Inter-channel crosstalk in densely aligned multimode polymer parallel optical waveguides

We theoretically estimate the inter-channel crosstalk in densely aligned multimode polymer parallel optical waveguides using a beam propagation method, and compare the results of graded-index (GI)-core waveguides with those of conventional step-index (SI)-core counterpart. In particular, we simulate the crosstalk in bridged core waveguides. Here, the bridged core is sometimes observed in the waveguides fabricated using the imprinting method. The inter-channel crosstalk in SI-core waveguide increases from-25 dB to-4 dB with increasing the bridge thickness. Contrastingly, the worst crosstalk in a GI-core is as low as-15 dB despite the bridged structure as long as the bridge of the core is not included in the index distribution of the GI-core core, namely SI bridged core. In addition, the crosstalk in the GI-core decreases when the multiple cores aligned in parallel have a different structure (core size, refractive index, etc.), because the difference in the core structure makes changes in the distribution of propagation constants, resulting in decreasing the mode coupling efficiency between the two cores. Hence, the worst crosstalk in the GI-core waveguide with a slightly different core structure is as low as-19 dB despite the bridged structure. Thus, the imprinting method should be utilized for GI-core waveguides: the inter-channel crosstalk is un-problematic even if a residual layer remains.