Polarization-insensitive MMI-coupled ring resonators in silicon-on-insulator using cladding stress engineering

Abstract

We review the design considerations and experimental results of a novel design of polarization insensitive ring resonators in silicon-on-insulator (SOI) ridge waveguides. The polarization insensitive coupling is achieved using a multi-mode-interference (MMI) coupler. Cladding stress-induced birefringence is used to correct the round trip phase difference between the TE and TM polarizations. Experimental demonstration is presented for such ring resonators fabricated on SOI with 1.5 µm Si waveguide core layer. For resonators with radius R = 200 µm, polarization insensitive operation is achieved in both the resonance wavelength and linewidth over a ~ 4 nm wavelength range using a 7.5 µm × 84 µm MMI coupler and 0.8 µm thick oxide cladding with -250 MPa stress, with the resonance wavelength shifts between TE and TM polarizations less than 3 pm. The quality factor Q of ~ 15,000 and free-spectral range (FSR) of 0.46 nm is measured. For resonators with a smaller radius of 50 µm, similar FRS of 1.3 nm and extinction of 13 dB are observed for TE and TM, although the resonance wavelengths are shifted, in agreement with the theoretical prediction. By choosing the proper combination of the cladding stress and thickness, zero-birefringence condition can be achieved for resonators of different cavity lengths.

Stress Induced Effects for Advanced Polarization Control in Silicon Photonics Components

“…First, we present polarization insensitive AWG demultiplexers, where the birefringence in the waveguide array is eliminated by the proper selection of the oxide cladding stress and thickness [13,17]. Then we introduce polarization insensitive ring resonators using multimode interference (MMI) couplers to achieve polarization insensitive coupling between the bus waveguide and the ring cavity [18,19]. Stress-induced birefringence is used to compensate the round-trip phase difference between the two polarizations.…”

“…We also applied the stress engineering technique to achieve polarization-insensitive ring resonators [18,19]. Ring res-onators have been used as the building blocks in applications including add-drop, switching, modulation, and sensing [24][25][26][27].…”

“…Another source is the radiation loss in waveguide bends, as well as possible losses at discontinuities such as the junctions between the straight and curved waveguides. These factors need to be considered in the device design, and detailed discussions can be found in [19].…”

“…In this paper, we review the characteristics of modal birefringence in SOI waveguides, with regard to the two main sources of polarization anisotropy, namely, the geometrical and the stress-induced birefringence [13,16]. Design methodologies and experimental demonstrations for using the stress-induced birefringence to achieve polarization-insensitive arrayed waveguide gratings (AWGs) [13,17], polarization insensitive ring resonators [18,19], and zero-order AWG-based broadband polarization splitters [20] are presented.…”

Mirror cavity MMI-coupled photonic wire resonator in SOI

“…Ring resonators have been realized in silicon waveguides [4], including racetrack designs [5], and polarization insensitive structures [6]. The use of an MMI as the coupling device in a ring resonator on SOI was reported in [7] and a polarization insensitive MMI resonator in [8,9]. MMI resonators have the added advantage of not requiring extended racetrack configurations or curved input/output channels.…”