Phase-controlled amplification of few-cycle laser pulses

Intense ultrashort waveforms of light that can be produced with an exactly predetermined electromagnetic field are essential in a number of applications of extreme nonlinear optics, most prominently in laser-driven sources of high-energy attosecond radiation. Field reproducibility in each laser shot requires stabilization of the carrier-envelope phase. The authors analyze different schemes of phase-stable pulse amplification and identify constraints limiting the precision with which the phase can be maintained. Next, they describe a phase-stabilized laser system based on a 20-fs multipass Ti:sapphire amplifier supplemented with a fiber compression stage for producing pulses in the few-cycle regime. It is shown that the amplifier introduces only a slow millihertz phase drift and, therefore, can be seeded by a standard phase-stabilized oscillator. This residual phase drift is assigned primarily to the beam pointing instability and can be precompensated in the phase-control loop of the seed oscillator using a feedback signal from a phase detector placed in the amplifier output. The phase stability of the resultant 5-fs 400-/spl mu/J pulses at a 1-kHz repetition rate is subsequently independently verified by higher order harmonic generation, in which different carrier-envelope phase settings are shown, both theoretically and experimentally, to produce distinctly different spectral shapes of the XUV radiation. From a series of such spectral patterns, the authors succeed in calibrating the value of the carrier envelope phase (with a /spl plusmn//spl pi/ ambiguity), which in turn allows them to fully characterize the temporal structure of the electric field of the laser pulses. The estimated precision of the phase control on the XUV target is better than /spl pi//5, which reduces the timing jitter between the driving laser pulse and the XUV bursts to /spl sim/ 250 as and opens the way to generate stable isolated attosecond pulses.     

Broad-bandwidth parametric amplification in the visible:femtosecond experiments and simulations

A general geometric condition, involving group velocities, for large bandwidth parametric amplification with all types of phase-matching and in certain wavelength ranges is derived. This condition has been exploited to produce visible widely tunable sub-20-fs pulses from an optical parametric oscillator and an optical parametric amplifier based on β-barium borate (BaB₂O₄) pumped in the blue. These results are interpreted in terms of a modified soliton model for the optical parametric oscillator (OPO) and detailed numerical simulations for both devices     

Generation and spatiotemporal evolution of optical vortices in femtosecond laser pulses

Topology has become one of the key concepts allowing one to understand the intrinsic, qualitative properties of phenomena throughout various scientific fields. To date, this concept has been extended to the field of material science and technology. On the other hand, we can now utilize the spatially controlled light defined by the topology (so‐called “optical vortices”) in order to characterize the topological properties of materials. In particular, optical vortices in femtosecond pulses will be invaluable for advanced topological spectroscopy. In this work, the authors created femtosecond optical vortices using a spatial light modulator. Their spatiotemporal properties were evaluated using interferogram and correlation measurements. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(4): 39–46, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20791     

Mechanism for self-synchronization of femtosecond pulses in atwo-color Ti:sapphire laser

An experimental and theoretical analysis of the nonlinear coupling mechanism between the two solitary pulses circulating in a two-color femtosecond laser is presented. Two operation regimes; synchronized; and nonsynchronized; and a hysteresis of the transition between the two regimes are clearly observed; while independent modelocking and tunability of the output pulse trains is found in both regimes. Pulses in the range from 15 to 100 fs are synchronized with a timing jitter below 2 fs. The combined effects of cross-phase modulation and negative group velocity dispersion are shown to be responsible for the strong pulse correlation in the synchronized regime. Our experimental observations are in agreement with numerical simulations, thus confirming the theoretical model     

In-situ diagnostic of femtosecond laser probe pulses for high resolution ultrafast imaging

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variat...     

Optical guiding of intense laser pulses by fast Z-pinch discharge

We have proposed the optical guiding of intense laser pulses by fast Z-pinch for channel-guided laser wakefield acceleration (LWFA). The method has been developed based on capillary discharge-pumped X-ray laser technique. A discharge through preionized helium gas driven by a current of 4.8 kA with a rise time of 15 ns proved able to produce a uniform guiding channel with good reproducibility, less than the time jitter of 1.8 ns. The observed guiding channel formation process was corroborated by 1D-MHD simulation. With this new guiding method, an intense Ti-sapphire laser pulse (λ = 790 nm, 2.2 TW, 90 fs, 1 × 10¹⁷ W/cm²) was transported through the channel over a distance of 2 cm, corresponding to 12.5 times the Rayleigh length. © 2001 Scripta Technica, Electr Eng Jpn, 136(3): 19–27, 2001     

Characterizing the nonlinear propagation of femtosecond pulses inbulk media

Frequency-resolved optical gating is used to characterize the amplitude and phase of intense femtosecond pulses propagating in nonlinear dispersive media. The combined effects of group velocity dispersion (GVD) and third-order nonlinearity (nz) lead to rapid broadening and splitting of the pulses. We present measurements at 800 and 1200 nm and investigate the influence of the chirp of the input field. Measurements are compared with the predictions of one- and three-dimensional nonlinear Schrodinger equations. The influence of the Raman contribution to the nonlinear index of refraction is also examined theoretically     

Femtosecond optical parametric amplification with dispersion precompensation

In this paper, we study a midinfrared femtosecond optical parametric amplifier (OPA) that is severely affected by group velocity dispersion (GVD). Both theoretical and experimental results show that GVDs in nonlinear crystals will significantly degrade the performance of a femtosecond OPA. By introducing a prechirp to the pump pulse, the effect of GVD can be effectively compensated. A lithium-niobate-crystal-based femtosecond OPA demonstrates that the conversion efficiency with optimally prechirped pumping is nearly twice that of the nonchirp case, and the output pulses can be further compressed to nearly their Fourier-transform limit by prism pairs.     

Trends in chirped pulse optical parametric amplification

Since the proof-of-principle demonstration of optical parametric amplifier to efficiently amplify chirped pulses in 1992, optical parametric chirped pulse amplification (OPCPA) became a widely recognized and rapidly developing technique for high-power femtosecond pulse generation. In the meantime, we are witnessing an exciting progress in the development of powerful and ultrashort pulse laser systems that employ chirped pulse parametric amplifiers. These systems cover a broad class of femtosecond lasers, with output power ranging from a few gigawatts to hundreds of terawatts, with a potential of generating few-optical-cycle pulses at the petawatt power level. In this paper, we discuss the main issues of optical parametric chirped pulse amplification and overview recent progress in the field.     

High-average-power femtosecond KrF excimer laser

Development of high-peak and high-average-power ultrashort pulse KrF excimer lasers is described. Technical issues of KrF excimer as the amplifying medium for the ultrashort pulses are dramatically improved, resulting in a 50 W average power with a pulse width of 480 fs at 200 Hz repetition rate     

Generation of attosecond X-ray laser pulses

We propose to generate as-X-ray laser pulses by beating of two or more X-ray laser lines with a frequency separation in the range of 10/sup 15/ Hz. We focus on nickel-like X-ray lasers, some of which have a few almost equidistant laser gain lines with an appropriate difference frequency. It is shown that in the case of three or more lines, these can be phase-locked by means of a Langmuir wave generated in the gain medium at a suitable electron density.     

Optical timing distribution system with femtosecond stability

Stable timing distribution is a key technology for the many fields that use timing signals for synchronization. The frequency of the timing signal depends on the field and multiple timing signals are sometimes required. In our DWDM‐based timing distribution system, phase deviation is detected by high‐speed round‐trip signal (10 GHz signal is used as this control signal). Fiber stretchers are controlled so as to minimize the phase deviation of the control signal. Multiple signals and the control signal are densely multiplexed and transmitted through the same stretchers. Therefore, the transmission of all signals is stabilized. This configuration provides a flexible platform for distributing various RF or microwave signals. As an example of arbitrary timing signals, a 1 GHz signal is transmitted over a length‐stabilized 400‐m fiber for more than 1 day. The recorded propagation time fluctuations are 10.4 and 2.8 fs rms for the 1 and 10 GHz signals, respectively. The Allan deviations are 2.7 × 10⁻¹⁹ (1 GHz) and 6.0 × 10⁻²⁰ (10 GHz) for the averaging time of 10⁵ s. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Measurement of 10-fs laser pulses

We report full characterization of the intensity and phase of ~10-fs optical pulses using second-harmonic-generation frequency-resolved-optical-gating (SHG FROG). We summarize the subtleties in such measurements, compare these measurements with predicted pulse shapes, and describe the implications of these measurements for the creation of even shorter pulses. We also discuss the problem of validating these measurements. Previous measurements of such short pulses using techniques such as autocorrelation have been difficult to validate because at best incomplete information is obtained and internal self-consistency checks are lacking. FROG measurements of these pulses, in contrast, can be validated, for several reasons. First, the complete pulse-shape information provided by FROG allows significantly better comparison of experimental data with theoretical models than do measurements of the autocorrelation trace of a pulse. Second, there exist internal self-consistency checks in FROG that are not present in other pulse-measurement techniques. Indeed, we show how to correct a FROG trace with systematic error using one of these checks     

Ce3+:LiCaAlF6 crystal for high-gain orhigh-peak-power amplification of ultraviolet femtosecond pulses and newpotential ultraviolet gain medium: Ce3+:LiSr0.8Ca0.2AlF6

To develop high-peak-power ultrashort pulse laser systems in the ultraviolet region, a large Ce³⁺:LiCaAlF₆ (Ce:LiCAF) crystal, a tunable ultraviolet laser medium with large saturation fluence and broad gain spectrum width, was grown successfully with a diameter of more than 70 mm. To demonstrate high small signal gain, a four-pass confocal amplifier with 60 dB gain and 54 μJ output energy was constructed. Chirped pulse amplification (CPA) in the ultraviolet region was demonstrated using Ce:LiCAF for higher energy extraction. A modified bow-tie-style four-pass amplifier pumped by 100-mJ 266-nm 10-Hz pulses from a Q-switched Nd:YAG laser had 370-times gain and delivered 6-mJ 290-nm pulses. After dispersion compensation, the output pulses can be compressed down to 115 fs. This is the first ultraviolet, all-solid-state high-peak-power CPA laser system using ultraviolet gain media, and this demonstration shows further scalability of the Ce:LiCAF CPA system. Additionally, a new gain medium, Ce³⁺ :LiSr_0.8Ca_0.2AlF₆, with longer fluorescence lifetime and sufficient gain spectrum width over 18 nm was grown to upgrade this system as a candidate for a final power amplifier gain module     

Broadband amplification of 800-nm pulses with a combination of negatively and positively chirped pulse amplification

It is experimentally proved that successive amplification of negatively and positively chirped laser pulses (NPCPA) counteracts the gain narrowing effect typical in chirped pulse amplification (CPA) lasers. The scheme is robust and easy to adopt to even petawatt (PW) level high power laser systems. As a demonstration, a multi-terawatt (TW) Ti:sapphire laser system was modified to the NPCPA. The bandwidth of the 150 mJ output pulses exceeds 50 nm without any additional spectral correction, which is 30% broader than those currently available from conventional CPA lasers. Moreover, the NPCPA scheme gives an opportunity to increase an intensity temporal contrast without any compromise in pulse energy.     

Spectral narrowing and temporal expanding of femtosecond pulses by use of quadratic nonlinear processes

A nonlinear system capable of expanding pulse with spectral narrowing, an analogy to a spatial beam expander in linear optics, is studied theoretically and experimentally. The system, with features of high efficiency and maintaining near Fourier-transform limit (FTL), is constructed by using quadratic nonlinear processes with chirped pulses. The spectral and temporal characteristics of such a pulse expander are investigated analytically and computationally. It shows that group-velocity mismatch of nonlinear crystal plays a detrimental role, which leads to a deviate operation of pulse expander from its ideal case, e.g., temporal shortening, spectral broadening, and a deviation from the FTL of the expanded pulses. The criteria for designing a near aberration-free pulse expander are given based on these analyses. As a demonstration, we experimentally expand broadband 70-fs pulses from a Ti:sapphire regenerative amplifier to narrowband 60-ps longer pulses. The conversion efficiency from pump to idler of the nonlinear pulse expander is currently limited to a few percent and can be practically improved to 10% to 20%.     

Characterization and applications of high repetition rate, broadlytunable, femtosecond optical parametric oscillators

In this paper, we review recent advances in optical parametric oscillators, with particular emphasis on high repetition rate, femtosecond optical parametric oscillators, We present results for several of these systems, and show that they offer a unique combination of short pulse durations, high average powers, high repetition rates; and broad tunability which have not been previously available from a single source. We conclude by looking at some applications in the field of ultrafast semiconductor spectroscopy, some of which are only now possible because of the femtosecond optical parametric oscillator. It is, therefore, apparent that this unique device offers greater flexibility than previously available sources, and will enable many new and novel experiments in many areas of science and technology     

Generation of coherent, femtosecond, X-ray pulses in the“water window”

We report experimental and theoretical results on high-harmonic generation with 25-fs laser excitation pulses. The shortest wavelength we observe, at 2.7 nm, is well within the “water window” region of X-ray transmission. In the case of all the noble gases, we obtain excellent agreement between theoretical predictions for the highest harmonic photon energy generated and our experimental observations. We also obtain excellent agreement between theory and experiment for the highest photon energy generated as a function of laser pulsewidth between 25 and 100 fs. Finally, we observe that the individual harmonic peaks near the cutoff are well resolved for positively chirped pump pulses, but are unresolved in the case of negatively chirped excitation pulses     

Process research on micro-machining diamond microgroove by femtosecond laser

Abstract

Using micro-nano processing system of 120 fs and 800?nm low-frequency femtosecond laser, the microgroove was processed on the CVD diamond. The effects of some processing parameters on the microgroove size were studied. The results showed that the depth and width of microgrooves increase with the increase of laser power, scanning times, while with the decrease of scanning speed. Moreover, the suitable scanning speed of femtosecond laser processing diamond is about 0.1?mm/s, and focusing on diamond can improve material removal efficiency and processing quality. It is indicated that these research work has certain guiding significance for processing diamond microchannels for heat dissipation of electronic chips.     

Compact geometry for diode-pumped Cr:LiSAF femtosecond laser

A novel geometry for dispersion compensation in femtosecond lasers without specific cut of optical parts in a flat/Brewster angle or Gires-Tournots mirrors is demonstrated for attaining a compact diode-pumped Cr:LiSAF femtosecond pulse laser. In this geometry, the laser medium lies between the dispersion-compensation prism pair. This approach enables the laser to operate at variable repetition rates from 163 to 235 MHz keeping the pulsewidth less than 90 fs, where the value of the time-bandwidth product are not larger than 0.34. An 89 fs pulse duration generated at the 235 MHz repetition rate is, to our knowledge, the shortest pulse achieved in an all-solid-state Kerr-lens mode-locked laser operating above the 200 MHz repetition rate