of 7
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
4
, Michael P. Fitzgerald
4
, G. Doppmann
5
, J. Wang
6
, D.
Mawet
6
,
2
, Scott B. Papp
3
, Scott A. Diddams
3
, C. Beichman
7
,
, 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
National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA
4
Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA
5
W.M. Keck Observatory, Kamuela, HI 96743, USA
6
Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
7
NASA Exoplanet Science Institute, California Institute of Technology, Pasadena, CA 91125, USA
Corresponding author: Kerry Vahala (email: vahala@caltech.edu) and C. Beichman (email: chas@ipac.caltech.edu)
Detection of weak radial velocity shifts of host
stars induced by orbiting planets is an important
technique for discovering and characterizing plan-
ets beyond our solar system. Optical frequency
combs enable calibration of stellar radial veloc-
ity shifts at levels required for detection of Earth
analogs. A new chip-based device, the Kerr soli-
ton microcomb, has properties ideal for ubiqui-
tous application outside the lab and even in fu-
ture space-borne instruments. Moreover, micro-
comb spectra are ideally suited for astronomi-
cal spectrograph calibration and eliminate filter-
ing steps required by conventional mode-locked-
laser frequency combs. Here, for the calibra-
tion of astronomical spectrographs, we demon-
strate an atomic/molecular line-referenced, near-
infrared soliton microcomb. Efforts to search for
the known exoplanet HD 187123b were conducted
at the Keck-II telescope as a first in-the-field
demonstration of microcombs.
A fundamental question of the human race, whether
life exists on other planets, has brought huge interest
in searching for Earth-like extrasolar planets (exoplan-
ets), especially in the ‘habitable zone’
1
where the orbital
separation is suitable for the presence of liquid water
at the planet’s surface. Since the discovery of an ex-
oplanet around a solar-type star
2,3
, thousands of exo-
planets have been reported
4
. In this quest, the radial
velocity (RV) method (Figure 1) employs high-precision
spectroscopic measurements of periodic Doppler shifts in
the stellar spectrum to infer the presence of an orbiting
exoplanet
5
. Importantly, the RV technique provides in-
formation about the exoplanet mass, which is unavailable
with the complementary technique of transit photometry.
However, RV detection of an earth-like planet in the hab-
itable zone requires extreme spectral precision of about
3
×
10
10
, equivalent to a recoil velocity of the star of
only
10 cm s
1
.
Paramount, therefore, to RV measurements is a
precisely-calibrated astronomical spectrometer
9
. A pow-
erful, new calibration technique is the use of laser fre-
quency combs (LFCs) to provide a spectrally broad
FIG. 1:
Concept of a microresonator astrocomb.
While the host star (red sphere) and exoplanet (blue
sphere) orbit their common center of mass, light waves
leaving the star experience a weak Doppler shift. The
frequency shift (∆
ν
) of the stellar spectral lines are
measured using a spectrograph calibrated using an
evenly spaced comb of frequencies. Here, the comb of
frequencies is produced by a soliton emission from a
microresonator.
‘comb’ of optical frequencies that are precisely stabilized
through the process of self-referencing
10
. 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 result-
ing in a remarkably accurate ‘optical ruler’. LFCs have
revolutionized spectroscopy, time standards, microwave
generation and a wide range of other applications
11
. For
RV detection of exoplanets, LFCs or simply astrocombs
have enabled spectrograph calibration that could enable
RV detection at the cm s
1
level
12
. This is both well
below requirements for detection of Earth-like planets in
the habitable zone of sun-like stars, and also at a level
arXiv:1801.05174v1 [physics.optics] 16 Jan 2018
2
FIG. 2:
Experimental schematic and atomic/molecular line-referenced soliton microcomb.
(a)
Continuous-wave (CW) fiber laser is coupled into a silica microresonator via a tapered fiber coupler
6,7
. 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
8
. The comb power is also tapped to detect and sta-
bilize the repetition frequency (
f
rep
). After dividing by 4,
f
rep
is frequency-locked to an oscillator and monitored
using a frequency counter. 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 fiber Bragg grating (FBG) filter and heterodyned with the reference laser. Finally, the soli-
ton 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.
not achievable by conventional calibration methods us-
ing the emission lines of hollow-cathode gas lamps. In
many cases, however, telluric atmospheric effects and/or
intrinsic stellar noise will impose detection limits above
this level.
The astrocombs used in these earlier experiments
12–18
are derived from femtosecond modelocked lasers that fea-
ture a comb line frequency spacing that is not resolvable
by most astronomical spectrographs
11
. This has neces-
sitated the addition of special spectral filters designed
to coarsen the line spacing
12–18
. The added complex-
ity 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
19,20
. Line referenced EO-astrocomb devices
21
and more recently, self-referenced EO-combs
22,23
have
been demonstrated. However, these devices require op-
tical 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
24,25
.
Driven by parametric oscillation and four-wave-mixing
26
,
millimeter-scale versions of these devices have line spac-
ings that are ideally suited for astronomical calibration
25
.
However, until recently microcombs operated in the so-
called modulation instability regime of comb formation
27
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 ap-
plications of microcombs
28–33
. Also observed in opti-
cal fiber
34
, soliton formation ensures highly stable mode
locking and reproducible spectral envelopes. For these
reasons soliton microcombs are being applied to fre-
quency synthesis
35
, dual comb spectroscopy
36–38
, laser-
ranging
39,40
, and optical communications
41
. Moreover,
their compact (often chip-based) form factor and low op-
erating power can enable implementation of these tech-
3
FIG. 3:
Data from testing at Keck II
. (a) Image of soliton comb projected onto the NIRSPEC echelle spec-
trometer from the echelle orders 44 to 51 with the corresponding wavelength ranges of each order indicated. The
white dashed box indicates soliton emission and has been strongly filtered to prevent potential damage of the spec-
trograph. 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) with line spacings indicated. (c) Gaussian profiles with variable ampli-
tude, centroid,
x
j
(
t
) and width were fitted (see Methods) to each soliton comb line in the echellogram order (#46).
An illustrative sample of 8 adjacent comb lines is shown. (d) Upper panel shows average centroid drift within Order
#46,
z
(
t
), relative to the first frame in the time series with both the soliton (blue) and EO combs (black) during
a quiescent portion of the on-sky observing time on 9/10/2017 UTC. Measurements were taken with the EO comb
bracketing the soliton comb over a 2.5 hour period. The solid black curve shows the drift of the NIRSPEC wave-
length scale in units of a NIRSPEC pixel (
4 km s
1
) measured with the EO comb. The zero-point of the soli-
ton measurement was adjusted to be approximately the same as the EO comb close to the beginning of the soliton
dataset (near UTC=10.22). 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 the Meth-
ods section). As discussed in the text the final wavelength calibration across the entire echellogram would be
5 m
s
1
.
nologies in remote and mobile environments beyond the
research lab, including satellites and other space-borne
platforms. In this work, we demonstrate a soliton mi-
crocomb as an astronomical spectrograph calibrator. We
discuss the experimental setup, laboratory results and
efforts to detect a previously known exoplanet (HD
187123b) at the W. M. Keck Observatory.
The on-site soliton microcomb demonstration was per-
formed at the 10 m Keck II telescope of the W.M. Keck
Observatory in order to calibrate the near-infrared spec-
4
trometer (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
42
. 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 plan-
etary signature. As a cross-check, the functionality of
the soliton microcomb was compared with a previously-
demonstrated line-referenced EO-astrocomb
21
. The ex-
perimental 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 subsys-
tem via single-mode fiber. 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 fiber
without disturbing NIRSPEC itself.
The primary elements of the soliton comb calibration
system are detailed in Figure 2a. The LFC light (soli-
ton microcomb or EO comb) is sent to the fiber acqui-
sition unit (light green box) to calibrate the NIRSPEC
spectrometer
43
. Soliton generation uses a silica microres-
onator fabricated on a silicon wafer
44
. The resonator fea-
tured 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. Figure 2b
shows the optical spectrum of the soliton microcomb.
For transport to the observatory, the microresonator was
mounted inside a brass package with FC/APC fiber con-
nectors. The package was placed inside an insulated foam
box and temperature-controlled using a thermoelectric
cooler to stay within an operating range. 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 shows 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 hy-
drogen 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., ac-
quisition time for a single spectrum) with standard devia-
tion less than 1 MHz. Over this time, the absolute optical
frequency of the HCN reference laser has an imprecision
of less than 1 MHz
21
. Because the soliton repetition rate
(i.e., comb line spacing) is frequency locked, the offset
frequency imprecision was the principal source of insta-
bility 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 fiber.
Figure 3a shows the echellogram of the soliton micro-
comb 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 ap-
proximately 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 consisted of
hundreds of frames taken every 20 seconds over the course
of many hours. As described in the Methods section, the
reduced echellograms for the two combs were analyzed in
a similar manner by fitting a Gaussian to each line (Fig-
ure 3c) to determine its pixel location. For this analysis
we chose Order #46 which spans 1.631 to 1.655
μ
m. The
centroids of each comb line,
x
j
, were determined across
a 2.5 hr interval when the telescope and instrument were
in a quiescent state and investigated for small drifts. The
average drift for the entire order,
z
(
t
), consisting of
N
=
122 (225) lines in the soliton (EO comb) dataset, was
computed with respect to the first frame in the time se-
ries 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 contin-
uously in
5 to 10 minute intervals over 2 hours was ex-
tremely 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). The soliton comb data re-
duced 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 pix-
els 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 inset in Figure 3d (see the Methods section)
demonstrates that the distribution of the differences be-
tween comb-line centroids from one time step to the next
is well represented by a Gaussian distribution, i.e. the
final precision is determined by the centroiding uncer-
tainty and the number of comb lines. The wavelength
precision obtained above is based only on order #46, but
there are four other orders in the echellogram with com-
parable amplitude and number of comb lines (Figure 3a).
Adding in these other lines would improve the wavelength
solution by
∼×
2, or roughly 3-5 m s
1
. 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 in-
herent high precision that have revealed the performance
of NIRSPEC at this level.
5
Observations of the target star, HD 187123, and a ref-
erence star were each bracketed with soliton comb mea-
surements. The analysis of the stellar spectra and of
telluric absorption lines within those spectra revealed
instrumental variations at the 100 m s
1
level (0.025-
0.05 pixel) which we attribute to wavelength shifts within
NIRSPEC which could not be corrected without simul-
taneous LFC-stellar data. These were not possible in the
present configuration. Some of these shifts were clearly
associated with telescope motions between different po-
sitions on the sky. Motions of the stellar image within
the NIRSPEC entrance slit and/or changes in illumina-
tion between the integrating sphere (LFC) and the slit
(starlight) can also result in shifts of this order. While
planet detection could not be achieved, we were able to
measure the two combs sequentially and either one with
respect to the arc lamps used for the absolute wavelength
calibration of NIRSPEC.
A funded upgrade presently underway will enhance
NIRSPEC’s internal thermal and mechanical stability
and future upgrades would enable simultaneous observa-
tion of an LFC and a stellar image stabilized via a single
mode fiber using Adaptive Optics. Finally, a new gen-
eration of spectrographs is in development by numerous
groups to take advantage of diffraction-limited Adaptive
Optics imaging to enable
R >
100
,
000 spectral resolution
and enhanced image stability using single mode fibers.
These new instruments will take full advantage of the
wavelength precision available with a new generation of
microresonator astrocombs.
In summary, we report in-situ astronomical spec-
trograph calibrations with a soliton microcomb. This
enables internal instrumental measurement precision at
the few m s
1
level when calibrating the NIRSPEC as-
tronomical spectrograph at the W. M. Keck observatory
(Keck II). The Kerr soliton microcomb we employ al-
ready possesses the desirable qualities of
20 GHz mode
spacing, low noise operation and short pulse generation.
And rapidly progressing research in this field has resulted
in microcomb spectral broadening and self-referencing
with integrated photonics
45
, as well as direct generation
at shorter wavelengths
46
. These advances will greatly
enhance the microcomb stability and bandwidth, which
increases the range of detectable astronomical objects.
The current prototype system occupies approximately
1.3 m in a standard instrument rack, but significant
effort towards system-level integration
35
, could ulti-
mately provide a microcomb system in a chip-integrated
package with a footprint measured in centimeters. 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 space-borne instrumentation.
Note: The authors would like to draw the read-
ers’ attention to another microresonator astrocomb
demonstration
47
, which was reported while preparing
this manuscript.
Methods
We calculate the
relative
drift in the NIRSPEC wavelength so-
lution,
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
=
N
j
=1
(
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 mi-
crocomb (blue line). The inset in Figure 3d demonstrates that the
distribution of differences in individual soliton comb-line positions
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 centroid-
ing 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 consid-
ered.
Acknowledgments
We wish to thank Josh Schlieder for agreeing to share
time between his night (2017-09-11) and our own (2017-
09-10) to allow a longer time baseline for the observa-
tions of HD187123. Prof. Andrew Howard generously
observed HD187123 with the Keck I HIRES instrument
at visible wavelengths to determine its RV signature in
the weeks immediately before our NIR observations.
We gratefully acknowledge the support of the entire
Keck summit team in making these tests possible. The
authors wish to recognize and acknowledge the very sig-
nificant cultural role and reverence that the summit of
Mauna Kea has always had within the indigenous Hawai-
ian community. We are most fortunate to have the oppor-
tunity to conduct observations from this mountain. The
data presented herein were obtained at the W.M. Keck
Observatory, which is operated as a scientific partner-
ship among the California Institute of Technology, the
University of California and the National Aeronautics
and Space Administration. The Observatory 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 Observa-
tory Archive. We thank Fred Hadaegh for his support
and encouragement which made this experiment possi-
ble. SD and SP acknowledge support from NIST. We
thank David Carlson and Henry Timmers for preparing
the highly nonlinear optical fiber. KV, MGS, XY and
YHL thank the Kavli Nanoscience Institute and the Na-
tional 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 Re-
search and Technology Development. Copyright 2017
California Institute of Technology. All rights reserved.
6
Author Information
Correspondence and requests
for materials should be addressed to KJV (va-
hala@caltech.edu ).
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