A study from the University of Washington in Seattle aimed to mitigate undesired polarization for TE and TM modes. Polarization can cause signal fading and performance degradation in fiber optic communications devices. To reduce this effect, the researchers wanted to make the optical components polarization independent. To achieve that, they targeted two of the common characteristics that lead to polarization: geometric asymmetry and stress-induced birefringence. Design and fine adjustment of the waveguide refractive index profile can enable a circular that eliminated the geometrical birefringence.
While it is impossible to eliminate variations in polarity in optical waveguides, the study notes that splitting the TE and TM modes and removing one orthogonal mode in the waveguides are two additional ways to create polarity independence in optical devices. The waveguide itself can be designed to be polarization-selective, which can actively target which polarity mode to target.
Key Parameters and Requirements
The experiments employed three waveguide EO material systems. Each consisted of a layer of EO-curable polymer that was spun-coated onto a silicon substrate and UV cured. The composite material was then baked in a vacuum oven between 65-85°C to remove undesired crosslinked monomers that would weaken the adhesion bond. Two samples used UV16, with the other employing OG125.
The primary performance metric used to assess the systems is the extinction ratio. The combination of host polymers and EO chromophores produces various optimal electron beam doses due to through differing sensitivities to the electron beam and poling-induced birefringences. As a result, the highest extinction ratio for a given material system has a respective beam dosage. In addition to peak extinction ratio, refractive index and poling temperature can help to confirm the better-performing material system.
The peak TE/TM extinction ratio was 21 dB at a beam dose between 90-100 μC/cm2 for the first material system (sample A), which used OG125 EO polymer. Peak ratios for the second and third material systems (samples B and C, both of which employed UV16 polymer), were 18 dB at a beam dose of 1000 μC/cm2 and 21 dB at a beam dose of 200 μC/cm2, respectively. The study confirmed that UV16 could be used to produce microrings that enable the highest attainable levels of resonance extinction ratios.