Gigahertz-repetition-rate soliton microcombs
M
YOUNG
-G
YUN
S
UH AND
K
ERRY
V
AHALA
*
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
*Corresponding author: vahala@caltech.edu
Received 4 December 2017; accepted 8 December 2017 (Doc. ID 314893); published 18 January 2018
Soliton microcombs with repetition rates as low as 1.86 GHz
are demonstrated, thereby entering a regime more typical of
table
–
top combs. Low rates are important in spectroscopy and
relax requirements on comb processing electronics.
© 2018
Optical Society of America under the terms of the
OSA Open Access
Publishing Agreement
OCIS codes:
(140.4780) Optical resonators; (190.4390) Nonlinear
optics, integrated optics; (190.5530) Pulse propagation and temporal
solitons.
https://doi.org/10.1364/OPTICA.5.000065
Since their invention, frequency combs have revolutionized a
wide range of applications including spectroscopy, time standards,
microwave generation, and laser ranging [
1
]. Conventional
frequency combs are table
–
top devices and emit ultrashort pulses
at repetition rates (i.e., comb line spacing) that typically lie be-
tween 100 MHz to 10 GHz [
1
]. An important recent develop-
ment has been soliton mode-locking in miniature, high-Q
microresonators [
2
–
6
]. Compared to earlier microcombs [
7
],
soliton microcombs are stable, offer reproducible spectral enve-
lopes and generate short pulses. Moreover, several conventional
comb applications have been demonstrated using soliton micro-
combs including dual-comb spectroscopy [
8
,
9
], dual-comb dis-
tance measurement [
10
,
11
], and optical frequency synthesis [
12
].
Because of their small size, soliton microcombs have much
higher pulse repetition rates (typically, tens of GHz to several
THz) than those of conventional mode-locked laser combs.
The small size also enables low parametric oscillation threshold
[
13
] and overall low operating power on account of the associated
small mode volume. However, while higher repetition rates
(
>
10 GHz
) are useful in certain applications [
7
,
10
,
14
], lower
repetitions rates (
<
10 GHz
) are desirable to resolve narrower
spectral lines [
8
], to create amplified high-peak-power pulses
for continuum generation [
15
], and to enable use of low-power
signal processing electronics [
1
]. Here, we report soliton micro-
combs with repetition rates as low as 1.859 GHz, which is
substantially lower than other rates reported to date and which
also overlaps with rates for conventional frequency combs. The
latter feature suggests that soliton microcombs can provide many
functions offered by conventional table
–
top comb technology.
To maintain low threshold and operating power in the
lower repetition rate (and larger mode volume) devices, we use
ultra-high quality factor silica wedge disks [
16
]. The thickness
of the silica disk is
∼
8
μ
m
, the wedge angle is typically in the
range of 10
–
40 deg and the soliton repetition frequency is deter-
mined by the disk diameter (
D
) which is controlled with precision
1:20,000. These resonator design parameters can be adjusted to
control resonator dispersion, minimize avoided-mode-crossings
and (for the rates
<
11 GHz
) avoid stimulated Brillouin scattering
[
16
]. In the experiment, a continuous-wave fiber laser at 1550 nm
is amplified by an erbium-doped fiber amplifier and coupled into
the microresonators via a tapered fiber coupler [
17
]. Stable soliton
generation uses the capture-lock method [
18
].
Typical experimental parameters for the soliton microcombs
are summarized in Table
1
. The intrinsic quality factors (
Q
0
)
of 4.358 GHz and 1.859 GHz soliton microcomb devices are
relatively lower because their large size (large exposure field) re-
quired use of contact photolithography during microfabrication.
Other devices are fabricated using 10:1 projection photolithogra-
phy (
1cm
2
field). The lower finesse (
F
) of the larger devices also
indicates fabication-induced differences. Using a larger-field pro-
jection tool [
16
] would improve
Q
factor, resulting in reduced
threshold power (
P
th
) and operation power (
P
pump
) for the
lowest-rate devices. Figure
1
shows the optical and electrical spec-
tra of the soliton microcombs with three different repetition rates
(
f
rep
) below 10 GHz. The squared hyperbolic secant envelope
(dashed red curve in upper panel) indicates single soliton states
with 200 fs
–
300 fs pulse width, which can be further compressed
by increasing operation power. The Fig.
1
lower-panel zoom-in
spectra and the upper-panel electrical spectra (insets) verify soliton
line spacing and repetition rate. The line contrast in the optical
spectra decreases as the soliton line spacing approaches 0.02 nm,
the spectrum analyzer resolution.
With further optical loss reduction through fabrication
process optimization,
<
1GHz
repetition rate operation is
Table 1. Experimental Parameters for Soliton Generation
f
rep
(GHz)
D
(mm)
Q
0
(×
10
6
)
F
(×
10
3
)
P
th
(mW)
P
pump
(mW)
33
a
2.0
180
31
2.2
>
23
22.10
a
3.0
300
34
1.8
>
17
14.61
a
4.5
340
26
1.7
>
13
9.355
a
7.0
670
32
1.2
>
25
4.358
b
15.0
380
8.6
6.7
>
300
1.859
b
35.0
460
4.4
14.5
>
415
a
Projection lithography.
b
Contact lithography.
Memorandum
Vol. 5, No. 1 / January 2018 /
Optica
65
2334-2536/18/010065-02 Journal © 2018 Optical Society of America
feasible, thereby allowing soliton microcomb operation at pre-
cisely controlled rates extending from hundreds of MHz to multi-
ple THz. Moreover, pulse-driven soliton generation [
19
]is
possible at the low rates demonstrated here and can reduce oper-
ating power. To reduce footprint in low-repetition-rate designs,
spiral resonators [
20
] could potentially be used. Also, the recent
demonstration of silicon-nitride waveguide-coupled silica-ridge
resonators can allow these silica soliton microcomb devices to
be integrated with other on-chip optical components [
21
].
Funding.
Defense Advanced Research Projects Agency
(DARPA) (HR0011-16-C-0118, W911NF-16-1-0548).
Acknowledgment.
This work was supported by DARPA
under the SCOUT and ACES programs. The authors also thank
the Kavli Nanoscience Institute.
REFERENCES
1. S. A. Diddams, J. Opt. Soc. Am. B
27
, B51 (2010).
2. T. Herr, V. Brasch, J. Jost, C. Wang, N. Kondratiev, M. Gorodetsky, and
T. Kippenberg, Nat. Photonics
8
, 145 (2014).
3. X. Yi, Q.-F. Yang, K. Y. Yang, M.-G. Suh, and K. Vahala, Optica
2
, 1078
(2015).
4. V. Brasch, M. Geiselmann, T. Herr, G. Lihachev, M. Pfeiffer, M.
Gorodetsky, and T. Kippenberg, Science
351
, 357 (2016).
5. C. Joshi, J. K. Jang, K. Luke, X. Ji, S. A. Miller, A. Klenner, Y. Okawachi,
M. Lipson, and A. L. Gaeta, Opt. Lett.
41
, 2565 (2016).
6. P.-H. Wang, J. A. Jaramillo-Villegas, Y. Xuan, X. Xue, C. Bao, D. E.
Leaird, M. Qi, and A. M. Weiner, Opt. Express
24
, 10890 (2016).
7. T. J. Kippenberg, R. Holzwarth, and S. Diddams, Science
332
, 555
(2011).
8. M.-G. Suh, Q.-F. Yang, K. Y. Yang, X. Yi, and K. J. Vahala, Science
354
,
600 (2016).
9. M. Yu, Y. Okawachi, A. G. Griffith, N. Picqué, M. Lipson, and A. L. Gaeta,
“
Silicon-chip-based
mid-infrared
dual-comb spectroscopy,
”
arXiv:1610.01121 (2016).
10. P. Trocha, D. Ganin, M. Karpov, M. H. Pfeiffer, A. Kordts, J.
Krockenberger, S. Wolf, P. Marin-Palomo, C. Weimann, S. Randel,
W. Freude, T. J. Kippenberg, and C. Koos,
“
Ultrafast optical ranging us-
ing microresonator soliton frequency combs,
”
arXiv:1707.05969 (2017).
11. M.-G. Suh and K. Vahala,
“
Soliton microcomb range measurement,
”
arXiv:1705.06697 (2017).
12. D. T. Spencer, T. Drake, T. C. Briles, J. Stone, L. C. Sinclair, C. Fredrick,
Q. Li, D. Westly, B. R. Ilic, A. Bluestone, N. Volet, T. Komljenovic,
L. Chang, S. H. Lee, D. Y. Oh, M.-G. Suh, K. Y. Yang, M. H. P.
Pfeiffer, T. J. Kippenberg, E. Norberg, L. Theogarajan, K. Vahala,
N. R. Newbury, K. Srinivasan, J. E. Bowers, S. A. Diddams, and
S. B. Papp,
“
An integrated-photonics optical-frequency synthesizer,
”
arXiv:1708.05228 (2017).
13. T. Kippenberg, S. Spillane, and K. Vahala, Phys. Rev. Lett.
93
, 083904
(2004).
14. P. Marin-Palomo, J. N. Kemal, M. Karpov, A. Kordts, J. Pfeifle,
M. H. Pfeiffer, P. Trocha, S. Wolf, V. Brasch, M. H. Anderson, R.
Rosenberger, K. Vijayan, W. Freude, T. J. Kippenberg, and C. Koos,
Nature
546
, 274 (2017).
15. J. M. Dudley, G. Genty, and S. Coen, Rev. Mod. Phys.
78
, 1135
(2006).
16. H. Lee, T. Chen, J. Li, K. Y. Yang, S. Jeon, O. Painter, and K. J. Vahala,
Nat. Photonics
6
, 369 (2012).
17. M. Cai, O. Painter, and K. J. Vahala, Phys. Rev. Lett.
85
, 74 (2000).
18. X. Yi, Q.-F. Yang, K. Youl, and K. Vahala, Opt. Lett.
41
, 2037 (2016).
19. E. Obrzud, S. Lecomte, and T. Herr, Nat. Photonics
11
, 600 (2017).
20. H. Lee, M.-G. Suh, T. Chen, J. Li, S. A. Diddams, and K. J. Vahala, Nat.
Commun.
4
, 2468 (2013).
21. K. Y. Yang, D. Y. Oh, S. H. Lee, Q.-F. Yang, X. Yi, and K. Vahala,
“
Integrated ultra-high-Q optical resonator,
”
arXiv:1702.05076 (2017).
(a)(b)(c)
Fig. 1.
Soliton microcomb optical spectra at repetition rates: (a) 9.355 GHz, (b) 4.358 GHz, and (c) 1.859 GHz. Upper panel shows optical spectra.
Lower panel shows zoom-in spectra of comb lines (resolution is 0.02 nm). Insets: Electrical spectra of the detected soliton pulse streams (resolutio
n
bandwidth is 500 Hz) and top-view of microresonators. PW, pulse width; D, duty cycle.
Memorandum
Vol. 5, No. 1 / January 2018 /
Optica
66