台大光电所所讯

Ultracompact Silicon Waveguide Bends Designed Using a Particle Swarm Optimization Algorithm

Professor Ding-Wei Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所黄定洧教授

In this study, the trajectory of a 90° bend is divided into two symmetric halves that are mirror images of each other as referenced to the symmetry axis at 45°, and each half is segmented into small curved sections (Fig. 1). The bending radius and waveguide width for every section are parameters to be determined using a particle swarm optimization algorithm. The optimization is performed to maximize the transmission of the waveguide bends, which is calculated by using the three-dimensional finite-difference time-domain technique (Figs. 2 and 3). The results indicate that the total bending loss of the optimized 90° bends with radii of 2, 3, 4, and 5 μm are 0.0106, 0.0051, 0.0025, and 0.0023 dB, respectively, at the wavelength λ = 1550 nm (Fig. 4). In addition, the optimal devices are fabrication tolerant, with fabrication errors in width and height within 10 nm, and less wavelength-dependent compared with circular bends.

Fig. 1 Schematic for the design method of a waveguide bend.

Fig. 2. Width versus length of the normalized light path S of the optimal design.

Fig. 3. Curvature versus length of the normalized light path S of the optimal design.

Fig. 4. Spectral response of the bending loss of circular and optimal waveguide bends for λ = 1500 – 1600 nm.

Analytical Modeling of Tunnel-Junction Transistor Lasers

Professor Chao-Hsin Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所吴肇欣教授

Compared with transistor lasers (TLs), tunnel-junction transistor lasers (TJTLs) are more easily modulated with the voltage across base-collector (BC) junction. In this work, we develop the charge-control model and modified rate equation of TJTLs which includes the Franz-Keldysh effect and direct tunneling. Then, we discuss the effects of the collector-doping concentration and the quantum well position on the performance of TJTL. The capability of the voltage modulation and the optical power are the conflict factors while designing the doping concentration since the heavy doping increases both the junction field and bias-sensitive F-K absorption (Fig. 1(a), Fig. 1(b)). Moreover, the bandwidth decreases when doping concentration increases due to stronger absorption and lower photon density (Fig. 1(c)). For the QW position, we need to find out the optimal position that we can get the best optical confinement to reach the maximum optical output. The result shows that the QW closer to the BC junction can obtain a higher confinement factor to achieve better output power and modulation bandwidth (Fig. 2). Therefore, for the design of TJTLs, the collector-doping concentration and the quantum well position need to be considered to a precise degree to achieve good capability of the voltage modulation with enough output power and bandwidth. [This work has been accepted by IEEE Journal of Selected Topics in Quantum Electronics]

Fig. 1. (a) The L-I family curves (b) The L-V family curves (c) Bandwidths of the different frequency responses of the TJTL with different doping concentrations.

Fig. 2. (a) The L-I family curves (b) The L-V family curves (c) Bandwidths of the different frequency responses of the TJTL with different positions of the QW.