Toru Nagai

Anomaly of Dielectric Constant of (Ba1-xSrx)(Mg1/3Ta2/3)O3 Solid Solution and Its Relation to Structural Change

The temperaure dependence of the dielectric constant has been studied in the (Ba1-xSrx)(Mg1/3Ta2/3)O3 solid solution. It is found by transmission electron microscopy and Raman spectroscopy that a new low-temperature phase exists in materials with strontium-rich compositions, and it is transformed into the ordered hexagonal perovskite phase with increasing temperature. The structural change is reversible by cooling and heating treatments. Because the dielectric constant of the low-temperature phase becomes large with increasing temperature, it is explained that materials with strontium-rich compositions have a negative temperature coefficient of resonant frequency. The anomaly of the dielectric constant with increasing temperature is discussed in relation to phase transformation.

Contribution of dielectric constant to change in temperature coefficient of resonant frequency in (Ba1-xSrx)(Mg1/3Ta2/3)O3 compounds

Dielectric resonator materials with the formula (Ba1-xSrx)(Mg1/3Ta2/3)O3 have been studied in order to clarify the factors controlling the resonant frequency as a function of temperature. Dielectric constant and thermal expansion are calculated from the resonant frequency measured by the dielectric resonator method and the capacitance obtained at 10 kHz. The results show that the temperature dependence of the resonant frequency originates mainly from the change in the dielectric constant with temperature. The dielectric constant exhibits a maximum at some critical temperature, which varies with increasing strontium content in the oxides.

Wave optics simulation of diode pumped alkali laser (DPAL)

A numerical simulation code for a diode pumped alkali laser (DPAL) was developed. The code employs the Fresnel- Kirchhoff diffraction integral for both laser mode and pump light propagations. A three-dimensional rate equation set was developed to determine the local gain. The spectral divergence of the pump beam was represented by a series of monochromatic beams with different wavelengths. The calculated results showed an excellent agreements with relevant experimental results. It was found that the main channel of the pump power drain is the spontaneous emission from the upper level of the lasing transition.

Output power characteristics of diode-pumped cesium vapor laser

We examine the output power as a function of the cesium vapor density in a diode-pumped cesium vapor laser. Since the pump light bandwidth of our apparatus is considerably wider than the absorption bandwidth, a fair amount of the pump power is unabsorbed. An optical-to-optical conversion efficiency of 70% is observed with respect to the absorbed pump power. Beyond a certain point, the output power starts to reduce despite linear increase in absorption power along with increase in the Cs number density. We perform a numerical simulation to study the observed phenomena, and it is found that spontaneous emission from the upper laser level is the main channel of the pump power loss.

Experimental study of the diode pumped alkali laser (DPAL)

A small-scale cesium diode-pumped alkali laser (DPAL) apparatus has been developed for fundamental researches. A commercial laser diode with volume Bragg grating outcoupler is used to pump the gain cell longitudinally. Both windows of the gain cell are set at Brewster’s angle for minimum loss and maximum durability. Output coupling coefficient is continuously variable from 13% to 85% by the slanted quartz plate outcoupler inserted in the optical resonator. Small signal gain is measured with a laser diode probe at various gain cell temperatures. A 6.5 W continuouswave output with 56% optical-to-optical conversion efficiency (based on the absorbed power) has been achieved. A numerical simulation code is developed and its calculation results are in good agreement with the experiments.

Experimental investigation and numerical simulation of exciplex pumped alkali lasers

An investigation of exciplex pumped alkali laser (XPAL) has been conducted. A 4-cm gain cell containing Cs as an active medium is pumped by a tunable Ti: sapphire pulse laser. Laser oscillation is observed in both five-level and four-level systems. Small signal gain and saturation intensity of the active medium is measured by both probe laser amplification and oscillation experiment with varied output coupling. A one-dimensional numerical simulation is developed and the result of calculations is compared with the experiments. The pulse shape and oscillation threshed agree reasonably while calculated optical-optical conversion efficiency is underestimated.

DIELECTRIC PROPERTIES OF PB(ZR, TI)O3-DISPERSED MGO NANOCOMPOSITE

Ceramic nanocomposites with perovskite-type ferroelectric dispersoids have been studied in order to introduce ferroelectricity and piezoelectricity into structural ceramics. First, MgO matrix/BaTiO3 dispersed ceramic nanocomposites were fabricated, but the piezoelectric coupling factor, kp, of the nanocomposites was reduced to less than 5%. In the present work, MgO matrix/Pb(Zr,Ti)O3 (PZT) dispersed ceramic nanocomposites were studied to realize ceramic nanocomposites with larger kp values than MgO/BaTiO3. MgO/PZT ceramic nanocomposites were successfully fabricated by a pressure-less sintering method. The MgO/PZT nanocomposites exhibited a D–E hysteresis loop, and the ferroelectricity of the PZT dispersoid was confirmed. The tetragonality of the PZT dispersoid was decreased in comparison to that of monolithic PZT ceramics, however, the tetragonal-cubic phase transformation temperature of the PZT dispersoid was higher than that of the monolithic PZT. The decrease in the tetragonality was correlated to the incorporation of Mg ions into the PZT dispersoid. The MgO/PZT nanocomposites exhibited piezoelectricity after poling, and the kp value was about twice that of the MgO/BaTiO3 nanocomposites.

Scalable pump beam arrangement for diode pumped alkali laser

A scalable pumping method for a diode-pumped alkali laser (DPAL) is proposed. In the past, pump beams were collinearly or transversely arranged to the optical axis of the optical resonator. In our proposal, pump sources are circularly arranged around the optical axis, and the pump beams irradiate the gain cell at a small (0.1 radians) angle from the optical axis. Tens of pump beams can be coupled to the gain medium with high circular symmetry and high coupling efficiency. The idea was verified by numerical simulation, and the results were validated by experimental investigation.