Photonic synthesis of radiofrequency revived the quest for unrivalled microwave purity by its seducing ability to convey the benefits of the optics to the microwave world 1-11 . In this work, we perform a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fiber-based frequency comb and cuttingedge photo-detection techniques. We demonstrate the generation of the purest microwave signal with a fractional frequency stability below 6.5 x 10 -16 at 1 s and a timing noise floor below 41 zs.Hz -1/2 (phase noise below -173 dBc.Hz -1 for a 12 GHz carrier). This outclasses existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation

Fluctuations of the optical power incident on a photodiode can be converted into 12 phase fluctuations of the resulting electronic signal due to nonlinear saturation in the 13 semiconductor. This impacts overall timing stability (phase noise) of microwave signals 14 generated from a photodetected optical pulse train. In this paper, we describe and utilize 15 techniques to characterize this conversion of amplitude noise to phase noise for several high-16 speed (>10 GHz) InGaAs P-I-N photodiodes operated at 900 nm. We focus on the impact of 17 this effect on the photonic generation of low phase noise 10 GHz microwave signals and show 18 that a combination of low laser amplitude noise, appropriate photodiode design, and optimum 19 average photocurrent is required to achieve phase noise at or below -100 dBc/Hz at 1 Hz offset 20 a 10 GHz carrier. In some photodiodes we find specific photocurrents where the power-to-21 phase conversion factor is observed to go to zero. 22 Index Terms: frequency combs, microwave photonics, photodetectors 23 24 30 emerging microwave photonics applications such as radio over fiber, phased-array radars [1], [2], 31 arbitrary waveform generation [3], [4], radio astronomy [5], large-scale free-electron lasers [6], [7], 32 and optical analog-to-digital conversion [8]. 33Our particular interest is the generation of ultra-low phase noise microwave tones and 34 waveforms using a frequency-stabilized mode-locked laser comb [6], [7], [9]- [12]. For this 35 application, we employ photodetection to convert the optical signals to electronic signals for use in 36 and analysis with standard electronic devices. While additional noise contributions from 37 components in the system can have a significant effect on the overall timing precision of these 38 ultra-low noise signals, excess noise in the photodetection process cannot be neglected. Besides 39 the fundamental shot noise of the photocurrent, the conversion of laser amplitude noise into 40 electronic phase noise during photodetection has been previously identified as a limiting noise

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