Recent advance in dense wavelength multiplexing division (DWDM) technology has provided tremendous bandwidth on optical fiber communications [ 11. However, the capability of switching and routing information at this high bandwidth (e.g., 1 Terabids) has been behind the transmission capability. Building a large-capacity switching system using only electronic technology may potentially lead a system bottleneck in interconnecting many electronic devices or modules, mainly caused by enormous interconnection wires and electromagnetic interference among them. We have explored the possibility of switching ATM cells in optical domain by prototyping a WDM ATM Multicast (3M) switch . By taking the advantages of both optical and electronic technologies, we have constructed two planes, optical switching plane and electronic control plane. ATM cells are routed through the optical switching plane, while their headers are extracted and processed in the electronic plane that controls the optical devices and routes the cells to proper output port(s). We have implemented and demonstrated a subsystem that performs the cell delineation based on ITU standards and overwrites VCWPI optically at 2.5 Gbids . In this paper, we present our recent work on aligning the phases of incoming ATM cells in optical domain. The subsystem can adjust the phases of optical cells at 2.5 Gbith to a reference cell clock with the adjustment range of 1 to 5 11 bits and with a precision of ?A bit (or 100 ps). We have adopted a novel sampling method to achieve the 100 ps precision without using a 10 GHz clock.