of 13
Searching for Exoplanets Using a Microresonator Astrocomb
Myoung-Gyun Suh
1
,
Xu Yi
1
,
Yu-Hung Lai
1
,
S. Leifer
2
,
Ivan S. Grudinin
2
,
G. Vasisht
2
,
Emily
C. Martin
3
,
Michael P. Fitzgerald
3
,
G. Doppmann
4
,
J. Wang
5
,
D. Mawet
2,5
,
Scott B. Papp
6
,
Scott A. Diddams
6
,
C. Beichman
7,*
, and
Kerry Vahala
1,*
1
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena,
California 91125, USA
2
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
3
Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, CA
90095, USA
4
W.M. Keck Observatory, Kamuela, HI 96743, USA
5
Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
6
National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA
7
NASA Exoplanet Science Institute, California Institute of Technology, Pasadena, CA 91125, USA
Abstract
Orbiting planets induce a weak radial velocity (RV) shift in the host star that provides a powerful
method of planet detection. Importantly, the RV technique provides information about the
exoplanet mass, which is unavailable with the complementary technique of transit photometry.
However, RV detection of an Earth-like planet in the ‘habitable zone’
1
requires extreme
spectroscopic precision that is only possible using a laser frequency comb (LFC)
2
. Conventional
LFCs require complex filtering steps to be compatible with astronomical spectrographs, but a new
chip-based microresonator device, the Kerr soliton microcomb
3
8
, is an ideal match for
astronomical spectrograph resolution and can eliminate these filtering steps. Here, we demonstrate
an atomic/molecular line-referenced soliton microcomb as a first in-the-field demonstration of
microcombs for calibration of astronomical spectrographs. These devices can ultimately provide
LFC systems that would occupy only a few cubic centimetres
9
,
10
, thereby greatly expanding
implementation of these technologies into remote and mobile environments beyond the research
lab.
Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research,
subject always to the full Conditions of use:http://www.nature.com/authors/editorial_policies/license.html#terms
*
Corresponding author: Kerry Vahala (vahala@caltech.edu) and C. Beichman (chas@ipac.caltech.edu).
Author contributions
M.G.S., S.L., G.V., M.P.F., D.M., C.B. and K.V. conceived the experiments. All co-authors designed and
performed experiments. M.G.S. and X.Y. built the soliton microcomb setup and EO comb setup with help from S.L., I.S.G., S.D., S.P.
and Y.H.L.. G.D. managed operation and experimental interface of the Keck II telescope. E.C.M., J.W., C.B. analyzed NIRSPEC data.
C.B. and K.V. supervised the experiment. M.G.S., C.B. and K.V. prepared the manuscript with input from all co-authors.
Competing interests
The authors declare no competing interests.
NASA Public Access
Author manuscript
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. Author manuscript; available in PMC 2019 July 01.
Published in final edited form as:
Nat Photonics
. 2019 ; 13: 25–30. doi:10.1038/s41566-018-0312-3.
NASA Author Manuscript
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NASA Author Manuscript
The radial velocity (RV) method (Figure 1) measures periodic Doppler shifts in the stellar
spectrum to infer the presence of an orbiting exoplanet
11
and relies upon a highly stable and
precisely calibrated spectrometer
12
. In astronomy, LFCs or simply astrocombs have enabled
spectrograph calibration at the few cm s
−1
level
13
. They do this by providing a spectrally
broad ‘comb’ of optical frequencies that are precisely stabilized through the process of self-
referencing
14
. Self-referencing ensures that both the comb’s spectral line spacing and the
common offset frequency of the spectral lines from the origin are locked to a radio
frequency standard resulting in a remarkably accurate ‘optical ruler’.
Astrocombs used in prior work
13
,
15
20
are derived from femtosecond modelocked lasers and
have a comb line frequency spacing that is not resolvable by astronomical spectrographs
2
.
This has necessitated the addition of special spectral filters designed to coarsen the line
spacing (typically 10-30 GHz)
13
,
15
20
. The added complexity of this filtering step has
created interest in frequency comb generation by other means that can intrinsically provide a
readily resolvable line spacing. For example, electro-optical (EO) modulation provides an
alternative approach for direct generation of > 10 GHz comb line spacings
21
,
22
. Line
referenced EO-astrocomb devices
23
and self-referenced EO-combs
24
,
25
have been
demonstrated, and more recently, 10 cm/s RV precision using a near-infrared EO-
astrocomb
26
has been reported. However, these devices require optical filtering to remove
amplified phase noise in the wings of the broadened comb. Another optical source that
produces wider comb line spacing is in the form of a tiny microresonator-based comb or
microcomb
27
,
28
. Driven by parametric oscillation and four-wave-mixing
29
, millimetre-scale
versions of these devices have line spacings that are ideally suited for astronomical
calibration
28
. However, until recently microcombs operated in the so-called modulation
instability regime of comb formation
30
and this severely limited their utility in frequency
comb applications.
The recent demonstration of soliton mode-locking in microresonators represents a major
turning point for applications of microcombs
3
8
. Also observed in optical fibre
31
, soliton
formation ensures highly stable mode locking and reproducible spectral envelopes. For these
reasons soliton microcombs are being applied to frequency synthesis
9
, dual comb
spectroscopy
32
34
, laser-ranging
35
,
36
, and optical communications
37
. Moreover, their
compact (often chip-based) form factor and low operating power are ideal for ubiquitous
application outside the lab and even in future space-borne instruments. In this work, we
demonstrate a soliton microcomb as an astronomical spectrograph calibrator. We discuss the
experimental setup, laboratory results and efforts to detect a previously known exoplanet.
The on-site soliton microcomb demonstration was performed at the 10 m Keck II telescope
of the W.M. Keck Observatory in order to calibrate the near-infrared spectrometer
(NIRSPEC). A secondary goal was to detect the RV signature of the 0.5 M
Jup
planet orbiting
the G3V star HD187123 in a 3.1 day period
38
. Calibrations and observations were
performed during the first half nights of 2017-09-10 and 2017-09-11 (UTC) in the hope of
detecting the 70 m s
−1
semi-amplitude of this planetary signature. As a cross-check, the
functionality of the soliton microcomb was compared with a previously-demonstrated line-
referenced EO-astrocomb
23
. The soliton and EO combs described here operate in the near-
infrared (NIR), centred at 1560 um with a usable breadth of ~250nm. NIR wavelengths are
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favored over visible wavelengths for the study of very cool stars, i.e. the M dwarfs. These
comprise about 70 percent of the stars in our galaxy and emit predominantly at long
wavelengths
26
,
39
,
40
.
The experimental apparatus for both combs was established in the computer room adjacent
to the telescope control room. Both combs were active simultaneously and the output of
either one could be fed into the integrating sphere at the input to the NIRSPEC calibration
subsystem via single-mode fibre. The switch between the two combs could be carried out in
the computer room within less than a minute by changing the input to the fibre without
disturbing NIRSPEC itself.
The primary elements of the soliton comb calibration system are detailed in Figure 2a. The
LFC light (soliton microcomb or EO comb) is sent to the fibre acquisition unit (light green
box) to calibrate the NIRSPEC spectrometer
41
. Soliton generation uses a silica
microresonator fabricated on a silicon wafer
42
. The resonator featured a 3 mm diameter
corresponding to an approximate 22.1 GHz soliton comb line spacing and had an unloaded
quality factor of approximately 300 million (see Methods). Figure 2b shows the optical
spectrum of the soliton microcomb. The soliton repetition frequency (
f
rep
) was locked to a
rubidium-stabilized local oscillator by servo control of the pump power using an AOM so as
to vary the soliton repetition rate. Allan deviation measurement of the locked and frequency-
divided signal show an instability of 10 mHz at 1000 s averaging time (Figure 2c). The
frequency of one of the soliton comb lines is monitored by heterodyne detection with a
reference laser, which is locked to a hydrogen cyanide (HCN) absorption line at 1559.9 nm.
The resulting offset frequency
f
0
is recorded at every second using a frequency counter
stabilized to the Rb clock with a time stamp for calibration of the frequency comb over time.
For calibration of the frequency comb over time,
f
0
was determined over a 20 second
averaging time (i.e., acquisition time for a single spectrum) with standard deviation less than
1 MHz. Over this time, the absolute optical frequency of the HCN reference laser has an
imprecision of less than 1 MHz
23
(see Methods). Because the soliton repetition rate (i.e.,
comb line spacing) is frequency locked, the offset frequency imprecision was the principal
source of instability in the comb calibration, equivalent to about 1 m s
−1
of RV imprecision.
Finally, the soliton microcomb is spectrally broadened using highly nonlinear optical fibre
(see Methods).
Figure 3a shows the echellogram of the soliton microcomb measured by NIRSPEC (8
Echelle orders ranging from 1471 nm to 1731 nm, which represents almost the entire
astronomical
H
-band). The raw echellograms were rectified spatially and spectrally.
Zoomed-in images of a single order from both the soliton and EO comb data (Figure 3b)
show that individual comb lines are resolved at the NIRSPEC resolution of R~25,000 and
spaced approximately 4 pixels apart (0.1 nm) for the EO comb and 8 pixels (0.2 nm) for the
soliton comb.
The soliton and EO comb time series data shown in Figure 3 consist of 450 data frames
taken every 20 seconds over the course of a 2.5 hr interval when the telescope and
instrument were in a quiescent state. The reduced echellograms were analyzed by fitting a
Gaussian to each comb line (Figure 3c) to determine its pixel location (see Methods). For
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this analysis we chose order #46 which spans 1.631 to 1.655
μ
m. The centroids of each
comb line were determined across this 2.5 hr interval. The average drift for the entire order
consisting of
N
= 122 (225) lines in the soliton (EO comb) dataset, was computed with
respect to the first frame in the time series and examined as a function of time to reveal drifts
within NIRSPEC.
In the absence of external disturbances such as telescope-induced vibrations, the drift
measured continuously in ~ 5 to 10 minute intervals over 2 hours was extremely regular and
could be removed by a simple first-order fit. Subtracting the linear drift from the soliton data
in the upper panel of Figure 3d results in the red dashed line (lower panel). Our calculation
(see Methods) shows that this level of drift corresponds to 3-5 m s
−1
precision in wavelength
solution. Thus the ability to calibrate NIRSPEC at the few m s
−1
level has been
demonstrated using the soliton microcomb near-infrared technology. We emphasize that this
wavelength precision is inherent to NIRSPEC’s resolution and stability, and it is only the
large number of LFC comb lines and their inherent high precision that have revealed the
performance of NIRSPEC at this level (see Methods).
In routine astronomical operation, i.e. without an LFC, NIRSPEC has achieved radial
velocity measurements at the 50 - 100 m/s level
43
. Sources of uncertainty previously known
or revealed during these observations include: changing illumination due to guiding errors of
the star within the slit or shifting from the slit to the integrating sphere; short and long term
drifts of 0.02-0.05 pixel due to internal vibrations and environmental effects; and sudden
grating offsets due to telescope motions (0.25 pixel).
Observations of HD 187123 were bracketed by soliton measurements, but analysis of the
stellar spectra and of telluric absorption lines within those spectra revealed variations at the
100 m s
−1
level (0.025-0.05 pixel) which we attribute to the sources of wavelength shifts as
described above. While planet detection could not be achieved, we were able to measure the
two combs sequentially with respect to the arc lamps used for absolute wavelength
calibration (see Extended Data Fig. 1).
A funded upgrade presently underway will enhance NIRSPEC’s thermal and mechanical
stability, and future upgrades would enable simultaneous observation of an LFC and a stellar
image stabilized via a single mode fibre using Adaptive Optics. Finally, a new generation of
spectrographs in development will utilize diffraction-limited Adaptive Optics imaging to
enable
R
> 100, 000 spectral resolution and enhanced image stability using single mode
fibres. These new instruments will be able to take full advantage of the wavelength precision
available with a new generation of microresonator astrocombs
44
.
In summary, we report in-situ astronomical spectrograph calibrations with a soliton
microcomb for the first time, which is an important milestone for future chip-based
astrocomb research. The internal instrumental precision at the few m s
−1
was limited by
internal drifts of NIRSPEC and not by the performance of the soliton microcomb, which
already possesses the desirable qualities of ~ 20 GHz mode spacing, low noise operation and
short pulse generation. Rapidly progressing research in this field has resulted in microcomb
spectral broadening and self-referencing with integrated photonics
45
, as well as direct
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generation at shorter wavelengths
46
. These advances will greatly enhance the microcomb
stability and bandwidth, and will eventually allow a new generation of astronomical
instruments to attain the precision needed to detect the 10 cm s
−1
RV signature of Earth-like
exoplanets orbiting solar type stars at visible wavelengths. The current prototype system
occupies approximately 1.3 m in a standard instrument rack, but significant effort towards
system-level integration
9
,
10
, could ultimately provide a microcomb system in a chip-
integrated package with a footprint measured in centimetres. Such dramatic reduction in size
is accompanied by reduced weight and power consumption, which would be an enabling
factor for ubiquitous frequency comb precision RV calibrations, and other metrology
applications in mobile and even space-borne
47
,
48
instrumentation.
Note: The authors would like to draw the readers’ attention to another microresonator
astrocomb demonstration
49
, which was reported while preparing this manuscript.
Methods
Silica microresonator and the device package.
The resonator is an 8 micron-thick disk resonator supporting whispering-gallery optical
modes at the ~ 30 degree-wedged perimeter
42
. For transport to the observatory, the
microresonator was mounted inside a brass package with FC/APC fibre connectors. The
package was temperature-controlled using a thermoelectric cooler to stay within an
operating range of 30 mK. The measured temperature stability was < 10 mK over an hour.
HCN reference laser.
Both centre-lock (reference) mode and side-lock (line narrowing) mode of the HCN
reference laser (Clarity laser manufactured by Wavelength References, Inc.) were tested in
the laboratory experiment. Stabilization of
f
0
to a Rubidium-referenced local oscillator was
possible by shifting entire soliton microcomb frequency using an acousto-optic frequency
shifter when the reference laser was in side-lock mode. However, centre-lock mode, which
provides better precision (<1 MHz), is used and free-running
f
0
is monitored for the on-site
demonstration at the W.M. Keck Observatory. Because the comb precision of <1 MHz is
sufficient to detect the RV signature of HD 187123b, further improvement of the precision
was not attempted to simplify the system as a first out-of-lab demonstration. In principle,
full-stabilization of soliton microcomb is possible using a better optical frequency reference
or via a self-referencing technique.
Spectral broadening of soliton microcomb.
The initial soliton microcomb out of the microresonator had ~ mW optical power and was
amplified above 1 Watt before entering the highly nonlinear optical fibre (HNLF). After
spectral broadening, the high power peaks near the pump laser frequency are filtered out to
prevent potential damage of the spectrograph. The broadened soliton microcomb had ~ 100
mW optical power and was further attenuated to ~ mW before entering into the integrating
sphere. The HNLF
25
,
45
,
53
, spectral broadening medium, had three sections of fibre fusion-
spliced together (5 metres of HNLF with −1.3 ps/nm/km normal dispersion, 60 centimetres
of SMF 28, 2 metres of HNLF with 1.5 ps/nm/km anomalous dispersion). The first piece of
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normal dispersion provides an efficient spectral broadening and a pulse output that can be
temporally compressed with SMF 28. The anomalous HNLF section provides
supercontinuum and dispersive-wave formation around 1200 nm.
Calibration of NIRSPEC.
We calculate the
relative
drift in the NIRSPEC wavelength solution,
z
(
t
), at time
t
by taking
the average difference in centroid positions of each comb line (
j
=1 to N) in Order #46 at
time
t
,
x
j
(
t
), (Figure 3c) relative to the first frame in the time series,
x
j
(
t
= 0) as defined in
eqn (1).
z
(
t
) =
1
N
j
= 1
j
=
N
(
x
j
(
t
) −
x
j
(
t
= 0)) .
(1)
z
(
t
) with its associated uncertainty,
σ
(
t
)
N
, is shown in Figure 3d as measured by both the
EO comb (black line) and the soliton microcomb (blue line). After subtracting the linear
drift from the soliton data in Figure 3d, the soliton comb data reduced the wavelength drift
over the two hour interval from 0.027± 0.002 pixel hr
−1
(120±10 m s
−1
hr
−1
) to zero ± 0.002
pixel hr
−1
(±10 m s
−1
hr
−1
). The 1
σ
residual around the linear fit in Figure 3d is 0.0034
pixels or 15 m s
−1
. Other soliton-only datasets taken during these two days showed residuals
as low as 0.0021 pixels after removal of a linear fit, or 9 m s
−1
. These values represent the
difference between two frames so that the wavelength precision in a single frame is
2
smaller or 10.6 m s
−1
and 6.5 m s
−1
.
The wavelength precision obtained above is based only on order #46, but there are four other
orders in the echellogram with comparable amplitude and number of comb lines (Figure 3a).
Adding in these other lines would improve the wavelength solution by ~ ×2, or roughly 3-5
ms
−1
.
The inset in Figure 3d demonstrates that the distribution of differences between comb-line
centroids from one time step to the next after the subtraction of the mean shift, (
x
j
(
t
) −
x
j
(
t
=
0)) −
z
(
t
) is well characterized by a Gaussian distribution. The precision in determining the
frame-to-frame shift is dominated by the centroiding uncertainty (0.038 pixel in the
differences, or
0.038
2 = 0.027
pixel in a single frame) and the total number of comb lines
considered. The mean value of this distribution over the two hour period is 0.0001±0.00079
pixels or 0.4±3.4 m s
−1
.
Performance limit of NIRSPEC.
A complete error-budget analysis for NIRSPEC shows that in an 900 sec observation,
NIRSPEC could achieve (1, 1.6, 4.4) m/s on an H =(6, 8, 10) mag M3 star after including
the following effects: photon noise, simultaneous single mode fibre feeds of AO-stabilized
starlight and the LFC, upgraded mechanical and thermal stability, stellar jitter and residual
telluric effects.
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Data availability.
The data that support the plots within this paper and other findings of this study are available
from the corresponding author upon reasonable request.
Extended Data
Extended Data Fig. 1: Arc lamp data for absolute wavelength calibration.
NIRSPEC Arc lamps with lines of Kr, Ar, and Xe (
https://www2.keck.hawaii.edu/inst/
nirspec/lines.html
) were used for the absolute wavelength calibration of the soliton comb
lines, alternately configuring NIRSPEC to observe the arcs and the soliton. The figure shows
a section of data from order 46. Gaussian fits were performed on 5 prominent lines from the
two nights (blue and black data points in the inset). The average difference between the 5
line centroids is 0.98 ± 1.3 × 10
−6
nm, which corresponds to 0.041 ± 0.025 pixel. This shift
is consistent with short term drifts seen through the two nights.
Acknowledgments
We gratefully acknowledge Josh Schlieder, Prof. Andrew Howard, Fred Hadaegh and the support of the entire Keck
summit team. We thank David Carlson and Henry Timmers for preparing the highly nonlinear optical fiber. The
authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of
Mauna Kea has always had within the indigenous Hawaiian community. The data presented herein were obtained at
the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of
Technology, the University of California and the National Aeronautics and Space Administration. The Observatory
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was made possible by the generous financial support of the W.M. Keck Foundation. This paper made use of data
available in the NASA Exoplanet Archive and the Keck Observatory Archive. S.D. and S.P. acknowledge support
from NIST. K.V., M.G.S., X.Y. and Y.H.L. thank the Kavli Nanoscience Institute and the National Aeronautics and
Space Administration for support under KJV.JPLNASA-1-JPL.1459106. This research was carried out at the Jet
Propulsion Laboratory and the California Institute of Technology under a contract with the National Aeronautics
and Space Administration and funded through the JPL Research and Technology Development.
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Methods references
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FIG. 1: Concept of a microresonator astrocomb.
While the host star (red sphere) and exoplanet (blue sphere) orbit their common centre of
mass, light waves leaving the star experience a weak Doppler shift. The frequency shift (Δ
ν
)
of the stellar spectral lines are measured with a spectrograph calibrated using an evenly
spaced comb of frequencies. Here, the comb of frequencies is produced by soliton emission
from a microresonator, which can be potentially integrated with a continuous-wave (CW)
laser, a photodetector (PD) and a field-programmable gate array (FPGA) on a chip-scale
device.
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FIG. 2: Experimental schematic and atomic/molecular line-referenced soliton microcomb.
(a) Continuous-wave (CW) fibre laser is coupled into a silica microresonator via a tapered
fibre coupler
50
,
51
. An acousto-optic modulator (AOM) controls pump power. The soliton
microcomb is long-term stabilized by servo control of the pump laser frequency to hold a
fixed soliton average power
52
. The comb power is also tapped to detect and stabilize the
repetition frequency (
f
rep
). After dividing by 4,
f
rep
is frequency-locked to an oscillator and
monitored using a frequency counter. The depicted control electronics can be potentially
replaced by a compact FPGA as shown in Fig.1. A rubidium (Rb) clock provides an external
frequency reference. The frequency offset (
f
0
) of a soliton comb line is measured relative to
a reference laser (stabilized to HCN at 1559.9 nm). This comb line is filtered-out by a fibre
Bragg grating (FBG) filter and heterodyned with the reference laser. Finally, the soliton
microcomb is spectrally broadened and sent to the integrating sphere of the NIRSPEC
instrument on the Keck II telescope for spectrograph calibration. As a cross check, an EO
comb (instead of soliton microcomb) is also used. (b) Optical spectrum of the soliton
microcomb. The hyperbolic-secant-square fit (red dotted curve) indicates that the soliton
pulse width is 145 fs. Inset : Zoom-in of the spectra showing 22.1 GHz line spacing. (c)
Allan deviation of the frequency-locked
f
rep
/4. PD : photodetector, OSA : optical spectrum
analyzer.
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FIG. 3: Data from testing at Keck II.
(a) Image of soliton comb projected onto the NIRSPEC from the echelle orders 44 to 51.
Soliton emission (white dashed box) has been strongly filtered to prevent potential damage
of the spectrograph. ADU: Analogue-to-Digital Units. (b) A zoom-in of the Echelle order 46
(red dashed box in panel a) of the EO comb (upper) and soliton (lower). (c) Gaussian
profiles of 8 adjacent soliton comb lines in Order #46 are shown (see Methods). (d) Upper
panel shows average centroid drift within Order #46 relative to the first frame in the time
series with both the soliton (blue) and EO combs (black) with the telescope in a quiescent
configuration on 9/10/2017 UTC. The EO comb data bracketing the soliton comb data
shows the drift of the NIRSPEC wavelength scale. The lower panel shows the NIRSPEC
drift after subtracting a linear trend and gives a residual of 0.0034 pixel which corresponds
to approximately 15 m s
−1
in a single order. The inset in the upper panel shows that the
distribution of centroid differences is well defined by a Gaussian distribution (see Methods).
As discussed in the text the final wavelength calibration across the entire echellogram would
be < 5 m s
−1
.
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