Optical Parametric Chirped-pulse Amplification System Research Articles

Ultra-broadband high conversion efficiency optical parametric chirped-pulse amplification based on YCOB crystals.

We present a high efficiency and ultra-broadband optical parametric chirped-pulse amplification (OPCPA) system fully based on yttrium calcium oxyborate (YCOB) crystals. The OPCPA properties of YCOB at 808 nm are studied for both high gain and saturated amplification. The non-collinear angle is finely tuned to study the variation of gain spectrum at a certain phase-matching angle of YCOB crystals. After amplification by four YCOB crystals, a total signal gain of 0.9×109 is obtained and the FWHM spectral bandwidth is still over 100 nm. An amplified signal pulse of 182 mJ is achieved with pump energy of 440 mJ in the saturated amplification stage and the conversion efficiency is about 40%. After a four-grating compressor, a pulse duration of 20 fs is measured by a second-order autocorrelator.

27 W 2.1 µm OPCPA system for coherent soft X-ray generation operating at 10 kHz.

We developed a high power optical parametric chirped-pulse amplification (OPCPA) system at 2.1 µm harnessing a 500 W Yb:YAG thin disk laser as the only pump and signal generation source. The OPCPA system operates at 10 kHz with a single pulse energy of up to 2.7 mJ and pulse duration of 30 fs. The maximum average output power of 27 W sets a new record for an OPCPA system in the 2 µm wavelength region. The soft X-ray continuum generated through high harmonic generation with this driver laser can extend to around 0.55 keV, thus covering the entire water window (284 eV - 543 eV). With a repetition rate still enabling pump-probe experiments on solid samples, the system can be used for many applications.

Water window soft x-ray source enabled by a 25 W few-cycle 2.2 µm OPCPA at 100 kHz

Coherent soft x-ray (SXR) sources enable fundamental studies in the important water window spectral region. Until now, such sources have been limited to repetition rates of 1 kHz or less, which restricts count rates and signal-to-noise ratio for a variety of experiments. SXR generation at high repetition rate has remained challenging because of the missing high-power infrared laser sources to drive the high-harmonic generation (HHG) process. Here we present an optical parametric chirped pulse amplifier (OPCPA) centered at a wavelength of 2.2 µm and generating 16.5 fs pulses (2.2 oscillation cycles of the carrier wave) with 25 W of average power and a peak power exceeding 14 GW at 100 kHz pulse repetition rate. This corresponds to the highest reported peak power for high-repetition-rate long-wavelength laser systems. The output of this 2.2 µm OPCPA system was used to generate a coherent SXR radiation extending beyond 0.6 keV through HHG in a high-pressure gas cell.

Comparative study of an ultrafast, CEP-stable, dual-channel mid-IR OPCPA system

In this theoretical work, the performances of ultrafast, carrier-envelope-phase-stable (CEP), mid-IR optical parametric chirped pulse amplification (OPCPA) systems were compared in two different amplification schemes. In the “idler scheme” the mid-IR pulse (idler) is produced in the first difference frequency generation stage of the OPCPA chain and then amplified in the following stages. In the “signal scheme,” the supercontinuum seed is amplified in the OPCPA chain and the mid-IR pulse is generated in the last amplifier stage. According to our results, the idler has higher energy and better energy stability in the idler scheme, while in the signal scheme the signal has the better characteristics in energy and stability. The CEP noise due to the pump intensity fluctuations was found to be lower in the signal scheme. Additionally, chirp optimization revealed that the peak power of the compressed idler and signal pulses at the output of the system is considerably higher if the chirp of the input signal pulse is positive.

Nonlinear Compression of Intense Optical Pulses at 1.55 μm by Multiple Plate Continuum Generation

Intense ultrashort laser pulses at 1.55 μ m are generated by a home-built light source and compressed to 20 fs by multiple plate continuum (MPC) and Fourier pulse shaping. The entire system consists of three building blocks, responsible for seed generation, optical parametric chirped pulse amplification (OPCPA), and nonlinear compression. A commercial Yb:KGW amplifier pumps a difference frequency generator and an optical parametric amplifier (OPA) to generate 1 kHz, 44 fs, 10 μ J, carrier-envelope phase stable seed pulses at 1.55 μ m. A grating–lens pair stretches the seed pulses to 50 ps by introducing a great amount of normal group delay dispersion (GDD). After that, the chirped pulse energy is boosted to 5 mJ by two OPA stages pumped by a commercial Nd:YAG amplifier (30 mJ, 85 ps). To compress the amplified pulses, a grating pair compressor providing an adequate amount of anomalous GDD is employed. The OPCPA system eventually delivers 3 mJ and 80 fs pulses, limited by the phase-matching bandwidth of nonlinear crystals. To further compress the pulse duration, pulses are sent to an MPC stage, which consisted of nine pieces of 200- μ m-thick quartz plates. The OPCPA output spectrum is broadened by four times and compressed to 20 fs (3.6 carrier cycles) by a femtosecond Fourier-transform pulse shaper. The pulses are characterized by polarization gating cross-correlation frequency-resolved optical gating. Due to the high pondermotive force, an intense and long-wavelength laser system like this is useful in seeding a MeV-level electron accelerator and generating coherent soft X-ray high-harmonic generation.

9 μm few-cycle optical parametric chirped-pulse amplifier based on LiGaS2.

We report a long-wavelength mid-infrared (mid-IR), few-cycle optical parametric chirped-pulse amplifier (OPCPA) based on LiGaS2 crystals, pumped by a 1μm Yb:YAG laser, at a 10kHz repetition rate. The mid-IR OPCPA system generates pulses centered at 9μm, with 14μJ pulse energy and 140mW average power. A 142fs pulse width, which corresponds to less than 5 optical cycles at 9μm, is measured by an interferometric autocorrelator. This is, to the best of our knowledge, the first long-wavelength mid-IR OPCPA pumped at 1μm wavelength. It paves the way for the energy and power scaling of the ultrafast long-wavelength mid-IR lasers by utilizing advanced high-energy, high-power 1μm pump lasers.

High average power 88 W OPCPA system for high-repetition-rate experiments at the LCLS x-ray free-electron laser.

We present a 100kHz, sub-20fs optical parametric chirped-pulse amplifier (OPCPA) system delivering 88.6W average power at a center wavelength of 800nm. The seed pulses are derived from the pump laser via white-light continuum generation and are amplified in three non-collinear OPCPA stages. The final two high-power stages are pumped with a 661W Yb:YAG InnoSlab amplifier. A simple and robust design is used for the OPCPA system to avoid thermal effects and enhance long-term stability, resulting in excellent beam quality and high conversion efficiency. To the best of our knowledge, this is the highest average power OPCPA system reported to date.

Toward terawatt few-cycle pulses via optical parametric chirped-pulse amplification with oxide crystals

High-power femtosecond lasers beyond $5~\unicode[STIX]{x03BC}\text{m}$ are attractive for strong-field physics with mid-infrared (IR) fields but are difficult to scale up. In optical parametric chirped-pulse amplification (OPCPA) at mid-IR wavelengths, a nonlinear crystal is vital, and its transmittance, dispersion, nonlinear coefficient and size determine the achievable power and wavelength. OPCPA beyond $5~\unicode[STIX]{x03BC}\text{m}$ routinely relies on semiconductor crystals because common oxide crystals are not transparent in this spectral range. However, the small size and low damage threshold of semiconductor crystals fundamentally limit the peak power to gigawatts. In this paper, we design a terawatt-class OPCPA system at $5.2~\unicode[STIX]{x03BC}\text{m}$ based on a new kind of oxide crystal of $\text{La}_{3}\text{Ga}_{5.5}\text{Nb}_{0.5}\text{O}_{14}$ (LGN). The extended transparent range, high damage threshold, superior phase-matching characteristics and large size of LGN enable the generation of 0.13 TW seven-cycle pulses at $5.2~\unicode[STIX]{x03BC}\text{m}$ . This design fully relies on the state-of-the-art OPCPA technology of an octave-spanning ultrafast Ti:sapphire laser and a thin-disk Yb:YAG laser, offering the performance characteristics of high power, a high repetition rate and a stable carrier–envelope phase.

Broadband main OPCPA amplifier at 808 nm wavelength in high deuterated DKDP crystals.

The optical aperture of ultrashort extreme intensity laser facilities, which reach 10 PW, will be beyond several hundred millimeters. DKDP is by now the only nonlinear crystal that can be grown to such diameter and used in the main optical parametric chirped-pulse amplification (OPCPA) amplifier of such a laser system. Here, at the signal wavelength of 808nm for the first time, we experimentally present a broadband OPCPA system that consists of a pre-amplifier in BBO crystals and a main OPCPA amplifier in two 95% deuterated DKDP crystals. The final amplified spectrum bandwidth exceeds 50nm, and a compressed pulse duration of 27fs has been measured. The conversion efficiency of the main OPCPA amplifier reached 24%, and a net signal gain of 13 was obtained. For the high energy OPCPA amplifier, the influence due to partial absorption on the idler pulses in DKDP crystal is theoretically analyzed. The results indicate the potential utilization of high deuterated DKDP for the main OPCPA amplifiers in a multi-petawatt laser system at 808nm wavelength.

Numerical Simulations of Transfer of Spatial Beam Aberrations in Optical Parametric Chirped-Pulse Amplification

In this paper, the spatial characteristics of the optical parametric chirped-pulse amplification (OPCPA) process were numerically studied when initial pump beam was aberrated. Numerical results showed that the spatial walk-off effect transferred phase modulation partly to the signal beam as the pump phase was modulated. Moreover, the modulation amplitude became increasingly severe as the nonlinear length extended. In the absence of phase aberration in the initial input signal, the induced phase aberration in the output signal was assumed as the differential form of the pump beam phase. As the pump beam intensity was modulated, the spatial walk-off effect reduced the influence of pump beam noise on beam quality and the angular spectrum but reduced signal gain simultaneously; thus, it may do more harm than good in the OPCPA process. In the case of a non-diffraction-limited pump beam, the greater the beam quality factor Mp2, the lower the conversion efficiency of the output signal in the OPCPA process. These results have important guiding significance for optimized design of an OPCPA system for high power laser.

High-power OPCPA generating 1.7 cycle pulses at 2.5 µm.

We present a high-power mid-infrared (mid-IR) optical parametric chirped-pulse amplifier (OPCPA) generating 14.4 fs pulses centered at 2.5 µm with an average power of 12.6 W and a repetition rate of 100 kHz. The short pulses are obtained without nonlinear pulse compression. This is in contrast to most few-cycle systems operating in the mid-IR. In our case, the ultrashort pulse duration is enabled by a careful design of the gain profile of each amplification stage as well as a precise control of the signal dispersion throughout the system. A pulse shaper is used in the seed beam to adjust the spectral phase at the output of the OPCPA system. This approach allows for a clean temporal profile leading to a high peak power of 6.3 GW.

Spectral pulse shaping of a 5 Hz, multi-joule, broadband optical parametric chirped pulse amplification frontend for a 10 PW laser system.

We present a broadband optical parametric chirped pulse amplification (OPCPA) system delivering 4J pulses at a repetition rate of 5Hz. It will serve as a frontend for the 1.5kJ, <150 fs, 10PW laser beamline currently under development by a consortium of National Energetics and Ekspla. The spectrum of the OPCPA system is precisely controlled by arbitrarily generated waveforms of the pump lasers. To fully exploit the high flexibility of the frontend, we have developed a 1D model of the system and an optimization algorithm that predicts suitable pump waveform settings for a desired output spectrum. The OPCPA system is shown to have high efficiency, a high-quality top-hat beam profile, and an output spectrum demonstrated to be shaped consistently with the theoretical model.

Chirped-pulse-amplification seed source through direct phase modulation.

This work presents integration of a directly chirped laser source (DCLS) into a high-energy optical parametric chirped-pulse-amplification (OPCPA) system. DCLS is an all-fiber, chirped laser source that produces nanosecond, linearly chirped laser pulses at 1053 nm for seeding high-energy chirped-pulse-amplification systems. DCLS produces a frequency chirp on an optical pulse through direct temporal phase modulation. A 1-ns, linearly chirped pulse with a 3-nm bandwidth is produced by applying an ~1000-rad (300π) quadratic temporal phase. The chirped pulse is amplified to 76 mJ in an OPCPA system and compressed to close to its Fourier transform limit, producing an intensity autocorrelation trace with a 1.5-ps width.

Contrast degradation in OPCPA systems induced by a pump with a post-pulse

A new mechanism that degrades the temporal contrast of pulses generated in optical parametric chirped-pulse amplification (OPCPA) systems is presented in this paper. Post-pulses exist in any laser system, including pump lasers. When a CPA scheme is used for pumping, a small-scale modulation of the pump signal may arise from the interference of a post-pulse with the main chirped pump pulse; the pre-pulses appear in the signal when the pump interacts with the signal, and following recompression. An OPCPA system was modelled using numerical methods to study this phenomenon. It is shown that several pre- and post-pulses appear in the signal after recompression, leading to contrast degradation. The contrast of the recompressed signal is proportional to the contrast of the pump, and depends on the time delay of the post-pulse. The temporal positions of the pre- and post-pulses are also affected by the pump parameters.

10 W CEP-stable few-cycle source at 2 µm with 100 kHz repetition rate.

We developed a high repetition rate optical parametric chirped-pulse amplification (OPCPA) laser system based on fiber-laser-seeded Innoslab to generate few-cycle pulses around 2 µm with passively stable carrier-envelope phase (CEP) by difference frequency generation (DFG). Incorporating a piezo mirror before the DFG stage permits rapid CEP control. The OPCPA system is seeded by a stable supercontinuum generated in bulk material with the picosecond Innoslab pulses. Few-cycle pulses with durations of 17 fs and energies of over 100 μJ were produced in a single OPCPA stage. Three different nonlinear crystals: BBO, BiBO, and LNB were tested in the final parametric amplifier, and their average power related limitations are addressed.

Spatio-spectral structures in high harmonic generation driven by tightly focused high repetition rate lasers

We investigate the spatio-spectral properties of extreme ultraviolet (XUV) high harmonic radiation driven by high repetition rate femtosecond laser systems. In the spatio-spectral domain, ring-shaped structures at each harmonic order associated with long-trajectory electrons are found to form arrow-shaped structures at the cutoff. These structures are observed with two different laser systems: an optical parametric chirped-pulse amplifier system at a central wavelength of 1.55 μm and 125 kHz repetition rate, and a temporally compressed femtosecond ytterbium fiber amplifier at 1.03 μm wavelength and 100 kHz repetition rate. As recently pointed out, the observed structures are well explained by considering the space–time atomic dipole-induced phase for short and long electron trajectories in the generation plane. The tighter focusing geometry and longer wavelength associated with these emerging driving laser systems increase the ring-like divergence and spectral broadening for high harmonics. Cutoff energies and photon fluxes obtained in argon and neon are also reported. Overall, these results shed new light on the properties of XUV radiation driven by these recently developed high average power laser systems, paving the way to high photon-flux XUV beamlines.

Spatial Control of Multiphoton Electron Excitations in InAs Nanowires by Varying Crystal Phase and Light Polarization.

We demonstrate the control of multiphoton electron excitations in InAs nanowires (NWs) by altering the crystal structure and the light polarization. Using few-cycle, near-infrared laser pulses from an optical parametric chirped-pulse amplification system, we induce multiphoton electron excitations in InAs nanowires with controlled wurtzite (WZ) and zincblende (ZB) segments. With a photoemission electron microscope, we show that we can selectively induce multiphoton electron emission from WZ or ZB segments of the same wire by varying the light polarization. Developing ab initio GW calculations of first to third order multiphoton excitations and using finite-difference time-domain simulations, we explain the experimental findings: While the electric-field enhancement due to the semiconductor/vacuum interface has a similar effect for all NW segments, the second and third order multiphoton transitions in the band structure of WZ InAs are highly anisotropic in contrast to ZB InAs. As the crystal phase of NWs can be precisely and reliably tailored, our findings open up for new semiconductor optoelectronics with controllable nanoscale emission of electrons through vacuum or dielectric barriers.

Active Supercontinuum Injection for Optical Parametric Chirped Pulse Amplification

The anticipant supercontinuum is a prerequisite for the pump seed generation in an optical parametric chirped pulse amplification (OPCPA) system. To achieve the anticipant supercontinuum, we develop an active method operated in a two-level control scheme according to the near- and far-field patterns and supercontinuum spectrum. We demonstrate this method by the experiments of spectral shifting to 1053 and 1060 nm, which show the good availability and flexibility. By this method, the anticipant supercontinuum is outputted, and then, the stable seed for the pump beam can be generated. This method has been successfully applied to a pure Petawatt-class OPCPA system.

Nonlinear spectral phase induced by optical parametric chirped-pulse amplification

Optical parametric phase (OPP), inherently induced in optical parametric chirped-pulse amplification (OPCPA), will significantly degrade pulse compression in the few-cycle pulse regime. While OPP is a well-known nonlinear phenomenon, its nonlinearity origin has remained unclear in physics. Here, we present a systematic theoretical investigation on OPP and demonstrate that OPP originates from the cascaded quadratic nonlinearity in a broadband OPCPA with intrinsic phase-mismatch. Because the intrinsic phase-mismatch is frequency dependent, OPP manifests itself as a nonlinear spectral phase. We show that OPP increases with the parametric gain, approaches to half the intrinsic phase-mismatch in the high-gain limit, and thus directly links with the crystal dispersion. We also find that the effect of OPP in the spatial domain is typically negligible, which makes the OPP compensation necessary only in the spectral domain. The results presented in this paper are of importance to guide the design of few-cycle intense OPCPA systems.

Femtosecond wavelength-tunable OPCPA system based on picosecond fiber laser seed and picosecond DPSS laser pump.

We present a compact and stable femtosecond wavelength-tunable optical parametric chirped pulse amplification (OPCPA) system. A novel OPCPA front-end was constructed using a multi-channel picosecond all-in-fiber source for seeding DPSS pump laser and white light supercontinuum generation. Broadband chirped pulses were parametrically amplified up to 1 mJ energy and compressed to less than 40 fs duration. Pulse wavelength tunability in the range from 680 nm to 930 nm was experimentally demonstrated.