Low-loss design and fabrication of multimode polymer optical waveguide circuit with crossings for high-density optical PCB

In this paper, we both theoretically and experimentally demonstrate that 90 % lower optical loss is realized with multimode crossed polymer optical waveguides by introducing graded-index (GI) cores, compared to conventional step-index (SI) core counterparts. First, the light leakage losses of crossed waveguides with SI and GI cores are simulated using a ray-trace simulation. Then, we show that a drastic reduction of the leakage loss is achieved in the GI-core crossed waveguide (only a loss of 0.095 dB for 50 perpendicular crossings), which is less than one-tenth of the leakage loss in the SI-core crossed waveguide (2.5-dB loss for the same 50 perpendicular crossings). Furthermore, even in the GI-core crossed waveguide with a 30-degree crossing angle, the leakage-loss is calculated to be only 0.009 dB/crossing, while 0.086 dB/crossing for the SI-core. This low loss of GI crossed core waveguides stems from the tight optical-field confinement in GI cores. Next, we confirm the validity of the calculated results. We fabricate the GI-core polymer crossed waveguides (multiple crossings) using the photo-address method for the first time to the best of our knowledge. In this method, we use a polynorbornene resin whose refractive index could be controlled by intensity variations of UV exposure during the curing process. The experimental results in the crossed waveguide with GI profile show a very good agreement with the calculated result.