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Microresonator frequency comb optical clock
S
COTT
B. P
APP
,
1,
*K
ATJA
B
EHA
,
1
P
ASCAL
D
EL
H
AYE
,
1
F
RANKLYN
Q
UINLAN
,
1
H
ANSUEK
L
EE
,
2
K
ERRY
J. V
AHALA
,
2
AND
S
COTT
A. D
IDDAMS
1
1
Time and Frequency Division 688, National Institute of Standards and Technology, Boulder, Colorado 80305, USA
2
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
*Corresponding author: scott.papp@nist.gov
Received 4 April 2014; revised 16 June 2014; accepted 17 June 2014 (Doc. ID 209505); published 22 July 2014
Optical frequency combs serve as the clockwork of optical clocks, which are now the best time-keeping
systems in existence. The use of precise optical time and frequency technology in various applications be-
yond the research lab remains a significant challenge, but one that integrated microresonator technology is
poised to address. Here, we report a silicon-chip-based microresonator comb optical clock that converts an
optical frequency reference to a microwave signal. A comb spectrum with a 25 THz span is generated with a
2 mm diameter silica disk and broadening in nonlinear fiber. This spectrum is stabilized to rubidium fre-
quency references separated by 3.5 THz by controlling two teeth 108 modes apart. The optical clock
s
output is the electronically countable 33 GHz microcomb line spacing, which features stability better than
the rubidium transitions by the expected factor of 108. Our work demonstrates the comprehensive set of
tools needed for interfacing microcombs to state-of-the-art optical clocks.
OCIS codes:
(140.3945) Microcavities; (190.4410) Nonlinear optics, parametric processes; (230.4910) Oscillators.
http://dx.doi.org/10.1364/OPTICA.1.000010
1. INTRODUCTION
Optical frequency combs enable extraordinary measurement
precision and accuracy entirely commensurate with their refer-
ence oscillator. A new direction in experiments is the creation
of ultracompact combs via parametric nonlinear optics in mi-
croresonators [
1
,
2
]. We refer to these as microcombs, and here
we report a silicon-chip-based microcomb optical clock that
phase-coherently converts an optical reference to a microwave
signal.
Optical clocks leverage the narrow, unvarying transitions of
atoms to realize exceptionally stable laser frequencies measured
at below the
10
17
level [
3
]. Optical frequency combs facilitate
the measurement and use of these atomic references by pro-
viding a set of clock-referenced lines that span more than
an octave [
4
]. Moreover, they have enabled advances in diverse
fields from spectroscopy of atoms and molecules [
5
,
6
]to
astronomy [
7
].
A new type of frequency comb has emerged based on op-
tical microresonators [
1
,
2
]. Here, the comb generation relies
on nonlinear parametric oscillation and cascaded four-
wave mixing driven by a CW laser. Such microcombs offer
revolutionary advantages over existing comb technology,
including chip-based photonic integration, uniquely large
comb-mode spacings in the tens of gigahertz range, and mono-
lithic construction with small size and power consumption.
Microcomb development has included frequency control of
their spectra [
8
11
], characterization of their noise properties
[
12
14
], a Rb-stabilized microcomb oscillator [
15
], and dem-
onstration of phase-locked [
12
,
16
,
17
] and mode-locked states
[
18
,
19
]. However, the milestone of all-optical frequency con-
trol of a microcomb to an atomic reference, including fre-
quency division to the microwave domain, has not been
achieved.
In this paper, we report the achievement of this goal by
demonstrating a functional optical clock based on full stabili-
zation of a microcomb to atomic Rb transitions. We generate a
low-noise, continuously equidistant microcomb spectrum by
use of an on-chip silica microresonator. The clock output is
the 33 GHz microcomb line spacing, which is electronically
measurable, and a traceable integer partition of the 3.5 THz
frequency spacing of the Rb references. Here, we explore the
basic features of this microcomb clock. Its
5
×
10
9
Allan
Research Article
Vol. 1, No. 1 / July 2014 /
Optica
10
deviation for 1 s averaging is completely dominated by the Rb
reference, and the microcomb contribution is only
<
2
×
10
14
at 1 s, indicating that much more stable clocks could be sup-
ported. Our results highlight an architecture for the integration
of microcombs with other high-performance and chip-scale
atomic frequency references [
20
].
2. EXPERIMENTAL METHODS
Figure
1(a)
shows a schematic of our microcomb optical clock.
A 2 mm diameter disk resonator with a 10° wedge side profile
provides parametric comb generation. The resonator, which
has an unloaded quality factor of 63 million, is fabricated
on a silicon chip using conventional semiconductor fabrication
techniques [
21
]. Hence, the core of our system is scalable and
could be integrated with other on-chip photonic elements, and
eventually atomic systems [
20
,
22
]. In these experiments we use
a tapered fiber for evanescent coupling [
23
]. We excite the disk
resonator with light from a CW laser (optical frequency
v
p
)
that is intensity modulated at frequency
f
eo
and amplified
to a maximum of 140 mW. The first-order sideband powers
are approximately 3 dB lower than the pump, and the piece of
highly nonlinear fiber (HNLF) before the disk resonator in-
creases the second-order (third-order) sidebands to 12 (25) dB
below the pump. The modulation implements our parametric
seeding technique [
11
], which enables unmatched control of
the microcomb line spacing. Here we further demonstrate that
parametric seeding enables the complete suppression of unde-
sirable, nonequidistant subcombs. Following generation in the
disk resonator, the microcomb output is optically filtered to
attenuate the pump laser and modulation sidebands; the result-
ing spectrum is shown by the top trace in Fig.
1(b)
. The micro-
comb bandwidth is approximately a factor of 10 higher than
the seeding comb. By amplifying the microcomb spectrum and
without any dispersion control, we broaden the initial 20 nm
bandwidth an additional factor of 10 to 200 nm. The
2ps
duration optical waveform obtained directly from the micro-
resonator offers sufficient peak power for our experiments and
is stable and repeatable even for different settings of pump
frequency and power, intensity modulation, taper
resonator
coupling, and pump polarization. The broadened spectrum
[Fig.
1(b)
] overlaps with the resonance frequencies of mole-
cules such as HCN,
C
2
H
2
,
CO
2
,
CH
4
, and atomic Rb and
K after second-harmonic generation.
For frequency stabilization, we heterodyne the microcomb
spectrum with telecom-grade semiconductor distributed feed-
back (DFB) lasers at 1560 and 1590 nm. These lasers are
frequency doubled and stabilized to well-known Rb transitions
[
24
26
]. Precise Rb spectroscopic data, especially near
780 nm, exist, and with attention to systematic effects a sta-
bility of
10
12
ffiffiffi
τ
p
has been demonstrated [
24
]. Therefore, we
focus only on salient details for controlling the microcomb
with these optical references. To operate an optical clock,
we stabilize the microcomb
stwo
independent
degrees of free-
dom to the Rb references by leveraging frequency control of its
spectrum. The central line of the microcomb is phase locked to
the 1560 nm DFB laser, which is separate from the pump la-
ser. [
9
,
15
]. Additionally, the 108th comb line from the center,
which we obtain via spectral broadening, is phase locked to the
1590 nm DFB laser by tuning the microcomb line spacing
through control of
f
eo
.
3. RESULTS AND DISCUSSION
The output of our microcomb optical clock is obtained via pho-
todetection of the
Δ
v

32.9819213 GHz
line spacing, which
reflects the frequency difference
Δ
Rb
of the D2- and D1-
stabilized lasers divided by 108, and a fixed
660
108 MHz
offset for phase stabilization. This specific offset arises because
the comb
s central mode is phase locked at a frequency
920 MHz higher than the 1560 nm laser, while mode 108 is
phase locked 260 MHz higher than the 1590 nm laser. The
offset could assume a range of predetermined values, including
zero, and the microresonator free spectral range could be tar-
geted to utilize a specific value. The data points in Fig.
1(c)
are a continuous
>
12 h
long record of the clock output. Here,
the vertical axis shows the difference between the clock output
and
Δ
Rb

3
;
561
;
387
;
470

180

kHz
, whose uncertainty
[
24
26
] is shown by the gray band. Although we have not sys-
tematically analyzed the accuracy of our clock, its output is in
reasonable agreement with these previous data. The 271 Hz
RMS fluctuation in a 20 s average of the clock output is signifi-
cantly reduced from those of the D2 and D1 reference lasers,
due to the principle of optical frequency division associated with
(a)
(b)
(c)
Fig. 1.
Microcomb optical clock with Rb atoms. (a) An intensity-
modulated pump laser excites a chip-based microresonator (see micro-
graph at right) to create a 33 GHz spacing comb. The comb is broadened
in highly nonlinear fiber (HNLF) following amplification to 1.4 W. Two
lines of the comb 108 modes apart are stabilized to Rb transitions by
control of the pump frequency and the intensity modulation
f
eo
.
The clock output is obtained via photodetection of the unbroadened
spectrum. Not shown are polarization controllers, which are needed be-
fore the intensity modulator, the microresonator, the HNLF, and all the
elements of the Rb spectrometers. Other components are an optical
bandpass filter (BPF), a bandreject filter (BRF), and two erbium-doped
fiber amplifiers (EDFA). (b) Optical spectrum after a filter to suppress the
pump (top) and following spectral broadening (bottom), (c) optical clock
output over 12 h. Each point is the average of twenty 1 s measurements.
For comparison, published Rb spectroscopic data on the D2
D1 differ-
ence divided by 108 has been subtracted. The solid [
25
] and hatched [
26
]
gray regions represent previous data.
Research Article
Vol. 1, No. 1 / July 2014 /
Optica
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