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

Abstract

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.

Silicon-based on-chip multiplexing technologies and devices for Peta-bit optical interconnects

“…For fiber-related applications, polarization-insensitive PICs are usually needed. Theoretically speaking, it is possible to make SOI nanowires non-birefringent by optimizing the waveguide dimensions, like those demonstrated for large SOI ridge waveguides [90][91][92][93][94]. However, it is very hard to achieve non-birefringent SOI nanowires experimentally because of the stringent fabrication tolerance.…”

The influence of light propagation direction on the stress-induced polarization dependence of silicon waveguides

“…A similar method as the previous section is used to obtain the components of photoelastic relation as a function of light propagation direction. Under the same coordinate system as Section II, the photoelastic relation can be expressed as (2), shown at the bottom of the page, where is the tensor of the stress-induced refractive index change. is the photoelastic tensor, which is determined by the light propagation direction ( -axis).…”

“…The major issue of the silicon waveguide is the large polarization dependence. One approach to tune the polarization dependence of the silicon ridge waveguide is to utilize the stresses caused by the upper cladding [2], [3]. Due to the photoelastic effect, the stress can change the refractive index, thus influencing the optical performance [4].…”

Design of polarization-insensitive SOI ring resonators using cladding stress-induced birefringence and MMI couplers

“…As we have reported previously [6,7], the waveguide birefringence (∆n eff = n eff TM -n eff TE ) is a combination of the geometrical and cladding stress-induced birefringence. For the waveguide dimensions given above, both the ring and the MMI waveguides have a large geometrical birefringence of ∆n eff = -2.65×10 -3 and -5.56×10 -3 respectively, as indicated by the circles in Fig.…”

“…Although only specific coupling ratios are possible using 2×2 MMI couplers, the coupling ratio and overall coupler loss are nearly polarization independent (even though the MMI waveguide itself may have significant birefringence), and the tolerances on coupler dimensions are well within the limits of standard photolithography. We have recently reported on the use of SiO 2 cladding stress induced birefringence to control or eliminate birefringence in SOI waveguides, and experimentally demonstrated polarization insensitive arrayed waveguide grating (AWG) devices using this method [6,7]. We demonstrate here that combining stress birefringence control with MMI couplers can result in a ring resonator design that has very modest fabrication requirements, yet has useful FSR (0.5 nm for the waveguide cross-sections used in this example) and resonance line shapes.…”