High-power Picosecond Pulses Research Articles

Propagation of High-Power Picosecond Pulse at 1064 nm Using Nodeless Anti-Resonant Hollow-Core Fibre

Propagation of High-Power Picosecond Pulse at 1064 nm Using Nodeless Anti-Resonant Hollow-Core Fibre

High-power DUV picosecond pulse laser with a gain-switched-LD-seeded MOPA and large CLBO crystal.

We report the generation of a 266 nm deep ultraviolet (DUV) picosecond pulse with an average output power of 14 W by the fourth-harmonic generation (FHG) from two consecutive frequency-doubling stages of a 1064 nm pulse based on a gain-switched-laser-diode (LD)-seeded hybrid fiber/solid-state master oscillator and power amplifier (MOPA) system. Through the gain-switched operation of a narrow-spectral-linewidth distributed-feedback laser diode and by using a Yb-doped fiber and a two-stage ${\rm Nd}:{{\rm YVO}_4}$Nd:YVO4 solid-state amplifier, we achieved an average power of 46.5 W near the Fourier transform limit for a 13 ps pulse with a repetition rate of 200 kHz. The narrow linewidth pulse characteristics enabled highly efficient frequency conversion, and the efficiency of conversion from 532 to 266 nm was 54%, and from 1064 to 266 nm was 31%. The beam quality factor ${M^2}$M2 of the generated DUV pulse was below 1.2. The highly efficient FHG process resulted in appeasing thermal stress caused by nonlinear absorption in the crystal, and more than 5000 h of continuous operation were achieved without any power down or beam profile degradation.

High-power mid-infrared picosecond pulse bunch generation through difference frequency generation

We report a high average power mid-infrared picosecond (ps) pulse bunch output by means of direct difference frequency generation (DFG) in periodically poled magnesium-doped lithium niobate between a linearly polarized ps pulse bunch Yb fiber laser and a synchronized Er fiber laser. The ps pulse bunch Yb fiber laser was composed of an all polarization maintained “figure of eight” structured mode-locked Yb fiber laser as the seed, a pulse multiplier, and two stages of Yb fiber amplifiers. The mode-locked Yb fiber laser has an output ps laser pulse at 1030 nm with a repetition rate of 16.32 MHz. The pulses were then transformed to the pulse bunches through the pulse multiplier. Within each bunch, there were 16 equally spaced pulses with pulse widths of 8.5 ps and time intervals around 300 ps. The Er fiber laser had a gain switched seed laser diode working at 1550 nm with a pulse width around 5.1 ns, which was synchronized to the mode-locked Yb fiber laser pulse bunch, and two stages of Er fiber amplifiers. Under the average power of 27.8 W of Yb fiber laser, we obtained an average output power of 3.1 W at 3.07 µm. This is, to the best of our knowledge, the highest average power mid-infrared ps pulse bunch obtained via DFG directly.

High pulse energy fiber/solid-slab hybrid picosecond pulse system for material processing on polycrystalline diamonds

We demonstrate an all polarization-maintaining (PM) fiber mode-locked laser seeded, hybrid fiber/solid-slab picosecond pulse laser system which outputs $40~\unicode[STIX]{x03BC}\text{J}$, 10 ps pulses at the central wavelength of 1064 nm. The beam quality factors $M^{2}$ in the unstable and stable directions are 1.35 and 1.31, respectively. $15~\unicode[STIX]{x03BC}\text{J}$ picosecond pulses at the central wavelength of 355 nm are generated through third harmonic generation (THG) by using two $\text{LiB}_{3}\text{O}_{5}$ (LBO) crystals, in order to get better processing efficiency on polycrystalline diamonds. The high pulse energy and beam quality of these ultraviolet (UV) picosecond pulses are confirmed by latter experiments of material processing on polycrystalline diamonds. This scheme which combines the advantages of the all PM fiber mode-locked laser and the solid-slab amplifier enables compact, robust and chirped pulse amplification-free amplification with high power picosecond pulses.

Model studies of THz-range generation via down conversion of the radiation of Ti:Sapphire lasers in LBO crystals

Results from model studies of the possibility of creating a source of high-power picosecond pulses of terahertz radiation via optical rectification and phase-matched down conversion of femtosecond pulses of Ti:Sapphire lasers in a nonlinear LBO crystal are presented. Modified Sellmeier equations are used in calculations. It is shown that the lengths of coherence for the generation of THz radiation at frequencies higher than 0.5 THz are more than 0.5 mm, allowing the technologically simple production of periodic structures. The maximum length of coherence is achieved for s–s type interactions in the XY plane. Phase-matched s–f type down conversion is possible only in the XY plane, including noncritical spectral matching conditions. Maximum efficiency can be expected for three-wave interactions with the polarization of interacting waves parallel to Z axis.

Spectral-temporal dynamics of high power Raman picosecond pulse using H2-filled Kagome HC-PCF.

We report on the spectral-temporal characterization of a 1.8μm wavelength and high power picosecond pulse Raman source. It is generated via frequency conversion to the first-order Stokes of a 27ps chirped pulse Yb-doped fiber laser inside a molecular hydrogen-filled Kagome hollow-core photonic crystal fiber (HC-PCF). Depending on the average power and chirp of the pump laser, the average power of this Raman source can be as high as 9.3W, and its pulse duration can be as short as ∼17 ps. In agreement with stimulated Raman scattering under the very high gain transient regime, the experimental results show the Stokes spectral structure to change following a three-stage sequence when the average pump power is increased. For a pump with a chirp corresponding to a bandwidth of 200GHz, we found that for a pump power lower than 7W, the Stokes spectrum is generated from the blue side of the pump spectrum, and then it exhibits a spectral replica of the pump spectrum for 7-14W pump power range. Finally,the Stokes spectrum is chiefly generated from the red side of the pump spectrum when the pump power is further increased. Conversely, the Stokes pulse temporal profile shows a strong dependence with the pump power. For a low pump power range, the Stokes pulse exhibits a single peak with a full width at half-maximum of ∼17 ps. For higher pump powers, the Stokes pulse presents a double-peak structure with each peak having a duration of less than 15ps. The present results can be used to develop compact and efficient frequency down-convertors to the increasingly widespread Yb-based picosecond lasers.

Stretching of picosecond laser pulses with uniform reflecting volume Bragg gratings

This study shows that a uniform reflecting volume Bragg grating (VBG) can be used as a compact monolithic stretcher of high-power picosecond laser pulses, which is important for cases in which chirped Bragg gratings with the required chirp rate are difficult to fabricate. When an incident short pulse propagates along a grating and experiences local Bragg diffraction, a chirp-free reflected stretched pulse with an almost rectangular shape is generated. The increase in the duration of the reflected pulse is approximately equal to twice the propagation time along the grating. We derive an analytic expression for the diffraction efficiency, which incorporates the incident pulse duration, grating thickness, and amplitude of the refractive index modulation, enabling selection of the optimum grating for pulse stretching. Theoretical models of the extended pulse profiles are found to be in good agreement with experimental autocorrelation measurements. We also propose a simple and reliable method to control the temporal parameters of high-power picosecond pulses using the same laser source and a VBG of variable thickness, which can simplify experiments requiring different pulse durations significantly.

Nonlinear processes upon two-photon interband picosecond excitation of crystal

A new experimental method is proposed to study the dynamics of nonlinear processes occurring upon two-photon interband picosecond excitation of a lead tungstate crystal and upon its excitation by cw probe radiation in a temporal range from several nanoseconds to several seconds. The method is applied to the case of crystal excitation by a sequence of 25 high-power picosecond pulses with a wavelength of and cw probe radiation. Measuring the probe beam transmittance during crystal excitation, one can investigate the influence of two-photon interband absorption and the thermal nonlinearity of the refractive index on the dynamics of nonlinear processes in a wide range of times (from several nanoseconds to several seconds). The time resolution of the measuring system makes it possible to distinguish fast and slow nonlinear processes of electronic or thermal nature, including the generation of a thermal lens and thermal diffusion. An alternative method is proposed to study the dynamics of induced absorption transformation and, therefore, the dynamics of the development of nonlinear rocesses upon degenerate two-photon excitation of the crystal in the absence of external probe radiation.

67.9 W high-power white supercontinuum all-fiber laser source.

We present a high-power white supercontinuum (SC) all-fiber laser source with average power of 67.9 W, spectrum ranging from 500 to 1700 nm, and spectral width exceeding 1000 nm for spectrum with flatness below 10 dB (except pump wavelength). Also, the visible waveband power (below 850 nm) occupies about 21% of the total SC power. A 145 W high-power picosecond pulse fiber laser is specially designed with high repetition frequency of 656 MHz to reduce nonlinear effects. Meanwhile, a homemade high-power mode field adaptor that can operate stably at hundreds of watts of pulse power has high coupling efficiency of 82%. To our knowledge, the 67.9 W white SC fiber laser source we achieved is the highest reported with such a wide and flat optical spectrum.

Hollow-core fibers for high power pulse delivery.

We investigate hollow-core fibers for fiber delivery of high power ultrashort laser pulses. We use numerical techniques to design an anti-resonant hollow-core fiber having one layer of non-touching tubes to determine which structures offer the best optical properties for the delivery of high power picosecond pulses. A novel fiber with 7 tubes and a core of 30µm was fabricated and it is here described and characterized, showing remarkable low loss, low bend loss, and good mode quality. Its optical properties are compared to both a 10µm and a 18µm core diameter photonic band gap hollow-core fiber. The three fibers are characterized experimentally for the delivery of 22 picosecond pulses at 1032nm. We demonstrate flexible, diffraction limited beam delivery with output average powers in excess of 70W.

Oscillations of absorption of a probe picosecond light pulse caused by its interaction with stimulated picosecond emission of GaAs

The self-modulation of absorption of a picosecond light pulse was observed earlier [1] in a thin (∼1-μm thick) GaAs layer pumped by a high-power picosecond pulse. Analysis of the characteristics of this self-modulation predicted [5] that the dependences of the probe pulse absorption on the pump pulse energy and picosecond delay between pump and probe pulses should be self-modulated by oscillations. Such self-modulation was experimentally observed in this work. Under certain conditions, absorption oscillations proved to be a function of part of the energy of picosecond stimulated emission of GaAs lying above a certain threshold in the region where the emission front overlapped the probe pulse front. Absorption oscillations are similar to self-modulation of the GaAs emission characteristics observed earlier [4]. This suggests that the self-modulation of absorption and emission is determined by the same type of interaction of light pulses in the active medium, the physical mechanism of which has yet to be determined.

Telecom to mid-infrared spanning supercontinuum generation in hydrogenated amorphous silicon waveguides using a Thulium doped fiber laser pump source

A 1,000 nm wide supercontinuum, spanning from 1470 nm in the telecom band to 2470 nm in the mid-infrared is demonstrated in a 800 nm x 220 nm 1 cm long hydrogenated amorphous silicon strip waveguide. The pump source was a picosecond Thulium doped fiber laser centered at 1950 nm. The real part of the nonlinear parameter of this waveguide at 1950 nm is measured to be 100 ± 10 W -1m-1, while the imaginary part of the nonlinear parameter is measured to be 1.2 ± 0.2 W-1m-1. The supercontinuum is stable over a period of at least several hours, as the hydrogenated amorphous silicon waveguides do not degrade when exposed to the high power picosecond pulse train.

Synchronously pumped OPO for picosecond pulse generation in the mid-infrared near 6.45 μm using AgGaS2 and CdSiP2: a comparative study

We compare the performance of two different mid-infrared nonlinear crystals, type I and type II AgGaS2 (AGS), and type I CdSiP2 (CSP), in a synchronously pumped optical parametric oscillator (SPOPO) pumped at 1064 nm. CSP showed a clear performance advantage at 6.5 μm idler wavelength in comparison to AGS. The oscillation threshold with CSP was three and five times lower compared to AGS type II and type I, respectively. The highest quantum conversion efficiency was obtained for CSP. It allowed us to reach 27.5 mW of idler average power, compared to 13 and 6 mW with type II and type I AGS, respectively. However, the tuning range was narrower with CSP than with AGS in both type I and type II configurations. The performance of the CSP SPOPO makes it suitable for generation of high power picosecond pulses at 6.45 μm for surgical applications.

Supercontinuum generated in all-normal dispersion photonic crystal fibers with picosecond pump pulses

The supercontinuum (SC) generation in all-normal dispersion (ANDi) photonic-crystal fiber (PCF) pumped by high power picosecond pulses are investigated in this paper. Our results show that an octave SC may be achieved by pumping the ANDi PCF with picosecond pump pulses. However, the PCF length required may have to be lengthened to several tens of centimeters, which is much longer than that with femtosecond pump pulses. The relatively long PCF gives rise to much higher Raman gain and stronger Raman frequency shift compared to those with femtosecond pump pulses, which in turn not only cause a distorted temporal waveform and an un-flattened spectrum, but also severely degrade the coherence of the generated SC.

Self-diffraction of ultrashort laser pulses under resonant excitation of excitons in a colloidal solution of CdSe/ZnS quantum dots

We report self-diffraction processes of two types under resonant excitation of the fundamental electron – hole (exciton) transition in a strongly absorbing colloidal solution of CdSe/ZnS quantum dots (QDs) by high-power picosecond laser pulses. In the first case the absorption saturation (bleaching) at the exciton transition frequency and the Stark shift of exciton absorption line lead to the formation of a transparency channel and self-diffraction of the laser beam from the thus induced round diaphragm. In the second case, self-diffraction of two laser beams, intersecting in a cell with a colloidal QD solution, occurs on the diffraction grating induced by these beams. The physical processes responsible for the nonlinear optical properties of CdSe/ZnS QDs and the found self-action effects are analysed.

Generation and emission of high-power ultrabroadband picosecond pulses

The generation and emission of high-power ultrabroadband electromagnetic pulses are studied. The possibility of high-voltage (about 100 kV) bipolar voltage pulses with a duration of about 200 ps is demonstrated. The electromagnetic pulses with a FWHM of less than 100 ps, an effective potential of up to 400 kV, and a repetition rate of 100 Hz are generated using the 16-element antenna array.

High-Power Versatile Picosecond Pulse Generation from Mode-Locked Quantum-Dot Laser Diodes

This paper presents the current status of our research in mode-locked quantum-dot edge-emitting laser diodes, particularly highlighting the recent progress in spectral and temporal versatility of both monolithic and external-cavity laser configurations. Spectral versatility is demonstrated through broadband tunability and novel mode-locking regimes that involve distinct spectral bands, such as dual-wavelength mode-locking, and robust high-power wavelength bistability. Broad tunability of the pulse repetition rate is also demonstrated for an external-cavity mode-locked quantum-dot laser, revealing a nearly constant pulse peak power at different pulse repetition rates. High-energy and low-noise pulse generations are demonstrated for low-pulse repetition rates. These recent advances confirm the potential of quantum-dot lasers as versatile, compact, and low-cost sources of ultrashort pulses.

Two-photon absorption of high-power picosecond pulses in PbWO4, ZnWO4, PbMoO4, and CaMoO4 crystals

The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm2. The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO4 and ZnWO4 crystals reaches 50–60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients β for the crystals vary over a wide range, namely, from β = 2.4 cm/GW for PbMoO4 to β = 0.14 cm/GW for CaMoO4, and the values of β can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.

High-power picosecond laser pulse recirculation

We demonstrate a nonlinear crystal-based short pulse recirculation cavity for trapping the second harmonic of an incident high-power laser pulse. This scheme aims to increase the efficiency and flux of Compton-scattering-based light sources. We demonstrate up to 40x average power enhancement of frequency-doubled submillijoule picosecond pulses, and 17x average power enhancement of 177 mJ, 10 ps, 10 Hz pulses.

Multiphoton avalanche generation of free carriers in a multiband crystal

A new mechanism for generating nonequilibrium electron-hole pairs in transparent wide-gap crystals under high-power picosecond light pulses of prebreakdown intensity is considered. The kinetics of free carrier generation is investigated by the numerical solution of the system of balance equations for particle concentrations within the multiband model of electron energy spectrum of the crystal. It is shown that in the intensity range of 1010–1012 W/cm2 the proposed way of nonequilibrium carrier generation is more effective in the case of picosecond pulses than the conventional multiphoton absorption. It is shown that the nonequilibrium carrier generation considered here may initiate a multiphoton avalanche.