Dong Hun Lee

We investigate the influence of a matching load resistance on frequency responses of a lumped-electro-absorption modulator-integrated lasers (EMLs) whose modulation band-widths are extended by LC resonance effect. An equivalent circuit modeling is performed to determine the optimal values of a matching load resistance enabling 112-Gbaud PAM4 operation for a lumped-EML chip with an electro-absorption modulator (EAM) length of 150 um. To verify the simulation results, we package a lumped-EML submodule and measure its high-frequency charac-teristics. Then, we experimentally demonstrate that the 3-dB bandwidth of the lumped-EML submodule can be enhanced to 55 GHz and beyond by optimizing the matching load resistance value as well as lengths of the bondwires, although the EAM length is as long as 150 um. The fabricated lumped-EML submodule exhibits clear optical eye patterns under 112-Gbaud PAM4 operation.

We have developed a compact 4 × 80-Gbps transmitter-receiver optical subassembly (TROSA) module using a chip-to-chip optical butt-coupling method. For a Tx submodule, four directly modulated laser (DML) chips precisely bonded on a silicon DML-carrier are simultaneously butt-coupled to a vertically polished silica-based arrayed waveguide grating (AWG) chip, revealing good optical characteristics insensitive to external optical feedback. A Rx submodule is realized by passive alignment between a 45°-polished AWG and four surface-illuminated (SI)-PD chips, and subsequently integrated with the Tx submodule into a single package case to form the compact TROSA module whose size is only 25 mm × 15 mm × 6.4 mm and compliant to fit into the QSFP-DD/OSFP transceiver. The fabricated TROSA module shows clear Tx optical eye patterns at 80-Gbps PAM4 signals for all channels, achieving the Rx sensitivities within −7.1∼−6 dBm at a bit error rate (BER) of 2.4 × 10 ⁻⁴ .

Nondestructive THz imaging technology is a promising technology in several industrial applications such as process monitoring and quality control. To further popularize the application of this technique in the industry, we developed photonics-based devices such as a dual-mode laser, uni-travelling carrier photodiode, and Schottky-barrier diode that enable the realization of a compact, low-cost, multifunctional continuous wave (CW) THz system. To develop CW THz reflective imaging systems with these devices, a telecentric f-θ lens and novel two-dimensional (2D) beam scanner were developed. A high-speed, low-cost 2D reflector with a 4-sided polygon mirror, a brushless DC (BLDC) motor rotating at approximately 2,990 revolutions per minute (RPM) and a stepping motor was designed to ensure that a THz image sized 500 × 400 pixels could be scanned in approximately 2 s. However, it is difficult to maintain a constant speed in high-speed rotating BLDC motors. The measured maximum deviation is approximately 0.7%, which results in distorted images. To compensate for this distortion, a sync signal reflector was designed to monitor the real-time RPM of the BLDC motor. Using the novel 2D beam scanner and sync signal reflector, the distorted images could be corrected simply, enabling the development of a 2D THz reflective imaging system using a single emitter and detector; this system could exhibit a higher speed and was more cost effective than the existing system. Although further improvements to photonics-based THz technologies are necessary, we believe that these efforts will begin an era of THz technologies as a widely used industrial technique.

We present a 10-Gb/s L-band reflective electro-absorption modulator integrated with a semiconductor optical amplifier (REAM-SOA) having improved transmission performance at very low input power of seed light. To decrease the input power of seed light, the absorption characteristics of the REAM are adjusted to reduce the amplified spontaneous emission light returned into the SOA, suppressing the gain saturation effect of the SOA. At a considerably low input power of -16 dBm, the REAM-SOA exhibits a low transmission penalty of about 1.2 dB after 50-km SMF transmission. Over a wide input power range from -16 dBm to 5 dBm, a penalty of less than 1.6 dB is achieved at 50-km transmission.