LETTERIEICE Electronics Express, Vol.12, No.22, 1–7

Simpliﬁed optical correlationdomain reﬂectometry using
polymer ﬁber
Neisei Hayashia), Makoto Shizuka, Kazunari Minakawa,
Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology,
Yokohama 226–8503, Japana) hayashi@sonic.pi.titech.ac.jp

Abstract: We develop a simpliﬁed conﬁguration of optical correlation(or coherence-) domain reﬂectometry (OCDR) using a polymer optical ﬁber(POF). The conventional reference light path is removed by using as
reference light the Fresnel-reﬂected light caused at the interface between asilica single-mode ﬁber and the POF. We demonstrate its basic operation andinvestigate the dependences of the spatial resolution and signal-to-noise ratio(SNR) on the sweep time of the laser modulation frequency. The optimal
sweep time is found to be 30 ms (corresponding to a repetition rate of 33 Hz),at which both the resolution and SNR are maintained at high values.
Keywords: polymer optical ﬁbers, optical correlation-domain reﬂectometry, Fresnel reﬂection
Classiﬁcation: Fiber optics, Microwave photonics, Optical interconnection, Photonic signal processing, Photonic integration and systems
References© IEICE 2015
DOI: 10.1587/elex.12.20150824
Received October 1, 2015
Accepted October 13, 2015
Publicized October 29, 2015
Copyedited November 25, 2015

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2009.2

1© IEICE 2015
DOI: 10.1587/elex.12.20150824
Received October 1, 2015
Accepted October 13, 2015
Publicized October 29, 2015
Copyedited November 25, 2015

Introduction

Detection of bad connections and splices in optical ﬁber networks has recentlybecome extremely important. One of the most widely used techniques is ﬁber-optic
reﬂectometry based on Fresnel reﬂection. Two of its major conﬁgurations are
optical time-domain reﬂectometry (OTDR) [1, 2, 3, 4, 5] and optical frequencydomain reﬂectometry (OFDR) [6, 7, 8, 9, 10]. OTDR, however, generally suffersfrom its insufﬁcient spatial resolution and relatively long measurement time,
whereas OFDR is susceptible to phase ﬂuctuations caused by environmental
disturbance. One promising technique for mitigating these problems is opticalcorrelation (or coherence)-domain reﬂectometry (OCDR) [11, 12, 13, 14, 15, 16,17, 18, 19, 20] using synthesized optical coherence function (SOCF) [18], which
operates based on optical correlation control through modulated optical frequencyof the laser output. Two methods have been reported to implement the frequency

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IEICE Electronics Express, Vol.12, No.22, 1–7

modulation: sinusoidal modulation [13, 14, 15] and stepwise modulation [16, 17,18]. The latter includes the use of an optical frequency comb [19, 20], which
improves the measurement stability. Of these two methods, sinusoidal modulation
can be implemented with the higher cost efﬁciency.
In a standard OCDR-SOCF system [13, 14, 15, 16, 17, 18, 19, 20], the Fresnel
spectrum is shifted by several tens of megahertz by optical heterodyne detection
using an acousto-optic modulator (AOM) to avoid the overlap of low-frequency
noise of the electrical devices. To perform electrical signal processing in thefrequency range near direct current (DC), and thus to reduce the cost of therelevant devices, we have recently succeeded in excluding the optical heterodynedetection (i.e. without the use of an AOM) by exploiting the foot of the Fresnel
spectrum [21]. High-speed operation has also been demonstrated by tracking amoving loss point. Further simpliﬁcation of this AOM-free conﬁguration will boost
the convenience in practical use.
In the meantime, ﬁbers under test (FUTs) used so far were basically silica-based
single-mode ﬁbers (SMFs). However, for the past decade, polymer optical ﬁbers(POFs) have emerged as candidate media for next-generation optical home networks due to their various advantages, such as high ﬂexibility (endurable for several
tens of percent of strain), low-cost connection, ease of handling, and high safety[22, 23]. By exploiting these merits, POFs have been already used to implement
some optical reﬂectometers [24, 25], but not yet employed in OCDR systems.
In this work, by the use of a POF, we further simplify the AOM-free OCDRSOCF. By using as reference light the Fresnel light reﬂected at the interface
between a silica SMF and the POF, the conventional reference light path can beremoved, leading to the further simpliﬁed setup. After demonstrating a distributed
reﬂectivity measurement, we investigate the dependences of the spatial resolution
and the signal-to-noise ratio (SNR) on the sweep time of the laser modulation
frequency. A repetition rate of 33 Hz is found to be optimal to accomplish a highspatial resolution and a high SNR simultaneously.
2© IEICE 2015
DOI: 10.1587/elex.12.20150824
Received October 1, 2015
Accepted October 13, 2015
Publicized October 29, 2015
Copyedited November 25, 2015

Principle

The experimental setup of standard OCDR-SOCF including an AOM [15] isdepicted in Fig. 1(a). The laser output is divided into two; one is used as incident
light and the other is used as reference light. After passing though the AOM for
frequency downshift by several tens of megahertz, the incident light is injected into
an FUT (silica SMF), the open end of which is angled cut to suppress the Fresnel
reﬂection. The reﬂected light is heterodyned with the reference light to generate anoptical beat signal, which is converted into an electrical signal using a photodetector (PD) and is then monitored using an electrical spectrum analyzer (ESA).
The inﬂuence of the low-frequency noise of the PD and ESA can be mitigated byusing the heterodyne detection. To measure the reﬂected light in a distributedmanner, the optical frequency of the laser output is sinusoidally modulated to forma so-called “correlation peak” in the FUT [18]. By controlling the modulation
frequency, the location of the correlation peak can be scanned along the FUT,
which enables distributed reﬂectivity measurement. As a sinusoidal waveform is a3

IEICE Electronics Express, Vol.12, No.22, 1–7

periodical function, the correlation peaks also appear periodically along the FUT,and therefore their interval determines the measurement range D, which is given by
D¼c;
2nfm

ð1Þ

where c is the optical velocity in vacuum, n is the refractive index of the ﬁber core,and fm is the modulation frequency. As for the spatial resolution z, it is deﬁned as
a 3-dB linewidth of the correlation peak, which is theoretically given by [26]
z ﬃ0:76c;nf

ð2Þ

where f is the modulation amplitude.
Subsequently, a recently developed setup of the OCDR-SOCF without the useof the AOM is shown in Fig. 1(b). This conﬁguration works properly because
the Fresnel spectrum in reality has some non-negligible bandwidth (determined bythe laser bandwidth); the foot of the Fresnel spectrum, which is not overlapped by
the low-frequency noise of the PD and ESA, is used for reﬂectivity sensing [21].
The measurement range D and the spatial resolution z are given as the same
expressions as Eq. (1) and Eq. (2), respectively [21].
Finally, a further simpliﬁed setup of the AOM-free OCDR is depicted inFig. 1(c); a POF with its open end angled cut is used as the FUT. At the buttcoupled interface between a silica SMF (n  1:46; the pigtail of an opticalcirculator) and the POF (n  1:35; perﬂuorinated, as detailed in the next section),
relatively high-power Fresnel-reﬂected light (theoretical reﬂectivity ∼0.2%) isgenerated. We utilize this Fresnel light as new reference light instead of theconventional reference light, and thus the conventional reference light path canbe removed from the system. Note that similar implementation has been reported ina Brillouin-based OCDR system [25, 27]. The 0-th correlation peak, i.e. the zerooptical-path-difference point, is always located at the SMF-to-POF interface, and(a)(b)

Fig. 1.

© IEICE 2015
DOI: 10.1587/elex.12.20150824
Received October 1, 2015
Accepted October 13, 2015
Publicized October 29, 2015
Copyedited November 25, 2015(c)

Schematic experimental setups of (a) standard optical correlation-domain reﬂectometry (OCDR) [15], (b) acousto-optic
modulator (AOM)-free OCDR [21], and (c) further simpliﬁed
AOM-free OCDR using a polymer optical ﬁber (POF). ESA,
electrical spectrum analyzer; FUT, ﬁber under test; PD, photodetector.

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IEICE Electronics Express, Vol.12, No.22, 1–7

the 1st correlation peak is used for distributed measurement. Note that when the 1stpeak reaches the midpoint of the POF, the 2nd peak starts to enter the POF at theopen end. Thus, the measurement range is limited to the distal half of the POF
length rather than Eq. (1).
3Experimental setup

The actual experimental setup of further simpliﬁed AOM-free OCDR using a POF
as an FUT is schematically shown in Fig. 2(a). All the optical paths except the FUT
were composed of silica SMFs. A laser diode at 1.55 µm with a 3-dB bandwidthof ∼1 MHz was used as a continuous-wave light source. The laser output waspolarization-scrambled, ampliﬁed to ∼16 dBm using an erbium-doped ﬁber ampliﬁer (EDFA), and was injected into the FUT via an optical circulator. The reﬂected
light from the FUT was ampliﬁed to ∼3 dBm using another EDFA. The optical beatsignal of this reﬂected light and the Fresnel-reﬂected light was converted into anelectrical signal using a PD, and was sent to an ESA. To achieve high-speed
operation, the spectral power at 3 MHz (selected so that the SNR becomes maximal[21]) was continuously output from the analog terminal of the ESA to an oscilloscope (OSC). The resolution bandwidth and the video bandwidth of the ESA were
set to 300 kHz and 1 kHz, respectively.
The structure of the FUT is shown in Fig. 2(b). A 1.0-m-long pigtail of thecirculator was connected to 10.0-m-, 2.9-m-, 2.9-m-, 2.0-m-, and 2.2-m-long POFs
sequentially using SC/PC connectors; the distal end of the FUT was kept open
(angled cut). Note that the POFs used here were perﬂuorinated graded-index POFs
[28]. The core and cladding layers, composed of doped and undoped polyperﬂuorobutenylvinyl ether, have diameters of 50 µm and 70 µm, respectively. The refractiveindex at the center of the core is 1.356, the numerical aperture is 0.185, and thepropagation loss at 1.55 µm is ∼250 dB/km. The polycarbonate reinforcement
overcladding layer (diameter: 490 µm) reduces micro-bending loss.
The laser output frequency was sinusoidally modulated by direct modulation of
the driving current [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. The modulation
frequency fm was swept from 5.52 MHz to 11.05 MHz with a repetition rate of
10 Hz. The measurement range D was half the POF length, i.e. 10.0 m. The
modulation amplitude f was ﬁxed at 0.75 GHz, resulting in the theoretical spatial
resolution z of approximately 71.5 mm (refer to Eq. (2)). The room temperature
was 24 °C.

(a)Fig. 2.
© IEICE 2015
DOI: 10.1587/elex.12.20150824
Received October 1, 2015
Accepted October 13, 2015
Publicized October 29, 2015
Copyedited November 25, 2015(b)(a) Actual experimental setup of further simpliﬁed AOM-free
OCDR using a POF, and (b) the structure of the ﬁber under
test (FUT). AC, alternating current; DC, direct current; EDFA,
erbium-doped ﬁber ampliﬁer; OSC, oscilloscope; PSCR,polarization scrambler.

5IEICE Electronics Express, Vol.12, No.22, 1–7

4

Experimental results

First, we conﬁrmed the basic operation of the system by measuring the reﬂectivity
distribution along the FUT. The sweep time of the modulation frequency was set to30 ms, and no averaging was performed. The result is shown in Fig. 3; note that thevertical axis is a Fresnel-reﬂected power. The relative position was here deﬁned as
the distance from the open end of the FUT. Three clear peaks were observed at therelative positions of 2.2, 4.2, and 7.1 m, which correspond to the locations of theconnectors. The peak power corresponds to the actual reﬂectivity of each connector.
The 3-dB bandwidth of the maximal peak observed at the relative position of 4.2 m,
which can be regarded as the spatial resolution, was approximately 0.11 m; thisvalue was in moderate agreement with the theory (note that the derivation ofEq. (2) includes some mathematical approximations).
Next, the effects of the sweep time on the spatial resolution and the SNR wereinvestigated. In this experiment, the SNR was deﬁned as the difference between themaximal peak power and the lowest power of the noise ﬂoor (at the relative
position of 8.9 m in Fig. 3); this deﬁnition is the same as that in Ref. [15]. Fig. 4(a)shows the spatial resolution plotted as a function of the sweep time. The resolution
grew higher with increasing sweep time, and it became almost constant when thesweep time was longer than ∼30 ms. Fig. 4(b) shows the SNR dependence on thesweep time. The SNR increased with increasing sweep time, and also becamealmost constant when the sweep time was longer than ∼30 ms, considering theSNR ﬂuctuations of þ= 1 dB. Thus, a ∼30-ms sweep time, corresponding to arepetition rate of ∼33 Hz, was found to be optimal in this condition.

Fig. 3. Measured distribution of the Fresnel-reﬂected power.

© IEICE 2015
DOI: 10.1587/elex.12.20150824
Received October 1, 2015
Accepted October 13, 2015
Publicized October 29, 2015
Copyedited November 25, 2015(a)Fig. 4.

(b)

Measured sweep-time dependences of (a) the spatial resolutionand (b) the signal-to-noise ratio (SNR).

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IEICE Electronics Express, Vol.12, No.22, 1–7

5Conclusion

By use of a POF, a standard AOM-free OCDR-SOCF system was further simpliﬁed. By using as reference light the Fresnel light reﬂected at the SMF-to-POFinterface, the conventional reference light path was removed. We conﬁrmed thedistributed reﬂectivity measurement capability, and investigated the effects of thesweep time of the laser modulation, i.e. the repetition rate, on the spatial resolution
and the SNR of the system. A repetition rate of 33 Hz was found to be optimal toachieve a high spatial resolution and a high SNR simultaneously. We expect that
our further simpliﬁed POF-based AOM-free OCDR-SOCF system will be one of
the most promising techniques for cost-effective distributed reﬂectivity sensing infuture.
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 Science and TechnologyFoundation.

© IEICE 2015
DOI: 10.1587/elex.12.20150824
Received October 1, 2015
Accepted October 13, 2015
Publicized October 29, 2015
Copyedited November 25, 20157