Key Technologies Of High Speed Photodiodes

With the demand for high speed and large capacity of modern communication systems growing rapidly, the optical network will be upgraded continuously to meet the requirements of high capacity for future data transmission systems. By employing wavelength division multiplexing and time division multiplexing techniques, the maximum capacity in single fiber has reached to102.3Tera-bit/s. High speed photodiodes, which can transform the modulated optical signals into high speed electrical signals, are the key devices on the receiver side of communication systems. Besides, high speed photodiodes are also widely used in analog optics links, such as the generation of millimeter or terahertz wave signal and optical oscillators, etc. This dissertation is mainly concerned with key technologies of InP based high speed photodiodes, related devices, and their significant applications. The main results are outlined as below:1. Multimode interference couplersThe comparison between performances of2x2restricted multimode interference (MMI) couplers and2x2general MMI couplers has been demonstrated. A novel comparison method has been proposed, drawing a conclusion that general multimode interference devices have more compact dimensions and similar performances. The fabricated2x2MMI couplers have low loss and high imbalance. Within the wavelength range(1540-1560nm), the insertion loss of the fabricated2x2MMI couplers ranges from-lOdB to-5dB, and the imbalance ranges from-1dB to1dB. A novel1.55μ m/1.31u m multi-section MMI wavelength splitter design has been presented. The MMI wavelength splitter, whose length is shorter than20%of the conventional design, has low loss and high extinction ratio. The insertion loss is22.4dB at1.55μ m, and-1.46dB at1.31um. The extinction ratio is22.4dB at1.55μm, and12dB at1.31μ m.2. High speed InP based photodiodesAccording to the coupling modes theory, A simple and effective method has been presented for evanescently coupled photodiodes design. The responsivity of fabricated photodiodes ranges from0.5A/W to0.7A/W, and bandwidth ranges from70GHz to120GHz. Without temperature control, the saturation current of photodiodes is higher than20mA at10GHz, about16mA at100GHz. The microwave output power of photodiodes is higher than OdBm at100GHz. The research on dark current of photodiodes has been successfully done by combining Atlas and various physical models. It is found that the doping concentration and thickness of the absorption layer of photodiodes are two main factors determining the dark current. The physical process of the photodiodes saturation behavior under high input optical power has also been analyzed. A novel circuit model for simulating the frequency response (DC-110GHz) of photodiodes has been developed by employing HFSS, ADS and drift-diffusion model. The simulation results agree well with measurement ones. Under zero bias voltage, the3dB bandwidth of photodiodes is about40G, the DC saturation current is about6mA, and the responsivity is similar with that under2V bias voltage. Two series connection photodiodes have been designed. Due to halved capacity induced by two photodiodes in series connection and larger total area, the saturation current-bandwidth product of the proposed photodiodes structure has been improved.3. Antenna-integrated photodiodes chipsThe circuit model for simulating the frequency response (50G-1T) of photodiodes chips integrating log period antennas or quasi dipole antennas has been developed successfully. Log period antenna-integrated self-bias photodiodes chips delivered about0.3μ W at200GHz, and about8nW at700GHz, and log period antenna-integrated photodiodes chips delivered about3μ W at200GHz, about30nW at700GHz. The fabricated devices are suitable for the generation of the millimeter wave signal. The investigation and analysis of the novel coupling packaging structure of quasi dipole antenna-integrated photodiodes chips and WR10waveguides have been performed, drawing the conclusion that the notch in the frequency response is caused by the interference of substrate modes. The coupling packaging structure is not sensitive to the length of WR10waveguide, and the optimal depth of the chips inserting the WR10waveguide is about1000-1200μm. According to the antenna theory, a novel dual dipole end fire antenna has been proposed, converting the radiation of the antenna from vertical to horizontal direction, simplifying the difficulty of packaging, improving the coupling efficiency between chips and WR10or WR8waveguide, and expanding the operating frequency range.