We successfully fabricate a polymer optical waveguide with multiple graded-index (GI) cores directly on a substrate utilizing the soft-lithography method. A UV-curable polymer (TPIR-202) supplied from Tokyo Ohka Kogyo Co., Ltd. is used, and the GI cores are formed during the curing process of the core region, which is similar to the preform process we previously reported. We experimentally confirm that near parabolic refractive index profiles are formed in the parallel cores (more than 50 channels) with 40 microm x 40 microm size at 250-microm pitch. Although the loss is still as high as 0.1 approximately 0.3 dB/cm at 850 nm, which is mainly due to scattering loss inherent to the polymer matrix, the scattering loss attributed to the waveguide's structural irregularity could be sufficiently reduced by a graded refractive index profile. For comparison, we fabricate step-index (SI)-core waveguides with the same materials by means of the same process. Then, we evaluate the inter-channel crosstalk in the SI- and GI-core waveguides under almost the same conditions. It is noteworthy that remarkable crosstalk reduction (5 dB and beyond) is confirmed in the GI-core waveguides, since the propagating modes in GI-cores are tightly confined near the core center and less optical power is found near the core cladding boundary. This significant improvement in the inter-channel crosstalk allows the GI-core waveguides to be utilized for extra high-density on-board optical interconnections.

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We present two methods for fabricating densely-aligned graded-index (GI) multiple-core polymer optical waveguides utilizing soft-lithography and dispensers. For on-board optical interconnects with polymer waveguides, much higher data rate and channel density have been required in the past few years. However, because of the high scattering loss of polymer waveguides, their inter-channel crosstalk is of great concern, when narrower pitch is required. Meanwhile, we have demonstrated low propagation loss and low inter-channel crosstalk of polymer optical waveguides with GI cores due to the optical confinement effect of the GI-core. Hence, the channel density could be increased for GI multiple-core waveguides. In this paper, we successfully fabricate a polymer waveguide with GI cores directly on a substrate utilizing the soft-lithography method or a dispenser. We experimentally confirm that near parabolic refractive index profiles are formed in the parallel cores with 50 μm x 30 μm size at 125-μm pitch in a length of 20 cm. We also use a copolymer for forming the GI profile, and then, confirm the high-temperature stability of the parabolic index profile compared to the one formed with a doped polymer. Finally, we discuss the crosstalk properties of the fabricated waveguides.

A multi-channel polymer optical waveguide with graded-index (GI) cores is successfully fabricated directly on a substrate by means of the soft-lithographic method. The great advantage in the inter-channel crosstalk of densely-aligned GI-core waveguides is demonstrated both theoretically and experimentally.