Du Hyun Beak

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.

Waveform preservation of the backscattered stimulated Brillouin scattering wave by using a prepulse injection

We have found that it is possible to preserve the temporal waveform of the reflected wave generated from stimulated Brillouin scattering (SBS) by using a prepulse technique. The waveform of the SBS wave usually shows a steep rising edge in the ordinary SBS process. It has been found that the waveform of the reflected wave depends on both the prepulse energy and the time delay between the main pulse and the prepulses. A prepulse energy of 5 mJ and a time delay of 5 ns have been measured to be the optimum values under the experimental conditions. This prepulse method is useful in developing a multistage system employing several SBS cells in series for high-power laser applications.

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.

Long-term stabilized two beam combination laser system with amplifiers using the phase controlled stimulated Brillouin scattering phase conjugate mirrors

The beam combination method using stimulated Brillouin scattering phase conjugate mirrors is a promising technique for solid state lasers of high power/energy operating with high repetition rate. The key technology of this method is the phase control of the SBS waves. In the previous works, the principle of this phase control technique was demonstrated experimentally. As a next step, in this work, amplifiers have been added to the beam combination system. Inserting the amplifiers, a stabilized phase difference has been obtained with a fluctuation less than λ/50 at 44 mJ total output energy and 10 Hz repetition rate.

Suggestion of a laser fusion driver using beam combination with stimulated Brillouin scattering phase conjugate mirrors

We developed new techniques for phase control and waveform conservation of a stimulated Brillouin scattering wave. Using these techniques, a laser fusion driver using beam combination, with which the output energy can be unlimitedly scaled up with high repetition rate over 10 Hz, can be achieved.

The effect of a prepulse technique in the stimulated Brillouin scattering and its applications

We have introduced the additional prepulse with main pulse to generate the stimulated Brillouin scattering (SBS) and investigated the effect of this technique. In general, temporal pulse shape deformation takes place when the pulse is reflected from the medium breeding SBS. This deformation of the SBS wave can cause optical breakdown in the optical components and consequently it leads to low reflectivity and low fidelity of the phase conjugated wave in the SBS medium. It has been shown that there is optimum prepulse time delay and minimum energy for preserving the SBS waveform. This method is so simple that it can be applied to other systems and utilized in many applications easily, such as high-power laser and optical isolator applications employing several SBS cells.

Prepulse Technique for preserving the pulse shape of the stimulated Brillouin Scattering

We have found that it is possible to preserve the temporal waveform of the reflected wave generated from stimulated Brillouin scattering (SBS) by using a prepulse technique. In this work, the fundamental research has been carried out to preserve the deformed pulse shape reflected from SBS medium. It is well known that the reflected SBS wave has a steep rising edge. If one employs SBS cells in series, the rising edge of the pulse shape becomes steeper every time it reflects at every SBS cell. This deformation of the SBS wave can cause the undesirable effects when we employ several SBS cells in series, such as an optical breakdown in the optical components and the lower reflectivity and lower fidelity of the phase conjugated wave in the SBS medium. A prepulse energy of 5 mJ and a time delay of 5 ns have been measured to be the optimum values under this experimental condition. This prepulse method is useful in developing a multistage system employing several SBS cells in series for high-power laser applications.