Control over the parameters of a laser beam such as intensity and phase provides an important basis of modern photonics. Established control schemes, however, cover only a limited parameter range. We employ intense ultrasound fields in ambient air, enabling control of laser light in extreme parameter regimes. We acousto-optically modulate ultrashort pulses at 1030 nm with a peak power of 20 GW efficiently (⪆ 50%) in ambient air. Further, we show excellent beam profile conservation and separability of diffracted and transmitted beams. Finally, our approaches show that light control can prospectively be translated from solid-state media to the gas phase by means of intense ultrasound, considerably widening the scope of established light control methods.
Efficiencies of nonlinear optical-to-terahertz (THz) conversion below one percent remain a limiting factor for applications of multicycle THz radiation like THz-driven acceleration and inspired the use of multi-line pump spectra. To overcome the difficulty of phase stabilization of multiple narrowband sources required by the multi-line approach, we exploit its temporal analog, i.e., regular pulse trains with THz repetition rate, in which the THz waves generated by rectifying the individual pulses add coherently. The optical setup producing the pulse trains consists of motorized interferometers and enables precise control over the pulse train parameters like pulse spacing and amplitude. It is operated with a laser providing 400 fs pulses and energies of up to 110 mJ, which is the highest yet attempted for a pulse-train-type experiment. Opposed to earlier work, pulse division is done after amplification making the system more flexible in terms of tuning the pulse number. We present initial results of an experimental campaign of multicycle THz generation in custom periodically poled crystals with large apertures up to 10x20 mm2. The available pump energy allows filling these apertures at high fluences, promising increased THz yields. We investigate the dependence of the conversion efficiency on the single pulse duration and aim to find the optimum pulse number for different crystal lengths to determine the efficiency limitations in a regime avoiding laser-induced damage. Since crystal length and pulse number define the bandwidth of the THz pulses, this work demonstrates a path to an optimized THz source tunable to different requirements of applications.
We demonstrate efficient generation of terahertz radiation (THz) by nonlinear down-conversion driven by trains of pulses of tunable duration, spacing, amplitude and number. The optical setup, based on cascaded interferometers, provides fine control over the pulse train parameters allowing precise control over the conversion efficiency of the process as well as over the THz frequency and pulse duration. Our approach enables thorough optimization of the conversion process, which theory predicts can reach multiple percent. We perform experiments and simulations of THz generation in periodically-poled lithium niobate and KTP to validate theory and determine conversion-efficiency limitations in a highly-optimized scenario.
The state of the art in multicycle THz generation relies on nonlinear optical conversion in commercial periodically-poled lithium niobate crystals. THz pulse energies, however, are limited by the small apertures of available crystals as well as the low damage thresholds connected to photorefractive effects, especially at low temperatures associated with optimum efficiency. We explore three options for increasing the THz pulse energy achievable: increasing the crystal aperture to allow use of higher energy driver lasers; tuning the crystal temperature to look for an optimum; and testing an alternate medium (KTP) to mitigate photorefractive effects and push to higher intensities.
Yb:YLF crystal with near-unity quantum efficiency attracts the attention of researchers in high-power laser development and laser-cooling communities. Here, we will review our recent efforts in energy and average/peak power scaling of room temperature and cryogenic Yb:YLF lasers and amplifiers. At first, we will present temperature dependence of important laser related parameters in Yb:YLF such as fluorescence lifetime, absorption cross section, emission cross section and gain in the 78-300 K range. We will then discuss the in situ optical temperature estimation methods that could be used to accurately estimate Yb:YLF crystal temperature. Later, we will review our recent lasing/amplification results with room temperature (RT) and cryogenic Yb:YLF systems, where we have achieved output powers exceeding 500 W in cw operation with efficiencies approaching 80% and pulse energies above 300 mJ at 10 Hz repetition rate.
Despite the popularity and ubiquitousness of the tilted-pulse-front technique for single-cycle terahertz (THz) generation, optimization of the experimental setup remains complex and difficult due to the sensitive dependence on and coupling between the optical pulse parameters, including fluence, beam size, angular dispersion and temporal compression. Here we present a systematic and robust method to tune the tilted pulse-front setup, based on use of selected multi-dimensional scans, which enables a straight-forward and accurate determination of optimum parameter values. Our methodology not only allows us to determine parameter sensitivities and achieve a robust optimum in the performance, but also enables a verification of certain physical properties of the lithium niobate prism, including the THz refractive index. The detailed step-by-step procedure is discussed and applied to a tilted-pulse-front THz setup at both room temperature and cryogenic temperatures. The procedure can be applied to any setup based on the tilted-pulse-front geometry and is important for the construction of high energy THz sources required for strong field terahertz applications such as novel particle acceleration schemes or beam manipulators.
The successful implementation of superconducting LINAC technology at the European XFEL will boost the time averaged X-RAY power density substantially above current values. In fact, the XFEL will operate at repetition rates of up to 4.5MHz. However, this high pulse rate occurs only during 600μs long bursts of 10Hz repetition rate, rendering up to 27000 pulses per second. Matching this peculiar burst mode operation is a requirement also for the pump-probe femtosecond laser used in experiments. There is currently no commercially available femtosecond laser technology offering this kind of output at close to mJ-level energies per pulse and sub-20fs pulse width. We will outline the scheme of the pump-probe laser currently under development at the European XFEL. The laser design is based on a noncollinear optical parametric amplifier (NOPA) which will be pumped by sub-picosecond pulses from a high power, frequency-doubled Yb:YAG slab amplifier, delivering up to 20kW of fundamental average power during a burst. The design aims at highest possible flexibility regarding intra-burst rep-rates and pulse energies. Also, the use of uncompressed as well as compressed NOPA pump pulses at fundamental wavelength will enhance the flexibility in experiments.
The information approach was applied to a determination of the actual properties of CCD-cameras. This approach is very useful when we have to establish from spectrograms, interferograms, holograms or other recorded data a set of functions that describe how the object under investigation behaves in space and time. Moreover this approach allows the actual properties of the measuring device be taken into account in order to determine the lower bound of experimental errors both before and after the measurement and to check how much use is made of the information that is contained in the recorded data. One of the best professional-quality CCD-cameras (ST-7I, Santa Barbara Instrument Group) was chosen for experimental investigation. Both the transfer (spread) function and the frequency-contrast function of this CCD-camera were determined experimentally and the optical channel fluctuation noise and the signal-to-noise ratio were measured as well. It has been shown that due to light scattering in the semiconductor base of the CCD matrix the amplitudes of high spatial frequency components of the image are reduced; the smaller the frequency of the light, the smaller are the amplitudes of the spatial frequency components of the output image. It has been ascertained that the relative error of the recorded image can be as small as one percent if the whole of the dynamic range is used (the full well capacity is equal to 40,000e) and if the highest spatial frequency of the image is small enough. But if the highest spatial frequency of the image fc is limited by pixel steps Δ (Δ approximately equals ½fc ), the relative error increases due to contrast reduction of the high spatial frequency and the relative error of the reconstructed input intensity distribution of the image more than doubles. This experimental result is very close to that derived from the calculated output image information. These experimental and simulation results are illustrated by recently obtained data.
A. Bugrov, I. Burdonskiy, V. Gavrilov, A. Goltsov, V. Kondrashov, S. Koptyaev, N. Kovalskiy, S. Medovchshikov, V. Nikolaevskiy, Michael Pergament, V. Petryakov, A. Sorokin, E. Zhuzhukalo
The interaction of powerful laser pulses with low-density materials is experimentally studied following our earlier experiments on 'Mishen' facility. Porous or foam-like materials considered as very important and suitable components of advanced ICF target designs. The low-density media seem to be very helpful for some other research programs of great importance including modeling of astrophysical phenomena, realization of x-ray lasers, determination of the EOS for matter in a multimegabar range.
A. Bugrov, I. Burdonskiy, V. Gavrilov, A. Goltsov, V. Kondrashov, N. Kovalskiy, Michael Pergament, V. Petryakov, A. Sorokin, G. Yankovskii, E. Zhuzhukalo
In the context of research on laser interaction with ultra low-density porous media, comparative experiments with thin, burning through plastic foils have been carried out. The structure of backscattered spectra near the laser fundamental frequency was observed and analyzed taking into account the simultaneously temporally and spectrally resolved measurements of the second harmonic emissions. Parametric instabilities responsible for the emissions are identified.
An approach is proposed to phase conjugation (PC) of broadband light that is a problem for traditional PC at stimulated Brillouin scattering. Experimental and simulation results of a work aimed to obtain a complex conjugated wave of a broadband laser beam in three-wave mixing in a nonlinear crystal are presented. It is demonstrated by direct wavefront measurements that in mixing of the broadband first harmonic signal wave with the single- frequency second harmonic pump wave having quasi-plane wavefront the arising broadband first harmonic idler wave has a wavefront complex conjugated to the one of the signal light.
The possibility to control an intensity distribution in the far field of a powerful laser system by rapid motion of a focal spot is considered. Quadruple electro optic deflector on the base of LiNgO3 crystal installed in resonance capacity with 1 cm clear aperture has been developed, constructed and tested both in high frequency and single pulse operation modes. The main parameters of the device are as follows: amplitude of the angular deflection +/- 4 dif. limits at 6.5 GHz operation frequency, total angular deflection 12 dif. limits in the single ns-pulse operation mode. Results of the Beam Wiggler dynamic testing are presented and discussed.
Zig-Zag-Slab-Amplifier had active element 4.5 X 40 X 43 cm3 in dimensions, six-bounce geometry of beam pass, pump cavity with transverse lamp orientation, and diffuse reflectors. Tests showed: the gain is equal to 8 per single pass at 77 kJ of pumping; its non-uniformity over about half of amplifier aperture is less than +/- 2.5 percent; the depolarization distortion over operating area is less than 0.5 percent.
R. Bikmatov, Charles Boley, I. Burdonsky, V. Chernyak, A. Fedorov, A. Goltsov, V. Kondrashov, S. Koptyaev, N. Kovalsky, V. Kuznetsov, David Milam, James Murray, Michael Pergament, V. Petryakov, Ruslan Smirnov, Victor Sokolov, E. Zhuzhukalo
Pinhole plasma effects on parameters of the laser beam passing through the spatial filter in conditions of interest for large scale ICF laser facilities were investigated. The experiments on pinhole irradiation were conducted at power density range 1010-1011 W/cm2 with approximately 15 ns laser pulses. Al, Fe, and Ta pinholes were used. The diagnostic approach was chosen based on probing the pinhole region with frequency doubled 3-ns-long laser pulse. Ablative-plasma dynamics was studied with shadowgraphy and interferometry. Also measured were the parameters of transmitted probing beam in the near- and far-fields. The rate of pinhole 'closure' is found to decrease with the increase in the atomic number of pinhole material. The rate o pinhole closure ranges from approximately 5*106 cm/s for aluminum pinhole down to approximately 2*106 cm/s for tantalum pinhole in experiments with power density at the pinhole edge of approximately 50 GW/cm2. For aluminum and steel pinholes the parameters of the transmitted probing beam deteriorate to unacceptable level for approximately 15-20 ns after the irradiation start. In the same experimental conditions the pinholes of tantalum exhibits acceptable performance till the end of the irradiation process. Fast plasma jets converging to the pinhole axis with velocities up to approximately 107 cm/s and significantly deteriorating transmitted probing beam quality are observed. Reasonable agreement was found between the data obtained in experiments with circular pinholes and linear edge experiments.
Streak camera based on two-stage streak image tube with 40 X 4 mm2 S-20 photocathode of the approximately 30 image compression factor for LIDAR Thomson scattering diagnostics has been developed, constructed and tested. Over 300 time resolved 60 ps elements at 10 dots of spatial resolution has been obtained. Results of the streak camera testing are presented and discussed.
The possibility to use information theory for determination of real optical channel features and optical storage capacities as well as for option of the best image reconstruction procedure is considered. The information approach to determination of required optical channels quality and needed data processing algorithms is developed and the mathematical descriptions necessary for those purpose have been obtained. The reconstruction accuracy is shown to be determined only by the difference between output image entropy and noise one. The computer simulation procedure to estimate lower bound of experimental errors that is possible to reach in principle is proposed.
The concept to use a slab as active element, working in zig- zag geometry, and also as Fresnel rhomb, seems to be rather attractive. However, in this case different depolarization effects in active element are of crucial importance. We have carrier out the estimation of depolarization effects arising both due to mechanical loading of an active element at its fastening and due to thermooptical distortions. To check up these rigid requirements to depolarization (0.1% - 0.01%) careful measurements of depolarization effects and their sources are being carried out. Mechanical loading gives one of the main contributions in depolarization at fastening of active element. Using model experiments with glass Fresnel rhomb under mechanical loading we have measured depolarization effects. It is proposed to use additional glass plate to compensate beam depolarization in zig-zag slab. The received results allow to expect successful use of the slab amplifier as a Fresnel rhomb providing rather high quality of optical materials of active element.
This report presents results of a study of reabsorbed spontaneous emission (RSE) influence on the spatial inversion distribution in an active medium of large phosphate slab. Experimental results are also presented for a model case of active medium pumping by monopulse second harmonic of Nd-glass laser having specific profile of spatial distribution. RSE is observed for this case. A model is proposed for RSE simulation in solid bodies of arbitrary shape. It is shown that RSE influence is taken into account completely by first approximation, considering the rest fluorescence part of active medium to be an additional source of pump in order to define inversion in the arbitrary point of active medium. Fluorescence kinetics coincides with numerical results of a model experiment. Calculations are carried out of inversion redistribution due to RSE in a large aperture slab of phosphate glass demonstrating that spatial distribution distortions of inversion are small by the moment when inversion maximum is achieved, however, 10 - 12% of stored energy may relate to RSE effect at the absence of amplified spontaneous emission (ASE). Fluorescence kinetics measurements are carried out over slab cross-section demonstrating good agreement with model results.
The main goal of this report is to analyze the feasibility to use phase conjugation (PC) in megajoule class lasers for inertial confinement fusion (ICF). Phase conjugation has the potential for relieving the fabrication requirements to any optical elements and for compensation of residual thermo-optical distortions. The key problems for phase conjugation are the dynamic range over which phase conjugation operates efficiently, the reflected energy limit, the effect of the laser light bandwidth, and the possibility to mount the PC subsystem into a real megajoule laser. Analytical results of the possibility of use of a classical stimulated Brillouin scattering (SBS) mirror and based on a nonlinear crystal phase conjugator in ICF- laser are presented.
R. Bikmatov, Alexander Charikov, V. Chernyak, L. Ignat'ev, V. Kondrashov, V. Kuznetsov, Kenneth Manes, V. Nikolaevskii, A. Nugumanov, Michael Pergament, A. Rozhkov
The work is devoted to the investigation of a wide-aperture amplifier which is intended to be installed in the laser system Nova upgrade. The amplifier should meet rather severe requirements. We have carried out the experimental investigation of gain nonuniformity over all the aperture of the amplifier and estimated depolarization and phase distortions to determine the sizes of the amplifier aperture operating zone.
The possibility to control an intensity distribution in the far field of a powerful laser system by high-frequency moving the position of a focal spot is considered. Quadruple electro-optic deflector (beam wiggler) on the base of LiNbO3 crystal installed in resonance cavity has been developed and constructed. The main parameters of the device are as follows: amplitude of the angular deflection plus or minus 4 dif.limit, clear aperture 1 cm, the deflector is designed for operation at 6.5 GHz powerful microwave source. Results of beam wiggler dynamic testing are presented and discussed.
Results of the tests of the universal image converter camera operating at streak and three- frame modes are described. Transition to the modular construction principle of the image converter tract makes it possible to design the 2-nd camera modification with flexible architecture that can be easily, and in the best manner, adapted to requirements of the concrete experiment. Camera construction and the testing methods are described.
There are a variety of vacuum ultraviolet and soft x ray radiation sources. They may be stationary (as a synchrotron) and of pulse nature (x-ray tubes with rotating anode, z-pinches, plasma focus, etc.). Present-day investigations prove that the ideas of laser technology may be used for producing sources over this range of photon energies. Here we should note x-ray lasing, laser driven plasma x ray sources, and nonlinear conversion of visible laser radiation. Though this variety gives researchers a wide latitude in the use of such sources, careful examination and optimization for certain experimental conditions must be done. From our point of view, a laser driven x ray source (LDS) has good scientific and commercial potential. LDS has the advantages of small size, short duration, sufficiently high energy, and an easily changeable spectral range. These performances are very suitable for lithography, high resolution microscopy of wet biological samples, and many applications in inertial confinement fusion researches.
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