Research Of Optical Modulation And Control In Silicon-Based Nano Devices And Couplers

For the last40years, the silicon chip has been the mainstay of theelectronics industry and has a revolutionary impact on the world. Thenumber of transistors on a single silicon electric chip with a size offingernail doubles every18months according to Moore’s Law, withimproving performance. However, As the Internet-based communicationcontinues to grow, the demand for broad-bandwidth devices is becomingurgent, pushing the limitations of copper-based interconnect technologies.All-optical interconnect offers a technologically feasible alternative toovercome such intrinsic limitations. Therefore, integrated photonics hasbeen a hot research topic in both the academy and industry.Silicon photonics, which is based on silicon-on-insulator (SOI)technology, has been attracting increasing attention. Recentbreakthroughs in silicon-based optical modulators make silicon photonicsa promising platform for next-generation photonic integrated circuits(PICs), owing to their compact size, compatibility with existing CMOSmicroelectronic technology, and potentially low power consumption.My thesis focuses on the research of optical modulation and controlin silicon-based nano devices and couplers. We first propose an activedevice, phase modulator, employing a silicon-based low-loss hybridplasmonic waveguide. Then we develop a new method to generate16-QAM signal by using the proposed plasmonic phase modulator. Themonolithic16-QAM modulator can be integrated with other silicon-based devices. At the third part, we achieve an optical parametric amplifier withan ultra-broad bandwidth and a high peak gain, using a silicon-basedhigh-confinement plasmonic waveguide. And at last, we focus on thecoupling issue between two different types of waveguides-strip and slotwaveguides.1. Silicon-based electro-optic phase modulatorPhase modulators are primary and essential elements of high-densityphotonic integrated circuits. Through a phase modulator, an optical phaseshift keying (PSK) signal can be obtained by applying an electric signal.Optical amplitude shift keying (ASK) signals can also be achieved usingtwo or more phase modulators. In this thesis, a silicon-based phasemodulator is proposed based on a low-loss hybrid plasmonic waveguidewith nano-scale mode area. Combining strong optical confnement abilityof plasmonic waveguide with highly nonlinear characteristic of polymer,the proposed phase modulator has a low voltage-length product VπL ofonly0.07Vmm, a potential ultrahigh modulation bandwidth, and a lowpropagation loss of0.078dB/μm at1550-nm wavelength, making itsuitable for on-chip integration.2. The approaches to on-chip16-QAM modulatorFor16quadrature amplitude modulation (16-QAM), a symbolcarries4information bits, much larger than that for amplitude shiftkeying (ASK) and differential phase-shifting keying (DPSK). Therefore,with the same baud rate, the bit rate of the16-QAM signal will be fourtimes of the bit rate of ASK and DPSK, greatly increasing the data rate ofthe signals. In this thesis, a compact silicon-based16-QAM modulator isachieved with a simple structure using two proposed plasmonic phaseshifters. By taking advantage of the loss in the plasmonic phase shifter toform different amplitudes of signals without introducing any additionalattenuator, the proposed16-QAM modulator has a very small footprint of less than100μm, a moderate insertion loss of about5.17dB, and canachieve high-speed operation, allowing for high-density on-chipintegration and broadband long-haul optical transmissions.3. On-chip optical parametric amplifierOptical parametric amplifiers (OPAs) relying on four-wave mixing(FWM) have evinced tremendous interest over the years since they canpotentially provide broader gain bandwidth than doped-fiber opticalamplifiers (D A’s) or Raman amplifiers. In principle, they can beoperated at an arbitrary center wavelength. In this thesis, we propose anOPA based on a step hybrid plasmonic waveguide (SHP). By particularlyintroducing the polymer with a negative nonlinear refractive-indexcoefficient, the phase-matching condition is satisfied. Thanks to theultra-high nonlinearity of the SHP waveguide, the proposed OPA couldpossess an ultra-broad gain bandwidth of~1.12μm with a comparablesignal peak gain of19.3dB for a much shorter propagation length of50μm and a1-W pump. Our proposed parametric amplifier occupies anano-scale mode area (~1×103μm2), which can be highly integrated intosilicon-based nano-photonic platform.4. Coupling light to the slot waveguideSilicon based nano-scale waveguides are essential elements ofhigh-density PICs because of their strong optical confinement. Siliconbased slot waveguide, where a nano-scale low-index slot is embeddedbetween two high-index slabs, has attracted much attention recently dueto its unique capability in enhancing and confining the optical field in thenano-scale slot. However, direct excitation of the eigen-mode of the slotwaveguide through coupling light from an optical fiber or external sourceis inefficient owing to the large mismatch of k vectors and field profilesof the two modes. On the other hand, coupling light into a silicon stripwaveguide has been sucessfully achieved. Therefore, it is expected that the eigen-mode of the slot waveguide can be excited through couplinglight from strip waveguide. To this end, efficient and ultra-compactcoupling between these two types of waveguides is of great significance.Here, we propose an ultra-compact orthogonal coupler for broadbandcoupling from the strip waveguide to the slot waveguide with a couplinglength as short as475nm and a3-dB coupling bandwidth of as large as700nm.