In a Nature Physics publication from 2021, scientists around Kerry J. Vahala from the Caltech in Pasadena demonstrated how to use a Balanced Optical Cross-Correlator (BOC) as an ultra-precise measurement device for optical timing jitter.
Their experiments precisely measured the timing jitter of soliton pairs in a microcavity and could determine the effects of quantum noise on the relative soliton movement.
A soliton or solitary wave is a self-reinforcing wave packet that maintains its shape while propagating at a constant velocity. The latter is possible by balancing nonlinear and dispersive effects in the medium. Furthermore, with its self-reinforcing properties, solitons are an exciting field as they might find their way into next-generation telecommunication systems.
The solitons analyzed by the Caltech team were coherently pumped (Kerr) solitons in an optical microcavity. Such a setup could be the basis for chip-based frequency combs. However, these optical solitons are expected to undergo random quantum diffusion that set a fundamental performance limit in applications of soliton microcombs.
Bao et al. (2021) built an experimental setup that can create co-propagating and counter-propagating solitons in a microcavity and releases solitons from the cavity to send them to a BOC. There, the timing jitter between soliton pairs from the microcavity can be analyzed. From the measurements, they were able to determine the quantum limit of counter-propagating solitons. However, by also measuring co-propagating soliton pairs, the team could confirm the stabilization effects of these co-propagating solitons. In their experiments, co-propagating solitons were found to have relative timing jitter well below the quantum limit of a single soliton on account of mutual solid motion correlation which was only a theoretical predictions so far.
For the Caltech team in Pasadena, the high precision of the BOC was a crucial part of their experiments. Furthermore, it allowed them to explore the fundamental limits of timing jitter in soliton microcombs and provide new insights on multi-soliton physics.