Letter
Vol. 48, No. 15 / 1 August 2023 /
Optics Letters
3853
AlGaAs soliton microcombs at room temperature
Lue Wu,
1,
†
Weiqiang Xie,
2,3,
†
Hao-Jing Chen,
1,
†
Kellan Colburn,
1
Chao Xiang,
2
Lin Chang,
2
Warren Jin,
2
Jin-Yu Liu,
1
Yan Yu,
1
Yoshihisa Yamamoto,
4
John E.
Bowers,
2,5
Myoung-Gyun Suh,
4,6
AND
Kerry J. Vahala
1,
∗
1
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California, 91125, USA
2
Department of Electrical and Computer Engineering, University of California, Santa Barbara, California, 93106, USA
3
Current address: Department of Electronic Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
4
Physics and Informatics Laboratories, NTT Research, Inc., Sunnyvale, California, 94085, USA
5
jbowers@ucsb.edu
6
myoung-gyun.suh@ntt-research.com
†
These authors contributed equally to this Letter.
*
vahala@caltech.edu
Received 3 January 2023; revised 18 May 2023; accepted 21 May 2023; posted 22 May 2023; published 18 July 2023
Soliton mode locking in high-
Q
microcavities provides a way
to integrate frequency comb systems. Among material plat-
forms, AlGaAs has one of the largest optical nonlinearity
coefficients, and is advantageous for low-pump-threshold
comb generation. However, AlGaAs also has a very large
thermo-optic effect that destabilizes soliton formation, and
femtosecond soliton pulse generation has only been possi-
ble at cryogenic temperatures. Here, soliton generation in
AlGaAs microresonators at room temperature is reported
for the first time, to the best of our knowledge. The destabiliz-
ing thermo-optic effect is shown to instead provide stability
in the high-repetition-rate soliton regime (corresponding
to a large, normalized second-order dispersion parameter
D
2
/
κ
). Single soliton and soliton crystal generation with
sub-milliwatt optical pump power are demonstrated. The
generality of this approach is verified in a high-
Q
silica
microtoroid where manual tuning into the soliton regime is
demonstrated. Besides the advantages of large optical non-
linearity, these AlGaAs devices are natural candidates for
integration with semiconductor pump lasers. Furthermore,
the approach should generalize to any high-
Q
resonator
material platform.
© 2023 Optica Publishing Group under the terms of the Optica Open
Access Publishing Agreement
https://doi.org/10.1364/OL.484552
Optical frequency combs have revolutionized precision time and
frequency metrology, and they find a wide range of applications
in areas as diverse as spectroscopy, optical communications,
distance measurement, and low-noise microwave generation [1].
Recent advances in chip integration of optical frequency combs
[2–4] could accelerate their widespread use even beyond the
laboratory environment. Taking advantage of chip-based high-
Q
-factor microresonators [5], dissipative Kerr soliton (DKS)
formation is a promising method of comb generation [2].
Because the Kerr nonlinearity exists in all optical materials,
DKSs have been demonstrated in many materials including sil-
icon nitride [6], silica [7] , aluminum nitride [8], and lithium
niobate [9].
Among various materials, AlGaAs offers a combination of
large nonlinearity and high refractive index, well suited for low
(
μ
W-level) pump threshold [10–12]. Moreover, because of its
compatibility with semiconductor lasers, it has the potential
to be integrated with pump lasers. However, the large thermo-
optic effect of AlGaAs has prohibited femtosecond soliton pulse
generation at room temperature, and either cooling to cryo-
genic temperature [13] or generation of dark soliton pulses
[14] has been necessary to achieve stable microcombs. Here,
we demonstrate stable room-temperature bright-soliton genera-
tion in AlGaAs microresonators for the first time. Single soliton
states are generated in waveguide-coupled AlGaAs microres-
onators at a repetition rate of 1 THz. The coherence of these
soliton microcombs is confirmed by beatnote measurements
with a self-referenced fiber comb system and observation of per-
fect soliton crystals (PSCs) [15]. The approach uses the scaling
of soliton power with the normalized second-order dispersion
parameter
D
2
/
κ
, and is further verified by soliton generation in
a high-
Q
silica microtoroid using only manual tuning (i.e., no
special triggering [7]) of the optical pumping frequency.
Room temperature generation of solitons in AlGaAs is frus-
trated by a large thermo-optic coefficient in combination with
an abrupt drop of intracavity power that usually accompanies
soliton formation [16]. The subsequent rapid cooling of the
mode volume causes the resonator frequency to quickly tune
away from the pumping frequency. If the thermo-optic effect
is large, this tuning prevents stable soliton formation. On the
other hand, the thermo-optic effect will also self-stabilize an
optically pumped system if the intracavity power increases with
laser blue-to-red tuning [17]. For soliton generation, the sta-
bility benefit of this increase has been noted in soliton crystal
formation [18,19].
To mitigate the decrease in mode temperature, a simple strat-
egy is to increase soliton power. The emitted soliton power
normalized by parametric oscillation threshold power is given
0146-9592/23/153853-04 Journal © 2023 Optica Publishing Group
3854 Vol. 48, No. 15/ 1 August 2023 /
Optics Letters
Letter
by [7]
P
sol
P
th
=
8
√
2
η
2
π
√︃
δω
κ
√︃
D
2
κ
,
(1)
where
δω
=
ω
0
−
ω
is the detuning of pumping frequency
ω
relative to the frequency
ω
0
of the cavity mode that is pumped,
D
2
is the second-order dispersion parameter [see Fig. 1(b) cap-
tion], and
κ
=
ω
0
/
Q
is the optical loss rate, where
Q
is the
optical
Q
factor. Also,
η
=
Q
/
Q
ex
is the waveguide-to-resonator
loading factor, where
Q
ex
is the external coupling
Q
factor
(1
/
Q
=
1
/
Q
0
+
1
/
Q
ex
, where
Q
0
is the intrinsic
Q
factor). In
Fig. 1(a), plots of Eq. (1) versus normalized detuning (2
δω
/
κ
)
at several values of
D
2
/
κ
are superimposed on numerical simu-
lations of normalized soliton power using the Lugiato–Lefever
equation [20]. Increasing
D
2
/
κ
results in higher soliton power at
a given pump detuning. Since
D
2
=
−(
c
/
n
g
)
β
2
D
2
1
(where
β
2
is the
group velocity dispersion, GVD), larger
D
1
(equivalently larger
free-spectral-range, FSR)
=
D
1
/
2
π
and larger soliton repetition
rate), larger
β
2
, and smaller
κ
will increase the normalized soliton
power and mitigate the photothermal instability. A comparison
of
D
2
/
κ
values taken from different devices reported in the liter-
ature as well as the devices studied here is provided in the table in
Fig. 1(a).
The Al
0.2
Ga
0.8
As devices had FSR
=
1 THz (12.46
μ
m radius)
and details of their fabrication are provided in Ref. [12]. The
microresonator waveguide cross section had 600 nm width and
400 nm height [see SEM image in the inset of Fig. 1(b)]. With
these dimensions, the fundamental TE mode has a calculated
GVD of
β
2
=
−
431 ps
2
/km at 1532 nm (the pumping wave-
length in this study). This value is in good agreement with
the measured microresonator GVD of
β
2
=
−
n
g
D
2
/
cD
2
1
≈−
455
ps
2
/km [see Fig. 1(b) and caption for dispersion measurement
details]. Intrinsic
Q
factor
Q
0
=
1.7
×
10
6
and total
Q
factor
Q
=
0.92
×
10
6
at the pumping wavelength were measured by
characterizing the transmission spectrum. The spectrum fea-
tured a full-width-at-half-maximum linewidth of
κ
/
2
π
=
210
MHz which was broadened slightly by mode splitting of 92
MHz [23]. The parametric oscillation threshold was measured
(in-waveguide) to be approximately 20
μ
W which is in good
agreement with the theoretical value of 22
μ
W [7,24], where
n
2
≈
1.7
×
10
−
17
m
2
/W [25] was assumed in addition to the fol-
lowing calculated parameters: effective index
n
eff
=
2.899, group
index
n
g
=
3.746, and effective mode area
A
eff
=
0.256
μ
m
2
.
The refractive index of Al
0.2
Ga
0.8
As is taken from the material
refractive index database [26].
Fig. 1.
AlGaAs and silica microresonator properties and soliton generation with increasing
D
2
/
κ
. (a) Numerical simulation of normalized
intracavity power versus normalized pump laser detuning (2
δω
/
κ
=
2
(
ω
0
−
ω
)/
κ
) for three microresonators having
D
2
/
κ
values of 1, 0.1,
and 0.01. The dashed lines are analytical results from Eq. (1). The pump power is set to 25 times parametric oscillation threshold for all three
traces. The table summarizes
D
2
/
κ
values in various soliton generation devices from Refs. [6,7,13,21,22] and the devices tested in this work.
(b) Measured integrated frequency dispersion for the AlGaAs resonator (red points) is plotted versus relative mode number,
μ
. To construct
this plot the center wavelength of each split mode is measured using an optical spectrum analyzer (OSA). The mode frequency is given by
ω
μ
=
ω
0
+
μ
D
1
+
1
2
D
2
μ
2
and the blue dashed curve is a fit using
D
1
/
2
π
=
1.0126 THz and
D
2
/
2
π
=
235 MHz. The measured modes span
wavelengths from 1516 to 1620 nm and
μ
=
0 corresponds to the pump mode wavelength at 1532 nm. A slight avoided mode crossing near
μ
=
−
5 originates from TE/TM mode hybridization. Upper right inset: optical micrograph of the device with scale bar indicated. Lower left
inset: SEM micrograph of AlGaAs waveguide cross section. (c) Silica microtoroid dispersion plot with similar content to (b). Upper right
inset: SEM micrograph of the device with scale bar indicated.