  In this paper, we describe Apollo, to the best of our knowledge, the world's
first large-scale production deployment of optical circuit switches (OCSes) for
datacenter networking. We will first describe the infrastructure challenges and
use cases that motivated optical switching inside datacenters. We then delve
into the requirements of OCSes for datacenter applications: balancing cost,
port count, switching time, and optical performance, which drive design choices
and implementation details of our internally developed 3D MEMS-based OCS. To
enable the Apollo optical switching layer, we employ circulators to realize
bidirectional links through the OCS, effectively doubling the OCS radix. The
OCS and circulator design choices were critical for meeting network bandwidth,
scale, and cost targets. We review the critical co-design of WDM transceiver
technology for these OCS plus circulator-based bidirectional links and their
corresponding physical impairments, delivered over four generations/speeds of
optical interconnect. Finally, we conclude with thoughts on future directions
in hardware development and associated applications.

Networking and Internet Architecture