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Optical-frequency combs enable measurement precision at the 20 th digit, and accuracy entirely commensurate with their reference oscillator. A new direction in experiments is the creation of ultracompact frequency combs by way of nonlinear parametric optics in microresonators. We refer to these as microcombs, and here we report a silicon-chip-based microcomb optical clock that phase-coherently converts an optical-frequency reference to a microwave signal. A low-noise comb spectrum with 25 THz span is generated with a 2 mm diameter silica disk and broadening in nonlinear fiber. This spectrum is stabilized to rubidium frequency references separated by 3.5 THz by controlling two teeth 108 modes apart. The optical clock's output is the electronically countable 33 GHz microcomb line spacing, which features an absolute stability better than the rubidium transitions by the expected factor of 108. Our work demonstrates the comprehensive set of tools needed for interfacing microcombs to state-of-the-art optical clocks.Optical clocks leverage the narrow, unvarying transitions of atoms to realize exceptionally stable laser frequencies measured at below the 10 -17 level (1, 2). Optical-frequency combs facilitate the measurement and use of these atomic references by providing a dense set of clock-referenced lines that span more than an octave (3). By way of their extraordinary measurement precision, frequency combs have enabled advances in diverse fields from spectroscopy of atoms and molecules (4, 5) to astronomy (6). Some of the most exciting, but yet unrealized, applications call for operation in environments that are unsuitable for existing comb technology.A new class of frequency combs has emerged based on optical microresonators (7,8). Here the comb generation relies on nonlinear parametric oscillation and cascaded four-wave mixing driven by a single CW laser. Such microcombs offer revolutionary advantages including chip-based photonic integration, uniquely large comb-mode spacings in the 10's of GHz range, and monolithic construction with small size and power consumption. Progress in microcomb development has included frequency control of their spectra (9-12), characterization of their noise properties (13-15), a Rb-stabilized microcomb oscillator (16), and demonstration of phase-locked (13,17,18) and modelocked states (19,20). However, the milestone of all-optical frequency control of a microcomb to an

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

The discovery and characterization of exoplanets around nearby stars is driven by profound scientific questions about the uniqueness of Earth and our Solar System, and the conditions under which life could exist elsewhere in our Galaxy. Doppler spectroscopy, or the radial velocity (RV) technique, has been used extensively to identify hundreds of exoplanets, but with notable challenges in detecting terrestrial mass planets orbiting within habitable zones. We describe infrared RV spectroscopy at the 10 m Hobby-Eberly telescope that leverages a 30 GHz electro-optic laser frequency comb with nanophotonic supercontinuum to calibrate the Habitable Zone Planet Finder spectrograph. Demonstrated instrument precision <10 cm/s and stellar RVs approaching 1 m/s open the path to discovery and confirmation of habitable zone planets around M-dwarfs, the most ubiquitous type of stars in our Galaxy. Fig.1. Instrumentation for precision infrared astronomical RV spectroscopy. (A) Starlight is collected by the Hobby-Eberly telescope and directed to an optical fiber. Lasers, electro-optics and nanophotonics are used to generate an optical frequency comb with teeth spaced by 30 GHz and stabilized to an atomic clock. Both the starlight and frequency comb light are coupled to the highly-stabilized Habitable Zone Planet Finder (HPF) spectrograph where minute wavelength changes in the stellar spectrum are tracked with the precise calibration grid provided by the laser frequency comb. (B) Components for frequency comb generation. (upper) A fiber-optic integrated electro-optic modulator and (lower) silicon nitride chip (5 mm × 3 mm) on which nanophotonic waveguides are patterned. Light is coupled into a waveguide from the left and supercontinuum is extracted from the right with a lensed fiber. (C) The HPF spectrograph, opened and showing the camera optics on the left, echelle grating on the right, and relay mirrors in front. The spectrograph footprint is approximately 1.5 m × 3 m. (D) The 10 m Hobby-Eberly telescope at the McDonald Observatory in southwest Texas.

We present an investigation of the white noise plateau in residual phase noise measurement of an optical frequency comb for short and long external laser cavity lengths. OCIS codes: (140.5960) Semiconductor Lasers; (140.4050) Mode-locked lasers 1. Motivation Semiconductor harmonically mode-locked lasers have a number of features which make them attractive for use in communications, sampling, and metrology applications [1,2] For some of these applications, low timing jitter and narrow optical comb linewidth is required for good sampling resolution or increased stability. In semiconductor phase noise performance plots, we generally distinct characteristics at low, middling, and high offset frequencies relative to the carrier [3,4]. For good phase noise performance at high offset frequencies, we can implement a high finesse intra-cavity to suppress competing longitudinal mode-groups [5], and we can lower the shot noise floor by increasing photodetected optical power. In this submission, we address phase noise performance at middling offset frequencies corresponding to the white noise plateau. By decreasing optical linewidth, the corner, or "knee" frequency of the white noise plateau should also decrease proportionately [6], and we aim to hit the shot noise floor at lower offset frequencies and thereby reduce timing jitter. 2. Background It is well known that laser cavity length determines longitudinal mode linewidth [7]. It is also known that a mode-locked pulse train's temporal coherence is inversely related to longitudinal mode linewidth [8]. For pulses separated in time by more than the coherence time, we expect to see no coherence, and, importantly, no noise correlation. Thus the noise contribution at low offset frequencies is dominated by uncorrelated white noise. This result has been shown in a previous publication without the use of the etalon to suppress supermode contributions to phase noise [6], and here we will attempt to implement the concept into our existing laser architecture [9]. 3. Setup and Data The laser's setup is shown in Fig. 1 and a similar architecture is discussed in [9]. Here we perform measurements at the shortest manageable cavity length (22m, f0 = 9MHz) and for a longer cavity length (351 m, f0 = 570 kHz). Fig. 2 shows the change in knee location for the residual phase noise measurement when bypassing the etalon in the laser (not PDH-locked). Fig. 3 shows the residual phase noise measurement for the short cavity. As of the time of this submission, the PDH lock on the longer cavity is limited due to the higher required dynamic range for the fiber stretcher, but the dashed line in Fig. 3 shows where we expect to measure the new knee location once we have ability to stabilize the laser. For equal shot noise floor and noise power at 1 Hz offset, we expect a reduction in timing jitter between 10% and 20%. This represents a promising improvement in the timing jitter of our optical frequency comb sources. Fig. 1 Laser Setup Fig. 2 Residual phase noise of short and long etalon-less cavities Fig...