Session P2a: Advanced and Future Clocks

Long-Term and Environmental Testing of Critical Subcomponents of an Acetylene Optical Clock with Pathways to Space Deployment
Henry Timmers, Jose Valencia, Nate Phillips, Vescent; Jan Hald, Michael Kjaer, DFM; Bennett Sodergren, Andrew Attar, Kurt Vogel, and Kevin Knabe, Vescent
Location: Seaview A/B
Date/Time: Wednesday, Jan. 29, 9:20 a.m.

Optical atomics clocks have recently become commercially available due to the miniaturization and ruggedization of critical optical systems such as optical frequency combs, narrow linewidth lasers, and warm-vapor physics packages. These systems are capable of supporting femtosecond level timing instabilities and maintaining better-than-GPS performance at day or even week timescales. Such systems are finding interest in in critical application spaces such as assured positioning, navigation, and timing (PNT), time and frequency transfer (TFT), distributed radar networks, and in advanced metrology laboratories. Reliability, ruggedization, and miniaturization continue to be development areas that are critical for these systems to operate in real-world environments. Previously, an optical clock based on commercial-off-the-shelf (COTS) components was demonstrated to have maser-like performance at short timescales while having similar environmental ruggedness to cesium beam tube frequency references. This system was based on an optical frequency comb (OFC) from Vescent Technologies, Inc. (Vescent) and an acetylene optical frequency reference (OFR) from the Danish National Metrology Institute (DFM). Here we present long term and environmental testing of these critical subsystems.
The DFM Stabilaser OFR system (https://stabilaser.dk/stabilized-laser/) offers an optical output of more than 10 mW of frequency-stabilized light in a PM1550 fiber. The narrow linewidth laser is stabilized to an overtone transition in acetylene near 1542 nm using frequency modulation (FM) saturated absorption spectroscopy. The laser, spectroscopy cell, electro-optic actuators, and control electronics are all contained in a 3U rack mount chassis with a volume of approximately 30 L. Low-noise detection of the FM spectroscopy signal with high signal-to-noise ratio enables a short-term linewidth of approximately 300 Hz and optical-domain Allan deviations (ADEVs) of less than 3E-13/(tau)^(1/2) out past tau = 100 s and with long term stability reaching at or below 8E-15 around tau = 2,000 s. The fractional frequency drift of this system has been estimated to be below 2E-12/year (or < 7E-20/s), which is within an order of magnitude of high-performance masers and obtained without large environmental enclosures to control temperature and vibrations. Recently, an optical heterodyne between two DFM Stabilasers (mixing down the optical stability to radio-frequencies) was counted for 30+ days against a rubidium GPS reference. During this glitch-free measurement no user intervention was required (and the length of the measurement was limited by user-initiated interventions to perform different experiments), and long term Allan deviations (ADEVs) stayed below 1E-13 even beyond 1,000,000 seconds.
Vescent’s FFC-100 OFC system (https://vescent.com/us/ffc-100-frequency-comb.html) is based on robust telecom fiber technologies and is enclosed in a 2U rack mount chassis. Out-of-loop characterization of the fully stabilized comb shows ADEVs < 5E 14/(tau)^(3/2). This level of performance ensures that Vescent’s OFC can support next-generation warm-vapor atomic and molecular clocks; additional testing and characterization is on-going to ensure that this system is suitable for even higher performance cold-atom systems. Vescent’s OFCs have been designed and tested to operate reliably over large temperature ranges and have been proven to maintain continuous, glitch-free frequency division over several months (thus far limited by user-initiated interventions to disable the measurement). Additionally, phase-locked operation of these OFCs has been shown from 0 – 50 °C, under vibrations with >1 kHz of excitation bandwidth and > 0.5 g_RMS amplitudes, and even shock events lasting milliseconds with amplitudes of > 5 g_peak. Finally, progress towards space-deployed optical frequency combs including miniaturization, size, weight, and power reductions, and thermal management for NASA and Space Force applications will be discussed.
In summary, we have presented long term testing of critical optical clock subsystems including OFCs and an acetylene OFR that were previously demonstrated to have maser-like performance. Due to the higher technology readiness level of the optical frequency comb, phase-locked operation of the OFC has been shown in challenging terrestrial environments. Progress and pathways to space deployment of the OFC system will be reviewed.