臺大光電所所訊

High Speed 850-nm Vertical Cavity Surface Emitting Lasers (VCSELs)

Professor Chao-Hsin Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所吳肇欣教授

850 nm vertical-cavity surface-emitting lasers (VCSELs) have been extensively applied on high-speed short reach optical network systems due to their characteristics like low divergence angle, cost-effective fabrication, small footprint, low power consumption and high modulation speed at low currents, which can be applied on high-speed datacenter and giga bit Ethernet.

We accomplished a 50 Gb/s operation of an 850 nm oxide-confined VCSEL with a 3.3 μm aperture diameter. The light- current- voltage curve of 850nm VCSEL with a 0.8 mA threshold current (I_th) and the 0.6 W/A slope efficiency, and the high differential resistance of 135 Ω due to the small aperture, which is shown in Fig.1. Fig.2 shows the optical response of the VCSEL with injected current of 4, 6, and 8mA, respectively, and the modulation bandwidth increases from 22.7 to 25 GHz. Fig.3(a) shows 50 Gb/s BER measurement at I/I_th of 13. The error-free data transmission (BER < 10^-12) is achieved with received optical power of 1.7 mW and the corresponding eye diagram is shown in Fig. 3(b). The VCSEL shows an open eye at 50 Gb/s data rate without overshoot which means it can provide higher signal integrity without using pre-emphasis, equalization, and forward error correction. The energy/data efficiency is 748 fJ/bit at 50 Gb/s at room temperature.

Fig. 1. The light-current-voltage (L-I-V) characteristics of 850 nm VCSEL.

Fig. 2. Optical modulation of the VCSEL at various bias current. A 25 GHz bandwidth is obtained at I = 8 mA.

Fig. 3. (a) The 50 Gb/s bit-error-rate-test (BERT) results of the 850 nm VCSEL biased at 11 mA (I/I_th~ 13) with PRBS7 bit sequence and V_pp = 1252 mV. (b) is the corresponding eye diagram.

Three Dimensional Corrugated Electrode Structure for Low-voltage High-transmittance Blue-Phase Liquid Crystal Displays

Professor Wing-Kit Choi

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所蔡永傑教授

We have recently proposed new electrode designs for corrugated Polymer-Stabilized Blue-Phase Liquid Crystal Display (PSBP-LCD). These new designs are based on the concept of three dimensional (3D) electrode structure which can help create extra electric field in the transverse direction (y-axis). These new designs can help enhance the potential transmission by reducing the dead zones that exist along the turning edges of these devices. By using suitable device parameters, it is possible to obtain high transmission T of > 90% at low operation voltage of ~10V. PSBP-LCD has many attractive features such as sub-millisecond fast response time, no need of alignment layer, wide-viewing angle etc., and has been actively researched as a potential technology for next-generation displays in recent years. Results of this research have recently been published in Liquid Crystals [1].

[1] Bing-Han Chan & Wing-Kit Choi (2018): Three-dimensional corrugated electrode structure for low-voltage high-transmittance blue-phase liquid crystal displays, Liquid Crystals, DOI: 10.1080/02678292.2018.1530385