Olivero, Massimo

Perrone, Guido

Tosi, D.

Vallan, Alberto

English

PORTO Publications Open Repository TOrino

 DGaO Proceedings 2009 – http://www.dgao-proceedings.de – ISSN: 1614-8436  
Improved Fibre Bragg Grating Interrogation for Dynamic Strain Measurement 
Daniele Tosi, Massimo Olivero, Guido Perrone, Alberto Vallan 
Department of Electronics, Politecnico di Torino 
mailto:daniele.tosi@polito.it 
A cost-effective fiber Bragg grating sensor capable of measuring 
small vibrations also at high frequencies, with an accuracy of few 
nanostrains, is presented.  Low cost and high sensitivity are 
obtained using an interrogation scheme based on intensity 
modulation and signal processing to reduce of the impact of noise. 
1 Introduction 
Fiber Bragg gratings (FBGs) are the most 
popular technology for the measurement 
of deformations, strains and vibrations 
using photonics [1]. For instance, FBGs 
are typically employed in monitoring large 
civil infrastructures or historical buildings, 
but applications can be found also in bio-
medical devices and acoustics. In spite of 
this, the expensiveness intrinsic in many 
FBG interrogation techniques severely 
limits the range of potential application of 
such optical sensors. For this reason, 
among the several alternatives resumed in 
Fig. 1, we focused on low-cost ap-
proaches, based on intensity modulation 
and simple power detection, to open up to 
a broader range of applications in the field 
of structural engineering. 
 
Fig. 1 The market of fiber Bragg grating sensor: accu-
racy vs cost for the main sensing techniques, and the 
effect of signal processing. 
However, intensity based methods usually 
have a poor resolution, but this limitation 
can be overcome using suitable signal 
processing (SP) techniques, which allow a 
dramatic increase of performances, im-
proving the sensitivity of about two orders 
of magnitude, with just minimum additional 
costs due to processing units [2,3]. The 
combination of the two techniques pushes 
this kind of sensor up to the same per-
formance level of more expensive alterna-
tive configuration, making it a really com-
petitive solution.  
 
Fig. 2 Layout of the FBG interrogation system. 
2 The optical system 
The optical system has an extremely sim-
ple configuration, as sketched in Fig. 2. It 
uses a fixed-wavelength laser driven by a 
thermo-electric controller; then the laser is 
connected to the FBG transducers 
through a highly isolating stage. When the 
FBG is strained, the output power 
changes as a result of the induced wave-
length shift. The receiver is made by a 
photodiode, a conditioning circuit and a 
data acquisition card. Multiple FBG inter-
rogation is possible by splitting the laser 
 DGaO Proceedings 2009 – http://www.dgao-proceedings.de – ISSN: 1614-8436  
source into multiple channels, all connect-
ing to wavelength-matched FBGs and 
terminating into a receiving circuit. 
 
Fig. 3 Layout of the FBG interrogation system. 
3 Signal processing 
With the ”pure” optical system, perform-
ances are limited by internal noise, due to 
the backreflection from FBGs into the la-
ser cavity. Using adaptive filters, it is pos-
sible to reduce the internal noise without 
having prior knowledge of its spectral 
properties. By using a RLS filter trained in 
static conditions, we can improve the sig-
nal-to-noise ratio (SNR) by 14 dB. 
We can have an additional gain by using 
an efficient spectral estimator. Using the 
Capon estimator as in [5], we obtained 
correct detection with SNR as low as -25 
dB, corresponding to a net gain of 15 dB 
with respect to standard estimators such 
as FFT. 
The result is that SP produces an out-
standing increase of 29 dB in terms of 
SNR; with this value, the minimum oscilla-
tion amplitude that can be correctly proc-
essed in the frequency domain is 5.6 nε. 
4 Experimental results 
The proposed sensor has been used to 
measure vibrations in some simple struc-
tures as well as for the detection of acous-
tic waves, in order to validate perform-
ances for a broad spectrum of applica-
tions. Fig. 3 reports some results, ob-
tained comparing the FBG sensor with a 
MEMS for the measurement of resonance 
frequency of a sample structure, and reg-
istering voice from a recorded speech. 
 
 
Fig. 4 Experimental results: (a) measurement of the 
resonance frequency of an aluminum bar, comparing the 
FBG sensor with a MEMS; (b) spectral measurement of 
recorded speech. 
5 Conclusions 
Performances of the sensing system, and 
the effect of SP, are resumed in Table 1. 
 
Performances Optical Optical + SP 
Minimum SNR -10 dB -39 dB 
Accuracy 800 nε 5.6 nε 
Working range 60 µε 60 µε 
Resolution << 1 nε << 1 nε 
Tab. 1 Performances of the optical system, with and 
without the application of signal processing. 
References 
[1] Y. J. Rao: „In-fibre Bragg grating sensors“ in Meas. 
Sci. Technol., 8: 355-75 (1997). 
[2] D. Tosi, M. Olivero, G. Perrone, „Low-cost fiber 
Bragg grating vibroacoustic sensor for voice and 
heartbeat detection“ in Appl. Opt.,47: 5123-29 
(2008). 
[3] D. Tosi, M. Olivero, G. Perrone, „Dynamic strain 
measurement system with fiber Bragg gratings and 
noise mitigation techniques“ in Meas. Sci. Technol., 
20: (2009). 
[4] S. Haykin: Adaptive Filters Theory – 4th edition, 
Prentice Hall, 2001). 
[5] T. Ekman, A. Jakobsson, P. Stoica, „On efficient 
implementation of the Capon algorithm“ in Proc. 
European Sign. Proc. Conference (2000). 
(a) 
(b) 


Adaptive Filters Theory

Dynamic strain measurement system with fiber Bragg gratings and noise mitigation techniques“ in Meas.

In-fibre Bragg grating sensors“ in Meas.

Low-cost fiber Bragg grating vibroacoustic sensor for voice and heartbeat detection“ in Appl. Opt.,47:

On efficient implementation of the Capon algorithm“ in

Improved Fibre Bragg Grating Interrogation for Dynamic Strain Measurement

Improved Fibre Bragg Grating Interrogation for Dynamic Strain Measurement

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