Seong Ku Lee

Beam combined laser fusion driver with high power and high repetition rate using stimulated Brillouin scattering phase conjugation mirrors and self-phase-locking

For achieving practically useful laser fusion energy generation, it is necessary to have a M-J laser system with a repetition rate over 10 Hz. We believe that a beam combination method using stimulated Brillouin scattering phase conjugate mirrors ~SBS-PCM! is one of the most practical techniques for achieving the high repetition rate of the high power laser. In this paper, we present the recent results about the beam combination laser, such as SBS reflectivity depending on the mode structure, the cross type isolator, and the phase-locking technique. For the phase-locking technique, especially, the self-phase-locking is proposed and its result is discussed.

Phase control of a stimulated Brillouin scattering phase conjugate mirror by a self-generated density modulation

We report that the phase of a backward wave by stimulated Brillouin scattering (SBS) can be controlled by a self-generated density modulation without any backward Stokes seed beam, which is generated due to electrostriction induced by electromagnetic standing wave through the interferences between the main beam and the low intensity counter-propagating beam. The density modulation ignites the position of the Brillouin grating so that the phase of the backward SBS can be controlled. We obtained 96% success with the phase error less than a quarter wave under a proper scheme of the self-seeding methods. This phase control technique is so simple, scalable, and very efficient that it can be applied to many beam combination systems for a high power laser system with a high repetition rate over 10Hz.

Three-dimensional microfabrication using two-photon absorption by femtosecond laser

As a femtosecond laser has recently been developed, both of high power and high photon density are easily obtained. The high photon density results in photopolymerization of urethane acrylate resin whose absorption spectrum is shorter than that of the femtosecond laser. The stereo-lithography using the two-photon absorption (TPA) makes micro structures with great resolution. We used this phenomenon to make micron-sized structures with sub-micron resolution. Before fabricating 3-D structures, precise 2-D structures were preceded. The TPA photopolymerization was applied of poly-dimethyl siloxane (PDMS) molding. In this paper, we report the recent progress and application of this technology in our laboratory.

Fabrication of 3D microstructure and analysis of voxel generation by ultrafast-laser-induced two-photon absorption

Multi-photon absorption phenomena induced by ultra fast laser have been considered for many applications of microfabrications such as metal ablation, glass etching and photopolymerization. Among the applications, the photopolymerization by two-photon absorption (TPA) has been regarded as a new microfabricating method. It is possible to be used in photo mask correcting, diffractive optical element and micro machining. The TPA photopolymerization is made possible to fabricate a complicated three dimensional (3D) micro-structure which the conventional photomask technology has not been able to make. In fact, the shape of the voxel (volume pixel: a unit structure of TPA fabrication) is an important factor which could affect the microfabrication process. In this paper, we have reported that 3D micro-structures were fabricated and the generation of voxel shape was analyzed for various optical conditions.

Fabrication of PDMS (poly-dimethyl siloxane) molding and 3D structure by two-photon absorption induced by an ultrafast laser

Multi-photon absorption phenomena induced by ultra fast laser have been considered for many applications of microfabrications such as metal ablation, glass etching and photopolymerization. Among the applications, the photopolymerization by two-photon absorption (TPA) has been regarded as a new microfabricating method. It is possible to be used in photo mask correcting, diffractive optical element and micro machining. The TPA photopolymerization is made possible to fabricate a complicated three dimensional structure which the conventional photomask technology has not been able to make. Furthermore the TPA photopolymerization process applied to a two dimensional structure fabrication may take shorter time than the old process since the absence of etching and deposition processes. Recently we have made a simple 3D structure and applied the technique to PDMS(poly-dimethyl siloxane) molding.

Phase control of backward stimulated Brillouin scattering wave by a self-generated density modulation

We shows that the phase of a backward stimulated Brillouin scattering (SBS) wave can be controlled, which is necessary for a beam combination laser using SBS phase conjugate mirrors. The phase control is achieved by a self-generated density modulation without seeding the Stokes beam. Theoretical analysis shows that the phase fluctuation is mainly due to the pump energy fluctuation and is inversely proportional to the pump energy. We have achieved that the relative phase difference between two SBS waves is smaller than λ/4 for all the pump pulses.

Independent phase control of Stokes waves for beam combination

We report that the phase of backward Stokes waves can be independently controlled, which is necessary for a beam combination laser using stimulated Brillouin scattering-phase conjugate mirrors (SBS-PCM). The phase control is achieved by a self-generated density modulation without seeding the Stokes beam. Theoretical analysis shows that the phase fluctuation is mainly due to the pump energy fluctuation and is inversely proportional to the pump energy. We have achieved that the relative phase difference between two Stokes waves via SBS is smaller than λ/4 for all the pump pulses.