Jae Sung Shin

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.

Phase stabilization of a wave-front dividing four-beam combined amplifier with stimulated Brillouin scattering phase conjugate mirrors

Beam combining method is a promising technique for achieving energy scaling of solid-state lasers. The key technique of the coherent beam combining is the phase control of the stimulated Brillouin scattering wave. In previous works, the amplitude dividing scheme has shown more effective phase control than the wave-front dividing scheme. A recent experiment demonstrated that the phase stabilization in the wave-front dividing scheme can be improved by reducing the beam pointing fluctuation. In this work, a phase stabilization experiment is performed with the wave-front dividing four-beam combined amplifier. The phases are stabilized within λ/25 with an amplified gain of 5.25.

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.

Hollow conic beam generator using a cylindrical rod and its performances

By numerical simulations, it has been shown that a conven- tional cylindrical rod can be used as a hollow conic beam generator by illuminating a parallel laser beam inclined to the axis of the rod. Half of the conic beam is formed by the reflection at the surface of the cylindrical rod, and the opposite side of the conic beam by its transmission. We discuss the parameters to determine the size of the conic beam and the effect of the dielectric multilayer coating on the intensity distribution of the conic beam. The line beams of the shapes such as circle, ellipse, parabola, or hyperbola can be generated by this hollow conic beam gen- erator, depending on the position and orientation of the observing plane.

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.

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

Current status of the SBS PCM approach to self-navigation of lasers on injected IFE pellets

Current status of SBS PCM based IFE approach proposed recently as an alternative to the IFE classical approach is presented. This technology is of particular importance to the direct drive scheme taking care of automatic self-navigation of every individual laser beam on the injected pellets with no need for any final optics adjustment. Conceptual design of one typical laser driver is shown and its features discussed. In comparison with the earlier design an upgraded scheme was developed with the low energy illumination laser beam (glint) entering the reactor chamber through the same entrance window as used by the corresponding high energy irradiation laser beam. Results of experimental verification of this improved design are reported. In these experiments for the fist time a complete setup including the pellet (realized by the static steel ball) was employed. The pellet survival conditions in the period between its low energy illumination and subsequent high energy irradiation were studied and the upper limits on the allowed energies absorbed were found for both DD and DT fuels.