  We introduce PULSE, a sub-microsecond optical circuit-switched data centre
network architecture controlled by distributed hardware schedulers. PULSE is a
flat architecture that uses parallel passive coupler-based broadcast and select
networks. We employ a novel transceiver architecture, for dynamic
wavelength-timeslot selection, to achieve a reconfiguration time down to
O(100ps), establishing timeslots of O(10ns). A novel scheduling algorithm that
has a clock period of 2.3ns performs multiple iterations to maximize
throughput, wavelength usage and reduce latency, enhancing the overall
performance. In order to scale, the single-hop PULSE architecture uses
sub-networks that are disjoint by using multiple transceivers for each node in
64 node racks. At the reconfiguration circuit duration (epoch = 120 ns), the
scheduling algorithm is shown to achieve up to 93% throughput and 100%
wavelength usage of 64 wavelengths, incurring an average latency that ranges
from 0.7-1.2 microseconds with best-case 0.4 microsecond median and 5
microsecond tail latency, limited by the timeslot (20 ns) and epoch size (120
ns). We show how the 4096-node PULSE architecture allows up to 260k optical
channels to be re-used across sub-networks achieving a capacity of 25.6 Pbps
with an energy consumption of 85 pJ/bit.
