Bridging ultra-high-Q devices and photonic circuits
Ki Youl Yang
1
*, Dong Yoon Oh
1
*, Seung Hoon Lee
1
*, Qi-Fan Yang
1
, Xu Yi
1
and Kerry Vahala
1
†
1
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA.
*These authors contributed equally to this work.
†
Corresponding author: vahala@caltech.edu
Optical microcavities are essential in numerous
technologies and scientific disciplines. However,
their application in many areas relies exclusively
upon discrete microcavities in order to satisfy
challenging combinations of ultra-low-loss perfor-
mance (high cavity-Q-factor) and cavity design
requirements. Indeed, finding a microfabrication
bridge connecting ultra-high-Q device functions
with micro and nanophotonic circuits has been a
long-term priority of the microcavity field. Here,
an integrated ridge resonator having a record Q
factor over 200 million is presented. Its ultra-
low-loss and flexible cavity design brings perfor-
mance that has been the exclusive domain of dis-
crete silica and crytalline microcavity devices to
integrated systems. Two distinctly different de-
vices are demonstrated: soliton sources with elec-
tronic repetition rates and high-coherence Bril-
louin lasers. This multi-device capability and per-
formance from a single integrated cavity platform
represents a critical advance for future nanopho-
tonic circuits and systems.
Optical microcavities
1
provide diverse device functions
that include frequency microcombs
2,3
, soliton mode-
locked microcombs
4–8
, Brillouin lasers
9–13
, bio and nano-
particle sensors
14–16
, cavity optomechanical oscillators
17
,
parametric oscillators
18,19
, Raman lasers
20
, reference
cavities/sources
21–24
, and quantum optical devices
25,26
.
Key performance metrics improve with increasing Q-
factor across all applications areas
1
. For example, higher
Q factors dramatically reduce power consumption as
well as phase and intensity noise in signal sources, be-
cause these quantities scale inverse quadratically with
Q factor. Also, higher Q improves the ability to re-
solve a resonance for sensing or for frequency stabiliza-
tion. Such favorable scalings of performance with Q fac-
tor have accounted for a sustained period of progress
in boosting Q factor by reducing optical loss in res-
onators across a range of materials
27–31
. Likewise, the
need for complex microcavity systems that leverage high-
Q factors has driven interest in low-loss monolithically
integrated resonators
28,29,32–38
. For example, Q values
in waveguide-integrated devices to values as high as 80
million
35
and 67 million
38
in strongly-confined resonators
have been attained.
Nonetheless, the highest Q-factor resonators re-
main discrete devices that are crystalline
39
or silica
based
1,11,40,41
. These discrete resonators are moreover
unique in the microcavity world in terms of overall per-
formance and breadth of capability. This includes gener-
ation of electronic-repetition-rate soliton streams as re-
quired in optical clocks
42–44
and optical synthesizers
45
,
rotation measurement at near-earth-rate sensitivity in
micro-optical-gyros
46,47
, synthesis of high-performance
microwave signals
48–51
, and operation as high-stability
optical frequency references
21–23
and reference sources
24
.
Functions such as these belong to a new class of compact
photonic systems that rely upon ultra-high-Q fabrication
methods which have so far defied photonic integration.
In this work a monolithic microcavity having a record
high Q factor is demonstrated. The materials, process
steps and, in particular, the use of a PECVD silicon
nitride waveguide enable full integration of these ultra-
high-Q resonators with other photonic devices. Criti-
cally, and as required for new system-on-a-chip applica-
tions, the resonator supports design controls required to
realize many device functions previously possible using
only discrete (non-waveguide-integrated) devices. To de-
montrate its capability, electronics-rate solitons (15 GHz)
are generated at a low pumping power level. Also, as a
distinctly different capability, high-coherence stimulated
Brillouin laser oscillation is demonstrated. Beyond the
necessity of ultra-high-Q factor in these demonstrations,
each device presents additional challenges. The soliton
source requires resonator dispersion control to support
dissipative Kerr solitons and the Brillouin laser requires
precise diameter control to phase match the Brillouin
laser process. As representative photonic circuits based
on these devices, Fig. 1a conceptualizes a dual Brillouin
optical gyroscope and a dual-comb spectrometer modeled
after recent demonstrations using discrete ultra-high-Q
devices
46,52
.
A cross-sectional rendering of the ridge resonator
structure is provided as the inset in Fig. 1a. A mode
field is also included. The resonator is composed of ther-
mal silica and has a maximum thickness of approximately
8.5 microns. In Fig. 1b, a scanning electron micro-
scope (SEM) image shows two of the silica ridge res-
onators connected by a common silicon nitride waveguide
(false color red in the image). White arrows indicate how
light guided in the silicon nitride couples into the ridge
resonator where it circulates and then recouples to the
waveguide. The silicon nitride waveguide has a thickness
of approximately 250 nm. It is initially 3 to 3.5 microns
in width at the edge of the wafer and is tapered to about
900 nm near the resonator so as to phase match to the res-
onator optical mode. The lightwave evanescently couples
between the silicon nitride waveguide and the microcav-
arXiv:1702.05076v3 [physics.optics] 17 Sep 2017
2
FIG. 1:
Integrated ultra-high-Q microresonator
(
a
) Artists conception of a dual-gyro chip (left) and micro-
fluidic
53
dual-comb spectrometer (right) based on recent discrete Brillouin gyro
46
and dual-comb spectroscopy
demonstrations
52
. Component color codings are as follows: silicon nitride waveguides (red lines), electrical traces
(gold lines), laser sources (black), detectors (brown), modulators (turquoise), loop-control electronics (gold), and
readout processing electronics (blue). Both chips include Pound-Drever-Hall (PDH) locking loops and thermal con-
trol for the resonators. Note: gyros are counter-pumped to reduce possible systematic errors. First and second Bril-
louin Stokes waves (blue and red) are shown in the gyroscope resonators. Soliton pulses (white) appear in the spec-
trometer resonators. Inset: Rendering showing silica ridge resonator with nearby silicon nitride waveguide. Fun-
damental spatial mode is illustrated in color. (
b
) SEM image of two ridge resonators with a common silicon ni-
tride waveguide (false red color). White arrows show the direction of circulation within the top resonator for the
indicated direction of coupling from the waveguide. Dashed white box is the region for the zoom-in image in 1c.
Dashed white line segment gives the location of cleavage plane used for preparation of the SEM image in 1d. (
c
)
SEM zoom-in image of the waveguide-resonator coupling region shown within the dashed white box in 1b. (
d
) SEM
image of resonator cross section prepared by cleaving at the dashed white line in 1b.
ity through the silica layer which supports the waveguide.
The waveguide was designed in both straight versions and
pulley versions to vary coupling strength
35
. It is noted
that because the waveguide has a higher index than the
silica resonator, phase matching to resonator occurs when
the waveguide supports only a single transverse mode.
This improves waveguide-resonator coupling ideality
55
.
Details on the fabrication process and phase matching of
3
FIG. 2:
Spectral scan of integrated ridge resonator and ring-down measurement
(
a
) Spectral scan en-
compassing three free-spectral-ranges. Fundamental TE and TM modes are indicated. (
b
) High resolution zoom-in
scan for the fundamental TM and TE modes with intrinsic quality factor (
Q
o
) indicated (M = million). Red curve
is a Lorentzian fit. The green sinusoidal signal is a frequency calibration scan using a radio-frequency calibrated
fiber Mach-Zehnder interferometer (free-spectral range is 5.979 MHz). (
c
) Superposition of 10 cavity-ringdown sig-
nal scans for the fundamental TM mode with the corresponding decay time and the loaded and intrinsic Q factors.
the waveguide to the resonator are provided in the Meth-
ods Section. SEM zoom-in and cross-sectional views of
portions of the resonator are shown in Fig. 1c and in Fig.
1d.
Spectral measurements of the integrated microcavity
were performed by end-fire coupling to a tunable external
cavity laser and monitoring of the transmission through
the waveguide-coupled device as the laser is scanned. Ap-
proximately 25 % of the power could be coupled from
the lensed optical fiber to the silicon nitride waveguide.
No attempt to improve coupling was made, but in the
future a tapered waveguide end can be introduced to im-
prove the coupling efficiency
56
. The devices measured
had a free-spectral-range (FSR) of approximately 15 GHz
(ridge ring diameter of approximately 4.3 mm) and Fig.
2a presents a spectral scan containing over three FSRs.
Several transverse modes appear in the scan. These
transverse mode families were identified by measuring
dispersion curves for the resonator modes as described
below and comparing to numerical modeling. The waveg-
uide used in this scan (and in Fig. 2b and 2c) was a pul-
ley version. High-resolution scans of the fundamental TE
and TM resonator modes are presented in Fig. 2b. The
frequency scales in the scans are calibrated using a fiber
Mach-Zehnder interferometer which appears as the green
sinusoid in the lower half of each panel. Spectral calibra-
tion of the interferometer is performed using microwave
sidebands generated by phase modulation of the laser. A
linewidth fitting algorithm gives an intrinsic Q factor of
120 million for the TE mode and 205 million for the TM
mode. To further confirm the high optical Q factor of
the TM mode, cavity ring down was performed
57
. Fig 2c
is a superposition of ten ring-down traces of the highest
Q-factor measured from the fundamental TM mode fam-
ily, and the ring-down gives an intrinsic Q factor of 230
million. The ring-down of the fundamental TM mode in
Fig 2b gives an intrinsic Q factor of 216 million in close
agreement with the linewidth data.
Microcombs and in particular soliton mode-locked
microcombs
4
represent a major new application area of
microcavities. These tiny systems are being studied as
a way to transfer large-scale frequency comb technology
to an integrated photonic chip. They provide highly re-
producible optical spectra, have achieved 2/3 and full
octave span coverage
6,58
and have highly stable repe-
tition rates
5
.
However, mode-locked pulse repetition
rates that are both detectable and readily processed by
electronics are required in all frequency comb systems
in order to self-reference the comb
59
. To achieve self-
referenced octave-span operation in microcombs at prac-
4
FIG. 3:
Demonstration of 15 GHz repetition rate
temporal solitons in an integrated optical mi-
croresonator
(
a
) Measured mode frequencies plot-
ted versus mode number for the fundamental TE mode
family with linear dispersion removed. The red curve
is a parabola showing that the dispersion is primarily
second order over 400 modes. Legend gives the free-
spectral range (
D
1
) for mode
μ
= 0 and the second-
order dispersion (
D
2
). The inset is a photograph of two
ridge resonators. (
b
) Optical spectrum of soliton with
pump line indicated. Left inset illustrates continuous-
wave pumping to produce soliton mode locking of res-
onator. Right inset shows the electrical spectrum of
the detected soliton pulse stream (RBW: resolution
bandwidth) (
c
) FROG scan of single soliton state in
3b showing the single pulse signal with pulse period of
66.5 ps corresponding to resonator round-trip time.
tical power levels, frequency comb formation is divided
into a THz-rate comb (micrometer-scale resonator di-
ameter) and an electronics-rate comb (centimeter-scale
resonator diameter)
45
.
However, while the smaller-
diameter, THz-repetition-rate, soliton combs have been
demonstrated with integrated waveguides
58,60
, the large-
diameter electronics-rate soliton microcomb has so far
only been possible using discrete silica and crystalline
FIG. 4:
Demonstration of Brillouin lasing in an
integrated optical microresonator
Optical spec-
trum of Brillouin laser with pump and Stokes waves
spectral peaks indicated. The left inset shows the out-
put laser power vs. input coupled power and shows
laser threshold behavior. All powers are in the waveg-
uide. The clamping of the Stokes power occurs as a
result of second-Stokes laser oscillation (i.e., Stokes
cascade
54
). The right inset shows the microwave beat-
note signal of the back-reflected pump and lasing sig-
nal. RBW is resolution bandwidth. Lower left inset
shows pump (blue) and Brillouin laser action on first
Stokes wave (red).
resonators
4,5
. Part of the challenge here is achieving
a sufficiently high Q-factor to overcome the increased
pumping volume of the larger electronics-rate soliton
comb. Also compounding this problem are resonator de-
sign requirements imposed by the soliton physics. These
include minimization of avoided mode crossings and
anomalous modal dispersion. Attaining the combined
features of ultra-high-Q factor to overcome a large opti-
cal pumping volume, while incorporating these resonator
design features within an integrated microcavity has not
been possible. Here, the integrated ridge resonator is
used to demonstrate a new capability for an integrated
design, 15 GHz soliton generation. The pumping power
level is also low.
Using modeling and measurement techniques described
elsewhere
4,5
, the ridge resonator was designed to mini-
mize avoided mode crossings as required for soliton gen-
eration. Along these lines, the fundamental TE mode
family dispersion was characterized by measuring the
frequency of all modes between 1530 nm and 1580 nm
using an external cavity laser calibrated by the Mach-
Zehnder interferometer described above
5
. This data was
then plotted by removing both an offset frequency (
ω
0
)
and a linear dispersion term proportional to the FSR
5
(
D
1
) of the mode family at
ω
0
(
D
1
/
2
π
= 15.2 GHz was
measured). Modes are indexed using the label
μ
with
μ
= 0 assigned to the mode at frequency
ω
0
, which
is also the mode that is pumped to generate the soli-
tons. The resulting data are plotted in Fig. 3a. The
red curve is a parabola showing that the mode family is
dominated by second-order dispersion over the range of
modes measured. A fitting gives
D
2
/
2
π
= 6.4 kHz where
ω
μ
=
ω
0
+
D
1
μ
+
D
2
μ
2
/
2. Significantly, there is no ob-
servable mode-crossing-induced distortion of the mode
family, therefore making the mode family well suited for
soliton formation
4,5
.
Solitons were triggered and stabilized using the power
kick
6
and capture-lock technique
61
. An optical spectrum
is shown for a single soliton state in Fig. 3b. Visible in
the spectrum is the optical pump wave. Despite the large
(4.3 mm diameter) resonator required to attain the 15
GHz repetition rate, the ultra-high-Q ensured a low para-
metric oscillation threshold
18
power near 2 mW (waveg-
uide coupled) and pump power as low 61 mW for stable
soliton operation. To confirm the soliton repetition rate,
the soliton pulse stream was detected and analyzed on
an electrical spectrum analyzer. The electrical spectrum
is provided as an inset to Fig. 3b and shows a measured
repetition rate near 15.2 GHz. The pulse-like nature of
the soliton stream was measured using the frequency re-
solved optical gate (FROG) method. The FROG data is
presented in Fig. 3c.
The Brillouin process has attracted considerable inter-
est in micro devices
62
. Brillouin laser action has been
demonstrated in discrete resonators based on silica
9,11,54
and CaF
2
10
.
Laser action has also been realized in
integrated resonators using silicon
13
and chalcogenide
waveguides
12
. However, reference sources
24
, microwave
synthesizers
49,50
and Brillouin gyroscopes
46
require the
highest possible optical Q factors for generation of
narrow-linewidth signals and have so far relied upon
discrete devices.
Moreover, ultra-high-Q also allows
these devices to function at low power despite their
increased size. For example, discrete centimeter-scale
silica
46
devices have recently achieved rotation rate sen-
sitivities near the Earth’s rotation rate using an active
Brillouin-laser
46
. In this approach sub-Hertz Brillouin
laser linewidths result from the inverse quadratic scaling
of their fundamental phase noise on Q factor
54
. These
narrow linewidths endow the device with high sensitiv-
ity. However, the gyroscope performance is also boosted
by increased resonator size (increased Sagnac effect), and
here the ultra-high-Q resonance provides an additional
benefit by enabling Brillouin gyro operation at milliWatt-
scale power levels.
As a second device demonstration, the integrated ridge
resonator is applied to generate high coherence Brillouin
laser action. Devices were fabricated to phase match
the Brillouin process when pumped near 1550 nm. De-
vice diameters of approximately 6.0 mm were fabricated
and tested. Fig. 4 shows the optical spectrum of the
lasing Stokes wave. The weaker pump signal peak in
the spectrum results from the need to collect the lasing
Stokes wave in the propagation direction opposite to the
pumping direction. Its strength is determined by residual
backscattering in the measurement. The upper left inset
in Fig. 4 shows the Stokes power versus pumping and
gives a threshold power of approximately 800
μ
W in the
waveguide. The upper right inset is the microwave beat
signal between the pump wave and the Stokes wave. It
has a high coherence as evidenced by the resolution band-
width (RBW) and has a Hertz-level, short-term funda-
mental linewidth.
Beyond the specific device demonstrations provided
here, performance considerations relating to noise and
fluctuations frequently require combination of high-Q
and large mode volume that present challenges for scal-
able fabrication processes. For example, thermorefrac-
tive and photothermal fluctuations
21,63
destabilize cav-
ity frequency and vary inversely with resonator volume.
Likewise, the fundamental timing jitter noise of a micro-
comb soliton stream is predicted to scale as
n
2
/V
where
n
2
is the Kerr nonlinear coefficient and
V
is the mode
volume
64
. To the extent these sources of fluctuation can
be managed or reduced in active devices by increased cav-
ity volume (or reduced
n
2
as in the case of soliton timing
jitter), higher Q factors can counteract the accompanying
effect on pumping power. Finally, dual-comb spectrome-
ters require reduced free-spectral-range (large diameter)
devices so as to prevent under-sampling of chemical spec-
tra. These larger microcomb devices also benefit from
the highest possible Q factor so as to enable operation at
practical power levels.
In summary, we have demonstrated an integrated
resonator having a record optical Q factor.
Low-
pump-power soliton generation at 15 GHz as well as
high-coherence Brillouin laser action were demonstrated
to illustrate functionality previously possible in only dis-
crete devices, but nonetheless required for self-referenced
microcomb photonic systems and for integrated systems
requiring high-coherence signal sources. The waveguide
material, PECVD silicon nitride, is among the most
widely used waveguide materials in the photonics indus-
try and provides a nearly universal interface to other
photonic devices fabricated on a common silicon wafer.
Methods
Referring to Fig. 5, the fabrication process
begins by growing a thermal silica layer on a high-purity
float-zone silicon wafer. This layer is then patterned and
wet etched using buffered hydrofluoric acid (HF) accord-
ing to a process used to create wedge resonators
11
. The
resulting etched oxide forms a circular disk structure that
defines the exterior of the resonator. A subsequent ther-
mal oxidation grows the oxide layer beneath the oxide
disk. This growth is followed by deposition of 500 nm
of PECVD silicon nitride. Lithography and shallow ICP
RIE dry-etch steps are then applied to partially etch the
silicon nitride. Phosphoric acid is used to fully define the
silicon nitride waveguide. A thin layer (20 nm) of silica is
applied by atomic layer deposition to protect the silicon