Study On Silicon-based Optical Tunable Delay Line Chips With A Wide Tuning Ragnge

Tunable delay lines have been widely used in optical communication systems and microwave photonic signal processing. Integrated optical tunable delay line, as one of the basic units in optical on-chip system, is a key development direction. Among various photonic integration technologies, silicon photonics has become a hot research topic due to its compatiblity with complementary-metal-oxide-semiconductor(CMOS) technologies. Hence, silicon integrated optical delay line chips with a wide tuning range have important academic value and extensive applications in optoelectronics. It has become a research hotspot in recent years.For the increasing demand on a large tuning range, high tuning precision, low optical loss, and low electrical power consumption, we propose two novel optical tunable delay lines based on microring resonators and characterize their performances. In order to break the delay-bandwidth product limitation in optical delay lines based on slow light effect, we present and demonstrate a silicon reconfigurable optical delay line. In this desertation, we will discuss in detail the design methods of these structures along with their novelties.For microring resonators, we analyze the key factors, i.e., mode properties, propagation loss, and coupling of silicon waveguides. The transmission spectrum and tuning principle of group velocity based on slow light effect in microrings are elaborated. Based on these analyses, we present a new microring structure with a pulley coupler in which the coupling dispersion characteristics are theoretical analyzed and verified by experiments. Compared with traditional microring resonators, the coupling transmission coefficient of the new structure is tunable. Furthermore, near the critical coupling wavelength, we can set the microring at either the fast light or the slow light regime via thermo-optic effect to get a wide tuning range of delay. These new features add flexibility to microring design and expand the applications of microring delay lines.The delay-bandwidth product of a single microring resonator is fixed, which can be increased by cascading microrings. We analyzed the delay characteristics of cascading microrings. The principle of achieving a tunable delay at the operation wavelength by changing the phase detuning of multiple microrings is investigated. In order to reduce the signal distortion and intersymbol interference upon tuning, we adopt a balanced tuning method. Then, we present a novel reflective-type delay line consisting of cascaded microrings to improve the delay-bandwidth. This scheme makes incoming light pass microrings twice via sagnac loop reflector and doubles the delay-bandwidth product. As a result, the device footprint and power consumption on tuning is reduced by half. In order to achieve active tuning of device based on thermal effect, we design a novel p-i-p resisitive heater on the basis of preceding work of our group. Its thermal tuning efficiency and optical loss induced by free carrier absorption is studied by numerical simulation. According to the analysis results and fabrication process, we design the chip layout and fabricate the device. The electrical package is achieved by wire bonding and a test platform for reflective-type integrated device is built. To evaluate the tuning performance, we compare the group delay spectra in different delay configurations. Meanwhile, the power consumption is measured under different delays. In order to characterize device in practical optical communication systems, transmission measurement is carried out using a 20 Gbps 251-1 PRBS signal to confirm the signal fidelity after 10 and 110 ps delays. The tuning stability results show that the device can work stably for a long time.To avoid the limitation of bandwidth in microring delay lines, we demonstrate a novel silicon N-bit reconfigurable optical delay line which consists of several 2×2 optical switches and waveguide pairs with different lengthes. Variable optical time delays are obtained upon selection of the signal transmission path. To obtain a wide tuning range, the length of delay waveguide needs to be more than 100 millimeter. Hence, the waveguide loss become one of the main limiting factors for insertion loss. We analyze the optical modes of delay waveguides to reduce the scattering loss induced by fabrication. Mach-Zehnder interferometer switches with large bandwidth and high stability are used. As the bandwidth and delay performance of Nbit reconfigurable optical delay lines is limited by the switch crosstalk, we design the optical structure of switch and investigate the switch crosstalk deteriorated by fabrication error. To actively tune the switch, we embed a pi-n phase shifter which is faster and has lower crosstalk than thermal tuning. We study the p-i-n phase shifter which is based on free carrier dispersion effect. We analyze the switch crosstalk due to loss imbalance between the two arms of switch induced by free carrier absorption. To solve the signal distortion caused by these problems, p-i-n diodes, which are used as variable optical attenuators(VOAs) based on free carrier absorption effect, are inserted in the waveguides and selectively turned on according to signal transmission path. We design the chip layout and experimentally characterize the N-bit reconfigurable optical delay line. After wire-bonding the delay line chip, we measure the transmission spectrum on an optical test platform with optical fiber array coupling. The results indicate that the delay performance is improved by VOAs. 7-bit reconfigurable optical delay is successfully achieved, with only 4 VOAs used to reduce the interference induced power fluctuation in spectrum from more than 10 d B to 4 d B. The power consumption of each VOA is 320 m W with 10 d B attenuation. Optical signal delay is measured by transmitting non-return-to zero 27-1 PRBS signal through the device under different bit rates. We examine the quality of the delayed optical signal by measuring the eye diagrams of a 25 Gbps PRBS signal and the unwrapped phase using an RF vector network analyzer.At last, we summarize the work and innovations. The research and technology foresight for the silicon optical delay line with wide range is pointed out.