✪FURTHER READING

Real-time distributed strain sensing by BOCDR:
www.youtube.com/watch?v=vZ3SoIt7TWE

Optical reﬂectometry implemented using only one ﬁbre ampliﬁer for distributedstrain sensing➔
Page 754, ‘Low-cost
Brillouin opticalcorrelation-domain
reﬂectometry involving
merely one ﬁbre
ampliﬁer’, Y. Mizuno,G. Han, K. Noda,
H. Lee andK. Nakamura.

reﬂect on this✪Researchers from the Tokyo Institute of Technology,
Japan, have shown that in Brillouin opticalcorrelation-domain reflectometry (BOCDR),
implemented using three erbium-doped fibre
amplifiers (EDFAs), the one EDFA in the Stokes
path plays the greatest role in enhancing the signal tonoise ratio (SNR) of the measurement.

Structural healthcare
Age degradation and seismic damage to civil
infrastructure poses a serious problem for society.
One promising technology for monitoring thecondition of these structures is optical fibre sensing.
By embedding long optical fibres into a structure,strain and temperature distributions along the fibres
can be detected. Among the various types of opticalfibre sensors available, distributed strain andtemperature sensors based on Brillouin scattering
have received considerable attention due to theirhigh sensitivity and stability. One such sensor is thegroup’s advanced technology, called “BOCDR”,
which operates based on the correlation control ofcontinuous light waves.
Co-author Doctor Mizuno describes BOCDR: “Itis known to be an intrinsically one-end-accessdistributed sensing technique with high spatialresolution. This means that millimetre-long strains
can be detected by injecting light into only one endof a sensing fibre. Another advantage of BOCDR isits so-called random accessibility, with which we
can measure strain or temperature at any point (orpoints) along the sensing fibre at high speed. Inaddition, we have recently succeeded in increasing
its sampling rate to >100 kHz and demonstrated
one-end-access real-time distributed measurement
for the first time to the best of our knowledge.”

TOP: Ms. Heeyoung Lee (ﬁrst,from front), Mr. Kohei Noda(second), Asst. Prof. Yosuke
Mizuno (third), and Prof.
Kentaro Nakamura (fourth)measuring strain distribution
by BOCDR at Tokyo Institute ofTechnology. A relevant video isavailable online.
BOTTOM: Example of Brillouingain spectrum distribution
measured when strain waslocally applied.

Illuminating
The major drawbacks of conventional BOCDR
include its relatively high cost and large system size,
which limit the range of practical applications withreduced portability. In their Letter, to tackle thisissue, they work to minimise the number ofhigh-cost, large-scale optical amplifiers conventionally employed in the system. As a result ofcomprehensive evaluation, they showed that the useof merely one amplifier, which plays the mostsignificant role in enhancing the SNR of the system, issufficient to perform distributed strain measurement.
In addition to this, it was noted that the light source
does not need to be a high-power laser; acost-effective standard laser can be used. These
results are beneficial from the viewpoints of size andcost and will boost the practicality of BOCDR.
Real-time distributed Brillouin sensing has also
been demonstrated by Brillouin optical time-domain
analysis (BOTDA) and Brillouin opticalcorrelation-domain analysis (BOCDA). Althoughthese techniques have much higher SNRs, two lightbeams need to be injected into both ends of a sensing
fibre. Note that the two-end-access nature is notbeneficial to users, not only because the degree of
freedom in deploying the fibre is reduced but also
because the measurement can no longer becontinued if the fibre has one breakage point. In

720contrast, though the SNR is lower, BOCDR is moreuser-friendly with its one-end accessibility, which
can mitigate such shortcomings.
The team’s work has made it possible for the firsttime to achieve one-end-access real-time
distributed measurement at considerably low cost.
This proves the feasibility, and will lead to theeventual implementation, of practical distributedvibration sensing.

Just the one
BOCDR is a one-end-access system, which meansthat extremely weak spontaneous Brillouinscattering needs to be exploited, leading to a low
SNR in general. Note that two-end-access systems
allow the use of stimulated Brillouin scattering,which is much stronger. To improve the SNR inBOCDR, three or more optical amplifiers have beenused in conventional systems, resulting in relatively
large system size and high cost. In this work, by
carefully investigating the role of each amplifier,implementing BOCDR utilising only one amplifier
was achieved.
The proposed low-cost BOCDR can be used as ahealth monitoring technique for various structures,

ELECTRONICS LETTERS 27th June 2019 Vol.55 No.13

such as buildings, tunnels, bridges, dams, levees,
windmill blades, and aircraft wings. It also has
potential applications in robotics, acting as artificial“nerves” for detecting touch, distortion, andtemperature change. Therefore, this work will
impact research in many different topics ranging
from optical sensing to robotic, medical,
environmental, architectural, and civilengineering fields.

Future sensing
Other large-scale and/or high-cost devices used in
the system are an electrical spectrum analyser (ESA)
and a high-speed photodetector (PD), which are used
to observe Brillouin gain spectra for strain andtemperature measurement. To reduce the size andcost of the system further, the group are planning toreplace these devices with a voltage-controlled
oscillator, an electrical notch filter, and a low-speedPD. They are hopeful that the ESA-free configuration of BOCDR will also enhance the operating
speed of the system.
Doctor Mizuno outlines his expectations for thenear future: “We expect that distributed optical fibre
sensors will be practically used in society to provide
safe and secure environments all over the world. As
for technological progress, performance ofdistributed sensors, such as spatial resolution,measurement range, sampling rate, accuracy, andcost will be drastically improved beyond ouranticipation. Some physical parameters that have notbeen measured accurately in a distributed manner,including acidity, acoustic impedance, andradioactive concentration, will be possible.
Complete discrimination of multiple parameters, notonly strain and temperature but also many othercombinations, will also be feasible.”doi: 10.1049/el.2019.1950