Sarvagya Dwivedi

Ge-on-Si and Ge-on-SOI thermo-optic phase shifters for the mid-infrared

Germanium-on-silicon thermo-optic phase shifters are demonstrated in the 5 μm wavelength range. Basic phase shifters require 700 mW of power for a 2π phase shift. The required power is brought down to 80 mW by complete undercut using focused ion beam. Finally an efficient thermo-optic phase shifter is demonstrated on the germanium on SOI platform. A tuning power (for a 2π phase shift) of 105 mW is achieved for a Ge-on-SOI structure which is lowered to 16 mW for a free standing phase shifter.

A Compact All-Silicon Temperature Insensitive Filter for WDM and Bio-Sensing Applications

We propose a compact, temperature-insensitive, and all-silicon Mach-Zehnder interferometer filter that uses the polarization-rotating asymmetrical directional couplers. Temperature sensitivity of the filter is for a wavelength range of 30 nm. The device achieves a reduced footprint by making use of different polarizations, which is made possible by the asymmetric directional couplers that act both as a splitter/combiner and as a polarization rotator. Simulation of the device shows that it can also be useful for gas sensing and bio-sensing applications with three times larger response to cladding changes while keeping a thermally robust behavior.

Compact Thermally Tunable Silicon Racetrack Modulators Based on an Asymmetric Waveguide

A compact wavelength-tunable 10-Gb/s silicon racetrack modulator with integrated thermo-optic heater is demonstrated by using a waveguide with an asymmetric cross section, combining the compact footprint of microdisk modulators with the design simplicity of regular racetrack or ring modulators. The outer perimeter of the asymmetric racetrack modulator is fully etched to maximize optical confinement, and the inner waveguide edge is shallowly etched to maintain an electrically conductive path to the embedded p-n diode and to control the propagation of the asymmetric optical mode and its coupling to the bus waveguide. The resistive heating elements based on highly doped Si strips are implemented at the outer edge of the modulator for thermo-optic control. The asymmetric modulators can be fabricated along with Si wire waveguides and shallowly etched fiber-grating couplers using a simple process flow involving just two Si-patterning steps. Devices with a bending radius of 10 μm and a novel “T”-shaped p-n diode layout have been fabricated, and exhibit electro-optic modulation and heater efficiencies of 28 pm/V and 42 pm/mW, respectively. At 10 Gb/s, a stable extinction ratio of 10 dB is demonstrated from a 2Vpp drive swing, which can be maintained over a wavelength range of 4.6 nm by thermally tuning the modulator. This is equivalent with a temperature variation of about 62°C.

Maximizing Fabrication and Thermal Tolerances of All-Silicon FIR Wavelength Filters

We propose a method to make silicon optical finite impulse response filters tolerant to fabrication (waveguide geometry) and ambient thermal variations. We experimentally demonstrate a Mach-Zehnder interferometer filter with fabrication and thermal tolerance, both separately and together. The fabrication-tolerant device measurements show a 20-fold improved tolerance to systematic waveguide linewidth variations with a wavelength shift of <;60 pm/nm linewidth change. The fabrication- and thermal-tolerant device is possible using orthogonal polarizations in the two arms. The fabricated device shows a shift of less than ±65 pm/nm and a thermal drift smaller than ±15 pm/K over a wavelength range of 40 nm. Simulations show that this concept can be extended to multichannel filters.

Experimental Extraction of Effective Refractive Index and Thermo-Optic Coefficients of Silicon-on-Insulator Waveguides Using Interferometers

We propose and demonstrate an accurate method of measuring the effective refractive index and thermo-optic coefficient of silicon-on-insulator waveguides in the entire C-band using three Mach-Zehnder interferometers. The method allows for accurate extraction of the wavelength dispersion and takes into account fabrication variability. Wafer scale measurements are performed and the effective refractive index variations are presented for three different waveguide widths: 450, 600, and 800 nm, for the TE polarization. The presented method is generic and can be applied to other waveguide geometries and material systems and for different wavelengths and polarizations.

Silicon photonics non-resonant wavelength filters: comparison between AWGs, echelle gratings, and cascaded Mach-Zehnder filters

We present a comparison of different silicon photonics-based wavelength filters for different design criteria (e.g. channel spacing, number of channels, ...) and different performance metrics (e.g. insertion loss or crosstalk ). In this paper we compare only non-resonant filters, or finite-impulse response (FIR) filters, such as Arrayed Waveguide Gratings, Echelle Gratings and higher-order cascades of Mach-Zehnder filters. We derive the strengths and weaknesses from their operational principles and confirm those with experimental data from fabricated devices and extrapolated simulations.

Measurements of Effective Refractive Index of SOI Waveguides using Interferometers

We demonstrate an accurate method of measuring the effective refractive index of SOI waveguides in the C-band using three Mach-Zehnder Interferometers. Over wafer the average extraction error of effective index and group index is 0.003 and 0.004.

Compact athermal filter in silicon waveguides for WDM and bio-sensing applications

We present a compact MZI on silicon platform with temperature sensitivity less than 0.2pm/K and total length around 100μm. It makes the device a promising candidate for wide range of WDM and bio-sensing applications.