Polarization-independent optical racetrack resonators using rib waveguides on silicon-on-insulator

Abstract

In an effort to find low-cost alternatives for components currently used in dense wavelength division multiplexing, optical ring resonators fabricated on silicon on insulator are currently being investigated. Their performance can be further enhanced if they are polarization independent. Herein we use rib waveguides to control the polarization properties of the devices and hence produce polarization-independent racetrack ring resonators. Transverse electric and transverse magnetic resonant peaks are measured to within 2 pm of one another over four cycles of the free spectral range. The racetrack resonators also exhibit measured An optical ring resonator is a versatile device that can then be utilized to create many different optical devices, including filters, 1,2 switches, 3,4 and add-drop multiplexers 5 for use in dense wavelength division multiplexing systems. In high index media such as silicon on insulator, (SOI), most ring resonator devices realized thus far are made using strip, or photonic wire waveguides.6,7 This is to allow for the small bend radii necessary to minimize the loss associated with devices and hence to produce a large free spectral range (FSR). However, the use of strip waveguides makes it much more difficult to control the polarization properties of the resultant devices. In this work we take an alternative approach, designing optical racetrack resonators based upon rib waveguides, to allow us to optimize control of the polarization properties and therefore realize a polarizationindependent ring resonator. While a polarization-independent ring resonator has been demonstrated in the low index contrast material of silicon oxynitride, our devices are fabricated in single-crystal SOI. The design and fabrication is discussed first, followed by presentation and discussion of experimental results.In order to design a polarization-independent ring resonator, the first step was to investigate the polarization characteristics of the rib waveguide due to its geometry. The BEAMPROP 8 simulation package was used to determine the dimensions necessary to create such a device. The initial height and width were set to 1.35 and 1 µm, respectively. These dimensions were chosen in order to maintain as large a device as possible (within the constraints of the rib height) for ease of coupling light to the device. Using BEAMPROP, the effective indices of the fundamental TE and TM waveguide modes were monitored as the etch depth was varied. Figure 1(a) shows a plot of the effective index as a function of etch depth for both polarizations. The etch depth at which the two curves cross indicates that both TE and TM polarizations have the same effective index and therefore the waveguide is polarization independent in terms of propagation constant.Next it was necessary to design a polarizationindependent directional coupler. Figure 1(b) shows a typical result of modeling a directional coupler using BEAMPROP. For the purposes of the modeling, the left arm of the coupler was excited with an optical field.

Silicon photonic waveguides for different wavelength regions

“…These waveguide dimensions are chosen as we have previously reported optical filters based on such waveguides [e.g. 26,27]. The propagation loss of these waveguides were <1 dB/cm [28], and we have recently shown that 0.7 μm high rib waveguides can have the propagation loss as low as ~0.2 dB/cm [29].…”

A continuous-wave Raman silicon laser

“…This would reduce the saturation effect and thus increase the laser output power. The multilayer coating approach used to form the cavity mirrors could also be replaced with waveguide Bragg reflectors 20,21 , ring or microdisk resonator architectures [22][23][24][25] ; this could provide a platform for monolithic integration of silicon-based optoelectronics.…”

Tailoring the Response of Silicon Photonics Devices

“…It is also clear that rib waveguides offers better control for satisfying both single mode and polarisation independent propagation [6]. We have already shown that polarisation independent ring resonator filters based upon relatively small rib waveguides can be achieved [7]. However, the main limitation of a rib waveguide with cross sectional dimensions of the order of 1 µm, is loss in resonator bends when the radius of the curvature is of the order of 10 µm or less.…”