APPLICATION OF FIBER-OPTIC DOPPLER SENSOR TO ROCK STRUCTURES
Yuichi Machijima1, Seiji Kikuyama1, and Masahiro Tanaka1, Tsuyoshi Ishida2¹)

2)

LAZOC Inc.
Kyoto University

Abstract: Fiber Optic Doppler (FOD) sensor that can measure AE (Acoustic Emission) has been inventedand developed. FOD sensor is made by looping an optical fiber and it is attached on an object or embedded.
It measures a displacement rate (strain speed) as the frequency changes of laser light due to the Doppler
Effect. FOD sensor has the following advantages; (1) small diameter and small weight, (2) long-distancetransmission, (3) immunity from the electro-magnetic noise, (4) usability under high temperature, (5) widefrequency coverage to ultrasonic, (6) durability and anti-corrosive. In order to examine applicability of FOD
sensor to monitoring of rock structures, the authors compared the sensitivity and the frequency response of
FOD sensor to those of the conventional PZT sensor in laboratory experiments. The results indicated thatFOD sensor has strong possibility to be used for real rock structures.

1. INTRODUCTION
During and after excavation/construction ofunderground hydro-power stations, tunnels, dams
or rock slopes, rock engineers are monitoring
displacement and strain for safety and intelligent
construction management. The authors group havebeen engaged with such monitoring, using
Acoustic Emission (AE) technology which isdetecting rock fracture, as well as displacement
and strain measurement. In view of Japanese future
project, such as the “High Level Nuclear Waste”
into the depth of more than 500m, the sensor musthave longer-term reliability. The sensor must be
explosion-proof to monitor rock stability of thehuge underground storage of LPG (liquefied
petroleum gas), governed by the regulation.
Meanwhile, the fiber optic sensing technology instrain or temperature has been developed worldwide. Recently, the innovative fiber optical sensor
is invented and developed to detect very fast elastic
wave inside a solid 1) 2). Fiber optic is made of silica,
and it provides superior durability and corrosion
resistance in comparison with electric cable. The
sensor component without using electricity causes
few troubles like insulating failure, and it has
longer-term durability than electrical sensors. In
addition, as the fiber optic sensor does not needelectricity to be activated, it is essentiallyexplosion-proof. Therefore, the fiber optic sensor
will be utilized in environments where theelectrical PZT sensor could hardly be used.
Hereafter, this sensor is designated “Fiber
Optical Doppler (FOD) sensor”. FOD sensor can
detect the fine AE as the conventional PZT sensor
does. However, these two sensors’ measuring
principles are completely different. The PZT sensor
is a velocity sensor that the ceramic motion

corresponding to vibration is converted to theelectric voltage. On the other hand, FOD sensor isa velocity sensor that the Doppler effect causes thelaser frequency modulation, corresponding to fiber
optic’s elongating and shortening motion, i.e.
change in length of fiber optic in time domain.
FOD sensor has the following characteristics:(1)Small-size and light-weight with O.D.20-30mm(2)Long distance transmittable due to little
attenuation of laser signals through the fiber optic(3)Free from electromagnetic noise(4)Heat resistance to approx. 600 degrees
centigrade(5)Wide frequency coverage with a few Hz -1MHz(6)Durability and corrosion resistance
The authors carried out the experiment by rockspecimen, and made comparison of sensitivity andfrequency response between FOD sensor and PZTsensor in view of utilization at the future
underground construction projects. The result waspromising and here are reports:

2. PRINCIPLE
2.1 Summary of principle
A sensing region of the fiber optic is attached ona measured object as shown in Fig.1. When theobject vibrates with some elastic motion on surface,
the attached fiber optic elongates and shortens
simultaneously. If the light wave of frequency fo isemitted into a corresponding fiber optic region,change in length of fiber optic causes a laser
frequency modulation, because the number of lightwaves in that region is constant in a moment. In
other words, the frequency modulates by fd because
the light velocity is constant. This is called “Laser

Doppler effect” that the frequency of light wave
transmitted from other part is given “fo - fd”. The
frequency modulation fd is proportional to thevelocity of fiber optical length, i.e., thedisplacement rate of attached region.

– f d”. At detecting optical path, frequency f M (80MHz) is added by AOM (Acousto OpticalModulator) to create the frequency “f o + f M”. The
difference in frequency given as “f M + f d” isconverted into the voltage.

Figure 1.

Figure 2.

Sensing elements of FOD

3. FUNDAMENTAL TEST

2.2 Doppler effect in fiber opticFrequency modulation by Doppler effect isdescribed as formula (1). fd as modulatedfrequency, λ as light wavelength, and dL/dt as
displacement rate. The negative (-) indicates thatthe frequency modulation is opposite todisplacement rate in terms of phase analysis (+),(-).
fd = −

1 dL
λ dt

(1)

As shown in the formula above, the frequency
modulation fd and the displacement rate dL/dt areproportional. The frequency modulation fd isdetected using the heterodyne interferometry and isconverted into the voltage V with thefrequency/voltage converter. The relation between
the converted voltage V and the displacement ratedL/dt is indicated in formula (2) including theproportional constant K.
k dLdL=Kλ dtdt
kwhere, K = −V = kf d = −Laser Doppler Interferometer

3.1 Experimental method
3.1.1 Sensitivity test
In this test, the rectangular wave (pulse wave)
was added by PZT sensor with the pulse generator
through a rock of granite of 75 mm (H) x 25 mm(W) x 25 mm (L) as the transmitting media. BothFOD sensor and PZT sensor were compared inmaximum amplitudes of waveforms. As shown inFig. 3, pulsing PZT was glued to a side of a
specimen with instantaneous adhesive. And in thesame way, FOD sensor and PZT sensor forreceiving were glued to the opposite side. PZTsensors both for transmitting and receiving had 150
kHz resonance. FOD sensor was the 30-coiled,which is approx. 0.9 meter gauge long. The input
voltage was 15 V as the maximum. The received
waveform was not amplified at both sensors andwas recorded into the digital oscilloscope hard disk
with 100 kHz high-pass filter.

(2)

λ2.3 Method of detecting frequency
modulation
Fig. 2 shows the Laser Doppler Interferometry
system (LAZOC Inc.). The laser with frequency fo
is emitted and divided into the sensing optical pathand detecting optical path. Where measured object
has a certain displacement at sensing region
dynamically, the length of optical path changes
accordingly. Then the proportional frequency
modulation fd occurs to the laser light, which is “f oFigure 3.

Experimental Set-up3.1.2 Frequency response test
In this test, the range of frequency was between
50 kHz – 200 kHz in accordance with authors 1), 2)
worksite experience. There are several methods,
which are relative calibration method, soundpressure method, and contact method. Contact
method was chosen due to simplicity.
Contact method is that the sensing surfaces of
two sensors are attached to each other and that thesine-wave with the same amplitude is input withchange in frequency. The amplitude (sensitivity)for each frequency was recorded. Both PZT sensors
for pulsing and receiving were broadband type withfrequency 100 kHz – 1 MHz. FOD sensor was 30coiled.
PZT sensor was greased by silicon paste to eachother, while FOD sensor was attached to pulsing
PZT via an aluminum plate of 40 mm x 40 mm, 0.5mm thick, in order for FOD sensor to have a wider
contact with very narrow pulse point of PZT.

Figure 4

Sensitivity test result

Figure 5

FFT analysis

Figure 6

Signal to Noise ratio

3.2 Result
3.2.1 Sensitivity test
Fig. 4 shows the sensitivity test results; (a) input
pulse, (b) received FOD waveform, (c) received
PZT waveform. From the data (b) and (c), it wasknown that the maximum amplitude of bothsensors is ±0.05V and they have almost the samesensitivity. Also the duration of waveform wasalmost equal.
Fig. 5 shows the FFT (Fast Fourier Transform)
analysis of received waveform by both sensors.
According to the results of FOD sensor shown infigure (a), spectrum peak was clearly displayed at
140 kHz, 190 kHz and 280 kHz. Looking into
figure (b), PZT sensor had shown the similarfrequency characteristics. The reason of distinct
150 kHz was estimated due to PZT resonance.
Next, Fig. 6 shows the S/N (signal to noise) ratio.
This figure is an enlarged section of primary wave,
so that the background noise (N) can be comparedwith signal level (s) clearly. For simplicity, here
was defined S/N ratio as an amplitude value against
background noise. It was found that the S/N ratio
of FOD sensor was approx. 10, while that of PZT
sensor was approx. 80. FOD showed less S/N ratio
due to shorter fiber optic gauge length (0.9 meter atthis experiment).

3.2.2 Frequency response testFig. 7 shows the test results. This figure displays
the relation between pulse frequency and amplitude
of both sensors in the range of 50 kHz – 200 kHz,and the solid line and perforated line show FODsensor and PZT sensor respectively. Accordingly,
FOD sensor at frequency range higher than 70 kHz
has a similar frequency response. From the above,it is concluded that the frequency characteristics ofFOD sensor in 70 kHz – 200 kHz are almost equal
to those of the broadband PZT sensor.

Figure 7

The amplifying value of PZT sensor was 80 dB(preamplifier 40 dB, plus main amplifier 40 dB).
FOD sensor was amplified by only 20 dB. Thethreshold was set a little higher than the noise level.
It automatically transmitted only the waveforms,
which exceeded the threshold, into the AE logging
unit for both FOD sensor and PZT sensor. At thesame time of waveform recording, the time, ringdown counting, and maximum amplitude were also
recorded. Sampling interval of waveform,
recording duration and pre-trigger duration were
0.2 micro sec, 2,048 words and 512 words,respectively. After measurement, the frequency
analysis (Fast Fourier Transform) and AE
parameters were analyzed.

Frequency response

4. UNIAXIAL COMPRESSION TESTThe test to compare FOD sensor with PZT
sensor in the previous section enabled to makeclear, to some degree, the characteristics of FOD
sensor in the high frequency range. Thus, weproceeded to actual AE measurement generated byrock destruction at the uniaxial compression test ofthe granite.

4.1 Experimental method
A rock of granite of 75 mm (H) x 25 mm (W) x25 mm (L) was used as a specimen. The hydraulicservo type fatigue testing machine with maximum
capacity 100 kN was used. Loading was controlled
at constant distortion speed of 0.5 mm/min. Fig. 8
shows the measuring systems, having PZT block
flow and FOD block flow. The measuring systemfor PZT sensor consists of the preamplifier, signal
conditioner, AE waveform recorder and analyzer.
FOD system consists of the light source, photodetector and frequency/voltage converter (FVconverter). Both systems are connected to AE
logging unit and analyzer. Three broadband PZTsensors (100 kHz – 1 MHz) were used inconsideration of analyzing the frequency ofwaveform and were glued to the upper part, middle
part and lower part of side face of a specimen. OneFOD sensor was glued, next to the PZT type sensor,
to the center of side face of a specimen. AE data
was compared between PZT sensor and FOD
sensor that were at the center of side face.

Figure 8Measuring set-up

4.2 Result
Fig. 9 shows the AE event (counts/sec) and its
frequency (kHz) caused by destruction of aspecimen of granite. (a) and (b) are FOD data andPZT data respectively. The frequency analysis wascalculated from the measured waveform and thecenter gravity was determined from the spectrum
form with frequency in horizontal axis andspectrum power in vertical axis. Small circle in Fig.
9 shows the averaged AE frequency over 10
waveforms along with the elapsed time. First,FOD sensor in Fig. 9 (a) indicates that thespecimen was broken at approx. 130 MPa after
1,300 seconds from the beginning of loading. The
generation of AE started rapidly at 96% (125 MPa)of destruction load and the maximum AE eventswas 200 counts/sec for 1,300 seconds. The
frequency of AE was in the range of 200 kHz to500 kHz. Large circle is the frequency averaged
over 50 waveforms from 10 small circled
frequency data. The frequency becomes lower as itis nearing toward the destruction of a specimen.

Meanwhile, PZT sensor in Fig. 9 (b) indicates
AE event in the same tendency as FOD sensor, but
the maximum AE events was recorded 130
counts/sec. The main frequency range was 200 kHz
to 500 kHz as well as FOD sensor. The frequency,
which is averaged over 100 waveforms that 10events enclosed with a small circle, 2 times of FOD
sensor, are calculated on the average in order,
displays the tendency to lower as it is nearing along
toward the destruction.
As above, the AE characteristics measured byboth sensors indicate almost the same tendency, sothat it is concluded that FOD sensor has the samemechanical performance as PZT sensor in such an
AE measurement.

Figure 9Compression test data

5. CONCLUSIONSIn order to investigate the application of FOD
sensor to the rock engineering, comparison test wascarried out against the conventional PZT-AEsensor. From the experiment, we confirm that FOD
sensor has the same performance as PZT-AE
sensor in terms of sensitivity and frequency
response. We shall plan to apply this innovative
sensor to the filed application where PZT sensor
cannot be used and where explosion-proof, deepand remote, long-term durable requirement isobserved.

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