臺大光電所所訊

30 GHz Highly Damped Oxide Confined Vertical-Cavity Surface-Emitting Laser

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

Oxide-confined 850-nm vertical-cavity surface-emitting lasers (VCSELs) are reliable light sources for short-reach multi-mode optical communication applications due to their high efficiency, ease of fabrication, reliability, and small foot-print. 850 nm VCSELs with non-return-to-zero (NRZ) and 4-level pulse amplitude modulation (PAM-4) modulation have provided backbone optical interconnects for 400 and 800 Gigabit Ethernet networking technologies. The dramatically increasing data demand drives the VCSEL-based multi-mode fiber (MMF) links to equip advanced modulation schemes such as PAM-4, which puts specific emphasis on the dynamics performance of VCSELs, such as high bandwidth at low current, sharp edge in S21 response, and low intrinsic noise. Various methods have been investigated to reduce the device parasitics, increase modulation bandwidth and yield highly damped modulation responses.

Our device employs six-pairs thick oxide aperture for a reduced device parasitics capacitance and an optimized relaxation oscillation damping with currents for high quality eyes at the targeted data rates. The highly-damped VCSEL exhibits a low thermal impedance, ultra-high bandwidth, and delivers a flat modulation response. The L-I-V curve of our high damped VCSEL with 3.5 μm oxide aperture at room temperature (RT) and 75°C is shown in Fig. 1. The VCSEL exhibit a peak optical power of 3.88 mW and roll-over current over 9 mA. Fig. 2 shows the measured modulation responses and the VCSEL exhibits flat modulation responses at various biases at RT and 75°C. When biased at 5 mA, the VCSEL reached 27.3 GHz and 25.2 GHz at RT and 75°C respectively, and showed a mere 2.1 GHz decrease in modulation bandwidth at the elevated temperature. The fitted MCEF is 12.2 GHz / mA^1/2 and is illustrated in Fig. 3. We have demonstrated an ultra-fast highly damped VCSEL with a flat modulation response for up to 30 GHz. We employed several optimizations in the design of the VCSEL, including the intrinsic dynamic performance, RC parasitics, and photon lifetime tuning, which enable a thermally-stable operation for up to 75°C. The flat modulation characteristics of the VCSEL could enable over 100 Gb/s line rates PAM-4 modulation for 400G and 800G high-capacity optical interconnect systems.

Fig. 1. The L-I-V characteristics for a ~3.5 μm oxide aperture diameter at RT and 75°C.

Fig. 2. The S₂₁ optical response when 3 mA, 5 mA, and 7 mA at RT and 75°C.

Fig. 3. 3 dB bandwidth plotted against square root of biased current and MCEF fits.

Step-shaped electrode for Blue Phase transflective 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

In recent years, our laboratory has been proposing new electrode designs for blue phase liquid crystal displays (LCDs) with high light efficiency and low operation voltage. One such design is shown in figure 1 below for a transflective LCD. We named this design “step-shaped electrode”. By using this design, it is possible to produce a blue-phase transflective LCD with very high light efficiency of > ~ 95% at low operation voltage of ~ 9 to 13V with well-matched T-V and R-V electro-optic curves as shown in figure 2. This design is based on the concept of forming fairly strong and uniform horizontal electric fields above the reflective (R) region which has a step-shaped reflective electrode and also above transmissive (T) region which is enhanced by the vertical wall-like electrode. These strong electric fields help minimize the dead-zones which normally exist above electrodes (as illustrated in figure 3) and hence enhance the transmission/reflection and lower the operation voltage.

Figure 1. Structure of a blue-phase transflective LCD with step-shaped electrodes

Figure 2. Normalized transmission (T) and reflection (R) of step-shaped electrode blue-phase transflective LCD with two matched designs A and B

Figure 3. Top-view transmission (T) and reflection (R) profiles of a step-shaped electrode blue-phase transflective LCD showing almost no dead zone exist above electrodes