HIGH PRECISION OPERATION OF FIBER BRAGG GRATING SENSOR WITH
INTENSITY-MODULATED LIGHT SOURCE
Nobuaki Takahashi, Hiroki Yokosuka, Kiyoyuki Inamoto and Satoshi Tanaka
Department of Communications Engineering, National Defense Academy
1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686 Japan
[email protected]
ABSTRACT
High precision operation of a fiber Bragg gratin (FBG) vibration sensor is examined for accurate measurement of signal phase
in noisy environment by using electro-optically (EO) modulated light source and demodulation technique with two approaches:
one with the lock-in detection technique and another with the envelope detection technique. The experimental demonstrations
are successful and definite signal waveforms are obtained in the sensor output in real time without performing average of the
signals. The advantage of the former approach is the ability of obtaining extremely definite signal waveforms while the latter
provides us with wider frequency range in the sensor operation. Even with the EO modulated light source in the sensor
system, shifting the source wavelength when the temperature change causes shift in the Bragg wavelength of the FBG
sensing element would thermally stabilize the sensor.
Introduction
An optical fiber sensor is characterized by high sensitivity, wide dynamic range, distributed sensing, immunity to
electromagnetic interference, resistivity to chemical corrosion, compact size, light weight and so on [1]. Because it can
operate in harsh environment such as high temperature [2,3] and high pressure [4], one can use it for in-well imaging and
monitoring applications [5] and expect to use it even in a down hole drilled into the earth mantle. Furthermore its small size
and light weight make it possible to be buried in a material and attached to a structure without affecting their nature for real
time health monitoring [6,7]. Among fiber optic sensors, a sensor using a fiber Bragg grating (FBG) as a sensing element is
especially expected to play an important role in various applications because of its easy operation, quasi-point sensing and
inherent wavelength-division-multiplicity.
Fundamental principle of FBG sensing is based on the Bragg wavelength shift induced by physical influence applied to the
FBG [8]. Interrogation methods of FBG sensor systems may be divided into two schemes: One is to determine the shift of the
Bragg wavelength by measuring the reflection peak wavelength of the FBG shined by broad spectrum light [6]. Another is to
measure change in intensity of the light either reflected from or transmitted through a FBG while illuminating the FBG by
narrow spectrum light [9]. In the latter scheme, the wavelength of the light is tuned to the slope of either reflectance or
transmittance spectrum curve of the FBG and then a FBG works as an optical intensity modulator. We adopt this method (the
intensity-modulation method) because of the advantages that the interrogation system is simple and the observation is rather
direct, namely, the output of the optical detection is directly proportional to the applied physical influence and one can observe
amplitude and phase of the signal in real time in the case of dynamic behavior measurement. Furthermore, the sensor can be
operated in either reflection or transmission mode though only the reflected light is made use of in the measurement of the
peak-wavelength shift.
Using the intensity-modulation method, clear signal waveforms are obtained, for example, in the underwater acoustic
measurement so that the direction of the sound source can be determined by measuring phase difference between two
observation points when a signal is sufficiently strong [10]. When a signal is not strong, however, the waveform of the signal
becomes ambiguous due to various kinds of noises and it is difficult to perform the phase detection of the signal. In the
present work, therefore, we propose a new method to clarify a signal of a FBG vibration and/or underwater acoustic sensor for
precise measurement of signal phase by reducing noises involved in the system. The method is based on the use of a laser
light source that is intensity-modulated at high frequency. Using two techniques, the demodulation of the signal is carried out,
depending on the range of the signal frequency. In a lower frequency range the lock-in detection is conducted while the
envelope detection of amplitude-modulated (AM) signal is examined in a higher frequency range.
An experimental
demonstration is carried out for the case of vibration sensing
Principle
When one interrogates a FBG sensor with the intensity-modulation method, as has been said above, narrow spectrum light is
launched into the FBG and its wavelength is tuned to the slope region of the FBG reflection spectrum curve, for example, in
the reflection mode. We here suppose that the intensity of the incident light is modulated at high frequency. If the intensity of
the light is given by
Iin(t) = I0(1 + mcosωmt),
(1)
where I0 is the average intensity, m is the modulation index and ωm is the angular frequency of the modulation, then the
intensity of the reflected light is given by
Ir(t) = R(λin)I0 (1 + mcosωmt),
(2)
where λin is the wavelength of the incident light, R(λ) is the reflection spectrum curve, i.e., reflectance of the FBG as a function
of optical wavelength and in turn R(λin) is the reflectance of the FBG at λin. Suppose the FBG is under influence of vibration,
for example, the FBG spectrum curve is shifted along the wavelength axis in synchronization of the vibration. When the
vibration-induced strain across the FBG is uniform and not large, the shift is in proportion to the vibration and the shape of the
FBG spectrum curve is kept unchanged, which is indeed the case in most of practical applications. When the wavelength shift
is small, we can then rewrite the above equation as
Ir(t) = (R0 = I0R0
- I0
!
"R
"#
"R
"#
$v% 0 sin &t )I0 (1 + mcosωmt)
$v% 0 sin &t
+ I0(R0 -
"R
"#
$v% 0 sin &t )mcosωmt,
(3)
! there is no vibration applied to the FBG, ηv is the Bragg wavelength shift rate per
where R0 is the reflectance of the FBG when
unit strain due to the vibration, ξ0 and ω are the amplitude and angular frequency of the strain [11]. The frequency of the
optical source modulation is chosen in such a way that it is to be far higher than that of the vibration of interest. It is thus seen
!
from Eq. (3) that the optical detection of the reflected
light gives us a dc component, ω frequency component and ωm frequency
component whose amplitude is amplitude-modulated by the strain. The third term is utilized to extract the vibration signal
from the detector output. One can use a lock-in amplifier to obtain the signal because the lock-in detection of the detector
output locked at the frequency ωm yields a signal proportional to the amplitude of the third term. Although this is powerful
method to extract the signal in a noisy environment, the operation frequency of the method is rather limited due to the
bandwidth of an available instrument. In a higher frequency range, therefore, we apply the demodulation technique for an AM
signal to the interrogation of the FBG signal.
.
Experiments and Results
A) In lower frequency
When the frequency of vibration is low, the lock-in detection technique is applied to extract the signal from the output of the
optical detector. Figure 1 shows the experimental setup for the proposed FBG vibration sensor. In this configuration then the
sensor is in the reflection mode. Figure 2 shows the reflection spectrum of the FBG used in the experiment along with the
spectrum of the optical source. The Bragg wavelength and bandwidth of the FBG are 1556.23 and 1.3 nm, respectively. The
output light from the DFB laser diode is electro-optically (EO) modulated. The function generator provides the driving signal
for the EO modulator and the reference signal for the lock-in detection. An optical isolator is here inserted for stabilization of
the optical source operation. After transmitting through an optical circulator, the light is launched into an FBG sensing
element that is under influence of mechanical vibration. A PZT mechanical vibrator is used in the experiment and induces
expansion and contraction in the FBG length. The vibration of the vibrator is monitored with a laser Doppler vibrometer.
Since we assume that there is no slip between the FBG and vibrator, i.e., the strain of the FBG is equal to that of the vibrator,
we can practically measure the dynamic strain of the FBG in real time. The length change and associated photoelastic effect
in the FBG shifts the Bragg wavelength of the FBG periodically. Since the perturbation is usually small, as said above, the
Bragg wavelength shift is proportional to the vibration amplitude applied to the FBG and the shape of the FBG reflection
spectrum curve is then kept unchanged. When the wavelength of the light is tuned to the slope of the FBG reflection
spectrum curve as seen in Fig. 2, the envelope of the EO modulated light intensity is modulated by the vibration-induced strain
on reflecting back from the FBG. The output of the detector therefore contains the vibration signal as an envelope of the highfrequency modulated signal.
Optical Circulator
FBG
DFB
Laser Diode
Optical
Isolator
Electro-Optic
Modulator
PZT
Vibrator
Function
Generator
Fiber
Optical
Detector
PZT
Driver
Electric Wire
Ref. Input
RF Lock-in
Amplifier
OUTPUT
Figure 1. Experimental Setup for FBG Vibration Sensor: Lock-in Detection Scheme
1.0
1.0
0.5
0.5
0.0
0.0
1558
1556
1557
㻃
wavelength
Figure 2. FBG Reflection Spectrum and Light Source
Figure 3 shows a typical waveform of the optical detector output when 110 Hz vibration is applied to the FBG. The amplitude
of the dynamic strain due to the vibration is 6.2 µε. Although one can see a sinusoidal waveform, the signal is noisy and we
suffer from ambiguity in determining phase of the signal. Figure 4 shows the output of the sensor when the EO modulated
light and lock-in detection technique are used with the other conditions kept identical to those in Fig. 3. The frequency of the
EO modulation is 100 kHz. We did not perform averaging for signal processing in observing the waveform. It is clear from
the figure that the signal waveform is far more definite and the noise associated with the signal is far less than that in Fig. 3.
Varying the frequency of the vibration while keeping the strain amplitude constant, the frequency characteristic of the sensor is
measured. The result is shown in Fig. 5. We may say that the proposed system can be used up to about 1 kHz. Since a
FBG is known to operate at even more than a few MHz [12], the frequency limit of the sensor in this configuration is caused by
the bandwidth of the RF lock-in amplifier used in the experiment.
Figure 3. Sensor Output Signal without Lock-In Detection
Figure 4. Sensor Output Signal with Lock-In Detection
Figure 5. Vibration Frequency Dependence of Sensor; Lock-in Detection Scheme
B) In higher frequency
From the above discussion, the FBG sensor with the lock-in detection technique suffers from some inaccuracy if the frequency
of the vibration is a few kHz and it can no longer operate at the frequency higher than several kHz. We then apply the
envelope detection technique that is commonly used for demodulating an amplitude-modulated (AM) signal. Figure 6 shows
the experimental setup for the proposed sensor.
The optical detector output is here sent to the process of the AM
demodulation instead of the lock-in detection. The detected signal is first filtered out by passing it through an intermediate
frequency filter. The frequency of the filter is 10.7 MHz. So is the frequency of the EO modulation. The signal is then
amplified by an intermediate frequency amplifier.
The output of the sensor is finally extracted from the envelope of the
amplified signal.
10.7 MHz
DFB
Laser Diode
Optical
Isolator
Electric Wire
Optical Circulator
Fiber
Electro-Optic
Modulator
Function
Generator
Fiber
PZT
Vibrator
Optical
Detector
10.7 MHz
Band-Pass Filter
Amplifier
Output
FBG
Envelope Detection
Figure 6. Experimental Setup for FBG Vibration Sensor: Envelope Detection Scheme
Figure 7. Sensor Output Signal without Envelope Detection
PZT
Driver
Figures 7 and 8 show typical experimental results without and with the proposed technique, respectively. The frequency of
the vibration is 5 kHz and strain amplitude is 25 µε. The signal is clear and its phase is definite when the envelope detection
technique is applied, although without the technique it is perturbed by noise and its phase is indefinite. The output of the
sensor is again measured as a function of vibration frequency. Figure 9 shows the vibration frequency dependence of the
sensor output. (The measurement was done up to 30 kHz because of the availability of the equipment in the laboratory.
Since we could not get a signal when the frequency is 50 kHz, the cut-off frequency is considered to be located between 30
and 50 kHz.) The upper limit of the operation frequency is extended and higher than that with the lock-in detection scheme by
more than an order.
Figure 8. Sensor Output Signal with Envelope Detection
Figure 9. Vibration Frequency Dependence of Sensor; Envelope Detection scheme
Discussion and Summary
Both the lock-in and envelope detection schemes are successfully demonstrated for precise operation of the FBG vibration
sensor. The former is better in obtaining more definite signal in the noisy environment. Because its operation frequency is
rather limited and can work up to about 1 kHz, however, it should be used for lower frequency application. The latter scheme
provides us with wider frequency range and much more flexibility in the choice of the operation frequency at lower cost. The
proper choice of an intermediate frequency filter specification would give us a proper frequency range for the sensor operation.
The wider frequency bandwidth of the filter would give rise to a sensor that operates at higher frequency. However, the output
waveform is not so definite as in the former scheme. It should then be used either for the case that the signal is relatively
large or for the case that a wider frequency range is required. It is also better when the cost effectiveness is important.
When the FBG sensor with the intensity-modulation method is to be used in temperature varying environment, we need to
stabilize the sensor thermally because the temperature change causes large change in the sensor sensitivity.
The dc
component of the optical detector output has the information about the operation point of the sensor on the FBG reflectance
spectrum curve and can be used to stabilize the sensor sensitivity. Namely, the sensor sensitivity would be kept unchanged if
the wavelength of the light source were shifted when the temperature varies so that the operation point on the curve should
stay at the same position [13].
All the analyses and experiments are done for the reflection mode of a FBG sensor. Because the similar analyses and
techniques hold also in the transmission mode of the sensor, however, it should also be possible to repeat the same
experiments in the transmission mode. The reflection mode provides us with a compact design because a lead fiber can be
used as a two-way road for light propagation, whereas in the transmission mode one needs less number of optical
components and the optical source power can also be made less since the loss involved in the system is smaller compared to
that in the reflection mode.
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