femtosecond optical parametric oscillation(OPO) are calculated theoretically and analysed. The analytical expression of the spatial chirp in the nonlinear crystal(BBO) is presented. And the analytical expressions of second-and third-order dispersions including angular dispersion of the nonlinear crystal for the noncollinearly phase-matched femtosecond BBO OPO in type-Ⅰ configuration with angle tuning. Moreover, variations of second- and third-order dispersions with the angular dispersion are calculated exactly.