Applied Physics Express 9, 032702 (2016)
http://doi.org/10.7567/APEX.9.032702

Optical correlation-domain reﬂectometry without optical frequency shifter
Makoto Shizuka, Shumpei Shimada, Neisei Hayashi, Yosuke Mizuno*, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan*E-mail: ymizuno@sonic.pi.titech.ac.jp
Received November 12, 2015; accepted January 26, 2016; published online February 12, 2016
To achieve distributed reﬂectivity measurements along an optical ﬁber, we developed a simpliﬁed cost-effective conﬁguration for optical correlation(or coherence-) domain reﬂectometry based on a synthesized optical coherence function using sinusoidal modulation. By excluding theconventional optical heterodyne detection that accompanies the frequency shift (without using an acousto-optic modulator) and by exploiting thefoot of the electrical Fresnel reﬂection spectrum, the electrical bandwidth required for signal processing is lowered to several megahertz. We
evaluate the basic system performance and demonstrate its high-speed operation (10 ms for one scan) by tracking a moving reﬂection point in realtime. © 2016 The Japan Society of Applied Physics

ptical reﬂectometry is a useful technology for monitoring the health of optical components, modules,
and ﬁber networks, and it serves as a fundamental
technique for multiplexed and distributed sensing systems1–5)
and optical coherence tomography.6–8) To detect badconnections=splices and other reﬂection points along ﬁbers
under test (FUTs), two types of Fresnel-based ﬁber-optic
reﬂectometry have been developed and extensively studied:
optical time-domain reﬂectometry (OTDR)9–13) and opticalfrequency-domain reﬂectometry (OFDR).14–18) However,OTDR generally suﬀers from a relatively low spatial
resolution and a low sampling rate (or a long measurementtime), while OFDR commonly suﬀers from phase ﬂuctuations caused by environmental disturbance. One promising
method for overcoming these drawbacks is the so-called
optical correlation- (or coherence-) domain reﬂectometry(OCDR)19–28) exploiting a synthesized optical coherencefunction (SOCF),26) which operates on the basis of correlation control of propagating light waves by modulating thelaser output frequency. Two methods for frequency modulation have been implemented: sinusoidal21–23) and stepwise
modulations.24–26) The latter includes modulations using an
optical frequency comb,27,28) which can enhance the measurement stability. Of these two frequency modulation methods,
sinusoidal modulation is more suitable for cost-eﬀective
implementation.21–23)
In conventional SOCF–OCDR systems,21–28) the measured
Fresnel spectra are shifted by several tens of megahertz by
optical heterodyne detection using acousto-optic modulators(AOMs). This shift is required because otherwise the lowfrequency noise from the electrical devices overlaps theelectrical Fresnel spectra. If the system can be implemented
without the use of an optical frequency shifter, electricalsignal processing can be performed in the frequency range
near DC (up to several megahertz), leading to cost reductionof relevant devices and thus of the whole system.
In this work, by excluding optical heterodyne detection,
we developed a simple SOCF–OCDR conﬁguration based on
sinusoidal modulation. By exploiting the foot of the Fresnel
reﬂection spectrum, the system was simpliﬁed with reduced
cost. We further demonstrated its high-speed operation.
The experimental setup of a standard SOCF–OCDR system containing an AOM21–23) is depicted in Fig. 1(a). The
laser output is divided with a coupler; one beam is injected
into an FUT, and the other beam is used as the reference
light. The reﬂected light from the FUT is mixed with the

O

(a)

(b)
Fig. 1. Experimental setup for OCDR. (a) Conventional system and(b) new system without an AOM. EDFA: erbium-doped ﬁber ampliﬁer; ESA:
electrical spectrum analyzer; FG: function generator; OSC: oscilloscope; PD:
photodetector.

reference light, the frequency of which is downshifted byseveral tens of megahertz using the AOM for optical
heterodyne detection. The signal is then converted into anelectrical signal with a photodetector (PD) and monitored
using an electrical spectrum analyzer (ESA). Owing to
optical heterodyne detection, the low-frequency noiseinﬂuence of the PD and ESA can be mitigated. To achieve
high-speed operation, the spectral peak power is continuously
recorded using the analog output terminal of the ESA.
Repetition rates of 0.1–1 Hz have been obtained by using the“peripheral component interconnect extensions for instrumentation” (PXI) platform.23)
To spatially resolve the measurement positions, we
sinusoidally modulate the frequency of the laser output toform the so-called “correlation peak” in the FUT. By using
the correlation peak, the reﬂected light generated at a speciﬁc
position can be selectively observed.26) By sweeping themodulation frequency, the correlation peak is scanned along
the FUT, and thus the reﬂectivity can be measured in a

032702-1© 2016 The Japan Society of Applied Physics

Appl. Phys. Express 9, 032702 (2016)

M. Shizuka et al.

distributed manner. Sinusoidal frequency modulation periodically generates multiple correlation peaks, and the interval of
the periodicity determines the measurement range. According
to detailed calculations, the spatial resolution Δz (i.e., the3 dB linewidth of the correlation peak) and the measurement
range D are theoretically given by29)
0:76c
;
n f
c
D¼
;
2nfm

z ﬃ

Fig. 2. Structure of the ﬁber under test for basic characterization.

ð1Þ
ð2Þ

respectively, where c is the light velocity in vacuum, n is therefractive index of the ﬁber core, Δ f is the modulation
amplitude, and fm is the modulation frequency.
In contrast, the experimental setup of a new SOCF–OCDR
system without optical heterodyne detection is shown in
Fig. 1(b), which is similar to that in Fig. 1(a) except thatthe new system does not contain an AOM. If the optical
spectrum of Fresnel reﬂection has a delta-function-like shape,
this new conﬁguration does not work properly. In reality,
because the Fresnel spectrum has some non-negligible
bandwidth (determined by the laser bandwidth),30) the
foot of the electrical Fresnel reﬂection spectrum can avoid
the overlap of the low-frequency noise of the PD and ESA
without the use of an optical frequency shifter. In this conﬁguration, the frequency used for measuring the opticalpower ranges from several hundreds of kilohertz to several
megahertz. In this frequency range, the power level is lower
than that at DC, and the dynamic reﬂectivity range islowered. The theoretical expressions for the spatial resolution and measurement range are the same as those in the
aforementioned standard SOCF–OCDR system. In thisexperiment, the ESA output is sent to a personal computer
via an inexpensive sound card instead of the PXI platform23)
and monitored using a virtual oscilloscope, leading to arepetition rate of >10 Hz with a less expensive system.
In the experiments below, we employed a laser diode at1550 nm with a 3 dB bandwidth of ∼1 MHz. The laser output
was ampliﬁed to ∼19 dBm and then injected into the FUTs.
The modulation amplitude Δ f was ﬁxed at 0.75 GHz (toavoid damage to the laser), resulting in a theoretical spatial
resolution of ∼66 mm [see Eq. (1)]. Note that a spatial
resolution of <100 µm is theoretically achievable by employing a high-speed broadband tunable laser, such as a superstructure-grating distributed Bragg reﬂector laser.
First, to conﬁrm the basic operation of OCDR without
optical heterodyne detection, distributed reﬂectivity measurements were performed using the simply structured FUT
depicted in Fig. 2. A 1.0-m-long pigtail of a circulator
composed of a silica single-mode ﬁber (SMF) was sequentially connected to 1.0- and 3.0-m-long silica SMFs using
angled physical contact (APC) connectors; the end of the
3.0-m-long SMF was also kept open with an APC connector.
Here, we deﬁne fz as the central frequency of the band-passﬁlter of the ESA; the electrical spectral power at thisfrequency is selectively transmitted to an oscilloscope (OSC)
to derive the reﬂectivity distribution along the FUT. The
resolution bandwidth (RBW) and the video bandwidth(VBW) of the ESA were set to 300 and 1 kHz, respectively.
The modulation frequency fm was swept from 5.01 to
5.16 MHz with a repetition rate of 10 Hz, corresponding to

Fig. 3. Electrical spectrum measured when fm was 5.1 MHz.

a measurement range of approximately 20 m according to
Eq. (2).
The measured electrical spectrum in the range from DC to
12.0 MHz (when fm = 5.1 MHz) is shown in Fig. 3. In addition to the low-frequency noise around DC, two peaks wereobserved at 5.1 and 10.2 MHz, which correspond to fm and its
second harmonic component, respectively. The reﬂectivity
distributions obtained when fz = 1.2, 2.0, and 5.0 MHz are
shown in Figs. 4(a)–4(c), respectively. The relative positiond was deﬁned to be 0 at the circulator. In Figs. 4(a) and 4(b),three clear peaks were observed with reﬂectivities higher than
−52 dB, and the locations of the peaks corresponded well tothose of the APC connectors (including the open end). The
relatively small peak (at d = 0) corresponds to the reﬂection
at the circulator (approximately −52.5 dB). In contrast, inFig. 4(c), no peaks were observed because of the overlap of
the noise component induced by the modulation frequency
fm. Next, the signal-to-noise ratio (SNR), which is deﬁned
as the diﬀerence between the maximal peak power and thenoise ﬂoor level, was plotted as a function of fz (Fig. 5).
When no peaks corresponding to the APC connectors
were observed, the SNR was deﬁned to be 0. The SNR
deteriorated at fz < ∼1 MHz under these experimental conditions (dependent on the RBW and VBW) because of thelow-frequency noise of the ESA and PD. The SNR was
also reduced at fz of around 5 MHz because of the noisecaused by the laser modulation. The maximal SNR was
obtained at fz ∼ 2 MHz.
The relationship between the peak power obtained by thismethod and the actual reﬂection power (measured using a
power meter) was also investigated (Fig. 6). As the reﬂection
power increased, the peak power increased monotonically.
The nonlinear dependence was probably due to the nonlinear
characteristics of the PD. Nevertheless, using this one-to-one
correspondence, the actual reﬂection power can be inferredfrom the measured peak power. Note that, if diﬀerent fz
values are employed, these conversion characteristics willchange and need to be reinvestigated.
We subsequently performed the same measurement using
another FUT with a more complicated structure (Fig. 7). The
1.0-m-long pigtail of the circulator was sequentially con-032702-2© 2016 The Japan Society of Applied Physics

Appl. Phys. Express 9, 032702 (2016)

M. Shizuka et al.

(a)Fig. 6. Actual reﬂection power plotted with respect to the peak powerobtained by this method.

Fig. 7. Structure of the ﬁber under test for demonstrating multiple
reﬂection point detection.

(b)

Fig. 8. Reﬂectivity distribution measured when fz was 2.0 MHz.

(c)Fig. 4. Reﬂectivity distributions measured when fz was (a) 1.2, (b) 2.0,
and (c) 5.0 MHz.

Fig. 9. Structure of the ﬁber under test for demonstrating high-speed
operation.

Fig. 5. SNR plotted as a function of fz.

nected to 1.0-, 1.0-, 3.0-, and 1.0-m-long silica SMFs using
APC connectors; the distal end of the FUT was also kept
open with an APC connector. The modulation frequency fm
was swept from 5.01 to 5.20 MHz with a repetition rate of
10 Hz (D ≃ 20 m). Figure 8 shows the reﬂectivity distribution( fz = 2.0 MHz). Five clear peaks were observed, which corresponded to the locations of the APC connectors (including

the open end). A relatively small peak corresponding to thereﬂection at the circulator was also observed.
Finally, the high-speed operation of this system was
veriﬁed by tracking a moving reﬂection point. The structureof the FUT is shown in Fig. 9. Part of a 7.0-m-long silica
SMF (with a 0.3-mm-thick jacket) was wound on a screw for
one turn with an outer diameter of 6.4 mm (ﬁxed on a stage),and the stage was moved along the SMF for ∼3.0 m at aconstant speed of 0.2 m=s. Thus, a moving reﬂection point
can be implemented even though the reﬂectivity (∼53 dB) is
not suﬃciently stable. The reﬂectivity distributions measured
when time t = 0, 3.0, 6.0, and 9.0 s are shown in Fig. 10(a)(t was deﬁned to be 0 when the screw started to move). In
addition to the ﬁxed peak at the APC connector, other peaks
were clearly observed at diﬀerent positions. The peak powerof ∼53.5 dB agreed well with the actual value (∼53 dB). The
peak power ﬂuctuated within approximately ±1 dB, which
could be partially attributed to the actual reﬂectivity ﬂuctua-

032702-3© 2016 The Japan Society of Applied Physics

Appl. Phys. Express 9, 032702 (2016)

M. Shizuka et al.

possible by replacing the ESA with an electrical circuit withan equivalent function, such as a narrow band-pass ﬁlter. We
believe that this conﬁguration will improve the convenienceof the SOCF–OCDR technology for practical applications.
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.

(a)

(b)
Fig. 10. (a) Reﬂectivity distributions measured when time t = 0, 3.0, 6.0,
and 9.0 s. (b) Temporal variation of the measured reﬂectivity distribution.

tions. The temporal variation of the measured reﬂectivity
distribution is shown in Fig. 10(b). We clearly recognize thelinearly moving reﬂection point; from the measured data, themoving speed was calculated to be 0.19 m=s, which agrees
well with the actual value of 0.2 m=s.
In conclusion, a simpliﬁed cost-eﬀective SOCF–OCDR
conﬁguration based on sinusoidal modulation was developedby excluding conventional optical heterodyne detection, andits basic performance was evaluated. By utilizing the foot of
the electrical Fresnel reﬂection spectrum, the electrical bandwidth required for signal processing was lowered to several
megahertz. One drawback is the reduced dynamic reﬂectivity
range. A high-speed reﬂectivity measurement (10 ms foracquiring one distribution) was also demonstrated by tracking
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