Optical frequency comb technology has been revolutionizing optical fields of ultra-precision metrology over two decades, connecting the SI unit standards of various optical clocks with utmost accuracy and stability. With an increasing demand for free-space communication and time/frequency transfer applications, the frequency comb is stretching its versatility even the outside of laboratory. Still, the environmental disturbances of free space degrade the excellent phase noise of the optical frequencies, hindering the reliable utilization of frequency comb at any desirable location. This research performs comb-to-comb synchronization over a 1.3-km Free-Space Optical link (FSO) with frequency stability of 3.80×10-15 at 0.1 s averaging time throughout the entire comb spectrum. Phase-locked loop compensates the Doppler shift of optical frequencies propagating atmospheric free-space channel, providing optical references for synchronizing distant frequency comb.
The terahertz spectral regime finds applications in terahertz spectroscopy and next-generation wireless communications, necessitating improved frequency stability and accuracy. This study presents a novel approach to achieve ultra-stable terahertz continuous-wave generation by referencing an optical cavity using an optical frequency comb. The results demonstrate an extended operating frequency range of 0.1-1.1 THz with frequency stability of about 10-15 at 1 s. This exceptional level of stability has significant implications for terahertz wave research, holding the potential for groundbreaking advancements in both fundamental and industrial applications.
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