||High-speed high-efficiency optical modulation has become one of the key technologies in enabling next-generation optical data transmission, motivating fundamental research into optical modulators. In this dissertation, a cascaded integration (CI) of electroabsorption modulator (EAM) and semiconductor optical amplifier (SOA) with a by-passing high-impedance transmission line (HITL) is developed for broadband, high-efficiency, and pre-chirp optical processing.|
As the modulation frequency is increased to the point that the microwave wavelength is of the order of the length of the device, the effect of the propagation of an electrical wave on the electrical-to-optical (EO) interaction will become significant, so designing to take into account the its distributive effect on optical modulation is important. Unlike the conventional EO response of a point-like structure design, wave interactions, such as propagation loss, phase mismatch, and wave reflection that is induced by impedance mismatch at boundaries require that a design trade-off be made between modulation speed and efficiency. An impedance mismatch between the intrinsic waveguide structure (typically ~20Ω) and the 50Ω loaded impedance can result in the high field standing wave effect and strong phase distortion, limiting the properties of the modulator. In the CI structure, the series integration of EAM and HITL supports phase and impedance matching in the distributive EO interaction. Therefore, the EO bandwidth can be pushed to the limit for microwaves, i.e. propagation loss. Also the CI structure allows segmental SOAs to be monolithically integrated with segmental EAMs and performs optical gain processing. Experimental and theoretical results have demonstrated a flat EO response in the form of a -3dB drop at 47GHz, a -10dB microwave reflection from DC to 65 GHz, an optical gain of 13.5dB, all of which represent a large improvement over the conventional EAM. Such a design yields a high modulation efficiency of over 15dB/V and a modulation of 40Gb/s can be obtained without sacrificing the speed.
With the integration of SOA into EAM, the nonlinear effect of SOA, self-phase modulation, can be exploited for transmit data, allowing pattern control and pre-chirp optical modulation. In a field-driven EAM, the frequency chirp can be controlled by setting the bias points of the EAM, and the inherently contrary sweeping dynamic in the EAM can balance the carrier-recombination limit in a current driving SOA. A total negative chirp of 6.2GHz is obtained from the EAM and the SOA, and a -2.6GHz frequency shift from SOA is observed. The low-pattern-effect signal process improves the SNR from 8.8 to 10.8. Finally, a 10Gb/s non-return-to-zero with a 43 km error-free data transmission is demonstrated.
The signal process of the eliminated pattern effect is used in signal regeneration. All-optical reamplification and reshaping (2R) regeneration is developed, and the limitation on the material speed from carrier dynamic in this field can be eliminated. An S-sharp transfer function to improve SNR and a 40Gb/s data transmission is demonstrated, revealing that the method is suitable for use in a high-speed all-optical network.