µJ-level multi-cycle terahertz generation in a periodically poled Rb:KTP crystal.

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 900mJ/cm2, showing the great potential of Rb:PPKTP for multi-millijoule-level MC-THz generation.

87-W 1018-nm Yb-fiber ultrafast seeding source for cryogenic Yb: yttrium lithium fluoride amplifier.

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.

Narrowband terahertz generation with chirped-and-delayed laser pulses in periodically poled lithium niobate.

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.

Pulse sequences for efficient multi-cycle terahertz generation in periodically poled lithium niobate.

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 (100 K) periodically poled Lithium Niobate is proposed and studied. Detailed simulations considering the coupled nonlinear interaction of terahertz and optical waves (or pump depletion), 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 limitations at Joule-level pumping by 1µm 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. The physics governing terahertz generation using pulse sequences in this high conversion efficiency regime, limitations and practical considerations are discussed. It is shown that varying the poling period along the crystal length and further reduction of absorption can lead to even higher energy conversion efficiencies >>10%. In addition to numerical calculations, 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.

Cascaded parametric amplification for highly efficient terahertz generation.

A highly efficient, practical approach to high-energy multi-cycle terahertz (THz) generation based on spectrally cascaded optical parametric amplification (THz-COPA) is introduced. Feasible designs are presented that enable the THz wave, initially generated by difference frequency generation between a narrowband optical pump and optical seed (0.1-10% of pump energy), to self-start a cascaded (or repeated) energy downconversion of pump photons in a single pass through a single crystal. In cryogenically cooled, periodically poled lithium niobate, unprecedented energy conversion efficiencies >8% achievable with existing pump laser technology are predicted using realistic simulations. The calculations account for cascading effects, absorption, dispersion, and laser-induced damage. Due to the simultaneous, coupled nonlinear evolution of multiple phase-matched three-wave mixing processes, THz-COPA exhibits physics distinctly different from conventional three-wave mixing parametric amplifiers. This, in turn, governs optimal phase-matching conditions, evolution of optical spectra, and limitations of the nonlinear process. Circumventing these limitations is shown to yield conversion efficiencies ≫10%.

Efficient narrowband terahertz generation in cryogenically cooled periodically poled lithium niobate.

We present an efficiency scaling study of optical rectification in cryogenically cooled periodically poled lithium niobate for the generation of narrowband terahertz radiation using ultrashort pulses. The results show an efficiency and brilliance increase compared to previous schemes of up to 2 orders of magnitude by exploring the optimal pump pulse format at around 800 nm, and reveal saturation mechanisms limiting the conversion efficiency. We achieve >10⁻³ energy conversion efficiencies, µJ-level energies, and bandwidths <20 GHz at ∼0.5 THz, thereby showing unprecedented spectral brightness in the 0.1-1 THz range relevant to terahertz science and technology.

Theory of terahertz generation by optical rectification using tilted-pulse-fronts.

A model for terahertz (THz) generation by optical rectification using tilted-pulse-fronts is developed. It simultaneously accounts for in two spatial dimensions (2-D) (i) the spatio-temporal variations of the optical pump pulse imparted by the tilted-pulse-front setup, (ii) the nonlinear coupled interaction of THz and optical radiation, (iii) self-phase modulation and (iv) stimulated Raman scattering. The model is validated by quantitative agreement with experiments and analytic calculations. We show that the optical pump beam is significantly broadened in the transverse-momentum (kx) domain as a consequence of its spectral broadening due to THz generation. In the presence of this large frequency and transverse-momentum (or angular) spread, group velocity dispersion causes a spatio-temporal break-up of the optical pump pulse which inhibits further THz generation. The implications of these effects on energy scaling and optimization of optical-to-THz conversion efficiency are discussed. This suggests the use of optical pump pulses with elliptical beam profiles for large optical pump energies. Furthermore, it is seen that optimization of the setup is highly dependent on optical pump conditions. Trade-offs in optimizing the optical-to-THz conversion efficiency on the spatial and spectral properties of THz radiation are discussed to guide the development of such sources.

Pre-chirp managed nonlinear amplification in fibers delivering 100 W, 60 fs pulses.

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.

Radially polarized Bessel-Gauss beams: decentered Gaussian beam analysis and experimental verification.

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.

Optimization of femtosecond Yb-doped fiber amplifiers for high-quality pulse compression.

We both theoretically and experimentally investigate the optimization of femtosecond Yb-doped fiber amplifiers (YDFAs) to achieve high-quality, high-power, compressed pulses. Ultrashort pulses amplified inside YDFAs are modeled by the generalized nonlinear Schrödinger equation coupled to the steady-state propagation-rate equations. We use this model to study the dependence of compressed-pulse quality on the YDFA parameters, such as the gain fiber's doping concentration and length, and input pulse pre-chirp, duration, and power. The modeling results confirmed by experiments show that an optimum negative pre-chirp for the input pulse exists to achieve the best compression.

Bessel-Gauss beam enhancement cavities for high-intensity applications.

We introduce Bessel-Gauss beam enhancement cavities that may circumvent the major obstacles to more efficient cavity-enhanced high-field physics such as high-harmonic generation. The basic properties of Bessel-Gauss beams are reviewed and their transformation properties through simple optical systems (consisting of spherical and conical elements) are presented. A general Bessel-Gauss cavity design strategy is outlined, and a particular geometry, the confocal Bessel-Gauss cavity, is analyzed in detail. We numerically simulate the confocal Bessel-Gauss cavity and present an example cavity with 300 MHz repetition rate supporting an effective waist of 33 µm at the focus and an intensity ratio from the focus to the cavity mirror surfaces of 1.5 × 10(4).

Generalizing higher-order Bessel-Gauss beams: analytical description and demonstration.

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.

Self-phase modulation compensated by positive dispersion in chirped-pulse systems.

We experimentally demonstrate compensation of the impact of Kerr-nonlinearity with positive dispersion in chirped-pulse systems. The condition for the phase-compensation is derived and design guidelines are presented. The technique is shown with a fiber-based system employing conventional diffraction gratings as well in a system that is based on chirped volume Bragg-gratings. Practical requirements on the stretching unit are discussed.

Circular versus linear polarization in laser-amplifiers with Kerr-nonlinearity.

In this contribution it is reported that circularly polarized light is advantageous if the Kerr-effect has to be minimized during laser-amplification. The experimental demonstration is based on a fiber CPA-system. The different polarization states 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 verified. In laser-amplifiers circularly polarized light reduces the detrimental impact of the Kerr-nonlinearity, and thus, increases the peak-power and the self-focussing threshold.

Compensation of pulse-distortion in saturated laser amplifiers.

We derive an expression describing pre-compensation of pulse-distortion due to saturation effects in short pulse laser-amplifiers. The analytical solution determines the optimum input pulse-shape required to obtain any arbitrary target pulse-shape 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. The method will prove useful in applications of high power, high energy laser-amplifier systems that need particular pulse-shapes to be efficient, e.g. micromachining and scientific laser-matter-interactions.

The impact of spectral modulations on the contrast of pulses of nonlinear chirped-pulse amplification systems.

A detrimental pulse distortion mechanism inherent to nonlinear chirped-pulse amplification systems is revealed and analyzed. When seeding the nonlinear amplification stage with pulses possessing weak side-pulses, the Kerr-nonlinearity causes a transfer of energy from the main pulse to side pulses. The resulting decrease in pulse contrast is determined by the accumulated nonlinear phase-shift (i.e., the B-integral) and the initial pulse-contrast. The energy transfer can be described by Bessel-functions. Thus, applications relying on a high pulse-contrast demand a low B-integral of the amplification system and a master-oscillator that exhibits an excellent pulse-contrast. In particular, nonlinear fiber CPA-systems operated at B-integrals far beyond pi have to be revised in this context.

Controlling the influence of SPM in fiber-based chirped-pulse amplification systems by using an actively shaped parabolic spectrum.

We report on the experimental demonstration of the control of the influence of nonlinearity in fiber-based chirped-pulse amplification (CPA) using active spectral amplitude shaping. By applying a liquid crystal spatial light modulator, the influence of the spectral profile on the recompressed pulse quality is experimentally revealed. The parabolic spectrum is experimentally determined to be very suitable for CPA-systems in which nonlinearity is present. The corresponding nonlinear phase contribution can be efficiently compensated by a conventional grating compressor. In a proof-of-principle experiment using an Yb-doped fiber- CPA-system, control at a B-integral as high as 16 rad is demonstrated. The method allows significant performance improvement of fiber-based chirpedpulse amplification.