Laser-interrogated optical-fiber mechanical
and chemical sensors
G. Gagliardi, M. Salza, P. Ferraro, P. De Natale
CNR - Istituto Nazionale di Ottica Applicata (INOA), sezione di Napoli
Comprensorio “A. Olivetti”, via Campi Flegrei 34, 80078 Pozzuoli (NA)
Different systems using fiber-optic structures as chemical and mechanical sensors are shown. The interrogation approach is based on laser-frequency modulation techniques with
single fiber Bragg-gratings (FBGs), or resonators made from high-reflectivity FBGs, and fiber rings. In the case of fiber resonators the laser is frequency locked to the cavity
resonance by an in-fiber Pound-Drever-Hall (PDH) scheme. For strain detection in static and dynamic regime, the locking-loop error and correction signals fed back to the laser
both serve as strain monitor with sub-picostrain sensitivity. On the other hand, the resonator’s fiber optical field may be evanescently coupled to the external environment,
providing information on the external medium refractive index and/or concentration. Minimally-invasive, selective chemical sensing can be thus carried out on liquid and gas species
absorbing in the laser’s wavelength range.
FBG principle
Two different systems devoted to chemical sensing in liquids are presented. A first setBragg max reflectivity: λB = 2n eff L
Here, we show our experimental work based on a frequency-modulation spectroscopy
scheme. The idea is to superimpose sidebands to the laser carrier to obtain three
different frequencies interrogating an FBG structure. Using this method, both static
and dynamic strain measurements can be performed, with a sensitivity of the order
of 150 n/√Hz in the quasi-static domain (2Hz) and 1.6 n/√Hz at higher frequencies
(1 kHz). A further improvement of the first apparatus relies on resonant in-fiber
optical resonators, such as -shifted FBGs or FBG Fabry-Pérot. The basic principle is to
interrogate it by a frequency-locked laser using Pound-Drever-Hall (PDH) technique
(at 2-10 MHz). The reflected signal from the cavity is demodulated by a mixer that
gives a dispersive-like signal containing information about the strain. The idea is that
the laser follows any shift of the resonance eventually caused by changes of the
intra-cavity optical pathlength. Test strain signals can be applied to the fiber in a
controlled manner via a piezo actuator. Similarly, acceleration measurements can be
performed by a proper mechanical mount.
up exploits the sensitivity enhancement of a whispering-gallery microsphere resonator
made from the tip of a common silica fiber. Another one relies on evanescent-wave
spectroscopy in a fiber-loop resonator. The detection principle is to probe the
”resonant” absorption due to an external medium that induces extra losses in the
cavities. This can be accomplished measuring the change in the intrinsic cavity-decay
time by either phase-shift ring-down spectroscopy (PS CRDS) or direct mode-locked
cavity-enhanced spectroscopy (CEAS).
FBG
input signal
transmitted signal
reflected signal
I
I
I
l
l
l
l
l
B
B
Whispering-gallery-mode spherical resonator sensor
-shifted FBG
7
1550.00 1550.02 1550.04 1550.06 1550.08 1550.10
4
3
Intensity /arb. units
-shifted-FBG 3D optical accelerometer
Transmission and backscattered spectra of a silica microsphere
5
2
1
0
-1
0
20
40
60
80
100
Laser frequency scan (GHz)
Strain-to-acceleration
6( L  x)m
 ( x) 
a
2
bd E
3-axis simultaneous acceleration
2
4
0
3
-2
2
c
-4
a
1
0
Phase-shift spectra
From Rayleigh scattering
800 MHz
PZT
FBG
FBG
0,025
DBM
0
1000
2000
3000
4000
5000
0,015
PD
0,010
Splitter
   tan (2 )
 2
2 neff L
1

c
2ln    L
1
Cavity transmission (arb. un.)
0,020
PD
1550.00 1550.02 1550.04 1550.06 1550.08 1550.10
0,005
Servo
Phase
Intensity
AM at 500 kHz
0
Overlap of the FBG reflection bandwidths
PM 3-dB
coupler
b
300-micron Silica sphere
Cavity-based strain sensor
PR
(B)
Wavelength /nm
Lock-in phase shift (degree)
Cantilever’s
excitation
-6
0
0,0
-1
-1
-0,5
FWHM = 900 kHz
-1,0
-2
-2
-1,5
168
172
176
180
-3
-3
-4
-4
-50
0
50
100
150
200
250
300
Intensity (V)
Sensor’s performance:
•3D sensing
•1.5-kHz bandwidth
•20 µg/Hz sensitivity
•100 g dynamic range
Phase 
/ deg
Shift,
Transmission signal (V)
6
350
Laser frequency scan (MHz)
0,000
20000
40000
60000
Evanescent-wave ring-down spectroscopy of ETDA
from the sphere’s backscattered field:
80000
Frequency detuning (MHz)
0.00
150
125
0,08
100
0,06
75
0,04
50
0,02
25
0,00
0
0
5
10
15
-0.10
a)
0,8
0,6
0,4
-0.15
-0.20
0,2
-0.25
0,0
-0.30
-0,4
0
-0,6
-0,8
-3
-2
-1
0
1
2
1
2
3
4
1/2
Strain noise ( rms/Hz )
1/2
6
1E-9
1/2
1E-10
7
Angular Frequency,  / rad s-1
8
-25
1E-10
20
25
Var coupler
Diode laser
1E-11
±
EOM
min< 1 p/Hz
1/2
POL
l/2
l/4
0
30
500
50 % PM
IN1
BVA+Rb clock-GPS
IN2
100 MHz
PC
BEAT & LOCK
UNIT
S
Fast MOD
D
D.A. PLL
30 MHz ± ()
Distribution
100
150
200
Laser frequency scan (MHz)
0,025
0,020
0,020
0,015
0,015
0,010
0,010
0,005
0,005
0,000
0,000
0,0
0,1
0,2
0,3
0,4
Time (sec)
Applications:
•Real-time analysis of biological compounds
•Environmental monitoring
9.4 MHz
Related publications
G
G. Gagliardi, M. Salza, P. Ferraro, P. De Natale, "Fiber Bragg-grating strain
sensor interrogation using laser radio-frequency modulation," Opt. Express 13,
2377-2384 (2005)
to
counter
J. Barnes, B. Carver, J. Fraser, G. Gagliardi, H.-P. Loock, Z. Tian, M. Wilson,
S.Yam, O. Yastrubshak, “Loss determination in microsphere resonators by phaseshift cavity ring-down measurements”, Opt. Express 16, 13158-13167 (2008).
10 MHz
Osc/Analyser
STRAIN
MONITOR
Servo
IF
9.4 MHz
Acoustic shield
NOISE EATER
T-controlled fibre cavity
PC
AOM
+ -
Vref
10 %
K. Bescherer, S. Dias, G. Gagliardi, H.P. Loock, N.R. Trefiak, H. Waechter, S. Yam,
“Measurement of Multi-Exponential Decays by Phase-Shift Cavity Ring-Down”,
Appl. Phys. B 96, 390-397 (2009)
SENSOR UNIT
MOD IN
Applications:
•Seismic monitoring
•Gravimetry
•Giroscopic sensing
10-MHz
reference
50
PD
PC
EOM
192.1
THz
Er-fibre fs-laser
MenloSystems FC1500
Laser & Comb
…towards the 10-14 level
Rep. Rate
&
CEO
control
Cavity’s transmission decrease
and shift due to
evanescent-wave absorption
by liquid EDA
Comparison of Resonator signals with and without Diamine0.5%
and Glycerin+D2O solution 99.5%, 10th July 2008
Resonator signal with Diamine0.5% (volts)
Optical-frequency comb
for laser’s noise reduction
0,025
OPTICAL FREQUENCY COMB UNIT
0,04
0
Frequency (Hz)
PZT
5
A 1560-nm diode laser is locked on the
fiber ring resonance:
0,00
The transmission spectrum contains information
on the RI and loss change caused by the sample (EDA)
1000 1500 2000 2500 3000 3500
Frequency (Hz)
1560-nm ECDL
Wavelength Shift, l/ pm
0
0,08
Resonator reference signal for Diamine. (volts)
15
-5
loop
Acoustic sensing
Isolato
r
10
-10
sample
1E-11
5
-15
Fiber-loop evanescent-wave sensor
1E-12
0
-20
3
1E-9
min< 50 p/Hz
5
Frequency detuning (MHz)
Quasi-static sensing
-0.5
-0.35
-4
1E-8
0.0
-1.0
-0,2
Time (s)
Strain noise ( rms/Hz )
EDA = 1.5 1015 cm-2
0.5
Cavity signals (V)
0,10
Correction signal (n)
PZT signal (V)
b)
Frequency locking
1,0
tan-
(0)
0,12
Cavity tranmitted and reflected signals (V)
-0.05
Phase angle / arb units
0
Piezo
driver
Circ
PZT
F300
G. Gagliardi, M. Salza, P. Ferraro, P. De Natale, “Interrogation of FBG-based strain
sensors by means of laser radio-frequency modulation techniques”
J. Opt. A: Pure Appl. Opt. 8 (2006) S507–S513
G. Gagliardi, P. Ferraro, S. De Nicola, P. De Natale “Interrogation of fiber Bragg-grating
resonators by polarization-spectroscopy laser-frequency locking”, Opt. Express 15, 37153728 (2007).
G. Gagliardi, M. Salza, P. Ferraro, P. De Natale, A. Di Maio, S. Carlino , G. De Natale and E.
Boschi, “Design and test of a laser-based optical-fiber Bragg-grating accelerometer for
seismic applications”, Meas. Sci. Technol. 19 (2008) 085306
Patents
IT patent on “fiber cavity laser locking”. RM2006A000279. Inventors: G. Gagliardi, P. Ferraro, S. De Nicola, P. De Natale (2007).
IT patent on “Fiber-optic seismic sensor”. RM2007A000589. Inventors (CNR-INOA): G. Gagliardi, M. Salza, A. Di Maio, P. Ferraro, P. De Natale. (2007)
EU patent on “fiber cavity laser locking”. PCT/IT2007/000365. Inventors: G. Gagliardi, P. Ferraro, S. De Nicola, P. De Natale (2008).
International collaborations
-Department of Chemistry, Queen's University, Kingston, ON (Canada)
-Centre for Photonics and Optical Engineering, Optical Sensors Group, Cranfield
University, Bedford (UK)
-Fibre Optics Laboratory, Central Glass & Ceramic Research Institute (CSIR),
Calcutta (India)
-Centre for Gravitational Wave, Australian National University, Canberra (AU)