Generation of mid-infrared combs (3.3 micron band) with GigaHertz line spacing is demonstrated by interleaved difference-frequency-generation. The method, applied to a 22 GHz repetition-rate microcomb, is useful for spectral densification of sparse microcomb spectra

Bao, Chengying

Leifer, Stephanie

Lin, Qiang

Shen, Boqiang

Sung, Keeyoon

Vahala, Kerry

Wang, Heming

Wu, Lue

Yuan, Zhiquan

English

Caltech Authors - Main

SW3J.1.pdf CLEO 2020 © OSA 2020
Interleaved difference-frequency-generation for
mid-infrared microcomb spectral densification
Chengying Bao,1,† Zhiquan Yuan,1,† Heming Wang,1 Lue Wu,1 Boqiang Shen,1 
Keeyoon Sung,2 Stephanie Leifer,2 Qiang Lin,3 and Kerry Vahala1,∗
1T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA. 
2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
3Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY 14627, USA.
†These authors contributed equally to this work.
∗vahala@caltech.edu
Abstract: Generation of mid-infrared combs (3.3 micron band) with GigaHertz line spac-
ing is demonstrated by interleaved difference-frequency-generation. The method, applied 
to a 22 GHz repetition-rate microcomb, is useful for spectral densification of sparse micro-
comb spectra. 
Temporal soliton formation in coherently pumped microresonators provides a reliable method to generate fully
coherent microcombs that can be used in a wide range of applications including spectroscopy, optical frequency
synthesis, and optical clocks [1]. In application to spectroscopy, microcombs can be an invaluable tool for detection
of chemical threats and field observation of green-house gases (e.g., CO2 and CH4). However, on account of
their small form factors, microcombs usually feature wide comb line spacings (>10 GHz). Such a wide line
spacing may cause spectral undersampling when using microcombs for gas spectroscopy. Hence, it is desirable
to spectrally densify the microcombs (see Fig. 1(a) right-top panel). Soliton microcombs with GigaHertz line
spacing have only been possible in the near-infrared (IR) [2]. And although there has been progress on mid-IR
soliton microcombs [3], generating mid-IR microcombs with GigaHertz line spacing still presents a significant
challenge. Resolving this challenge can be an important step towards scalable and portable microcomb-based
mid-IR gas sensor modules.
Here, we introduce interleaved difference-frequency-generation (iDFG) (i.e., DFG between two combs with
different repetition rates) to spectrally densify microcomb line spacing upon conversion to the mid-IR band. DFG
is a well-known technique that leverages more mature visible and near-IR light sources to generate mid-IR light
PD
BPF
WDM
10s GHz
10s GHz
(b)
(a)
NTS
4 inch
1.5 μm CW
1 μm CW
 fr/N
fr
÷Nm
ic
ro
w
av
e 
lightwave
(N-1)TEO
f r=
1/
T S
PPLN
1.4 THz In
te
n.
 (1
0 
dB
/d
iv
)
Frequency (GHz)
20.5 20.6 21.85 21.95
fr15fr/16
sech²
1061 1062 1063 1064 1065 1066 1067 1500 1520 1540 1560 1580 1600
Wavelength (nm)
In
te
ns
ity
 (1
0 
dB
/d
iv
)
EOMs
3 mm 
Micro- 
cavity
Fig. 1. (a) Experimental layout for iDFG. EOM: electro-optical modulator, BPF: bandpass filter,
WDM: wavelength division multiplexer. (b) The optical spectra of the 1.55 micron soliton micro-
comb and the 1.06 micron EO-comb. The inset shows the repetition rate fr of the soliton microcomb
and the derived frequency of 15 fr/16 that drives the EO-comb.
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3420
Wavelength (nm)
In
te
ns
ity
 (l
ie
na
r a
.u
.)
fr soliton + 
15fr/16 EO-comb
(a)
PPLN Temperature 1
PPLN Temperature 2
EO comb for iDFG
Frequency (GHz)
In
te
ns
ity
 (1
0 
dB
/d
iv
) 15fr/16
15fr/16+δf
16δf
(b)
fr/16-δf
fr/16-17δf fr/16
1.4 GHz
3300 3320 3340 3360 3380 3400 0 0.5 1 1.5
Fig. 2. (a) Spectra of the iDFG generated mid-IR comb. The comb center wavelength is shifted by
changing the temperature of the PPLN crystal. Due to the limited resolution of the spectrometer,
individual comb lines are not resolved. (b) Electrical spectrum of the photo detected mid-IR comb
in panel (a) showing a repetition rate of fr/16=1.4 GHz (blue line) resulting from driving the EO-
comb at 15 fr/16. When driving the EO-comb at a frequency slightly offset from 15 fr/16, there are
additional peaks in the electrical spectrum (red lines).
usually from combs having equal repetition rates [4, 5]. In contrast to this conventional DFG, we apply two near-
IR combs with different repetition rates. If rates fr and (N − 1) fr/N (N is an integer) are used then a temporal
interleaving effect is created whereby the repetition rate of the generated mid-IR comb is reduced to fr/N. A more
general analysis shows that for ∆ fr = m fr/n (∆ fr is the repetition rate rate difference between the two near-IR
combs) that m, n must be mutually-prime to ensure a uniform comb line spacing in the mid-IR.
In the experiment, a 1.55 micron soliton microcomb (rate fr = 22 GHz) is used as both a lightwave source and
a microwave source. A 1.06 micron electro-optic frequency comb (EO-comb) is also used (see Fig. 1(a)). Figure
1(b) shows the optical and electrical spectra of combs. By processing the 22 GHz microcomb signal, a microwave
frequency of 15 fr/16 is generated and is used to drive the EO-comb (see Fig. 1(b)). The soliton microcomb is
silcon-chip based and the LiNbO3 EO-comb could also be integrated [6]. When mixing the soliton microcomb and
EO-comb in a PPLN crystal, a mid-IR comb at 3.3 microns is generated (Fig. 2(a)). The mid-IR comb spans about
80 nm which includes more than 1500 lines with a spacing of 1.4 GHz (i.e., reduced to fr/16 through the iDFG
process). The center wavelength of the mid-IR comb can be tuned by changing the temperature of the crystal.
Due to the limited spectral resolution of the mid-IR spectrometer, a discrete comb structure is not resolvable in the
spectrum. Therefore, to verify that the generated comb has a uniform line spacing, a fast mid-IR detector is used to
measure the line spacing by photodetection of the comb. The electrical spectrum of the detected 3.3 micron comb
shows a single peak at fr/16 = 1.4 GHz (blue line Fig. 2(b)). There are no additional RF peaks which suggests
the line spacing is uniform in the mid-IR. In contrast, when choosing a non-optimal EO-comb line spacing, there
are additional RF peaks observed indicating that the 3.3 micron light has a non-uniform spacing (red lines in Fig.
2(b)). In this test, the EO-comb drive frequency is derived from an independent microwave oscillator (as opposed
to the microcomb) and the frequency is slightly offset (δ f ) from 15 fr/16.
In summary, we have introduced iDFG for spectral densification of mid-IR comb light generated by difference-
frequency generation. 3.3 micron comb light having GigaHertz line spacing was produced from K-band rate near-
IR combs. The method can be integrated using compact microcombs either alone or in combination with thin-film
LiNbO3 waveguides and modulators. Such a system would be useful for field deployable and compact methane
and green-house gas sensor systems.
We acknowledge support from the DTRA (HD-TRA11810047); the AFSOR (FA9550-18-1-0353) and the
Resnick Institute at Caltech.
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Interleaved difference-frequency-generation for mid-infrared microcomb spectral densification

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Interleaved difference-frequency-generation for mid-infrared microcomb spectral densification

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