Adiabatic mode evolution is an important concept in photonic devices and is utilized in several applications including spot size conversion, mode coupling between the two waveguides, and broadband power splitters/couplers. In such devices, the linear adiabatic tapers are not always the best choice in terms of the device footprint and hence the taper profile needs to be optimized to provide small footprint while ensuring adiabatic mode evolution. In many instances, the best taper profile can be quite complex, and a parametric design might require a lot of parameters to accurately define the taper profile. In such cases, the traditional optimization using parameter sweep can be challenging. Several techniques such as Fast Quasi Adiabatic (FAQUAD) dynamics and constant loss approach have been proposed in the literature for such optimization. We implement these techniques and conduct a brief comparison using two different integrated photonic components. We also briefly discuss the challenges associated with relying solely on the optimization algorithms (inverse design) without utilizing constraints associated with these techniques.
We use a coaxial fiber, which is a cylindrical coupled waveguide structure consisting of two concentric cores, the inner rod and an outer ring core as a first order dispersive media to achieve temporal Talbot effect for pulse repetition rate multiplication (PRRM) in high bit rate optical fiber communication. It is observed that for an input Gaussian pulse train with pulse width, 2τ0=1ps at a repetition rate of 40 Gbps (repetition period, T=25ps), an output repetition rate of 640 Gbps can be achieved without significant distortion at a length of 40.92 m.
Design of electro-optic ON-OFF switches based on well-known phase change material Ge2Sb2Te5 (GST) is presented. The electro-optic switch is achieved by implementing by co-directional coupling between a 220 nm thick silicon nanowire and a silicon waveguide topped with ITO-GST-ITO layers at the 1.55μm wavelength. By introducing the electric field via the ITO electrodes, the GST layer can be changed between the amorphous and crystalline states. As the modal loss in the crystalline state is much higher than the amorphous state, through a rigorous modal analysis of the coupled silicon nanowire and GST waveguide by using the finite element method, the optimal ITO spacing is obtained at 75nm which is less sensitive to device parameter variations and thus offering better tolerances. The GST thickness is also optimized for the phase matching point at 25 nm in order to efficiently transfer power from silicon nanowire to GST waveguide to attain the OFF state. Once the device is phase matched in crystalline state, the power in the amorphous state will pass with very little interaction with the GST waveguide resulting in an ON state. The Eigenmode Expansion Method of Fimmprop is used as a junction analysis approach to calculate the optical power coupling efficiencies to the output silicon nanowire. The extinction ratio of the electro-optic switch and insertion loss in ON state at phase matching can be obtained as a function of the device length. A compact 1.75 μm long device shows a high extinction ratio of 22 dB with an insertion loss of only 0.56 dB.
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