The Study Of Silicon Photonic-Wires Based Arrayed Waveguide Grating

Arrayed waveguide grating (AWG) has been a key component in dense wavelength division multiplexing systems. With the development of large scale photonic integration, the miniaturization and athermalization of AWGs are important issues. The silicon photonic wire (Si-wire) with high refractive index contrast has been attracted much attention in these years, since silicon has low loss in the optical communication window and it is compatible with the Complementary Metal-Oxide-Semiconductor (CMOS) technology. In this paper, the Si-wire based AWGs (Si-AWGs) are designed, simulated, and optimized. The temperature-insensitive AWGs (TI-AWGs) using the thermal stress plates or combination of multiple types of waveguides are also studied.The theory of optical waveguides is the design basis of AWGs. Both the planar waveguide and buried rectangular waveguide are studied. The effective index method (EIM), the finite-difference beam-propagation method (FDBPM), and the finite element method (FEM) are used to analyze the buried rectangular waveguide. It is shown that the coupling between different polarized field components is strong in the Si-wires, and the full-vector FEM should be used. The FDBPM and the finite-difference time-domain (FDTD) method are introduced to simulate the light propagation in the Si-wires. The decoupling between straight Si-wires, the bend loss, and the transmission loss between straight and bent waveguides are analyzed. The stress effects on the optical waveguides are studied. The FEM is used to estimate the stresses in the core layer, and the stress-induced birefringence is illustrated.The design, simulation, and optimization of Si-AWGs are the first goals of this paper. The horse-shoe shape is selected as the layout of AWGs. The detailed design theory is developed, and the general design flow is also shown. For the sake of simulation, the three-dimensional waveguide structure is simplified to two-dimensional (2D) planar waveguides by use of EIM. Then, the simulation method based on the 2D fresnel-kirchhoff diffraction formula (2D-FKDF) is developed, and the 2D-FDTD is also studied. It is shown that the results of these two methods are consistent. The structural parameters of Si-AWGs are optimized using the 2D-FKDF and the 2D-FDTD. For the design example, the loss of the peak wavelength in the central channel is about 1.5 dB, and the crosstalk is less than 20 dB. The low loss Si-AWG based on the combination of narrow and wide Si-wires are studied. Two design methods are given and verified by 2D-FDTD simulation.The study of the thermal-stress compensated TI-AWGs is the second goal of this paper. The effects of the stresses on the anisotropic crystalline silicon and the isotropic fused silica are studied at first, but only the homogenous and isotropic parameters are considered later. Based on the elastic multilayer thermal-stress theory and the stress concentration effect in the buried rectangular optical waveguide, the thermal stresses in the core layer of arrayed waveguide are obtained. The results are consistent with the 3D-FEM. The influence factors of stress birefringence are analyzed, and the tuning effects of stress plate is studied. It is shown that the parallel stress-components can be largely adjusted, but the effects on the perpendicular stress-components are week. A polarization-independent AWG can be obtained when a stress-plate with large positive thermal expansion coefficient (TEC) is attached on the bottom of AWG chips, or one with large negative TEC is attached on the top. The upper cladding is divided to two parts, i.e. the inter-cladding layer and the cover-cladding layer. It is shown that the inter-cladding layer plays the most important role in determining thermal stresses of the core layer, and the influences of the cover-cladding layer can be neglected. The effective refractive index and its temperature coefficient of the anisotropic planar waveguide are derived, and then the EIM is used to estimate the temperature coefficient of the effective refractive index in the rectangular core layer of arrayed waveguide. The results are consistent with 3D-FEM. The effects of stress plates on the temperature coefficient of central wavelength (TCCW) are completely studied. It is shown that the TCCW can be effectively adjusted by the structural parameters and the thickness of plates. The TCCW can be significantly reduced when a plate with large positive TEC is attached on the bottom of AWG chips, or one with large negative TEC is attached on the top. The effects of brass thermal-stress plates are demonstrated by the experimental results of SiO2-AWGs. The study of TI-AWGs based on the combination of multiple types of waveguides is the third goal of this paper. All the waveguides are with the thickness of 260 nm. The mode fields of 2D and 3D slot waveguides, the connection loss between Si-wire and slot waveguide, the decoupling between straight waveguides, and the thermal performance of Si-wires and slot waveguides with different structural parameters are studied. Then the Si-wire with the width of 400 nm is selected as the first type waveguide, and the thermal performance of the combination of two types of waveguides are studied, while the second type waveguide is a Si-wire with the width of 250 nm or a slot waveguide with the width of Si-rib is 150 nm and the width of slot is 100 nm. The Mach-Zehnder Interferometer based optical filter (MZI-OF) and the AWG are designed according to the design theory of combination of two types of waveguides. The tuning effects of the different path-length of the two types of waveguides on the thermal sensitivity, the dispersion performance, and the device size are analyzed. The 2D-FDTD simulation is used for verification. It is shown that a TI-MZI-OF or TI-AWG that designed according to the TI point of central wavelength will be over-compensated and under-compensated for the long-wavelength and short-wavelength, respectively. Moreover, the long-wavelength regime has small dispersion enhancement factor than the short-wavelength regime. It is also shown that the type II waveguide should be 250 nm-wide Si-wire for the sake of compact device size, or should be slot waveguide with the width of Si-rib is 150 nm and the width of slot is 100 nm for the sake of small dispersion effect and stable thermal performance. The complete design theory is also extended to the combined use of arbitrary types of waveguides, which is useful for the design of a TI-AWG that satisfied multiple requirements.