Effects of the Thickness Ratio of Zirconia-Lithium Disilicate Bilayered Ceramics on the Translucency and Flexural Strength.

PURPOSE: To determine the effects of the total thickness and core/veneer thickness ratio of bilayered ceramics on their three-point flexural strength and translucency. MATERIALS AND METHODS: Two groups of specimens were prepared with two different total thicknesses, 1.0 mm and 0.6 mm. These groups were divided into five and three subgroups with ten specimens each, respectively, having different core/veneer thickness ratios. Lithium disilicate was pressed on the zirconia surface using the "lost-wax technique." The translucency and three-point flexural strength were measured, and a one-way analysis of variance test (p < 0.05) was performed to determine whether the translucency and three-point flexural strength were affected by the total thickness and core/veneer thickness ratio of the specimens. RESULTS: For the same total thickness, translucency decreased with a increase in the zirconia core proportion (p < 0.001). For the same core/veneer ratio, the translucency decreased with an increase in the total thickness (p < 0.001). If the zirconia thickness was less than half of the total thickness, the bilayered ceramic specimens containing zirconia cores and monolithic lithium disilicate specimens exhibited a similar translucency. The three-point flexural strength increased with the increasing proportion of zirconia; however, it did not change significantly with the total thickness. However, for the same total thickness, the flexural strengths of the bilayered ceramics were higher than those of the monolithic specimens. CONCLUSIONS: At the same total thickness, increasing the zirconia core thickness decreased the translucency and increased the three-point flexural strength. Bilayered ceramic specimens with low zirconia ratios exhibited translucency similar to that of monolithic lithium disilicate ceramic but a higher flexural strength.

Cost-effective 400-Gbps micro-intradyne coherent receiver using optical butt-coupling and FPCB wirings.

We present a cost-effective and bandwidth-enhanced 64-Gbaud micro-intradyne coherent receiver based on hybrid integration of InP waveguide-photodetector (WG-PD) and silica planar lightwave circuit (PLC). InP waveguide-photodetector (WG-PD) arrays are simply chip-to-chip bonded and optically butt-coupled to a silica-based dual-polarization optical hybrid chip. Multiple flexible printed circuit boards are adapted for electrical RF and DC wirings, which provide low-cost integration and good RF performance of the receiver. A 3-dB bandwidth of the fabricated coherent receiver is extended to ~36 GHz by optimization of bondwire inductance between the WG-PD array and the transimpedance amplifier (TIA), even when commercial TIAs with a typical bandwidth of ~29 GHz are used. Through optimization of the silica hybrid integrated coherent receiver, 64-Gbaud DP-16QAM signal transmission over 1050-km standard single-mode fiber is successfully demonstrated below a bit error rate of 2 × 10-3. This is the threshold for a soft decision-based forward error correction, at the optical signal to noise ratio of 23.8 dB.

Experimental filtering effect on the daylight operation of a free-space quantum key distribution.

One of the challenges of implementing free-space quantum key distribution (QKD) systems working in daylight is to remove unwanted background noise photons from sunlight. Elaborate elimination of background photons in the spectral, temporal, and spatial domains is an indispensable requirement to decrease the quantum bit error rate (QBER), which guarantees the security of the systems. However, quantitative effects of different filtering techniques and performance optimization in terms of the secure key rate have not been investigated. In this study, we quantitatively analyze how the performance of the QBER and the key rates changes for different combinations of filtering techniques in a free-space BB84 QKD system in daylight. Moreover, we optimize the conditions of filtering techniques in order to obtain the maximum secure key rate.

High-speed waveguide photodetector for 64 Gbaud coherent receiver.

A high-speed waveguide photodetector has been successfully fabricated for an integrated coherent receiver. Dual laterally tapered structures are introduced for a spot-size converter. We optimize the responsivity and the polarization-dependent loss of the spot-size converter-integrated waveguide photodetector through the beam propagation method simulation. The waveguide photodetector is designed with electrical as well as optical optimizations. The photodetector provides sufficient alignment tolerance, high responsivity of 0.73 A/W, and low polarization-dependent loss of 0.27 dB, which is in good agreement with the simulation results. By increasing the thickness of the matching layer and the n-doped upper taper, the electrical properties of the photodetector are enhanced. The photodetector has a 3 dB bandwidth of 45 GHz, providing high-speed operation. Through the electrical and optical optimizations, we successfully obtain the high-speed waveguide photodetector for a 64 Gbaud integrated coherent receiver.

Critical side channel effects in random bit generation with multiple semiconductor lasers in a polarization-based quantum key distribution system.

Most polarization-based BB84 quantum key distribution (QKD) systems utilize multiple lasers to generate one of four polarization quantum states randomly. However, random bit generation with multiple lasers can potentially open critical side channels that significantly endangers the security of QKD systems. In this paper, we show unnoticed side channels of temporal disparity and intensity fluctuation, which possibly exist in the operation of multiple semiconductor laser diodes. Experimental results show that the side channels can enormously degrade security performance of QKD systems. An important system issue for the improvement of quantum bit error rate (QBER) related with laser driving condition is further addressed with experimental results.

Optimization of spot-size converter for low polarization dependent loss of waveguide photodetector.

We present an optimization of spot-size converter (SSC) of waveguide photodetector (PD) for small polarization dependent loss (PDL). Beam-propagation method simulation gives responsivity for each polarization and SSC structure. From the calculated responsivity data, optimum structure of SSC is determined that can be implemented with a sufficient process tolerance. We confirm the optimization by measuring PDL of waveguide PD designed according to the structure obtained through the simulation.

Directional single mode emission in a microcavity laser.

We report directional single mode emission in an InGaAsP semiconductor microcavity laser, which is composed of a circle and an isosceles trapezoid. When exciting a whole cavity, the laser generates a single mode without hopping in two directions over a wide range of continuous injection currents. In the emission spectrum, it is confirmed from the equidistant mode spacing that a scar mode becomes a single lasing mode above the lasing threshold. A numerical analysis of the boundary element method shows that the far-field pattern of the resonance agrees well with that of the experiment.

Channel estimation and synchronization for polarization-division multiplexed CO-OFDM using subcarrier/polarization interleaved training symbols.

We propose and demonstrate the use of subcarrier/polarization-interleaved training symbols for channel estimation and synchronization in polarization-division multiplexed (PDM) coherent optical orthogonal frequency-division multiplexed (CO-OFDM) transmission. The principle, the computational efficiency, and the frequency-offset tolerance of the proposed method are described. We show that the use of subcarrier/polarization interleaving doubles the tolerance to the frequency offset between the transmit laser and the receiver's optical local oscillator as compared to conventional schemes. Using this method, we demonstrate 43-Gb/s PDM CO-OFDM transmission with 16-QAM subcarrier modulation over 5,200-km of ultra-large-area fiber.

The effect of gain medium length on dynamic mode stability in semiconductor lasers with a long intra-cavity filter.

We investigate theoretically and experimentally the effect of the physical length of gain medium on dynamic mode stability in semiconductor lasers with an intra-cavity filter. In simulation, two types of analysis models were used to examine the lasing properties and to analyze the dynamic mode stability of the external-cavity system, respectively. In experiment, two different kinds of the structures were fabricated and their spectra were analyzed. Both simulation and measurement results show clearly the length of the gain medium has a critical influence on the stability around the peak wavelength of the filter.

Asymmetric multiple-quantum-well laser diodes with wide and flat gain.

Asymmetric multiple-quantum-well laser diodes with wide and flat gain spectra were designed, fabricated, and analyzed. The active layer was composed of three 10-nm, one 8-nm, and two 6-nm 0.5% compressive strained wells and four 10-nm and one 5-nm 0.4% tensile strained barrier layer. Measured spectra of antireflection-coated ridge waveguide laser diodes with such quantum-well structures have shown that -1-dB spectral gain bandwidth can be as large as 90 nm.