Sub-femtosecond optical timing distribution for next-generation light sources

Abstract

Precise timing distribution is critical for realizing a new regime of light control in next-generation X-ray free-electron lasers. These facilities aim to generate sub-femtosecond (fs) X-ray pulses with unprecedented brightness to realize the long-standing scientific dream to capture chemical and physical reactions with atomic-level spatiotemporal resolution. To achieve this, a high-precision timing system is required to synchronize dozens of radio frequency (RF) and optical sources across kilometer distances with sub-fs precision. Since conventional RF timing systems have already reached a practical limit of 50 fs, next-generation systems are adopting optical technology to achieve superior performance. In this thesis, an optical timing distribution system (TDS) is developed using ultrafast mode-locked laser technology to deliver sub-fs timing stability. Optical domain components of the TDS are first presented. The timing jitter of commercial mode-locked lasers is characterized to confirm their viability as optical master oscillators for timing distribution. Stabilization of a 1.2-km dispersion-compensated polarization-maintaining fiber link is demonstrated as a proof-of-concept for eliminating polarization-induced timing drifts. The link is then enhanced to achieve state-of-the-art timing distribution across a 4.7-km fiber network with 0.58 fs RMS residual drift for over 52 hours. For a complete end-to-end TDS, a remote laser is stabilized at the output of a 3.5 km fiber link with 0.2 fs RMS residual drift. All demonstrations depend critically on the balanced optical cross-correlator for high-precision optical timing measurements. Second, the coverage of the TDS is extended into the RF domain using balanced optical-microwave phase detectors (BOMPD). Two generations of BOMPDs are developed to achieve sub-fs noise performance with MHz-level bandwidth capabilities and robust AM-PM suppression ratios (>50 dB). Optical-to-RF synchronization is demonstrated with 0.98 fs RMS drift for over 24 hours, while RF-to-optical synchronization is demonstrated with 0.5 fs RMS. Lastly, an Erbium Silicon Photonics Integrated OscillatoR (ESPIOR) based on optical frequency division (OFD) is developed for ultralow-noise microwave generation. Since f-2f interferometry is unavailable on-chip, an alternative fCEO control scheme called quasi-OFD is proposed to improve stabilization of an integrated frequency comb. The ESPIOR concept is demonstrated in a discrete testbed to achieve low-noise RF generation with -63 dBc/Hz phase noise at 10 Hz offset for a 6-GHz carrier frequency. This corresponds to an OFD ratio of 85 dB, which is close to the ideal OFD ratio of 90 dB.