Abstract
A fully stabilized EO comb is demonstrated by phase locking the two degrees of freedom of an EO comb to a low noise mode-locked fiber comb. Division/magnification of residual phase noise of locked beats is observed by measuring an out-of-loop beat. By phase locking the 200 th harmonics of the EO comb and a driving cw frequency to a fiber comb, a record low phase noise EO comb across +/- 200 harmonics (from 1544.8 nm to 1577.3 nm) is demonstrated.
Highlights
Optical frequency combs have been applied to many fields such as optical atomic clocks [1,2], lidar [3], biology [4], microwave generation [5], and astronomical calibration [6]
We demonstrate phase locking of an electro-optic combs (EO comb) to a fiber comb to overcome the phase noise limitations of the RF source of the EO comb. fcw and nfm are phase locked to selected fiber comb teeth, while measuring an out-of-loop beat between the EO comb and the fiber comb
From these results, when the EO comb is fully stabilized to the ML comb to reduce phase noise, large harmonic numbers should be used for phase locking, as observed by Ishizawa et al [17,18]
Summary
Optical frequency combs have been applied to many fields such as optical atomic clocks [1,2], lidar [3], biology [4], microwave generation [5], and astronomical calibration [6]. The phase noise of fn(ML) is much lower than that of fn(EO) at low frequency offset, once ML combs are stabilized to a cw laser which is referenced to an ultra stable optical cavity [5]. In our recent conference paper [20], we demonstrated the proof-of-concept, but, here, a broader spectrum EO comb is generated and feedback bandwidth is improved. Because of these improvement, we clearly show division/magnification of the residual phase noise of nfm, depending on the wavelength of the out-of-loop beat. We propose and demonstrate coherent addition of optical beats to obtain a lower noise floor, which is useful for low noise microwave generation or phase noise measurement when using an EO comb
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