Research Article

Vol. 55, No. 15 / May 20 2016 / Applied Optics

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Simplified optical correlation-domainreflectometry without reference path
MAKOTO SHIZUKA,1 NEISEI HAYASHI,2 YOSUKE MIZUNO,1,* AND KENTARO NAKAMURA1
1

Laboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute of Technology, 4259, Nagatsuta-cho, Midori-ku,
Yokohama 226-8503, Japan
2
Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan*Corresponding author: ymizuno@sonic.pi.titech.ac.jp
Received 7 March 2016; revised 17 April 2016; accepted 21 April 2016; posted 21 April 2016 (Doc. ID 260502); published 12 May 2016

We develop a simplified configuration for optical correlation-domain reflectometry (OCDR) without an explicit
reference path. Instead, the Fresnel-reflected light generated at the distal open end of the sensing fiber is exploited
as a reference light. After the fundamental demonstration, the optimal incident power is found to be approximately 8 dBm. We also show that the loss near the distal end should not be applied, unlike in the case of
Brillouin-based OCDR. © 2016 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (120.1840) Densitometers, reflectometers; (120.5700) Reflection.
http://dx.doi.org/10.1364/AO.55.003925

1. INTRODUCTION
Fiber-optic reflectometry is a fundamental technique for implementing multiplexed and distributed sensing systems [1–8],
and its various configurations have been developed to detect avariety of physical parameters, such as strain [2–4], temperature[3–6], pressure [7], and humidity [8]. Among these various
configurations, in order to detect bad connections (or splices)and other reflection points along fibers under test (FUTs) in a
distributed manner, three types of fiber-optic reflectometry
based on Fresnel reflection have been developed: optical timedomain reflectometry (OTDR) [9–13], optical frequencydomain reflectometry (OFDR) [14–18], and optical correlation(or coherence)-domain reflectometry (OCDR) [19–28]. It has
been reported that OTDR commonly suffers from a relatively
low spatial resolution and a low sampling rate, while OFDR
generally suffers from phase fluctuations caused by environmental disturbances. Thus, here we focus on OCDR, which
can mitigate these shortcomings.
OCDR operates by exploiting a synthesized optical coherence function (SOCF) [26]—i.e., by controlling the correlationof propagating light beams through optical frequency modulation. The modulation methods can be broken into two categories: sinusoidal modulation [21–23] and stepwise modulation[24–26] (including frequency-comb-based modulation [27,28]).
As the latter approach requires accurate frequency adjustment
and/or frequency-comb generation, sinusoidal modulation ismore cost-efficient [21–23]. In a standard SOCF-OCDR system[21–28], an optical frequency shifter such as an acousto-optic
modulator (AOM) is utilized so that the heterodyned Fresnel spectrum is shifted from DC by several dozen megahertz; otherwise,1559-128X/16/153925-04 Journal © 2016 Optical Society of America

the Fresnel reflection spectrum to be detected overlaps with the
low-frequency noise of the electrical devices.
Additionally, in order to reduce the cost of the system, we
have recently developed a new SOCF-OCDR configuration
without using an AOM [29]. By exploiting the foot of theFresnel reflection spectrum, a sufficiently high signal-to-noise
ratio (SNR) was obtained. However, in both the standard andsimplified SOCF-OCDR configurations mentioned above, two
optical paths—the incident optical path including an FUT andthe reference optical path for optical interference—were required. Removing the reference path will further simplify
the system and boost its practical convenience.
By using a polymer optical fiber (POF) as an FUT, we havealready demonstrated SOCF-OCDR operations without an
explicit reference path [30]. In this case, the Fresnel-reflected
light generated at the boundary between the POF and a silica
single-mode fiber (SMF; a pigtail of an optical circulator) wasused as a reference light. However, when the FUT is composed
not of a POF but of silica SMF, Fresnel-reflected light is not
generated at the boundary of the two silica SMFs, which makes
it difficult for us to demonstrate the similar SOCF-OCDR
operations.
In this work, we develop a simplified configuration of
AOM-free SOCF-OCDR without an explicit reference pathwhen standard silica SMF is used as an FUT. Instead, theFresnel-reflected light generated at the distal open end of
the FUT is exploited as a reference light. In addition to demonstrating the basic operation of this approach, we investigate
the optimal incident power and the influence of the loss near
the distal end on the measured results.

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2. PRINCIPLE AND EXPERIMENTAL SETUP
Figure 1 depicts the experimental setup of the AOM-free
SOCF-OCDR without a reference path, which is extremely
simple compared to previous configurations [21–29]. The laser
output at 1550 nm was amplified using an erbium-doped fiber
amplifier (EDFA) and was injected into an FUT composed ofsequentially connected multiple silica SMFs (detailed below inthis section). The reflected light, which contained the opticalbeat signal between the light beams reflected at the SMFto-SMF boundaries and the light beam Fresnel-reflected at
the distal open end of the FUT (∼4% reflectivity), was guided
to a photodetector (note that some structural analogy can befound in optical coherence tomography [31]). The beat signal
was then converted into an electrical signal and input to anelectrical spectrum analyzer (ESA). By using the ESA as an electrical narrow bandpass filter, the electrical spectral power at
2 MHz (at which a maximal SNR can be obtained [29])was selectively transmitted to an oscilloscope. The resolution
bandwidth and the video bandwidth of the ESA were set to
300 kHz and 1 kHz, respectively.
To perform distributed reflectivity (or reflection power)
measurements, the output frequency of the laser was sinusoidally modulated by directly modulating the driving current,
leading to the formation of a correlation peak in the FUT[26] (here we denote the modulation frequency and amplitudeof the optical frequency by f m and Δf , respectively). The lightreflected at a specific position along the FUT can be selectively
observed using the correlation peak. By sweeping the modulation frequency, the correlation peak is scanned along the FUT,
and in this manner the reflectivity distribution can be obtained.
In a conventional SOCF-OCDR system involving a reference
path (either with or without an AOM), sinusoidal frequency
modulation generates multiple correlation peaks periodically.
The measurement range D is then given by the spacing between
the correlation peaks as follows [32],
c
;
(1)
D
2nf m
where c is the velocity of light in a vacuum, and n is the refractive index of the fiber core. The spatial resolution Δz(which equals the 3 dB linewidth of the correlation peak) istheoretically given by [32]
0.76c
:
(2)
Δz ≅
πnΔf
However, in an SOCF-OCDR system without a reference
path, the measurement range D is limited to the proximal halfof the FUT length, rather than the range expressed by Eq. (1).

Fig. 1. Experimental setup of simplified optical correlation-domainreflectometry (OCDR) without a reference path. EDFA, erbiumdoped fiber amplifier; ESA, electrical spectrum analyzer; FG, function
generator; OSC, oscilloscope; PD, photodiode.

Fig. 2. Schematic structure of the fiber under test (FUT).

This is because in this configuration, the 0th correlation peak—i.e., the zero-optical-path-difference point—is constantly located at the distal open end of the FUT, and the first correlation
peak is utilized for distributed measurement. When the firstpeak reaches the midpoint of the FUT, the second peak starts
to enter the FUT at the optical circulator. Note that a similar
configuration has been implemented in a Brillouin-based
OCDR system [33]. Although the spatial resolution of this simplified Brillouin OCDR is a function of the sensing position,that of the SOCF-OCDR without a reference path is notdependent on the sensing position.
The detailed structure of the FUT is shown in Fig. 2. A
1.0 m long pigtail (silica SMF) of the circulator was sequentiallyconnected to 2.9, 3.0, 7.0, 3.0, 3.0, and 2.9 m long silica
SMFs using fiber-channel/physical-contact (FC/PC) or fiberchannel/angled-physical-contact (FC/APC) connectors. The
distal PC end of the FUT was kept open. The modulation frequency f m was swept from 4.67 to 9.34 MHz with a repetition
rate of 33 Hz, which corresponds to the measurement range ofd  0–10.9 m (where d was defined as the length between
Connector A and the distal end). The modulation amplitude
Δf was set to 0.75 GHz (to avoid the damage of the laser),which corresponded to the spatial resolution of 66 mm according to Eq. (2). The range-to-resolution ratio was currently 167,
which can be further enhanced by employing a laser speciallydesigned for high-amplitude modulation use, such as a superstructure-grating distributed-Bragg-reflector laser [34,35]. A
temporal gating technique [36,37] may be another method
for improving the range-to-resolution ratio.

3. EXPERIMENTAL RESULTSFirst, the reflection power distributions along the FUT were
measured with varying incident power P in, as shown in Fig. 3.
When P in was higher than ∼0 dBm, clear peaks corresponding
to Connectors B and C were observed at d  2.9 and 5.9 m,
respectively, which verified the basic operation of this system.
However, when P in was lower than ∼0 dBm, the peak corresponding to Connector C was buried by the noise floor.
We then evaluated the P in dependence of the SNR of eachpeak, which was defined as the ratio between the reflection
power and the noise floor at a certain position. As shown in
Fig. 4, as P in increased, the SNR became maximal and thendecreased for both peaks. Under these experimental conditions,the optimal P in value that yielded the maximal SNR was approximately 8 dBm, irrespective of the connector in question.
Another finding is that the measurement was correctly performed with a moderate SNR even when P in was lower than
0 dBm. This indicates that the EDFA is not required in theResearch Article

Vol. 55, No. 15 / May 20 2016 / Applied Optics

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Fig. 3. Measured distributions of reflection power as a function of
position for various incident powers.

Fig. 5. Measured distributions of reflection power for various bending loss values.

Fig. 4. Signal-to-noise ratio (SNR) measured as a function of theincident power.

Fig. 6. Signal-to-noise ratio (SNR) measured as a function of thebending loss.

incident path, which is desirable from the standpoint of system
simplification.
At this point, we evaluated the influence of the noise causedby the 0th correlation peak. Note that in a Brillouin-based
OCDR system without a reference path, some amount of loss
must be artificially applied near the distal open end in order tomitigate the influence of the 0th correlation peak, and thus toobtain a sufficiently high SNR [33]. Accordingly, a tunablebending loss was applied to the point 0.1 m away from the
distal end. The P in value was 8 dBm. The dependence ofthe measured reflection power distribution on the bending loss
is shown in Fig. 5. As the bending loss increased, the power of
the two peaks corresponding to Connectors B and C decreased,
while new peaks started to grow at d  ∼9 and ∼10 m, where
no connectors existed. These ghost peaks can be explained by
the fact that, as the reflection power from the original 0th correlation peak located at the distal end decreases, one of the
SMF-to-SMF boundaries with relatively weak reflectivity starts
to function as a new 0th correlation peak; in this manner, themultiple reflections among the connectors result in the appearance of the ghost peaks.

Finally, the bending loss dependence of the SNR (the ghost
peaks were also included in the SNR measurements) atd  2.9, ∼9, and ∼10 m was measured (Fig. 6). As the bending loss increased, the SNR of the desired peak at d  2.9 m
decreased, while those of the ghost peaks increased. This result
indicates that, unlike in the case of the Brillouin OCDR systemthat is based on frequency information, we need not (or shouldnot) apply an artificial loss near the open end in the Fresnel
OCDR system that is based on power information.
4. CONCLUSION
A simplified SOCF-OCDR configuration without an explicit
reference path was developed. As standard silica SMF was used
as an FUT, and the Fresnel-reflected light generated at the distal open end of the FUT was exploited as a reference light.
Distributed reflection power measurements were demonstrated, and the optimal incident power was found to be approximately 8 dBm. We also showed that the loss near the
distal end should not be applied, unlike in the case of3928

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Brillouin-based OCDR. We believe that although our simplified AOM-free SOCF-OCDR system has a disadvantage thatits measurement range is limited to the proximal half of the
FUT length, it will still be of great use in implementing
cost-efficient distributed reflectivity sensors in the future.
Funding. Japan Society for the Promotion of Science(JSPS) KAKENHI (25709032, 26630180, 25007652).
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