Abstract The sealing of sintered ceramics with metal often results in the formation of huge voids that reduce the strength of the final structure. This study therefore investigates the mechanism of void formation during Nb-sealing of an Al2O3 capillary using Nd:YAG laser irradiation. It is found that during the initial stages of cooling the outer surface of the Al2O3 capillary solidifies rapidly, but the Nb wire used as the heat source remains at a higher temperature for longer. This causes any gases emitted from the sealing section at the time of melting to be constrained within the capillary, which eventually leads to the formation of huge voids.

Abstract Based on the analysis of the quasi-three-level side pumped Tm:YAG laser system, the oscillating conditions of this laser are predicted from the point of pump threshold. The laser oscillation at 2.02 µm with larger stimulated emission sections is suppressed when the crystal length is larger than ~85 mm with a 5% output coupling and then an efficient side-diode-pumped rod Tm:YAG laser operating at 2.07 µm is realized. Experimental results are reported from single-module and two-modules Tm:YAG laser and compared with the predictions obtained from the model. The experimental results show that the center wavelength of the Tm:YAG laser is shifted to 2.07 μm when the Tm:YAG crystal length is switched from 69 mm to 138 mm, which is in well agreement with the predictions obtained from the model.

c -axis alignment in a polymer or crystal structure has drawn attention in numerous scientific and technological applications, including crystals, thin film growth, electro-optic devices, and phase difference optics. We here demonstrate a new approach based on retardation measurement that can obtain the direction of the c -axis alignment. This is employed to visualize the three-dimensional direction of continuous crystals as a thin optical film utilizing a liquid crystal panel, using retardation analysis equipment with high resolution measurement capability. The direction of the c -axis alignment is shown so as to allow a detailed characterization of the direction perpendicular to the plane. In this analysis, the direction of the c -axis alignment is identified, and differences between molecules at inequivalent sites are quantified. The results suggest that the excellent lubrication properties of the c -axis alignment may be due to a significant localization in lateral directions. • There was no report to analyze c -axis alignment directly. • A new analysis approach is demonstrated with a direction of the c -axis alignment. • We could visualize the three-dimensional direction of optical axes in a thin film.

Abstract The photorefractive response in semi-insulating GaAs:Cr is theoretically studied when an alternating electric field is applied to a crystal. The calculations are based on material equations including electron and hole currents, nonlinear hot-electron transport and field-enhanced recombination rates. The generalized solutions for the fundamental amplitude of the space-charge field are given. It is shown that for an AC sine-wave field the hot-electron effect reveals in a very small degree. A comparison of the obtained solutions with available experimental data indicates recombination coefficient values of the order of 10−6–10−5 cm3/s which can be attributed to the field-enhanced capture cross section effect.

Abstract Synchronous operation of the closure of two spark gap switches was studied by triggering them under capacitively and optically (UV) coupled conditions. The combined action of electrical coupling and optical coupling resulted in effective synchronization between the closures of the two gaps yielding controllable delay and near jitter-free operation as compared to the case of only electrical coupling. This is attributed to the generation of photoelectrons in the inter-electrode gap in the case of optical coupling that reduced the formation time lag, and in turn, the delay and jitter.

Abstract The large-area monolayer graphene film was grown on Cu foil by chemical vapor deposition (CVD) technique. After transferring it to a quartz substrate, the graphene film was used as a saturable absorber (SA) in an Nd:YVO4 laser. Mode-locked laser pulses of 7.6 ps were obtained at a central wavelength of 1064 nm. Under a pump power of 6.7 W, the average output power reached a high value of 2.5 W, corresponding to an optical-to-optical efficiency of 37.3% and a slope efficiency of 40.3%. The pulse repetition rate was 87.4 MHz and the maximum pulse energy was as high as 28.6 nJ. To date, this might be the highest average output power for mode-locked laser by using graphene SA. Our results illustrate that graphene can potentially be employed in the practical low-cost and high-power mode-locking laser system.

Abstract We report an active Q-switching with different coded schemes generated in the high power diode end pumped Nd:YVO 4 laser (DEPSS) using two different techniques of special combination of optical choppers and acousto-optic modulator (AOM) at higher repetition rates of 100 kHz for the first time to the best of our knowledge in any practical laser target designation (LTD) systems. The highest peak power of 95.4 kW with the lowest pulse repetition frequency of 222 Hz and pulse width of 46 ns at FWHM with the pulse-to-pulse stability of ~95% was measured at certain regime of operation. Special modulation and coding of high stable, high peak power Nd:YVO 4 laser pulses at 1064 nm using the combination of different optical chopper blades were achieved. A high peak power of more than 4.84 kW with the shortest pulse width of 25 ns at FWHM has been obtained using an acousto-optic modulator (AOM) as an active Q-switched. The pulse-to-pulse stability was measured and improved to be ~97% at high repetition rate ranges from 10 kHz to 100 kHz. The correlation between the coded laser irradiation (transmitted) and the scattered (received) laser signals for three different targets materials of sand, wood and metal using this laser designation system are examined and discussed.

Abstract We investigate, using a heterostructure consisting of Cd 0.97 Mn 0.03 Te/Cd 0.98 Mg 0.02 Te layers the optical spin injection process from the Cd 0.97 Mn 0.03 Te spin aligner into the non-magnetic Cd 0.98 Mg 0.02 Te neighboring layer. We find that the spin injection process can be controlled by an external magnetic field (~0.5 T). Utilizing the circular polarization of the photoluminescence emitted from the Cd 0.98 Mg 0.02 Te layer, we monitor the injected spins of the carriers. As the magnetic field is increased, the degree of the circular polarization changes its sign from positive (with the σ + intensity dominating), indicating occurrence of the spin injection, to a negative value (with the σ − intensity dominating), indicating the inhibition of the spin injection. We study also the temperature dependence of the degree of the circular polarization of Cd 0.98 Mg 0.02 Te emissions at a fixed magnetic field, and thus of the injection efficiency.

Detailed experimental and theoretical analyses of the dispersion compensation effect in a femtosecond laser amplifier are presented. It is confirmed that the temporal structures in the vicinity of the central peak of the amplified laser pulse are primarily caused by the uncompensated third- and/or fourth-order dispersion. The specific detrimental roles played by the third- and fourth-order dispersions such as resulting in the formation of asymmetrical pulse shapes and satellite pulses are revealed and experimentally verified with third-order autocorrelation measurements. With the help of a third-order autocorrelator , it is more efficient and accurate to optimize the third- and fourth-order dispersion compensation when the roundtrip times of a laser pulse inside the regenerative amplifier changes. For practical applications, in order to achieve laser pulses with highest quality, namely with minimum pulse energy in their wings, it is imperative to optimize the dispersion-control parameters while monitoring the laser pulses with a third-order autocorrelator. • We give detailed analyses of optimization of dispersion compensation in a CPA system. • TOD and FOD normally lead to the asymmetrical pulse shapes and satellite pulses. • The effects of HOD are revealed by 3rd-order autocorrelation measurement. • It is efficient to optimize TOD and FOD under a 3rd-order autocorrelator's monitoring. • The measurement helps to guide the adjustment of parameters for high quality pulses.