arXiv subjects

Skevos F. E. Karpathakis

Publications and source records attributed to Skevos F. E. Karpathakis.

3 recordsLinked to original sources

arXiv2024

Free Space Optical Frequency Comparison Over Rapidly Moving Links

The comparison of optical reference frequency signals over free-space optical links is limited by the relative motion between local and remote sites. For ground to low earth orbit comparison, the expected Doppler shift and Doppler rate typically reach 4 GHz at 100 MHz/s, which prevents the narrow-band detection required to compare optical frequencies at the highest levels of stability. We demonstrate a system capable of optical frequency comparison in the presence of these significant Doppler shifts, using an electro-optic phase modulator with an actuation bandwidth of 10 GHz, which will enable ground-to-space frequency comparison. This system was demonstrated over a retro-reflected drone link, with a maximum line-of-sight velocity of 15 m/s and Doppler shift of 19 MHz at a Doppler rate of 1 MHz/s. The best fractional frequency stability obtained was 7E-18 at an integration time of 5s. These results are an important step toward ground to low earth orbit optical frequency comparison, providing a scalable terrestrial test bed.

physics.ins-det↗

arXiv2021

Ultra-wideband free-space optical phase stabilisation

Free-space optical (FSO) communications has the potential to revolutionize wireless communications due to its advantages of inherent security, high-directionality, high available bandwidth and small physical footprint. The effects of atmospheric turbulence currently limit the performance of FSO communications. In this letter, we demonstrate a system capable of indiscriminately suppressing the atmospheric phase noise encountered by independent optical signals spread over a range of 7.2 THz (encompassing the full optical C-Band), by actively phase stabilizing a primary optical signal at 193.1 THz (1552 nm). We show ~30 dB of indiscriminate phase stabilization over the full range, down to average phase noise at 10 Hz of -39.6 dBc/Hz when using an acousto-optic modulator (AOM) as a Doppler actuator, and -39.9 dBc/Hz when using a fiber-stretcher as group-delay actuator to provide the phase-stabilization system's feedback. We demonstrate that this suppression is limited by the noise of the independent optical signals, and that the expected achievable suppression is more than 40 dB greater, reaching around -90 dB/Hz at 10 Hz. We conclude that 40 Tbps ground-to-space FSO transmission would be made possible with the combination of our stabilization system and other demonstrated technologies.

eess.SP↗

arXiv2020

Phase synchronisation system receiver module for the Mid-Frequency Square Kilometre Array

Next generation radio telescopes, such as the Square Kilometre Array (SKA) and Next Generation Very Large Array (ngVLA), require precise microwave frequency reference signals to be transmitted over fiber links to each dish to coherently sample astronomical signals. Such telescopes employ phase stabilization systems to suppress the phase noise imparted on the reference signals by environmental perturbations on the links; however, the stabilization systems are bandwidth limited by the round-trip time of light travelling on the fiber links. A phase-locked Receiver Module (RM) is employed on each dish to suppress residual phase noise outside of the round-trip bandwidth. The SKA RM must deliver a 3.96 GHz output signal with 4 MHz of tuning range and less than 100 fs of timing jitter. We present an RM architecture to meet both requirements. Analytical modelling of the RM predicts 30 fs of output jitter when the reference signal is integrated between 1 Hz and 2.8 GHz. The proposed RM was conceived with best practice electromagnetic compatibility in mind, and to meet size, weight and power requirements for the SKA dish indexer. As the ngVLA reference design also incorporates a round-trip phase stabilization system, this RM may be applicable to future ngVLA design.

astro-ph.IM↗