Laboratory optical atomic clocks achieve remarkable accuracy (now counted to 18 digits or more), opening possibilities for exploring fundamental physics and enabling new measurements. However, their size and the use of bulk components prevent them from being more widely adopted in applications that require precision timing. By leveraging silicon-chip photonics for integration and to reduce component size and complexity, we demonstrate a compact optical-clock architecture. Here a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 1013. These results demonstrate key concepts of how to use silicon-chip devices in future portable and ultraprecise optical clocks.

Architecture for the photonic integration of an optical atomic clock

Frequency-stable laser systems are a key technology in
precision experiments and have recently become applicable
in space. Optical frequency references based on cavities,
atoms, or molecules allow for precise laser frequency
stabilization and thus enable, for example, high-precision
laser ranging in space missions. Examples are the gravity
recovery and climate experiment-follow-on (GRACEFO)
mission [1,2] that was recently put in operation, the
planned laser interferometer gravitational wave observatory
LISA [3], and future global satellite navigation systems
based on optical clocks [4,5]. In the context of these
and other missions, frequency references based on optical
cavities [6,7] and unequal-arm-length interferometers
[8,9] have been developed as compact, ruggedized instruments
for application in space, featuring good short-term
instability.
One way to also realize low long-term instability is
the stabilization to atomic or molecular transitions, providing
an advantage in accuracy over relative frequency
references. Together with optical frequency combs, such
absolute references can further be operated as optical
clocks with instability relevant for global navigation satellite
systems (GNSS). Several such systems are under
*klaus.doeringshoff@physik.hu-berlin.de
investigation today at various wavelengths, based on
thermal calcium beams [10–12] or hot vapor cells using
single- [13] or two-photon transitions in rubidium [14,15].
Another, already matured frequency reference is based
on saturation spectroscopy of molecular iodine using the
second harmonic of the narrow linewidth Nd:yttriumaluminum-
garnet (YAG) laser at 1064 nm. These systems
rely on the modulation-transfer spectroscopy (MTS)
technique applied to the rovibronic transition R(56)32-
0, featuring narrow transitions with a natural linewidth
of 300 kHz [16]. The hyperfine spectrum of this transition
was studied in detail [17], the absolute frequency
was accurately measured with an uncertainty of 1.1 kHz
by Nevsky et al. [18], and Ye et al. demonstrated operation
of a molecular iodine optical clock over the course
of a full year [19]. In the past decade, several groups
realized compact and portable iodine references with fractional
frequency instability in the low 10−15 regime [20,21]
and subjected their setups to environmental tests [22]. We
believe that, using available technology at 1064 nm, developed
in the context of the LISA and the GRACE-FO
mission and established in satellite laser communication
terminals, such systems can be developed for spaceborne
instruments on relatively short time scales.
Here, we present a stand-alone iodine frequency reference
at 1064 nm, named JOKARUS, that is based on
a microintegrated extended-cavity diode laser (ECDL)
2331-7019/19/11(5)/054068(9) 054068-1 © 2019 American Physical Society
KLAUS DORINGSHOFF et al. PHYS. REV. APPLIED 11, 054068 (2019)
[23,24] in a master oscillator plus power amplifier (MOPA)
configuration. JOKARUS was built to demonstrate the
maturity of our technology and its applicability in space
missions. To this end, we operate JOKARUS on a sounding
rocket mission and thereby prove the autonomous operation
of an optical iodine frequency reference.We compare
the optical frequency to a chip-scale atomic clock (CSAC)
via a self-referenced frequency comb on the ground and in
space.
This paper is organized as follows. Section II describes
the optical iodine frequency reference and its autonomous
operation, as well as the optical frequency comb used
to verify the frequency instability of the iodine reference
aboard the sounding rocket. Section III presents results
on the characterization of the frequency instability of
the iodine frequency reference obtained on the ground
and from the operation in space. Finally, in Sec. IV, we
summarize the results and give a conclusion.

Iodine Frequency Reference on a Sounding Rocket

Lasers with high spectral purity can enable a diverse application space, including precision spectroscopy, coherent high-speed communications, physical sensing, and manipulation of quantum systems. Already, meticulous design and construction of bench Fabry-Perot cavities has made possible dramatic achievements in active laser-linewidth reduction, predominantly for optical-atomic clocks. Yet there is increasing demand for miniaturized laser systems operating with high performance in ambient environments. Here, we report a compact and robust photonic-atomic laser comprising a 2.5 cm long, 20,000 finesse, monolithic Fabry-Perot cavity integrated with a micromachined rubidium vapor cell. By leveraging the short-time frequency stability of the cavity and the long-time frequency stability of atoms, we realize an ultranarrow-linewidth laser that enables integration for extended measurements. Specifically, our laser supports a fractional-frequency stability of $1\times 10^{-13}$ at an averaging time of 20 ms, $7 \times 10^{-13}$ at 300 s, an integrated linewidth of 25 Hz that results from thermal noise, a Lorentzian linewidth as low as 0.06 Hz$^2$/Hz, and a passive vibration immunity as low as $10^{-10}$/g. Our work explores hybrid laser systems with monolithic photonic and atomic packages based on physical design.

Ultranarrow Linewidth Photonic‐Atomic Laser

Atomic frequency standards are used to generate accurate and precise time and frequency, enabling many communications, synchronization, and navigation systems in modern life. GPS and other satellite navigation systems, voice and data telecommunications, and timestamping of financial transactions all rely on precise time and frequency enabled by atomic frequency standards. This review provides a snapshot and an outlook of commercial and emerging atomic frequency standards. We provide a concise summary of the performance and physics of operation of current atomic frequency standards. In addition, we discuss examples of emerging frequency standard technologies and prototype demonstrations with a focus on technologies expected to provide commercial or military utility within the next decade. We include a comparison of performance versus size and power for current atomic frequency standards. We develop and discuss an empirical relationship between frequency standard performance and product size.

A Review of Commercial and Emerging Atomic Frequency Standards

Optical frequency standards, or lasers stabilized to atomic or molecular transitions, are widely used in length metrology and laser ranging, provide a backbone for optical communications and lie at the heart of next-generation optical atomic clocks. Here we demonstrate a compact, low-power optical frequency reference based on the Doppler-free, two-photon transition in rubidium-87 at 778 nm implemented on a micro-optics breadboard. Our optical reference achieves a fractional frequency instability of 2.9×10−12/τ for averaging times τ less than 103 s, has a volume of ≈35 cm3 and operates on ≈450 mW of electrical power. The advanced optical integration presented here demonstrates a key step towards the development of compact optical clocks and the broad dissemination of SI-traceable wavelength references.

pdf/miniaturized-optical-frequency-reference-for-next-generation-2rq31bq8ub.pdf

Miniaturized optical frequency reference for next-generation portable optical clocks

Optical frequency standards surpass their microwave counterparts in both stability and accuracy, yet they are often bulky, power-hungry, and unable to operate outside of a well-controlled laboratory environment. Leveraging a two-photon transition in ${}^{87}$Rb vapor and recent advances in fiber frequency combs, the authors build an optical clock to beat the current portable standards, with an architecture that can be made compact and low-power. These results point the way to a real-world optical frequency standard of even higher stability, for applications such as satellite navigation.

Compact Optical Atomic Clock Based on a Two-Photon Transition in Rubidium

Extra-laboratory atomic clocks are necessary for a wide array of applications (e.g. satellite-based navigation and communication). Building upon existing vapor cell and laser technologies, we describe an optical atomic clock, designed around a simple and manufacturable architecture, that utilizes the 778~nm two-photon transition in rubidium and yields fractional frequency instabilities of $3\times10^{-13}/\sqrt{\tau (s)}$ for $\tau$ from 1~s to 10000~s. We present a complete stability budget for this system and explore the required conditions under which a fractional frequency instability of $1\times 10^{-15}$ can be maintained on long timescales. We provide precise characterization of the leading sensitivities to external processes including magnetic fields and fluctuations of the vapor cell temperature and 778~nm laser power. The system is constructed primarily from commercially-available components, an attractive feature from the standpoint of commercialization and deployment of optical frequency standards.

It is highly desirable for future measurements of Newton’s gravitational constant G to use test/source masses that allow nondestructive, quantitative internal density gradient measurements. High density optically transparent materials are ideally suited for this purpose since their density gradient can be measured with laser interferometry, and they allow in-situ optical metrology methods for the critical distance measurements often needed in a G apparatus. We present an upper bound on possible internal density gradients in lead tungstate (PbWO4) crystals determined using a laser interferometer. We placed an upper bound on the fractional atomic density gradient in two PbWO4 test crystals of 1ρdρdx<2.1×10−8 cm−1. This value is more than two orders of magnitude smaller than what is required for G measurements. They are also consistent with but more sensitive than a recently reported measurements of the same samples, using neutron interferometry. These results indicate that PbWO4 crystals are well suited to be used as test masses in G experiments. Future measurements of internal density gradients of test masses used for measurements of G can now be conducted non-destructively for a wide range of possible test masses.

pdf/imaging-of-pbwo4-crystals-for-g-experiment-test-masses-using-2xbxi0sg.pdf

Imaging of PbWO4 crystals for G experiment test masses using a laser interferometer

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Compact Optical Atomic Clock Based on a Two-Photon Transition in Rubidium

Compact Optical Atomic Clock Based on a Two-Photon Transition in Rubidium

Imaging of PbWO4 crystals for G experiment test masses using a laser interferometer