BRIEF NOTEJapanese Journal of Applied Physics 55, 128003 (2016)
http://doi.org/10.7567/JJAP.55.128003

Simpliﬁed optical correlation-domain reﬂectometry
employing proximal reﬂection point
Makoto Shizuka1, Heeyoung Lee1, Neisei Hayashi2, Yosuke Mizuno1*, and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Research Center for Advanced Science and Technology, The University of Tokyo, Meguro, Tokyo 153-8904, Japan2*E-mail: ymizuno@sonic.pi.titech.ac.jp
Received August 9, 2016; accepted October 9, 2016; published online November 16, 2016
We develop a new conﬁguration of simpliﬁed optical correlation-domain reﬂectometry using a proximal reﬂection point. The experimental setup isbasically composed of standard silica ﬁbers and includes neither an optical frequency shifter such as an acousto-optic modulator nor an explicit
reference path. The light reﬂected at the proximal reﬂection point, which is artiﬁcially fabricated near an optical circulator, is used as the reference
light. Unlike the conventional silica-based setup, this conﬁguration can perform a distributed reﬂectivity measurement along the distal half of thesensing ﬁber. After demonstration of the basic operation, the incident optical power dependence of the reﬂectivity distribution is investigated, andthe existence of the optimal incident power is clariﬁed. © 2016 The Japan Society of Applied Physics

Fiber-optic distributed reﬂectometry has been vigorously
studied as one of the key methods for developing smart
materials and structures.1–4) Among various types of reﬂectometry, those based on Fresnel reﬂection enable us to detect
bad connections (or splices) and other reﬂection points alongﬁbers under test (FUTs) in a distributed manner. Two of its
successful conﬁgurations are optical time-domain reﬂectometry (OTDR)5–9) and optical frequency-domain reﬂectometry(OFDR),10–14) both of which have the limitations of insuﬃcient spatial resolution and relatively low measurement
speed, and phase ﬂuctuations caused by environmental
disturbance, respectively. To mitigate these shortcomings,
optical correlation (or coherence)-domain reﬂectometry(OCDR)15–25) based on a synthesized optical coherencefunction (SOCF)22) has been developed and extensively
studied. Among the many SOCF-OCDR conﬁgurations using
directly modulated laser outputs17–22) and optical frequency
combs,23,24) those based on direct sinusoidal modulation of thelaser outputs17–19) can be most cost-eﬃciently implemented.
To further reduce the implementation cost and boost convenience in practical applications, we have been focusing on
the simpliﬁcation of the SOCF-OCDR systems by sinusoidal
modulation. First, by exploiting the foot of the Fresnel
reﬂection spectrum, we developed a conﬁguration that did
not require an optical frequency shifter such as an acoustooptic modulator (AOM).26) We subsequently implemented
this AOM-free SOCF-OCDR without using an explicit
reference path.27) In this conﬁguration, the Fresnel-reﬂected
light generated at the distal open end of an FUT [silica singlemode ﬁber (SMF)] was exploited as the reference light. This
conﬁguration, however, poses two problems: 1) the measurement range is limited to the proximal half of the FUT lengthand 2) the measurement cannot be continued when the
FUT has even one breakage point. In order to alleviate these
problems, a polymer optical ﬁber (POF) was used as an FUTin the same conﬁguration without a reference path,28) whichenables us to exploit the Fresnel reﬂection at the proximal
boundary between the silica SMF and the POF (instead ofthat at the distal open end of the FUT). The reﬂectivity
measurement along the distal half of the FUT, which is moreconvenient for practical applications, was successfully
demonstrated. Besides, the measurement was not interrupted
even when part of the FUT was broken. However, the useof non-silica ﬁbers is not ideal in some cases, because they

are not compatible with the ﬁber-optic telecommunication
systems, which are usually composed of silica SMFs.
In this work, we develop a new silica-SMF-based conﬁguration of AOM-free SOCF-OCDR without an explicit
reference path. As the reference light, we use the Fresnelreﬂected light at a partial reﬂection point artiﬁcially producednear an optical circulator. A distributed reﬂectivity measurement along the distal half of the FUT is demonstrated, andthen the incident optical power dependence of the reﬂectivity
distribution is investigated.
In general, distributed reﬂectivity measurements by SOCFOCDR using explicit reference paths (either with or without
an AOM) are performed on the basis of optical correlation
control.26) Speciﬁcally, the optical frequency of the laser
output is sinusoidally modulated to generate what is called acorrelation peak in the FUT.22) By controlling modulation
frequency, the position of the correlation peak can be scannedalong the FUT, and thus the reﬂected light can be measured
in a distributed manner. On account of the periodicity ofcorrelation peaks, the measurement range D of standard
SOCF-OCDR systems is determined by their interval as
D¼

c
;
2nfm

ð1Þ

where c is the optical velocity in a vacuum, n is the corerefractive index, and fm is the modulation frequency of thelaser output. The bandwidth of the correlation peak determines the spatial resolution Δz as
z ﬃ

0:76c
;
nf

ð2Þ

where Δ f is the modulation amplitude.
Subsequently, consider the performance of reference-pathfree SOCF-OCDR systems. As described in detail in Ref. 27,
when the Fresnel-reﬂected light at the open end of the FUT isused as the reference light, the measurement range is limited
only to the proximal half of the FUT [not determined by
Eq. (1)]. Here, we instead produce an artiﬁcial reﬂection
point at the proximal end of the FUT (near an opticalcirculator) and suppress the reﬂection at the distal end. Thenthe Fresnel-reﬂected light at the proximal end of the FUT canbe used as a reference light. In this case, the zeroth-ordercorrelation peak (i.e., the zero-optical-path-diﬀerence point)
is constantly located at the proximal reﬂection point, and the128003-1© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 128003 (2016)

Measurement range

Laser
EDFA
Function
generator

M. Shizuka et al.

Reflec.

Fiber under testPC

EDFA
PhotodiodeElectrical
spectrum
analyzer

Oscilloscope

PC

PC

5m
4m
Midpoint

7m
Reflec.

PC

3m

Fig. 2. (Color online) Structure of the ﬁber under test.

Fig. 1. (Color online) Experimental setup of the silica-based referencepath-free SOCF-OCDR system. EDFA, erbium-doped ﬁber ampliﬁer.

Incident power = 0 dBm

Power [dBm]

-92.4
-92.8
-93.2
-93.6
-94.0
10

12

14

16

18

20

Relative position [m]Fig. 3. Reﬂection power distribution measured along the distal half of the
FUT. The incident power was 0 dBm. The horizontal axis indicates the
distance from the reﬂection point where the 0th correlation peak was located.

-92.2

Power [dBm]ﬁrst-order correlation peak is exploited for distributed measurement. Considering that the second-order correlation peakstarts to enter the FUT at the distal end of the FUT when theﬁrst-order correlation peak is scanned from the distal end tothe midpoint, the measurement range is limited to the distal
half of the FUT. This operating principle has been describedin more detail in the case of a POF-based system.28) Note thata similar conﬁguration has also been reported in Brillouinbased OCDR systems.29,30)
The experimental setup of the reference-path-free SOCFOCDR is shown in Fig. 1. All the optical paths were mainlycomposed of silica SMFs. The output of a laser at 1.55 µm
with a 3 dB bandwidth of ∼1 MHz was ampliﬁed using anerbium-doped ﬁber ampliﬁer (EDFA) and was injected into
the FUT via an optical circulator. The light reﬂected from the
FUT was ampliﬁed to ∼3 dBm using another EDFA. The
optical beat signals of this reﬂected light and the Fresnelreﬂected light were converted to electrical signals using aphoto diode and was sent to an electrical spectrum analyzer(ESA). The resolution bandwidth and video bandwidth of the
ESA were set to 300 and 1 kHz, respectively. To enable highspeed operation, the spectral power at 2 MHz [with a maximal signal-to-noise ratio (SNR)]26) was continuously output
from the analog terminal of the ESA to an oscilloscope.
The proximal partial reﬂection point was, in this experiment, composed of a 650-µm-long air gap ﬁlled with cyclic
transparent optical polymer (CYTOP) liquid, which has beenused as the core material for perﬂuorinated graded-index
POFs with a relatively low propagation loss (∼250 dB=km at
1.55 µm).31–33) The reﬂection point was fabricated between
the open end of a 1-m-long pigtail (silica SMF; second port)of the optical circulator and the proximal end of the FUT.
The structure of the FUT is shown in Fig. 2, where 7-, 5-, 4-,
and 3-m-long silica SMFs were sequentially connected using“ferrule connector=physical contact (FC=PC)” connectors(loosely connected to imitate weak reﬂection points). The
distal PC end of the FUT was kept open. The modulation
amplitude Δ f was 3.9 GHz and the modulation frequency was
swept from 5.15 to 10.3 MHz, corresponding to the nominal
spatial resolution of ∼30 mm according to Eq. (2), which is>40 times larger than the gap of the partial reﬂection point.
The repetition rate was 20 Hz, and averaging was performed
32 times.
Figure 3 shows the measured reﬂection power distribution
along the distal half of the FUT when the incident power was
0 dBm. The horizontal axis indicates the distance from thereﬂection point where the 0th correlation peak was located.
The three weak reﬂection points, including the open end,
were successfully detected at the correct positions. Note that
because of the total loss of the proximal reﬂection points
is high, the light reﬂected at the distal open end of the FUT-92.0-6 dBm
-3 dBm
-2 dBm

-92.6

0 dBm
1 dBm
2 dBm
3 dBm

-93.0
-93.4
-93.8
10

12

14

16

18

20

Relative position [m]Fig. 4. (Color online) Reﬂectivity distributions measured at various
incident powers from −6 to 3 dBm. Each distribution is shifted by 0.25 m; thehorizontal scale is true only for the result at −6 dBm.

becomes relatively low in power, and thus cannot serve as thereference light.
We then measured the incident optical power dependenceof the reﬂectivity distribution. Figure 4 shows the reﬂectivity
distributions at various incident powers from −6 to 3 dBm.
Each result is shifted by 0.25 m. The power of each peakincreased when the incident power was increased from −6 to
0 dBm, but it decreased when the incident power was further
increased. Here, we deﬁne the SNR of each peak detection as
the diﬀerence between the peak power and the noise ﬂoor
(the power at a position of 14 m was used). Figure 5 shows
the SNRs of the each peak plotted as functions of the incident
optical power. Irrespective of the peaks, the SNR was maximal when the incident power was 0 dBm. This behavior is thesame as that reported in Ref. 27. Thus, the optimal incident
power was shown to exist from the viewpoint of the SNR.
In conclusion, a new conﬁguration of AOM-free SOCFOCDR without an explicit reference path was implemented
using a silica SMF as the FUT. A proximal partial reﬂection

128003-2© 2016 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 55, 128003 (2016)
1.4

19 m
16 m
12 m

1.2
1.0
SNR [dB]

M. Shizuka et al.

0.8
0.6
0.4

0.2
0.0
-6

-4

-2

0

2

4

Incident power [dBm]
Fig. 5. (Color online) Incident optical power dependence of the SNR of
the three peaks at the relative positions of 12, 16, and 19 m.

point was artiﬁcially fabricated near an optical circulator,
and the light reﬂected at this point was used as a reference
light. The measurement capability of a distributed reﬂectivity
measurement along the distal half of the FUT was experimentally proved. In addition, the incident optical power
dependence of the reﬂectivity distribution was investigated,
and the existence of the optimal incident power was clariﬁed.
We anticipate that our silica-based AOM-free SOCF-OCDR
system without an explicit reference path will be of greatuse for implementing cost-eﬀective distributed reﬂectivity
sensors in the near future.
Acknowledgments This work was supported by JSPS KAKENHI Grant
Numbers 25709032, 26630180, and 25007652, and by research grants from the
Iwatani Naoji Foundation, the SCAT Foundation, and the Konica Minolta Scienceand Technology Foundation.

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128003-3© 2016 The Japan Society of Applied Physics