Although optical sensors incorporating grating inscribed and etched fibres are now sufficiently mature and well established in the market, however, designs based on more exotic nanowires and photonic crystal fibres are becoming increasingly important and showing much improved sensitivity by accessing a larger evanescent field. Similarly, novel planar design concepts, such as the silicon slot guide-based design is showing even greater promise, allowing the exploitation of well-developed CMOS fabrication technologies for potentially low-cost sensor elements. In compact Integrated Optic format, dielectric slots, plasmonic slots, Mach-Zehnder interferometer, and ring resonators are also emerging as novel photonic sensors. However, high index contrast also makes the modes in such sensing structures fully hybrid in nature and in such a case, full-vectorial rigorous numerical approaches will be necessary for their design optimization. Some selected results for silicon based compact photonic sensors will be presented illustrating the value and potential of the computationally efficient finite element method in such designs.
A full-vectorial numerically efficient Finite Element Method (FEM) based computer code is developed to study complex light-sound interactions in a single mode fiber (SMF). The SBS gain or SBS threshold in a fiber is highly related to the overlap between the optical and acoustic modes. For a typical SMF the acoustic-optic overlap strongly depends on the optical and acoustic mode profiles and it is observed that the acoustic mode is more confined in the core than the optical mode and reported overlap is around 94 % between these fundamental optical and acoustic modes. However, it is shown here that selective co-doping of Aluminum and Germanium in core reduces the acoustic index while keeping the optical index of the same value and thus results in increased acoustic- optic overlap of 99.7%. On the other hand, a design of acoustic anti-guide fiber for high-power transmission systems is also proposed, where the overlap between acoustic and optical modes is reduced. Here, we show that by keeping the optical properties same as a standard SMF and introducing a Boron doped 2nd layer in the cladding, a very low value of 2.7% overlap is achieved. Boron doping in cladding 2nd layer results in a high acoustic index and acoustic modes shifts in the cladding from the core, allowing much high power delivery through this SMF.
Following the Industrial advancements in the last few decades, highly flammable chemicals, such as ethanol (CH3CH2OH) and methanol (CH3OH) are widely being used in daily life. Ethanol have some degrees of carcinogenic effects in human whereas acute and chronic exposer of methanol results blurred vision and nausea. Therefore, accurate and efficient sensing of these two vapors in industrial environment are of high priorities. We have designed a novel, ultra-compact chemical vapor sensor based on composite plasmonic horizontal slot waveguide (CPHSW) where a low-index porous-ZnO (P-ZnO) layer is sandwiched in between top silver metal and lower silicon layers. Different P-ZnO templates, such as nano-spheres, nano-sheets and nanoplates could be used for high-selectivity of ethanol and methanol at different temperatures. The Lorentz-Lorenz model is used to determine the variation of P-ZnO refractive index (RI) with porosity and equivalent RI of P-ZnO layer for capillary condensation of different percentage of absorbed vapor. An in-house, new divergence modified finite element method is used to calculate effective index and attenuation sensitivity. Plasmonic modal analyses of dominant quasi-TM mode shows a high 42% power confinement in the slot. Next, an ultra-compact MZI incorporating a few micrometres long CPHSW is designed and analysed as a transducer device for accurate detection of effective index change. The device performance has been studied for different percentage of ethanol into P-ZnO with different porosity and a maximum phase sensitivity of >0.35 a.u. is achieved for both the chemical vapors at a mid-IR operating wavelength of 1550 nm.
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.
A mathematical model to study the phase difference introduced between the two orthogonal components of light wave
due to electro-optic effect in a trapezoidal shaped lithium niobate single crystal has been made. Here, the electrodes are
placed on the two opposite non-parallel faces such that the field is nearly perpendicular to the light beam direction. It is
found that for the said geometry of the device structure, the maximum phase difference is a function of oblique angle
between the two non-parallel faces, which has a critical value. Using random optimization technique based on genetic
algorithm, the device parameters can be optimized for any required phase change between the two components of light
wave. The phase difference between the two orthogonal components of light can be made tunable by using a multi-strip
electrode structure. The advantage of this technique is that tunability can be obtained with a constant voltage source.