Search

We demonstrate multi-cycle terahertz (MC-THz) generation in a 15.5 mm long periodically poled rubidium (Rb)-doped potassium titanyl phosphate (Rb:PPKTP) crystal with a poling period of 300 µm. By cryogenically cooling the crystal to 77 K, up to 0.72 µJ terahertz energy is obtained at a frequency of 0.5 THz with a 3 GHz bandwidth. A maximum internal optical-to-terahertz conversion efficiency of 0.16% is achieved, which is comparable with results achieved using periodically poled lithium niobate crystal. Neither photorefractive effects nor damage was observed with up to 900 m J / c m 2 , showing the great potential of Rb:PPKTP for multi-millijoule-level MC-THz generation.

We demonstrate efficient multicycle THz generation with periodically poled lithium niobate crystals via difference frequency generation using a multiple-line source. The THz yield for different crystal lengths as well as pump pulse durations is investigated. Our experimental results show excellent agreement with the theoretical predictions. By employing a 20-mm PPLN crystal, up to 2.7 μJ at 0.5 THz was obtained with 6.2 mJ input pump pulse energy (130 m.J/cm2 peak fluence) at 77 K, corresponding to an internal conversion efficiency of 0.3%. The record internal efficiency at the peak fluence of 130 m.l/cm? shows the great potential for high-energy multicycle THz generation with a multiple-line source.

Nuclear instruments & methods in physics research / A 909, 27 - 32 (2018). doi:10.1016/j.nima.2018.01.074, We describe initial steps in the development of the technology for a THz-driven accelerator that will drive a compact attosecond X-ray light source. THz- driven structures represent a promising emerging technology for compact acceleration of sub-femtosecond electron bunches. The millimeter scale of the driving field offers a favorable compromise between conventional accelerators which are proven and reliable but large and costly, and other advanced accelerators like plasma-based or laser-driven devices where the microscopic accelerator structuresmake device control difficult and limit the charge payload. By contrast the THz-driven structures are large enough to be fabricated by conventional means leading to a high degree of repeatability and control, can support field gradients that are significantly higher than in conventional accelerators, promising capabilities to produce sub-femtosecond electron bunches. In addition, the strong fields in THz based devices offer potential for compact, strong-field manipulation and diagnosis of electron bunches. Our results pave the way for development of a THz-based light source for sub-femtosecond investigation of material structure., Published by Elsevier, Amsterdam

We demonstrated efficient multi-cycle terahertz generation centered at 0.5 THz with periodically poled lithium niobate crystals via difference frequency generation with a multiple-frequency-line source. At 130 mJ/cm2 pump fluence, the internal optical-to-THz conversion efficiency reaches 0.3%.

We investigate multi-cycle 0.5-Terahertz generation in a periodically poled Rb-doped potassium titanyl phosphate (PPKTP) crystal. Up to 0.65-gJ terahertz energy was obtained at 77 K with maximum internal optical-to-terahertz efficiency of 0.09%.

We report on an 1.035-μ m, all-fiber, femtosecond chirped pulse amplifier with 55 uJ pulse energy, 1 MHz repetition rate, producing 800 ps chirped pulses. The output is compressed to 245 fs, 1.3x it's transform limit.

We demonstrate a compact and robust Yb-fiber master-oscillator power-amplifier system operating at 1018 nm with 2.5-nm bandwidth and 1-ns stretched pulse duration. It produces 87-W average power and 4.9-μJ pulse energy, constituting a powerful seed source for cryogenically cooled ultrafast Yb: yttrium lithium fluoride (Yb:YLF) amplifiers.

We report a turnkey timing distribution system able to serve multiple remote optical and microwave sources. The system exhibits only 0.3-fs timing jitter at the outputs of stabilized fiber links.

We generate narrowband terahertz radiation in periodically poled lithium niobate crystals using a train of chirped-and-delayed pulses from a broadband high energy Ti:sapphire laser. We achieve a multi-cycle THz energy of 40 μJ at 0.544 THz.

The potential of a linear THz accelerator technology is discussed. Theoretical and first experimental results on THz-driven guns and accelerators are presented with a focus on laser based THz generation to drive these devices.

High-peak-field, narrowband terahertz (THz) sources are well suited for compact electron acceleration [1], however, these sources are still under development and yet to reach optimal energies. A high conversion efficiency via difference frequency generation is established by quasi-phase matching in periodically poled lithium niobate (PPLN). Moreover, to achieve the required mJ-level THz pulses high energy input pulses are inevitable [2]. Chirp-and-delay pumping offers a method to increase the NIR pulse duration and to scale up the energy without damaging the crystal. Here, we present chirp-and-delay in PPLN of varying poling periods and study the effect of crystal length and temperature.

We propose and investigate a high power superluminescence diode (SLD) as a pulsed seed source for a highly dynamic and versatile pulse fiber amplifier system. The SLD provides, contrary to conventional Fabry–Perot laser diodes, a smooth and broad output spectrum which is independent of the input pulse parameters. The output pulses from the SLD are as short as 10 ns with up to 150 mW peak power. Moreover, the pulses can be directly shaped by modulating the injection current of the SLD. Pulse shaping in an amplifier configuration is demonstrated without the observation of stimulated Brillouin scattering (SBS) due to the provided spectral bandwidth of 10 nm FWHM. Further spectral shaping was realized with a band pass filter in the amplifier chain.

We review the main challenges and give design guidelines for high-peak-power high-average-power fiber-based chirped-pulse amplification (CPA) systems. It is clearly pointed out that the lowest order fiber nonlinearity (NL), namely the self-phase modulation, limits the scalability of high-energy ultrashort pulse fiber amplifiers. Therefore, a distinguished difference arises between the consequences of accumulated nonlinear phase originating from the pulse envelope and initial weak modulations, resulting in a strong recommendation to operate an amplification system as linearly as possible in order to generate high-contrast pulses. Low-NL rare-earth-doped fibers, such as the recently available designs of photonic crystal fibers, are the key element for successful peak power scaling in fiber laser systems. In this paper, we present a detailed analysis and optimization of the extraction characteristics in connection with the accumulated nonlinear phase in such extreme fiber dimensions. Consequently, millijoule pulse energy femtosecond pulses at repetition rates in the 100 kHz range have already been demonstrated experimentally in a Yb-fiber-based CPA system that has even further scaling potential.

We demonstrate experimentally the onset of cascaded optical parametric amplification (COPA) in periodically-poled lithium niobate. This technique permits narrowband terahertz wave generation beyond the Manley-Rowe limit.

The use of laser pulse sequences to drive the cascaded difference frequency generation of high energy, high peak-power and multi-cycle terahertz pulses in cryogenically cooled periodically poled lithium niobate is proposed. Detailed simulations considering the coupled nonlinear interaction of terahertz and optical waves show that unprecedented optical-to-terahertz energy conversion efficiencies > 5%, peak electric fields of hundred(s) of Mega volts/meter at terahertz pulse durations of hundred(s) of picoseconds can be achieved. The proposed methods are shown to circumvent laser-induced damage at Joule-level pumping by 1$��$m lasers to enable multi-cycle terahertz sources with pulse energies >> 10 milli-joules. Various pulse sequence formats are proposed and analyzed. Numerical calculations for periodically poled structures accounting for cascaded difference frequency generation, self-phase-modulation, cascaded second harmonic generation and laser induced damage are introduced. Unprecedented studies of the physics governing terahertz generation in this high conversion efficiency regime, limitations and practical considerations are discussed. Varying the poling period along the crystal length and further reduction of absorption is shown to lead to even higher energy conversion efficiencies >>10%. An analytic formulation valid for arbitrary pulse formats and closed-form expressions for important cases are presented. Parameters optimizing conversion efficiency in the 0.1-1 THz range, the corresponding peak electric fields, crystal lengths and terahertz pulse properties are furnished., 24 pages, 11 figures

Optics letters 43(8), 1686 - 1689 (2018). doi:10.1364/OL.43.001686, We demonstrate a compact and robust Yb-fiber master-oscillator power-amplifier system operating at 1018 nm with 2.5-nm bandwidth and 1-ns stretched pulse duration. It produces 87-W average power and 4.9-μJ pulse energy, constituting a powerful seed source for cryogenically cooled ultrafast Yb: yttrium lithium fluoride (Yb:YLF) amplifiers., Published by OSA, Washington, DC

A highly efficient, practical approach to high-energy terahertz (THz) generation based on spectrally cascaded optical parametric amplification (THz-COPA) is introduced. The THz wave initially generated by difference frequency generation between a strong narrowband optical pump and optical seed (0.1-10% of pump energy) kick-starts a repeated or cascaded energy down-conversion of pump photons. This helps to greatly surpass the quantum-defect efficiency and results in exponential growth of THz energy over crystal length. In cryogenically cooled periodically poled lithium niobate, energy conversion efficiencies >8% for 100 ps pulses are predicted. The calculations account for cascading effects, absorption, dispersion and laser-induced damage. Due to the coupled nonlinear interaction of multiple triplets of waves, THz-COPA exhibits physics distinct from conventional three-wave mixing parametric amplifiers. This in turn governs optimal phase-matching conditions, evolution of optical spectra as well as limitations of the nonlinear process., 5 pages, double column

We describe cascaded difference-frequency generation (DFG) between multiple laser-lines generating terahertz pulses at >10% energy-conversion efficiency. DFG initiated by a laser-line and weak laser-seed rapidly cascades to self-generate multiple laser-lines and produce exponential terahertz growth.

International Conference on Ultrafast Phenomena, UP2016, Santa Fe, USA, 17 Jul 2016 - 22 Jul 2016 ; (2016)., We experimentally demonstrate cascaded optical parametric chirped-pulse amplification, an efficient method for the amplification of multi-cycle THz-frequency waves. Experimental results are supported by numerical simulations and promise THz efficiencies at the percent level.

We proposed and demonstrated pre-chirp managed amplification (PCMA), in which the seeding pulse was nonlinearly amplified such that the amplified spectrum was substantially broadened. By properly pre-chirping the seeding pulse, the amplified pulse can be compressed with the duration much shorter than the transform-limited duration allowed by the seeding spectrum. Using an Yb-doped rod-type large-pitch fiber as the power amplifier, PCMA has enabled us to generate 75 MHz, ~60 fs, linearly-polarized pulses with >100-W average power. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We demonstrate that seeding pre-chirp managed amplification by circularly polarized seeding pulses can increase the amplified pulse energies. Using this method, we generate 100-W, 26-fs pulses at 23.7-MHz repetition rate.

We demonstrate a pre-chirp managed Yb-doped fiber laser system that outputs 75 MHz, 130 W spectrally broadened pulses, which are compressed by a diffraction-grating pair to 60 fs with average powers as high as 100 W. Fine tuning the pulse chirp prior to amplification leads to high-quality compressed pulses. Detailed experiments and numerical simulation reveal that the optimum pre-chirp group-delay dispersion increases from negative to positive with increasing output power for rod-type high-power fiber amplifiers. The resulting laser parameters are suitable for extreme nonlinear optics applications such as frequency conversion in femtosecond enhancement cavities.

We experimentally characterize amplitude and phase fluctuations of a femtosecond fiber oscillator close to the standard quantum limit. A passive cavity is employed to convert frequency noise to RIN with close to quantum-limited sensitivity.

We demonstrate a 1.5×10−4 room temperature conversion efficiency and 0.29 μJ pulse energy for narrowband terahertz generation in PPLN pumped with ultrafast pulses at 800 nm by experimentally and numerically optimizing the pump-pulse format.

Pre-chirp managed nonlinear amplification delivering high quality 60 fs pulses is compared to conventionally employed CPA. We present a compact rod-type system that outputs >100 W at 75 MHz.

Optics letters 42(11), 2118 - 2121 (2017). doi:10.1364/OL.42.002118, We generate narrowband terahertz (THz) radiation in periodically poled lithium niobate (PPLN) crystals using two chirped-and-delayed driver pulses from a high-energy Ti:sapphire laser. The generated frequency is determined by the phase-matching condition in the PPLN and influences the temporal delay of the two pulses for efficient terahertz generation. We achieve internal conversion efficiencies up to 0.13% as well as a record multicycle THz energy of 40 μJ at 0.544 THz in a cryogenically cooled PPLN., Published by OSA, Washington, DC

We report on efficient generation of millijoule-level, kilohertz-repetition-rate few-cycle laser pulses with radial polarization by combining a gas-filled hollow-waveguide compression technique with a suitable polarization mode converter. Peak power levels >85 GW are routinely achieved, capable of reaching relativistic intensities >10(19) W/cm2 with carrier-envelope-phase control, by employing readily accessible ultrafast high-energy laser technology.

We generate radially polarized ultra-intense few-cycle pulses with 62 GW peak-power at 3 kHz capable of reaching intensities >1E19 W/cm^2 and perform simulations predicting acceleration of non-relativistic electrons in vacuum by up to 300 keV

We demonstrate a pre-chirp managed Yb-fiber laser system that outputs 75-MHz, 130-W spectrally broadened pulses, which are compressed to 60 fs with 100-W average power. The source is suitable for cavity-enhanced ultrafast optics.

We propose and experimentally demonstrate new beam solutions in form of azimuthally symmetric higher-order Bessel-Gauss beams. We experimentally observe these beams as higher-order eigenmodes in optical resonators consisting of aspheric mirrors.

We derive solutions for radially polarized Bessel-Gauss beams in free-space by superimposing decentered Gaussian beams with differing polarization states. We numerically show that the analytical result is applicable even for large semi-aperture angles, and we experimentally confirm the analytical expression by employing a fiber-based mode-converter.

We propose azimuthally symmetric higher-order Bessel-Gauss beams, and experimentally demonstrate them as higher-order eigenmodes in optical resonators comprising aspheric mirrors

We derive solutions for radially polarized Bessel-Gauss beams in ABCD optical systems by superimposing decentered Gaussian beams with linear polarization states. We experimentally confirm the expression by employing a fiber-based mode-converter.

We report on a novel class of higher-order Bessel-Gauss beams in which the well-known Bessel-Gauss beam is the fundamental mode and the azimuthally symmetric Laguerre-Gaussian beams are special cases. We find these higher-order Bessel-Gauss beams by superimposing decentered Hermite-Gaussian beams. We show analytically and experimentally that these higher-order Bessel-Gauss beams resemble higher-order eigenmodes of optical resonators consisting of aspheric mirrors. This work is relevant for the many applications of Bessel-Gauss beams in particular the more recently proposed high-intensity Bessel-Gauss enhancement cavities for strong-field physics applications.

We describe polarization-sensitive measurements of the modal content using wide-field optical low-coherence interferometry. This polarization-sensitive cross-correlation (C2) imaging technique is well suited for characterizing the impact of polarization on the amplification dynamics in high-power fiber lasers.

We combine steady-state propagation-rate equation and generalized nonlinear Schrodinger equation to accurately model high-repetition rate femtosecond Yb-fiber amplifiers. Such modeling reveals the nonlinear-evolution dynamics of the amplified pulse and allows the optimization of the compressed-pulse quality.

We experimentally demonstrate that circular polarization state is beneficial if the Kerr-nonlinearity has to be lowered during the amplification of laser pulses. It can be shown that in a fiber-based chirped pulse amplification (CPA) system, the use of circularly and linearly polarized light result in different B-integrals, which are measured using phase-only pulse-shaping. The theoretical value of 2/3 for the ratio of the B-integrals of circularly and linearly polarized light is experimentally confirmed. Circularly polarized light facilitates peak-power scaling, moreover, the self-focussing threshold can be enhanced.

We experimentally demonstrate phase-shaping in fiber CPA-systems, providing pulse-energies at the mJ-level. The applied method is based on an analytical model describing the impact of SPM in CPA-systems. Using this phase-shaping technique nearly transform limited pulses are produced at B-integrals up to 10 rad. Compared to a nonlinear CPAsystem with the best performance being achieved by adjusting the compressor, operation of the same system using the phase-shaping method permits peak-power enhancement by a factor better than 2.

To further scale the peak-power of state-of-the-art fiber CPA-systems, a careful optimization of the spectral as well as temporal dynamics is required. The wavelength dependence of the small-signal gain, as well as the saturation of the amplifier, strongly affect the signal bandwidth. For unsaturated amplifiers only a spectral optimization is required. It can be shown that both the spectral center and the width of the input spectrum strongly affect the output bandwidth. An optimization regarding these two parameters will be given. Design guidelines are presented. We develop a simple yet efficient model to simulate the impact of saturation in broadband Ytterbium-doped fiber CPA-systems. Using this model, we reveal that significant peak-power scaling up to 10 GW of current fiber CPA-systems is possible.

In this work we present a new method for peak-power scaling in nonlinear CPA-systems. By clipping the tails of the spectrum we demonstrate pulse quality enhancement and an increase of peak-power at the output of the CPA-system. A theoretical model allows us to determine an optimal ratio between the spectral clipping bandwidth and the pulse bandwidth at a certain B-integral. Additionally, a simple redesign of a grating based stretcher unit, applying our new spectral clipping technique, would significantly increase the output peak-power of such nonlinear CPA-systems by a factor up to six due to the higher stretching ratio.

Based on an analytical model for the impact of SPM on stretched ultrashort pulses, the influence of SPM is experimentally controlled in a fiber-based CPA-system. Nearly transform-limited pulses with energies as high as 1 mJ are produced at a B-integral as high as 8 rad. The value of the B-integral is measured by the phase-compensation method. The technique neither relies on complex phase-measurements nor on time-consuming optimization routines.

We experimentally present the advantage of using circularly polarized light during laser amplification to reduce the accumulated Kerr-nonlinearity.The theoretical value of 2/3 for the reduction of the B-integral is verified in a fiber CPA-system. The demonstrated technique is very attractive since it can be employed in nearly all kind of ultrafast laser systems to scale their performance, when they are limited by Kerr-nonlinearity.

We report on a high repetition rate noncollinear optical parametric amplifier system (NOPA) based on a cavity dumped Ti:Sapphire oscillator providing the signal, and an Ytterbium-doped fiber amplifier pumping the device. Temporally synchronized NOPA pump pulses are created via soliton generation in a highly nonlinear photonic crystal fiber. This soliton is fiber amplified to high pulse-energies at high repetition rates. The broadband Ti:Sapphire laser pulses are parametrically amplified either directly or after additional spectral broadening. The approach of fiber-based pump-pulse generation from a femtosecond laser, that emits in the spectral region of NOPA-gain, offers enhanced long-term stability and pulse quality compared to conventional techniques, such as signal pulse generation from a high power laser system via filamentation in bulk media. The presented system produces high-energy ultra-short pulses with pulse-durations down to 15.6 fs and pulse-energies up to 500 nJ at a repetition rate as high as 2 MHz.

We report on an optical parametric amplification system which is pumped and seeded by fiber generated laser radiation. Due to its low broadening threshold, high spatial beam quality and high stability, the fiber based broad bandwidth signal generation is a promising alternative to white light generation in bulky glass or sapphire plates. We demonstrate a novel and successful signal engineering implemented in a setup for parametric amplification and subsequent recompression of resonant linear waves resulting from soliton fission in a highly nonlinear photonic crystal fiber. The applied pump source is a high repetition rate ytterbium-doped fiber chirped pulse amplification system. The presented approach results in the generation of ~50 fs pulses at MHz repetition rate. The potential of generating even shorter pulse duration and higher pulse energies will be discussed.

We investigate supercontinuum generation in photonic crystal fibers under femtosecond single and dual wavelength pumping experimentally and by numerical simulations. Details about the expansion of the blue but also the red side of the continuum due to cross-phase modulation (XPM) and transfer of energy to dispersive waves are revealed and experimentally confirmed. Additionally, simple guidelines are given to predicte the maximum bandwidth of supercontinuum generation only by the use of the dispersion curve of the fiber.

In this work we present a new method for peak-power scaling in nonlinear CPA-systems. In conventional CPA systems temporal stretching is used to avoid high peak powers developing during the amplification process. However, if the peak power of the stretched pulse is high enough, a considerable amount of nonlinear spectral phase shift (as described by the B-integral) can be accumulated due to self-phase modulation. Consequently, reduced peak powers and pulse shape degradation will be obtained after compression. Such systems are referred to as nonlinear CPA-systems.

The generation of ultra-short pulses is typically governed by the interplay of dispersion and nonlinearity. Ultrashort pulses have high peak-powers which cause nonlinearities. In particular, nonlinearity can cause wave-breaking of ultra-short pulses. This effect imposes limitations on the generation and amplification of femtosecond laser pulses. Chirping of the pulses reduces the peak-power during amplification, and thus, increases the onset of nonlinearities. Since the chirping of the pulses is limited due to practical reasons, the impact of nonlinearity plays a role again with increasing pulse-energies. To further scale the energy of such systems, there is need for operation in the nonlinear CPA-regime.

We derive an expression describing pre-compensation of pulse-distortion due to saturation effects in short pulse laseramplifiers. The analytical solution determines the optimum input pulse required to obtain any arbitrary target pulse at the output of the saturated laser-amplifier. The relation is experimentally verified using an all-fiber amplifier chain that is seeded by a directly modulated laser-diode.

Pulse-contrast degradation at the CPA-system's output is analyzed. If Kerr-nonlinearity is present, weak initial spectral phase and amplitude modulations are responsible for the decrease. The pulse is split into several sub-pulses. Bessel-functions describe the intensities of the side-pulses relative to the principal pulse. We provide the governing physical quantities.