This paper describes a low-cost, two-layer optical interconnection network for a computer cluster that was designed and implemented with a virtual-parallel transmission technique. The top layer of the network is a bi-directional optical bus; the sub-layer is a synchronous, pipelined optical ring. The virtual-parallel transmission technique was used in the optical ring to decrease total system cost. The network has two-dimensional scalability — meaning that it can be developed either via the bus or via the ring.