Silicon-based Microring Resonator And The Appli- Cations

With the increasing demands for high-speed data processing as well as data transmission, the technology of optical interconnects has been developed very well because of the advantages like low power consumption, high bandwidth. Silicon photonics is one of the most popular technologies to enable optical interconnects since the silicon-on-insulator (SOI) platform has a ultra-high index-contrast and transparent window from near-infrared to mid-infrared. More importantly, the CMOS compatibility makes silicon photonics very attractive to be low-cost potentially. Among various functionality elements, a microring resonator is one of the most popular integrated photonic devices regarding the small footprint as well as the versatility and flexibility for many applications.It is well known that the spectrum of single microring resonator is typically a Lorentzian curve, for which the 3dB bandwidth is narrow and the rising/the falling edge is not sharp. For some applica-tions like optical filtering, it is desired to achieve a box-like spectral response, which can be realized by using cascaded multi-microrings. For the design of cascaded multi-microring, one should opti-mize the power coupling ratios between the adjacent mircroring and the ratios between the input/ output waveguide and the microring. For example, for the ultra-compact 5th order MRR optical filters, the power coupling coefficients κ2while κ itself defined as the field coupling coefficient for all the couplers can be designed to be 0.45,0.09,0.05,0.05,0.09, and 0.45, respectively. In or-der to achieve the sufficient power coupling coefficient of 0.45 between the access waveguide and the side-microring, one usually uses a multimode interference (MMI) coupler, or a long directional coupler. This causes a smaller free-spectral range (FSR) and introduces some excess loss. In order to solve these problems, this thesis introduces bent directional couplers for high-order MRR optical filters to have a box-like filtering response. In the present design, bent couplers are used for light coupling between the access waveguides and the side-microring. With bent couplers, the coupling ratio can be chosen flexibly by choosing the length of the coupling region appropriately even when the gap width is relatively large. Furthermore, there is no excess loss theoretically and the cavity length is the same as a regular microring. The measured spectral responses for optical filters with two-, three-, and five-rings have a 3dB bandwidth of 2.0 nm,2.6 nm and 2.38 nm, respectively. And the measured out-of-band rejection ratios are 25dB,30dB,36dB respectively.The microring resonator is applied popularly in the WDM system. The problem is that the width of the ring is not possible to be exactly the same as the design value due to some fluctuation introduced during the fabrication processes. Such a width fluctuation impact the effective index of the guided mode in the ring and thus the resonant wavelength drifted randomly. In a bad case, the resonant wavelength shift is might be very serious and the device does not work. In this paper, an improved method is introduced by using wide ring. When the waveguide is wider, the effective index of the waveguide becomes less sensitive to the width variation. The simulation shows that the dependence of (?)λ/(?)w decreases from 1.07 to 0.0263, when the width of the microring increases from 0.40 μm to 1.50 μm. On the other hand, the wide ring waveguide supports not only the fun-damental mode but also high order modes. Correspondingly, these are some unexpected resonant wavelengths for these higher-order modes. Fortunately, a bent coupler can suppress these high order modes through selecting suitable structure parameters. The simulation and the experimental result shows the random deviation fo the resonant wavelengths is reduced effectively.Since the thermal-optical coefficient of Si is as large as 1.8×10-4/℃, silicon photonic in-tegrated devices are temperature-sensitive. In order to solve this problem, TiO2 is introduced to be the upper-cladding of the SOI nanowire as a material with a negative temperature coefficient. Compared with polymer material which also has negative temperature coefficient, TiO2 has better stability. In this thesis, silicon micoring resonators with a TiO2 upper-cladding layer is designed and realized. The experiment result shows that the temperature-sensitivity is reduced from 70pm/℃ to 30pm/℃.