Min Hwan Kwak

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.

We report the complex dielectric characteristics of the Co doped ZnO thin films by terahertz time-domain spectroscopy. The Co doped ZnO thin films are prepared by sol-gel spin coating process on glass substrate. The crystal structure and morphology of the Zn_1-xCo_xO films are characterized by high resolution X-ray diffraction and scanning electron microscopy, respectively. The Zn_1-xCo_xO thin film with 10 at.% Co concentration exhibits highly c-axis orientation and the lowest electrical resistivity. The measured THz pulse and the complex dielectric constant of Zn_0.8Co_0.2O film exhibit different behavior to others due to the decrease of the crystallinity of the film.

Terahertz time-domain spectroscopy has been used to investigate the dielectric properties of ferroelectric thin Ba_xSr_1-xTiO₃ films in the frequency range of 0.3 to 2.5 THz. The Ba_xSr_1-xTiO₃ films were deposited on (001) MgO substrate by pulsed laser deposition. The measured complex dielectric function was compared with the Cole-Cole relaxation model and agreed well with the theoretical fit.

A phased array antenna using ferroelectric coplanar waveguide (CPW) phase shifter active modules is presented based on a Ba_0.6Sr_0.4TiO₃ (BST) thin film. The ferroelectric BST thin film was deposited on an MgO substrate by using pulsed laser deposition. The active module amplifies an RF signal with low noise and changes the phase of a received RF signal to control the main beam of the array antenna. It is composed of low noise amplifier (LNA), dc block, and ferroelectric CPW phase shifter. The prototype of 6×12 phased array patch antenna with the active modules using a ferroelectric phase shifter is proposed. The proposed array antenna continuously scanned its main beam in elevation plane to 32° from its broadside direction at 11.85 GHz by applying the continuous dc voltage to the ferroelectric phase shifter.

Stress controlled epitaxial ferroelectric Ba_0.5Sr_0.5TiO₃ (BST) films have been deposited on Gd₂O₃/SrTiO₃ by pulsed laser deposition with oxygen background pressure of 200 mTorr at the deposition temperature of 750°C. In order to control the stress in BST films, oxygen pressures for Gd₂O₃ buffer layers have been varied from 0.1 to 100 mTorr, while that of BST films have been fixed at 200 mTorr. It has been found that the lattice parameters of the BST films deposited on Gd₂O₃ were changed. Furthermore, microwave properties of co-planar waveguide (CPW) fabricated on BST films were investigated by a HP 8510C vector network analyzer from 1–20 GHz. Large dielectric tunabilities were observed from the CPW's fabricated on BST films deposited on Gd₂O₃ layers deposited at low and high oxygen pressures, 0.1 and 100 mTorr, respectively.

We report on the design, fabrication and microwave performance of coupled microstrip line phase shifters (CMPS) based on (Ba,Sr)TiO₃ (BST) thin films for Ka-band phased array antenna. Single phase BST thin films were deposited by pulsed laser deposition (PLD) on (001) MgO substrates. The CMPS were analyzed and designed by using an exact analysis in terms of even and odd mode phase velocities rather than the approach in which the velocities are averaged. These CMPS were tested in Ka-band, at room temperatures, and between the bias ranges of 0∼200 V. The measured differential phase shift was 458° and the insertion loss varied between 7.8 dB and 10.5 dB at 29 GHz in the bias ranges of 5∼200 V. The CMPS exhibited a figure of merit of about 44°/dB.