US 11,515,994 B2

11

the relative time offset of one pulse from another without
distortion of measurement by amplitude variations by one of
two methods. First, a dither can be introduced in timing of
the tracking frequency comb 202 with a dither frequency
higher than the typical amplitude fluctuation frequency, and
an average time offset over one cycle of this dither that can
be extracted via processing by the signal processor-control-
ler 204. Second, with the optional inclusion of the CW laser
242 and CW heterodyne detector 208, then simultaneous
amplitude measurements of the signal from the CW hetero-
dyne detector 208 are combined with the signals from the
comb timing discriminator 206 in the time-frequency offset
signals 219 that can be analyzed by the signal processor-
controller 204 to generate the relative time offset of the
incoming clock comb pulse and local tracking comb pulse
without distortion of measurement by amplitude variations.

Synchronizer 200 provides initial acquisition of the
received clock frequency comb 203 pulses from remote site
synchronizer 200.2 at synchronizer 200 relative to the local

tracking frequency comb 202 pulses. ‘The acquisition mode 2

is controlled by algorithms implemented on the signal
processor-controller 204, in which the timing of the tracking
frequency comb 202 pulses is repeatedly scanned over up to
the full time window set by the inverse repetition frequency

of the frequency combs. The signal processor-controller 204 2

provide rapid switching between this acquisition mode and
the steady-state tracking operational mode wherein the
tracking frequency comb 202 is locked onto the clock
frequency comb 203 as described in the above two para-
graphs. In the event of long signal dropouts from turbulence,
or obstructions or any other event, the signal processor-
controller 204 will revert to acquisition mode.
Synchronizer 200 provides absolute time comparison of
the clock frequency comb 203 pulses at Synchronizer
200 and the clock frequency comb 203 pulses at the
remote site synchronizer 200.2 by initial calibration of
the fixed time delay offsets within the free-space or
fiber optic paths of the Synchronizer 200 and by
periodic measurements of the time offset between the
clock frequency comb 203 and the tracking frequency
comb 202 through a second comb timing discriminator
206, whose signals are recorded and processed in the
signal processor-controller 204. In this way, any phase
slips in the phase locks of either the clock frequency
comb 203 or the tracking frequency comb 202 at either
site are removed and do not cause ambiguity in the time
comparison. In addition, to avoid the time ambiguity
equal to the inverse comb repetition frequency, which
is otherwise inherent in the time-frequency offset sig-

nals 219 output by the comb timing discriminator 206, s

the communicator 215 system is used to implement a
standard two-way time transfer protocol that measures
the relative time offset without ambiguity and with a
precision that is below the inverse repetition frequency
of the clock frequency comb 203.

‘Components of synchronizer 200 can be made from and
include various materials. Frequency reference oscillator
201 can an optical or radiofrequency (RF) oscillator. Exem-
plary optical oscillators include a narrow-linewidth laser,
cavity-stabilized laser, cavity-stabilized laser steered to an
atomic transition, and the like. Exemplary RF oscillators
include a quartz oscillator, dielectric resonator oscillator
(DRO), RF oscillator optionally steered to an atomic tran-
sition, and the like. Various oscillators are described in U.S.
Pat. Nos. 10,389,514 and 9,557,625, the disclosures of each
are incorporated by reference in their entirety. Frequency
reference oscillator 201 produces frequency reference signal

&

&

12

227 that can have a frequency from 20 kilohertz (kHz) to
700 terahertz (THz), specifically from 1 gigahertz (GHz) to
500 THz, and more specifically from 500 GHz to 400 THz.

It should be appreciated that synchronizer 200 performs
time-frequency transfer over optical link 218. It can be
operated in one of four modes: frequency comparison, time
comparison, frequency syntonization, and time synchroni-
zation. The latter two require the real-time communication
channel 214 between the two sites, the two-way processor
241, and clock comb feedback 213. In certain embodiments,
the outgoing optical timing signals 216 includes both the
clock comb signal 224 and the CW optical signal 226 that is
transferred over optical link 218 as synchronization data 270
between connected synchronizers 200. For time comparison
or synchronization, the communication channel 214 is also
used to implement a conventional two-way time transfer
protocol between the synchronizers 200.

To accomplish synchronizing transfer over optical link
218, synchronizer 200 includes clock frequency comb 203.
Clock frequency comb 203 is a self-referenced optical
frequency comb that produces clock comb signal 224
Exemplary components of the clock frequency comb 203 are
shown in FIG. 9. Clock comb signal 224 is an optical pulse
train output from clock frequency comb 203 that can be
filtered to a selected bandwidth and can have a wavelength
that is, e.g., around 1550 nm. It is contemplated that clock
comb signal 224 can be amplified to provide additional
power to the outgoing clock comb signal 224 or to an
external user. Additionally, clock frequency comb 203 pro-
duces clock output 210. Clock output 210 is an optical pulse
train or RF pulse train output signal and can include data for
comb locking parameters that define RF or optical frequen-
cies with respect to frequency reference signal 227. Clock
output 210 can include a counter for the number of optical
or RF pulses that is used to define the local time scale.

Tracking frequency comb 202 also receives frequency
reference signal 227 from frequency reference oscillator 201
and can include control electronics that deterministically
alter a phase or timing of output of tracking frequency comb
202 relative to frequency reference signal 227. Exemplary
components of tracking frequency comb 202 are shown in
FIG. 8. Here, tracking frequency comb 202 produces track-
ing comb signal 225. It is contemplated that tracking fre-
quency comb 202 also can produce a parallel output signal
that contains data, wherein the data can be corrections for a
paper clock (e.g., real-time or delayed). An exemplary paper
clock, outputs tracking comb signal 225 and a similar
parallel output signal containing data. Tracking comb signal
225 is an optical pulse train output from tracking frequency
comb 202 that can be filtered to match a bandwidth of clock
comb signal 224.

With reference, e.g., to FIG. 1, FIG. 2, FIG. 4, FIG. 6, and
FIG. 7, comb timing discriminator 206 receives a plurality
of signals and interferes input comb pulse streams (e.g.,
incoming clock comb signal 229 and tracking comb signal
225 or the clock frequency comb 203 and tracking comb
signal 225) to produce signals (e.g.. time offset signal 232 or
the clock comb-tracking comb time offset 212,) that are
analyzed to determine a relative time offset of incoming
comb pulse streams. In this regard, clock comb-tracking
comb time offset 212 can be such an output of comb timing
discriminator 206.

Tracking optical timing source 209 can be locked onto an
incoming optical timing signal (¢.g., clock comb signal 224,
frequency reference signal 227). Locking can be actively
locking or virtually locking by tracking any difference.
Tracking optical timing source 209 can include tracking



U.S. Patent Nov. 29, 2022 Sheet 13 of 13 US 11,515,994 B2

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FIG. 13


US 11,515,994 B2

19

coupled into the fiber optic carrying the incoming optical
timing signals 217 at the other synchronizer or over single-
mode fiber optic and connecting communicator 215 to
second synchronizer 200.2 over the communication channel
214 by a free-space terminal for a free-space link or over
fiber optic for a fiber link or over coaxial cable for a wired
link. The fiber optic connections can be made by polariza-
tion maintaining single-mode fiber or single-mode fiber.

The process for making synchronizer 200 also can include
calibration of the time delays for the signals throughout
synchronizer 200 by use of an optical time domain reflec-
tometer, or equivalent measurement instrument, in both the
optical domain for fiber-optic or free-space optical connec-
tions and an time domain reflectometer, or similar instru-
ment, in the RF domain for coaxial or wired connections
The process for making synchronizer 200 also can include
combining the function of the communicator 215 with the
outgoing optical timing signals 216 by inserting a phase or
amplitude modulator on either the outgoing clock comb
signal 224 or outgoing CW optical signal 226 prior to the
bidirectional terminal 207.

Synchronizer 200 has numerous advantageous and unex-
pected benefits and uses. In an embodiment, a process for
synchronizing transfer over an optical link with synchro-
nizer 200 includes: producing, by frequency reference oscil-
lator 201, frequency reference signal 227; receives, by
tracking frequency comb 202, frequency reference signal
227 from frequency reference oscillator 201; produces, by
tracking frequency comb 202, tracking comb signal 225:
receiving, by tracking optical timing source 209, tracking
comb feedback 211 from signal processor-controller 204;
producing, by tracking optical timing source 209, tracking
optical timing signal 220 based on tracking comb feedback
211; receiving, by comb timing discriminator 206, tracking
comb signal 225 from tracking optical timing source 209:
receiving by comb timing discriminator 206, clock comb
signal 224 from clock frequency comb 203; producing, by
comb timing discriminator 206, clock comb-tracking comb
time offset 212 from clock comb signal 224 and tracking
comb signal 225; receiving, by clock frequency comb 203,
frequency referen nal 227 from frequency reft
oscillator 201; producing, by clock frequency comb 203,
clock comb signal 224 based on frequency reference signal
227; producing, by clock frequency comb 203, clock output
210 based on frequency reference signal 227; communicat-
ing, by clock frequency comb 203, clock comb signal 224 to
comb timing discriminator 206 and bidirectional terminal
207; receiving, by signal processor-controller 204, clock
comb-tracking comb time offset 212 from comb timing

discriminator 206; receiving, by signal processor-controller_ 5

204, time-frequency offset signals 219 from time-frequency
offset measurement system 205; producing, by signal pro-
cessor-controller 204 based on clock comb-tracking comb
time offset 212 and time-frequency offset signals 219, track-

ing comb feedback 211 and local one-way data 221; receiv- 5

ing, by time-frequency offset measurement system 205,
tracking optical timing signal 220 from tracking optical
timing source 209; receiving, by time-frequency offset mea-
surement system 205, incoming optical timing signals 217
from bidirectional terminal 207; producing, by time-fre-
quency offset measurement system 205, time-frequency
offset signals 219 from tracking optical timing signal 220
and incoming optical timing signals 217; communicating, by
time-frequency offset measurement system 205, time-fre-
quency offset signals 219 to signal processor-controller 204;
receiving, by bidirectional terminal 207, clock comb signal
224 from clock frequency comb 203; receiving, by bidirec-

20

tional terminal 207, synchronization data 270 from optical
link 218, synchronization data 270 including incoming
optical timing signals 217; communicating, by bidirectional
terminal 207, incoming optical timing signals 217 to time-
frequency offset measurement system 205 from optical link
218; and communicating, by bidirectional terminal 207,
clock comb signal 224 to optical link 218 from clock
frequency comb 203 as part of synchronization data 270.

In an embodiment, synchronizing transfer over an optical
link further includes receiving, by communicator 215, local
one-way data 221 from signal processor-controller 204;
communicating, by communicator 215, side information
228 with signal processor-controller 204; and producing, by
communicator 215, remote one-way data 222 from local
one-way data 221.

Synchronizing transfer over an optical link can include
communicating, by communicator 215 via communication
channel 214, with second synchronizer 200.2.

Synchronizing transfer over an optical link can include
receiving, by two-way processor 241, local one-way data
221 from signal processor-controller 204; receiving, by the
two-way processor 241, the remote one-way data 222 from
the communicator 215; producing, by the two-way proces-
sor 241, two-way output data 223 and clock comb feedback
213 based on the local one-way data 221 and the remote
data 222; and communicating, by the two-way
sor 241, the clock comb feedback 213 to the clock

Synchronizing transfer over an optical link can include
receiving, by dispersion compensation unit 256 of second
comb timing discriminator 206, incoming clock comb signal
229 from incoming optical timing signals 217 and tracking
comb signal 225 from tracking optical timing signal 220;
communicating, by dispersion compensation unit 256,
incoming clock comb signal 229 and tracking comb signal
225 to signal combiner-splitter 252 of second comb timing
discriminator 206; receiving, by signal combiner-splitter
252, incoming clock comb signal 229 and tracking comb
signal 225 from dispersion compensation unit 256; produc-
ing, by signal combiner-splitter 252, combined coherent
optical pulse trains 257 from incoming clock comb signal
229 and tracking comb signal 225; receiving, by leading arm
delay stage 254 of second comb timing discriminator 206.2,
combined coherent optical pulse trains 257 from signal
combiner-splitter 252; producing, by leading arm delay
stage 254, leading coherent optical pulse trains 259 from
combined coherent optical pulse trains 257; receiving, by
lagging arm delay stage 253 of second comb timing dis-
criminator 206.2, combined coherent optical pulse trains 257
from signal combiner-splitter 252; producing, by lagging
arm delay stage 253, lagging coherent optical pulse trains
258 from combined coherent optical pulse trains 257;
receiving, by leading arm delay stage 254 of second comb
timing discriminator 206, the combined coherent optical
pulse trains 257 from signal combiner-splitter 252; produc-
ing, by leading arm delay stage 254, leading coherent optical
pulse trains 259 from combined coherent optical pulse trains
257; receiving, by second signal combiner-splitter 252 of
second comb timing discriminator 206, leading coherent
optical pulse trains 259 from leading arm delay stage 254;
receiving, by second signal combiner-splitter 252, lagging
coherent optical pulse trains 258 from lagging arm delay
stage 253; producing, by second signal combiner-splitter
252, optical timing signals 260 from leading coherent opti-
cal pulse trains 259 and lagging coherent optical pulse trains
258; receiving, by fixed delay stage 255 of second comb
timing discriminator 206.2, optical timing signals 260 from



U.S. Patent Nov. 29, 2022 Sheet 4 of 13 US 11,515,994 B2

| 220
2174
205
206.2
229-——| +995
252.1 «eH 252.2
2291—4 229.2 225.14 225.2
255.1 255.2 253 || 254
229.24 258
229.1-— 259 — 231
252.3 252.4
257.1—| 257.3—|
257.24 257.44
261.1 261.2



US 11,515,994 B2

13

frequency comb 202 and optionally a continuous wave
(CW) coherent laser 242 that can be phase-locked with a
known frequency to frequency reference oscillator 201 or
tracking frequency comb 202. Tracking optical timing
source 209 produces tracking optical timing signal 220 that
can include tracking comb signal 225 and optionally CW
optical signal 226.

Time-frequency offset measurement system 205 measures
a relative time and frequency of incoming optical timing
signals such as incoming optical timing signals 217 relative
to tracking optical timing signal 220. In time-frequency
offset measurement system 205, comb timing discriminator
206 measures the relative time offset between the incoming
clock comb signal 229 of incoming optical timing signals
217 and the tracking comb signal 225 of tracking optical
timing signal 220 from tracking frequency comb 202. In
time-frequency offset measurement system 205, CW hetero-
dyne detector 208 can be a photoreceiver (e.g.. balanced or
otherwise) that detects a heterodyne signal between an

output of CW laser 242 carried by CW optical signal 226 and. 2

incoming signal from second synchronizer 200 of incoming
CW optical signal 230 carried by the incoming optical
timing signals 217. Time-frequency offset measurement
system 205 produces time-frequency offset signals 219 that
is processed by signal processor-controller 204, Time-fre-
quency offset signals 219 includes time offset signal 232
from second comb timing discriminator 206.2 and frequency
offset signal 231 from CW heterodyne detector 208.

Bidirectional terminal 207 is a free-space terminal for
physical acquisition and tracking of a free-space optical link
218 (c.g., a single-mode optical link) among first synchro-
nizer 200.1 and second synchronizer 200.2. Bidirectional
terminal 207 can also include a beacon laser or other article
for acquiring and tracking another synchronizer. Bidirec-
tional terminal 207 can combine outgoing optical timing
signals 216 (e.g., clock comb signal 224, CW optical signal
226) by a splitter, combiner, or wavelength division multi-
plexer or can divide incoming optical timing signals 217 for
input to time-frequency offset measurement system 205. If
optical link 218 is over fiber optics rather than free-space,
Bidirectional terminal 207 is a fiber-optic terminal that
similarly combine outgoing optical timing signals 216 and
divides incoming optical timing signals 217.

Signal processor-controller 204 receives time-frequency
offset signals 219 from time-frequency offset measurement
system 205. Signal processor-controller 204 performs vari-
ous functions that include communicating control signals to
adjust the frequency or phase (e.g., timing) of tracking
frequency comb 202 or clock frequency comb 203; com-

municating local one-way data 221 and side information 228 5

from communicator 215, wherein side information 228 can
include data such as current comb parameters; receiving
one-way data and side information from a remote site (e.g..
second synchronizer 200.2). Signal processor-controller 204

also can transmits and receive data from two-way processor 5

241 to provide information on relative time-of-flight, clock
timing difference, or doppler shift between the local and
remote site. Signal processor-controller 204 can apply a
tunable low-pass filter to time-frequency offset signals 219
to improve signal-to-noise ratio. A center frequency of the
low-pass filter can be tracked dynamically based on the
available information a priori or from data received via
communication channel 214 or based on time-frequency
offset signals 219. Data can be decimated to reduce com-
putational effort after filtering. The bandwidth of the low-
pass filter can be adjusted depending on a current measured
optical coherence time across optical link 218. Optionally,

&

&

14

comb time discriminator signals can have an increased
bandwidth by using knowledge from the CW heterodyne
beat signal to extend the optical coherence time. Signal
processor-controller 204 can process signals from time-
frequency offset signals 219 to estimate the time shift in
seconds between tracking frequency comb 202 and incom-
ing clock comb signal 229. Signal processor-controller 204
process signals from time-frequency offset signals 219 and
estimate an excess frequency shift (ie., a measured fre-
quency shift compared to an expected center frequency of
the low-pass filter) between incoming optical timing signals
217 and tracking optical timing signal 220. This frequency
shift can be present between comb signals and CW hetero-
dyne signals. Signal processor-controller 204 can implement
a tracking mode, wherein the processed time-frequency
signals are used to feedback to tracking frequency comb 202
so that its time offset against the incoming clock comb signal
is driven towards zero. Signal processor-controller 204 can
implement an acquisition mode, wherein a search algorithm
is performed over frequency and time for initial acquisition
of clock comb-tracking comb time offset 212 and time-
frequency offset signals 219. The search algorithm can use
side information 228 received from communicator 215.
Signal processor-controller 204 tracks signal loss on time-

5 frequency offset signals 219 and determines when to switch

from tracking mode to search mode. Depending on a dura-
tion of signal loss, the signal process can select different
search mode patterns to reduce the acquisition time.

Signal processor-controller 204 produces tracking comb
feedback 211. Tracking comb feedback 211 are control
signals that are used to adjust phase and time of tracking
frequency comb 202, e.g., as provided with reference to
tracking frequency comb 202 in FIG. 8. As a frequency
comb, tracking frequency comb 202 has two degrees-of-
freedom that can be its carrier phase/frequency (ie., a
frequency of one defined comb tooth in the center of the
transmitted optical frequency band) and its pulse repetition
phase/timing. The Tracking comb feedback 211 controls
both these degrees of freedom, e.g., to adjust the phase/
timing of the pulses from tracking frequency comb 202 to
align with the incoming clock comb signal 229 pulses and to
simultaneously adjust the carrier frequency of the tracking
frequency comb 202 to be offset from the carrier frequency
of the tracking frequency comb 202 in order to match the
center frequency of the low pass filter on the time-frequency
offset signals 219, as implemented in the signal processor-
controller 204.

Signal processor-controller 204 also produces local one-
way data 221. Local one-way data 221 is a combined data
stream from the site, including the clock comb-tracking
comb time offset 212, time-frequency offset signals 219,
diagnostics and phase-lock detection signals 237 of tracking
frequency comb 202 and clock frequency comb 203, track-
ing comb feedback 211, clock comb feedback 213, and
additional timing data from the Communicator 215 if imple-
mented as a two-way time transfer over the communication
channel 214 to remove ambiguities. Tracking comb feed-
back 211, time offset signal 232 from the time-frequency
offset signals 219, clock comb-tracking comb time offset
212, and any transceiver calibration offsets are combined to
give the one-way time-of-arrival of the incoming clock
comb signal 229 compared to the clock frequency comb 203.

Communicator 215 communicates with signal processor-
controller 204 and provides communication along commu-
nication channel 214 between first synchronizer 200.1 and.
second synchronizer 200.2. Communicator 215 provides
coarse timing data to remove time or frequency ambiguities



US 11,515,994 B2

25

combs 202 that both exclude clock frequency comb 203. The
relative timing from the two tracking frequency combs is
recorded by the signal processer and transmitted over the
communication link to the two remote synchronizers 200.1
and 200.2. This information generates timing information to
achieve frequency comparison, frequency syntonization, or
time synchronization between synchronizer 200.1 and syn-
chronizer 200.2.

It should be appreciated that synchronizer 200 and syn-
chronizing transfer over an optical link can be used for a
network of optical or rf clocks that are synchronized for use
in navigation or coherent sensing. It should be appreciated
that synchronizer 200 and synchronizing transfer over an
optical link also provides the time-of-flight and range infor-
mation between sites with high precision so that it can
support multi-vehicle formation flying for generation of
coherent synthetized apertures for active or passive detec-
tion. The system has the benefit that it provides timing,
frequency, and range information simultaneously between
defined reference planes at both sites. This information is
available at 1000 times higher precision and accuracy than
with conventional microwave-based methods. The timing
precision is less than one femtosecond. The ranging preci-
sion is less than a micron, and the frequency precision is less
than one part in 10!° in fractional units, wherein all these
precisions include any systematic uncertainties. Unlike con-
ventional optical time transfer approaches, this method
achieves this level of precision without a power penalty, i.e.,
at the shot noise limit. Furthermore, as the system can use
the same fiber optic technology as that used by the optical
communication industry, it is compatible with future optical
free-space communication links and can be implemented in
a low size-weight and power package suitable for operation
outside of the laboratory, on airborne platforms, or on
spaceborne platforms.

Synchronizer 200 and processes disclosed herein have
numerous beneficial uses, including establishing free-space
optical clock networks that can distribute time and fre-
quency either regionally or globally. Because the system can
operate at the shot-noise limit without excess power penalty,
this network can extend across long distances at low tran
mit power and low transmit aperture size, while simultane-
ously providing 1000 times greater higher performance than
conventional microwave-based time-frequency transfer and
provides long distance optical clock networks. If the optical
clock network uses high performance optical clocks at each
node of the network, it supports fundamental scientific
experiments in relativity, in geodesy, or in investigation of
violation of current understanding of the physical world.

Alternatively, the clock network could have a single main 5

optical clock, and the synchronizer could synchronize mul-
tiple secondary optical clocks. In this configuration, the
optical clock network could be used for time-frequency
distribution and to support precision navigation. The syn-

chronizer could also be used to establish distributed timing s

across a network for coherent sensing, either passive or
active, in either the optical or radiofrequency domain. For
this coherent sensing application, the synchronizer addition-
ally provides precise ranging information between sites and
including orientation. The synchronizer could compare the
time between two distant optical clocks for geodetic appli-
cations with accuracy at the sub-millimeter level after incor-
porating atmospheric corrections. Advantageously, for these
applications, the synchronizer 200 overcomes limitations
and technical deficiencies of conventional devices that use
radiofrequency or microwave signals. These conventional
microwave-based approaches only provide timing precision

20

3

&

&

26

at the picosecond level because of systematic errors and only
achieve ranging precision at the cm level. Similarly, con-
ventional optically-based approaches in which an radiofre-
quency or microwave modulation is applied to an optical
carrier, as for example in optical communication, also only
provide timing precision at the picosecond level because of
systematic errors, and only achieve ranging precision at the
cm level. Synchronizer 200 overcomes these limitations by
a factor of 1000 because it maintains the timing in the optical
domain. Other optical timing approaches that exploit linear
optical sampling have been demonstrated over kilometer
distances. Compared to these approaches, the synchronizer
can operate at 1000 times lower received power without any
penalty in performance because the synchronizer does not
suffer from the power penalty associated with linear optical
sampling. This reduction in received power has significant
advantages since it can translate to much longer distance
operation, for example from the ground to a satellite. Alter-
natively, the lower received power can allow the use of 1000
times lower transmit power for the same range of operation,
dramatically reducing the physical size and electrical power
consumption required for the time-frequency transfer. This
reduction in the required received power is accomplished by
use of the tracking comb, which is unique to this implemen-
tation of optical time transfer.

Synchronizer 200 and processes herein unexpectedly the
ability to operate at the standard quantum limit for timing
distribution. By use of the combination of the tracking comb
and coherent integration over the coherence time of the
atmosphere, the synchronizer can successfully operate at
less than one photon per optical pulse from the comb.
Synchronizer 200 was operated at a fractional occupation
number as low as 2s”. The system can operate even if most
(24 out of 25) comb pulses do not contain a photon. This is
possible through the use of the tracking comb and coherent
integration over the coherence time of the atmosphere. As a
result, this power threshold for operation is over 1000 times
lower than the power threshold for previous optical time
transfer approaches. Unexpectedly, operation at this low
power does not cause additional performance degradation,
and the synchronizer can still provide timing comparison or
synchronization at the femtosecond level, frequency com-
parison or syntonization at fractional levels of one part in
10°, and ranging information at the micron-level. The
synchronizer provides the unexpected ability to establish
satellite-based optical time transfer for global time distribu-
tion, large scale optical clock networks for ultra-precise
navigation and timing as well as scientific experiments, and
coherent sensing through distributed arrays at high micro-
wave frequencies or optical frequencies. The low power
threshold and correspondingly extended range provide the
novel capability of a compact synchronizer compatible with
the constraints of airborne or spaceborne instruments, not
provided by conventional devices or conventional processes.
Moreover, by inclusion of optical atmospheric spectroscopy
on the same timing signals, the system provides the unex-
pected capability of atmosphere-corrected geodetic ranging
at the sub-millimeter level.

While one or more embodiments have been shown and
described, modifications and substitutions may be made
thereto without departing from the spirit and scope of the
invention. Accordingly, it is to be understood that the present
invention has been described by way of illustrations and not
limitation. Embodiments herein can be used independently
or can be combined.

All ranges disclosed herein are inclusive of the endpoints,
and the endpoints are independently combinable with each


US 11,515,994 B2

17

comparator 269 compares the phase of the two input signals
where this phase is the comparator data 273.

As part of the time-frequency offset measurement system
205 shown in FIG. 4, FIG. 6, and FIG. 7, the signal
combiner-splitter 252 is a four-port free-space or fiber-optic
50/50 combiner. With a single input (e.g. the incoming clock
comb signal 229 or the tracking comb signal 225), its output
is two copies of the input signals (the incoming clock comb
signal 229.1 and incoming clock comb signal 229.2 or
tracking comb signal 225.1 and tracking comb signal 225.2).
With two inputs (e.g. the incoming clock comb signal 229
and the tracking comb signal 225), its output is two copies
‘of combined coherent optical pulse trains 257. The disper-
sion compensation unit 256 is a fiber-optic or other medium
with dispersion whose length is adjusted such that the
relative dispersion between the two input comb pulses (from
incoming clock comb signal 229 and tracking comb signal
225 or clock comb signal 224 and tracking comb signal
225), is close to zero.

As part of the time-frequency offset measurement system
205 embodied in FIG. 4, the lagging arm delay stage 253 and
leading arm delay stage 254 are both just delay lines in fiber
optic or free space where the difference in their delay is
equal to some fraction of the pulse width, which is typically
on the order of 0.5 psec. the fixed delay stage 255 is a delay
line where all instances have the same delay. The delayed
optical output of the lagging arm delay stage 253 is the
lagging coherent optical pulse trains 258 and the delayed
optical output of the leading arm delay stage 254 is the
leading coherent optical pulse trains 259. The output of
signal combiner-splitter 252 is combined coherent optical
pulse trains 257 of the two input signals. The photodetector
unit 261 and be a single or balanced photodetector.

As part of the time-frequency offset measurement system
205, ninety-degree polarization rotator 243 rotates the polar-
ization of the input incoming clock comb signal 229 by
ninety degrees to generate rotated comb pulse train 248. The
polarization preserving combiner 244 combines the tracking
and clock comb pulse trains with now perpendicular polar-
izations to generate two copies of the polarization combined
pulse trains 249.1 and polarization combined pulse trains
249.2. The polarization combined pulse trains 249.1 is
transmitted through the birefringent lagging arm delay stage
245 to create a fixed delay between the tracking and comb
pulse trains. Similarly, the polarization combined pulse
trains 249.2 is transmitted through the birefringent leading
arm delay stage 246 to create a different fixed delay between
the tracking and comb pulse trains. The lengths of the

birefringent lagging arm delay stage 245 and birefringent s

leading arm delay stage 246 are set such that their differ-
ential delay between the tracking and comb pulse trains is
fraction of the pulse width, typically around 0.5 psec. The
output lagging polarization combined pulse trains 250 is sent

into a forty-five-degree polarization splitter 247 that com- 5

bines the orthogonally polarized tracking and clock combs at
45 degrees to ge polarization aligned combined coher-
ent optical pulse trains 257.1 that are photo detected by a
balanced (or non-balanced) photodetector unit 261. The
output leading polarization combined pulse trains 251 is sent
into a second forty-five-degree polarization splitter 247 that
combines the orthogonally polarized tracking and clock
combs at 45 degrees to generate polarization aligned com-
bined coherent optical pulse trains 257.2 that are photo
detected by a balanced (or non-balanced) photodetector unit
261.2. The outputs of the photodetector unit 261.1 and
photodetector unit 261.1 are the time offset signal 232.

18

As part of the time-frequency offset measurement system
205 embodied in FIG. 6, the low-band BPF 262 is a low pass
optical bandpass filter that transmits optical frequencies
below some center frequency and does not transmit optical
frequencies above some center frequency. The filtered out-
put of the low-band BPF 262 is the low-band optical signal
264. The high-band BPF 263 is a high pass optical bandpass
filter that transmits optical frequencies below some center
frequency and does not transmit optical frequencies above
some center frequency. The filtered output of the high-band
BPF 263 is the high-band optical signal 265. The center
frequency of both bandpass filters is selected to be near the
center of the optical frequency spectrum of both the clock
and tracking comb pulse trains. The pulse interference
detector 266 is a balanced or single photoreceiver whose
voltage output is the low-band interference signal 267 or
high-band interference signal 268.

‘Synchronizer 200 can be made in various ways. Synchro-
nizer 200 includes a number of optical and electrical com-
ponents, wherein such components can be interconnected
and placed in communication (e.g., optical communication,
electrical communication, and the like) by physical or free-
space interconnects. The components can be disposed on
mounts that can be disposed on a bulkhead for alignment and

s physical compartmentalization. As a result, synchronizer

200 can be disposed in a terrestrial environment or space
environment.

In an embodiment, a process for making synchronizer 200
includes: providing frequency reference oscillator 201,
clock frequency comb 203, tracking frequency comb 202,
bidirectional terminal 207, comb timing discriminator 206,
tracking optical timing source 209, time-frequency offset
measurement system 208, signal processor-controlier 204,
optionally two-way processor 241 and communicator 215;
connecting clock frequency comb 203 to frequency refer-
ence oscillator 201 by free-space or fiber optic path; con-
necting tracking optical timing source 209 to frequency
reference oscillator 201 by free-space or fiber optic path;
connecting tracking optical timing source 209 to comb
timing discriminator 206 by free-space or fiber optic path;
connecting clock frequency comb 203 to comb timing
discriminator 206 by free-space or fiber optic path; connect-
ing comb timing discriminator 206 to signal processor-
controller 204 by co-axial or other wired connection; con-
necting clock frequency comb 203 to bidirectional terminal
207 by free-space or fiber optic path; connecting bidirec-
tional terminal 207 to tracking optical timing source 209 by
free-space or fiber optic path; connecting signal processor
controller 204 to time-frequency offset measurement system
205 by co-axial or other wired connection; connecting signal
processor-controller 204 to tracking optical timing source
209 by co-axial or other wired connection; connecting
time-frequency offset measurement system 205 to tracking
optical timing source 209 by free-space or fiber optic path;
connecting time-frequency offset measurement system 205
to bidirectional terminal 207 by free-space or fiber optic
path; connecting two-way processor 241 to signal processor-
controller 204 by co-axial or other wired connection; con-
necting two-way processor 241 to clock frequency comb
203 by co-axial or other wired connection; connecting signal
processor-controller 204 to communicator 215 by co-axial
or other wired connection; connecting two-way processor
241 to communicator 215 by co-axial or other wired con-
nection; connecting bidirectional terminal 207 to second
synchronizer 200.2 either over free space by pointing and
tracking of the bidirectional terminal 207 such that the
outgoing optical timing signals 216 at one synchronizer is



US 11,515,994 B2

7

output of the lagging arm delay stage 253 is the lagging
coherent optical pulse trains 258 that is in communication
with the signal combiner-splitter 252.3 that received the
incoming clock comb signal 229.1 from the Fixed delay
stage 255.1. The output of the leading arm delay stage 254
is the leading coherent optical pulse trains 259 that is in
communication with the second signal combiner-splitter
252.4 that received the second copy of the incoming clock
comb signal 229.2 from the Fixed delay stage 255.2. The
output of signal combiner-splitter 252.3 and signal com-
biner-splitter 252.4 are combined coherent optical pulse
trains 257 that are in communication with independent
photodetector unit 261.1 and photodetector unit 261.2,
respectively. The outputs of photodetector unit 261.1 and
photodetector unit 261.2 produces time offset signal 232. In
addition, time-frequency offset measurement system 205
can further includes an optional CW heterodyne detector
208. If the optional CW heterodyne detector 208 is included,
it receives incoming CW optical signal 230 from incoming

optical timing signals 217 and CW optical signal 226 from 2

tracking optical timing signal 220 and produces frequency
offset signal 231 from incoming CW optical signal 230 and
CW optical signal 226. Time-frequency offset measurement
system 205 combines time offset signal 232 from second
comb timing discriminator 206.2 and frequency offset signal
231 from CW heterodyne detector 208 into time-frequency
offset signals 219.

In an embodiment, with reference to FIG. 6, time-fre-
quency offset measurement system 205 further includes CW
heterodyne detector 208, and second comb timing discrimi-
nator 206.2 that includes dispersion compensation unit 256
that receives incoming clock comb signal 229 from incom-
ing optical timing signals 217 and tracking comb signal 225
from tracking optical timing signal 220. Dispersion com-
pensation unit 256 also communicates incoming clock comb
signal 229 and tracking comb signal 225 to signal combiner-
splitter 252. Signal combiner-splitter 252 is in communica-
tion with dispersion compensation unit 256 and receives
incoming clock comb signal 229 and tracking comb signal
225 from dispersion compensation unit 256, and produces
combined coherent optical pulse trains 257 from incoming
clock comb signal 229 and tracking comb signal 225.
High-band band pass filter (BPF) 263 is in communication
with signal combiner-splitter 252 and receives combined
coherent optical pulse trains 257 from signal combiner-
splitter 252 and produces high-band optical signal 265 from
combined coherent optical pulse trains 257. Low-band BPF
262 is in communication with signal combiner-splitter 252
and receives combined coherent optical pulse trains 257

from signal combiner-splitter 252 and produces low-band

optical signal 264 from combined coherent optical pulse
trains 257. Pulse interference detector 266 is in communi-
cation with high-band BPF 263 and receives high-band
optical signal 265 from high-band BPF 263 and produces

high-band interference signal 268 from high-band optical

signal 265. Second pulse interference detector 266.2 is in
communication with low-band BPF 262 and receives low-
band optical signal 264 from low-band BPF 262, and
produces low-band interference signal 267 from low-band
optical signal 264. Phase comparator 269 is in communica-
tion with pulse interference detector 266 and second pulse
interference detector 266.2 and receives high-band interfer-
ence signal 268 from pulse interference detector 266,
receives low-band interference signal 267 from second pulse
interference detector 266.2, and produces time offset signal
232 from high-band interference signal 268 and low-band
interference signal 267. CW heterodyne detector 208

8

receives incoming CW optical signal 230 from incoming
optical timing signals 217 and CW optical signal 226 from
tracking optical timing signal 220 and produces frequency
offset signal 231 from incoming CW optical signal 230 and
CW optical signal 226. Time-frequency offset measurement
system 205 combines time offset signal 232 from second
comb timing discriminator 206.2 and frequency offset signal
231 from CW heterodyne detector 208 into time-frequency
offset signals 219.

In an embodiment, with reference to FIG. 7, time-fre-
quency offset measurement system 205 includes an optional
CW heterodyne detector 208, and second comb timing
discriminator 206.2 that includes dispersion compensation
unit 256 that receives incoming clock comb signal 229 from
incoming optical timing signals 217 and tracking comb
signal 225 from tracking optical timing signal 220 and
communicates incoming clock comb signal 229 and tracking
comb signal 225 to signal combiner-splitter 252. Signal
combiner-splitter 252 is in communication with dispersion
compensation unit 256 and receives incoming clock comb
signal 229 and tracking comb signal 225 from dispersion
compensation unit 256, and produces combined coherent
optical pulse trains 257 from incoming clock comb signal
229 and tracking comb signal 225. Photodetector unit 261 is
in communication with signal combiner-splitter 252 and
receives combined coherent optical pulse trains 257 from
signal combiner-splitter 252 and produces time offset signal
232 from combined coherent optical pulse trains 257. If the
optional CW heterodyne detector 208 is included, it receives
incoming CW optical signal 230 from incoming optical
timing signals 217 and CW optical signal 226 from tracking
optical timing signal 220 and produces frequency offset
signal 231 from incoming CW optical signal 230 and CW
optical signal 226. Time-frequency offset measurement sys-
tem 205 combines time offset signal 232 from second comb
timing discriminator 206.2 and the optional frequency offset
signal 231 from CW heterodyne detector 208 into time-
frequency offset signals 219.

In an embodiment, with reference to FIG. 8, tracking
frequency comb 202 includes comb digital controller 234 in
communication with phase-lock detection unit 236 and that
receives tracking comb feedback 211, receives phase-lock
detection signals 237 from phase-lock detection signals 237,
and produces actuator control signals 235 from tracking
comb feedback 211 and phase-lock detection signals 237.
Frequency comb optics package 233 is in communication
with comb digital controller 234 and receives actuator
control signals 235 from comb digital controller 234 and
produces tracking comb signal 225 based on actuator control
signals 235. Phase-lock detection unit 236 is in communi-
cation with frequency comb optics package 233 and comb
digital controller 234 and receives tracking comb signal 225
from frequency comb optics package 233, receives fre-
quency reference signal 227 from frequency reference oscil-
lator 201, and produces phase-lock detection signals 237
from tracking comb signal 225 and frequency reference
signal 227.

In an embodiment, with reference to FIG. 9, clock fre-
quency comb 203 includes comb digital controller 234 in
communication with frequency reference oscillator 201,
phase-lock detection unit 236, and frequency comb optics
package 233 and that optionally receives clock comb feed-
back 213 from two-way processor 241, receives phase-lock
detection signals 237 from phase-lock detection unit 236,
and produces actuator control signals 235 from clock comb
feedback 213 and phase-lock detection signals 237. Fre-
quency comb optics package 233 is in communication with



U.S. Patent Nov. 29, 2022 Sheet 9 of 13 US 11,515,994 B2

203
224
a3 ) .
235—~|
237
234 ) 236
2B 2I~

FIG. 9


US 11,515,994 B2

37

receiving, by the phase comparator, the synthesizer data
from the phase-frequency agile synthesizer and the
conditioned phase-lock detection signals from the sig-
nal conditioning:

comparing, by the phase comparator, the synthesizer data
to the conditioned phase-lock detection signals;

producing, by the phase comparator, comparator data
from comparing the synthesizer data to the conditioned
phase-lock detection signals;

communicating the comparator data to a loop filter of the
comb digital controller from the phase comparator; and

receiving, by the loop filter, the comparator data from the
phase comparator;

producing, by the loop filter, the actuator control signals
from the comparator data; and

communicating the actuator control signals from the loop
filter to the frequency comb optics package.

ee eee

0

38


US 11,515,994 B2

31

communicates the incoming clock comb signal and
the tracking comb signal to a signal combiner-
splitter;
the signal combiner-splitter in communication with the
dispersion compensation unit and that receives the
incoming clock comb signal and the tracking comb
signal from the dispersion compensation unit, and
produces a combined coherent optical pulse trains
from the incoming clock comb signal and the track-
ing comb signal; and
a photodetector unit in communication with the signal
combiner-splitter and that receives the combined
coherent optical pulse trains from the signal com-
biner-splitter and produces a time offset signal from
the combined coherent optical pulse trains; and
the CW heterodyne detector receives an incoming CW
optical signal from the incoming optical timing signals
and CW optical signal from the tracking optical timing

signal and produces a frequency offset signal from the 2

incoming CW optical signal and the CW optical signal,
wherein the time-frequency offset measurement system
combines the time offset signal from the second comb
timing discriminator and the frequency offset signal
from the CW heterodyne detector into the time-fre-
quency offset signals.

8. The synchronizer of claim 1, wherein the tracking

frequency comb compris:
acomb digital controller in communication with a phase-
Jock detection unit and that receives the tracking comb
feedback, receives a phase-lock detection signals from
the phase-lock detection signals, and produces a actua-
tor control signals from the tracking comb feedback

and the phase-lock detection signals;

a frequency comb optics package in communication with
the comb digital controller and that receives the actua-
tor control signals from the comb digital controller and
produces the tracking comb signal from the actuator
control signals; and

the phase-lock detection unit in communication with the
frequency comb optics package and the comb digital
controller and that receives the tracking comb signal
from the frequency comb optics package. receives the
frequency reference signal, and produces the phase-
lock detection signals from the tracking comb signal
and the frequency reference signal.

9. The synchronizer of claim 1, wherein the clock fre-

quency comb comprises:

comb digital controller in communication with the
frequency reference oscillator, a phase-lock detection
unit, and a frequency comb optics package and that
optionally receives a clock comb feedback from a
two-way processor, receives a phase-lock detection

signals from the phase-lock detection unit, and pro- s

duces an actuator control signals from the clock comb
feedback and the phase-lock detection signals;

the frequency comb optics package in communication
with the comb digital controller and that receives the
actuator control signals from the comb digital control-
ler and produces the clock comb signal from the
actuator control signals; and

the phase-lock detection unit in communication with the
frequency comb optics package and the comb digital
controller and that receives the clock comb signal from
the frequency comb optics package, receives the fre-
quency reference signal from the frequency reference

5 controller compris

40

&

50

0

32

oscillator, and produces the phase-lock detection sig-
nals from the clock comb signal and the frequency
reference signal

10. The synchronizer of claim 1, wherein the comb digital

s:

a phase-frequency agile synthesizer that receives the
tracking comb feedback or clock comb feedback and
produces synthesizer data from the tracking comb
feedback;

a signal conditioning that receives the phase-lock detec-
tion signals, conditions the phase-lock detection signals
to produce conditioned phase-lock detection signals,
and communicates the conditioned phase-lock detec-
tion signals to a phase comparator;

the phase comparator in communication with the phase-
frequency agile synthesizer and the signal conditioning
and that receives the synthesizer data from the phase-
frequency agile synthesizer and conditioned phase-lock
detection signals from the signal conditioning, com-
pares the synthesizer data to the conditioned phase-lock
detection signals to produce comparator data, and com-
municates the comparator data to a loop filter; and

the loop filter in communication with the phase compara-
tor and that receives the comparator data from the
phase comparator and produces the actuator control
signals from the comparator data.

11. A process for synchronizing transfer over an optical

link with the synchronizer, the process comprising:
producing, by the frequency reference oscillator, the fre-
quency reference signal;

receiving, by the tracking frequency comb, the frequency
reference signal from the frequency reference oscilla-
tor;

producing, by the tracking frequency comb, a tracking
comb signal;

receiving, by the tracking optical timing source, the
tracking comb feedback from the signal processor-
controller;

producing, by the tracking optical timing source, the
tracking optical timing signal based on the tracking
comb feedback;

receiving, by the comb timing discriminator, the tracking
comb signal from the tracking optical timing source;

receiving by the comb timing discriminator, the clock
comb signal from the clock frequency comb;

producing, by the comb timing discriminator, the clock
comb-tracking comb time offset from the clock comb
signal and the tracking comb signal;

receiving, by the clock frequency comb, the frequency
reference signal from the frequency reference oscilla-
tor;

producing, by the clock frequency comb, the clock comb
signal based on the frequency reference signal;

producing, by the clock frequency comb, the clock output
based on the frequency reference signal;

communicating, by the clock frequency comb, the clock
comb signal to the comb timing discriminator and the
bidirectional terminal;

receiving, by the signal processor-controller, the clock
comb-tracking comb time offset from the comb timing
discriminator;

receiving, by the signal processor-controller, the time-
frequency offset signals from the time-frequency offset
measurement system;



U.S. Patent Nov. 29, 2022 Sheet 6 of 13 US 11,515,994 B2

| 220
217
205
206.2 229
229
252
257—| 257
263 262
t+ 265 264— 231
266 266
P68 267—|
269
2324
219-4

FIG. 6


US 11,515,994 B2

Sheet 5 of 13

Nov. 29, 2022

U.S. Patent

—~ 234

\249

246

244

243

245

57

247

|

261

248

249

247

1257 2571

261

S71

4
4

FIG. 5


US 11,515,994 B2

9

comb digital controller 234 and receives actuator control
signals 235 from comb digital controller 234 and produces
clock comb signal 224 based on actuator control signals 235.
Phase-lock detection unit 236 is in communication with
frequency comb optics package 233 and comb digital con-
troller 234 and receives clock comb signal 224 from fre-
quency comb optics package 233, receives frequency refer-
ence signal 227 from frequency reference oscillator 201, and
produces phase-lock detection signals 237 from clock comb
signal 224 and frequency reference signal 227.

In an embodiment, with reference to FIG. 10, comb digital
controller 234 includes phase-frequency agile synthesizer
239 that receives tracking comb feedback 211 or clock comb
feedback 213 and produces synthesizer data 271 from track-
ing comb feedback 211 or clock comb feedback 213. Signal
conditioning 238 receives phase-lock detection signals 237,
conditions phase-lock detection signals 237 to produce
conditioned phase-lock detection signals 272, and commu-
nicates conditioned phase-lock detection signals 272 to
phase comparator 269. Phase comparator 269 is in commu-
nication with phase-frequency agile synthesizer 239 and
signal conditioning 238 and receives synthesizer data 271
from phase-frequency agile synthesizer 239 and conditioned
phase-lock detection signals 272 from signal conditioning
238, compares synthesizer data 271 to conditioned phase-
lock detection signals 272 to produce comparator data 273,
and communicates comparator data 273 to loop filter 240.
Loop filter 240 is in communication with phase comparator
269 and receives comparator data 273 from phase compara-
tor 269 and produces actuator control signals 235 from
comparator data 273.

With reference to FIG, 2 and FIG. 3, synchronizer 200,
also referred to herein as photon-efficient agile comb optical
clock synchronizer (PEACOCS), can be connected to other
synchronizers 200 in a network of synchronizer that
includes at least two PEACOCSs as timing units, wherein
each synchronizer 200 includes a clock and tracking fre-
quency comb (202, 203), comb timing discriminator 206,
and signal processor-controller 204. With reference to FIG.
11, one of synchronizers 200 is at each end of free-space
optical link 218. Within each synchronizer 200, one fre-
quency comb is the clock frequency comb 203, and the
second comb is the tracking frequency comb 202. Pulses
from the clock frequency comb 203 are sent over the optical
link 218 to synchronizer 200 located at the opposite side of
optical link 218. The clock frequency comb 203 signal from
the link are received in the time-frequency offset measure-
ment system 205 and optically combined with the tracking
frequency comb 202 pulses. The time-frequency offset sig-

nals 219 from the comb timing discriminator 206 in the s

time-frequency offset measurement system 205 are directed
to the signal processor-controller 204 which returns a rela-
tive timing of the received clock comb pulses in the incom-
ing optical timing signals 217 against the tracking frequency

comb 202 pulses. These time offsets are recorded and, s

processed in the signal processor-controller 204 to generate
a tracking comb feedback 211 to the tacking frequency
comb 202 so that pulses from tracking frequency comb 202
remain overlapped in time and frequency with the received
clock comb pulses in the incoming optical timing signals
217. The tracking comb feedback 211 from the signal
processor-controller 204 can adjust the phase (time) of the
tracking frequency comb 202 arbitrarily from femtoseconds
to nanoseconds and its carrier frequency over the full range
of the repetition frequency in order to track the incoming
clock comb pulses. By performing this operation at both
ends of optical link 218, the local one-way data 221 at each

20

&

&

10

site, and specifically the time offset of the tracking frequency
comb 202 from the incoming clock comb as measured by the
sum of tracking comb feedback 211 and time-frequency
offset signals 219 from the comb timing discriminator 206,
can be combined to suppress the time-of-flight fluctuations
(from atmospheric turbulence or platform motion). This
operation yields the time offset between the clock frequency
comb 203 at each site to better than a femtosecond. This
operation also yields the relative frequencies between the
frequency reference oscillator 201 of the two sites at a
fractional uncertainty of better than 107'*. In the case of
synchronization, the measured relative frequency offset is
used to apply a clock comb feedback 213 to the clock
frequency comb 203 at one site to correct the frequency. In
the case of synchronization, the measured relative time
offset between the clock combs is used apply to a clock
comb feedback 213 to the clock frequency comb 203 at one
site to correct the time.

Synchronizer 200 achieves this level of performance by
the tracking frequency comb 202, time-frequency offset
measurement system 205, and the signal processor-control-
ler 204 that together allow the tracking frequency comb 202
to lock onto the incoming clock frequency comb 203 signal
from the opposite site at extremely low received powers,

5 corresponding to much less than one photon per comb pulse.

This is accomplished by shot-noise limited detection within
the time-frequency offset measurement system 205, fol-
lowed by coherent integration of the tracking optical timing
signal 220 over a time window of the turbulence-limited
optical coherence time across the optical link 218. With
optional inclusion of CW laser 242 and CW heterodyne
detector 208, the integration time of the signal from comb
timing discriminator 206 can be extended to longer integra-
tion times than the turbulence-limited optical coherence
time. The combination of the shot-noise limited detection
and coherent integration yields an enhancement of 5000-
times signal-to-noise over conventional optical-based time
transfer without penalty in timing performance. The signal-
to-noise enhancement is advantageous for long distance
operation, wherein an operational range of the optical link
218 is increased by 70-times for a given transmit power
budget as a result of the tracking frequency comb 202,
time-frequency offset measurement system 205, and the
signal processor-controller 204.

Detection of an arrival time of the received clock fre-
quency comb 203 pulses from remote site synchronizer
200.2 at synchronizer 200 relative to the local tracking
frequency comb 202 pulses can be accomplished in syn-
chronizer 200 by timing discriminator comb timing dis-
criminator 206. Comb timing discriminator 206 can include
two-tap interferometer (FIG. 4) constructed from free-space
or fiber-optic components. The two-tap interferometer (FIG.
4) detects relative time offset of one pulse from another
without distortion of measurement by amplitude variations
by comparison of the heterodyne detection from the lagging
coherent optical pulse trains 258 and leading coherent
optical pulse trains 259. In an embodiment, comb timing
discriminator 206 includes two-color interferometer (FIG. 6)
constructed from free-space or fiber-optic components. The
two-color interferometer (FIG. 6) detects the relative time
offset of one pulse from another without distortion of
measurement by amplitude variations by comparison of the
heterodyne detection from the low-band optical signal 264
and high-band optical signal 265. In an embodiment, comb
timing discriminator 206 includes a single-tap interferom-
eter (FIG. 7) constructed from free-space or fiber-optic
components. The single-tap interferometer (FIG. 7) detects



U.S. Patent Nov. 29, 2022 Sheet 8 of 13 US 11,515,994 B2

202
225
a ) .
235—~\
237
234 ) 236
21 27-~

FIG. 8


US 11,515,994 B2

21

signal combiner-splitter 252; producing, by fixed delay stage
255, time offset signal 232 from optical timing signals 260;
receiving, by CW heterodyne detector 208, incoming CW
optical signal 230 from incoming optical timing signals 217
and CW optical signal 226 from tracking optical timing
signal 220; producing, by CW heterodyne detector 208,
frequency offset signal 231 from incoming CW optical
signal 230 and CW optical signal 226; and combining, by
time-frequency offset measurement system 205, time offset
signal 232 from second comb timing discriminator 206.2
and frequency offset signal 231 from CW heterodyne detec-
tor 208 into time-frequency offset signals 219.
Synchronizing transfer over an optical link can include
receiving, by dispersion compensation unit 256 of second
comb timing discriminator 206.2, incoming clock comb
signal 229 from incoming optical timing signals 217 and
tracking comb signal 225 from tracking optical timing signal
220; communicating, by dispersion compensation unit 256
of second comb timing discriminator 206.2, incoming clock
comb signal 229 and tracking comb signal 225 to signal
combiner-splitter 252 of second comb timing discriminator
206.2; receiving, by signal combiner-splitter 252, incoming
clock comb signal 229 and tracking comb signal 225 from
dispersion compensation unit 256; producing, by signal
combiner-splitter 252, combined coherent optical pulse
trains 257 from incoming clock comb signal 229 and track-
ing comb signal 225; receiving, by high-band BPF 263 of
second comb timing discriminator 206.2, combined coher-
ent optical pulse trains 257 from signal combiner-splitter
252; producing, by high-band BPF 263, a high-band optical
signal 265 from combined coherent optical pulse trains 257;
receiving, by low-band BPF 262 of second comb timing
discriminator 206.2, combined coherent optical pulse trains
257 from signal combiner-splitter 252; producing, by low-
band BPF 262, low-band optical signal 264 from combined
coherent optical pulse trains 257; receiving, by pulse inter-
ference detector 266 of second comb timing discriminator
206.2, high-band optical signal 265 from high-band BPF
263; producing, by pulse interference detector 266, high-
band interference signal 268 from high-band optical signal
265; receiving, by second pulse interference detector 266.2
of second comb timing discriminator 206.2, low-band opti-
cal signal 264 from low-band BPF 262; producing, by
second pulse interference detector 266.2, low-band interfer-
ence signal 267 from low-band optical signal 264; receiving,
by phase comparator 269 of second comb timing discrimi-
nator 206.2, high-band interference signal 268 from pulse
interference detector 266; receiving, by phase comparator
269, low-band interference signal 267 from second pulse

interference detector 266.2: producing, by phase comparator 5

269, time offset signal 232 from high-band interference
signal 268 and low-band interference signal 267; receiving,
by CW heterodyne detector 208 of time-frequency offset
measurement system 205, incoming CW optical signal 230

from incoming optical timing signals 217 and CW optical

signal 226 from tracking optical timing signal 220; produc-
ing, by CW heterodyne detector 208, frequency offset signal
231 from incoming CW optical signal 230 and CW optical
signal 226; and combining, by time-frequency offset mea-
surement system 205, time offset signal 232 from second
comb timing discriminator 206.2 and frequency offset signal
231 from CW heterodyne detector 208 into time-frequency
offset signals 219.

Synchronizing transfer over an optical link can include
receiving, by dispersion compensation unit 256 of second
comb timing discriminator 206.2, incoming clock comb
signal 229 from incoming optical timing signals 217 and

20

at

22

tracking comb signal 225 from tracking optical timing signal
220; communicating, by dispersion compensation unit 256,
incoming clock comb signal 229 and tracking comb signal
225 to signal combiner-splitter 252 dispersion compensation
unit 256 of second comb timing discriminator 206.2; receiv-
ing, by signal combiner-splitter 252, incoming clock comb
signal 229 and tracking comb signal 225 from dispersion
compensation unit 256; producing, by signal combiner-
splitter 252, combined coherent optical pulse trains 257
from the incoming clock comb signal 229 and tracking comb
signal 225; receiving, by high-band BPF 263 of second
comb timing discriminator 206.2, combined coherent optical
pulse trains 257 from signal combiner-splitter 252; produe-
ing, by high-band BPF 263, time offset signal 232 from
combined coherent optical pulse trains 257; receiving, by
CW heterodyne detector 208 of time-frequency offset mea-
surement system 205, incoming CW optical signal 230 from
incoming optical timing signals 217 and CW optical signal
226 from tracking optical timing signal 220; producing, by
CW heterodyne detector 208, frequency offset signal 231
from incoming CW optical signal 230 and CW optical signal
226; and combining, by time-frequency offset measurement
system 208, time offset signal 232 from second comb timing,

5 discriminator 206.2 and frequency offset signal 231 from

CW heterodyne detector 208 into time-frequency offset
signals 219.

Synchronizing transfer over an optical link can include
receiving, by comb digital controller 234 of tracking fre-
quency comb 202, tracking comb feedback 211; receiving,
by comb digital controller 234, phase-lock detection signals
237 from phase-lock detection signals 237; producing, by
comb digital controller 234, actuator control signals 235
from tracking comb feedback 211 and phase-lock detection
signals 237; receiving, by frequency comb optics package
233 of tracking frequency comb 202, the actuator control
signals 235 from comb digital controller 234; producing, by
frequency comb optics package 233, tracking comb signal
225 from actuator control signals 235; receiving, by phase-
lock detection unit 236 of tracking frequency comb 202
tracking comb signal 225 from frequency comb optic:
package 233; receiving, by phase-lock detection unit 236,
frequency reference signal 227; and producing, by phase-
lock detection unit 236, phase-lock detection signals 237
from tracking comb signal 225 and frequency reference
signal 227

‘Synchronizing transfer over an optical link can include
receiving, by comb digital controller 234 of clock frequency
comb 203, clock comb feedback 213 from two-way proces-
sor 241; receiving, by comb digital controller 234, phase-
lock detection signals 237 from phase-lock detection unit
236 of clock frequency comb 203; producing, by comb
digital controller 234, actuator control signals 235 from
clock comb feedback 213 and phase-lock detection signals
237; receiving, by frequency comb optics package 233 of
clock frequency comb 203, actuator control signals 235
from comb digital controller 234; producing, by frequency
comb optics package 233, clock comb signal 224 from
actuator control signals 235; receiving, by phase-lock detec-
tion unit 236, clock comb signal 224 from frequency comb
optics package 233; receiving, by phase-lock detection unit
236, frequency reference signal 227 from frequency refer-
ence oscillator 201; producing, by phase-lock detection unit
236, phase-lock detection signals 237 from clock comb
signal 224 and frequency reference signal 227; and com-
municating clock comb signal 224 from frequency comb
optics package 233 to bidirectional terminal 207.



a2) United States Patent

Sinclair et al.

US 11,515,994 B2
Nov. 29, 2022

US011515994B:

2

(10) Patent No.:
(45) Date of Patent:

(54) SYNCHRONIZER AND SYN

TRANSFER OVER AN OP’

‘HRONIZING
SAL LINK

(71) Applicant: Government of the United States of
America, as represented by the
Secretary of Commerce, Gaithersburg,
MD (US)

(72) Inventors: Laura Cathleen Sinclair, Boulder, CO

(US); Nathan Reynolds Newbury,

Boulder, CO (US); m Charles

Swann, Boulder, CO (US); Jean-Daniel

Deschenes, Quebec (CA)

(73) GOVERNMENT OF THE UNITED
STATES OF AMERICA, AS
REPRESENTED BY THE
SECRETARY OF COMMERCE,

Gaithersburg, MD (US)

Assignee:

Notice: Subject to any disclaimer, the term of this,
patent is extended or adjusted under 35
U.S.C. 154(b) by 4 days.

(21)

Appl. No.: 17/209,285

(22) Filed: Mar. 23, 2021

(65) Prior Publi

US 2022/0026192 Al

tion Data

Jan. 27, 2022

Related U.S. Application Data

(60) Provisional application No. 63/054,993, filed on Jul.

22, 2020.
(51) Int. CL.
HOB 10/00
HOAL 7/00
US. CL
cpc

(2013.01)
(2006.01)
(52)
HOAL 7/0008 (2013.01)

(58) Field of Classification Search

CPC veessseeee HO4L 7/0008; HO4L 7/0075; H04B
10/0775; HO4B 10/112
USPC .. 375/354, 356; 398/154

See application file for complete search history.

(56)

References d

U.S. PATENT DOCUMENTS

9,557,625 B2 1/2017 Coddington et all
10,389,514 B2__ 8/2019 Sinclair et al

2006/0120733 AL* 6/2006 Tucker HO4B 10/60

398/204
2012/0154062 A1* 6/2012 Wilkinson GO4F 13/02

331/94.
201810294946 A1* 10/2018. Sinclair HO4B 10/61
2021/0099230 Al* 4/2021 Nishioka . HO4B 10/2575

OTHER PUBLICATIONS,

Deschenes, J-D., et al., “Synchronization of Distant Optical Clocks
at the Femtosecond Level”, Physical Review X, 2016, p. 021016,
vol. 6.

Sinclair, L., et al., “Optical system design for femtosecondlevel
synchronization of clocks’ IE, 2016.

Giorgetta, FR., et al.,"Optical two-way time and frequency transfer
over free-space”, Nature Photonics, 2013, p. 434-438, vol. 7

* cited by examiner

Primary Examiner — Don N Vo

(74) Attorney, Agent, or Firm — Office of Chief Counsel
for National Institute of Standards and Technology

(57) ABSTRACT

A synchronizer for synchronizing transfer over an optical
link includes a frequency reference oscillator; a tracking
optical timing source; a tracking comb signal; a signal
processor-controller; a comb timing discriminator; a clock
frequency comb; a bidirectional terminal; a time-frequency
offset measurement system; and a second comb timing
discriminator.

20 Claims, 13 Drawing Sheets

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Novy. 29, 2022 Sheet 10 of 13

US 11,515,994 B2

211/213-—\\ 237
234
239 238
271 22—~|
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FIG. 10


US 11,515,994 B2

35

producing, by the low-band BPF, a low-band optical
signal from the combined coherent optical pulse trains;

receiving, by a pulse interference detector of the second
comb timing discriminator, the high-band optical signal
from the high-band BPF;

producing, by the pulse interference detector, a high-band
interference signal from the high-band optical signal;

receiving, by a second pulse interference detector of the
second comb timing discriminator, the low-band opti-
cal signal from the low-band BPF;

producing, by the second pulse interference detector, a
Jow-band interference signal from the low-band optical
signal;

receiving, by a phase comparator of the second comb
timing discriminator, the high-band interference signal
from the pulse interference detector;

receiving, by the phase comparator, the low-band inter-
ference signal from the second pulse interference detec-
tor;

producing, by the phase comparator, a time offset signal
from the high-band interference signal and the low-
band interference signal;

receiving, by a CW heterodyne detector of the time-
frequency offset measurement system, an incoming
CW optical signal from the incoming optical timing
signals and CW optical signal from the tracking optical
timing signal;

producing, by the CW heterodyne detector, a frequency
offset signal from the incoming CW optical signal and
the CW optical signal; and

combining, by the time-frequency offset measurement
system, the time offset signal from the second comb
timing discriminator and the frequency offset signal
from the CW heterodyne detector into the time-fre-
quency offset signals.

17. The process of claim 11, further comprising:

receiving, by a dispersion compensation unit of the sec-
ond comb timing discriminator, an incoming clock
comb signal from the incoming optical timing signals

and tracking comb signal from the tracking optical
timing signal; and

communicating, by the dispersion compensation unit, the
incoming clock comb signal and the tracking comb
signal to a signal combiner-splitter dispersion compen-
sation unit of the second comb timing discriminator;

receiving, by the signal combiner-splitter, the incoming
clock comb signal and the tracking comb signal from
the dispersion compensation unit;

producing, by the signal combiner-splitter, a combined 5

coherent optical pulse trains from the incoming clock
comb signal and the tracking comb signal;

receiving, by a high-band BPF of the second comb timing
discriminator, the combined coherent optical pulse
trains from the signal combiner-splitter;

producing, by the high-band BPF, a time offset signal
from the combined coherent optical pulse trains;

receiving, by a CW heterodyne detector of the time-
frequency offset measurement system, an incoming
CW optical signal from the incoming optical timing
signals and CW optical signal from the tracking optical
timing signal;

producing, by the CW heterodyne detector, a frequency
offset signal from the incoming CW optical signal and
the CW optical signal; and

combining, by the time-frequency offset measurement
system, the time offset signal from the second comb

28

0

35

65

36

timing discriminator and the frequency offset signal
from the CW heterodyne detector into the time-fre-
quency offset signals.

18. The process of claim 11, further comprising:

receiving, by a comb digital controller of the tracking
frequency comb, the tracking comb feedback:

receiving, by the comb digital controller, a phase-lock
detection signals from the phase-lock detection signals;

producing, by the comb digital controller, a actuator
control signals from the tracking comb feedback and
the phase-lock detection signals;

receiving, by a frequency comb optics package of the
tracking frequency comb, the actuator control signals
from the comb digital controller;

producing, by the frequency comb optics package, the
tracking comb signal from the actuator control signals;

receiving, by a phase-lock detection unit of the tracking
frequency comb, the tracking comb signal from the
frequency comb optics package;

receiving, by the phase-lock detection unit, the frequency
reference signal; and

producing, by the phase-lock detection unit, the phase-
lock detection signals from the tracking comb signal
and the frequency reference signal.

19. The process of claim 11, wherein the clock frequency

comb comprises:

receiving, by a comb digital controller of the clock
frequency comb, a clock comb feedback from a two-
way processor:

receiving, by the comb digital controller, a phase-lock
detection signals from a phase-lock detection unit of
the clock frequency comb;

producing, by the comb digital controller, an actuator
control signals from the clock comb feedback and the
phase-lock detection signals;

receiving, by a frequency comb optics package of the
clock frequency comb, the actuator control signals
from the comb digital controller;

producing, by the frequency comb optics package, the
clock comb signal from the actuator control signals;

receiving, by the phase-lock detection unit, the clock
comb signal from the frequency comb optics package:

receiving, by the phase-lock detection unit, the frequency
reference signal from the frequency reference oscilla-
tor;

producing, by the phase-lock detection unit, the phase-
Jock detection signals from the clock comb signal and
the frequency reference signal; and

communicating the clock comb signal from the frequency
comb optics package to the bidirectional terminal.

20. The process of claim 11, further comprising:

receiving, by a phase-frequency agile synthesizer of the
comb digital controller, the tracking comb feedback or
clock comb feedback;

producing, by the phase-frequency agile synthesize
thesizer data from the tracking comb feedback;

receiving, by a signal conditioning of the comb digital
controller, the phase-lock detection signals;

conditioning, by the signal conditioning, the phase-lock
detection signals to produce conditioned phase-lock
detection signals;

communicating, by the signal conditioning, the condi-
tioned phase-lock detection signals to a phase com-
parator of the comb digital controller;

syn-



US 11,515,994 B2

33

producing, by the signal processor-controller based on the
clock comb-tracking comb time offset and the time-
frequency offset signals, the tracking comb feedback
and local one-way data;

receiving, by the time-frequency offset measurement sys-
tem, the tracking optical timing signal from the track-
ing optical timing source;

receiving, by the time-frequency offset measurement sys-
tem, the incoming optical timing signals from the
bidirectional terminal;

producing, by the time-frequency offset measurement
system, the time-frequency offset signals from the
tracking optical timing signal and the incoming optical
timing signals;

communicating, by the time-frequency offset measure-
ment system, the time-frequency offset signals to the
signal processor-controller;

receiving, by the bidirectional terminal, the clock comb
signal from the clock frequency comb;

receiving, by the bidirectional terminal, synchronization 2

data from the optical link, the synchronization data
comprising incoming optical timing signals;
communicating, by the bidirectional terminal, the incom-
ing optical timing signals to time-frequency offset
measurement system from the optical link; and
communicating, by the bidirectional terminal, the clock
comb signal to the optical link from the clock fre-
quency comb as part of the synchronization data.

12. The process of claim 11, further comprising:

receiving, by the communicator, the local one-way data

from the signal processor-controller;

communicating, by the communicator, the side informa-

tion with signal processor-controller; and

producing, by the communicator, remote one-way data

from the local one-way data.

13. The process of claim 12, further comprising commu-
nicating, by the communicator via a communication chan-
nel, with a second synchronizer.

14. The process of claim 12, further comprising:

receiving, by a two-way processor, local one-way data

from the signal processor-controller;

receiving, by the two-way processor, the remote one-way

data from the communicator;
producing, by the two-way processor, two-way output
data and clock comb feedback based on the local
one-way data and the remote one-way data; and

communicating, by the two-way processor, the clock
comb feedback to the clock frequency comb.

15. The process of claim 12, further comprising:

receiving, by a dispersion compensation unit of the sec-

ond comb timing discriminator, an incoming clock
comb signal from the incoming optical timing signals
and tracking comb signal from the tracking optical
timing signal;

communicating, by the dispersion compensation unit, the 5

incoming clock comb signal and the tracking comb
signal to a signal combiner-splitter of the second comb
timing discriminator;

receiving, by the signal combiner-splitter, the incoming
clock comb signal and the tracking comb signal from
the dispersion compensation unit;

producing, by the signal combiner-splitter, a combined
coherent optical pulse trains from the incoming clock
comb signal and the tracking comb signal;

receiving, by a leading arm delay stage of the second
comb timing discriminator, the combined coherent
optical pulse trains from the signal combiner-splitter;

0

38

&

0

&

50

0

34

producing, by the leading arm delay stage, a leading
coherent optical pulse trains from the combined coher-
ent optical pulse trains;

receiving, by a lagging arm delay stage of the second
comb timing discriminator, the combined coherent
optical pulse trains from the signal combiner-splitter;

producing, by the lagging arm delay stage, a lagging
coherent optical pulse trains from the combined coher-
ent optical pulse trains;

receiving, by a leading arm delay stage of the second
comb timing discriminator, the combined coherent
optical pulse trains from the signal combiner-splitter;

producing, by the leading arm delay stage, a leading
coherent optical pulse trains from the combined coher-
ent optical pulse trains;

receiving, by a second signal combiner-splitter of the
second comb timing discriminator, the leading coherent
optical pulse trains from the leading arm delay stage;

receiving, by the second signal combiner-splitter, the
lagging coherent optical pulse trains from the lagging
arm delay stage;

producing, by the second signal combiner-splitter, an
optical timing signals from the leading coherent optical
pulse trains and the lagging coherent optical pulse
trains;

receiving, by a fixed delay stage of the second comb
timing discriminator, the optical timing signals from
the signal combiner-splitter;

producing, by the fixed delay stage, a time offset signal
from the optical timing signals;

receiving, by a CW heterodyne detector, an incoming CW
optical signal from the incoming optical timing signals
and CW optical signal from the tracking optical timing
sign:

producing, by the CW heterodyne detector, a frequency
offset signal from the incoming CW optical signal and
the CW optical signal; and

combining, by the time-frequency offset measurement
system, the time offset signal from the second comb
timing discriminator and the frequency offset signal
from the CW heterodyne detector into the time-fre-
quency offset signals.

16. The process of claim 11, further comprising:

receiving, by a dispersion compensation unit of the sec-
ond comb timing discriminator, an incoming clock
comb signal from the incoming optical timing signals
and tracking comb signal from the tracking optical
timing signal;

communicating, by the dispersion compensation unit of
the second comb timing discriminator, the incoming
clock comb signal and the tracking comb signal to a
signal combiner-splitter of the second comb timing
discriminator;

receiving, by the signal combiner-splitter, the incoming
clock comb signal and the tracking comb signal from
the dispersion compensation uni

producing, by the signal combiner-splitter, a combined
coherent optical pulse trains from the incoming clock
comb signal and the tracking comb signal;

receiving, by a high-band BPF of the second comb timing
discriminator, the combined coherent optical pulse
trains from the signal combiner-splitter;

producing, by the high-band BPF, a high-band optical
signal from the combined coherent optical pulse trains;

receiving, by a low-band BPF of the second comb timing
discriminator, the combined coherent optical pulse
trains from the signal combiner-splitter:



U.S. Patent Nov. 29, 2022 Sheet 7 of 13 US 11,515,994 B2

| 220.
217

205

206.2

Ino

219—]

FIG. 7


US 11,515,994 B2

15

in outgoing optical timing signals 216 by two-way time
transfer over a direct RF channel or an optical communica-
tion channel. Communicator 215 exchanges one-way time
data at each site (e.g., synchronizer 200.1, synchronizer
200.2, . . . , synchronizer 200,m). Communicator 215 and
communication channel 214 can be implemented in various
ways, including as separate RF or microwave communica-
tion links: by a separate optical communication link that can
optionally use bidirectional terminal 207 and optical link
218 as the optical timing signals; by inserting a phase or
amplitude modulator into a communication path of CW
optical signal 226 to apply data formatting to the outgoing
signal at each site, where the data is demodulated from the
incoming CW signal in CW heterodyne detector 208 by
communicator 215; by inserting a phase or amplitude modu-
lator into the communication path for clock comb signal 224
to bidirectional terminal 207 to data format outgoing clock
comb light at each site, where the data can be demodulated
in comb timing discriminator 206 by communicator 215.

Remote one-way data 222 is produced by communicator
215 from local one-way data 221 from the other side (e.g.,
synchronizer 200.2) that is sent over communication chan-
nel 214 via communicator 215.

‘Two-way processor 241 receives local one-way data 221
from signal processor-controller 204. Two-way processor
241 combines the two one-way data streams (local one-way
data 221, remote one-way data 222) to generate two-way
output data 223 by applying two-way time transfer equ:
with corrections for relativistic and systematic effect
way output data 223 includes data related to two-way time
transfer including outputs for time and frequency offset
between clock frequency comb 203 at synchronizer 200 and
at the remote site (e.g., synchronizer 200.1, synchronizer
200.2, . . . , synchronizer 200.m), and the time-of-flight
between synchronizer 200 and the remote site (e.g., syn-
chronizer 200.1, synchronizer 200.2, . . . , synchronizer
200.m) over optical link 218, or the relative velocity
between synchronizer 200 and the remote site (e.g., syn-
chronizer 200.1, synchronizer 200.2, .. . , synchronizer
200.m). Two-way output data 223 also includes data on
signal amplitudes, diagnostic data, and the like for synchro-
nizer 200, synchronizer 200.1, . . . , synchronizer 200.m.

‘Two-way processor 241 also produces clock comb feed-
back 213 that includes control signals to adjust a phase/
timing and frequency of clock frequency comb 203. It is
contemplated that clock comb feedback 213 can include
control of both degrees-of-freedom of the clock frequency
comb 203, which we can take as its carrier phase/frequency
(ie. frequency of one defined comb tooth in the center of the

transmitted optical frequency band) and its pulse repetition 5

phase/timing as for tracking frequency comb 202. The
feedback could be used to adjust the clock comb signal 224
itself or to provide additional correction data to the clock
output 210.

In an embodiment, synchronizer 200 includes tracking s

frequency comb 202 and clock frequency comb 203 that are
identical with mapping of input control signals to provide an
agile comb, as shown in FIGS. 7 and 8. Here, for tracking
frequency comb 202 or clock frequency comb 203, a com-
mon element is that the agile comb is phase-locked to a sum
of frequency reference signal 227 and arbitrary control
signals, e.g. clock comb feedback 213 or tracking comb
feedback 211. The phase-lock can be performed by comb
digital controller 234, by a phase synthesizer or frequency
agile synthesizer. Such a synthesizer can be implemented by
analog electronics, digital electronics as a direct-digital
synthesizer (DDS), or a combination thereof. In this con-

20

&

45

16

figuration, control signals distinguish tracking frequency
comb 202 and clock frequency comb 203, wherein tracking
frequency comb 202 tracks incoming clock comb signal
229, which itself is the clock comb signal 224 from the
remote site combined with any time-of-flight delay over the
optical link 218, and clock frequency comb 203 is adjusted
to track just the timing of the clock comb signal 224 at the
remote clock, as calculated by the two-way processor 241.

Further, the side information 228 at each site consists of
the local one-way data 221 and other information available
at the site including but not limited to its location, orienta-
tion and velocity from navigational information, time-of-
day, environmental conditions, and the like.

The incoming clock comb signal 229 is clock comb signal
224 of the outgoing optical timing signals 216 at the remote
site after traversing the optical link 218. The incoming CW
optical signal 230 is the CW optical signal 226 of the
outgoing optical timing signals 216 at the remote site after
traversing the optical link 218.

The frequency offset signal 231 is the heterodyne signal
from the heterodyne detection of the incoming CW optical
signal 230 and CW optical signal 226 by the CW heterodyne
detector 208. The time offset signal 232 is the output of the

5 comb timing discriminator 206 that provides the time and

frequency offset between the pulses of the two frequency
combs input to the comb timing discriminator 206.

The continuous wave CW laser 242 is a single-mode laser
(eg., a fiber laser or external cavity semiconductor laser or
diode laser) that is phase-locked to tooth of clock frequency
comb 203 so that it has a known and stable optical fre-
quency.

The frequency comb optics package 233 is the physical
optical system that generates the tracking comb signal 225
or the clock comb signal 224 (e.g. a mode-locked laser
including any pump lasers, followed by a supercontinuum
generation model and including any actuators for control of
the frequency comb output). The actuator control signals
235 are the voltage signals that are sent to the actuators in
the frequency comb optics package 233 to adjust the two
degrees-of-freedom of the frequency comb. The phase-lock
detection unit 236 includes the opto-electronics for detection
of the carrier-envelope offset frequency of the comb. If the
frequency reference signal 227 is optical, the phase-lock
detection unit 236 includes the heterodyne detection of a
comb tooth compared to the frequency reference signal 227.
If the frequency reference signal 227 is a microwave signal,
the phase-lock detection unit 236 includes the detection of a
harmonic of the comb repetition frequency and its phase
comparison with the frequency reference signal 227. If
frequency reference signal 227 includes a microwave and
optical reference signal, phase-lock detection unit 236
includes both the detection of a harmonic of the comb
repetition frequency and its phase comparison with the
microwave reference in the frequency reference signal 227,
and heterodyne detection of a comb tooth compared to the
optical reference frequency re! signal 227. The pha:
lock detection signals 237 are the two measured beat signals
output by the phase-lock detection unit 236 that are then
processed in the comb digital controller 234.

As part of the comb digital controller 234, (FIG. 10) the
signal conditioning 238 includes any linear filtering to the
input signals to produce the conditioned phase-lock detec-
tion signals 272, the phase-frequency agile synthesizer 239
generates an rf synthesizer data 271 with an adjustable phase
and frequency, the loop filter 240 is a proportional-integral-
derivative loop filter as used in feedback systems, the phase



US 11,515,994 B2

27

other, The ranges are continuous and thus contain every
value and subset thereof in the range. Unless otherwise
stated or contextually inapplicable, all percentages, when
expressing a quantity, are weight percentages. The suflix
“(s)” as used herein is intended to include both the singular
and the plural of the term that it modifies, thereby including
at least one of that term (e.g., the colorant(s) includes at least
one colorants). “Optional” or “optionally” means that the
subsequently described event or circumstance can or cannot
occur, and that the description includes instances where the
event occurs and instances where it does not. As used herein,
“combination” is inclusive of blends, mixtures, alloys, rea
tion products, and the like.

As used herein, “a combination thereof” refers to a
combination comprising at least one of the named constitu-
ents, components, compounds, or elements, optionally
together with one or more of the same class of constituents,
components, compounds, or elements.

All references are incorporated herein by reference.

The use of the terms “a” and “an” and “the” and similar
referents in the context of describing the invention (esp
cially in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by con-
text. “Or” means “and/or.” It should further be noted that the
terms “first,” “second,” “primary,” “secondary,” and the like
herein do not denote any order, quantity, or importance, but
rather are used to distinguish one element from another. The
modifier “about” used in connection with a quantity is
inclusive of the stated value and has the meaning dictated by
the context (e.g., it includes the degree of error associated
with measurement of the particular quantity). The conjunc-
tion “or” is used to link objects of a list or alternatives and
is not disjunctive; rather the elements can be used separately
‘or can be combined together under appropriate circum-
stances.

What is claimed is:
1. A synchronizer for synchronizing transfer over an
optical link, the synchronizer comprising:
a frequency reference oscillator that produces a frequency
reference signal;
tracking optical timing source in communication with
the frequency reference oscillator and comprises a
tracking frequency comb and that:
receives, by the tracking frequency comb, the fre-
quency reference signal from the frequency refer-
ence oscillator;
produces, by the tracking frequency comb, a tracking
comb signal:
receives, tracking comb feedback from a signal pro-
cessor-controller;
produces a tracking optical timing signal based on the
tracking comb feedback:

2

2

tracking optical timing source and a clock frequency
comb and that:
receives the tracking comb signal from the tracking
optical timing source:
receives a clock comb signal from the clock frequency
comb; and
produces an clock comb-tracking comb time offset
from the clock comb signal and the tracking comb
signal;
the clock frequency comb in communication with the
clock frequency comb, the frequency reference oscil-
lator, and a bidirectional terminal and that:

comb timing discriminator in communication with the

0

20

0

40

45

0

tor further communi
a second synchronizer.

28

receives the frequency reference signal from the fre-
quency reference oscillator;
produces the clock comb signal based on the frequency
reference signal;
produces a clock output based on the frequency refer-
ence signal; and
communicates the clock comb signal to the comb
timing discriminator and a bidirectional terminal;
the signal processor-controller in communication with the
tracking optical timing source, a time-frequency offset
measurement system, and the comb timing discrimina-
tor and that:
receives the clock comb-tracking comb time offset
from the comb timing discriminator;
receives a time-frequency offset signals from the time-
frequency offset measurement systen
produces, based on the clock comb-tracking comb time
offset and the time-frequency offset signals, the
tracking comb feedback and local one-way data;
the time-frequency offset measurement system in com-
munication with the tracking optical timing source, the
bidirectional terminal, and the signal processor-control-
ler; comprises a second comb timing discriminator; and
that:
receives the tracking optical timing signal from the
tracking optical timing source;
receives an incoming optical timing signals from the
bidirectional terminal;
produces the time-frequency offset signals from the
tracking optical timing signal and the incoming
optical timing signals; and
communicates the time-frequency offset signals to the
signal processor-controller; and
the bidirectional terminal in communication with the
clock frequency comb and the time-frequency offset
measurement system and that:
receives the clock comb signal from the clock fre-
quency comb;
receives synchronization data from an optical link, the
synchronization data comprising incoming optical
timing signals;
communicates the incoming optical timing signals to
time-frequency offset measurement system from the
optical link; and
communicates the clock comb signal to the optical link
from the clock frequency comb as part of the syn-
chronization data.
2. The synchronizer of claim 1, further comprising:
a communicator in communication with the signal pro-
cessor-controller and that:
receives the local one-way data from the signal pro-
cessor-controller;
communicates side information with signal processor-
controller; and
produces remote one-way data from the local one-way
data.
. The synchronizer of claim 2, wherein the communica-
ites Via a communication channel with

4. The synchronizer of claim 2, further comprising:

a two-way processor in communication with the clock
frequency comb, the signal processor-controller, and
the communicator and that:
receives local one-way data from the signal processor-

controller;
receives the remote one-way data from the communi-
cator;


U.S. Patent Nov. 29, 2022 Sheet 3 of 13 US 11,515,994 B2

200.1
207 200.2
207 200.3
207 200.4
e
e
e
e
e
207 e
200.m

FIG. 3


US 11,515,994 B2

1
SYNCHRONIZER AND SYNCHRONIZIN¢
TRANSFER OVER AN OPTICAL LIN

CROSS REFERENCE TO RELATED,
APPLICATIONS

‘The application claims priority to U.S. Provisional Patent
Application Ser. No. 63/054,993, filed Jul. 22, 2020, the
disclosure of which is incorporated herein by reference in its
entirety.

STATEMENT REGARDING FEDERALLY,
SPONSORED RESEARCH

This invention was made with United States Government
support from the National Institute of Standards and Tech-
nology (NIST), an agency of the United States Department
of Commerce and under Agreement Nos. HR0011832582
and HR001118P002 awarded by DARPA; Agreement No.
F2KBAA9030G103 awarded by Air Force Research Labo-
ratories; and under contract number 1333ND19PNB680111
awarded by NIST. The Government has certain rights in the
invention. Licensing inquiries may be directed to the Tech-
nology Partnerships Office, NIST, Gaithersburg, Md..
20899; voice 301-975-2573; email tpo@nist gov; reference
NIST Docket Number 20-033US1.

BRIEF DESCRIPTION

Disclosed is a synchronizer for synchronizing transfer
over an optical link, the synchronizer comprising: a fre-
quency reference oscillator that produces a frequency ref-
erence signal; a tracking optical timing source in commu-
nication with the frequency reference oscillator and
comprises a tracking frequency comb and that: receives, by
the tracking frequency comb, the frequency reference signal
from the frequency reference oscillator; produces, by the
tracking frequency comb, a tracking comb signal; receives,
tracking comb feedback from a signal processor-controller;
produces a tracking optical timing signal based on the
tracking comb feedback; a comb timing discriminator in
communication with the tracking optical timing source and
aclock frequency comb and that: receives the tracking comb
signal from the tracking optical timing source: receives a
clock comb signal from the clock frequency comb; and
produces an initial clock comb-tracking comb time offset
from the clock comb signal and the tracking comb signal; the
clock frequency comb in communication with the clock
frequency comb, the frequency reference oscillator, and a

bidirectional terminal and that: receives the frequency ref- 5

erence signal from the frequency reference oscillator; pro-
duces the clock comb signal based on the frequency refer-
ence signal; produces a clock output based on the frequency
reference signal; and communicates the clock comb signal to

the comb timing discriminator and a bidirectional terminal; s

the signal processor-controller in communication with the
tracking optical timing source, a time-frequency offs
surement system, and the comb timing discriminator and
that: receives the initial clock comb-tracking comb time
offset from the comb timing discriminator; receives a time-
frequency offset signals from the time-frequency offset
measurement system; produces, based on the initial clock
comb-tracking comb time offset and the time-frequency
offset signals, the tracking comb feedback and local one-way
data; the time-frequency offset measurement system in com-
munication with the tracking optical timing source, the
bidirectional terminal, and the signal processor-controller;

20

&

&

2

comprises a second comb timing discriminator.2; and that:
receives the tracking optical timing signal from the tracking
optical timing source; receives an incoming optical timing
signals from the bidirectional terminal; produces the time-
frequency offset signals from the tracking optical timing
signal and the incoming optical timing signals; and com-
municates the time-frequency offset signals to the signal
processor-controller; and the bidirectional terminal in com-
munication with the clock frequency comb and the time-
frequency offset measurement system and that: receives the
clock comb signal from the clock frequency comb; receives
synchronization data from an optical link, the synchroniza-
tion data comprising incoming optical timing signals; com-
municates the incoming optical timing signals to time-
frequency offset measurement system from the optical link;
and communicates the clock comb signal to the optical link
from the clock frequency comb as part of the synchroniza-
tion data.

Disclosed is a process for synchronizing transfer over an
optical link with the synchronizer, the process comprising:
producing, by the frequency reference oscillator, the fre-
quency reference signal; receiving, by the tracking fre-
quency comb, the frequency reference signal from the
frequency reference oscillator; producing, by the tracking

5 frequency comb, a tracking comb signal: receiving, by the

tracking optical timing source, the tracking comb feedback
from the signal processor-controller; producing, by the
king optical timing source, the tracking optical timing
signal based on the tracking comb feedback; receiving, by
the comb timing discriminator, the tracking comb signal
from the tracking optical timing source; receiving by the
comb timing discriminator, the clock comb signal from the
clock frequency comb: producing, by the comb timing
discriminator, the initial clock comb-tracking comb time
offset from the clock comb signal and the tracking comb
signal; receiving, by the clock frequency comb, the fre-
quency reference signal from the frequency reference oscil-
lator; producing, by the clock frequency comb, the clock
comb signal based on the frequency reference signal; pro-
ducing, by the clock frequency comb, the clock output based
on the frequency reference signal; communicating, by the
clock frequency comb, the clock comb signal to the comb
timing discriminator and the bidirectional terminal; receiv-
ing, by the signal processor-controller, the initial clock
comb-tracking comb time offset from the comb timing
discriminator; receiving, by the signal processor-controller,
the time-frequency offset signals from the time-frequency
offset measurement system; producing, by the signal pro-
cessor-controller based on the initial clock comb-tracking
comb time offset and the time-frequency offset signals, the
tracking comb feedback and local one-way data; receiving,
by the time-frequency offset measurement system, the track-
ing optical timing signal from the tracking optical timing
source; receiving, by the time-frequency offset measurement
system, the incoming optical timing signals from the bidi-
rectional terminal; producing, by the time-frequency offset
-m, the time-frequency offset signals from
the tracking optical timing signal and the incoming optical
timing signals; communicating, by the time-frequency offset
measurement system, the time-frequency offset signals to
the signal processor-controller; receiving, by the bidirec-
tional terminal, the clock comb signal from the clock fre-
quency comb; receives, by the bidirectional terminal, syn-
chronization data from the optical link, the synchronization
data comprising incoming optical timing signals; commu-
nicating, by the bidirectional terminal, the incoming optical
timing signals to time-frequency offset measurement system



US 11,515,994 B2

5
signal between CW optical signal 226 and the incoming CW
optical signal from the opposite site within incoming optical
timing signals 217.

Tn an embodiment, with reference to FIG. 1 and FIG. 2,
synchronizer 200 includes communicator 215 in communi-
cation with signal processor-controller 204. Communicator
215 receives local one-way data 221 from signal processor-
controller 204, communicates side information 228 with
signal processor-controller 204; and produces remote one-
way data 222 via communication channel 214 with second
synchronizer 200.2. Communicator 215 also transmits con-
ventional two-way time transfer protocols via communi
tion channel 214 with second synchronizer 200 to remove
time ambiguities associated with outgoing optical timing
signals 216 and incoming optical timing signals 217. Com-
municator 215 can a radio-frequency communication chan-
nel or optical communication channel. If it is the optical
communication channel, communication channel 214 can be
identical to optical link 218 and be transmitted out of
bidirectional terminal 207. If it is an optical communication
channel, communicator 215 can be implemented by direct
amplitude or phase modulation of outgoing optical timing
signals 216, clock comb signal 224, or CW optical signal
226.

In an embodiment, with reference to FIG. 1 and FIG. 2,
synchronizer 200 includes two-way processor 241 in com-
munication with clock frequency comb 203, signal proces-
sor-controller 204, and communicator 215. Two-way pro-
cessor 241 receives local one-way data 221 from signal
processor-controller 204, receives remote one-way data 222
from communicator 215, produces two-way output data 223
and clock comb feedback 213 based on local one-way data
221 and remote one-way data 222, and communicates clock
comb feedback 213 to clock frequency comb 203.

Tn an embodiment, with reference to FIG. 2 and FIG. 3,
first synchronizer 200.1 is interconnected and in communi-
cation with second synchronizer 200.2 via optical link 218
and communication channel 214. Although, only two syn-
chronizers (200.1, 200.2) are shown in FIG. 2, first synchro-
nizer 200.1 can be connected to an arbitrary number, m, of
other synchronizers, wherein m number of synchronizers
can be, eg., single, tens, hundreds, thousands, or millions of
other synchronizers as shown in FIG. 3. In the case of a
plurality of synchronizers 200 being connected, first syn-
chronizer 200.1 is present and connected to m synchronizers
(200.2, 200.3, ... ., 200.m). It should appreciated, as shown
in FIG. 2, that clock comb feedback 213 is produced in in
synchronizers (200.2, .. . , 200.3) that are connected to first
synchronizer 200.1 but absent in first synchronizer 200.1. In

this manner, first synchronizer 200.1 operates a lead syn- 5

chronizer while other synchronizers (200.2, . . . , 200.2)
operate as followers to first synchronizer 200.1. It further
should be appreciated that among synchronizers
(200.1... . , 200.m) synchronization data 270 includes

outgoing optical timing signals 216 (including clock comb

signal 224 and CW optical signal 226) that is sent from one
to another synchronizer and incoming optical timing signals
217 that is received from one to another synchronizer.
Accordingly, outgoing optical timing signals 216 sent from
one synchronizer (e.g., synchronizer 200.1) is received as
incoming optical timing signals 217 by another synchronizer
(eg. synchronizer 200.2), and incoming optical timing
signals 217 received by one synchronizer (e.g., synchronizer
200.1) is sent as outgoing optical timing signals 216 by
another synchronizer (e.g., synchronizer 200.2). It further
should be appreciated that, although second synchronizer
200.2 is used to describe a synchronizer 200 that is different

20

&

&

6

that first 200.1, such second synchronizer could be any of or
a plurality of arbitrary synchronizers (200.2, 200.3, ... ,
200.m).

In an embodiment, with reference to FIG. 4, second comb
timing discriminator 206.2 that includes dispersion compen-
sation unit 256 that receives incoming clock comb signal
229 from incoming optical timing signals 217 and tracking
comb signal 225 from tracking optical timing signal 220;
and communicates incoming clock comb signal 229 and
tracking comb signal 225 each to two separate signal com-
biner-splitter 252.1 and signal combiner-splitter 252.2. The
signal combiner-splitter 252.1 outputs two identical copies
of incoming clock comb signal 229.1 and incoming clock
comb signal 229.2. The signal combiner-splitter 252.2 out-
puts two identical copies of tracking comb signal 225.1 and
tracking comb signal 225.2. The incoming clock comb
signal 229.1 and incoming clock comb signal 229.1 are
transmitted to the signal combiner-splitter 252.3 and signal
combiner-splitter 252.4 through Fixed delay stage 255.1 and
Fixed delay stage 255.2. The tracking comb signal 225.1 is
sent to lagging arm delay stage 253 and the tracking comb
signal 225.2 is sent to a leading arm delay stage 254. The
output of the lagging arm delay stage 253 is the lagging
coherent optical pulse trains 258 that is in communication

5 with the signal combiner-splitter 252.3 that received the

incoming clock comb signal 229.1 from the Fixed delay
stage 255.1. The output of the leading arm delay stage 254
is the leading coherent optical pulse trains 259 that is in
communication with the second signal combiner-splitter
252.4 that received the second copy of the incoming clock
comb signal 229.2 from the fixed delay stage 255.2. The
output of signal combiner-splitter 252.3 and signal com-
biner-splitter 252.4 are combined coherent optical pulse
trains 257 that are in communication with independent
photodetector unit 261.1 and photodetector unit 261.2,
respectively. The outputs of photodetector unit 261.1 and
photodetector unit 261.2 produces time offset signal 232. In
addition, time-frequency offset measurement system 205
can include CW heterodyne detector 208. CW heterodyne
detector 208 can receive incoming CW optical signal 230
from incoming optical timing signals 217 and CW optical
signal 226 from tracking optical timing signal 220 and
produces frequency offset signal 231 from incoming CW
optical signal 230 and CW optical signal 226. Time-fre-
quency offset measurement system 205 combines time offset
signal 232 from second comb timing discriminator 206.2
and frequency offset signal 231 from CW heterodyne detec-
tor 208 into time-frequency offset signals 219.

In an embodiment, with reference to FIG. 5, a second
comb timing discriminator 206.2 that includes dispersion
compensation unit 256 that receives incoming clock comb
signal 229 from incoming optical timing signals 217 and
tracking comb signal 225 from tracking optical timing signal
220; and communicates incoming clock comb signal 229
and tracking comb signal 225 each to two separate signal
combiner-splitter 252.1 and signal combiner-splitter 252.2
The signal combiner-splitter 252.1 outputs two identical
copies of incoming clock comb signal 229.1 and incoming
clock comb signal 229.2. The signal combiner-splitter 252.2
outputs two identical copies of tracking comb signal 225.1
and tracking comb signal 225.2. The incoming clock comb
signal 229.1 and incoming clock comb signal 229.1 are
transmitted to the signal combiner-splitter 252.3 and signal
combiner-splitter 252.4 through Fixed delay stage 255.1 and
Fixed delay stage 255.2. The tracking comb signal 225.1 is
sent to lagging arm delay stage 253 and the tracking comb
signal 225.2 is sent to a leading arm delay stage 254. The



US 11,515,994 B2

Sheet 2 of 13

Nov. 29, 2022

U.S. Patent



US 11,515,994 B2

23

Synchronizing transfer over an optical link can include
receiving, by phase-frequency agile synthesizer 239 of comb
digital controller 234, tracking comb feedback 211 or clock
comb feedback 213; producing, by phase-frequency agile
synthesizer 239, synthesizer data 271 from tracking comb
feedback 211; receiving, by signal conditioning 238 of comb
digital controller 234, phase-lock detection signals 237;
conditioning, by signal conditioning 238, phase-lock detec-
tion signals 237 to produce conditioned phase-lock detection

signals 272; communicating, by signal conditioning 238, |

conditioned phase-lock detection signals 272 to phase com-
parator 269 of comb digital controller 234; receiving, by
phase comparator 269, synthesizer data 271 from phase-
frequency agile synthesizer 239 and conditioned phase-lock
detection signals 272 from signal conditioning 238; com-
paring, by phase comparator 269, synthesizer data 271 to
conditioned phase-lock detection signals 272; producing, by
phase comparator 269, comparator data 273 from comparing

synthesizer data 271 to conditioned phase-lock detection 2

signals 272; communicating comparator data 273 to loop
filter 240 of comb digital controller 234 from phase com-
parator 269; receiving, by loop filter 240, comparator data
273 from phase comparator 269; producing, by loop filter
240, actuator control signals 235 from comparator data 273;
and communicating actuator control signals 235 from loop
filter 240 to frequency comb optics package 233.

In an embodiment, with reference to FIG. 12, a process
for synchronizing transfer over an optical link (300) pro-
vides frequency comparison, time comparison, frequency
synchronization, or full time synchronization. For frequency
comparison, the process can include: phase-locking optical
timing signals and clock frequency comb to reference oscil-
lator (step 301); acquiring temporal-frequency overlap
between incoming optical timing signal and tracking optical
timing signal with a search pattern; repeating step if long
duration link loss occurs (step 303); precisely measure
timing and frequency offset of tracking optical timing signal
relative to incoming optical timing signals (step 304); deter-
mining one-way timing data by combining the timing and
frequency offset measurements with current tracking optical
timing settings (step 305); communicating feedback to
tracking optical timing signals to lock onto incoming optical
timing signals (step 306); storing one-way timing data (step
307); later or in real time through a communication channel,
combining one-way timing data to produce two-way timing
data (e.g. clock ofiset, time-of-flight, and relative velocity)
including the relative frequency between the sites (step 309)

In an embodiment, with reference to FIG. 12, a process 5

for synchronizing transfer over an optical link (300) to
accomplish time comparison additionally includes at least
the following steps: phase-locking optical timing signals and
clock frequency comb to reference oscillator (step 301);

calibrating all time delays in the synchronizer; measuring. ;

time offset between clock comb and tracking comb; repeat-
ing this step if a phase slip occurs (step 302); acquiring
temporal-frequency overlap between incoming optical tim-
ing signal and tracking optical timing signal with a search
pattern; repeating step if long duration link loss occurs (step
303); precisely measure timing and frequency offset of
tracking optical timing signal relative to incoming optical
timing signals (step 304); determining one-way timing data
by combining measurements with current tracking optical
timing settings (step 305); storing one-way timing data (step
307); possibly communicating with the other synchronizer
(step 308); in real-time or later combining one-way timing

&

&

24

data to produce two-way timing data (e.g. clock offset,
time-of-flight, and relative velocity) including the time
offset (step 309).

In an embodiment, with reference to FIG. 12, a process
for synchronizing transfer over an optical link (300) for
frequency synchronization includes: phase-locking optical
timing signals and clock frequency comb to reference oscil-
lator (step 301); acquiring temporal-frequency overlap
between incoming optical timing signal and tracking optical
timing signal with a search pattern; repeating step if long
duration link loss occurs (step 303); precisely measure
timing and frequency offset of tracking optical timing signal
relative to incoming optical timing signals (step 304); deter-
mining one-way timing data by combining measurements
with current tracking optical timing settings (step 305);
communicating feedback to tracking optical timing signals
to lock onto incoming optical timing signals (step 306);
storing one-way timing data (step 307); communicating with
the other synchronizer (step 308); combining one-way tim-
ing data to produce two-way timing data (e.g. clock offset,
time-of-flight, and relative velocity) particularly including
any frequency offset (step 309); providing feedback to clock
comb (step 310); and communicating timing data to user
(step 311).

In an embodiment, with reference to FIG. 12, a process
for synchronizing transfer over an optical link (300) for full
time synchronization includes: phase-locking optical timing
signals and clock frequency comb to reference oscillator
(step 301); calibrating all time delays in the synchronizer;
measuring time offset between clock comb and tracking
comb; repeating this step if a phase slip occurs (step 302):
acquiring temporal-frequency overlap between incoming
optical timing signal and tracking optical timing signal with
a search pattern; repeating step if long duration link loss
occurs (step 303); precisely measure timing and frequency
offset of tracking optical timing signal relative to incoming
optical timing signals (step 304); determining one-way
timing data by combining measurements with current track-
ing optical timing settings (step 305); communicating feed-
back to tracking optical timing signals to lock onto incoming
optical timing signals (step 306); storing one-way timing
data (step 307); communicating with the other synchronizer
(step 308); combining one-way timing data to produce
two-way timing data (e.g. clock offset, time-of-flight, and
relative velocity) (step 309); providing feedback to clock
comb (step 310); and communicating timing data to user
(step 311).

Modified steps can be performed in synchronizing trans-
fer over an optical link comb timing discriminator 206 with
synchronizer 200 for a paper clock. In an embodiment, the
clock frequency comb 203 is not included within the syn-
chronizer but is replaced by a paper clock that includes
tracking comb signal 225 that is an optical pulse train output
from tracking frequency comb 202, and that is filtered to
match a bandwidth of clock comb signal 224. Tracking,
comb signal 225 can include a parallel data stream output
that contains timing corrections that can be applied to the
optical pulse train from tracking frequency comb 202 to
achieve frequency comparison, frequency syntonization, or
time synchronization as described in the previous para-
graphs.

Modified steps can be performed in synchronizing trans-
fer over an optical link comb timing discriminator 206 with
synchronizer 200 for a repeater configuration, e.g., as shown
in FIG. 13. In an embodiment, a coherent repeater unit
200.R, includes two synchronizers 200 that share a common
signal processor unit, but with separate tracking frequency



U.S. Patent Nov. 29, 2022 Sheet 1 of 13 US 11,515,994 B2

2104 218
203 207

Ibo
I>
IS

FIG. |


US 11,515,994 B2
29 30

produces two-way output data and clock comb feed- 6. The synchronizer of claim 1, wherein the time-fre-
back based on the local one-way data and the remote quency offset measurement system further comprises a CW
one-way data; and heterodyne detector, such that:
communicates the clock comb feedback to the clock the second comb timing discriminator comprises:
frequency comb. 5 a dispersion compensation unit that:

5. The synchronizer of claim 1, wherein the time-fre- receives an incoming clock comb signal from the
quency offset measurement system further comprises a CW incoming optical timing signals and tracking
heterodyne detector, such that: comb signal from the tracking optical timing sig-

the second comb timing discriminator comprises: nal; and

a dispersion compensation unit that: 10 communicates the incoming clock comb signal and
receives an incoming clock comb signal from the the tracking comb signal to a signal combiner-
incoming optical timing signals and tracking splitter;
comb signal from the tracking optical timing sig- the signal combiner-splitter in communication with the
nal; and 45 dispersion compensation unit and that receives the
communicates the incoming clock comb signal and incoming clock comb signal and the tracking comb
the tracking comb signal to a signal combiner- signal from the dispersion compensation unit, and
splitter; produces a combined coherent optical pulse trains
the signal combiner-splitter in communication with the from the incoming clock comb signal and the track-
dispersion compensation unit and that receives the 20 ing comb signal;
incoming clock comb signal and the tracking comb a high-band BPF in communication with the signal
signal from the dispersion compensation unit, and combiner-splitter and that receives the combined
produces a combined coherent optical pulse trains coherent optical pulse trains from the signal com-
from the incoming clock comb signal and the track- biner-splitter and produces a high-band optical sig-
ing comb signal: 2s nal from the combined coherent optical pulse trains;
a leading arm delay stage in communication with the a low-band BPF in communication with the signal
signal combiner-splitter and that receives the com- combiner-splitter and that receives the combined
bined coherent optical pulse trains from the signal coherent optical pulse trains from the signal com-
combiner-splitter and produces a leading coherent biner-splitter and produces a low-band optical signal
optical pulse trains from the combined coherent 30 from the combined coherent optical pulse trains;
optical pulse trains; a pulse interference detector in communication with the
a lagging arm delay stage in communication with the high-band BPF and that receives the high-band opti-
signal combiner-splitter and that receives the com- cal signal from the high-band BPF and produces a
bined coherent optical pulse trains from the signal high-band interference signal from the high-band
combiner-splitter and produces a lagging coherent 35 optical signal;
optical pulse trains from the combined coherent a second pulse interference detector in communication
optical pulse trains; with the low-band BPF and that receives the low-
a leading arm delay stage in communication with the band optical signal from the low-band BPF, and
signal combiner-splitter and that receives the com- produces a low-band interference signal from the
bined coherent optical pulse trains from the signal 40 low-band optical signal;
combiner-splitter and produces a leading coherent a phase comparator in communication with the pulse
optical pulse trains from the combined coherent interference detector and the second pulse interfer-
optical pulse trains; ence detector and that receives the high-band inter-
a second signal combiner-splitter in communication ference signal from the pulse interference detector,
with the leading arm delay stage and that receives the 45 receives the low-band interference signal from the
leading coherent optical pulse trains from the leading second pulse interference detector, and produces a
arm delay stage, receives the lagging coherent opti- time offset signal from the high-band interference
cal pulse trains from the lagging arm delay stage, and signal and the low-band interference signal; and
produces an optical timing signals from the leading the CW heterodyne detector receives an incoming CW
coherent optical pulse trains and the lagging coherent so optical signal from the incoming optical timing signals
optical pulse trains; and CW optical signal from the tracking optical timing
a fixed delay stage in communication with the signal signal and produces a frequency offset signal from the
combiner-splitter and that receives the optical timing incoming CW optical signal and the CW optical signal,
signals from the signal combiner-splitter and pro- wherein the time-frequency offset measurement system
duces a time offset signal from the optical timing ss combines the time offset signal from the second comb

signals; and

timing discriminator and the frequency offset signal

the CW heterodyne detector that receives an incoming
CW optical signal from the incoming optical timing quency offset signals.
signals and CW optical signal from the tracking optical 7. The synchronizer of claim 1, wherein the time-fre-
timing signal and produces a frequency offset signal 60 quency offset measurement system further comprises a CW
from the incoming CW optical signal and the CW _ heterodyne detector, such that:
optical signal, the second comb timing discriminator comprises:
wherein the time-frequency offset measurement system a dispersion compensation unit that:

from the CW heterodyne detector into the time-fre-

combines the time offset signal from the second comb
timing discriminator and the frequency offset signal
from the CW heterodyne detector into the time-fre-
quency offset signals.

receives an incoming clock comb signal from the
incoming optical timing signals and tracking
comb signal from the tracking optical timing sig-
nal; and


U.S. Patent Nov. 29, 2022 Sheet 12 of 13 US 11,515,994 B2

300 Process for performing synchronization with synchronizer

301 Phase-locking optical timing signals (clock comb, optional CW
laser) to reference oscillator and tracking comb to reference

oscillator
. . c i
302 Measuring time offset between clock comb and tracking
comb; repeating this step if a phase slip occurs
i
303. Acquiring temporal-frequency overlap between incoming

optical timing signal and tracking optical timing signal with a
search pattern; repeating step if long duration link loss occurs

we
ie

Precisely measure timing and frequency offset of tracking
optical timing signal relative to incoming optical timing signals

305 Determining one-way timing data by combining measurements
with current tracking optical timing settings

I | |

306 ati 307 308
Communicating feedback to .
tracking optical timing Storing one-way Communicating
signals to lock onto timing data with the other
incoming optical timing synchronizer
signals
Post-process Real-time

309 Combining one-way timing data to produce two-way timing
data (c.g. clock offset, time-of-flight, and relative velocity)

J

310 Providing feedback to clock comb

11 Communicating timing data to user



US 11,515,994 B2

3
from the optical link; and communicating, by the bidiree-
tional terminal, the clock comb signal to the optical link
from the clock frequency comb as part of the synchroniza-
tion data.

BRIEF DI

IPTION OF THE DRAWINGS

The following description should not be considered lim-
iting in any way. With reference to the accompanying
drawings, like elements are numbered alike.

FIG. 1 shows a synchronizer;

FIG. 2 shows a plurality of synchronizer in communi-
cation via an optical link and a communication channel;

FIG. 3 shows a plurality of synchronizers in communi-
cation via an optical link and a communication channel;

FIG. 4 shows a time-frequency offset measurement sys-
tem;

FIG. 5 show a time-frequency offset measurement sys-
tem;

FIG.
tem;

FIG. 7 shows a time-frequency offset measurement sys-
tem;

FIG. 8 shows a tracking frequency comb;

FIG. 9 shows a clock frequency comb;

FIG. 10 shows a comb digital controller;

FIG. 11 shows a plurality of synchronizers in communi-
cation via an optical link and a communication channel;

FIG. 12 shows steps in a process for synchronizing
transfer over an optical link; and

FIG. 13 shows a configuration for a coherent repeater
operation.

6 shows a time-frequency offset measurement s

DETAILED DESCRIPTION.

A detailed description of one or more embodiments is
presented herein by way of exemplification and not limita-
tion,

It has been discovered that a synchronizer with a tracking
comb provides photon-eflicient agile comb optical clock
synchronization for ultra-precise time or frequency transfer
cover an optical link, e.g., a free-space link, with a distance
‘of communication among the synchronizer and connected
devices from kilometers (km) to thousands of kilometers.
Frequency-comb-based optical two-way time-frequency
transfer provides frequency comparison, time comparison,
frequency synchronization or full time synchronization
between distance-separated clocks on a sub-femtosecond
timescale. The synchronizer provides synchronous operation

of a tracking frequency comb with an incoming clock s

frequency comb signal to retain sub-femtosecond timing and
to increase measurement signal-to-noise by over 5000 times
and distance of operation from the synchronizer to a con-
nected device by multiple orders of magnitude over previous

optically based hardware and optically-based methods. The s

frequency and timing transfer is accomplished with over
1000 times greater precision than is possible with conven-
tional microwave-based hardware or conventional micro-
wave-based methods.

Synchronizer 200 performs synchronizing transfer over
an optical link enabling frequency comparison, time com-
parison, frequency synchronization, or full time synchroni-
zation between sites at each end of the optical link. In an
embodiment, with reference to FIG. 1 and FIG, 2, synchro-
nizer 200 includes: frequency reference oscillator 201 that
produces frequency reference signal 227; a tracking optical
timing source 209 in communication with frequency refer-

3

3

&

&

4

ence oscillator 201 that includes tracking frequency comb
202 and that: receives, frequency reference signal 227 from
frequency reference oscillator 201 to phase lock the tracking
frequency comb 202; produces, by tracking frequency comb
202, tracking comb signal 225: receives, tracking comb
feedback 211 from signal processor-controller 204; produces
tracking optical timing signal 220 from the tracking fre-
quency comb 202; comb timing discriminator 206 in com-
munication with tracking optical timing source 209 and
clock frequency comb 203 and that: receives tracking comb
signal 225 from tracking frequency comb 202; receives
clock comb signal 224 from clock frequency comb 203; and
produces clock comb-tracking comb time offset 212 from
clock comb signal 224 and tracking comb signal 225; clock
frequency comb 203 in communication with frequency
reference oscillator 201, and bidirectional terminal 207 and
that: receives frequency reference signal 227 from frequency
reference oscillator 201; produces clock comb signal 224
based on frequency reference signal 227; produces clock
output 210 based on frequency reference signal 227; and
communicates clock comb signal 224 to comb timing dis-
criminator 206 and bidirectional terminal 207; signal pro-
cessor-controller 204 in communication with tracking opti-
cal timing source 209, time-frequency offset measurement

5 system 205, and comb timing discriminator 206 and that:

receives clock comb-tracking comb time offset 212 from
comb timing discriminator 206; receives time-frequency
offset signals 219 from time-frequency offset measurement
system 205; produces, based on clock comb-tracking comb
time offset 212 and time-frequency offset signals 219, track-
ing comb feedback 211 and local one-way data 221; time.
frequency offset measurement system 205 in communica-
tion with tracking optical timing source 209, bidirectional
terminal 207, and signal processor-controller 204; includes
second comb timing discriminator 206.2; and that: receives
tracking optical timing signal 220 from tracking optical
timing source 209; receives incoming optical timing signals
217 from bidirectional terminal 207; produces time-fre-
quency offset signals 219 from tracking optical timing signal
220 and incoming optical timing signals 217; and commu-
nicates time-frequency offset signals 219 to signal proces-
sor-controller 204; and bidirectional terminal 207 in com-
munication with clock frequency comb 203 and time-
frequency offset measurement system 205 and that: receives
clock comb signal 224 from clock frequency comb 203;
receives synchronization data 270 from optical link 218,
synchronization data 270 including incoming optical timing
signals 217; communicates incoming optical timing signals
217 to time-frequency offset measurement system 205 from
optical link 218; and communicates clock comb signal 224
to optical link 218 from clock frequency comb 203 as part
of synchronization data 270. The synchronization data 270
is the bidirectional data transmitted over the optical link 218
consisting of the outgoing optical timing signals 216 from
both Synchronizer 200 and Synchronizer 200.2.

In an embodiment, with reference to FIG. 1 and FIG. 2,
synchronizer 200 includes a CW laser 242 within tracking
optical timing source 209 and tracking frequency comb 202.
Tracking optical timing source 209 receives clock comb
signal 224 to phase lock the CW laser 242. Tracking optical
timing source 209 produces tracking optical timing signal
220 comprised of CW optical signal 226 and tracking comb
signal 225. Tracking optical timing source 209 produces CW
optical signal 226 for combination with clock comb signal
224 to generate outgoing optical timing signals 216. Time-
frequency offset measurement system 205 further includes
CW heterodyne detector 208 that measures the heterodyne