of 7
Microresonator soliton dual-comb imaging
C
HENGYING
B
AO
,
1,
M
YOUNG
-G
YUN
S
UH
,
1,2,
AND
K
ERRY
V
AHALA
1,
*
1
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
2
NTT Physics and Information Laboratory, 1950 University Ave., East Palo Alto, California 94303, USA
*Corresponding author: vahala@caltech.edu
Received 8 April 2019; revised 5 July 2019; accepted 24 July 2019 (Doc. ID 364448); published 27 August 2019
Fast-responding detector arrays are commonly used for imaging rapidly changing scenes. Besides array detectors, a
single-pixel detector combined with a broadband optical spectrum can also be used for rapid imaging by mapping the
spectrum into a spatial coordinate grid and then rapidly measuring the spectrum. Here, optical frequency combs
generated from high-
Q
silica microresonators are used to implement this method. The microcomb is dispersed in
two spatial dimensions to measure a test target. The target-encoded spectrum is then measured by multi-heterodyne
beating with another microcomb having a slightly different repetition rate, enabling an imaging frame rate up to
200 kHz and fill rates as high as 48 megapixels/s. The system is used to monitor the flow of microparticles in a fluid
cell. Microcombs in combination with a monolithic waveguide grating array imager could greatly magnify these results
by combining the spatial parallelism of detector arrays with spectral parallelism of optics.
© 2019 Optical Society of
America under the terms of the
OSA Open Access Publishing Agreement
https://doi.org/10.1364/OPTICA.6.001110
1. INTRODUCTION
The development of the rapid-frame-capture detector array sen-
sors based on charge-coupled device and complementary metal
oxide semiconductor technology has revolutionized imaging
(see, for example, Ref. [
1
]). Recently, there has also been interest
in new methods that leverage the massive bandwidth of optical
signals to perform imaging using a single-pixel detector. One such
approach uses the broadband spectrum of ultrashort optical pulses
[
2
] and works by mapping different optical frequencies into dis-
tinct spatial locations using spatial dispersers such as demon-
strated in the technique of femtosecond pulse shaping [
3
]. To
create a two-dimensional (2D) map, a conventional grating dis-
perses the spectrum in one spatial dimension, while a virtually
imaged phase array (VIPA) disperses light into the other spatial
dimension. As shown in Fig.
1(a)
, the grating and the VIPA create
a
2D spectral shower
in which distinct optical frequencies have
a one-to-one (spectral-spatial) correspondence with spatial coor-
dinates in two dimensions [
2
,
4
6
]. To recover the image, the
spectrum can be measured by the time-stretch method, which
converts the spectrally encoded spatial information into a tempo-
ral waveform measured on the single-pixel photodetector [
2
].
This approach measures the image on a shot-by-shot basis and
6 MHz frame rates have been demonstrated [
2
].
An alternative image recovery technique based on two
frequency combs has also been recently demonstrated [
6
]. This
approach, termed here dual-comb imaging, parallels the tech-
nique of dual-comb spectroscopy [
7
,
8
] by converting an optical
spectrum into a radio frequency (RF) electrical signal. In effect the
method maps the optical signal comb with target information into
these radio frequency components [see Fig.
1(b)
]. If the signal and
reference comb are phase locked then both amplitude and phase
information about the target can be retrieved, enabling acquisi-
tion of three-dimensional information [
6
]. Line-scan spectral-
spatial imaging using dual frequency combs has also been recently
reported [
9
11
]. To generate broadband optical pulses for imag-
ing, tabletop mode-locked lasers have so far been used. A recent
advance in optical pulse and frequency comb generation is based
on dissipative Kerr soliton mode locking in optical microcavities
[
12
17
]. The devices provide high repetition rate soliton streams
and their associated optical frequency combs feature smooth
spectral envelopes. These miniature frequency combs, or micro-
combs [
18
], are considered a possible way to dramatically reduce
the form factor of conventional frequency comb systems.
Accordingly, they are being studied for several applications includ-
ing dual-comb spectroscopy [
19
,
20
], ranging [
21
,
22
], optical
communications [
23
], optical frequency synthesis [
24
], and exo-
planet detection in astronomy [
25
,
26
].
The application of microcombs to the dual-comb imaging
method is considered here. These devices offer a system-on-a-chip
architecture that eliminates fiber optics (i.e., that required for the
time-stretch image recovery method and to generate mode-locked
pulses). A fully integrated platform that avoids the free space gra-
ting and VIPA elements is also possible [see Fig.
1(c)
]. This work
explores soliton microcomb dual-comb imaging by measuring a
USAF1951 test target and by monitoring microparticles in a
flow-cell. An important feature of microcombs is their very high
repetition rate as compared to conventional combs (typically mi-
crowave to terahertz rates as compared to radio frequency rates).
The impact of such high repetition rates on future dual-comb
imaging system performance is also considered.
2334-2536/19/091110-07 Journal © 2019 Optical Society of America
Research Article
Vol. 6, No. 9 / September 2019 /
Optica
1110
2. EXPERIMENTAL SETUP
High-
Q
silica-on-silicon wedge microresonators [
27
] are used to
generate the dual soliton streams. The devices feature repetition
rates of 1.86 and 9.39 GHz [
28
]. Details on methods used to
trigger and stabilize the soliton microcombs are presented else-
where [
13
]. The microcombs are coupled directly to optical fibers
using tapered-fiber couplers [
29
,
30
]. The signal comb and refer-
ence comb are conveyed along the optical train, as shown in
Fig.
1(a)
. The tapered-fiber couplers can be replaced by integrated
waveguides [
31
]. Moreover, fully integrated soliton microcombs
with an on-chip pump have also been reported [
32
].
The VIPA and grating act together to create the 2D spectral
shower with the VIPA dispersing the spectrum along the vertical
direction and the grating dispersing the spectrum along the hori-
zontal direction. More specifically, the VIPA disperses light only
within its free spectral range (FSR), which means that optical
Circulator
PD
PD
Reference
Signal
Grating
(a)
Microresonators
Spherical Lens
VIPA
Cylindrical Lens
Oscilloscope
Ch1
Ch2
Target
Collimator
(b)
‘2D spectral shower’
Monolithic Microcomb Array
2D Spectral Shower
(c)
f
rep2
f
rep1
rep
= f
rep1
- f
rep2
Optical Domain
RF Domain
Frequency Domain
Space Domain
Optical comb II
Signal RF comb
Optical comb I
Target-encoded modulation
frequency
X - direction
Y - direction
VIPA FSR
VIPA
FSR
Comb
spacing
‘2D spectral shower’
on a target pattern
Soliton Microcombs
Monolithic Grating Array
Reference
Signal
Fig. 1.
Dual-comb imaging using microresonator solitons. (a) A conceptual diagram showing the operational principle for spectral-spatial-mapping and
dual-microcomb imaging. Two soliton microcombs (signal and reference) having slightly different repetition rates are generated using two on-chip
microresonators. A 2D disperser (VIPA+grating) maps frequencies from the signal microcomb into a 2D grid of spatial locations (spectral shower) tha
t
are reflected by a target. The reflected signal spectrum is measured by multi-heterodyne detection with the reference microcomb. The chip is shown wi
th
small (high rate) and larger (low rate) comb pairs in both the signal and reference arms. These can enable different operational modes for the imaging s
ystem.
(b) Dual-comb imaging proceeds by illuminating the target (right panel) with the 2D spectral shower formed as shown in panel (a). As shown in the left
panel, the target reflection amplitude is encoded onto the signal comb (Optical comb II). The signal comb is then heterodyned with the reference comb
(Optical comb I) to generate the RF comb.
f
rep1
,
f
rep2
, and
Δ
f
rep
are the frequency line spacing of the reference comb, the signal comb, and the signal RF
comb. (c) Dual-comb imaging concept based on integrated waveguide grating antennas. Microcomb outputs are divided into multiple waveguides that dr
ive
the grating antennas. Comb light is dispersed by a corresponding waveguide grating antenna (eliminates VIPA and grating) to create one imaging dimen
sion
in the spectral shower (right). The second imaging dimension is provided by the spatial location of each grating antenna. This approach combines spec
tral
parallelism of photonics with spatial parallelism of detector arrays to greatly magnify performance. A single (shared) pump is shown, but the microc
ombs
could also be individually pumped so as to create frequency combs that are spectrally displaced. Receiver combs and antennas are not shown.
Research Article
Vol. 6, No. 9 / September 2019 /
Optica
1111