Received 1 April 2025; revised 21 July 2025; accepted 31 July 2025.
Date of publication 8 August 2025; date of current version 20 August 2025.
Digital Object Identifier 10.1109/OJUFFC.2025.3596866
Radio Frequency From Optical With
Instabilities Below 10
−15
-Generation
and Measurement
ARCHITA HATI
1
(Member, IEEE), MARCO POMPONIO
1,2
, NICHOLAS V. NARDELLI
1
,
TANNER GROGAN
1,3
, KYUNGTAE KIM
1,3,4
, DAHYEON LEE
1,3,4
, JUN YE
1,3,4
,
TARA M. FORTIER
1
, ANDREW LUDLOW
1,2,3
, AND CRAIG W. NELSON
1
(Member, IEEE)
1
National Institute of Standards and Technology, Boulder, CO 80305 USA
2
Department of Electrical, Computer and Energy Engineering, University of Colorado Boulder, Boulder, CO 80309 USA
3
Department of Physics, University of Colorado Boulder, Boulder, CO 80309 USA
4
JILA, University of Colorado Boulder, Boulder, CO 80309 USA
CORRESPONDING AUTHOR: A. HATI (archita.hati@nist.gov)
ABSTRACT This paper presents a frequency synthesis that achieves exceptional stability by transferring
optical signals to the radio frequency (RF) domain at 100 MHz. We describe and characterize two synthesis
chains composed of a cryogenic silicon cavity-stabilized laser at 1542 nm and an ultra-low expansion (ULE)
glass cavity at 1157 nm, both converted to 10 GHz signals via Ti:Sapphire and Er/Yb:glass optical frequency
combs (OFCs). The 10 GHz microwave outputs are further divided down to 100 MHz using a commercial
microwave prescaler, which exhibits a residual frequency instability of σ
y
(1 s) < 10
−15
and low 10
−18
level
at a few thousand seconds. Measurements are performed using a newly developed custom ultra-low-noise
digital measurement system and are compared to the carrier-suppression technique. The new system enables
high-sensitivity evaluation across the entire synthesis chain, from the optical and microwave heterodynes as
well as the direct RF signals. Results show an absolute instability of σ
y
(1 s) ≈ 4.7 × 10
−16
at 100 MHz.
This represents the first demonstration of such low instability at 100 MHz, corresponding to a phase noise of
−140 dBc/Hz at a 1 Hz offset and significantly surpassing earlier systems. These advancements open new
opportunities for precision metrology and timing systems.
INDEX TERMS Allan deviation, digital measurement system, frequency instability, phase noise, prescalers,
optical clocks, optical frequency divider, stability transfer.
I. INTRODUCTION
G
ENERATING extremely stable radio frequency (RF)
signals from optical sources is an important capa-
bility that benefits high-precision radar, navigation, com-
munication systems, and metrology. Optical clocks and
cavity-stabilized lasers currently set the benchmark for fre-
quency stability and accuracy, outperforming conventional
microwave standards by two orders of magnitude both in
short and long-term fractional frequency instability [1], [2].
However, translating the extraordinary stability of these opti-
cal systems to more accessible RF frequencies, such as
10 MHz and 100 MHz, poses unique challenges. Optical
clocks operate at frequencies in the hundreds of terahertz and
achieve fractional frequency stabilities below 10
−16
on short
integration times. This remarkable precision will underpin the
redefinition of the SI second [3] and extend the application
of optical systems beyond their intrinsic domain. The optical
frequency comb (OFC) is central to this effort because it
enables phase-coherent division of optical frequencies into
the RF and microwave regimes with an exceptional level
of spectral purity and stability [4], [5], [6]. While previous
developments have mainly focused on generating 10 GHz
signals [7], there remains a demand for equally stable sig-
nals at lower frequencies, such as 10 MHz and 100 MHz,
for applications requiring long-term temporal coherence, and
high spectral purity. Currently, 10 MHz signals are widely
used as a standard reference frequency in many electronic
devices and test instruments, serving as the stable timing
source for precise measurements. Additionally, distributing
signals at 10 MHz and 100 MHz via coaxial cables is more
VOLUME 5, 2025
2025 The Authors. This work is licensed under a Creative Commons Attribution 4.0 License.
For more information, see https://creativecommons.org/licenses/by/4.0/
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