| Analog fiber-optic links are very useful in many applications including cable television, antenna remoting, and hybrid fiber-radio systems. This dissertation investigates various methods to improve the modulation and detection efficiency for high-performance analog fiber-optic links.; As an attempt to improve the detection efficiency of the conventional dual-function electroabsorption modulators (EAMs), the npin structure is incorporated to utilize the phototransistor gain. As a consequence, the resulting EAM/phtotransistor shows a superior detector responsivity in the photodetector mode. The effect due to the recombination centers at the emitter-base junction is studied in the DC as well as the RF responsivity. The phototransistor behavior is further investigated with a waveguide heterojunction phototransistor.; To improve the modulation efficiency, two different approaches are studied. First, utilization of the reflection grating as an intensity optical modulator is proposed and studied. Two electro-optic materials, namely, the electro-optic polymer and LiNbO3, are considered with realistic electro-optic coefficients, waveguide propagation losses, and grating coupling coefficients. The simulation results identify that the waveguide propagation loss is the biggest challenge to overcome in achieving an extremely low Vπ.; The second approach to improve the modulation efficiency uses the bandgap engineering in the semiconductor quantum well. It is shown that by incorporating a step barrier inside the well, the red shift of the quantum-confined Stark effect is delayed to a higher electric field. Since the screening effect due to photogenerated carriers is reduced at high electric field, the saturation optical power of the EAM with these intra-step-barrier quantum wells is expected to increase significantly. Both the InGa(Al)As/InAlAs and the InGaAsP/InGaAsP material system are analyzed.; Finally, a new utilization of the EAM is proposed and characterized. The operation of the EAM as a photodetector/mixer is shown to be superior to the conventional operation as a modulator/mixer for both the MQW and the bulk EAMs. RF-signal mixing with high dynamic range and low added phase noise is demonstrated with this approach. |
