Sungil Kim

Designing a traveling wave electrode (TWE), which feeds a high‐speed electrical signal to modulate an optical signal, is a key issue of high‐speed Mach‐Zehnder electro‐optic modulator (MZM) development fields. In this article, we have presented TWE design and characterization results for an MZM on an n‐type InP substrate operating frequency range from DC to 25 GHz. A coplanar waveguide with ground structure has been adapted to the TWE to overcome lossy characteristics of the n‐type InP substrate and increase noise immunity. The designed results are analyzed as comparing with equivalent circuits extracted by the data, which is measured by a vector network analyzer. Consequently, we hope our results can be helpful in the fields of design and fabrication of high‐speed opto‐electric components.

We have demonstrated fixed-current operation of a vertical-cavity surface-emitting laser (VCSEL) transmitter operating at 1.25 Gb/s over a wide temperature range by using an elevated-oxide-layer structure. Four different types of oxide VCSELs have been prepared for the transmitter modules; conventional-oxide and elevated-oxide-layer VCSELs with the oxide-aperture diameter of 6.6 and 10 μm, and their temperature dependence of the threshold current and turn-off transient responses have been measured. Approximately 10%-reduced both fall time and timing jitter mainly originating from the alleviated turn-off-induced abnormalities in the elevated-oxide-layer VCSEL transmitters contributed to achieve the fixed-current operation in the temperature range from 20 °C to 80 °C. Although the operating current of the elevated-oxide-layer VCSEL transmitter should be increased as the threshold current increases, a simple driving circuit owing to the fixed-current operating transmitter over a wide temperature range will bring a drastic energy saving in the optical data transmission system that incorporates VCSEL as a light source.

Lead zirconium titanate (PbZr_0.52Ti_0.48 O₃ and PbZr_0.30Ti_0.70O₃) films were prepared on MgO (100) substrate by the sol-gel method. Film Thickness, microstructure and crystalline structure of the films were investigated by scanning electron microscope (SEM) and X-ray diffraction analysis, respectively. Terahertz time-domain spectroscopy has been used to investigate the dielectric properties of ferroelectric Pb(Zr, Ti)O₃ thin films in the frequency range of 0.2 to 2.0 THz.

Sodium tungsten oxide nanobundles are synthesized by acid precipitation method with the ammonium tungstate and sodium hydroxide. The crystal structure, morphology and optical properties of the sodium tungsten oxide (Na_xWO₃, x = 0.33, 0.5, 0.75, and 1.0) nanobundles are analyzed by X-ray diffraction, field-emission scanning electron microscopy, and UV-visible spectroscopy, respectively.