Attosecond timing in optical-to-electrical conversion
Frederick N. Baynes, Franklyn J. Quinlan, Tara M. Fortier, Qiugui Zhou, Andreas Beling, Joe C. Campbell, Scott A. Diddams
The most frequency-stable sources of electromagnetic radiation are produced optically, and optical frequency combs provide the means for high fidelity frequency transfer across hundreds of terahertz and into the microwave domain. A critical step in the photonic-based synthesis of microwave signals is the optical-to-electrical conversion process. Here we report the residual phase, timing, and frequency stability of microwaves generated by short optical pulse illumination of high-power photodiodes. For a generated 10 GHz carrier, measurements covered over 7 decades of offset frequencies, from 1 mHz to greater than 10 kHz. The phase noise was found to largely follow a flicker (1/f) dependence over the entire range, reaching -135 dBc/Hz (4 as/√Hz) at 1 Hz, and -170 dBc/Hz (0.07 as/√Hz) at 10 kHz offset. The corresponding Allan deviation was measured at 1.4 x 10-17 at 1 second and 5.5 x 10-20 at 1000 seconds. The implications for the transfer of state-of-the art optical frequency references to the microwave domain are discussed.