Jin Woo Yoon

Generation of high-contrast, 30 fs, 1.5 PW laser pulses from chirped-pulse amplification Ti:sapphire laser

A high-contrast, 30 fs, 1.5 PW Ti:sapphire laser has been developed for research on high field physics. The maximum output energy of 60.2 J was obtained from a booster amplifier pumped by four frenquency-doubled Nd:glass laser systems. Parasitic lasing was suppressed by index matching fluid with absorption dye and the careful manipulation of the time delay between the seed and pump pulses. After compression, the measured pulse duration was 30.2 ± 1.8 fs, and the output energy was 44.5 J, yielding a peak power of about 1.5 PW. A saturable absorber and two ultrafast Pockels cells were installed in the front-end system for the minimization of the amplified spontaneous emission (ASE) and pre-pulse intensity. An adaptive optics system was implemented for obtaining the near diffraction-limited focal spot.

Laser fusion driver using stimulated Brillouin scattering phase conjugate mirrors by a self-density modulation

A new concept of laser fusion driver is proposed, which uses a beam combination technique with stimulated Brillouin scattering phase conjugate mirror (SBS-PCM). It is constructed systematically with a cross-type amplifier as a basic unit. In the first part of this paper, we introduce the cross-type laser amplifier using SBS-PCM, with several advantages by experimental results. These advantages are the ideal properties for practical laser fusion driver, such as the perfect isolation of leak beam, the compensation of thermally induced birefringence through the amplifiers, the easy maintenance and alignment insensitiveness, and the freely-scale-up energy. Next, some successful results for the phase control of SBS-PCM are presented, which is one of the main problems in the current beam combination laser using SBS-PCM. Particularly, a new technique for controlling the phase of SBS-PCM, “self-density modulation,” is introduced, which is the simplest ever among those reported. With the advantages of the cross-type amplifier using SBS-PCM and the novel method for controlling the phase of SBS-PCM, the proposed beam combination laser system is presented as the most promising one, which can contribute to the realization of high energy laser that can operate with high repetition rate over 10 Hz, even in the case of huge output energy over MJ.

Phase stabilization of the amplitude dividing four-beam combined laser system using stimulated Brillouin scattering phase conjugate mirrors

AbstractThe beam combination method using stimulated Brillouin scattering phase conjugate mirrors (SBS-PCMs) is a promisingtechnique for a high energy and high power laser output operating with a high repetition rate. The two-beam combinedsystem was previously demonstrated with an amplitude dividing method. A four-beam combined laser system withamplitude dividing method is demonstrated in this work, and the phase stabilization experiment of this system isperformed using the self phase control and the long-term stabilization technique. The phase differences between theSBS waves are stabilized with l/30 and the fluctuation of the four-beam combined output energy is 6.16% during2000 shots (200 s).Keywords: Beam combination; High energy laser; Long-term stabilization; Phase conjugate mirror; Phase control;Stimulated Brillouin scattering INTRODUCTIONApplication of Brillouin scattering methods have attractedgreat attention in recent years (Hasi et al., 2007; Kappeet al., 2007; Lontano et al., 2006; Meister et al., 2007;Ostermeyer et al., 2008; Wang et al., 2007; Yoshida et al.,2007). A very prominent application is laser fusion energy(LFE), which requires very high energy and high powerlaser output of several megajoules in a few tens of nano-seconds with a high repetition rate around 10 Hz (Nakai M Kuehl et al., 2007),PALS (Jungwirth, 2005; Batani et al., 2007; Laska et al.,2006; Torrisi et al., 2008), and Vulcan Petawatt (Dansonet al., 2005), are operated with a low repetition rate or asingleshotduetothethermalproblemsofthelasermaterials.The beam combination method using stimulated Brillouinscattering phase conjugate mirrors (SBS-PCMs) is a promis-ing one for the high energy output with a high repetition rate(Kong et al., 1997; 1999; Basov et al., 1979; Rockwell G Valley et al., 1986; Loree et al., 1987;Bower & Boyd, 1998; Riesbeck et al., 2001; Riesbeck E Kappe et al., 2007; Ostermeyer et al.,2008). This beam combined system can resolve the thermalproblems by combining beams of small energies after separ-ateamplifications.Furthermore,thehighqualityoutputbeamcan also be obtained from the PCMs of this system, whichcompensatethermaldistortionsinthelaseramplifiersbygen-erating the phase conjugate waves.The SBS wave of the PCM has a random phase because itis naturally ignited by thermal noise (Boyd et al., 1990). Fora coherent beam combined output with SBS-PCMs, there-fore, the phase relations between the SBS beams should belocked.Forthisreason,manypreviousresearchersdevelopedtheir own techniques to lock the phase difference betweenSBS beams (Basov et al., 1979; Rockwell & Giuliano,1986; Valley et al., 1986; Loree et al., 1987; Bower &Boyd, 1998). However, their systems have a structural limit-ation when combining many beams, due tovery complicatedcomposition with large number of optical components. Toovercome this limitation, Kong et al. (2004, 2005a, 2005b,2005c) proposed the self phase control technique, whichcan independently lock and control the phases of SBSwaves from each phase conjugate mirrors, with the simplecomposition of few optical components. Therefore, the179

Long-term stabilized two-beam combination laser amplifier with stimulated Brillouin scattering mirrors

The beam combination method is the promising technique for constructing a very high energy laser with a high repetition rate over 10Hz such as a real fusion driver. In our previous works, the phase control technique essential for realizing this system was proposed and demonstrated experimentally. However, these previous works were done without amplifiers. In this work, we employed amplifiers to test the real beam combination system and obtained a well stabilized phase controlling with λ∕51 fluctuation by standard deviation during 5000 laser shots (500s) at 204mJ total output energy.

Relativistic frequency upshift to the extreme ultraviolet regime using self-induced oscillatory flying mirrors

Coherent short-wavelength radiation from laser–plasma interactions is of increasing interest in disciplines including ultrafast biomolecular imaging and attosecond physics. Using solid targets instead of atomic gases could enable the generation of coherent extreme ultraviolet radiation with higher energy and more energetic photons. Here we present the generation of extreme ultraviolet radiation through coherent high-harmonic generation from self-induced oscillatory flying mirrors—a new-generation mechanism established in a long underdense plasma on a solid target. Using a 30-fs, 100-TW Ti:sapphire laser, we obtain wavelengths as short as 4.9 nm for an optimized level of amplified spontaneous emission. Particle-in-cell simulations show that oscillatory flying electron nanosheets form in a long underdense plasma, and suggest that the high-harmonic generation is caused by reflection of the laser pulse from electron nanosheets. We expect this extreme ultraviolet radiation to be valuable in realizing a compact X-ray instrument for research in biomolecular imaging and attosecond physics.

Restoration of high spatial frequency at the image formed by stimulated Brillouin scattering with a prepulse

We have found experimentally that it is possible to restore the high spatial frequency of optical images by using stimulated Brillouin scattering (SBS) with a prepulse. The Stokes wave usually has to lose the high spatial frequency because of the necessary energy to generate the acoustic grating. However, this problem has been resolved by using a prepulse method. We have achieved an amplitude increase of ∼41% compared to the normal SBS scheme at a spatial frequency of ∼0.027 mm−1. This method is easy to apply to a system using the optical image by the SBS process.

Proposal of the practical laser fusion driver operating at high repetition rate over 10Hz by a beam combination technique using phase controlled stimulated brillouin scattering mirrors

The laser fusion requires a laser having a high repetition rate/high energy/high power. We developed a new phase control technique for beam combination and have demonstrated successfully its feasibility on the base of the low energy level. Its operational principle has enough possibility for scale-up to the required fusion driver level without any fundamental problem. Here we report the successful phase controlling of the practical beam combination system with amplifiers. We obtained the stabilized phase controlling within λ/50 fluctuation during 5000 laser shots (500 sec) at 204 mJ total output pulse energy and 10 Hz repetition rate.

High efficient amplification in a PW Ti:Sapphire laser

We report that 60-J output energy from a Ti:sapphire amplifier was achieved with 120-J pump energy under the condition of the strong transverse parasitic lasing. To suppress the transverse parasitic lasing, the time delay between an IR pulse and a pump pulse was optimized.

Long-term stabilization of the phase control technique of the stimulated Brillouin scattering wave for the beam combination technique

The beam combination technique using stimulated Brillouin scattering (SBS) phase conjugate mirrors (PCMs) proposed by one of the authors, H. J. Kong, is a promising one for realization of high energy/power laser system with high repetition rate. However, phase controlling of the SBS waves is essentially required for beam combination system, since the SBS-PCM generates the random phase. Recently, we have achieved successful results for phase locking by the self-generated density modulation method. But it showed a long-term phase fluctuation due to the long-term fluctuation of the density of the liquid SBS medium. To compensate this long-term phase fluctuation, we have designed new phase stabilization system. In this paper, we will introduce this system and show successful experimental results.

Stimulated Brillouin Scattering Phase Conjugate Mirror and its Application to Coherent Beam Combined Laser System Producing a High Energy, High Power, High Beam Quality, and High Repetition Rate Output

Stimulated Brillouin scattering (SBS) is a nonlinear optical process that generates backward scattered phase conjugate wave (Zel’dovich et al., 1972; Zel’dovich et al., 1985; Damzen et al., 2003; Brignon & Huignard, 2004). A device that generates the phase conjugate wave by the SBS process is called SBS phase conjugate mirror (PCM). An SBS-PCM can compensate wavefront distortion induced by a phase aberrator, such as a laser gain medium; hence, it is widely used in high-energy laser systems to obtain a high-quality beam. Efficient heat dissipation is a major issue in high-energy laser systems, particularly with regard to the high repetition rate. The combination of beams from small laser systems is a constructive approach to this issue. Of the various beam combined systems using SBS-PCMs, the crosstype beam combined system has many outstanding advantages, such as perfect isolation of leak beam, compensation of thermal birefringence, easy alignment and convenient maintenance (Kong et al., 1997, 2005a). Since the SBS wave generates from a thermal noise, it naturally has a random phase with respect to the incident beam. Therefore the phase controlling of the SBS wave is a key technology in the realization of a coherent beam combined system. For this reason, the self-phase control method was proposed and has been developed by Kong et al. (2005a, 2005b, 2005c), which can control the phase of the SBS wave with the simplest composition as well as ease of alignment, no limitations on the number of combined beams, and excellent phase conjugation. Furthermore, the active phase control with a piezoelectric translator (PZT) enables long-term phase stabilization (Kong et al., 2006, 2008). In addition to a random phase characteristic, the distortion that generally occurs in a pulse waveform of an SBS wave is another negative characteristic in terms of the beam combination. Kong et al. (2005d) has overcome this problem with the SBS waveform preservation technique, which is called the prepulse injection method. These works are expected to boost the development of laser systems in term of a high level of energy and