Pump Pulse Parameters Research Articles

Highly coherent mid-infrared supercontinuum generations in a strip titanium dioxide waveguide with three zero-dispersion wavelengths

A strip titanium dioxide (TiO2) waveguide is designed for highly coherent mid-infrared (MIR) supercontinuum (SC) generation. For the designed TiO2 waveguide, three zero-dispersion wavelengths (ZDWs) are obtained through adjusting the waveguide structure parameters. The three ZDWs are located at 1.53, 3.96, and 5.43 μm, respectively. The nonlinearity coefficient γ is calculated as 1.12 W − 1 m − 1 at wavelength 3.1 μm. By optimizing the pump pulse parameters, the highly coherent MIR SCs are generated when the hyperbolic secant pump pulse with a duration of 80 fs, peak power of 1 kW, and wavelength of 3.1 μm is launched into the TiO2 waveguide and propagated 4.2-mm in length. The obtained SC covers a wavelength range from 1.71 to 9.90 μm (more than 2.5 octaves). Our research results can find important applications in MIR photonics and spectroscopy, biophotonics, optical precision measurement, etc.

Electronic origin of x-ray absorption peak shifts

Encoded in the transient x-ray absorption (XAS) and magnetic circular (MCD) response functions resides a wealth of information of the microscopic processes of ultrafast demagnetization. Employing state-of-the-art first-principles dynamical simulations we show that the experimentally observed energy shift of the ${L}_{3}$ XAS peak in Ni, and the absence of a corresponding shift in the dichroic MCD response, can be explained in terms of laser-induced changes in band occupation. Strikingly, we predict that for the same ultrashort pump pulse applied to Co the opposite effect will occur: a substantial shift upward in energy of the MCD peaks will be accompanied by very small change in the position of XAS peaks, a fact we relate to the reduced $d$-band filling of Co that allows a greater energetic range above the Fermi energy into which charge can be excited. We also carefully elucidate the dependence of this effect on pump pulse parameters. These findings (i) establish an electronic origin for early-time peak shifts in transient XAS and MCD spectroscopy and (ii) illustrate the rich information that may be extracted from transient response functions of the underlying dynamical system.

Broadband Coherent Mid-Infrared Supercontinuum Generation in All-Chalcogenide Microstructured Fiber With All-Normal Dispersion

We demonstrated numerically the generation of broadband, coherent supercontinuum (SC) spectra in the mid-infrared region using dispersion-engineered all-chalcogenide microstructured fibers (MOFs). The 1-cm-long hexagonal fiber can be made with Ge11.5As24S64.5 glass as a low-refractive-index material embedded in a Ge11.5As24Se64.5 glass matrix. By optimizing the structural parameters, we determined a broad and flat all-normal-dispersion characteristic up to 14 μm. A highly coherent broadband SC with an intensity greater than −3 dB in the range from 5973 to 8695 nm is obtained when the fiber is pumped by a 7 μm laser with pulse duration of 50 fs and peak power of 6 kW. Flat-top SC of −30 dB level can be observed utilizing identical pump pulse parameters, covering wavelengths from 3823 to 13577 nm (>1.5 octaves). This broadband coherent MIR SC source can be applied in frequency metrology, optical coherence tomography, biomedical imaging, and few-cycle pulse compression.

Gain-switched watt-level thulium-doped fiber laser and amplifier operating at 1.7 μm

Abstract A 1.7 μm gain-switched thulium-doped all-fiber laser with a master oscillator power amplifier (MOPA) configuration, utilizing a bandpass fiber filter and a 1550 nm erbium/ytterbium-codoped fiber MOPA, is demonstrated. The influences of pump pulse parameters (repetition rate and pulse duration) and laser cavity structures (ring and linear) on the laser performances were experimentally investigated. To the best of our knowledge, the power quenching and drop were observed in the 1.7 μm gain-switched thulium-doped fiber lasers for the first time, resulting from the mode-locked-resembling operation and nonlinear effects. Moreover, the fiber ring-cavity laser was more stable than the linear-cavity laser in the time domain and power. Finally, a laser with a maximum average power of 1.687 W, a slope efficiency of 19.7%, a single-pulse energy of 16.87 μJ, a pulse width of 425 ns, a repetition rate of 100 kHz and a peak power of 39.69 W was obtained.

Highly coherent and multi-octave mid-infrared supercontinuum generations in a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths.

In this paper, a reverse-strip AlGaAs waveguide with three zero-dispersion wavelengths (ZDWs) is designed. The corresponding three ZDWs are located at 3.74, 6.56, and 8.89 µm. The nonlinearity coefficient of the proposed reverse-strip AlGaAs waveguide is calculated as 2.09W-1m-1 at wavelength 4.9 µm. The effects of pump pulse parameters, waveguide length, and noise coefficient on the nonlinear dynamics of supercontinuum (SC) generation are investigated. When the hyperbolic secant pump pulse with a wavelength of 4.9 µm, peak power of 900 W, and duration of 100 fs is launched into the proposed waveguide and propagated after a 3 mm length, highly coherent and multi-octave mid-infrared (MIR) SC spanning from 2.2 to 14.5 µm (more than 2.7 octaves, at -40dB level) is generated. Finally, a possible fabrication process of the reverse-strip AlGaAs waveguide is introduced. Our research results have important applications in MIR photonics, MIR spectroscopy, optical precision measurement, and more.

Broadband ultraviolet-visible frequency combs from cascaded high-harmonic generation in quasi-phase-matched waveguides

High-harmonic generation (HHG) provides short-wavelength light that is useful for precision spectroscopy and probing ultrafast dynamics. We report efficient, phase-coherent harmonic generation up to the ninth order (333 nm) in chirped periodically poled lithium niobate waveguides driven by phase-stable ≤ 12 n J , 100 fs pulses at 3 µm with 100 MHz repetition rate. A mid-infrared to ultraviolet-visible conversion efficiency as high as 10% is observed, among an overall 23% conversion of the fundamental to all harmonics. We verify the coherence of the harmonic frequency combs despite the complex highly nonlinear process. Accommodating the extreme spectral bandwidth, numerical simulations based on a single broadband envelope equation with only quadratic nonlinearity give estimates for the conversion efficiency within approximately 1 order of magnitude over a wide range of experimental parameters. From this comparison between theory and experiment, we identify a dimensionless parameter capturing the competition between three-wave mixing and group-velocity walk-off of the harmonics that governs the cascaded HHG physics. We also gain insights into spectral optimization via tuning the waveguide poling profile and pump pulse parameters. These results can inform cascaded HHG in a range of different platforms.

Excitation of molecular rotational levels by unipolar subcycle pulses

We study theoretically the action of subcycle unipolar pulses on the rotational dynamics of polar molecules. We show that efficient excitation of molecular rotational resonances by subcycle pump pulses with respect to bipolar single-cycle one is achieved, provided that duration of excitation pulses is much smaller than the rotational period of the molecule. The excitation probability for different rotational levels is analysed in dependence on the parameters of molecules and pump pulses. Remarkably, our analysis revealed the possibility of the almost single level excitation and the formation of the population inversion in the rotational levels excited by unipolar pulses, in spite of the nonresonant interaction and ultrabroad spectrum of subcycle pulses. This approach is entirely different from the traditional resonant excitation techniques with long quasi-monochromatic pulses tuned to the molecular transition. And from the other methods, when multi-color laser pulses or single-cycle THz pulses are used for control of the molecular properties. This opens new avenues in applications related to control of molecular systems and lasing creation, since it allows efficient ultrafast control over different rotational levels.

Ring-shaped microstructured chalcogenide optical fiber for octave-spanning flat-top mid-infrared supercontinuum generation

Broadband flat-top mid-infrared supercontinuum (SC) generation in optical fibers is of great interest for many applications. Here, we designed a microstructured chalcogenide optical fiber composed of hexagonal rings. By optimizing the structure parameters, it is demonstrated that the fiber is characterized by an all-normal dispersion profile. Numerical simulation results show that the generated SC spectrum with intensity above − 3 d B ranges from 3450 nm to 8015 nm corresponding to more than one octave when the fiber is pumped with 150 fs pulses at 5 µm. By solving the generalized nonlinear Schrödinger equation, we investigated the influence of pump pulse parameters including pump wavelength, peak power, and pulse duration on the spectral broadening. The results turn out that the optimal pump wavelength is around 5 µm for obtaining SC spectra with sufficient flatness. High peak power and short duration enhance the spectral flatness and bandwidth. We also investigated SC generation in the optimized fiber pumped at 3 µm. The generated SC at − 6 d B level covers wavelength from 1970 to 4370 nm, corresponding to more than one octave.

Suppression of stimulated Brillouin scattering in pulsed erbium-doped fiber amplifier through intensity-modulated counter pumping

We suppress stimulated Brillouin scattering in an erbium-doped optical fiber amplifier for 50-ns-long transform-limited signal pulses by counter-directional pumping with a pulse burst. The pump pulse burst is codirectional with the parasitic Brillouin Stokes wave, which, therefore, undergoes cross-phase modulation and thus spectral broadening due to the intensity-modulated pump. The broadening inhibits its growth. We experimentally study the effect of pump pulse parameters and improve the SBS threshold by up to 4 dB when amplifying signal pulses at a wavelength of 1565 nm with pumping at 1536 nm.

Highly coherent supercontinuum generation in a polarization-maintaining CS2-core photonic crystal fiber.

In this paper, we design a polarization-maintaining CS2-core photonic crystal fiber (PM-CCPCF). The two air holes in the x direction are infiltrated with C2H5OH in order to introduce birefringence. By optimizing the structure parameters of the PM-CCPCF, it is demonstrated that the x-polarization fundamental mode has an all-normal dispersion profile and the corresponding y-polarization fundamental mode has an anomalous dispersion profile for a pump wavelength of 1.76μm. Then, we investigate the supercontinuum (SC) generations when different fiber lengths, pump peak powers, and pump pulse widths are chosen, respectively. Simulation results show that for the x-polarization and y-polarization fundamental modes, highly coherent SCs can be generated by appropriately choosing the fiber length and pump pulse parameters. Finally, nonlinear propagation dynamics are analysed when the optimized fiber length and pump pulse parameters are used. The bandwidth of the SCs generated for the x-polarization and y-polarization fundamental modes can be up to 0.82 and 1.26octave, respectively.

Improved sensitivity for W-band Gd(III)-Gd(III) and nitroxide-nitroxide DEER measurements with shaped pulses

Chirp and shaped pulses have been recently shown to be highly advantageous for improving sensitivity in DEER (double electron–electron resonance, also called PELDOR) measurements due to their large excitation bandwidth. The implementation of such pulses for pulse EPR has become feasible due to the availability of arbitrary waveform generators (AWG) with high sampling rates to support pulse shaping for pulses with tens of nanoseconds duration. Here we present a setup for obtaining chirp pulses on our home-built W-band (95GHz) spectrometer and demonstrate its performance on Gd(III)-Gd(III) and nitroxide-nitroxide DEER measurements.We carried out an extensive optimization procedure on two model systems, Gd(III)-PyMTA–spacer–Gd(III)-PyMTA (Gd-PyMTA ruler; zero-field splitting parameter (ZFS) D∼1150MHz) as well as nitroxide–spacer–nitroxide (nitroxide ruler) to evaluate the applicability of shaped pulses to Gd(III) complexes and nitroxides, which are two important classes of spin labels used in modern DEER/EPR experiments. We applied our findings to ubiquitin, doubly labeled with Gd-DOTA-monoamide (D∼550MHz) asa model for a system with a small ZFS. Our experiments were focused on the questions (i) what are the best conditions for positioning of the detection frequency, (ii) which pump pulse parameters (bandwidth, positioning in the spectrum, length) yield the best signal-to-noise ratio (SNR) improvements when compared to classical DEER, and (iii) how do the sample’s spectral parameters influence the experiment.For the nitroxide ruler, we report an improvement of up to 1.9 in total SNR, while for the Gd-PyMTA ruler the improvement was 3.1–3.4 and for Gd-DOTA-monoamide labeled ubiquitin it was a factor of 1.8. Whereas for the Gd-PyMTA ruler the two setups pump on maximum and observe on maximum gave about the same improvement, for Gd-DOTA-monoamide a significant difference was found. In general the choice of the best set of parameters depends on the D parameter of the Gd(III) complex.

Generation of unipolar optical pulses in a Raman-active medium

Response of a Raman-active media (RAM) to the excitation by a series of ultrashort (few-cycle) optical pulses propagating at a superluminal velocity is studied theoretically. It is shown that under certain conditions rectangular unipolar pulses (video-pulses) can be generated as the RAM response. The duration, shape and amplitude of these video-pulses can be widely tuned by modifying the pump pulse parameters.

High peak power continuum generation by high-order solitons at the mid-infrared wavelength in a photonic crystal fiber

A photonic crystal fiber (PCF) is fabricated from silica material purified by the complexing method of rectification and adsorption in our laboratory. The parameters of pump pulses and fiber length are optimized in the experiment, and the nonlinear optical process of the high-order soliton generation is studied. The high peak power continuums at the mid-infrared wavelength are generated by the overlap among the high-order soliton peaks. Specifically, when a PCF with a length of 60 cm is used, the conversion efficiency η and bandwidth B of the continuum generated can be up to 65% and 358 nm, and the corresponding peak power Ppeak-o and pulse width Wp are 144 kW and 30 fs. It is believed that the high peak power and broadband mid-infrared continuum can be used as short pulse sources, which will find important applications in biophotonics and ultrafast science and technology.

Numerical simulations of spectral broadening in all-normal dispersion photonic crystal fiber at various pump pulse conditions

Supercontinuum (SC) generation contained in the normal dispersion range of an optical fiber has been shown to be limited primarily by the available peak power and length of the pump pulse. In this work, we numerically investigate the SC spectral width and flatness for various pump pulse conditions in a nonlinear, all-solid, soft-glass, photonic crystal fiber (PCF) with a flattened dispersion profile. We assume a range of pump pulse parameters with pulse lengths between 250 and 100 fs (60 to 150 kW of peak power), and input pulse energies between 10 and 30 nJ, numerically reaching a maximum SC width of 800 to 2600 nm. The presented theoretical study provides a guideline for the selection of a fiber laser pump source, or in other words, it enables one to expect the extent of spectral broadening in the developed, all-normal dispersion PCF, when presently available fiber laser pump pulse parameters are assumed.

Generation of anti-Stokes pulses through phase-matched four wave mixing in birefringent photonic crystal fibers

An analysis into the phase matching condition of four-wave-mixing in highly birefringent photonic crystal fiber is presented to yield twelve independent processes combining the nonlinear terms with those of linear terms. The pump wavelength, power and pump pulse incident angles effects on the anti-Stokes frequency shifts are analyzed. The results show that the incident polarization angles along the slow-axis of the PCF, anti-Stokes wavelength is shorter than the case of along the fast-axis. In the case of a divided pump polarization, the frequency shifts decrease gradually with the increase of the incident angles. The pump wavelength is much more important factor in the phase-matched processes, but not sensitive to change of power. Experimental results link power, incident angles and phase-matched wavelengths are presented. Experiments demonstrate that the variation of the pump pulse parameters results in the anti-Stokes lines generation at 550 and 620 nm, which is in good agreement with the theoretical analysis. Furthermore, there exists the polarization-, power-independent anti-Stokes emissions.

Supercontinuum generation at 800 nm in all-normal dispersion photonic crystal fiber

We have numerically investigated the supercontinuum generation and pulse compression in a specially designed all-normal dispersion photonic crystal fiber with a flat-top dispersion curve, pumped by typical pulses from state of the art Ti:Sapphire lasers at 800 nm. The optimal combination of pump pulse parameters for a given fiber was found, which provides a wide octave-spanning spectrum with superb spectral flatness (a drop in spectral intensity of ~1.7 dB). With regard to the pulse compression for these spectra, multiple-cycle pulses (~8 fs) can be obtained with the use of a simple quadratic compressor and nearly single-cycle pulses (3.3 fs) can be obtained with the application of full phase compensation. The impact of pump pulse wavelength-shifting relative to the top of the dispersion curve on the generated SC and pulse compression was also investigated. The optimal pump pulse wavelength range was found to be 750 nm < λp < 850 nm, where the distortions of pulse shape are quite small (< -3.3 dB). The influences of realistic fiber fabrication errors on the SC generation and pulse compression were investigated systematically. We propose that the spectral shape distortions generated by fiber fabrication errors can be significantly attenuated by properly manipulating the pump.

Ultrafast coherent control of giant oscillating molecular dipoles in the presence of static electric fields

We propose a scheme to generate electric dipole moments in homonuclear molecular cations by creating, with an ultrashort pump pulse, a quantum superposition of vibrational states on electronic states strongly perturbed by very strong static electric fields. By field-induced molecular stabilization, the dipoles can reach values as large as 50 Debyes and oscillate on a time-scale comparable to that of the slow vibrational motion. We show that both the electric field and the pump pulse parameters can be used to control the amplitude and period of the oscillation, while preventing the molecule from ionizing or dissociating.

Highly efficient Cherenkov radiation in photonic crystal fibers for broadband visible wavelength generation

We investigate the dependence of Cherenkov radiation (CR) on pump pulse parameters and its evolution along the propagation distance. Using a Ti:sapphire laser emitting 10 fs pulses as the pump source, we demonstrate highly efficient (>40%), broadband (>50 nm) CR in the visible-wavelength range with a threshold energy less than 100 pJ and a tuning range over 100 nm.

Optimization of a tabletop high-repetition-rate soft x-ray laser pumped in double-pulse single-beam grazing incidence

This Letter reports on the optimization of a tabletop nickel-like molybdenum transient collisionally excited soft x-ray laser (SXRL) at 18.9 nm performed by a double-pulse single-beam grazing incidence pumping (DGRIP). This scheme allows for the first time, to our knowledge, the full control of the pump laser parameters including the pre-pulse duration optimally generating the SXRL amplifier under a grazing incidence. The single-beam geometry of the collinear double-pulse propagation guarantees the ideal overlap of the pre-pulse and main pulse from shot to shot resulting in a more efficient and highly stable SXRL output. SXRL energies up to 2.2 microJ are obtained with a total pump energy less than 1 J for several hours at a 10 Hz repetition rate without realignment under once optimized double pumping pulse parameters including energy ratio, time delay, pre-pulse and main pulse durations, and line focus width.

Supercontinuum generation in a single-mode optical fiber with novel dispersion profile

A novel dispersion model different from the conventional one is introduced to enhance supercontinuum generation in dispersion-flattened/decreasing fiber. The results show that supercontinuum spectrum generated in this fiber is by far wider than that in the conventional one,whose flat width can be nearly 1000 nm. The results further show that both the fiber parameters such as peak chromatic dispersion, dispersion differential constant and dispersion decreasing coefficient and the pump pulse parameters including pulse width, soliton order, frequency chirp are crucial to the flat ultra-wideband supercontinuum spectrum generated in this fiber.