Winnie N. Ye

Birefringence control using stress engineering in silicon-on-insulator (SOI) waveguides

We demonstrate that stress engineering is an effective tool to modify or eliminate polarization dispersion in silicon-on-insulator (SOI) waveguide devices, for a wide range of waveguide cross-section shapes and dimensions. The stress-induced effects on the modal birefringence of SOI waveguides are investigated numerically and experimentally. Finite-element simulations show that while the birefringence of ridge waveguides with both slanted and vertical sidewalls can be effectively modified using cladding stress, the birefringence becomes much less sensitive to dimension fluctuations with decreasing sidewall slope. To efficiently simulate the stress-induced effects we propose a normalized plane-strain model which can achieve comparable accuracy as a fully generalized plane-strain model but requires significantly less computational resources. Excellent agreement is achieved between the calculated and measured birefringence tuning using SiO/sub 2/ cladding induced stress. Finally, both calculations and experiments confirm that cladding induced stress can be used to eliminate the birefringence in SOI waveguides of arbitrary shapes, for typical SiO/sub 2/ film stress values (/spl sigma//sub film//spl ap/-100 to -300 MPa) and cladding thicknesses of the order of 1 /spl mu/m or less.

Athermal operation of Silicon waveguides: spectral, second order and footprint dependencies

We report the design criteria and performance of Si ring resonators for passive athermal applications in wavelength division multiplexing (WDM). The waveguide design rules address i) positive-negative thermo-optic (TO) composite structures, ii) resonant wavelength dependent geometry to achieve constant confinement factor (Gamma), and iii) observation of small residual second order effects. We develop exact design requirements for a temperature dependent resonant wavelength shift (TDWS) of 0 pm/K and present prototype TDWS performance of 0.5 pm/K. We evaluate the materials selection tradeoffs between high-index contrast (HIC) and low-index contrast (LIC) systems and show, remarkably, that FSR and footprint become comparable under the constraint of athermal design.

Eliminating the birefringence in silicon-on-insulator ridge waveguides by use of cladding stress

We propose and demonstrate the use of the cladding stress-induced photoelastic effect to eliminate modal birefringence in silicon-on-insulator (SOI) ridge waveguides. Birefringence-free operation was achieved for waveguides with otherwise large birefringence by use of properly chosen thickness and stress of the upper cladding layer. With the stress levels typically found in cladding materials such as SiO2, the birefringence modification range can be as large as 10(-3). In arrayed waveguide grating demultiplexers that were fabricated in a SOI platform, we demonstrated the reduction of the birefringence from 1.2 x 10(-3) (without the upper cladding) to 4.5 x 10(-5) when a 0.8-microm oxide upper cladding with a stress of -320 MPa (compressive) was used. Because the index changes induced by the stress are orders of magnitude smaller than the waveguide core-cladding index contrast, the associated mode mismatch loss is negligible.

Athermal High-Index-Contrast Waveguide Design

We present generalized design rules for athermal performance and materials compatibility in high-index-contrast (HIC) waveguides. Thermal stability of integrated photonic devices is one critical limitation in the development of commercially viable integrated optoelectronic circuits. Thermooptically neutral designs are achieved by choosing a cladding material whose thermooptic coefficient is opposite to that of the waveguide core. We derive analytical expressions of athermal conditions for both symmetric and asymmetric channel waveguide structures. The equations apply to general HIC systems such as silicon-on-insulator and SiN-based structures.

Two-mode division multiplexing in a silicon-on-insulator ring resonator.

Mode-division multiplexing (MDM) is an emerging multiple-input multiple-output method, utilizing multimode waveguides to increase channel numbers. In the past, silicon-on-insulator (SOI) devices have been primarily focused on single-mode waveguides. We present the design and fabrication of a two-mode SOI ring resonator for MDM systems. By optimizing the device parameters, we have ensured that each mode is treated equally within the ring. Using adiabatic Bezier curves in the ring bends, our ring demonstrated a signal-to-crosstalk ratio above 18 dB for both modes at the through and drop ports. We conclude that the ring resonator has the potential for filtering and switching for MDM systems on SOI.

Passive broadband silicon-on-insulator polarization splitter

We present the implementation of a novel wavelength independent polarization splitter on a silicon-on-insulator platform. The waveguide splitter is based on a zero-order arrayed waveguide grating (AWG) configuration. The splitting function is realized by employing cladding stress-induced birefringence. The device demonstrated a TE to TM splitting ratio better than -15 dB over a 20 nm tuning range centered around lambda=1550 nm and better than -10 dB over our entire accessible wavelength range from lambda=1465 nm to 1580 nm. The highest splitting extinction ratio achieved was -20 dB. To our knowledge, this is the first reported passive broadband polarization splitter based on AWG.

Fabrication tolerant and broadband polarization splitter and rotator based on a taper-etched directional coupler

We propose a fabrication tolerant polarization splitter and rotator (PSR) on the silicon-on-insulator platform based on the mode-coupling mechanism. The PSR consists of a silicon wire waveguide coupled to a taper-etched waveguide. Compared to previously reported PSRs based on directional couplers which are sensitive to fabrication variations, the partially etched taper structure can compensate for fabrication inaccuracies. In addition, the taper-etched geometry breaks both the horizontal and vertical symmetries of the waveguide, introducing an additional degree of design freedom to accommodate different upper cladding layers. The proposed PSR can be readily integrated in a planar waveguide circuit using e.g. SiO(2) cladding, making it compatible with typical metal back-end-of-line processes. Our simulation results show that the PSR has a low TM-to-TE polarization conversion loss of -0.09 dB in the C-band (or a conversion efficiency of 98%). A low TE-to-TE through insertion loss (-0.07 dB) and a very low polarization crosstalk (-30 dB) over a wide wavelength range exceeding 160 nm with a large fabrication tolerance (>50 nm) are numerically demonstrated.

Microphotonic Elements for Integration on the Silicon-on-Insulator Waveguide Platform

This paper presents an overview of our recent work on several fundamental optical elements and their integration on the silicon-on-insulator (SOI) waveguide platform. Theory, design and experimental results are presented for monolithically integrated asymmetric graded-index waveguide couplers, as well as output couplers based on total internal reflection mirrors. Design strategies for dispersive elements on SOI, for example, ring resonators and arrayed waveguide gratings, are discussed with special emphasis on methods to eliminate the polarization sensitivity. Finally, the properties and applications of evanescent fields in SOI waveguides are reviewed

Fabrication techniques of high aspect ratio vertical lightpipes using a dielectric photomask

We report the development of new fabrication techniques for creating high aspect ratio optical lightpipes in SiO2 layers of 10μm thickness and above. A dielectric photo mask was used for deep reactive ion etching. Our experiments show that CF4-based reaction gases were best for deep etching with high selectivity and etch rate. Trenches with diameters or width of 1.5μm were demonstrated, with an aspect ratio of 7.2:1 and a sidewall angle of 87.4 degrees. We also present the lift-off process of the etch masks and the via-filling procedures for the lightpipes. These structures are useful for image sensors, vertical interconnect and waveguiding applications.

Temperature-independent silicon subwavelength grating waveguides.

We demonstrate, by experiment and numerical calculations, temperature-independent subwavelength grating waveguides with a periodic composite core composed of alternating regions of silicon and SU-8 polymer. The polymer has a negative thermo-optic (TO) material coefficient that cancels the large positive TO effect of the silicon. Measurements and Bloch mode calculations were carried out over a range of silicon-polymer duty ratios. The lowest measured TO coefficient at a wavelength of 1550 nm is 1.8×10(-6) K(-1); 2 orders of magnitude smaller than a conventional silicon photonic wire waveguide. Calculations predict the possibility of complete cancellation of the silicon waveguide temperature dependence.