Design and fabrication of restricted mode launching device for high-speed multimode fiber link

The IP traffic in datacenter has rapidly increased over the last couple of years because of the wide deployment of cloud computing services. Hence, multimode fiber (MMF) links are already deployed in the rack-to-rack networks particularly in large-scale datacenters to sustain the rapid growth of data traffic. In recent years, the network topology in datacenters shifts from the legacy tree-type to fabric-type: the leaf and spine structure is requiring higher-bandwidth distant product for the MMF links to maintain low latency. However, the modal dispersion has been a large issue to realize high bandwidth links.To address the problem, we focus on the restricted mode launch (RML) approach, by which optical power is coupled to limited numbers of propagating modes in MMFs. Key parameters for the RML are the numerical aperture (NA) and the spot size on the MMF launch end: both of them should be small to reduce the modal dispersion. These are difficult to achieve with small-size launch systems composed of microlenses. Thus, in this paper, a new compact RML device without using microlenses is designed and fabricated: an axially tapered graded-index (GI) core polymer optical waveguide is applied to the RML exciter. Firstly, the launch condition for a 1000-m long OM3 fiber is investigated by calculating the effective modal bandwidth (EMBc). To realize OM4-grade EMBc, we find that the NA and the spot size of the launching beam should be lower than 0.118 and 33.2 μm, respectively. Also, the polymer waveguides are designed to meet the above conditions. The optimum refractive index profile of the tapered core is calculated by solving the Fick's diffusion equation, and then using the beam propagation method (BPM), the spot sizes and NAs from the tapered polymer waveguides are simulated. Finally, we fabricate GI core tapered waveguides using the imprint method.