Study On All-silicon High-order Electrooptic Modulators And Photodetectors

In future optical communication systems, hybrid integration of photonic and electronic components is one of the development trends. As key components for EO/OE convention in optical transmission networks, the performances of modulators and detectors will affect the optical communication systems. Benefiting from the merits of low power consumption, low cost, tiny size and compatibility with CMOS fabrication process, silicon based modulators and photodetectors are becoming the essential parts in the development of optical communication technologies. However, both index modulation and absorption effects are very weak in the intrinsic silicon when compared to other materials(such as In P, Ga As). Therefore, it will have high academic value and application prospects if the two effects can be improved. To solve the problems existing in the previous modulators and detectors, this dissertation mainly focuses on the development of high speed, low power consumption and small size all silicon modulators and detectors, and meanwhile takes into consideration of device fabrication feasibility, simplicity and stability. All of the devices discussed in this dissertation are based on silicon material(the fabrication process uses only silicon, silicon oxide and aluminum electrode material, making the process simpler and cost lower). These studies are the fundamental work for future high quality all silicon transceivers.All silicon electro-optic modulators are used to convert electrical signals into optical signals. The modulators studied in this dissertation are based on the Mach-Zehnder interferometer(MZI) structure. The design goal is to achieve small device size, low power consumption, high bandwidth, high modulation efficiency, and low bit error rate. Meanwhile, the study also covers the device package and integration with other silicon photonic devices. The design of the MZI modulator can be divided into the optical part, the electrical part, and the thermo-optic part. The optical part consists of modulation unit and power splitter. For the modulation unit, as the silicon material lacks inversion symmetry, there is no linear electrooptic effect. Therefore p-n junction is embedded in the silicon waveguide to utilize the free carrier plasma dispersion effect. By using the reverse-biased p-n junction, high speed and high efficiency modulation can be achieved. For the power splitter, the optical power on each of the MZI arms should be equal to maximize the modulation depth. Therefore, the multimode interferometer(MMI) structure is designed. By using the self-imaging effect, the insertion loss is lower, the device is smaller and the power splitting is more uniform. The electrical part consists of microwave electrode and terminal resistor. For the microwave electrode, as the MZI arm is long(~mm), phase match and mode field overlap between microwave and optical wave should be considered. Therefore, a single-drive push-pull traveling wave electrode(TWE) is designed to lower microwave signal reflection and to improve modulation bandwidth. For the embedded terminal resistor, its impedance should be matched to the characteristic impedance of the TWE to reduce signal reflection at the end of the electrode. Designed on a heavily doped silicon slab, the impedance of the on-chip resistor after fabrication can approach 50Ω upon optimizing device length under the same doping concentration, device width and height with other waveguide devices. For the phase shifter in the thermo-optic part, this dissertation mainly focuses on how to reduce the numbers of phase shifters in high order modulation and how to reduce the power consumption in one phase shifter. The high order modulation(Quadrature Phase Shift Keying, QPSK; Quadrature Amplitude Modulation, QAM) is defined as multi-level phase or amplitude modulation. Compared with the traditional double-level phase or amplitude modulation(Binary Phase Shift Keying, BPSK; On-Off Keying, OOK), the modulation speed is greatly improved. The phase shifter is a key component in the high order modulation. This dissertation studies the phase relationship among different phase shifters in the QPSK modulator. By carefully setting the phase difference, the numbers of phase shifters are reduced by one third when comparing to the commonly used QPSK modulators. On the other hand, in order to lower theπ shift power, deep air trenches are used to concentrate the heat in the waveguide, improving the thermal tuning efficiency. At last, this dissertation analyzes modulation signal quality by using different modulation formats such as OOK, BPSK and QPSK, and different driving signal speeds of 10Gb/s, 20Gb/s, 25Gb/s and 32Gb/s. This dissertation also tests the 64Gb/s QPSK modulation signal after 10 km standard single mode fiber transmission. Compared to the previous all silicon QPSK modulators, the proposed devices possess smaller size(<5mm2), lower power consumption(~7.1pJ/bit) and a simplified system for integration and package under the same modulation speed and bit error rate.The all silicon photodetector is used to convert the optical signal into electrical signal. The detection responsivity and bandwidth are the key parameters for all silicon photodetectors. In order to improve these two parameters, this dissertation mainly studies the silicon material property, the detection principles(Surface-State Absorption, SSA; Two Photon Absorption, TPA) and the junction types(interleaved p-n junctions, the n-p-n junction and lateral p-n junction). Because the silicon is an indirect bandgap material, and the bandgap is larger than the photon energy in the telecommunication wavelength, the photon absorption is very weak. However, due to the interruption of the periodic potential on the waveguide surface, midbandgap states are present and photocurrent can be generated by SSA process. On the other hand, in the silicon microdisk structure, the stored optical power is strong enough to induce nonlinear effect and photocurrent can be generated by TPA process. The responsivity of all silicon photodetectors is still low when compared to the commercial used photodetectors. Current gain process therefore needs to be introduced to increase the photocurrent. This dissertation designs and fabricates four kinds of all silicon photodetectors: waveguide photodetector embedded with interleaved p-n junctions; waveguide photodetector embedded with n-p-n junction; microdisk photodetector embedded with interleaved p-n junctions; microdisk photodetector embedded with lateral p-n junction. First, this dissertation discusses waveguide photodetector embedded with interleaved p-n junctions. The interleaved p-n junction is a kind of p-n junctions which is embedded vertically along the direction of light propagation in the waveguide. Under the same reverse bias, the electric field in the junction has a better coverage on the waveguide surface, which makes the photocurrent multiplication more effective. As a result, the measured and calculated normalized responsivity is higher than that of waveguide photodetector embedded with p-i-n or lateral p-n junction. In order to improve the performance of the SSA effect based silicon waveguide photodetector, the waveguide width is narrowed to have more optical power residing on the sidewall for stronger photon absorption. The doping concentration is heavier to introduce avalanche effect at a lower bias. The length of the device is shorter to achieve higher bandwidth. Meanwhile, the balance between the device performance and stability affected by the temperature under different biases are considered. Second, the phototransistor based on SSA effect, where the silicon waveguide is embedded with n-p-n junction, is studied. Compared with the former device based on avalanche process, this kind of device can get linear current gain under even lower biases without excess noise, as there is no ionization accumulation process. Third, this dissertation studies the microdisk photodetector embedded with interleaved p-n junctions based on TPA effect. Under the strong electric field and wide depletion region of the interleaved p-n junction with relatively low doping concentration, the free carrier density is significantly reduced. Therefore the free carrier absorption(FCA) effect is greatly suppressed, resulting in stronger photon absorption. Finally, also based on the TPA effect, the microdisk photodetector embedded with lateral p-n junction is discussed. As the positions of the lateral p-n junction are specially designed to make the maximum overlap between the optical field and electric field, the detection of whispering gallery modes(WGMs) can be much more efficient by resonance enhancement. Compared with previous all silicon photodetectors, the proposed devices have a higher responsivity and a higher GBP. Moreover, they also possess the merits of simple fabrication process, flexible design and easy integration.Finally, this dissertation summarizes the work and discusses the development trend of silicon photonics for next generation optical communications.