Kazuo Imasaki

Total-reflection active-mirror laser with cryogenic Yb:YAG ceramics.

An efficient high-power laser operation has been demonstrated by using a cryogenic Yb:YAG composite ceramic with a total-reflection active-mirror arrangement. The composite ceramic, which had no high-reflection coating and was cooled with liquid nitrogen directly, showed four-level operation even at 67 kW/cm(3) of high pump density. A 273 W cw output power was obtained with 65% optical efficiency and 72% slope efficiency.

Three-dimensional simulation of super-radiant Smith-Purcell radiation

A simulation of coherent and super-radiant Smith-Purcell radiation is performed in the gigahertz regime using a three-dimensional particle-in-cell code. The simulation model supposes a rectangular grating to be driven by a single electron bunch and a train of periodic bunches, respectively. The true Smith-Purcell radiation is distinguished from the evanescent wave, which has an angle independent frequency lower than the minimum allowed Smith-Purcell frequency. We also find that the super-radiant radiations excited by periodic bunches are emitted at higher harmonics of the bunching frequency and at the corresponding Smith-Purcell angles.

Reduce the start current of Smith-Purcell backward wave oscillator by sidewall grating

A sidewall grating for the Smith-Purcell device is proposed to enhance the coupling of the optical mode with the electron beam and, consequently, relax the stringent requirements to the electron beam. With the help of three-dimensional particle-in-cell simulations, it has been shown that, comparing with the general grating, the usage of a sidewall grating improves the growth rate and dramatically shortens the time for the device to reach saturation. It is also found that the sidewall grating holds the potential to reduce the start current for the operation of a Smith-Purcell backward wave oscillator.

Nd/Cr:YAG Ceramic Rod Laser Pumped Using Arc-Metal-Halide-Lamp

We observed laser oscillations of rod-type Nd/Cr:YAG ceramics experimentally pumped using an arc-metal-halide lamp having a similar spectrum to solar light. An optical–optical conversion efficiency of 43% was obtained by chromium-ion codoping of Nd:YAG with an efficiency of 26%.

Experimental study of a waveguide free‐electron laser using the coherent synchrotron radiation emitted from electron bunches

We have observed the oscillation of a waveguide mode free‐electron laser using the coherent synchrotron radiation emitted from electron bunches. The oscillation was observed at 2.73 mm with a linewidth of 0.4%. This spectral selection was attributed to the dispersion of the radiation in the waveguide. We found that the oscillating radiation had to have the same group velocity as the velocity of the electrons passing through the wiggler field.

Analysis on Coupling Gamma-ray to Nuclear Giant Resonance

Some nuclei from nuclear waste are hazardous to the environment because of a very long decay time more than 106 years that is longer than the life of artificial barrier. A scheme for shortening the radioactive time of long-lived fission product, through the method of realizing nuclear transmutation withγ -ray irradiation, has been proposed. 1,2) The system comprises a storage ring circulating electron beam, and a super cavity accumulating laser photons, γ -ray can be generated by the way of Compton back scattering. 4) Technology of super cavity for increasing the intensity of laser field has been experimentally researched by us 1,3) in recent years. As we know, besides nuclear giant resonance, there are many other forms of absorption to γ -ray in a nuclear target, such as electron-positron pair creation, Compton scattering and photoelectric effect. The efficiency of transmutation depends critically on the absorption efficiency of nuclear giant resonance to γ -ray photons, which is defined as coupling efficiency in this paper. Coupling efficiency also plays an important role in considerations on designing the whole system such as energy recovery, energy balance and so on. To investigate the coupling efficiency is the main aim of this paper.

Effective Fluorescence Lifetime and Stimulated Emission Cross-Section of Nd/Cr:YAG Ceramics under CW Lamplight Pumping

Remarkable improvements in the lifetime of the Nd upper level and in the effective stimulated emission cross-section of Nd/Cr:YAG ceramics have been theoretically and experimentally studied. Until recently, it had been thought that the long energy transition time from Cr ions to Nd ions of Nd/Cr:YAG adversely affects laser action, degrading optical–optical conversion efficiency under CW and flash lamp pumping. However, current research showed that high-efficiency energy transition has a positive effect on laser action. The effective lifetime is increased from 0.23 to 1.1 ms and the emission cross-section is effectively increased to three times for that of the conventional Nd:YAG. A small signal gain is significantly improved, and the saturation power density is reduced to 1/10 that of the Nd:YAG for the same pumping power density. A CW laser light generated in a laser diode (LD)-pumped 1064 nm Nd:YAG laser oscillator was amplified, and the measured output power was saturated. The output laser power calculated using theoretical saturation power density was consistent with the experimental results.

Zig-zag active-mirror laser with cryogenic Yb 3+ :YAG/YAG composite ceramics

We report on a novel amplifier configuration concept for a 10 kW laser system using a zig-zag optical path based on a cryogenic Yb:YAG Total-Reflection Active-Mirror (TRAM) laser. The laser material is a compact composite ceramic, in which three Yb:YAG TRAMs are combined in series to increase the output power. Output powers of up to 214 W with a slope efficiency of 63% have been demonstrated for CW operation, even at a quite low pump intensity of less than 170 W/cm2. Further scaling could achieve output powers of more than 10 kW.

Focusing permanent magnet undulator

Abstract A plane polarized undulator does not have the ability to focus an electron beam in the undulating plane (X-Z plane) because the magnetic field decreases away from the X-axis. To focus in the bending plane, a quadrupole field must be added to the undulating field. This can be achieved using curved magnets or trapezoid-shaped magnets. In this article, we analyze a few magnet configurations which produce X-Z focusing. It has been found that semicircular magnetics or trapezoid magnets having 1.2° angle are suitable as focusing permanent magnet undulator.

Highly Efficient Lasing Action of Nd3+- and Cr3+-Doped Yttrium Aluminum Garnet Ceramics Based on Phonon-Assisted Cross-Relaxation Using Solar Light Source

We constructed a theory to explain the mechanism of laser generation with a high optical–optical conversion efficiency for Nd3+- and Cr3+-doped yttrium aluminum ceramics when sunlight or lamplight sources are used for pumping. As a result, a unique mechanism of laser action was found where the solar or lamp-light power could be converted to laser power with a high efficiency close to 80%, which has not previously been observed. The high conversion efficiency was not only considered to be based on one-to-one photon conversion but on two-photon excitation by a single photon with phonon assistance. Thus, the mechanism of lasing action should include a process where thermal energy is converted to photon energy. The theoretical results we obtained were consistent with those of the experiments.