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

We demonstrate a self-referenced fiber frequency comb that can operate outside the wellcontrolled optical laboratory. The frequency comb has residual optical linewidths of < 1 Hz, subradian residual optical phase noise, and residual pulse-to-pulse timing jitter of 2.4 -5 fs, when locked to an optical reference. This fully phase-locked frequency comb has been successfully operated in a moving vehicle with 0.5 g peak accelerations and on a shaker table with a sustained 0.5 g rms integrated acceleration, while retaining its optical coherence and 5-fs-level timing jitter. This frequency comb should enable metrological measurements outside the laboratory with the precision and accuracy that are the hallmarks of comb-based systems.Work of the U.S.

We present a frequency domain model of shot noise in the photodetection of ultrashort optical pulse trains using a time-varying analysis. Shot noise-limited photocurrent power spectral densities, signal-tonoise expressions, and shot noise spectral correlations are derived that explicitly include the finite response of the photodetector. It is shown that the strength of the spectral correlations in the shot noise depends on the optical pulse width, and that these correlations can create orders-of-magnitude imbalance between the shot noise-limited amplitude and phase noise of photonically generated microwave carriers. It is also shown that only by accounting for spectral correlations can shot noise be equated with the fundamental quantum limit in the detection of optical pulse-to-pulse timing jitter.

High-quality frequency references are the cornerstones in position, navigation and timing applications of both scientific and commercial domains. Optomechanical oscillators, with direct coupling to continuous-wave light and non-material-limited f × Q product, are long regarded as a potential platform for frequency reference in radio-frequency-photonic architectures. However, one major challenge is the compatibility with standard CMOS fabrication processes while maintaining optomechanical high quality performance. Here we demonstrate the monolithic integration of photonic crystal optomechanical oscillators and on-chip high speed Ge detectors based on the silicon CMOS platform. With the generation of both high harmonics (up to 59th order) and subharmonics (down to 1/4), our chipset provides multiple frequency tones for applications in both frequency multipliers and dividers. The phase noise is measured down to −125 dBc/Hz at 10 kHz offset at ~400 μW dropped-in powers, one of the lowest noise optomechanical oscillators to date and in room-temperature and atmospheric non-vacuum operating conditions. These characteristics enable optomechanical oscillators as a frequency reference platform for radio-frequency-photonic information processing.

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX We utilize and characterize high-power, high-linearity modified uni-traveling carrier (MUTC) photodiodes for low-phasenoise photonic microwave generation based on optical frequency division. When illuminated with picosecond pulses from a repetition-rate-multiplied gigahertz Ti:sapphire modelocked laser, the photodiodes can achieve 10 GHz signal power +14 dBm. Using these diodes, a 10 GHz microwave tone is generated with less than 500 attoseconds absolute integrated timing jitter (1 Hz -10 MHz) and a phase noise floor of -177 dBc/Hz. We also characterize the electrical response, amplitude-to-phase conversion, saturation and residual noise of the MUTC photodiodes.