of 8
ARTICLE
Towards visible soliton microcomb generation
Seung Hoon Lee
1
, Dong Yoon Oh
1
, Qi-Fan Yang
1
, Boqiang Shen
1
, Heming Wang
1
, Ki Youl Yang
1
,
Yu-Hung Lai
1
,XuYi
1
, Xinbai Li
1,2
& Kerry Vahala
1
Frequency combs have applications that extend from the ultra-violet into the mid-infrared
bands. Microcombs, a miniature and often semiconductor-chip-based device, can potentially
access most of these applications, but are currently more limited in spectral reach. Here, we
demonstrate mode-locked silica microcombs with emission near the edge of the visible
spectrum. By using both geometrical and mode-hybridization dispersion control, devices are
engineered for soliton generation while also maintaining optical
Q
factors as high as
80 million. Electronics-bandwidth-compatible (20 GHz) soliton mode locking is achieved with
low pumping powers (parametric oscillation threshold powers as low as 5.4 mW). These are
the shortest wavelength soliton microcombs demonstrated to date and could be used in
miniature optical clocks. The results should also extend to visible and potentially ultra-violet
bands.
DOI: 10.1038/s41467-017-01473-9
OPEN
1
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA.
2
State Key Laboratory of Advanced Optical
Communication Systems and Networks, School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China. Seung Hoon
Lee,
Dong Yoon Oh, Qi-Fan Yang, Boqiang Shen, and Heming Wang contributed equally to this work. Correspondence and requests for materials should be
addressed to K.V. (email:
vahala@caltech.edu
)
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1
1234567890
S
oliton mode locking
1
5
in frequency microcombs
6
provides
a pathway to miniaturize many conventional comb
applications. It has also opened investigations into new
nonlinear physics associated with dissipative Kerr solitons
1
and Stokes solitons
7
. In contrast to early microcombs
6
,
soliton microcombs eliminate instabilities, provide stable
(low-phase-noise) mode locking, and feature a highly repro-
ducible spectral envelope. Many applications of these devices are
being studied, including chip-based optical frequency synthesis
8
,
secondary time standards
9
, and dual-comb spectroscopy
10
12
.
Also, a range of operating wavelengths is opening up by use
of several low-optical-loss dielectric materials for resonator
fabrication. In the near-infrared (IR), microcombs based on
magnesium
fl
uoride
1
, silica
2
,
13
, and silicon nitride
3
5
,
14
,
15
are
being studied for frequency metrology and frequency synthesis.
In the mid-IR spectral region silicon nitride
16
, crystalline
17
, and
silicon-based
18
Kerr microcombs, as well as quantum-cascade
microcombs
19
are being studied for application to molecular
fi
ngerprinting.
At shorter wavelengths below 1
μ
m microcomb technology
would bene
fi
t optical atomic clock technology
20
, particularly
efforts to miniaturize these clocks. For example, microcomb
optical clocks based on the D1 transition (795 nm) and the
two-photon clock transition
21
(798 nm) in rubidium have been
proposed
9
,
22
. Also, a microcomb clock using two-point locking to
rubidium D1 and D2 lines has been demonstrated
23
by frequency
doubling from the near-IR. More generally, microcomb sources
in the visible and ultra-violet bands could provide a miniature
alternative to larger mode-locked systems such as titanium
sapphire lasers in cases where high power is not required. It is
also possible that these shorter wavelength systems could be
applied in optical coherence tomography systems
24
26
. Efforts
directed toward short wavelength microcomb operation include
1
μ
m microcombs in silicon nitride microresonators
27
as well
as harmonically generated combs. The latter have successfully
converted near-IR comb light to shorter wavelength bands
28
and
even into the visible band
29
,
30
within the same resonator used to
create the initial comb of near-IR frequencies. Also, crystalline
t
(
μ
m)
700
1000
1300
1600
2
4
6
8

Wavelength (nm)
Pump at 1550 nm
Pump at 1064 nm
Pump at 778 nm
I
b
e
d
a
II
III
c
t
185
190
195
Power
260
280
380
385
390
Frequency (THz)
0
10
20
P
th
(mW)
0
100
200
300
Q
(M)
2468
0
20
40
t
(
μ
m)
A
eff
(
μ
m
2
)
778
1064
1550
Wavelength (nm)
Pump

ZDW
(TM1)
I
II
III
Normal
Anomalous
Bulk silica
TM1-TE2 hybrid band
20 dB
Fig. 1
Soliton frequency comb generation in dispersion-engineered silica resonators.
a
A rendering of a silica resonator with the calculated TM1 mode
pro
fi
le superimposed.
b
Regions of normal and anomalous dispersion are shown vs. silica resonator thickness (
t
) and pump wavelength. The zero
dispersion wavelength (
λ
ZDW
) for the TM1 mode appears as a blue curve. The dark green band shows the 10-dB bandwidth of anomalous dispersion
created by TM1-TE2 mode hybridization. The plot is made for a 3.2-mm diameter silica resonator with a 40° wedge angle. Three different device types I, I
I,
and III (corresponding to
t
=
7.9, 3.4, and 1.5
μ
m) are indicated for soliton generation at 1550, 1064, and 778 nm.
c
Measured
Q
factors and parametric
oscillation threshold powers vs. thickness and pump wavelength for the three device types. Powers are measured in the tapered
fi
ber coupler under critical
coupling. Effective mode area (
A
eff
) of the TM1 mode family is also plotted as a function of wavelength and thickness.
d
A photograph of a silica resonator
(Type III device pumped at 778 nm) while generating a soliton stream. The pump light is coupled via a tapered
fi
ber from the left side of the resonator. The
red light along the circumference of the resonator and at the right side of the taper is believed to result from short wavelength components of the solit
on
comb.
e
Soliton frequency comb spectra measured from the devices. The red, green, and blue soliton spectra correspond to device types I, II, and III
designed for pump wavelengths 1550, 1064, and 778 nm, respectively. Pump frequency location is indicated by a dashed vertical line. The soliton pulse
repetition rate of all devices is about 20 GHz. Differences in SNR of the spectra originate from the resolution of the optical spectrum analyzer (OSA).
In particular, the 778 nm comb spectrum was measured using the second-order diffracted spectrum of the OSA, while other comb spectra were measured
as
fi
rst-order diffracted spectra. Insets: cross-sectional SEM images of the fabricated resonators. White scale bar is 5
μ
m
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resonators
31
and silica microbubble resonators
32
have been
dispersion-engineered for comb generation in the 700 nm band.
Finally, diamond-based microcombs afford the possibility of
broad wavelength coverage
33
. However, none of the short wave-
length microcomb systems have so far been able to generate stable
mode-locked microcombs as required in all comb applications.
A key impediment to mode-locked microcomb operation at
short wavelengths is material dispersion associated with the
various dielectric materials used for microresonator fabrication.
At shorter wavelengths, these materials feature large normal
dispersion that dramatically increases into the visible and ultra-
violet bands. While dark soliton pulses can be generated in a
regime of normal dispersion
34
, bright solitons require anomalous
dispersion. Dispersion engineering by proper design of the
resonator geometry
22
,
31
,
32
,
35
41
offers a possible way to offset the
normal dispersion. Typically, by compressing the waveguide
dimension of a resonator, geometrical dispersion will ultimately
compensate a large normal material dispersion component to
produce overall anomalous dispersion. For example, in silica,
strong con
fi
nement in bubble resonators
32
and straight wave-
guides
42
has been used to push the anomalous dispersion tran-
sition wavelength from the near-IR into the visible band. Phase
matching to ultra-violet dispersive waves has also been demon-
strated using this technique
42
. However, to compensate the rising
material dispersion this compression must increase as the
operational wavelength is decreased, and as a side effect, highly
con
fi
ned waveguides tend to suffer increased optical losses. This
happens because mode overlap with the dielectric waveguide
interface is greater with reduced waveguide cross-section. Con-
sequently, the residual fabrication-induced roughness of that
interface degrades the resonator
Q
factor and increases pumping
power (e.g., comb threshold power varies inverse quadratically
with
Q
factor
43
).
Minimizing material dispersion provides one way to ease the
impact of these constraints. In this sense, silica offers an excellent
material for short wavelength operation, because it has the lowest
dispersion among all on-chip integrable materials. For example,
at 778 nm, silica has a group velocity dispersion (GVD) equal to
38 ps
2
km
1
, which is over
fi
ve times smaller than the GVD of
silicon nitride at this wavelength (
>
200 ps
2
km
1
)
44
. Other
integrable materials that are also transparent in the visible, such
as diamond
33
and aluminum nitride
45
, have dispersion that is
similar to or higher than silicon nitride. Silica also features a
spectrally broad low-optical-loss window so that optical
Q
factors
can be high at short wavelengths. Here, we demonstrate soliton
microcombs with pump wavelengths of 1064 and 778 nm. These
are the shortest soliton microcomb wavelengths demonstrated to
date. By engineering geometrical dispersion and by employing
mode hybridization, a net anomalous dispersion is achieved at
these wavelengths while also maintaining high optical
Q
factors
(80 million at 778 nm, 90 million at 1064 nm). The devices have
large (millimeter-scale) diameters and produce single soliton
pulse streams at rates that are both detectable and processable by
low-cost electronic circuits. Besides illustrating the
fl
exibility of
silica for soliton microcomb generation across a range of short
wavelengths, these results are relevant to potential secondary time
standards based on transitions in rubidium
9
,
22
,
23
. Using
dispersive-wave engineering in silica it might also be possible to
extend the emission of these combs into the ultra-violet as
recently demonstrated in compact silica waveguides
42
.
20 dB
d
b
0
50
0
100
Relative mode number
μ
50
100
–50
–100
1100
Wavelength (nm)
1150
1050
1000
Power (dBm)
–70
–60
–20
–30
–40
–50
Frequency (MHz
+ 20.28535 GHz)
–1
0
1
Soliton A
Pump
Sech
2
fitting (44 fs)
Soliton B
Sech
2
fitting (52 fs)
YDFA
PM
μ
disk
FBG
CW laser
PD
PC
Feedback loop
90/10
a
c
0
5
10
–5
–10
TM1
TM2
TM3
Thickness (
μ
m)
4
3.5
3
Normal
Anomalous

2
(ps
2
km
–1
)
Servo
(

μ

0

D
1
)/2

(MHz)
Fig. 2
Microresonator dispersion engineering and soliton generation at 1064 nm.
a
Simulated GVD of TM mode families vs. resonator thickness. The angle
of the wedge ranges from 30° to 40° in the colored regions. Measured data points are indicated and agree well with the simulation. The error bars depict
standard deviations obtained from measurement of eight samples having the same thickness.
b
Measured relative mode frequencies (blue points) plotted
vs. relative mode number of a soliton-forming TM1 mode family in a 3.4
μ
m thick resonator. The red curve is a parabolic
fi
t yielding
D
2
/2
π
=
3.3 kHz.
c
Experimental setup for soliton generation. A CW
fi
ber laser is modulated by an electro-optic PM before coupling to a YDFA. The pump light is then
coupled to the resonator using a tapered
fi
ber. Part of the comb power is used to servo-control the pump laser frequency.
d
Optical spectra of solitons at
1064 nm generated from the mode family shown in
b
. The two soliton spectra correspond to different power levels with the blue spectrum being a higher
power and wider bandwidth soliton. The dashed vertical line shows the location of the pump frequency. The solid curves are sech
2
fi
ttings. Inset: typical
detected electrical beatnote showing soliton repetition rate. The weak sidebands are induced by the feedback loop used to stabilize the soliton. The
resolution bandwidth is 1 kHz. FBG
fi
ber Bragg grating, PD photodetector, PC polarization controller
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3
Results
Silica resonator design
. The silica resonator used in this work
is shown schematically in Fig.
1
a. A fundamental mode pro
fi
le
is overlaid onto the cross-sectional rendering. The resonator
design is a variation on the wedge resonator
46
, and its geometry
can be fully characterized by its resonator diameter, silica thick-
ness (
t
), and wedge angle (
θ
) (see Fig.
1
a). The diameter of all-
resonators in this work (and the assumed diameter in all
simulations) is 3.2 mm, which corresponds to a free spectral range
(FSR) of approximately 20 GHz, and the resonator thickness is
controlled to obtain net anomalous dispersion at the design
wavelengths, as described in detail below. Further details on
fabrication are given elsewhere
46
. As an aside, we note that a
waveguide-integrated version of this design is also possible
47
.
Adaptation of that device using the methods described
here would enable full integration with other photonic elements
on the silicon chip.
Figure
1
b illustrates how the geometrical dispersion induced by
varying resonator thickness
t
offsets the material dispersion.
Regions of anomalous and normal dispersion are shown for the
TM1 mode family of a resonator having a wedge angle of 40°. The
plots show that thinner resonators enable shorter wavelength
solitons. Accordingly, three device types (I, II, and III shown as
the colored dots in Fig.
1
b) are selected for soliton frequency
comb operation at three different pump wavelengths. At a pump
wavelength of 1550 nm, the anomalous dispersion window is
wide because bulk silica possesses anomalous dispersion at
wavelengths above 1270 nm. For this type I device, a 7.9-
μ
m
thickness was used. Devices of type II and III have thicknesses
near 3.4 and 1.5
μ
m for operation with pump wavelengths of 1064
and 778 nm, respectively. Beyond geometrical control of disper-
sion, the type III design also uses mode hybridization to
substantially boost the anomalous dispersion. This hybridization
occurs within a relatively narrow wavelength band which tunes
with
t
(darker green region in Fig.
1
b) and is discussed in detail
below. Measured
Q
factors for the three device types are plotted
in the upper panel of Fig.
1
c. Maximum
Q
factors at thicknesses
which also produce anomalous dispersion were: 280 million
(Type I, 1550 nm), 90 million (Type II, 1064 nm), and 80 million
(Type III, 778 nm).
Using these three designs, soliton frequency combs were
successfully generated with low threshold pump power. Shown in
Fig.
1
d is a photograph of a type III device under conditions
where it is generating solitons. Figure
1
e shows optical spectra of
the soliton microcombs generated for each device type. A slight
Raman-induced soliton self-frequency-shift is observable in the
type I and type II devices
2
,
48
50
. The pulse width of the type III
device is longer and has a relatively smaller Raman shift, which is
consistent with theory
50
. The presence of a dispersive wave in this
spectrum also somewhat offsets the smaller Raman shift
3
.
Scanning electron microscope (SEM) images appear as insets in
Fig.
1
e and provide cross-sectional views of the three device types.
It is worthwhile to note that microcomb threshold power,
expressed as
P
th

A
eff
=
λ
P
Q
2
(
λ
P
is pump wavelength and
A
eff
is
effective mode area) remains within a close range of powers for all
devices (lower panel of Fig.
1
c). This can be understood to result
from a partial compensation of reduced
Q
factor in the shorter
wavelength devices by reduced optical mode area (see plot in
Fig.
1
c). For example, from 1550 to 778 nm the mode area is
reduced by roughly a factor of 9 and this helps to offset a three
times decrease in
Q
factor. The resulting
P
th
increase (5.4 mW at
778 nm vs. approximately 2.5 mW at 1550 nm) is therefore
caused primarily by the decrease in pump wavelength
λ
P
. In the
following sections additional details on the device design,
dispersion, and experimental techniques used to generate these
solitons are presented.
Soliton generation at 1064 nm
. Dispersion simulations for TM
modes near 1064 nm are presented in Fig.
2
aandshowthatTM
modes with anomalous dispersion occur in silica resonators having
oxide thicknesses less than 3.7
μ
m. Aside from the thickness con-
trol, a secondary method to manipulate dispersion is by changing
the wedge angle (see Fig.
2
a). Both thickness and wedge angle are
well controlled in the fabrication process
41
. Precise thickness con-
trol is possible because this layer is formed through calibrated
oxidation of the silicon wafer. Wedge angles between 30° and 40°
were chosen in order to maximize the
Q
factors
46
. The resonator
dispersion is characterized by measuring mode frequencies using a
scanning external-cavity diode laser (ECDL) whose frequency is
calibrated using a Mach
Zehnder interferometer. As described
elsewhere
1
,
2
the mode frequencies,
ω
μ
, are Taylor expanded as
ω
μ
¼
ω
0
þ
μ
D
1
þ
μ
2
D
2
=
2
þ
μ
3
D
3
=
6, where
ω
0
denotes the
pumped mode frequency,
D
1
/2
π
is the FSR, and
D
2
is proportional
to the GVD,
β
2
(
D
2
¼
cD
2
1
β
2
=
n
0
,where
c
and
n
0
are the speed of
light and material refractive index).
D
3
is a third-order expansion
term that is sometimes necessary to adequately
fi
t the spectra (see
discussion of 778 nm soliton below). The measured frequency
spectrum of the TM1 mode family in a 3.4
μ
m thick resonator is
plotted in Fig.
2
b. The plot gives the frequency as relative frequency
(i.e.,
ω
μ

ω
0

μ
D
1
) to make clear the second-order dispersion
contribution. The frequencies are measured using a radio-frequency
calibrated Mach
Zehnder interferometer having a FSR of
approximately 40 MHz. Also shown is a
fi
tted parabola (red curve)
revealing
D
2
/2
π
=
3.3 kHz (positive parabolic curvature indicates
anomalous dispersion). Some avoided mode crossings are observed
in the spectrum. The dispersion measured in resonators of different
thicknesses, marked as solid dots in Fig.
2
a, is in good agreement
with numerical simulations.
The experimental setup for generation of 1064 nm pumped
solitons is shown in Fig.
2
c. The microresonator is pumped by a
continuous wave (CW) laser ampli
fi
ed by a ytterbium-doped
fi
ber
ampli
fi
er (YDFA). The pump light and comb power are coupled
to and from the resonator by a tapered
fi
ber
51
,
52
. Typical
pumping power is around 100 mW. Solitons are generated while
scanning the laser from higher frequencies to lower frequencies
across the pump mode
1
3
. The pump light is modulated by an
electro-optic phase modulator (PM) to overcome the thermal
transient during soliton generation
2
,
3
,
53
. A servo control
referenced to the soliton power is employed to capture and
stabilize the solitons
53
. Shown in Fig.
2
d are the optical spectra of
solitons pumped at 1064 nm. These solitons are generated using
the mode family whose dispersion is characterized in Fig.
2
b. Due
to the relatively low dispersion (small
D
2
), these solitons have a
short temporal pulse width. Using the hyperbolic-secant-squared
fi
tting method
2
(see orange and green curves in Fig.
2
d), a soliton
pulse width of 52 fs is estimated for the red spectrum. By
increasing the soliton power (blue spectrum) the soliton can be
further compressed to 44 fs, which corresponds to a duty cycle of
0.09% at the 20 GHz repetition rate. Finally, the inset in Fig.
2
d
shows the electrical spectrum of the photo-detected soliton pulse
stream. Besides con
fi
rming the repetition frequency, the spectrum
is very stable with excellent signal-to-noise ratio (SNR) greater
than 70 dB at 1 kHz resolution bandwidth.
Soliton generation at 778 nm
. As the operational wavelength
shifts further toward the visible band, normal material dispersion
increases. To generate solitons at 778 nm an additional dispersion
engineering method, TM1-TE2 mode hybridization, is therefore
added to supplement the geometrical dispersion control. The
green band region in Fig.
1
b gives the oxide thicknesses and
wavelengths where this hybridization is prominent. Polarization
mode hybridization is a form of mode coupling-induced
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dispersion control
22
,
38
,
39
,
54
. The coupling of the TM1 and TE2
modes creates two hybrid mode families, one of which features
strong anomalous dispersion. This hybridization is caused when a
degeneracy in the TM1 and TE2 effective indices is lifted by a
broken re
fl
ection symmetry of the resonator
55
. The wavelength at
which the degeneracy occurs is controlled by the oxide thickness
and determines the soliton operation wavelength. Finite element
method simulation in Fig.
3
a shows that at 778 nm the TM1 and
TE2 modes are expected to have the same effective index at the
oxide thickness 1.48
μ
m when the resonator features a re
fl
ection
symmetry through a plane that is both parallel to the resonator
surface and that lies at the center of the resonator. Such a sym-
metry exists when the resonator has vertical sidewalls or
equivalently a wedge angle
θ
=
90° (note: the wet-etch process
used to fabricate the wedge resonators does not support a vertical
side wall). A zoom-in of the effective index crossing is provided in
Fig.
3
b. In this re
fl
ection symmetric case, the two modes cross in
the effective-index plot without hybridization. However, in the
case of
θ
=
40° (Fig.
3
c), the symmetry is broken and the effective
index degeneracy is lifted. The resulting avoided crossing causes a
sudden transition in the GVD as shown in Fig.
3
d, and one of the
hybrid modes experiences enhanced anomalous dispersion.
To verify this effect, resonators having four different
thicknesses (
θ
=
40°) were fabricated and their dispersion was
characterized using the same method as for the 1064 nm soliton
device. The measured second-order dispersion values are plotted
as solid circles in Fig.
3
d and agree with the calculated values
given by the solid curves. Figure
3
e, f shows the measured relative
mode frequencies vs. mode number of the two modes for devices
with
t
=
1.47
μ
m and
t
=
1.49
μ
m. As before, upward curvature in
the data indicates anomalous dispersion. The dominant polariza-
tion component of the hybrid mode is also indicated on both
mode-family branches. The polarization mode hybridization
produces a strong anomalous dispersion component that can
compensate normal material dispersion over the entire band.
Moreover, the tuning of this component occurs over a range of
larger oxide thicknesses for which it would be impossible to
compensate material dispersion using geometrical control alone.
To project the application of this hybridization method to yet
shorter soliton wavelengths, Fig.
3
g summarizes calculations of
second-order dispersion at a series of oxide thicknesses. At a
thickness close to 1 micron, it should be possible to generate
solitons at the blue end of the visible spectrum. Moreover, wedge
resonators having these oxide
fi
lm thicknesses have been
fabricated during the course of this work. They are mechanically
stable with respect to stress-induced buckling
56
at silicon
undercut values that are suf
fi
cient for high-
Q
operation.
For soliton generation, the microresonator is pumped at
778 nm by frequency-doubling a CW ECDL operating at 1557 nm
(see Fig.
4
a). The 1557 nm laser is modulated by a quadrature
phase-shift keying (QPSK) modulator for frequency-kicking
57
and then ampli
fi
ed by an erbium-doped
fi
ber ampli
fi
er (EDFA).
The ampli
fi
ed light is sent into a periodically poled lithium
niobate (PPLN) device for second-harmonic generation. The
frequency-doubled output pump power at 778 nm is coupled to
the microresonator using a tapered
fi
ber. The pump power is
typically about 135 mW. The soliton capture and locking method
was again used to stabilize the solitons
53
. A zoom-in of the TM1
mode spectrum for
t
=
1.47
μ
m with a
fi
t that includes third-order
dispersion (red curve) is shown in Fig.
4
b. The impact of higher-
order dispersion on dissipative soliton formation has been
studied
58
,
59
. In the present case, the dispersion curve is well suited
for soliton formation. The optical spectrum of a 778 nm pumped
soliton formed on this mode family is shown in Fig.
4
c. It features a
TE1
TM1
TE2
TM2
1.45
1.5
1.4
1.45
1.5
1.55
1.427
1.430
1.433
1.436
t
(
μ
m)
t
(
μ
m)
n
eff
ab
c
d
TM1-TE2
hybrid
TM1
1.4
Normal
Anomalous
1.45
1.5
1.55
–200
0
200
TM1
TE2
TM1
TE2
0
–10
–20
0
–10
–20
0
200
TM1
TE2
TM1
TE2
0
200
Relative mode number
μ
e
g
t
= 1.49
μ
m
1.432
1.431
1.431
1.432
t
(
μ
m)
1.45
1.5
t
(
μ
m)
Relative mode number
μ
f
t
= 1.47
μ
m
TE2
Wavelength (nm)
380
400
460
480
540
560
580 660
680
700
200
100
0
–100
–200
800
750
660 640 620
540
520
450 440 430
Frequency (THz)
1.47
μ
m
1.27
μ
m
1.10
μ
m
0.96
μ
m
n
eff
n
eff

= 40
°
0
1

= 90
°
(

μ

0

D
1
)/2

(GHz)
(

μ

0

D
1
)/2

(GHz)

2
(ps
2
km
–1
)

2
(ps
2
km
–1
)

= 40
°

= 90
°
Fig. 3
Dispersion engineering and solition generation at 778 nm.
a
Calculated effective indices
n
eff
for TE1, TE2, TM1, and TM2 modes at 778 nm plotted vs.
thickness for a silica resonator with re
fl
ection symmetry (i.e.,
θ
=
90°). The TM1 and TE2 modes cross each other without hybridization.
b
Zoom-in of the
dashed box in
a
.
c
As in
b
but for a resonator with
θ
=
40°. An avoided crossing of TM1 and TE2 occurs due to mode hybridization. Insets of
b
,
c
show
simulated mode pro
fi
les (normalized electric
fi
eld) in resonators with
θ
=
90° and
θ
=
40°, respectively. The color bar is shown to the right.
d
Calculated
GVD of the two modes. For the
θ
=
40° case, hybridization causes a transition in the dispersion around the thickness 1.48
μ
m. The points are the measured
dispersion values.
e
,
f
Measured relative mode frequencies of the TM1 and TE2 mode families vs. relative mode number
μ
for devices with
t
=
1.47
μ
m and
t
=
1.49
μ
m.
g
Calculated total second-order dispersion vs. frequency (below) and wavelength (above) at four different oxide thicknesses (number in lower
left of each panel). Red and blue curves correspond to the two hybridized mode families. Anomalous dispersion is negative and shifts progressively to
bluer
wavelengths as thickness decreases. Background color gives the approximate corresponding color spectrum
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5
temporal pulse width of 145 fs as derived from a sech
2
fi
t(red
curve). The electrical spectrum of the photo-detected soliton
stream is provided in the inset in Fig.
4
c and exhibits high stability.
Figure
4
d gives the measured mode spectrum and
fi
tting under
conditions of slightly thicker oxide (
t
=
1.53
μ
m). In this case, the
polarization of the hybrid mode more strongly resembles the TE2
mode family. The overall magnitude of second-order dispersion is
also much lower than for the more strongly hybridized soliton in
Fig.
4
b, c. The corresponding measured soliton spectrum is
shown in Fig.
4
e and features a dispersive wave near 758 nm. The
location of the wave is predicted from the
fi
tting in Fig.
4
d (see
dashed vertical and horizontal lines). The dispersive wave exists
in a spectral region of overall normal dispersion, thereby
illustrating that dispersion engineering can provide a way to
further extend the soliton spectrum toward the visible band. As
an aside, the plot in Fig.
4
d has incorporated a correction to the
FSR (
D
1
) so that the soliton line is given as the horizontal dashed
black line. This correction results from the soliton red spectral
shift relative to the pump that is apparent in Fig.
4
e. This shift is a
combination of the Raman self shift
49
,
50
and some additional
dispersive wave recoil
3
. Finally, the detected beat note of the
soliton and dispersive wave is shown as the inset in Fig.
4
e. It is
overall somewhat broader than the beatnote of the other solitons,
but is nonetheless quite stable.
Discussion
We have demonstrated soliton microcombs at 778 and 1064 nm
using on-chip high-
Q
silica resonators. Material-limited normal
dispersion, which is dominant at these wavelengths, was com-
pensated by using geometrical dispersion through control of the
resonator thickness and wedge angle. At the shortest wavelength,
778 nm, mode hybridization was also utilized to achieve anom-
alous dispersion while maintaining high optical
Q
. These results
are the shortest wavelength soliton microcombs demonstrated to
date. Moreover, the hybridization method can be readily extended
so as to produce solitons over the entire visible band. The gen-
erated solitons have pulse repetition rates of 20 GHz at both
wavelengths. Such detectable and electronics-compatible repeti-
tion rate soliton microcombs at short wavelengths have direct
applications in the development of miniature optical clocks
9
,
22
,
23
and potentially optical coherence tomography
24
26
. Also, any
application requiring low-power near-visible mode-locked laser
sources will bene
fi
t. The same dispersion control methods used
here should be transferable to silica ridge resonator designs that
contain silicon nitride waveguides for on-chip coupling to
other photonic devices
47
. Dispersive-wave generation at 758 nm
was also demonstrated. It could be possible to design devices that
use solitons formed at either 778 or 1064 nm for dispersive-wave
generation into the visible and potentially into the ultra-violet
as has been recently demonstrated using straight silica
waveguides
42
.
Data availability
. The data that support the plots within this
paper and other
fi
ndings of this study are available from the
corresponding author upon reasonable request.
770
775
780
785
790
Wavelength (nm)
–60
–40
–20
Power (dBm)
770
775
780
785
790
Wavelength (nm)
0
2
4
750
760
770
780
790
800
Wavelength (nm)
–60
–40
–20
Power (dBm)
ab
c
d
778 nm
PD
Feedback Loop
Pump
Pump
1
–1
20 dB
Frequency (MHz
+ 19.76 GHz)
0
1
–1
20 dB
Frequency (MHz
+ 19.98 GHz)
0
e
750
760
770
780
790
800
Wavelength (nm)
–0.1
0
0.1
0.2
1557 nm
CW laser
QPSK
EDFA
PPLN
μ
disk
FBG
Servo
90/10
t
= 1.47
μ
m
t
= 1.53
μ
m
(

μ

0

D
1
)/2

(GHz)
(

μ

0

D
1
)/2

(GHz)
Fig. 4
Solition generation at 778 nm.
a
Experimental setup for soliton generation. A 1557 nm tunable laser is sent to a QPSK modulator to utilize frequency-
kicking
57
and is then ampli
fi
ed by an EDFA. Then, a PPLN waveguide frequency doubles the 1557 nm input into 778 nm output. The 778 nm pump light is
coupled to the resonator for soliton generation. A servo loop is used to maintain pump locking
53
.
b
Measured relative mode frequencies of the TM1 mode
family vs. wavelength for devices with
t
=
1.47
μ
m. A number of crossing mode families are visible. The red curve is a numerical
fi
t using
D
2
/2
π
=
49.8 kHz
and
D
3
/2
π
=
340 Hz.
c
Optical spectrum of a 778 nm soliton generated using the device measured in
b
with pump line indicated by the dashed vertical line.
The red curve is a spectral
fi
tting which reveals a pulse width of 145 fs. Most of the spurs in the spectrum correspond to the mode crossings visible in
b
.
Inset shows the electrical spectrum of the detected soliton pulse stream. The resolution bandwidth is 1 kHz.
d
Measured relative mode frequencies of the
TE2 mode family vs. wavelength for devices with
t
=
1.53
μ
m. The red curve is a
fi
t with
D
2
/2
π
=
4.70 kHz and
D
3
/2
π
=
51.6 Hz.
e
Optical spectrum of a
soliton generated using the device measured in
d
with pump line indicated as the dashed vertical line. A dispersive wave is visible near 758 nm. Inset shows
the electrical spectrum of the detected soliton pulse stream. The resolution bandwidth is 1 kHz
ARTICLE
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DOI: 10.1038/s41467-017-01473-9
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Received: 29 May 2017 Accepted: 20 September 2017
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Acknowledgements
The authors thank Scott Diddams and Andrey Matsko for helpful comments on this
work. The authors gratefully acknowledge the Defense Advanced Research Projects
Agency under the ACES program (Award No. HR0011-16-C-0118) and the SCOUT
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7
program (Award No. W911NF-16-1-0548). The authors also thank the Kavli
Nanoscience Institute.
Author contributions
S.H.L., D.Y.O., Q.-F.Y., B.S., H.W. and K.V. conceived the experiment. S.H.L. fabricated
devices with assistance from D.Y.O., B.S., H.W. and K.Y.Y. D.Y.O., Q.-F.Y., B.S. and
H.W. tested the resonator structures with assistance from S.H.L., K.Y.Y., Y.H.L. and
X.Y. S.H.L., D.Y.O., Q.-F.Y., B.S., H.W. and X.L. modeled the device designs. All authors
analyzed the data and contributed to writing the manuscript.
Additional information
Supplementary Information
accompanies this paper at doi:
10.1038/s41467-017-01473-9
.
Competing interests:
The authors declare no competing
fi
nancial interests.
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