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Improvement of dynamic range and repeatability in a refractive-indexsensing optical comb by combining saturable-absorber-mirror modelocking with an intracavity multimode interference fiber sensor
To cite this article: Ryo Oe et al 2019 Jpn. J. Appl. Phys. 58 060912

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Japanese Journal of Applied Physics 58, 060912 (2019)https://doi.org/10.7567/1347-4065/ab219a

Improvement of dynamic range and repeatability in a refractive-index-sensing
optical comb by combining saturable-absorber-mirror mode-locking with an
intracavity multimode interference fiber sensor
Ryo Oe1,2, Takeo Minamikawa2,3,4 , Shuji Taue5, Takuya Nakahara1, Hidenori Koresawa1,2, Takahiko Mizuno2,3,4,
Masatomo Yamagiwa2,3,4, Yasuhiro Mizutani2,6, Hirotsugu Yamamoto2,7, Tetsuo Iwata2,4, Yoshiaki Nakajima2,8,
Kaoru Minoshima2,3,8 , and Takeshi Yasui2,3,4*
1

Graduate School of Advanced Technology and Science, Tokushima University, Tokushima, Tokushima 770-8506, Japan
JST, ERATO, MINOSHIMA Intelligent Optical Synthesizer Project, Tokushima, Tokushima 770-8506, Japan
3
Institute of Post-LED Photonics (pLED), Tokushima University, Tokushima, Tokushima 770-8506, Japan
4
Graduate School of Technology, Industrial and Social Sciences, Tokushima University, Tokushima, Tokushima 770-8506, Japan
5
School of System Engineering, Kochi University of Technology, Kami, Kochi 782-8502, Japan
6
Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
7
Center for Optical Research and Education, Utsunomiya University, Utsunomiya, Tochigi 321-8585, Japan
8
Graduate School of Informatics and Engineering, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan2*

E-mail: yasui.takeshi@tokushima-u.ac.jp

Received April 20, 2019; revised May 6, 2019; accepted May 13, 2019; published online May 31, 2019
A mode-locked fiber comb equipped with a multimode interference fiber sensor functions as a high-precision refractive-index (RI) sensor
benefitting from precise RF measurement. However, its dynamic range and repeatability are hampered by the inherent characteristics of nonlinearpolarization-rotation mode-locking oscillation. In this article, we introduce saturable-absorber-mirror mode-locking for RI sensing with a wide
dynamic range and high repeatability. While the RI dynamic range was expanded to 41.4 dB due to high robustness against cavity disturbance, theself-starting capability without the need for polarization control improves the RI sensing repeatability to 1.10 × 10−8 for each mode-locking
activation. The improved dynamic range and repeatability will be useful for enhancing the performance of RI sensing.
© 2019 The Japan Society of Applied Physics

T

he refractive-index (RI) is an important physicalvalue of materials. Therefore, RI sensing has been
widely used for the identiﬁcation or characterizationof materials. Among various RI sensors, the RI ﬁber sensor
beneﬁts from compactness, simplicity, ﬂexibility, robustness
against noise and availability in various environments; hence,
this sensor has been used for various applications, including
ethanol sensing,1) glucose sensing,2) biosensing,3) and gas
sensing.4) In most cases, a change in the RI of the sample isconverted into a shift of optical spectrum via the surface
plasmon resonance5–8) or the multimode interference(MMI).9–11) However, the precision of RI sensing is limitedby the sharpness of the maximum or minimum of the optical
spectrum as well as by the instrumentational resolution.
If a change in the RI of a sample is transformed into a
photonic RF signal in combination with a sharpened spectrum, the RI sensing beneﬁts from high-precision, highfunctionality, convenience, and low cost by making use of
frequency standards and precise measurement apparatuses in
the RF region. Such photonic RF ﬁber sensors have beeneffectively applied for strain sensing12) and ultrasoundsensing13,14) by using multiple-longitudinal-mode or multiple-polarization-mode spacing in a continuous-wave (CW)ﬁber laser or a CW ﬁber-Bragg-grating laser. However, theinherent frequency ﬂuctuation of mode spacing hampers
high-precision RI sensing.
Recently, an optical frequency comb (OFC) appeared as a
new photonic RF ﬁber sensor for determining liquid RI15)
as well as for strain,16) acoustic wave,17) and ultrasoundsensing.18) Although OFC has been widely used as an opticalfrequency ruler secured by a frequency standard,19–21) the
OFC was used here as a photonic RF converter, converting
the sample RI into an OFC mode spacing frep via a
combination of RI-dependent tunable bandpass ﬁltering by
an intracavity MMI ﬁber sensor and wavelength dispersion

by a cavity ﬁber. Due to the ultranarrow linewidth and highstability in the mode-locking oscillation, this MMI-OFC
enables us to precisely measure the RI-dependent frep shiftby using an RF frequency counter synchronized with arubidium frequency standard, leading to an RI resolution of
4.9 × 10−6 refractive-index units (RIU) and an RI accuracyof 5.4 × 10−5 RIU.
Notable problems with the previous MMI-OFC include its
limited dynamic range and the low repeatability of RI
sensing, which are caused by the nonlinear-polarization
rotation (NPR)22) used for mode-locking oscillation in the
MMI-OFC. Although NPR has been widely used for theﬁber-based optical comb with broad optical bandwidth, it isless robust against external disturbance to the cavity becauseof high sensitivity to the ﬁber birefringence. In the MMIOFC, a large change in the sample RI causes a change in theintracavity polarization condition via the intracavity MMI
ﬁber sensor, leading to a nonnegligible disturbance to thecavity and the disruption of mode-locking oscillation. This is
the reason for the limited dynamic range of RI sensing. On
the other hand, NPR-based mode-locking oscillation is
activated by polarization adjustment in the ﬁber cavity witha polarization controller. While such polarization adjustment
enables ﬂexible mode-locking oscillation, frep changes withevery mode-locking activation, leading to low repeatability.
If high repeatability of frep can be achieved in an RI sensing
MMI-OFC, the RI sensing repeatability will be enhanced.
To overcome the limited dynamic range and the low
repeatability of RI sensing, a robust mode-locking mechanism with fewer degrees of freedom is required in the
RI sensing MMI-OFC. One possible candidate for such a
purpose is mode-locking oscillation with a saturable-absorbermirror (SAM).23) SAM enables easy, stable, and robustmode-locking oscillation without the need for polarization
control. While such characteristics in SAM enable us to060912-1© 2019 The Japan Society of Applied Physics

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R. Oe et al.

Fig. 1. (Color online) (a) Principle of operation for MMI-OFC.
(b) Experimental setup of SAM-MMI-OFC. Pump LD, pumping laser diode;
WDM, wavelength-division-multiplexing coupler; SMF, single-mode ﬁber;
EDF, erbium-doped ﬁber; SAM module, saturable absorbed mirror module;ISO, polarization-insensitive isolator; FC, 70:30 ﬁber coupler; PD, photodiode.

expand the dynamic range of RI sensing in MMI-OFC,
mode-locking oscillation with fewer degrees of freedom willcontribute to obtaining high repeatability for both frep and RI
sensing. In this article, we evaluate the validity of SAMbased MMI-OFC (SAM-MMI-OFC) by comparing it withthe previous NPR-based MMI-OFC (NPR-MMI-OFC).
Figure 1(a) shows the principle of operation for a
MMI-OFC.15) The MMI-OFC has a ring-type ﬁber cavity
containing a mode-locker and a MMI ﬁber sensor. The
intracavity of the MMI ﬁber sensor functions as an RIdependent tunable optical bandpass ﬁlter (bandpass center
wavelength = λMMI) via the MMI and the Goos–Hänchen
shift on the surface of the MMI ﬁber sensor. In other words,
the intracavity MMI ﬁber sensor shifts the optical spectrum(λMMI) of the MMI-OFC depending on the sample RI.
The wavelength-shifted MMI-OFC spectrum experiences
the wavelength dispersion of the cavity ﬁber, resulting in
the conversion from the RI-dependent optical spectral shift toan RI-dependent shift in the optical cavity length nL. Since
frep of OFC is given by c/nL, where c is the velocity of lightin a vacuum, a change in the RI of a sample can be read out
as an RI-dependent frep shift (Δfrep).
We modiﬁed a mode-locked Er:ﬁber laser oscillator for the
SAM-MMI-OFC, as shown in Fig. 1(b). This oscillator
had a ring cavity, including a 3.3 m length of singlemode ﬁber (SMF, SMF-28, Corning, dispersion at

1550 nm = 17 ps km−1 nm−1), a 0.5 m length of erbiumdoped ﬁber (ER80-4/125, LIEKKI, dispersion at 1550 nm =−65 ps km−1 nm−1), a ﬁber-coupled saturable absorbed
mirror (FC-SAM, SAM-1550-4-4ps-FC, BATOP, highreﬂection band = 1480–1580 nm, absorbance = 4%, modulation depth = 2.4%, relaxation time constant ∼4 ps, mounted
on a 1 m SMF cable with FC connector), a polarizationinsensitive isolator (ISO, PSSI-55-P-I-N-B-I, AFR), a 70:30
ﬁber coupler (FC, SBC-1-55-30-1-B-1, AFR), a wavelengthdivision-multiplexing coupler (WDM, WDM-1-9855-1-L-1F, AFR), a pumping laser diode (BL976-PAG900, Thorlabs,
wavelength = 980 nm, power = 900 mW), a ﬁber circulator(FCIR-55-1-B-1-1, AFR), and an intracavity MMI ﬁber
sensor. The intracavity MMI ﬁber sensor was composed ofa clad-less MMF (FG125LA, Thorlabs, core diameter =125 μm, ﬁber length = 58 mm) with a pair of SMFs at bothends (core diameter = 6 μm, clad diameter = 125 μm, ﬁber
length = 54 mm), the detail of which is given elsewhere.15)
Here, we set m to 4 for the use of the intracavity MMI ﬁber
sensor as the RI-dependent tunable bandpass ﬁlter. The SAM
functions as a stable mode-locker via a ﬁber circulator. The
ﬁber cavity was enclosed in an aluminum box, and its
temperature was controlled to 26.0 °C by a combination ofa Peltier heater (TEC1-12708, Kaito Denshi, power = 76 W),
a thermistor (PB7-42H-K1, Yamaki) and a temperature
controller (TED200, Thorlabs, PID control) [not shown in
Fig. 1(b)]. The output light from the oscillator was detectedby a photodetector, and frep was measured by an RF
frequency counter (53 230 A, Keysight Technologies, frequency resolution = 12 digit s−1) and an RF spectrum
analyzer (E4402B, Keysight Technologies, frequency
resolution = 1 Hz), both of which were synchronized to arubidium frequency standard (FS725, Stanford Research
Systems, accuracy = 5 × 10−11 and instability = 2 × 10−11
at 1 s). Additionally, its optical spectrum was measured
by an optical spectrum analyzer (AQ6315A, Yokogawa
Electric Corp., wavelength accuracy = 0.02 nm, wavelength
resolution = 0.02 nm). For comparison, we prepared the
NPR-MMI-OFC15) and enclosed it in the same temperature-controlled box. The speciﬁcation of the NPR-MMI-OFC
was set to be similar to the SAM-MMI-OFC, as shown later.
The same MMI ﬁber sensor was used in the NPR-MMI-OFC.
Before evaluating the performance of RI sensing, we
compared an optical spectrum and an RF spectrum between
the SAM-MMI-OFC and NPR-MMI-OFC. Pure water wasused here as a liquid sample. The red and blue plots in
Fig. 2(a) show an optical spectrum of the SAM-MMI-OFC(center wavelength = 1558.8 nm, spectral bandwidth =0.8 nm, mean power = 1.43 mW) and the NPR-MMI-OFC(center wavelength = 1552.2 nm, spectral bandwidth =7.7 nm, mean power = 1.46 mW), respectively. The soliton
mode-locking oscillation was achieved near the zero-dispersion region of the cavity (−0.0338 ps2 for the SAM-MMIOFC and −0.0339 ps2 for the NPR-MMI-OFC). The spectral
bandwidth in the SAM-MMI-OFC was signiﬁcantly narrower
than that in the NPR-MMI-OFC. While the SAM-MMI-OFC
can be easily mode-locked by the narrower spectral light and,hence, by the longer pulse light, the mode-locking oscillationof the NPR-MMI-OFC needs the broader spectral light and theshorter pulse light together with precise polarization control.
Such ease of mode-locking oscillation in the SAM-MMI-OFC

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Fig. 3. (Color online) RI-dependent frep shift in (a) SAM-MMI-OFC and(b) NPR-MMI-OFC. (c) Relation between ethanol concentration or sample
RI and frep shift (Δfrep) in SAM-MMI-OFC (red plot) and NPR-MMI-OFC(blue plot).
Fig. 2. (Color online) Comparison of (a) optical spectrum, (b) RF spectrumof frep, and (c) frequency instability of frep between SAM-MMI-OFC and
NPR-MMI-OFC.

will contribute to the robustness against the cavity disturbancecaused by the intracavity MMI ﬁber sensor. Figure 2(b)compares the RF spectrum of frep between the SAM-MMIOFC (red plot) and the NPR-MMI-OFC (blue plot). frep was
54.86 MHz for the SAM-MMI-OFC and 54.79 MHz for the
NPR-MMI-OFC, which are approximately equal. Both RF
spectra have a similar linewidth of approximately 1 Hz, which
is limited by the instrumentational resolution of the RF
spectrum analyzer rather than the actual RF spectrum of frep.
We also compare the frequency instability of frep between the
SAM-MMI-OFC and the NPR-MMI-OFC. The red and blue
plots in Fig. 2(c) show the frequency instabilities of frep withrespect to the gate time, which is deﬁned as the ratio of the frep
ﬂuctuation to the mean frep value. Little difference wasobserved between them; in other words, the use of SAM in
MMI-OFC does not degrade the frep instability and has thepotential to achieve the same RI resolution as the NPR-MMIOFC.
To conﬁrm the RI sensing capability in the optical region,
we ﬁrst investigated the RI-dependent λMMI shift in the
SAM-MMI-OFC and the NPR-MMI-OFC. Mixtures of
ethanol and pure water were used here as liquid samples.
The sample RI was adjusted by changing the ethanol andwater mixture ratio. Furthermore, the temperature of thesample was controlled at 22 °C by a combination of a K-type
thermocouple (TJA-550K, AS ONE), a cord heater (603-6069-01, Tokyo Glass Kikai, power = 15 W), and a temperature
controller (TJA-550, AS ONE, PID control, display

resolution = 0.1 °C). The relationship between the ethanol
volume concentration EC (unit: EtOH%) and the sample RI(unit: RIU) is given by RI = 1.3326 + 4.90 × 10−4 × EC.24)
The resulting RI-dependent λMMI shift was determined to be
94.5 nm/RIU for the SAM-MMI-OFC and 67.8 nm/RIU for
NPR-MMI-OFC, respectively (not shown). We next evaluated the relation between the sample RI and the frep shift(Δfrep) in the SAM-MMI-OFC and NPR-MMI-OFC.
Figures 3(a) and 3(b) respectively show the RI-dependent
RF spectrum shift of frep obtained by the SAM-MMI-OFC
and the NPR-MMI-OFC for the ethanol/water samples withdifferent mixture ratios (=0–20 EtOH%, corresponding to1.333–1.342 RIU) and acquired by an RF spectrum analyzer.
The magnitude of the spectral shift was signiﬁcantly larger
than that of the spectral linewidth in both. Then, we measured
the RI-dependent Δfrep more precisely by using an RF
frequency counter, as shown in Fig. 3(c). Although the linear
relation was conﬁrmed in the SAM-MMI-OFC and the NPRMMI-OFC, the former shows better linearity than the latter.
The RF slope coefﬁcient was determined to be −5.82 Hz/
EtOH% for the SAM-MMI-OFC and −4.01 Hz/EtOH%
for the NPR-MMI-OFC. The RF slope coefﬁcient ratio[= (−5.82 Hz/EtOH%)/(−4.01 Hz/EtOH%) = 1.45] signiﬁcantly agrees with the optical slope coefﬁcient ratio of the
SAM-MMI-OFC to the NPR-MMI-OFC [= (94.5 nm/RIU)/(67.8 nm/RIU) = 1.39]; the difference in the RF slope coefﬁcient between the SAM-MMI-OFC and the NPR-MMI-OFC
is mainly due to the difference in the optical slope coefﬁcient
between the two. From these slope coefﬁcients and thefrequency ﬂuctuation of frep, the RI resolution was

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Fig. 5. (Color online) Temporal change in frep when the mode-locking
oscillation was disrupted. (a) SAM-MMI-OFC and (b) NPR-MMI-OFC.
Fig. 4. (Color online) Relation between ethanol concentration or sample RI
and frep shift (Δfrep) in (a) SAM-MMI-OFC and (b) NPR-MMI-OFC.

determined to be 3.37 × 10−6 for the SAM-MMI-OFC and6.92 × 10−6 for the NPR-MMI-OFC at a measurement timeof 0.5 s.
We further investigated the RI-dependent Δfrep of water/
ethanol samples within a wider range of mixture ratio (0−100
EtOH%, corresponding to 1.333–1.382 RIU). Figure 4(a)shows the RI-dependent Δfrep of the water/ethanol sample in
the SAM-MMI-OFC. The SAM-based mode-locking oscillation was maintained in all ethanol concentrations. The
resulting slope coefﬁcient at the lower concentrations waslarger than that at the higher concentrations because the RI
value of the water/ethanol mixture increased from the low
concentration (= 0−80 EtOH%), reached a plateau approximately 80%, and decreased in the higher concentrations(= 80−100 EtOH%).11) Figure 4(b) shows the RI-dependentΔfrep of the water/ethanol sample in the NPR-MMI-OFC. In
contrast to the SAM-MMI-OFC, the NPR-based modelocking oscillation was lost four times due to the lower
robustness against the cavity disturbance caused by the
intracavity MMI ﬁber sensor. When we activated the modelocking oscillation again by ﬁne adjustment of an intracavity
polarization controller, the frep value signiﬁcantly jumped
from the frep value before the mode-locking oscillation was
lost. As a result, the RI-dependent Δfrep slope shows several
discontinuity points. Such discontinuity points hamper thewide dynamic range of RI sensing in the NPR-MMI-OFC.
The dynamic range of RI sensing in the SAM-MMI-OFC andthe NPR-MMI-OFC is given by
DR SAM = 10 log [(RImax - RImin ) / (RI resolution)]= 10 log [(1.3792–1.3326) / (3.37 ´ 10-6)]= 41.4dB,(1 )DRNPR = 10 log [(1.3473 – 1.3326) / (6.92 ´ 10-6)]= 33.3dB,(2 )where RImax and RImin are the maximum and minimum RI
values. This comparison clearly indicates the superiority of

the SAM-MMI-OFC over the NPR-MMI-OFC for a wide
dynamic range of RI sensing. The maximum RI will belimited by the RI of the clad-less MMF (= 1.444 RIU)
because the MMI ﬁber sensor is based on the total reﬂection
at a boundary between the clad-less MMF surface and thesample.
We evaluated the repeatability of frep in the SAMMMI-OFC and the NPR-MMI-OFC. Figure 5(a) shows
the temporal change of frep in the SAM-MMI-OFC whenthe mode-locking oscillation was disrupted by turning thepumping LD off. Due to the self-starting of the mode-locking
oscillation without the need for additional adjustment of theintracavity component, the frep values were recovered withhigh repeatability before and after the disruption. The slow
temporal change of frep was mainly due to the residual
thermal drift of nL. However, the temporal behavior of frep
was almost continuous even though the mode-locking
oscillation was disrupted. Frequency deviation before andafter disruption of the mode-locking oscillation was
0.60 ± 0.66 Hz for 4 disruptions, corresponding to a repeatability of (1.10 ± 1.21) × 10−8 in frep. This repeatability of frep
is equivalent to the repeatability of the RI measurement. On
the other hand, with the NPR-MMI-OFC, it is difﬁcult toactivate the mode-locking oscillation as self-starting when themode-locking oscillation is disrupted by turning the pumping
LD off. The NPR-MMI-OFC needs the precise adjustment of
the polarization controller for the activation of the modelocking oscillation. As a result, frep increased by several tensof Hz at every disruption point, even though the scale of thepolarization controller was set to that before the disruption,
as shown in Fig. 5(b). Frequency deviation before andafter disruption of the mode-locking oscillation was
46.0 ± 4.67 Hz for 3 disruptions, corresponding to the frep
repeatability of (8.40 ± 0.85) × 10−7. Thus, the SAM-MMIOFC has better frep repeatability f than the NPR-MMI-OFC.
We here discuss the possibility of the absolute measurement of the sample RI based on one-to-one correspondence
between the sample RI and frep. In the previous research on
MMI-OFC,15) the low repeatability of frep in the NPRMMI-OFC [see Fig. 5(b)] hampers such an absolute RI

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measurement. Therefore, the relative measurement was
performed based on a one-to-one correspondence between
the sample RI and the frep shift (Δfrep). The use of SAM in
MMI-OFC beneﬁts from the high repeatability of frep [seeFig. 5(a)] in addition to the wide dynamic range of RI
sensing. However, a slow temporal change of frep remained
due to the residual thermal drift of nL, even though
the temperature control of the ﬁber cavity was activated.
This remaining change is the last barrier preventing theabsolute measurement of the sample RI. One possible
method to get rid of this barrier is to compensate the drifted
frep by another measured parameter (for example, thecarrier-envelope-offset frequency of OFC, optical power,
optical spectrum, and so on) related to the thermal
condition of the ﬁber cavity. Currently, work is in progressto achieve the absolute measurement of the sample RI
based on a one-to-one correspondence between the sample
RI and frep.
In summary, we proposed the use of SAM-based modelocking, in place of NPR-based mode-locking, in MMIOFCs. An improved dynamic range and repeatability wereeffectively demonstrated in RI sensing of the ethanol/water
sample. While the RI dynamic range was signiﬁcantly
increased due to the high robustness against the cavity
disturbance caused by the MMI ﬁber sensor, the selfstarting capability without the need for polarization control
signiﬁcantly improves the repeatability of frep-based
RI sensing at every mode-locking activation. RI sensing
based on SAM-MMI-OFC will be a powerful tool for thequality control of liquid products, biosensing, and gas
sensing.
Acknowledgments This work is supported by Exploratory Research for
Advanced Technology (ERATO, MINOSHIMA Intelligent Optical Synthesizer
Project, JPMJER1304) from Japanese Science and Technology Agency and
Grant-in-Aid for Exploratory Research (19H00871) from the Japan Society for
the Promotion of Science.

https://orcid.org/0000-0003-3528ORCID iDs Takeo Minamikawa
7163 Kaoru Minoshima
https://orcid.org/0000-0003-13011406 Takeshi Yasui
https://orcid.org/0000-0001-7742-6155

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