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We developed a longitudinally excited N 2 laser with a simple driver circuit and a simple power supply. The N 2 laser consisted of a 20 cm-long glass tube with an inner diameter of 2.5 mm, a normal stable resonator formed by flat mirrors, a variable transformer, a neon sign transformer, a spark gap, and a 200 pF capacitance. The N 2 laser produced a laser pulse with an energy of 379 nJ and a pulse width of 7.5 ns at a repetition rate of 100 Hz. The laser beam was circular and had a Gaussian profile with a correlation factor of 0.992 93.

To realize an ultraviolet laser with high beam quality and high output power, we are developing a master oscillator power amplifier system for a longitudinally excited gas laser. In this system, high beam quality is produced by an oscillator stage, and high output power is produced by an amplifier stage. Here, the beam profile of the longitudinally excited gas N 2 laser had a quasi-Gaussian shape when using a large-diameter discharge tube, which enabled the use of the amplifier stage. The discharge tube consisted of a dielectric pipe with an inner diameter of 14 mm and a length of 30 cm. The discharge tube was covered with a metallic tube for pre-ionization. The strength and spatial distribution of pre-ionization were optimized by adjusting the gap between the metallic tube and the discharge tube. The uniform discharge achieved by controlling the pre-ionization produced a quasi-Gaussian laser beam profile with a correlation factor of 0.99513, a laser pulse energy of 343 μJ, and an input energy of 2.95 J.

A series of experiments were carried out to evaluate the energy-coupling efficiency from heating laser to a fuel core in the fast-ignition scheme of laser-driven inertial confinement fusion. Although the efficiency is determined by a wide variety of complex physics, from intense laser plasma interactions to the properties of high-energy density plasmas and the transport of relativistic electron beams (REB), here we simplify the physics by breaking down the efficiency into three measurable parameters: (i) energy conversion ratio from laser to REB, (ii) probability of collision between the REB and the fusion fuel core, and (iii) fraction of energy deposited in the fuel core from the REB. These three parameters were measured with the newly developed experimental platform designed for mimicking the plasma conditions of a realistic integrated fast-ignition experiment. The experimental results indicate that the high-energy tail of REB must be suppressed to heat the fuel core efficiently.

The temperature dependence of the laser-induced damage threshold on optical coatings was studied in detail for laser pulses from 123 K to 473 K at different temperature. For pulses longer than a few picoseconds, the laser-induced damage threshold of coated substrates increased with decreasing temperature. This temperature dependence was reversed for pulses shorter than a few picoseconds. We describe the physics models to explain the observed scaling. The electron avalanche is essential to explain the differences in the temperature dependence.

We developed a modified driver circuit composed of a capacitance and a spark gap, called a direct-drive circuit, for a longitudinally excited gas laser. The direct-drive circuit uses a large discharge impedance caused by a long discharge length of the longitudinal excitation scheme and eliminates the buffer capacitance used in the traditional capacitor-transfer circuit. We compared the direct-drive circuit and the capacitor-transfer circuit in a longitudinally excited N2 laser (wavelength: 337 nm). Producing high output energy with the capacitor-transfer circuit requires a large storage capacitance and a discharge tube with optimum dimensions (an inner diameter of 4 mm and a length of 10 cm in this work); in contrast, the direct-drive circuit requires a high breakdown voltage, achieved with a small storage capacitance and a large discharge tube. Additionally, for the same input energy of 792 mJ, the maximum output energy of the capacitor-transfer circuit was 174.2 μJ, and that of the direct-drive circuit was 344.7 μJ.

Segmented (tiled) grating arrays are being intensively investigated for petawatt-scale pulse compression due to the expense and technical challenges of fabricating monolithic diffraction gratings with apertures of over 1m. However, the considerable freedom of motion among grating segments complicates compression and laser focusing. We constructed a real compressor system using a segmented grating for an 18cm aperture laser beam of the Gekko MII 100TW laser system at Osaka University. To produce clean pulse shapes and single focal spots tolerant of misalignment and groove density difference of grating tiles, we applied a new compressor scheme with image rotation in which each beam segment samples each grating segment but from opposite sides. In high-energy shots of up to 50J, we demonstrated nearly Fourier-transform-limited pulse compression (0.5ps) with an almost diffraction-limited spot size (20microm).

We introduce a four-pass laser pulse compressor design based on two grating apertures with two gratings per aperture that is tolerant to some alignment errors and, importantly, to grating-to-grating period variations. Each half-beam samples each grating in a diamond-shaped compressor that is symmetric about a central bisecting plane. For any given grating, the two half-beams impinge on opposite sides of its surface normal. It is shown that the two split beams have no pointing difference from paired gratings with different periods. Furthermore, no phase shift between half-beams is incurred as long as the planes containing a grating line and the surface normal for each grating of the pair are parallel. For grating pairs satisfying this condition, gratings surfaces need not be on the same plane, as changes in the gap between the two can compensate to bring the beams back in phase.

We have developed novel excitation circuits without high-voltage switches for two longitudinally excited N(2) lasers (wavelength: 337 nm). One uses a single tube without a trigger and the other uses a tandem tube with a trigger. In both systems, the discharge tube acts as a switch. In the single-tube system, the laser output energy was 125.8 microJ and the efficiency was 0.16% at 18 Torr (2.4 kPa) when a slow-rising voltage pulse of -28 kV was applied (rise time: 21.3 micros). In the tandem-tube system, the laser output energy was 259.4 microJ and the efficiency was 0.11% at 18 Torr when a slow-rising voltage pulse of -48 kV was applied (rise time: 27 micros).

A new scheme for phase control of optical components with laser ablation has been developed. One can ablate the surface shape of optical plastic material coated on a glass plate by using 193-nm laser light to control the transmission wave front. The surface shape is monitored in situ and corrected to attain the desired aberration level. The irradiation fluence is approximately 40 mJ/cm(2), and the ablation depth/pulse is approximately 0.01 microm/pulse for UV-cured resin. A wave-front aberration of 3.0 lambda is reduced to 0.17 lambda for flat surface shaping. For spherical surface generation, an aberration of 2.5 lambda is reduced to 0.2 lambda. The increase in surface roughness is kept within acceptable levels.

To design a fully continuous wave-front distribution suitable for focused beam shaping by a deformable mirror, we modify the phase-retrieval algorithm by employing a uniformly distributed phase as a starting phase screen and spatial filtering for the near-field phase retrieved during the iteration process. A special phase unwrapping algorithm is not required to obtain a continuous phase distribution from the retrieved phase since the boundary of the 2pi-phase-jumped region in the designed phase distribution is perfectly closed. From the computational result producing a uniform square beam transformation from a circular defocused beam, this algorithm has provided a fully continuous wave-front distribution with a lower spatial frequency for a deformable mirror. The transformed square beam has a normalized intensity nonuniformity of varsigma(rms) = 0.14 with respect to a desired flat-topped square beam pattern. This beam-shaping method also provides a high energy-concentration rate of more than 98%.

A high-aspect-ratio line focus is required on a plane target in x-ray laser experiments for obtaining a high gain-length product. Inherent wave-front aberrations in line-focusing optics, which consist of a cylindrical lens and a spherical lens, are discussed with respect to beam diameter. The nonuniformity of the linewidth that is due to the aberrations is also calculated by the ABCD matrix method. A deformable mirror of a continuous plate type with a diameter of 185 mm provides an adequate wave-front distribution for compensating for the wave-front aberration. The wave-front control by the deformable mirror realizes a fine linewidth of 25 microm and 18.2 mm long, corresponding to the aspect ratio of 728. The linewidth is three times the diffraction limit. The intensity distribution along the line focus is also improved.

A new plastic microlens array, consisting of 900 lenslets, has been developed for the Shack Hartmann wave-front sensor.The individual lens is 300 µm × 300µm and has a focal length of 10 mm, which provides the same focal size, 60 µm in diameter, with a constant peak intensity. One can improve thewave-front measurement accuracy by reducing the spot centroiding error by averaging a few frame memories of an image processor. A deformable mirror for testing the wave-front sensor gives anappropriate defocus and astigmatism, and the laser wave front is measured with a Shack Hartmann wave-front sensor. The measurement accuracy and reproducibility of our wave-front sensor are better than λ/20 and λ/50 (λ = 632.8 nm),respectively, in rms.