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Optical Clock Technologies for Global Navigation Satellite Systems


Abstract and Figures

Future generations of global navigation satellite systems (GNSS) can benefit from optical technologies. Especially optical clocks could back-up or replace the currently used microwave clocks, having the potential to improve GNSS position determination enabled by their lower frequency instabilities. Furthermore, optical clock technologies - in combination with optical inter-satellite links - enable new GNSS architectures, e.g. by synchronization of distant optical frequency references within the constellation using time and frequency transfer techniques. Optical frequency references based on Doppler-free spectroscopy of molecular iodine are seen as a promising candidate for a future GNSS optical clock. Compact and ruggedized setups have been developed, showing frequency instabilities at the 10^(-15) level for averaging times between 1s and 10.000s. In this paper, we introduce optical clock technologies for applications in future GNSS and present the current status of our developments of iodine-based optical frequency references.
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Optical clock technologies forglobal navigation satellite systems
ThiloSchuldt1 · MartinGohlke1· MarkusOswald1,2· JanWüst1· TimBlomberg1· KlausDöringsho3,4·
AhmadBawamia3· AndreasWicht3· MatthiasLezius5· KaiVoss6· MarkusKrutzik3,4· SvenHerrmann2·
EvgenyKovalchuk3,4· AchimPeters3,4· ClausBraxmaier1,2
Received: 5 October 2020 / Accepted: 3 March 2021 / Published online: 5 April 2021
© The Author(s) 2021
Future generations of global navigation satellite systems (GNSSs) can benefit from optical technologies. Especially opti-
cal clocks could back-up or replace the currently used microwave clocks, having the potential to improve GNSS position
determination enabled by their lower frequency instabilities. Furthermore, optical clock technologies—in combination with
optical inter-satellite links—enable new GNSS architectures, e.g., by synchronization of distant optical frequency references
within the constellation using time and frequency transfer techniques. Optical frequency references based on Doppler-free
spectroscopy of molecular iodine are seen as a promising candidate for a future GNSS optical clock. Compact and ruggedized
setups have been developed, showing frequency instabilities at the 10–15 level for averaging times between 1s and 10,000s.
We introduce optical clock technologies for applications in future GNSS and present the current status of our developments
of iodine-based optical frequency references.
Keywords Optical clock· Iodine reference· Space instrumentation· Future GNSS
Over the last decades, optical clock technologies evolved,
recently demonstrating frequency instabilities at the 10–18
level for integration times of a few thousand seconds (Ushi-
jima etal. 2015; McGrew etal. 2018). While becoming more
and more widespread technology in and outside laboratories
on Earth, also space applications—including GNSS—can
benefit from the recent advancement of optical technologies.
Optical clocks surpass the performance of the currently used
GNSS microwave clocks by several orders of magnitude.
On the one hand, optical clocks could back-up or replace
the currently used microwave clocks (Droz etal. 2006), on
the other hand, optical clock technologies—in combination
with optical inter-satellite links—enable new GNSS archi-
tectures. One example is the proposed Kepler architecture
(Giorgi etal. 2019) which foresees 24 satellites in medium-
earth orbits (MEO, in three orbital planes, similar to the cur-
rent Galileo system), together with 6 satellites in low-Earth
orbit (LEO). All Kepler satellites carry optical frequency
references which are intra-system synchronized using bi-
directional optical inter-satellite links together with time and
frequency transfer techniques. In the current baseline, the
satellites are equipped with frequency references based on
optical resonators, providing the required high short-term
stability where the round-trip time of the light within one
orbital plane (of about 0.1s) is relevant. The LEO satellites
additionally carry mid- to long-term stable absolute opti-
cal clocks based on Doppler-free spectroscopy of molecular
iodine for the definition of the system time.
In the following, we first give a short overview of opti-
cal clock technologies for space, together with their current
technology development status. We then detail our develop-
ments with respect to iodine-based optical frequency ref-
erences for applications in space and present the mission
* Thilo Schuldt
1 German Aerospace Center (DLR), Institute ofSpace
Systems, Bremen, Germany
2 Center ofApplied Space Technology andMicrogravity,
University ofBremen, Bremen, Germany
3 Ferdinand-Braun-Institut gGmbH, Leibniz-Institut Für
Höchstfrequenztechnik, Berlin, Germany
4 Institute ofPhysics, Humboldt-Universität zu Berlin, Berlin,
5 Menlo Systems GmbH, Martinsried, Germany
6 SpaceTech GmbH, Immenstaad, Germany
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GPS Solutions (2021) 25:83
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concept COMPASSO, a proposed DLR in-orbit verification
mission on the International Space Station (ISS).
Please note that we refer to a “clock” for a system which
delivers a countable frequency signal in the radio frequency
range, and where a time signal can be derived from, as it is
e.g., the case in GNSS applications. When being based on an
absolute and known frequency, such a system is often also
referred to as a “frequency standard”.
Optical clock technologies forspace
Optical clock technologies include a variety of different
implementations, all having their own assets and drawbacks.
Looking at space applications, in addition to the demon-
strated performance such as frequency stability and accu-
racy, it is necessary to consider the size, weight, power con-
sumption, robustness, and reliability of the optical reference.
Ultimate frequency stability at the 10–18 level is shown
by optical lattice and single-ion clocks in rather complex
laboratory setups (Ushijima etal. 2015; McGrew etal.
2018; Delehay and Lacroute 2018). Technology develop-
ment with respect to transportable setups has been initiated
(Koller etal. 2017; Cao etal. 2017; Brewer etal. 2019; Han-
nig etal. 2019), and a compact setup of a 88Sr lattice clock
has been realized, where space-related design criteria have
been considered (Bongs etal. 2015; Origlia etal. 2018).
Such optical clocks require several lasers, a vacuum chamber
and a cavity pre-stabilization of the clock laser to achieve
their outstanding frequency instability enabled by millihertz
linewidth transitions.
Optical atomic beam standards, e.g., using Ca or Sr atoms
show a lower complexity and can be realized in more com-
pact setups. However, similar to optical lattice and single-
ion clocks, they require a vacuum chamber and use cavity
pre-stabilization of the clock laser. With a compact setup of
a Ca beam standard, frequency instabilities of 1.8 × 10–15 at
an integration time of 1600s have been demonstrated (Shang
etal. 2017). A compact and ruggedized Sr beam standard
for application on a sounding rocket is currently developed
at HU Berlin (Gutsch etal. 2019).
Gas-cell-based optical frequency references have mod-
est complexity without need of a vacuum chamber or pre-
stabilization and can be realized with small dimensions,
weight and power budgets. They typically employ modula-
tion transfer spectroscopy (MTS) or frequency modulation
spectroscopy (FMS) of optical transitions with linewidths
of the order of MHz. With a compact Rubidium-based fre-
quency reference using MTS near 420nm, frequency insta-
bilities of 2.1 × 10–15 at an integration time of 80s have been
claimed, deduced from the error signal (Zhang etal. 2017;
Chang etal. 2019). Iodine-based frequency references near
532nm have been realized for many decades, resulting in
compact and ruggedized setups, also with respect to appli-
cations in space (Nyholm etal. 2003; Leonhard and Camp
2006; Zang etal. 2007; Argence etal. 2010; Schuldt etal.
2017; Döringshoff etal. 2017), showing frequency insta-
bilities at the 10–15 level for integration times between 1
and 1000s. A very compact setup has been successfully
flown on a sounding rocket, together with a frequency comb
(Schkolnik etal. 2017; Döringshoff etal. 2019). Frequency
stabilization to iodine transitions near 515nm is investi-
gated within the Japanese proposed space gravitational wave
detector DECIGO (Deci-Hertz Interferometer Gravitational
Wave Observatory) where a compact setup has been realized
(Suemasa etal. 2017) and by a French collaboration using
a frequency tripled output at 514nm of Telecom laser tech-
nology at a wavelength near 1542nm (Barbarat etal. 2018).
The Rb two-photon transition (TPT) at 778nm is often
used as a frequency standard, providing Doppler-free spec-
troscopy. No laser pre-stabilization and no vacuum chamber
are required. While several setups have been realized in the
past, recent developments include a compact setup for appli-
cations as a successor to the atomic frequency standard in
GPS with demonstrated frequency instabilities at the 10–15
level (Martin etal. 2018). Furthermore, an integrated Rb
clock has been realized, using a micro-fabricated rubidium
gas cell in combination with a microcomb (Newman etal.
2019; Maurice etal. 2020).
Optical clocks require a frequency comb to transfer the
stability of the clock laser to a radio frequency and thus to
provide a countable clock signal. Developments for space
applications are already initiated, and compact frequency
combs have been successfully flown on a sounding rocket
(Lezius etal. 2016; Döringshoff etal. 2019; Pröbster etal.
Table1 summarizes the key figures of the technolo-
gies detailed above. For comparison, it also includes the
space-grade microwave references currently used on Galileo
[rubidium atomic frequency reference (RAFS) and passive
hydrogen maser (PHM)] where data are taken from the pub-
licly available data sheets. For the optical references, the
given values on performance, i.e., frequency stability, are
taken from the corresponding publications, together with
the values on size, weight and power (SWaP) budgets, if
given. As the technology development status of the tech-
nologies is quite different—ranging from transportable and
compact setups to implementations dedicated for space—the
entries cannot directly be compared. It is e.g., assumed that
within a dedicated development, the budgets of lattice and
ion clocks can be significantly reduced with respect to the
current transportable setups (Takamoto etal. 2020). Also,
long-term stability of the optical reference is often not yet
investigated. However, the summary in Table1 can be taken
as the basis for necessary trade-offs, e.g., between required
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GPS Solutions (2021) 25:83
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Table 1 Summary of the key figures of the different optical clock technologies, together with the corresponding figures of the Galileo RAFS and PHM
a Values on frequency instabilities deduced from the error signal (Zhang etal. 2017).
b The values for the frequency comb are explicitly given for the optical references (10kg, 66W, 7l) (Döringshoff etal. 2019; Pröbster etal. 2021). It is assumed that the values on mass and
power consumption can be further reduced in a design upgrade of the frequency comb
c The SWaP budgets for the optical clock technologies include the laser(s)
d Estimation based on current state-of-the-art implementation techniques using a similar design as for the iodine references
e Estimation based on the QUEEN study by HU Berlin, which includes 2 ECDL-MOPA laser systems and is not yet designed for highest frequency stability at the 10–15 level
f At + 10°C baseplate; 70W at -5C baseplate
g Iodine reference successfully flown on a sounding rocket (not designed for highest performance), component level space heritage within other space missions (LISA, LISA Pathfinder, GRACE
follow-on, NGGM)
Galileo RAFS Galileo PHM Ca beam I2 MTS Rb MTS Rb TPT Sr Lattice clock Ca single ion clock
References Orolia datasheet
(2016) Leonardo data-
sheet (2017) Shang etal.
(2017)Schuldt etal.
(2017); Döring-
shoff etal.
Zhang etal.
(2017)Martin etal.
(2018)Bongs etal.
(2015); Origlia
etal. (2018)
(Delehay and Lac-
route 2018; Cao
etal. 2017)
Frequency stabil-
ity (in RAV
@ integration
time τ)
1s 3 × 10–12 2 × 10–12 5 × 10–14 6 × 10–15 1 × 10−14a 4 × 10–13 n/s n/s
10s 1 × 10–12 3 × 10–13 2 × 10–14 3 × 10–15 4 × 10−15a 1 × 10–13 1 × 10–16 6 × 10–15
102s 3 × 10–13 7 × 10–14 5 × 10–15 2 × 10–15 3 × 10−15a 4 × 10–14 4 × 10–17 2 × 10–15
103s 6 × 10–14 2 × 10–14 2 × 10–15 2 × 10–15 n/s 1 × 10–14 1 × 10–17 6 × 10–16
104s 3 × 10–14 7 × 10–15 n/s 3 × 10–15 n/s 5 × 10–15 4 × 10–18 2 × 10–16
105s Long-term
drift < 10–10 /
drift < 10–15 /
n/s < 2 × 10–14 n/s n/s n/s n/s
106s n/s n/s n/s n/s n/s n/s
Longest reported
τ (s)
1600 700,000 600 180,000 30,000 30,000
Clock transition frequency/wave-
length 6.8GHz 1.4GHz 657nm 532nm 420nm 778nm 698nm 729nm
Clock transition natural linewidth 0.4kHz 300kHz 1450kHz 330kHz 6mHz 140mHz
SWaP Budgetsb,c Mass (kg) 3.4 18.2 n/s 21 + 10b10d + 10b12e + 10b < 250 n/s
Power (W) 35 60fn/s 44 + 66b20d + 66b25e + 66bn/s n/s
Volume (l) 3.2 26.3 300 + 7b33 + 7bn/s 8e + 7b < 1000 540
Complexity # Lasers n/a n/a 2 1 1 1 5 6
Vacuum chamber Yes No No No Yes Yes
Cavity pre-stabi-
lization n/a n/a Yes No No No Yes Yes
TRL 9 9 4 4-5g 4 4 4 4
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GPS Solutions (2021) 25:83
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frequency stability, SWaP budgets and robustness/complex-
ity, and also concerning the necessary time frame to develop
a space-qualified optical clock.
In Table1, also the technology maturity is assessed and
quantified by the so-called Technology Readiness Level
(TRL). The highest level (TRL9) is reached for flight-proven
components and systems with demonstrated performance
in space operation. The lowest level (TRL1) corresponds to
the observation and reporting of basic principles. Functional
verification in a typical laboratory environment corresponds
to TRL4, the full-scale engineering model with successful
environmental testing to TRL6.
At DLR and the University of Bremen, iodine-based
frequency references using modulation transfer spectros-
copy have been investigated for several years with respect
to applications in space, including missions to measure the
earth’s gravity field (Nicklaus etal. 2017) and space-borne
gravitational wave detection (Schuldt etal. 2019). Such ref-
erences can be realized compact and ruggedized, with small
dimensions, mass and power consumption. Typically, the
a10 component of the R(56)32–0 transition in 127I2 near a
wavelength of 532nm is used for frequency stabilization.
It is a standard frequency, recommended by the Interna-
tional Bureau of Weights and Measures (Bureau Interna-
tional des Poids et Mesures,BIPM) with a relative standard
uncertainty of 8.9 × 10–12 (Riehle etal. 2018). As the laser
is operated at 1064nm, iodine-based frequency references
can rely on space heritage of the laser and laser components,
developed e.g., within the missions LISA (Laser Interferom-
eter Space Antenna), LISA Pathfinder and GRACE (Grav-
ity Recovery and Climate Experiment) follow-on as well as
mission concepts developed within the NGGM (Next Gen-
eration Gravity Mission) program by the European Space
Agency (ESA). Furthermore, commercial laser communica-
tion terminals operate at the same wavelength. Iodine-based
optical frequency references are seen as a promising can-
didate for an optical clock for future GNSS showing lower
frequency instabilities as the currently used PHM. Relying
on an extensive heritage, a flight model fulfilling the Gali-
leo requirements could be realized on short timescale and
at moderate costs. As a perspective, this technology is seen
as an initiating step toward a routinely applied and reliable
optical clock technology in space, also paving the way for
future ultra-high performance optical single-ion and lattice
clocks in space.
Absolute frequency references based
onDoppler‑free spectroscopy ofmolecular
iodine near532nm
Molecular iodine offers very strong absorption lines near
532nm which can easily be accessed with a laser system at
a wavelength of 1064nm using second harmonic generation
(SHG), also realized in compact and ruggedized setups for
applications in space.
Over the last years, DLR—in collaboration with the Hum-
boldt-Universität zu Berlin and the University Bremen—has
developed several setups of iodine-based frequency refer-
ences with a roadmap toward space applications. The opti-
cal setups for modulation transfer spectroscopy are realized
using an adhesive bonding technology where the optical
components are joint to a baseplate made of glass (or glass
ceramics, respectively) with a space-qualified two-compo-
nent epoxy, see Fig.1. This ensures the high thermal and
mechanical stability of the optical system needed for opera-
tion in space.
With a setup on Elegant Breadboard (EBB) level (Fig.1,
left), frequency instabilities of 6× 10–15 at 1s integration
time and below 3× 10–15 for integration times between
100s and 10,000s have been demonstrated (Schuldt etal.
2017). This is, to our knowledge, the best published per-
formance for such an iodine-based frequency reference.
The frequency instabilities of the EBB setup—given in
Fig. 1 Several implementations of the iodine spectroscopy unit, real-
ized at DLR in collaboration with the universities HU Berlin and
Bremen. Left: Elegant Breadboard (EBB, 25 cm × 55 cm) (Schuldt
et al. 2017); Middle: Engineering Model (EM, 18 cm × 38 cm)
(Döringshoff etal. 2017); Right: Setup used on the sounding rocket
mission JOKARUS (15cm × 25cm) (Schkolnik etal. 2017; Döring-
shoff etal. 2019)
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GPS Solutions (2021) 25:83
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Allan deviation—are shown in Fig.2, evaluated from a
beat measurement with an ultra-low expansion (ULE) cav-
ity setup. A linear and a second-order polynomial drift
have been removed from the corresponding time records
where the linear drift is attributed to the iso-thermal creep
of the cavity. The Allan deviation shows white frequency
noise for integration times between 1 and 10s and Flicker
noise of about 3× 10–15 at integration times > 1000s. For
integration times up to 1000s, the frequency instabilities
are 1–2 orders of magnitude lower than the one of the
current Galileo clocks, i.e., RAFS and PHM. At longer
integration times, the iodine reference approaches the per-
formance of the Space Hydrogen Maser (SHM), an active
hydrogen maser which is currently implemented within
the ACES (Atomic Clock Ensemble in Space) mission on
the ISS (Goujon etal. 2010). As our measurement of the
iodine performance is most probably limited by the cav-
ity reference, we started an investigation of the long-term
stability of the EBB setup which is currently ongoing. A
first frequency stability evaluation where the iodine EBB
is compared to a hydrogen maser via an optical frequency
comb is shown in Fig.2 (blue curve). It is based on a
16days continuous operation of the iodine reference.
The engineering model (EM) setup was further devel-
oped with respect to compactness and uses a specifically
designed compact iodine cell, see Fig.1, middle (Döring-
shoff etal. 2017). The EM spectroscopy unit was subjected
to thermal cycling from -20°C to + 60°C and vibrational
loads with sine vibration up to 30g and random vibration
up to 25.1 grms. The frequency stability was measured before
and after the tests where no degradation was observed. The
frequency offset between the EBB and EM setups is below
1.5kHz with a reproducibility below 250Hz (Döringshoff
etal. 2017). While gas cells typically use a cold finger for
setting the pressure inside the cell, the EM setup was also
operated with a gas cell filled at an unsaturated vapor pres-
sure of about 1Pa, showing similar performance. This
allows lowering the complexity of the setup by omitting a
temperature control and reducing the SWaP budgets.
An iodine-based frequency reference has been success-
fully operated on a sounding rocket (Schkolnik etal. 2017;
Döringshoff etal. 2019) as part of the JOKARUS mis-
sion, showing autonomous operation during the 6min long
space flight. The setup was optimized with respect to the
specific sounding rocket requirements, especially regarding
dimensions. The compact spectroscopy unit with a 15cm
long iodine cell, is shown in Fig.1, right, using a micro-
integrated extended cavity diode laser (ECDL) including a
semiconductor power amplifier as light source (Kürbis etal.
2020). A short-term instability of the iodine reference of
1.5 × 10–13/√τ has been demonstrated (Döringshoff etal.
2019), probably limited due to its very compact design,
intermodulation noise and the lack of intensity and RAM
Within the ongoing project ADVANTAGE (Advanced
Technologies for Navigation and Geodesy), a next iteration
of the iodine setup is developed, based on the EBB-, EM-
and JOKARUS developments. The next step toward space
instrumentation is carried out by a system level design where
all components for the laser system, and the spectroscopy
are integrated within one physical box. For the design, an
absolute temperature of 15°C with a stability of ± 5°C is
Fig. 2 Measured frequency
stability of the Iodine-EBB ref-
erence, given in Allan deviation.
Shown is a beat measurement
with a cavity setup (Schuldt
etal. 2017), together with the
frequency stabilities of the
current Galileo clocks (RAFS,
PHM) and the active hydrogen
maser for ACES (SHM). Also
shown is a beat measurement
with a hydrogen maser (first
measurement, not yet opti-
mized, the degradation > 1000s
is believed to be an artefact).
The measurement is limited by
the H-maser performance for
integration times < 1000s
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assumed for the spacecraft interface plate. An actively con-
trolled thermal shield within the iodine reference guarantees
an operation of the spectroscopy board at (22 ± 0.1)°C. The
laser is fiber-coupled to this unit in the current design, where
a Nd:YAG solid state laser is assumed baseline. In a design
upgrade, a compact ECDL module will be integrated within
the unit. Figure3 shows a photograph of the integrated spec-
troscopy board, the corresponding optical layout for modula-
tion transfer spectroscopy and a CAD model of the overall
system level design of the reference.
The design of the ADVANTAGE setup is taken as basis
for an optical reference developed for future GNSS. The
corresponding schematic is shown in Fig.4 where the fre-
quency reference is split into two functional units: the iodine
spectroscopy unit (including laser, fiber-optic components
for beam preparation and frequency doubling) and the iodine
control electronics (including laser driver, temperature
controllers, AOM and EOM drivers, servo control loops for
intensity, RAM and frequency stabilization). The stable light
at a wavelength of 1064nm is input to an optical frequency
comb, which delivers a stable10MHz clock signal.
A component-level breakdown, based on the design
shown in Fig.4 results in an overall mass of 20.1kg (includ-
ing a 20% component level margin), a power consumption
of 43.1W (including a 10% component level margin) and a
volume of 32.3l (without margin) of the optical reference
(without frequency comb).
In‑orbit verication mission
As part of a general technology development roadmap for
future Galileo, DLR plans an in-orbit verification mission,
called COMPASSO. This mission will demonstrate optical
Fig. 3 ADVANTAGE setup
of the iodine reference. Top:
Photograph and schematic of
the spectroscopy board using a
22cm long iodine cell in 4-pass
configuration. Bottom: CAD
at system level, showing the
spectroscopy board within a
thermal shield. The fiber-optic
components for the laser system
(including AOM and EOM)
are mounted to the bottom
side of the thermal shield, the
SHG modules at one of its side
plates. The laser is fiber coupled
to this unit
Fig. 4 Layout of the iodine
reference with interfaces to the
frequency comb, which deliv-
ers the stable10MHz output
signal. ECDL extended cavity
diode laser, AOM acousto-
optical modulator, EOM electro-
optics phase modulator, SHG
second harmonic generation,
VHBG volume holographic
bragg grating, RAM residual
amplitude modulation, NC
noise-canceling photo-detector,
PD photo-detector, DDS direct
digital synthesizer, MO master
oscillator, PA power amplifier,
PID proportional-integral-
derivative servo control, TM/TC
telemetry and telecommand
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GPS Solutions (2021) 25:83
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clock and optical link technologies on the Bartolomeo plat-
form (Steimle etal. 2019) which is externally attached to the
Columbus module of the ISS. Based on the mission concept
presented by Schuldt etal. (2019), the mission feasibility
of COMPASSO is currently investigated within a Phase A
study at DLR where a mission lifetime of 2years is assumed.
The payload consists of two iodine-based frequency refer-
ences as detailed above, together with an optical frequency
comb, a microwave frequency reference and an optical laser
communication and ranging terminal, cf. the architecture
shown in Fig.5. The optical link is used for time- and fre-
quency transfer and synchronization of a ground-based (micro-
wave or optical) frequency reference to the space-based optical
frequency references. Furthermore, it enables data communi-
cation and high-accuracy ranging.
The two iodine-based frequency references can be stabi-
lized to the same or to different (nearby) ro-vibronic transi-
tions. Their frequency stabilities are evaluated by compar-
ing both references in the optical frequency range, i.e., near
282THz (corresponding to a wavelength of 1064nm). The
optical frequency comb can be referenced to the iodine ref-
erence and transfers its frequency stability from the optical
frequency range to the radio frequency range. Furthermore, the
frequency comb can be referenced to an onboard microwave
reference (e.g., a Galileo PHM or RAFS) and thus enables
multiple comparison measurements with which the frequency
stability in the relevant time period of the references can be
evaluated. Using the two-way optical laser communication
and ranging terminal (LCRT) the performance of the optical
references onboard the ISS can additionally be compared to
ground-based clocks.
Acknowledgements This work is supported by the Helmholtz-Gemein-
schaft Deutscher Forschungszentren e.V. under grant number ZT-0007
(ADVANTAGE, Advanced Technologies for Navigation and Geod-
esy) and by the German Space Agency DLR with funds provided by
the Federal Ministry of Economic Affairs and Energy (BMWi) under
grant numbers 50QT1102, 50QT1201, 50WM1646 and 50NA1905.
The iodine reference is investigated within the “Iodine Optical Clock
(IOC)” project under lead of SpaceTech GmbH, Immenstaad, Germany,
with funding by the EU Horizon 2020 program.
Funding Open Access funding enabled and organized by Projekt
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Argence B, Halloin H, Jeannin O, Prat P, Turazza O, de Vismes
E, Auger G, Plagnol E (2010) Molecular laser stabilization at
low frequencies for the LISA mission. Phys Rev D 81:082002.
https:// doi. org/ 10. 1103/ PhysR evD. 81. 082002
Barbarat J, Gillot J, Alvarez-Martinez H etal (2018) Compact and
Transportable Iodine Frequency-Stabilized Laser. In: Proc SPIE
11180, International Conference on Space Optics, ICSO 2018,
111800T. https:// doi. org/ 10. 1117/ 12. 25359 48
Bongs K, Sing Y, Smith L etal (2015) Development of a strontium
optical lattice clock for the SOC mission on the ISS. C R Phys
16:553–564. https:// doi. org/ 10. 1016/j. crhy. 2015. 03. 009
Brewer AM, Chen JS, Hankin AM, Clements ER, Chou CW, Wine-
land DJ, Hume DB, Leibrandt DR (2019) 27Al+ Quantum-logic
clock with a systematic uncertainty below 10–18. Phys Rev Lett
123:033201. https:// doi. org/ 10. 1103/ PhysR evLett. 123. 033201
Cao J, Zhang P, Shang J, Cui K, Yuan J, Chao S, Wang S, Shu H,
Huang X (2017) A compact, transportable single-ion optical
clock with 7.8 × 10–17 systematic uncertainty. Appl Phys B
123:112. https:// doi. org/ 10. 1007/ s00340- 017- 6671-5
Chang P, Zhang S, Shang H, Chen J (2019) Stabilizing didoe laser
to 1 Hz-level Allan deviation with atomic spectroscopy for
Rb four-level active optical frequency standard. Appl Phys B
125:196. https:// doi. org/ 10. 1007/ s00340- 019- 7313-x
Fig. 5 High-level architecture of
the COMPASSO payload
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
GPS Solutions (2021) 25:83
1 3
83 Page 8 of 11
Delehay M, Lacroute C (2018) Single-ion, transportable optical
atomic clocks. J Mod Opt 65(5–6):622–639. https:// doi. org/ 10.
1080/ 09500 340. 2018. 14419 17
Döringshoff K, Schuldt T, Kovalchuk E, Stühler J, Braxmaier C,
Peters A (2017) A flight-like absolute optical frequency refer-
ence based on iodine for laser systems at 1064 nm. Appl Phys B
123:183. https:// doi. org/ 10. 1007/ s00340- 017- 6756-1
Döringshoff K etal (2019) Iodine frequency reference on a sound-
ing rocket. Phys Rev Appl 11:054068. https:// doi. org/ 10. 1103/
PhysR evApp lied. 11. 054068
Droz F, etal. (2006) The onboard Galileo clocks: Rubidium standard
and Passive Hydrogen Maser—current status and performance.
Proceedings of the 20th European Frequency and Time Forum,
pp. 420–426.
Giorgi G etal (2019) Advanced technologies for satellite navigation
and geodesy. Adv Space Res 64(6):1256–1273. https:// doi. org/ 10.
1016/j. asr. 2019. 06. 010
Goujon D etal (2010) Development of the space active hydrogen maser
for the ACES mission.In: Proceedings of the 24th European Fre-
quency and Time Forum, Noordwijk, pp 1–6. https:// doi. org/ 10.
1109/ EFTF. 2010. 65337 30
Gutsch etal (2019) Towards a strontium beam optical reference based
on the 1S0 3P1 intercombination line on a sounding rocket. In:
2019 Joint Conference of the IEEE international frequency control
symposium and european frequency and time forum (EFTF/IFC),
Orlando. https:// doi. org/ 10. 1109/ FCS. 2019. 88560 31
Hannig S etal (2019) Towards a transportable aluminium ion quantum
logic optical clock. Rev Sci Instrum 90:053204. https:// doi. org/
10. 1063/1. 50905 83
Koller SB, Grotty J, Vogt S, Al-Masoudi A, Dörscher S, Häfner S,
Sterr U, Lisdat C (2017) Transportable optical lattice clock with
7 x 10–17 uncertainty. Phys Rev Lett 118:073601. https:// doi. org/
10. 1103/ PhysR evLett. 118. 073601
Kürbis C, Bawamia A, Krüger M, Smol R, Peters A, Wicht A, Tränkle
G (2020) Extended cavity diode laser master-oscillator-power-
amplifier for operation of an iodine frequency reference on a
sounding rocket. Appl Opt 50(2):253–262. https:// doi. org/ 10.
1364/ AO. 379955
Leonhard V, Camp JB (2006) Space interferometry application of
laser frequency stabilization with molecular iodine. Appl Opt
45(17):4142–4146. https:// doi. org/ 10. 1364/ AO. 45. 004142
Lezius M etal (2016) Space-borne frequency comb metrology. Optica
3(12):1381–1387. https:// doi. org/ 10. 1364/ OPTICA. 3. 001381
Martin KW etal (2018) Compact optical atomic clock based on a two-
photon transition in rubidium. Phys Rev Appl 9:014019. https://
doi. org/ 10. 1103/ PhysR evApp lied.9. 014019
Maurice V et al (2020) Miniaturized optical frequency refer-
ence for next-generation portable optical clocks. Opt Express
28(17):24708–24720. https:// doi. org/ 10. 1364/ OE. 396296
McGrew WF etal (2018) Atomic clock performance enabling geodesy
below the centimetre level. Nature 564:87–90. https:// doi. org/ 10.
1038/ s41586- 018- 0738-2
Newman ZL etal (2019) Architecture for the photonic integration of
an optical atomic clock. Optica 6(5):680–685. https:// doi. org/ 10.
1364/ OPTICA. 6. 000680
Nicklaus K etal (2017) High stability laser for next generation gravity
missions. In: Proc SPIE 10563, International Conference on Space
Optics, ICSO 2014, 105632T. https:// doi. org/ 10. 1117/ 12. 23041 61
Nyholm K, Merimaa M, Ahola T, Lassila A (2003) Frequency Stabi-
lization of a Diode-Pumped Nd: Yag Laser at 532 nm to Iodine
by using third-harmonic technique. IEEE Trans Instrum Meas
52(2):284–287. https:// doi. org/ 10. 1109/ TIM. 2003. 811679
Origlia S etal (2018) Towards an optical clock for space: Compact,
high-performance optical lattice clock based on bosonic atoms.
Phys Rev A 98:053443. https:// doi. org/ 10. 1103/ PhysR evA. 98.
Pröbster BJ, Lezius M, Mandel O, Braxmaier C, Holzwarth R (2021)
FOKUS II—space flight of a compact and vacuum compatible
dual frequency comb system. Accepted for publication by JOSA B
Riehle F, Gill P, Arias F, Robertsson L (2018) The CIPM list of recom-
mended frequency standard values: guidelines and procedures.
Metrologia 55(2):188–200. https:// doi. org/ 10. 1088/ 1681- 7575/
Schkolnik V etal (2017) JOKARUS—design of a compact opti-
cal iodine frequency reference for a sounding rocket mis-
sion. EPJ Quantum Technol 4:9. https:// doi. org/ 10. 1140/ epjqt/
s40507- 017- 0063-y
Schuldt T etal (2019) In-orbit-verification of optical clock technolo-
gies. In: Proc. 70th International Aeronautics Conference (IAC),
Schuldt T, Döringshoff K, Kovalchuk E, Keetman A, Pahl J, Peters A,
Braxmaier C (2017) Development of a compact optical absolute
frequency reference for space with 10–15 instability. Appl Opt
56(4):1101–1106. https:// doi. org/ 10. 1364/ AO. 56. 001101
Schuldt T, Döringshoff K, Oswald M, Kovalchuk EV, Peters A, Brax-
maier C (2019) Absolute laser frequency stabilization for LISA.
Int J Mod Phys D 28:12. https:// doi. org/ 10. 1142/ S0218 27181
84500 25
Shang H, Zhang X, Zhang S, Pan D, Chen H, Chen J (2017) Miniatur-
ized calcium beam optical frequency standard using fully-sealed
vacuum tube with 10–15 instability. Opt Express 25(24):30459–
30467. https:// doi. org/ 10. 1364/ OE. 25. 030459
Steimle C, Walz C, Fuchs C, Pedersen D, Lombardi CM (2019) Bar-
tolomeo external platform entering into commercial service.
In: Proc. 70th International Aeronautics Conference (IAC),
Suemasa A, Shimo-oku A, Nakagawa K, Musha M (2017) Develop-
ments of high frequency and intensity stabilized lasers for space
gravitational wave detector DECIGO/B-DECIGO. CEAS Space J
9:485–491. https:// doi. org/ 10. 1007/ s12567- 017- 0151-y
Takamoto M, Ushijima I, Ohmae N, Yahagi T, Kokado K, Shinkai H,
Katori H (2020) Test of general relativity by a pair of transport-
able optical lattice clocks. Nat Photonics 14:411–415. https:// doi.
org/ 10. 1038/ s41566- 020- 0619-8
Ushijima I, Takamoto M, Das M, Ohkubo T, Katori H (2015) Cryo-
genic optical lattice clocks. Nat Photonics 9:185–189. https:// doi.
org/ 10. 1038/ nphot on. 2015.5
Zang EJ, Cao JP, Li Y, Li CY, Deng YK, Gao CQ (2007) Realization of
four-pass i2 absorption cell in 532-nm optical frequency standard.
IEEE Trans Instrum Meas 56(2):673–676. https:// doi. org/ 10. 1109/
TIM. 2007. 890816
Zhang S etal (2017) Compact Rb optical frequency standard with 10–15
stability. Rev Sci Instrum 88:103106. https:// doi. org/ 10. 1063/1.
50069 62
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GPS Solutions (2021) 25:83
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Thilo Schuldt joined the German
Aerospace Center (DLR), Insti-
tute of Space Systems (Bremen)
in 2013 and is group leader for
“System Enabling Technolo-
gies”. He received his Ph.D.
from the Humboldt-Universität
zu Berlin for his work on the
space-based gravitational wave
detector LISA. Thilo Schuldt has
been working on many ESA and
DLR funded projects in optical
metrology, focusing on optical
frequency references and high-
sensitivity laser interferometry.
Martin Gohlke joined the German
Aerospace Center (DLR), Insti-
tute of Space Systems (Bremen)
in 2012. He worked on the opti-
cal ground support equipment
for the GRACE follow-on mis-
sion in cooperation with DLR
institutes and space companies
(Airbus, STI and JPL). LISA
related topics such as laser inter-
ferometry, optical frequency ref-
erences and temperature sensors
are part of his current activities.
Markus Oswald is doing his
Ph.D. on iodine frequency refer-
ences for space at the University
of Bremen. After his master’s
degree from the HTWG Kon-
stanz – University of Applied
Sciences, in 2014, he worked on
an FPGA-based digital laser
locking system. Within the scope
of his Ph.D. program, he has
contributed to the development
and realization of various iodine
frequency reference setups for
Jan Wüst joined the German
Aerospace Center (DLR), Insti-
tute of Space Systems (Bremen)
in 2019. He received his master’s
degree from the HTWG Kon-
stanz – University of Applied
Sciences. The thesis was about
residual amplitude modulation in
modulation transfer spectroscopy
for frequency stabilized laser
systems. Within the ADVAN-
TAGE project, he is responsible
for the mechanical and thermal
design of the iodine spectros-
copy subsystem.
Tim Blomberg joined the Ger-
man Aerospace Center (DLR),
Institute of Space Systems
(Bremen) in 2018. He received
his master’s degree from the
RWTH Aachen University for
the development, integration and
test of an isostatic mounting for
the iodine spectroscopy board of
ADVANTAGE. Assembly, inte-
gration and test activities within
ADVANTAGE are part of his
current responsibilities.
Klaus Döringshoff joined the
Ferdinand-Braun-Institut, Leib-
niz-Institut für Höchstfrequenz-
technik (Berlin) in 2020, and
works on the development of
optical frequency references
based on high resolution laser
spectroscopy. He received his
Ph.D. from the Humboldt-Uni-
versität zu Berlin in 2018 for his
work on the optical frequency
references based on ro-virbronic
transitions in molecular iodine.
Ahmad Bawamia received a
bachelor’s degree in mathemat-
ics and physics at Université
Bordeaux I in Bordeaux, France
in 2001 and his master’s in opto-
electronics engineering at école
nationale supérieure des sciences
appliquées et de technologie in
Lannion, France in 2004. He
then joined the Ferdinand-
Braun-Institut, Leibniz-Institut
für Höchstfrequenztechnik
(FBH) in Berlin, Germany in
2005, working on the improve-
ment of the spatial beam charac-
teristics of broad-area semicon-
ductor lasers. He obtained his doctoral degree at the technical
university of Berlin in 2011. Since 2010, Ahmad is a full-time scientist
at FBH, where he specialized in the development of micro-integrated
semiconductor laser modules for precision spectroscopy applications,
especially in space.
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
GPS Solutions (2021) 25:83
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Andreas Wicht heads the Joint
Lab Quantum Photonic Compo-
nents at FBH. He is an expert in
the development of micro-inte-
grated diode laser modules for
applications that require stable
lasers, including quantum optics
with ultra-cold atoms. After
receiving his Ph.D. from the
University of Hannover, he
joined Stanford University as a
postdoc in 1999 to work on pre-
cision atom interferometers.
Between 2002 and 2008 he was
affiliated with the Universität of
Düsseldorf as an assistant pro-
fessor. In 2008 he joined the FBH to establish the Joint Lab Quantum
Photonic Components, a Joint Lab between the FBH and the Hum-
boldt-Universität zu Berlin.
Matthias Lezius is group leader
and senior scientist for “Space
Combs” at Menlo Systems
GmbH in Martinsried. He
received his Ph.D. at the Inns-
bruck University in Austria for
laser-interaction with rare-gas
clusters. Since 2010 he has man-
aged various DLR and ESA
funded projects advancing the
technology readiness, robustness
and compactness of optical fre-
quency combs. These combs
have subsequently been used for
scientific experiments on
TEXUS sounding rockets.
Kai Voss is the head of optical
instruments group at SpaceTech
GmbH (STI). He received his
Phd. in physics in 2004 and is
working on laser-optical systems
for space since 2011. He has
been the system engineer at STI
for the Laser ranging interferom-
eter on GRACE FO, has led sev-
eral technology studies and hard-
ware projects.
Markus Krutzik is head of the
Joint Lab Integrated Quantum
Sensors (IQS) operated by Ferdi-
nand-Braun-Institut, Leibniz-
Institut für Höchstfrequenztech-
nik (FBH) and the
Humboldt-Universität zu Berlin.
R&D activities focus on the
development of integrated
atomic systems for frequency
metrology, timing and field sens-
ing applications. He got his
Ph.D. in 2014 (HU Berlin),
worked at the University of Cali-
fornia, Berkeley, and NASA Jet
Propulsion Laboratory,
Sven Herrmann is a research
scientist at the Center of Applied
Space Technology and Micro-
gravity at the University of
Bremen. He received his Ph.D.
from the Humboldt-Universität
zu Berlin for his work on optical
cavity-based frequency refer-
ences for tests of fundamental
physics. He has since contrib-
uted to several DLR and ESA
funded activities in the field of
fundamental physics, quantum
sensing and cold atoms.
Evgeny Kovalchuk is a research
scientist at the Humboldt-Uni-
versität zu Berlin, where he
received his Ph.D. for his work
on optical parametric oscillators
for precision IR spectroscopy
and metrology. His main
research interests are laser stabi-
lization, femtosecond combs,
optical clock development, pre-
cision measurements, optical
frequency metrology and phase
noise measurements.
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
GPS Solutions (2021) 25:83
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Achim Peters is professor for
Optical Metrology at the physics
department of the Humboldt-
Universität zu Berlin. Since 2008
he is also affiliated with the Fer-
dinand-Braun-Institut, Leibniz-
Institut für Höchstfrequenztech-
nik (FBH) and the Joint Lab
Quantum Photonic Components.
He obtained his Ph.D. in physics
at Stanford University in 1998
and has since then worked on
many projects pursuing a variety
of precision measurements as
well as advancing the related
Claus Braxmaier is chair of
Space Technology, University of
Bremen, in cooperation with
DLR. Braxmaier is head of the
DLR-department “System Ena-
bling Technologies” in Bremen.
He received a Ph.D. from the
University of Konstanz and has
been working on many ESA and
DLR funded projects in space
sciences, gravity missions, cold
atom optics, quantum metrology,
frequency references and laser
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... But, to achieve an error in a range below 1 m, for example, the timing error must be better than three nanoseconds. Numerous studies have discussed the possibility of optical clocks being employed onboard GNSS satellites [10][11][12][13][14][15][16][17][18]; however, an optical clock is yet to be deployed on a GNSS satellite mission. ...
... [10,11] demonstrated a compact rubidium optical clock (CROC) based on two-photon transition achieving 4 × 10 −13 / τ(s) frequency stability over averaging times (τ) up to 10,000 s, which was attractive meeting the technical requirement of GPS Block III satellites, though its long-term stability after that period was not appealing. Whereas German Aerospace Center (DLR) developed a different set-up of an optical clock named Iodine Modulation Transfer Spectroscopy (MTS) with exceptional short-term stability of approximately 10 −15 from 1 to 10,000 s, and they advocated the clock for the next generation of Galileo satellites (Kepler) with an inter-satellite link and orbit verification applications [12,17,18]. Further, a miniaturised Strontium (Sr) Lattice optical clock based on bosonic atoms was introduced by [13] with 4.1 × 10 −16 / τ(s), which experienced almost 10 −18 stability after a couple of hours. ...
... For example, optical lattice clocks that require hundreds to thousands of 87,88 Strontium, 171,174 Ytterbium, or 199 Mercury atoms in the system while 27,40 Aluminum, 88 Calcium + , 171 Ytterbium + , and 199 Mercury + ions are used in making trapped ion optical clocks [3]. There are other clock types simultaneously developed as well, namely Rubidium twophoton transition and modulation transfer spectroscopy clocks [12]. ...
Full-text available
Atomic clocks are highly precise timing devices used in numerous Positioning, Navigation, and Timing (PNT) applications on the ground and in outer space. In recent years, however, more precise timing solutions based on optical technology have been introduced as current technology capabilities advance. State-of-the-art optical clocks—predicted to be the next level of their predecessor atomic clocks—have achieved ultimate uncertainty of 1 × 10−18 and beyond, which exceeds the best atomic clock’s performance by two orders of magnitude. Hence, the successful development of optical clocks has drawn significant attention in academia and industry to exploit many more opportunities. This paper first provides an overview of the emerging optical clock technology, its current development, and characteristics, followed by a clock stability analysis of some of the successfully developed optical clocks against current Global Navigation Satellite System (GNSS) satellite clocks to discuss the optical clock potentiality in GNSS positioning. The overlapping Allan Deviation (ADEV) method is applied to estimate the satellite clock stability from International GNSS Service (IGS) clock products, whereas the optical clock details are sourced from the existing literature. The findings are (a) the optical clocks are more stable than that of atomic clocks onboard GNSS satellites, though they may require further technological maturity to meet spacecraft payload requirements, and (b) in GNSS positioning, optical clocks could potentially offer less than a 1 mm range error (clock-related) in 30 s and at least 10 times better timing performance after 900 s in contrast to the Galileo satellite atomic clocks—which is determined in this study as the most stable GNSS atomic clock type used in satellite positioning.
... Solutions can be found in already existing technology such as laser communication terminals [146,149,150] and optical clocks [151,152]. ...
Investigating and verifying the connections between the foundations of quantum mechanics and general relativity will require extremely sensitive quantum experiments. To provide ultimate insight into this fascinating area of physics, the realization of dedicated experiments in space will sooner or later become a necessity. Quantum technologies, and among them quantum memories in particular, are providing novel approaches to reach conclusive experimental results due to their advanced state of development backed by decades of progress. Storing quantum states for prolonged time will make it possible to study Bell tests on astronomical baselines, to increase measurement precision for investigations of gravitational effects on quantum systems, or enable distributed networks of quantum sensors and clocks. We here promote the case of exploiting quantum memories for fundamental physics in space, and discuss both distinct experiments as well as potential quantum memory platforms and their performance.
... While most of existing laboratories have developed their own solutions for operating the complex optical clock, there are few papers that report their electronic circuits in details [11][12][13][14]. At the same time, autonomously operated optical atomic clocks are becoming more common and needed for new technology applications [15][16][17] and new definition of the SI second [18,19]. ...
Full-text available
We present an open-source hardware and software ecosystem for optical atomic clocks. We provide PCB schematics and fabrication files for manufacturing the most important electronic systems together with the required software. The boards are designed for an active bad-cavity superradiant strontium clock (ASC) and a passive optical lattice strontium clock (OLC), but they can be easily adapted to other atomic species optical atomic clocks or ultra-cold atoms’ systems like magneto-optical traps (MOT) or Bose-Einstein Condensate (BEC) setups.
We investigate the general relativistic phase of an electromagnetic wave as it propagates in the gravitational field of the Earth, which is modeled as an isolated, weakly aspherical gravitating body. We introduce coordinate systems to describe light propagation in the Earth’s vicinity along with the relevant coordinate transformations, and discuss the transformations between proper and coordinate times. We represent the Earth’s gravitational field using Cartesian symmetric trace-free (STF) mass multipole moments. The light propagation equation is solvable along the trajectory of a light ray to all STF orders ℓ. Although we focus primarily on the quadrupole (ℓ=2), octupole (ℓ=3), and hexadecapole (ℓ=4) cases, our approach is valid to all orders. We express the STF moments via spherical harmonic coefficients of various degree and order, Cℓk,Sℓk. The result is the gravitational phase shift expressed in terms of the spherical harmonics. These results are new. We also consider contributions due to tides and the Earth’s rotation. We estimate the characteristic magnitudes of each term of the resulting overall gravitational phase shift. The terms assessed are either large enough to impact current-generation clocks or will become significant as future-generation clocks offer greater sensitivity. Our formulation is useful for many practical and scientific applications, including space-based time and frequency transfers, relativistic geodesy and navigation, as well as quantum communication links and space-based tests of fundamental physics.
Full-text available
This article reports on the use of an FPGA platform for local ultra-stable optical frequency distribution through a typically 90 m-long fiber network. This platform is used to implement a fully digital treatment of the Doppler-cancellation scheme required by fiber links to be able to distribute ultra-stable frequencies. We present a novel protocol that uses aliased images of a digital synthesizer output to directly generate signals above the Nyquist frequency. This approach significantly simplifies the set-up, making it easy to duplicate within a local fiber network. We demonstrate performances enabling the distribution of an optical signal with an instability below 10^(-17) at one second at the receiver end. We also use the board to implement an original characterization method. It leads to an efficient characterization of the disturbance rejection of the system, that can be realized without accessing the remote output of the fiber link.
We describe a high-performance optical frequency reference based on dual-frequency sub-Doppler spectroscopy (DFSDS) using a Cs vapor microfabricated cell and an external-cavity diode laser at 895 nm. Measured against a reference optical signal extracted from a cavity-stabilized laser, the microcell-stabilized laser demonstrates an instability of 3 × 10 ⁻¹³ at 1 s, in agreement with a phase noise of +40 dBrad ² /Hz at 1-Hz offset frequency, and below 5 × 10 ⁻¹⁴ at 10 ² s. The laser short-term stability limit is in good agreement with the intermodulation effect from the laser frequency noise. These results suggest that DFSDS is a valuable approach for the development of ultra-stable microcell-based optical standards.
Optical atomic clocks produce highly stable frequency standards and frequency combs bridge clock frequencies with hundreds of terahertz difference. In this paper, we propose a hybrid clock scheme, where a light source pumps an active optical clock through a microresonator-based nonlinear third harmonic process, serves as a passive optical clock via indirectly locking its frequency to an atomic transition, and drives a chip-scale microcomb whose mode spacing is stabilized using the active optical clock. The operation of the whole hybrid system is investigated through simulation analysis. The numerical results show: (i) The short-term frequency stability of the passive optical clock follows an Allan deviation of σ y ( τ ) = 9.3 × 10 ⁻¹⁴ τ −1/2 with the averaging time τ , limited by the population fluctuations of interrogated atoms. (ii) The frequency stability of the active optical clock reaches σ y ( τ ) = 6.2 × 10 ⁻¹⁵ τ −1/2 , which is close to the quantum noise limit. (iii) The mode spacing of the stabilized microcomb has a shot-noise-limited Allan deviation of σ y ( τ ) = 1.9 × 10 ⁻¹¹ τ −1/2 . Our hybrid scheme may be realized using recently developed technologies in (micro)photonics and atomic physics, paving the way towards on-chip optical frequency comparison, synthesis, and synchronization.
We present a high-performance laser frequency stabilization method using modulation transfer spectroscopy (MTS) on the rubidium ⁸⁷ D 2 transition line. A substantial improvement of the laser frequency stability was achieved by searching for the optimal diameter and intensity settings of the probe and pump beam. The frequency instability measured from the beat frequency of two locked external cavity diode lasers (ECDLs) reached a short-term stability of 4.5 × 10 − 14 / τ and did not exceed 2 × 10 ⁻¹² until 10 ⁵ s, which is the best performance reported thus far with a D 2 transition. The long-term stability is limited by the offset fluctuations of the baseline induced by the residual amplitude modulation (RAM), which can be further improved by reducing the current temperature variation of about 0.2 K by means of temperature stabilization or through a further reduction of the RAM.
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Bartolomeo is a new external payload hosting platform on the International Space Station's Columbus module. Bartolomeo offers 12 new external payload sites, all of them at the forward-facing side of Columbus. Payloads are accommodated using the General-purpose Oceaneering Latching Device 2 (GOLD-2) which enables full robotic servicing of the facility. As a standard Bartolomeo offers to host payloads in a range of 3 Cubesat units up to 0.56 cubic meters corresponding to 450 kg. Smaller payloads down to 3U size are accommodated in the ArgUS multi-payload frame installed on one standard slot. Designed to user requirements from the commercial and institutional sector, Bartolomeo complements the space station with its unique capabilities and resupply logistics with unique features: access to best viewing angles in nadir, zenith and limb directions with minimal obstructions from other ISS elements, choice between unpressurized and pressurized launch of payloads to ISS, compatibility with all ISS payload airlocks, return option, enhanced data downlink capability through optical communication, and easy access to space with standardized payload interfaces. Payload sites on the new facility are accessible to customers worldwide through a commercial contract. With a lead time of 18 months, the Bartolomeo Mission Service offers end-to-end mission integration with standardized interfaces definition to the user. Payloads, thereby, benefit directly from the partnership with the ISS program providing frequent access to space. The Bartolomeo platform will enable customers to use LEO more frequently, quicker and at lower cost supporting competitiveness and growth of the industrial sector, especially for small and medium enterprises and academic institutions who are yet unexperienced in using space. With the Bartolomeo platform scheduled for launch in March 2020 and the installation scheduled thereafter, the paper will focus on the preparation of the operational phase. Bartolomeo introduces a new operational concept to external payloads on the space station: all payloads can be operated by the customer from ground through a web-based console using the functionality of the Columbus MultiPurpose Computer & Communication system. Customer payload operations are supported by the platform level monitoring and commanding by the Bartolomeo Control Center at Airbus. Both the payload and the platform-level commanding functions are implemented through the new Airbus Cloud which is part of Airbus' digitalization initiative, using the Columbus MultiPurpose Computer and Communication capabilities All platform and payload operations are monitored by the Columbus Control Center which remains in charge of resources and activities planning. With this new operations scenario Bartolomeo introduces a new, payload operator-oriented service of conducting a space mission in LEO in a low effort and cost-efficient way.
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A clock at a higher altitude ticks faster than one at a lower altitude, in accordance with Einstein’s theory of general relativity. The outstanding stability and accuracy of optical clocks, at 10−18 levels1–5, allows height differences6 of a centimetre to be measured. However, such state-of-the-art clocks have been demonstrated only in well-conditioned laboratories. Here, we demonstrate an 18-digit-precision frequency comparison in a broadcasting tower, Tokyo Skytree, by developing transportable optical lattice clocks. The tower provides the clocks with adverse conditions to test the robustness and a 450 m height difference to test the gravitational redshift at (1.4 ± 9.1) × 10−5. The result improves ground-based clock comparisons7–9 by an order of magnitude and is comparable with space experiments10,11. Our demonstration shows that optical clocks resolving centimetres are technically ready for field applications, such as monitoring spatiotemporal changes of geopotentials caused by active volcanoes or crustal deformation12 and for defining the geoid13,14, which will have an immense impact on future society. A pair of transportable optical lattice clocks with 10−18 uncertainty is developed. The relativistic redshift predicted by the theory of general relativity has been tested at the 10–5 level by the two optical clocks with a height difference of 450 m on the ground.
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We achieve a compact ultra-stable 420 nm blue diode laser system by immediately stabilizing the laser on the hyperfine transition line of Rb atom. The Allan deviation of the residual error signal reaches 1 Hz-level Allan deviation within 1 s averaging time, and the fractional frequency Allan deviation is \(1.4\times 10^{-15}/\sqrt{\tau }\), which shows the best result of frequency-stabilized lasers based on the atomic spectroscopy without Pound–Drever–Hall (PDH) system. The signal-to-noise ratio of the atomic spectroscopy is evaluated to be 3,000,000 from the Allan deviation formula, which is the highest record, to the best of our knowledge. The frequency noise suppression characterization is demonstrated and the maximal noise suppression can be near 40 dB at 6 Hz. As a good candidate of pumping source, the ultra-stable 420 nm diode laser is successfully used in our Rb four-level active optical frequency standard system. The method can be easily extended to other wavelengths ultra-stable lasers with a Allan deviation of \(10^{-15}\) level retaining an atomic reference with low cost and low complexity while in the absence of an expensive and complicated PDH system.
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Within the OPUS collaboration, we develop optical frequency references based on spectroscopy of the 1S0→3P1 intercombination line of 88Sr at 689 nm in thermal and laser-cooled strontium beams for operation on sounding rockets. One main objective is to identify suitable experimental setups and core components for a compact and robust strontium beam clock, providing a stepping stone for future spaceborne devices. Recent results demonstrate successful laser stabilization to the inter-combination line in a thermal beam breadboard setup.
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This manuscript reviews recent progress in optical frequency references and optical communication systems and discusses their utilizations in global satellite navigation systems and satellite geodesy. Lasers stabilized with optical cavities or spectroscopy of molecular iodine are analyzed, and a hybrid architecture is proposed to combine both forms of stabilization with the aim of achieving a target frequency stability of 1e-15 [s/s] over a wide range of sampling intervals. The synchronization between two optical frequency references in real-time is realized by means of time and frequency transfer on optical carriers. The technologies enabling coherent optical links are reviewed, and the development of an optical communication system for synchronization, ranging and data communication in space is described. An infrastructure exploiting the capabilities of both optical technologies for the realization of a modernized constellation of navigation satellites emitting highly synchronized signals is reviewed. Such infrastructure, named Kepler system, improves satellite navigation in terms intra-system synchronization, orbit determination accuracy, as well as system monitoring and integrity. The potential impact on geodetic key parameters is addressed.
Frequency combs downconvert absolute optical frequency references and thereby can significantly advance time and frequency precision in satellite-based navigation systems, fundamental science, earth observation, and many other spaceborne applications. We have developed a compact and vacuum compatible double comb system, thereby minimizing volume, mass, and power consumption compared to its precursor. Apart from redundancy aspects, the two combs enable autonomous mode number determination. Here, we report on the space comb system design and experimental results of a zero-gravity parabolic space flight in connection with an iodine-referenced cw laser system.
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.
Conference Paper
Optical frequency references are an essential tool for many applications in space, including Earth observation, fundamental physics and navigation and ranging. They are either needed as light source for a high-sensitivity inter-spacecraft optical metrology system, as part of a payload enabling tests of fundamental physics or as a high accuracy timebase for global navigation satellite systems (GNSS). Examples are the space-based gravitational wave detector LISA (Laser Interferometer Space Antenna), future missions measuring the Earth’s gravitational field within the NGGM (Next Generation Gravity Mission) program and the proposed space-based test of Special Relativity BOOST (BOOst Symmetry Test). Furthermore, new concepts for satellite navigation foresee optical frequency references in combination with optical links used for synchronization, communication and ranging. Technology development for future operation of optical frequency references in space is currently carried out where crucial design parameters are compactness and rigidity. First setups of iodine-based frequency references on Elegant Breadboard (EBB) and Engineering Model (EM) level have been realized in a cooperation of DLR Bremen, University of Bremen and Humboldt-University Berlin and a compact iodine reference was successfully flown on a sounding rocket within the project JOKARUS, lead by HU Berlin. A long-term stable cavity-based system is currently developed at DLR Bremen/ University of Bremen in the context of the BOOST mission; development of an Engineering Model in the context of NGGM and as clock laser for an optical clock is currently ongoing within the OSRC (Optical Stabilizing Reference Cavity) project by ESA. Optical terminals are commercially provided by Tesat GmbH, developed in cooperation with DLR Oberpfaffenhofen. We present a specific mission concept for in-orbit verification of optical frequency references on a small satellite in low-Earth orbit, called COFROS (Compact Optical Frequency References on a Satellite). The payload consists of an iodine- and a cavity-based frequency reference, together with an optical frequency comb and a GNSS receiver and, eventually, an optical terminal. The optical frequency references are operated at a wavelength of 1064 nm and directly compared in a beat measurement. Additionally, both references are compared to GNSS (GPS; Galileo) via the optical frequency comb and the GNSS receiver. The optical link is used for synchronization of a ground-based oscillator to the space-based optical frequency reference. A first mission scenario was worked out and a preliminary budget estimation shows compatibility with the DLR compact satellite bus.
We describe an optical atomic clock based on quantum-logic spectroscopy of the S01↔P30 transition in Al+27 with a systematic uncertainty of 9.4×10−19 and a frequency stability of 1.2×10−15/τ. A Mg+25 ion is simultaneously trapped with the Al+27 ion and used for sympathetic cooling and state readout. Improvements in a new trap have led to reduced secular motion heating, compared to previous Al+27 clocks, enabling clock operation with ion secular motion near the three-dimensional ground state. Operating the clock with a lower trap drive frequency has reduced excess micromotion compared to previous Al+27 clocks. Both of these improvements have led to a reduced time-dilation shift uncertainty. Other systematic uncertainties including those due to blackbody radiation and the second-order Zeeman effect have also been reduced.