This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTQE.2017.2770098, IEEE Journalof Selected Topics in Quantum Electronics> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <1

Dead-band-free, high-resolution microwave
frequency measurement using a free-running
triple-comb fiber laser
Xin Zhao, Cui Li, Ting Li, Guoqing Hu, Ruixiao Li, Ming Bai, Takeshi Yasui, and Zheng Zheng,
Member, IEEE

Abstract—Microwave photonic solutions of frequency

measurement have advantages in broad frequency
coverage, but achieving high-resolution measurement
remains a challenge. Those schemes based on opticalfrequency
combs
could
achievehigh-resolution
measurement over a broad frequency range. Here adead-band-free, high-resolution microwave frequency
measurement scheme based on undersampling the
microwave signal by three pulse sequences generated froma triple-wavelength mode-locked fiber laser is proposedand demonstrated. The triple-wavelength ultrashort pulses
generated in one laser cavity have slightly different
repetition rates due to chromatic dispersion. Thiseliminates the needs of multiple mode-locked lasers andfrequency control between them and drastically reduces
the system complexity. The absolute frequency of the
microwave signal can be determined based on threedown-converted low-frequency beat notes of the microwave
signal with the nearest comb lines without ambiguity. A
10-10 relative measurement precision at a sampling speed of
100 Hz is demonstrated from 1 GHz to 20 GHz, and themeasurement accuracy remains within 0.3 Hz. Microwave
signal with a RF power as low as -75dBm can be measured
with a 10Hz precision at 10 GHz by using RF frontend
amplifiers. The simple and compact triple-comb fiber laser
would enable the development of low-complexity,
high-performance microwave characterization instrument.
Index Terms—Mode locked lasers, optical fiber lasers,
frequency measurement, optical pulses

Manuscript received August1, 2017. This work was supported by the NSFC
(61435002/61521091/61675014/61675015) and Fundamental Research Funds
for the Central Universities.
X. Zhao, C. Li, R. Li, T. Li, G. Hu, M. Bai and Z. Zheng are with the Schoolof Electronic and Information Engineering, Beihang University, Beijing,
100083,China(e-mail:x.zhao@buaa.edu.cn;
licui@buaa.edu.cn;
liruixiao94@126.com;
litingshd@163.com;
guoqing2014@buaa.edu.cn;
Corresponding Author: zhengzheng@buaa.edu.cn). Z. Zheng is also with the
Collaborative Innovation Center of Geospatial Technology, 129 Luoyu Road,
Wuhan 430079, China
T. Yasui is with the Graduate School of Science and Technology,
Tokushima University, 2-1 Minami-Josanjima, Tokushima 770-8506, Japanand JST, ERATO MINOSHIMA Intelligent Optical Synthesizer (IOS), 2-1,
Minami-Josanjima, Tokushima, Tokushima 770-8506, Japan (e-mail:yasui.takeshi@tokushima-u.ac.jp).

P

I. INTRODUCTION

recise determination of the frequency of microwave (MW)signals is of great importance, since such signals covering
an increasing broad electromagnetic wave spectrum, frommegahertz to even terahertz, have been prevalently used in our
information
technology
infrastructures,
suchascommunications, remote sensing, radar, navigation and manyother areas. Electronic solutions for real-time MW signal
measurement may be limited by the bandwidth of theacquisition frontend’s and are facing challenges in dealing withunknown signals that may reside over an increasingly broadfrequency range . Microwave photonic techniques have been
investigated as a promising solution for high-frequency
microwave signal characterization and processing, due to theirpotential in broadband operation and immunity toelectromagnetic interferences [1-3]. Numerous photonics
schemes for MW frequency measurement had been proposed.
Frequency-to-power conversion can be realized when the
sideband of an optical CW carrier generated by MW
modulation is filtered by narrow optical filters [4-6]. Opticalmixing method could also map the MW signal frequency to
optical power by mixing an optical signal modulated by MW
signal and its replica with a given time delay. The mixing
process could be realized by two cascaded modulators [7] or
nonlinear optical process [8]. Similarly, the power fading of themodulated double-sideband optical signal propagating througha dispersive medium can provide information about themodulation frequency [9-11]. These methods can cover MW
frequencies up to several tens of gigahertz, but theirmeasurement resolution is no better than MHz level.
Instead of the schemes based on using CW lasers, opticalfrequency combs could also be used in MW signal
characteristics measurement. When a highly stable periodic
pulse train generated by a mode-locked laser, often with arelatively low repetition rate, is modulated by a high-frequency
MW signal, the MW signal is undersampled by the short pulse
sequence. When detected and filtered by a low-pass filter, adown-converted copy of the MW wave with the same
amplitude and phase characteristics is generated [12]. From
another point of view, the down-converted signal corresponds
to the beat note between the MW signal and the closest
neighboring frequency comb line. If two pulse trains withslightly different repetition rates are used to sample the same
MW signal, the aliasing caused by the undersampling could be

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTQE.2017.2770098, IEEE Journalof Selected Topics in Quantum Electronics> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <resolved under certain assumptions, and the absolute frequencyof the MW signal can be recovered. Such a scheme had beensuccessfully
demonstratedfor
real-time
frequency
measurement of continuous-wave THz radiation using dual
THz combs, and very high measurement accuracy andprecision had been achieved [13]. Similarly, two sets of combs
with slightly different comb tooth spacings generated byelectro-optic modulation of light from a CW laser have been
proposed to down-convert MW signals for characterizations[14-16].
Recently, single-cavity, dual-comb mode-locked fiber lasers
have been proposed and demonstrated, which can generate two
asynchronous pulse sequences simultaneously by leveraging
multiplexing in the dimensions of center wavelength,polarization state, or mode-locking mechanism [17-21].
Among these schemes, dual-wavelength mode-locked laser
could generate dual pulsed trains with different center
wavelengths by gain tilt tuning [17] or birefringence-induced
Lyot filtering effects [19, 21]. Due to the cavity dispersion, therepetition rates of the dual-wavelength pulse trains are slightly
different. These kinds of lasers have emerged as an attractive,
low-complexity dual-comb laser source, due to the inherent
mutual coherence between the dual pulse trains. Besides theirapplication in asynchronous optical sampling and spectroscopy[19], THz frequency measurement with a free-running,dual-wavelength laser had been demonstrated [22].
However, in the current dual-comb frequency measurement
schemes, ‘dead bands’ exist due to ambiguity in determining
the relative position between the beat signal and comb teeth[22]. Our preliminary results had shown that using triple-comb
instead of dual-comb could eliminate such dead-bands [23].
However, if using two mode-locked lasers are cumbersome,
implementation of such a system with three similar but slightly
different ultrafast lasers would be prohibitively complicatedand impractical for microwave applications. A simple
multi-comb laser source would be the key to enabledead-band-free high-resolution MW frequency measurement.
Here, we propose and demonstrate a MW frequency
measurement scheme based on a single-cavity, triple-comb
mode-locked fiber laser, which overcomes the ‘dead band’
limitation and achieve gap-less frequency measurement.
High-resolution measurement over a frequency range of tens of
GHz is realized with a precision of 10-10 and sub-Hz accuracy.
II. EXPERIMENTAL SETUP AND PRINCIPLE OF OPERATIONIn our experiment demonstration, a triple-wavelength
mode-locked fiber laser is used to generate three pulse trains
with different repetition rates. The free running,
triple-wavelength mode-locked laser in Fig. 1(a) is constructedbased on a ring cavity configuration. The relatively simple
design consists of a 980-nm pumped laser diode (LD), a
980/1550 nm wavelength-division multiplexer (WDM),
0.45-meter of erbium-doped fiber (Er110), an optical isolator(ISO), a single-wall carbon nanotube (SWNT) saturable
absorber, an in-line polarizer (ILP) with two 1-meter-longpolarization maintaining fiber (PMF) pigtails at both ends, afiber output coupler and a fiber-squeezer-based polarization
controller (PC). The SWNT saturable absorber is fabricated on
an FC/APC ferrule by using the optical deposition method. The

2

total length of fiber in the cavity is estimated to 6.5m, and thetotal dispersion is estimated to be ~103fs/nm. Because of therelatively large anomalous dispersion, the laser is expected tooperate in the soliton regime. By introducing the ILP with PMF
pigtails into the ring fiber laser, birefringence-induced Lyot
filtering effects can provide periodic spectral filtering within
the gain profile of EDF and could enable simultaneously mode
locking at more than one wavelengths, when the intra cavity
polarization state is properly adjusted [19, 22]. Based on thewidth of the EDF gain window around 1550nm and the lengthof PMF, i.e. the amount of birefringence introduced, it isexpected that around three filtered gain windows could bepresent with relatively equalized peak gains. Therefore,
simultaneous triple-wavelength mode-locked ultrashort pulse
generation could be realized in such a compact andcost-effective platform. Due to the difference in their center
wavelengths and the intra cavity dispersion of the fiber laser,
the repetition rates of the triple-wavelength pulses (frep,i, i=1,2,3)
would be slightly different, and therefore triple-comb samplingsignals could be generated from a single mode-locked fiber
laser. As all pulses propagate through the same cavity and sharealmost the same disturbance and noises, such as temperaturefluctuation and pump power RIN noise, it had been
demonstrated that good mutual coherence and stable repetition
rate difference between the multi-wavelength pulses can be
maintained without active feedback control of the laser [19].
Yet, as the laser is operated in a free-running mode, theserepetition rates themselves would still drift over time. It isnecessary to monitor those repetition rates in real time forhigh-resolution measurement of the MW frequencies. A
scheme similar to our triple-comb MW frequency measurement
method is applied. As seen in Fig. 1(b), the output of the laseris split by an optical coupler, and 20% of it is used to for
repetition rate monitoring. The pulses are modulated by thereference signal of a known frequency (fref = 5.985GHz) from a
6 GHz-bandwidth, RF signal generator (Agilent N5162A)
through a 10 GHz-bandwidth electro-optic modulator. The
modulated triple-comb pulses are detected by a 8.5
MHz-bandwidth photodetector (Thorlabs PDA10CS). As
described later, the beat notes between the reference frequency
and the triple-comb signals, denoted as fbeat1_ref, fbeat2_ref, andfbeat3_ref, are obtained. fref is considered to be very accurate andstable, and its ratios to these repetition rates are also known andunchanged as the drifts in the repetition rates are relatively
small. Since these repetition rates are already roughly known
based on the prior knowledge of the laser source, one candetermine the repetition rates with high accuracy in real-timeby just monitoring the frequency changes in the
down-converted beat notes. These measured repetition rates
will be used later in determining the MW frequency.
On the other hand, 80% of the laser output is sent to beat with
the MW signal to be measured. Triple-wavelength pulses are
simultaneously modulated by the target signal generated by aRF signal generator (CETC AV1487B) with a frequency rangeof 0.01 GHz to 20 GHz through a 40–GHz-bandwidthelectro-optic intensity modulator. Unlikely the repetition ratemonitoring setup, the modulated optical pulses from the1077-260X (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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modulator are separated by a dense wavelength divisionmultiplexer (DWDM) and detected by three 150
MHz-bandwidth photodetectors (Thorlabs PDA10CF)
respectively. The electrical signals from the photodetectors arefurther filtered by low-pass filters (Mini-circuits SLP-21.4)
before being acquired by analog-digital converters (ADC, NI
5122) at a sampling rate of 50 MHz, higher than twice themaximumofbeatfrequencies.
By
using
the
instantaneous-frequency-calculation algorithm [22], thefrequencies of the beat notes fbeat1, fbeat2, and fbeat3 can be
determined. The repetition rates of the three combs could bedetermined based on the principle and algorithms given below.
In our demonstration, the two RF signal generators and ADC
are synchronized by the same 10 MHz clock. This avoids theinfluence of the absolute precision of the clocks in these
instruments on the frequency readings from them, to which ourmeasurement results will be compared in order to determine theperformance of our scheme. In future applications, only thereference signal and the ADC are required to be synchronized
to a high-quality clock.

(2)

While fbeat can be easily measured with high accuracy, mremains to be determined, as well as the choice of the sign in (1),depending on the relatively position between mfrep and fx.
As shown in Fig. 2, dual-comb-based scheme provides aneffective solution to this problem to identify m. With two pulse
sequences with slightly different repetition rates, by assuming
the lowest beat notes fbeat1and fbeat2are generated by beating withthe comb lines of the same m-th order, the comb mode order mcan be determined based on:

where frep1and frep2 are the repetition rate of the dual combs
respectively and it is assumed that frep2>frep1. The sign of fbeat
and fbeat1 also is determined by the relative position of fx, frep1,
and frep2 [22].
However, there are ‘dead bands’ in dual-comb-basedfrequency measurement, due to the limitation of the above
assumption [22]. When the frequency to be measured falls
close to the midpoint of a pair of comb teeth and may beat withcomb lines of different mode numbers, (2) becomes no longer
applicable to determine m. Therefore, the absolute frequency ofthe signal can’t be determined in these dead bands. Using athird comb to beat with the signal can overcome the mode
number ambiguity as illustrated in Fig. 2. It is assumed therepetition rate of the third comb is larger than the previous dual
combs, which is frep3>frep2>frep1. If only Comb1 and Comb2 areused, the frequency range of A and B (marked grey in the figure)are dead bands because WM signal beats with the m-th ordermode of Comb1 but m-1-th order mode of Comb2. On the other
hand, if only Comb2 and Comb3 are used, the range C and D
are dead bands. However, as these frequency ranges are disjoint,dead-band-free frequency measurement could be realized by
selecting a right pair of dual combs among the triple combs. For
example, when in the range of A and B, Comb2 and Comb3 canbe used to give the correct results

Fig. 1. (a) Configuration of the free-running triple-comb mode-locked laser; (b)Schematic of the experimental setup. OC: optical coupler; EOM: electro-optic
intensity modulator; LBW-PD: low-bandwidth photodetector.

The principle of our MW frequency measurement scheme inthis article is similar to that of dual-terahertz-comb real-timefrequency measurement, but further leverages thedown-conversion of high-frequency MW signal through EO
modulation as well as the triple-comb sampling pulses. Unlikein the conventional microwave photonic schemes where CW
light is modulated by a MW signal, when a sequence of
periodic short optical pulses with a repetition rate are used, theMW frequency can be down shifted to a much lower frequency
given by its beating with the nearest adjacent frequency
component, i.e. comb line, of the pulse sequence [12]. The
relationship between the frequency of the MW signal fx and thatof the beat signal fbeat is given by(1)
where frep is the repetition rate of the pulse train and m is themode number of the comb line that beats with the MW signal.

Fig. 2. Illustration of the dead-band issue with dual-comb and triple-comb
frequency measurement schemes.

In the dead-band-free frequency measurement based on the
triple-comb, different formulas for calculating m are applied todifferent relative position between fx, mfrep1, mfrep2 and mfrep3. If
the WM signal beats with the m-th order of Comb2, whichmeans (m−1/2)frep2≤fx<(m+1/2)frep2, the frequency region can bedivided to six disjoint sets of frequency bands, as shown in Fig.
3(a).

These

bands,

namely,

band

Ⅰ

:

(m−1/2)frep2≤fx<(m−1/2)frep3, band Ⅱ: (m−1/2)frep3≤fx<mfrep1;

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Fig. 3. Schematic of the six bands and their corresponding senarios in triple-comb sampling. (a) six bands; (b) band I; (c) band II; (d) band III; (e) band IV; (f) band
V; and (g) band VI.

band Ⅲ: mfrep1≤fx<mfrep2; band Ⅳ: mfrep2≤fx<mfrep3; band Ⅴ:
mfrep3≤fx<(m+1/2)frep1; and bandⅥ: (m+1/2)frep1≤fx<(m+1/2)frep2,
correspond to different combinations of the lowest beating
frequencies between the MW signal and the triple-comb, asshown in Fig. 3(b-g).
Once frep1, frep2, frep3, fbeat1, fbeat2, and fbeat3 are obtained
simultaneously, the frequency band and m can be determinedby the following (3) to (8), where the subscript of m indicates1077-260X (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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the corresponding frequency band that the MW signal falls into.
f− f beat1 f rep 3 − f beat 3 − fbeat1 f rep 3 − fbeat 3 − fbeat 2(3)
m = beat 2==Ⅰ

f rep 2 − f rep1

f rep 3 − f rep1

f rep 3 − f rep 2

f
−f
f
−f
f
−f
mⅡ = beat 2 beat1 = beat 3 beat1 = beat 3 beat 2
f rep 2 − f rep1
f rep 3 − f rep1
f rep 3 − f rep 2

(4)

mⅢ =

fbeat 2 + fbeat1 fbeat 3 + fbeat1 fbeat 3 − fbeat 2==
f rep 2 − f rep1
f rep 3 − f rep1
f rep 3 − f rep 2

(5)

mⅣ =

− fbeat 2 + fbeat1 fbeat 3 + fbeat1 fbeat 3 + fbeat 2==
f rep 2 − f rep1
f rep 3 − f rep1
f rep 3 − f rep 2

(6)

mⅤ =

− fbeat 2 + fbeat1 − fbeat 3 + fbeat1 − fbeat 3 + fbeat 2==
f rep 2 − f rep1
f rep 3 − f rep1
f rep 3 − f rep 2(7)− fbeat 2 + f rep1 − fbeat1

(8)

mⅥ =

f rep 2 − f rep1=

− fbeat 3 + f rep1 − fbeat1
f rep 3 − f rep1=

− fbeat 3 + fbeat 2
f rep 3 − f rep 2

The frequency of MW signal can then be calculated using:fx =mf rep 2 − f beat 2band Ⅰ,Ⅱ,Ⅲ

mf rep 2 + f beat 2band Ⅳ,Ⅴ,Ⅵ(9)

For any unknown single-frequency signal, the above
equations can unequivocally determine its relatively position to
the closest neighboring comb lines and identify the mode
number and, thus, the absolute frequency of the signal.
III. EXPERIMENTAL RESULTS AND DISCUSSIONS
When the fiber laser is pumped above its mode-locking
threshold and the polarization state is properly adjusted, threepulse sequences are simultaneously generated with similar peak
spectral intensity in the optical spectrum, as shown in Fig. 4.
Their spectra are centered at 1549.36 nm, 1555.18 nm, and1560.85 nm with 3-dB bandwidths of 0.76 nm, 0.71 nm, and0.98 nm, respectively. The intervals between their central
wavelengths are 5.67 nm and 5.82 nm, which are consistent
with the birefringence introduced by the PM fiber. The small
difference between the two intervals is caused by the non-flat
gain profile of the erbium fiber. After filtered by adense-wavelength-division-multiplexing (DWDM) filter, thespectra of the triple-wavelength pulses are well-separated withno overlap, and are named as Comb3, Comb2, and Comb1,
from shorter to longer wavelength (also shown in Fig. 4). Theirrepetition rates are ~31.8166 MHz with differences of 527 Hz
and 610 Hz, respectively, as in Fig. 5. Because of theanomalous dispersion in the cavity, Comb1, which has thelowest repetition rate, has the longest central wavelength.

Fig.4. Output optical spectrum of the triple-comb laser and the spectra of thefiltered three combs.

Fig.5. RF spectrum of the triple-comb pulses.

In our current setup, the total output power of thetriple-wavelength ultrafast pulse laser is ~300 µW. Due to theinsertion loss of the splitter, modulators and filters, the received
optical power at LBW-PD1 is ~15 µW and the optical power at
LBW-PD 2, 3, 4 is ~7 µW/each. We note that since thehigh-frequency signal is down converted to rather low beatfrequencies and high-gain low-bandwidth photodetectors canbe used instead of high-bandwidth ones, µW-level opticalpower is sufficient for the following high-resolution
measurements.
To demonstrate dead-band-free measurement across a broad
microwave frequency range, the frequency of the MW signal tobe measured is scanned from 1 to 20 GHz, first, with 1 GHz
step. The signal power is set to 0 dBm. The data acquiring timefor each measurement is 10 ms. As shown in Fig. 6(a), themeasured frequency matches the real frequency of the signal in
every measurement. To prove the capability of dead-band-freemeasurement of the proposed triple-comb scheme, around
10GHz, the frequency of the signal is scanned from 10.006055
GHz to 10.006235 GHz over a 180 kHz range with a small
frequency step. Within this frequency range, there is a deadband for the conventional dual-comb measurement scheme. Yet,the frequency of the MW signal can still be resolved accurately
without any ambiguity, seen in the zoom-in inset of Fig. 6(a).
The measurement accuracy and precision are also evaluated.
Accuracy is defined as the difference between the mean of themeasurement results and the nominal value, and precision isdefined as the standard deviation (SD) of the measured values.
As shown in Fig. 6(b), the accuracy is less than 0.3 Hz over thewhole frequency range when averaged over 100 continuous
measurements, which is much higher compared to otherproposed microwave photonic methods [3]. On the other hand,the precision increases roughly linearly with the signal
frequency and the relative precision defined as σ (fx)/fx is ~10-10.
The dependence of the precision value on the frequency can be
explained as following: as seen in (9), the error of the measured
fx is given by that of frep and fbeat, and the former is also
indirectly related to the error in measuring the beat notefrequency in the repetition rate setup measurement. In ourcurrent configuration, the relationship can be given as:

σ ( fx ) =m
σ ( fbeat 2 _ ref ) + σ ( fbeat 2 )
188

(10)

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10.006145

15 10.006055
10+With amplifier
Without amplifier

45 dB

0

10

Signal Power (dBm)
Fig. 7. Frequency precision changes with signal power w/o electrical amplifier.

0
0

5

10

15

Signal frequency (GHz)

2.0

Accuracy
Precision

1.5

1.5

(b)

1.0

20

1.0

0.5

0.5

0.0

0.0
2

4

6

-0.5
8 10 12 14 16 18 20

Signal Frequency (GHz)Fig.6. (a) Measured frequency of the MW signal, the inset: dead-band-freemeasurement demonstration; (b) frequency measurement accuracy andprecision.

In order to determine the detection limit of our scheme forweak microwave signal, the measurement precision isevaluated under different RF powers. The frequency of the MW
signal is set to 10 GHz and the sampling time remains at 10 ms.
When we gradually reduce the input power, after the power
drops between -10 dBm, the measurement precision getssignificantly worse, because of increased errors in determining
the beat frequencies due to the lower signal to noise ratio (SNR),
as shown in Fig. 7 (black line). At -25dBm input RF power tothe EO modulator, the precision is ~6 Hz. By introducing
broadband microwave amplifiers that are widely available forhigh-speed optical communications, the required RF power canbe significantly reduced. When two modulator drivers (onewith 32 dB gain @10 GHz and the other with ~21 dB gain) arecascaded and used, the required power to reach the samemeasurement precision is reduced by ~45 dB. This is slightly
lower than the nominal cascaded gain of ~53 dB, likely due tothe introduced amplifier noise. With the amplifiers, at -55dBm
input power, the precision can be still around 10-10 level asshown in Fig. 6(b). Even at the input RF power of -75 dBm, aprecision of about 10 Hz for a 10GHz input can still be realized.

Finally, for the purpose of fast, real-time frequency
measurements, the dependence of measurement precision on
the sampling speed is investigated. The MW signal is set to 10
GHz and 0 dBm power. As shown in Fig. 8, at a sampling speedof 25 Hz, the precision can reach less than 0.25Hz. The
precision get worse when the sampling time get shorter. This isbecause errors increase in determining the beat frequencies
when the sampling time is decreased. This would lead to
reduced measurement precision as well. However, even if themeasurement is operated at a fast speed, for example 1 kHz, a
precision of about 50 Hz at 10 GHz can still be achieved.
100

Precision (Hz)

5

-0.5
0

+

32dB 21dB@10GHz1-80 -70 -60 -50 -40 -30 -20 -10(a)10.006055 10.006145 10.006235

2.0

Accuracy (Hz)10

Precision (Hz)20

Precision (Hz)Measured Frequency (GHz)

Equation 10 shows that SD of the beat frequency
measurement contributes to the error in determining themeasured frequency, and the measurement SD would increaselinearly as m increases. 188 is the mode number of the mostadjacent comb line to the reference frequency (5.985GHz),
given the repetition rate is ~31.8166 MHz.

6

10

1

0.1

1E-3

0.01

Sampling Time (s)0.1

Fig.8. Frequency measurement precision vs. sampling time.

IV. CONCLUSIONS
A triple-comb-assisted high-resolution microwave frequency
measurement scheme is demonstrated for the first time using asingle simple and cost-effective fiber laser source as well asother fiber-optic components. The frequency of microwave
signal between 1 and 20 GHz can be unequivocally determinedwith a 10-10 precision at a sampling speed of 100 Hz at an input
power of -55dBm, and the measurement accuracy remains
within 0.3 Hz. The precision could be further improved at anincreased sampling time. Though limited by the bandwidth of
the EO modulators (~40 GHz) and the RF source (20 GHz)
used here, our demonstration is limited to the current frequency
range. Nevertheless, the modulators with 100 GHz bandwidthor higher and lower driving voltage are already available.
Considering the bandwidth of the triple-comb laser pulses andthe potential to further tailoring the repetition rates and theirdifferences, it is easy to see that our scheme can readily cover

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTQE.2017.2770098, IEEE Journalof Selected Topics in Quantum Electronics> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <microwave frequencies to 100 GHz and even beyond.
We note that the multi-wavelength, multi-comb fiber source
has negligible complexity increase compared to a traditionalmode-locked fiber laser. Compact and portable microwave
photonic real-time frequency characterization systems capableof covering a broad microwave range with excellent accuracy
could be implemented. One could further expect the emergenceof more multi-comb-based microwave photonic schemes
enabled by such accessible single-laser-based multi-comb
sources.

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Xin Zhao received the Ph.D. degree from Beihang University,
Beijing, China in 2010. From 2011 to 2012, she worked as a
Post-Doctoral Researcher at Beihang University, where she hassince been a Lecturer. She had been a visiting researcher in
National Physical Laboratory, UK between 2016 and 2017. Her
research interests include ultrafast fiber lasers, metrology,
microresonator combs, and nonlinear optics.
Cui Li received the B.S. degree in electronic and informationengineering from Beihang University, Beijing, China, in 2015.
He is currently pursuing the M.S. degree in Electronic
Engineering at Beihang University, Beijing, China. His current
research interests include microwave photonics and signal
processing.
Ting Li received the B.S. degree in applied physics from
Shandong Normal University, Shandong, China, in 2009 andthe M.S. degree in condensed matter physics from Ningxia
University, Ningxia, China, in 2013. She is currently pursuing
the Ph.D. degree in Optical Engineering at Beihang University,
Beijing, China. She is a student member of the Optical Societyof America (OSA). Her current research interest includes fiber
lasers and their applications.
Guoqing Hu received the Ph.D. degree in optical engineeringfrom Beihang University, Beijng, China, in 2017. He had beena visiting student at the University of Tokushima, Japan, for six
months between 2015-2017. His current research interests
include fiber laser, nanomaterial, optical comb, and THz
spectroscopy.
Ruixiao Li received the B.S. degree in electronic andinformation engineering from Beihang University, Beijing,
China, in 2017. He is to pursue his M.S. degree in electric andelectronic engineering at Tokyo Institute of Technology, Tokyo,
Japan. His current research interests include opticalcommunication and integrated optoelectronic devices.
Ming Bai received the Ph.D. degree from University of Scienceand Technology of China, Hefei, China in 2001. From 2002 to

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTQE.2017.2770098, IEEE Journalof Selected Topics in Quantum Electronics> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <8

2006, he worked as a Post-Doctoral Researcher at the Spanish
National Research Council. He is currently a Professor at
Beihang University. His research interests include
electromagnetic technology, microwave technology, andcomputational electromagnetic.
Takeshi Yasui received the Ph.D. degree in mechanicalengineering from the University of Tokushima, Tokushima,
Japan, in 1997. From 1997 to 1999, he worked as a
Post-Doctoral Research Fellow in the National ResearchLaboratory of Metrology, Japan. He was with the Graduate
School of Engineering Science, Osaka University from 1999 to2010, and was briefly with the University of Bordeaux I in 2007
and 2012, and with the University of Littoral Côted'Opale in2010 as an Invited Professor. He is currently a Professor in theInstitute of Technology and Science, The University ofTokushima, Tokushima, Japan, and an Invited Professor in the
Graduate School of Engineering Science, Osaka University,
Toyonaka, Japan. His research interests include THz
instrumentation and metrology, laser stabilization, nonlinear
optical microscopy, and ultrafast time-resolved measurement.
Prof. Yasui is a member of the Optical Society (OSA), theJapan Society of Applied Physics, the Optical Society of Japan,
the Laser Society of Japan, the Japan Society of Medical
Electronics and Biological Engineering, the Japan Society of
Mechanical Engineers, and the Institute of Electronics,
Information and Communication Engineers. He received theAward for the Most Promising Young Scientist from the
Optical Society of Japan in 1998, the Sakamoto Award from theJapan Society of Medical Electronics and BiologicalEngineering in 2006, the Optics Paper Award from the Japan
Society of Applied Physics, and the Funai Award from theJapan Society of Mechanical Engineers in 2009.
Zheng Zheng (S’97–M’01) received the B. Eng. degree inelectronic engineering from Tsinghua University, Beijing,
China, in 1995, and the M.S.E.E. and Ph.D. degrees from
Purdue University, West Lafayette, IN, in 1997 and 2000,
respectively.
He was with the Optical Networking Group of Lucent
Technologies, Holmdel, NJ, as a Member of Technical Staff
between 2001 and 2002, where he had conducted research on
advanced optical fiber communications technologies. In
December 2003, he joined Beihang University as a Professor.
He has published more than 230 technical journal papers andconference talks and holds 6 US patents. His current researchinterests include various areas in photonics and opticalcommunications, such as ultrafast and nonlinear optics,
microwave photonics, nanophotonics, and optical sensing.
Prof. Zheng received the Li Foundation Heritage Prize in
2004. He is a member of IEEE Photonics Society and the
Optical Society of America.

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